Multistage organic redox systems. Aromaticity as a guiding principle for materials chemistry

Abstract

Organic π-conjugated molecules capable of undergoing multistage redox processes have emerged as versatile platforms for applications in energy storage, molecular electronics, and optical devices. This review highlights the critical role of aromaticity in shaping the redox behavior of such systems. Structural factors including topology of the ring system, heteroatom incorporation, and conjugation pathways are examined with respect to their influence on redox ranges, stability, and properties of oxidation levels. The discussion encompasses monomeric aromatic systems and their linked and fused covalent assemblies with linear, branched, and cyclic topologies. The survey provides a unifying perspective on multiredox design principles, offering insights for future development of functional organic redox materials.

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Marcin Stępień (left), Ishfaq A. Bhat (top), Ankit Kumar Gaur (right), Natasza Sprutta (center)

Marcin Stępień (PhD 2003, habilitation 2010, titular professorship 2017) works at the Faculty of Chemistry, University of Wrocław (UWr), Poland. His team studies the synthesis, properties, and theory of π-conjugated systems, including nanographene analogues, curved aromatics, and open-shell organics, with a focus on aromaticity-driven functions. In 2023, his contributions to the field were recognized with the Foundation for Polish Science Prize. Ankit Kumar Gaur (PhD 2023 IISER Mohali) and Ishfaq A. Bhat (PhD 2023, Hyderabad) have joined Marcin's research group at UWr as NCN postdoctoral fellows to develop next-generation multiredox macrocycles. Natasza Sprutta (PhD 2001) teaches organic chemistry at UWr; conducting research on porphyrin analogues and azulene-containing aromatics.

1. Introduction

The concept of the oxidation state can be traced back to Friedrich Wöhler's 1835 textbook Unorganische Chemie,^1,2 in which he used the term Oxydationsstufe (“oxidation grade”) to denote the stoichiometric ratios found in different oxides of the same element. While the existence of multiple oxidation states in inorganic compounds was recognized early on, the development of organic redox chemistry progressed more slowly. This delay was likely due to the structural complexity of organic compounds and the broader range of redox processes they can undergo. Over the course of the XIX century, it was appreciated that oxidations and reductions were associated with changes in the amount of either oxygen or hydrogen in an organic compound. However, although the first redox processes in which electron transfer was not tied to atom transfer were discovered around the same time, their full understanding was only achieved in the 20th century.³ For instance, Wurster's blue, containing the radical cation [S1.1]˙⁺ (Scheme 1), was discovered in 1879,⁴ but its identity as a discrete oxidation state of S1.1 was only confirmed in 1926.^5–7

Scheme 1 Relationship between oxidation level and aromaticity in Weitz- and Wurster-type multistage redox systems according to Deuchert and Hünig's classification.⁷ Clar sextets are indicated in light red. The semiquinone [S1.1]˙⁺ is Wurster's blue. In the schemes and figures of this review, fully localized structures are typically shown for charged and radical states, to accurately represent the electron counts in the π systems.

In organic chemistry, systems comprising π bonds are the most effective electron donors and acceptors, and even relatively small π-conjugated molecules can exist in multiple redox states. However, the thermodynamic accessibility and properties of these states depend on the size, topology, and geometrical distortion of the π system, as well as on the inclusion of heteroatom dopants and peripheral substitution. All these structural features directly affect the key characteristics that define multiredox behavior, i.e., the HOMO–LUMO gap, redox potentials, and spectral properties of individual redox levels. These relationships are relevant to diverse classes of compounds and various fields of application, which are seldom discussed from a unified perspective. The objective of this review is to explore strategies developed across these diverse research areas to create multiredox π-conjugated molecules. We aim to provide a broader look at the relationship between multiredox behavior and π-conjugation with a particular focus on aromaticity, which can change dramatically upon either oxidation or reduction.

Our title is a reference to Deuchert and Hünig's classic review,⁷ which summarized the state of the field in the late 1970s, and formulated general structural principles underlying many of the then-known multiredox designs. Since then, the area has expanded considerably, driven by advances in synthetic organic chemistry and by growing interest in materials science. This progress is evident from some of the recently published reviews covering selected aspects of multiredox organics,^8–11 and their application in such fields as energy storage and conversion,^12–17 supramolecular chemistry,^9,18 near-infrared absorbers,¹¹ superconductivity,¹⁹ and electrochromism.²⁰ The aim of this review is not to compile an exhaustive bibliography, but rather to provide a comprehensive overview of recent developments, emphasizing the diversity of molecular structures, properties, and applications. To that end, we focus primarily on literature from the past two decades, with occasional references to earlier key contributions for context.

We will begin by discussing general principles underlying the design of multistage redox systems. After that, we will take a detailed look at the most prominent classes of multiredox organic compounds. We will start with monocyclic aromatics, which not only provide a simple illustration of principles underlying multiredox behavior but also serve as structural components of larger assemblies. Different types of connectivity between redox-active building blocks, i.e., linear, two-dimensional, and cyclic, will be discussed in subsequent chapters. Because of our focus on aromaticity, the scope of the review will be limited to systems in which the multiredox behavior emerges in a contiguously conjugated π system. Covalent and non-covalent assemblies of multiple π-conjugated units, have been reviewed elsewhere,^9,17,18 and will generally be excluded from this review, except for some recent examples of hyperconjugatively coupled systems.

In the context of this review, a multistage redox (multiredox) system is a species that has at least three accessible oxidation levels. We will avoid the ambiguous term “multivalent”, which is occasionally used to describe multiredox organics^21,22 but can also refer to e.g. Mⁿ⁺ cations (n > 1) in metal-ion batteries,¹⁶ or to supramolecular interactions.²³ The terms redox state, oxidation state, and oxidation level will be used interchangeably, and will be identified with the charge of the species, provided that the composition of the system does not change upon reduction or oxidation. The latter rule may not hold when proton transfer or metal coordination occurs to (partially) compensate the change of total charge caused by the redox process, as often seen e.g. in porphyrin analogues (see section 5).

The redox range of an organic system encompasses all its accessible oxidation states. In the graphics illustrating this review, redox ranges are highlighted in rounded boxes accompanying molecular structures. The accurate determination of the redox range of an organic system is complicated by the different ways in which the existence and stability of an oxidation state can be determined. Ideally, all oxidation levels should be isolable as pure compounds; however, such a requirement would be overly restrictive given the experimental challenges associated with the isolation of highly charged organic ions. The redox ranges given in this review attempt to include all states whose stability is sufficiently documented by the reported chemical and electrochemical experiments performed in solution. It should be noted, however, that these two experimental approaches often produce somewhat different redox ranges, and in some cases, the different states may not be stable under a single set of conditions. Larger ranges are often observed electrochemically, however, the reversibility of the outer oxidations and reductions is not always clearly determined in the original papers. It can also happen that oxidation states that do not seem to be accessible via reversible electrochemical events, have been obtained using chemical means. If not indicated otherwise, all potentials are referenced to the ferrocene/ferrocenium couple (Fc/Fc⁺).

The multistage redox behavior of organic molecules most often relies on the ability of a π-electron system to act as a reservoir for electrons or holes. While the resulting redox ranges are occasionally spectacular, π-conjugated molecules usually show a strong preference for a single oxidation level, which is typically characterized by the neutral charge, closed-shell configuration, and aromaticity. The inability of a system to simultaneously satisfy all three requirements often leads to redox amphoterism, e.g. when the system can switch between a diradicaloid neutral state and an aromatic yet charged state. For larger systems, usually containing multiple rings, heteroatoms, labile hydrogens, multiple oxidation levels can be observed, which derive their stability from extensive charge and spin delocalization, favorable substituent effects, and changes in their aromatic character. The oxidation levels may differ only in the number of electrons, but in some cases, especially in porphyrinoids, the changes of total charge may be partly or completely compensated by protonation or deprotonation. Usually, the geometry and topology of the π system is insignificantly affected by the redox state, although exceptions are known. Planarization of the cyclooctatetraene dianion is a classic example; in more complex cases, recently termed dynamic redox (DYREX) systems,²⁴ a change of the oxidation state can induce large torsional changes,^24–26 or rearrangements of the ring system.^27–29

Given their great diversity, it is hardly possible to provide a general an unambiguous structural classification of multiredox organic molecules. A simple, yet useful typology differentiates between Wurster- and Weitz-type multiredox organics.⁷ Wurster systems, named after Wurster's blue [S1.1]˙⁺, feature charge-carrying sites (“end groups”) outside the ring system, and are aromatic in the neutral state. Inverse Wurster systems (e.g. S1.2) also have exocyclic redox-active groups, but they gain aromaticity in the oxidized state. In contrast, Weitz systems contain endocyclic end groups and are quinoidal in the reduced state, as exemplified by viologen S1.3. S1.4 is an example of an inverse Weitz system which becomes quinoidal upon oxidation.

The classification of multiredox organics used in this review focuses on the subunit-level connectivity of the π system. Indeed, the majority of systems discussed herein can be interpreted as covalent assemblies of smaller π-conjugated units, often monocyclic, each with at least two accessible redox states. The subunits can be linked into linear, two-dimensional (branched, radial), or macrocyclic assemblies (Fig. 1). The coupling between subunits depends on the linker type (bold red lines), which may be a direct bond, consist of one or more atoms (e.g. methine or vinylene bridges). A similar classification can be used for some fused ring systems, for which, however, delineation of subunits may be somewhat arbitrary.

Fig. 1 Topology-based classification of multiredox π-conjugated molecules used in this review. Shaded rings represent π-conjugated subunits with arbitrary internal topology (number and size of fused rings), and heteroatom content. Linking bonds and fused ring edges are indicated in red and green, respectively.

The multiredox behavior critically depends on the interaction strength between subunits: in very weakly coupled systems, all subunits will be reduced or oxidized in unison. Conversely, in the limit of very strong coupling, achieved, e.g., by cross-conjugated linkers such as meso bridges in porphyrins, the number of accessible oxidation states is usually limited, because of the greater separation of MO levels. In such a case, the π system starts behaving as a single “larger subunit”. The multiredox behavior is however difficult to predict in general, because the interaction strength depends not only on the connectivity between the subunits but also on the oxidation state of the system. Furthermore, while the neutral state (and some of the limiting oxidized or reduced states) often feature closed-shell configurations and large energy gaps, the electronic structure of intermediate redox levels is often additionally complicated by their mixed-valence character.^30,31

2. π-Conjugated monomers

Even though small aromatic molecules typically show a limited number of oxidation states, their redox chemistry is of considerable fundamental and practical importance. Small monocyclic aromatics serve as models for the behavior of more complex systems. Additionally, given their low molecular weights and ease of preparation, small molecules are suitable for charge-storage¹³ and electrochromic applications.²⁰ Redox chemistry of monocyclic π-conjugated systems typically evolves from the 6-electron aromaticity found in benzene, cyclopentadienyl anion, and tropylium cation, with the stability of oxidized and reduced states being affected by their open-shell character or antiaromaticity.^32,33 In this section we highlight some recent cases of multiredox behavior observed in simple, often monocyclic π-conjugated structures.

Benzene and its simple derivatives are inherently difficult to reduce or oxidize because of their aromaticity and large energy gaps. In sandwich complexes of d-electron metals, ηⁿ-coordinated benzene rings are typically considered to be neutral ligands, while s-block metals such as magnesium can produce fairly localized bonding that disrupts π-conjugation.³⁴ Highly charged benzene anions can, however, be stabilized by the more electropositive lanthanide cations. The dinuclear lanthanum(II) complex [2.1]⁻, reported by Lappert et al., contained a formally monoanionic benzene ring (Fig. 2), providing an early example of such stabilization.³⁵ Benzene dianion complexes can be stabilized by both mono- and dinuclear complexes of various Ln^II/III ions, such as [2.2]⁻ (ref. 36 and 37) and [2.3]²⁻,³⁸ respectively. By judicious choice of ligands and metal ions, it was possible to obtain further inverse-sandwich complexes containing benzene tetraanion [C₆H₆]⁴⁻ and related arenes as bridging ligands.^39–43 Dithorium(IV) species such as 2.4 (Fig. 2) are chargeless and act as four-electron reductants toward reactive arenes such as anthracene. These species provide unique experimental insight into the properties of the benzene tetraanion, which as a 10π-electron system, is predicted to be Hückel-aromatic. In contrast, The 8 π-electron benzene dianion [C₆H₆]²⁻ is expected to be Hückel-antiaromatic in its singlet state and Baird-aromatic in the triplet state. The two multiplicities are inherently close in energy and the ground state of the dianion is apparently controlled by the nature of the interacting cations and ancillary ligands. For instance, the Y^III-containing dianion [S2.2-Y]²⁻ (Scheme 2), reported by Long et al.,⁴⁴ was found to be a ground-state singlet with a very small ΔE_ST = 0.023(4) kcal mol⁻¹. In comparison, the gadolinium analogue [S2.2-Gd]²⁻ exhibited an S = 6 ground state, which was interpreted as originating from antiferromagnetic coupling between the two Gd³⁺ ions (S = 7/2) and ³[C₆H₆]²⁻ (S = 1). In contrast, the neutral complex S2.4-Y (Scheme 2), reported recently by Delano IV and Demir,⁴⁵ contains a well defined singlet ¹[C₆H₆]²⁻, displaying quinone-like distortion and paratropicity.

Fig. 2 Sandwich complexes of benzene anions. Countercations are not shown for clarity.

Scheme 2 Synthesis of dinuclear lanthanide complexes stabilizing a benzene dianion ring. Reagents and conditions: (a)⁴⁴ 1. M(CH₂SiMe₃)₃(THF)₂; 2. n KC₈, n 18-crown-6; (b)⁴⁵ 20 equiv. KC₈, C₆H₆.

Oxidation states of benzene and other small rings are occasionally “virtual”, i.e. they appear in specifically substituted or coordinated species, but have not been generated via a sequence of redox processes for a single derivative. Indeed, the chemistry of π-conjugated seven-membered carbocycles is largely dominated by the most stable oxidation level, the 6π-aromatic tropylium cation [3.1]⁺ (Fig. 3).^46,47 The 7π-electron tropyl radical is unstable,⁴⁸ but its fused oligocyclic analogues, such as 3.2a, can be isolated, some of them being redox-active.^49–53 Similarly, the cycloheptatrienyl anion is rarely observed in solution chemistry: it can be stabilized by benzannulation, as in [3.3]⁻,⁵⁴ or by substitution with electron withdrawing groups as in [3.4]⁻.⁵⁵ The former system showed magnetic characteristics consistent with paratropicity of the seven-membered ring. In the crystal of the potassium salt K[3.4], the anion was non-planar, showing significant localization of one endocyclic double bond. The first evidence for the formation of the cycloheptatrienyl trianion [3.1]³⁻ was obtained in 1977 by Bates et al.⁵⁶ As a 10π-electron species, [3.1]³⁻ is expected to be Hückel-aromatic, which however has been considered too unstable to exist without the stabilization provided by lithium cations.⁵⁷ C₇H₇ rings bound to early transition metals have been described as 7-electron donors,⁵⁸ however, the trianionic state is believed to exist in the inverse-sandwich lanthanide complexes with the general structure of 3.5.^59–61 The elusive [C₆H₆]⁴⁻ and [C₇H₇]³⁻ ions may be seen as lower homologues of the more easily accessible cyclooctatetraene (COT) dianion [3.6]²⁻, obtainable directly via two-electron reduction of COT.⁶² COT dianions have enjoyed continued interest as ligands in lanthanide-based single-molecule magnets.^63–65 The recently reported the diaza-COT derivative 3.7 could similarly be reduced to the corresponding dianion using potassium metal.²⁹ However, the presence of two nitrogens made 3.7 susceptible to a reductive transannular contraction reaction, which was induced by a Co(I) complex used as an alternative reductant. Interestingly, similar bicyclo rearrangements can be induced in tetrabenzo-COTs by either two-electron oxidation²⁷ or reduction²⁸ (cf. 14.2, Fig. 14).

Fig. 3 Examples of charged states of 7- and 8-membered carbocycles.

Redox chemistry of higher annulenes was extensively investigated in the second half of the 20^th century, greatly contributing to our current understanding of aromaticity and π-electron delocalization.⁶⁶ This field still offers unexpected research opportunities, as illustrated by the recent reinvestigation of the reduction chemistry of [18]annulene (4.1, Fig. 4), carried out by the groups of Petrukhina and Anderson.⁶⁷ The mono-, di- and tetraanions of 4.1 were obtained by reduction with lithium metal; ¹H NMR spectroscopy showed that [4.1]²⁻ and [4.1]⁴⁻ were respectively para- and diatropic, in line with their 20π and 22π electron counts. The crystal structure of the Li₄[4.1] salt revealed a quasi-dimeric sandwich structure, with five of the eight Li cations intercalated between the two annulene ligands. The new analysis further revealed, in contrast to the original report,⁶⁸ that the (−2) and (−4) states of 4.1 adopt a C_2v symmetric conformation.

Fig. 4 Top: Reduction chemistry of [18]annulene 4.1. Arbitrary localized valence structures are shown for [4.1]²⁻ and [4.1]⁴⁻. Bottom: Single-crystal X-ray structure of Li₈[4.1]₂. The asymmetric unit (left, interstitial solvent, hydrogen atoms and minor disorder components omitted for clarity). Two orthogonal views on the sandwich part (right). Color code: C, grey; O, red and Li, blue. Adapted from ref. 67, licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0, https://creativecommons.org/licenses/by/4.0/).

Reduced states of benzenes can be stabilized by introduction of substituents that can delocalize the negative charge, thus compensating for the loss of aromatic stabilization. Aromatic carboxylic esters such as diethyl terephthalate 5.1a and tetraethyl pyromellitate 5.1b can be sequentially reduced to the corresponding radical anions and dianions, providing a working principle for multi-color electrochromic devices (Fig. 5 and 6).⁶⁹ The radical anions [5.1a]˙⁻ and [5.1b]˙⁻ are respectively pink- and magenta-colored, whereas the dianion [5.1b]²⁻ is yellow. A similar reduction sequence is thought to occur in pyromellitic diimide salts, which have been employed as electrode materials in Li-ion and Na-ion batteries.^70,71 The initial diimide dianion [5.2]²⁻ (Fig. 5) is aromatic and undergoes a two-electron reduction to the quinoidal [5.2]⁴⁻ state. Interestingly, the latter structure may be considered to have some s-indacene conjugation, a feature that may affect the relative stability of the tetraanion. The performance of these imides was found to be better in Li-ion setups, while in their sodium-based counterparts, prolonged cycling resulted in faster decomposition, which was attributed to overreduction of the imides beyond the [5.2]⁴⁻ state. The range of accessible oxidation levels can be extended by simultaneously substituting the system with electron-donating and electron-withdrawing groups. For instance, the tetraanion of 2,5-dihydroxyterephthalic acid, [5.3]⁴⁻, can be both reduced to the quinodimethane-like hexaanion [5.3]⁶⁻ and oxidized to the benzoquinone dianion [5.3]²⁻ (Fig. 5). These complementary capabilities were employed in the construction of a symmetrical rocking-chair sodium-ion cell, capable of delivering an average voltage of +1.8 V and energy density of ca. 65 Wh kg⁻¹.⁷²

Fig. 5 The use of electron-withdrawing and electron-donating groups for stabilization of charged oxidation states in simple benzene derivatives.

Fig. 6 Color changes of electrochromic devices containing 5.1a, 5.1b, 8.15, and 15.2 as a function of applied potential. Adapted with permission of the Royal Society of Chemistry, from ref. 69; permission conveyed through Copyright Clearance Center, Inc.

In contrast to the above carboxylic derivatives, which gain aromaticity upon reduction, the systems 5.4⁷³ and 5.5,⁷⁴ reported by the group of Jana, are aromatic in the oxidized state, and can thus be classified as inverse Wurster species. The benzenoid state [5.4]²⁺ is additionally stabilized by delocalization of positive charges into the iminium groups away from the central ring, but it is easily reduced to the corresponding radical cation and neutral state at relatively high potentials. The dipolar 5.5 is a radical in the neutral state, with the spin delocalized across the aromatic backbone. The radical is easily oxidized to the cation [5.5]⁺, which has a conventional aminidium-like structure. In contrast, the monoanion [5.5]⁻, which was accessed by further reduction of 5.5 with KC₈, is a ground-state singlet diradical with a ΔE_ST = –0.73 kcal mol⁻¹.

Oxocarbons⁷⁵ C_nO_n (S3.1-n, n = 3 to 6, Scheme 3) may be viewed as fully oxidized analogues of monocyclic hydrocarbons and as formal precursors of radialenes (section 4). Neutral oxocarbons tend to exist only in covalently hydrated forms, although syntheses of pristine S3.1–4, S3.1–5, and S3.1–6 were recently claimed.^76,77 These results need to be taken with caution given the known thermodynamic instability of the lowest oxocarbons and the triplet ground state of S3.1–4.^78,79 All known oxocarbons are most stable as dianions [S3.1-n]²⁻ which are conjugate bases of the corresponding acids (deltic, squaric, croconic, and rhodizonic). The dianions are fully conjugated and symmetric, but aromatic stabilization is present only in the deltate dianion [S3.1–3]²⁻.⁸⁰ The redox activity of the oxocarbons is complex and dependent on the ring size, but it has received considerable interest because of the possible application of these materials in energy storage.^81,82 Oxocarbon-based batteries are attractive because they can potentially be made from renewable sources, but their capacity retentions are at present relatively low. While neutral states of some oxocarbons, e.g., S3.1–5,⁸³ can be electrochemically generated in solution, charge storage is typically realized by reduction of the dianions to more highly charged states, i.e., [S3.1–5]⁴⁻ and the presumably aromatic [S3.1–6]⁶⁻.^77,84

Scheme 3 Structure and selected redox transformations of tripak (S3.2). Reagents and conditions:²² (a) Br₂, DCM; (b) [Fe^III(phen)₃][PF₆]₃; (c) 1 equiv. KC₈, THF; (d) 2.2 equiv. KC₈, THF.

A remarkable way of converting a single benzenoid ring into a multiredox system was devised by Pakulski, Pinkowicz et al.²² Their design, called tripak (S3.2, Scheme 3) consists of three thiadiazole dioxide rings fused to a central C₆ core, and was found to stabilize the (−2) oxidation level. Tripak can be electrochemically switched between oxidation levels from 0 to −6, in a potential window of ca. +0.7 V through −2.7 V (in THF, vs. Fc/Fc⁺). Oxidation levels from 0 through −4 were also generated chemically and isolated (Scheme 3). Computational investigations showed that S3.2 has three low-lying virtual orbitals, the upper two showing twofold degeneracy. As a consequence, while [S3.2]⁰ and [S3.2]²⁻ were closed shell singlets, [S3.2]⁴⁻ was predicted to be a ground-state triplet with a relatively small singlet–triplet gap (+2.3 kcal mol⁻¹), in agreement with the experimentally observed paramagnetism of the tetraanion. On the basis of computational data, the ¹[S3.2]⁰, ¹[S3.2]²⁻, and ³[S3.2]⁴⁻ states were classified as non-aromatic, weakly Hückel-aromatic, and Baird-aromatic, respectively. Bond length variations occurring during stepwise reduction suggested that the tripak π system is gradually transformed from the fully quinoidal conjugation in the neutral state toward benzenoid conjugation in the central ring in the negatively charged states. While the −6 oxidation state was not investigated, it is possible that it contains a fully aromatic benzene ring, which is however destabilized by the significant accumulation of negative charge at the periphery.

3. Linear systems

Linearly linked systems

One of the oldest-known redox-active aromatic molecules,⁸⁵ the parent viologen system ([S4.1]²⁺, Scheme 4, various R groups), undergoes sequential one-electron reductions to the highly colored radical cation [S4.1]˙⁺ and the weakly colored neutral S4.1. The viologens were recognized early as useful redox indicators,⁸⁶ and they have remained of high continuing interest because of their electrochromic and energy-storage applications.^20,87,88 Viologen subunits have also been extensively explored as components of mechanically interlocked molecules,⁸⁹ which are outside the scope of this review. The properties of the viologen can be modified by inserting conjugated linkers between the two pyridinium moieties, as in, e.g., [S4.2]²⁺ through [S4.11]²⁺ (Scheme 4).^90–92 The reduced states of extended viologens show highly variable properties, dependent on the length and nature of the linker. Upon reduction to the neutral state, the linker switches from an aromatic (A) to a cross-conjugated or quinoidal form (Q, Scheme 4). For weakly coupled systems, the quinoidal neutral state may have a considerable open-shell character, and equilibrate between singlet and triplet configurations, as experimentally observed for S4.8.⁹³

Scheme 4 Extended viologens and other linearly linked systems. The charges correspond only to the viologen units (charges located on R groups, if present, are not included). Clar sextets and their heterocyclic equivalents are indicated in light red.

An illustration of such dependences is provided by the series of bispyridinium electrolytes [S4.1]²⁺–[S4.10]²⁺ (R = CH₂CH₂CH₂NMe₃⁺, Cl⁻ counteranions) developed by Grey, Scherman et al.⁹² These systems were employed in redox flow batteries, whose remarkable air tolerance was explained by the stabilizing effect of π-dimerization of viologen radical cations. In Scheme 4, these viologens are ordered according to the increasing singlet–triplet gap of the neutral state, which provides a measure of the interaction strength between the pyridine subunits. In electrochemical measurements, [S4.1]²⁺ and [S4.2]²⁺ showed two reversible one-electron reductions, characteristic of a strongly conjugated π system. A single two-electron reduction was observed for [S4.3]²⁺ through [S4.6]²⁺, in line with the greater separation between the pyridinium ends in these viologens. The remaining four systems showed non-reversible electrochemical reductions, suggesting relatively low stability of the neutral states, which was attributed to their open-shell character, which may be expressed both in the singlet and triplet state. In particular, S4.8 bears a structural similarity to the Chichibabin hydrocarbon, whereas the meta-linked S4.9 and S4.10 contain no closed-shell quinoidal forms and may be seen as analogues of the Schlenk hydrocarbon.

Because their redox properties are coupled to color changes, viologens can double as both electrolytes and electrochromic materials. For instance, when paired with a soluble TEMPO derivative, thiazolothiazole [S4.11]²⁺ (R = CH₂CH₂CH₂NMe₃⁺, Scheme 4) was used as a two-electron storage anolyte in all-organic aqueous redox flow batteries (Fig. 7).⁹⁴ When substituted with chargeless groups, [S4.11]²⁺ was employed in electrochromic devices, wherein it was switchable from the yellow, and strongly fluorescent dicationic state to the deep-blue and non-emissive neutral form.⁹⁵ A related case of electrofluorochromism was demonstrated for liquid-crystalline derivatives of [S4.5]²⁺, whose strong fluorescence in the bulk state, could be switched by application of reductive voltage or electric field.⁹⁶

Fig. 7 Aqueous organic redox flow battery based on the extended viologen motif [S4.11]²⁺ (cf. Scheme 4). (A) Cyclic voltammograms of 4.0 mM [(NPr)₂TTz]Cl₄ and 4.0 mM N^Me-TEMPO in 0.5 m NaCl solution. The gray dash curve is the cyclic voltammogram of only the 0.5 m NaCl electrolyte, with labels for the onset potentials for the hydrogen evolution reaction (HER, −1.00 V) and oxygen evolution reaction (OER, +1.50 V). The red and green dash curves are the fitted redox waves for the 1^st and 2^nd electron reductions, respectively. (B) Schematic representation of the [(NPr)₂TTz]⁴⁺/N^Me-TEMPO battery and its anodic and cathodic half-cell reactions. Reprinted with permission from ref. 94. Copyright © 2018 Wiley-VCH GmbH.

Ring fusion provides modified viologens that do not contain an extended conjugation pathway between the pyridinium units, but can benefit from the greater conformational rigidity and heteroatom effects imparted by the additional rings. Phosphaviologens [S4.12]²⁺ (R = alkyl, benzyl, aryl), developed by Baumgartner et al.,^97,98 are reduced to the corresponding [S4.12]˙⁺ and S4.12 states at relatively high potentials (−0.60 V and −1.0 V, respectively, for R = Me), benefiting from the hyperconjugative electron-withdrawing effect of the phosphole oxide ring. The large potential separation of the two reductions enabled two-stage electrochromism: for instance, the benzyl-substituted [S4.12]²⁺ switches from colorless through blue-purple, to orange upon sequential electrochemical or chemical reduction. The similarly structured chalcogenoviologens [S4.13-E]²⁺ (E = S, Se, Te), showed a remarkable dependence of the optical energy gap on the chalcogen center (3.00, 2.78, and 2.33 eV, respectively, for R = Bn, TfO⁻ counteranions).⁹⁹ The latter systems could be used not only in electrochromic devices, but also for visible light-driven hydrogen evolution. In those experiments, the chalcogenoviologens played a dual role of a photosensitizer and an electron mediator.

Typically, in extended viologens, the accessible oxidation levels remain limited to 0, +1, and +2, although anionic states are occasionally observed, as in S4.8 (R = n-octyl), which was electrochemically reduced to [S4.8]˙⁻ and [S4.8]²⁻.⁹¹ Limited redox ranges are typical in linear systems, as illustrated by Reynolds’ electrochromic oligomers, which consist of up to six thiophene or benzene subunits but were oxidized only to +1 and +2 states.¹⁰⁰ Extending the redox range is possible by connecting a greater number of electroactive subunits, e.g. using radial designs described below. Linear multiredox systems can be obtained by judiciously linking multiple charged rings, as illustrated by [S4.14]⁴⁺, reported by Hansmann et al. (Scheme 4).¹⁰¹ The nearly colorless tetracation could be electrochemically reduced to the +3, +2, and 0 states, which were respectively green blue and purple in solution, thus providing a particularly rich electrochromic response. The radical trication [S4.14]˙³⁺ additionally absorbed in the near-infrared region, with the strongest maximum at 1513 nm). A phenanthroline analogue of [S4.14]⁴⁺ produced a complete range of discrete oxidation levels from 0 to +4, and showed high stability when tested as a potential anolyte material.

Given the redox amphotericity of many organic radicals, building conjugated oligoradical oligomers is an attractive route toward multiredox organics. The principal challenge of achieving sufficient chemical stability of such assemblies, was successfully addressed by Wu et al. in their work on fluorenyl oligomers S4.15–n (n = 1 through 6).¹⁰² In these systems, the inter-subunit coupling is ensured by quinoidal contributions, such as S4.15–2′ (Scheme 4). In electrochemical measurements, each of these systems showed n one-electron oxidations and n one-electron reductions, although in the longer oligomers some of these events considerably overlapped. In the limit of complete reduction to the [S4.15–n]ⁿ⁻ level, each subunit bears a unit negative charge, and can be treated as an aromatic fluorenyl monoanion, decoupled from the adjacent subunits.

In a somewhat similar fashion, oligo(biindenylidene)s S4.16–n, developed by Fukazawa et al. as linearized substructures of C₆₀,¹⁰³ are capable of stabilizing up to one negative charge per indenyl subunit. The largest member of the series with n = 3, could be reduced up to the pentaanion [S4.16–3]⁵⁻. Further reduction was likely limited by the accessible potential range. The difference between the first two reductions decreased with the increasing length of the oligomer (+0.31 V, +0.15 V, and 0.00 V, for n = 1, 2, and 3, respectively), reflecting the diminishing interaction between the charges. The stability of the oligoanions can be attributed to locally aromatic 6π/10π-electron cyclopentadienyl/indenyl anion resonance contributions emerging upon reduction.

Linearly fused systems

Acenes are the simplest class of aromatics with one-dimensional fusion. Their redox chemistry typically covers oxidation levels from −2 to +2, and underlies potential applications of acene-based materials as semiconductors,^104,105 superconductors,¹⁰⁶ and quantum spin liquids.¹⁰⁷ Linearly fused acenes (e.g., 8.1–4 through 8.1–7, Fig. 8) contain a single “migrating” Clar sextet regardless of the fusion length, a feature that contributes to the decreasing stability of the higher members of the series (n > 4). Since acenes are formally (4n + 2)-electron systems (where n coincides with number of fused rings), their dications could in principle be expected to show antiaromatic behavior associated with the 4n-electron count. However, two-electron oxidation of 8.1–4–8.1–7, which occurs spontaneously in fuming sulfuric acid, produces persistent and strongly diatropic dications [8.1–n]²⁺ (Fig. 8).¹⁰⁸ This counterintuitive enhancement of aromaticity was rationalized by the presence of two Clar sextets in the dications. Dianions of higher acenes, e.g., [8.1–5]²⁻ (ref. 109 and 110) and [8.1–6]²⁻,¹¹¹ can be generated under strongly reducing conditions. In contrast to the corresponding dications, the dianions show global aromaticity, apparently originating from the peripheral (4n + 4)-electron circuit.

Fig. 8 Aromaticity changes in acenes and their derivatives. Red arrows indicate possible relocations of Clar sextet in the acene ring system. Experimental discharge voltages and experimental (theoretical) discharge capacities are given for 8.12–8.14.

A similar switching between acene and oligoquinone conjugation can be found in some linear heteroacenes, such as 5,6,11,12-tetraazanaphthacene 8.2. The latter species can be reduced to the dihydro derivative, fluoflavine,¹¹² whose dianion [8.2]²⁻ can act as a bridging ligand.¹¹³ The range of oxidation levels attainable in metal complexes of 8.2 extends from −1 to −3, as recently demonstrated by Benner and Demir.¹¹⁴ Dithiaacenes 8.3, 8.6, and 8.7, developed by Chi et al.,¹¹⁵ show a reversed charge–aromaticity relationship relative to the corresponding hydrocarbons. The former species are oligoquinoidal in their neutral states, with a progressively increasing diradicaloid character. Their dications, in contrast, are closed-shell species, isoelectronic with neutral acenes. Analogs of 8.3 containing selenium, nitrogen, and boron sites were reported respectively by the groups of Nagahora,¹¹⁶ Bunz (8.4),¹¹⁷ and Yamaguchi (8.8).¹¹⁸ 8.4 has three oxidation states, isoelectronic with those of 8.3, with significant diatropicity of the radical cation [8.4]˙⁺ and dication [8.4]²⁺ determined in NICS calculations. In the B₂O₂ system 8.8, notable for its sharp NIR emission, low-lying oxidations (−0.07 V, +0.27 V) are observed in addition to reductions (−1.44 V, −1.83 V). In the latter system the stabilization of cationic and anionic states was associated with the oxygen and boron sites, respectively.

In the linear acene-like and oligoquinone conjugation patterns discussed above, the number of Clar sextets remains 1 and 2, respectively, regardless of the acene length. A larger number of Clar sextets can be produced by introduction of multiple benzoquinone units into the acene structure, leading to greater aromatic stabilization. This well-established strategy also works with benzoquinodimethane units, which were implemented by Ishigaki, Suzuki et al. in pentacene derivatives such as 8.5.^25,26 The latter system shows an unusual hysteretic redox behavior: the neutral 8.5 is oxidized to the tetracation [8.5]⁴⁺ in a single four-electron step (+1.12 V vs. SCE, R = p-anisyl), whereas the reduction of the tetracation takes place in two two-electron steps (+0.55 and +0.27 V). This remarkable characteristic may originate from the significant conformational difference between the quinodimethane and dicarbocationic sections and the associated isomerization barriers.

The properties of acenes can be significantly altered by heteroatom doping or by substitution. A simple substitution with ester groups is sufficient to produce a useful shift of reduction potentials to higher voltages, as exemplified by the tetraester 8.15 which was found to be a useful electrochromic material.⁶⁹ The nitrogen-doped azaacenes,¹¹⁹ are typically more electron deficient than the parent hydrocarbons, and are consequently of interest as n-type semiconductors. For instance, 5,7,12,14-tetraazapentacene 8.9¹²⁰ shows two one-electron reductions at relatively high potentials (E_red1 = −0.79 V and E_red2 = −1.23 V, vs. E_red1 = −1.87 V for 8.1–5¹²¹) to produce a NIR-absorbing radical anion and a photostable and highly fluorescent dianion (λ_em = 598 nm, QY = 95%).¹²² Interestingly, while 8.9 turned out to be an excellent n-type organic semiconductor, with an electron mobility of up to 3.3 cm² V⁻¹ s⁻¹, the related pentacene derivatives 8.10 and 8.11 showed respectively ambipolar and p-type behavior.¹²³ 8.11 is a doubly hydrogenated derivative of 8.10 and is thus not isoelectronic with pentacene. In fact, 8.11 is quite electron-rich, with HOMO and LUMO levels at −5.06 eV and −2.66 eV, respectively.

Acene-derived quinones have been considered as potential cathode materials for Li-ion batteries, as they can potentially provide higher discharge capacities than conventional inorganic cathodes. Their practical application is currently limited by relatively low discharge voltages and rapid capacity fade. For instance, anthraquinone 8.12, a simple and inexpensive material, produced a high discharge capacity in the first cycle (up to 251 mAh g⁻¹), but the discharge voltage was only ca. +2.2 V.¹²⁴ In 5,7,12,14-pentacenetetraone 8.13, which can be reduced to the tetraanionic state [8.13]⁴⁻, the number of quinone groups per mass unit is increased, leading to a higher discharge capacity; however, the attainable voltage (+2.1 V) was even lower than for anthraquinone.¹²⁵ In both, 8.12 and 8.13, the neutral form of the quinone (corresponding to the charged state of the battery) is more aromatic than the fully reduced form, as can be inferred from the number of Clar sextets in both states. Chen et al. proposed that a higher discharge potential can be achieved if the relationship is reversed, i.e. if the aromaticity increases in the reduced (discharged) state.¹²⁶ This assumption was confirmed experimentally by using 1,4,5,8-phenanthrenediquinone 8.14 as the cathode material, which yielded a discharge voltage of +2.77 V. As a consequence of the angular fusion present in 8.14, the number of Clar sextets increases from one to two upon reduction of the tetraanion [8.14]⁴⁻.

Reduced states of acenes can display unusual magnetic and electronic properties in the solid state.¹⁹ In 2010, Kubozono et al. reported the first hydrocarbon-based superconductor, K_x[8.17], obtained by annealing picene with potassium.¹⁰⁶ The properties of the material strongly depended on the potassium content, varying from Pauli-like for x < 2, through superconducting, with a maximum T_c = 18 K obtained for x = 3.3, to Curie-like, for x ≥ 4. Superconductors based on doped phenanthrene, dibenzopentacene, and other hydrocarbons have also been reported.¹⁹ While their behavior is still incompletely understood, the doping factor x dramatically affects the solid-state interactions between the metal and the hydrocarbon.^107,109 Significant changes can occur also at low doping levels, for instance phenanthrene salts Cs₂[8.17] and Cs[8.17] are respectively diamagnetic and antiferromagnetic, the latter material displaying features of a quantum spin liquid.¹⁰⁷

Reduction of the energy gap of an acene molecule may be seen as a way of simultaneously bringing the oxidation and reduction events into a chemically accessible potential regime. Such redox amphoterism is most readily achievable in open-shell derivatives, such as diindenoarenes.^127,128 In such systems, each indene unit can be considered as a carrier of either positive or negative charge, potentially stabilizing oxidation states from (−2) to (+2). For smaller diindenoarenes, such as 8.18 (E_ox1 = +0.13 V, E_red1 = −1.13 V),¹²⁹ the stability of the (+2) and (−2) states is usually limited. A full range of expected oxidation states can however be observed in systems containing larger carbocyclic^130–134 (e.g. 8.19¹³⁰) or heterocyclic^135,136 motifs. Similar behavior is observed in oligoradicaloids based on other structural motifs, e.g. phenalenyls.¹³⁷

4. Two-dimensional systems

[6]Radialenes

A multiredox organic system can be built by radially connecting multiple redox-active units around a common “hub” moiety. The electronic structure such as assembly depends on the properties of the subcomponents and nature of the possible interactions between these redox units and the hub. Benzene is a particularly useful hub motif, offering the possibility of connecting six redox units in a symmetrical and chemically robust fashion. Vaid's [6]radialene S5.1¹³⁸ provides an example of a strongly coupled system, which adopts radialene-like conjugation in its neutral state. S5.1, which revealed a chair-like conformation in the crystal structure, is air-sensitive. It was electrochemically oxidized to [S5.1]⁴⁺ and then to [S5.1]⁶⁺ in 4- and 2-electron processes, respectively (−1.33 V and −1.14). This behavior, notably the simultaneous transfer of four electrons, is unique, and is apparently related to the interplay between π-conjugation and conformational changes caused by oxidation. It is worth noting that in hexaferrocenylbenzene, which is expected to be weakly π-coupled across the hub, the electrochemical oxidation to the corresponding hexacation occurs in a sequence of 1-, 2- and 3-electron events.¹³⁹

Hexaaniline S5.2, investigated by Kochi et al.,^140,141 was sequentially oxidized to [S5.2]˙⁺, [S5.2]²⁺, [S5.2]³⁺, and [S5.2]⁶⁺ at potentials in the 0.5–0.9 V range (vs. SCE). The radical monocation of this species, was of particular interest as a potential example of toroidal delocalization of the hole among the six aniline arms (green arrows, Scheme 5).¹⁴² All cationic states [S5.2]ⁿ⁺ (n = 1 to 6) could apparently be observed in chemical oxidation experiments. A radialene-like conjugation can be envisaged for the hexacationic state [S5.2]⁶⁺, in an interesting opposition to S5.1, the latter having a radialene structure in its neutral state. Kochi's group also investigated the redox properties of hexacarbazolylbenzene S5.3,^141,143 from which only the first four electrons could be removed reversibly. Because of the large torsional twist of the carbazoles relative to the central benzene, S5.3 is likely to exhibit limited π conjugation across the hub unit.

Scheme 5 [6]Radialenes and related structures based on benzene hubs.

Some features of the radialene design can be extended to other systems. Hexakis(guanidino)benzene S5.4, developed by Himmel et al.,¹⁴⁴ contains no peripheral aromatic units and is a very strong electron donor. It is quadruply oxidized via two-electron low-potential events (−0.94, −0.41 V) and can form a pentacation at ca. +0.85 V. The latter oxidation level corresponds to a high theoretical charge capacity of 180 Ah kg⁻¹, comparable with capacities achievable by Li-ion batteries (ca. 150–170 Ah kg⁻¹).

The extent of coupling between substituents in radialene-like systems depends on their intrinsic redox properties, their number, and sterics. This dependence is illustrated by Diederich's charge-transfer systems S5.5 and S5.6:²¹ the former system showed significant differentiation of the first six reductions (−0.46 to −1.07 V), but further electrochemical reduction occurred as a single six-electron event (−1.57 V, Scheme 5). Similarly, a six-electron oxidation, likely involving the amino groups, was observed for S5.5 at +0.89 V. In the less densely substituted S5.6, there were three two-electron events, corresponding to the oxidation of the 1,2-di(1,3-dithiol-2-ylidene)ethane moieties (+0.41, +0.65, and +0.81 V) followed by a three-electron event at +0.96 V, likely involving the amine termini. Similarly to S5.5, compound S5.6 appeared to undergo sequential reduction, but these electrochemical events were somewhat less resolved in the latter case. Since S5.6 contains meta-connected arms and is relatively crowded, the differentiation of oxidation and reduction events may be largely caused by through-space electrostatic interactions, especially for the oxidations which occur closer to the benzene hub. When the charge is delocalized further away from the ring as in the oxidation of hexa- and tripyrrolylbenzenes S5.7 and S5.8, all TTF-pyrrole arms are oxidized almost simultaneously.¹⁴⁵ In particular, S5.7 was oxidized to the NIR-absorbing hexa(radical cation) [S5.7]⁶⁺ and hexa(dication) [S5.7]¹²⁺. For S5.8, additional mixed-valence states were observed, which were attributed to the formation of aggregates, likely facilitated by the more planarized geometry of this species.

Non-benzenoid radialenes and related systems

The 2π-electron cyclopropenyl cation is the smallest entity exhibiting aromatic stabilization. Its diminutive π-conjugated system needs to be appropriately functionalized to exhibit multiredox behavior. Because of their small size and stability of their cationic state, such substituted cyclopropenyl cations have been researched as organic catholytes in redox-flow batteries, a field recently reviewed by Sigman, Sanford et al.¹⁴⁶ The triaminocyclopropenyl cations typically used for this purpose ([9.1]⁺, Fig. 9) undergo just a single oxidation to the corresponding radical dication [9.1]˙²⁺, however, systems with a more extended redox behavior, such as [9.2]⁺,¹⁴⁷ can provide higher energy densities.

Fig. 9 Non-benzenoid radialenes and related systems with a multiple redox states. Hückel-aromatic rings are shaded in color.

[3]Radialenes, the smallest members of the radialene family,¹⁴⁸ provide another example of using cyclopropenyl cation contributions for stabilization of charged states. In particular, the electron-deficient hexacyano[3]radialene 9.3, reported by Fukunaga in 1976, can stabilize oxidation levels from 0 to −2 (Fig. 9).^149,150 The dianion derives its unique stability from the aromaticity of the cyclopropenyl cation. 9.3 and related derivatives have been recently explored as soluble p-dopants in conductive polymer films,¹⁵¹ organic catholytes in redox flow batteries,¹⁵² and multimodal information switches.¹⁵³ The electron-rich [3]radialenes 9.4a and 9.4b, reported by Matsumoto et al.,¹⁵⁴ display four reversible one-electron oxidations and one reversible one-electron reduction in cyclic voltammograms. While detailed investigations of the tetracations [9.4a]⁴⁺ and [9.4b]⁴⁺ were not reported, their stability may likewise originate from the local aromaticity of the three-membered ring (Fig. 9). The deep-red [4]radialene 9.5, described by Horner and Hünig in 1977,^155,156 can be oxidized up to the (+4) level. The initial oxidation is effectively a two-electron process (∼0.1 V vs. AgCl/Ag), leading to a stable dication [9.5]²⁺ characterized by a sharp red-shifted absorption. Formation of [9.5]⁴⁺ required a relatively high potential of +1.39 V (vs. AgCl/Ag), implying that the tetracation is strongly destabilized by the antiaromaticity of the central cyclobutadiene ring.

The radialene design principle can be applied to more complex core motifs and peripheral units, containing, e.g., multiple fused rings, triple bonds or heteroatoms. By judicious combination of two redox-active motifs, i.e. tetrathiafulvalene (TTF) and radiannulene, the group of Nielsen produced the hybrid system 9.6 with a significant redox range (Fig. 9).¹⁵⁷ The system was oxidizable up to the tetracation level, relying on the ability of the TTF units to easily release two electrons each. The dianion [9.6]²⁻ achieved its stability by producing an aromatic [14]annulene pathway in the macrocycle and delocalizing the negative charges in the outer branches of the oligoyne π system. The radialene-like 9.7, reported by the Yamaguchi group,¹⁵⁸ contains an azatriangulene core and three dibenzo[c,g]fluorenylidene arms. The system has a relatively small energy gap of ca. 1.3 eV, and shows two reversible electrochemical one electron oxidations at potentials lower than 1 V. 9.7 undergoes a two-electron reduction at −1.13 V, which is followed by two one electron events.

Multiredox alkenes

The C=C double bond can be seen as the smallest conjugated entity enabling quasi-radial linking of multiple redox-active groups. Tetraarylethylenes bearing electron rich groups such as 10.1 (Fig. 10) can be reversibly oxidized to the corresponding radical cations and dications.¹⁵⁹ The structurally related 9,9′-bifluorenylidene 10.2 forms an antiaromatic dication,¹⁶⁰ as well as the anionic states, [10.2]˙⁻ and [10.2]²⁻,¹⁶¹ the latter stabilized by fluorenyl anion-like aromaticity. The redox range of 10.2 can be extended by fusion of additional rings, as in the recently reported tetrafluorenofulvalene 10.3, for which all oxidation levels from (−4) to (+3) could be generated electrochemically.¹⁶² The cross-conjugated structure of 10.3 leads to a complex relationship between the oxidation state, aromaticity and the strength of the central carbon–carbon bond. For instance, in the [10.3]⁴⁻ tetraanion, the central bond becomes the strongest, and the system behaves as a union of four aromatic fluorenyl anions. Conversely, in [10.3]²⁺, the two monocationic halves of the molecule become almost completely decoupled, each showing considerable antiaromaticity, reminiscent of the fluorenyl cation.

Fig. 10 Multiredox alkenes.

Compound 10.4, reported by Hein, Feringa et al.,²⁴ provides an example of a dynamic redox (DYREX) system in which changes of the oxidation state are coupled to conformational dynamics of the overcrowded alkene bond. The neutral 10.4 adopts an anti-folded conformation. When electrochemically oxidized, it undergoes a three-step process consisting of (1) the formation of the transient anti-folded radical cation [10.4]˙⁺, (2) its rapid isomerization to the most stable twisted form, and (3) immediate oxidation to the dication [10.4]²⁺, resulting in an apparent 2-electron electrochemical event. The dication has an orthogonal conformation similar to the twisted conformation of [10.4]˙⁺, and redox switching between the two species occurs with a minimal steric constraint. The reduction of [10.4]˙⁺ to the neutral 10.4 involves a final conformational change, which makes the process electrochemically non-reversible.

Through-space conjugated systems

In [6]radialene systems, such as hexacarbazolyl S5.3 (Scheme 5), the redox behavior is affected by a fairly significant through space component, but the inner core of the radialene is, at least in principle, π-conjugated. Multiredox systems relying solely on through-space interactions have been constructed using triptycene and trinaphtho[3.3.3]propellane (TNP) cores (Fig. 11). The principle is illustrated by triptycene triquinone 11.1, explored by Kwon et al. as a cathode material for Li-ion batteries.¹⁶³ This species undergoes three redox events at −0.58, −0.77, and −0.94 V vs. Ag/Ag⁺, corresponding consecutive reductions of the three quinone arms. Interestingly, the first reduction occurs at a higher potential than in benzoquinone (+0.79 V), suggesting that the radical anion [11.1]˙⁻ is stabilized by electron sharing, while the relative lowering of the third reduction potential may originate from electrostatic repulsion. Further reduction of 11.1 (−1.44 and −1.59 V vs. Ag/Ag⁺) was limited to two one-electron events, suggesting that complete reduction of all three quinone units was thermodynamically unfeasible. In Li-ion half-cell tests, the overall transfer of up to five electrons was confirmed by the measured initial discharge capacity of 387 mAh g⁻¹, very close to the theoretical value of 389 mAh g⁻¹.

Fig. 11 Multiredox systems with radial through-space conjugation.

Triradical 11.2 consisting of three Blatter-radical units attached to a triptycene core (Fig. 11) was obtained by Matsuda, Shimizu, et al.,¹⁶⁴ as two separable isomers, syn-11.2 and anti-11.2, with nearly identical properties. In electrochemical measurements, the C_3v-symmetric syn-11.2 showed three-closely spaced oxidations (−0.08, −0.15, −0.27 V) and three reductions (−1.23, −1.31, −1.38 V), all corresponding to reversible events. For larger systems, such as the family of 3D nanostructures 11.3-n, described by Sisto et al.,¹⁶⁵ the interaction between the three π-conjugated arms is hardly discernible. Thus, 11.3–3 underwent six consecutive electrochemical reductions in a small voltage window from approximately −1 to −2 V. Each of these events corresponded to simultaneous transfer of three electrons. The number of observed reduction events was 2n, for each member of the 11.3-n series, suggesting that each PDI units behaves as a 2-electron acceptor that is strongly coupled to other subunits in the ribbon.

The TNP triradical 11.4, reported by Kubo et al.,¹⁶⁶ featured three reversible one-electron oxidations (−0.47, −0.02, +0.22 V) and three reductions (−1.46, −1.71, −2.00 V). The observed differentiation of potentials implies a relatively strong communication between the three phenalenyl subunits. The effectiveness of TNP as a hyperconjugative coupler unit has also been demonstrated in the triimide 11.5, developed by Zhang et al.¹⁶⁷ This electron-deficient system consisting of three naphthalenemonoimide (NMI) arms showed three one-electron reductions, the first one occurring at −1.36 V. i.e. at a potential higher by +0.29 V than the reduction of the NMI parent. This difference was interpreted as a sign of additional stabilization of the monoanion by electron sharing between the three NMI subunits.

2D-fused hydrocarbons with peripheral conjugation

Two-dimensionally fused aromatics, often containing peri-condensed rings, provide more opportunities for multiredox behavior because of the typically large size of their π systems, and a greater number of potential functionalization sites. Furthermore, complex ring fusion patterns can lead to unusual conjugation effects, involving a combination of local and global contributions, which may variously affect the stability of neutral and charged states.

Corannulene 12.1, the first curved fullerene fragment synthesized, apparently has no stable oxidized states,¹⁶⁸ but it can accept up to four electrons via both electrochemical and chemical reduction. Both the dianion [12.1]²⁻ (ref. 169) and tetraanion [12.1]⁴⁻ (ref. 170–172) can be described using the annulene-within-the-annulene (AWA) model, with the 6π-electron cyclopentadienide anion as the inner annulene circuit. In [12.1]²⁻, the outer circuit may be interpreted as the 16π-electron [15]annulene anion, in line with the paratropic character of the dianion. By analogy, the tetraanion [12.1]⁴⁻ contains a 18π-electron [15]annulene-trianion peripheral pathway, which explains its diatropicity and unusual stability. [12.1]⁴⁻ is capable binding multiple lithium cations in solution and in the solid state, indicating a potential utility of corannulene and related systems in lithium-ion batteries. Interestingly, the bicorannulenyl 12.2 forms a dianion [12.2]²⁻ with two local six-electron Cp rings and an oligoene periphery. The tetraanion [12.2]⁴⁻ behaves as a dimer of two corannulene dianions, as inferred from its paratropic character.

The bis-pentannulated bisanthene diradicaloid 12.3, reported by Chi et al.,¹⁷⁴ showed stable oxidation states in the (−2,+2) range, forming at clearly separated redox potentials. The neutral state was globally paratropic and can formally be characterized as a 10π + 20π AWA system, although according to ACID calculations, the inner naphthalene substructure was negligibly aromatic. The inner 10π-electron circulation was similarly weak in the moderately aromatic closed-shell dianion [12.3]²⁻, which contains a 22π-electron periphery. In contrast, the diradicaloid dication [12.3]²⁺ featured strong diatropicity in ¹H NMR, while the ACID analysis revealed the presence of both 10π and 18π circulations confirming an AWA structure. Similar relationships between the oxidation level and aromaticity were established for Wu's 12.4,¹⁷⁵ a smaller homologue of 12.3. The magnetism of the neutral state appeared to combine features of a globally antiaromatic AWA contribution and a diradicaloid contribution with local benzenoid aromaticity. The dianion [12.4]²⁻ showed moderate peripheral aromaticity with no significant inner 6π contribution. The [12.4]²⁺ dication, which was not experimentally accessible, showed AWA aromaticity analogous to that of [12.3]²⁺. Tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene 12.5, described by Tobe et al., has an open-shell quinoid structure of D_2h symmetry, and does not exhibit AWA conjugation.¹⁷⁶ In the doubly charged states, the AWA conjugation is predicted by theoretical methods, leading to significant decoupling of the periphery from the inner 8π circuit.¹⁷⁷ The open-shell dianion [12.5]²⁻ and closed-shell dication [12.5]²⁺ are both globally aromatic, with their peripheral circuits containing respectively, 22 and 18 π electrons.

The fullerenes, whose spherical shapes provided inspiration for much of the research on curved aromatics, are fundamentally important as multistage electron acceptors and as examples of three-dimensional electron delocalization. The two most studied fullerenes, C₆₀ and C₇₀, can be oxidized to the (+1) state with considerable difficulty,^178,179 but each of them can reversibly accept up to 6 electrons in electrochemical experiments.¹⁸⁰ While the initial reductions occur at relatively high potentials (E_red1 = −0.98 V and −0.97 V for C₆₀ and C₇₀, respectively), the last reductions require potentials lower than −3 V vs. Fc/Fc⁺. In C₆₀, the six reductions correspond to sequentially populating the low-lying triply degenerate LUMO level (t_1u, Fig. 12, inset). The (−3) state, containing a half-filled LUMO, is responsible for the metallic character of the A_xA′_3−x[C₆₀] solids (A, A′ = Li, Na, K, Rb, Cs, 3 ≥ x ≥ 0), many of which show superconducting properties at low temperatures.^181–183 When generated electrochemically, the lower oxidation levels of fullerenes show only limited stability in solution;¹⁷⁸ however, bulk amounts of [C₆₀]⁶⁻ and [C₇₀]⁶⁻ were reported to form upon reduction with lithium metal.¹⁸⁴ Reduction of C₆₀ with magnesium metal was recently found to produce polymeric polyfullerides, which open a promising path to new carbon allotropes.^185,186

Fig. 12 Multiredox aromatics with two- and three-dimensional fusion. Substituents are omitted for clarity. Relevant conjugated circuits are indicated with bold bonds: aromatic in red, and antiaromatic in blue. Open-shell contributors are not shown. Relative contributions of various conjugation types are discussed in the text. For clarity, π-conjugation is not indicated for the fullerenes. The molecular orbital diagram for neutral C₆₀ (bottom right) is based on published Hückel energy levels.¹⁷³

With a completely filled t_1u level, the hexaanion [C₆₀]⁶⁻ is a closed-shell species, yielding insulating solids, such as K₆[C₆₀].¹⁸² The relatively low energy of the L + 1 level in C₆₀, suggests that further reduction, down to the (−12) state, could in principle be possible. The [C₆₀]¹²⁻ anion can be envisaged as being aromatic both in the local sense (12 aromatic sextets associated with the five-membered rings) and in terms of the spherical aromaticity model (2·(n + 1)² π electrons).^173,187 Clusters with a composition Li₁₂[C₆₀], potentially containing [C₆₀]¹²⁻, were indeed observed using mass spectrometry.^188,189 However, the solid Li₁₂[C₆₀] was found to contain Li₄ clusters, with the [C₆₀]⁶⁻ oxidation level indicated by NMR and Raman spectroscopic data.^190,191 Overall, from the practical viewpoint, the stability of fulleride oligoanions is relatively low, limiting the use of pristine fullerenes in rechargeable batteries.¹⁹²

Azacoronenes

Hexapyrrolohexaazacoronenes (HPHACs, e.g. 13.1a–b, Fig. 13) may be viewed as analogues of hexa-peri-hexabenzocoronene (p-HBC), in which the peri-fused outer benzene rings are replaced with pyrroles. The majority of known derivatives posses 12 peripheral aryl^193,194 or alkyl¹⁹⁵ substituents, which affect the redox potentials of these species. Except for NMI-fused analogues^51,196,197 (see below), these pyrrole-based nanographenoids are highly electron-rich and stabilize a variety of oxidized states, while being relatively unsusceptible to reduction. Both 13.1a ¹⁹⁴ and 13.1b ¹⁹⁵ showed three one-electron oxidations in electrochemical experiments. In each case, the oxidation states [13.1]˙⁺ and [13.1]²⁺ are stable and can be generated by chemical oxidation. The stability of these dications is derived from the presence of a 22-electron Hückel-aromatic peripheral conjugation pathway (Fig. 13). Likely because of this stabilization, the third oxidation requires a much higher potential than the second one (ΔE ≈ +0.6 V and +1.0 V for 13.1b and 13.1a, respectively).

Fig. 13 Azacoronenes and their selected oxidized states. Peripheral conjugation paths are shown in red. Aromatic sextets are shaded in light red.

The accessibility of higher oxidation states in azacoronenes can be influenced by various modifications of the fused ring system. The indole-containing 13.2 ¹⁹⁸ shows a fairly similar redox behavior, even though the accessible conjugation pathway in the dication encompasses 26 π-electrons. Other expanded HPHACs were however found to produce monoradical states and switch between global aromaticity and antiaromaticity.^51,199 The juxtaposition of pyrrole and benzene rings 13.3 and related systems²⁰⁰ leads to partial decoupling of the fused subunits, which interestingly made the higher oxidation states more accessible than in unmodified HPHACs. Closed-shell configurations, such as that shown in Fig. 13, can be constructed the [13.3]²⁺ dication, however, its topology precludes the existence of peripheral aromatic pathways. Experimental data suggested that the latter dication could exist in open-shell singlet and triplet states. An even stronger case of decoupling was achieved in the doubly bridged azacoronene analogue 13.4,¹⁹³ for which four reversible oxidation events could be observed. In fact, the tetracation [13.4]⁴⁺ could be obtained by chemical oxidation and was proven to be diamagnetic. The core-expanded azacoronene analogues 13.5 and 13.6 reveal an effective strategy for increasing the redox range of the π system. In the naphthalene-based 13.5, four oxidations were observed,²⁰¹ the first two located at very low potentials (−0.52 V and −0.32 V respectively). The highly contorted anthracene analogue 13.6²⁰² showed four reversible and two pseudoreversible oxidations. The dications [13.5]²⁺ and [13.6]²⁺ may be seen as globally aromatic, with respectively 30- and 38-electron peripheral pathways analogous to that found in [13.1]²⁺.

Eight-membered rings

Peripheral fusion of π-conjugated subunits can be used to extend the intrinsic range of redox states of cyclooctatetraene (vide supra). The outcome may depend on the extent of planarization induced by fusion²⁰³ and the chemical stability of the charged states. The saddle-shaped thiazole cyclotetramer 14.1 (Fig. 14), reported by the group of Yamaguchi,²⁰⁴ showed a one-electron oxidation, apparently producing the corresponding radical cation, and could be reduced in two steps to the nearly planar aromatic dianion [14.1]²⁻. The heavily substituted tetraphenylene 14.2 was explored by the teams of Petrukhina and Kivala,²⁰⁵ who showed that it could be reduced with lithium to yield [14.2]⁴⁻. In the solid state, the latter tetraanion was highly contorted, with two internally coordinated Li⁺ ions. The Kivala group subsequently reported the indenoannulated tridecacyclene 14.3,²⁰⁶ which is notable for two reversible one-electron oxidations occurring at relatively low potentials (+0.33 V and +0.56 V). The system undergoes four consecutive reductions above −2.2 V, with an additional, probably also reversible event at −2.44 V.

Fig. 14 Redox-active cyclooctatetraene and [8]circulene derivatives.

The rich redox chemistry of corannulene ([5]circulene) and derivatives of coronene ([6]circulene), such as HPHACs, suggests that higher [n]circulenes might also act as suitable multiredox platforms. With the notable exception of Tobe's 12.5 (see above), carbocyclic [8]circulenes typically show just one electrochemically reversible oxidation,^207,208 and their reduction chemistry has not been explored. The electron-rich tetraoxa[8]circulenes appear to be more redox active, and a low-symmetry derivative 14.4 (Fig. 14) was reported by Pittelkow et al. to undergo three electrochemical oxidations and three reductions in a potential range between −2.75 and +1.20 V, the first oxidation occurring at +0.76 V.²⁰⁹ Tetraaza[8]circulenes developed by Tanaka et al.²¹⁰ are apparently more electron rich than their tetraoxa congeners. For instance, in the desymmetrized derivative 14.5, three reversible one-electron oxidations were observed at +0.22, +0.63, and +1.00 V, and the radical cation [14.5]˙⁺ was isolated and structurally characterized.²¹¹ As with other systems, the redox activity of circulenes can be considerably extended by fusion of π-conjugated units. The PDI-fused system 14.6 featured two reversible reduction events (−1.21, −1.50 V), which were interpreted as corresponding to four simultaneous electron transfers.²¹² Such behavior is consistent with a weak interaction between the PDI fragments.

Electron deficient 2D-fused systems

In comparison with linearly fused aromatics, 2D-fused systems offer, perhaps non-intuitively, a smaller number of functionalization sites per π-electron center. Consequently, introduction of electron-withdrawing functional groups can produce high theoretical charge capacities only for relatively small π-conjugated cores. Pyrene-4,5,9,10-tetraone 15.1 (Fig. 15) can be reduced to the tetraanion [15.1]⁴⁻, which has a greater flexibility in the placement of Clar sextets than the neutral state. The resulting increase of aromaticity in the tetraanion may account for a relatively high discharge potential achieved in Li-ion batteries based on 15.1 (+2.59 V).²¹³ Quadruple reduction of the relatively small 15.1 corresponds to a high theoretical specific capacity of these batteries (409 mAh g⁻¹), with experimentally observed values of up to 360 mAh g⁻¹. The trioxotriangulene motif (TOT, [15.5]˙, Fig. 15), explored by the groups of Morita and Takui as a material for organic Li-ion batteries,²¹⁴ provides an instructive example of engineering a multielectron system using keto groups. The neutral [15.5]˙ is an ambient-stable monoradical, which may be viewed as an electron-poor, flattened version of the trityl radical. [15.5]˙ has a doubly degenerate LUMO level that creates a relatively small energy gap above the SOMO (0.82 eV in [15.5a]˙ and 0.56 eV in [15.5b]˙). These features could in principle make [15.5]˙ a 5-electron acceptor, however, a maximum of four reductions were observed in actual experiments. The first electron is transferred with particular ease, leading to the monoanion [15.5]⁻, which is stabilized by a combination of carbanionic (I, N_CS = 3) and phenolate resonance contributors (II, N_CS = 2). Further electrochemical reduction in solution leads to three one-electron events at closely spaced potentials. In the tetraanion [15.5]⁴⁻, the three carbonyls are likely completely reduced and mostly decoupled from the aromatic core, which attains triangulene-like conjugation. The use of [15.5a]˙ in Li-ion batteries led to high discharge capacities (>300 Ah kg⁻¹). In comparison, batteries based on the more electron deficient [15.5b]˙ produced a higher output voltage and improved cycle performance.

Fig. 15 Electron-deficient 2D-fused aromatics. Aromatic sextets are shaded in light red.

A different type of stabilization of reduced states is achieved in rylene tetraesters (e.g. 15.2) and diimides (e.g. 15.3a–d). These systems form easily accessible radical anions and dianions. The particular stability of the dianions originates from cross-conjugation of the aromatic core with the electron efficient substituents, which can occur without decreasing the Clar sextet count (Fig. 15). Because the ester groups are twisted away from the plane of the rylene, the conjugation is weaker, but the reductions nevertheless occur at relatively high potentials, making e.g. 15.2 useful in electrochromic applications (Fig. 6).⁶⁹ The smallest members of the series, naphthalene tetraesters and diimides (NDIs),²¹⁵ are of particular interest in battery applications, as they can provide the highest theoretical specific capacities. Their redox potentials can be tuned over a substantial range via substitution of the naphthalene core (e.g., 15.3a–d).²¹⁶ Thanks to their chemical robustness, and good crystallinity, NDIs found use as electrode materials in Li-ion batteries,²¹⁶ and as anolytes in aqueous organic redox flow batteries.²¹⁷ According to NICS calculations, the NDI dianion [15.3b]²⁻ shows enhanced diatropicity in the imide rings,²¹⁷ suggesting that the stability of the dianion is derived from the charge-separated bis-pyridinium contributions (II), which have a higher Clar-sextet count than the quinoidal contributors such as I (Fig. 15). Such structures resemble those proposed for reduced pentacosacyclene tetraimide (see below).

By simultaneous application of two or more design principles, it is possible to dramatically extend the multiredox character of 2D-fused aromatics. Simultaneous use of imide fusion, nitrogen doping and oligomerization is illustrated by the tetraimide 15.4, reported by Shinokubo et al.,²¹⁸ which contains two diazazethrene diimide subunits. Even though linked via a single bond and twisted out of planarity, the two units are apparently quite strongly conjugated, yielding four well-resolved one-electron reductions in the −0.36 V to −0.91 V range, and several non-reversible oxidation events. Radial fusion of redox-active units is a potentially more efficient strategy, as it can capitalize on the higher symmetry of the resulting structures, and provide stronger inter-subunit communication than the complementary radial linking discussed above. These advantages are evident even in relatively small molecules such as diquinoxalino[2,3-a:2′,3′-c]phenazine 15.6, which is susceptible to six consecutive one-electron reductions,²¹⁹ a feature that has recently been exploited in a variety of battery setups.^219–222

Multiredox nanographenoids have been obtained by fusion of naphthalimide (NMI) units. The Würthner group developed efficient benzannulation methods suitable for making both planar and curved aromatic molecules, such as the pyrene-based C₆₄ system 16.1 ²²³ or the C₈₀N₅ nanocone 16.2 containing a corannulene core²²⁴ (Fig. 16). Pentannulation of NMI units, explored by our group, produced a variety of extended systems, such azacoronenes (16.3),^196,197 metalloporphyrins (16.4),²²⁵ and cyclooctatetraenes (16.5 ²²⁶ and 16.6 ²²⁷). In these systems, the oxidation behavior usually reflects the properties of the fusion-free core, for instance, 16.3 and its derivatives, inherited the +1 and +2 oxidation states from the parent HPHAC motif. The accessibility of reduced states was found to correlate with the number of low-lying virtual molecular orbitals (MOs). Thus, 16.2, which has three low-energy empty MO levels, underwent six consecutive one-electron reductions, and as many as 10 reductions were observed for 16.3, matching its five low-energy virtual MOs. As shown for 16.4, in the initial reduction products, i.e. [16.4]˙⁻ and [16.4]²⁻, most of the negative charge is located in the porphyrin core, and the contribution of NMI units to charge delocalization gradually increases upon progressive reduction.²²⁵ Theoretical data obtained for 16.5 and 16.6 showed that the stability of anionic states in the pentannulated systems can be linked to an increase of local aromaticity caused by reduction. For instance, in the octaanionic [16.6]⁸⁻, the conjugation in each of the four subunits can be described using local 6e, 10e, and 14e contributions (I–V, Fig. 16). The role of the five membered ring is particularly important, as it can accept one of the two negative charges, yielding an aromatic cyclopentadienyl anion. Anionic states can also be stabilized by multiple cyanation, instead of imide fusion, as shown recently by the Kivala group for an octacyanotridecacyclene derivative.²²⁸

Fig. 16 Imide-fused nanographenoids. dipp = 2,6-di(isopropyl)phenyl.

5. Macrocycles

Porphyrinoids

Oxidation and reduction of free-base porphyrinoids are often associated with a change in the protonation status of the pyrrolic nitrogens. Protonation-coupled redox reactions are of significant interest in their own right,²²⁹ but are outside the scope of this review. Proton transfer can be suppressed, e.g., by complete protonation of the macrocycle,²³⁰ by metal coordination, or by structural modifications at the pyrrolic nitrogens (see below). In porphyrins and their analogues, a two electron oxidation of reduction results in switching of macrocyclic aromaticity (between Hückel-aromatic and either Hückel-antiaromatic or Möbius-aromatic), although analogous changes can occur also in intrinsically non-aromatic macrocycles.²³¹ Metal complexation typically prevents redox-coupled proton transfer, leading to well-defined oxidation and reduction behavior. Typical metalloporphyrins have stable oxidation states in the (−2) to (+2) range,²³² although electrochemical reduction to the hexaanion was achieved for zinc(II) 5,10,15,20-tetra(p-tolyl)porphyrin (17.1a, Fig. 17).²³³ The neutral state of porphyrins is globally aromatic, with an 18π-electron conjugation pathway. The dicationic and dianionic states feature respectively 16π- and 20π-electron pathways, both of which correspond to Hückel antiaromaticity. Both of these redox regimes can be employed in battery applications, as shown by Fichtner et al. in their work on a diethynyl-substituted copper(II) porphyrin 17.1b.²³⁴ The latter compound was employed in two different configurations: as a cathode in a Li–metal cell, operating in the (+2) to (−2) redox range; and as an anode coupled with a graphite cathode, with a working redox range from (0) to (−2). Both of these configurations yielded supercapacitor-like power densities and long cycle life.

Fig. 17 Tuning of multiredox behavior in porphyrins and their analogues. Aromatic, antiaromatic, and odd-electron conjugation pathways are shown respectively in red, blue, and green. The conjugation pathway shown for 17.8 corresponds to the proposed [16]annulene dianion-like conjugation. In the structure of 17.10a and 17.10b, π-bonds are not shown for clarity.

Redox ranges of porphyrins can be controlled by diverse structural modifications, including macrocycle expansion and contraction, peripheral substitution and fusion, heteroatom doping, subunit replacement, and metal coordination. Heteroatom doping at the meso positions has been recently employed by the group of Matano in their metallo-5,15-diazaporphyrins 17.3 (M = Ni, Zn, Cu).^235,236 The neutral state of the latter species is antiaromatic and is very easily oxidized to the corresponding radical cation and dication. The dication, isoelectronic with an undoped neutral metalloporphyrin, is globally aromatic. The related oxygen-doped system 17.4 (M = Ni), reported by Shimizu et al.,²³⁷ features somewhat diminished paratropicity in the neutral state but largely reproduces the redox behavior of 17.3.

The use of non-pyrrolic subunits for tailoring the redox properties of porphyrinoids is illustrated by the chalcogen-containing diazuliporphyrins 17.4 and 17.5, synthesized by the group of Latos-Grażyński.^238,239 In their neutral states, these systems retain the local aromaticity of individual azulene units. When treated with relatively mild oxidants, such as elemental bromine, they undergo sequential oxidation to the respective radical cations and dications, all of which absorbed intensely in the visible and NIR regions. The dications are stabilized by the combination of porphyrin-like 18π-electron macrocyclic aromaticity with tropylium-like aromaticity of the outer seven-membered rings. The color changes observed on going from 17.4 (dark red), through [17.4]˙⁺ (purple) to [17.4]²⁺ (navy blue) were proposed to be useful in electrochromic applications.

From the viewpoint of porphyrinoid chemistry, nitridomanganese(V) phthalocyanine 17.8, reported by Ménard et al.,²⁴⁰ may be interpreted as combining peripheral fusion, heteroatom doping and metal coordination. Similarly to typical metalloporphyrins, the redox range of 17.8 covers states from (−2) to (+2), which are separated by electrochemical events at −1.75, −1.38, +0.02, and +0.45 V. All five oxidation levels were characterized structurally in the solid state and the even-electron states were shown to be diamagnetic by ¹H NMR. The coordinated Mn^VN was found to insignificantly participate in the redox chemistry of 17.8. The aromaticity of the neutral 17.8 was interpreted as corresponding to either neutral [18]annulene or [16]annulene dianion, whereas the dication [17.8]²⁺ was found to be mostly non-aromatic in spite of its formally [16]annulene structure. In contrast, the dianion [17.8]²⁻ was decisively antiaromatic, showing a NICS pattern consistent with an [18]annulene-dianion conjugation.

Radial fusion has been used to produce porphyrin analogues both electron-deficient, such as 16.4 (Fig. 16), or electron-rich. For the latter purpose, fusion of tetrathiafulvalene (TTF) and related units is particularly attractive, because of their easy oxidation to the corresponding radical cations and dications.²⁴¹ Such porphyrins, containing from one to four TTF subunits fused at the β positions as in 17.11a–b, were originally developed by the groups of Jeppsen and Sessler.^242,243 Each of these systems showed two one-electron reductions, likely associated with the metalloporphyrin core, and an increasing number of low-potential oxidation events, occurring in the 0.0–0.7 V range. The tetra-TTF macrocycle 17.11a, underwent at least 5 overlapping oxidations, but the total number of transferred electrons may have been higher. Expanded porphyrins containing benzo-TTF units showed a similar redox behavior.²⁴⁴

Resizing of the macrocycle affects the redox range in the expected way, however, no specific correlation seems to exist between the size of the porphyrinoid and the number of accessible stable oxidation levels. Metal complexes of norcorroles, the smallest contracted porphyrin analogues, are chemically stable in spite of their pronounced 16π-electron antiaromaticity.^245,246 Just as typical metalloporphyrins, nickel(II) norcorrole 17.6 features two reversible reductions and two reversible oxidations. However, all these events occur over a fairly narrow potential range of ca. +2.5 V. The relative ease of oxidation and reduction is rationalized by the global aromaticity of both the dication and dianion (respectively 14π and 18π-electron). 17.6 was utilized in rechargeable batteries both as a cathode active material with a Li metal anode and in lithium-free batteries exploiting the bipolar character of the norcorrole (Fig. 18).²⁴⁷ In the former setup, a maximum discharge capacity of about 207 mAh g⁻¹ was maintained after 100 charge/discharge cycles.

Fig. 18 Charge/discharge reactions for (a) Li-NiNC batteries and (b) NiNC–NiNC batteries (NiNC = 17.6). Reprinted with permission from ref. 247. Copyright © 2014 Wiley-VCH GmbH.

Porphyrinoids containing solely furan or thiophene subunits, cannot undergo deprotonation when oxidized, which makes them useful for controllable redox switching. In such electron-rich systems, two or three neutral and cationic states can be chemically accessed,^248,249 but the range may be extended in larger macrocycles. For instance, electrochemical data obtained for a [38]octaphyrin diradicaloid 17.9, reported by Wu et al.,²⁵⁰ featured two one-electron reductions and four one-electron oxidations at potentials lower than +0.9 V. In contrast, pyrrole-based expanded porphyrins are usually quite electron-deficient, a feature that may be partly caused by the usual substitution with electron-withdrawing pentafluorophenyl groups. Achieving well-defined redox behavior in these systems is however, challenging, because of their conformational flexibility and potentially limited chemical stability. For instance, voltammetric data reported for a mononuclear Zn^II complex of [62]tetradecaphyrin by Yoneda, Osuka et al.²⁵¹ were somewhat poorly resolved, but they nevertheless showed up to three oxidations and up to 8 reductions in the +0.64 V to −2.13 V potential range. Fusing expanded porphyrins into aromatic tapes such as 17.7 ²⁵² apparently yields more robust redox systems, capable of accepting up to 5 electrons. Less obvious fusion patterns, such as the recently reported COT fusion of orangarins,²⁵³ can also lead to oligomacrocyclic systems showing simultaneously multiple oxidation and reduction events.

In the examples of metal complexes highlighted above, the coordinated ions played mainly a structural role, stabilizing the geometry of the macrocycle and preventing proton transfer reactions. In the triple-decker complexes 17.10a and 17.10b, investigated by Ogawa et al.,²⁵⁴ terbium(III) and yttrium(III), respectively, act as linkers between the outer porphyrin decks and the inner fused bisporphyrin. The neutral states of these complexes featured remarkably small energy gaps (ca. 0.33 eV) and were characterized as diradicaloids with singlet ground states and thermally accessible triplets. Voltammograms recorded for 17.10a revealed seven reversible one-electron events: two reductions and five oxidations in the range of −0.73 to +1.44 V. At least 10 electron transfers were observed for a DBU salt of the dianionic [17.10a]²⁻, implying that the redox range may extend to much lower potentials. While the role of the metal ions in the redox processes was not clarified, it seems likely that the reductions and oxidations are primarily associated with the π-systems, with a good electronic communication ensured by the triple-decker structure of the complexes.

Nanohoops and nanorings

[n]Cycloparaphenylenes (CPPs, 19.1–n, n ≥ 5, Fig. 19), first synthesized by Jasti, Bertozzi et al. (n = 9, 12, 18),²⁵⁵ may be viewed as highly strained paracyclophanes containing no bridges between the para-phenylene rings. The smallest of these systems, [5]CPP (19.1–5),^256,257 has a remarkably narrow electrochemical gap and shows two pseudoreversible one-electron oxidations (at +0.25 and +0.46 V) and two reductions (at −2.27 and −2.55 V), but none of the corresponding charged states has been isolated. Radical cations [19.1–n]˙⁺ and dications [19.1–n]²⁺ can be chemically generated^258–260 by oxidation with, e.g., [NO][SbF₆], and they were subsequently characterized using Raman spectroscopy (5 ≤ n ≤ 12),²⁶¹ magnetic circular dichroism (n = 8),²⁶² and NIR fluorescence spectroscopy (5 ≤ n ≤ 9).²⁶³ Unlike the neutral states, which have benzenoid conjugation, the dications show global in-plane aromaticity, corresponding to the (4n − 2)π pathways emerging upon oxidation. For n > 9, the dications undergo symmetry breaking, yielding diradicaloid configurations.²⁶¹ It can be expected that most cycloparaphenylenes should stabilize negative states down to at least (−2). CPP anions characterized to date include [19.1–6]˙⁻, [19.1–6]²⁻,²⁶⁴ and [19.1–8]⁴⁻.²⁶⁵ The latter species shows a significant elliptical distortion in the solid state, which is partly attributable to interactions with the internally bound potassium cations.

Fig. 19 Multiredox nanohoops and nanorings. For clarity, π-conjugation is not shown in porphyrin units of 19.2.

Remarkable instances of multiredox systems are found among Anderson's porphyrin nanorings, exemplified here by 19.2.²⁶⁶ In this system, the macrocycle consisting of 12 zinc(II) 5,15-di(ethynyl)porphyrin subunits is rigidified by coordination of two stacked star-shaped hexapyridine ligands. 19.2 could be oxidized chemically up to the dodecacation [19.2]¹²⁺. Such oxidations tend to occur sequentially, in one-electron steps, and using ¹⁹F NMR spectroscopy of the CF₃ groups installed on the inner ligands, it is possible to directly observe the majority of intermediate states [19.2]ⁿ⁺ (0 < n < 12), including the odd-electron ones.^266,267 Given the extraordinary dimensions of 19.2, the charge density achieved upon oxidation is relatively low; however, the electronic properties of the oligocations strongly depend on the oxidation level, providing evidence for global aromaticity at a previously unexpected scale. While local aromaticity of individual porphyrins prevails in the neutral 19.2 (168π) and the fully charged [19.2]¹²⁺ (156π) intermediate states, i.e. [19.2]⁶⁺ (162π), [19.2]⁸⁺ (160π), and [19.2]¹⁰⁺ (158π) are, respectively, globally aromatic, antiaromatic, and again aromatic.

Other bridgeless cyclophanes

In spite of their structural simplicity, CPPs are rather unusual cyclophanes, because of their high internal strain and in-plane conjugation. Direct linking of larger, angular subunits leads to less strained and potentially more robust systems. Phenanthrenequinone cyclotrimer S6.1 (Scheme 6), was implemented in rechargeable aluminum–organic batteries by the groups of Stoddart and Choi.²⁶⁸ In this particular application, S6.1 underwent three-electron reduction to the complex S6.2, containing the Al-coordinated triradical trianion [S6.1]³⁽˙⁻⁾. S6.1 formed layered superstructures which facilitated reversible insertion and extraction of aluminum during, respectively, discharging and charging of the battery. This feature led to outstanding electrochemical performance, with a reversible capacity of 110 mAh g⁻¹ and cyclability of up to 5000 cycles.

Scheme 6 Multiredox cyclophanes containing no conjugated meso bridges. R = 10-(3,5-di-tert-butylphenyl)anthracen-9-yl in S6.4–n.

The accessible redox range typically increases with the size of the macrocycle, but in order to observe multiple redox events, the interaction between subunits must be sufficiently strong. This requirement is met by the series of oligoradicaloid fluorenyl macrocycles S6.4–n (n = 4 through 6), synthesized by the Wu group.²⁶⁹ Linking the fluorenyls at positions 3,6 enables quinoidal conjugation reminiscent of the Chichibabin hydrocarbon. Electrochemical oxidation and reduction of S6.4–n occurred sequentially, up to, respectively, [S6.4–n]ⁿ⁺ and [S6.4–n]ⁿ⁻ oxidation levels, and the n-cations could in all cases be observed using spectroelectrochemistry. A particularly well resolved cyclic voltammogram was obtained for S6.4–6, revealing a number of redox events at +0.03 V, +0.12 V, +0.25 V, +0.44 V (3e) and at −1.07 V (2e), −1.23 V, 1.30 V, and −1.51 V (2e).

To ensure well-defined redox behavior, it is advantageous to stabilize the conformation of very large macrocycles, e.g., by coordinating templates, as in porphyrin nanorings, or by covalent linking. The latter approach is illustrated by the hexacarbazole bis-macrocycle S6.5, reported recently by Jin, Chen et al.²⁷⁰ In this species, the π system is rigidified by inclusion of a 2,2′,7,7′-spirobifluorenyl subunit, which interconnects two tercarbazole sections. In voltammetric experiments, up to six electrons can be removed from S6.5 (2 e at +0.42 V, 1 e at +0.60 V, 1 e at +0.71 V, 2 e at +1.10 V), and the presence of one-electron events indicated an electronic interaction between the two halves of the molecule. The even-electron cationic states [S6.5]²⁺, [S6.5]⁴⁺, and [S6.5]⁶⁺ were all predicted to have singlet ground states with open-shell oligoradicaloid configurations.

Through-space coupling in cyclophanes

In appropriately rigidified cyclophanes, through-space interactions between π-conjugated subunits provide a mechanism of electronic communication in the charged states, resulting in a multistage redox behavior. This concept has recently been reviewed by Stoddart et al.²⁷¹ and is only highlighted here, using the molecular triangle S6.3 ^272,273 (Scheme 6) as a representative example. S6.3 undergoes six one-electron electrochemical reductions (−0.61, −0.70, −0.77, −1.33, −1.55, −1.72 V). Electron sharing among subunits in such reduced states can be explained in terms of spatial overlap of the lowest virtual orbitals, occurring between adjacent imide moieties at the corners of the triangle.²⁷⁴ When used as a cathode in Li-ion batteries, S6.3 showed a galvanostatic voltage profile with a single plateau at +2.33 V vs. Li⁺/Li, and a specific capacity of 163 mAh g⁻¹, indicating acceptance of 5.8 Li⁺ per molecule.²⁷² It was proposed that the unique performance of S6.3 originated from its strain-free structure, minimization of electrostatic repulsion in the anions, and from the ability to chelate lithium cations by pairs of adjacent imide oxygens.

Phenylene cyclophanes with meso bridges

[2₄]Paracyclophanetetraene 20.1 is a classic example of the interplay between the oxidation level and aromaticity.^275–279 The neutral state of 20.1 features locally aromatic benzene rings, while its dianion [20.1]²⁻ and tetraanion [20.1]⁴⁻ contain macrocyclic conjugation pathways corresponding respectively to [26]annulenoid aromaticity and [28]annulenoid antiaromaticity. The compound was recently revisited by the groups of Petrukhina and Anderson, who provided crystallographic evidence for aromaticity changes in the dianionic and tetraanionic states of 20.1.²⁸⁰ The switching between local and global aromaticity of 20.1 and [20.1]²⁻ was exploited by Choi, Glöcklhofer et al. in organic sodium-ion battery anodes.²⁸¹ The resulting batteries combined stable cycling behavior and showed excellent performance during fast charging and discharging. Squarephaneic tetraimide 20.2, obtained by the Glöcklhofer group,²⁸² features significantly elevated redox potentials relative to those of 20.1, and it could be implemented in Li-ion batteries, albeit, with a moderate cycling performance. Interestingly, the tetraanion [20.2]⁴⁻ was computationally predicted to have a Baird-aromatic 28π-electron ground state.

Tetraaza[1.1.1.1]m,p,m,p-cyclophane, reported by the groups of Hartwig²⁸³ and Tanaka (20.3, Fig. 20),²⁸⁴ may be viewed as a macrocyclic analog of Wurster's blue. Consequently, its most stable oxidation level is the diradical dication [20.3]²⁺: when aryl-substituted, it is air-stable and has a triplet ground state with ΔE_ST ≈ 0.5 kcal mol⁻¹.²⁸⁵ In electrochemical experiments, 20.3 was found to reversibly undergo four one-electron oxidations at −0.03, +0.16, +0.47, and +0.62 V. 20.3 is a versatile building block for the construction of multiredox aromatics, and covalent assemblies containing two copies of 20.3 were subsequently shown to be oxidizable up to a +12 oxidation level.²⁸⁶ 20.4, synthesized by Ito, Tanaka et al.,²⁸⁷ is another example of such a structure: it contains two tetraazacyclophane moieties bound into a macrocycle, and can be oxidized up to a hexacation. In general, cage-like structures containing multiple phenylene rings, aza bridges, and electron rich substituents, often achieve multiple redox states, as exemplified by Ito's double-decker hexaamine 20.5.²⁸⁸ The latter system showed a range of oxidized states in electrochemical experiments: the lowest three species, i.e., [20.5]⁺, [20.5]²⁺, [20.5]⁴⁺ (the latter formed via a two-electron oxidation), were studied in detail and found to have a delocalized intervalence character. The cage design found in 20.4 and 20.5 apparently offers a general approach to multiredox systems, as illustrated by a bis-macrocyclic oligothiophene cage, recently reported by the Wu group, which yielded oxidation states up to +6.²⁸⁹

Fig. 20 Multiredox phenylene macrocycles containing meso bridges.

While nitrogen meso bridges, present e.g. in 20.3–20.5, are relatively weakly coupled with phenylene rings in neutral macrocycles, the single-carbon meso bridges promote strong cross-conjugation, leading to enhanced interactions between subunits in the cyclophane. Such phenylene rings can sustain global aromaticity in neutral macrocycles^290,291 but can also increase the open-shell characteristics. Both features are evident in [6]cyclo-para-phenylmethine 20.6–3, reported by the groups of Casanova and Wu,²⁹² which has a singlet ground state combining 30π-electron global aromaticity with a diradicaloid character. The species stabilizes oxidation levels from −2 through +3, as evidenced by electrochemical and chemical experiments. In analogy to 20.6–3, which could be viewed as an expanded benzene, its larger congeners, 20.6–4 ²⁹³ and 20.6–5,²⁹⁴ may be likened to cyclooctatetraene and [10]annulene, respectively. Both macrocycles display localized quinoidal structures in their neutral states and can both be oxidized up to a tetracation level, with concomitant changes of their global aromatic character.

Replacing some of the methine carbons with nitrogens increases the number of Clar sextets in the neutral state of the cyclophane, as illustrated by the nitrogen-doped superbenzene 20.7 ²⁹⁵ and supernaphthalene 20.8,²⁹⁶ reported by the Chi group. The first of these systems could be oxidized up to the tetracation level, and [20.7]²⁺, [20.7]³⁺ (30π-aromatic) and [20.7]⁴⁺ (28π-antiaromatic) were characterized crystallographically in the solid state. For the bis-macrocyclic 20.8, only the even-electron states could be isolated, i.e., [20.8]²⁺ (diradicaloid), [20.8]⁴⁺ (52π-antiaromatic) and [20.8]⁶⁺ (50π-aromatic, diradicaloid).

[n]Cyclo-para-biphenylmethines 20.9-n, synthesized by the Wu group,²⁹⁷ can be viewed as higher benzologues of the 20.6–n series, or as macrocyclic analogues of the Chichibabin hydrocarbon. The latter relationship is reflected in their oligoradicaloid configurations containing only Clar sextets in the macrocycle, and methine-localized radical sites. For even n values, closed-shell configurations 20.9–n′ can also be constructed; indeed, 20.9–4 is actually quinoidally localized, undergoing rapid valence tautomerization via a tetraradicaloid transition state. Because of their open-shell nature, the 20.6–n macrocycles show redox amphoterism, and higher members of the series (n > 4) have shown up to four oxidations and up to four reductions in electrochemical measurements.

Coronoids and related systems

Coronoids (cycloarenes) consist of contiguously fused rings fully enclosing a macrocyclic opening.^298,299 Investigations of non-functionalized coronoids, such as kekulene (21.1a, Fig. 21),³⁰⁰ have often been hampered by their very low solubility; however, electrochemical data reported for substituted kekulenes and octulenes (21.1b–c, 21.2b–c) show several reductions and oxidations at relatively high absolute potentials.^301,302 To date, no charged states have been isolated for such benzenoid cycloarenes, perhaps because the available substitution patterns are insufficient to protect the resulting coronoid ions from decomposition. Cycloarene analogues featuring five-membered rings, heteroatoms, or incomplete fusion (e.g., 21.3–21.8, Fig. 21) are often much easier to oxidize and reduce than their benzenoid congeners. These features can often be linked to the open-shell character of the neutral state, which results in the narrowing of the energy gap, and to aromatic stabilization achieved by certain charged levels. Because of their sizes and limited stability, cycloarene oligoradicaloids remain challenging to analyze, especially using electrochemical methods.

Fig. 21 Redox-active coronoids. Fully radicaloid configurations are shown for the neutral states of 21.3–21.7.

[4]Chrysaorene 21.7, reported independently by the group of Wu³⁰³ and ourselves,³⁰⁴ is possibly the best-studied case of multiredox behavior in coronoids. The neutral state of 21.7 has a singlet ground state with significant tetraradicaloid character. A theoretical analysis showed that the experimentally observed diatropicity of the neutral 21.7 stems from the global 36π-electron contribution 21.7′, rather than from AWA conjugation. In electrochemical experiments, 21.7 showed four one-electron oxidations and four one-electron oxidations. The radical cation [21.7]˙⁺ and dication [21.7]²⁺ could be generated chemically using elemental iodine, which acts not only as a mild oxidant but also as a source of iodides which were bound in the cavity of the cationic states.³⁰⁴ Reduction with sodium anthracenide in turn produced the anionic states [21.7]²⁻, [21.7]˙³⁻, and [21.7]⁴⁻.³⁰³ Both the dication and the dianion were diamagnetic and globally diatropic, consistent with the 38π-electron aromatic pathway shown in Fig. 21. Unlike all the other attainable redox states, the tetraanion [21.7]⁴⁻ showed a ¹H NMR spectrum indicative of local aromaticity, and a large optical energy gap. Both of these features were consistent with the benzenoid character of the tetraanion, isoelectronic with octulene.

The above results suggest that for the open-shell coronoids, the lower reduction limit can be expected to equal the number of formal radical sites (i.e., 4, 8, 10, and 6 for 21.3,³⁰⁵ 21.4, 21.5,³⁰⁶ and 21.6,³⁰⁷ respectively), since the corresponding oligoanions are strongly stabilized by local aromaticity. An analogous argument will not, however, work for the corresponding cations, which will be destabilized by local antiaromaticity, similar to that of the fluorenyl cation. In many cases, however, the first two oxidations occur at very low potentials, and the corresponding cation, such as [21.7]²⁺ or [21.6]²⁺, is ambient-stable and globally aromatic. The actual range of accessible oxidation levels will depend on the global aromatic/antiaromatic contributions, charge density (which should be higher in systems such as 21.4 and 21.5), and the stability of the cations and anions against decomposition.

The nitrogen-doped coronoid 21.8, obtained by Lu, Zhao et al.,³⁰⁸ is a higher homologue of the bowl-shaped [3]chrysaorole.³⁰⁹ Unlike the tetraradicaloid 21.7, compound 21.8 is a closed-shell species, featuring up to 6 low-potential electrochemical oxidations (+0.08, +0.30, +0.38, +0.60, +0.75 and +0.93 V), yielding hole mobilities of up to 2.06 cm² V⁻¹ s⁻¹ in single-crystal transistors. In this regard, it significantly outperformed the related carbazole cyclooligomer, suggesting potential applications of appropriately designed cycloarenes as organic semiconductors.

7. Conclusions and outlook

Multistage redox behavior represents a compelling expression of π-electron delocalization in organic molecules. As this review illustrates, aromaticity serves as a fundamental organizing principle for understanding and engineering such systems. The emergence of distinct redox states is closely tied to topological, electronic, and conformational properties of the π framework. Linear, cyclic, and radial arrays of redox-active units provide access to particularly diverse and effective multiredox systems. Recent structural developments, which range from new monomeric motifs to complex multiredox arrays based on, e.g. nanographenes and oligoradicaloids such as, rely on the judicious use of global and local aromatic stabilization at specific oxidation levels and on proper tuning of inter-subunit coupling. Continued exploration of these themes promises new materials with tailored redox characteristics for energy, optoelectronic, and molecular switching applications.

Author contributions

M.S. writing – original draft. I. A. B, A. K. G. N. S. writing – review & editing, visualization.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

Financial support was kindly provided by the National Science Center of Poland (UMO-2022/47/B/ST4/00798 to M. S.).

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