Carbolong complexes: an emerging class of metallaaromatics for next-generation functional materials

Abstract

Aromaticity and antiaromaticity represent fundamental pillars of chemical stability, reactivity, and performance. As distinctive aromatic complexes with metallaaromaticity, carbolong complexes featuring metal–carbon conjugated polycyclic metallacycles have emerged as an intriguing class of compounds in organometallic chemistry, biomedical science, and materials science. Characterized by a long carbon chain chelated to a metal center via multiple metal–carbon bonds, these complexes exhibit rare Craig aromaticity in addition to Hückel topologies. This unique electronic structure confers exceptional stability, extensive electron delocalization, and highly tunable optoelectronic properties. This review provides a comprehensive analysis of the synthetic strategies and design principles governing carbolong chemistry. We have highlighted recent advances in the single-molecule charge transport of carbolong molecules, the integration of carbolong complexes into high-efficiency solar cells, and their burgeoning potential in photothermal therapy and molecular optoelectronics. By bridging the gap between fundamental metallaaromaticity and functional materials science, this work serves as a strategic roadmap for researchers leveraging carbolong chemistry and related design principles to drive the development of next-generation optoelectronic, energy conversion, and biomedical technologies.

---

Haixin Zhang

Haixin Zhang received a B.S. in Applied Physics from the Zhengzhou University in 2018 and an M.S. in Materials Engineering from the University of Dayton in 2021. Currently, he is pursuing a Ph.D. degree under the supervision of Dr Kun Wang at the University of Miami. His current research is mainly focused on the molecule-scale charge transport, energy conversion, self-assembled supramolecules, and AFM-related material characterization.

Shiyan Chen

Shiyan Chen is currently a Research Assistant Professor in the Shenzhen Grubbs Institute at the Southern University of Science and Technology. He received a B.S. in Material Chemistry from Guizhou University in 2016 and a Ph.D. in Energy Chemistry from the Xiamen University in 2021. After that, he joined the Southern University of Science and Technology as a Postdoc Researcher. His current research is mainly focused on the rational design and development of the main-chain metal-containing conjugated polymers.

Haiping Xia

Haiping Xia is a Chair Professor in the Department of Chemistry at the Southern University of Science and Technology (SUSTech). He obtained his B.S. in Chemistry in 1983, M.S. in Polymer Science in 1986, and Ph.D. in Chemistry in 2002, all from Xiamen University, where he was promoted to Full Professor in 1999. In 2018, he moved to SUSTech and was appointed as the Executive Dean of Shenzhen Grubbs Institute in 2019. His work encompasses the synthesis, reactivities, and applications of metallaaromatics; especially, the chemistry of metallapentalynes and metallapentalenes, also named as “Carbolong Chemistry”, was developed by him.

Kun Wang

Kun Wang is an Assistant Professor in the Department of Physics with a joint appointment in the Department of Chemistry at the University of Miami (Coral Gables, Florida, USA). He earned his B.S. in Physics from Shandong University in 2011 and his Ph.D. from the University of Georgia in 2016. His research group operates at the intersection of physics, chemistry, and nanoengineering, focusing on probing quantum effects, charge transport, optoelectronics, and energy conversion at the atomic and molecular scales. The group aims to translate these insights into innovative technologies for molecular quantum sciences, next-generation optoelectronics, energy harvesting, and sensing.

---

1. Introduction

Aromatic compounds, with cyclic delocalized π conjugation, constitute one of the most fundamental chemical families.^1–5 Due to their enhanced stability compared with that of other saturated compounds, organic aromatics have been widely used in chemical synthesis,^6,7 materials engineering,^8–10 biochemistry,^11–13 nanotechnology,¹⁴ and industries.^15–17 In addition, antiaromatic compounds with similar cyclic structures but different numbers of delocalized π electrons exhibit a relatively narrow HOMO–LUMO gap and efficient charge transport,^18–20 offering new opportunities for applications in organic semiconductors,^21,22 nanoelectronics,^23,24 and energy-storage materials.^25,26 Therefore, aromaticity and antiaromaticity constitute the conceptual basis for structure–property relationships in π-systems, governing stability, reactivity, and electronic response across organic and organometallic chemistry.

As a unique class of aromatic compounds, metallaaromatics include one or more metal atoms in the aromatic cycle(s).^27–29 Metallabenzene, the first, simplest, and most researched metallaaromatic compound, was theoretically predicted by Thorn and Hoffman³⁰ and synthesized and characterized by Roper et al.³¹ in the 1900s. Over the past few decades, besides metallabenzenes,^32,33 a variety of metallaaromatics have been synthesized and investigated, including metallabenzynes,^34–37 heterometallaaromatics,^38–42 spiro metalloles,⁴³ dianion metalloles,^44–46 metallapentalenes,^47,48 and metallapentalynes.^48–50 Different from conventional molecules only containing pπ–pπ conjugation, metallaaromatics with embedded metal atoms provide dπ electrons that conjugate with carbon atoms, creating dπ–pπ conjugation; this uniquely combines the properties of organic aromatics and organometallics.^28,51–53 This dπ–pπ conjugation modulates molecular orbital profiles and, consequently, optical absorption and charge transport behaviors.^48,54 These exceptional properties enable their broad and substantial applications in solar cells,^55,56 memory materials,^57,58 optoelectronics,^59,60 and phototherapy materials.^61,62 Furthermore, compared with traditional organometallics, metal-engaged aromatic rings effectively enhance stability due to aromaticity.^2,63–65 Therefore, in catalytic chemistry, metallaaromatics, especially multi-metal-atoms metallaaromatics with synergism between the multiple metal centers, often exhibit superior catalytic performance.^66,67

Metal–carbon bonds, as the most fundamental features of organometallic chemistry, have been broadly applied in various fields, including materials science,^68,69 applied physics,^70,71 biology,^72–74 and industrial engineering.^75–77 However, polydentate metal–carbon chelation is rare since most metals prefer to chelate with heteroatoms at lower energy rather than with carbon atoms.⁴⁸ In 2013, Xia's group synthesized a planar metallapentalyne that belongs to the class of Craig-type aromatic systems,^49,78 thereby demonstrating aromatic stabilization in a [4n] π-electron framework, in contrast to the classical Hückel⁷⁹ [4n + 2] aromatic rule.^80,81 This molecule has three metal–carbon bonds as a 7-carbon (7C) chain coordinated with a metal atom in the center, forming tridentate chelation. A series of novel polycyclic frameworks were recently created with an extended carbon chain from 7C to 12C or even 15C.⁴⁸ Notably, the 15C complex represents the first metal-centered [15]annulene. Today, the planar conjugation systems of the long unsaturated carbon chain (not less than 7C) coordinated to a transition metal atom with at least three metal–carbon bonds are termed carbolong complexes.^82,83 Due to the unique dπ–pπ conjugation from the embedded metal atom, carbolong complexes yield outstanding chemical and physical features, such as chemical and structural stability,⁴⁹ high electron transmission,⁸⁴ broad light absorption,⁸⁵ and stable photothermal effect.⁸⁶ These special properties hold promise for expanding the application of carbolong chemistry in nanoelectronics,^59,87 synthetic chemistry,⁸⁸ materials science,⁸⁹ and biomedicine.^90–92

To highlight the development of and stimulate interest in emerging carbolong complexes for emerging functional applications, this review examines recent advances in both the synthesis and applications of carbolong chemistry, with a special focus on nanoelectronics, energy conversion, and optoelectronics (Fig. 1). The synthesis and design sections delve into the synthetic strategies employed, highlighting key achievements and the tunable modification of carbolong complexes. The carbolong electronics section focuses on recent advancements in single-molecule charge transport investigations and applications in solar cells, providing insights into the electronic properties of carbolong complexes. Furthermore, the photoresponsive application section explores the application in photothermal devices and the potential of employing carbolong chemistry in optoelectronics and thermoelectric molecular devices.

Fig. 1 Scheme of the structure of this review.

2. Design and synthesis of carbolong complexes

From the first discovery of benzene by Michael Faraday in 1825,⁹³ aromaticity has a history of around 200 years. As one of the most fundamental chemical concepts and properties, it depicts the special stability of some cyclic compounds, for example, benzene and its derivatives,⁹⁴ annulenes,⁹⁵ heteroaromatics,⁹⁶ and fullerenes.⁹⁷ Although aromaticity is an inherent property of molecules, it is related to some typical characteristics,^{28,81,98–100} such as planarity, Hückel rule, bond-length equalization, and the deshielding of exocyclic protons in the ¹H NMR spectrum. In planar cyclic compounds, Hückel's rule explains that a [4n + 2] delocalized π-electron structure in the ring indicates aromaticity, while [4n] delocalized π electrons indicate antiaromaticity. However, carbolong complexes, by replacing one of the bridgehead carbons with a transition-metal atom, change the antiaromaticity of parent organic analogues to Craig-type aromaticity. The inserted d orbital from the metal atom into the conjugation ring system renders Craig aromaticity with the opposite electron rule: the compound has aromaticity with [4n] delocalized π electrons and antiaromaticity with [4n + 2] π electrons. For example, pentalyne, as the parent organic compound of metallapentalyne, is Hückel antiaromatic, but metallapentalyne shows Craig aromaticity.⁴⁹ Therefore, carbolong complexes exhibit stability attributed to their intrinsic aromaticity.

There are two major synthesis strategies for constructing carbolong frameworks. One is a step-by-step synthesis, in which the number of skeleton carbon atoms is increased by adding alkynes stepwise.⁴⁸ As a result, the carbon chain coordinated with metal atoms in the center can be extended from 7C to 12C or even 15C. However, the metal variety can only be osmium (Os) in this synthesis method. The other is the one-pot approach, which uses multiyne chains to chelate metals.¹⁰¹ Besides the simple and convenient procedures, the metal variety can extend to osmium, ruthenium, rhodium, and iridium. In this section, these two synthesis strategies will be elaborated in detail.

2.1 Step-by-step synthesis of carbolong frameworks

Carbolong complexes are categorized according to the number of chelated carbon atoms in their carbolong skeletons, spanning 7C carbolong complexes, 8C carbolong complexes, and so on (Scheme 1). The additional carbon atom source can be an alkyne or isonitrile.

Scheme 1 .

In 2013, Xia and Zhu et al. reported the synthesis of metallapentalyne (7C carbolong complex).⁴⁹ The treatment of osmium precursor complex S1 with methyl propiolate in dichloromethane at room temperature (RT) could yield the 7C carbolong complex 1 in high isolated yield. It is worth noting that it contains a carbyne bond with the M≡C–C angle of ∼130° in an extremely strained structure. The inherent Craig aromaticity could overcome its high ring strain, which may account for the stability of 1. After adding an acid, there was a tautomeric shift of the metal–carbon triple bond from the initial ring to a different five-membered ring (Scheme 2).¹⁰²

Scheme 2 .

The metal–carbon triple bond of metallapentalyne 1 can be reacted with quantitative isocyanides through [2 + 1] cycloaddition, which directs the formation of η²-iminoketenyl complexes 3 (8C carbolong framework).¹⁰³ This species has been postulated as the intermediate in nucleophile-induced carbyne–isocyanide C–C coupling processes. A similar 8C carbolong framework 4 could also be obtained by the treatment of osmium precursor complex S1 with allenylboronic acid pinacol ester.¹⁰⁴ There is σ-aromaticity in the unsaturated three-membered ring, which is supported by experimental observations and theoretical calculations (Scheme 3).

Scheme 3 .

The 9C carbolong framework 5 could be constructed by the [2 + 2] cycloaddition of metallapentalynes and alkynes.¹⁰⁵ Cyclobutadiene and pentalene are antiaromatic species, which are substantially less thermodynamically stable than aromatic moieties. However, using one transition metal could stabilize these two antiaromatic frameworks simultaneously, as reflected by the good stability of carbolong complexes 5. Interestingly, metallacyclobutadiene 6 could be converted into metallacyclopropene 7 upon adding trifluoroacetic acid. The realization of a photoresponsive structurally defined ring contraction was driven by π- and σ-aromaticity relays (Scheme 4).¹⁰⁶

Scheme 4 .

Unsaturated metallacyclopropenes are usually regarded as “three-atom synthons” with high reactivities, which could undergo [3 + 2] cycloaddition with various unsaturated substrates.¹⁰⁷ The chlorine ligand of complex 4 is removed by treatment with silver tetrafluoroborate. Subsequently, the strained three-membered ring of osmapentalenes undergoes [3 + 2] cycloaddition with alkynones, leading to a 10C carbolong framework 8. The three fused five-membered rings are both aromatic. However, they are non-aromatic in t-BuNC substituted complex 9 (Scheme 5).

Scheme 5 .

The 11C carbolong framework 11 could be obtained by the [2 + 2 + 2] cycloaddition of metallapentalynes and alkynes.¹⁰⁸ The [2 + 2] cycloaddition intermediate A was first formed with the insertion of an alkyne. Then, intermediate A could react with another molecule of alkyne to generate an 11C carbolong framework. The resulting osmium six-membered ring structure does not have a stable planar spatial configuration. It could be converted to a more thermodynamically stable complex 12 under heating conditions (Scheme 6).

Scheme 6 .

Using the carbon chain-growing strategy, the 12C carbolong frameworks 13 and 14 could be easily constructed by adding alkynes or allenylboronic acid pinacol ester to the 8C carbolong framework 4 in the presence of AgBF₄.⁹² This type of osmium complex represents the highest number of coordinated carbon atoms in the equatorial plane. The large π-conjugation with good aromaticity exhibits broad and strong ultraviolet-visible-near-infrared absorption bands and excellent photothermal properties (Scheme 7).

Scheme 7 .

The 15C carbolong framework 20 consists of five fused five-membered aromatic rings and exhibits overall D_5h symmetry (Scheme 8).¹⁰⁹ These compounds were synthesized from 11C carbolong complexes.^110–112 The alkynyl-substituted complex 16 was obtained by treating complex 15 with alkynyl lithium reagents. Subsequently, complex 16 underwent a cycloaddition reaction with TsC≡CH to afford the metal-centered [15]annulene 17. Interestingly, a series of skeleton transformations based on the 15C carbolong framework were observed under different reaction conditions. Ultimately, a molecule with a “Chinese plum blossom”–like architecture is successfully synthesized.

Scheme 8 .

2.2 One-pot synthesis of carbolong frameworks

Chelating transition metals with multiyne chains could generate carbolong frameworks in a one-pot synthesis.¹⁰¹ The direct reactions of these multiyne chains with commercially available substrates (complexes of osmium, ruthenium, rhodium, and iridium), even simple metal salts, afford an atom-economic, convenient, and efficient one-pot strategy for constructing tridentate carbon-ligated chelates under ambient conditions on a gram scale. The core structure of multiyne chains ligands consists of three parts: 1,4-pentane-3-ol, flexible alkyl linking groups, and terminal alkynes (Scheme 9).

Scheme 9 .

As shown in Scheme 10, in the structure of multiyne L1, three C≡C triple bonds are separated by sp³-hybridized carbon atoms. Specifically, the three consecutive sp³ carbon atoms were designed to form a five-membered carbocycle with the other two sp carbon atoms, which may facilitate the binding of the carbons to the metal center. The readily feasible combination of multiyne chain L1 with metals gives rise to a versatile carbon-chelating platform that can produce carbolong frameworks with four different metal centers (Osmium, Ruthenium, Iridium, and Rhodium).^{101,110,113,114} In short, multiyne chains act as a carbon scarf to access chelates with three metal–carbon bonds.

Scheme 10 .

By modifying the terminal alkynes of the multiyne chains with 1,3-butyldyne (L2), dienyl (L3), and ortho-phenol (L4) substituents, different carbolong derivatives (25, 26, and 27) could be easily generated, respectively.¹⁰¹ Specifically, during the conversion process of compounds L2 to 26, all eight sp-hybridized carbon atoms and 1 sp³ hybridized carbon atom were converted into sp² hybridized carbons, demonstrating the strong chelating ability of the multiyne chains ligand. In a word, the multiyne chains exhibit strong chelating ability reflected by the one-pot strategy via multiyne chains chelating transition metals, providing a facile, efficient methodology for preparing carbolong frameworks (Scheme 11).^52,115,116

Scheme 11 .

2.3 Fundamental properties relevant to applications

Carbolong frameworks arise when a conjugated carbon chain chelates a transition metal through multiple independent M–C bonds. The resulting metallacycles display Craig-type aromaticity in [4n] π systems and retain planarity through polydentate carbon coordination. These structural features stabilize unusually high carbon coordination at the metal center and create a rigid, delocalized pathway across the metal–carbon scaffold, which is essential for reproducible transport and processing stability.⁸³

The electronic structure couples metal d orbitals with the π manifold and compresses the optical gap. As a result, carbolong complexes commonly show strong and broad absorption that extends from the ultraviolet to the near-infrared (NIR) region. The absorbed energy can relax radiatively or dissipate as heat or ultrasound, giving photoluminescence together with efficient photothermal and photoacoustic responses.⁵²

Charge-transport and interfacial characteristics connect these molecular traits to device performance. Carbolong motifs function as electron-transmission building blocks and as interlayers in photovoltaic devices. Their interfacial dipoles can downshift electrode work functions and promote charge extraction, supporting both efficiency and operational stability.¹¹⁷ The same chelating design can be embedded in polymer backbones to deliver processable films that retain the underlying electronic features. These properties provide a direct bridge from molecular architecture to the transport, optical, and interfacial functions discussed in the following application sections.⁸³

3. Optoelectronic applications of carbolong complexes

Efficient charge transport and robust chemical stability represent two fundamental prerequisites in the design and development of advanced optoelectronic devices. As an emerging class of metallaaromatics, carbolong complexes, by virtue of their extended π-conjugation and high aromaticity, exhibit both excellent charge-transport characteristics and exceptional chemical stability. This combination of properties, coupled with their tunable charge-transport behavior, renders them promising for applications in next-generation nanoelectronics, including sensors, molecular switches, and diodes.^118–120 Consequently, the application-driven synthesis of functional metallaaromatics is significantly expanding the horizon of advanced functional electronics and sustainable energy technologies, such as solar cells.⁵⁴ The unique charge-transport properties of carbolong complexes, stemming from the dπ–pπ conjugation inherent in their multiple metal–carbon bonds and polydentate chelation, are particularly favorable for their integration into single-molecule electronics,^59,87 organic solar cells (OSCs),^55,121 and perovskite solar cells (PSCs).^56,122,123 Furthermore, the enhanced stability conferred by the complexes’ aromaticity often leads to a longer operational lifetime and sustained high efficiency in solar cell devices.⁵⁴ This section will delve into the distinct advantages and current applications of carbolong complexes within the fields of molecular electronics and photovoltaic devices.

3.1 Carbolong-based single-molecule electronics

Device scaling is approaching fundamental limits from quantum tunneling and power dissipation, pushing conventional solid-state materials toward the bounds of Moore's Law.¹²⁴ To address these challenges, single-molecule electronics have emerged over the past decades, with several demonstrated functional elements, including molecular diodes,¹²⁵ molecular switches,¹²⁶ memory devices,¹²⁷ bio- and chemical sensors,¹²⁸ and molecular transistors.¹²⁹ Conjugated molecules typically serve as the active building blocks, and hydrocarbon π systems and other light-element analogues featuring pπ–pπ conjugation have been widely explored in numerous single-molecule charge-transport studies.¹³⁰ Within the carbolong framework, the incorporation of a metal center chelated by multiple carbon atoms establishes dπ–pπ conjugation, modifies frontier orbital alignment and transmission pathways, and confers aromatic stabilization and chemical robustness. Accordingly, elucidating charge transport through dπ–pπ frameworks in carbolong complexes is essential for identifying faster and more efficient transmission concepts and for broadening the scope of single-molecule electronics. Relative to conventional metallaaromatics, such as metallabenzenes, carbolong frameworks present a single, linear metal–carbon conduction path with strong dπ–pπ delocalization and superior chemical stability.^29,83 This architecture supports reproducible single-molecule conductance, clean protonation-controlled rectification, and robust chemical stability under bias. In contrast, many previously reported metallaaromatic molecules rely on aryl-based ligand frameworks, where rotation between aromatic units and changes in substituents significantly alter conjugation and energy-level alignment, leading to variable molecule–electrode coupling and broad conductance distributions.¹³¹

The construction of a metal–molecule–metal junction device using break junction (BJ) techniques provides a robust approach for characterizing the electrical conductance of individual molecules and elucidating the charge-transport mechanisms at the molecular scale.¹³² Mechanically controlled break junction¹³³ (MCBJ) and scanning tunneling microscope break junction¹³⁴ (STMBJ) are widely adopted BJ techniques. In both methods, a single molecule bridges two metal electrodes that are gradually separated until the junction ruptures. Conductance is extracted from the resulting conductance vs. junction separation traces and statistical analyses, which reveal the most probable conductance of a single molecule. These approaches enable a detailed evaluation of the impact of dπ–pπ conjugation on electron transport.

Primarily attributed to the quantum interference effect, the conductance of single-molecule junctions is contingent upon the connecting sites and the charge transport pathway within the aromatic system.¹³⁵ Conversely, in carbolong metallaaromatic compounds, dπ–pπ conjugated aromatic rings regulate charge transport through the molecular bridge by unique electron delocalization. The investigation of charge transport through carbolong complexes at the single-molecule level was only made possible recently by Li et al.,⁵⁹ using the MCBJ technique. They measured three Osmium (Os)-centered alkene carbolong complexes, 2, 3, and 4, shown in Fig. 2a–c. As shown in the one-dimensional (1D) conductance histograms, the conductance of complex 2 is higher than that of complex 3 by almost one order of magnitude. The conductance of complex 4 only shows an exponential drop without any molecular plateau features (Fig. 2d), implying a conductance lower than the detection limitation. Compared to complex 4, the osmafuran unit in complexes 2 and 3 contributes to a more delocalized structure due to its higher planarity with the fused lactone ring. The opened osmafuran also disrupts the electronic delocalization through the molecular backbone, forcing the charge to migrate through an oxygen atom with a weaker conjugation, thereby resulting in a lower conductance. Meanwhile, sidechain engineering is commonly used to alter the molecular conductance by manipulating the orbital alignment. In complex 3, due to the destructive quantum interference effect, charge transport preferentially proceeds through the carbon chain around the metalacyclic ring rather than through the metal–carbon carbene bond. The phosphonium group in complex 2, however, enhances the carbene character of the Os–C double bond by involving the resonant structure, leading to a higher degree of electron delocalization. These findings indicate that the conductance of carbolong complexes is predominantly governed by the dπ–pπ conjugation, which can be effectively tuned by manipulating the main conjugation backbone and/or side group.

Fig. 2 (a)–(c) Structures of carbolong complexes 2, 3, and 4. (d) Conductance histograms of carbolong complexes. Reproduced with permission from ref. 57. Copyright 2017, American Chemical Society. (e) and (f) Molecular structures of carbolong complexes A-H and A. (g) Conductance histograms of A and A-H. (h) I–V measurements of A and A-H. Reproduced with permission from ref. 58. Copyright 2023, American Chemical Society.

Besides the metal–carbon carbene bond, the metallocarbyne carbolong complex has also exhibited potential for molecular electronic devices due to the stronger connectivity between the metal atom and carbon atoms. Owing to the d-orbital electron activity, a metal atom can participate in multiple bonding reactions, including distinct secondary interactions. For example, the hyperconjugation effect from the metal–carbon center enhances aromaticity for the main molecular backbone and offers an additional way for charge transport. Using the STMBJ technique, Tang et al.⁸⁷ recently investigated the molecular conductance of carbolong complex A (shown in Fig. 2f), including the osmium–carbon triple bond in the central fused ring. The osmium-embedded carbolong carbyne complex yields a higher conductance than its pπ–pπ conjugated counterparts. Moreover, the metal carbyne bond can be protonated to a metal carbene bond, which enables the hyperconjugation with the secondary interaction induced by the metal atom. Although the unprotonated carbyne complex has a higher degree of planarity, the protonated carbolong complex A-H (Fig. 2e) exhibits higher transmission efficiency. Due to the σ-type interaction with the protonated hydrocarbon, the Os-d_xy orbital from the metal atom is further stabilized, which facilitates an energy-level crossing between the LUMO and LUMO+1. Consequently, the osmium center exhibits enhanced electron-accepting character, leading to stronger interactions with the neighboring carbon atoms. The compact A-H molecule renders a twisting angle between the C7–C8 bond and the metal center, causing a smaller spatial gap between the C8 and the osmium atom. The reduced gap makes it possible for charge transport through space rather than through bonds in the molecule, which enhances the conductance of the protonated A-H complex (Fig. 2g). Furthermore, as shown in Fig. 2h, complex A-H shows an asymmetric I–V behavior, a signature of the rectification effect, contrasting the symmetric I–V curve of unprotonated complex A. DFT calculations suggest that the frontier molecular orbital (FMO) of complex A is evenly distributed through the whole backbone, but complex A-H exhibits a more localized FMO resulting from protonation. Since the LUMO of A-H is closer to one of the anchors, this asymmetric orbital distribution facilitates bias-dependent transmission, leading to higher current in the forward bias direction and more suppressed electron transmission in the reverse bias region. Therefore, chemically tailoring transition metals through hyperconjugation represents a promising strategy for regulating charge flow in metallaaromatic molecules.

3.2 Application in solar cells

Solar cells are photovoltaic devices that convert sunlight directly into electricity.¹³⁶ To address resource constraints, environmental pollution, and the inefficiencies of fossil-fuel power generation, photovoltaic technologies have been commercialized and are expected to play a central role in the energy transition and storage landscape. The conversion of solar energy into electrical power proceeds through three steps, namely absorption and exciton generation, charge separation, and carrier collection at the electrodes.¹³⁷ Carbolong complexes are promising electron-transport layers in both OSCs and PSCs because metal–carbon multiple-bond dπ–pπ conjugation is enhanced through backbone transmission within a metallaaromatic framework.⁵⁴ Their aromatic stabilization also promotes chemical robustness, which can extend the operating lifetime at high efficiencies. Taken together, carbolong complexes could be potential electron transport layer (ETL) candidates for next-generation energy-conversion devices due to their improved performance.

Over the past few decades, OSCs have garnered broad attention owing to their lightweight nature, mechanical flexibility, solution processability, and potential for low-cost manufacturing.¹³⁸ These non-fullerene acceptor OSCs typically adopt an acceptor–donor–acceptor molecular architecture with π-conjugated linkers, which enables efficient charge separation and transport. However, such conjugated frameworks are chemically vulnerable when interfaced with low-work-function interlayers or basic cathode interfacial materials (CIMs).¹³⁹ Undesirable chemical interactions at these interfaces can induce structural or electronic degradation of the non-fullerene acceptors, leading to reduced device efficiency and poor operational stability. As a result, the long-term performance of OSCs is often limited by the chemical instability of non-fullerene acceptors in contact with CIMs. Because the photovoltaic efficiency of OSCs critically depends on preserving the electronic integrity of the acceptor materials, the development of CIMs that combine effective energy-level alignment with high structural and chemical stability is essential for realizing commercially viable, long-lived OSC devices.

Previous works have validated that metallic chelation can create high-stability CIMs with improved photovoltaic performance.^55,89,140 Thus, polydentate metal–carbon chelated carbolong compounds are promising candidates for novel CIMs in OSCs. Recently, Lai et al.¹²¹ reported two carbolong-based alcohol-soluble CIMs, single-phenanthroline-carbolong (SPC) and double-phenanthroline-carbolong (DPC). As shown in Fig. 3a, for stabilizing the metal atoms, they adopted the same phenanthroline group as a multi-functional chelating agent connected with one and two carbolong metallapentalene complex(s), respectively. Due to the π delocalization between the phenanthroline and introduced carbolong substitution, both SPC and DPC exhibit lower HOMO and LUMO levels compared to the commonly used CIM bathocuproine. The lower HOMO levels of SPC and DPC can potently hinder the transmission of hole carriers from the active layer to the cathode, avoiding the recombination with electrons. On the other hand, the decreased LUMO level is closer to the work function of the cathode material Ag, which boosts the charge transport at the interface between the active layer and the ETL (Fig. 3b). As a result, the efficiencies of OSCs with SPC and DPC are improved to 17.8% and 18.2%, respectively. Moreover, DPC effectively inhibits the chemical reaction between the cathode interlayer and acceptor on account of its powerful electron-withdrawing properties and large steric hindrance. Consequently, DPC-based devices retain approximately 80% of their initial efficiency after 96 h of thermal aging at 85 °C (Fig. 3d) and exhibit an extended storage T₈₀ lifetime of about 2170 h in the dark (Fig. 3e). Under illumination, DPC-based devices also exhibit enhanced operational stability. This improvement is attributed to the chemically robust and sterically hindered DPC interlayer, which mitigates interfacial degradation at the cathode and thereby prolongs the device lifetime.

Fig. 3 (a) Molecular structures of SPC and DPC. (b) Energy levels of the functional layer with different materials in OSCs. (c) Device structure of a single-junction OSC. (d) Thermal stability of devices with bathocuproine, SPC and DPC cathode interlayers. (e) Storage lifetime of devices with bathocuproine and DPC cathode interlayers. Reproduced with permission from ref. 116. Copyright 2023, Springer Nature.

In addition, organic–inorganic halide PSCs, as another key branch of photovoltaic devices, attract enormous interest due to their superb light absorption properties, ease of fabrication, low cost, and low carrier recombination rates.¹⁴¹ Previous works have demonstrated that the embedded π conjugation between donor and acceptor can optimize the electron transportation to the ETL by applying dipole moments at the interface of perovskite and ETL.⁸⁹ Recently, Wang et al.¹⁴⁰ introduced a series of carbolong-derived complexes as the cathode interlayer in inverted PSCs. Owing to the dipole moments of carbolong derivatives, the work function of cathode materials (e.g., Ag and Au) coated with carbolong molecules can be significantly decreased, enhancing electron transport from the ETL to the external metal cathode. Therefore, using carbolong-derived complexes, the inverted PSCs reached a power conversion efficiency as high as 21.29%. Another recent work also explored the integration of carbolong complexes as an electron-transfer bridge to enhance the interaction between perovskite films and C60 layers. An increased efficiency of up to 25.80% with long-term stability was observed.¹⁴² Conclusively, the incorporation of carbolong complexes enhances solar-cell performance by simultaneously stabilizing the cathode interface and optimizing interfacial charge extraction. Compared with conventional organic cathode interfacial materials, such as bathocuproine, carbolong-based interlayers exhibit downshifted HOMO and LUMO energy levels, which suppress hole leakage and interfacial recombination. At the same time, their LUMO levels align more favorably with the cathode Fermi level, enabling more efficient electron extraction and improved operational stability.

4. Photoresponsive applications

Metal d-orbital participation in dπ–pπ conjugation narrows the HOMO–LUMO gap in carbolong complexes, which produces broad absorption.^49,54 As a result, these complexes exhibit favorable charge-transport characteristics and pronounced photoresponsive behavior, including photothermal conversion,¹⁴³ photoluminescent,⁴⁹ and photoacoustic effect.¹⁴⁴ The broad and intense UV-vis-NIR bands enable efficient photothermal heating that has been applied to phase-transition materials,¹⁴³ self-healing devices,¹⁴⁵ shape memory materials,⁵⁷ and phototherapy.¹⁴⁶ In this section, we use photoresponsive to denote light-induced thermal, optical, or mechanical changes in carbolong materials. Zhu et al.⁹² first synthesized the 12C carbolong complex (Fig. 4a), where all chelating agents are carbon atoms in the equatorial plane. The resulting strong metal–carbon conjugation and high thermodynamic stability give rise to a broad and tunable absorption band spanning the UV-vis to near-infrared region. Owing to this broad NIR absorption, the 12C carbolong complex exhibits efficient and stable photothermal conversion, enabling its application in photothermal cancer therapy. When formulated into polymeric micelles and intravenously injected into tumor-bearing mice, the carbolong derivatives accumulate at the tumor site and generate sufficient localized heating under NIR irradiation. As shown in Fig. 4b, this photothermal treatment leads to pronounced tumor regression, with near-complete tumor elimination observed within 14 days.

Fig. 4 (a) Molecular structures of the 12C carbolong complex. (b) Regression of the tumor after injection of carbolong derivatives. Reproduced with permission from ref. 88. Copyright 2016, American Association for the Advancement of Science. (c) Molecular structure of osmapentalenofuran 12. (d) Molecular structure of metallopolymer 13. (e) Temperature curves of the carbolong-based polymer 13 with five laser on–off cycles. Reproduced with permission from ref. 137. Copyright 2018, Royal Society of Chemistry.

In addition, the integration of carbolong complexes into polymeric architectures has sparked significant interest in metallopolymers within the fields of polymer science. For instance, Lu et al.¹⁴³ in 2018 reported a metallopolymer 13, a system derived based on osmapentalenofuran (Fig. 4c and d). Owing to the strong metal–carbon conjugation and extended π-system of the carbolong motif, polymer 13 exhibits efficient NIR light absorption and rapid photothermal conversion. Under 808 nm laser irradiation (1.0 W cm⁻²), an aqueous solution of polymer 13 shows a temperature increase of approximately 34 °C within 8 min. The photothermal response remains stable over multiple laser on–off cycles, demonstrating excellent photothermal durability (Fig. 4e). In a parallel effort, Zhang et al.¹⁴⁷ developed NIR photothermally healable carbolong polyurethanes (CLPUs) based on the 12C carbolong framework (Fig. 5a–c). The broad UV-vis-NIR absorption of the carbolong units enables efficient photothermal conversion, with the heating efficiency readily tuned by varying the fraction of carbolong moieties incorporated into the polymer backbone (Fig. 5d). Upon NIR irradiation, the localized photothermal heating activates polymer chain mobility at the damaged regions, allowing rapid (<30 s) and repeatable (>5 cycles) self-healing with precise spatial and temporal control (Fig. 5e). Together, these studies demonstrate that carbolong motifs can serve as robust photothermal transducers within polymer matrices, providing a versatile platform for adaptive and self-healing polymeric materials.

Fig. 5 (a) Molecular structure of carbolong polyurethane. (b) Molecular structure of the polymerizable groups. (c) Molecular structure of CLPU containing carbolong polyurethane and the polymerizable groups. (d) Increase in temperature with varying carbolong contents in CLPU. (e) Optical images of the healing process after 15 seconds. Reproduced with permission from ref. 141. Copyright 2019, Royal Society of Chemistry.

As evidenced by the aforementioned studies, the distinctive chemical and physical properties arising from the metal d orbital endow carbolong complexes with superior photothermal and photosensitive properties. These traits not only underscore their burgeoning potential in materials science and biomedical treatments but also position them as compelling candidates in future optoelectronics and thermoelectric molecular devices.

Building on the synthesis–property–application correlations summarized above, recent advances have transitioned carbolong chemistry from a fundamental molecular class into a versatile functional platform. While broad synthetic access has only been realized within the last decade, enabled by stepwise chain-growth and one-pot multiyne chelation strategies, these methodologies now facilitate the gram-scale production of stable frameworks across Os, Ru, Rh, and Ir.^112,148 Although device-level integration remains in its nascent stages, current results establish clear advantages when carbolong complexes are embedded into functional architectures. These include robust photothermal responses, efficient interfacial electron transport, high single-molecule conductance, and compatibility with stable, self-healing elastomers. These recent findings indicate the significant scope for carbolong-based electronics, energy harvesting, and biomedical technologies, necessitating systematic investigations that bridge molecular design with device-level performance metrics.

5. Conclusion and outlook

Emerging as a distinct class of compounds with metallaaromaticity, carbolong complexes exhibit exceptional features such as rare Craig aromaticity, providing enhanced stability, extensive electron transmission, broad absorption, and photothermal characteristics. We have reviewed various synthesis methods, novel molecular designs, optoelectronics applications, and photoresponsive applications of carbolong complexes. The synthesis and design strategies have allowed more accessible and faster carbolong complex fabrication with a high yield, which supports deeper fundamental investigation in physics and chemistry, as well as broader application exploration for catalysis, molecular optoelectronics, energy conversion, and biomedicine. In addition, as discussed in this review, the nature of the unique dπ–pπ conjugation from the embedded metal atom positions carbolong complexes as promising candidates for improving the technology in nanoelectronics, energy conversion, optoelectronics, biomedicine, and intelligent materials.

Looking forward, the future of carbolong chemistry appears vibrant and full of potential. On the synthetic side, progress will come from six aspects: (i) varying the metal center (Os, Ru, Rh, Ir) to tune redox windows and spin–orbit coupling,^110–112 (ii) lengthening or fusing the carbon backbone to enforce full sp² conjugation and narrow the gap,¹⁴⁹ (iii) tailoring ancillary ligands and terminal anchors to adjust interfacial dipoles and contact geometry and coupling,¹⁵⁰ (iv) introducing core protonation or redox switches to modulate conductance and work function while retaining conjugation,⁶⁰ and (v) developing a polymerizable or nanographene hybrid carbolong to yield stable and scalable films;¹⁴⁷ (vi) the fused 15C framework 20 featuring D_5h symmetry is a promising building block for π-conjugated functional molecules and may be considered as a metal-doped graphene fragment.¹⁰⁹ The dπ–pπ conjugation enables intriguing charge transport and presents opportunities for single-molecule electronics within metallaaromatic systems, particularly those exhibiting Craig-type aromaticity or antiaromaticity. Tunable charge pathways in carbolong molecular junctions point to advances in molecular switches, diodes, and sensors. The application of carbolong complexes in organic and perovskite solar cells signals a shift toward clean and highly efficient energy-conversion devices. As research progresses, optimizing and tailoring carbolong derivatives for interlayer and interface design will likely open new frontiers in solar energy technology. Their photothermal and photosensitive properties also suggest promising applications in materials science and biomedical treatment. These design levers can be translated directly to function by controlling single-molecule transport, stabilizing and aligning interfaces in OSC and PSC interlayers, and setting the optical bandwidth that underpins photothermal and other photoresponsive uses, thereby linking molecular edits to device performance.

Translating these opportunities into practical technologies requires addressing several challenges. First, interfacial and operational stability must be demonstrated under illumination, heat, humidity, and bias, since low-work-function contacts can induce parasitic reactions and energy-level drift. Second, the control and reproducibility of the electrical contact remain difficult in single-molecule and thin-film platforms. Variations in anchor geometry, local environment, and electrode coupling can mask intrinsic transport. Third, scalable processing must deliver continuous and oriented films with minimal defects such that charge extraction and optical response are preserved during manufacturing. Addressing these issues in concert with the design levers above will create a clear path from molecular discovery to reliable devices.

Finally, this article captures the current state of carbolong chemistry and propels it into a future where these complexes stand poised to make significant contributions across a spectrum of scientific and technological domains at the molecular scale. Investigations are encouraged to delve deeper into the synthesis, design, and applications of carbolong complexes, fostering innovation and shaping the landscape of cutting-edge materials and molecular devices.

Author contributions

H. Z. and S. C. conceived the project under the guidance of K. W. H. Z. and S. C. performed the analysis and drafted the manuscript in collaboration with K. W. All the authors have reviewed the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

Acknowledgements

We gratefully thank the financial support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0024924 (Conceptualization, analysis, and drafting). K. W. acknowledges the support from the University of Miami Provost's Research Award (UM-PRA-2025-5714). S. C. acknowledges the support from the National Natural Science Foundation of China (No. 22305113 and 22350009).

References

1. W. A. Hofmann, Proc. R. Soc. London, 1857, 8, 1–3.
2. P. von Ragué Schleyer, Chem. Rev., 2001, 101, 1115–1117.
3. P. von Ragué Schleyer, Chem. Rev., 2005, 105, 3433–3435.
4. A. T. Balaban, P. v. R. Schleyer and H. S. Rzepa, Chem. Rev., 2005, 105, 3436–3447.
5. A. Sekiguchi, T. Matsuo and H. Watanabe, J. Am. Chem. Soc., 2000, 122, 5652–5653.
6. K. Parmar, C. S. Blaquiere, B. E. Lukan, S. N. Gengler and M. Gravel, Nat. Synth., 2022, 1, 696–700.
7. N. M.-W. Wu, M. C. Warndorf, A. Alexander-Katz and T. M. Swager, Macromolecules, 2024, 19, 41.
8. J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney and Z. Bao, J. Am. Chem. Soc., 2011, 133, 20130–20133.
9. T. Lei, Y. Cao, Y. Fan, C. J. Liu, S. C. Yuan and J. Pei, J. Am. Chem. Soc., 2011, 133, 6099–6101.
10. J. Huang, Z. Mao, Z. Chen, D. Gao, C. Wei, W. Zhang and G. Yu, Chem. Mater., 2016, 28, 2209–2218.
11. A. B. Okey, D. S. Riddick and P. A. Harper, Trends Pharmacol. Sci., 1994, 15, 226–232.
12. E. A. Meyer, R. K. Castellano and F. Diederich, Angew. Chem., Int. Ed., 2003, 42, 1210–1250.
13. T. Kawakami, K. Murakami and K. Itami, J. Am. Chem. Soc., 2015, 137, 2460–2463.
14. X. Liu, M. He, D. Calvani, H. Qi, K. B. S. S. Gupta, H. J. M. de Groot, G. J. A. Sevink, F. Buda, U. Kaiser and G. F. Schneider, Nat. Nanotechnol., 2020, 15, 307–312.
15. A. S. Dias, S. Lima, M. Pillinger and A. A. Valente, Furfural and Furfural-Based Industrial Chemicals, in Ideas in Chemistry and Molecular Sciences, ed. B. Pignataro, 2010,  DOI:10.1002/9783527630554.ch8.
16. J. P. Lange, E. van der Heide, J. van Buijtenen and R. Price, ChemSusChem, 2012, 5, 150–166.
17. R. J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J. Heeres and J. G. de Vries, Chem. Rev., 2013, 113, 1499–1597.
18. W. Chen, H. Li, J. R. Widawsky, C. Appayee, L. Venkataraman and R. Breslow, J. Am. Chem. Soc., 2014, 136, 918–920.
19. Z. Xie, X. L. Ji, Y. Song, M. Z. Wei and C. K. Wang, Chem. Phys. Lett., 2015, 639, 131–134.
20. L. J. Karas and J. I. C. Wu, Aromaticity, Elsevier, 2021, pp. 319–338.
21. G. Dai, J. Chang, W. Zhang, S. Bai, K.-W. Huang, J. Xu and C. Chi, Chem. Commun., 2014, 51, 503–506.
22. Y. Sun, Y. Guo and Y. Liu, Mater. Sci. Eng., R, 2019, 136, 13–26.
23. S. Fujii, S. Marqués-González, J. Y. Shin, H. Shinokubo, T. Masuda, T. Nishino, N. P. Arasu, H. Vázquez and M. Kiguchi, Nat. Commun., 2017, 8, 1–8.
24. Z. Jin, Z. F. Yao, K. P. Barker, J. Pei and Y. Xia, Angew. Chem., Int. Ed., 2019, 58, 2034–2039.
25. H. U. Kim, J. H. Kim, H. Suh, J. Kwak, D. Kim, A. C. Grimsdale, S. C. Yoon and D. H. Hwang, Chem. Commun., 2013, 49, 10950–10952.
26. Z. Zhang, H. Fan and X. Zhu, Org. Chem. Front., 2017, 4, 711–716.
27. H. Masui, Coord. Chem. Rev., 2001, 219–221, 957–992.
28. B. J. Frogley and L. J. Wright, Chem. – Eur. J., 2018, 24, 2025–2038.
29. D. Chen, Y. Hua and H. Xia, Chem. Rev., 2020, 120, 12994–13086.
30. D. L. Thorn and R. Hoffmann, Delocalization in metallocycles, 1979, 3, 1–1979.
31. G. P. Elliott, W. R. Roper and J. M. Waters, J. Chem. Soc., Chem. Commun., 1982, 811–813.
32. C. E. F. Rickard, W. R. Roper, S. D. Woodgate and L. J. Wright, Angew. Chem., Int. Ed., 2000, 39, 750–752.
33. K. C. Poon, L. Liu, T. Guo, J. Li, H. H. Y. Sung, I. D. Williams, Z. Lin and G. Jia, Angew. Chem., Int. Ed., 2010, 49, 2759–2762.
34. T. B. Wen, Z. Y. Zhou and G. Jia, Collect. Czech. Chem. Commun., 2001, 66, 116.
35. S. M. Ng, X. Huang, T. B. Wen, G. Jia and Z. Lin, Organometallics, 2003, 22, 3898–3904.
36. G. Jia, Acc. Chem. Res., 2004, 37, 479–486.
37. J. Chen and G. Jia, Coord. Chem. Rev., 2013, 257, 2491–2521.
38. R. J. Angelici, F. Ruette, N. Valencia, R. Sanchez-Delgado, L. Latos-Grazynski and L. Lisowski, Organometallics, 1990, 112, 14.
39. R. M. Chin and W. D. Jones, Angew. Chem., Int. Ed., 1992, 31, 357–358.
40. J. R. Bleeke, Acc. Chem. Res., 2007, 40, 1035–1047.
41. B. J. Frogley and L. J. Wright, Coord. Chem. Rev., 2014, 270–271, 151–166.
42. H. Wang, X. Zhou and H. Xia, Chin. J. Chem., 2018, 36, 93–105.
43. Y. Zhang, J. Wei, Y. Chi, X. Zhang, W.-X. Zhang and Z. Xi, J. Am. Chem. Soc., 2017, 139, 5039–5042.
44. M. Saito and M. Yoshioka, Coord. Chem. Rev., 2005, 249, 765–780.
45. M. Saito, M. Nakada, T. Kuwabara and M. Minoura, Chem. Commun., 2015, 51, 4674–4676.
46. Y. Zhang, Y. Chi, J. Wei, Q. Yang, Z. Yang, H. Chen, R. Yang, W. X. Zhang and Z. Xi, Organometallics, 2017, 36, 2982–2986.
47. X. Zhou, J. Wu, Y. Hao, C. Zhu, Q. Zhuo, H. Xia and J. Zhu, Chem. – Eur. J., 2018, 24, 2389–2395.
48. C. Zhu and H. Xia, Acc. Chem. Res., 2018, 51, 1691–1700.
49. C. Zhu, S. Li, M. Luo, X. Zhou, Y. Niu, M. Lin, J. Zhu, Z. Cao, X. Lu, T. Wen, Z. Xie, P. v. R. Schleyer and H. Xia, Nat. Chem., 2013, 5, 698–703.
50. H. Xia, G. He, H. Zhang, T. B. Wen, H. H. Y. Sung, I. D. Williams and G. Jia, J. Am. Chem. Soc., 2004, 126, 6862–6863.
51. S. Chen, L. Feng, L. Peng, X. Gao, Y. Zhu, L. Yang, D. Chen, K. Zhang, X. Guo, F. Huang and H. Xia, Angew. Chem., Int. Ed., 2023, 62, e202305489.
52. M. Luo, D. Chen, Q. Li and H. Xia, Acc. Chem. Res., 2023, 56, 924–937.
53. C. Zhu and H. Xia, Acc. Chem. Res., 2018, 51, 1691–1700.
54. M. Luo, D. Chen, Q. Li and H. Xia, Acc. Chem. Res., 2023, 56, 924–937.
55. L. Liu, S. Chen, Y. Qu, X. Gao, L. Han, Z. Lin, L. Yang, W. Wang, N. Zheng, Y. Liang, Y. Tan, H. Xia and F. He, Adv. Mater., 2021, 33, 2101279.
56. J. Wang, J. Li, Y. Zhou, C. Yu, Y. Hua, Y. Yu, R. Li, X. Lin, R. Chen, H. Wu, H. Xia and H. L. Wang, J. Am. Chem. Soc., 2021, 143, 7759–7768.
57. L. Yang, P. Ouyang, Y. Chen, S. Xiang, Y. Ruan, W. Weng, X. He and H. Xia, Giant, 2021, 8, 100069.
58. L. Yang, H. Zhao, Y. Xie, P. Ouyang, Y. Ruan, J. Chen, W. Weng, X. He and H. Xia, Polym. Chem., 2022, 13, 1844–1851.
59. R. Li, Z. Lu, Y. Cai, F. Jiang, C. Tang, Z. Chen, J. Zheng, J. Pi, R. Zhang, J. Liu, Z.-B. Chen, Y. Yang, J. Shi, W. Hong and H. Xia, J. Am. Chem. Soc., 2017, 139, 14344–14347.
60. C. Tang, X. L. Jiang, S. Chen, W. Hong, J. Li and H. Xia, J. Am. Chem. Soc., 2023, 145, 10404–10410.
61. M. Faustova, E. Nikolskaya, M. Sokol, M. Fomicheva, R. Petrov and N. Yabbarov, ACS Appl. Bio Mater., 2020, 3, 8146–8171.
62. N. Lu, Z. Deng, J. Gao, C. Liang, H. Xia and P. Zhang, Nat. Commun., 2022, 13, 1–11.
63. P. v. R. Schleyer and H. Jiao, Pure Appl. Chem., 1996, 68, 209–218.
64. T. M. Krygowski, M. K. Cyrański, Z. Czarnocki, G. Häfelinger and A. R. Katritzky, Tetrahedron, 2000, 56, 1783–1796.
65. M. Rosenberg, C. Dahlstrand, K. Kilså and H. Ottosson, Chem. Rev., 2014, 114, 5379–5425.
66. C. Gunanathan and D. Milstein, Acc. Chem. Res., 2011, 44, 588–602.
67. T. P. Gonçalves, I. Dutta and K.-W. Huang, Chem. Commun., 2021, 57, 3070–3082.
68. J. S. Miller and A. J. Epstein, Angew. Chem., Int. Ed., 1994, 33, 385–415.
69. Z. Xiao, Y. Yuan, Q. Wang, Y. Shao, Y. Bai, Y. Deng, Q. Dong, M. Hu, C. Bi and J. Huang, Mater. Sci. Eng., R, 2016, 101, 1–38.
70. H. S. Nalwa, Appl. Organomet. Chem., 1991, 5, 349–377.
71. O. H. Nam, M. D. Bremser, B. L. Ward, R. J. Nemanich and R. F. Davis, Jpn. J. Appl. Phys., 1997, 36, 532.
72. M. P. Coogan, P. J. Dyson and M. Bochmann, Organometallics, 2012, 31, 5671–5672.
73. M. Patra and G. Gasser, ChemBioChem, 2012, 13, 1232–1252.
74. E. V. Vinogradova, Pure Appl. Chem., 2017, 89, 1619–1640.
75. G. W. Parshall, R. E. Putscher, K. Ziegler, G. Natta, G. Wilkinson and E. Fischer, J. Chem. Educ., 1986, 63, 189–191.
76. G. W. Parshall, Organometallics, 1987, 6, 687–692.
77. B. M. Gardner, C. C. C. J. Seechurn and T. J. Colacot, Industrial Milestones in Organometallic Chemistry, in Organometallic Chemistry in Industry, ed. T. J. Colacot and C. C. C. J. Seechurn, 2020,  DOI:10.1002/9783527819201.ch1.
78. D. P. Craig and N. L. Paddock, Nature, 1958, 181, 1052–1053.
79. E. Hückel, Quantentheoretische Beiträge zum Benzolproblem, Z. Physik, 1931, 70, 204–286.
80. D. Chen, Q. Xie and J. Zhu, Acc. Chem. Res., 2019, 52, 1449–1460.
81. D. Chen, Y. Hua and H. Xia, Chem. Rev., 2020, 120, 12994–13086.
82. C. Zhu, C. Yang, Y. Wang, G. Lin, Y. Yang, X. Wang, J. Zhu, X. Chen, X. Lu, G. Liu and H. Xia, Sci. Adv., 2016, 2, 1.
83. X. Zhou and Q. Zhuo, Chin. J. Chem., 2025, 44, 583–603.
84. S. Chen, C. Cao, Z. Yu, A. Zhang, X. Lai, L. Peng, Y. Hua, L. Yang, W. Jiang, D. Chen, Z. Wang, F. He and H. Xia, Adv. Funct. Mater., 2023, 33, 2300359.
85. Z. Lu, Q. Lin, Y. Cai, S. Chen, J. Chen, W. Wu, X. He and H. Xia, ACS Macro Lett., 2018, 7, 1034–1038.
86. Z. Lu, Y. Cai, Y. Wei, Q. Lin, J. Chen, X. He, S. Li, W. Wu and H. Xia, Polym. Chem., 2018, 9, 2092–2100.
87. C. Tang, X. L. Jiang, S. Chen, W. Hong, J. Li and H. Xia, J. Am. Chem. Soc., 2023, 145, 10404–10410.
88. Q. Li, Y. Hua, C. Tang, D. Chen, M. Luo and H. Xia, J. Am. Chem. Soc., 2023, 145, 7580–7591.
89. S. Chen, L. Liu, X. Gao, Y. Hua, L. Peng, Y. Zhang, L. Yang, Y. Tan, F. He and H. Xia, Nat. Commun., 2020, 11, 1–11.
90. C. Yang, G. Lin, C. Zhu, X. Pang, Y. Zhang, X. Wang, X. Li, B. Wang, H. Xia and G. Liu, J. Mater. Chem. B, 2018, 6, 2528–2535.
91. X. He, X. He, S. Li, K. Zhuo, W. Qin, S. Dong, J. Chen, L. Ren, G. Liu and H. Xia, Polym. Chem., 2017, 8, 3674–3678.
92. C. Zhu, C. Yang, Y. Wang, G. Lin, Y. Yang, X. Wang, J. Zhu, X. Chen, X. Lu, G. Liu and H. Xia, Sci. Adv., 2016, 2, 1.
93. M. Faraday, Philos. Trans. R. Soc. London, 1825, 115, 440–466.
94. B. Brutschy, Chem. Rev., 2000, 100, 3891–3920.
95. E. L. Spitler, C. A. Johnson and M. M. Haley, Chem. Rev., 2006, 106, 5344–5386.
96. Y. Ouyang, X. H. Xu and F. L. Qing, Angew. Chem., Int. Ed., 2022, 61, e202114048.
97. R. Taylor and D. R. M. Walton, Nature, 1993, 363, 685–693.
98. T. M. Krygowski, M. K. Cyrański, Z. Czarnocki, G. Häfelinger and A. R. Katritzky, Tetrahedron, 2000, 56, 1783–1796.
99. R. H. Mitchell, Chem. Rev., 2001, 101, 1301–1315.
100. T. M. Krygowski and M. K. Cyrański, Chem. Rev., 2001, 101, 1385–1419.
101. Q. Zhuo, J. Lin, Y. Hua, X. Zhou, Y. Shao, S. Chen, Z. Chen, J. Zhu, H. Zhang and H. Xia, Nat. Commun., 2017, 8, 1–7.
102. C. Zhu, M. Luo, Q. Zhu, J. Zhu, P. v. R. Schleyer, J. I. C. Wu, X. Lu and H. Xia, Nat. Commun., 2014, 5, 1–7.
103. M. Luo, L. Long, H. Zhang, Y. Yang, Y. Hua, G. Liu, Z. Lin and H. Xia, J. Am. Chem. Soc., 2017, 139, 1822–1825.
104. C. Zhu, Y. Yang, J. Wu, M. Luo, J. Fan, J. Zhu and H. Xia, Angew. Chem., Int. Ed., 2015, 54, 7189–7192.
105. C. Zhu, Y. Yang, I. Luo, A. Yang, J. Wu, L. Chen, G. Liu, T. Wen, H. Xia, C. Z. Hu, Y. Yang, M. Luo, J. Wu, L. Chen, T. W. En, J. Zhu, H. Xia, C. Yang and G. L. Iu, Angew. Chem., Int. Ed., 2015, 54, 6181–6185.
106. K. Zhuo, Y. Liu, K. Ruan, Y. Hua, Y. M. Lin and H. Xia, Nat. Synth., 2022, 2, 67–75.
107. C. Zhu, J. Wu, S. Li, Y. Yang, J. Zhu, X. Lu and H. Xia, Angew. Chem., Int. Ed., 2017, 56, 9067–9071.
108. C. Zhu, J. Zhu, X. Zhou, Q. Zhu, Y. Yang, T. B. Wen and H. Xia, Angew. Chem., Int. Ed., 2018, 130, 3208–3211.
109. B. Xu, D. Chen, K. Ruan, M. Luo, Y. Cai, J. Qiu, W. Zhou, B. Cao, Z. Lin, J. L. Sessler and H. Xia, Nature, 2025, 641, 106–111.
110. Q. Zhuo, H. Zhang, Y. Hua, H. Kang, X. Zhou, X. Lin, Z. Chen, J. Lin, K. Zhuo and H. Xia, Sci. Adv., 2018, 4, 336–358.
111. J. Guo, Z. Lu, W. Zhou, Q. Zhuo, D. Chen and H. Xia, Inorg. Chem. Front., 2025, 12, 7226.
112. J. Li, Z. Chu, Z. Lu, M. Luo, D. Chen and H. Xia, Organometallics, 2022, 41, 2589–2596.
113. Q. Zhuo, H. Zhang, L. Ding, J. Lin, X. Zhou, Y. Hua, J. Zhu and H. Xia, iScience, 2019, 19, 1214–1224.
114. J. Li, Q. Zhuo, K. Zhuo, D. Chen and H. Xia, Acta Chim. Sin., 2021, 79, 71.
115. Z. Lu, Q. Lin, Y. Cai, S. Chen, J. Chen, W. Wu, X. He and H. Xia, ACS Macro Lett., 2018, 7, 1034–1038.
116. C. Yang, G. Lin, C. Zhu, X. Pang, Y. Zhang, X. Wang, X. Li, B. Wang, H. Xia and G. Liu, J. Mater. Chem. B, 2018, 6, 2528–2535.
117. J. Li, J. Wang, Y. Zhou, C. Yu, H. Liu, X. Qi, R. Li, Y. Hua, Y. Yu, R. Chen, D. Chen, L. Mao, H. Xia and H. L. Wang, Mater. Chem. Front., 2022, 6, 2211–2218.
118. N. Xin, J. Guan, C. Zhou, X. Chen, C. Gu, Y. Li, M. A. Ratner, A. Nitzan, J. F. Stoddart and X. Guo, Nat. Rev. Phys., 2019, 1, 211–230.
119. C. Tang, M. Shiri, H. Zhang, R. T. Ayinla and K. Wang, Nanomaterials, 2022, 12, 698.
120. H. Zhang, M. Shiri, R. T. Ayinla, Z. Qiang and K. Wang, MRS Commun., 2022, 12, 495–509.
121. X. Lai, S. Chen, X. Gu, H. Lai, Y. Wang, Y. Zhu, H. Wang, J. Qu, A. K. K. Kyaw, H. Xia and F. He, Nat. Commun., 2023, 14, 1–11.
122. H. Liu, Z. Lu, W. Zhang, J. Wang, Z. Lu, Q. Dai, X. Qi, Y. Shi, Y. Hua, R. Chen, T. Shi, H. Xia and H. L. Wang, Adv. Sci., 2022, 9, 2203640.
123. J. Li, J. Wang, Y. Zhou, C. Yu, H. Liu, X. Qi, R. Li, Y. Hua, Y. Yu, R. Chen, D. Chen, L. Mao, H. Xia and H. L. Wang, Mater. Chem. Front., 2022, 6, 2211–2218.
124. C. E. Leiserson, N. C. Thompson, J. S. Emer, B. C. Kuszmaul, B. W. Lampson, D. Sanchez and T. B. Schardl, Science, 2020, 368, 1–7.
125. B. Capozzi, J. Xia, O. Adak, E. J. Dell, Z. F. Liu, J. C. Taylor, J. B. Neaton, L. M. Campos and L. Venkataraman, Nat. Nanotechnol., 2015, 10, 522–527.
126. C. D. Frisbie, Science, 2016, 352, 1394–1395.
127. T. Miyamachi, M. Gruber, V. Davesne, M. Bowen, S. Boukari, L. Joly, F. Scheurer, G. Rogez, T. K. Yamada, P. Ohresser, E. Beaurepaire and W. Wulfhekel, Nat. Commun., 2012, 3, 1–6.
128. S. Dey, M. Dolci and P. Zijlstra, ACS Phys. Chem. Au, 2023, 3, 143–156.
129. H. Fu, X. Zhu, P. Li, M. Li, L. Yang, C. Jia and X. Guo, J. Mater. Chem. C, 2022, 10, 2375–2389.
130. Y. Lv, J. Lin, K. Song, X. Song, H. Zang, Y. Zang and D. Zhu, Sci. Adv., 2021, 7, 3095.
131. D. Y. Zubarev, B. B. Averkiev, H. J. Zhai, L. S. Wang and A. I. Boldyrev, Phys. Chem. Chem. Phys., 2007, 10, 257–267.
132. X. Xie, P. Li, Y. Xu, L. Zhou, Y. Yan, L. Xie, C. Jia and X. Guo, ACS Nano, 2022, 16, 3476–3505.
133. D. Xiang, H. Jeong, T. Lee and D. Mayer, Adv. Mater., 2013, 25, 4845–4867.
134. B. Xu and N. J. Tao, Science, 2003, 301, 1221–1223.
135. Y. Zhu, Y. Zhou, L. Ren, J. Ye, H. Wang, X. Liu, R. Huang, H. Liu, J. Liu, J. Shi, P. Gao and W. Hong, Angew. Chem., Int. Ed., 2023, 62, e202302693.
136. A. Goetzberger, J. Luther and G. Willeke, Sol. Energy Mater. Sol. Cells, 2002, 74, 1–11.
137. A. S. Al-Ezzi and M. N. M. Ansari, Appl. Syst. Innov., 2022, 5, 67.
138. D. J. Lipomi, Joule, 2018, 2, 195–198.
139. Y. Ji, L. Xu, X. Hao and K. Gao, Sol. RRL, 2020, 4, 2000130.
140. J. Wang, J. Li, Y. Zhou, C. Yu, Y. Hua, Y. Yu, R. Li, X. Lin, R. Chen, H. Wu, H. Xia and H. L. Wang, J. Am. Chem. Soc., 2021, 143, 7759–7768.
141. J.-P. Correa-Baena, M. Saliba, T. Buonassisi, M. Grätzel, A. Abate, W. Tress and A. Hagfeldt, Science, 2017, 358, 739–744.
142. Y. Yang, S. Chen, Z. Dai, H. Wei, S. Wan, Y. Chen, J. Sun, Z. Liu, L. Ding, H. Xia, R. Chen and H. Wang, Angew. Chem., Int. Ed., 2025, 64, e202420262.
143. Z. Lu, Y. Cai, Y. Wei, Q. Lin, J. Chen, X. He, S. Li, W. Wu and H. Xia, Polym. Chem., 2018, 9, 2092–2100.
144. C. Yang, G. Lin, C. Zhu, X. Pang, Y. Zhang, X. Wang, X. Li, B. Wang, H. Xia and G. Liu, J. Mater. Chem. B, 2018, 6, 2528–2535.
145. H. Zhang, H. Zhao, K. Zhuo, Y. Hua, J. Chen, X. He, W. Weng and H. Xia, Polym. Chem., 2019, 10, 386–394.
146. N. Lu, Z. Deng, J. Gao, C. Liang, H. Xia and P. Zhang, Nat. Commun., 2022, 13, 1–11.
147. H. Zhang, H. Zhao, K. Zhuo, Y. Hua, J. Chen, X. He, W. Weng and H. Xia, Polym. Chem., 2019, 10, 386–394.
148. B. Xu, W. Mao, C. Wu, J. Li, Z. Lu, M. Luo, L. L. Liu, L. Mao, D. Chen and H. Xia, Chin. J. Chem., 2022, 40, 1777–1784.
149. B. Xu, W. Mao, Z. Lu, Y. Cai, D. Chen and H. Xia, Nat. Commun., 2024, 15, 1–8.
150. X. Lai, S. Chen, X. Gu, H. Lai, Y. Wang, Y. Zhu, H. Wang, J. Qu, A. K. K. Kyaw, H. Xia and F. He, Nat. Commun., 2023, 14, 1–11.

---