An exocyclic π-system extension of the phenanthriporphyrin framework: towards azaaceneporphyrinoids

Bartosz Szyszko*a, Damian Dróżdża, Aleksandra Sarwaa, Sebastian G. Muchabc, Agata Białońskaa, Michał J. Białeka, Katarzyna Matczyszync and Lechosław Latos-Grażyński*a
aDepartment of Chemistry, University of Wrocław, 14F. Joliot-Curie St., 50-383 Wrocław, Poland. E-mail: bartosz.szyszko@chem.uni.wroc.pl; lechoslaw.latos-grazynski@chem.uni.wroc.pl
bLaboratoire Charles Coulomb, UMR5221, University of Montpellier – CNRS, Montpellier, France
cAdvanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, Wyb. Wyspiańskiego 27, 50-370 Wrocław, Poland

Received 9th April 2020 , Accepted 29th April 2020

First published on 30th April 2020


A facile method for the exocyclic π-extension of the carbaporphyrinoid embedding phenanthrene moiety – phenanthriporphyrin – has been described. The reaction between 5,6-dioxophenanthriporphyrin and a series of aliphatic and aromatic amines provided a family of carbaporphyrinoids incorporating azaacene components. The 1H NMR spectroscopic, electrochemical, and optical properties of azaaceneporphyrinoids strongly depend on the structure of the incorporated moiety. The π-system extension of the attached azaacene subunit reduces the macrocyclic antiaromatic character.


The extension of the π-electron system is an effective strategy allowing for the alteration of the electronic structure of porphyrins and porphyrinoids. Peripheral annulation was demonstrated to strongly influence the electrochemical, optical, and magnetic properties1–5 of macrocycles and often results in a bathochromic shift of the absorption and a red-shift of fluorescence emission.6 Linear and non-linear optical properties of various porphyrinic systems demonstrated large sensitivity to peripheral modifications, with the magnitude of alteration highly dependent on the type and the extent of π-conjugation.7,8 For this reason π-extended porphyrins and porphyrinoids have emerged as a class of organic molecules that may potentially serve as functional materials for the construction of electron- and (or) energy-transferring devices.9 π-Extended macrocycles are being considered as promising candidates for NIR-absorbing and emitting dyes,10,11 dye-sensitized solar cells,12 optical materials,13 organic semiconductors14 and photosensitizing agents for photodynamic therapy.15

One of the most straightforward approaches providing an access to π-enlarged porphyrins is the method pioneered and consequently developed by Crossley and co-workers.16 In this synthetic scenario the respective dioxochlorin I2, which can be obtained from a properly substituted porphyrin of choice (I1),17–19 serves as a fundamental, macrocyclic building block that can be condensed with various amines, providing a family of pyrazine/quinoxaline extended products (Scheme 1A).20–22 Following this methodology, a variety of heterocycle-fused porphyrinoids (e.g. I3 and I4, Scheme 1B) have been prepared.


image file: d0qo00436g-s1.tif
Scheme 1 (A) Synthesis of pyrazine/quinoxaline-fused porphyrins, as reported by Crossley and co-workers,16 and (B) selected examples of pyrazine- and quinoxaline-fused porphyrins and boron(III) subporphyrins.40

Despite considerable synthetic efforts devoted towards the formation of π-extended (hetero)porphyrins and expanded porphyrins, the intentional enlargement of carbocyclic moieties originally incorporated into carbaporphyrinoids was seldom encountered.23–29 In fact, even the formation of π-extended analogues of relatively simple carbaporphyrinoids such as benziporphyrins,30–32 e.g. naphthiporphyrins33–36 or anthriporphyrins,37–39 requires the preparation of specifically tailored precursors incorporating the respective PAH subunit, e.g. naphthalene or anthracene, into the macrocycle.

In 2019 Osuka and co-workers demonstrated that adaptation of Crossley's method to the respective boron(III) subporphyrin-based di-(tetra-)ones can provide mono- and bis-quinoxaline fused boron(III) subporphyrins (I5, I6) showing that the scope of the approach exceeds beyond the porphyrin class, and might be applied for the modification of other porphyrin-inspired systems.40 Lash and co-workers introduced a family of peripherally extended porphyrins bearing, among others, quinoline and isoquinoline motifs attached to pyrrole(s) by the C–C bond, rather than the C[double bond, length as m-dash]N bond.41

In this contribution we report on a facile method for the peripheral π-system extension of the carbocyclic entity embedded in phenanthriporphyrin 1,42–44 the archetypical carbaporphyrinoid incorporating phenanthrene subunit.45–47 Furthermore, spectroscopic studies supported by the DFT calculations performed for a series of new macrocycles allowed for the assessment of the influence of the π-extension of the phenanthrene subunit on the magnetic and electronic properties of the phenanthriporphyrin framework.

In search for a facile strategy of carbaporphyrinoid extension we envisioned that adaptation of Crossley's method for phenanthriporphyrinoids can provide an access to a family of new macrocycles. In fact, the resemblance between dioxochlorin I2 and 5,6-dioxophenanthriporphyrin 2 encouraged the use of the latter as a suitable macrocyclic building block for the carbocycle π-extension (Scheme 2A). The readily available 1, synthesized using the well-optimized procedure, served as the direct macrocyclic precursor of 2, namely, cleavage of methoxy groups in 1 followed by spontaneous oxidation afforded 2 in a very good yield (ca. 85%) as demonstrated earlier.43


image file: d0qo00436g-s2.tif
Scheme 2 (A) Synthesis of azaaceneporphyrinoids, and (B) the macrocycles obtained by this method. Yields correspond to the final condensation step.

With the suitable building block 2 in hand, the general synthetic procedure to form π-extended phenanthriporphyrinoids was elaborated. The reaction between 2 and the appropriately selected 1,2-diamine (5–50-fold excess) proceeded relatively smoothly in pyridine, when carried out at reflux under inert gas conditions. The isolation of a product requires chromatographic separation followed by recrystallization from a suitable solvent system. The relatively wide scope of aliphatic and aromatic diamines has been probed, providing targeted azaacene-incorporated48,49 macrocycles with moderate-to-good yields (Scheme 2B). Altogether with the elongation of the heterocyclic subunit from 3, encompassing the dibenzo[f,h]quinoxaline motif, to 9 containing the tribenzo[a,c,i]phenazine ring, a considerable drop of yields was observed. Eventually, under the given conditions the targeted fusion of the subunit originating from 2,3-diaminophenazine and 9,10-diaminophenanthrene failed. The attempted synthesis of the dimeric structure incorporating the 3,3′-diaminobenzidine motif produced a monomeric structure 8, as the sole macrocyclic product. Analogously, in the reaction between 2 and 1,2,4,5-tetraminebenzene tetrahydrochloride, only monomeric species 7 was detected. The systematic variation of the reaction conditions (time, concentration, stoichiometry, and sequence of reagents addition) did not result in the targeted dimer formation. Nevertheless, diamine 7 was found to constitute a valuable building block allowing for the widening of the scope of accessible heterocyclic motifs by employing the respective dicarbonyl reagents for the reaction. Thus, the condensation of 7 with butano-2,3-dione produced the macrocycle 10 embedding dimethyldibenzo[a,c]pyrazino[2,3-i]phenazine subunit.

The identity of the obtained compounds was confirmed by means of high-resolution mass spectrometry (Fig. S43–S50, ESI) and 1H NMR (Fig. S1–S27, ESI) and 13C NMR spectroscopy (Fig. S28–S42, ESI).

Porphyrinoids 3–10 are soluble in chloroform and dichloromethane, but with increasing size their solubility considerably decreases, likely due to their aggregation in solution.

The single crystals suitable for X-ray crystallographic studies were grown for 3–6 by slow evaporation of the respective solutions in dichloromethane (3–5) or chloroform (6).50

The geometry of 3 in the solid state (Fig. 1) closely resembles the one observed originally for 1 hydrochloride.42 In particular, the molecule adopted a slightly folded conformation with characteristic bowl-like deformation of the dibenzo[f,h]quinoxaline subunit, expressed by a 20.6° dihedral angle between rings A and C. The H(22,25) protons and two nitrogen atoms are pointed in opposite directions of the plane of three-meso-carbon atoms (C3meso). Interestingly, the attachment of two nitrile groups to the pyrazine ring in 4 resulted in nearly complete planarization of the macrocycle. Still, the small twist of the heterocyclic subunit likely helps to reduce the steric repulsion between inner core hydrogen atoms. The torsion angle C(22)–C(23)–C(24)–C(25) equals −6.7°. This angle rises to −13.0° in 5 but it is smaller (−4.5°) in the case of pyridine fused 6. The molecules of 5 and 6 demonstrate a slight twist between the dipyrrin part and the phenanthrene-based heterocyclic segment, so that the dihedral angle between the mean plane of two pyrrole rings and that of the G ring ranges from 8.0° in 6 to 13.7° in 5. The C(21)–C(1)/C(10)–C(11) bond lengths measure 1.477(8)/1.453(8) (3), 1.477(9)/1.464(8) (4), 1.464(3)/1.472(3) (5) and 1.468(4)/1.470(3) (6) indicating that the modified phenanthrenylene subunit is connected with the dipyrrin part through C(sp2)–C(sp2) single bonds.51 The bond lengths are alternated in the dipyrromethene part.


image file: d0qo00436g-f1.tif
Fig. 1 X-ray molecular structures of (A) 3, (B) 4, (C) 5 and (D) 6. Top: front views, bottom: side views with meso-phenyl groups and protons omitted for clarity. Displacement ellipsoids are shown at the 50% probability level.

For further analysis it is necessary to address the feasible communication (coupling or π-delocalization) between carbaporphyrinoid and fused quinoxaline, or quinoxaline-related units. The π-electron conjugation within 1–10 can be, in principle, represented with the use of three fundamental structures corresponding to distinct types of potentially accessible π-electron delocalization (Scheme 3). Thus, class A reflects the electronic features of bridged biphenylcorroles52 being characterized by lack of global π-electron conjugation. In case B, a heterocyclic subunit is directly incorporated in macrocylic π-conjugation, generating the paratropic properties. Finally, in class C, the antiaromatic character of the original phenathriporphyrin 1 is expected to be conserved, presuming the minor electronic impact of annulation.


image file: d0qo00436g-s3.tif
Scheme 3 Feasible π-electron conjugation routes in azaaceneporphyrinoids.

The 1H NMR spectra of 3–5 and 7, 9 and 10 recorded at 300 K are consistent with the effective C2v symmetry, resembling the fundamental features of 1 (Fig. 2 and S5, S15, and S25, ESI).42 The number of resonances in combination with their multiplicity reflects the structure of the macrocycles composed of two pyrrole rings and the respective azaacene-annulated phenanthrene moiety. Lowering the symmetry of the molecule by introducing a pyridine ring (6) or attaching the diaminophenyl group (8) results in doubling of resonances (Fig. S9 and S17, ESI).


image file: d0qo00436g-f2.tif
Fig. 2 The 1H NMR spectra of phenanthriporphyrin 1, and linearly extended 3, 5 and 9 (300 K, CDCl3). The relocation of H(22,25) (phenin) marked in red, H(2,9) and H(3,8) (phenout) in blue and β-pyrrolic signals in green. The relative intensity of the +17 to 11 ppm region in traces was increased 8 times relative to the right part of the spectrum. In this fragment the signal-to-noise ratio was enhanced by exponential apodization which introduced a negligible line broadening.

In the 1H NMR spectra (CDCl3, 300 K) of 3–10, the most down-field shifted signal corresponds to the H(22,25) protons of the π-extended phenanthrene subunit. The NH resonance is buried in the baseline, accompanying the H(22,25) line. Lowering the temperature allows the observation of three separate signals corresponding to one NH and two CH protons located within the cavity (e.g. for 3, Fig. S3 and S4, ESI) reflecting the freezing of the tautomeric NH–N exchange.

The H(22,25) was found to be the most sensitive to peripheral modification of the macrocycle. In fact, the relative position of its signal in the series of phenanthriporphyrinoids spectra can be used as a spectroscopic probe illustrating the changes in the magnetic properties of the molecule (Fig. 2 and S83, ESI). In the case of clearly antiaromatic 1 the chemical shift of H(22,25) equals 16.70 ppm.42 A formal removal of the dimethoxyethene bridge linking the biphenylene moiety in 1 provides biphenylcorrole S2, reported by Srinivasan and co-workers.52 Such a seemingly subtle structural modification strikingly affects the magnetic properties of the macrocyclic system. Consequently, S2 demonstrated the 1H NMR spectroscopic features characteristic of non-aromatic porphyrinoids as its H(22,25) resonance appeared at 9.09 ppm (CD2Cl2, 300 K), clearly indicating the disruption of π-electron delocalization. Therefore, antiaromatic phenanthriporphyrin 1 and non-aromatic biphenylcorrole S2 can be considered to be a pair of structurally-close model compounds determining two limits on a scale of phenanthriporphyrinoid magnetic properties.

Alteration of π-electron conjugation of 1, accomplished by fusing a phenanthrene subunit with the pyrazine ring at 3, is reflected by >2.5 ppm up-field relocation of the H(22,25) resonance, which was detected at 14.15 ppm for 3, and at 13.25 ppm for 4. Benzologation of 3 resulted in a further up-field shift of the H(22,25) peak to 12.90 ppm (5) although a higher value was detected for the electron-rich diamine derivative 7 (13.83 ppm). Replacement of the phenylene ring in 5 with o-fused pyridine (6) practically did not affect the position of the H(22,25) line. Further extension of the attached azaacene moiety resulted in continuous movement of the signals of the internal CH protons up to 11.14 (9)–11.12 ppm (10). Consistently, the resonances of peripheral H(2,9), H(3,8) and β-pyrrolic protons experience a down-field shift, yet the effect is much less pronounced.

It is also clear that the number of nitrogen atoms introduced with the azaacene subunit seems to be irrelevant (5/6 and 9/10). The minor effect reflected the impact of the substitution with electron-withdrawing (3/4) and electron-donating groups (5/7).

The changes in the 1H NMR spectra reflect the gradual reduction of the macrocyclic paratropic ring current upon the π-system extension of the phenanthrene subunit (series 1, 3, 5, 9).

Interestingly, the reverse effect was demonstrated for norcorrole,53,54 when the π-extension via fusion of pyrrole(s) with benzene,55 and pyrazole56 enhanced the overall paratropicity. The antiaromaticity enhancement was also demonstrated for the pyridine-fused bis(norcorrole) system.57 On the other hand, the β,β′-fusion of the quinoxaline-type unit to meso-tetraarylporphyrin, reported by Spence58 and Crossley,16 resulted in a rather minor impact on their macrocyclic aromaticity as reflected by the magnetic criterion based on 1H NMR.59 In fact, it was eventually concluded that quinoxalinoporphyrin acquired a chlorin-like π-delocalization pathway.60–62 Boron(III) quinoxalinosubporphyrin demonstrated the analogous behaviour as well.40

The experimentally detected trend was reproduced in the GIAO/B3LYP/6-31G(d,p) simulated 1H NMR spectra calculated for the DFT-optimized models of phenanthriporphyrinoids (Fig. S71–S82, ESI). In particular the significant drop in the H(22,25) chemical shifts in the function of the π-extension was theoretically reproduced (Fig. S83, ESI).

In order to assess the influence of π-extension on the magnetic properties of phenanthriporphyrinoids the Nucleus-Independent Chemical Shifts (NICS) analysis was performed for macrocycles 1 and 3–10. In addition the hypothetical non-substituted phenanthriporphyrin S1 and biphenylcorrole S2 were included in the theoretical studies serving as the suitable reference compounds. The NICS(0) values were determined in the geometric centre of the cavity defined by 16 atoms which form the smallest macrocyclic circuit (Fig. S84, ESI). A systematic reduction of the NICS(0) value was observed for a series of linearly extended phenanthriporphyrinoids. The values ranged from +11 to +12 ppm for 1 and S1, through +7 to +5 ppm for 3–7, eventually reaching +3.7 ppm for the most extended 9 and 10, a value that approaches the one determined for the non-aromatic biphenylcorrole S2 (+2.7 ppm).

Compounds 3–10 dissolved in chlorinated solvents or toluene form solutions with colours ranging from yellowish green to dark brown. While the electronic absorption characteristics in toluene of 3 and 4 resemble that of phenanthriporphyrin 1, further modifications strongly alter the optical properties of the generated chromophores (Fig. S51–S66, ESI). Phenanthriporphyrinoids vary greatly in their absorption spectra (Fig. 3), although most of them feature the sharp peak located in the UV region (i.e. 359 nm – 9, 360 nm – 3, 365 nm – 1, 373 nm – 6, 380 nm – 4, and 385 nm – 2) while the major band of 7 is rather red-shifted (ca. 448 nm) (Fig. S55 and S65, ESI). In turn, 5 does not show clearly a single band in the high energy region but rather multiple peaks centred at 365 nm, 377 nm, 396 nm, and 419 nm. Besides, each compound reveals the abundance of broad and complex characteristic peaks ranging from the blue (e.g. 9) to near-infrared (e.g. 1) region.


image file: d0qo00436g-f3.tif
Fig. 3 The normalized UV-Vis-NIR absorption spectra of selected phenanthriporphyrinoids in toluene at 298 K.

The cyclic and differential pulse voltammetry measurements were performed in dichloromethane solutions of selected phenanthriporphyrinoids, namely 1 and three simplest, linearly extended derivatives 3, 5, and 9 (Fig. S67–S70, ESI). The representative data are presented in Table 1. The electrochemical HOMO–LUMO gaps demonstrated ca. 0.1 V increase upon linear extension, i.e. 1 – 1.52 eV, 3–5 (1.63–1.69), and ca. 0.1–0.2 V anodic shift of the first oxidation potential. The electrochemically determined HOMO/LUMO gap corresponds favourably with the values calculated by DFT methods (B3LYP/6-31G(d,p), Fig. S85, ESI).

Table 1 First oxidation and reduction potentials, electrochemical HOMO–LUMO gaps, and DFT-calculated HOMO–LUMO gaps of 1, 3, 5, and 9
Compound Eox.1a [V] Ered.1a [V] Eox.1Ered.1 [eV] DFT calc. ELUMOEHOMO [eV]
a Determined by differential pulse voltammetry.
1 0.11 −1.41 1.52 1.87
3 0.29 −1.34 1.63 1.96
5 0.25 −1.44 1.69 2.00
9 0.21 −1.47 1.68 2.01


In conclusion, a facile method for phenanthriporphyrin peripheral π-extension was developed, providing an access to a variety of carbaporphyrinoids incorporating azaarene moieties. Linear π-extension of the fused heterocycle was demonstrated to affect the magnetic properties of the macrocycle, resulting in reduction of the antiaromatic character of phenanthriporphyrin. In fact, the largest of the extended macrocycles demonstrated spectroscopic features typical of non-aromatic carbaporphyrinoids.

The described method for phenanthriporphyrin π-extension is believed to allow for a facile functionalization of the carbaporphyrinoid skeleton with more complex organic architectures providing macrocycles with finely tuned physicochemical properties resulting from the subunit communication via coupling or π-delocalization. Furthermore, azaaceneporphyrinoids 3–10 can be envisaged as a novel type of carbaporphyrinoid which can act as ligands of dual functionalities due to the inner core and perimeter coordination, providing intriguing systems with potential for the construction of ligands for multi-metal catalysis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The project was funded by the National Science Centre of Poland (2017/26/D/ST5/00184 to B. S. and 2016/23/B/ST5/00161 to L. L.-G.). DFT calculations were carried out using resources provided by the Wroclaw Centre for Networking and Supercomputing, Grant 329.

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Footnote

Electronic supplementary information (ESI) available. CCDC 1991712–1991715. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qo00436g

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