Five-Fold Symmetric Pentaindolo- and Pentakis(benzoindolo)Corannulenes: Unique Structural Dynamics Derived from the Combination of Helical and Bowl Inversions.

Peripherally π-extended corannulenes bearing quintuple azahelicene units, 10 and 11, were prepared and their dynamic behaviors were studied experimentally and theoretically. The fused corannulenes were synthesized from sym-pentabromocorannulene in three steps. X-Ray diffraction analysis for 10 displayed a conformer possessing a P(M) bowl chirality and a PPMPM (PMPMM) helical chirality, which was found to be the most stable conformer(s). Variable-temperature NMR measurements of 10 and 11 revealed that their structural isomers can be interconvertible in solution, depending on the steric congestion around the helical scaffolds. Automated search for conformers in the equilibrium and transition states by Artificial Force Induced Reaction (AFIR) method revealed their interconversion networks, including bowl-inversion and helical-inversion. This analysis indicated that the co-existing corannulene and azahelicene moieties influence the conformational dynamics, which leads to mitigation of the activation energy barriers for isomerization.

meso-Oxoisocorroles: Tunable Antiaromaticity by Metalation and Coordination of Lewis Acids as Well as Aromaticity Reversal in the Triplet Excited State.

The corrole derivative meso-oxoisocorrole has been theoretically predicted to be antiaromatic, despite its formally cross conjugated electronic system. In this study, this prediction has been experimentally proven by the facile preparation of meso-oxoisocorrole via the oxidation of a meso free corrole with MnO2 and its comprehensive characterization using NMR, UV/vis absorption, FT-IR, and transient-absorption spectroscopy, cyclic voltammetry, and X-ray diffraction analysis. Furthermore, the free base meso-oxoisocorrole was metalated by treatment with Ni(acac)2, PdCl2(PhCN)2, and Zn(OAc)2 to give the corresponding metal complexes. These complexes are more strongly antiaromatic, and their degree of paratropicity depends on their planarity. Thus, fine tuning of their antiaromaticity was achieved with concomitant modulation of their HOMO-LUMO gaps. In the presence of tris(pentafluorophenyl)borane, their antiaromaticity is significantly enhanced due to the elongation of the C═O bond, which promotes the polarized C+-O- resonance state. Furthermore, a distinct frequency shift of the C═O vibrational mode in the triplet state was observed in the time-resolved IR spectra in accordance with the Baird rule, which indicates aromaticity reversal in the excited state.

Stable meso-meso-Linked 2NH-Corrole Radical Dimers as a Key Intermediate to Corrole Tape.

While oxidation of 5,5',15,15'-tetramesityl-10-10'-linked 3NH-corrole dimer with DDQ gave the corresponding triply linked 2NH-corrole tape, the use of an equimolar amount of p-chloranil as a milder oxidant resulted in the formation of a 10-10'-linked neutral 2NH-corrole radical dimer as a stable product. The stability of this peculiar product is ascribed largely to strong antiferromagnetic interaction of the two spins. Further oxidation of this diradical produced corrole tape, suggesting its involvement as a reaction intermediate to the corrole tape. Oxidation of 10-10'-linked bis-pyridine-coordinated CoIII corrole dimer with DDQ produced a cobalt corrole radical dimer and a doubly linked corrole dimer both as stable compounds bearing pyridine and cyanide axial ligands. This type of oxidative transformation involving neutral diradical intermediates is a unique reaction mechanism specific for corrole dimers.

Bis-copper(II) Complex of Triply-linked Corrole Dimer and Its Dication.

Copper complexes of corroles have recently been a subject of keen interest due to their ligand non-innocent character and unique redox properties. Here we investigated bis-copper complex of a triply-linked corrole dimer that serves as a pair of divalent metal ligands but can be reduced to a pair of trivalent metal ligands. Reaction of triply-linked corrole dimer 2 with Cu(acac)2 (acac=acetylacetonate) gave bis-copper(II) complex 2Cu as a highly planar molecule with a mean-plane deviation value of 0.020 Å, where the two copper ions were revealed to be divalent by ESR, SQUID, and XPS methods. Oxidation of 2Cu with two equivalents of AgBF4 gave complex 3Cu, which was characterized as a bis-copper(II) complex of a dicationic triply-linked corrole dimer not as the corresponding bis-copper(III) complex. In accord with this assignment, the structural parameters around the copper ions were revealed to be quite similar for 2Cu and 3Cu. Importantly, the magnetic spin-spin interaction differs depending on the redox-state of the ligand, being weak ferromagnetic in 2Cu and antiferromagnetic in 3Cu.

Singly, Doubly, and Triply Linked Corrole Oligomers: Synthesis, Structures, and Linking Position Dependent Properties.

Direct covalent connections of porphyrinoids have been demonstrated as an effective way to expand their π-electronic networks and perturb their optical and electrochemical properties. Covalent connections are also useful for studies on the precise electronic interaction between two porphyrinoid units. However, the chemistry of covalently linked corrole oligomers lags far behind that of the porphyrin counterparts in spite of the importance of corrole as a ring-contracted porphyrinoid. The lower symmetry of corroles naturally leads to many possible corrole dimers, which have been found to display different optical and electrochemical properties depending on the linking position. In this review, we summarize the synthesis, structures, and properties of corrole oligomers such as singly linked oligomers and fused dimers as well as their metal complexes with a particular focus on their linking position dependent electronic properties.

Stable Face-to-Face Singlet Diradicaloids: Triply Linked Corrole Dimer Gallium(III) Complexes with Two µ-Hydroxo-Bridges.

In known singlet diradicaloid molecules, two inherent radicals are interacted antiferromagnetically as a result of delicate energetic balances between closed-shell and open-shell states. Reported herein is that gallium(III) metalations of triply linked corrole dimers provide face-to-face dimers, with two µ-hydroxo-bridges, and they display curved π-planes nearing each other at about 3.24 Å. Based on ESR and SQUID measurements, and theoretical calculations, the dimers have been assigned as singlet diradicaloids that consist of two monoradicals each delocalized on the triply linked corrole dimer planes. Therefore, it is concluded that the dimers have unusual open-shell states stabilized by large through-space spin-exchange interactions.

Unprecedented rearrangement of diketopyrrolopyrroles leads to structurally unique chromophores.

Diketopyrrolopyrroles possessing thienyl, furyl and benzofuryl substituents undergo unprecedented skeletal rearrangement in the presence of trimethylsilyl bromide resulting in the formation of thieno[2,3-f]isoindole-5,8-diones and furo[2,3-f]isoindole-5,8-diones. These relatively small dyes possess favorable photophysical properties with the emission maxima within the range of 573-624 nm, large fluorescence quantum yields, moderate sensitivity of emission to solvent polarity and a HOMO-LUMO gap of ca. 1.8 eV.

meso-Cumulenic 2H-Corroles from meso-Ethynyl-3H-corroles.

Alkynyl-substituted 3H-corrole 9 a was converted to [3]cumulenic 2H-corrole 10 a by treatment with trimethylsilyl chloride (TMSCl), and 1,3-butadiyne-bridged 3H-corrole dimer 11 b was transformed into [5]cumulene-bridged 2H-corrole dimer 12 b by oxidation with PbO2 . Both 10 a and 12 b were metalated to form ZnII complexes 10 a-Zn and 12 b-Zn. The structures of 10 a-Zn and 12 b-Zn show planar conformations with bond-length alternations that are analogous to those of tetraaryl [n]cumulenes. The cumulenic corrole dimers 12 b and 12 b-Zn display large NIR absorption bands in the range of 700-1400 nm (maximum ϵ≈1.0×105 m-1 cm-1 ) owing to the effective π-conjugation between the two corrole units through the [5]cumulene bridge.

Excited-state torsional relaxation dynamics of meso-meso directly linked corrole dimers: importance of linking position.

Herein, we present a combined spectroscopic and computational study on 5,5'-linked and 10,10'-linked corrole dimers, 5CD and 10CD, respectively, to reveal their strikingly different excited-state dynamics. The excited-state dynamics of 10CD indicate relatively small electronic interactions between the two corrole units, which is similar to the case of the meso-meso directly linked Zn(ii) porphyrin dimer. On the other hand, 5CD exhibits characteristic excitation-wavelength-dependent dual fluorescence. Transient absorption spectra of 10CD on the picosecond timescale showed torsional relaxation with a time constant of 25 ps, whereas the torsional relaxation of 5CD was faster, exhibiting a time constant of 10 ps. Quantum calculations have indicated that the eccentric dual fluorescence and the faster torsional relaxation process of 5CD are consequences of a large π-orbital coefficient at the connecting 5-position, which enhances the conjugative stabilization in the excited-state. In contrast, a small π-orbital coefficient at the 10-position and a larger torsional barrier of 10CD cause monomer-like characters in the excited-state.

Metal Complexes of meso-meso Linked Corrole Dimers.

Cobalt, gallium, silver, and copper complexes of 5,5'-linked corrole dimer 1 and 10,10'-linked corrole dimer 2 were synthesized by metalations with Co(OAc)2·4H2O, GaCl3, AgOAc, and Cu(OAc)2·H2O, respectively, in good yields. The structures of cobalt(III), gallium(III), and silver(III) complexes have been unambiguously revealed by X-ray diffraction analysis. Their optical and electrochemical properties have been studied, which revealed different electronic interactions between the two corrole units depending upon the positions of meso-meso linkage and axial-ligand coordination modes.

Triply Linked Corrole Dimers.

The oxidation of 10-10' singly linked corrole dimers with DDQ at low concentration in CHCl3 afforded meso-meso, ß-ß, ß-ß triply linked 2H-corrole dimers (with two inner NH groups in each corrole unit), which exhibited characteristic (1) H NMR and absorption spectra attributable to their nonaromatic electronic networks. These 2H-corrole dimers were reduced with NaBH4 to aromatic 3H-corrole dimers, which were unstable and easily oxidized back to the 2H-corrole dimers upon exposure to air. Bis(zinc(II)) complexes of the 2H-corrole dimers were synthesized and characterized as rare examples of nonaromatic zinc(II) corrole complexes.

meso-Free Corroles: Syntheses, Structures, Properties, and Chemical Reactivities.

5,10-Bis(pentafluorophenyl)corrole (5) and 5,15-bis(pentafluorophenyl)corrole (9) have been synthesized as meso-free corroles by rational synthetic routes. Both the structures of these corroles have been unambiguously revealed by X-ray diffraction analysis and their optical and electrochemical properties have been studied. Chlorination and oxidative dimerization of 5 and 9 have been explored, which revealed a marked different reactivity of the free meso-positions in 5 and 9. 10-Chlorinated corrole 11 was effectively prepared by the reaction of 9 with Palau'chlor in the presence of 1 % pyridine whereas 5-chlorinated corrole 12 was obtained in a trace amount from similar chlorination of 5. 5,5'-Linked corrole dimer 13 was produced by reaction of 5 with AgNO2 in a good yield, whereas 10,10'-linked corrole dimer 14 was formed in a moderate yield by the reaction of 9 with [bis(trifluoroacetoxy)iodo]benzene. Observed large electronic interaction between the two corroles in 13 as compared with that in 14 has been ascribed mainly to conformational flexibility of the former, which allows more coplanar conformation.

Fused corrole dimers interconvert between nonaromatic and aromatic states through two-electron redox reactions.

A singly linked corrole dimer was synthesized by condensation of a dipyrromethane-1-carbinol with 1,1,2,2-tetrapyrroethane. Oxidation of the dimer gave doubly linked corrole dimers 9 and 10 as the first examples of fused corrole dimers involving a meso-meso linkage. Dimers 9 and 10 exhibit characteristic (1)H NMR spectra, absorption spectra, excited-state dynamics, and two-photon absorption (TPA) values, which indicate the nonaromatic nature of 9 and the aromatic nature of 10. Interestingly, 9 is fairly stable despite its unusual 2H-corrole structure, which has been ascribed to the presence of two direct connections between the individual corrole units.