A Green-to-Near-Infrared Photoswitch Based on a Blended Subporphyrazine-Dithienylethene System.

A subporphyrazine (SubPz)-dithienylethene (DTE) photochromic device with 1o and 1c states, was developed and characterized. In this device, the DTE unit can reversibly switch the SubPz absorbance from green to near-infrared [λmax (o/c) = 527 nm/740 nm], as well as the SubPz fluorescence and singlet oxygen quantum yields. The core of this design involves using a highly tunable SubPz chromophore that shares its quasi-isolated ethene moiety with a DTE photoswitch.

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.

Thienylquinonoidal Porphyrins and Hexaphyrins with Singlet Diradical Ground States.

To explore stable organic diradicaloids, meso-thienylquinonoid-substituted porphyrins Pn and hexaphyrins Hn, where "n" denotes the number of thienyl units in the meso-substituents, were synthesized. P0 was identified as a closed-shell quinonoid, whereas P1 was shown to possess significant diradical character with diradical character index (y) of 0.99 and quite small singlet-triplet energy gap (ΔES-T ) of -0.13 kcal mol-1 . P1 was certainly stable, allowing its isolation, but decomposed gradually in solution. In the hexaphyrin series, it was shown that H0 and H1 were closed-shell quinonoids, but H2 was a highly stable diradicaloid with y=0.85 and ΔES-T of -3.72 kcal mol-1 . The high stability of H2 was ascribed to effective spin delocalization over the entire conjugated network. Characteristically, H2 displays an intense absorption band in NIR region at λmax =1175 nm with molar absorption coefficient (ϵ) of 8.81×104  mol-1 L cm-1 , a narrow HOMO-LUMO gap of 0.69 eV, and nine reversible redox potential waves.

A Doubly Zwitterionic Antiaromatic [28]Hexaphyrin Formed upon Deprotonation of 5,20-Di(N-methyl-4-pyridinium)-Substituted [28]Hexaphyrin.

A rectangular 5,20-di(4-pyridyl) [26]hexaphyrin was reduced with NaBH4 to give the corresponding twisted Möbius aromatic [28]hexaphyrin. Subsequent double N-methylation gave a dicationic 5,20-di(N-methyl-4-pyridinium) [28]hexaphyrin, which was converted to a doubly zwitterionic and Hückel antiaromatic [28]hexaphyrin upon deprotonation with sodium methoxide.

A Description of Vibrational Modes in Hexaphyrins: Understanding the Aromaticity Reversal in the Lowest Triplet State.

Aromaticity reversal in the lowest triplet state, or Baird's rule, has been postulated for the past few decades. Despite numerous theoretical works on aromaticity reversal, experimental study is still at a rudimentary stage. Herein, we investigate the aromaticity reversal in the lowest excited triplet state using a comparable set of [26]- and [28]hexaphyrins by femtosecond time-resolved infrared (IR) spectroscopy. Compared to the relatively simple IR spectra of [26]bis(rhodium) hexaphyrin (R26H), those of [28]bis(rhodium) hexaphyrin (R28H) show complex IR spectra the region for the stretching modes of conjugated rings. Whereas time-resolved IR spectra of R26H in the excited triplet state are dominated by excited state IR absorption peaks, while those of R28H largely show ground state IR bleaching peaks, reflecting the aromaticity reversal in the lowest triplet state. These contrasting IR spectral features serve as new experimental aromaticity indices for Baird's rule.

Synthesis of Di-peri-dinaphthoporphyrins by PtCl2 -Mediated Cyclization of Quinodimethane-type Porphyrins.

Di-peri-dinaphthoporphyrins can be regarded as a key and common substructure of fused porphyrinoids. PtCl2 -mediated cycloisomerization reaction of quinodimethane-type porphyrins provided these doubly fused porphyrins, which exhibit characteristic paratropic ring currents that presumably arise from 24π antiaromatic circuit as a dominant resonance contributor. UV/Vis absorption spectra, cyclic voltammetry, and excited-state dynamics as well as theoretical calculation support this conclusion.

5,20-Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1): A Stable Hückel Antiaromatic Hexaphyrin Stabilized by Intramolecular Hydrogen Bonding and Protonation-Induced Conformational Twist To Gain Möbius Aromaticity.

5,20-Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1) 7 was prepared and characterized as a stable Hückel antiaromatic molecule with a dumbbell-like structure stabilized by effective intramolecular hydrogen bonding interactions involving the 2-pyridyl nitrogen atoms. Pd(II) metalation of 7 afforded two bis-Pd(II) complexes, 9-syn and 9-anti, whose structures are rigidly held by Pd(II) coordination, rendering 9-syn to be nonaromatic because of its highly distorted structure and 9-anti to be Hückel antiaromatic because of its enforced planar dumbbell structure. In contrast, protonation of 7 with methanesulfonic acid (MSA) led to the formation of its triprotonated species 7H(3), which has been shown to take on twisted conformations with Möbius aromaticity in CH(2)Cl(2), while the structure was held to be a planar rectangular conformation in the crystal. Excited-state dynamics were measured for 7, 7H(3), 9-syn, and 9-anti, which indicated their electronic nature to be antiaromatic, aromatic, nonaromatic, and antiaromatic, respectively.

Reversal of Hückel (anti)aromaticity in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird's rule.

The reversal of (anti)aromaticity in a molecule's triplet excited state compared with its closed-shell singlet ground state is known as Baird's rule and has attracted the interest of synthetic, physical organic chemists and theorists because of the potential to modulate the fundamental properties of highly conjugated molecules. Here we show that two closely related bis-rhodium hexaphyrins (R26H and R28H) containing [26] and [28] π-electron peripheries, respectively, exhibit properties consistent with Baird's rule. In the ground state, R26H exhibits a sharp Soret-like band and distinct Q-like bands characteristic of an aromatic porphyrinoid, whereas R28H exhibits a broad absorption spectrum without Q-like bands, which is typical of an antiaromatic porphyrinoid. In contrast, the T-T absorption of R26H is broad, weak and featureless, whereas that of R28H displays an intense and sharp Soret-like band. These spectral signatures, in combination with quantum chemical calculations, are in line with qualitative expectations based on Baird's rule.

Ni(II) metalations of [40]- and [42]nonaphyrins(1.1.1.1.1.1.1.1.1): the largest doubly twisted Hückel antiaromatic molecule.

Ni(II) metalation of [42]nonaphyrin gave two Hückel aromatic [42]nonaphyrin Ni(II) complexes that possess different doubly twisted figure-eight conformations. While these complexes are conformers, the activation barriers for conformational interconversion were found to be exceptionally high. However, one-way isomerization from the higher-energy conformer to the lower-energy conformer can be induced upon treatment with trifluoroacetic acid (TFA). This conformational isomerization has been suggested to proceed via a protonation-induced caterpillar-like conformational rotation of the metal-free hexaphyrin-like segment by examining the similar isomerization of an A3 B6 -type [42]nonaphyrin Ni(II) complex. Rh(I) metalation of the lower-energy [42]nonaphyrin Ni(II) complex afforded a [42]nonaphyrin Ni(II) -Rh(I) hybrid complex, which was oxidized with p-chloranil to produce a [40]nonaphyrin Ni(II) -Rh(I) hybrid complex as the largest doubly twisted Hückel antiaromatic molecule to date.

Cross-conjugated hexaphyrins and their bis-rhodium complexes.

A cross-conjugated hexaphyrin that carries two meso-oxacyclohexadienylidenyl (OCH) groups 9 was synthesized from the condensation of 5,10-bis(pentafluorophenyl)tripyrrane with 3,5-di-tert-butyl-4-hydroxybenzaldehyde. The reduction of 9 with NaBH4 afforded the Möbius aromatic [28]hexaphyrin 10. Bis-rhodium complex 11, prepared from the reaction of 10 with [{RhCl(CO)2}2], displays strong Hückel antiaromatic character because of the 28 π electrons that occupy the conjugated circuit on the enforced planar structure. The oxidation of 11 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yielded complexes 12 and 13 depending upon the reaction conditions. Both 12 and 13 are planar owing to bis-rhodium metalation. Although complex 12 bears two meso-OCH groups at the long sides and is quinonoidal and nonaromatic in nature, complex 13 bears 3,5-di-tert-butyl-4-hydroxyphenyl and OCH groups and exhibits a moderate diatropic ring current despite its cross-conjugated electronic circuit. The diatropic ring current increases upon increasing the solvent polarity, most likely due to an increased contribution of an aromatic zwitterionic resonance hybrid.

Gold(III)-iridium(III) hybrid complexes of hexaphyrin(1.1.1.1.1.1).

Au revoIR: Hexaphyrin Au(III)-Ir(III) complexes were prepared from the reaction of a mono-Au(III) [26]hexaphyrin complex with [IrCl(cod)]2 followed by treatment with pyridine. N-Fusion reaction of this complex gave a syn-doubly N-fused hexaphyrin as the major product, which displayed antiaromatic properties such as strong paratropic ring current, a small HOMO-LUMO gap and a low-energy forbidden absorption. The oxidation of this fused product with MnO2 afforded an oxidatively cleaved product.

Hexaphyrin fused to two anthracenes.

Gold standard: A bis(Au(III)) complex containing the title compound was prepared and characterized (see scheme; DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone, Tf=trifluoromethanesulfonyl). Owing to the effective conjugative network over the flat and elongated rectangular molecular frame, this complex displays a remarkably red-shifted and sharp Q-band-like band at 1467 nm, multiple reversible redox potentials, and a large TPA cross-section value.