Deprotonation‐Induced Aromaticity Enhancement and New Conjugated Networks in meso‐Hexakis(pentafluorophenyl)[26]hexaphyrin

Abstract

meso-Hexakis(pentafluorophenyl)-substituted neutral hexaphyrin with a 26π-electronic circuit can be regarded as a real homolog of porphyrin with an 18π-electronic circuit with respect to a quite flat molecular structure and strong aromaticity. We have investigated additional aromaticity enhancement of meso-hexakis(pentafluorophenyl)[26]hexaphyrin(1.1.1.1.1.1) by deprotonation of the inner N-H groups in the macrocyclic molecular cavity to try to induce further structural planarization. Deprotonated mono- and dianions of [26]hexaphyrin display sharp B-like bands, remarkably strong fluorescence, and long-lived singlet and triplet excited-states, which indicate enhanced aromaticity. Structural, spectroscopic, and computational studies have revealed that deprotonation induces structural deformations, which lead to a change in the main conjugated π-electronic circuit and cause enhanced aromaticity.

Synthesis of a [26]Hexaphyrin Bis‐PdII Complex with a Characteristic Aromatic Circuit

“…Similar deprotonation was observed for hexaphyrin 1, [14,15] but it would appear that 12 is more acidic than 1, as double deprotonation of 12 was accomplished with the addition of only three equivalents of 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) in CH 2 Cl 2 , whereas single and double deprotonation of 1 requires about 10 2 and 10 4 equivalents of DBU, respectively. The 1 H NMR spectrum of 13 shows two signals at d = 9.33 and 9.15 ppm, which correspond to the outer b-protons, indicating a slightly enhanced diatropic ring current than that of 12.…”

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

“…To understand the effect of distorted and asymmetric structures of hexaphyrins on their three distinctive vibrational modes, we have analyzed the mean‐plane deviation which represents the degree of structural distortions (Figure ) . The mean plane deviation of R26H (0.266 Å) in the ground state shows smaller values compared to that of R28H (0.381 Å).…”

Structural and Aromaticity Control of [34]Octaphyrin(1.1.1.0.1.1.1.0) by Protonation and Deprotonation

“…Among these means, protonation and deprotonation are expected to be quite effective, since the conformations of expanded porphyrins are usually held by intramolecular hydrogen bonding between the amine‐ and imine‐type pyrroles. Actually, protonation and deprotonation of meso ‐hexakis(pentafluorophenyl) [26]hexaphyrin (1.1.1.1.1.1) and [28]hexaphyrin(1.1.1.1.1.1), meso ‐heptakis(pentafluorophenyl) [32]heptaphyrin(1.1.1.1.1.1.1), and meso ‐octakis(pentafluorophenyl) [36]octaphyrin (1.1.1.1.1.1.1.1) have been reported to lead to changes in conformation and aromaticity. Protonation‐ and deprotonation‐induced conformational changes are naturally dependent upon conformational flexibilities.…”