Effect of Solvent and Protonation/Deprotonation on Electrochemistry, Spectroelectrochemistry and Electron‐Transfer Mechanisms of N‐Confused Tetraarylporphyrins in Nonaqueous Media

Abstract

A series of N-confused free-base meso-substituted tetraarylporphyrins was investigated by electrochemistry and spectroelectrochemistry in nonaqueous media containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) and added acid or base. The investigated compounds are represented as (XPh)4 NcpH2 , in which "Ncp" is the N-confused porphyrin macrocycle and X is a OCH3 , CH3 , H, or Cl substituent on the para position of each meso-phenyl ring of the macrocycle. Two distinct types of UV/Vis spectra are initially observed depending upon solvent, one corresponding to an inner-2H form and the other to an inner-3H form of the porphyrin. Both forms have an inverted pyrrole with a carbon inside the cavity and a nitrogen on the periphery of the π-system. Each porphyrin undergoes multiple irreversible reductions and oxidations. The first one-electron addition and first one-electron abstraction are located on the porphyrin π-ring system to give π-anion and π-cation radicals with a potential separation of 1.52 to 1.65 V between the two processes, but both electrogenerated products are unstable and undergo a rapid chemical reaction to give new electroactive species, which were characterized in the present study. The effect of the solvent and protonation/deprotonation reactions on the UV/Vis spectra, redox potentials and reduction/oxidation mechanisms is discussed with comparisons made to data and mechanisms for the structurally related free-base corroles and porphyrins.

Electrochemistry and spectroelectrochemistry of metallohexaphyrins containing bis-copper or bis-zinc central metal ions

“…In contrast to these values, smaller HOMO-LUMO gaps have been reported for the non-planar copper porphyrins, (OETPP)Cu and (DPP)Cu [50], the copper tetrabenzotetraarylporphyrin [51], the N-confused porphyrins [52][53][54] and the non-planar free-base porphyrins [55], where the separation ranges from 1.5-1.8 V under similar solution conditions.…”

Solvent Effects on the UV–Vis Absorption Properties and Tautomerism of N-Confused Tetraphenylporphyrin

“…20,[24][25] A variety of subsequent experiments support the solvent dependent tautomerization, and several groups have used this information to study the various properties of 1e vs. 1i, as well as the tautomeric forms of other NCTPP derivatives. [22][23][26][27][28] Recently, the construction of self-assembled supramolecular systems was investigated as a function of NCTPP tautomer, and a proposed mechanism of tautomeric structures in a combination of four protic and aprotic solvents using SEM and AFM analysis was described. 29 Computational work by Marchand and Jacquemin, 30 and separately by us, 21,31 using the polarization continuum model in Gaussian09 and density functional theory (DFT), confirmed energetic differences as a function of solvent polarity, but also showed a switch in tautomer preference did not occur under these conditions.…”

Electrochemistry of Methylated N‐Confused Free‐Base Tetraarylporphyrins in Nonaqueous Media

“…N‐confused porphyrins are structurally similar to “normal” porphyrins, but have an inverted pyrrole with a carbon inside the cavity and a nitrogen on the periphery of the π‐system (see structures in Figure ). These compounds have attracted much attention because of their unique spectral and electrochemical properties as compared to “normal” porphyrins …”

“…We recently described the electrochemistry and protonation reactions of N‐confused porphyrins with and without an inner NO 2 group, (Ar) 4 NcpH 2 and NO 2 (Ar) 4 NcpH 2 , and have now turned our attention to the same type of N‐confused free‐base porphyrins with an exterior N‐methylated pyrrole. The structures of these newly investigated compounds are shown in Figure a and represented as N‐CH 3 (Ar) 4 NcpH 2 , where Ncp is the dianion of the N‐confused porphyrin and Ar is a CH 3 OPh, CH 3 Ph, Ph or ClPh substituent on the four meso‐positions of the macrocycle.…”