Effects of Structural Deformations on Optical Properties of Tetrabenzoporphyrins: Free-Bases and Pd Complexes

Abstract

A recently developed method of synthesis of π-extended porphyrins made it possible to prepare a series of tetrabenzoporphyrins (TBP) with different numbers of meso-aryl substituents. The photophysical parameters of free-bases and Pd complexes of meso-unsubstituted TBP’s, 5,15-diaryl-TBP’s (Ar2TBP’s) and 5,10,15,20-tetraaryl-TBP’s (Ar4TBP’s) were measured. For comparison, similarly meso-arylsubstituted porphyrins fused with nonaromatic cyclohexeno-rings, i.e. Arn-tetracyclohexenoporphyrins (ArnTCHP’s, n = 0, 2, 4), were also synthesized and studied. Structural information was obtained by ab initio (DFT) calculations and X-ray crystallography. It was found that: 1) Free-base Ar4TBP’s are strongly distorted out-of-plane (saddled), possess broadened, red-shifted spectra, short excited-state lifetimes and low fluorescence quantum yields (τfl = 2–3 ns, ϕfl = 0.02–0.03). These features are characteristic of other nonplanar free-base porphyrins, including Ar4TCHP’s. 2) Ar2TBP free-bases possess completely planar geometries, although with significant in-plane deformations. These deformations have practically no effect on the singlet excited-state properties of Ar2TBP’s as compared to planar meso-unsubstituted TBP’s. Both types of porphyrins retain strong fluorescence (τfl = 10–12 ns, ϕfl = 0.3–0.4), and their radiative rate constants (kr) are 3–4 times higher than those of planar H2TCHP’s. 3) Nonplanar deformations dramatically enhance nonradiative decay of triplet states of regular Pd porphyrins. For example, planar PdTCHP phosphoresces with high quantum yield (ϕphos = 0.45, τphos = 1118 µs), while saddled PdPh4TCHP is practically nonemissive. In contrast, both ruffled and saddled PdArnTBP’s retain strong phosphorescence at ambient temperatures (PdPh2TBP: τphos = 496 µs, ϕphos = 0.15; PdPh4TBP: τphos = 258 µs, ϕphos = 0.08). It appears that π-extension is capable of counterbalancing deleterious effects of nonplanar deformations on triplet emissivity of Pd porphyrins.

Ab initio electronic structure calculations of solid, solution-processed metallotetrabenzoporphyrins

“…31,42,[44][45][46][47] Determination of the HOMO and LUMO orbitals yields similar structure to that observed here in the solid-state. The D 2h symmetry of TBP, versus D 4h symmetry in the metal-substituted molecules, is known to lift the molecular orbital degeneracy, as also shown here in the solid-state for k between Z-Γ-Y.…”

Electrochemical study of the dication of porphyrins with carboxylic acids: Shift of the absorption bands compared to that of the redox potentials

“…The out-of-plane deformations of porphyrins and their influences on the electronic and NMR spectroscopy of the aromatic macrocycle have been of interest for chemists for more than two decades [1][2][3][4][5][6]. Chemical properties such as the basicity of two internal pyrrolenine nitrogen atoms, oxidation potentials of porphyrins and their metal complexes and binding affinity of the metal center to axial ligands are known to be modified by the out-of-plane deformations of the porphyrin core [3,7,8].…”

“…Accordingly, the dibasic properties of porphyrins may be utilized to induce different nonplanar distortions of the macrocycle, causing the shifts of the absorption bands in the UV-vis spectra and the chemicals shift of different protons in the NMR spectra. The HOMO/LUMO gap of porphyrins has been shown to be affected by the out-of-plane deformations of the macrocycle, leading to the shift of the Soret and Q bands [1][2][3][4][14][15][16][17]. The formation of donor-acceptor complexes between porphyrins and different acceptors is accompanied with the shift of electron density from the pyrrolenine nitrogens to the acceptor molecules and a decrease in the dihedral angles between the meso-aryl substituents and the porphyrin mean plane [2,14,17] which may also lead to the change of the energy level of the frontier orbitals of the porphyrins and the HOMO/LUMO gap.…”

Synthesis of Metallated (Cu, Pd) Crown Substituted Tetracyclohexenoporphyrins. Crown-Containing Pd Tetrabenzoporphyrin and Its Phosphorescence

“…[28] Нами был снят спектр испускания впервые синтезированного краунированного тетра-бензопорфирина палладия 12-Pd (Рисунок 4). Сравнение полученного спектра со спектром испускания некраунированного аналога PdPh 4 TBP показало, что Рисунок 3.…”