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. Ar_n-tetracyclohexenoporphyrins (Ar_nTCHP’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 (τ_fl = 2−3 ns, ϕ_fl = 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 (τ_fl = 10−12 ns, ϕ_fl = 0.3−0.4), and their radiative rate constants (k_r) 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 (ϕ_phos = 0.45, τ_phos = 1118 μs), while saddled PdPh₄TCHP is practically nonemissive. In contrast, both ruffled and saddled PdAr_nTBP’s retain strong phosphorescence at ambient temperatures (PdPh₂TBP: τ_phos = 496 μs, ϕ_phos = 0.15; PdPh₄TBP: τ_phos = 258 μs, ϕ_phos = 0.08). It appears that π-extension is capable of counterbalancing deleterious effects of nonplanar deformations on triplet emissivity of Pd porphyrins.