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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 112; no. 33; pp. 7723 - 7733
• Main Authors: Lebedev, Artem Y, Filatov, Mikhail A, Cheprakov, Andrei V, Vinogradov, Sergei A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 21-08-2008
• Subjects: A: Molecular Structure, Quantum Chemistry, General Theory; Absorption; Crystallography, X-Ray; Metalloporphyrins - chemistry; Optics and Photonics; Palladium - chemistry; Quantum Theory; Spectrometry, Fluorescence
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp8043626

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 n TCHP’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 (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 (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.