Photophysical Properties and Electronic Structure of Porphyrins Bearing Zero to Four meso-Phenyl Substituents: New Insights into Seemingly Well Understood Tetrapyrroles.

Six free base porphyrins bearing 0-4 meso substituents have been examined by steady-state and time-resolved absorption and fluorescence spectroscopy in both toluene and N,N-dimethylformamide (DMF). The lifetime of the lowest singlet excited state (S) decreases with an increase in the number of meso-phenyl groups; the values in toluene are HP-0 (15.5 ns) > HP-1 (14.9 ns) > HP-2c (14.4 ns) > HP-2t (13.8 ns) ∼ HP-3 (13.8 ns) > HP-4 (12.8 ns), where "HP" refers to the core free base porphyrin, the numerical suffix indicates the number of meso-phenyl groups, and "c" and "t" refer to cis and trans, respectively. The opposite trend is found for the fluorescence quantum yield; the values in toluene are HP-0 (0.049) < HP-1 (0.063) ∼ HP-2c (0.063) < HP-2t (0.071) < HP-3 (0.073) < HP-4 (0.090). Similar trends occur in DMF. All radiative and nonradiative (internal conversion and intersystem crossing) rate constants for S decay increase with the increasing number of meso-phenyl groups. The increase in the rate constant for fluorescence parallels an increase in oscillator strength of the S → S absorption manifold. The trend is reproduced by time-dependent density functional theory calculations. The calculations within the context of the four-orbital model reveal that the enhanced S ↔ S radiative probabilities derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital, which underpins a parallel bathochromic shift in the S → S absorption wavelength. Polarizations of the S and S excited states with respect to molecular structural features (e.g., the central proton axis) are analyzed in the context of historical conventions for porphyrins versus chlorins and bacteriochlorins, where some ambiguity exists, including for porphine, one of the simplest tetrapyrroles. Collectively, the study affords fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins that should aid their continued widespread use as benchmarks for tetrapyrrole-based architectures in chemical, solar-energy, and life-sciences research.