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The ground- and excited-state properties of a series of meso -tetraphenylporphyrin (H 2 TPP) diacids, H 4 TPP(X) 2 (X = F, Cl, Br, I), ad hoc synthesized and characterized by 1 H NMR, RLS, and UV−vis spectroscopies, have been studied theoretically using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Several conformations corresponding to different deformations of the porphyrin core have been explored. The nearly degenerate purely saddled ( sad ) and hybrid (saddled with a small superimposed ruffling: sadruf ) conformations are the preferred “gas phase” conformations. The type and degree of distortion of the macrocycle and the orientation of the phenyl rings compare well to X-ray data available for H 2 TPP diacids. Two electronic structure features are key to an understanding of the optical and photophysical properties. (1) Strong interaction of the π-system of the phenyls with the π-system of the porphyrin leads to an upshift of the G-a 2u (G = Gouterman) orbital and, hence, to a significant splitting of the occupied pair of a 2u /a 1u Gouterman orbitals. The diminished G-a 2u /G-e g * gap and the lifting of the a 2u /a 1u degeneracy explain the red shift of the Q and B bands and the hyperchromicity of the Q-band in the diacids. (2) The highest occupied orbitals of the diacids comprise the set of halide lone pair orbitals, which move from completely above the Gouterman orbitals (I - counterion) to below them (F - ). The lowest halide to porphyrin charge-transfer (HPCT) transitions are therefore predicted at very low energy (to the red of the Q-band) for Cl - −I -, but with very low intensity. Weak measured absorptions to the red of the Q-band support these theoretical findings. Quenching of the S1 (Q) state via these low-lying singlet HPCT excited states accounts for the decrease of the fluorescence quantum yield and for the measured trend along the series.
Rosa et al. (Wed,) studied this question.