Effects of porphyrin core saddling, meso-phenyl twisting and counterions on the optical properties of meso-tetraphenylporphyrin diacids: The [H4TPP](X)(2) (X = F, Cl, B, I) series as a case study

The ground- and excited-state properties of a series of meso-tetraphenylporphyrin (H2TPP) diacids, [H4TPP](X)(2) (X = F, Cl, Br, I), ad hoc synthesized and characterized by H-1 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 H2TPP diacids. Two electronic structure features are key to an understanding of the optical and photophysical properties. (1) Strong interaction of the pi-system of the phenyls with the pi-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 (V 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.