Excited states of a significantly ruffled porphyrin: computational study on structure-induced rapid decay mechanism via intersystem crossing.

The compound meso-tetra-tert-butylporphyrin (H2T(t-Bu)P) is a significantly ruffled porphyrin and known as a good quencher. Compared with planar porphyrins, H2T(t-Bu)P showed bathochromic shift and rapid radiationless decay of the (1)(π, π*) excited state. Density functional theory, approximated coupled-cluster theory, and complete active space self-consistent field method level calculations were performed for the potential energy surface (PES) of the low-lying singlet and triplet states of H2T(t-Bu)P. The origin of the bathochromic shift in the absorption and fluorescence spectra was attributed to both steric distortions of the ring and electronic effects of the substituents. The nonradiative deactivation process of H2T(t-Bu)P via intersystem crossing (ISC) is proposed as (S1 → T2 → T1 → S0). Along a nonplanar distortion angle, the PESs of the S1 and T2 states are very close to each other, which suggests that many channels exist for ISC. For the T1 → S0 transition, minimum energy ISC points were located, and spin-orbit coupling (SOC) was evaluated. The present results indicate that the ISC can also occur at the T1/S0 intersection, in addition to the vibrational SOC promoted by specific normal modes.