The compound meso-tetra-tert-butylporphyrin(H2T(t-Bu)P)is a significantly ruffled porphyrin and known 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(DFT),approximated coupled-cluster theory,and complete active space self-consistent field(CASSCF)method level calculations were performed for the potential energy surface(PES)reproduction of the low-lying singlet and triplet states of H2T(t-Bu)P to investigate its excited-state properties variation.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 course of the nonradiative deactivation of H2T(t-Bu)P is proposed as(S1 → T2 → T1 → S0).Different from planer porphyrin,along a non-planer distortion angle,the PES of the S1 and T2 states of H2T(t-Bu)P are very close to each other,which suggests many channels for intersystem crossing(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.