Theoretical study of the absorption and emission spectrum and non-adiabatic excited state dynamics of gas-phase xanthone

The ground, singlet, and triplet excited state structures (S-1, S-2, T-1, and T-2) of xanthone have been calculated and characterized in the adiabatic representation by using time-dependent density functional theory (TDDFT). However, the fast intramolecular transition mechanisms of xanthone are still under debate, and so we perform non-adiabatic excited state dynamics of the photochemistry of xanthone gas phase and find that it follows El-Sayed's rule. Electronic transition mechanism of xanthone is sequential from the S-2 state: the singlet internal conversion (IC) time from S-2 ((1)pi pi*) to S-1 ((1)n pi*) is 3.85 ps, the intersystem crossing (ISC) from S-1 ((1)n pi*) to T-2 ((3)pi pi*) takes 4.76 ps, and the triplet internal conversion from T-2 ((3)pi pi*) to T-1 ((3)n pi*) takes 472 fs. The displaced oscillator, Franck-Condon approximation, and one-photon excitation equations were used to simulate the absorption spectra of S-0 -> S-2 transition, with v(55) being most crucial for S-0 structure; the fluorescence spectra of S-1 -> S-0 transition with v(47) for S-1; and the phosphorescence spectra of T-1 -> S-0 transition with v(4) for T-1. Our method can reproduce the experimental absorption, fluorescence, and phosphorescence spectra of gas-phase xanthone.