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 (S₁, S₂, T₁, and T₂) 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 S₂ state: the singlet internal conversion (IC) time from S₂ (¹ππ*) to S₁ (¹nπ*) is 3.85 ps, the intersystem crossing (ISC) from S₁ (¹nπ*) to T₂ (³ππ*) takes 4.76 ps, and the triplet internal conversion from T₂ (³ππ*) to T₁ (³nπ*) takes 472 fs. The displaced oscillator, Franck–Condon approximation, and one-photon excitation equations were used to simulate the absorption spectra of S₀ → S₂ transition, with v₅₅ being most crucial for S₀ structure; the fluorescence spectra of S₁ → S₀ transition with v₄₇ for S₁; and the phosphorescence spectra of T₁ → S₀ transition with v₄ for T₁. Our method can reproduce the experimental absorption, fluorescence, and phosphorescence spectra of gas-phase xanthone.