Theoretical study on the singlet excited state of pterin and its deactivation pathway.

The excited-state properties and related photophysical processes of the acidic and basic forms of pterin have been investigated by the density functional theory and ab initio methodologies. The solvent effects on the low-lying states have been estimated by the polarized continuum model and combined QM/MM calculations. Calculations reveal that the observed two strong absorptions arise from the strong pi --> pi* transitions to 1(pipiL(a)) and 1(pipiL(b)) in the acidic and basic forms of pterin. The first 1(pipiL(a)) excited state is exclusively responsible for the experimental emission band. The vertical 1(n(N)pi) state with a small oscillator strength, slightly higher in energy than the 1(pipiL(a)) state, is less accessible by the direct electronic transition. The 1(n(N)pi) state may be involved in the photophysical process of the excited pterin via the 1(pipiL(a)/n(N)pi) conical intersection. The radiationless decay of the excited PT to the ground state experiences a barrier of 13.8 kcal/mol for the acidic form to reach the (S(1)/S(0)) conical intersection. Such internal conversion can be enhanced with the increase in excitation energy, which will reduce the fluorescence intensity as observed experimentally.