Ultrafast radiationless transition pathways through conical intersections in photo-excited 9H-adenine

Abstract

We performed CASSCF and MRCI calculations for determination of the effective pathways of ultrafast radiationless transitions from the optically allowed ππ* ¹L_a state to the ground state S₀ of 9H-adenine. The nπ*, πσ*, and two ππ* states were taken into account as states involved in the radiationless process. Optimized geometry and conical intersections were searched in the full dimensional space for the vibrational degrees of freedom by using the suite of quantum chemistry codes MOLPRO. The MRCI transition energies to excited states are in good agreement with the experimental values. The mechanisms of three competing pathways, two indirect pathways via the πσ* and nπ* states, ¹L_a → πσ* → S₀ and ¹L_a →nπ* → S₀, and a direct pathway ¹L_a→ S₀, were examined on the basis of the structures and energies of conical intersections involved in ultrafast radiationless transitions from ¹L_a to S₀. Any conical intersection between the πσ* and nπ* states was not found. This suggests that the two indirect pathways are independent of each other. The ππ* ¹L_a–πσ* conical intersection lies higher than the ππ* ¹L_a state at the Franck–Condon geometry by 0.19 eV according to the present MRCI calculation, which is consistent with the experimental observation that a new channel is open at the excess energy of ∼0.2 eV above the band origin of the ππ* ¹L_a state. It is concluded that relaxation from the ππ* ¹L_a–πσ* conical intersection to S₀ occurs mainly through the πσ*–S₀ conical intersection. The ππ* ¹L_a–nπ* conical intersection lies higher by 0.1 eV (MRCI value) than the ππ* ¹L_a state at the Franck–Condon geometry. The fast decay component in time-resolved spectra of 9H-adenine is attributed to rapid radiationless transitions to the nπ* state via this conical intersection followed by the transition to S₀via the nπ*–S₀ (or ππ* ¹L_a–S₀) conical intersection. The ππ* ¹L_a–S₀ conical intersection of large out-of-plane distortion has the lowest energy among the conical intersections found in this study. We identified the transition state between the ππ* ¹L_a at the Franck–Condon geometry and the ππ* ¹L_a–S₀ conical intersection. The MRCI energy of the transition state on the ¹L_a potential surface is higher by 0.21 eV than the vertical excitation energy. The possibility of strong coupling between the two close-lying states ¹L_a and nπ* indicates that, besides this direct pathway, radiationless transitions to S₀via the ππ* ¹L_a–S₀ conical intersection can also occur after rapid relaxations between ¹L_a and nπ*. The analysis of the h-vector for each conical intersection has shown that the active coupling for the πσ* pathway is dominated by the out-of-plane normal mode ν₁₀, while the active coupling for the nπ* pathway is distributed among many normal modes. Control of the branching ratio of the two indirect pathways can be achieved by selective excitation of single vibronic levels involving active coupling modes such as the mode ν₁₀.