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 pi pi* (1)L(a) state to the ground state S(0) of 9H-adenine. The n pi*, pi sigma*, and two pp* 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 ps* and np* states, 1 La-ps*-S0 and 1 La-np*-S0, and a direct pathway (1)L(a)-S(0), were examined on the basis of the structures and energies of conical intersections involved in ultrafast radiationless transitions from 1 La to S0. Any conical intersection between the ps* and np* states was not found. This suggests that the two indirect pathways are independent of each other. The pp* 1 La-ps* conical intersection lies higher than the pp* (1)L(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 B0.2 eV above the band origin of the pp* 1 La state. It is concluded that relaxation from the pp* (1)L(a)-ps* conical intersection to S0 occurs mainly through the ps*-S(0) conical intersection. The pp* 1 La-np* conical intersection lies higher by 0.1 eV ( MRCI value) than the pp* 1 La 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 np* state via this conical intersection followed by the transition to S0 via the np*-S(0) ( or pp*(1)L(a)-S0) conical intersection. The pp* (1)L(a)-S0 conical intersection of large out-of-plane distortion has the lowest energy among the conical intersections found in this study. We identi. ed the transition state between the pp* (1)L(a) at the Franck-Condon geometry and the pp* (1)L(a)-S0 conical intersection. The MRCI energy of the transition state on the (1)L(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 (1)L(a) and np* indicates that, besides this direct pathway, radiationless transitions to S(0) via the pp* (1)L(a)-S(0) conical intersection can also occur after rapid relaxations between (1)L(a) and np*. The analysis of the h-vector for each conical intersection has shown that the active coupling for the ps* pathway is dominated by the out-of-plane normal mode n 10, while the active coupling for the np* 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 n v(10).