A joint experimental/theoretical study of the ultrafast excited state deactivation of deoxyadenosine and 9-methyladenine in water and acetonitrile

Abstract

The excited states of deoxyadenosine (dA) and 9-methyladenine (9Me-Ade) were studied in water and acetonitrile by a combination of steady-state and time-resolved spectroscopy and quantum chemical calculations. Femtosecond fluorescence upconversion experiments show that the decays of dA and 9Me-Ade after excitation at 267 nm are very similar, confirming that 9Me-Ade is a valid model for the calculations. The fluorescence decays can be described by an ultrafast component (<100 fs) and a slower one (≈ 300–500 fs); they are slightly slower in acetonitrile than in water. Time-dependent DFT calculations on 9Me-Ade, using PBE0 and M052X functionals and including both bulk and specific solvent effects, provide absorption and emission spectra in good agreement with experiments, giving a comprehensive description of the decay mechanism. It is shown that, in the Franck–Condon region, the lowest in energy state is the optically bright L_a state, with the L_b state situated about 2000 cm⁻¹ higher. Both states are populated when excited at 267 nm, but the L_b state undergoes an ultrafast L_b → L_a decay, too fast for our time-resolution (≈ 80 fs). This is confirmed by the experimentally observed fluorescence anisotropies, attaining values lower than 0.4 already at time zero. Consequently, the ensuing excited state relaxation mechanism can be described as the evolution along an almost barrierless path from the Franck–Condon region of the L_a potential energy surface towards a conical intersection with the ground state. This internal conversion mechanism proceeds without any significant involvement of any near-lying nπ* state.