Non-radiative deactivation of cytosine derivatives at elevated temperature

Abstract

<p>In this work, we simulate the non-radiative deactivation process of three cytosine derivatives with known S0/S1 conical intersections (cytosine, 5-fluorocytosine and 5-methylcytosine). We use quantum chemistry methods to compute the potential energy profile of each derivatives and estimate the energy barrier height between the minimum of the S1 state and the conical intersection. Although the topology of the potential surface seems to play a role in the deactivation process, we show that the magnitude of the barrier is too high to explain the picosecond timescale reported for this reaction. Instead, rates in agreement with experiments are predicted only when incorporating dynamical factors via <i>ab-initio</i> molecular dynamics and a generalised master equation approach. In particular, we find that the energy fluctuations experienced by the system after photoexcitation are key to realistically model the relaxation dynamics. In gas phase, the cytosine derivatives remain vibrationally ‘hot’ for long after the excitation, raising the effective temperature of the system. We argue that it is this elevated temperature that allows for the crossing of the energy barrier. Further, we show that the reaction kinetics are not actually dominated by the conical intersection as it is enough for the system to find an avoided crossing.</p