Non-radiative deactivation of cytosine derivatives at elevated temperature

ABSTRACT

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 ab-initio 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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Non-radiative deactivation
• conical intersection
• non-adiabatics
• ab-initio molecular dynamics

Acknowledgements

This work was funded by a grant from the US Department of Energy, Basic Energy Sciences (BES ER46474). T. V. Voorhis is a David and Lucile Packard Foundation Fellow.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

US Department of Energy, Basic Energy Sciences [grant number ER46474].