Nonadiabatic dynamics simulations of photoexcited urocanic acid

Urocanic acid (UA) is a UV filter found in human skin, which has been linked to photoimmunosuppression and the formation of skin cancer. Its UV-light-induced photoisomerization and radiationless deactivation mechanisms have been addressed previously by static calculations. In this paper, we present nonadiabatic trajectory-surface-hopping dynamics simulations of photoexcited UA using the semiempirical OM2/MRCI methodology and an adaptive-timestep algorithm. We have simulated almost 6000 trajectories, each for a simulation time of 1.6 ps, covering the entire conformational space of the E and Z isomers of both possible tautomers of the isolated neutral form of UA (overall 32 conformers). Initial conditions for the excited-state dynamics were obtained from 1 ns ground-state dynamics simulations. We find that UA has an ultrashort excited-state lifetime, which is due to ultrafast radiationless excited-state deactivation driven by E↔Z photoisomerization and excited-state intramolecular proton-transfer (ESIPT) processes. The computed S1 excited-state lifetimes for the E and Z isomers of the N1H and N3H tautomers range from 271 to 506 fs. The photoisomerization quantum yield is calculated to be 43% (32%) for the combined E (Z) isomers of both tautomers. The shorter lifetime and the lower photoisomerization quantum yield of the Z isomers can be rationalized by the larger number of available excited-state deactivation processes: the Z isomers can undergo ESIPT and photoisomerization, whereas the E isomers can only deactivate via the latter process. The intramolecular hydrogen bond that is present in many Z conformers can prevent successful photoisomerization to an E isomer. We find no evidence for an excitation-energy-dependent quantum yield for photoisomerization (EEDQY-PI) in isolated (E)-UA, which has previously been detected spectroscopically in aqueous solution. However, we do find an EEDQY-PI as well as a complementary excitation-energy-dependent quantum yield for ESIPT (EEDQY-ESIPT) for the N1H-Z isomers, which demonstrates the competition of the photoisomerization and ESIPT processes. The present comprehensive study lays the groundwork for future photodynamics simulations of UA in the aqueous phase.