In this work, different levels of quantum computational
models
such as MP2, ADC(2), CASSCF/CASPT2, and DFT/TD-DFT have been employed
to investigate the photophysics and photostability of a mycosporine
system, mycosporine glycine (MyG). First of all, a molecular mechanics
approach based on the Monte Carlo conformational search has been employed
to investigate the possible geometry structures of MyG. Then, comprehensive
studies on the electronic excited states and deactivation mechanism
have been conducted on the most stable conformer. The first optically
bright electronic transition responsible for the UV absorption of
MyG has been assigned as the S2(1ππ*)
owing to the large oscillator strength (0.450). The first excited
electronic state (S1) has been assigned as an optically
dark (1nπ*) state. From the nonadiabatic dynamics
simulation model, we propose that the initial population in the S2(1ππ*) state transfers to the S1state in under 100 fs, through an S2/S1conical intersection (CI). The barrierless S1potential
energy curves then drive the excited system to the S1/S0CI. This latter CI provides a significant route for ultrafast
deactivation of the system to the ground stateviainternal conversion.