Photoinduced nonadiabatic dynamics of 9H-guanine.

Surface-hopping simulations are used to study the nonradiative relaxation of 9H-guanine. Two distinct S(1)-->S(0) (pipi*-->gs) decay channels, both of which pass through a conical intersection (CI), are found to be responsible for the experimentally observed double-decay behavior [schematic diagram: see text].The photoinduced nonadiabatic decay dynamics of 9H-guanine is investigated by surface-hopping calculations at the semiempirical OM2/MRCI level of theory. Following excitation, fast internal conversion from the pipi* (L(a)) excited state to the ground state is observed within 800 fs. Relaxation proceeds through two distinct S(1)-->S(0) pathways. The first channel goes through a conical intersection with pronounced out-of-plane displacement of the C2 atom and yields ultrafast decay with a time constant of 190 fs. The second channel evolves through a conical intersection with strong out-of-plane distortion of the amino group and leads to slower decay with a lifetime of 400 fs. These decay mechanisms and the computed decay times are consistent with the available experimental evidence and previous theoretical studies.