The Position of the N Atom Plays a Significant Role for Excited-State Decay of Heterocycles

We have employed combined electronic structure calculations and nonadiabatic dynamics simulations to study the S-1 radiationless deactivation mechanism of pyrazole. In terms of MS-CASPT2 computed results, we propose that the (1)pi sigma* state-driven nonadiabatic N-N dissociation is a major relaxation path; the ring-puckering deformation path as well as the (1)pi sigma* state-driven N-H dissociation are less favorable. This excited-state decay mechanism is supported by MS-CASPT2 nonadiabatic dynamics simulations. The present study demonstrates that pyrazole has a different excited-state radiationless deactivation mechanism compared with its structural isomer imidazole, in which the (1)pi sigma* state driven nonadiabatic N-H dissociation plays a more important role. However, such a channel is suppressed in pyrazole; instead, the (1)pi sigma* state driven nonadiabatic N-N dissociation is dominant.