Excited-State Dynamics of a Benzotriazole Photostabilizer: 2-(2'-Hydroxy-5'-methylphenyl)benzotriazole

A large number of common photostabilizers are based on the 2-(2'-hydroxyphenyl)benzotriazole structure. One common example is 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, or TINUVIN-P. The excited-state dynamics of this molecule have been extensively characterized by ultrafast spectroscopies. These experiments have established that upon photoexcitation TINUVIN-P exhibits excited-state proton transfer followed by a remarkably fast internal conversion. We simulate the excited-state dynamics using ab initio multiple spawning (AIMS) and a complete active space configuration interaction (CASCI) wave function with a correction from density functional theory (DFT) to generate the potential energy surfaces. We predict ultrafast proton transfer on the order of 20 fs followed by simultaneous twisting and pyramidalization until a seam of conical intersection is reached. Near the intersection seam population transfer to the ground state is highly efficient. The process is best described as ballistic wavepacket motion from the Franck-Condon point along a barrierless coordinate leading to the seam of intersection. Internal conversion is primarily mediated by a minimum-energy conical intersection (MECI) with a high degree of pyramidalization. We posit that the presence of a nitrogen atom in the bond linking the phenyl to the benzotriazole allows for the rapid pyramidalization and the short excited-state lifetime.