Radiationless deactivation pathways versus H-atom elimination from the N-H bond photodissociation in PhNH-(Py) (n = 1,2) complexes.

Minimum energy structures of the ground and lowest excited states of aniline (PhNH) solvated by pyridine (Py) show that the clusters formed are stabilized by hydrogen bonds in which only one or both hydrogen atoms of the NH group take part. Two different N-H bonds photodissociation in PhNH-(Py) (n = 1,2) complexes, free and hydrogen bonded have been studied by analyzing excited state potential energy surfaces. In the first one, only N-H bonds engaged in hydrogen bonding in these complexes are considered. RICC2 calculations of potential energy (PE) profiles indicate that all photochemical reaction paths along N-H stretching occur mainly via the proton-coupled electron transfer (PCET) mechanism. The repulsive charge transfer ππ*(CT) state dominates the PE profiles, leading to low-lying ππ*(CT)/S conical intersections and thus provide channels for ultrafast radiationless deactivation of the electronic excitation or stabilization to biradical complexes. The second photoreaction consists of a direct dissociation along the free N-H bond of the NH group. It has been shown that this process is played by excited singlet states of πσ* character having repulsive potential energy profiles with respect to the stretching of N-H bond, which dissociates over an exit barrier about 0.5 eV giving rise to the formation of a πσ*/S conical intersection. This may cause an internal conversion to the ground state or may lead to H-atom elimination. This photophysical process is the same in both planar and T-shaped conformers of the PhNH-Py monomer complex. Our findings reveal that there is no single dominating path in the photodissociation of N-H bonds in PhNH-(Py) complexes, but rather a variety of paths involving H-atom elimination and several quenching mechanisms.