Theoretical Studies on Excited-State Photodynamics: N-H Dissociation in Aniline and Intermolecular H atom Transfer in the 2-Amino Pyridine Dimer

Abstract

DNA is composed of light-absorbing molecular units like adenine, guanine, thymine, cytosine, etc. It is well-known that DNA as a whole is photostable upon exposure to sunlight. The photostability of biomolecules suggests that the relaxation process from the electronically excited state to the ground state (GS) is very efficient, and the excited state lifetime is very short. Several experimental and theoretical studies in the past couple of decades have investigated such excited state dynamics. Early studies by Domcke, Sobolewski and coworkers explored excited-state processes in prototypical molecules like phenol, pyrrole and indole and have proposed that one of the dominant processes on the excited state is the 1πσ∗ state mediated N-H/O-H dissociation via 1ππ∗/1πσ∗ and 1πσ∗/GS conical intersections (CIs) 1. They also carried out computational calculations to understand the underlying reason for the photostability of biomolecules 2. It is now understood that they can undergo a charge transfer (CT) state-mediated intermolecular proton transfer on the excited state and relax to the ground electronic state through CT/GS CI very efficiently on an ultrafast timescale. The study of excited-state dynamics of prototypical molecules to the biomolecules is essential to understand photodamage and photostability of complex biomolecules. The present thesis explores these aspects in aniline and the 2-amino pyridine dimer, respectively