A computational insight into the photophysics of a potent UV absorber Tinuvin P: Critical evaluation of the role of charge transfer interaction and topological properties of the intramolecular hydrogen bonding

In the present contribution, we report a computational investigation using Density Functional Theory (DFT) method on the photophysics of a potent UV absorber (UVA) Tinuvin P (2-(2-hydroxy-5-methylphenyl)benzotriazole). Since the exceptional photostability of such UVAs is principally banked upon their ability to undergo excited-state deactivation via an ultrafast excited-state proton transfer (ESIPT) mechanism, emphasis has been rested on this particular mode of excited-state deactivation of Tinuvin P (TIN P). The operation of ESIPT has been most critically argued on the lexicon of potential energy surface (PES) across the reaction coordinate and cross-validated from frontier molecular orbital (MO) analysis. Evolution of other geometrical parameters during the course of the ESIPT process in TIN P has been crucial to delve into the mechanistic details of the process. Major emphasis of the work is rendered on the analysis of the intramolecular hydrogen bonding (IMHB) interaction in TIN P. The IMHB interaction has been explored by calculation of electron density ρ(r) and Laplacian ∇2ρ(r) at the bond critical point (BCP) using Atoms-In-Molecule (AIM). Concomitantly, the role of charge transfer interaction in the IMHB has been critically evaluated and addressed under the provision of natural bond orbital (NBO) analysis. Simulated IR spectra also provide reinforcing evidence for IMHB interaction on the basis of OH stretching frequency shift.