Density functional investigation of photo induced Intramolecular Proton Transfer (IPT) in Indole-7-carboxaldehyde and its experimental verification

Abstract

A detail theoretical study has been performed using Density functional theory (DFT) and Time dependent DFT (TDDFT) to investigate the Intramolecular Proton Transfer (IPT) mechanism in Indole-7-carboxaldehyde (I7C) from its normal (I*) to zwitterion (II*) form. B3LYP/6-311++G (d, p) basis set has been used to obtain structural parameters and relative energies in the ground state (S0) and excited state (S1). Atoms in Molecules (AIMs), Mulliken and Natural bond orbitals (NBOs) analysis proves the existence of intramolecular hydrogen bonding (IHB). The electron density (ρ) at Bond critical points (BCPs) on a hydrogen bridge (N15H12⋯O18) certify IHB and possibility of IPT from acidic (N15H12) to basic (C16O18) group and creation of II*. Transition state (TS) with dual minima in the Potential energy surface (PES) confirms the I*→TS→II* transition due to excited state Intramolecular Proton Transfer (ESIPT). Photo-physical pathway from I*→II* agrees well with computed/experimental emission peaks.