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

• Stability of molecule has been analyzed with AIM and NBO analysis.
• Existence of intramolecular hydrogen bonding promotes ESIPT.
• Dual minima in potential energy surface in S1 state verify the ESIPT.
• Computed absorption/emission spectra perfectly match with the experimental results.

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.

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