Role of hydrogen bonding in excited state intramolecular proton transfer of Indole-7-Carboxaldehyde: A theoretical and experimental study

► ESIPT occurs in Indole-7-Carboxaldehyde in the form of dual fluorescence (S1 → S0). ► Two minima in PES for S1 occurs due to Z∗(zwitterion) and Nt∗(normal) conformers. ► Z∗ in S1 is due to proton transfer from acidic (-N-H) to basic (>CO) moiety. ► Electron density shows the existence of ESIPT by π-bond shift from HOMO to LUMO. ► Observed UV-VIS absorption/emission spectra matches with that of computed spectra.