White light generation by carbonyl based indole derivatives due to proton transfer: an efficient fluorescence sensor

Abstract

The motivation of the present work is to understand the optical, chemical, and electrical aspects of the proton transfer mechanism of indole (I) and some carbonyl based indole derivatives: indole-3-carboxaldehyde (I3C) and indole-7-carboxaldehyde (I7C) for both powder form and their liquid solution. Structural information for indole derivatives (isolated molecule and in solution) is obtained with Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT) methods. Calculated transition energies are used to generate UV–vis, FTIR, Raman and NMR spectra which are later verified with the experimental spectra. The occurrence of different conformers [cis (N_c), trans (N_t) and zwitterion (Z*)] have been interpreted by Mulliken charge, Natural Bond Orbital (NBO) analysis and polarization versus electric field (P-E loop) studies. ¹H and ¹³C NMR and molecular vibrational frequencies of the fundamental modes established the stability of N_c due to the presence of Intramolecular Hydrogen Bonding (IHB) in the ground state (S₀). Computed/experimental UV–vis absorption/emission studies reveal the creation of new species: zwitterion (Z*) and anion (A*) in the excited state (S₁) due to excited state intramolecular and intermolecular proton transfer (ESI_raPT and ESI_erPT). Increased electrical conductivity (σ_ac) with temperature and increased ferroelectric polarization at higher field verifies proton conduction in I7C.