Study of prototropic reactions of indole chalcone derivatives in ground and excited states using absorption and fluorescence spectroscopy

Abstract

The prototropic behavior of biologically active intramolecular charge transfer (ICT) probes namely, indole chalcone (IC) derivatives in aqueous media of Ho/pH/H_ from −3.03 to 17.95, was studied using absorption and steady-state fluorescence spectroscopy. This study reveals the presence of monocation (MC), neutral (N) and monoanion (MA) prototropic species in ground and excited states. Prototropic equilibrium between the ionic species is found to be dependent on the nature of substituents. These molecules show the formation of MC via protonation of the carbonyl oxygen atom, conversely a predictable deprotonation takes place from the >NH group of indole ring to form the MA in the ground (S0) and the first excited singlet states (S1). The presence of OH and NH2 group enhances the prototropic behavior of these ICT probes leading to the formation of dianion species (DA). The highest occupied molecular orbitals (HOMO), lowest unoccupied molecular orbitals (LUMO) and molecular electrostatic potential (MEP) surfaces were generated from the optimized geometries of IC derivatives obtained using Density Functional Theory (DFT) calculations with restricted hybrid functional B3LYP using 6-31G (d) basis set to account for ICT. The MEP surfaces also show the possible protonation and deportation sites in these molecules. The dissociation constants, pKa and pKa⁎, for all the prototropic equilibria were determined in S0 and S1 states, respectively using different methods such as Henderson-Hasselbalch (HH), Photometric Titration (PT), Fluorimetric Titration (FT) and Förster Cycle (FC). For MC ⇌ N equilibrium, the excited state pKa⁎ values are found to be greater than the ground state pKa values, which suggests a more basic nature of these molecules in the excited state.