ESIPT reaction of potential bioactive heterocyclic Schiff base: Atomic visualization coupled with in vitro spectroscopy

The present study epitomizes the identification of ground state geometry, determination of X-ray structure and solvent dependent excited state intramolecular proton transfer (ESIPT) process of a potential bioactive benzothiazole Schiff base ligand viz (E)-2-((6-chlorobenzo[d]thiazol-2-ylimino)methyl)-5-(diethylamino)phenol (CBMDP) using ¹H and ¹³C NMR, FTIR, single crystal XRD, electronic absorption, steady state emission, time resolved and 3D-fluorescence spectroscopic techniques. The experimental observations have further been corroborated with quantum chemical calculations using Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) methods. Single crystal X-ray structure confirms the existence of enol-imine (N) form of CBMDP in the ground state due to strong intramolecular hydrogen bond between phenolic hydrogen and azomethine nitrogen. The photophysical studies reveal that on photoexcitation, the molecule undergoes structural changes via ESIPT reaction. Dual emission have been identified from the excited enol-imine (N∗) and tautomeric keto-amine (T∗) species in non-polar heptane. The redistribution of the electron density in the photoexcited CBMDP molecule is visualised from DFT and TDDFT calculations. This electronic redistribution enhances the in basic character of azomethine nitrogen which is reflected in the Mulliken partial atomic charges increases from -0.21 to -0.56, which ultimately leads to an intramolecular hydroxyl proton transfer in the excited state. The simultaneous emission from two states are also theoretically supported by the low energy difference (0.66 kcal/mol) between the N∗and T∗form in the potential energy surface diagram. The consolidated spectroscopic research, described herein, provides enormous information to open up the new avenues in designing and synthesizing potential bioactive benzothiazole Schiff bases for medicinal chemistry research.