Experimental Investigation by Laser Flash Photolysis To Reveal the Optical and Electron-Donating Properties of Benzothiazole Derivatives and a Theoretical Approach by Using Time-Dependent Density Functional Theory

Abstract

The photophysics and electron-donating nature of the three substituted benzothiazoles (BTs) have been studied by electrochemical, steady state, time-resolved spectroscopic techniques. Calculations on isolated molecules in the gas phase as well as solvent environment were performed with use of the density functional theory (DFT) to correlate with the observed polarized spectra. The time-dependent density functional theory (TDDFT) has been shown to give reasonable singlet and triplet vertical excitation energies for all the BTs studied. The electrochemical measurements demonstrate that the photoinduced electron-transfer (PET) reactions between the donor BT and excited (S1) acceptor 9-cyanoanthracene (9CNA) are energetically favorable from the thermodynamic point of view. Steady state fluorescence and flash photolysis measurements indicate that the PET is involved only in the excited singlet state of 9CNA in the presence of BTs. It is inferred that the triplet state of 9CNA, being formed by the charge recombination mechanism, would not be involved in the electrontransfer (ET) reaction as the free energy of the triplet state of 9CNA appears to be less than the free energy of the ion-pair (products).