In-Depth Insights Into the Excited State Intramolecular Proton Transfer of Biologically Active Triazene Derivatives of Oxazole

Abstract

Phenyltriaz-1-en-1-yl-4-phenyloxazoles (PTOs) have been identified as photoinduced tautomeric photophysical probes. A series of PTOs has been synthesized and studied for their absorption and fluorescence spectral behavior in different solvent media with varying polarities. PTOs exhibited dual fluorescence behavior and large Stoke's shifts suggesting the presence of tautomeric forms (N* and T*). Dipole moments of PTOs in their excited states were calculated using Bakhshiev, Lippert–Mataga, Reichardt, and Kawski solvatochromic shift methods and were correlated with the theoretical dipole moments for N* and T* forms. The frontier molecular orbitals (HOMO-LUMO) and various physicochemical parameters (χ, η, σ, μ_p, ω, ΔE^II, N_max) were calculated using density functional theory (ground state-(CIS)/B3LYP/6–311++G(d,p)) computations to analyze the excited state intramolecular proton transfer (ESIPT) within these probes. The electrostatic potential (ESP) maps and Mulliken/NBO charges established the electrophilic and nucleophilic sites within molecules and intramolecular proton transfer from one end of the triazeno-bridge to the other end. DFT incorporated dispersion correction (D3(BJ)) accounts for significant van der Waals interactions. In vitro antioxidant activity of PTOs studied using 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging assay revealed them as potential antioxidants. Two multi-parametric scales based on solvent polarity, namely, (i) Catálan and (ii) Abboud-Kamlet-Taft, were employed to analyze the acidity and basicity of PTOs in ground and excited states to provide a plausible mechanism for ESIPT.