Excited-state Relaxation Mechanisms of Janus-type proton in benzimidazole-conjugated aminomaleonitrile: Single or Double Proton Transfer?

Abstract

The excited-state intramolecular proton transfer (ESIPT) dynamics of the ratiometric fluorescent benzimidazole-conjugated aminomalanitrile-based probe 1, an asymmetric structure synthesized by Gong et al. (RSC Adv. 2019, 9, 30943–30951), were investigated using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The presence of strengthened dual-facet hydrogen bonds in the excited state, along with charge redistribution, facilitates the ESIPT process. The geometrical optimization of six conformations identified C3 and C4 as the most stable at S₀ state (58.8% and 33.6%, respectively). Theoretical calculations align well with experimental absorption spectra and excitation values within the 350–425 nm range. FTIR analysis confirmed enhanced intramolecular hydrogen bonding (intraHBs) in C3 and C4, evidenced by redshifts and reduced interaction distances. The relative free energy profiles of tautomeric forms indicate stable states in S₀ and S₁, with low energy barriers enabling proton transfer in the excited state. The vertical emission peaks closely match experimental spectra, underscoring the role of dual-facet hydrogen bonding in photophysical behaviour. The S₁-state potential barriers suggest an excited-state single proton transfer (ESPT) rather than a double proton transfer. Upon OCl⁻ addition, the ESIPT process remains uninhibited in probe 1, confirming its OCl⁻ sensing mechanism via imidazole derivative formation. This study not only elucidates the ESIPT mechanisms of probe 1 but also enhances the understanding of HOCl detection, contributing to the development of novel fluorescent probes.