Unveiling Excited-State Dynamics of 2-(4’-(Diethylamino)-2’-hydroxyphenyl)benzimidazole-5-carbonitrile in Solutions, Powder, and Polymer Films: Intramolecular Proton Transfer and Charge Transfer

Abstract Excited-state intramolecular proton transfer (ESIPT) processes have attracted much attention due to their applications in sensing, imaging, and solid-state optoelectronic devices. The effect of substitution on ESIPT was studied by synthesizing 2-(4’-(N,N-diethylamino)-2’-hydroxyphenyl)benzimidazole-5-carbonitrile (DEAHBIC). The fluorophore was investigated experimentally in solution and as a powder sample. Theoretical calculations were performed to understand the effect of solvent polarity on the emission behavior. The theoretical studies revealed that not only the shift in equilibrium but also a reduction in ESIPT decrease the tautomer emission in polar environments. The fluorescence quantum yield of DEAHBIC was significantly lower than 2-(2’-hydroxyphenyl)benzimidazole (HBI). The formation of an intramolecular charge-transfer-induced twisted keto form reduced the fluorescence. Time-resolved fluorescence studies revealed that the tautomer emission lifetime was longer than the normal emission in less polar solvents. In contrast to HBI, in more polar solvents the lifetime of the normal emission of DEAHBIC was longer than its tautomer emission. Unlike the electron-withdrawing NO2 group, the cyano substitution reduced ESIPT. In the powder sample, ESIPT was increased, which enhanced the relative intensity of the tautomer emission, but the overall fluorescence quantum yield was reduced. In addition, ESIPT was also investigated in polymer films. The tautomer fluorescence quantum yield was improved in thin films. Studies in polymer thin films are also useful for understanding the microenvironment experienced by the molecule in the polymer film. The tautomer emission wavelength red-shifted and the ϕT/ϕN ratio increased in the order polystyrene (PS) < polysulfone (PSF) < poly(methyl methacrylate) (PMMA) < polyvinyl acetate (PVAc) < polyvinyl chloride (PVC).