Excited-state intramolecular proton transfer and charge transfer in 2-(2′-hydroxyphenyl)benzimidazole crystals studied by polymorphs-selected electronic spectroscopy

Abstract

Crystal structures of polymorphs of 2-(2′-hydroxyphenyl)benzimidazole (HPBI), Forms α and β, are analyzed by X-ray crystallography. The fluorescence excitation (FE) and fluorescence spectra of the polymorphs are separately observed at temperatures 77–298 K. It has been found that the electronic spectra of the two crystal forms are significantly different from each other. Photo-excitation of the enol forms in Forms α and β induces the excited-state intramolecular proton transfer (ESIPT) to produce the S₁ state of the keto forms. In the FE spectra of Forms α and β, the S₁ ← S₀ (ππ*) transition of the keto form is observed in the 360–420 nm region in addition to that of the enol form in the 250–420 nm region. In the FE spectrum of Form β a new band peaking at 305 nm is observed, which is assigned to the S₁ ← S₀ transition of a non-planar enol form based on the observation of dual fluorescence in the UV and visible regions and quantum chemical calculation on the transition energy against the twisted angle between the benzimidazole and hydroxyphenyl rings. The fluorescence quantum yield (ϕ_T) for the keto form is remarkably dependent on polymorphs at room temperature; ϕ_T = 0.53 for Form α is much larger than ϕ_T ≤ 0.23 for Form β. At 77 K the ϕ_T values for Forms α and β increase to 0.67 and ≤0.57, respectively. The changes in the ϕ_T values are associated with the intramolecular charge transfer (ICT) state. The potential barrier height between the S₁-keto and S₁-ICT states is significantly lower for Form β than for Form α. At 77 K the S₁-keto → S₁-ICT process followed by S₁-ICT → S₀-keto internal conversion is significantly suppressed in Form β. We compare difference in the dynamics between Forms α and β in the electronic ground and excited states.