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

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 (ϕT) for the keto form is remarkably dependent on polymorphs at room temperature; ϕT = 0.53 for Form α is much larger than ϕT ≤ 0.23 for Form β. At 77 K the ϕT values for Forms α and β increase to 0.67 and ≤0.57, respectively. The changes in the ϕT 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.