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 alpha and beta, 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 alpha and beta induces the excited-state intramolecular proton transfer (ESIPT) to produce the S-1 state of the keto forms. In the FE spectra of Form alpha and beta, the S-1 <- S-0 (pi pi*) 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 beta a new band peaking at 305 nm is observed, which is assigned to the S-1 <- S-0 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 (phi(T)) for the keto form is remarkably dependent on polymorphs at room temperature; phi(T) = 0.53 for Form alpha is much larger than phi(T) <= 0.23 for Form b. At 77 K the phi(T) values for Forms alpha and beta increase to 0.67 and <= 0.57, respectively. The changes in the phi(T) values are associated with the intramolecular charge transfer (ICT) state. The potential barrier height between the S-1-keto and S-1-ICT states is significantly lower for Form beta than for Form alpha. At 77 K the S-1-keto -> S-1-ICT process followed by S-1-ICT -> S-0-keto internal conversion is significantly suppressed in Form beta. We compare difference in the dynamics between Forms alpha and beta in the electronic ground and excited states.