Enol or keto? Interplay between solvents and substituents as a factor controlling ESIPT

Abstract

The emission properties associated with excited state intramolecular proton transfer (ESIPT) were theoretically studied in solution and in the solid state for a series of 2-(2′-hydroxyphenyl)benzimidazole-based (HBI) ESIPT-fluorophores 3a–3e having different halogen substituents in the ortho and para positions to the proton-donating hydroxy group. DFT and TDDFT computations were performed for understanding experimental trends in the absorption and fluorescence spectra on going from a non-halogenated compound 3a to monohalogenated compounds 3b–3d and a dihalogenated compound 3e. Quantum chemical calculations identified an interplay of solvent and halogen atoms as a factor controlling the photophysics and photochemistry of ESIPT-fluorophores 3a–3e. ESIPT and emission from the keto form can be observed in solution if at least one of the following conditions is satisfied. (i) Intermolecular hydrogen bonding of HBI to solvent molecules is relatively weak. This favors the formation of OH⋯N intramolecular hydrogen bonds in HBI and facilitates ESIPT. (ii) A halogen atom is introduced in the ortho position to the OH–group. This stabilizes the keto form and decreases the energy barrier for ESIPT due to the strong negative inductive effect. If neither of these two conditions is met, the ESIPT process is hindered and the emission originates from the enol form. Compared to solution, the potential energy curve of the S1 state has only one minimum in the solid state, which leads to barrierless ESIPT and fluorescence of the keto form. According to DFT and TDDFT calculations, twisted intramolecular charge transfer state (TICT), which is often coupled with ESIPT, is thermodynamically and kinetically unfavorable in the case of HBI, resulting in moderate to high photoluminescence quantum yields for 3a–3e.