First 1-hydroxy-1H-imidazole-based ESIPT emitter with an O-H•••O intramolecular hydrogen bond: ESIPT-triggered TICT and speciation in solution

Search for new types of ESIPT-capable molecules is crucial for understanding the factors controlling the ESIPT photoreaction. In this paper, we present combined experimental and theoretical studies of the proton transfer capabilities of [5-(4-fluorophenyl)-1-hydroxy-4-methyl-1H-imidazol-2-yl](phenyl)-methanone (HL) featuring a short intramolecular O-H center dot center dot center dot O hydrogen bond (O center dot center dot center dot O 2.57 angstrom) between the proton-donating 1-hydroxy-1H-imidazole moiety and the proton-accepting benzoyl group as a pre-requisite for the ESIPT process. This compound is the first representative of a new class of ESIPT-capable molecules, 1-hydroxy-1H-imidazoles with oxygen-containing proton-accepting groups. In solutions, HL can exist in a variety of ESIPT-capable and ESIPT-incapable species. The emissions of HL in non-protic solvents, CH2O2 and MeCN, and in the solid state occur with lambda(max) in a narrow range of 455-470 nm. The emission of HL in MeCN is excitation wavelength dependent with a shoulder appearing near 400 nm under high energy excitation. This emission is dominated by the S-1 -> S-0 transition in the ESIPT-capable (N-hydroxy) form of HL and is contributed by the Luminescence of the ESIPT-incapable forms. In a protic solvent, EtOH, the emission maximum shifts to lambda(max) = 413 nm. In solutions, the photoluminescence quantum yield (PLQY) of HL is on the order of 0.1%. In contradistinction to solutions, a rigid molecular environment in the solid state prevents the HL molecules from being conformationally converted and the only form of HL to be found in crystals is the ESIPT-capable one, which Leads to an excitation wavelength independent emission with an enhanced PLQY of ca. 5%. The ESIPT photoreaction in the ESIPT-capable form of HL proceeds after overcoming a small energy barrier and ends up at an ESIPT-triggered TICT state followed by a non-radiative deactivation through an S-1/S-0 conical intersection.

Automated summary

In this study, the proton transfer capabilities of a new class of ESIPT-capable molecules were investigated through experimental and theoretical studies. Different forms of the molecule with various emission wavelengths were observed in solutions, while only the ESIPT-capable form was found in the solid state. The ESIPT photoreaction in the ESIPT-capable form proceeded through a non-radiative deactivation process after overcoming a small energy barrier.