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⋯O hydrogen bond (O⋯O 2.57 Å) 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, CH2Cl2 and MeCN, and in the solid state occur with λmax 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 S1 → S0 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 λmax = 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 S1/S0 conical intersection.