Issue 47, 2022

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

Abstract

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.

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

Supplementary files

Article information

Article type
Paper
Submitted
07 Sep 2022
Accepted
03 Nov 2022
First published
03 Nov 2022

New J. Chem., 2022,46, 22804-22817

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

N. A. Shekhovtsov, A. A. Ryadun, V. F. Plyusnin, E. B. Nikolaenkova, A. Ya. Tikhonov and M. B. Bushuev, New J. Chem., 2022, 46, 22804 DOI: 10.1039/D2NJ04463C

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