ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence

1H-Imidazole derivatives establish one of the iconic classes of ESIPT-capable compounds (ESIPT = excited state intramolecular proton transfer). This work presents the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L(OH,OH)) as the first example of ESIPT-capable imidazole derivatives wherein the imidazole moiety simultaneously acts as a proton acceptor and a proton donor. The reaction of L(OH,OH) with chloroacetone leads to the selective reduction of the imidazolic OH group (whereas the phenolic OH group remains unaffected) and to the isolation of 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L(H,OH)), a monohydroxy congener of L(OH,OH). Both L(OH,OH) and L(H,OH) demonstrate luminescence in the solid state. The number of OH···N proton transfer sites in these compounds (one for L(H,OH) and two for L(OH,OH)) strongly affects the luminescence mechanism and color of the emission: L(H,OH) emits in the light green region, whereas L(OH,OH) luminesces in the orange region. According to joint experimental and theoretical studies, the main emission pathway of both compounds is associated with T(1) → S(0) phosphorescence and not related to ESIPT. At the same time, L(OH,OH) also exhibits S(1) → S(0) fluorescence associated with ESIPT with one proton transferred from the hydroxyimidazole moiety to the pyridine moiety, which is not possible for L(H,OH) due to the absence of the hydroxy group in the imidazole moiety.