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

1 H -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)-1 H -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)-1 H -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.