ESIPT emission behavior of methoxy-substituted 2-hydroxyphenylbenzimidazole isomers†
Three 2-hydroxyphenylbenzimidazole isomers 1, 2, and 3 with the methoxy group at the 6, 4, and 3-position, respectively, were synthesized in short steps. 1 exhibited ESIPT emission irrespective of the solvent character. While 2 only showed ESIPT emission in THF and MeCN, LE emission was also observed in MeOH and a THF/water mixed solvent. These phenomena were nicely accounted for by intramolecular hydrogen bonding interactions. The fluorescence spectra of 3 always consisted of two emission bands and it exhibited strange behavior particularly in protic solvents. The fluorescence emissions in the solid state were found to be closely related to the crystal packing structure.