Photoinduced Generation of the π-Conjugated Zwitterionic State in the ESIPT Fluorophore of 2,4-Bisimidazolylphenol

We demonstrate that 2,4-bis(4,5-diphenyl-1H-imidazol-2-yl)phenol (2,4-bImP) undergoes photoinduced conversion into the so-called "π-conjugated zwitterion" after causing an excited-state intramolecular proton transfer (ESIPT) reaction. The powder sample of 2,4-bImP exhibits largely Stokes-shifted fluorescence characteristics to ESIPT fluorophores. On the other hand, its originally colorless solutions become colored when exposed to UV light for several minutes, whose color depends on the type of solvent. In particular, the CHCl3 solution rapidly turns dark green with the absorption maximum around 700 nm, and the colored solution is nearly restored to original by alternating addition of acid and base. To explain such drastic and reversible color changes, we hypothesized that the occurrence of ESIPT (i.e., deprotonation of the phenol and protonation of the imidazolyl group at its 2-position) triggered the charge-separated structure between the negatively charged phenolate and the positively charged imidazoliumyl group at its 4-position, which allowed resonance with the neutral p-quinoid structure. The formation of this π-conjugated zwitterion was strongly supported by the results of 1H and 15N NMR and Raman measurements.