Excited state intramolecular proton transfer involving a four-membered system: Photophysical exploration of an isoindole fused imidazole system

The spectral behaviour of a pharmaceutically important molecule, 1-(2-Hydroxy-5-bromo-phenyl)-3,5-dioxo-1H-imidazo-[3,4-b] isoindole (ADBR), has been reported. The photophysical properties of ADBR reveal the occurrence of excited state intramolecular proton transfer (ESIPT) phenomenon involving a four-membered system, resulting in a conversion from lactim to lactam form of ADBR in the excited state. Excitation at shorter wavelength resulted in dual emission from ADBR in water due to the presence of the proton transferred (lactam) form along with the excited local (lactim) form, although, only PT emission was observed in all other solvents of varying polarity used in this study. The ESIPT process was found to be hindered in water probably due to intermolecular hydrogen bonding between the water molecules and the functional groups of lactim form of ADBR involved in the ESIPT phenomenon. The time-resolved studies corroborated the steady state data and confirmed the operation of an ultrafast ESIPT process.