A novel structure for ESIPT emission: Experimental and theoretical investigations

A number of substituted imidazole-based molecules were synthesized via a two-step one-pot reaction and their UV/vis and fluorescence spectra were analyzed. Emissions of the imidazole-based molecules were attributed to the excited state intramolecular proton transfer (ESIPT) process. Imidazole-based sensor molecules have high Stokes shifts which is good clue for the ESIPT mechanism. Furthermore, the ESIPT mechanism was confirmed by some control experiments in which acceptor and donor groups were functionalized, resulting in hampered ESIPT emission. Time-resolved experiments showed that the skeleton synthesized had different exponential decays depending on the functional groups (OMe and OH). According to the time-resolved experiments, the ESIPT reaction occurred between 1.0 and 3.10 ns. Furthermore, the lifetime of molecules in EtOH increased when the phenyl ring possessed OMe or OH groups. Dihedral angles between the NH and carbonyl groups were calculated and found to be important for a stable configuration. The methyl substituted NH group on the imidazole skeleton in which the ESIPT reaction should be interrupted, revealed a 0.3 ns lifetime in DMSO. Potential energy curves for all compounds were analyzed using TD-DFT. It was seen that there is no barrier for excited state proton transfer resulting in ultrafast ESIPT process.