Dual fluorescence of 2-(4′- N, N-dimethylaminophenyl)pyrido[3,4-d]imidazole: effect of solvents

Abstract

Spectral characteristics of 2-(4′- N, N-dimethylaminophenyl)pyrido[3,4-d]imidazole (DMAPPI) have been studied in different solvents. Dual fluorescence is observed in polar/aprotic and polar/protic solvents. Short wavelength (SW) emission band is assigned to the locally excited (B*) state, whereas the long wavelength (LW) emission band is due to the twisted intramolecular charge transfer (TICT, A*) state. Fluorescence excitation spectra recorded at SW and LW emissions suggest that both these emission originate from the same ground state species. Time correlated fluorescence studies have suggested that there is no equilibrium between these emissions in the excited state. The reverse reaction of TICT state to the B* state is much slower than the forward reaction of B* state to A* state. AM1 calculations were carried out on the isolated and solvated DMAPPI molecules. These studies have shown that the hydrogen bonding made the pyridoimidazole (PI) ring more planar with benzene ring and this facilitates the flow of charge from the N, N-dimethylaminophenyl (DMAP) ring to the electron withdrawing group (PI).