Synthesis and a TD-DFT study of a series of novel 3-imidazolyl-substituted coumarin molecules with large Stokes shifts

3-Imidazo[1,2-& alpha;]pyridine coumarin molecules 3a-3d with four different substituents were synthesised from salicylic aldehyde derivatives, ethyl acetoacetate and 2-aminopyridine derivatives, and exhibited strong blue fluorescence in non-polar solutions and large Stokes shift (184-210 nm) in polar solutions. All these were validated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Furthermore, 3a-3d exhibited a solvatochromism effect and solid state fluorescence. The results indicated that the introduction of the 3-imidazo[1,2-& alpha;]pyridine group at the 3 positions of coumarin constructed an asymmetric electronic structure, which induced intramolecular charge transfer (ICT) with non-radiative energy enhancement, leading to large Stokes shifts. Meanwhile, by introducing an electron-donating group into imidazo[1,2-& alpha;]pyridine, the HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) gap can be narrowed to enhance ICT, further increasing the Stokes shift, suggesting a promising and novel model for achieving large Stokes shifts for the development of coumarin dyes. 3-Imidazo[1,2-& alpha;]pyridine coumarin 3 with four different substituents were synthesised from salicylic aldehyde derivatives, ethyl acetoacetate and 2-aminopyridine, and exhibited strong blue fluorescence in non-polar solutions and large Stokes shifts (184-210 nm) in polar solutions.