A New Series of Highly Fluorescent Blue-Green Emitting, Imidazole-Based ICT-ESIPT Compounds: Detail Experimental and DFT Study of Structural and Donating Group Effects on Fluorescence Properties

We designed and synthesized different imidazole and benzil based donor-acceptor compounds and studied their intramolecular charge transfer (ICT) as well as excited state intramolecular proton transfer (ESIPT) behavior simultaneously. Compound CHPI exhibited uniquely very high Stokes shift and negative solvatochromism in its keto tautomer emission, while all remaining compound exhibited small Stokes shift with positive solvatochromism and usual charge transfer characteristics. The ESIPT process of CHPI was further confirmed through the boron complexation of ESIPT core to exhibit highly blue shifted emissions for CHPI-BF2 compound, while opposite results were obtained for non-ESIPT N-Ph DHPI derivative. Optimized structures of these compounds showed variable twist in dihedral angle in their excited state, supporting our prediction of ICT interference in the ESIPT process.