A low-molecular-weight, long-wavelength emissive Coumarin: Green synthesis, spectral properties, fluorescence enhancement and solvatochromism application

In this study, a novel low-molecular-weight coumarin fluorescent dye, 3,4-dicyano-7-diethylaminocoumarin, was designed. This dye features a donor-π-bridge-acceptor (D-π-A) conjugated structure, which confers superior fluorescence properties.The dye was synthesized using a simple, efficient, and environmentally friendly two-step process, facilitating the direct conversion of the key intermediate into the final product via simultaneous addition and elimination reactions. A comprehensive structural characterization of the dye was undertaken, and its UV absorption and fluorescence emission properties were extensively analyzed using spectroscopic techniques, with structural explanations of the fluorescence activities provided by the Density Functional Theory (DFT) method. Experimental results revealed that this coumarin dye emits strong fluorescence, characterized by a long wavelength, a large Stokes shift, and high fluorescence intensity. It exhibits excellent fluorescent performance in various solvents. Despite experiencing fluorescence quenching in its solid state, the formation of an inclusion complex with β-cyclodextrin restores strong red fluorescence, presenting new possibilities for biofluorescence imaging. Notably, this coumarin dye also demonstrates a significant solvatochromic effect. This effect enables rapid visual discrimination among multiple sets of structurally and chemically similar organic solvents. Consequently, the dye holds promise as a versatile solvatochromic fluorescent probe for a broad spectrum of applications.