Dye Aggregation and Complex Formation Effects in 7-(Diethylamino)-coumarin-3-carboxylic Acid

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7-(Diethylamino)-coumarin-3-carboxylic acid (1) has been used as a laser dye, fluorescent label, and biomedical inhibitor in many different applications. Although this dye is typically used in the solution phase, it is prone to molecular aggregation, resulting in many inconsistent optoelectronic properties being reported in the literature. In this paper, the ultraviolet–visible absorption and fluorescence spectra of 1 are investigated in three representative solvents: cyclohexane [nonpolar and non-hydrogen bonding (NHB)], ethanol (moderately polar and hydrogen bond accepting and donating), and dimethyl sulfoxide (DMSO) (strongly polar and hydrogen bond accepting). These experimental results, in conjunction with (time-dependent) density functional theory (DFT/TD-DFT)-based quantum calculations, have led to the identification of the J-aggregates of 1 and rationalized its different aggregation characteristic in cyclohexane in contrast to that of another similar compound, coumarin 343. We show here that these aggregates are largely responsible for the anomalous optoelectronic properties of this compound. In addition, DFT calculations and 1H NMR spectroscopy measurements suggest that the intramolecular hydrogen bond in 1 could be “opened up” in hydrogen bond accepting solvents, affording significant molecular conformational changes and complex formation effects. The comprehensive understanding of the molecular aggregation and complex formation mechanisms of 1 acquired through this work forms a foundation for the knowledge-based molecular design of organic dyes with tailored aggregation tendencies or antiaggregation characteristics catering to different optoelectronic applications.