7‑(Dimethylamino)coumarin-3-carbaldehyde and Its Phenylsemicarbazone: TICT Excited State Modulation, Fluorescent H‑Aggregates, and Preferential Solvation

The photophysical properties of 7-(dimethylamino)­coumarin-3-carbaldehyde <b>3</b> and its phenylsemicarbazone <b>4</b> were investigated in solvents of various polarity and in differing solvent mixtures. The different fluorescent quantum yield (Φ_F) behavior of <b>3</b> and <b>4</b> in highly polar solvents is discussed in terms of Twisted Intramolecular Charge-Tranfer (TICT) state formation and the specific solute–solvent interactions. Because of the weak intermolecular hydrogen bonding ability of both the radiative ICT and nonradiative TICT excited state of <b>3</b> and the linear steep decrease in Φ_F from a medium to high polarity region, coumarin <b>3</b> could be a useful polarity probe for microenvironments containing hydrogen bonding groups. Compared to <b>3</b>, coumarin <b>4</b> exhibits the highest Φ_F values in highly polar solvents with strong hydrogen bond acceptor ability. The high quantum yield of fluorescence in DMSO, DMF, and alcohols qualifies coumarin <b>4</b> as a laser dye in the given medium, with <i>k</i>_F higher than <i>k</i>_nr. Contrary to previous reports that many H-aggregates are nonfluorescent in nature, coumarin <b>3</b> forms highly fluorescent H-aggregates in MeOH and EtOH. On the basis of the restrictions of the Kasha-exciton theory model, we assume that the formation of fluorescent H-dimer aggregates of <b>3</b> is driven by π⁺–π^– interactions. To the best of our knowledge, this is the first report on aggregation of coumarin dye in alcoholic solutions. In addition, restrictions in the fitting procedure relating to determination of the solvation number, <i>n</i>, using the Covington–Newman model of preferential solvation and also the solvent nonideality parameter, <i>h</i>′, are discussed in this article