Structure and Solvent Effects on Nonconventional Fluorescence in Choline–Carboxylic Acid Ionic Liquids Hybrid Solvents

Abstract The molecular parameters that influence emissive-state formation and excited-state relaxation in nonconventional fluorophores remain poorly understood. Here, we use structurally related choline–carboxylic acid ionic liquids (ILs) derived from valeric acid (CAVA), levulinic acid (CALE), and 4-pentenoic acid (CAPE) to investigate how the structure, composition, and solvent environment of the carboxylic acid influence nonconventional fluorescence. CAVA exhibits negligible fluorescence, whereas CALE and CAPE display pronounced and composition-dependent emission in aqueous solution. Increasing the choline:carboxylic acid ratio from 1:1 to 1:4 enhances the observed fluorescence intensity in both emissive series, while replacing the carbonyl-containing levulinic acid with alkenyl-containing 4-pentenoic acid markedly alters the emission profile, excitation dependence, and excited-state dynamics. SAXS reveals no detectable nanoscale aggregation associated with fluorescence enhancement, while viscosity and 1H NMR measurements show composition-dependent changes in bulk and local intermolecular environments. Neither properties nor differences in excitationlight absorption consistently track the fluorescence behavior across CALE and CAPE. Solvent- and excitation-dependent fluorescence, ultrafast transient absorption, and time-resolved fluorescence measurements reveal excitation-dependent emission, sub-picosecond spectral evolution, and biexponential nanosecond decay, consistent with a complex excited-state landscape. Together, these results demonstrate that carboxylic acid structure, composition, and solvent environment substantially influence the photophysical behavior of these ILs and establish choline–carboxylic acid ILs as a useful platform for investigating structure–photophysics relationships in nonconventional fluorescence.