Salt-cocrystal continuum for photofunction modulation: Stimuli-responsive fluorescence color-tuning of pyridine-modified intramolecular charge-transfer dyes and acid complexes

The regulation of proton transfer dynamics between acid-base complexes is of significant interest in the pharmaceutical industry and materials chemistry. However, the extent of proton transfer in the solid state is difficult to predict, and identifying the salts (protonation), cocrystals (hydrogen bond), and salt-cocrystal continuum (partially protonated states) remain challenging topics. Here we report that the three states (salts/cocrystals/salt-cocrystal continuum) can be distinguished by photoluminescent color changes based on acid-base complexes consisting of a pyridine-modified pyrrolopyrrole dye and organic acids. Structure-property relationships of 10 complexes indicated that ΔpK_a value (pK_a (protonated base)-pK_a (acid)) and the crystalline environment determine the extent of proton transfer, which governs the intramolecular charge-transfer (ICT) strength of the complexes and tunes the photoluminescence properties. Enhancement of the ICT strength leads to a bathochromic shift of emission from blue to green to yellow under UV light. The salt-cocrystal continuum (0 < ΔpK_a < 3) showed vapochromism/vapofluorochromism against CH₂Cl₂, owing to the extent of proton transfer from the acid to the pyridine moiety of the dye being modulated by inclusion and desorption of CH₂Cl₂. This study suggests that the rational design of photoluminescent acid-base complexes is useful for obtaining novel photofunctional materials.