Counterion-Mediated Luminophore Dimerization

Emissive organic salts have long been integral to the discovery of fluorescence phenomena and functional luminescent dyes. Typically, one component of the salt acts as the photoactive unit (luminophore) and its nonemissive counterion is selected to independently tune bulk physical properties, such as solubility. However, the impact of counterion choice on the aggregation and resulting emissive state of organic salts in solution has not been widely investigated. Here, we report that a single cationic luminophore gives rise to either monomer, dimer, excimer, or multichromatic emission under otherwise identical conditions by varying only its counterion. We employ N-methyl quininium (MeQn⁺) as a permanently charged cationic luminophore, which we pair with a series of monovalent anions. At low solution-state concentrations, all the salts give identical absorption and emission spectra that correlate with the MeQn⁺ monomer. However, at higher concentrations, the emission, excitation, and absorption data differ, revealing the presence of monomer, dimer, excimer, or all three, depending on the structure of the anion. Understanding and modulating the formation of dimeric or other well-defined aggregated species by specific ion effects could be exploited in the design of molecular probes for biological systems or emissive thin-film dispersions for optoelectronic devices.