Counteranion-Driven Dealkylation of Photoluminescent Cationic Chromophores

Highly photoluminescent organic materials are of interest in various domains, such as sensors, bioimaging, and optoelectronic devices. Typically, counterions in charged chromophores are perceived as loosely bound to their counterparts and have minimal influence on their photophysical properties. Herein, we uncovered that counteranions can induce a chemical transformation of cationic chromophores and completely alter their chemical and photophysical properties. Detailed photophysical studies using an archetypal N-alkylpyridinium-based cationic chromophore (thiazolo[5,4-d]thiazole) showed that under conditions favoring contact-ion-pair formation (small counteranion in nonpolar solvent), dealkylation takes place with fully altered photophysical properties. However, under conditions that favor solvent-separated ion-pair formation (bulky counteranions or polar solvents), the photophysical properties remain unchanged, and high photoluminescence quantum yields were observed. Dealkylation was also observed in other cationic chromophores and in dye-embedded thin films, indicating its generality. These findings warrant caution when using N-alkylpyridinium-based cationic dyes for applications in hydrophobic environments or in thin-film-based materials.