2025

Strada, Rebecca; Dunlop, David; Vorba, Michal; Raj, Amar; Tütüncü, Büşra Buse; Myllyperkiö, Pasi; Slanina, Tomáš; Kumpulainen, Tatu; Sebej, Peter

Bucking the trend of all other polymethines, semicroconaines are virtually nonfluorescent dyes. But their fluorescence could be increased through external physicochemical factors, thereby converting them into turn-on fluorescent probes. To test this hypothesis, we analyzed excited-state dynamics of a small library of semicroconaines bearing various auxochrome substituents to determine how they lose their excited-state energy. Using a combined experimental/quantum-chemical approach, we found that isomerization of their methine bridge leads to non-radiative S1-S0 relaxation through a conical intersection. This relaxation pattern was consistently identified in semicroconaines bearing various auxochrome substituents (-F, -I, -OCH3, -SO3H and -NO2). Just as consistently, their fluorescence in solution significantly increased when increasing solvent viscosity and inducing complexation with two macromolecules, namely glycoluril dimer and DNA. Therefore, semicroconaine dyes display turn-on aggregation-induced emission, a mechanism that may be exploited for macromolecular sensing under physiological and pathological conditions.