Restricting intramolecular motion converts non-fluorescent semicroconaine dyes into turn-on aggregation-induced emission probes

Abstract

Bucking the trend of all other polymethines, semicroconaines are virtually non-fluorescent (Φ_f < 0.1%) 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 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 with 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.