Aggregation-Induced Emission in Phenothiazine-Based Fluorophores: An Insight into the Excited State and Aggregate Formation Mechanism

Abstract

In this study, we report evidence for emissive aggregates of push-pull phenothiazine and phenothiazine dioxide derivatives produced both in water dispersions and in the solid state, highly promising for applications in both biology and material science. An insightful investigation of the aggregation-induced emission (AIE) mechanism was carried out via time-resolved spectroscopies, with nanosecond and femtosecond temporal resolution, coupled with advanced data analysis. Our steady-state and time-resolved spectroscopic results unambiguously show that a significant AIE behavior is activated by the restriction of intramolecular rotations for the phenothiazine derivatives. Investigation of the nonlinear optical properties also revealed that aggregates exhibit notable emission upon two-photon excitation. In particular, the phenothiazine dioxide-based aggregates exhibit remarkable fluorescence efficiencies and large two-photon absorption cross sections, as well as the capability to be internalized in HeLa cells exerting no cytotoxic effect. These aggregate species thus prove to be promising as novel fluorophores for bioimaging.