Phenothiazine versus Phenoxazine: Structural Effects on the Photophysical Properties of NIR-II AIE Fluorophores

Aggregation-induced emission (AIE) fluorophores with second near-infrared window (NIR-II) fluorescence are very promising for in vivo imaging because they emit fluorescence in an aggregated state and provide desirable imaging resolution and depth. Up to now, only a limited number of NIR-II AIE fluorophores have been developed. Therefore, synthesizing novel NIR-II AIE fluorophores and investigating structural effects on their photophysical properties are very important for the development of AIE probes. In this work, we synthesized two donor-acceptor-donor-type NIR fluorophores with emissions extending into the NIR-II window named DPTQ-PhPTZ and DPTQ-PhPXZ with phenothiazine (PTZ) and phenoxazine (PXZ) derivatives as the electron donors, respectively, and studied their photophysical properties via theoretical and experimental approaches as well as the properties in NIR-II in vivo imaging. The PTZ and PXZ moieties provided typical AIE characteristics. Despite the very similar chemical structures of PTZ and PXZ, DPTQ-PhPTZ and DPTQ-PhPXZ exhibited rather different photophysical properties, for example, compared to DPTQ-PhPTZ, DPTQ-PhPXZ had higher quantum yield (QY) both in solution and in the aggregated state and its QY was less sensitive to solvent polarity. After being coated with an amphiphilic copolymer F-127, the fluorophores maintained fluorescence, and the formed fluorescent polymer nanoparticles (NPs) had satisfactory tumor accumulation and biocompatibility, implying that they are applicable for in vivo tumor detection.