Interplay between Donor Identity and Sulfur Oxidation in Regulating the Photophysics of Phenothiazine-Based D–A Molecules

Phenothiazine-based donor-acceptor (D-A) molecules represent an important class of luminogens owing to their butterfly shaped geometry and rich excited-state behavior. Sulfur oxidation of the phenothiazine core has been widely employed as an effective strategy to tune their photophysical properties; however, the generality of this approach across different donor frameworks remains unclear. Herein, we report a systematic comparative study on the interplay between donor identity and sulfur oxidation in regulating the photophysics of phenothiazine-based D-A molecules. A carbazole-based molecular series with stepwise sulfur oxidation (CZ-S, CZ-SO, and CZ-SOO) was designed as a direct analogue of a previously reported diphenylamine-based system, enabling an unambiguous evaluation of donor-dependent effects under identical structural and oxidation conditions. Comprehensive photophysical investigations reveal that replacing diphenylamine with the more rigid carbazole donor fundamentally reshapes the impact of sulfur oxidation on emission behavior. In solution, carbazole-based derivatives exhibit attenuated solvatochromism and moderated intramolecular charge transfer (ICT), as confirmed by Lippert-Mataga analysis. In the aggregated and solid states, the carbazole system displays diversified emission behaviors, including aggregation-induced emission (AIE), aggregation-induced emission enhancement (AIEE), aggregation-caused quenching (ACQ), nonmonotonic solid-state emission shifts, and distinct mechanochromic mechanisms. Single-crystal X-ray diffraction analysis demonstrates that these donor-dependent photophysical differences originate primarily from enhanced conformational rigidity and restricted packing adaptability introduced by the carbazole donor, rather than from changes in intermolecular interaction motifs. This work establishes that sulfur oxidation alone does not universally dictate photophysical outcomes in phenothiazine-based systems; instead, donor identity plays a decisive and cooperative role. These findings provide new insights into the rational design of butterfly shaped luminogens with programmable photophysical responses.