Electronically Controlled Conformational Equilibria in Symmetrically Substituted 3,7,10-Triarylphenothiazines: Tuning Redox and Emission Properties

The combination of Suzuki arylation and Buchwald–Hartwig amination provides a sequentially Pd-catalyzed pseudo-four-component strategy for the synthesis of symmetrically substituted 3,7,10-triarylphenothiazines in moderate to good yields. Using p-anisyl-derived donor units and p-benzonitrile-derived acceptor units, the electronic and photophysical properties of four representative derivatives were investigated by cyclic voltammetry, absorption and emission spectroscopy, and (TD-)DFT calculations. The calculated electronic transitions are in good agreement with the experimental absorption spectra and enable assignment of the underlying optical transitions, while the observed photophysical behavior is interpreted in the context of previous studies on related 3,10-diarylphenothiazines. A p-anisyl donor substituent at the phenothiazine nitrogen atom favors the intra-oriented ground-state conformation, resulting in intense low-energy absorption bands and high fluorescence quantum yields. In contrast, a p-benzonitrile substituent at this position shifts the conformational equilibrium toward the extra-oriented conformation, leading to altered electronic transitions and reduced fluorescence efficiency. Combined with p-anisyl donor units at the 3,7-positions, the extra-oriented conformation becomes predominant, resulting in pronounced emission quenching. These findings demonstrate that 3,7,10-triarylphenothiazines represent a class of redox-active luminophores in which electronic substitution and conformational preferences provide complementary handles for tuning ground- and excited-state properties.