<i>Ad-Hoc</i> Functionalization of Phenothiazine Dioxide Derivatives to Achieve Blue Thermally Activated Delayed Fluorescence in Organic Nanoaggregates

In this study, four new phenothiazine dioxide derivatives were synthesized. They were functionalized with rotatable electron donor substituents characterized by different degree of rotatability (such as triphenylamine, phenothiazine and tetraphenylethylene) symmetrically attached to the central core. On the one hand, the triphenylamine- and phenothiazine-based molecules exhibited efficient fluorescence in the blue portion of the visible spectrum, consistent with their modest push–pull character. On the other hand, the tetraphenylethylene-functionalized compound showed very low fluorescence quantum yields and an ultrafast nonradiative deactivation to the ground state via intramolecular rotations. Efficient intersystem crossing (ISC) was revealed through transient absorption experiments with femtosecond and nanosecond temporal resolution for the triphenylamine and phenothiazine derivatives. Organic nanoaggregates of the investigated fluorophores with mean diameters of 50–150 nm were prepared in water dispersion and intriguingly exhibited either blue thermally activated delayed fluorescence (TADF, for the triphenylamine-based dye) or aggregation-induced emission (AIE, for the tetraphenylethylene-based dye). Such organic nanoparticles were successfully internalized in human melanoma and lung cancer cells, exhibiting nuclear localization via fluorescence imaging and some cytotoxicity. TADF was interestingly revealed for the triphenylamine-functionalized molecule also in the solid state, in host–guest powders where triphenylphosphine was employed as the host matrix. This new blue TADF emitter holds significant potential for applications in third- and fourth-generation OLED devices.