Stimuli-Responsive Phenothiazine-<i>S</i>,<i>S</i>-dioxide-Based Nondoped OLEDs with Color-Changeable Electroluminescence
Faizal Khan, Malek Mahmoudi, Dmytro Volyniuk, Juozas V. Gražulevičius, Rajneesh Misra
Abstract
Oxidized phenothiazines have gained significant research interest due to their excellent fluorescence and phosphorescence properties and high photoluminescence quantum yields in the solid state. Herein, we report the development of aggregation-induced emission (AIE) active mechanochromic materials, namely, PTZ(H), PTZ(CN), PTZ(CHO), and PTZ(PhCN), in which the phenothiazine core was oxidized to explore the impact of changing the electron-rich sulfur atom of phenothiazine to electron-withdrawing sulfones on the AIE and mechanochromism. The compounds were synthesized by the Suzuki cross-coupling reactions of the boronate ester of tetraphenylethylene with the appropriate bromo derivatives of phenothiazine-S,S-dioxides. The PTZ(CHO) and PTZ(CN) possessed reversible switching of emission color by the virtue of conformational changes in the molecular structure. The computational investigations demonstrated well-separated electronic density in the HOMO and LUMO energy levels of PTZ(CN), PTZ(CHO), and PTZ(PhCN), whereas in PTZ(H), the HOMO and LUMO were found to be located on the same moiety. These derivatives were used as fluorescent emitters for nondoped organic light-emitting diodes (OLEDs) which showed maximum external quantum efficiency of 2% and brightness of 10,000 cd/m2. Because of the stimulus-responsive emission and thermal stability, one derivative demonstrated sky-blue or whitish-yellow electroluminescence with CIE coordinates (0.22,0.37) or (0.34,0.45) in nonannealed and thermally annealed OLEDs, respectively, without efficiency loss.