Mechanochromic Materials Based on Tetraphenylethylene-Substituted Phenothiazines: Substituent-Dependent Hypsochromic and Bathochromic Switching of Emission

Phenothiazine- and tetraphenylethylene (TPE)-based mechanochromic materials (PTZTPE-1CHO, PTZTPE-4CHO, and PTZTPE-4CN) with different electron-withdrawing substituents (−CHO and −CN) were designed and synthesized by the Suzuki cross-coupling reaction of bromo-TPEs with the corresponding boronate esters of phenothiazine. The photophysical, aggregation-induced emission, mechanochromic, thermal, and electroluminescence properties of the synthesized compounds were investigated. The cyano group-bearing derivative PTZTPE-4CN exhibited reversible mechanochromism, and its emission maximum shifted bathochromically when mechanically ground, whereas the formyl group-containing derivatives PTZTPE-1CHO and PTZTPE-4CHO exhibited hypsochromically shifted mechanochromism. The solution of PTZTPE-4CN was weakly emissive, and PTZTPE-1CHO and PTZTPE-4CHO exhibit strong fluorescence in the solution state due to the intramolecular charge transfer transitions. The electroluminescence properties of the synthesized compounds as non-annealed and annealed non-doped prompt fluorescent emitters were studied in organic light-emitting diodes, which showed a maximum external quantum efficiency of 1.2% and a luminance of 8762 cd·m^–2.