Multifunctional 2-(2′-hydroxyphenyl)benzoxazoles: Ready synthesis, mechanochromism, fluorescence imaging, and OLEDs

Triphenylamine (TPA)-containing 2-(2′-hydroxyphenyl)benzoxazoles (2a-2c) have been synthesized via a highly efficient rhodium-catalyzed C–H/C–H cross-coupling reaction. Compound 2a is a novel mechanofluorochromic material with blue-shifted mechanochromic properties. Compounds 2b and 2c presented opposite mechanochromic trends. For 2b, the enol-form emission enhanced, and the keto-form emission blue-shift after grinding. In contrast, 2c exhibited the weak enol-form emission disappeared and the keto-form emission slightly red-shift after grinding treatments. The estrone-containing 2b-based water-dispersed nanoparticles (NPs) exhibit apparent dual-emission and were applied for fluorescence images. In addition, bis(TPA)-containing 2c-based devices exhibit dual-emission with good performance and a singlet exciton yield of 92%, which breaks through the theoretical upper limit of 25% in conventional fluorescent OLEDs. This is one of the highest exciton utilization values recorded for the ESIPT molecules with a dual emission system.