Mechanochromism and electroluminescence in positional isomers of tetraphenylethylene substituted phenanthroimidazoles

The study of aggregation-induced emission (AIE) luminogens has gained momentum due to their remarkable luminogenic properties and applications in mechano-sensors and organic light-emitting diodes (OLEDs). In this article we have studied three positional isomers (ortho, meta, and para) of phenanthroimidazoles 3a-3c and explored their AIE, mechanochromic and electroluminescence behavior. The phenanthroimidazoles 3a-3c were synthesized by the Suzuki cross-coupling reaction of (2-bromo/3-bromo/4-bromo)phenathroimidazoles 2a-2c with 4-(1,2,2-triphenylvinyl)phenylboronic acid pinacol ester in good yields. The phenanthroimidazoles 3a-3c exhibit strong AIE. The mechanochromic study reveals reversible mechanochromism with good color contrast between blue and green colors. The ortho (3a) and meta (3b) isomers exhibit a grinding induced spectral shift of 98 nm while the para-isomer (3c) exhibits a spectral shift of 43 nm. Moreover, 3a-3c were explored as non-doped blue emitters in efficient organic light-emitting diodes. Among the three emitters, 3c provided a high quantum efficiency of 4.0% in a non-doped blue device.