Effect of End Groups on Mechanochromism and Electroluminescence in Tetraphenylethylene Substituted Phenanthroimidazoles

The design and synthesis of aggregation-induced emission (AIE) luminogens have attracted rapidly growing interest owing to the fundamental significance and diverse applications in mechanosensors, optical recording, security papers, and optoelectronic devices. In this contribution we report design and synthesis of four phenanthroimidazoles <b>3a</b>–<b>3d</b> and explored their mechanochromic and electroluminescence properties. The phenanthroimidazoles <b>3a</b>–<b>3d</b> were synthesized by Suzuki coupling reaction of iodophenathroimidazoles <b>2a</b>–<b>2d</b> having different end groups (H, CH₃, CF₃, and CN) with 4-(1,2,2-triphenylvinyl)­phenylboronic acid pinacol ester in good yields. The single crystal X-ray structures of <b>2b</b>, <b>2c</b>, <b>3a</b>, and <b>3d</b> are reported and show that the multiple phenyl rings in the TPE unit adopt nonplanar orientation. Their photophysical, aggregation induced emission (AIE), mechanochromic, electrochemical, and electroluminescence properties were studied. The phenanthroimidazoles <b>3a</b>–<b>3d</b> exhibit strong AIE. High color contrast reversible mechanochromism between blue and green color was observed for phenanthroimidazoles <b>3a</b>–<b>3d</b>. It was found that the solid state emission and mechanochromic behavior of tetraphenylethylene substituted phenanthroimidazoles <b>3a</b>–<b>3d</b> are functions of the end group on phenanthroimidazole. In addition to the AIE and mechnochromic behavior, <b>3a</b> and <b>3d</b> compounds performed well as nondoped blue emitters in the organic light-emitting diodes by affording 3.9% and 4.0% external quantum efficiency, respectively