Mechanochromic luminescence from N,O-Chelated diphenylborinates

A series of N,O-bidentated BPh2 complexes (B1-B3) have been designed and synthesized through the direct reaction of N,O-chelate ligands and diaryl borate solutions. Their crystal structures, photophysical and mechanochromic properties were systematically studied. As verified by both experimental and calculated results, all these compounds possess inherent D-A structures, and adopt highly distorted molecular conformations. Multiple weak intermolecular interactions and relatively loose packing are prevailing in their crystal structures. These favorable structure features promise high solid-state fluorescence efficiency and high color contrast reversible mechanochromic for the three dyes. Moreover, their photophysical and mechanical responsive properties have been found structurally dependent, which are mainly modulated by electronic and steric effects derived from different substitutions in the ligand and the boron atom. As a result, B2 with the most twisted and loosely packed structure shows a most sensitive and pronounced mechanochromic response. Our study thus shows that selection of a N,O-bidentated BPh2 based emitter enables the development of excellent MCL-active materials. The current design strategy will be beneficial for the facile access to various advanced BPh2 based dyes for promising applications in optical force sensors, optoelectronic devices and so on.