Mechanofluorochromism of D–π–A‐Type Difluoroboron β‐Carbonyl Cyclic Ketonate Complexes that Contain a Carbazole Unit

Four new D–π–A‐type difluoroboron β‐carbonyl cyclic ketonate complexes that contain carbazole units ( TCCH , TCCP , CCH , and CCP ) have been synthesized that exhibited reversible mechanofluorochromic (MFC) behaviors. Interestingly, TCCH and TCCP , which contained a tert ‐butyl carbazole group, exhibited high‐contrast mechanofluorochromism compared with CCH and CCP , which just contained a carbazole group. For example, shifts of Δ δ =49 and 20 nm were detected during the mechanofluorochromism of TCCH and CCH , respectively. We found that tert ‐butyl carbazole derivatives existed as isolated molecules in the as‐synthesized crystals, on account of the steric hindrance of the tert ‐butyl groups, and that J ‐aggregates were formed upon grinding. Transformation between the isolated and aggregated molecules, which was stimulated by mechanical force, led to clear changes in the emission properties. However, J ‐aggregates were formed in the as‐synthesized crystals of carbazole derivatives, and grinding only weakened the strength of the π–π interactions, thereby resulting in low‐contrast mechanofluorochromism. Therefore, this approach provides a useful strategy for the design of MFC dyes with high performance.