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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 Δδ=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. |
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