Mechanofluorochromism of NIR-emitting dyes based on difluoroboron β-carbonyl cyclic ketonate complexesl

New difluoroboron beta-carbonyl cyclic ketonate complexes CnB (n = 2, 16), in which carbazole was linked to difluoroboron beta-diketonate directly, and DCnB (n = 2, 16), where two terminal carbazole units were bridged by vinyl groups to link to difluoroboron beta-diketonate core, were synthesized. They exhibited reversible mechanofluorochromic (MFC) behavior under grinding/fuming or heating treatment. Particularly, the emission of DC2B and DC16B emerged in the range of 650-850 nm in the solid-state. They gave red and rose-red luminescence in the as-synthesized crystals and in the ground powders, respectively, during MFC processes. Such MFC materials emitting NIR (near-infrared) light were seldom reported. In addition, compared with C2B, CI6B showed high-contrast mechanofluorochromism because the long alkyl chain might decrease the strength of pi-pi interactions in the as-synthesized crystals, leading to the emission appeared at the high-energy region (483 nm). The disassembling of parts of pi-aggregates and the improvement of the molecular planarity led to red-shift of the emission (509 nm) upon grinding.