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A method of determining individual keto-enol equilibrium and acid dissociation constants for systems in which unstable enols can exist in cis and trans isomeric forms is developed and is applied to phenylacetaldehyde in aqueous solution; the results give equilibrium cis: trans ratios of 35 : 65 in acidic and neutral solutions and 20 : 80 in a basic solution (where the enols are converted to enolate ions), but show considerably less cis: trans differentiation for enols formed under kinetic control.

Supramolecular coordination complexes with solidstate stimuli-responsive characteristics are highly desirable but are rarely reported. Herein, we describe two coordination-driven selfassembled monoanthracene or dianthracene-based hexagonal metallacycles by subtle structure modification. Notably, the dianthracenecontaining hexagon 1 exhibits tricolor mechanochromic and vapochromic characteristics, while the monoanthracene-containing hexagon 4 does not show obvious changes toward mechanical force. Further studies have indicated that changes in hexagon 1, especially the ulterior anthracene of hexagon 1 in the molecular stacking through intermolecular interactions toward external stimuli, are responsible for the above behavioral differences. Furthermore, the present work also demonstrates a novel light-harvesting strategy for achieving high-contrast mechanochromic fluorescence involving solidstate energy transfer from hexagon 1 to an organic carbazole derivant 6 without mechanofluorochromism or tetraphenylethylene derivant 7 exhibiting inconspicuous mechanofluorochromism.

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Six 1,8-naphthalimide-based tetraphenylethene derivatives were reported. These highly solid-state emissive compounds showed aggregation-induced emission enhancement or aggregate fluorescence change characteristics. Furthermore, these aggregation-induced green light-emitting or aggregation-induced yellow light-emitting luminophors also exhibited various mechanofluorochromism behaviors involving different fluorescent color changes. To determine the mechanochromic mechanism of these six mechanical stimulus-responsive luminogens, the powder X-ray diffraction (XRD) and X-ray single-crystal measurements were carried out. The powder XRD test results indicated that their interesting mechanofluorochromic behaviors were attributed to the interconversion between crystalline and amorphous states. Meanwhile, the X-ray single-crystal analyses results implied that the twisted molecular conformation and the absence of an intense intermolecular acting force led to a loose packing motif, and the alteration of molecular packing was responsible for the observed mechanofluorochromic effect.