Photoluminescent materials that exhibit tunable emission properties when subjected to mechanical stimuli have numerous potential applications. Although many organic/inorganic and organometallic compounds display this property, called mechanochromic luminescence, most of these materials undergo a crystalline-to-amorphous (C → A) phase transition; examples of crystalline-to-crystalline (C1 → C2) transformation are rare. Single-crystal X-ray diffraction may allow direct analysis of the molecular packing of mechanochromic luminescence materials before and after C1 → C2 transformation, which may help to understand the underlying mechanism of this transformation. Reported herein is a mechanochromic luminescence material that displays an unprecedented type of C1 → C2 transformation mediated by a transient amorphous phase (C1 → [A] → C2). This mechanochromic luminescence material was developed by introducing soft triethylene glycol side chains in a crystalline gold­(I) complex that exhibits mechanochromic luminescence based on a C → A phase transition. When this new gold­(I) complex bearing triethylene glycol chains was subjected to a mechanical or thermal stimulus, dynamic phase changes were observed with irreversible luminescence color changes from blue to yellow to green in both the cases. The crystallinity of the mechanically generated C2 phase was lower than that of the thermally generated C2 phase. This is because the mechanically induced C1 → [A] → C2 process was finished within seconds, whereas the thermal C1 → [A] → C2 process occurred over a few minutes. To control the C1 → [A] → C2 transformation, we doped the complex with an inactive soft component. This successfully made the transformation reversible (from green to blue) upon thermal annealing of the mechanically obtained C2 phase. This approach allowed the development of an imaging process involving invisible information storage even under UV illumination.

A simple protocol to create nanofibers and -rings through a rational self-assembly approach is described. Whereas the melamine-oligo(p-phenylenevinylene) conjugate 1a self-aggregates to form ill-defined nanostructures, conjugate 1b, which possesses an amide group as an additional interactive site, self-aggregates to form 1D nanofibers that induce gelation of the solvent. AFM and XRD studies have shown that dimerization through the melamine-melamine hydrogen-bonding interaction occurs only for 1b. Upon complexation with 1/3 equivalents of cyanuric acid (CA), conjugate 1a provides well-defined, ring-shaped nanostructures at micromolar concentrations, which open to form fibrous assemblies at submillimolar concentrations and organogels in the millimolar concentration range. Apparently, the enhanced aggregation ability of 1a by CA is a consequence of columnar organization of the resulting discotic complex 1a(3).CA. In contrast, coaggregation of 1b with CA does not provide well-defined nanostructures, probably due to the interference of complementary hydrogen-bonding interactions by the amide group.

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