A Mechanochromic and Vapochromic Luminescent Cuprous Complex Based on a Switchable Intramolecular pi center dot center dot center dot pi Interaction

An in-depth study on a stimuli-responsive tetranuclear cuprous luminescent complex is reported and gives new insights into the origin and possible use of the observed stimuli-responsive luminescence. Its crystalline polymorphs with two different shapes are obtained by using different crystallization solvents and show distinct emissions, with one being blue emissive and the other being yellow emissive. Upon grinding, only the blue-emitting polymorph has a marked change in the emission color from blue to yellow, and its ground sample exhibits a yellow emission similar to that of the yellow-emitting polymorph. Interestingly, the yellow-emitting polymorph after exposure to acetone vapor can emit a blue emission and display luminescence mechanochromism similar to that of the blue-emitting polymorph. Single-crystal structural analyses of the two different polymorphs reveal the relationship between the mechanochromic luminescence and the geometrical configuration of the {Cu(mu-dppm)(2)Cu} unit and intramolecular "pyridyl/phenyl" pi center dot center dot center dot pi interactions, supported as well by their PXRD, FT-IR, TGA, and PL studies in various states and by TD-DFT analyses. The results demonstrate the different roles of switchable intramolecular pi center dot center dot center dot pi interactions and the geometrical configuration of the {Cu(mu-dppm)(2)Cu} unit in this stimuli-responsive luminescence and potential applications of such stimuli-responsive luminescence in optical sensing and anticounterfeiting encryption technologies and deepen the understanding of such stimuli-responsive luminescence originating from switchable intramolecular pi center dot center dot center dot pi interactions. In addition, it is clearly suggested that the rational utilization of switchable intramolecular pi center dot center dot center dot pi interactions is a feasible route for developing stimuli-responsive intelligent luminescent materials and devices.