The Role of Weak Interactions in the Mechano-induced Single-Crystal-to-Single-Crystal Phase Transition of 8-Hydroxyquinoline-Based Co-crystals

Mechano-induced single-crystal-to-single-crystal (SCSC) phase transitions in crystalline materials that change their properties have received more and more attention. However, there are still too few examples to study molecular-level mechanisms in the mechano-induced SCSC phase transitions, making the systematic and in-depth understanding very difficult. We report that bis(8-hydroxyquinolinato) palladium(II)-tetracyanoquinodimethane (PdQ(2)-TCNQ) and bis-(8-hydroxyquinolinato) copper(II)-tetracyanoquinodimethane (CuQ(2)-TCNQ) show very different mechano-response behaviors during the SCSC phase transition. Phase transition in CuQ(2)-TCNQ can be triggered by pricking on the crystal surface, while in PdQ(2)-TCNQ it can only be induced by applying pressure uniformly over the whole crystal face. The crystallography data and Hirshfeld surface analysis indicate that the weak intra-layer C-H center dot center dot center dot O, C-H center dot center dot center dot N hydrogen bonds and interlayer stacking interactions determine the feasibility of the SCSC phase transition by mechanical stimuli. Weaker intralayer interactions and looser inter-layer stacking make the SCSC phase transition occur much more easily in the CuQ(2)-TCNQ.