Solvo-Mechanical Flexing in Cryogenically 2D Flexible Single-Crystal-to-Single-Crystal Polymorphic Switch

Stimuli-responsive molecular crystals that couple mechanical flexibility with structural phase transformations are rare and hold great promise for smart material applications. Here, we report a solvo-mechanical bending-induced single-crystal-to-single-crystal (SCSC) transformation in an indomethacin (IND) polymorph that integrates two-dimensional (2D) elastic flexibility with polymorphic switching. The 2D elastically bendable methanol solvate crystal of indomethacin (INDM) undergoes a desolvation-driven transformation into the IND-δ polymorph upon heating (∼84°C), while retaining both crystallinity and 2D elasticity. In addition, IND-δ exhibits mechanical flexibility not only at room temperature but also under cryogenic and elevated thermal conditions. In situ time-resolved atomic force microscopy (AFM) captures the spontaneous transformation of INDM to IND-δ at room temperature over 10 days, revealing solvent-directed surface pore formation. Upon further heating, IND-δ undergoes a SCSC phase transition into the mechanically brittle IND-γ form, establishing a rare flexible-to-flexible-to-brittle polymorphic pathway. Mechanical strain analysis and energy framework calculations highlight the role of interlocked hydrogen bonding, π-π stacking, and halogen interactions in enabling directional elasticity. Comparative studies with IND-α reveal contrasting 1D elastic behavior. Contrary to earlier reports of IND-δ and IND-α as plastic and brittle, respectively, this work demonstrates their elasticity and dynamic mechanical characteristics, offering new directions for designing adaptive crystalline materials.