From Strain to Pressure: Atomically Resolved Mechanisms of Stress Dissipation in Emissive, Elastically Deformable Molecular Crystals Under Ambient and High Pressure

ABSTRACT Elastically deformable molecular crystals are attractive for sensors, actuators, and flexible photonic devices, yet conventional tests such as three‐point bending and nanoindentation probe only their anisotropic stress response, overlooking hydrostatic and interfacial stresses relevant to device operation. Here, we combine in situ high‐pressure single‐crystal X‐ray diffraction with dispersion‐corrected density functional theory to map, with atomic precision, the stress‐dissipation mechanisms in a herringbone‐packed, emissive crystal that bends elastically at ambient‐pressure and guides light as a low‐optical‐loss single‐crystal waveguide. The crystal can also withstand hydrostatic pressures of up to 3.63 GPa, among the highest recorded for elastically deformable molecular crystals. The material exhibits multicolor luminescence, a pronounced piezochromic color change under compression and robust emission over multiple bending cycles. Stress‐normalized crystallographic analysis reveals a mode‐dependent deformation mechanism. Under uniaxial bending, molecular tilting and π···π (“beams”) interactions dissipate the added stress across the crystal (∼11.5%·GPa ⁻¹ ), whereas under hydrostatic compression, the weak C─H···π and C─H···Br short contacts (“joints”) absorb most of the added stress (−3%·GPa ⁻¹ to −4%·GPa ⁻¹ ) with negligible molecular tilt (∼0.8%·GPa ⁻¹ ). Together, these insights establish a “beam‐and‐joint” design principle that connects supramolecular interactions to the observed optical and mechanical behavior, enabling the rational design of stress‐tolerant multifunctional molecular crystals for high‐pressure optoelectronic applications.