Mechanistic Insight Into the Polymorph-Dependent Photosalient Effect of a White Light or Two-Photon Activated Photoreactive Molecular Salt.

The photosalient effect (PE) provides a visually striking demonstration of the direct transduction of light energy into macroscopic mechanical work. However, establishing a predictive, atomic-level understanding of how the underlying crystal packing governs the accumulation and release of macroscopic stress remains a challenge. Here, we report two photoreactive polymorphs of a stilbene-type molecular salt (OHT-TI and OHT-TII) that undergo the identical topochemical [2 + 2] photocycloaddition but exhibit divergently different mechanical behaviors: while OHT-TII reacts statically, OHT-TI displays a violent, explosive photosalient actuation. By combining single-crystal x-ray diffraction with advanced periodic density functional theory (DFT) calculations, we reveal the mechanistic origin of this phase-dependent selectivity. We demonstrate that the specific shell-like packing of OHT-TI completely frustrates the structural relaxation of the newly formed cyclobutane dimer, leading to accumulation of elastic strain. Conversely, the uninterrupted stacking in OHT-TII allows for the unhindered dissipation of this strain. Furthermore, the pronounced push-pull character of the chromophore effectively overcomes the traditional reliance on UV light, enabling this actuation to be cleanly triggered by low-energy visible light and, unprecedentedly, via near-infrared two-photon absorption. Supported by complete thermal reversibility, this study provides a predictive structural and computational blueprint for the rational design of next-generation dynamic materials.