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

ABSTRACT 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‐T I and OHT‐T II ) that undergo the identical topochemical [2 + 2] photocycloaddition but exhibit divergently different mechanical behaviors: while OHT‐T II reacts statically, OHT‐T I 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‐T I completely frustrates the structural relaxation of the newly formed cyclobutane dimer, leading to accumulation of elastic strain. Conversely, the uninterrupted stacking in OHT‐T II 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.