Photosalient behavior of photoreactive crystals

Being documented with only about a dozen serendipitous observations, the photosalient effect, where crystals leap when exposed to light, is considered a very rare phenomenon. Here, with a set of structurally related materials that undergo [2 + 2] photocycloaddition we present evidence that this effect is more common than it has been realized in the past, and we seek to establish correlations with the kinematics and the crystal structure toward rational design of photosalient materials. To that end, nine photoreactive complexes AgL₂X₂ (L = 4-styrylpyridine, 2-fluoro-4-styrylpyridine, and 3-fluoro-4-styrylpyridine, X = BF₄^-, ClO₄^- and NO₃^-) were prepared. The [AgL₂]⁺ cations in these structures pack by both head-to-head and head-to-tail alignment of the styrylpyridine ligands. Crystals of six out of the nine complexes were photosalient and popped, hopped, and/or leaped when exposed to UV light. It is concluded that the occurrence of the photosalient effect is determined not only by the nature of the ligand but also by the crystal packing which directs the magnitude, direction, and rate of volume expansion during the photoreaction.