Photosalient Behavior of Photoreactive Crystals

Abstract

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