Single Crystals Popping Under UV Light: A Photosalient Effect Triggered by a [2+2] Cycloaddition Reaction

Abstract

The extremely rare examples of dynamic single crystals where excitation by light or heat induces macroscopic motility present not only a visually appealing demonstration of the utility of molecular materials for conversion of energy to work, but they also provide a unique opportunity to explore the mechanistic link between collective molecular processes and their consequences at a macroscopic level. Here, we report the first example of a photosalient effect (photoinduced leaping) observed with crystals of three coordination complexes which is induced by a [2+2] photocycloaddition reaction. Unlike a plethora of other dimerization reactions, when exposed to even weak UV light, single crystals of these materials burst violently, whereby they are propelled to travel several millimeters. The results point to a multistep mechanism where the strain energy that has been accumulated during the dimerization triggers a rapid structure transformation which ultimately results in crystal disintegration.Popping of popcorn is a mechanistically unique phenomenon. [1] As the hard shell of corn kernels breaks at high temperature, the thermal energy is converted into mechanical energy whereupon the kernels are self-actuated. The underlying theory invoked to explain this process calls upon the abrupt increase in temperature which causes strong thermal expansion of sugar/starch in the interior of the kernels and of the cellulose of the cover peel, resulting from rapid accumulation of very high strain. The pressure of the superheated water in the interior of the sealed kernel increases up to ca. 9.2 atm (931 kPa) and adds up to the strain on the cellulose cover. Around 450 K, the strain induced by thermal expansion exceeds the elastic threshold of the cover and the pericarp bursts out, whereby the endosperm turns out as a soft and less dense, spongy foam. If the kernel is heated slowly, the fierce popping is not observed as the strain created during the slow heating is dissipated before it reaches the threshold. Despite the burgeoning research on mechanical effects in crystals, [2] a synthetic mimic of such a unique ballistic event of efficient self-actuation has not yet been designed.Herein we report the first instance of an extremely rare phenomenon, photosalient effect, [2a,n] where a mechanical response that visually (and to some extent, mechanistically) resembles the popping of popcorn kernels on a hot surface occurs in single crystals in response to a solid state [2+2] cycloaddition reaction. [3] The photosalient effect is a sudden and rapid actuation of crystals when they are excited with UV light whereby the crystals are propelled to distances many times their own size. [2a] While studying the photoreactivity of the coordination metal complexes [Zn 2 (benzoate) 4 (L) 2 ] (1-3) [L = 2F-4spy (2'-fluoro-4-styrylpyridine) in 1, 3F-4spy (3'fluoro-4-styrylpyridine) in 2 and 4spy (4-styrylpyridine) in 3] we serendipitously noted that when crystals of these materials are exposed to even weak UV light, they pop violently (Movie SV1 an...

Theoretical and experimental studies of photomechanical materials [Invited]

“…We note that the time constant in Equation 46 for pure heating is of the same functional form as that for population conversion between spring types given by Equation 39. Similarly, the onset heating time dependence given by Equation 48 and its amplitude given by Equation 49 Parallels the time dependence given by Equation 35 for the spring-conversion model and its associated steady state population given by Equation 38.…”

“…However, molecular crystals offer great promise due to their high density of photomechanical molecules. [35][36][37][38][39] Crystals therefore have the potential for a much larger photomechanical response due the absence of parasitic passive components and higher young's modulus. The theory presented above would apply with the unit cell acting as the photomorphon.…”

“…[20] This implies that β αI for the power range used in those experiments. If this is so, Equation 35 yields the long-time amplitude…”

“…where we have used Equation 35. Thus, the time constant of the time-dependence under illumination yields β + αI via Equation 35 and β can be determined from the decay time constant according to Equation 37. These two time constants can be used to determine α if the power is known.…”

Light‐Induced Bending of Needle‐Like Crystals of Naphthylvinylbenzoxazole Triggered by trans–cis Isomerization

“…[5] Some molecularc rystals formed from photochemically active compounds showed interesting light-induced bending, [6] curling, [7] twisting, [8] spinning, leaping,b lasting, [9] crawling, [10] and salient. [11] The above photomechanical effects could be triggered by photocyclization reactions of diarylethenes, [12] [2+ +2] cycloaddition reactions of olefins, [13] [4+ +4] photodimerization of anthracenes, [14] tautomerization of salicylideneanilines, [15] and trans-cis isomerization of azobenzene. [16] In our previous work, we found the bending, curling, and salientb ehavior of molecular crystals of (E)-2- (2,4-dichlorostyryl)benzo [d]oxazole driven by [2+ +2] cycloaddition.…”