Photomechanical Motions in Organoboron-Based Phosphorescent Molecular Crystals Driven by a Crystal-State [2 + 2] Cycloaddition Reaction

Abstract

Photoluminescent molecular crystals integrated with the ability to transform light energy into macroscopic mechanical motions are a promising choice of materials for both actuating and photonic devices. However, such dynamic photomechanical effects, based on molecular organoboron compounds as well as phosphorescent crystalline materials, are not yet known. Here we present an intriguing example of photomechanical molecular single crystals of a newly synthesized organoboron containing Lewis acid−base molecular adduct (BN1, substituted triphenylboroxine and 1,2-di(4-pyridyl)ethylene) having a capsule shape molecular geometry. The single crystals of BN1 under UV light exhibit controllable rapid bending−shape recovery, delamination, violent splitting−jumping, and expanding features. The detailed structural investigation by single-crystal X-ray diffraction and 1 H NMR spectroscopy reveals that the photosalient behavior of the BN1 single crystals is driven by a crystal-to-crystal [2 + 2] cycloaddition reaction, supported by four donor−acceptor type B←N bonds. The instant photomechanical reaction in the BN1 crystals occurs under UV on account of sudden release of stress associated with the strained molecular geometry, significant solid-state molecular movements (supramolecular change), and cleavage of half intermolecular B←N linkages to result in a complete photodimerized singlecrystalline product via the existence of two other intermediate photoproducts. In addition, the BN1 crystals display short-lived room temperature phosphorescence, and the photodynamic events are accompanied by the enhancement of their phosphorescence intensity to yield the photoproduct. Interestingly, the molecular crystals of the final photoproduct polymerize at ambient conditions when recrystallized from the solution forming a 2D supramolecular crystalline polymer stabilized by the retention of all B←N coordination modes.

Photopolymerization‐Driven Macroscopic Mechanical Motions of a Composite Film Containing a Vinyl Coordination Polymer

“…[26] In order to develop devices that can perform photomechanical movements on a macroscopic level, the most important issue is to find or design suitable compounds that can undergo photoreactions such as photoisomerization, photocycloaddition. [27] To date, only a few kinds of organic compounds such as azobenzenes, [28][29][30] anthracenes, [31,32] diarylethenes, [33][34][35][36] furylfulgides [37] and spiropyran [38] are known to trigger photoactuation behaviors.…”

“…Due to the microscopic atomic displacements and the strong structural resistance inside the materials, it is still a great challenge to achieve the direct photomechanical motions of photoresponsive materials or devices on a macroscopic scale [26] . In order to develop devices that can perform photomechanical movements on a macroscopic level, the most important issue is to find or design suitable compounds that can undergo photoreactions such as photoisomerization, photocycloaddition [27] . To date, only a few kinds of organic compounds such as azobenzenes, [28–30] anthracenes, [31, 32] diarylethenes, [33–36] furylfulgides [37] and spiropyran [38] are known to trigger photoactuation behaviors.…”

Cocrystal Strategy to Harmonize the Photochemical Reactivity of Styrylquinolines and the Photoactuation Performance of the Molecular Crystals

“…20,25 Accordingly, the template-assisted method has been employed for the prearrangement of styrylpyridine units in crystals so as to make the packing meet Schmidt's criteria. 26,27 Vittal and Rath presented the photosalient behavior of the crystal based on the complexes where the distorted octahedral Pb(II) center is coordinated to the N atom in styrylpyridine derivatives and the strain is generated during the photo-induced [2 + 2] cycloaddition reaction. 28,29 MacGillivray et al reported the supramolecular alignment between resorcinol and styrylpyridine via multiple H-bonds of O−H•••N so as to realize the photochemical reaction-powered mechanical response of the crystals.…”

“…As for the olefin-based molecular crystals, [2 + 2] cycloaddition between adjacent CC double bonds would trigger the macroscopic mechanical motions. Although styrylpyridine is one of the widely studied building blocks in photomechanical systems, , the distance between adjacent CC double bonds was often greater than 4.2 Å in crystals. , Accordingly, the template-assisted method has been employed for the prearrangement of styrylpyridine units in crystals so as to make the packing meet Schmidt’s criteria. , Vittal and Rath presented the photosalient behavior of the crystal based on the complexes where the distorted octahedral Pb­(II) center is coordinated to the N atom in styrylpyridine derivatives and the strain is generated during the photo-induced [2 + 2] cycloaddition reaction. , MacGillivray et al reported the supramolecular alignment between resorcinol and styrylpyridine via multiple H-bonds of O–H···N so as to realize the photochemical reaction-powered mechanical response of the crystals . Besides, much effort is also made to optimize the olefin geometry, such as cocrystal engineering.…”

Visible Light‐fueled Mechanical Motions with Dynamic Phosphorescence Induced by Topochemical [2+2] Reactions in Organoboron Crystals

“…[6][7][8][9][10] When exposed to light, photomechanical crystals may exhibit various macroscopic mechanical effects such as bending, twisting, cracking, expanding, splitting, and jumping due to their molecular-level transformation in terms of isomerization, [11][12][13] cyclization, [2a, 14, 15] and various topochemical reactions. [16][17][18][19][20][21][22][23][24][25][26][27][28][29] However, macroscopic photodynamic motions of single crystals mostly require activation with damaging and high energy UV light which limits their practical applications. [10,23,29] In this context, the photomechanical bending with solid-state reactions in a few molecular crystals under visible light has previously been explored.…”