Reversible Photoinduced Shape Changes of Crystalline Organic Nanorods

Abstract

Mechanical devices that function on sub-micrometer length scales are expected to find applications in fields ranging from medicine to manufacturing. Biology provides many examples of nanoscale machines, for example, the adenosine triphosphate (ATP)-fueled motion of myosin along the actin filament. [1] In such systems, the motion is fueled by random encounters with high-energy chemical species in the surrounding medium and occurs stochastically, because there is no way to externally control diffusive encounters at the molecular level. An alternative is to use external energy sources (e.g., photons) to activate motion at the nanoscale. Photochemically powered mechanical motion is attractive because it does not require the presence of a second chemical species and external control of the motion can be achieved by manipulating the illumination conditions. These advantages have propelled research into photoactivated mechanical motion on the molecular level, and there have been many examples of photoactivated molecular switching, translation, and geometrical transformations. [2][3][4] In general, these schemes rely on intramolecular events-bond formation, cyclization, cis-trans isomerization-to drive a structural change on the order of 1 Å or less. This molecular-level change can be amplified by coupling multiple molecules together using a liquid-crystal polymeric host. Recent progress in the development of photodeformable polymer fibers and strips has led to micrometer-scale materials that exhibit light-induced shape memory effects, [5,6] reversible bending under anisotropic illumination conditions, [7][8][9][10][11][12][13] and photocontrol of the crosslink density. [14] Recently, we demonstrated the use of an intermolecular photochemical reaction to achieve large physical displacements in a different type of structure: organic molecular crystal nanorods.[15] By taking advantage of the well-known anthracene [4+4] photocycloaddition reaction in 9-tertbutyl-anthracene ester (9-TBAE) molecular crystal nanorods, a large (15 %) change in rod length could be generated. In most cases, the photochemical changes drive a reconstructive crystal-to-crystal phase transition that leads to crystal fragmentation and disintegration. [16][17][18] Apparently, the high surface-to-volume ratio of the nanorods provides sufficient strain relief to avoid cracking and fragmentation during this transition. Although the expansion of the 9-TBAE rods was ca. 100 times larger than that observed in other photoisomerizable molecular crystals, [19] it was largely irreversible. If a reversible system could be found, significant amounts of useful work could be generated by repeatedly inducing such mechanical motion. In the following, we demonstrate that molecular crystal nanorods composed of a related molecule, 9-anthracene carboxylic acid (9-AC), can undergo reversible photoinduced cycling between well-defined shapes after spatially localized excitation. The kinetics of recovery, its dependence on illumination conditions, and the long-term stability ...

Quantitative Evaluation of Photoinduced Bending Speed of Diarylethene Crystals

“…Various types of photomechanical behaviors such as contraction [8,10], expansion [11], OPEN ACCESS bending [8,[11][12][13][14][15][16][17], separation [18], twisting [19], and rolling [12] have been reported so far. Although photochromic compounds such as diarylethenes [8,19], furylfulgide [20], azobenzene [21,22], anthracene carboxylates [23][24][25][26][27][28], salicylideneaniline [29], 4-chlorocinnamic acid [30], 1,2-bis(4-pyridyl)ethylene salt [31], benzylidenedimethylimidazolinone [32], and others [33] exhibit photochemical reactions in the crystalline phase, their photomechanical motions in the crystal are almost bending. It is essential for the development of the research field of photomechanical crystals to investigate the crystal bending behavior in detail.…”

Photomobile polymers from commercially available compounds: photoinduced deformation of side-chain polymers containing hydrogen-bonded photoreactive compounds

“…Photoreactive compounds have also attracted much attention for application as photomobile materials. [8][9][10][11] The change in physical properties of photoreactive compounds can be easily traced using spectrometry, owing to significant changes in their chemical structure and thermal stability. Anthracene is a conventional photoreactive compound that can undergo [4+4] photodimerization upon exposure to UV light.…”

“…The change in the chemical structure is sufficiently large to induce macroscopic movement in the crystal state. 8 We have previously studied photomobile polymers and achieved light-driven deformation of anthracene polymers. The photomechanical effects of the polymers were strongly affected by the glass transition temperature, which was successfully exploited to program deformation of the shape.…”

Optical and Physical Applications of Photocontrollable Materials: Azobenzene-Containing and Liquid Crystalline Polymers

“…Research reports have been presented for both photoisomerizable and photocrosslinkable molecules. Azobenzene [7][8][9], diarylethene [10] and spiropyran [11] are examples of photoisomerizable molecules, while anthracene [12,13] is a photocrosslinkable molecule. In this section, azobenzene polymers that show macroscopic changes upon exposure to light are introduced.…”