RESUMO
Here we present a study on the solid state properties of trans tetra-ortho-bromo azobenzene (4Br-Azo). Two distinct crystal polymorphs were identified: the α-phase and ß-phase. Notably, only the ß-phase exhibited an extraordinary photosalient effect (jumping/breaking) upon exposure to a wide range of visible light. Powder X-ray diffraction and Raman spectroscopy revealed that the ß-phase is metastable and can transition to the α-phase when subjected to specific stimuli like heat and light. Furthermore, single crystal X-ray diffraction and density functional theory calculations highlighted the significance of a highly strained conformer in the ß-phase, showing that the metastability of the phase potentially arises from relieving this strain. This metastability leads to a light induced phase transition, which appears to be the cause of the photosalient effect in these crystals. Interestingly the polymorphism at the core of 4Br-Azo's dynamic behavior is based on different arrangements of halogen based intermolecular interactions. It is possible that continued study on combining visible light capturing chromophores with halogen interaction-based polymorphism will lead to the discovery of even more visible light controlled dynamic crystal materials.
RESUMO
The photoinduced crawling motion of crystals is a continuous motion that azobenzene molecular crystals exhibit under light irradiation. Such motion enables object manipulation at the microscale with a simple setup of fixed LED light sources. Transportation of nano-/micromaterials using photoinduced crawling motion has recently been reported. However, the details of the motion mechanism have not been revealed so far. Herein, we report visualization of the dynamics of fine particles in 4-(methylamino)azobenzene (4-MAAB) crystals under light irradiation via diffracted X-ray tracking (DXT). Continuously repeated melting and recrystallization of 4-MAAB crystals under light irradiation results in the flow of liquid 4-MAAB. Zinc oxide (ZnO) particles were introduced inside the 4-MAAB crystals to detect diffracted X-rays. The ZnO particles rotate with the flow of liquid 4-MAAB. By using white X-rays with a wide energy width, the rotation of each zinc oxide nanoparticle was detected as the movement of a bright spot in the X-ray diffraction pattern. It was clearly shown that the ZnO particles rotated increasingly as the irradiation light intensity increased. Furthermore, we also found anisotropy in the rotational direction of ZnO particles that occurred during the crawling motion of 4-MAAB crystals. It has become clear that the flow perpendicular to the supporting film of 4-MAAB crystals is enhanced inside the crystal during the crawling motion. DXT provides a unique means to elucidate the mechanism of photoinduced crawling motion of crystals.
Assuntos
Óxido de Zinco , Raios X , Compostos Azo/química , RotaçãoRESUMO
Organic crystals of 3,3'-dimethylazobenzene (DMAB) exhibit photo-induced crawling motion on solid surfaces when they are simultaneously irradiated with ultraviolet and visible light from opposite directions. DMAB crystals are candidates for light-driven cargo transporters, having simple chemical compositions and material structures. However, fast crawling motion without significant shape deformation has not yet been achieved. In this study, compared with hydrophilic glass and conventional hydrophobic surfaces with alkyl chains, siloxane-based hybrid surfaces, which are "liquid-like surfaces," result in the fastest crawling motion (4.2 µm min-1) while the droplet-like shape of DMAB crystals is maintained. Additionally, we successfully demonstrate that the DMAB crystals are capable of capturing and carrying silica particles on the hybrid surface. The transport direction is changed on demand without releasing the particles by simply changing the irradiation direction. The particles can be left on the substrate by removing the DMAB crystals via sublimation at room temperature. This result showcases a new concept of "photo-controllable microcleaner" that can operate a series of cargo capture-carry-release tasks. We expect this transporter to contribute to the development of crystal actuators, microfluidics, and microscale molecular flasks/reactors.
RESUMO
The control of droplet motion is a significant challenge, as there has been no simple method for effective manipulation. Utilizing light for the control of droplets offers a promising solution due to its non-contact nature and high degree of controllability. In this study, we present our findings on the translational motion of pre-photomelted droplets composed of azobenzene derivatives on a glass surface when exposed to UV and visible light sources from different directions. These droplets exhibited directional and continuous motion upon light irradiation and this motion was size-dependent. Only droplets with diameters less than 10 µm moved with a maximum velocity of 300 µm min-1. In addition, the direction of the movement was controllable by the direction of the light. The motion is driven by a change in contact angle, where UV or visible light switched the contact angle to approximately 50° or 35°, respectively. In addition, these droplets were also found to be capable carriers for fluorescent quantum dots. As such, droplets composed of photoresponsive molecules offer unique opportunities for designing novel light-driven open-surface microfluidic systems.