Strange crystals studied using atomic force microscopy (ATM) have been found by researchers to change shape upon exposure to ultraviolet (UV) light. The surface of the colorless crystal also underwent a thermally stable color change to blue, but returned to its original shape and clear color upon re-radiation with visible light. Such morphological change may have application in switching or redirecting optical signals without the need for electronics or microelectromechanical systems with moving parts.
Masahiro Irie, Seiya Kobatake, and Masashi Horichi of the Department of Chemistry and Biochemistry at Kyushu Univeristy, and CREST, Japan Science and Technology Corporation (Fukoka, Japan), illuminated the surface of diarylethene single crystals with UV light (366 nm) for a minimum of 10 s at 12 mW/cm2. This drove reversible photocyclization reactions that created a series of 5 to 10 steps on the (100) surface. The step height of 1.0 ± 0.1 nm corresponds to one molecular layer. With prolonged irradiation time, more steps with heights of 2.0 ± 0.2 nm and 3.0 ± 0.3 nm appeared. The height was always a multiple of the minimum step height (1.0 ± 0.1 nm), and the team did not observe any steps with a height lower than one molecular layer (see figure).
The steps remained stable until exposed to visible light (500 nm), which restored the colorless, smooth-surface topology. The light-dependent coloration-decoloration cycles were repeatable more than 10,000 times without changing the shape of the crystal. In addition, the photo-generated color states remained stable at temperatures up to 100°C.
Using absorption anisotropy (polar plot) analysis, Irie and colleages found that the UV-irradiated single crystal undergoes a shortening in the distance between the phenyl ring edges. The shrinking of molecular layers produces vacancies in the crystal bulk, which allows the surface to sink as much as one layer. The reactions take place as deep as 500 µm.
Photochromism is the color change of a chemical species by photoirradiation. Although various kinds of photochromic crystals have been developed, the reversible quality is very rare. Photochromic single crystals may have use in holographic and three-dimensional memories. The reversible morphological changes discovered may have use in photodriven, nanometer-scale actuators.