Crystal chemistry and Photochemistry using Optical Forces

Optical trapping has been widely employed as optical tweezers to trap and manipulate micrometer-sized substances such as a living cell three dimensionally in solution, since Ashkin and co-workers firstly demonstrated the trapping of dielectric particles with a tightly focused laser beam in 1986 [1]. Chemists had hardly paid attention to this method because molecules are generally too small to be trapped stably in solution at room temperature. However, in 2007, we applied the laser trapping method to supersaturated glycine solution, and succeeded in the first demonstration of its laser trapping-induced crystal nucleation [2]. Thereafter, we confirmed that the crystallization is available even in unsaturated solution by this method, meaning that laser trapping of molecules/clusters in solution surely causes increase in local concentration enough to trigger nucleation [3]. Nobody can predict when and where spontaneous nucleation takes place, while spatiotemporally- controlled crystallization is made possible by this crystallization method.

For the past decade after our first success of the trapping-induced crystallization, we have extended this crystallization method to various kinds of compounds such as amino acids, cyclodextrins, alkali halides, and so on. Recently, we conducted the crystallization experiments for hen egg white lysozyme (HEWL), and succeeded in demonstrating its crystal nucleation and crystal growth control [4]. The most interesting result in these works is that a highly concentrated domain of HEWL liquid-like clusters, in which solutes and solvents are weakly linked with each other, is formed by laser trapping prior to crystal nucleation, and furthermore that the domain size is in a millimeter order. Concentration and orientation of the clusters in domain are dependent on laser power and polarization, by which HEWL crystal nucleation and growth are controlled. We now consider that the mechanism involving such a large highly concentrated domain of liquid-like clusters in aqueous solution should be general on laser trapping experiments, so that the cluster domain has a high potential in chemical application of laser trapping.

In this paper, we will also introduce our recent research results about control of chiral crystallization of NaClO₃ [5] with laser. Chiral crystallization, which occurs because of the chiral self-assembly of achiral molecules, is a fascinating phenomenon for the elucidation of the origin of biohomochirality since the crystallization process leads to a spontaneous transition from the achiral state to the chiral state. Laser irradiation provides a chiral bias to achiral molecule (NaClO₃), leading to its chiral crystallization depending on laser polarization. Thus, optical forces are currently opening a new door for novel crystal chemistry and photochemistry