In nature, control of chirality at different hierarchical levels is manifested in processes such as protein folding and enzyme catalysis. Efforts at achieving similar control in synthetic systems are aimed not only at gaining insight into how nature does it but also at applying such control to the design of reactions and materials.
Researchers have now reported a reversible, photoresponsive, self-assembling system in which chiral information is exchanged between molecular and supramolecular units [Science, 304, 278 (2004)]. Designed by chemistry professors Ben L. Feringa and Jan H. van Esch at the University of Groningen, the Netherlands, and coworkers, the system is based on a dithienylethene unit functionalized with chiral amide groups.
In the presence of light, the molecules can switch between two states: ring-open or ring-closed. Because of the flexibility of the dithienylethene core, the open form can assume either a left- or right-handed helical conformation. When a solution of the open form is irradiated with UV light, a 1:1 mixture of left- and right-handed closed forms is produced.
At concentrations that promote aggregation, the molecules self-assemble to form fibers of one particular helicity (handedness). When the fibers are irradiated, that handedness is locked in as the individual molecules are converted from the open to the closed form. If the fibers are then disturbed--for example, by heating--individual molecules with identical helicity are released, yielding an optically active solution. The chiral information of the supramolecular structure has been transmitted to the molecular level.
"This photoactive system cannot be achieved through common thermal aggregation processes," Feringa says. Two points are key. First, the UV-light-induced changes in the system can be reversed by irradiation with visible light. "There's a lot of interest in control of chirality at different hierarchical levels. That we can control chirality through light in such a fully reversible way is unprecedented," he says.
And second, both hierarchical levels of the system contribute to the control of chirality. The specific chiralities of the amide groups attached to the dithienylethene core determine the helicity of the fiber, which in turn determines the helicity of the molecules released from the fiber when the aggregate is allowed to collapse.
"This mutual ability to control chirality at different hierarchical levels in a synthetic system has great potential for new approaches in asymmetric synthesis, the understanding of molecular communication processes, and the development of molecular memory systems and smart materials," the researchers write.