Reversible molecular photoswitches: A key technology for nanoscience and fluorescence imaging

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Abstract
In recent years, molecular switches have attracted considerable interest because they hold great promises as molecular electronic and photonic devices. In contrast to commonplace switches that turn electric appliances on and off, molecular switches enable the storage of information on a molecular level, and their application in nanotechnology, biomedicine, and computer chip design opens up whole new horizons. Usually, molecular switches are addressed by an electrical field, a scanning tunneling microscope tip, or a chemical or electrochemical reaction to specifically switch the physical properties between two states (1–3). Alternatively, molecules might be switched optically between two stable forms, and quite an effort has been put into the synthesis of reversibly photoswitchable fulgides and diarylethenes (4). A photoswitch exhibits two stable and selectively addressable states, a fluorescent and a nonfluorescent, which can be conveyed into another in a reversible fashion upon irradiation with different wavelengths of light. Although highly reproducible optical switching of individual chromophores could be achieved in liquid helium temperature experiments (5), the synthetic approach based on a photoswitchable diarylethene derivative was crowned with success only recently (6, 7). Very recently (8, 9), it was demonstrated that even conventional unmodified carbocyanine derivatives such as Cy5 can function as efficient reversible single-molecule photoswitches. In this issue of PNAS, Habuchi et al. (10) report about photoswitching of a mutant of a GFP-like fluorescent protein that was cloned from the coral Pectiniidae. They demonstrate reversible photoswitching between dim and bright states of individual molecules embedded in poly-(vinyl alcohol) by using 488 and 405 nm laser light with a response time in the millisecond range and a repeatability of >100 times (Fig. 1). So far, only a few successful reversible photoswitching events have been reported for a GFP mutant with a response time of several minutes (11). The intriguing …