A consortium of scientists from five European institutions has developed a new class of molecular photoswitches that are operated with visible light only. The new switches show large separation of absorption bands and function in various solvents including water. The researchers believe that the new switches could lead to the development of improved photocontrolled systems for a variety of applications that require fast, responsive functions. The new class of photoswitches combines the photochromic dyes thioindigo and azobenzene into a photoswitch called iminothioindoxyl (ITI). Researchers at the University Medical Center Groningnen surmised that the fusion of these two dyes, which are used extensively in molecular switches, could provide better functionality as a photoswitch than either chemical by itself. “However, the initial results were very disappointing,” said researcher Mark Hoorens, who synthesized the compound. But when the irradiation experiments were repeated at facilities at the University of Amsterdam, where there is better time resolution, the results were positive. The optical equipment that was used to make the first discovery that ITI in fact works. Courtesy of Wybren Jan Buma, University of Amsterdam. One of ITI’s “parents” absorbs in the UV region and has band separation, while the other “parent” absorbs in the visible light region but does not have good band separation. At the Amsterdam facility, the researchers saw a completely separated absorption band appear 100 nm to the red of the steady-state absorption band of ITI, with a lifetime of about 10 to 20 milliseconds. This is an illustration depicting how ITI is switched between states. Courtesy of Wiktor Szymanski, University Medical Center Groningen. Additional experiments on femto- and picosecond timescales, performed at the European Laboratory of Non-Linear Spectroscopy at the University of Florence, provided the basis for further mechanistic studies. “From these studies, it became clear that ITI switches on an ultrafast timescale of a few hundreds of femtoseconds, similar to how fast the visual pigment in our eyes is switched when light falls on it,” said researcher Mariangela Di Donato. Researchers at the University of Nantes and Palacký University performed quantum chemical calculations to confirm that ITI is a fully visible-light photoswitch. The calculations predicted absorption maxima of the two photo-isomers that were similar to those observed experimentally, but also predicted a barrier for switching back to the original form that fitted the observed lifetime. “In the first instance, we were quite puzzled by this gigantic 100-nm band separation,” said researchers Adèle Laurent and Miroslav Medved’. “But our calculations now provide a logical explanation for this. What is even better is that they allow us to predict how ITI can be modified to meet the specific requirements of its users.” Hoorens has synthesized several varieties of the photoswitch that have been further characterized in labs at the University of Amsterdam, University of Florence, University of Nantes, and Palacký University. From these studies, the researchers say it has become clear that ITI is a versatile photoswitch that can be operated under a variety of experimental conditions including biological, and that it is relatively easy to tune. ITI can switch in solid state and in solvents ranging in polarity from cyclohexane to water, making it suitable for a range of applications, from responsive materials to photopharmacology. The research was published in Nature Communications (https://doi.org/10.1038/s41467-019-10251-8). See also “Better Understanding of Photoswitch Pathway Could Lead to New Applications,” Photonics.com, May 10, 2019.