With applications in molecular machines and protein probes, azobenzene might have one of the most useful cis-trans isomerizations in chemistry. Yin Yang, Russell P. Hughes, and Ivan Aprahamian of Dartmouth College have now used azobenzene’s skill to cut a path to a visible light switch (J. Am. Chem. Soc., DOI: 10.1021/ja306030d). To minimize damage to living samples, researchers have been pushing to make azobenzene’s reversible isomerization occur by visible light instead of the ultraviolet light typically required. Most attempts have been bedeviled by heat-induced cis-trans isomerization or by side reactions. The Dartmouth team extended the conjugation of azobenzene by tacking on a larger aromatic group and then complexed that molecule with the Lewis acid BF2. Shining 570-nm light on the trans isomer converts it to cis, accompanied by a color change from magenta to orange; 450-nm light accomplishes the reverse. The probe undergoes faster cis-trans isomerization in an oxygenated solvent versus a deoxygenated solvent. The researchers are trying to understand and control this oxygen effect and plan to explore the stability of related compounds in water. “There’s a lot of room to play around with the molecule,” Aprahamian says, including modifications to the aromatic skeleton and the Lewis acid.