Single molecule force spectroscopy of azobenzene polymers: Switching elasticity of single photochromic macromolecules

The reversible, optical switching of individual polymer molecules was observed using molecular force spectroscopy. We synthesized a polypeptide with multiple photoactive azobenzene groups incorporated in the backbone. The contour length of the polymer could be selectively lengthened or shortened by switching between the trans- and cis-azo configurations with 420 and 365 nm wavelength light, respectively. This cis- to trans-azo configurational transition induced by ultraviolet light resulted in a measurable change in polymer contour length. The contour length change was observed at low force and under external loads of up to 400 pN using a modified force spectrometer, in which the sample could be irradiated in total internal reflectance. The ability to shorten the polymer against an external load is the first demonstration of photomechanical energy conversion in an individual molecule. This is a significant milestone in the road toward molecular level machines.