Photoisomerization in dendrimers by harvesting of low-energy photons

Infrared radiation can induce low-frequency molecular vibrations, but, with the exception of hydrogen-bond reorganization, the excitation energy tends to be dissipated rapidly through molecular collisions rather than inducing photochemical changes. Here we show that in a macromolecular system that is designed to be insulated against collisional energy scattering, infrared absorption can excite photoisomerization by multiphoton intramolecular energy transfer. We have prepared highly branched dendrimers from aryl ethers with a photoisomerizable azobenzene core, in which infrared excitation of the aromatic units is apparently followed by a channelling of the absorbed energy to the core while the dendrimer matrix protects against collisional de-excitation. These findings suggest a strategy for harvesting low-energy photons to effect chemical transformations.