Infrared radiation can induce low-frequency molecular vibrations, but, with the exception of hydrogen-bond reorganization(1-3), 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(4-6) 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.