Large aryl ether azodendrimers (LnAZO) with the number of the aromatic layers (n) larger than 4 showed a fis-to-trans isomerization upon infrared excitation (1597 cm(-1)) of the dendrimer matrix. whereas smaller families such us LIAZO-L3AZO were totally silent to the infrared light. The observed dependence of the isomerization rate constant on the irradiation distance suggests an unusual 'multiphotonic' isomerization mechanism which can overcome the activation free energy of 0.84 eV for the cis-to-trans isomerization. (C) 1998 Elsevier Science S.A. All rights reserved.