Photochemistry and Photoorientational Phenomena in Carbosilane Dendrimers with Terminal Azobenzene Groups Alexey Bobrovsky, 1 Sergey Ponomarenko, 1 Natalia Boiko, 1 Valery Shibaev,* 1 Eugeny Rebrov, 2 Aziz Muzafarov, 2 Joachim Stumpe 3 1 Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119899 Russia Fax: þ7 (095) 939 0174; E-mail: lcp@libro.genebee.msu.su 2 Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, Profsoyuznaya st. 70, Moscow, 117393 Russia 3 Fraunhofer-Institute for Applied Polymer Research, Geiselbergstr.69, 14641 Golm, Germany Keywords: azobenzene terminal groups; dendrimers; photochemistry; E/Z photoisomerization; photoorientation phenomena Introduction Recently, photochromism has gained appreciable attention, and now it is an active research area because of its tre- mendous importance in biological phenomena and poten- tial applications in the fields of linear and nonlinear optics. [1] Photochromism, that is a change in the color under light irradiation, is also accompanied by changes in the number of physical characteristics, such as refractive index, dielectric constant, etc. There are numerous photochemical reactions that can lead to photochromism, among which E/Z photoisomerization of azobenzene fragments is the well- known photoreaction. Presently, numerous low molecular mass and polymer compounds which are able to undergo various photochemical irradiation-induced transformations have been synthesized. [2–7] Dendrimers being a distinguished class of superbranched molecules, which are intermediate between polymeric and low-molar-mass compounds, hold a special place among a large number of photochromic compounds. During the last ten years, a lot of reports concerning the study of their photo- physical and photochemical properties have been pub- lished. [8–24] Special attention has been paid to effects of energy transfer in light-harvesting antenna systems based Full Paper: Photo-optical properties of carbosilane dendri- mers of the first, third, and fifth generations with terminal azobenzene groups are presented. The dendrimers synthesiz- ed display a crystalline phase, which melts into isotropic liquid at 81–91 8C (depending on the generation number). A comparative analysis of the photo-optical behavior of the dendrimers in dilute solutions and films was carried out. It was shown that under the action of UV irradiation (365 nm) an E/Z photoinduced isomerization of azobenzene groups takes place in the solutions and amorphous films. This process was found to be thermally reversible: annealing leads to a back Z/E isomerization. The processes of photoinduced orientation of terminal azobenzene groups of the dendrimers under the action of polarized UV light (365 nm) were studied. Changes of absorbance spectra of the dendrimer G1AzoEt solution (dichloroethane, 8.13  10 3 mg/mL) during UV irradiation (365 nm, intensity 1.9  10 8 einstein/(cm 2  s)). Spectra were recorded each 10 s of irradiation. Dashed line corresponds to the photostationary state ( 10 min of irradiation). Macromol. Chem. Phys. 2002, 203, 1539–1546 1539 Macromol. Chem. Phys. 2002, 203, No. 10/11 ß WILEY-VCH Verlag GmbH, 69451 Weinheim 2002 1022-1352/2002/10-1107–1539$17.50+.50/0