Pure &App/. Chem., Vol. 70, No. 11, pp. 2157-2166, 1998. Printed in Great Britain. Q 1998 IUPAC Photoinduced manipulations of photochromes in polymers: Anisotropy, modulation of the NLO properties and creation of surface gratings Yomen Atassi, Jerome Chauvin, Jacques A. Delaire, Jean-Francois Delouis, Isabelle Fanton-Maltey and Keitaro Nakatani PPSM, URA 1906 of CNRS, Ecole Normale Supe'rieure de Cachan, 61, avenue du Prdt Wilson 94235 Cachan Cedex, France Abstract: Photochromes in solid matrices such as polymers or sol-gels are interesting because of their potential applications as optical filters or optical memories. Besides their reversible color change which is used in these applications, they have also raised a new interest because other optical or physicochemical properties can be changed under light control. We review here the optical properties induced by irradiation with polarized light. First, it has been known for more than two decades that birefringence and UV-visible dichrolsm can be induced in azo polymers. More recently, such phenomena have also been found with spiropyrans. Secondly, when thin polymer films are irradiated with an interference pattern, it is possible to create both refractive index and thickness gratings. Other optical properties arise from the coupling between photochromism and nonlinear optical (NLO) properties. First, when the polarized excitation beam is superimposed with a DC electric field directed perpendicular to it, a non centrosymmetric arrangement of dipoles can be created in a polymeric medium, and second order NLO susceptibilities can be measured. Secondly, it is possible to photoswitch the NLO response of previously oriented photochromic and NLO active polymers (photoswitching) by irradiating alternatively each form of the photochrome. Thirdly, when two coherent beams at frequencies o and 2 61 are superimposed on the same NLO sample, it is also possible to induce non centrosymmetry. This method, called "all-optical poling" is very promising to create NLO devices such as electro-optical modulators or frequency converters. Keywords: photochromes, birefringence, photoassisted poling, photoswitching of NLO properties, surface gratings, azobenzene, spiropyrans, fulgides, diarylethenes, anils. INTRODUCTION Photochromism has been widely studied in polymers as well as in solutions (ref. 1,2). It is usually defined as the reversible photoinduced transformation between two molecular states whose absorption spectra are significantly different. Photochromic molecules have received an increasing interest over the last ten years: new photochromic systems have been found, with increased photochemical stability, and new applications have been developed (ref.3). Photochromic polymers offer advantages because they are easier to use in practical applications, as in photochromic sunglasses or optical memories. Our work deals with optical applications of photochromic polymers other than color change, but connected with it, namely dichroism and birefringence, photoinduced surface gratings, photoassisted orientation and disorientation and photoswitching of the nonlinear optical (NLO) response. We will develop below the different optical (and structural) phenomena which can be induced by irradiation with polarized light of thin polymer films containing photochromic molecules. Among these molecules, azo dyes play a dominant role probably due to some specific features of the trans + cis isomerization reaction which occurs in a good yield in solid state matrices ( @trans+cis = 0.1 1 and @cis+trans = 0.70 for Disperse Red One in PMMA matrix at room temperature) and to the very fast back thermal cisjtrans reaction (k = 0.2 s-1 for DR1 in PMMA at room temperature). *Lecture presented at the 17th IUPAC Symposium on Photochemistry, Sitges, Barcelona, Spain, 19-24 July 1998. Other presentations are published in this issue, pp. 2147-2232. 2157