New photocrosslinkable polymers for second-order nonlinear optics

Among all the strategies developed by chemists to stabilize NLO properties of poled amorphous polymeric films, one of the most efficient is the formation of a network into which NLO moieties are covalently bound by both ends after their alignment under a DC electric field. For that purpose, complementary chemical groups can be used, reacting either thermally or photochemically. In our work, the use of photocrosslinkable groups (cinnamate and furylacrylate) has been investigated. In that case, photocrosslinking reactions occur between the photocrosslinkable groups functionalized into NLO molecules and the same photocrosslinkable groups appended via an alkyl spacer group into a polymer acting as the matrix. After synthesis of the different compounds and film realization, the photocrosslinking process was characterized by UV-visible spectroscopy, thus giving evidence of the higher efficiency of the furylacrylate-based system with respect to the cinnamate-based one. The alkyl spacer group was recognized as playing a crucial role in furthering the dimerization reactions between the polymer and the NLO moieties. Evolution of the maximal absorption band corresponding to the NLO moiety itself (around 500 nm) was interpreted through UV-visible spectroscopic studies using different model compounds. This was not due to NLO moiety damage, as first feared, but only to a modification in the surrounding polarity during the crosslinking process. The furylacrylate-based system was then poled and photocrosslinked in the poled state, yielding a material with excellent optical quality and very stable optical nonlinearity.