Properties and Design Criteria of Second‐order Nonlinear Optical Materials Based on Side Chain Polyesters with an Aromatic Main Chain

Abstract

Abstract Polymers composed of an aromatic polyester backbone to which NLO‐active chromophores and flexible alkyl side chains are attached have been developed as a class of second order nonlinear optical materials. Thin films suitable to be used as waveguides could be produced by spin coating technique and were investigated for their NLO‐properties. The polymers were investigated by optical microscopy, DSC, X‐ray diffraction, 2 H‐NMR, dielectric relaxation spectroscopy and — for their optical properties — by the method of attenuated total reflection of visible light. The solid state structure is best described by an arrangement of alternating layers of polyester backbones and flexible side chains with good layer correlation, especially for derivatives with long side chains. The low nonlinear optical response of these derivatives seems to be due to dipole pair formation of the chromophores. Polyesters with short side chains show a similar main chain packing but substantially different optical properties. In addition, they could be macroscopically aligned in strong magnetic fields. Anisotropic samples prepared in this way indicated a restricted orientational freedom of the chromophores in the space available between the backbone layers. Consequently the ratio of the second order susceptibilities χ versus χ deviated significantly from the expected isotropic value of three.