Molecular design and properties of side chain liquid crystal polymers for applications in optoelectronics

The synthesis of a number of side chain liquid crystalline polymers for quadratic non-linear optical (NLO) applications is reported, The active chromophore is a charge transfer (CT) biphenyl derivative which possesses mesogenic properties itself, The liquid crystalline behaviour of these polymers was established by differential scanning calorimetry (DSC), optical microscopy and X-ray diffraction. The active chromophores were oriented by the standard corona poling technique and the degree of axial ordering was determined as a function of poling conditions by linear optical absorption. Growth of the second-harmonic generation (SHG) signal was used to probe the induction of polar order. These experiments clearly indicated that liquid crystallinity results in an enhancement of the polar order over that of isotropic materials, The second-order NLO susceptibility tensor components d(31) and d(33) were measured by the SHG technique, The ratio d(33)/d(31) was found to be much larger than 3, in agreement with molecular statistical models, Values of d(33) up to 30-35 pm V(-1) were obtained at 1064 nm fundamental wavelength. These values are essentially not resonance-enhanced, since the chromophore absorption occurs below 350 nm. (C) 1998 John Wiley & Sons, Ltd.