A new optically active photochromic polymethacrylate containing the carbazole moiety, deriving from the chiral
monomer (<i>S</i>)-(4-cyanophenyl)-[3-[9-[2-(2-methacryloyloxypropanoyloxy) ethyl]carbazolyl]]diazene [(<i>S</i>)-MLECA] has
been prepared and fully characterized with the aim to obtain a multifunctional material which can be considered at the
same time as a photonic material for NLO and optical storage, for chiroptical switches, and for photorefractive and
The complete reversibility of the photoinduced linear birefringence, which is related to fatigue resistance properties
seems to be promising for use in optical storage or more generally in the field of photoresponsive systems, and it is not
necessary to add dopants in order to observe photoconduction thanks to the presence of the carbazole moiety, which is
well known for its hole conducting properties.
The photorefractive properties of SmC* phases are usually studied in samples which are not intrinsically photoconductive and in which light sensitization is achieved by doping. We propose two different approaches: the first one is the use of a liquid crystal which is intrinsically photoconducting and photorefractive without any doping. In this system a PR performance larger than what previously reported for doped systems was measured, as well as the amplitude of the photoinduced space-charge field. A further development of this research is the control of the polarization switching in a bistable surface-stabilized ferroelectric liquid crystal (SSFLC) device via a photorefractive (PR) mechanism. In this case, the space-charge field was generated on the surfaces of the SSFLC device, in two photoconducting layers coated on the sample substrates. The formation of a stable grating was observed.
We present results on the photorefractive performance of cyclometalated complexes in which a central metal atom (Pd or Pt) coordinates two different molecular sub-units in a single species. Depending on the details of their structure, these molecules aggregate in crystals, glasses or liquid crystalline phases. The photorefractive properties of the complexes are discussed by treating separately results obtained in different phases. Crystalline compounds can be dissolved in suitable polymers and we show how phase separation in polymeric composites, which is usually detrimental for sample stability, can be controlled and used to increase photorefractive performance parameters by orders of magnitude. In addition, we present a method for estimating the itensity of the space-charge field in chiral smectic phases without using any of the standard models developed for crystalline or amorphous materials.
We report on the photorefractive properties of two polymer composites that utilize a new bis-triarylamine side-chain polymer matrix. Correctly locating the frontier orbitals of the new transport manifold with respect to the HOMO levels of chromophores, allows stable continuous operation over exposure levels of more that 4 kJ/cm<sup>2</sup> when samples are electrically biased at 57 V/μm. This operational stability is combined with video-rate compatible grating build-up times and a dynamic range that allows index modulations of 3 x 10<sup>-3</sup> and gain coefficients on the order of 100 cm<sup>-1</sup> at moderate fields. The thermal stability of one of the composites reported is excellent, showing no signs of phase separation even after one week at 60°C. A comparison with the stability of composites where the new matrix was replaced by PVK is also presented.
We present results on the photorefractive performance of two different classes of organic materials. One of them is based on the space-charge field induced reorientation of the optical axis of chiral smectic A phases. In this case the orientational effect is linear in the field and it is due to the so-called electroclinic effect, in contrast with the quadratic effect present in nematics and associated with dielectric anisotropy. Besides presenting data on the photorefractive properties of these new mesophases, we will consider a simple model which describes their performance as a function of several material and geometrical parameters. In the second part of the paper we introduce cyclopalladated complexes as a new class of multifunctional photorefractive materials. Such molecules form amorphous phases which are photoconducting and exhibit a field dependent refractive index. Their efficiency is among the best known to date for organic materials and the simple synthetic route makes us foresee a fast optimization of cyclometallated compounds for photorefractive applications.