Advanced Photorefractive and Light-Emitting Organic Materials
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Electroluminescent polymers combine transport and light-emitting properties. In contrast to organic molecules dissolved in a solvent (like in dye lasers) or in an inert polymer binder or solgel (like in solid state dye lasers), conjugated polymers do not exhibit any strong quenching of their light-emitting properties at high concentration or even in neat films. Due to their semiconducting properties, they have transport properties that make them suitable for electroluminescent devices. In contrast to inorganic semiconductors, the emission properties of organics can be tuned through the entire visible part of the spectrum. These materials are therefore suitable for full-color displays. With the recent evidence of stimulated emission [12-16] and optical gain in conjugated polymers and organic molecules, electrically injected organic lasers are under investigation. This paper is divided into two sections. In the first part, we will report some recent advances in two different classes of organic photorefractive materials: polymers and polymer dispersed liquid crystals [17]. In polymers, the dynamic range could be improved by a factor of four compared with previous materials by using chromophores that were designed according to a new design rationale [18-20]. New polymers with high dynamic range in the near IR are discussed. We also report on new guest/host polymer composites that are thermally stable and where the dopant chromophore has a triple functionality. Then we describe a new class of organic photorefractive materials: photorefractive polymer dispersed liquid crystals (PDLC) [17]. In the second part of this paper, we report on several optically pumped laser structures using a conjugated polymer in devices where feedback is provided in different ways. 39 Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
PHOTOREFRACTIVE POLYMERS New polymers with improved dynamic range and near IR spectral sensitivity In low glass transition temperature (T.) photorefractive polymer composites, both electrooptic and orientational birefringence contribute to the total refractive index modulation [18,6]. For such materials, a figure of merit FOM for the design of chromophores for photorefractive applications is [18-20]: FOM = A(T) Aapp2
/J+
(1)
where A(T) = 2/9kT is a numerical scaling factor. Recently, we used the BLA (Bond Length
Alternation) model to define new design guidelines of chromophores for photorefractive applications [20,21]. Within that model, molecular quantities such as the dipole moment, the polarizability, as well as the hyperpolarizability can be correlated with the degree of groundstate polarization [22,23]. Donor-acceptor substituted molecules with a n-electron conjugation path have a ground-state structure that can be viewed as a linear combination of two limiting resonance forms: a neutral form (Fig. la) and a charge-separated form (Fig. lb). The relative
a)
DA
b)
D
+
Fig. 1: Two limiting charge transfer resonanceforms ofa donor-acceptorpolyene molecule: the neutralform a) and th
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