Journal of Physical Chemistry B, Vol.108, No.25, 8702-8715, 2004
Near-infrared optical absorption Behavior in high-beta nonlinear optical chromophore-polymer guest-host materials. 1. Continuum dielectric effects in polycarbonate hosts
Optical waveguide technologies have been identified for broadband applications related to fiber network, airborne, and space-based communications. Active waveguide technologies developed around nonlinear optical (NLO) dye/polymer-based electrooptic devices have distinct advantages over existing RF communications in terms of cost, weight, size, bandwidth, and immunity to electromagnetic interference, but their ultimate applicability may be governed by optical loss. Though much attention has been given to the influences of the NLO chromophore optical nonlinearity, geometry, concentration, and poling effects on the bulk material nonlinearity and device performance, less attention has been directed at the effects of the component material properties and dye concentration on fundamental near-IR optical absorption. We investigate here the effects of polymer structural variations within the Bisphenol A polycarbonate family on near-IR absorption behavior of guest-host materials, holding the dye constant, over a range of dye concentrations. Solvatochromism plays an important role in the near-IR absorption loss of a particular NLO dye, (2-(3-cyano-4-{2-[5-(2-{4[ethyl(2-methoxyethyl)amino]phenyl}vinyl)-3,4-d iethylthiophen-2-yl]vinyl}-5,5-dimethyl-5H-furan-2-ylidene)-malononitrile). Near-IR loss vs dye absorption spectral shifts can be understood in terms of dye-polymer interaction energies within the context of Marcus' theory of polar contributions to initial and final electronic state free energies. The peak shift behavior can be described by the solvent polarity function of the polymer host, consistent with the Onsager continuum dielectric model. Analysis of the geometric parameters intrinsic to the generalized Kawski solvent polarity correlation suggests a more spherical dye shape can lead to reduced near-IR loss. The loss vs concentration results show that selection of a low-loss host polymer is a necessary but insufficient condition for establishing acceptable loss in a doped NLO dye-polymer system.