Polymer host effects

Farid and co-workers88 have investigated the effect of a glassy polymer host on the spectral position of the excimer emission peak produced by high concentrations of the compound methyl 4-(l-pyrenyl)-butyrate. The excimer peak position in a glassy polymer host was compared to the peak position in fluid solution for the following polymer hosts (and solvents) PS(toluene), PMMA(methyl isobutyrate), and poly-(vinyl benzoate) (methyl benzoate). The excimer emission peak of the pyrene compound in all three solvents occurred at about 20,800 cm-1, but the emission peak in all three polymer hosts was blue-shifted about 1900 cm-1 relative to the solution value. This is in contrast to the behavior of unsubstituted pyrene in PMMA 82) and PS 83), whose excimer peak does not shift from the solution value. 

One of the manifestations of orientational effect in LC polymers is presented by a so called guest-host effect, which is well-known for low-molecular liquid crystals. 

In the case of LC polymers, the polymeric matrix performs as a host, while the guest is a dye, whose molecules are elongated in shape, and the absorption oscillator is parallel (or perpendicular) to the big axis of the molecule 65,163-165>. The experiments investigating guest-host effect in nematic polymers with dichroic dyes covalently attached to the polymer 163) (type I) and mechanically incorporated65) (type II) reveal the possibility to obtain regulated color indicators (see page 60). 

In addition to characterization of molecular and macroscopic electro-optic activity, it is important to define optical loss. Optical loss can be influenced both by absorption and by scattering effects. In order to minimize overall loss, it is important to understand the independent contributions made by scattering and absorption. To separate these effects, we need to determine the contributions made by both chromophore and polymer host to the optical absorption at device operating wavelengths. Chromophore interband electronic absorption can be measured on resonance by traditional UV-Visible spectrometry however, we will typically be concerned with optical absorption at telecommunication wavelengths of 1.3 and 1.55 microns where such techniques do not provide accurate information. Total optical absorption at 1.3 microns is occasionally determined by both the interband electronic absorption of the chromophore and by C-H vi-... 

Kaino and co-workers [255] have investigated the effect of polymer polydis-persity on electro-optic materials properties. No dependence on polydispersity was observed for guest host materials but for Disperse Red chromophores covalently attached to monodisperse polystyrene weaker absorption tails were observed. This result suggests that chromophore-chromophore interactions are modified by the polymer host. 

Yuh and Pai argued that the role of the polymer was related to the activation energy. Borsenberger and Bassler explained their results on a model based on dipolar disorder. According to the model, a is determined by the dipole moment of both the dopant molecule and the polymer repeat unit. The effect of the polymer host is then related to the difference in dipole moments of the dopant molecule and the polymer repeat unit as well as the dopant concentration. Most recent studies have been described by dipolar disorder arguments. 

Polymer hosts continue to exhibit unusual effects on the excited states of guest molecules and/or vice versa. Several molecules containing the aryl—C=N—N—aryl chromophore exhibit anomalously high fluorescence quantum yields when doped into a polyfmethyl methacrylate) matrix. Under... 

Studies continue on the effects of a polymer host matrix on the excited-state properties of guest molecules. For example, the lifetime of excited singlet-state species may be greatly prolonged through restrictions of molecular motions (Gusten and Meisner, inter alia). Accurate information on the blend miscibility of polymers is provided by studies of excimer emission (Mikes et al.), and George et al. claim that the service life of many polymers can be predicted from their luminescence properties see also Martin. 

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