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, and tris(2-dimethylaminoetheny1)-methinium perchlorate (6)-were studied by polarized hyper-Rayleigh scattering (HRS). The NLO activity of these two-dimensional NLO chromophores is attributed to three-level contributions to the

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Communications refractive index of calcium are not readily available, the values n = 0.5 and k = 2% independent of wavelength, were used. The values are typical for alkaline earth metals."41It should be pointed out that the PDDPT is particularly useful in the present results, since the index of refraction then varies across the polymer-1TO-Si02-glass structure as "low-high-low-high". On the other hand, for poly(pphenylenevinylene), which is probably the most studied EL polymer, the sequence would be "high-high-low-high", and the high degree of interference effects reported here are not expected to occur.The inherent optical properties of multilayer light-emitting devices based upon conjugated polymers may be utilized in designing a self-interference-filter type of lightemitting device with very narrow spectral output. This type of device does not rely upon the use of either a dielectric mirror or the presence of two metal (reflecting) electrodes, which serve to reduce light output. Consequently, unlike in the case of microcavity devices, the color selection is achieved through the selection of appropriate parameters of the LED elements, including thicknesses and refractive indices. In the present example, the use of regioregular poly(4,4'-didecyL2,2'-bithiophene) in connection with commercially available 1TO/Si02/glass substrates, results in an output band width of about 30 nm. These results demonstrate the importance of a global or "systems" approach to the design of polymer-based light-emitting devices, and that the single-polymer-layer concept is too simple for understanding the details of light emission in these devices.

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