Host-guest approach

The stability of conjugated polymer-fullerene devices embedded in conventional polymers (guest-host approach) are higher due to encapsulation against environmental influences. 

In Fig. 3, the values for the electron-number-related static hyperpolarizability fiJN312 obtained for these ionic chromophores (open symbols) have been compared with the same values for the best dipolar, neutral chromophores reported so far (diamonds).31 32 These chromophores, with a reduced number of electrons N equal to 20, have dynamic first hyperpolarizabilities approaching 3000 x 10 30 esu at a fundamental wavelength of 1.064 pm, in combination with a charge transfer (CT) absorption band around 650 nm. It is clear that at this point, the neutral NLOphores surpass the available ionic stilbazolium chromophores for second-order NLO applications, however, only at the molecular level. The chromophore number density that can be achieved in ionic crystals is larger than the optimal chromophore density in guest-host systems. 

An alternative approach is to dissolve an efficient electroluminescent chro-mophore in an inert polymer host in a guest-host system or polymer blend. Unfortunately, phase separation and demixing often limits the amount of low-molar-mass electroluminescent chromophores that can be dissolved in polymer hosts. Therefore, the relative brightness is lower for such guest-host systems. This problem can be overcome by fixing the chromophore chemically to the polymer itself as a pendent group on a side-chain polymer separated by spacer units. Bulk phase separation is then impossible, although microphase separation may still take place. This is illustrated by the structures collated in Table 6.10 for some typical side-chain polymers 53-55. ... 

NLO polymers are designed by incorporating nonlinear chromophores into a polymer matrix. The simplest approach is the use of polymer solutions, so-called guest-host systems, in which the nonlinear chromophore is dissolved in a compatible polymer matrix. Unfortunately, the solubility of guest molecules in a polymer matrix is usually low, which limits the magnitude of the NLO response. This problem is solved by attaching the chromophores covalently to the polymer backbone (side chain polymers) or by incorporating them into the backbone of the polymer (main chain polymers). 

Fig. 16a,b. Schematics of different chemical designs for photorefractive polymers a guest/host b fully functionalized approach... 

In general, the placement of reactive functionalities on the exterior surface of the dendrimers allows introduction of a wide variety of terminal moieties. In alternate synthetic approaches, spacer groups have been deliberately introduced to relieve the steric hindrance in order to faciUtate construction of the next generation. This may provide the possibility of enhancing interior cargo spaces for guest-host type chemistry. ... 

Another approach is cocrystallization of a substituted urea (host) and a diacetylene (guest) [48]. Substituted ureas are used to prepare layered diacetylene crystals. Two examples are shown in Scheme 21. Such a host-guest/cocrystal approach to supramolecular synthesis should be general. In this strategy, the host is used to control the structure and the guest provides the function (optical, electrical, chemical, or physical). 

This chapter concentrates on the design of efficient dipolar NLO chromophores and the different approaches for their incorporation in non-centrosymmetric materials, including guest-host polymer systems, chromophore-functionalized polymers (side-chain and main-chain), cross-linked chromophore-macromolecule matrices, dendrimers, and intrinsically acentric self-assembled chromophoric superlattices. The different architectures will be compared together with the requirements (e.g., large EO coefficient, low optical absorption, high stability, and processability) for their incorporation into practical EO devices. First, a brief introduction to nonlinear optics is presented. 

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