Donor-acceptor groups, nonlinear optics

Polysilanes are cr-conjugated polymers composed of Si-Si skeletons and organic pendant groups. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong cr conjugation, they have rather narrow band gaps of less than 4 eV [24,25] and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Recently they have attracted much attention because of their potential utility as one-dimensional conductors, nonlinear optical materials, and electroluminescent materials [26-28]. 

If a highly polarizable group is introduced into a receptor molecule, substrate binding should cause substantial perturbations, so that the recognition event would be converted into a non-linear optical signal. Such recognition-dependent nonlinear optical probes may be derived for instance from polyenes such as those shown in Figure 20, from inclusion complexes [8.94a] or from donor-acceptor calixarenes [8.94b]. 

Coe, B. J., Jones, L. A., Harris, J. A., et al., Syntheses and spectroscopic and quadratic nonlinear optical properties of extended dipolar complexes with ruthenium(II) ammine electron donor and N-methylpyridinium acceptor groups. J. Am. 

Fig. 19. Molecular structure of polytriacetylenes (PTA). The conjugation ends are substituted by donor (dimethylaniline) and acceptor (nitrophenyl) groups to modify the nonlinear optical properties. The lateral OTBDMS ((ferf-butyl)dimethylsilyloxyl) groups serve for improved solubility in the solvent (typically chloroform)...

Second order, polyazomethine possesses both donor and acceptor groups at the molecular level, which when aligned gives a poled polymer. Tatsuura et al. fabricated an as-deposited poled polymer film by applying anin-situ electric field during CVD and reported on the second order nonlinear optical properties of the obtained film. The polymer film refractive index was reported to be about 1.68 at 632.8 nm. Second order NLO susceptability, p, was found to be... 

Second order optical nonlinearity can be induced in polymeric systems containing dipolar (donor-acceptor) chromophores. The chromophore can be a molecular species attached to the host chain or it can be incorporated in the polymeric structure itself. In general, a good chromophore has an electron donating group connected to an electron... 

In the simplest case of a donor-acceptor (D-A) molecule, the nonlinear optical activity arises from the electric-field-induced mixing of electronic states such as D-A and D+-A . This makes the response (polarizability) of the molecule different according to the sense of the electric field, and a second-order hyperpolarizability fi coefficient) is observed. If D and A are connected by some bridge, its role in promoting the electronic interaction will be quite similar to the bridge role in mixed-valence complexes. Metal complexes can play the role of donor or acceptor groups. Recent examples have been described with ferrocene or ruthenium(pentaammine) groups [48], but they are either monometallic or too short to be considered in this review. 

Linking of donors such as ferrocenyl, OMe or NMe2 to a W(NO)LX 2+ acceptor via azo or stilbene groups has produced interesting nonlinear optical effects.183, 84... 

In this contribution we review the connection between the nonlinear optical (NLO) response and the solvatochromic behavior of the important class of organic molecules, namely donor-acceptor -tr-conjugated compounds (D-tr-A). In these compounds, also called push-pull chromophores, an electron-donating group D is conjugated to an electron-acceptor substituent A through a system of the localized tr-bonds (Scheme 1). It is well established that such compounds exhibit the following properties [1-7] ... 

Since in molecular materials the nonlinear effect occurs fundamentally at the molecular level, it is possible to assemble new molecules by substituting groups of donors or acceptors to optimize the nonlinear optical response. The optimization of optical nonlinearities at the molecular level constitutes a field that incorporates elements of chemistry, physics, polymer science and material science, known as organic molecular engineering. 

---