Organic superlattices

Fig. 10. The leh-hand diagram shows an organic superlattice with a unique polar axis. The two types of molecule involved could be a fatty acid and a fatty amine. The insert is designed to show that these two materials have dipole moments in opposite senses with respect lo the hydrophobic chain. Thus, the V-lype film has a resultant dipole moment...

Figure 4.4. Top A combination of two molecules A and B enables a Y type film to be produced of noncentrosymmetric character. Bottom A rotating substrate is used to alternately pick up different molecules from separate areas of the trough and thus form an organic superlattice.

To illustrate the potential of LB films in nonlinear optics, two examples will be given to illustrate both quadratic- and cubic-order effects. The motivation for the first example came from early accounts of nonlinearity in a specific merocyanine dye molecule (49). No good crystals of this material are available, but an extremely large value of P was predicted on the basis of measurements with powdered samples. The reliability of such data is not high because powder efiSciency is a function of particle size distributions. The initial experiments were with a simple merocyanine dye alternated with o)-TA. However, the best results (50) were obtained by using organic superlattices based on the two molecules shown by Structures 4. la and 4. lb. 

Structures 4.1a and 4.1b. These two molecules, one (a) a hemicyanine and the other (b) a nitrostilbene dye can be used to form an organic superlattice displaying a high coefficient for second harmonic generation (see reference 50). 

Fermi resonance interface modes in organic superlattices 9.2.1 Fermi resonance in molecules... 

Cumulative photovoltage in asymmetrical donor—acceptor organic superlattices... 

Chromophoric self-assembled multilayers. Organic superlattice approaches to thin-film nonlinear optical materials, J. Amer. Chem. Soc. 112 7389 (1990). 

Li, D. Ratner, M.A. Marks, T.J. Zhang, C.Yang, J. Wong. G.K. Chromophoric self-assembled multilayers. Organic superlattice approaches to thin-film nonlinear optical materials. J. Am. Chem. Soc. 1990. 112 (20), 7389. 

Figure 2 shows the band structures of several homopolymers and pyrrole-bithiophene copolymers estimated by electrochemical and optical methods as examples. A combination of these homopolymers and/or copolymers implies various kinds of superlattice structures. The electrochemical preparation of both homopolymer multiheterolayers and/or copolymer multiheterolayers results in a superlattices. The electrochemical copolymerization method as used to prepare heterolayers was easier than in the homopolymer heterolayers. The copolymer multi heterolayers are prepared by simply changing the applied electrode potential. On the contrary, the latter needs exchange of the mother solutions. The present electrocopolymerization method which makes compositionally modulated copolymer heterolayers possible is considered to be one of the most fascinating methods to fabricate organic superlattices. 

Synthesis of a Photoresponsive Polymer and Its Incorporation into an Organic Superlattice... 

Two routes were explored to fabricate organic superlattices a modified spin casting procedure and multilayer coextrusion. Spin casting was selected as a relatively quick, low tech way to select the most promising photoresponsive polymer. 

The different kinds of molecules can be incorporated in an LB multilayer. The resultant LB film has the structure of a organic superlattice if each kind of molecule is positioned within different molecular layers. It is also possible to incorporate different kinds of molecule within the same layer by forming mixed monolayers at the air-water interface (Figure 14.4). In the case of mixed monolayers, poorly surface active or even non-surface active molecules can be incorporated into films with the aid of highly surface active molecules such as long-chain fatty acids. 

Marks and co-workers have su ested a new organic superlattice approach for developing covalently linked self-assembled, chromophore-containing multilayer sys-tems. In this approach, self-assemblies of intrinsically acentric multilayers of high fip chromophores on inorganic oxide substrates are formed. These multilayers show excellent adhering properties and are insoluble in common organic... 

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