Ladder Polymers: The new generation

Quantum dots based on semiconductors have acquired increasing interest recently. Band filling and screening of excitons by free carriers are the most important mechanisms for the large resonant nonlinearity of semiconductors. The information in the literature is mostly concerned with semiconductor crystals or conventional filter glasses doped by semiconductor particles. Due to the spatial confinement of the photogenerated charged carriers, the electronic levels of small size particles rise to higher energies than in bigger particles, In such a case, one can treat the problem in a one-dimensional box; whereas, the energetic band continuum splits into discrete quantized levels which can be detected in the absorption and excitation spectra.

Semiconductor Quantum Dots in Amorphous Materials

Although there has been a high level of interest in nonlinear optical effects for more than 20 years now, most of the work in this field has concentrated on the development of noncentrosymmetric inorganic compounds such as LiNbO3 and analogues of potassium dihydrogen phosphate (KDP). Many of these inorganic crystals have been commercialized in the form of single-crystal frequency convertors, electro-optic modulators, and more recently as simple waveguide couplers and switches. Since about 1975 there has been increasing interest in organic compounds with nonlinear optical properties as potential alternatives to the inorganics mentioned above (Williams, 1983, 1984; Chemla and Zyss, 1986). Much of the work has concentrated on second-order properties, although more recently third-order effects have also been described.

Nonlinearities in organic solids

The sections in this article are


Introduction
Synthesis
Classical Routes–A Critical Perspective
Diels-Alder Route


Characterization
Molecular Structure
Shape: Three-Dimensional Versus Two-Dimensional Coils
Molecular Weights


Aromatization
Monodisperse Ladders in the Nanometer Range
Summary and Outlook
Acknowledgements

Diels-Alder Ladder Polymers: Synthesis and Aromatization

Electroactive molecules including fused ring (ladder oligomer) dyes, phenylpolyenes, tetraazaannulenes, carbocyanine dyes, squarylium-heterocyclic moieties, etc. have been synthetically incorporated into a variety of traditional polymers both as pendants and as part of the polymer backbone. Such incorporation has permitted a means of addressing the intrinsic poor solubility of these materials and has permitted the realization of optical quality films by spin casting techniques. The approach has also permitted a comparative survey of a significant number of electroactive moieties and has permitted the systematic investigation of the dependence of second and third order optical nonlinearity upon the presence and nature of electron donating and/or withdrawing substituents, upon the concentration of electroactive units (effects of intermolecular interactions), upon the presence of oxidizing or reducing dopants, etc. Data obtained from copolymer studies are compared, where possible, with data from solution studies and studies of electroactive molecule/polymer composites. A new phase conjugate interferometric method is used to simultaneously measure the real and imaginary parts of the third order susceptibility over the frequency range 532–720 nm. These measurements are complemented by studies of the temporal, spatial, and concentration dependence of the phase conjugate signals. DFWM signals are also investigated as a function of pulse intensity and width and as a function of beam polarizations. Finally, the imaginary part of the third order susceptibility from these measurements correlates well with photobleaching/photo-induced absorption data demonstrating a contribution to optical nonlinearity from photo-induced bipolaron species for ladder copolymers. Analysis of the temporal and frequency dependence establish the existence of excitonic, bipolaronic, and four photon parametric mixing contributions to optical nonlinearity.

Advances in the Synthesis of Electroactive Materials and in the Characterization of Mechanisms of NLO Activity

It is well-known that electron delocalization associated with π-orbital overlap contributes to third order susceptibility (1–6). It is thus reasonable to attempt to optimize nonlinear optical activity in centrosymmetric polymers by control of conformation to maximize n-orbital overlap between adjacent sp2 centers and to minimize interruptions of delocalization by minimizing sp3 defects. Steric interactions frequently force nonplanar conformations in linear-chain polymers and many systems such as polyacetylene have yet to be prepared in a form free of sp3 defects such as cross bridges. Ladder polymers, such as shown in Fig. 1, represent structures which should lead to extensive electron delocalization and interesting nonlinear optical activity. Some theoretical evidence exists (6,7) to support this contention. Clearly, orbital overlap is optimized in ladder polymers with a zero dihedral angle between adjacent phenyl rings and for systems where nitrogen atoms exist in the deprotonated (imine) form.

L. R. Dalton

Papers

Synthesis of New Nonlinear Optical Ladder Polymers