Electrostatic interactions chromophores

Harper, A.W., S. Sun, L.R. Dalton, S.M. Garner, A. Chen, S. Kalluri, W.H. Steier, and B.H. Robinson. 1998. Translating microscopic optical nonhnearity to macroscopic optical nonlinearity The role of chromophore-chromophore electrostatic interactions. / Opt Soc Amer B 15 329-337. 

The term in brackets represents the attenuation of the order parameter associated with chromophore-chromophore electrostatic interactions. 

Theory, which explicitly includes chromophore-chromophore electrostatic interactions, certainly makes a number of predictions that can be readily verified. The predicted maxima in graphs of the electro-optic coefficient versus number density indicate that there is an ideal size for spherically shaped chromophores characterized by specific dipole moments, polarizabilities, and ionization potentials. More advanced calculations suggest that prolate ellipsoidal shapes are the least favorable for optimizing optical nonlinearily. Both spherical and oblate ellipsoidal shapes offer advantages. Finally, calculations suggest that to understand fully the observed poling behavior, including temporal behavior, molecular dynamics must be taken into account. 

Theory also predicts that chromophore-chromophore electrostatic interactions will influence the ordering of the chromophore in lattices prepared by sequential synthesis. In particular, the angle of tilt from the normal to the substrate surface should increase with increasing chromophore concentration and increasing electrostatic interactions as increasing the tilt angle will reduce unfavorable electrostatic interactions according to the cos (f> dependence of Eq. 20. Recall that in sequential... 

Conductivity, photoconductivity, and chromophore electrostatic interactions almost certainly account for the disappointing and erratic results that were initially obtained for the high-/x/3 chromophores of Table 2. Utilization of the data of recent studies has permitted rectification of these problems. 

PP chromophores into electro-optically active materials suitable for fabrication of prototype devices. Our initial focus is a review of problems associated with intermolecular chromophore electrostatic interactions and how these can be niimmuzed by simple structural modification of chromophores. We summarize our experiences with defining maximum achievable optic nonlinearity, minimum optical loss (including processing associated loss), and maximum material stability. This... 

As is demonstrated in Figure 1, electro-optic coefficients increase linearly with chromophore number density (with a slope proportional to pP) only at low chromophore loading. At higher concentrations, deviation from linearity is observed and the problem becomes more severe as chromophores with higher pp are studied. In particular, maximum in plots of r versus N are observed and these shift to lower N for the higher pp chromophores. The problem can be traced to increasing chromophore electrostatic interactions as is illustrated in Table II. In this Table, we illustrate the variation of dipole moment, polarizability, and ionization potential (the parameters which define chromophore-chromophore electrostatic interactions-London forces) with molecular hyperpolarizability. 

Our point in Ms communication is not to discuss the quantitative simulation of a large number of effects that can be attributed to chromophore-chromophore electrostatic interactions but rather to explore a simple prediction from the more rigorous theoretical analysis, namely, that attaching steric substituents to chromophores to increase the dimensions of the minor axes of prolate ellipsoidal chromophores should decrease chromophore-chromophore electrostatic interactions. Here we report are first crude efforts to exploit this observation. 

Reduction in chromophore chromophore electrostatic interactions by derivatization with bulky substituents. 

As is evident from a consideration of Figs. 7-9, each of these chromophores has exhibited electro-optic activity exceeding that of lithium niobate while at the same time exhibiting auxiliary properties of chemical stability (Td >300 °C) and solubility that permits preparation of device quality materials [183,210-212]. These materials also illustrate another major direction in the preparation of electro-optic materials namely, the development of bridging segments that lead to improved chemical stability, improved solubility in spin-casting solvents, improved compatibility with polymer host materials, and which inhibit unwanted intermolecular electrostatic interactions (we shall discuss such interactions... 

In the following sections, we shah demonstrate that the observed behavior of electro-optic activity with chromophore number density can be quantitatively explained in terms of intermolecular electrostatic interactions treated within a self-consistent framework. We shall consider such interactions at various levels to provide detailed insight into the role of both electronic and nuclear (molecular shape) interactions. Treatments at several levels of mathematical sophistication will be discussed and both analytical and numerical results will be presented. The theoretical approaches presented here also provide a bridge to the fast-developing area of ferro- and antiferroelectric liquid crystals [219-222]. Let us start with the simplest description of our system possible, namely, that of the Ising model [223,224]. This model is a simple two-state representation of the to-... 

Fig. 10. Definition of unit cell structures for closely packed chromophores. This figure illustrates the problem of packing chromophores at high loading. Both shape (nuclear repulsive interactions) and electronic electrostatic interactions come into play and can be treated using the insights from this simple diagram...

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