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In the course of our research on excited state enhancements of nonlinear optical processes in it conjugated structures1, we discovered new linear electro-optic (EO) effects2 in chiral systems. It has long been known that because of symmetry conditions, the macroscopic susceptibility x7(—W;O, (0) for the linear EO effect as in the case of second harmonic generation vanishes in an isotropic chiral system3'4. We found, however, that if the electronic excited state of the chiral system is optically pumped, then permutation symmetry is broken and the macroscopic linear EO susceptibility is non-zero, given by x(—co;O,w) =EaiyX2 where is an antisymmetric third rank unit tensor. We have further observed that the new EO effect depends on the molecular spatial dimensionality of the chiral system and is non-zero only for three-dimensional structures. This dependence is important to molecular design rules for candidate it-conjugated structures. We have studied and designed several classes of molecular structures possessing the required three dimensional noncentrosymmetry. Included, for example, are donor-acceptor substituted chiral cyclophanes having nonvanishing transition moments in three dimensions. There are several additional advantages associated with our new chiral EO effect. Among these is the fact that no electric filed poling is necessary to achieve the noncentrosymmetry required for observing the linear EO effect. Moreover, the new EO property avoids the long term decay of the EO coefficient commonly observed in poled EO materials. The new EO coefficient can remain thermally stable up to temperatures approaching the material decomposition temperature. Our studies also show that simple structural alignment of otherwise isotropic chiral centers leads to further enhancement of the linear £0 coefficients5. Additionally, our findings are general, extending directly to other second order optical processes such as second harmonic generation (o + (i)= 2o) and parametric rectification (o — o =0). In the present report, we will center our discussion on basic theory of the new chiral EO effect in section II. A many-electron sum-over-states calculation of the microscopic EO susceptibility for a model chiral donor-acceptor cyclophane is presented in section ifi as an illustration of both what a new class of EO chiral organic structures might look like and how to begin formulating design rules for new optimized structures.
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