Toward optimal EO response from ONLO chromophores: a statistical mechanics study of optimizing shape

Organic nonlinear optical (ONLO) chromophores are key components in electro-optic (EO) devices, particularly on chip. They have the potential to have footprints compatible with silicon-based devices. Materials based on ONLO chromophores are extremely easy to process, being plastics. The development of better chromophores requires the study of how strong the EO properties are of individual chromophores and how well they can be organized in a host material and, ultimately, how densely they can be packed in a neat material. We now assess how well the existing chromophores perform as neat materials and compare with how well the optimal chromophore could perform. By comparison with idealized structures the nature of the packing of such structures is examined. Furthermore, we consider how to augment core chromophores to improve the order and the packing density to improve the EO performance for bulk materials. The optimal chromophore for a neat material requires efficient utilization of its volume; groups pendant to a chromophore core can be designed to improve the overall order under poling, but eventually become self-defeating when they become too large. Adding pendant groups can lead to desired chemical and physical material properties although they may not necessarily improve the EO performance. Now, simulations show how pendant groups can be placed strategically to optimize EO performance. Estimates are developed for the theoretical maximum EO performance and compared to hypothetical and existing molecules. These comparisons lead to design principles that can be applied in molecular synthesis. Determining the effect of local environment (beyond just the general dielectric effect) is the next major challenge for quantum mechanics/molecular mechanics theory.