As both the number and type of side chain substituents increase and, additionally, as synthetic capabilities continue to expand the possibilities for new polymers appear to be almost endless. Only a limited subset of already existing $\pi $ and $\sigma $ conjugated polymer families actually yield highly ordered materials. Polymers with substantial side chain crystallization and/or mesogenic activity are clearly preferable from this perspective but crystallization is not a panacea for achieving the highest level performance of optical and electronic properties. Improved ordering at larger length scales does not necessarily guaranty the best behavior at the shortest length scales. Thus any and all strategies which provide control of structure and functionality at the molecular level are desirable. Still the presence of molecular ordering and crystallization represents an important reference frame for understanding the fundamental underlying structure/properties relationship governing conjugated polymer behavior.

This review has shown that, in a very limited cross-section of conjugated polymers, a combination of direct structural studies and molecular level modeling can better elucidate molecular level relationships that impact charge transport and photophysics. More exotic types of applications require different molecular level attributes. For use as photovoltaics[126] and in light harvesting technologies[127] these electronic polymers (or polymer blends) require good electron transfer coupled with high ``electron'' and/or ``hole'' mobilities. Preventing recombination is often essential so, in general, it is necessary to incorporate two distinct components, one for transporting positive charge and one for negative charge at the molecular level. High-performance sensors are another rapidly emerging application area[128]. In this case the main chain and side chain architectures may require complex functionalization[129]. Both these examples introduce even greater microscopic complexity and yet another level for addressing structure/property relationships. In the arena of conjugated polymer studies much work remain to be done.

Acknowledgments: We gratefully acknowledge continuing support through NSF grant DMR-0077698 and, previously, that of former students Guomin Mao and Ty Prosa. Valuable contributions by Jason Slinker and Boy Tanto in the PFO studies are acknowledged. One of the authors (M.J.W.) thanks the efforts of numerous collaborators over the years including A.J. Heeger, P. Smith, F.E. Karasz, R. West, J. Reynolds and J.E. Fischer. We also thank many colleagues for their input and insight including M. Ediger, D. Bradley, S. Guha, E. Zojer, T. Bjornholm, Frank Weinhold, J. Michl and many others as well.

Winokur 2004-01-28