Stacked conjugated oligomers as molecular models to examine interchain interactions in conjugated materials

Conjugated oligomers and polymers have attracted attention for their potential use in organic field effect transistors and light emitting diodes by virtue of their semiconducting properties. However, the movement of delocalized charge carriers along individual conjugated chains (i.e., a one dimensional pathway) does not provide for a complete understanding of conduction in the solid state. The importance of interchain hopping in conjugated materials led us to propose an extension of one-dimensional models into the second dimension by preparation and analysis of well-defined oligomers held in permanent cofacial arrangements. Thieno-fused bicyclo[4.4.1]undecanes provide a convenient scaffold to prepare oligomers that are held in a covalent pi-stack, affording control of the distance and orientation of the conjugated segments and the study of the nature of charge transport by UV-vis-NIR spectroscopy and electrochemical measurements.;We have substituted the bis(thieno)fused bicyclo[4.4.1]undecane core to prepare a series of stacked oligothiophenes including terthiophene and a quinquethiophene, along with their linear (unstacked) analogs. Preparation of this first series of compounds provided insights into synthetic methodology as well as preliminary understanding of the pi-stacking effects the electronic structure of radical cations and dication. With this series we were able to determine trends in the spectroscopic and electrochemical behavior of conjugated molecules and relate the influence of pi-stacking to the stability of delocalized radical cations, and the potential role for pi-dimers in charge transport.;Expanding this synthetic methodology, we were able to devise compounds that were covalently stacked with phenyl-capped oligothiophenes and a stacked ferrocene hybrid compound. Although less electron rich and with slightly higher oxidation potentials, the phenyl-capped series allowed us to examine the formation of pi-dimers of conjugated radical cations. The stacked ferrocene compound and its unstacked model also demonstrated effective stacking by through space charge delocalization illustrated in electrochemistry measurements.;With this bis(thieno)fused bicyclo[4.4.1]undecane core, numerous other types of pi-stacked derivatives may be synthesized to provide further insight into organic semiconducting materials (i.e. alkyne substituted oligomers, 3,4-ethylenedioxythiophenes, and n-dopable substituents). Varying the degree of pi-overlap of terthiophene also may allow us to examine the stability of radical cations held in different orientations by use of the bis(thieno)fused bicyclo[4.4.1]undecane core.