π-Stacked Polymers and Molecules

Synthesis, structure, and function of poly(dibenzofulvene) and its derivatives and analogues are π-stacked vinyl polymers. π-Stacked polymers indicate characteristic properties such as remarkable hypochromism in absorbance spectra and upfield shifts of signals in NMR spectra and also reduced oxidation potential and charge mobility. These properties are based on strong interactions between stacked aromatic groups that are not seen in isolated molecular systems. As π-stacked polymers can mediate charge mobility, they have a potential in applications for organic electronics. π-Stacked polymers are advantageous over main-chain conjugated polymers and can find wider applications due to the facts that they are colorless and that, in general, they have higher solubility compared with conjugated polymers. We envisage that π-stacked polymers can be excellent complements to main-chain conjugation polymers in practical polymer materials science and, further, that they could exhibit their own characteristic properties that are yet to be explored.

This chapter focused on the synthesis and fundamental properties of π-stacked polymers, which consist of the stacked π-electron systems created by the [m.n]cyclophane, in particular [2.2]paracyclophane, in the polymer main chain. Our recent works in this field are mainly focused on due to the limited examples, and their characteristic features are summarized. Incorporation of the [2.2]paracyclophane skeleton into a π-conjugated polymer backbone leads to a π-stacked structure. Pseudo-para-, pseudo-ortho-, and pseudo-geminal-disubstituted [2.2]paracyclophanes allow the construction of various π-stacked conformations such as straight, zigzag, and fully stacked structures. Optically active π-stacked polymers comprising the planar chiral pseudo-ortho-disubstituted [2.2]paracyclophanes open a new frontier in chiral polymer chemistry. Common π-conjugated polymers have a set of HOMO (valence band) and LUMO (conduction band) energy bandgaps, whereas in the π-stacked polymers each π-electron system has its own HOMO–LUMO energy bandgap. Various aromatic groups can be incorporated into polymers; therefore, energy and charge transfer through the polymer chain can be controlled by appropriate tuning of the bandgaps and energy levels of the stacked π-electron system. We hope that this new class of π-stacked polymers can make a fundamental contribution to the field of molecular electronics in the form of single molecular wires.

Intermolecular interactions between π systems have a significant effect on the electronic structure and physical properties of thin films of semiconducting conjugated oligomers and polymers. The solid-state packing of these chains may be mimicked within molecules in which conjugated segments are held in a stacked arrangement by their attachment to a rigid scaffold. Electrochemical and spectroscopic characterization of such molecules provide insights into the effect of the structure and length of the conjugated segment, the position at which the segments interact, the length over which the segments are stacked, and the number of segments that are held in such an arrangement. In this review particular emphasis is placed on the use of substituted [2.2] paracyclophanes and arene-fused bicyclo[4.4.1]undecanes as scaffolds.

In line with increasing use of density functional theory (DFT) in quantum chemistry, it is presently employed in more than 80 % of van der Waals calculations. Since most van der Waals calculations target at large-scale systems such as biomolecules and nanomaterials, it is natural to use DFT having features of both high speed and high accuracy. Nevertheless, it has been reported that DFT provides poor van der Waals bonds for many years [1]. For example, until recently, no exchange-correlation functional gives meaningful potential energy curves for the van der Waals bonds of rare gas dimers in Kohn–Sham calculations [1]. The main cause for the poor DFT results of van der Waals bonds is the neglect of van der Waals interactions in conventional exchange-correlation functionals [2].