Li Jia , The University of Akron
The goal of the PRF-supported research is to design and synthesize p-conjugated polymers with superior n-type semiconducting property (i.e., high electron conductance, s), which has been long sought after but remained sparsely achieved. Two parameters, charge carrier density (n) and mobility (m), must be taken into consideration since s = nm. In the first year under PRF-ND support, we explored two venues to achieve this goal. First, we used 9,10-diboroanthracene as a key building block that induces high electron affinity. In the second direct, we attempted to take advantage of the high ambipolar charge mobility in regioregular polythiophene and incorporated electron-withdrawing groups to the side chain to increase n. The efforts have resulted in two published papers but also have led us to realize that the approaches focusing on the LUMO energy level for achieving high m are likely not the right choice.
We have come to the conclusion that the conjugated aromatic rings in most of the conjugated polymers known to date are too electronically and structurally isolated. The exceptions are a few ladder-type conjugated polymers, where the repeat units are structurally coplanar, but the aromaticity of each repeat unit dictates that they are electronically isolated from each other. Based on the above reasoning, we have been working on chemical synthesis of boron-doped graphenes.
The specific synthetic targets are shown in Scheme 1 along with their synthetic routes. We have accomplished the synthesis of A in high yield. Current focus is on the dehydrogenative cyclization of A. Direct cyclization using the Scholl reaction conditions has proven not successful. We are now attempting other methods such as the reductive cyclization using alkali
metals. Meanwhile, we are also attempting to brominate X and then realize the cyclization via conventional transition metal-catalyzed coupling reactions (Scheme 2).
Impact to Career and Future Research Direction
The PRF-ND support allowed the Jia group to enter the area of conducting polymers. Collaboration with Dr. Meador at NASA-Glenn Research Center has been established based on the mutual interest in anthracene-type structures and polyaromatic hydrocarbons in general. The collaboration focuses on the chemical synthesis of graphene nanoribbons, which has not been achieved to date despite of many excellent efforts as well as misleading claims in the literature. A proposal has been submitted last April to NASA to support this collaborative effort.