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45830-GB7
Probing Through-Space Charge Migration in Segmented Conducting Polymers

Jocelyn M. Nadeau, Marist College

The discovery of conducting polymers in the 1970s has led to countless important technological applications including organic light-emitting diodes, chemical sensors, and corrosion inhibitors.  Using segmented conducting polymers composed of electroactive segments connected by insulating segments, our goal is to study how the relative orientation of the electroactive segments modulates the electrical and optical properties of the polymers.  Toward this aim, undergraduate coworkers and I have established a successful route to C-shaped electroactive monomer 1 and are currently working on the synthesis of electroactive monomer 2 (Figure 1).  Electropolymerization of these monomers is expected to lead to segmented conducting polymers with well-defined structures that will be used to study through-space charge migration.   

Figure 1. C-Shaped electroactive monomers, 1 and 2.

Our first, and perhaps most important, task was to synthesize dibromide 3, where C-shaped 3 is the basic building block from which a family of desired C-shaped electroactive monomers will be obtained (Figure 2; left).  Dibromide 3 was produced as a mixture of diastereomers, where the major isomer comprising 71% of the mixture is C-shaped (NMR integration).   

Evidence supporting the C-shaped topology of the major diastereomer of 3 comes from preliminary X-ray diffraction data (Figure 2; right).  A molecule of CDCl₃, the crystallization solvent, can be seen occupying the cleft between the aromatic rings, which is a promising result because we anticipate that poly(1) and poly(2) might show promise in sensing applications.  Although some progress has been made toward isolating pure C-shaped 3 using column chromatography and recrystallization, studies are currently underway to determine conditions for separating the diastereomers by preparative HPLC.  Pd-catalyzed Suzuki coupling of 3 with 2-thiopheneboronic acid gave C-shaped monomer 1 in 60% yield.  Preliminary electrochemical studies showed that the redox potential of 1 is too high to allow for electropolymerization within the solvent window, and our solvent selection is limited due to poor monomer solubility.  Monomer 2 is expected to have a lower redox potential due to the additional thiophene rings, which should favor electropolymerization at a lower potential.  Although our current focus is on the synthesis of 2, we will also explore alternatives for obtaining poly(1) either chemically or electrochemically.   

We became interested in another family of electroactive monomers, 4 and 5, that we have synthesized and characterized (Figure 3).  These axially chiral monomers are intriguing because their electropolymerization leads to chiral segmented conducting polymers wherein the electroactive segments are constrained to be perpendicular to each other along the polymer backbone.       

Figure 3. Structures of 4 and 5.

Electropolymerization of 4 and 5 using cyclic voltammetry led to poly(4) and poly(5), respectively, where both polymers exhibited robust growth with each successive scan.  Spectroelectrochemistry studies demonstrated that both polymers are electrochromic.  We will continue to study charge migration in poly(4) and poly(5) to assess the effect this new polymer topology has on their electrical and optical properties.

This funding has had a huge impact on my research career and on the blossoming careers of my undergraduate coworkers.  Although I received start-up support from Marist, this money was quickly exhausted on glassware, chemicals, and supplies for synthesis.  In addition to summer stipends, including a SUMR supplement, the PRF funding has allowed us to initiate the electrochemistry thrust of the research.  I purchased the electrochemistry equipment and associated accessories, including several costly precious metal electrodes, using a combination of PRF and matching funds from the College.  The four undergraduates who have worked on the project gained valuable experience by their direct participation at all levels of the research, including critical analysis of the primary spectroscopic and electrochemical data.  They have not only been enriched by the lab experience, but they have all had opportunities to present their research at off-campus venues.  Most notably, my three summer 2008 researchers presented posters at the 236^th National ACS Meeting in Philadelphia, PA.  All four undergraduates intend to pursue a PhD in chemistry, where one is already at the U. of Cincinnati and two are applying fall 2008.

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