60th Annual Report on Research 2015

We are aiming to answer four main questions about the structure-property relationships of spin-charge and spin-exciton interactions in radical-substituted thiophenes and phthalocyanines:

1.     What are the effects of through-bond spin coupling on charge transport and excitonic properties?

2.     What are the effects of through-space dipolar spin coupling on charge transport and excitonic properties and what are the spatial limits of this type of interaction?

3.     What are the rules and limits for coupling between the radical SOMO and semiconductor HOMO/LUMO?

4.     How does the structural geometry of radical connection to the organic semiconductor skeleton affect charge transport and excitonic properties?

The structures with which we will answer these aforementioned questions are shown in Chart 1.

To date, we have synthesized a small series of regioregular, alternating side-chain radical functionalized poly(thiophene)s (Figure 1). Our modular approach uses a common synthetic intermediate, PTa, and a simple nucleophilic substitution reaction to produce chemically pure derivatives via post-polymerization functionalization. Polymer PTa is accessed via a robust GRIM polymerization reaction and is therefore isolated in >95% yield as a regioregular, medium molecular-weight polymer (20-30 KDa, depending on reaction batch). Polymers P1-P4 were all isolated in 100-500 mg quantities as metallic-orange powders that are soluble in chloroform, THF, toluene, dichlorobenzene and other nonpolar organic solvents.

We confirmed that post-polymerization functionalization of PTa did not change the molecular weight of the starting material using calibrated gel permeation chromatography. The presence of persistent radical moieties in P1-4 after post-polymerization functionalization of PTa was confirmed using CW EPR spectroscopy. Further, all radical materials were succesfully reacted with either ascorbic acid or phenylhydrazine to quantitatively quench the free radical moieties and produce diamagnetic compounds whose ¹H and ¹³C NMR spectra were recorded to confirm structures and sample purity. However, more thorough EPR and ¹H/¹³C NMR experiments need to be completed to quantitate spin densities and percent yield of functionalization.

The observed oxidation onset potentials (aka valence band edges) and optical band gaps of P1, P3 and P4 were similar to those of PTa and P3HT in dilute solutions, confirming that the radical side-chain substituents are electronically isolated from the polymer backbone. We can therefore tentatively assume that the band edges of P1, P3 and P4 are equivalent to those of the unsubstituted parent polymer, P3HT.

We are also concurrently working to synthesize and isolate through-bond spin coupled conjugated organic radicals (Scheme 1); however, this synthetic endeavor remains challenging. Our initial target molecule, seen in Scheme 1a, could not be accessed because all attempts to synthesize the necessary 3,4-bis(N-hydroxylamino)thiophene intermediate failed. Therefore, we redesigned our target molecule to a phenanthronitronyl nitroxide derivative, seen in Scheme 1b. However, this radical was only persistent for approximately 20 minutes at ambient conditions, after which an insoluble diamagnetic species was formed. DFT/UB3LYP calculations revealed that the phenanthrylnitronyl nitroxide radical shown in Scheme 1b possesses significant spin density on the bridging carbons of the phenanthrene ring system, which may lead to facile dimerization. Therefore, we are currently synthesizing a sterically-hindered through-bond coupled radical (Scheme 1c) that we propose will display greater chemical stability and radical persistence time.

Research Impact

All the materials shown in Figure 1 will be useful in characterzing through-space spin exchange interactions in pi-conjugated organic compounds. Polymers P1-4 are the first known examples of medium molecular weight, side-chain radical substituted regioregular polythiophenes. We note that poly(3-arylthiophene)s containing pendant radical groups were synthesized over a decade earlier by Lahti and Nishide. However, since the GRIM polymerization method was unknown at the time, these previously-reported radical substituted polymers were either low molecular-weight (reported DP of 6) oligomers or regiorandom polymers that were possibly contaminated with paramagnetic iron ions. In comparison, the polymers synthesized herein are medium-to-high molecular weight (DP >100), regioregular, alternating poly(thiophene)s and the simple post-polymerization functionalization method that we employ to introduce radical moieties into the side chain is known to proceed in >98% yield. Therefore, the materials used herein promise to display superior electronic and magnetic properties compared to previous literature examples.

Career Impact

Dr. Brandon Kobilka, the postdoctoral researcher who worked on the synthesis of radical substituted poly(thiophene)s, succesfully obtained a job with IBM and started his position in February 2015. Further, the materials developed with ACS PRF support allowed the PI to write a follow-up spintronics grant proposal to the NSF.