Charge Transport in Conjugated Polymers with Pendent Stable Radical Groups

Abstract

Synthesizing a stable radical polymer with a conjugated backbone seems like a natural way to introduce conductivity to radical polymers, which are traditionally synthesized with insulating, nonconjugated backbones. For charge storage applications that take advantage of the redox-active nature of stable radical polymers, enhanced conductivity would improve performance. To explore the interplay between stable radicals and a conjugated backbone, we prepared and studied soluble polythiophene with high regioregularity and various concentrations of pendent radical groups to systematically examine any change in conductivity with radical incorporation. Using electron paramagnetic resonance and electrical conductivity measurements, we show that there is an exponential decrease in conductivity as we increase the percentage of pendent groups attached to repeating units, which changes the conductivity by 6 orders of magnitude between the nonradical control polythiophene material and the material with the highest radical content (∼80%). These findings serve as an important guide to the future design of radical polymers on conjugated backbones with the goal of tuning conductivity as a function of stable radical content in redox-active energy storage applications.

Quantifying internal charge transfer and mixed ion-electron transfer in conjugated radical polymers

“…1) has been recently reported by Zhang et al 30 TEMPO units were clicked onto a polythiophene backbone made from controlled amounts of 2,5-dibromo-3-hexylthiophene (HT) and 2,5-dibromo-3-(6bromohexyl)thiophene (BrHT), modied with an azide. 30 The nal TEMPO-bearing P3HT was denoted as P3HT-TEMPO-X, in which X refers to the intended percent radical loading. The authors found that the solid-state conductivity decreased with increasing TEMPO loading for two reasons.…”

“…The authors found that the solid-state conductivity decreased with increasing TEMPO loading for two reasons. 30 First, bulky TEMPO groups twisted the polymer backbone, making it less planar and reducing the conjugation length. 30 As a result, intrachain electron transfer through the backbone was impeded.…”

“…30 First, bulky TEMPO groups twisted the polymer backbone, making it less planar and reducing the conjugation length. 30 As a result, intrachain electron transfer through the backbone was impeded. 30 Second, the steric hinderance of TEMPO side groups disrupted polymer packing and crystallization.…”

“…30 As a result, intrachain electron transfer through the backbone was impeded. 30 Second, the steric hinderance of TEMPO side groups disrupted polymer packing and crystallization. 30 At low TEMPO loading (25%), P3HT-TEMPO was semi-crystalline, which allowed for interchain electron transfer; at high TEMPO loading (50-75%), P3HT-TEMPO formed amorphous aggregates, causing a higher energy barrier for interchain electron transfer.…”

Aqueous one-pot synthesis of epoxy-functional diblock copolymer worms from a single monomer: new anisotropic scaffolds for potential charge storage applications

“…Non-conjugated redox-active organic radical polymers have recently gained considerable interest owing to their potential applications for energy storage and conversion. [1][2][3][4] In particular, the stable free radical species 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) has been widely investigated since the first TEMPO-based active electrode material reported by Nakahara et al in 2002. 5 TEMPO-functional polymers can be prepared by (i) polymerization of a nitroxide precursor, (ii) direct polymerization of a nitroxide-functional monomer, or (iii) post-polymerization derivatization.…”