Relationship between Molecular Structure and Electron Transfer in a Polymeric Nitroxyl-Radical Energy Storage Material

In recent years, stable organic radical functional groups have been incorporated into a variety of polymeric materials for use as electrodes within energy storage devices, for example, batteries and capacitors. With the complex nature of the charge-transfer processes in a polymer matrix, the morphologies of the polymer films can have a significant impact on the redox behavior of the organic-based radical. To elucidate possible effects of packing on electron-transport mechanisms, theoretical modeling of the well-characterized cathode material poly­(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) was conducted. Polymer morphologies were modeled using classical molecular dynamics simulations, and subsequently, the electronic-coupling matrix element between each radical site was calculated. Building on a previously derived treatment of diffusion in inhomogeneous materials, an expression for an effective electron diffusion length and an effective electron diffusion rate was derived in terms of an electronic-coupling-weighted radial distribution function. Two primary distances were found to contribute to the effective electron transfer length of 5.5 Å with a majority of the electron transfer, nearly 85%, occurring between radical sites on different polymer chains. Finally, we point out that this analysis of charge transfer using an electronic-coupling-weighted radial distribution function has application beyond the specific system addressed here and that it may prove useful more generally for simulating electron-transfer processes in disordered molecular materials

Quenching of the Perylene Fluorophore by Stable Nitroxide Radical-Containing Macromolecules

Stable nitroxide radical bearing organic polymer materials are attracting much attention for their application as next generation energy storage materials. A greater understanding of the inherent charge transfer mechanisms in such systems will ultimately be paramount to further advancements in the understanding of both intrafilm and interfacial ion- and electron-transfer reactions. This work is focused on advancing the fundamental understanding of these dynamic charge transfer properties by exploiting the fact that these species are efficient fluorescence quenchers. We systematically incorporated fluorescent perylene dyes into solutions containing the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical and controlled their interaction by binding the TEMPO moiety into macromolecules with varying morphologies (e.g., chain length, density of radical pendant groups). In the case of the model compound, 4-oxo-TEMPO, quenching of the perylene excited state was found to be dominated by a dynamic (collisional) process, with a contribution from an apparent static process that is described by an ∼2 nm quenching sphere of action. When we incorporated the TEMPO unit into a macromolecule, the quenching behavior was altered significantly. The results can be described by using two models: (A) a collisional quenching process that becomes less efficient, presumably due to a reduction in the diffusion constant of the quenching entity, with a quenching sphere of action similar to 4-oxo-TEMPO or (B) a collisional quenching process that becomes more efficient as the radius of interaction grows larger with increasing oligomer length. This is the first study that definitively illustrates that fluorophore quenching by a polymer system cannot be explained using merely a classical Stern–Volmer approach but rather necessitates a more complex model

Close Packing of Nitroxide Radicals in Stable Organic Radical Polymeric Materials

The relationship between the polymer network and electronic transport properties for stable radical polymeric materials has come under investigation owing to their potential application in electronic devices. For the radical polymer poly­(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl methacrylate), it is unclear whether the radical packing is optimal for charge transport partially because the relationship between radical packing and molecular structure is not well-understood. Using the paramagnetic nitroxide radical as a probe of the polymer and synthetic techniques to control the radical concentration on the methyl methacrylate backbone, we investigate the dependence of radical concentration on molecular structure. The electron paramagnetic resonance data indicate that radicals in the PTMA assume a closest approach distance to each other when more than 60% of the backbone is populated with radical pendant groups. Below 60% coverage, the polymer rearranges to accommodate larger radical–radical spacing. These findings are consistent with theoretical calculations and help explain some experimentally determined electron-transport properties

Simplified Models for Accelerated Structural Prediction of Conjugated Semiconducting Polymers

We perform molecular dynamics simulations of poly­(benzodithiophene-thienopyrrolodione) (BDT-TPD) oligomers in order to evaluate the accuracy with which unoptimized molecular models can predict experimentally characterized morphologies. The predicted morphologies are characterized using simulated grazing-incidence X-ray scattering (GIXS) and compared to the experimental scattering patterns. We find that approximating the aromatic rings in BDT-TPD with rigid bodies, rather than combinations of bond, angle, and dihedral constraints, results in 14% lower computational cost and provides nearly equivalent structural predictions compared to the flexible model case. The predicted glass transition temperature of BDT-TPD (410 ± 32 K) is found to be in agreement with experiments. Predicted morphologies demonstrate short-range structural order due to stacking of the chain backbones (π–π stacking around 3.9 Å), and long-range spatial correlations due to the self-organization of backbone stacks into “ribbons” (lamellar ordering around 20.9 Å), representing the best-to-date computational predictions of structure of complex conjugated oligomers. We find that expensive simulated annealing schedules are not needed to predict experimental structures here, with instantaneous quenches providing nearly equivalent predictions at a fraction of the computational cost of annealing. We therefore suggest utilizing rigid bodies and fast cooling schedules for high-throughput screening studies of semiflexible polymers and oligomers to utilize their significant computational benefits where appropriate