Molecular Design Features for Charge Transport in Nonconjugated Radical Polymers

Abstract

Conducting polymers based on open-shell radical moieties exhibit potentially advantageous processing, stability, and optical attributes compared with conventional doped conjugated polymers. Despite their ascendance, reported radical conductors have been based almost exclusively on (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl (TEMPO), which raises fundamental questions regarding the ultimate limits of charge transport in these materials and whether some of the deficiencies exhibited by contemporary materials are due to the choice of radical chemistry. To address these questions, we have performed a density functional theory (DFT) study of the charge transfer characteristics of a broad range of open-shell chemistries relevant to radical conductors, including p-type, n-type, and ambipolar open-shell chemistries. We observe that far from being representative, TEMPO exhibits anomalously high reorganization energies due to strong charge localization. This, in turn, limits charge transfer in TEMPO compared with more delocalized open-shell species. By comprehensively mapping the dependence of charge transfer on radical–radical orientation, we have also identified a large mismatch between the conformations that are favored by intermolecular interactions and the conformations that maximize charge transfer in all of the open-shell chemistries investigated. These results suggest that significant opportunities exist to exploit directing interactions to promote charge transport in radical polymers.

Effect of Strong Intermolecular Interaction in 2D Inorganic Molecular Crystals

“…55−57 Regardless of the transport mechanism, two important molecular parameters governing charge transport are the transfer integral and the reorganization energy (λ). 58 Therefore, to acquire a rational assessment of the charge transport property in our as-synthesized 2D P 4 Se 3 nanoflakes, we calculated the reorganization energy of the 16 P 4 Se 3 molecules in the unit cell, as shown in Figure 3d and Table S8. The reorganization energy of electrons is larger than that of holes, which directly lowers the electron mobility by 2 orders of magnitude in comparison to hole mobility, as exhibited in Figure 3e,f and Table S9.…”

“…Inspired by the strong intermolecular coupling in each layer, we were very interested in exploring the in-plane charge transport of α-P 4 Se 3 nanoflakes, because in-plane charge transport for inorganic molecular crystals has rarely been reported and they have always been regarded as insulating solids. − Regardless of the transport mechanism, two important molecular parameters governing charge transport are the transfer integral and the reorganization energy (λ) . Therefore, to acquire a rational assessment of the charge transport property in our as-synthesized 2D P 4 Se 3 nanoflakes, we calculated the reorganization energy of the 16 P 4 Se 3 molecules in the unit cell, as shown in Figure d and Table S8.…”

Bypassing Backmapping: Coarse-Grained Electronic Property Distributions Using Heteroscedastic Gaussian Processes

“…For example, in TEMPO, the important difference between the 5-bead and 4-bead CG representations used in Figure 4b is the decimation of the N-O bond degree of freedom. The length of this bond is strongly associated with the molecular orbital energy of the radical, 56 and consequently choosing a CG mapping operator that decimates this bond information introduces a large amount of CG noise (Figure 4b). Further coarsening of the CG resolution produces less significant changes in the noise.…”

Stable organic radicals and their untapped potential in ionic liquids

“…Specifically, TEMPO has a strong localisation of charge, which can limit charge transfer and reduce electronic conductivity in the radical polymer system. [53] Alternative open shell species (Fig. 5) with a high degree of delocalisation could overcome the prevalence of mismatched intermolecular conformations and their use in a conducting polymer system may alleviate and improve this deficiency.…”