Air-stable open-shell organic radical polymers are an emerging class of materials for future electronic applications including energy storage devices,1-3 transparent displays,4 memory devices,5-8 and spintronics.9,10 Compared to the conventional conjugated organic materials that require chemical doping or a high degree of crystallinity for high electrical conductivity, amorphous, undoped, and nonconjugated radical polymers can enable tunable and robust conductivity and optical transparency for myriad applications. In radical polymers, the electronic charge transport occurs through the high-density active radical sites upon reduction-oxidation self-exchange reactions.3,11,12 The tunable chemical design of polymeric radicals allows a potentially enormous range of open-shell groups to be combined with varying backbones to tune electronic properties in a powerful manner. PTMA has been used as the initial organic radical conductor. The champion electronic conductivity (~20 S m-1) of radical polymers was reported in 2018 with the ether-oxygen-based organic radical polymer within the channel distance of 600 nm or less.13-15 While this poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) exhibits the champion performance, the ultimate performance of radical polymers still needs to improve.3,14,16 To address this point, it is essential to establish the charge transport properties of open-shell chemistries with the understanding of structure-function relationships and design strategies of the alternative chemical structure of radical polymers for the quantification of their potential impact.
Here, we demonstrate the improvement of charge transport of radical polymers by overcoming the channel dimension limitation over a larger length scale up to microscale through the appropriate molecular design. In particular, polymer poly-(TEMPO-substituted ethylene sulfide) (PTES) was synthesized using a ring-opening anionic polymerization methodology to form a radical polymer with a flexible macromolecular backbone and a relatively low glass transition temperature. The thermal annealing of PTES radical polymer leads to the formation of percolation domains for macroscopic conductivity with fast charge transfer reactions. The reversible electronic communication of an open-shell active site can be realized by repeating the percolation network above the glass transition temperature (Tg) and insulating the system below Tg in amorphous regimes. This concept is similar to the previous observation for other radical polymers of PTEO, however, the advanced chemical design of radical macromolecule is additionally demonstrated by using the organosulfur backbone in the radical polymer, where the sulfur group proceeds the self-redox-reaction by itself in a rational manner.17 Furthermore, it enhances the overall redox reaction of the radical polymer with the promotion of a nitroxide pendant group that can strongly interact between sulfur (S) of the backbone resulting in the enhancement of active units close enough for self-exchange redox reactions of open-shell moieties in the microscale channel device. This not only contrasts strongly general π-conjugated organic electronics but also represents an advanced design paradigm in the radical polymer electronic society.
The high density of radical active sites is a critical factor in the synthetic design parameters for high-electrical conductivity radical polymers.18,19 It is because the conductivity of radical polymers increased exponentially as the density of the radical region increased.20 The well-known strategy to maintain the high density of radical site is to polymerize the monomer that included an open-shell active site directly for the formation of radical polymer instead of polymerizing a closed-shell monomer and subsequently converting this closed-shell to open-shell species.6,13,15,21,22 To do so, the small radical molecule monomer including nitroxide functionality (thiirane, 1, Figure 1A) is prepared through the reaction between 4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (GTEMPO) and thiourea (Detail in Experimental Section). This small molecular product can be refined to a high level to secure a single pendant radical group per monomer unit. This monomer can participate in an anionic ring-opening polymerization (AROP) in high yield (~90%) with the base (nBuSH) as the initiator, generating the well-defined PTES radical polymer with a number-average molecular weight of 19.8 kg mol-1. The security of the pendant radical group was clued with the UV-vis absorption peak (Figure 1B). The PTES maintained a high radical density, above 99%, because the polymerization does not contain the radical intermediate. In addition, with the strong nucleophilicity of thiolate anion, polymerization of the episulfide monomer was carried out at ambient temperature without the need for a strong base (tBuOK) as an initiator (Figure 1A). A high spin concentration of radical in the polymer was determined using electron paramagnetic resonance (EPR) spectroscopy (Figure 1C, D). This result indicated a negligible loss of radical density during the polymerization process and the thermal stability of the radical site in the temperature range of 288 – 232 K for solid-state device tests during the heating and cooling of materials.
In addition, another critical design criterion for the high-electrical conducting radical polymer is that there should be a large window between the flow transition temperature of the open-shell radical polymer and the degradation temperatures of the pendant group. In this aspect, PTES exhibits a low glass transition temperature of 50 oC (Figure 2A) which is far below the degradation temperature of PTES (180 oC, Figure 2B).
Due to the high reorganization energies and relatively low electronic coupling to other types of radical moieties, the nitroxide radicals are considered to have some limitation to a certain level of charge transfer with the coupling of the active site in the non-equilibrium amorphous packing structure (Figure 2C).23 In the solid state, the radical site is required to be close enough for the rapid redox kinetics. To do so, a molecular design approach to the redox-active radical polymer is required to promote the high charge-transfer orientation. The PTES material with a low glass transition temperature (Tg) undergoes a phase change from solid-state to molten-state to create the percolating nitroxide network for charge transport while keeping it away from the degradation temperature of radical. This type of design can be tailored with radical polymers, as the macromolecular backbone dominates the mechanical and thermal properties, and pendant chains separated from the backbone dictate the electrochemical properties of the material.13 However, XPS provides evidence of coupling between backbone-backbone (S-S), backbone-pedant group (S-NO˙), pedant group-pendant group (NO˙-NO˙) with the reversible cleavage of C-S-S-C bond (164.21 eV),17,24 and charge transfer, and -NO˙ (399.6 eV) to C-S-C (163.35 eV) and of NO˙ to N=O+, respectively (Figure 3A, B, C).16,21,25 The strong couplings trigger the favorable orientation of the pendant group with annealing for redox reaction for electronic communication in an amorphous regime. The energetically preferred orientation due to the strong coupling between the backbone and the pedant group (Figure 3A, B) can potentially be substantially regulated by the details of the polymer packing.26
The origins of PTES's ability to maintain high conductivity on a micrometer (μm) length scale were further explored through density functional theory (DFT) simulations. We performed geometry optimization on repeat units of PTES and identically on repeat units of PTEO as a control (Figure 4). Upon examining the charge distribution in the optimized structures, it was found that while the oxygen atoms in the PTEO repeat unit had a relatively greater concentration of electrons, the overall distribution of electrons was even throughout the unit (Figure 4A). Compared to the electron distribution observed in PTEO, the distribution near the sulfur atom of PTES's backbone chain exhibited the most significant difference, with almost no electrons detected near the sulfur atom and a noticeable concentration of electrons toward the adjacent carbon atoms on both sides (Figure 4B). The electron-donating nature of the sulfur-containing backbone in PTES has the potential to impact intermolecular interactions between polymer chains, which could lead to improved packing and overall material stability. In addition, when comparing the bond and dihedral angles of PTES and PTEO backbones (Figures 4C and 4D), it was observed that there was no significant difference in the bond angle. However, in terms of the dihedral angle, the PTEO backbone containing oxygen was curved at around 70°, while the PTES backbone containing sulfur was almost flat at approximately 175°. That is to say, the PTES backbone containing flexible and planar thioethers (C-S-C) is believed to provide greater structural diversity by virtue of its ability to adopt various conformations. This is in line with molecular simulation results that PTES shows a very low conformational energy compared to PTEO as a result of optimizing polymer structures with geometric variables for dihedral angles and tacticity between repeat units based on the DFT-optimized structure (Figure S6).
The thermal-activated conductivity behavior of PTES was scanned from the low- to high-charge transport regime versus time during the real transition state of PTES from the glassy state into the molten state. PTES thin film stayed at its glassy state below Tg before raising the temperature. When the temperature was greater than Tg, the conductivity of PTES increased by more than nine orders of magnitudes; that is, it went from ~ 1.8 ×10-9 S m-1 to 1.92 S m-1 (Pt electrode) (Figure 5A). The conductivity was calculated with Ohm’s law of ( is conductivity, is resistance, is channel length, and is channel area) extracted by the slope of the IV curve from DC measurement. The device construction is shown on set in Figure 5B with a thickness channel size of 1.5 µm. Interestingly, the radical polymer exhibited reversible electronic conducting behavior following the heating and cooling process. It demonstrates that charge transport regimes occur through the favorable packing of active radical sites mainly due to the non-covalent, and covalent coupling of backbone and functional nitroxide group above Tg, which can vanish at low-temperature (Figure 3, XPS).26 The conductivity was further scanned at the high–charge transport region of PTES in a range temperature of 373- 333 K. The activation energy was also extrapolated to be 0.19 eV when the temperature was above Tg (Figure 5B), which is in good agreement with the thermally activated transport behavior of radical polymer as the result of molecular motion.13,16 The high optical transparency of PTES-thin film was clued with > 98% transmission at range wavelengths smaller than 500 nm and 100 % transmission at a longer wavelength (Figure 5C). The inset in Figure 5C with the upside-down device where the electrode can be easily readable, clearly demonstrate the high transparency of the PTES-on-glass sample.
Lastly, we investigate the conductivity of nonconjugated radical polymer PTES by changing the electrode material, where the working function based on differences in the band diagram between PTES and metal electrodes could govern the conductivity. Alternative electrode materials were used to observe the varying conductivity of PTES. The electrical conductivity of PTES was experimentally extracted to be 1.92, 12.1, 26.4, and 32.0 S m-1 for electrode materials of platinum (Pt), copper (Cu), silver (Ag), and gold (Au), respectively (Figure 6A), which exceed the conductivity of radical polymer of PTEO even in the micrometer channel. To explain this observation, the SOMO of PTES was extrapolated to be 5.09 eV through the cyclic voltammetry plot (Figure S4). In general, besides the molecular structure, band/energy level modulation in the junction of metal/molecules is a critical factor governing the current molecular electronics.27,28 As shown in the schematic energy band diagram Figure 6B, the lowest difference in barrier height of Au working function and the SOMO energy of radical polymer attributed to the favor for charge transport, resulting in the high collected current for PTES under junction contact. Such a large electronic conductivity value also provides evidence of the good electron transfer of the PTES radical polymer to the Au electrode.