Magnetic Characterization of Open-Shell Donor–Acceptor Conjugated Polymers

Abstract

Donor–acceptor (DA) conjugated polymers (CPs) with narrow bandgaps and open-shell electronic structures offer a fundamentally new paradigm for integrating the spin degree of freedom within emerging functional devices. Recent advancements have demonstrated that control of long-range electronic correlations enables low-spin (S = 0) and high-spin (S = 1) DA CPs, in which extended π-conjugation overcomes the intrinsic instability of these electronic configurations in light-element materials. While design strategies that articulate mechanisms of spin alignment, topology control, and quantum mechanical exchange are emerging, dedicated studies of the magnetic behavior of these materials remain rare. Here, we utilize sensitive magnetometry techniques to analyze the magnetic properties of open-shell DA CPs with low- and high-spin ground states. We demonstrate improved measurement accuracy through combining vibrating sample magnetometry and superconducting quantum interference device magnetometry. This serves to overcome challenges associated with the inherently weak magnetic moments of these materials and a measurement environment in which the background signal is always significant and must be carefully removed. Analyzing the results following established models for paramagnetic materials enables precise quantification of the spin quantum number and temperature-dependent spin alignment. These studies articulate approaches that enable precise characterization of the bulk magnetic features of these heterogeneous and disordered materials systems, providing a path for rational property elucidation that will enable the integration of these materials within emerging technologies.

Strong Acceptor Annulation Enables Control of Electronic Structure and Spin Configuration in Donor–Acceptor Conjugated Polymers

“…Fielddependent magnetization isotherms show the expected diamagnetic behavior (Figure 6a, inset). 34 In sharp contrast, P1 shows a positive χ m for temperatures below 160 K, decreasing as temperature is increased and following a Curie−Weiss trend over the measured temperature range of 2−400 K (Figure 6b), consistent with the results obtained using EPR. The data were fit to the Curie−Weiss law, χ m = C/(T − θ) + χ 0 , where C is the material-dependent Curie constant, θ is the Weiss constant, and χ 0 accounts for any observable offset in the susceptibility.…”

“…The source of such a rise in χ m is often considered extrinsic, attributed to trace amounts of radical impurities present in the sample, which likely account for the very small signal in EPR. Field-dependent magnetization isotherms show the expected diamagnetic behavior (Figure a, inset) …”

“…The data were fit to the Curie−Weiss law, χ m = C/(T − θ) + χ 0 , where C is the material-dependent Curie constant, θ is the Weiss constant, and χ 0 accounts for any observable offset in the susceptibility. The best fit gives θ = −0.3 K, indicative of short-range antiferromagnetic interactions among small populations of unpaired spins, 3,24,34 and a negligible χ 0 . A paramagnetic susceptibility is observed at lower temperatures, with a subtle transition to a diamagnetic state near 160 K, which prevails at higher temperatures.…”

Charge transport and antiferromagnetic ordering in nitroxide radical crystals

“…The magnetic transition around T N corresponds to a singlet-triplet transition, as the spins were aligned in pairs of opposite directions in antiferromagnetic behavior, while the spins were aligned into the same direction under the applied magnetic field in paramagnetic behavior. 27,40,41 Therefore, this transition can be approximately described using the Bleaney-Bowers equation (eqn (2)) when S = 1/2.…”

“…Stable organic radicals have been used extensively in electronic, [1][2][3][4][5][6][7][8][9] electrochemical, [10][11][12][13][14][15][16][17][18][19] and magnetic field-responsive applications [20][21][22][23][24][25][26][27][28][29] because radical-based polymers and small molecules are proven electronic and ionic conductors. [30][31][32][33][34][35] Additionally, stable radical molecules have intriguing magnetic properties because the unpaired electrons of the radical sites interact with applied magnetic fields, and intermolecular spin-spin interactions inside these open-shell materials can result in magnetic effects.…”