A high-spin ground-state donor-acceptor conjugated polymer

Abstract

Interest in high-spin organic materials is driven by opportunities to enable far-reaching fundamental science and develop technologies that integrate light element spin, magnetic, and quantum functionalities. Although extensively studied, the intrinsic instability of these materials complicates synthesis and precludes an understanding of how fundamental properties associated with the nature of the chemical bond and electron pairing in organic materials systems manifest in practical applications. Here, we demonstrate a conjugated polymer semiconductor, based on alternating cyclopentadithiophene and thiadiazoloquinoxaline units, that is a ground-state triplet in its neutral form. Electron paramagnetic resonance and magnetic susceptibility measurements are consistent with a high-to-low spin energy gap of 9.30 × 10^-3 kcal mol^-1. The strongly correlated electronic structure, very narrow bandgap, intramolecular ferromagnetic coupling, high electrical conductivity, solution processability, and robust stability open access to a broad variety of technologically relevant applications once thought of as beyond the current scope of organic semiconductors.

Fig. 1. Synthesis of high-spin DA CP using a Stille cross-coupling copolymerization reaction.

(A) The molecular building blocks and rapid polymerization approach used to synthesize the polymer. (B) Measured magnetic properties exhibiting intramolecular FM exchange coupling and a high-to-low spin energy gap of 9.30 × 10⁻³ kcal mol⁻¹.

Fig. 2. Solid-state properties of polymer thin films.

(A) Absorption spectra of thin film cast from chlorobenzene onto a NaCl substrate and (B) two-dimensional GIWAXS profile obtained using the same processing conditions and a silicon substrate. (C) CV indicates a HOMO-LUMO energy gap of 0.56 eV. (D) Current-voltage characteristics in a 30-μm channel with σ of ~10⁻² S cm⁻¹. (E) One-dimensional line cuts of the integrated in-plane and out-of-plane two-dimensional GIWAXS profile mainly showing a peak at q ~1.79 Å⁻¹ that is related to interchain spacing of 3.51 Å. a.u., arbitrary units.

Fig. 3. Magnetic properties of the polymer.

(A) EPR (X band) spectra from 4 to 50 K and (B) temperature-dependent fit to the Bleaney-Bowers equation with ΔE_ST of 9.30 × 10⁻³ kcal mol⁻¹. (C) SQUID magnetometry of solid sample. Main plot: Magnetic susceptibility, χ versus T, from 3 to 225 K fit to the Curie-Weiss law (blue line). Inset: Observed χT versus T dependence. (D) Magnetic field (H) dependence of the magnetization (M) at 2, 3 and 5 K, plotted as M/M_sat versus H, with Brillouin functions for S = 1/2, 0.94, and 3/2. (E) Log-log plot of the X band 1/T_1e recovery rates versus temperature fit by the temperature dependence of the Orbach-Aminov process with an energy gap of 4.67 K. (F) Two-pulse electron spin echo instantaneous diffusion data at 10 K indicate a one-dimensional spin distribution (d = 1) along a linear chain. ESEEM, electron spin echo envelope modulation.

Fig. 4. Electronic structure of the oligomer (n = 8) at the UB3LYP/6-31G** level of theory.

(A) Calculated α-SOMO (top) and β-SOMO (bottom) profiles of the open-shell singlet. (B) Spin density distribution of the singlet and (C) triplet states with most probable locations for the unpaired electrons highlighted with open circles (red, up spin; blue, down spin).