High-Spin (S = 1) Blatter-Based Diradical with Robust Stability and Electrical Conductivity

Abstract

Triplet ground-state organic molecules are of interest with respect to several emerging technologies but usually show limited stability, especially as thin films. We report an organic diradical, consisting of two Blatter radicals, that possesses a triplet ground state with a singlet-triplet energy gap, ΔE_ST ≈ 0.4-0.5 kcal mol^-1 (2J/k ≈ 220-275 K). The diradical possesses robust thermal stability, with an onset of decomposition above 264 °C (TGA). In toluene/chloroform, glassy matrix, and fluid solution, an equilibrium between two conformations with ΔE_ST ≈ 0.4 kcal mol^-1 and ΔE_ST ≈ -0.7 kcal mol^-1 is observed, favoring the triplet ground state over the singlet ground-state conformation in the 110-330 K temperature range. The diradical with the triplet ground-state conformation is found exclusively in crystals and in a polystyrene matrix. The crystalline neutral diradical is a good electrical conductor with conductivity comparable to the thoroughly optimized bis(thiazolyl)-related monoradicals. This is surprising because the triplet ground state implies that the underlying π-system is cross-conjugated and thus is not compatible with either good conductance or electron delocalization. The diradical is evaporated under ultra-high vacuum to form thin films, which are stable in air for at least 18 h, as demonstrated by X-ray photoelectron and electron paramagnetic resonance (EPR) spectroscopies.

Figure 1.

Blatter-based diradicals: TGA onset of decomposition ≈ 1% mass loss. Blatter radical and its spin density map at the UB3LYP/6–31G(d,p) level of theory; positive (blue) and negative (green) spin densities are shown at the isodensity level of 0.002 electron/Bohr.

Figure 2.

Single crystal X-ray structure of diradical 4 at 100 K, with carbon and nitrogen atoms depicted using thermal ellipsoids set at the 50% probability level (A and B); the Bravais, Friedel, Donnay and Harker (BFDH) crystal morphoplogy of 4 (C), confirmed with experimental face index; needle and stacking direction are along the a-axis. Additional data, e.g., for radical 10, can be found in the SI: Figs. S1–S6 and Tables S1–S6.

Figure 3.

EPR (110 K, ν = 9.3269 GHz) spectrum for 0.54 mM diradical 4 in toluene/chloroform, 3:1 glass; a small center peak corresponds to monoradical impurity (ca. 5%). Inset: the |Δm_s| = 2 transition. Spectral simulation of the |Δm_s| = 1 region (rmsd = 0.0197, see: Table 1 and Figs. S18–S20, SI.

Figure 4.

EPR spectroscopy of diradical 4: plots and numerical fits of χT vs T in toluene/chloroform (4:1) (A) and polystyrene (B). Further details are reported in the SI: Table S7, Figs. S16, S21–S23, Eqs. S1 and S2.

Figure 5.

Solid state characterization of diradical 4. A, main plot: single crystal conductivity, σ, of diradical 4 (plotted on a logarithmic scale) as a function of the reciprocal temperature (1000/T), with the fits in the high- and low-temperature ranges, showing effective activation energies (E_a/k). Inset plot: single crystal σ vs T, showing near-linear relationship. B, SQUID magnetometry of polycrystalline 4: magnetic susceptibility, χ vs T for T = 1.8 – 320 K. C and D, EPR spectroscopy of polycrystalline 4 with particle size of <75 μm: DI/Q ~ χ vs T and representative EPR spectra, showing Dysonian line-shape, where DI is a double integrated intensity and Q is a microwave cavity quality factor. Further details are reported in the SI: Tables S8 and S9, Figs. S12, S13, S15, S17, S24–S40.

Figure 6.

Cyclic voltammetry (CV, A) and differential pulse voltammetry (DPV, B) of diradical 4 in 0.1 M tetrabutylammonium hexafluorophosphate in dichloromethane. Scan rates (SR) are 100 and 10 mV s⁻¹. Redox potentials are given as mean ± stddev with n = 7–8 (CV) and n = 4 (DPV). For further details, including square wave voltammetry, see: SI.

Figure 7.

UV–vis–NIR (294 K) absorption spectrum for 0.2 mM diradical 4. Bands at λ_max = 303, 384, 520, and 720 nm have the following extinction coefficients: ε₃₀₃ = 6.0 × 10⁴ L mol^–1 cm^–1, ε₃₈₄ = 1.6 × 10⁴ L mol^–1 cm^–1, ε₅₂₀ = 5.3 × 10³ L mol^–1 cm^–1, and ε₇₂₀ = 5.8 × 10² L mol^–1 cm^–1. Feature at λ ≈ 900 nm, marked with a red asterisk is an instrumental artefact (change of grating).

Figure 8.

Crystalline diradical 4 at the B3LYP level of theory. Fermi energy (E_F) level is indicated with dashed lines. Top: Band structure. Single occupied (S_O1, S_O2) and single unoccupied (S_U1, S_U2) bands are marked in green. Bottom: Spin-resolved density-of-states (DOS), for neutral (Q = 0, black line) and charged (Q = +1, red line) crystals. Peak A identifies the defect level in the charged system.

Figure 9.

Top: Thermogravimetric analysis with IR spectra of diradicals 2 and 4 under N₂; heating rate = 5 °C min⁻¹. For further details, including IR spectra, see: SI, Figs. S41 and S42. Bottom: C 1s and N 1s core level spectra of a multilayer of 4 deposited on SiO₂/Si(111) substrate, compared to the powder spectra.

Figure 10.

Top: Attenuation of the Si 2p XPS signal, normalized to the corresponding saturation signal at zero film thickness as a function of the film nominal thickness, deposition at room temperature. The line is a guide to the eye. Bottom: A typical 3 μm × 3 μm AFM image.

Scheme 1.

Synthesis of diradical 4 and monoradical 10.