Thermally Ultra-Robust S = ½ Tetrazolinyl Radicals: Synthesis, Electronic Structure, Magnetism, and Nanoneedle Assemblies on Silicon Surface.

Open-shell organic molecules, including S = ½ radicals, may provide enhanced properties for several emerging technologies, however, relatively few synthesized to date possess robust thermal stability and processability. We report synthesis of S = ½ biphenylene-fused tetrazolinyl radicals 1 and 2. Both radicals possess near-perfect planar structures based on their X-ray structures and DFT computations. Radical 1 possesses an outstanding thermal stability as indicated by the onset of decomposition at 269 °C, based on thermogravimetric analysis (TGA) data. Both radicals possess very low oxidation potentials < 0 V (vs. SCE) and their electrochemical energy gaps, Ecell 0.9 eV, are rather low. Magnetic properties of polycrystalline 1 are characterized by SQUID magnetometry revealing a one-dimensional S = ½ antiferromagnetic Heisenberg chain with exchange coupling constant J’/k –22.0 K. Radical 1 in toluene glass possesses a long electron spin coherence time, Tm ≈ 7 μs in the 40 – 80 K temperature range, a property advantageous for potential applications as a molecular spin qubit. Radical 1 is evaporated under ultrahigh vacuum (UHV) forming assemblies of intact radicals on a silicon substrate, as confirmed by high-resolution X-ray photoelectron spectroscopy (XPS). Scanning electron microscope (SEM) images indicate that the radical molecules form nanoneedles on the substrate. The nanoneedles are stable for at least 64 hours under air as monitored by using X-ray photoelectron spectroscopy. EPR studies of the thicker assemblies, prepared by UHV evaporation, indicate radical decay according to first-order kinetics with a long half-life of 50 +/-4 days at ambient conditions.