Thermally Ultrarobust S = 1/2 Tetrazolinyl Radicals: Synthesis, Electronic Structure, Magnetism, and Nanoneedle Assemblies on Silicon Surface

Abstract

Open-shell organic molecules, including S = 1/2 radicals, may provide enhanced properties for several emerging technologies; however, relatively few synthesized to date possess robust thermal stability and processability. We report the synthesis of S = 1/2 biphenylene-fused tetrazolinyl radicals 1 and 2. Both radicals possess near-perfect planar structures based on their X-ray structures and density-functional theory (DFT) computations. Radical 1 possesses 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, E cell ≈ 0.9 eV, are rather low. Magnetic properties of polycrystalline 1 are characterized by superconducting quantum interference device (SQUID) magnetometry revealing a one-dimensional S = 1/2 antiferromagnetic Heisenberg chain with exchange coupling constant J′/k ≈ −22.0 K. Radical 1 in toluene glass possesses a long electron spin coherence time, T m ≈ 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. Electron paramagnetic resonance (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.

From Stable Radicals to Thermally Robust High-Spin Diradicals and Triradicals

“…Another family of bishydrazyl radicals, such as tetrazolinyl and the Kuhn–Jerchel radical (phototetrazolinyl, PHT ), have a long history, − but their thermal robustness and other properties were explored only recently. − For example, the dicyano-substituted derivative of phototetrazolinyl ( DiCN-PHT ) is found to be stable under inert atmosphere up to 260+ °C (polycrystalline), and, in benzene solution at room temperature, its estimated half-life is a few months . In polycrystalline DiCN-PHT , uniform one-dimensional S = 1/2 antiferromagnetic Heisenberg chains ( J / k = −22 K) are formed . Upon controlled evaporation on SiO 2 /Si substrate under ultrahigh vacuum, DiCN-PHT forms nanoneedle assemblies .…”

“…Because heteroatom substituted extended π-systems typically have E 1 ° on the order of +0.5 V (vs SCE), nitronyl nitroxide with E 1 ° ≈ +0.9 V for nitronyl nitroxide or even oxoverdazyl with E 1 ° ≈ +0.6 V (vs SCE) is a good choice. We note that the Blatter radical ( E 1 ° = +0.10 V) and phototetrazolinyl radical DiCN-PHT ( E 1 ° = −0.14 V) (Figure ) are too easy to oxidize, and thus they are not suitable.…”

“…159−161 For example, the dicyano-substituted derivative of phototetrazolinyl (DiCN-PHT) is found to be stable under inert atmosphere up to 260+ °C (polycrystalline), and, in benzene solution at room temperature, its estimated half-life is a few months. 161 In polycrystalline DiCN-PHT, uniform one-dimensional S = 1/2 antiferromagnetic Heisenberg chains (J/k = −22 K) are formed. 161 Upon controlled evaporation on SiO 2 /Si substrate under ultrahigh vacuum, 139 DiCN-PHT forms nanoneedle assemblies.…”

“…161 In polycrystalline DiCN-PHT, uniform one-dimensional S = 1/2 antiferromagnetic Heisenberg chains (J/k = −22 K) are formed. 161 Upon controlled evaporation on SiO 2 /Si substrate under ultrahigh vacuum, 139 DiCN-PHT forms nanoneedle assemblies. 161 The stability of this planar radical is unusual because of the relatively large spin densities at the ortho-(and para-) positions with respect to the nitrogens of the tetrazolinyl moiety, as indicated by the hyperfine coupling constant, A( 1 H) = 5.8 MHz, which is greater by a factor of 3 than that in the 1,3,5triphenyl-6-oxoverdazyl radical (section 4.3).…”

“…161 The stability of this planar radical is unusual because of the relatively large spin densities at the ortho-(and para-) positions with respect to the nitrogens of the tetrazolinyl moiety, as indicated by the hyperfine coupling constant, A( 1 H) = 5.8 MHz, which is greater by a factor of 3 than that in the 1,3,5triphenyl-6-oxoverdazyl radical (section 4.3). 161…”