(η8-Cyclooctatetraene)(η5-fluorenyl)titanium: a processable molecular spin qubit with optimized control of the molecule-substrate interface

Abstract

Depositing single paramagnetic molecules on surfaces for sensing and quantum computing applications requires subtle topological control. To overcome issues that are often encountered with sandwich metal complexes, we exploit here the low symmetry architecture and suitable vaporability of mixed-sandwich [FluTi(cot)], Flu = fluorenyl, cot = cyclooctatetraene, to drive submonolayer coverage and select an adsorption configuration that preserves the spin of molecules deposited on Au(111). Electron paramagnetic resonance spectroscopy and ab initio quantum computation evidence a d _{z ²} ground state that protects the spin from phonon-induced relaxation. Additionally, computed and measured spin coherence times exceed 10 μs despite the molecules being rich in hydrogen. A thorough submonolayer investigation by scanning tunneling microscopy, X-ray photoelectron and absorption spectrocopies and X-ray magnetic circular dichroism measurements supported by DFT calculations reveals that the most stable configuration, with the fluorenyl in contact with the metal surface, prevents titanium(iii) oxidation and spin delocalization to the surface. This is a necessary condition for single molecular spin qubit addressing on surfaces.

Fig. 1. (a) Structural representation of [FluTi(cot)], with the atom numbering scheme. Thermal ellipsoids encompass 50% of the electron density probability. (b) Unit cell representation down the crystallographic b axis. Symmetry code (i): x, −y + 1/2, z.

Fig. 2. Energies and contour surfaces (50%) of frontier molecular orbitals in [FluTi(cot)] (spin α).

Fig. 3. (a and b) X-band CW EPR spectra recorded for a 1.0 mmol L⁻¹ solution of [FluTi(cot)] in toluene at two temperatures (298 K and 77 K); (c) Q-band echo-detected spectrum of a 0.5 mmol L⁻¹ solution in toluene-d₈ at T = 15 K. Simulated spectra are shown as thin black or grey lines. A common g scale is reported on the top to facilitate the comparison.

Fig. 4. Temperature dependence of the T₁ and T_m of a 0.5 mmol L⁻¹ solution of [FluTi(cot)] in toluene-d₈, superimposed to those of [CpTi(cot)] we reported in ref. . The black solid line corresponds to the computed (T₁) values (see text).

Fig. 5. STM images (T = 35 K) of a submonolayer (a–c) and a 2 ML deposit (e–g) of [FluTi(cot)] on Au(111). (d) Line profiles taken across the red dashed lines in panel c.

Fig. 6. Ti 2p (a) and C 1s (b) XPS spectra of [FluTi(cot)] deposited on Au(111) as a sub-monolayer. Peak color code: blue, Ti^III main peaks; magenta, Ti^III satellite peaks; green, C 1s main component; brown, C 1s satellite/shake-up component.

Fig. 7. Results of the pDFT calculations: optimized geometries, adsorption energies, Ti spin density, and simulated STM images at −2 V bias (filled states).

Fig. 8. XAS (top) and XMCD (bottom) spectra recorded at θ = 0°, B = 6 T, and T = 2.0 K on a sub-monolayer of [FluTi(cot)] on Au(111) surface.