Cyclophane with eclipsed pyrene units enables construction of spin interfaces with chemical accuracy

Abstract

Advanced functionality in molecular electronics and spintronics is orchestrated by exact molecular arrangements at metal surfaces, but the strategies for constructing such arrangements remain limited. Here, we report the synthesis and surface hybridization of a cyclophane that comprises two pyrene groups fastened together by two ferrocene pillars. Crystallographic structure analysis revealed pyrene planes separated by ∼352 pm and stacked in an eclipsed geometry that approximates the rare configuration of AA-stacked bilayer graphene. We deposited this cyclophane onto surfaces of Cu(111) and Co(111) at submonolayer coverage and studied the resulting hybrid entities with scanning tunnelling microscopy (STM). We found distinct characteristics of this cyclophane on each metal surface: on non-magnetic Cu(111), physisorption occurred and the two pyrene groups remained electronically coupled to each other; on ferromagnetic Co(111) nanoislands, chemisorption occurred and the two pyrene groups became electronically decoupled. Spin-polarized STM measurements revealed that the ferrocene groups had spin polarization opposite to that of the surrounding Co metal, while the pyrene stack had no spin polarization. Comparisons to the non-stacked analogue comprising only one pyrene group bolster our interpretation of the cyclophane's STM features. The design strategy presented herein can be extended to realize versatile, three-dimensional platforms in single-molecule electronics and spintronics.

Fig. 1. Dependence of the energy gap that separates the two highest occupied molecular orbitals (HOMO and HOMO−1) upon the lateral offset between pyrene molecules in a face-to-face dimer. Two optimized structures of pyrene were positioned in space with 352 pm vertical separation, and the lateral offset was varied from perfectly eclipsed (the configuration of AA graphene) along the coordinate through semi-eclipsed (the configuration of AB graphene) by increments of ∼28 pm. At each position we report the energy gap E_HOMO − E_HOMO−1, which is proportional to the electronic coupling between the pyrene groups, determined by single-point calculations using density functional theory, B3LYP functional, 6-311G(2d) basis set. We plotted ±0.04 (e bohr⁻³)^1/2 isodensity surfaces of the HOMOs at the indicated offsets.

Scheme 1. Synthesis of 1 and 2via Suzuki–Miyaura cross-coupling.

Fig. 2. Crystal structure of cyclophane 2 represented with thermal ellipsoids (50% probability); important structural features are indicated in grey with standard uncertainties given in parenthesis.

Fig. 3. Comparison of topographic constant-current STM images showing (a) cyclophane molecules 2 (V_b = 500 mV, I_t = 0.5 nA) and (b) non-stacked molecules 1 (V_b = 100 mV, I_t = 0.1 nA) on Cu(111) substrates (e.g. red ellipses), and intact symmetric (e.g. green ellipses) as well as modified asymmetric (e.g. black ellipses) molecules on Co(111) nanoislands. The white ellipse highlights aggregated molecules 2 on Cu(111) and the blue ellipse molecular fragments. (c and d) Show representative cross-sections of apparent height surfaces along the longitudinal molecular axes of several molecules of (c) cyclophane 2 and (d) non-stacked molecule 1 on Cu (red) and Co (green) as well as for modified molecules on Co (black). Depths of the dips measured for 1 and 2 and peak height differences Δh of the double-peak profiles measured for modified molecules are indicated.

Fig. 4. Proposed adsorption geometry for 1 and 2 on the topmost atomic layer of Co nanoislands on Cu(111). The border of the hexagonal Co lattice with the lattice constant a_Co = 251 pm (ref. 62) indicates the directions of the energetically most favourable islands edges. The longitudinal axis of the molecule (red arrow) points along the [2̄11] direction. Rotation by ±60° yields symmetry-equivalent, energetically degenerate adsorption geometries for which the longitudinal axes of the molecule point in [12̄1] and [1̄1̄2] directions.

Fig. 5. (a) Spin asymmetry map of cyclophane 2 chemisorbed to a Co nanoisland, generated from two spin-polarized conductance maps (Fig. S11†) recorded at V_b = −0.5 V and I_t = 1.0 nA while applying an external magnetic field of ±1 T, respectively, perpendicular to the surface. The definition and sign convention for the spin asymmetry A is given in eqn (S1) of the ESI. (b) Spin asymmetry profiles along the longitudinal molecular axes of two molecules 2 (red and black) and for comparison on the bare Cu(111) surface (green). According to the discussion of eqn (S2) in the ESI, the profiles show no spin polarization on Cu(111), inverted hybridization-induced spin polarization over the Fc sites nearly equal in magnitude to that on Co(111), and no significant spin polarization over the upper pyrene sites.