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

Chemical Science 12 (2021) 8430-8437

Marvin Metzelaars, Sebastian Schleicher, Takuma Hattori, Bogdana Borca, Frank Matthes, Sergio Sanz, Daniel E. Bürgler, Jeff Rawson, Claus M. Schneider and Paul Kögerler

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 2 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 2 onto surfaces of Cu(111) and Co(111) at submonolayer coverage and studied the resulting hybrid entities with scanning tunneling 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.

Figure: 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. Representative topographic cross-sections along the longitudinal molecular axes of several molecules of (c) cyclophane 2 and (e) non-stacked molecule 1 on Cu (red) and Co (green) as well as for modified molecules on Co (black). (d) 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 indicates the directions of the energetically most favorable islands edges. The longitudinal axis of the molecule (red arrow) points along the direction. Rotation by ±60° yields symmetry-equivalent, energetically degenerate adsorption geometries for which the longitudinal axes of the molecule point in and directions.

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.

Figure: (a) Spin asymmetry map of cyclophane 2 chemisorbed to a Co nanoisland, generated from two spin-polarized conductance maps 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. (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). The profiles show no spin polarization on Cu(111), inverted hybridization-induced spin polarization over the ferrocene sites nearly equal in magnitude to that on Co(111), and no significant spin polarization over upper pyrene sites.

The design strategy presented in this work can be extended to realize versatile, three-dimensional platforms in single-molecule electronics and spintronics.