Electron-Induced Spin-Crossover in Self-Assembled Tetramers

Abstract

The spin crossover compound Fe­(H2B­(pyrazole)­(pyridylpyrazole))2 was investigated in detail on Ag(111) with scanning tunneling microscopy (STM). A large fraction of the deposited molecules condenses into gridlike tetramers. Two molecules of each tetramer may be converted between two states by current injection. We attribute this effect to a spin transition. This interpretation is supported by control experiments on the analogous, magnetically passive Zn compound that forms virtually identical tetramers but exhibits no switching. The switching yields were studied for various electron energies, and the resulting values exceed those reported from other SCO systems by 2 orders of magnitude. The other two molecules of a tetramer were immutable. However, they may be used as contacts for current injection that leads to conversion of one of their neighbors. This “remote” switching is fairly efficient with yields reduced by only one to two orders of magnitude compared to direct excitation of a switchable molecule. We present a model of the tetramer structure that reproduces key observations from the experiments. In particular, sterical blocking prevents spin crossover of two molecules of a tetramer. Density functional theory calculations show that the model indeed represents a minimum energy structure. They also reproduce STM images and corroborate a remote-switching mechanism that is based on electron transfer between molecules.

Spin Crossover in a Cobalt Complex on Ag(111)

“…The complexes aggregate into rectangular tetramers although a degree of disorder is present. A detailed image of a tetramer (Figure 1e) reveals separations between topographic maxima of ≈1 and ≈1.15 nm, which are virtually identical to those observed of [Fe(H 2 B(pz)(pypz)) 2 ] tetramers [33] . Even the apparent heights, which are fairly bias‐independent in the range | V | < 1 V, are similar to the Fe case.…”

“…The other two molecules, however, remain stable, even at more harsh conditions. This striking effect was previously observed from [Fe(H 2 B(pz)(pypz)) 2 ] tetramers and attributed to their geometrical structure [33] . The terminal‐pyrazole moieties, which display the largest geometrical change during the Fe spin transition from S =0 to S =2, are sterically blocked for two molecules while the other two pz are free to move.…”

“…This striking effect was previously observed from [Fe(H 2 B(pz)(pypz)) 2 ] tetramers and attributed to their geometrical structure. [33] The terminal-pyrazole moieties, which display the largest geometrical change during the Fe spin transition from S = 0 to S = 2, are sterically blocked for two molecules while the other two pz are free to move. Similar to the case of [Fe(H 2 B(pz)(pypz)) 2 ] tetramers, injecting current into the stable molecules induces switching of their closest neighbors.…”

“…Similar to the case of [Fe(H 2 B(pz)(pypz)) 2 ] tetramers, injecting current into the stable molecules induces switching of their closest neighbors. [33] Figures 3 a and d present time series of the current for negative and positive sample voltages. The data were recorded after placing the tip above a switchable molecule.…”

“…Coordination bonding in Co complexes is expected to be even weaker owing to the larger population of the antibonding e g orbitals. To increase the chance of success, we designed and synthesized a Co complex (Figure 1a) inspired by one of the most robust Fe 2+ SCO compounds, [37] which is functional on Ag(111) [33] . Here we show that the Co 2+ compound, where spin states S = 1 / 2 and S = 3 / 2 are expected, aggregates into tetramers on Ag(111).…”