Anthoula C. Papageorgiou

arXiv2026

Interplay between Isomerization and Spin Crossover in 1D Fe-Indigo Coordination Polymers on Ag substrates

Spin-crossover (SCO) compounds offer a route to switchable molecular functionality in reduced dimensions. However, one-dimensional (1D) SCO chains, which offer the possibility to study ligand fields other than the paradigmatic octahedral field, remain comparatively little studied. Here, we use first-principles density functional theory (DFT+$U$) to investigate Fe-indigo coordination-polymer chains synthesized experimentally on Ag(111) and Ag(100) substrates. These display a rich interplay between changes in ligand field (isomerization) and spin crossover. On-surface isomerization on Ag(111) interconverts (N,O)-chelated \textit{trans} configuration and (N,N)-/(O,O)-chelated \textit{cis} configurations at the Fe centers. The lowest-energy \textit{trans} and \textit{cis} solutions on Ag(111) have different spin configurations over the interval $0.66<U<3.00$~eV. At the reference value $U=1$~eV, the preferred \textit{trans} solution is the mixed LS--LS--HS configuration, whereas the preferred \textit{cis} solution is LS--LS--LS. The experimentally observed preference for \textit{cis} chains on Ag(111) and \textit{trans} chains on Ag(100) is reproduced for the range $0.88<U<3.75$~eV. To interpret these results, toy models and spin-resolved Fe $3d$ projected densities of states are used, while freestanding-chain calculations reveal a strain-sensitive LS--HS competition. These results provide a microscopic explanation for isomerization-controlled spin-state switching in a 1D coordination polymer.

cond-mat.mtrl-sci↗

arXiv2014

Temperature-dependent templated growth of porphine thin films on the (111) facets of copper and silver

The templated growth of the basic porphyrin unit, free-base porphine (2H-P), is characterized by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy measurements and density functional theory (DFT). The DFT simulations allow the deconvolution of the complex XPS and NEXAFS signatures into contributions originating from five inequivalent carbon atoms, which can be grouped into C-N and C-C bonded species. Polarization-dependent NEXAFS measurements reveal an intriguing organizational behavior: On both Cu(111) and Ag(111), for coverages up to one monolayer, the molecules adsorb undeformed and parallel to the respective metal surface. Upon increasing the coverage, however, the orientation of the molecules in the thin films depends on the growth conditions. Multilayers deposited at low temperatures (LT) exhibit a similar average tilting angle (30 degree relative to the surface plane) on both substrates. Conversely, for multilayers grown at room temperature a markedly different scenario exists. On Cu(111) the film thickness is self-limited to a coverage of approximately two layers, while on Ag(111) multilayers can be grown easily and, in contrast to the bulk 2H-P crystal, the molecules are oriented perpendicular to the surface. This difference in molecular orientation results in a modified line-shape of the C 1s XPS signatures that is dependent on the incident photon energy, which is explained by comparison with depth-resolved DFT calculations. Simulations of ionization energies for differently stacked molecules show no indication for a packing-induced modification of the multilayer XP spectra, thus indicating that the comparison of single molecule calculations to multilayer data is justified.

cond-mat.mtrl-sci↗