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.