Spin-Crossover Iron(II) Coordination Polymer with Fluorescent Properties: Correlation between Emission Properties and Spin State

A spin-crossover
coordination polymer [Fe­(L1)­(bipy)]n (where
L = a N2O22– coordinating
Schiff base-like ligand bearing a phenazine fluorophore
and bipy = 4,4′-bipyridine) was synthesized and exhibits a
48 K wide thermal hysteresis above room temperature (T1/2↑ = 371 K and T1/2↓ = 323 K) that is stable for several cycles. The spin transition
was characterized using magnetic measurements, Mössbauer spectroscopy,
and DSC measurements. T-dependent X-ray powder diffraction
reveals a structural phase transition coupled with the spin transition
phenomenon. The dimeric excerpt {(μ-bipy)­[FeL1­(MeOH)]2}·​2MeOH of the coordination polymer chain crystallizes
in the triclinic space group P1̅ and reveals
that the packing of the molecules in the crystal is dominated by hydrogen
bonds. Investigation of the emission properties of the complexes with
regard to temperature shows that the spin crossover can be tracked
by monitoring the emission spectra, since the emission color changes
from greenish to a yellow color upon the low spin-to-high spin transition.