Being able to easily determine the Cu(II) affinity for biomolecules of moderate affinity is important. Such biomolecules include amyloidogenic peptides, such as the well-known amyloid-β peptide involved in Alzheimer’s disease. Here, we report the synthesis of a new water-soluble ratiometric Cu(II) dye with a moderate affinity (10⁹ M^–1 at pH 7.1) and the characterizations of the Cu(II) corresponding complex by X-ray crystallography, EPR, and XAS spectroscopic methods. UV–vis competition was performed on the Aβ peptide as well as on a wide series of modified peptides, leading to an affinity value of 1.6 × 10⁹ M^–1 at pH 7.1 for the Aβ peptide and to a coordination model for the Cu(II) site within the Aβ peptide that agrees with the one mostly accepted currently.

The magnetic properties of three spin‐crossover complexes, [Fe^II(R‐bik)₃](BF₄)₂·nH₂O (1–3), based on bis(imidazolyl)ketone ligands, were investigated in solution and the solid state. Their properties were compared with those of the ketone‐free analogue, [Fe^II(bim)₃](OTf)₂ (4). The alkyl and vinyl R groups have weak influence on the transition temperature, T_1/2, in solution, while stronger differences are observed in the solid state, because different intermolecular interactions occur in 1–3. The spin‐state equilibria in solution were followed by SQUID magnetometry and the Evans NMR spectroscopy method. Interestingly, the equilibria can also be simply and efficiently probed by following the temperature dependence of an adequately chosen ¹H chemical shift. Overall, these experiments give coherent results, with T_1/2 located between 320 and 335 K, a narrow range, in comparison with the solid state. DFT calculations have allowed the rationalization of the magnetic differences. The molecular‐orbital and spin‐density calculations reveal that the presence of the C=O group between the imidazolyl units in the ligands of 1–3 leads to an extended aromatic system, an effective π‐acceptor effect, stabilizing the LS state and reducing the LS–HS gap, in comparison with 4.