Spin crossover
[2] Spin crossover is commonly observed with first row transition metal complexes with a d4 through d7 electron configuration in an octahedral ligand geometry.
[2] SCO was first observed in 1931 by Cambi et al. who discovered anomalous magnetic behavior for the tris(N,N-dialkyldithiocarbamatoiron(III) complexes.
[5] Magnetic measurements and Mössbauer spectroscopic studies established the nature of the spin transition in iron(II) SCO complexes.
The magnetic susceptibility as a function of temperature, (χT) is the principal technique used to characterize SCO complexes.
When spectra are recorded as a function of temperature, the areas under the curves of the absorption peaks are proportional to the fraction of HS and LS states in the sample.
SCO induces changes in metal-to-ligand bond distances due to the population or depopulation of the eg orbitals that have a slight antibonding character.
Consequently X-ray crystallography above and below transition temperatures will generally reveal changes in metal-ligand bond lengths.
The spin crossover phenomenon is very sensitive to grinding, milling and pressure, but Raman spectroscopy has the advantage that the sample does not require further preparation, in contrast to Fourier Transform Infrared spectroscopy, FT-IR, techniques; highly colored samples may affect the measurements however.
[9] Raman spectroscopy is also advantageous because it allows perturbation of the sample with external stimuli to induce SCO.
Around 25% of the total entropy gain from the LS to HS transition originates from the increase in spin multiplicity according to the relationship:
As a result of the application of pressure on the Fe(phen)2(SCN)2 compound, the bond lengths are affected.
In Light Induced Excited Spin State Trapping (LIESST), the HS-LS transition is triggered by irradiating the sample.
Irradiation of d-d transitions of the LS metal complex or MLCT absorption bands leads to population of HS states.
Due to the aim to design photoswitchable materials that have higher working temperatures than those reported to date (~80 K), along with long-lifetime photoexcited states, another strategy for SCO called Ligand-Driven Light Induced Spin Change (LD-LISC) has been studied.
The prerequisite for LD-LISC to be observed is that the two complexes formed with the ligand photoisomers, must exhibit different magnetic behaviors as a function of temperature.
Upon successive irradiations of the system at two different wavelengths within a temperature range where the metal ion can either be LS or HS, a spin-state interconversion should occur.
In order to achieve this, it is convenient to design a metal environment to where at least one of the complexes exhibits a thermally induced SCO.
These potential applications would exploit the bistability (HS and LS) which leads to changes in the colour and magnetism of samples.
In order for the size of data storage devices to be reduced while the capacity of them increase, smaller units (such as molecules) that exhibit a bistability and thermal hysteresis are required.
