Room temperature (photo)-switching of Fe(II) spin crossover coordination complexes

In the current era of growth of modern technologies, the demand to develop new nanomaterials operating in the solid state at room temperature and able to offer multifunctionality is increasing drastically. In particular, stimuli responsive materials which offer possibilities to combine various functionalities in one single compound have attracted much attention. Among them spin crossover (SCO) Fe(II) compounds, for which the spin state is temperature induced are well represented, which actually restricts their practical use in powerless devices. A SCO induced by light irradiation with an easy processing and ability to trigger the switching remotely and reversibly would however be a good alternative to thermal spin crossover. Investigations of several light effects, namely the Light Induced Excited Spin State Trapping (LIESST), the Ligand-Driven Light Induced Spin Change (LD-LISC) or the Guest Driven Light Induced Spin Change (GD-LISC), despite their huge interest, have however shown their limitations in terms of room temperature photo-switching. The development of innovative strategies is therefore awaited to open new pathways for successful room temperature photo-switching. In this context, a new strategy was proposed by our laboratory to photo-induce a spin state change through incorporation of a photo-responsive anionic subunit in the crystal lattice of a given SCO compound. The first observation of a Anion Driven Light Induced Spin Change (AD-LISC) was obtained by working on p-sulfocinnamic acid (psca) which we reported to undergo a 2+2 photodimerization to yield 4,4’-disulfonate-truxillic acid (dsta) when irradiated with UV light. The anion was included in a Fe(II) SCO compound, as non-coordinated species, i.e. at a remote position compared to the metallic center, responsible of the spin switching effect. Such an anion driven strategy relies however on a long-range effect, through the light-induced structural changes occurring in non-coordinated anions, which are expected to induce a spin state change at the metal center.