Does the solid-liquid crystal phase transition provoke the spin-state change in spin-crossover metallomesogens?

Three types of interplay/synergy between spin-crossover (SCO) and liquid crystalline (LC) phase transitions can be predicted: (i) systems with coupled phase transitions, where the structural changes associated to the Cr <-> LC phase transition drives the spin-state transition, (ii) systems where both transitions coexist in the same temperature region but are not coupled, and (iii) systems with uncoupled phase transitions. Here we present a new family of Fe(II) metallomesogens based on the ligand tris[3-aza-4-((5-C-n)(6-R)(2-pyridyl))but-3-enyl]amine, with C-n = hexyloxy, dodecyloxy, hexadecyloxy, octadecyloxy, eicosyloxy, R = hydrogen or methyl (C-n-trenH or C-n-trenMe), which affords examples of systems of types i, ii, and iii. Self-assembly of the ligands C-n-trenH and C-n-trenMe with Fe(A)(2)center dot xH(2)O salts have afforded a family of complexes with general formula [Fe(C-n-trenR)](A)(2)center dot sH(2)O (S >= 0), with A = ClO4-, F-, Cl-, Br- and I-. Single-crystal X-ray diffraction measurements have been performed on two derivatives of this family, named as [Fe(C-6-trenH)](ClO4)(2) (C-6-1) and [Fe(C-6-trenMe)](ClO4)(2) (C-6-2), at 150 K for C-6-1 and at 90 and 298 K for C-6-2. At 150 K, C-6-1 displays the triclinic space group P (1) over bar, whereas at 90 and at 298 K C-6-2 adopts the monoclinic P2(1)/c space group. In both compounds the iron atoms adopt a pseudo-octahedral symmetry and are surrounded by six nitrogen atoms belonging to imino groups and pyridines of the ligands C-n-trenH and C-n-trenMe. The average Fe(II)-N bonds (1.963(2) angstrom) at 150 K denote that C-6-1 is in the low-spin (LS) state. For C-6-2 the average Fe(II)-N bonds (2.007(1) angstrom) at 90 K are characteristic of the LS state, while at 298 K they are typical for the high-spin (HS) state (2.234(3) angstrom). Compound C-6-1 and [Fe(C-18-trenH)](ClO4)(2) (C-18-1) adopts the LS state in the temperature region between 10 and 400 K, while compound C-6-2 and [Fe(C-n-trenMe)](ClO4)(2) (n = 12 (C-12-2), 18 (C-18-2)) exhibit spin crossover behavior at T-1/2 centered around 140 K. The thermal spin transition is accompanied by a pronounced change of color from dark red (LS) to orange (HS). The light-induced excited spin state trapping (LIESST) effect has been investigated in compounds C-6-2, C-12-2 and C-18-2. The T-1/2(LIESST) is 56 K (C-6-2), 48 K (C-16-2), and 56 K (C-18-2). On the basis of differential scanning calorimetry, optical polarizing microscopy, and X-ray diffraction findings for C-18-1, C-12-2, and C-18-2 at high temperature a smectic mesophase S-x has been identified with layered structures similar to C-6-1 and C-6-2. The compounds [Fe(C-n-trenH)](Cl)(2)center dot sH(2)O (n = 16 (C-16-3, s = 3.5, C-16-4, s = 0.5, C-16-5, s = 0), 18 (C-18-3, s = 3.5, C-18-4, s = 0.5, C-18-5, s = 0), 20 (C-20-3, s = 3.5, C-20-4, s = 0. 5, C-20-5, s = 0)) and [Fe(C(18)tren)](F)(2)center dot sH(2)O (C-18-6, s = 3.5, C-18-7, s = 0) show a very particular spin-state change, while [Fe(C-18-tren)](Br)(2)center dot 3H(2)O (C-18-8) together with [Fe(C-18-tren)](I)(2) (C-18-9) are in the LS state (10-400 K) and present mesomorphic behavior like that observed for the complexes C-18-1, C-12-2, and C-18-2. In compounds C-n-3 50% of the Fe(II) ions undergo spin-state change at T-1/2 = 375 K induced by releasing water, and in partially dehydrated compounds (s = 0.5) the Cr -> S-A phase transition occurs at 287 K (C-16-4), 301 K (C-18-4) and 330 K (C-20-4). For the fully dehydrated materials C-n-5 50% of the Fe(II) ions are in the HS state and show paramagnetic behavior between 10 and 400 K. In the partially dehydrated C-n-4 the spin transition is induced by the change of the aggregate state of matter (solid-liquid crystal). For compound C-18-6 the full dehydration to C-18-7 provokes the spin-state change of nearly 50% of the Fe(II) ions. The compounds C-n-3 and C-18-6 are dark purple in the LS state and become light purple-brown when 50% of the Fe(II) atoms are in the HS state.