| Sumario: | 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(ll) metallomesogens based on the ligand tris[3-aza-4-((5-C)(6-R)(2-pyridyl))but-3-enyl]amine, with C = hexyloxy, dodecyloxy, hexadecyloxy, octadecyloxy, eicosyloxy, R = hydrogen or methyl (C-trenH or C-trenMe), which affords examples of systems of types i, ii, and iii. Self-assembly of the ligands C-trenH and C-trenMe with Fe(A)·xHO salts have afforded a family of complexes with general formula [Fe(C-trenR)](A)·HO (S ⩾ O), with A = CIO, F, Cl, Br and I. Single-crystal X-ray diffraction measurements have been performed on two derivatives of this family, named as [Fe(C-trenH)](CIO) (C-1) and [Fe(C-trenMe)](CIO) (C-2), at 150 K for C-1 and at 90 and 298 K for C-2. At 150 K, C-1 displays the triclinic space group P1, whereas at 90 and at 298 K C-2 adopts the monoclinic P2/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-trenH and C-trenMe. The average Fe(ll)—N bonds (1.963(2) Å) at 150 K denote that C-1 is in the low-spin (LS) state. For C-2 the average Fe(ll)-N bonds (2.007(1) Å) at 90K are characteristic of the LS state, while at 298 K they are typical for the high-spin (HS) state (2.234(3) Å). Compound C-1 and [Fe-(C-trenH)](CIO) (C-1) adopts the LS state in the temperature region between 10 and 400K, while compound C-2 and [Fe(C-trenMe)](ClO) (n = 12 (C-2), 18 (C-2)) exhibit spin crossover behavior at T 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-2, C-2 and C-2. The T is 56 K (C-2), 48 K (C-2), and 56 K (C-2). On the basis of differential scanning calorimetry, optical polarizing microscopy, and X-ray diffraction findings for C-1, C-2, and C-2 at high temperature a smectic mesophase S has been identified with layered structures similar to C-1 and C-2.…
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