Three types of interplay/synergy
between spin-crosso
ver (SCO) and liquid crystalline (LC) phasetransitions can
be predicted: (i) systems with coupled phase transitions, where the structural changes associatedto the Cr
![](/images/entities/harr.gif)
LC phase transition dri
ves the spin-state transition, (ii) systems where
both transitions coexist in thesame temperature region
but are not coupled, and (iii) systems with uncoupled phase transitions. Here we presenta 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 orC
n-trenMe), which affords examples of systems of types i, ii, and iii. Self-assem
bly of the ligands C
n-trenH andC
n-trenMe with Fe(A)
2·
xH
2O salts ha
ve afforded a family of complexes with general formula [Fe(C
n-trenR)](A)
2·
sH
2O (
s ![](/images/entities/ge.gif)
0), with A = ClO
4-, F
-, Cl
-, Br
- and I
-. Single-crystal X-ray diffraction measurements ha
ve
beenperformed on two deri
vati
ves of this family, named as [Fe(C
6-trenH)](ClO
4)
2 (C
6-
1) and [Fe(C
6-trenMe)](ClO
4)
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![](/images/entities/onemacr.gif)
,whereas at 90 and at 298 K C
6-
2 adopts the monoclinic
P2
1/
c space group. In
both compounds the iron atomsadopt a pseudo-octahedral symmetry and are surrounded
by six nitrogen atoms
belonging to imino groups andpyridines of the ligands C
n-trenH and C
n-trenMe. The a
verage Fe(II)-N
bonds (1.963(2) Å) at 150 K denote thatC
6-
1 is in the low-spin (LS) state. For C
6-
2 the a
verage Fe(II)-N
bonds (2.007(1) Å) at 90 K are characteristic ofthe LS state, while at 298 K they are typical for the high-spin (HS) state (2.234(3) Å). Compound C
6-
1 and [Fe(C
18-trenH)](ClO
4)
2 (C
18-
1) adopts the LS state in the temperature region
between 10 and 400 K, while compoundC
6-
2 and [Fe(C
n-trenMe)](ClO
4)
2 (
n = 12 (C
12-
2), 18 (C
18-
2)) exhi
bit spin crosso
ver
beha
vior at
T1/2 centeredaround 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 in
vestigated in compoundsC
6-
2, C
12-
2 and C
18-
2. The
T1/2LIESST is 56 K (C
6-
2), 48 K (C
16-
2), and 56 K (C
18-
2). On the
basis of differentialscanning calorimetry, optical polarizing microscopy, and X-ray diffraction findings for C
18-
1, C
12-
2, and C
18-
2 athigh 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·
sH
2O (
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·
sH
2O(C
18-
6,
s = 3.5, C
18-
7,
s = 0) show a
very particular spin-state change, while [Fe(C
18-tren)](Br)
2·3H
2O (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
beha
vior likethat o
bser
ved for the complexes C
18-
1, C
12-
2, and C
18-
2. In compounds C
n-
3 50% of the Fe(II) ions undergospin-state change at
T1/2 = 375 K induced
by releasing water, and in partially dehydrated compounds (
s = 0.5)the Cr
![](/images/entities/rarr.gif)
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 dehydratedmaterials C
n-
5 50% of the Fe(II) ions are in the HS state and show paramagnetic
beha
vior
between 10 and 400K. In the partially dehydrated C
n-
4 the spin transition is induced
by the change of the aggregate state of matter(solid
![](/images/entities/harr.gif)
liquid crystal). For compound C
18-
6 the full dehydration to C
18-
7 pro
vokes the spin-state change of nearly50% 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.