(SiW11O39)2]13 anion with an azobenzene-containing cationic surfactant (Li et al., 2005b). No mesophase was observed for the corresponding complex with dioctadecyldimethylammonium counter cations.
12. MISCELLANEOUS THERMOTROPIC
lanthanide complex of the starting habbe ligands. Instead, rearrangement of these ligands was observed, giving symmetrical azines (L43) which showed nematic and/or smectic A phases. A synthetic pathway was proposed, in which the rare-earth ion promotes the decomposition of the habbe ligand into the sym- metrical azine. The structures of moleculesL39–L43is shown inFig. 57.
Lanthanide complexes of a steroid-substituted benzocrown ether have been synthesized (Binnemans and Gu¨ndogan, 2002). All the complexes had a 1:1 metal-to-ligand ratio. The ligand 40-[(cholesteryloxy)carbonyl]-benzo- 15-crown-5L44exhibited a monotropic cholesteric phase (Fig. 58). The lan- thanide(III) complexes with nitrate counter ions formed a highly viscous mesophase, which decomposed at the clearing point. The transition tempera- tures changed as a function of the lanthanide ion: the mesophase stability range became narrower when the size of the lanthanide ion decreased. The corresponding lanthanide complexes with dodecylsulfate (DOS) counter-ions did not form a mesophase. Bu¨nzli and coworkers observed a mesophase for europium(III) and terbium(III) complexes of a diaza-18-crown-6 with meso- genic pendant arms (Suarez et al., 2003). Although the mesophase was not
+
C12H25 N+ C12H25
O
NHC18H37 C18H37HN
L39 L40 L41
N H O
O H NN
HO OCnH2n+1 L42
H2n+1CnO
OH N N
HO
OCnH2n+1 L43
N N
FIGURE 57 Structures of 1-dodecyl-3-methylimidazolium C12mim (L39), N-dodecyl-N- C12mpyr methylpyrrolidinium (L40), 1,3-dioctadecylurea (L41),N-(2-hydroxy-4-alkoxybenzalde- hydeimino)-2-benzamidoethanamides (habbe,L42), and azines (L43).
O O
O O
O O
O
L44
FIGURE 58 40-[(Cholesteryloxy)carbonyl]-benzo-15-crown-5 (L44).
authors were able to detect the crystal-to-mesophase transition of the euro- pium(III) complex by temperature-dependent luminescence measurements.
In a follow-up study the authors made complexes of different tetracatenar or hexacatenar diaza-18-crown-6 ligandsL45–L48with lanthanide(III) nitrates, chlorides, and triflates (Suarez et al., 2005;Fig. 59). The complexes exhibited hexagonal columnar mesophases. The widest mesophase range was found for the nitrate complexes. It should be noted that the free ligands are not liquid- crystalline. The introduction of the lanthanide ion in the macrocyclic cavity leads to rigidification of the macrocyclic system and favors the formation of a mesophase.
N O
O N O
O O
O O
O C12H25O
O O
OC12H25
N O
O N O
O O
O C10H21O
O O
OC10H21
N O
O N O
O O
O C16H33O
O O
OC16H33
N O
O N O
O O
O C10H21O
O O
OC10H21 OC10H21
OC10H21 C10H21O
C10H21O
L45
L46
L47
L48 C12H25O
OC12H25
C10H21O
OC10H21
OC16H33 C16H33O
FIGURE 59 Azacrown ligandsL45–L48.
2-Aryl-substituted imidazo[4,5-f]-1,10-phenanthrolines with long alkyl chains on the aryl group form monohydrated complexes with lanthanide(III) chlorides (Cardinaels et al., 2008;Fig. 60). The metal-to-ligand ratio in these complexes is 1:2. Although the complexes of ligands with only one or two alkyl chains on the aryl group were not liquid-crystalline, some of the com- plexes of a ligand with three tetradecyloxy chains were mesomorphic. The complexes of the light lanthanides (R¼La, Pr, Nd, Sm, Eu) formed a cubic mesophase, while the complexes of the heavy lanthanides (R¼Gd, Tb, Dy, Ho) did not show any thermotropic mesomorphism. On the basis of XRD experiments, it was concluded that the most probable space group for the cubic mesophase isPm3n. The structure of this cubic phase is intermediate between micellar and bicontinuous topologies, with an infinite 3D interlock- ing network of mutually perpendicular, evenly pinched columns, compatible with the symmetry of the space group (three pairs of column sites evenly spaced along the bisectors of the cubic cell faces and two interstitial sites at the centers and corners of the cubic lattice). The 2-aryl-substituted imidazo [4,5-f]-1,10-phenanthroline ligand was modified by attaching one, two, or three cyanobiphenyl groups at the end of the long alkyl chains (Cardinaels et al., 2005a; Fig. 61, ligands L49–L51). The basic idea behind this ligand design was to decouple the mesophase-inducing group and the coordination group via a long flexible alkyl chain. This concept is different from other studies on metallomesogens, where the key idea to obtain high coordination number metallomesogens is to increase the number of long alkyl chains attached to the central metal core. Whereas metallomesogens with a large number of long alkyl chains often form columnar mesophases, the compounds with decoupled mesogenic and coordinating groups can form more disordered mesophases. This is illustrated by the fact that the complexes formed by adduct formation of these ligands with lanthanide tris 2-thenoyltrifluoroacetonate complexes R(tta)3 all exhibited a nematic meso- phase (Table 22). However, because of the large size of the metal complexes, the nematic phase was rather viscous in comparison of nematic phases formed by small organic molecules and the applicability of these complexes to be aligned by external electric or magnetic fields is limited. The compounds were strongly luminescent in the solid phase and as a solution in the LC host
N N
N NH
OC14H29 OC14H29
OC14H29 N
N N
NH C14H29O
C14H29O
C14H29O
R Cl
Cl
OH2 Cl
FIGURE 60 Monohydrated complexes of lanthanide(III) chlorides with 2-aryl-substituted imidazo[4,5-f]-1,10-phenanthroline ligands.
matrix 4-pentyl-40-cyanobiphenyl (5CB). The importance of this work lies in the fact that these complexes represent the first reported examples of nemato- genic lanthanidomesogens.
Co-condensates between metallic samarium and 4-pentyl-40-cyanobiphenyl (5CB) in the solid phase have been obtained via joint atomic/molecular beam deposition on a cooled calcium fluoride surface at liquid nitrogen temperature (Shabatina et al., 2000, 2001, 2005). The film samples have been studied by IR and UV/VIS spectroscopy in the temperature range from 80 to 300 K.
Two types of complexes were detected, one complex with a metal to ligand
N N
NH
N O O CN
N N
NH
N O O CN
N N
NH
N O O CN
O
O CN
O
O
O CN
O CN
L49
L50
L51
FIGURE 61 2-Aryl-substituted imidazo[4,5-f]-1,10-phenanthroline ligands bearing terminal cyanobiphenyl groups.
ratio of 1:1, [Sm2(5CB)2], and another complex with a 1:2 ratio, [Sm(5CB)2].
The solid phase transformation of the 1:2 complex to the 1:1 complex took place when the compound was heated to 183–200 K. The complexes are samar- iumpcomplexes, in which a samarium atom interacts with the aromatic ring of 5CB. Similar complexes were formed between metallic europium and 5CB (Vlasov et al., 2005).