Actinidomesogenscan be defined an actinide-containing LCs. In comparison to the lanthanidomesogens, examples of actinidomesogens are very scarce.
This is not a surprise, given the fact that actinides are radioactive. All actini- domesogens described in the literature are uranium-containing LCs (uranome- sogens). No thorium-containing LCs (thoromesogens) are known yet. In theory, one could imagine the design of LCs containing neptunium, pluto- nium, or americium, but these metals are much more difficult to handle than uranium or thorium due to their higher radioactivity. From a materials point of view, it does not make much sense to work with these elements, because these radioactive compounds have to be handled in specialized laboratories. For the same reason, it is unlikely that metallomesogens containing actinium, protac- tinium, or the heavy actinides will ever be prepared. It should be realized that organic complexes of highly reactive compounds suffer from severe radioly- sis, so that it is extremely difficult, if not impossible to prepare such com- pounds in a high purity. Uranium has a rich redox chemistry, but the most
stable oxidation state is theþVI state. In general, hexavalent uranium occurs under the form of thedioxouranium(VI) ion, which is better known as theura- nylion (Denning, 1992, 2007). The uranyl ion is an oxocation of hexavalent uranium, with chemical formula [UO2]2þ. It has a linear structure with short U-O bonds, which are represented as double bonds, although the molecular orbital description of the chemical bonds in uranyl is quite complicated. Four or more ligands (or two or three bidentate ligands) can bind in the equatorial plane to the uranyl ion. The uranyl ion is chemically a very stable oxocation, and it is very difficult to remove one or both of the oxygen atoms.
The first examples of uranium-containing LCs were the uranyl b-diketonate and tropolonate complexes reported by Sinn and coworkers (Clark et al., 2002;Fig. 69). The ligands are not liquid-crystalline themselves, but the corresponding uranyl complexes exhibit a mesophase. The tropolonate complex forms a highly ordered crystal B (or E) phase over a small tempera- ture range between 207 and 209C. The exact nature of the mesophase (B or E) could not be determined with certainty. Extension of the chain lengths leads to a wider mesophase stability range, but otherwise the properties of these complexes are similar (Elliott et al., 2002). The b-diketonate complex forms a smectic C phase at much lower temperatures (between 59 and 75C; Clark et al., 2002). However, upon cooling the smectic C phase is reformed from the isotropic liquid at 64C and can be supercooled down to room temperature. The uranylb-diketonate complex is thus an example of a room-temperature metallomesogen. Although the uranyl group (UO2) has some structural similarity to the vanadyl group (VOIV), but there are striking differences. Whereas the vanadyl ion can form weak intermolecular links
O
O O
O
C16H33O U OC16H33
O
O
O O C13H27
C10H21O
O HO
C13H27 OC10H21
U O
O
FIGURE 69 Liquid-crystalline uranyl complexes with tropolonate (top) and b-diketonate (bottom) ligands.
uranyl complexes of a series of mesomorphic N,N0-salicyliden(3,30-diamine- N-methyldipropylamine) ligandsL52H2–L55H2(Aiello et al., 2005;Fig. 70).
In these Schiff’s base complexes the uranyl ion is coordinated by 5 atoms in the equatorial plane (N3O2coordination). Some of the complexes showed a smectic C phase at high temperatures. Uranyl complexes of expanded por- phyrins are the first examples of discotic uranyl-containing metallomesogens (Sessler et al., 2004, 2006). The ligand is a so-calledalaskaphyrin, with eight peripheral alkoxy chains (Fig. 71). The uranyl ion acted as a template for the synthesis of the metal complex. The complexes occur as an anisotropic glass at room temperature (vitrified mesophase) and exhibited a hexagonal columnar mesophase over a temperature range of more than 100C.
Binnemans and coworkers prepared a uranyl mesogen with a propeller shape by coordination of three imidazo[4,5-f]-1,10-phenanthroline to the uranyl ion
N OH O
O C12H25O
N N
HO
O O
OC12H25 N
OH N
N C14H29
N N
HO
N N
C14H29
N OH N
N C14H29
N N
HO
N N
C14H29
C18H37O OC18H37
N OH
N N
HO
O O
O
C12H25O
O
OC12H25 L52H2
L53H2
L54H2
L55H2
FIGURE 70 N,N0-salicyliden(3,30-diamine-N-methyldipropylamine) ligands.
in the equatorial plane (Cardinaels et al., 2005b; Fig. 72). Unlike all other uranyl mesogens reported before, this compound is a cationic complex with triflate counter anions. The complex forms a hexagonal columnar phase between 95 and 181C. A rich mesomorphism is displayed by [UO2Br4]2 complexes with N-alkyl-N-methylpyrrolidinium cations (Goossens et al., 2009). Depending on the alkyl chain length, a smectic A, crystal smectic E or a hexagonal columnar phase was observed. The uranyl complexes were not photoluminescent in their pure form due to auto-quenching, but lumines- cence in the visible spectral region could be observed upon dilution of the complex in an ionic liquid. TheN-alkyl-N-methylpyrrolidinium bromide com- pounds exhibit a rare smectic phase, a T phase with tetragonal symmetry.
C14H29O C14H29O
N N
N N
OC14H29 OC14H29 N
N C14H29O OC14H29
C14H29O OC14H29 U
O O
FIGURE 71 Liquid-crystalline uranyl alaskaphyrin complex.
N N N N
N N
N HN
N NH N
HN
OC14H29 OC14H29 OC14H29
OC14H29 OC14H29 C14H29O
C14H29O C14H29O
OC14H29
U O
O
2+
FIGURE 72 Liquid-crystalline uranyl complex with imidazo[4,5-f]-1,10-phenanthroline ligands.
The two noncoordinating triflate counter ions have been omitted.
the formation of smectic phases with lower symmetry.
Uranyl fluoride, UO2F2, forms a lyotropic nematic phase in mixtures of acetone and (heavy) water (Lychev et al., 1990; Mikhalev and Shcherba- kov, 1985a,b, 1986). This behavior is unusual, because the uranyl com- pound does not contain long alkyl chains and cannot be considered as an amphiphilic. Nevertheless, compounds such as V2O5, FeOOH, AlOOH, Li2Mo6Se6,and H2WO4are other examples of purely inorganic compounds that can form lyomesophases in water, in organic solvents or in mixtures thereof. These are the so-calledinorganic lyotropic liquid crystals(Sonin, 1998). It was observed that upon addition of acetone to solution of uranyl fluoride in (heavy) water light scattering occurred. However, the presence of a nematic mesophase was evident from the quadrupole splitting pattern in the1H NMR spectrum of acetone. It is assumed that the uranyl fluoride molecules are present in the mesophase as rigid dimers. When the uranyl fluoride concentration was lowered, the clearing temperature decreased.
For uranyl fluoride concentrations lower than 0.30 mol (kg D2O)1, the nematic phase was no longer observed. Decrease of the acetone concentra- tion, led to a narrowing of the temperature range of existence of the nematic mesophase. Although there is evidence for the existence of lamellar of hex- agonal mesophases in the region of high acetone concentrations (ca. 28–
40 mol (kg D2O)1), no detailed studies of these systems have been carried out yet (Sonin, 1998).