organization of the original mesogenic molecules (Tirelli et al. 2000). This is due to
the big spherical structure of C 60 which is not easy to construct molecules with
aspect ratio >4. In fact, fullerenes belong to the family of plastic crystals, which is in
contrary to LCs by definition (André et al. 1992; Heiney 1992).
A plastic crystal is a crystal composed of weakly interacting molecules that
possess some orientational or conformational degree of freedom, i.e., has positional
order but no orientation order, while liquid crystals have orientation order and no
long-range positional order. The shape structures of plastic crystal and liquid crystal
are totally different: for liquid crystals, they have anisotropic shape with rod, disk, or
bowl-like structure, while for plastic crystals, the shape is highly symmetric (usually
sphere-like). The question then rise: can we construct LCs from typical plastic
crystal moiety like fullerene?
To now, there are typically two strategies for the design of [60]fullerene LCs,
namely, the molecular LC approach and the supramolecular LC approach. The
molecular LC approach has two different ways to meet the aspect ratio requirement
of LC: (1) by connecting fullerenes with a very long mesogen or (2) by multiaddition
of mesogens on fullerene cage. In the following, we will first discuss the synthesis of
[60]fullerene LCs based on the above design strategy, then the properties from these
[60]fullerene LCs, and last our perspectives for these materials.
Design Strategies for Fullerene LCs
Molecular LC Approach
Taking into account the size and shape of the C 60 unite, the elaboration of mesomorphic fullerene derivatives represents a conceptual and synthetic challenge. [60]
Fullerene (C 60 ) was successfully incorporated into liquid crystal systems (Chuard
and Deschenaux 1996; Dardel et al. 1999; Tirelli et al. 2000; Felder-Flesch et al.
2000, 2006; Even et al. 2001; Dardel et al. 2001; Sawamura et al. 2002; Kimura et al.
2002; Campidelli et al. 2002, 2003, 2004, 2005, 2006a, b; Matsuo et al. 2004; Allard
et al. 2005; Bushby et al. 2005; Lenoble et al. 2006) to give nematic, chiral nematic,
smectic A, smectic B, columnar, and cubic phases. Recently, it was shown that hexaaddition of a malonate derivative, which possessed a monotropic nematic phase,
yielded a fullerene derivative, which exhibited an enantiotropic smectic A phase.
This result indicated that C 60 can be used as a platform for the synthesis of liquid
crystalline materials from a non-mesomorphic addend. A bischolesterol derivative
was used to prepare a hexa-adduct of C 60 . As expected (due to the cholesterol units),
a smectic A phase was obtained.
Linking Long Mesogens with Fullerene
The first [60]fullerene-containing thermotropic liquid crystal 1 was synthesized in
1996 (Fig. 2). The following structural requirements were applied for the successful
design of the first [60]fullerene-containing LC derivative: (1) to generate strong
intermolecular interactions between the mesogenic units, a twin cholesterol
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X. Yang et al.
the big spherical structure of C 60 which is not easy to construct molecules with
aspect ratio >4. In fact, fullerenes belong to the family of plastic crystals, which is in
contrary to LCs by definition (André et al. 1992; Heiney 1992).
A plastic crystal is a crystal composed of weakly interacting molecules that
possess some orientational or conformational degree of freedom, i.e., has positional
order but no orientation order, while liquid crystals have orientation order and no
long-range positional order. The shape structures of plastic crystal and liquid crystal
are totally different: for liquid crystals, they have anisotropic shape with rod, disk, or
bowl-like structure, while for plastic crystals, the shape is highly symmetric (usually
sphere-like). The question then rise: can we construct LCs from typical plastic
crystal moiety like fullerene?
To now, there are typically two strategies for the design of [60]fullerene LCs,
namely, the molecular LC approach and the supramolecular LC approach. The
molecular LC approach has two different ways to meet the aspect ratio requirement
of LC: (1) by connecting fullerenes with a very long mesogen or (2) by multiaddition
of mesogens on fullerene cage. In the following, we will first discuss the synthesis of
[60]fullerene LCs based on the above design strategy, then the properties from these
[60]fullerene LCs, and last our perspectives for these materials.
Design Strategies for Fullerene LCs
Molecular LC Approach
Taking into account the size and shape of the C 60 unite, the elaboration of mesomorphic fullerene derivatives represents a conceptual and synthetic challenge. [60]
Fullerene (C 60 ) was successfully incorporated into liquid crystal systems (Chuard
and Deschenaux 1996; Dardel et al. 1999; Tirelli et al. 2000; Felder-Flesch et al.
2000, 2006; Even et al. 2001; Dardel et al. 2001; Sawamura et al. 2002; Kimura et al.
2002; Campidelli et al. 2002, 2003, 2004, 2005, 2006a, b; Matsuo et al. 2004; Allard
et al. 2005; Bushby et al. 2005; Lenoble et al. 2006) to give nematic, chiral nematic,
smectic A, smectic B, columnar, and cubic phases. Recently, it was shown that hexaaddition of a malonate derivative, which possessed a monotropic nematic phase,
yielded a fullerene derivative, which exhibited an enantiotropic smectic A phase.
This result indicated that C 60 can be used as a platform for the synthesis of liquid
crystalline materials from a non-mesomorphic addend. A bischolesterol derivative
was used to prepare a hexa-adduct of C 60 . As expected (due to the cholesterol units),
a smectic A phase was obtained.
Linking Long Mesogens with Fullerene
The first [60]fullerene-containing thermotropic liquid crystal 1 was synthesized in
1996 (Fig. 2). The following structural requirements were applied for the successful
design of the first [60]fullerene-containing LC derivative: (1) to generate strong
intermolecular interactions between the mesogenic units, a twin cholesterol
152
X. Yang et al.