Keywords
Discotic liquid crystals · Columnar self-assembly · Hydrogen bonding ·
Triphenylene · Phthalocyanine · Porphyrin · Polyhedral oligomeric
silsesquioxane (POSS)
Definition
Discotic liquid crystalline LEGOs refer to supermolecules composed of at least two
types of discotic components linked via flexible spacers. These discotic components
include, but not limited to, triphenylene, porphyrin, phthalocyanine, and polyhedral
oligomeric silsesquioxane (POSS). In addition to the π-π stacking interactions
between the discotic moieties, other intermolecular interactions such as hydrogen
bonding play an important role in the supramolecular self-assembly of these discotic
supermolecules. Finally, the spacer length determines the connectivity between the
discotic core and the peripheral disks.
Introduction
Discotic liquid crystals, which consist of rigid polyaromatic cores surrounded by
flexible arms (Hoeben et al. 2005; Sergeyev et al. 2007; Laschat et al. 2007), are an
interesting class of functional materials potential for photovoltaic cells (SchmidtMende et al. 2001; Nelson 2001; Oukachmih et al. 2005), light-emitting diodes
(Meier 1992; Christ et al. 1997; Bacher et al. 1997; Seguy et al. 2000; Hertmanowski
et al. 2008; Saleh et al. 2009), and field-effect transistors (van de Craats et al. 2003;
Pisula et al. 2005) due to the high charge carrier mobility along their
one-dimensional (1D) aromatic π-π stacking (Schmidt-Mende et al. 2001; Adam
et al. 1994). Besides the advantages of low cost, ability to scale up, and tunable
energy gap for organic photovoltaics over inorganic materials, columnar discotic
liquid crystals have unique properties such as easy processing, self-healing, and
lacking of defects (Simpson et al. 2004). To apply the columnar structure for 1D
charge transport, the area of π-orbital overlapping (van de Craats and Warman 2001)
and the intra-columnar distance (Liu and Bard 2002) are key factors affecting the
charge carrier mobility. To increase the overlapping area of π-π stacking, a large
conjugated polyaromatic core is required, such as hexa-peri-hexabenzocoronene
(HBC) (Simpson et al. 2004; van de Craats et al. 1998, 1999) and its derivatives
with a charger carrier mobility as high as 0.3–1 cm
2 V
À1 s
À1 , which is comparable to
that of graphite in the π-π stacking direction (i.e., 3 cm
2 V
À1 s
À1 ) (van de Craats et al.
1998). For organic semiconductors, the interdisk distance usually ranges from 0.33
to 0.4 nm (van de Craats and Warman 2001; Zamir et al. 1994; Ito et al. 2000;
Pieterse et al. 2001; Ichihara et al. 2007). To decrease the interdisk distance and
stabilize the columnar structure, various non-covalent interactions have been utilized, including electrostatic interactions, hydrophobic-hydrophilic effects, metal
coordination, and hydrogen bonding. For example, an interdisk distance of
218
L. Zhu
Discotic liquid crystals · Columnar self-assembly · Hydrogen bonding ·
Triphenylene · Phthalocyanine · Porphyrin · Polyhedral oligomeric
silsesquioxane (POSS)
Definition
Discotic liquid crystalline LEGOs refer to supermolecules composed of at least two
types of discotic components linked via flexible spacers. These discotic components
include, but not limited to, triphenylene, porphyrin, phthalocyanine, and polyhedral
oligomeric silsesquioxane (POSS). In addition to the π-π stacking interactions
between the discotic moieties, other intermolecular interactions such as hydrogen
bonding play an important role in the supramolecular self-assembly of these discotic
supermolecules. Finally, the spacer length determines the connectivity between the
discotic core and the peripheral disks.
Introduction
Discotic liquid crystals, which consist of rigid polyaromatic cores surrounded by
flexible arms (Hoeben et al. 2005; Sergeyev et al. 2007; Laschat et al. 2007), are an
interesting class of functional materials potential for photovoltaic cells (SchmidtMende et al. 2001; Nelson 2001; Oukachmih et al. 2005), light-emitting diodes
(Meier 1992; Christ et al. 1997; Bacher et al. 1997; Seguy et al. 2000; Hertmanowski
et al. 2008; Saleh et al. 2009), and field-effect transistors (van de Craats et al. 2003;
Pisula et al. 2005) due to the high charge carrier mobility along their
one-dimensional (1D) aromatic π-π stacking (Schmidt-Mende et al. 2001; Adam
et al. 1994). Besides the advantages of low cost, ability to scale up, and tunable
energy gap for organic photovoltaics over inorganic materials, columnar discotic
liquid crystals have unique properties such as easy processing, self-healing, and
lacking of defects (Simpson et al. 2004). To apply the columnar structure for 1D
charge transport, the area of π-orbital overlapping (van de Craats and Warman 2001)
and the intra-columnar distance (Liu and Bard 2002) are key factors affecting the
charge carrier mobility. To increase the overlapping area of π-π stacking, a large
conjugated polyaromatic core is required, such as hexa-peri-hexabenzocoronene
(HBC) (Simpson et al. 2004; van de Craats et al. 1998, 1999) and its derivatives
with a charger carrier mobility as high as 0.3–1 cm
2 V
À1 s
À1 , which is comparable to
that of graphite in the π-π stacking direction (i.e., 3 cm
2 V
À1 s
À1 ) (van de Craats et al.
1998). For organic semiconductors, the interdisk distance usually ranges from 0.33
to 0.4 nm (van de Craats and Warman 2001; Zamir et al. 1994; Ito et al. 2000;
Pieterse et al. 2001; Ichihara et al. 2007). To decrease the interdisk distance and
stabilize the columnar structure, various non-covalent interactions have been utilized, including electrostatic interactions, hydrophobic-hydrophilic effects, metal
coordination, and hydrogen bonding. For example, an interdisk distance of
218
L. Zhu