0.329 nm was reported for a tricycloquinazoline core (0.329 nm) (Kumar et al. 1999)
and 0.326 nm for lutetium phthalocyanine dimers (van de Craats et al. 1997). Ivanov
et al. reported the smallest intra-columnar stacking distance (0.318~0.32 nm) and a
mobility as high as 0.04~0.08 cm
2 V
À1 s
À1 in a hexaazatriphenylene with six amide
groups at the peripheral positions of the discotic core (Gearba et al. 2003), and it was
concluded that hydrogen bonding was responsible for the decreased interdisk
distance.
Another advantage of large polyaromatic molecules in organic photovoltaic applications is their intercolumnar distance in a range of a few to 10 nm, which is considered
to be the optimal distance for the excitons diffusion (Terao et al. 2007; Gommans et al.
2009). Therefore, there has been substantial interest in applying p-n-dyad-type discotic
molecules for heterojunction photovoltaics (Tchebotareva et al. 2003; Wasserfallen
et al. 2005; Samori et al. 2006; Bu et al. 2009; Bullock et al. 2009; Charvet et al. 2012;
Dossel et al. 2012; Hayashi et al. 2011; Hizume et al. 2010; Li et al. 2008; Mativetsky
et al. 2009; Peeters et al. 2002; Zeng et al. 2014; Zhao et al. 2015). However,
p- and n-dyad discotic molecules tend to form alternating stacks within a column due
to strong charge-transfer interactions between p- and n-moieties (Janietz 1998;
Cooke et al. 2000; Schultz et al. 2000; Mahlstedt et al. 2000). Therefore, it is important
to induce mesophase separation between the p and n columns for practical applications
(Hoeben et al. 2005). To achieve this, various strategies have been utilized (Dong
et al. 2011); (1) solubility difference-induced microphase separation during solution
processing (Schmidt-Mende et al. 2001), (2) antagonistic interactions among
side chains (Benanti et al. 2007), and (3) hydrogen bonding (Tschierske 2001;
Beginn 2003).
An elegant example of hydrogen bond-induced columnar liquid crystals in an
otherwise nonliquid crystalline discotic molecules is 1,3,5-benzenetrisamides
(Kawada et al. 1988; Hanabusa et al. 1997; Bushey et al. 2001, 2002). A C 3
symmetry was observed within a column, where the 120
rotation between neighboring benzenetrisamide molecules was stabilized by intermolecular hydrogen
bonding along the column direction. In addition, a sergeants-and-soldiers principle
was presented by Meijer et al. in these systems (Palmans et al. 1997; van Gorp et al.
2002; Smulders et al. 2008). Paraschiv et al. reported a series of 1,3,5benzenetrisamide with three pendant hexaalkoxytriphenylene groups by varying
the spacer length and the arm length of the triphenylenes (Tps) (Paraschiv et al.
2006, 2008). Totally, four columnar hexagonal phases were identified, where
intermolecular hydrogen bonds connected the central benzene cores in such a way
that the adjacent benzene cores rotated 120
apart, therefore leading to a helical
orientation of the resulting columnar stacks. A high mobility of 0.23 cm
2 V
À1 s
À1
was found in one of those Tp benzenetrisamide samples (Paraschiv et al. 2008),
although it did not show the ordered intra-columnar stacking peak at 0.35 nm. The
high mobility might be a result of the chirality, which induced a more favorable
overlap among the pendant Tps.
In this entry, we have synthesized a series of doubly discotic liquid crystals based
on Tp arms with phthalocyanine (Pc), porphyrin (Py), and polyhedral oligomeric
silsesquioxane (POSS) cores. By varying the lengths of the alkyl spacer between the
Tp arms and the core, different columnar liquid crystalline self-assemblies are
8 Supramolecular Self-Assembly of Discotic Liquid Crystalline LEGOs
219
and 0.326 nm for lutetium phthalocyanine dimers (van de Craats et al. 1997). Ivanov
et al. reported the smallest intra-columnar stacking distance (0.318~0.32 nm) and a
mobility as high as 0.04~0.08 cm
2 V
À1 s
À1 in a hexaazatriphenylene with six amide
groups at the peripheral positions of the discotic core (Gearba et al. 2003), and it was
concluded that hydrogen bonding was responsible for the decreased interdisk
distance.
Another advantage of large polyaromatic molecules in organic photovoltaic applications is their intercolumnar distance in a range of a few to 10 nm, which is considered
to be the optimal distance for the excitons diffusion (Terao et al. 2007; Gommans et al.
2009). Therefore, there has been substantial interest in applying p-n-dyad-type discotic
molecules for heterojunction photovoltaics (Tchebotareva et al. 2003; Wasserfallen
et al. 2005; Samori et al. 2006; Bu et al. 2009; Bullock et al. 2009; Charvet et al. 2012;
Dossel et al. 2012; Hayashi et al. 2011; Hizume et al. 2010; Li et al. 2008; Mativetsky
et al. 2009; Peeters et al. 2002; Zeng et al. 2014; Zhao et al. 2015). However,
p- and n-dyad discotic molecules tend to form alternating stacks within a column due
to strong charge-transfer interactions between p- and n-moieties (Janietz 1998;
Cooke et al. 2000; Schultz et al. 2000; Mahlstedt et al. 2000). Therefore, it is important
to induce mesophase separation between the p and n columns for practical applications
(Hoeben et al. 2005). To achieve this, various strategies have been utilized (Dong
et al. 2011); (1) solubility difference-induced microphase separation during solution
processing (Schmidt-Mende et al. 2001), (2) antagonistic interactions among
side chains (Benanti et al. 2007), and (3) hydrogen bonding (Tschierske 2001;
Beginn 2003).
An elegant example of hydrogen bond-induced columnar liquid crystals in an
otherwise nonliquid crystalline discotic molecules is 1,3,5-benzenetrisamides
(Kawada et al. 1988; Hanabusa et al. 1997; Bushey et al. 2001, 2002). A C 3
symmetry was observed within a column, where the 120
rotation between neighboring benzenetrisamide molecules was stabilized by intermolecular hydrogen
bonding along the column direction. In addition, a sergeants-and-soldiers principle
was presented by Meijer et al. in these systems (Palmans et al. 1997; van Gorp et al.
2002; Smulders et al. 2008). Paraschiv et al. reported a series of 1,3,5benzenetrisamide with three pendant hexaalkoxytriphenylene groups by varying
the spacer length and the arm length of the triphenylenes (Tps) (Paraschiv et al.
2006, 2008). Totally, four columnar hexagonal phases were identified, where
intermolecular hydrogen bonds connected the central benzene cores in such a way
that the adjacent benzene cores rotated 120
apart, therefore leading to a helical
orientation of the resulting columnar stacks. A high mobility of 0.23 cm
2 V
À1 s
À1
was found in one of those Tp benzenetrisamide samples (Paraschiv et al. 2008),
although it did not show the ordered intra-columnar stacking peak at 0.35 nm. The
high mobility might be a result of the chirality, which induced a more favorable
overlap among the pendant Tps.
In this entry, we have synthesized a series of doubly discotic liquid crystals based
on Tp arms with phthalocyanine (Pc), porphyrin (Py), and polyhedral oligomeric
silsesquioxane (POSS) cores. By varying the lengths of the alkyl spacer between the
Tp arms and the core, different columnar liquid crystalline self-assemblies are
8 Supramolecular Self-Assembly of Discotic Liquid Crystalline LEGOs
219