complexation-induced chemical shifts varying from 3 to 5 ppm. All guest molecules
undergo distinct motions, ranging from mere C 6 -rotations of benzene at low
temperatures to rather ill-defined 180
phenyl flips of fluorobenzene. In addition, by
combination of both computed NICS maps and explicit
19
F and
2
H ab initio chemical
shift calculations by density functional theory, the preferred orientation of the guest
molecules within the host was derived. Although benzene populates the center plane
formed by the stabilizing hydrogen bonds of the urea units, fluorobenzene prefers an
equatorial position. In the case of 1,4-difluorobenzene, an axial position is suggested.
Off-centered orientations towards the “walls” of the host, however, can be safely ruled
out [203].
4.6 Columnar Structures from Discotic Liquid Crystals
The disk-shaped polycyclic aromatic hydrocarbons (PAH) form π-stacks and are
currently of broad scientific interest due to their potential application as conducting
molecular wires [204]. Charge mobility can occur parallel to the stacking axis.
Critical for the charge-carrier properties are disk size, shape, and periphery
[205]. We have studied two such disk-shaped molecules that form molecular
wires in their crystalline and LC phases. We determined the packing and specific
molecular dynamics of the disks, including the intriguing kinetics of self organization and self-healing.
The most prominent examples of such molecular π-stacks are based on hexa-perihexabenzocoronenes (HBC) [160, 204]. Here, the kinetics of phase transformation
from the high temperature discotic liquid crystalline mesophase to the crystalline
phase at lower temperatures has been studied in a model dipole-functionalized HBC
derivative. Complementary structural (X-ray diffraction, solid-state NMR) and
dynamic (solid-state NMR, DS) methods were employed. These experiments
revealed long-lived metastability, slow kinetics, as well as an intermediate state
that involves a change in unit cell prior to crystallization. The barrier properties for
the unit cell transformation and crystallization amount to 1 and 2.5 eV, respectively.
Although crystallization bears some similarity to nucleation and growth, the LC to
crystalline transformation is more complex and involves fractional exponents. In
addition, the selective probing of the alkyl chains and disk dynamics by NMR
allowed identifying the role of the latter on inducing crystallization [206].
Among the different discotic liquid crystals, perylenediimide (PDI) derivatives
have received considerable attention, originally because of their industrial applications
as pigments. Efforts to optimize pigment colors resulted in high-grade industrial
applications including automotive coatings [207]. These applications made use of
the high tinctorial strength, light and weather stability, insolubility, and chemical
inertness of PDIs. Other major applications of PDI derivatives are as organic electronics in all-organic photovoltaic solar cells and field-effect transistors. These
applications rely on the high charge carrier mobilities that made PDI the best n-type
semiconductors available to date [208]. Central to these applications of HBCs
174
K. Binder et al.
undergo distinct motions, ranging from mere C 6 -rotations of benzene at low
temperatures to rather ill-defined 180
phenyl flips of fluorobenzene. In addition, by
combination of both computed NICS maps and explicit
19
F and
2
H ab initio chemical
shift calculations by density functional theory, the preferred orientation of the guest
molecules within the host was derived. Although benzene populates the center plane
formed by the stabilizing hydrogen bonds of the urea units, fluorobenzene prefers an
equatorial position. In the case of 1,4-difluorobenzene, an axial position is suggested.
Off-centered orientations towards the “walls” of the host, however, can be safely ruled
out [203].
4.6 Columnar Structures from Discotic Liquid Crystals
The disk-shaped polycyclic aromatic hydrocarbons (PAH) form π-stacks and are
currently of broad scientific interest due to their potential application as conducting
molecular wires [204]. Charge mobility can occur parallel to the stacking axis.
Critical for the charge-carrier properties are disk size, shape, and periphery
[205]. We have studied two such disk-shaped molecules that form molecular
wires in their crystalline and LC phases. We determined the packing and specific
molecular dynamics of the disks, including the intriguing kinetics of self organization and self-healing.
The most prominent examples of such molecular π-stacks are based on hexa-perihexabenzocoronenes (HBC) [160, 204]. Here, the kinetics of phase transformation
from the high temperature discotic liquid crystalline mesophase to the crystalline
phase at lower temperatures has been studied in a model dipole-functionalized HBC
derivative. Complementary structural (X-ray diffraction, solid-state NMR) and
dynamic (solid-state NMR, DS) methods were employed. These experiments
revealed long-lived metastability, slow kinetics, as well as an intermediate state
that involves a change in unit cell prior to crystallization. The barrier properties for
the unit cell transformation and crystallization amount to 1 and 2.5 eV, respectively.
Although crystallization bears some similarity to nucleation and growth, the LC to
crystalline transformation is more complex and involves fractional exponents. In
addition, the selective probing of the alkyl chains and disk dynamics by NMR
allowed identifying the role of the latter on inducing crystallization [206].
Among the different discotic liquid crystals, perylenediimide (PDI) derivatives
have received considerable attention, originally because of their industrial applications
as pigments. Efforts to optimize pigment colors resulted in high-grade industrial
applications including automotive coatings [207]. These applications made use of
the high tinctorial strength, light and weather stability, insolubility, and chemical
inertness of PDIs. Other major applications of PDI derivatives are as organic electronics in all-organic photovoltaic solar cells and field-effect transistors. These
applications rely on the high charge carrier mobilities that made PDI the best n-type
semiconductors available to date [208]. Central to these applications of HBCs
174
K. Binder et al.