π-stacking, indicating correlations of adjacent TEG-PDI molecules perpendicular to
each other. The dynamics of these systems was studied by different solid state NMR
techniques. These showed that TEG-PDI in its frozen state performs angular fluctuations with amplitudes up to Æ40
, reflecting the rather fragile packing of the
elongated PDI units perpendicular to each other. In the liquid crystal phase, additional
motional averaging in the NMR spectra is observed. The easiest motional process
consistent with the observed averaging involves cooperative rotation of the PDI
molecules by 90
around the column axis. Thus, whereas the restricted angular
fluctuations in the solid phase can be considered as local processes, the increased
dynamics in the liquid crystal phase must be highly cooperative in nature. Such
cooperative dynamic modes are, of course, particularly important in processing such
systems to align the columns on surfaces [115].
Moreover, slow molecular dynamics and very slow phase transformation [116]
hamper the formation of the equilibrium phases of DLCs and the different packing
in equilibrium and nonequilibrium phases can have pronounced effects on the
charge carrier mobilities. This was studied in detail in perylene bis(diimide)s
(PBIs) functionalized with dendritic groups [117, 118]. These dendronized PBIs
self-assemble into complex helical columns generated from tetramers containing a
pair of two molecules arranged side-by-side and another pair in the next stratum of
the column, turned upside-down and rotated around the column axis at an
intratetramer angle that is different from that of the intertetramer angle (Fig. 9).
In most cases, the intratetramer stacking distance in this column is 0. 41 nm, while
the intertetramer distance is 0. 35 nm The architecture of this complex helical
column, the structure of its 3D periodic array, and its kinetically controlled selforganization with such a long intratetramer distance are not ideal for the design of
supramolecular structures with high charge-carrier mobility. In fact, the mobility of
electrons is only moderate. However, in some cases, heating above 100
C in the
liquid crystal phase optimizes the packing and results in shorter intratetramer distances and much higher charge mobilities [117, 118]. This is accompanied by
substantial narrowing of the
1
H NMR lines. Computer simulation showed that this
narrowing of the NMR spectra indicates a complex reorganization mechanism,
whereby the PBI molecules leave the supramolecular column, flip over, and reenter
a column at a later time (Fig. 9b, c).
4.3 Pi-Conjugated Macromolecules for Organic Electronics
Likewise, polymers with extended π-conjugation and low band gaps are of broad
scientific interest because of their promising applications as semiconductors in
organic electronic devices. Examples include organic photovoltaic (OPV) cells,
organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs)
with optimized properties toward light harvesting, charge-carrier mobility, and light
emission, respectively [119–121]. Such polymers with lamellar π-stacks are often
semicrystalline [53], i.e., they exhibit phase separation with regions of high and low
Probing Macromolecular and Supramolecular Structure, Dynamics, and Function. . .
313
each other. The dynamics of these systems was studied by different solid state NMR
techniques. These showed that TEG-PDI in its frozen state performs angular fluctuations with amplitudes up to Æ40
, reflecting the rather fragile packing of the
elongated PDI units perpendicular to each other. In the liquid crystal phase, additional
motional averaging in the NMR spectra is observed. The easiest motional process
consistent with the observed averaging involves cooperative rotation of the PDI
molecules by 90
around the column axis. Thus, whereas the restricted angular
fluctuations in the solid phase can be considered as local processes, the increased
dynamics in the liquid crystal phase must be highly cooperative in nature. Such
cooperative dynamic modes are, of course, particularly important in processing such
systems to align the columns on surfaces [115].
Moreover, slow molecular dynamics and very slow phase transformation [116]
hamper the formation of the equilibrium phases of DLCs and the different packing
in equilibrium and nonequilibrium phases can have pronounced effects on the
charge carrier mobilities. This was studied in detail in perylene bis(diimide)s
(PBIs) functionalized with dendritic groups [117, 118]. These dendronized PBIs
self-assemble into complex helical columns generated from tetramers containing a
pair of two molecules arranged side-by-side and another pair in the next stratum of
the column, turned upside-down and rotated around the column axis at an
intratetramer angle that is different from that of the intertetramer angle (Fig. 9).
In most cases, the intratetramer stacking distance in this column is 0. 41 nm, while
the intertetramer distance is 0. 35 nm The architecture of this complex helical
column, the structure of its 3D periodic array, and its kinetically controlled selforganization with such a long intratetramer distance are not ideal for the design of
supramolecular structures with high charge-carrier mobility. In fact, the mobility of
electrons is only moderate. However, in some cases, heating above 100
C in the
liquid crystal phase optimizes the packing and results in shorter intratetramer distances and much higher charge mobilities [117, 118]. This is accompanied by
substantial narrowing of the
1
H NMR lines. Computer simulation showed that this
narrowing of the NMR spectra indicates a complex reorganization mechanism,
whereby the PBI molecules leave the supramolecular column, flip over, and reenter
a column at a later time (Fig. 9b, c).
4.3 Pi-Conjugated Macromolecules for Organic Electronics
Likewise, polymers with extended π-conjugation and low band gaps are of broad
scientific interest because of their promising applications as semiconductors in
organic electronic devices. Examples include organic photovoltaic (OPV) cells,
organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs)
with optimized properties toward light harvesting, charge-carrier mobility, and light
emission, respectively [119–121]. Such polymers with lamellar π-stacks are often
semicrystalline [53], i.e., they exhibit phase separation with regions of high and low
Probing Macromolecular and Supramolecular Structure, Dynamics, and Function. . .
313