PREFACE
1.3. Perylene as a core for the liquid crystalline organic semiconductor
1.3.3. Liquid crystals based on perylene bisimides
1.3.3.6. Symmetrical core-substituted perylene bisimides
while compound 10c with rigid aromatic spacer turned to be crystalline in nature.50 Asha et al. reported PBIs 10d and 10e, where the 3,4,5-tridodecyloxy benzoyl units are connected through a pentadecyl phenol (PDP) or cardanol.51 The compound 10d which had saturated side chain exhibited more crystalline order. This compound displayed a low temperature Colh plastic phase and a high temperature Colh phase, while the compound 10e with an unsaturated side chain exhibited Colh phase spanning over a broad thermal range. The presence of the long pentadecyl chain in the ortho-position to the imide linkage did not harm the ability of these PBIs to arrange in the form of H-type aggregates. Remarkably, a bend in the alkyl chain introduced through the cis-double bond did not cause any change in the type of the resulting aggregates but led to a drastic reduction in the aggregate length.
(6 × 10-6 cm2 V-1 s-1) in the Col phase similar to its nonchlorinated compound 6b (5 × 10-6 cm2 V-1 s-1). Remarkably, the charge carrier lifetime was over 100 times greater,
than that observed in the non-chlorinated parent compound (Figure 1.17).31 This dramatic enhancement in the lifetime was explained in terms of the increased order in the case of chlorinated PBI. This enhanced lifetime endures when the material is blended with p-type hexabenzocoronene (HBC) derivatives and hints the chlorinated derivative may find usefulness in device architectures, where an extended carrier lifetime is much desired.
However, the width of the Colh phase was reduced with the decrease in clearing and an increase in the melting points with respect to the parent compound. They have also reported PBIs, which are substituted with different phenoxy derivatives in a bay region (12b-12e).42 Compounds 12b-12d stabilized wide range Colh phase including room temperature, while compound 12e turned to be crystalline because of the lack of space filling and nanoseggregation of the aromatic cores and the flexible peripheral alkyl chain. These compounds exhibit high thermal stability and bright fluorescence making them promising materials for optoelectronics applications. The high fluorescence efficiency is attributed to the J-type aggregation of these molecules.42 They have also reported several PBIs with two
Figure 1.16. Structures and Graphical representation of the thermal behavior of unsymmetrical PBIs with aliphatic chains (11a-k).
or four substitutions at a bay area which bear 3,4,5-tridodecylphenyl groups at imide positions. These compounds exhibited either Colr (for compound 12f) or Colh phases (for compounds 12g-i) with low clearing point and room temperature liquid crystallinity.
Compounds 12j and 12k were crystalline. Contrary to the extended oligomeric π-stacks formed for planar unsubstituted PBIs, here the aggregation was limited to π-π stacked dimers in the apolar solvent due to the twisted aromatic structure. The fluorescence observed in solution was due to the dimeric aggregates. Interestingly, these PBIs exhibited fluorescence even in the mesophase. The core twisting brings a substantial effect on the π-π stacking manner of the molecules in solution, as well in the Col phases. In both cases, the more twisted tetrasubstituted PBIs prefer longitudinally slipped stacks with J-type emissive behaviors, whilst for the less twisted disubstituted PBIs, the rotational displacement between the PBIs in cofacial aggregates was confirmed.43
Figure 1.17. Structures and Graphical representation of the thermal behavior of unsymmetrical PBIs with aliphatic chains (12a-k).
Zhang et al. reported a series of symmetrical and unsymmetrical bay-chlorinated PBIs with siloxane terminal chains at imide positions.55 Symmetrical PBI Si/1OEt bearing one ethoxy group in the terminal silyl end exhibited an enatiotropic Colh phase, while all
the other symmetrical (with two or three ethoxy groups) and unsymmetrical PBIs (with an n-octyl chain at another terminus) exhibited monotropic Colh phase. They also exhibited a strong aggregation in concentrated THF solution along with a lyotropic LC behavior, which was also confirmed by red shifted absorption spectra. (Figure 1.18).
Figure 1.18. Structures and bar graph of symmetrical and unsymmetrical PBIs with silyl chains (Si/1OEt- 8Si/3OEt).
Figure 1.19. Structures and bar graph of core-twisted bay-substituted PBIs.
Würthner et al. reported56 the three-dimensional (3D) self-assembly of a racemic core-twisted PBI and its pure atropo-enantiomers ((P)-PBI and (M)-PBI) (Figure 1.19). In the condensed state, they studied the influence of core chirality in such materials. These studies showed characteristic differences in the bulk state properties of the racemic and enantiopure PBIs. The racemic material forms a soft crystalline phase, while the pure enantiomers ((P)-PBI and (M)-PBI stabilized a smectic phase with far lower viscosity. The presence of equal amounts of each enantiomer at the same time in the case of (rac)-PBI permits this material to self-assemble in a soft columnar crystalline phase with higher thermodynamic stability in comparison to the enantiopure (P)-PBI and (M)-PBI, which
organizes in lamellar/smectic LC phase. Here nano-segregation and interaction between the bridging units are the two crucial factors that lead to an observed difference in their bulk state behaviors. Alternation of MM and PP homochiral dimers in the condensed LC phase of (rac)-PBI provides an excellent nanosegregation and dense packing of the dimeric PBI building blocks into columns. In contrast, for enantiomerically pure (P)-PBI and (M)-PBI, the bridging units sterically prevent their packing into columns, which finally leads to the lamellar organization of the PBIs and a less viscous LC phase.
Figure 1.20. (a) Structures of symmetrical PBIs with aliphatic chains (12c,13a-d); (b) Graphical representation of the thermal behavior of bay-substituted PBIs (12c,13a-d); (c) Illustration of the molecular self-assembly of 13d and 12c into columnar hexagonal LC phases that exhibit orthogonal orientation of the PBIs. Blue arrows indicate the direction of the main transition dipole moments (mag) of the PBI molecules for 13d (Reproduced from reference [57] with the permission of Wiley VCH).
Yang et al. further reported several PBIs with H, Br, phenyl and tert-butylphenyl substituents on bay-positions and studied their mesomorphic and photophysical properties (Figure 1.20).50b The PBIs 13a-c with Br, phenyl and tert-butylphenyl at bay-positions
stabilized Colh phase in comparison with similar PBI 10a with no substituents on bay-position. Compounds 13a and 13b exhibited a short range (12-19 oC) nematic phase
before reaching the clearing point. The photophysical studies revealed that the compounds 13a-c possess stronger luminescence intensities, larger Stokes shifts and higher fluorescence quantum yields than that of PBI 10a. Lehmann and Würthner reported a new PBI derivative 13d that self-assembles through hydrogen bonds and π-π interactions into
J-aggregates. These J-aggregates in turn self-assemble to form Colh phase (Figure 20c).57 The PBI cores self-organized with their transition dipole moments parallel to the columnar axis, which is a unique type of structural organization in the case of Col LCs. XRD studies unravel a helical structure comprising of three self-assembled hydrogen-bonded PBI strands that create a single column of the Colh phase. Interestingly this Colh phase spans over a wide thermal range including RT.