PREFACE
1.3. Perylene as a core for the liquid crystalline organic semiconductor
1.3.3. Liquid crystals based on perylene bisimides
1.3.3.4. Symmetrical perylene bisimides with aryl groups
Würthner et al. reported the PBI 6b, which is obtained by the condensation of 3,4,5-tridodecyloxy aniline with perylene bisanhydride (Figure 1.13). This stabilized a
wide range Colh phase from RT to 373 oC, with a high thermal stability.42 Corresponding PBI with trialkyl phenyl groups (6a) also exhibited a wide range Colh phase but with a reduced clearing temperature by ≈70 oC.43 PBI 6c with chiral peripheral chains exhibited an ordered Colh phase where the molecules are packed in a helical fashion. The chiral peripheral chains bring a higher order within the Col phase leading to an increased charge-carrier mobility in the Col phase.44 Substitution with other aromatic groups with flexible chains as in the case of dialkyl fluorenyl and carbazolyl groups did not induce any mesomorphic behavior because of the lack of space filling in such molecular design.45 When the 3,4,5-trialkoxy phenyl group is connected through a flexible –CH2-, as in the case of PBI 7a, brings a lowering in the clearing temperature while keeping the mesomorphic behavior intact. The Colh phase is disordered as in the case of 6a and 6b. The charge carrier mobility measured by steady-state space-charge limited current obtained for 6b in the LC
state was 0.2 cm2 V–1 s–1, while that obtained for compound 7a was found to be 1.3 cm2 V–1 s–1.46 This value is higher than that of amorphous silicon. Use of suitable
alignment methods may enhance the mobility further in such materials along with a proper choice of electrodes with suitable work functions to improve the charge injections.
Percec et al. studied dendronized PBIs, where the PBIs are derived by connecting the 3,4,5-tridodecyloxyphenyl units through spacers of different length (Figure 1.13).47 The PBIs 7a-d exhibited Colh phase but with a reduced clearing point in comparison to compound 6b. The clearing points have reduced with the increase in the spacer length.
Compounds 7a and 7b were liquid crystalline at RT, while compounds 7c and 7d were crystalline at RT. This shows that the PBI 6a (without any spacer), 7b-7d with di, tri and tetramethylene spacers self-assembled into complex helical columns generated from tetramers of PBI. Dendronized PBI 7a with one methylene spacer, self-assembled into helical columns formed from the PBI dimers. At high temperature, all helical columns self-organize to form thermodynamically stable 2D Colh lattices with the intracolumnar order, which is due to fast self-assembly process. At low temperatures, dendronized PBIs self-organize in the thermodynamically stable 3D columnar simple orthorhombic (for 6a, 7b-7d) and in 3D columnar monoclinic (for 6b) lattices. This is through a very slow self-assembly process, that takes place from the closely related kinetic 2D product. These studies demonstrate the self-assembly of complex helical columns created from dimers and tetramers of dendronized PBIs that are persistent in 2D and 3D lattices formed in the solid state. In the 2D Colh phase with the intracolumnar order, the dendronized PBI exhibits an extraordinary dynamics able to leave the columns without the disorder. This provides a self-healing mechanism of structural defects. These complex helical columnar 2D and 3D periodic arrays are first reported for self-assembling dendritic structures. The complex helical columns observed here are racemic in nature. The general self-assembly processes reported in this class of molecules may help in explaining the mechanism of charge carriers transport. Further, this may improve the design methodology to obtain supramolecular electronic materials based on PBIs and other electron-deficient structures, which are of primary importance in the fabrication of OSCs and other organic electronic devices.
Usually, reported PBIs are known to stabilize only Col or lamellar assemblies.
Percec et al. shown that suitably functionalized PBI (7e) can be self-assembled into a supramolecular sphere.48 This PBI functionalized at its imide groups with a second
generation dendron, self-assembled into supramolecular spheres. These spheres self-organized into a body-centered cubic (BCC) lattice, which is rarely seen for self-assembling dendrons but often noticed in the case of block copolymers (Figure 1.13b).
These supramolecular spheres also assembled into a Colh array at a lower temperature and thus exhibited a rare transition from Colh to cubic phase.
Figure 1.13. (a) Structures of symmetrical PBIs with aryl (6a-6e) and dendron substitution (7a-7e); (b) Schematic showing the self-assembly of dendronized PBI 7e in colh phase and transformation to BCC phase (Reproduced from reference [48] with the permission of American Chemical Society); (c) Graphical representation of the thermal behavior of PBIs 6a-6e and dendron substitution 7a-7e.
Asha et al. reported a series of highly fluorescent LC PBI molecules having amide/ester linkage with an end-cap of phenyl, monododecyloxy phenyl, or tridodecyloxy phenyl moieties (Figure 1.14).49 The amide-functionalized series self-assembled to form H-type or head-on aggregates irrespective of their end-capping in polar organic solvents (for example, like tetrahydrofuran (THF), toluene, and dichloromethane). On the other hand, only the monododecyloxy phenyl end-capped molecule in the ester series showed a tendency to self-organize with a typical J-type or slip-stacked aggregation in toluene.
Figure 1.14. Structures and bar graph of symmetrical PBIs with aryl groups connected with a long spacer (8a-8c, 9a-9e).
Usually connecting the trialkoxyphenyl group directly to the perylene core to form PBIs quench the luminescence and authors suggested that connecting them through the spacers would stabilize liquid crystallinity and also preserve the luminescence in an
aggregated state. The ester and amide molecules here can be classified into three groups:
(a) molecules without a terminal substitution, (b) molecules with monododecyloxy terminal substitution, and (c) molecules with tridodecyloxy terminal substitution. In the first case, for example, consider ester 8a and amide 9a: the melting and clearing points have been reduced in the case of amide 9a. Increase in the spacer length from 6 to 12 as in the case of amide 9d reduced the clearing point but increased the melting point in comparison to 9a.
However, in the case of compound 9d, the mesophase was frozen in a glassy state in cooling cycle. Increase in the number of flexible chains on the periphery of the benzene end cap of the esters (8b and 8c) reduced the melting and clearing points, with the tridodecyloxy derivative 8c showing RT LC phase. In the case of corresponding amide derivatives, this trend was not there. Compound 9b exhibited higher melting and clearing point, while the tridodecyloxy derivative 9c exhibited a lower melting and clearing temperature. Increase in the spacer length to 12 (9e), led to an increase in the melting and clearing point in comparison to 9c but reduced the same in comparison to 9d.
Figure 1.15. Structures and Graphical representation of the thermal behavior of symmetrical PBIs with aryl groups connected with different linking groups (10a-e).
Yang et al. reported symmetrical PBIs, where the 3,4,5-tridodecyloxy benzoyl units are connected through different spacers containing amines at their termini (Figure 1.15).
PBIs 10a and 10b with soft-alkylene spacers with amine units exhibited RT Col phase,
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.