PREFACE
1.3. Perylene as a core for the liquid crystalline organic semiconductor
1.3.1. Liquid crystals based on perylene tetraesters and their extended derivatives
PBIs form the majority among the perylene derivatives utilized as organic semiconductors.
Their structurally related PTEs are less electron deficient and possess higher LUMO levels than PBIs. However, they are still electron deficient because of the four carboxylate groups and when connected with a suitable donor molecule to form donor-acceptor (D-A) polymers, such solar cells exhibited higher open circuit voltage (VOC) in comparison to the PDI analogue.10 In addition, they exhibited good solution processability and a good thermal stability. Inspite of that, liquid crystals based PTEs are less in number. Kitzerow et al.
reported a homologous series of LC 3,4,9,10-tetra-(n-alkoxycarbonyl)-perylenes, where the alkyl tails were varied from ethyl to n-decyl11 (Compounds T1-T10, Figure 1.4). Except for the lowest and highest homologue, all of these compounds exhibited Colh phase with a good homeotropic (face-on) alignment (Figure 1.2c). An increase in the chain length noticed the lowering of melting and clearing points, consequently the reduced mesophase width. They show intense green fluorescence in dilute solution, while a red-shifted orange to red emission in the solid state.12 This red-shifted emission is accounted for the excimer emission.
Figure 1.4. Bargraph showing the thermal behavior of PTEs with respect to the chain length.
The incorporation of rac-2-ethylhexyl chain lowered the melting point of the PTE, widened the range Col phase starting from the room temperature (RT).13 Yang et al.
investigated a series of perylene tetra methyl alkyl esters (secondary alkyl esters). They observed that the PTE with four sec-octyl (Ts-8) chains showed a decrease in the melting point, along with a fall in the clearing point.14 Substitution with 2-ethylhexyl chains also had a similar effect with lowered clearing point. PTEs with sec-butyl (Ts-4) and sec-hexyl (Ts-6) chains also exhibited room temperature Colh phase, but they decomposed before reaching the clearing point (Figure 1.5).13 In comparison to Ts-8, the PTE with four 3,7-dimethyl octyl tails (T-3,7) showed a much-lowered clearing point along with an RT Colh phase.15
Figure 1.5. Bargraph showing the effect of branching on the thermal behavior of PTEs.
Further investigations on PTEs with expanding the core in the bay-region was carried out to alter the thermal and electronic properties. Bock et al. reported benzoperylene-hexa- and tetracarboxylic esters, by expanding the PTE core in the bay region (Figure 1.6).16 The hexa and tetraesters showed strikingly different behavior in the mesophase formation. Surprisingly, the compound with four alkyl tails was far more mesogenic than the hexaester. However, the electronic properties have been found to change as expected, with the hexaester being more electron-accepting in nature than the tetraester. For example, compounds EE-2 to EE-4 (hexaesters with ethyl to n-butyl chains) exhibited monotropic Colh phase. Substitution with 2-ethylhexyl chains (EE-EH) turned the compound to be liquid. However, the tetraester with ethyl chains (EE-TE) exhibited a Colh phase of around 100 oC, while the corresponding ethyl hexyl derivative (EE-TEH) exhibited RT Colh phase over a broad thermal range. Tetraesters bearing slightly lengthier n-alkyl chains like propyl and butyl chains too exhibited monotropic Colh phase with a
reduced mesophase range (Figure 1.6). On comparison of the thermal behavior of bay-extended hexaesters with bay-extended tetraesters Bock et al. derived some
conclusions.16 For example, the tetraethyl ester EE-TE in comparison with the
hexaethylester EE-2 exhibited a greatly enhanced mesomorphism. Although both show nearly the same melting points the mesophase of EE-TE was enantiotropic and with wider thermal range. The effect was even more pronounced on comparing the 2-ethylhexyl derivatives EE-EH and EE-TEH. Although none of them are crystalline at room temperature (RT), compound EE-TEH assumes a Colh phase over a wide thermal range, which is a clear contrast to the room temperature liquid EE-EH. Further, the Colh phase of EE-TEH exhibits highly ordered homeotropic non-birefringent domains on annealing the isotropic melt. The dramatic enhancement in the mesomorphic behavior on going from bay-extended hexaesters to corresponding tetraesters specifies that at least with medium sized aromatic cores, it is extremely difficult to accommodate sterically demanding six alkoxycarbonyl side chains without any out-of-plane orientation. This steric bulk is detrimental in achieving the overall disc-like shape that promotes the columnar packing.
This is completely contrasting to alkoxy- or alkanoyloxy-substituted mesogens, especially hexaalkoxy- and hexaalkanoyloxy-triphenylenes, for which six is the usual number of flexible chains that are used to imbue mesomorphism.
Bock et al. further extended the bay region of the tetraethyl PTE with a dimethyl ester of naphthalene dicarboxylic acid to obtain a hexaester (EE-N2), which showed an exceptional mesomorphic behavior.17 From literature it is learnt that, in the case of alkyl-, alkoxy-, alkylthio-, and alkanoyloxy-substituted DLCs, alkyl chains containing a minimum of five carbon atoms connected linear fashion is essential to stabilizing columnar mesomorphism.2 However in the case of EE-N2 and in many alkoxycarbonyl-substituted PTEs, the ethyl derivatives also stabilize Col phase, although at higher temperatures. The dimethyltetraethyl derivative EE-N2 also exhibited a high-temperature mesophase on heating. But in complete contrast to all other known short-chain columnar PTEs, here the Colh phase was maintained at RT, which was stable even after 2 months as evidenced from XRD. Notably, this molecule showed a good homeotropic alignment. Here, the influence of the distortions in the arene core, which are mainly caused by the three pairs of ester groups that are in close proximity to each other and the axially chiral overcrowded helicene structure formed by the naphthalene extension at bay region stabilizes the mesophase.
However, dinaphtho[1,2-a:1′,2′-j] coronene-8,9,18,19-tetracarboxylic tetraesters (with straight and branched chains), which are bilaterally extended PTEs with naphthalene moieties turned to be crystalline with high melting temperature (EE-DN1 to EEDNBO, Figure 1.6).18
Figure 1.6. Structures and bar graph of other bay-extended perylene esters.
In general, Col PTEs exhibit homeotropic alignment on slow cooling from isotropic liquid state.11 However, Wolarz et al. reported planar alignment of T-6, on depositing its THF solution on a hydrophilic glass substrate using the zone-casting technique.19 They observed dendritic or flower-like structures at RT. It was established that such exclusive structures were not formed by the recrystallization of T-6 from a Col phase. The molecular aggregation in the dendritic structures was confirmed by the absorption and fluorescence studies. WAXS study proved the crystallization of T-6 in the monoclinic arrangement. The dendritic branches were birefringent and with a characteristic angle of 60o between them.
These observations led to an explanation that the dendrites were built with the molecules packed in a columnar fashion. Further, these columns were aligned along the dendrite
branches. The perylene units inside the columns were aligned with an edge-on or homogeneous orientation with respect to the substrate and with an inclination angle of about 30o with respect to the column axis. These dendritic structures vanished on annealing at the thermal range of the Col phase and were not rebuilt even after an extended annealing at RT. The homogeneous alignment of these molecules in the dendrites, growing at a temperature below the crystallization point is in contrast to the usual homeotropic alignment of the dendritic structures obtained in the Colh phase on annealing from the isotropic liquid.