PREFACE
1.3. Perylene as a core for the liquid crystalline organic semiconductor
1.3.3. Liquid crystals based on perylene bisimides
1.3.3.1. Symmetrical perylene bisimides with oligoethoxy chains
PBIs form a class of the most studied organic semiconductors. Cormier et al. reported the first LC PBIs in 1997.23 PBIs 1b-1c are based on the well-known perylenebis(phenethylimide) structure (1a, where R = H), while PBIs 2a-2h have purely linear aliphatic side chains (2a-2c, 2h) or branched aliphatic side chains (2d, 2e, 2f-g). All these compounds exhibited thermotropic LC phases as evidenced from DSC and POM.
Compound 1b and 1c showed a solid phase (crystalline or highly viscous LC) before transforming to a LC phase. PBI 2d is a RT LC with a clearing point of ∼55 oC. The spin-coated thin film undergoes a slow (∼24 h) crystallization on standing. On rapid cooling, it freezes to form an isotropic glassy state, which undergoes slow crystallization.
When it is spin-coated on untreated glass slides, the thin films of 2d spontaneously organize to form long, ribbon-like crystals. Both absorption and emission studies suggested that this spontaneous transformation is complemented by a reduced energetic disorder in the film, which in turn resulted in a decreased density of the exciton quenching sites. The propensity of self-healing of defects and glass formation is promising from the viewpoint of organic semiconductors. Similarly, several polyoxyethylene derivatives of PBIs have been investigated for their thermal behavior with the help of DSC and POM.24 Most of these PBIs exhibited a LC phase over a wide thermal range. The thin films of 2e, a RT LC, self-organizes into a well ordered crystalline phase that possesses superior photophysical properties in comparison to thermally evaporated, solvent vapor annealed films of the prototype PBI, 1a. Compounds 2f and 2g were crystalline in nature (Figure 1.8 and 1.9).
Cyclic voltammograms of thin polycrystalline films of LC PBI, 2b showed the evidence for strong attractive interactions between the PBI molecules and implied that the film undergoes two structural rearrangements to accommodate reduction to the anionic and dianionic states, which is also supported by spectroelectrochemical measurements. The redox conductivity of the film with respect to electrochemical potential was measured with the use of interdigitated array electrodes. The conductivity reaches the semiconducting level before the appearance of the first noticeable reduction wave. The maximum conductivity of 4.4 × 10-2 S/cm was observed when the film was reduced by 1 equivalent of electrons, in contrast to the anticipation that this state should be a Mott insulator.25
Figure 1.8. Structures of symmetrical PBIs with oligoethoxy chains.
Three different LC PBIs (2b,24 2e23-24 and 2h) were studied with respect to the optical and physical characteristics in their thin film state.26 These films were prepared by different techniques like spin-coating, thermal evaporation under vacuum and Langmuir-Blodgett (LB) techniques on wide variety of substrates like glass slides, indium tin oxide (ITO) coated glass slides and highly oriented pyrolytic graphite (HOPG). All the films were characterized by POM, UV-Vis and fluorescence spectroscopy as well as XRD
studies. It was noted that the changing of the preparation procedures/conditions, or using different substrates did not significantly alter the properties of the resultant thin films (Figure 1.8 and 1.9). The self-organizing ability of these LC PBIs permits them to quickly reach a stable, low-energy configuration, unlike many other thin film forming materials and discloses that they are compelled to self-assemble and orient in a highly specific fashion. Notably, such ordering is independent of the substrate or deposition method. The molecules incline to form a J-type organization that takes advantage of attractive π-π interactions, where the π-π stacking axis is in parallel orientation to the substrate. The LC PBI (2b) perylene cores are oriented to the substrate with a tilt angle of ∼47 oC along the stacking axis and ∼58 oC perpendicular to this direction. The two other LC PBIs (2e and 2h) also exhibited comparable structures. An investigation of the intermolecular electronic and steric interactions and the interactions between the molecules and the substrates were proposed to explain this strongly preferred orientation. For comparison, thin films of compounds 2b and 1a were also prepared by vacuum deposition with similar thickness, and LC PBI 2b was shown to exhibit highly ordered films due to its liquid crystalline nature.25
Figure 1.9. Bargraph showing the thermal behavior of symmetrical PBIs.
Thelakkat et al. reported swallowtail substituted symmetrical PBIs (2i and 2j) based on oligoethoxy chains (with two and three elthyleneoxy units), which stabilized Colh phase.
In contrast to symmetrical dialkyl swallowtail PBIs (3g 27 and 3h, Figure 1.10), the compounds 2i and 2j exhibited a much broader enantiotropic thermal behavior. Moreover, the length of the oligoethyleneglycol swallowtail substituent affects the clearing temperature of PBIs. As expected PBI 2j with longer oligoethoxy chains (three elthyleneoxy units in each branch) exhibited a lower clearing point, whereas the enthalpy change associated with Colh-I transition remains nearly the same (Figure 1.8 and 1.9).28
Bijak et al. reported RT thermotropic PBIs (2l, 2m and 2n) with wide mesophase range.29 PBIs 2m and 2n had a number of ethyleneoxy repeating units. They exhibited high relative photoluminescence quantum yield. The cyclic voltammogram of these PBIs showed two reversible reductions and one irreversible (except for PBI-2l) oxidation signals.
These compounds also showed a low electrochemical band gap (1.46–2.11 eV). The combination of the high fluorescence, excellent processability, low band gap, moreover a single RT mesophase structure makes these PBIs promising materials for optoelectronic applications. PBI with chiral oligoethoxy chains (four ethyleneoxy units connected to a chiral ethyleneoxy unit) exhibited a wide range Col phase including RT.30 Thus, in general the substitution with oligoethoxy chains improved the mesomorphic behavior and often stabilized the RT mesophases. But, the only problem with glycolic ether chains is their chelating nature with various cations, making such PBIs to be contaminated with ionic impurities for subsequent application in electronic devices (Figure 1.8 and 1.9).