Polymers with pendant perylene bisimide side chains (PPBI) were first reported by Lindner et al.83 in 2004. The authors synthesized an asymmetrical PBI monomer with a solubilizing swallow-tail substituent and an acrylate group for polymerization.
This acrylate monomer was polymerized with nitroxide-mediated radical polymerization (NMRP).84 It was possible to obtain moderate molecular weights by this method, and even block copolymers were obtained via sequential polymerization.40 The homo-polymerization of this high-molecular-weight PBI monomer is not optimal.
Narrow distributions, which are characteristic for controlled radical polymerizations, could not be obtained. In addition, the polymerization has to be performed at high monomer concentrations and the resulting polymer shows a limited solubility. This makes it difficult to achieve high molecular weights and also limits the choice of side chains that can be introduced. These synthetic restrictions were overcome by introducing the Copper-catalyzed azide-alkyne cycloaddition (CuAAC) “click” chemistry concept85 and combining it with controlled radical polymerization. The first attempt to obtain PPBI polymer using this concept was reported by Tao et al.86 in 2009.
Lang et al.87 investigated this concept in detail and compared it to the conventional synthesis approach. The polymer backbone is synthesized independently and afterward decorated with the PBI moieties. The authors synthesized poly(propargyloxystyrene) via NMRP with alkyne functionality at each monomer unit and also PBIs with azide functionality. Alkynes can undergo a very efficient reaction at room temperature with azides in the presence of a Cu(I) catalyst and form stable 1,4-substituted triazoles.86 It was reported that the polymer backbone can be almost quantitatively decorated with the PBI moieties as proven by Fourier transform infrared and proton NMR spectroscopy. These polymers had very narrow distributions as good as 1.16 and molecular weights of up to 15.000 g/mol.
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Fig. 14 Left: Pendant perylene side chains synthesized via “click chemistry” and right: a) Schematic of an electron-only device and (b) J–V characteristics for electron-only devices of compounds PPBI 1, PPBI 2, PPDB, and PPDI with active layer thickness L. (Reproduced with permission from 89)
The “clicked” polymers were compared with poly(PBI acrylates) (synthesized directly via NMRP) with different alkyl spacers. The comparison of the phase behavior showed a strong dependence on spacer lengths. In a further study, Lang et al.88 investigated the effect of the spacer length and different phase behavior of PPBI polymers with hydrophilic side chains. Two sets of polymers were synthesized as shown in Fig. 14: PPBI 1 - polymers with PPBIs with hydrophobic alky swallow tails;
and PPBI 2 - a set with hydrophilic oligo ethylene glycol swallow tails. In both cases three polymers were synthesized, using three different spacers [(CH2)6, (CH2)8, and (CH2)11]. Systematically investigating the influence of the PBI substitutions could be done because the approach of a polymer-analogous introduction of different pendant groups to a single precursor polymer made it possible to obtain a set of highly comparable polymers. All polymers had molecular weights up to 60.000 g/mol (SEC) and narrow distributions below 1.09. This also showed that this modular synthesis method is reliable even for higher molecular weights. A marked difference between hydrophilic PPBIs with OEG- and hydrophobic PPBIs with alkyl swallow tails was reported. The hydrophilic PPBIs were all found to be amorphous with the spacer length influencing the glass transition temperature, Tg, of the polymers. An increase in spacer length reduced Tg. A similar trend was observed for the Tg of the hydrophobic polymers with generally higher transition temperatures. If the spacer length did not exceed
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(CH2)8, the polymers were not amorphous but liquid crystalline. The introduction of the new hydrophilic side chains claimed to increase the χ-parameter through a polar-apolar driving force and could be interesting for a supposed block copolymer implementation.
Two other new perylene derivatives were introduced utilizing this reliable concept.89 In this case, the electronically active perylene core was modified itself, tuning the absorption properties of the polymers. A poly(perylene diester benzimidazole) (PPDB) and poly(perylene diesterimide) (PPDI) with a blue - respectively, redshifted - absorption with respect to PBIs were successfully synthesized (see Fig. 14).
3.2 Charge Carrier Transport in Polymeric PBIs
As the same parent scaffold polymer was used for “clicking” the different perylene derivatives, the influence on charge-transport properties by modifying the pendant perylene core and the substituents on PBI could be well compared.89 In organic field-effect transistor (OFET) devices, poor performance with high threshold voltages, hysteresis, and low on/off ratios were reported for each material. Electron mobility values of the OFET measurements are summarized in Table 1. Since the charge transport in OFET geometry is determined by a thin channel of charge at the gate–
dielectric interface, the results can be heavily influenced by the interface effects, wetting/dewetting issues, and unfavorable alignment of the polymers within the channel. Thus, the SCLC method is better suited to compare the bulk charge transport properties of these polymers. The SCLC electron mobilities are usually determined by fitting measured J–V characteristics using the Mott–Gurney equation in electron-only SCLC devices.90 The mobility values obtained from the SCLC devices are also mentioned in Table 1.
Table 1 Organic field-effect transistor (OFET) and space charge limited current (SCLC) (obtained from single-carrier devices with an active layer thickness of ca. 350 nm) mobility values for PDEB, PDEI,
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The typical J–V curves of electron-only devices for the four polymers mentioned above are depicted in Fig. 14 together with a schematic of an electron only device. A comparison of the mobilities shows that except for PPDI, all other polymers are good electron-transport materials. The electron mobility in PPDI, 5 x 10-6 cm2/Vs, was two orders of magnitude lower than that of in PPDB, 6 x 10-4 cm2/Vs. Nevertheless, in comparison to PPDB, better electron transport was reported for both PPBI polymers, that is, PPBI 1 and PPBI 2. A direct comparison of PPBI 1 and PPBI 2 showed that not only the core of the π-conjugation system, but also the substituent have an impact on the charge-transport properties of the material. PPBIs with hydrophilic OEG, PPBI 2, showed a major increase of one order of magnitude in electron mobility over PPBI 1 with hydrophobic alky tails. The reported electron mobility in PPBI 2 was 1 x 10-2 cm2/Vs, which is among the highest bulk electron mobility values ever reported for polymers.91,92 However, the X-ray diffraction data suggested a liquid crystalline SmC
structure for PPBI 1, whereas it suggested an amorphous phase for PPBI 2.88,89 Thus, the less-ordered PPBI 2 was surprisingly superior in terms of electron mobility.
3.3 Effect of Polymer Architecture on the Structure of PBIs
From the different types of acceptor polymers introduced above, one, namely an acrylate with PPBIs with hydrophobic alkyl swallow tails, was selected for detailed structural investigations. This type of polymer was used later in the D–A block copolymers. To study the effect of the polymer architecture on structure, we included a low-molecular-weight model compound, PBI, as a reference material (cf. Fig. 15).93 The results of polarized light microscopy had already suggested that there are structural differences between the low-molecular-weight model compound PBI and the corresponding polymer poly(perylene bisimide acrylate) (PPerAcr) (cf. Fig. 15, right).
Through a combination of DSC, optical microscopy, and temperature dependent SAXS/WAXS, it was shown that both compounds display a lamello-columnar packing.
While the PBI crystallizes, the PPerAcr suppresses order, leading to only a 2D lamello-columnar liquid-crystalline phase as schematically shown in Fig. 15. Most likely the reduced order in the polymeric compound is because of the quenched chemical disorder in the atactic polymer.