Tailor-made synthesis and structure-property relationship of several swallow-tail N-substituted perylene bisimide (PBI) dyes are presented. PBI derivatives were synthesized by two distinct synthetic approaches, the details being evaluated herein. All the PBIs carry either alkyl swallow-tail or oligoethylenglycolether (OEG) swallow-tail moieties as N-substituents, and many of them are unsymmetrically substituted. We avoided substitution at bay positions of the perylene core to maintain the planarity and strong pi-pi interactions, which favor intermolecular order and charge carrier transport. The thermotropic behavior, which is strongly influenced by the nature of the substituents was investigated using differential scanning calorimetry (DSC), polarization optical microscopy (POM), and X-ray diffraction measurements (XRD). The introduction of OEG swallow-tail units facilitates thermotropic liquid crystalline behavior in most cases and the unsymmetrical substitution allowed the tuning of the mesophase-width. The mesophases exhibit characteristic columnar hexagonal (Col(h)) packing arising from pi-pi interactions between cofacially orientated perylene molecules. Thus, the inherent tendency of PBI molecules for crystallization could be effectively suppressed by incorporating flexible OEG swallow-tail units only at imide positions. This molecular design was crucial to obtain liquid crystallinity and intracolumnar long-range order. The substituents did not influence the electronic energy levels such as HOMO and LUMO.

Efficient combination of two polymerization reactions allowed various complex issues in photovoltaic devices, such as light absorption, the presence of a donor–acceptor heterojunction, photoluminescence quenching, crystallinity, and microphase separation, to all be addressed in a single block copolymer (see picture).

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Microphase-separated donor–acceptor block copolymers have been discussed as ideal systems for morphology control in organic photovoltaics. Typical microphases as known from coil–coil systems were not observed in such systems due to crystallization dominating over microphase separation. We show how this problem can be overcome by the synthesis of high molecular weight block copolymers leading to a high enough χN parameter and microphase separation in the melt. A combination of copper-catalyzed azide-alkyne click reaction and nitroxide mediated radical polymerization (NMRP) was used for the synthesis of donor–acceptor poly(3-hexylthiophene)-block-poly perylene bisimide acrylate (P3HT-b-PPerAcr) block copolymers. With this synthetic strategy, high molecular weights are possible and no triblock copolymer byproducts are formed, as observed with former methods. Two different block copolymers with a high molecular weight P3HT block of 19.7 kg/mol and a PPerAcr content of 47 and 64 wt % were obtained. X-ray scattering measurements show that the diblock copolymers exhibit microphase separation in the melt state. Furthermore, upon cooling confined crystallization occurs inside the microphase separated domains without destroying the microphase order. The observed microstructures fit well to the respective volume fractions and the crystalline packing within the individual blocks is analogous to those in the respective homopolymers. For the first time, typical lamellar or cylindrical phase separated structures as known for amorphous coil–coil systems are realized for a crystalline–liquid crystalline, donor–acceptor block copolymer. A similar block copolymer synthesized with an earlier method exhibits a crystallization-induced microphase separation.

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