Crystalline-crystalline donor-acceptor block copolymers.

One of the challenging issues in designing new materials for organic electronics, such as photovoltaics or field-effect transistors, is that film-forming properties must be combined with functional and well-defined nanostructured morphologies to fulfill the complex requirements of light absorption, charge separation, and charge transport in confined geometries. Morphological control on the nanoscale is also required to tune the interface between the functional domains and to ensure long-term stability of such devices. Block copolymers can meet these requirements, as they undergo phase segregation into versatile equilibrium microdomains such as cylinders, gyroids, or lamellae, thus offering the possibility to control the type, size, and orientation of microstructure in the device. We have demonstrated the chain of control on all length scales—from molecular through mesoscopic to macroscopic—using the principle of selfassembly of donor–acceptor block copolymers for photovoltaic (PV) applications. A polymerizable perylene bisimide derivative was used as electron acceptor and crystalline microdomains in an amorphous poly(triaryl amine) matrix (donor) were observed as a result of strong p–p interactions between adjacent perylene bisimide moieties. Block copolymers that contain conjugated donor segments and electron-acceptor segments have also been presented, but microphase separation was not reported. The design of the Grignard metathesis polymerization (GRIM) of poly(3-hexylthiophene), P3HT, and the in situ introduction of defined end groups has stimulated the synthesis of P3HT block copolymers. Recently, nitroxidemediated radical polymerization (NMRP) starting from P3HT macroinitiators was demonstrated. Herein we report on the first synthesis of crystalline– crystalline donor–acceptor block copolymers with P3HT as donor block and poly(perylene bisimide acrylate) (PPerAcr) as acceptor block (Figure 1). The crystallinity of both blocks not only can give rise to rich phase behavior in the bulk of these materials, but is also advantageous for charge-carrier mobility in the domains. We show that all important prerequisites for PV applications such as a high optical density, photoluminescence quenching in film, and microphase separation can be achieved in one molecule at the same time. A set of donor–acceptor block copolymers poly(3-hexylthiophene)-block-poly(perylene bisimide acrylate) (P3HTblock-PPerAcr) was prepared in only two steps (Scheme 1). In the first step, we used a modified procedure of Yokozawa et al. starting from the thiophene derivative 1. In the last stage of polymerization, quenching of the active P3HT chain end in situ with 2 yields well-defined and narrowly distributed macroinitiator P3HT-I (3 ; polydispersity index (PDI) 1.1, Mn,SEC = 8900 gmol ). The introduction of 2 at the end of the P3HT block was verified by H NMR spectroscopy, which showed 84% of the chains to be functionalized. However, this is outweighed by the simple and straightforward one-pot procedure for 3. Nonfunctionalized P3HT and the resulting block copolymers with perylene bisimide acrylate can be separated in a later purification process which is necessary anyway. In the second step, 3 was used to polymerize perylene bisimide acrylate (PerAcr, 4). To obtain a series of block copolymers with different segment lengths of 4, the reaction time and [3]/[4] ratio were varied. All other reaction parameters, including the solvent, 0.2 equivalents TIPNO (5), and 5 mol% styrene (with respect to 4), were kept constant. Similar reaction conditions were already applied in earlier polymerizations of PerAcr. Addition of a small amount of styrene as comonomer results in improved control of the polymerization of PerAcr, whereby selfinitiation of styrene leading to possible homopolymerization of PerAcr 4 is not observed. Also, incorporation of a few styrene units into the chain does not affect the chargetransport properties of PPerAcr negatively. In this manner, Figure 1. Poly(3-hexylthiophene)-block-poly(perylene bisimide acrylate), P3HT-block-PPerAcr. The first block consists of a rigid-rod poly(3hexylthiophene) (blue); the second segment is built up of a flexible polyacrylate backbone with pendant side chains of crystalline perylene bisimides (red).