Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Copolymers of Diketopyrrolopyrrole and Benzothiadiazole: Design and Function from Simulations with Experimental Support Deyan Raychev,†,§ Rene ́ Daniel Meń dez Loṕ ez,‡,§ Anton Kiriy,‡ Gotthard Seifert,§,∥ Jens-Uwe Sommer,†,§,⊥ and Olga Guskova*,†,§ Institute Theory of Polymers and ‡Institute of Macromolecular Chemistry, Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, 01069 Dresden, Germany § Dresden Center for Computational Materials Science (DCMS) and ∥Theoretical Chemistry, Technische Universität Dresden, 01062 Dresden, Germany ⊥ Institute of Theoretical Physics, Technische Universität Dresden, Zellescher Weg 17, 01069 Dresden, Germany Macromolecules Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/19/19. For personal use only.
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S Supporting Information *
ABSTRACT: Alternating block copolymers consisting of diketopyrrolopyrrole and benzothiadiazole electron acceptor units linked together via aromatic five-membered donor heterocycles are studied using a combination of computer simulation techniques and experiments. Four copolymers are modeled starting from their monomers to stacked macromolecules: with two different linkersthiophene or furan, connecting electron-withdrawing core unitsand two different alkyl substituents at lactam nitrogens of diketopyrrolopyrrolelinear dodecyl and branched 2-octyldodecyl chains. In our experiments, we aim at characterization of the optical and electrochemical properties of two copolymers with branched side chains differing in the linker, since as the literature survey shows the data published on these copolymers are very sparse. These properties can be easily interpreted and later compared with theoretical predictions. The results of simulations supported by experiments show that monomers of these polymers have very similar electronic and optical properties, and the main difference between them consists in various chain curvature defined by the linker. More curved furan-containing monomers and more stretched thiophene-linked molecules are characterized by different energetics of the stack formation and diverse in charge carrier mobilities. The branching of the side chains affects the planarity of the macromolecules, leads to longer π−π stacking distance and lamellar interval in the ordered arrays of polymers, and defines the stacking patterns of the conjugated backbones. The ambipolar transport is predicted for the majority of considered copolymer morphologies, and a quantitatively satisfactory agreement between experiment and computation is achieved.
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INTRODUCTION
in the fabrication of single-component ambipolar OFETs is the narrow energy gap (the HOMO−LUMO gap 3.3 Å, the Ebind for Th molecules are below the values for Fu molecules, meaning that stacks of the first compound are more stable at larger d distances. In antiparallel stacks, the binding of the molecules is energetically favorable at all distances (except d < 3.1 Å for the Th molecule). The minima of the curves are more pronounced at 3.3 and 3.5 Å for Fu and Th stacks, respectively. The prediction of d = 3.5 Å for Th stacks coincides with experimentally measured distances for analogous polymers: 3.65 Å11 and 3.73 Å.7 For Fu-based polymers, however, the experiments give longer d distances of 4.20−4.66 Å,9 which can be explained by tilting molecules in a molecular stack, or by the effects coming from the side chains, or as pointed out by Müllen et al.46 due to higher curvature of the polymer influencing the π-stacking distance and increasing the interlayer distance. Specified d distances are utilized for the computations of the binding energies in stacks with transverse and longitudinal shifts (Figure 2c−f). The parallel configuration of two molecules in the molecular stack (Figure 2c,d) gives similar patterns with two minima for Fu- and Th-linked molecules. The k, l coordinates for the minimum points together with the illustrations of mutual molecular orientation are given in Figures S7 and S8. First, in parallel stacks the molecules
(1)
where λi and V are the inner reorganization energy and electronic coupling, respectively, obtained as aforementioned. The hopping mobilities μ are defined according to the Einstein−Smoluchowski equation (eq 2) for 1D transport in the direction of π-stacks: μ=
edh 2K CT 2kBT
(2)
where e is the elementary charge and dh is the hopping distance, which in our case is set as the π-stacking distance for the stacks of copolymer backbones. The values of V, KCT, and μ for polymers are presented as an average over the productive MD runs for preliminary annealed systems. The purpose of an annealing strategy is to find the different local minimum-energy structures for S and N-S starting configurations. Materials, Synthesis, and Experimental Characterization. Although the synthesis of Fu- and Th-containing polymers has been described in several papers,7−9,11 we briefly present the schemes for the synthetic routes and give an overview of the methods used and the measurements in the Supporting Information. The characterization of polymers, more precisely, the data from cyclic voltammetry (CV, electrochemical band gaps) and the absorption spectroscopy (optical band gaps and the UV−vis spectra) compared with both theoretical predictions and the data available in the literature, will be discussed in the next sections.
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RESULTS AND DISCUSSION Theoretical Approach to “Monomers”. The “monomers” in the ground state (Figure S2) represent the highly planar structures with dihedral angles between the aromatic rings equal to 180° independently of the linker. As a consequence, a complete delocalization of the electronic density along the conjugated backbone, as follows from the illustrations in Figure S4, is observed. The extensive delocalization of molecular orbitals over three to four monomeric units of copolymers is seen in Figure S5. For longer chains, the planarity is preserved with the deviation of
