Article pubs.acs.org/cm
Tuning Packing and Solubility of Donor (D)−Acceptor (A) Polymers by cis−trans Isomerization within Alkenyl Side Chains Felix Hinkel, Tomasz Marszalek, Wojciech Zajaczkowski, Sreenivasa Reddy Puniredd, Martin Baumgarten, Wojciech Pisula,* and Klaus Müllen* Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *
ABSTRACT: The impact of alkenyl substituents on the behavior of cyclopentadithiophene−benzothiadiazole (CDT− BTZ) donor (D)−acceptor (A) polymers in organic fieldeffect transistors (OFETs) and on the supramolecular organization was investigated. Linear cis- and trans-alkenes were attached to the donor unit of CDT−BTZ polymers to demonstrate the dependence of supramolecular ordering and solubility in organic solvents on chemical conformation. The layer interdigitation of the substituents differed due to shape disparities between cis- and trans-alkenes. While trans-alkenes exhibit zigzag structures that are beneficial for close packing, cis-alkenes are curved and thus possess a less regular shape that is disadvantageous to thin film ordering. This was proven by grazing incidence wide-angle X-ray scattering (GIWAXS) studies, which revealed shorter intermolecular distances for the polymer with trans-alkene substituents even in comparison to analogous polymers with saturated alkyl substituents. Furthermore, the isomerization of the cis-substituents toward their trans-conformers allowed improvement of the polymer crystallinity in thin films and was investigated in transistor devices and solubility studies. are π-conjugated polymers are rigid-rod type macromolecules1 possessing limited solubility in organic solvents. Therefore, alkyl substituents must be attached at the periphery of the polymer backbone2 in order to apply them in solution-processed organic electronics3 such as organic fieldeffect transistors (OFETs)4 and bulk heterojunction (BHJ) solar cells.5 As positive effects, solubilizing groups can promote phase formation6 and supramolecular ordering through selfassembly7 (hard core, soft chain) of the polymeric semiconductor in thin-films and in bulk. In regioregular P3HT,8 for instance, sequential interdigitation of the precisely positioned hexyl substituents leads to lamellar packing and high crystallinity. Regarding commonly employed alkyl substituents, branched alkanes ensure high solubility, whereas long linear ones favor lamellar packing through interdigitation of the aliphatic chains.2b Recent literature has presented a myriad of donor (D)−acceptor (A) polymers whose building blocks are varied and adjusted3c,9 but has often disregarded chemical or stimuli-induced modification of the alkyl substituents. While methods to drastically change the chemical structure of functional side chains (i.e., ketones and esters10) have been well established in organic electronic devices, there has only been limited attention paid toward manipulating alkyl substituents that are still important in tuning thin film morphology. The D−A polymer P1 (Figure 1) consisting of cyclopentadithiophene and benzothiadiazole (CDT−BTZ) revealed a close relation between morphology in thin films and transistor performance. Charge-carrier mobilities of up to 3.3 cm2 V−1 s−1
B
© 2014 American Chemical Society
Figure 1. Chemical structures of CDT−BTZ polymers P1 and targeted polymers P2 and P3.
(Ion/Ioff = 106) have been reported for this polymer in dependence on a high molecular weight (Mn = 35 kg mol−1).11 P1 showed good order and excellent film forming abilities. However, the question of the contribution of the hexadecyl substituent remained unanswered. The steric demand plays a key role, whereby the prevention of backfolding and curvature of the substituents is of high importance. Thus, the first synthesis of D−A polymers (P2 and P3) bearing stereoisomeric alkenyl substituents was accomplished. The zigzag shape of trans-alkenes attached at the periphery of CDT−BTZ (P3) enforces interdigitation of the solubilizing groups in the layer structure and prevents them from backfolding. cisSubstituents in P2, however, are curved and less regularly shaped, allowing no room for interdigitation of the substituents. Received: June 12, 2014 Revised: August 5, 2014 Published: August 5, 2014 4844
dx.doi.org/10.1021/cm5021355 | Chem. Mater. 2014, 26, 4844−4848
Chemistry of Materials
Article
cis-monomer, the alkenyl chain was obtained by a nucleophilic substitution reaction of 1,6-dibromohexane with lithiodec-1-yne followed by hydrogenation catalyzed by Lindlars catalyst. After the alkenyl chain was introduced into the CDT system, the 2positions were finally brominated with NBS at low temperature. The second monomer CDT(trans) was achieved by a pathway consisting of three reactions,13 as the trans-double bond was obtainable by a more convenient metathesis reaction. 1-Bromo8-octene was attached to the CDT unit, and the 2-positions were subsequently brominated. After bromination, the transcarbon−carbon double bond was generated by a metathesis reaction with Grubbs second generation catalyst. All compounds were characterized by 1H and 13C NMR spectroscopy, FD mass spectrometry, and high-resolution electrospray ionization (HR-ESI) mass spectrometry, which were in full agreement with the molecular structures. CDT(cis) and CDT(trans) were employed in the synthesis of D−A polymers with BTZ diboronic ester 8 (Scheme 3)
The impact of the carbon−carbon double bond configuration in the solubilizing groups on the organization was studied for thin films by grazing incidence wide-angle X-ray scattering (GIWAXS), while the charge carrier transport in the corresponding layers was monitored by transistor measurements. By comparing P2 and P3, we were able to correlate parameters such as chain-to-chain and π-stacking distances with OFET performance. Furthermore, cis−trans-isomerization studies of P2 focused on the modification of the thin film organization and subsequent solubility differences elucidated via UV−vis spectroscopy. These phenomena were in full agreement with the hypothesis that altering the configuration in the alkenyl substituent leads to a shape variation toward trans-isomers which is beneficial for the interlayer ordering. It has been reported that irradiation with UV−C light (wavelength of
