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Structural Models of Poly(cyclopentadithiophene- alt -benzothiadiazole) with Branched Side Chains: Impact of a Single Fluorine Atom on the Crystal Structure and Polymorphism of a Conjugated Polymer
Author(s)
Date Issued
June 23, 2015
Type
Article
Abstract
This study describes the impact of a single fluorine atom substitution in the conjugated backbone of poly(cyclopentadithiophene-alt-benzothiadiazole) on the structure and the polymorphism of the polymer bearing 2-ethylhexyl side chains. Controlled growth of the nonfluorinated and fluorinated polymers by solvent vapor annealing (SVA), melt-crystallization on oriented poly(tetrafluoroethylene), and higherature rubbing yield highly crystalline and/or oriented films of a thermodynamically stable polymorph called form I. The single-crystal hk0 and the oriented fiber patterns are obtained in films prepared by SVA and aligned films, respectively. Form I involves a high-symmetry packing of four chains in an orthorhombic cell (Pccn space group) with pseudohexagonal symmetry. The chains are paired into dimers with a 3.6-3.8 Å interchain π-stacking distance and a segregated mode of stacking of cyclopentadithiophene and benzothiadiazole (BT). A single H/F substitution on the BT unit modifies the orientation of the dimers in the unit cell of form I. Absorption and structural properties of this new polymorph are discussed with respect to the classical π-stacking structure obtained from solvent-additive processed films. Evidence of yet another polymorph (form II) highlights the polymorphism of these alternated donor-acceptor copolymers.
Citation
Macromolecules, 48(12), 3974-3982, 2015