C. David Heinricha, Sinem Tuncel Kostakoğlua,b and Mukundan Thelakkat*a
a Applied Functional Polymers, Macromolecular Chemistry I and Bavarian Polymer Institute, University of Bayreuth, Universitätsstr. 30, 95440 Bayreuth, Germany.
b Department of Chemistry, Gebze Technical University, 41400 Gebze, Turkey.
*E-mail of corresponding author: mukundan.thelakkat@uni-bayreuth.de
Published in Journal of Material Chemisty C,
Reproduced with permission from J. Mater. Chem. C, 2017, 5, 6259-6268.
Copyright 2017 The Royal Society of Chemistry
Densely Grafted Liquid Crystalline Copper Phthalocyanine Side Chain Polymer
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Abstract
A new polymeric donor material with pendant copper phthalocyanine side chains (PCuPc) was synthesized. The excellent suitability of a grafting-to approach, combining the controlled radical polymerization of propargyloxystyrene followed by “click”
chemistry, was demonstrated. A polymer with a high molecular weight (Mn = 88000 g mol-1) and a narrow distribution Đ = 1.20 was synthesized. FTIR and high resolution MALDI-ToF MS of PCuPc points towards quantitative grafting. The PCuPC is soluble in most of the commonly used solvents such as ethyl acetate, THF and acetone. The absorption behavior and electronic structure was investigated via UV-Vis spectroscopy and cyclic voltammetry. The thermal behavior could be elucidated via Flash-DSC and liquid crystalline behavior could be observed and confirmed via XRD and polarization microscopy. The bulk transport was determined by the SCLC method.
Introduction
The combination of controlled radical polymerization (CRP) and “click”-chemistry has been proved to be a highly versatile and robust way for the synthesis of novel highly defined polymeric materials.1 Our group successfully applied this strategy in the synthesis of electronically active materials designed for applications in organic electronics and organic solar cells. We reported pendant side chain polymers,2 block-copolymers3 and defined brush polymers.4 During our work we came to appreciate the comparability and versatility that arises from the modularity and high yield of grafting of this concept.4-6 The first step for the design of those materials is the synthesis of backbone polymers. These can be derivatives of standard thermoplastic polymers like polystyrene (PS) or polymethylmethacrylate (PMMA) which are accessible for the well-established types of CRP. Three methods, namely atom transfer radical polymerization (ATRP),7 reversible addition fragmentation chain transfer polymerization (RAFT)8 and nitroxide mediated radical polymerization (NMRP)9 are commonly used to control radical polymerizations. All three have in common that narrow distributions and a control over the molecular weight can be obtained by reducing the concentration of
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active radical species during the poly-merization.10 By this, the amount of termination reactions, which would otherwise lead to ill-defined polymers with broad distributions, is drastically reduced. We chose NMRP for our study as it is well established for styrene derivatives and the synthesis of propargyloxystyrene has been previously opti-mized.5 Propargyloxystyrene, a styrene derivative with alkyne functionality in para-position of the styrene, can be coupled to organic azides via the copper-catalyzed azide-alkyne cyclo-addition (CuAAC). Since the concept “click-chemistry” was proposed in 200211 it has been widely used not only in biochemistry but also materials science.12 The CuAAC, the copper catalyzed version13 of the Huisgen-reaction,14 is its most common representative and was instrumental in the recognition and spreading of the “click”
concept. The extra-ordinary efficiency of this reaction, even at low temperature, makes it an excellent choice for polymer analogous reactions with high sterical demand.4,12
Phthalocyanines (Pcs) are a long known class of macrocyclic compounds.15 Metal complexes of Pcs like the insoluble CuPc, also known as phthalocyanine blue, are widely used as blue or green pigments.16 The first reports on the synthesis of Pcs were published in the early 20th century15 and the semiconducting behavior has been known since 1948.17 In 1996 Pcs were used for the first time in organic photovoltaics (OPV) as the donor material in combination with fullerene as acceptor.18 Pcs were first used in vacuum deposited devices and optimized systems have reached power conversion efficiencies (PCEs) higher than 4 %.19 With solution-processible Pcs, which are desirable due the lower costs in device preparation by coating and printing methods, comparable PCEs could be achieved.20 Very recently, de Oliveira et al. reported on water-gated phthalocyanine transitors showing that Pcs may also be interesting materials in bio electronics.21
One great benefit of using a polymeric semiconductor material is the superior film forming ability of polymers in comparison to small molecules. The synthesis of Pc semiconductor polymers carrying Pc as side chains via a grafting-to process utilizing CuAAC is therefore an attractive way to obtain such materials. Earlier, two materials synthesized in this way were reported, but only low grafting densities were targeted and thus obtained.22 A third report by López-Duarte et al. is the only one that claims a
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densely grafted polymer.23 There are indications of the successful grafting (IR- and UV-Vis spectroscopy) but neither the polymer characteristic (degree of polymerization and distribution) nor the structure formation were investigated. The ring opening metathesis polymerization (ROMP) of norbornene is a competing strategy towards defined functional polymers. The ROMP of functional norbornene monomers is a representative for a grafting-through process which inherently leads to densely grafted polymers.24 A first report by Kimura et al. showed the shortcomings of this method though, as only oligomers could be synthesized.25 Escoursa et al. could achieve higher molecular weights but were limited by the low solubility of the resulting polymers and similar to the first report only poor control was achieved which resulted in a rather high dispersity of the polymers (Đ > 2).26 The solubility of Pcs can be tuned by the substitution of the PC-core with different side-groups. Both symmetrical and asymmetrical PCs, substituted with oligo ethylene glycol substituents, were reported.27 Our goal therefore was to establish a way to synthesize Pc side chain polymers with high grafting density, high solubility as well as a defined molecular weight.
Here we report the synthesis and characterization of a highly soluble, hydrophilic semiconductor using grafting-to of an unsymmetrical copper phthalocyanine derivative with azide functionality (CuPc-N3) to a styrene backbone. The hydrophilic swallow tail oligo ethylene glycol substituents allow the processing of the material from eco-friendly solvents such as THF, ethyl acetate etc. Additionally, this material is an interesting candidate for applications in bioelectronics.
Densely Grafted Liquid Crystalline Copper Phthalocyanine Side Chain Polymer