3.5 NMR Spectra
4.4.3 DFT calculations
The local, global minima of all transition states structures were optimised at the ωB97X-D/6-311+G* level of theory. Starting structures for the transition states were found us-ing the freezus-ing strus-ing method[39] at the same level of theory. Frequency analyses were performed to corroborate the saddle-point nature of the optimised transition states. In-trinsic reaction calculations were carried out in order to determine that the transition states correspond to the desired structures that connect their corresponding local min-ima. All calculations were performed with the Q-Chem 5.0 package.
4
4.5. NMR S
PECTRAFigure 4.819F NMR spectrum (376 MHz, chloroform-d) of compound 1F
-200
Figure 4.919F NMR spectrum (376 MHz, chloroform-d) of compound 2F
4
-200
Figure 4.1019F NMR spectrum (376 MHz, chloroform-d) of compound 3F
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Figure 4.1119F NMR spectrum (376 MHz, chloroform-d) of compound 4F
4
0.0
Figure 4.121H NMR spectrum (400 MHz, chloroform-d) of compound 6F
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Figure 4.1319F NMR spectrum (376 MHz, chloroform-d) of compound 6F
4
0.0
Figure 4.141H NMR spectrum (400 MHz, chloroform-d) of compound 7F
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Figure 4.1519F NMR spectrum (376 MHz, chloroform-d) of compound 7F
4
0.5
Figure 4.161H NMR spectrum (400 MHz, chloroform-d) of compound 8F
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Figure 4.1719F NMR spectrum (376 MHz, chloroform-d) of compound 8F
4
0.0
Figure 4.181H NMR spectrum (400 MHz, chloroform-d) of compound 9F
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Figure 4.1919F NMR spectrum (376 MHz, chloroform-d) of compound 9F
4
0.0
Figure 4.201H NMR spectrum (400 MHz, chloroform-d) of compound 11F
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Figure 4.2119F NMR spectrum (376 MHz, chloroform-d) of compound 11F
4
0.0
Figure 4.221H NMR spectrum (400 MHz, chloroform-d) of compound 12F
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Figure 4.2319F NMR spectrum (376 MHz, chloroform-d) of compound 12F
4
-190
Figure 4.2419F NMR spectrum (376 MHz, chloroform-d) of compound 5F
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f1 (ppm) -101.40 -101.45 -101.50 -101.55
f1 (ppm)
Figure 4.2519F NMR spectrum (376 MHz, chloroform-d) of compound 14F
4
-200
Figure 4.2619F NMR spectrum (376 MHz, chloroform-d) of compound 19F
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Figure 4.2719F NMR spectrum (376 MHz, chloroform-d) of compound 20F
4
B
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5
P URSUING P OLYMERIZATION OF
T HIOPHENES B EARING F LUORINATED S IDE C HAINS
Abstract: In this chapter, we attempt the polymerization of monomers bearing fluorin-ated side chains in order to draw a comparison with the model P3HT polymer (used as a benchmark donor material in BHJ OPV devices). We encountered difficulties introducing the hominal bis(gem-CF2) fragment, so using the ether-activation approach instead, we synthesized 3-(1,1-difluoro-2-methoxyethyl)thiophene, and used it as a starting point to develop monomers for test polymerizations. While the electropolymerization technique succeeded, conventional synthetic polymerization approaches failed. We postulate that attaching a CF2moiety next to the thiophene core leads to the withdrawal of electron dens-ity, which undermines the oxidative addition steps of synthetic polymerization. Possible solutions, such as placing the CF2group further from the core, using dimers or synthesiz-ing co-polymers with the fluorinated monomers, are proposed.
Parts of this chapter were used in MSc thesis of Ellen Kampinga (fse.studenttheses.ub.rug.nl/id/eprint/20283).
I would like to thank Ellen Kampinga for her contribution to this work.
5.1. I
NTRODUCTIONAs discussed in Chapter 1, in pursuit of further enhancement of the PCEs of organic solar cells, the idea of increasing the relative dielectric constant (εr) of organic materi-als is gaining broader attention [1]. This new approach was first proposed by Koster et al.[2], where the focus is drawn to the potential benefit of increasingεrof OPV materials to bridge the gap between the PCEs of organic and inorganic solar cells. In the work by Torabi et al.[3], the introduction of repeating units of ethylene glycol (EG) as side chains of donor polymers and part of acceptor materials resulted in doubling the value of the dielectric constant of resulting bulk organic material without affecting conjugation and other vital properties. Furthemore, in the work by Rousseva et al.[4], the introduction of EG units with increasing lenghts as side-chains on [60]PCBM resulted inεrvalue above 10, which is the first reported instance for the fullerene-based semiconductor mater-ial. This dramatic effect was attributed to the capability of EG chains to rapidly reorient their dipole moments together with the charge redistribution in the environment. The same effect on dielectric constant value can be expected when introducing side chains with ferroelectric properties, as they are also capable of reversible reorientation upon application of an electric field. Furthermore, they possess high dielectric constants, heat capacity, and low thermal loss. Such properties make ferroelectric materials attractive for a wide variety of applications, including thin film capacitors, artificial muscles, and electrocaloric cooling [5,6].
Among organic ferroelectric materials, polyvinylidene difluoride (PVDF) is the most commonly studied due to its unique set of features. Besides ferroelectricity, PVDF is also a flexible and light weight polymer with low thermal conductivity [7–10]. Furthermore, due to the presence of −(C H2− C F2)− repeating unit, it has a compact structure and large permanent dipole, which makes PVDF the polymer with the highest known piezo-and pyroelectric activity [6].
Blending PVDF into copolymers yielded materials that were used for ferroelectric memory devices [11]. Some attempts have also been made to study the effect of PVDF on the performance of OPVs by blending it into conjugated polymer systems [12] or in-corporating it as a thin ferroelectric polymer layer in conventional bulk heterojunction (BHJ) devices [13–15]. Although not fully understood [13], it is widely reported that such ferroelectric doping can enhance charge separation [15] and PCE [14] of OPV devices.
The effects of the backbone fluorination on the properties of organic semiconduct-ing materials are besemiconduct-ing widely studied, includsemiconduct-ing the recent work by Boufflet et al., which focuses on the positive influence of backbone fluorination on the dielectric constant of conjugated polythiophenes [16].
However, to the best of our knowledge, there are no examples of conjugated poly-mers that have PVDF fragment attached as a side chain. Literature sources report the high dielectric constant value of PVDF copolymers [17] and composites [18], along with fluorinated polyethers [19]. Thus, the introduction of the short PVDF-like fragment as a side chain into the conjugated materials might benefit the electronic properties of the polymer without breaking the conjugation pattern.
In our previous work [20] we pursued the synthesis of discrete chains containing a hominal bis(difluoromethyl) fragment (i.e., RCF2CH2CF2R’). As a result, we obtained a tosylate bearing the desired fragment, which was theoretically attachable to small
mo-5
lecules and monomers, introducing strong dipole moments in the 1,3 configuration, that enables their alignment in an electric field (akin to PVDF, where CF2groups reorient in the same direction upon poling in an electric field; see Chapter 3).
In this chapter, we attempted the synthesis and polymerization of the thiophene de-rivatives bearing fluorinated side chains, depicted in Figure5.1.
S
P3bTFT P3aDFOMeT P3aDFOHT P3aDFODEGT P3aDFObTFT
P3aDFOMeBT P3aDFOMeBTBDT PB3aDFOMeHT
Figure 5.1 Target thiophene-derived polymers bearing fluorinated side chains
Such polymers would allow direct comparison with the benchmark polymer, poly-3-hexylthiophene (P3HT), highlighting the effect of highly polarizable PVDF-like side chains on the electrochemical properties of polymers for the potential use in OPV.