3.5 NMR Spectra
5.2.5 Synthesis of P3aDFOMeT
After facing all the challenges throughout the previous polymerization attempts, we de-cided to pursue the synthesis of P3aDFOMeT as a model polymer, in a manner depicted in Scheme5.4.
With a sufficient amount of compound 5 in hand, we performed bromination toward compound 11 using bromine (5 Eq in CHCl3were required to achieve the complete con-version after 3–4 days), as milder reagents (such as NBS or DBDMH) allowed only partial bromination (compound 12 was obtained). Similar to our previous experience, the use of bromine as a halogenation agent led to partial de-etherification of 5, producing side-product 6b along with the desired compound 11. Next, organometallic intermediate 11a was obtained by refluxing a THF solution of 11 with Turbo Grignard. Quenching the reaction sample after 3 hours confirmed successful exchange, as compound 12 was isol-ated.
S THF, reflux, 3 hours
Scheme 5.4 Attempted synthesis of P3aDFOMeT
As was the case for P3aDFODEGT (see Scheme5.3), the exchange occurred in a less conventional 2-position (unlike a more common, and less sterically hindered 5-position). We speculate, that such halide bond position preference has to be influenced by the nature of the fluorinated sidechain. Since the electron density is withdrawn from the thiophene core, this makes it less susceptible to exchange (hence harsh conditions which were needed for bromination). Another possibility is a formation of a cyclic struc-ture incorporating the ether and the magnesium.
Unfortunately, GRIM polymerization did not yield the desired P3aDFOMeT. No pre-cipitation occurred after pouring the reaction mixture on methanol, and analysis of the resulting material by 19F NMR only showed traces of possible oligomers. Again, we suspected that the electron-withdrawing nature of the functional sidechain partially re-moved electron density from the thiophene core, making it electron-deficient and less prone to oxidative addition to the catalyst.
As GRIM polymerization did not show any promising results up to this point, we decided to test Negishi-type polymerization. A similar strategy was used, based on the halogen exchange of one of the bromines of precursor 11 first with Turbo Grignard (to form 11a), followed by the addition of ZnCl2. Quenching the sample of the reaction mix-ture proved the successful exchange at 2-position (compound 12 was formed). Much to our chagrin, after adding the catalyst (Ni(dppp)Cl2) and continuous refluxing of the res-ulting mixture, we did not observe the formation of the desired P3aDFOMeT, but only the monobromide 12.
Another tactic used to make the polymer P3aDFOMeT was to generate the organozinc reagent from 11 directly, omitting Grignard intermediate 11a. To achieve this, we used activated Rieke Zinc. This approach, however, failed again, as after forming the or-ganozinc intermediate, the successful formation of the desired polymer was not achieved.
All the aforementioned failed attempts made us concerned about the overall possib-ility of the polymerization of substrate 11. Since halogen exchange was observed spe-cifically at 2-position, it was proposed that the reactivity of the thiophene core was not fit for polymerization due to the strong electron-withdrawing effect of the pendant group.
As a simple test, we tried a coupling toward compound 13, using Stille cross-coupling.
Performing exchange on 11 with a trimethyltin-thiophene moiety, followed by a catalytic coupling using a Pd(PPh3)4allowed us to observe the formation of compound 13 by19F NMR and by1H NMR (characteristic aromatic peaks). Thus, both sides of the thiophene core of precursor 11 are able to couple to other thiophenes, and it is only the matter of finding a right polymerization conditions.
Following the successful test of Stille coupling conditions, our last synthetic attempt to make polymer P3aDFOMeT involved the formation of intermediate 11a, followed by the addition of trimethyltinchloride, and refluxing of the resulting mixture with Pd(PPh3)4
catalyst. Again, this attempt was in vain, but we believe that a broader, and more in-depth reaction study can lead to finding suitable polymerization conditions.
After gaining little to no success in obtaining P3aDFOMeT using the means of chem-ical polymerization we decided to test a fundamentally different method – electropoly-merization.
Electropolymerization is a process that is facilitated by reducing or oxidizing the monomer molecules in a solution . Polymer growth can proceed via different path-ways (free radical, anionic or cationic, or a combination of both). Direct redox events on the monomer molecule or interaction of another molecule with the reduced/oxidized monomer is usually the initiating step of the polymerization . Such polymers gener-ated in solution are usually very pure since there is only one sort of species present.
In order to conduct electropolymerization, a potential is applied to a solution of the monomer, containing a supportig electrolyte. The potential window ranges from (some-times) negative or zero toward positive value, and is swept back and forth for around 50 cycles. This leads to subsequent reduction and oxidation of the species in solution.
Throughout the polymerization process, there is a shift to more negative/positive poten-tial. This shift can be seen in Figure5.4, as well as the oxidation and reduction potential peaks [27,29].
After electropolymerization is done, the electrode with the polymer film is rinsed and put into a new vial of electrolyte solution, where no free monomer is present. Then, a cyclic voltammogram (CV) is recorded from the polymer, allowing the reduction and oxidation potentials to be determined, and the energy levels calculated [27,30]. After determining the onset slope of the oxidation potential one can estimate the energy of the HOMO using Equation5.1.
E (HOMO) = −e[Eonsetox + 4.4] (5.1)
In Equation5.1the 4.4 eV value is added, as it is the potential of the reference elec-trode Ag/AgCl (value can be changed according to the reference used) . The energy of the LUMO can be determined by adding the HOMO value to the optical band gap measured by UV-Vis. .
In the case of compound 5, electropolymerization was performed with a 100 mL of 0.1 M acetonitrile solution of TBAPF6as the supporting electrolyte. Glassy carbon was used as the working electrode, platinum as the counter-electrode, and Ag/AgCl as the reference electrode. After dissolving 3.5 mg of compound 5 in 5 mL of electrolyte solu-tion, 50 cycles between 0 and 1.7 V with a rate of 0.01 V/s were conducted. The CV of this process is depicted in Figure5.4.
Figure 5.4 Cyclic voltammogram of electropolymerization performed on compound 5, showing current re-sponse vs. applied potential referenced to an Ag/AgCl reference electrode
CV measurement of the resulting P3aDFOMeT polymer film is depicted in Figure5.5.
Figure 5.5 Cyclic voltammogram of generated P3aDFOMeT, showing current response vs. applied potential referenced to an Ag/AgCl reference electrode
As seen from this graph, an oxidation potential value of 0.643 V was determined from the peak onset. Using equation5.1, we calculated a HOMO energy of -5.04 eV for the resulting polymer film, which is lower compared to the -4.7 eV value commonly reported
for P3HT . The optical bandgap was not measured for the resulting material, so, unfortunately, the LUMO level cannot be estimated.
Despite the moderate success achieved using the electropolymerization technique, the synthesis of P3aDFOMeT remained as an open challenge, which we again attribute to the electron-withdrawing effect of the pendant chain. An interesting possibility for the future studies would be designing the monomer with the CF2group not connected to the thiophene core directly, but rather moved further down the pendant chain.