Azadipyrromethene Synthesis Essay

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  • Synthesis

    The synthesis of ADP was carried out according to literature procedures [9,33]. The ADP-analogs with fluorine at the proximal or distal phenyl positions (L1-ADP and L2-ADP) were synthesized in a similar fashion with the respective fluorinated chalcones (Scheme 1). In this case, the nitro intermediates of the fluorinated chalcones could not be isolated as a solid powder, so the synthesis was carried forward assuming complete conversion from the first reaction.

    To install phenylacetylene groups, iodination of the ADP derivatives was done according to literature procedures and purified by washing with chloroform in good yield [5,33,34]. Stille cross-coupling with the appropriate tributyltin-phenylacetylene analogs afforded the WS3 derivatives in good yield (Scheme 1). We chose to utilize Stille coupling instead of Sonogashira coupling because we had previously found that this method gives higher yields for installing phenylethynyl pyrrolic substituents [9]. The fluorinated tributyltinphenylacetylene analogs for the synthesis of L3 and L4 were synthesized according to literature procedures and used without purification [35,36]. The Stille cross coupling reactions for the synthesis of L3 and L4 were monitored by MALDI–TOF–MS and were found to not be complete after increasing the reaction time to 48 h, so the reaction time was increased to 96 h with the addition of more catalyst and tributyltin reactant after 48 h. These modifications were deemed necessary to push the reaction towards completion and aid in purification of the free ligand. The free ligands were isolated from the crude mixture by rotary evaporation and purified by trituration with cold methanol and the remaining solid was washed with cold ether. Due to the lowered solubility of the iodinated ADP derivatives and the free ligands in organic solvents, the identity of these compounds was confirmed only by MADLI–TOF–MS. These modifications allowed for the synthesis of all fluorinated WS3 derivatives in good yield with sufficient purity for chelation.

    The BF2+ chelation was carried out according to the literature procedures in moderate yields (Scheme 2) [2,5]. For the zinc(II) chelation, the reaction was changed from a reaction using Zn(OAc)2 to a 2-step, one pot reaction with sodium hydride in tetrahydrofuran, followed by the addition of zinc(II) chloride. Zinc(II) and BF2+ chelates were purified by silica gel column chromatography to isolate the chelates as blue solids and the identity and purity was confirmed by NMR spectroscopy, MALDI–TOF–MS and elemental analysis. In the case of L2, the pure BF2+ chelate could not be isolated by column chromatography, and will therefore be omitted from further analysis.

    The thermal stability of the zinc(II) complexes was examined by thermal gravity analysis and the results are shown in Figure 3. The fluorinated complexes had weight loss profiles similar to each other with a 5% loss between 438 °C and 474 °C, all lower than that of the unfluorinated Zn(WS3)2 at 517 °C.

    Optical properties

    Optical studies for the zinc(II) and BF2+ complexes were performed in chloroform solutions and with spun-cast films on microscope slides. Like the solid powders, all of the solutions and films were dark blue. Solution and film optical properties for zinc(II) and BF2+ chelates are summarized in Table 1. The molar absorptivity spectra in chloroform solutions for zinc(II) and BF2+ chelates are reported in Figure 4a and Figure 4b, respectively.

    The absorption spectra of the zinc(II) chelates are all similar. In most cases, the λmax and λonset values remain consistent with Zn(WS3)2 around 670 nm and 755 nm, respectively. An exception is Zn(L4)2, with λmax and λonset blue-shifted by 10 nm compared to Zn(WS3)2. This hypsochromic shift is consistent with other cases where a highly polarized CF3 group is added para to the conjugated structure [13,37]. Regardless of fluorination, the extinction coefficients are all near 100 × 103 M−1cm−1, showing the strong absorption properties of the WS3-core.

    Solutions of BF2+ chelates show a consistent trend compared with the solutions of zinc(II) chelates. The λmax and λonset values of BF2(WS3), BF2(L1), and BF2(L3) are ≈730 nm and ≈780 nm, respectively. Consistent with that of the zinc(II) chelate solutions, λmax of BF2(L4) is 15 nm blue shifted compared to BF2(WS3), showing a slightly greater effect from CF3 in the BF2+ chelate. Molar absorptivities of the compounds vary from 49 × 103 M−1cm−1 for BF2(WS3) to 66 × 103 M−1cm−1 for BF2(L4).

    Films of the zinc(II) and BF2+ chelates were made in order to better understand the optical properties of the materials in devices, and the properties are summarized in Table 1. Normalized absorption spectra of zinc(II) and BF2+ chelate films are reported in Figure 5a and Figure 5b, respectively. Following the same trend as the zinc(II) chelate solutions, the zinc(II) chelate films exhibited consistent λmax values of ≈695 nm, excluding Zn(L4)2 at 676 nm. The λonset values for films of Zn(L1)2, Zn(L2)2, Zn(L3)2, and Zn(L4)2 are 785, 780, 778, and 769 nm, respectively, all blue shifted compared to λonset of Zn(WS3)2 at 791 nm. The shapes of all the curves remain consistent with a broad absorption from 500 to 800 nm, good for OPV applications.

    All BF2+ chelate films exhibit broadening compared to solutions, with an absorbance ranging from 450 to 900 nm. The λmax values for BF2(WS3) and BF2(L1) are similar at 759 and 755 nm, respectively. The onset values for the two films are also similar at 835 and 829 nm, respectively. Films of BF2(L3) and BF2(L4) show marked differences from two different fluorine modifications at the same position. In BF2(L3), the λmax and λonset are recorded at 770 and 868 nm, respectively. Compared to BF2(WS3), there is a small 11 nm bathochromic shift of λmax. For BF2(L4), λmax and λonse are observed at 669 and 800 nm, respectively. Compared to λmax of BF2(WS3), BF2(L4) exhibits a large hypsochromic shift of 33 nm.

    Electrochemistry

    Cyclic voltammetry of the zinc(II) and BF2+ chelates was studied in dichloromethane solutions using ferrocene/ferricinium (Fc/Fc+) as an internal reference. The electrochemical properties of the complexes are summarized in Table 2 with the voltammograms of the zinc(II) and BF2+ chelates shown in Figure 6 and Figure 7, respectively. For all zinc(II) complexes, cyclic voltammograms reveal two reversible oxidations, while an irreversible oxidation occurs for all BF2+ chelates. The first oxidation potentials (E1/2 ox.) of the fluorinated zinc(II) chelates were higher than that of Zn(WS3)2 (0.50 V) by at least 0.04 V, with the highest value being 0.61 V. The increased E1/2 ox. values are consistent with the increased oxidative stability afforded by the addition of fluorine [13]. The second oxidation potential for all zinc(II) chelates, 0.77–0.79 V, showed little change except in two cases: Zn(L4)2 and Zn(L2)2 had second oxidation potentials of 0.84 V and 0.73 V, respectively. The differences between the first and second oxidation potential was 0.27 V for Zn(WS3)2, while it was 0.18 V and 0.19 V for Zn(L1)2 and Zn(L2)2, respectively. For both Zn(L3)2 and Zn(L4)2 the difference between oxidation potentials was 0.23 V, a slight decrease from Zn(WS3)2 at 0.27 V. All of the fluorinated zinc(II) complexes exhibit a rise of the first oxidation potential as well as a decrease between the first and second oxidation potentials, compared to Zn(WS3)2.

    Cyclic voltammograms of both the zinc(II) and BF2+ complexes showed two reversible reduction potentials. The reduction potentials (E1/2 red.) of Zn(L2)2 and Zn(L3)2 were similar to that of Zn(WS3)2, suggesting that the addition of one fluorine atom at the pyrrolic phenylacetylene or distal phenyl position does not stabilize the anion. On the other hand, there is significant increase of the reduction potentials going from −1.25 V to −1.16 V and −1.15 V for Zn(WS3)2, Zn(L1)2 and Zn(L4)2, respectively. This suggests that fluorine stabilizes the anion when at the proximal position or when a CF3 group is installed at the pyrrolic phenylacetylene moiety. The difference between the first oxidation and first reduction potentials of all the zinc(II) complexes are similar, 1.75 V to 1.79 V. This means that in the cases of Zn(L1)2 and Zn(L4)2, fluorine has a similar stabilizing effect on both the cation and anion.

    The differences between the first and second reduction potentials of each compound were similar, indicating that fluorine influences both reductions equally. For the fluorinated BF2+ complexes, the first and second reduction potentials were slightly more positive than those of BF2(WS3). The E1/2 red. values ranged from −0.79 V for BF2(WS3) to −0.69 V for BF2(L4). All the first reduction potentials of the fluorinated BF2+ complexes were −0.70 V with the difference between first and second E1/2 values being 0.77 V. Collectively, the fluorine-modified WS3 chelates showed higher oxidation potentials than those of unmodified chelates. The interpretation of electrochemical data shows that the addition of fluorine had the greatest effect on L1 and L4 chelates, while having minimal effects on the L2 and L3 chelates. The estimated HOMO and LUMO energy levels obtained from cyclic voltammetry are shown in Figure 8. The HOMO and LUMO of Zn(L1)2 and Zn(L4)2 approach those of PCBM.

    Crystallography

    Single crystals of the zinc(II) chelates were grown in order to better understand the structure of the materials. Only Zn(L2)2 produced crystals suitable for analysis. Figure 9 shows the ORTEP drawing of Zn(L2)2 with 50% ellipsoids and a partial labeling scheme for clarity. The crystal structure confirms the identity of the complex and gives an idea of the interactions in the complex. Like Zn(ADP)2, the structure is distorted tetrahedral with favorable π–π stacking distances between the proximal phenyl and pyrrole rings of the two separate ligands (Figure 10a and 10b). The distance between centroids is 3.56 Å for Zn(L2)2, compared to 3.63 Å for Zn(ADP)2[38]. The shorter distance found for Zn(L2)2 suggests a stronger interaction between the proximal phenyl and pyrrole rings than in Zn(ADP)2. Unfortunately, it cannot be determined whether the addition of fluorine or phenylacetylene contributed to the shorter π–π stacking distances without a crystal structure for Zn(WS3)2. Intermolecular favorable π–π stacking distances are observed between the pyrrolic phenylacetylene arms of two chelates seen on the outside of the unit cell, as well as between the distal phenyl rings of two chelates. Due to the crowded packing and difficulty in obtaining a clear image to convey these observations, the authors invite the reader to observe the intermolecular packing on their own using the cif file provided as Supporting Information File 2.

    Preliminary results in OPVs

    To test the potential of the new fluorinated zinc(II) complexes as electron acceptor, we fabricated bulk heterojunction OPVs in the inverted configuration using P3HT as the electron donor. The best results obtained so far are reported in Table 3. For comparison, we also included results for a typical P3HT:PCBM solar cell. First, we note that best PCEs for Zn(WS3)2 are lower than in our previous publication, 2.36% instead of 4.10% [10]. The main difference is that we now get lower Jsc, 5.2 mA/cm2 instead of 9.1 mA/cm2. While our previously reported results were reproducible at the time, we are no longer able to reproduce them with the new Zn(WS3)2 batches, even after extensive purification. We have therefore decided to report the results we now routinely obtain because they are obtained under similar conditions as the new results with the fluorinated compounds.

    Zn(L2–L4)2 showed an increase in PCE compared to Zn(WS3)2, due to an increase in JSC. This points to a generally positive effect of fluorination on device performance. The current best performance of 3.74% was obtained with Zn(L3)2 with an open circuit voltage (Voc) of 0.73V, a short-circuit current density (Jsc) of 8.54 mA/cm2 and a fill factor (FF) of 60%. While maintaining similar Voc and FF values, Zn(L2)2

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