Complexation of Fullerenes by Subphthalocyanine Dimers

Letter Cite This: Org. Lett. 2018, 20, 5821−5825

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Complexation of Fullerenes by Subphthalocyanine Dimers Henrik Gotfredsen, Thomas Holmstrøm, Alberto Viñas Muñoz, Freja Eilsø Storm, Christian G. Tortzen, Anders Kadziola, Kurt V. Mikkelsen, Ole Hammerich, and Mogens Brøndsted Nielsen* Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark

Org. Lett. 2018.20:5821-5825. Downloaded from pubs.acs.org by WASHINGTON UNIV on 09/21/18. For personal use only.

S Supporting Information *

ABSTRACT: Tweezer-like molecules comprised of two boron subphthalocyanine (SubPc) units were prepared by Sonogashira couplings and investigated using NMR spectroscopy for their ability to bind fullerenes (C60 and C70). The preorganization of the tweezers provided association constants of ca. 103 M−1 in toluene-d8, while a SubPc monomer did not show any association. Nevertheless, the SubPc monomer crystallized with the fullerenes as 2:1 complexes, supporting the favorable tweezer-like design for complexation, which was further corroborated by computations.

B

oron subphthalocyanines (SubPcs) are cone-shaped macrocycles comprised of three isoindole units bridged by aza-linkages and with a central sp3-hybridized boron atom containing an axial substituent. They were first reported by Meller and Ossko in 1974.1 On account of their optical and redox properties, they are potential light-harvesting or electron acceptor candidates in organic photovoltaics.2 In this field, fullerenes have played a prominent role as electron acceptors,3 and in some photovoltaic cells, SubPc and C60 have successfully been combined.2a With their unique curvature, SubPcs are interesting as host sites for fullerenes and hence for guiding self-assembly of functional units. Association between SubPc derivatives and C60 has been observed in the solid state by X-ray crystallographic analysis,4 and calculational studies reveal an excellent fit between the concave SubPc surface and C60.5 UV−vis absorption and fluorescence studies in solution have suggested association constants of 103−104 M−1 in toluene.6 Thus, SubPc 1 shown in Figure 1 (together with its X-ray crystal structure, obtained in this work, but also previously reported7) was reported to form a 1:1 complex with either C60 or C70 in toluene based on a Job plot analysis.6b This analysis was based on quite small changes in the absorbance at wavelengths of 340 or 386 nm. To our knowledge, a charge-transfer absorption red-shifted relative to the longest-wavelength absorption of SubPc has only been identified in the solid state (broad shoulder at 750 nm).4 By introducing electron-donating alkylthio chains at the periphery of SubPc, 2:1 SubPc·fullerene complexes were observed with enhanced binding of C60 and in particular of C70.6b Thus, a derivative with SBu chains was found to exhibit first and second association constants of 1.9 × 105 M−1 for binding of C60 in toluene (1 order of magnitude larger than the © 2018 American Chemical Society

Figure 1. Left: Boron subphthalocyanine (SubPc 1). Right: Molecular structure of 1 according to X-ray crystallography. Crystals were grown from toluene/heptane; solvent molecules were omitted for clarity. Ellipsoids displayed with 50% probability level. CCDC 1856086.

value reported for the 1:1 complex with 1) and first and second association constants of 1.1 × 104 M−1 for binding of C70 (2 orders of magnitude larger than the 1:1 complex with 1). The enhancement of binding by peripheral electron donors was also shown by introducing phenylthio substituents at the periphery;8 yet, this work suggested relatively weak interaction energies between unsubstituted SubPc and fullerenes based on mass spectrometry experiments. Herein we unequivocally show from NMR spectroscopic studies that UV−vis absorption/ fluorescence studies overestimate the fullerene binding affinity of SubPcs, a fact that may be of more general concern when using these methods, and that two units of 1 preorganized in a tweezer-like structure are needed for complexation. As the solution-state UV−vis absorption studies rely on very small absorbance changes at a wavelength where SubPc and also C60 absorb, we reasoned that NMR spectroscopy could be Received: August 7, 2018 Published: September 13, 2018 5821

DOI: 10.1021/acs.orglett.8b02518 Org. Lett. 2018, 20, 5821−5825

Letter

Organic Letters a more reliable method of quantifying complex formation. Nevertheless, we find that adding up to 10 equiv of C60 or C70 to a 10−4 M solution of 1 in toluene-d8 resulted in no changes in the SubPc chemical shifts (see Supporting Information (SI), Figure S29). We also varied concentrations to allow for Job plot analysis (total concentrations of 1 and C60 or C70 of 1.5 × 10−3 M and 7.4 × 10−4 M, respectively), but again with no sign of complexation within the uncertainty of the method. Considering the higher concentrations used for NMR spectroscopic studies (which should in fact promote association) in comparison to UV−vis absorption studies, it seems strange that no signs of complex formation are observed (neither corresponding to fast nor slow exchange regimes), and we therefore infer that the observed UV−vis spectral changes must originate from rather weak associations. Indeed, previous work by Torres and co-workers has shown that when C60 is included in an elegantly designed subphthalocyanine cage (with Pd(II)−pyridyl linkages) a broadening of the SubPc resonances does occur.9 To shed further light on the association between SubPc and fullerenes and the possibility for strengthening it, we decided to prepare SubPc tweezer-like structures where two units of SubPc 1 are placed in a preorganized arrangement for binding of C60 or C70. Indeed, linking together two complementary host sites has previously been shown to be a good strategy for binding of fullerenes.10 The synthesis of two diastereoisomeric SubPc dimers is shown in Scheme 1. The iodo-functionalized

when subjecting derivatives of SubPc to Sonogashira coupling reactions. The two diastereoisomers of 4 were isolated in about the same yields, and they were assigned by chiral HPLC analysis the racemic mixture 4a gave a trace with two signals (corresponding to the two enantiomers), while the meso form 4b only gave one signal. They have very similar UV−vis absorption spectra (Figure 2) and a red-shifted longest-

Scheme 1. Synthesis of SubPc Dimersa

wavelength absorption (two close peaks at λmax at 577 and 564 nm) relative to that of 1 (λmax at 562 nm) due to the expanded conjugation of the SubPc cores (peripheral functionalization). As expected, both diastereoisomers of 4 were fluorescent with an emission maximum at 590 nm (see SI, Figures S20−S21). Each of the two isolated tweezers was next subjected to complexation studies with C60 and C70 in toluene-d8 using NMR spectroscopy (500 MHz). The SubPc aromatic protons underwent significant changes in the presence of fullerene as illustrated in Figure 3 (while the resonances for the axial aryl group did not change to any significant degree, nor did the phenylene resonances of the spacer). A Job plot analysis revealed 1:1 complexation with C60 and with C70 for both tweezers (parabola with maximum at 0.5; one representative plot is shown in Figure 4). NMR titrations (see SI) provided similar association constants with C60: 1.1 × 103 M−1 for both 4a•C60 and 4b•C60 in toluene-d8 (298 K). The associations were stronger with C70, and host 4b (meso) formed a slightly stronger complex than 4a with this fullerene: 2.8 × 103 M−1 for 4a•C70 and 3.4 × 103 M−1 for 4b•C70. We also investigated the possibility of moving from the fast exchange regime on the NMR time scale to the slow exchange regime by cooling a mixture of 4b and C60 from 300 to 220 K; however, this only resulted in a broadening of signals (see SI, Figure S45), and no additional information about the binding event was thus obtained from this experiment. The fact that the signals for the axial aryl group remain relatively sharp agrees with the assumption that these groups are pointing outward, away from the pseudocavity, and thereby are not subject to hindered rotation. In agreement with the postulated binding of fullerene, the emission of 4b at 490 nm was found to decrease in intensity upon addition of either C60 or C70 (SI, Figures S22 and S24). Inner filter effects complicate the analysis, but when correcting for these, we achieve rough estimates of the association

Figure 2. UV−vis absorption spectra of 1 and the two diastereoisomers 4a and 4b in toluene.

a

dba = dibenzylidene acetone.

SubPc 2 (racemic mixture) and 1,2-diethynylbenzene (3)11 were subjected to Sonogashira coupling reactions to provide the two tweezers 4a (racemic mixture) and 4b (meso form) that were separated by flash column chromatography. As a catalyst system, we employed Pd2dba3/AsPh3/CuI; we have previously12 found it advantageous to employ triphenylarsine instead of the more commonly employed triphenylphosphine 5822


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Figure 5. Molecular structures of (1)2•C60 (top) and (1)2•C70 (bottom) according to X-ray crystallography. Solvent molecules were omitted for clarity. Ellipsoids displayed with 50% probability level. CCDCs 1856088 and 1856087.

but lacking significant association in solution, was previously reported by Shimizu et al.13 for a pyrene-fused SubPc monomer. On the other hand, despite the strong solutionstate associations, attempts of growing crystals of the complexes between 4a or 4b and fullerenes were unfortunately unsuccessful. Instead, we subjected the one enantiomer of 4a and the meso compound 4b to a computational study. The DFT-optimized structures (ωB97XD/cc-pVDZ) reveal a perfect match between the two individual concave SubPc faces and C60 (Figure 6). The meso-4b•C60 complex was found

Figure 3. Part of the 1H NMR spectrum (500 MHz, toluene-d8, 298 K) of the racemic mixture 4a (one enantiomer structure is shown on top) (2.3 × 10−4 M) in the presence of increasing amounts of C60. The spectra show the signals of the peripheral SubPc protons and of aryl protons of the axial group.

Figure 4. Job plot based on NMR spectrosocpic studies for the association between 4a and C60 in toluene-d8 at 298 K. The mole fraction χ4a corresponds to [4a]/[4a + C60], where the total concentration [4a + C60] = 7.3 × 10−4 M. The change (Δδ/ppm) in chemical shift of the SubPc HA proton labeled by red in Figure 3 was used. Data points are connected by red lines.

constants of 1.1 × 104 M−1 for 4b•C60 and 4.4 × 103 M−1 for 4b•C70. While the association constant with C70 is in good agreement with the one found by NMR titration (same order of magnitude), the one for C60 seems to be overestimated by 1 order of magnitude. UV−vis absorption spectra of 4b upon addition of either C60 or C70 are also shown in the SI (Figures S23 and S25), being less informative. We managed to cocrystallize 1 and C60 as well as 1 and C70 from toluene/heptane, and the structures obtained by X-ray crystallographic analyses are shown in Figure 5; the data set for the complex between 1 and C60 was quite poor, and this structure is therefore of low quality. Nevertheless, both reveal 2:1 stoichiometries with the concave π-surfaces of two SubPcs toward the fullerene. A similar ability to cocrystallize with C60,

Figure 6. Optimized structures (gas-phase DFT; ωB97XD/cc-pVDZ) of the fullerene complexes of 4a (one enantiomer selected) and 4b (meso).

to be slightly more stable than the 4a•C60 complex (by 7.7 kJ mol−1). The boron−boron distances between the two SubPc units are 9.2 and 12.5 Å for 4a•C60 and meso-4b•C60, respectively, while the distance between the two boron atoms of (1)2•C60 in the crystal is found to 14.9 Å. As expected, the organization of the two SubPc units in the complex with meso4b as host has the closest resemblance to the organization of the two SubPc units of (1)2•C60 in the solid state, and the geometry of this tweezer hence seems optimum for the 5823


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In conclusion, tweezer-like SubPc dimers exhibit strong 1:1 binding with fullerenes, with association constants on the order of 103 M−1 and with a stronger binding of C70 than of C60. It seems that previously reported association constants for the complexation between monomeric SubPc 1 and fullerenes C60 or C70 determined from minor changes in UV−vis absorption spectra are overestimated as we observed no associations according to NMR spectroscopic studies. In general, we believe that this result calls for some caution when advocating for associations in solution solely based on UV−vis absorption and fluorescence studies. Instead, SubPc 1 is fully capable of engaging in solid-state association with fullerenes as evident from the 2:1 cocrystallized structures of 1 and C60 and of 1 and C70. From a comparison of these crystal structures and the DFT-calculated tweezer•fullerene complexes, it seems likely that the π−π surface interactions between tweezer and guest can be further optimized through modifications of the linkage between the two SubPc units. Thus, stronger complexations of fullerenes by SubPc derivatives may thereby be achieved in future work.

complexation of C60, albeit no measurable difference in association constants for 4a and 4b were obtained experimentally. With C70, the calculations provided almost no difference in the stability of the complex with meso-4b and with 4a (only a difference of 1.8 kJ mol−1 in favor of 4b); optimized structures are shown in Figure 7. For the C70

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02518. Synthetic procedures, X-ray crystallographic data, UV− vis absorption and fluorescence spectra, NMR spectra, fluorescence and NMR titration studies, variabletemperature NMR studies, and computational details (PDF)

Figure 7. Cyclic voltammograms for the reduction of 0.5 mM solutions of 4a (black), C60 (blue), and the 1/1 mixture of 4a and C60 (red) recorded at a glassy carbon disk electrode (d = 3 mm) in toluene/MeCN (9/1) containing 0.1 M Bu4NPF6 at a voltage scan rate of 0.1 V s−1.

complexes, the boron−boron distances are 9.1 Å (4a•C70; gas phase), 12.6 Å (meso-4b•C70; gas phase), and 15.4 Å ((1)2•C70; crystal), which again indicates that meso-4b seems to have a more optimum tweezer geometry than 4a. Since C70 is of lower symmetry than C60, a detailed conformational search had to be performed from which a total of 1296 conformers were created by combining 36 orientations of C70 (rotation around the molecular axis) with 36 conformers of 4a or 4b (rotations of the 4-tert-butyl axial group) and were initially optimized with PM6. Unique structures (13 for 4a•C70 and 10 for 4b•C70) were reoptimized with B3LYP-D3/6-31G, and the lowest energy conformer was then finally reoptimized with ωB97XD/cc-pVDZ. The redox properties of 4a and 4b in CH2Cl2 and of 1:1 mixtures of 4a (or 4b) and C60 in toluene/MeCN (9/1), in both cases containing 0.1 M Bu4NPF6 as the supporting electrolyte, were studied by cyclic voltammetry (CV) at a voltage scan rate of 0.1 V s−1. The voltammograms of 4a and 4b were almost indistinguishable (see the SI) and similar to those obtained for structurally related SubPc derivatives.12b The reversible formation of the radical anions was observed during reduction at E°′ = −1.49 V (vs Fc/Fc+) followed by formation of the corresponding dianions at E°′ = −1.98 V. Reactive radical cations were observed at Ep = 0.61 V during oxidation. The presence of C60 complexes in toluene/MeCN (9/1) could not be detected by CV. As seen in Figure 7, the voltammogram of a 4a/C60 mixture (red curve) is essentially the sum of the voltammograms for 4a (black curve) and C60 (blue curve). However, it is also seen that C60 is reduced to the radical anion ∼0.5 V more easily than 4a (or 4b), and therefore it cannot be ruled out that a 1:1 complex present in solution dissociates upon reduction of C60. It was deemed beyond the scope of this study to investigate this aspect further.

Accession Codes

CCDC 1856086−1856088 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Freja Eilsø Storm: 0000-0001-6421-804X Kurt V. Mikkelsen: 0000-0003-4090-7697 Ole Hammerich: 0000-0002-2080-1206 Mogens Brøndsted Nielsen: 0000-0001-8377-0788 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS University of Copenhagen and the Independent Research Fund Denmark | Natural Sciences (#8021-00009B) are acknowledged for financial support. Dr. Steffen Bähring, University of Southern Denmark, is acknowledged for helpful discussions. 5824


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Complexation of Fullerenes by Subphthalocyanine Dimers

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