Article pubs.acs.org/JACS
Direct Covalent Coupling of Porphyrins to Graphene Daniela Dasler,† Ricarda A. Schaf̈ er,† Martin B. Minameyer,‡ Jakob F. Hitzenberger,‡ Frank Hauke,† Thomas Drewello,‡ and Andreas Hirsch*,† †
Department of Chemistry and Pharmacy and Joint Institute of Advanced Materials and Processes (ZMP), Friedrich-Alexander University Erlangen-Nürnberg, Chair of Organic Chemistry II, Henkestrasse 42, 91054 Erlangen, Germany ‡ Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg, Chair of Physical Chemistry I, Egerlandstrasse 3, 91058 Erlangen, Germany S Supporting Information *
ABSTRACT: Graphene−porphyrin nanohybrid materials with a direct covalent linkage between the graphene carbon network and the functional porphyrin unit have been successfully synthesized via a one-pot reductive diazotation approach. A graphite− potassium intercalation compound (KC8) was dispersed in THF, and different isolated porphyrin−diazonium salts were added. The direct covalent binding and the detailed characterization of the functional hybrid material were carried out by Raman spectroscopy, TG-MS, UV/vis, and fluorescence spectroscopy. LDI-ToF mass spectrometry was introduced as a new versatile and sensitive tool to investigate covalently functionalized graphene derivatives and to establish the composition of the respective nanohybrid materials.
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covalent functionalization,14 and therefore this carbon allotrope would represent a promising platform for the outlined applications. However, except for a structurally inconsistent reduced graphene oxide example,6 no directly coupled hybrids of graphene with porphyrins, which would represent a very fundamental compound class, have been reported up to now. Only a few approaches of an indirect, bridge-mediated binding have been followed so far. Zhang et al. presented a route of functionalizing graphene sheets with porphyrin molecules by initially attaching a trimethylsilyl-protected phenylacetylene group via an aryl diazonium salt reaction and subsequent addition of the porphyrin moiety by click chemistry.5 The group of Feringa used a 1,3-dipolar cycloaddition reaction in order to craft tetraphenylporphyrin moieties onto graphene sheets.15 Further, amide coupling reactions have been applied to covalently attach porphyrin building blocks to graphene oxide.16−19 An attractive way for the direct binding of porphyrins would be the use of aminoporphyrins, which after conversion into the corresponding diazonium intermediates could directly react with graphene/graphite. The general feasibility of this concept was demonstrated for simple amines.20 However, the drawback of this method is that the system itself has to deliverin a still unclear waythe required electrons for the diazonium reduction, which excludes highly efficient addition.21 To circumvent this problem, we have previously introduced the concept of reductive graphene activation/functionalization. Here, graphite intercalation com-
INTRODUCTION Covalent chemistry of graphene is currently an emerging field,1,2 since covalently attached addends modify the solubility and tune the physical properties of the parent system3 and allow for the combination of the unique properties4 of graphene with the large range of functions of other compound classes. However, the attachment of sophisticated functional building blocks to the graphene plane has been reported for only a few examples.5,6 This is due to (a) the difficulty of finding suitable and controllable reaction conditions for the covalent addend binding, (b) the very poor dispersibility of parent graphene in organic solvents, (c) the relatively low degrees of addition observed for covalent binding to nonactivated graphene/graphite, and (d) the difficulty of unequivocal product characterization, especially since standard characterization tools that are typically used in organic chemistry such as NMR spectroscopy and mass spectrometry could not be applied so far. The only exception in the latter regard is the solid-state NMR characterization of graphite oxides carried out in the Pumera group7 and of a highly hydrogenated graphene that we described recently.8 Very attractive building blocks for covalent graphene functionalization are certainly porphyrins due to their optical, electronic, and versatile metal complexation properties. Porphyrins are stable functional dyes with large extinction coefficients in the visible light region, photochemical electron-transfer ability, and rigid structures with potential applications in photoharvesting, sensing, and photoelectronic devices.5,6 Multiple examples based on other carbon allotropes have been investigated intensively.9−13 In graphene, the band gap can be fine-tuned by © 2017 American Chemical Society
Received: April 27, 2017 Published: August 1, 2017 11760
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The porphyrin diazonium salts employing the respective tetraphenylporphyrin (TPP) and tetra-tert-butyl-TPP units were synthesized (see Supporting Information) and isolated prior to the reaction in order to prevent further side reactions with the chemically activated graphene sheets. For the reductive functionalization sequence, 1/8 equiv of the corresponding diazonium compound was added under argon flow to the graphenide dispersion, respectively. The big advantage of this wet chemical reductive graphene functionalization approach is that the individualized graphene sheets can covalently be addressed by an attack of the intermediately generated aryl radicals in a ditopic fashion (Scheme 1).20,27,28 It has to be mentioned that in the case of an incomplete charge quenching of the graphenides by the oxidation with diazonium-porphyrins (R-TPP-N2+) side reactions such as protonation during the aqueous workup sequence could, in principle, be expected.29 In order to eliminate this possibility, the samples have also been treated with benzonitrile (PhCN) after the initial addition of the diazonium compound. We demonstrated recently that benzonitrile can quantitatively remove the negative charges from GICs and graphenides.30 All samples were finally washed several times with trifluoroacetic acid (TFA) in dichloromethane (DCM)/methanol (MeOH) (1:2) solution in order to remove byproducts such as noncovalently bound porphyrins. In a first characterization step, the resulting hybrid materials were analyzed by statistical Raman spectroscopy.
pounds (GICs) are used as starting materials, which can most efficiently be exfoliated in appropriate inert organic solvents.22−24 Their subsequent treatment with diazonium salts, iodonium salts, or alkyl iodides leads to very efficient covalent addend binding. In the present paper, this approach is successfully applied in the one-pot functionalization of graphene, leading to the first direct covalent binding of a tetraphenylporphyrin (TPP) addend. The porphyrin is linked in a perpendicular orientation by a single σ-bond to the graphene lattice, allowing a free rotational motion of the functional addend in combination with a fixed relative alignment of the π systems of the individual components. The corresponding diazonium precursors were isolated prior to the treatment with the GIC in order to prevent uncontrollable side reactions. The reaction products were characterized in detail by statistical Raman spectroscopy (SRS),21 by thermal analysis, by absorption and emission spectroscopy, and for the first time by laser desorption/ ionization (LDI) mass spectrometry.
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RESULTS AND DISCUSSION For the initial GIC formation (Scheme 1), pristine spherical graphite (SGN18) was reduced with a stoichiometric amount of potassium, yielding KC8. One equivalent of this highly activated intermediate was dispersed in 20 mL of dry and degassed tetrahydrofuran (THF) and sonicated for 2 min in order to support the delamination process of the graphenide sheets. Scheme 1. (a) Synthesis of the Diazotated Porphyrin Derivatives Starting from the Corresponding Aminoporphyrins;25,26 (b) Reaction Sequence for the Reductive Synthesis of the Graphene-Porphyrin Hybrid Materials (G-TPP, G-tBuTPP) with Direct σ-Bond Connection of the Functional Entities
Figure 1. Mean statistical Raman spectra of the covalently coupled graphene−porphyrin hybrids G-TPP, G-TPP quenched with benzonitrile (PhCN), and G-tBuTPP compared with pristine graphite. λexc = 532 nm.
Relative to the G-band at ∼1582 cm−1, the intensity of the D-band at ∼1350 cm−1 increases significantly to a mean ID/IG ratio of about 1.1 in all samples, indicating the conversion of sp2 to sp3 basal carbon atoms due to covalent binding of the porphyrin addend. The statistical analysis of the ID/IG ratio (Figure S1) provides further information about the homogeneity of the introduced defects, which is consistent in all samples. Due to the overlap of the D- and G-band with the characteristic porphyrin Raman bands (excitation in the Qband region), the full width at half-maximum of the D-band cannot be exactly determined, preventing the estimation of the degree of functionalization. The mean spectrum of G-TPP + PhCN is identical to the spectra recorded for G-TPP and G-tBuTPP obtained after aqueous workup without PhCN 11761
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Journal of the American Chemical Society treatment (Figure S2). This underlines the pronounced oxidation capability of the diazonium reagents and hence confirms thatif anyonly a very small amount of side reactions occurs during the final workup step. Furthermore, it can be noticed that a broad D+D′-band appears at ∼2884 cm−1, which overlaps with the characteristic 2D-band of graphene at ∼2680 cm−1. Moreover, new spectroscopic features (830, 1002, 1086, 1140, 1242, 1456, 1551 cm−1) can be detected and ascribed to TPP (Figure S3) and tBuTPP Raman bands, respectively. This phenomenon is based on the grapheneenhanced Raman scattering (GERS) effect, which allows identifying the spectroscopic patterns of functional molecules in close proximity to the graphene surface.31−33 In order to illustrate this effect further and to deconvolute the Raman contributions of both systems, additional Raman spectra of GTPP were recorded at different laser excitation wavelengths of 405, 457, and 473 nm, respectively (Figure 2). In comparison
Figure 3. Mean Raman spectrum of pristine SGN18, (a) SGN18/ TPP, (b) SGN18/TPP-NH2, (c) G + TPP, (d) CVD G/TPP, and (e) CVD G/TPP LB. λexc = 532 nm.
Figure 4. Temperature-dependent Raman investigation. Mean Raman spectra of G-TPP at different temperatures: room temperature to 450 °C.
Figure 2. Mean Raman spectra of G-TPP at different laser wavelengths.
to the spectrum recorded at 532 nm, the porphyrin spectral features obtained for 405 and 473 nm excitation become more pronounced. It has to be emphasized that the high intensity of the TPP bands in the spectrum recorded with 457 nm excitation is remarkable and allows a clearer attribution of the respective porphyrin bands. Moreover, when the excitation energy is tuned to 405 nm, which is close to the Soret-band absorption maximum of the porphyrin (cf. Figure 7), still a reasonably well resolved Raman spectrum is obtained. This is exceptional, as an excitation into the absorption maximum of the porphyrinor dye moleculeis generally accompanied by a prominent fluorescence of the system. This fluorescence background of dye molecules normally prevents the acquisition of a Raman spectrum with distinct features. It has been shown recently34 that the electronic communication between an excited dye molecule and graphene leads to a quenching of the
Figure 5. TG curves of pristine graphite (SGN18), G-TPP, and G-tBuTPP.
fluorescence and the detection of the characteristic Raman bands of the two individual components. On the basis of this finding, the well-resolved Raman spectrum obtained for our hybrid system with a laser excitation wavelength of 405 nm (Figure 2, top) can be taken as a first indication of an efficient electronic communication of the π systems of the individual components. 11762
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indication and a strong support of the successful covalent functionalization with TPP-N2+ and tBuTPP-N2+ by our onepot reductive functionalization approach. This finding is furthermore confirmed by the results of a temperature-dependent Raman investigation of G-TPP (Figure 4). Here, the mean ID/IG ratio clearly decreases with increasing temperature. At the beginning of the measurement, the mean Raman spectrum displays broad overlapping features for the 2D- and D+D′-bands. Due to the reversibility of the covalent binding, the porphyrin functionalization is detached at higher temperatures. This leads to a rehybridization of sp3 basal carbon atoms into the corresponding sp2 configuration and lowers the density of defects. As a consequence, the D+D′-band starts to decrease in intensity (it has to be emphasized that this process starts at around 350 °C), and finally the fully resolved characteristic spectroscopic feature of the 2D-band is obtained. Raman spectroscopy is not suited to draw conclusions about the chemical nature of the covalently attached moieties. Therefore, thermogravimetric analysis coupled with mass spectrometry (TG-MS) was carried out (Figure 5). The graphene−porphyrin nanohybrid materials G-TPP and G-tBuTPP exhibited a characteristic mass loss of 13% and 10%, respectively. This mass loss corresponds to a degree of functionalization (DOF) of approximately one TPP unit per 340 carbon atoms in G-TPP (0.29%) and one tBuTPP unit per 590 carbon atoms in G-tBuTPP (0.17%).15 Remarkably, these results are close to the values that have been found for a reductively arylated graphene sample in the course of a TGGC-MS quantification study.23 The TG analyses of the reference experiments display very different mass losses (Figure S4) in comparison to the respective G-TPP and G-tBuTPP samples. For the mixtures of SGN18 and (a) TPP or (b) TPP-NH2 a value of 3−4% is obtained, whereas the chemically activated graphene sample with TPP and the intermediately discharged sample (G + PhCN) exhibited a mass loss of around 7% (Figure S5). Due to the restricted mass range m/z 10 to m/z 310 of our coupled setup, the characteristic mass signals of the intact detached porphyrin ions could not be identified. Nevertheless, peaks attributed to pyrrole (m/z 64, 65), to an aryl moiety (m/z 77), and to a fragment of the porphyrin ring with an attached phenyl group (m/z 154) confirm the presence of porphyrins within the graphene−porphyrin hybrid materials (Figure S6). Strong support for the covalent attachment to the graphene lattice results from the fact that the fragment ions for the porphyrin units are detected only at higher temperatures. Due to the limitation of our TG-MS system, we decided to follow the classical mass spectrometric analysis approach of porphyrins and applied laser desorption/ionization time-offlight mass spectrometry (LDI-ToF-MS) to our graphene− porphyrin hybrid materials. This method employs laser activation for desorption and ionization of the materials and extends the mass range well beyond the intact porphyrins. Related applications of LDI have recently been published.35−37 While the use of graphene-based materials as a matrix for matrix-assisted LDI (MALDI) is under intense investigation,38−40 the analysis of covalently functionalized graphene derivatives by means of LDI is unprecedented. The LDI mass spectrum of G-TPP washed with TFA in DCM/MeOH (1:2) is depicted in Figure 6a. The intact TPP is observed in the form of its radical cation [M]•+ at m/z 614.2 and as the protonated quasi-molecular ion [M + H]+ at m/z 615.2. A signal for the porphyrin at m/z 613, intuitively
Figure 6. Positive-ion LDI-ToF mass spectra of (a) a TFA-washed GTPP and (b) a non-TFA-washed G-TPP sample.
Figure 7. UV/vis absorption spectra of G-TPP, TPP, and G-TPP with subtracted graphene baseline in THF.
To verify the covalent binding of the porphyrin moieties, several reference experiments with nonactivated reaction partners were carried out. For this purpose, pristine graphite (SGN18) was dispersed in THF, and (a) TPP or (b) TPP-NH2 was added. As a consequence of the missing diazonium group, the porphyrin can interact only noncovalently with the graphene π surface, establishing an equilibrium of adsorbed and solution-dissolved species. Additionally, (c) wet chemically activated graphene was also reacted with pure TPP under the same reaction conditions as applied for the covalent functionalization protocols of G-TPP and G-tBuTPP. For a more detailed understanding of the porphyrin/graphene interaction we also used CVD graphene and (d) let it directly react with the porphyrin diazonium salt (TPP-N2+ BF4−) without prior activation or (e) built a TPP Langmuir−Blodgett film on the CVD graphene substrate. In comparison to the spectrum of pristine graphite, the Raman spectra of (a), (b), and (c) do not exhibit any significant changes (Figure 3). In the Raman spectra of (d) and (e) only some minor contributions of the porphyrin bands can be identified, and no increase of the D-band intensity is present in these spectra. These reference experiments are a clear 11763
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715 nm with an excitation wavelength of 420 nm (Figure S11). In the covalently coupled adduct G-TPP, these bands are hypsochromically shifted to 654 and 717 nm, respectively.
expected for the release of covalently attached TPP, lacking a hydrogen atom at the position where it was bound to the graphene, could not be observed. We assume that efficient hydrogen uptake in the ablation plume or protonation, due to the acidic synthetic conditions, prevents the detection of such ions. Further signals for byproducts are almost undetectable. Hence, the noncovalently bound species are removed efficiently from the nanohybrid, with TPP remaining on the graphene. Thus, strong support is provided for the covalent nature of the bond between TPP and graphene. The resulting LDI mass spectrum (Figure 6b) of the G-TPP sample, which was not initially washed with the TFA solution, reveals also interesting insights into the functionalization mechanism. Compared to the TPP reference experiment, the G-TPP sample produced several additional signals (Figure S7). Here, a porphyrin bearing a potassium cation as the charge carrier can be identified. This species most likely originates from the complexation of deintercalated potassium cations. Moreover, several byproducts from the reaction are discernible in the obtained LDI mass spectra, and this underlines the versatility and sensitivity of LDI-ToF-MS as an analytical tool for covalently functionalized graphene derivatives. For instance, TPP-NH2 ([M]•+ at m/z 629.2, [M + H]+ at m/z 630.2) can be identified and most likely originates from a small amount of amino functionalities that did not react with the isoamyl nitrite during the diazotation of the precursor molecule. As the reference experiments have clarified, TPP-NH2 is not covalently attached to the graphene basal plane. Furthermore, the spectral signature of TPP-THF ([M]•+ at m/z 684.2, [M + H]+ at m/z 685.2) can be identified in the measured spectra, and this derivative is a potential side product of the reaction of an intermediately formed THF radical with a TPP core (Scheme S1). Most interestingly, at m/z 1226 and 1227, the radical cation and H+-adduct of a TPP-dimer can be found, respectively. Tandem mass spectrometry confirms a covalent bond between the two porphyrin units (Figure S8). This indicates that the dimer is a byproduct, formed during the reductive functionalization. At even higher masses, small signals of a TPP-trimer and even a tetramer are discernible (Figure S9). In an additional LDI reference experiment the G-TPP was compared to SGN18/TPP and SGN18/TPP-NH2 samples (Figure S10). For the two reference samples, it was difficult to obtain any TPP-related signals at all, demonstrating the efficiency of the reductive functionalization to immobilize the TPP on graphene. Finally, optical absorption (UV/vis spectroscopy) and emission studies (fluorescence spectroscopy) were also carried out for the conclusive characterization of the novel grapheneporphyrin hybrid materials. G-TPP was chosen as a representative example (Figure 7). The highly characteristic Soret-band of TPP in THF is detected at 416 nm along with several weaker Q-bands. The concentration of TPP was adjusted to 0.08 mg/mL for a better comparison with the hybrid material. For G-TPP, the Soretband appears 2 nm shifted at 418 nm due to the interaction between the π system of graphene and the porphyrin. In general, this result confirms that the porphyrin system stays intact despite the reductive activation pathway, which was also proven by LDI. For a further investigation, the functional hybrid material and the respective fluorophore were also compared by fluorescence spectroscopy. TPP displays two fluorescence bands at 651 and
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CONCLUSION In summary, we have successfully synthesized a novel graphene−porphyrin hybrid material with direct linkage via a one-pot wet chemical reductive activation/functionalization approach, starting from commercially available graphite. In the final adduct, the porphyrin is linked, in a perpendicular orientation, via only one σ-bond to the graphene lattice, allowing a free rotational motion of the functional moiety. This highly functional graphene−porphyrin hybrid system was characterized in detail by means of Raman, UV/vis, and fluorescence spectroscopy as well as TG-MS analysis. In addition, LDI-ToF-MS has been introduced as a versatile and highly sensitive tool for the analysis of covalently functionalized graphene derivatives. The combination of the applied techniques allows drawing conclusions on the binding and interaction of functional addends with graphene. Considering the individual properties of both graphene and porphyrin, this type of covalently coupled hybrid material has the potential to become an integral part in several applications in a number of areas, such as solar energy conversion cells and electronic devices.
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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/jacs.7b04122. Experimental details regarding the porphyrin synthesis and functionalization of graphene with porphyrin moieties, reference experiments, and additional Raman, TG-MS, and LDI-ToF-MS data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Frank Hauke: 0000-0001-9637-7299 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been carried out within the framework of the SFB953 “Synthetic Carbon Allotropes” (Project A1 & Project Z1). The research leading to these results has received partial funding from the European Union Seventh Framework Programme under grant agreement no. 604391 Graphene Flagship.
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