Basal Plane Fluorination of Graphene by XeF2 via a Radical Cation

Letter pubs.acs.org/JPCL

Basal Plane Fluorination of Graphene by XeF2 via a Radical Cation Mechanism Yijun Liu, Benjamin W. Noffke, Xiaoxiao Qiao, Qiqi Li, Xinfeng Gao, Krishnan Raghavachari, and Liang-shi Li* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States

Downloaded by UNIV OF WINNIPEG on September 4, 2015 | http://pubs.acs.org Publication Date (Web): September 3, 2015 | doi: 10.1021/acs.jpclett.5b01756

S Supporting Information *

ABSTRACT: Graphene fluorination with XeF2 is an attractive method to introduce a nonzero bandgap to graphene under mild conditions for potential electro-optical applications. Herein, we use well-defined graphene nanostructures as a model system to study the reaction mechanism of graphene fluorination by XeF2. Our combined experimental and theoretical studies show that the reaction can proceed through a radical cation mechanism, leading to fluorination and sp3-hybridized carbon in the basal plane.

C

onverting sp2-hybridized carbon atoms in graphitic carbon materials such as graphite, fullerenes, nanotubes, and graphene to sp3-carbon leads to significantly different chemical and physical properties. Early examples include synthesis of graphitic oxide (also known as “graphene oxide”)1,2 and graphite fluoride3,4 that both have drastically different optical and electrical properties from the starting material graphite. Recent emergence of graphene has stimulated further interest in this approach to modify properties of the zero-bandgap semiconductor for various potential applications;5−11 and various chemical reactions have been explored to produce sp3-carbon in its basal plane (i.e., interior) to introduce a finite bandgap. Toward this end, graphene fluorination with XeF2 is particularly attractive9−11 because it can proceed under mild reaction conditions. In addition, the fluorination product can be defluorinated to recover graphene, which when combined with lithographic techniques, can lead to graphene nanostructures with controlled patterns and dimensions.12,13 However, there are significant challenges in understanding the fluorination mechanisms within such large conjugated systems, preventing control of the graphene nanostructures with atomic precision.14−16 To more efficiently explore novel chemistry and better control the properties of graphitic carbon materials, it is imperative to conduct mechanistic studies on these systems. Herein, we use a well-defined colloidal nanographene as a model system to study the mechanism of graphene fluorination by XeF2. This reaction is generally assumed to involve atomic fluorine (i.e., free radical) that forms due to XeF 2 decomposition.8,11 In contrast, reactions of small aromatic hydrocarbons (such as benzene, naphthalene, pyrene, etc.) with XeF2 have been shown to start with electron transfer to XeF2 and proceed through a radical cation mechanism, resulting in fluorination, though only on the edge, and mostly maintaining the sp2-hybridization of the carbon atoms.17−20 Thus, understanding the origin of this apparent size-related difference in © XXXX American Chemical Society

reactivity is of fundamental interest as well. Using a well-defined colloidal nanographene to react with XeF2, we show for the first time that the radical cation mechanism provides an alternative route to basal-plane fluorination in large conjugated systems. Initiated by electron transfer from graphene to XeF2, a powerful oxidizing agent, fluorination occurs through fluoride addition followed by repetitive oxidation and fluorination. Combined experimental and theoretical studies show that the site selectivity of fluorination can be understood with frontier molecular orbital theory and may have practical significance in developing new methods to functionalize graphene and related materials. In our studies, a soluble hexabenzocoronene derivative (1) was used as the model compound for graphitic carbon materials. Studying such well-defined nanostructures greatly simplifies interpretation of experimental results and enables direct comparison between experiments and theoretical calculations for mechanistic studies.21 1 was synthesized with stepwise solution chemistry methods and characterized with conventional organic characterization techniques (details in the Supporting Information). In 1, we introduced three trialkylphenyl groups on the periphery22,23 to render both 1 and its fluorinated products soluble in common organic solvents. The three-dimensional arrangement22,23 of the flexible alkyl chains effectively prevents aggregation of the nanostructures, greatly facilitating characterization of the reaction with various readily available ensemble spectroscopic techniques. In addition, for the XeF2 reaction we study herein, the alkyl chains serve as effective scavengers for atomic fluorine,24 so that we can focus on fluorination through the radical cation mechanism. Received: August 11, 2015 Accepted: September 2, 2015

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DOI: 10.1021/acs.jpclett.5b01756 J. Phys. Chem. Lett. 2015, 6, 3645−3649

Letter

The Journal of Physical Chemistry Letters


Electron paramagnetic resonance spectroscopy (EPR) subsequently confirmed formation of radical cations during the reaction. Shown in Figure 2A are room-temperature in situ

For a typical reaction, 1 was dissolved in anhydrous dichloromethane, and the resultant yellow solution was transferred to a glass flask containing XeF2 powder. The procedure was done at room temperature under air-free conditions. As XeF2 dissolved the solution turned brown and got darker over time. After stirring overnight, the reaction mixture was dried under vacuum, leaving a product as a waxy solid. The color varies from dark brown to white with increasing XeF2/1 molar ratio used in the reaction. With XeF2/1 molar ratio of 4200:1, the product in dichloromethane forms a colorless solution, with an absorption spectrum (dotted curve, inset in Figure 1) significantly different from that of 1

Figure 2. (A) Room temperature EPR spectra of a reaction mixture at different reaction times (XeF2/1 molar ratio: 210:1). A sharp singlet can be readily seen in the 1 min spectrum. In the later spectra a different singlet can be discerned at a slightly higher g value with much lower amplitude. Inset: EPR spectrum in the early stage of the reaction measured at 250 K, showing a singlet at g = 2.0029 and width of 5.0 G. (B) The 30 min spectrum shown in panel A (black curve) and the simulated one (red solid curve) containing a set of double−doublets (blue dotted curve) and a singlet (red dotted curve). The singlet is different from that for 1•+, appearing at a slightly higher g value (2.0041).

EPR spectra obtained at different reaction times with a 210:1 XeF2/1 molar ratio in a Bruker EMX-X-band spectrometer (9.84 GHz). By comparing the 1 min spectrum with later ones, we can readily identify a sharp singlet peak that is present only in the early stage of the reaction, superimposed on a background mainly consisting of four broader ones (Figure 2A). Repeating the reaction at a lower temperature (250 K) revealed that the singlet precedes the four broader ones and can be temporally isolated (inset in Figure 2A) due to the lower reaction rate. It has a g-factor of 2.0029 and a width of 5.0 G, identical to the spectrum of a previously reported hexabenzocoronene radical cation,26 and thus, we attribute it to the radical cation of 1 (1•+). This assignment is consistent with previous work on fluorination of smaller aromatic compounds with XeF2,17,27−29 where radical cations form due to the high oxidizing power of XeF2.30 The large conjugation size of 1 makes it more prone to oxidation.25 Lack of a well-resolved hyperfine splitting pattern is due to the multiple magnetically nonequivalent protons and the small proton coupling constants (0.1−2.5 G, Supporting Information) as a result of the large conjugation size. Aromatic radical cations have been known to participate in reactions such as nucleophilic fluorine addition, deprotonation, and further oxidation if possible.31,32 Thus, it is likely that 1•+ can be fluorinated, that is

Figure 1. UV−vis absorption spectra of a reaction mixture at different reaction times (XeF2/1 molar ratio = 840:1). In the order of decreasing absorbance at 367 nm: ∼ 1 min, 3 h, 5 h, 9 h, 13 h, and 24 h. Inset: absorption spectra of 1 in DCM (solid curve) and the product for XeF2/1 molar ratio of 4200:1 and 24 h reaction time (dotted curve).

(solid curve, inset in Figure 1). The drastic change in the absorption spectrum clearly shows the disruption of the conjugated carbon framework, indicating sp2-to-sp3 conversion of the carbon atoms in the basal plane. The presence of fluorine in the final products is confirmed with fluorine NMR spectroscopy (Figure S1 in the Supporting Information). The color change during the reaction was studied with ultraviolet−visible (UV−vis) absorption spectroscopy in situ. The spectra obtained at different reaction times with an 840:1 XeF2/1 molar ratio are shown in Figure 1. Most distinctively, the absorption peak at 367 nm decreases in magnitude, indicating consumption of the starting material. This is accompanied by growth of a broad absorption feature at longer wavelengths (approximately 450 to 700 nm), suggesting emergence of radical species during the process.25

1•+ + F− → [1 − F]•

followed by further oxidation 3646

(1) 32 DOI: 10.1021/acs.jpclett.5b01756 J. Phys. Chem. Lett. 2015, 6, 3645−3649

Letter

The Journal of Physical Chemistry Letters [1 − F]• − 1e → [1 − F]+

neutral or cationic radical species of various possible fluorinated products of 1. As described in detail in the Supporting Information, our calculations clearly show that the splitting constant differs considerably between fluorine substitution (maintaining sp2-carbon) and fluorine addition (converting sp2 to sp3 carbon) products, the former being consistently smaller than 4 G and the latter varying widely depending on the molecular structure. In particular, addition of a fluorine atom to an sp2-carbon in 1 leads to a splitting of up to 110 G, because the overlap of fluorine 2p-orbitals with a singly occupied πorbital enhances the spin density at the fluorine atom (Figure 3A). Addition of more fluorine atoms drastically alters the spin

(2)

fluorine addition [1 − F]+ + F− → [1 − 2F]

(3)

and more oxidation and addition if conditions permit [1 − 2F] − 1e → [1 − 2F]•+ [1 − 2F]•+ + F− → [1 − 3F]• [1 − 3F]• − 1e → [1 − 3F]+


[1 − 3F]+ + F− → [1 − 4F]

and so forth. As shown with smaller aromatic compounds, fluorine addition to an edge carbon bonded to hydrogen can also be followed by deprotonation, resulting in fluorine substitution on the edge. The four broad peaks shown in Figure 2A are the predominant spectral features that persist throughout the later stage of the reactions. Their growth within the first 5 min of the reaction at room temperature (Figure 2A) coincides with rapid depletion of 1•+, thus indicating that they derive from the subsequent reactions of 1•+. Remarkably, despite the many reactions 1•+ can participate in, the positions and relative amplitudes of the four broad peaks remain invariant over time. Therefore, they are likely caused by the same structural features whose formation is highly preferred in the reactions. Numerical analysis showed that all the EPR spectra (e.g., Figure 2A) except for the 1 min one could be decomposed into an identical set of double−doublets at g = 2.0100 and an identical singlet at g = 2.0041 (Figure S2 and Table S1 in Supporting Information). The double−doublets have hyperfine splitting constants of 33.2 and 16.7 G, with a width of 24.0 G for each peak. The singlet appears at a g-factor higher than 1•+ with a larger width (10.0 G), indicating different species. Shown as an example in Figure 2B is the experimental 30 min spectrum (black curve) and a simulated one (red solid curve) containing the double−doublets (blue dotted curve) and the singlet (red dotted curve). Because in aromatic radicals with delocalized spin density, the hyperfine splitting due to proton is no greater than a few gauss,33 the magnitude of the splitting constants of the double−doublets indicates coupling to fluorine incorporated during the reaction. Further, the double−doublets indicate that two particular fluorine atoms that give rise to the observed splitting constants are the common structural features shared by radical species in the reaction mixture. The large width of the peaks is probably because of the many possible species present in the reaction mixture. It may also be due to hyperfine splitting caused by extra fluorine atoms present, as Xray photoelectron spectroscopy (XPS) analysis showed that the final products on average contained up to 20 fluorine atoms after 24 h of reaction (Supporting Information). Density functional theory (DFT) calculations using the widely used M06-2X functional34 (see details in the Supporting Information) were conducted to identify structures that can lead to the observed hyperfine splitting pattern, especially the distinct double−doublets. The hyperfine splitting constants due to paramagnetic nuclei such as proton and 19F are determined by the position and nature of the chemical bonds,33 and quantum chemical calculations have provided powerful tools to compute them with high accuracy.33,35 With DFT calculations,36 we screened fluorine hyperfine splitting constants in

Figure 3. (A) Schematics illustrating the large hyperfine splitting caused by fluorine addition products. The overlap of fluorine 2porbitals with a singly occupied π-orbital enhances the spin density at the fluorine atom. (B) Combination of difluoro addition sites that can yield the observed double−doublets. One fluorine atom is located in the central phenyl ring, at a site marked by a solid circle (red or green) or its equivalent by symmetry, and the other by an open circle of the same color. The calculated hyperfine splitting constants of these pairs are shown.

density distribution in the conjugated backbone, leading to splitting constants that range from 1 to 130 G depending on the positions of the fluorine atoms (Table S2 in Supporting Information). Further fluorine substitution on the edge, in contrast, barely modifies the spin density distribution and causes little change in the splitting due to pre-existing fluorine atoms. Our calculations reveal that the only structures consistent with the observed double−doublets in EPR are difluoro addition products of 1 depicted in Figure 3B and further fluorine substitution products. The difluoro addition products contain a fluorine atom in the central phenyl ring, at a site marked by a solid circle (red or green, Figure 3B) or its equivalent by symmetry, and the other by an open circle of the same color. The calculated splitting constants, labeled in Figure 3B, are insensitive to trans or cis arrangement of the two fluorine atoms (Supporting Information). All these combinations are in reasonable agreement with the experimental double−doublets with the large peak width. According to our calculations, further fluorine substitution of the difluoro addition products barely modifies the splitting constants of the two fluorine atoms and leads to an extra splitting smaller than 4 G, contributing only to the broadening of the double− doublets. However, the experimental splitting pattern does not agree with any mono fluoro addition products, because the neutral radicals are more readily oxidized to cations that have no unpaired electron and, thus, are not detectible by EPR.32 This is supported by the calculated ionization potential of [1 − F]• that is significantly lower than that of 1 (Supporting Information). Neither do the experiments agree with higher fluoro addition products, despite the observation that the final products on average contain many more fluorine atoms. This 3647


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the basal plane, explaining the preferential fluorination on the edge rather than in the interior. Even though a free radical mechanism has been postulated previously for XeF2 fluorination of graphene,11 our work suggests that electron transfer from graphene to the strongly oxidizing XeF2 and formation of radical cations can play a significant role. In addition, it has been recognized that residual moisture and even H+ adsorbed on a solid surface (such as that of glass vessels) can react with XeF2 to yield traces of HF to catalyze radical cation formation.41 Thus, with SiO2 being a common substrate for graphene, the radical cation mechanism could be quite important. Such a mechanism may enable us to modulate the fluorination reactivity to achieve patterned graphene nanostructures.12,42 We also envision that initiation of the reaction by other oxidants31,43 and even electrochemical means may enable a much wider range of functionalization of graphene for various applications.


may be attributed to the increasing difficulty of the highly fluorinated species to be oxidized into detectable radical species. As a result, their rates of consumption greatly exceed their rates of generation, resulting in too low a concentration of these radical species for the EPR measurements. This biases the EPR measurements toward the initial stage of the reactions and results in relatively simple spectra. To understand the observed preferential fluorine addition at the sites revealed by the EPR measurements, we examined the frontier molecular orbitals (FMOs) of the involved species.37,38 Shown in Figure 4A is the wave function of the β LUMO of 1•+

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ASSOCIATED CONTENT

S Supporting Information *

•+

Figure 4. (A) β LUMO of 1 as the FMO in the nucleophilic addition of F− to produce [1 − F]•. (B) Calculated electrostatic potential of 1•+. (C) LUMO of [1 − F]+ as the FMO in the fluoride addition reaction to produce [1 − 2F].

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01756. Experimental details, synthesis and characterization of 1, spectroscopic studies of fluorination and data analysis, theoretical calculations of hyperfine splitting constants. (PDF)

(we assume the unpaired electron has spin α), the FMO in the nucleophilic addition reaction with F− to produce [1 − F]•, the mono fluoro addition products. Clearly, the central phenyl ring has the highest electron density, and thus should be the preferred site for the first fluorine addition. This preference is further enhanced by Coulomb attraction between the species,39,40 with the calculated electrostatic potential of 1•+ (Figure 4B) being the most positive in the central ring. To understand the site preference in the second fluorine addition, in Figure 4C, we show the wave function of the LUMO of [1 − F]+ with a fluorine atom (marked by a red dot) in the central ring (red circle), the FMO in the fluoride addition reaction to produce [1 − 2F]. The electron density evidently indicates the sites at which the second fluorine addition would most likely occur. The six sites along the green circle are consistent with those depicted in Figure 3B that give rise to the observed double−doublets in EPR. In addition, the FMO suggests that the 2- and the 4-positions in the central ring (marked by the red circle) should be preferred sites as well, which may explain the singlet at g = 2.0041 observed in our EPR measurements (e.g., in Figure 2B and Figure S2 in Supporting Information). Our DFT calculations of the 1,2- and 1,4-difluoro addition products reveal that they have hyperfine splitting constants in the range of 4 to 9 G ( Supporting Information, Table S2). Thus, they may lead to a singlet with spectral broadening due to the multiple species present in the reaction mixture and the extra splitting from fluorine substitution. The varying amplitude of the singlet relative to that of the double−doublets (Figure S2 and Table S1 in Supporting Information) is likely due to the species having different generation or consumption rates. In summary, in this work, we show for the first time that graphene fluorination with XeF2 occurs via a radical cation mechanism, leading to fluorination in the basal plane. With this mechanism and the FMO theory, we can understand the apparent size-related reactivity that no basal plane fluorination was previously observed with small aromatic hydrocarbons. Calculated FMO orbitals (Figure S3 in Supporting Information) show that the radical cations of previously studied small polycyclic aromatic hydrocarbon have low electron density in

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

Y.L. and B.W.N. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Yaroslav Lozovyy for assistance with XPS measurements, and Professor Allen Siedle for helpful discussions. This work is supported by the National Science Foundation grants DMR-1105185 and CHE-1266154 at Indiana University. The XPS measurement was done at the Nanoscale Characterization Facility of Indiana University. This research was supported in part by Lilly Endowment, Inc., through its support for the Indiana University Pervasive Technology Institute, and in part by the Indiana METACyt Initiative. The Indiana METACyt Initiative at IU is also supported in part by Lilly Endowment, Inc.

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REFERENCES

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Basal Plane Fluorination of Graphene by XeF2 via a Radical Cation

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