Article pubs.acs.org/JPCC
Frustrated Lewis Pair Nanoribbons Benjamin G. Janesko* Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129, United States S Supporting Information *
ABSTRACT: This work proposes that zigzag graphene and graphane nanoribbons with alternating Lewis-acid and Lewis-base edge substituents can act as “frustrated Lewis pairs” (FLPs), with potential applications in heterogeneous catalysis and sensing. The nanoribbon scaffold prevents (frustrates) intramolecular Lewis pairing. Edge substitutents prevent inter-ribbon pairing and tune the Lewis acidity/basicity. Computational evidence suggests that graphene and graphane FLPs are synthetically feasible semiconductors, with heterolytic H2 splitting activity potentially comparable to molecular FLPs. These new materials show potential for extending FLPs to heterogeneous catalysis, coupling catalytic, electronic, and sensing activity on the nanoscale.
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INTRODUCTION Enzyme catalysts’ active sites, in which a well-defined spatial arrangement of chemical functional groups accelerates reactions of bound reactants, motivate synthesis of molecules and nanostructures with defined arrangements of reactive moieties. A notable recent example is “frustrated Lewis pairs” (FLPs), in which reactive Lewis acid and Lewis base functionalities are bound to a scaffold that prevents (frustrates) intra- and intermolecular Lewis pairing.1 Molecular FLPs have demonstrated activities including reversible heterolytic H2 cleavage,2 reversible carbon dioxide binding,3 and H2 activation for hydrogenation catalysis4 and hydrogenation of aromatics.5 These activities may be tuned by modifying the electronic properties and steric bulk of the Lewis acid and base substituents.1 Rigid and well-defined nanostructures could provide a scaffold to prevent Lewis pairing of Lewis acid and Lewis base substituents, similar to the scaffold of molecular FLPs. This work explores how the edges of graphane and graphene nanoribbons can act as such a scaffold. Graphene nanoribbons can be viewed as thin strips of graphene or as unrolled singlewall carbon nanotubes. They have been widely explored for their electronic properties.6,7 Their combination of electrical conductivity and chemical activity is applied in H2 adsorption,8 electrochemical sensors,9 and anodes for Li-ion batteries.10,11 Graphane (hydrogenated graphene) nanoribbons have been proposed as semiconducting organic electronic components.12−14 Recent experiments have demonstrated moderate control over functionalization of the nanoribbon edge.15,16 Dopants such as boron and nitrogen have been incorporated into graphene nanoribbons.17 Dopant effects on electronic18−26 and vibrational27 properties have been extensively investigated. However, less attention has been paid to the question addressed here: how the nanoribbon architecture can modulate dopants’ chemical reactivity. This work proposes that zigzag graphane and graphene nanoribbons with alternating Lewis-acid and Lewis-base edge substitution can act as “frustrated Lewis pair nanoribbons” © 2012 American Chemical Society
(Figure 1). I present computational evidence that FLP nanoribbons have reasonable energetic stabilities, semiconduct-
Figure 1. Structures of graphane and graphene FLP nanoribbons 1-2. Arrows denote the periodic direction. Zigzag graphane and graphene nanoribbons 3-4, boron nitride nanoribbon 5, and ladder polymer poly(pyrrolo[3,2-b;2′,3′-d]borole) 6 (ref 28) are shown for comparison.
ing electronic properties, tunable hydrogen-splitting activities potentially comparable to molecular FLPs, and coupling between electronic and chemical properties.
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THEORETICAL METHODS Calculations use the Gaussian 09 electronic structure package29 and density functional theory with the HSE06,30,31 M06, or Received: March 23, 2012 Revised: June 26, 2012 Published: July 26, 2012 16467
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M06-2X32,33 exchange-correlation functionals. The HSE06 screened hybrid provides reasonable bandgap predictions for many (but not all34) organic and inorganic semiconductors,35−38 and has been extensively applied to graphene nanostructures.24,25,39−42 M06-2X is particularly accurate for dative bonding,43 whereas M06 gives a reasonable balance between accuracy for dative bonding,43 band structures,35 and thermodynamic properties.32 Unless otherwise noted, nanoribbon calculations treat infinite, isolated, one-dimensionally periodic nanoribbons. Calculations use default values for selfconsistent field and geometry convergence, numerical integration grids in real and k space, and ranges of one-electron and exchange integrals.44 Calculations do not consider ferromagnetic states,6,45 which are not present in polyacene46 or edgepassivated graphane nanoribbons.14 Figures use color coding C (gray), H (white), N (blue), B (beige), and F (light blue). Bond orders are drawn as a guide to the eye.
band structure of poly(pyrrolo[3,2-b;2′,3′-d]borole) 6, a conjugated ladder polymer with alternating N and B substituents similar to 2a. (Ref 28 did not explore the system’s frustrated Lewis pair character.) Band Gaps and Band Structures. Table 1 gives HSE06/ 6-311G(2d,2p)//HSE06/6-31G(d) indirect band gaps calcuTable 1. Calculated Nanoribbon Band Gaps (eV)
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species
gap
3 5 1a 1b 2a 2b 6 4
7.17 6.10 4.21 3.74 1.23 1.09 0.27 0.19
RESULTS Stability and Synthetic Accessibility. The relative energetic stability of FLP nanoribbons can be quantified with isodesmic reactions. Figure 1 shows that each unit cell of graphene FLP nanoribbon 2a has the molecular formula C4B2N2H4. One unit cell of 2a may be formed from zigzag graphene (polyacene28,46) nanoribbon 4 (unit cell formula C8H4), and boron nitride nanoribbon 5 (unit cell formula B4N4H4) as 1 1 C8H4 + B4 N4H4 → C4 B2N2H4 2 2
(1)
(Calculations use the unit cells in Figure 1.) M06/6311G(2d,2p)//HSE06/6-31G(d) calculations predict that this isodesmic reaction has an energy change ΔE = +96.3 kcal/mol per unit cell (+0.95 kcal/g). Similarly, each unit cell of graphane FLP nanoribbon 1a has the molecular formula C4B2N2H8, and each unit cell of zigzag graphane nanoribbon 3 has rhe molecular formula C8H12. One unit cell of 1a may be formed from 3 and 5 as 1 1 C8H12 + B4 N4H4 → C4 B2N2H8 2 2
Figure 2. Calculated structures, lowest unoccupied (top), and highest occupied (bottom) crystal orbitals of graphane FLP nanoribbon 1b (left) and graphene FLP nanoribbon 2b (right). Orbitals are shown in one of two displayed unit cells.
(2)
M06/6-311G(2d,2p)//HSE06/6-31G(d) calculations predict that this isodesmic reaction has an energy change ΔE = +163.5 kcal/mol per unit cell (+1.55 kcal/g). Graphane FLP nanoribbon 1a has a more positive formation energy than graphene FLP nanoribbon 2a, and thus the former is expected to be more reactive. For comparison, the formation of one unit cell of graphene 3 from graphane 4 as C8H12 → C8H4 + 4H 2
lated for the nanoribbons in Figure 1. Figure 2 shows the HSE06/6-31G(d) frontier crystal orbitals of 1b and 2b. The small band gap of polyacene 4 is consistent with the ∼0.1 eV HF/6-31G* band gap reported in ref 46. The large band gaps of edge-passivated graphane and boron nitride nanoribbons 3 and 5 are consistent with refs 14 and 51. The 0.27 eV band gap of ladder polymer 6 is consistent with the 0.66 eV B3LYP gap reported in ref 28, subject to the nearly constant ∼0.38 eV difference between HSE06 and B3LYP conjugated polymer band gaps reported in ref 35. The most notable results in Table 1 are the relatively small band gaps of graphane FLP nanoribbons 1a and 1b. In contrast to graphane 3 or boron nitride 5, and despite their lack of π bonds, these nonconjugated nanoribbons have band gaps approaching the visible range. Such semiconducting behavior is rare in undoped52,53 nonconjugated polymers.54 Chemical substitution can yield even smaller predicted band gaps (Figure 5). (It is worth reiterating that the HSE06 functional accurately reproduces experimental values of many semiconductor band gaps.35−38) Graphene FLP nanoribbons 2a and 2b have calculated band gaps between graphene 4 and boron nitride 5 and somewhat higher than ladder polymer 6.
(3)
has a predicted total energy change ΔE = +105.4 kcal/mol per unit cell (+0.97 kcal per gram of 4). These calculations do not include entropic or other finite temperature effects. However, the trends indicate that FLP nanoribbons can be reasonably stable relative to other, previously synthesized nanostructures. Further evidence that FLP nanoribbons may be synthetically accessible comes from polymer chemistry. Molecules 1 and 2 can be viewed either as narrow nanoribbons or as ladder polymers similar to polyacene.28,46 While ladder polymers 1 and 2 have not to my knowledge been reported, conjugated and nonconjugated polymers containing main-chain boranes47−49 and conjugated polymers with alternating borane and amine functionalities50 have been synthesized. Ref 28 modeled the 16468
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Figure 3. Band structures of 1a, 2a, and 4.
Dihydrogen Activation and Electronic-Chemical Coupling. Table 2 reports M06/6-311G(2d,2p)//HSE06/6-
Table 1 also shows that substituents on the nanoribbon edges modulate FLP nanoribbons’ band gaps. Electronwithdrawing −CF3 substituents on boron lower the band gap of 1b by 0.48 eV relative to 1a and lower the band gap of 2b by 0.14 eV relative to 2a. Such “tuning” of FLP nanoribbons’ properties is further explored below. Figure 2 shows that the highest occupied and lowest unoccupied crystal orbitals of graphane FLP nanoribbon 1b are, respectively, localized to the Lewis acid and base, such that the relatively small band gaps in Table 1 arise from donor− acceptor transitions. This is evidence of significant frustrated Lewis pair character. For comparison, HSE06/6-311G(2d,2p)//HSE06/6-31G(d) calculations on the hydrogensplitting2 molecular FLP (C6H2Me3)2P(C6F4)B(C6F5)2 (molecule 7) give a highest occupied molecular orbital (HOMO) localized on P, a lowest unoccupied molecular orbital (LUMO) localized on B, and a 2.32 eV HOMO−LUMO gap close to the experimental2 absorbance λmax = 455 nm = 2.72 eV. Frontier orbitals of graphene FLP nanoribbon 2b have more B−N hybridization than graphane FLP nanoribbon 1b. This is consistent with the isodesmic reactions discussed above, which suggest that graphene FLP nanoribbons are more energetically stable than graphane FLP nanoribbons. Figure 3 shows HSE06/6-31G(d) band structures of graphane FLP nanoribbon 1a, graphene FLP nanoribbon 2a, and graphene (polyacene) nanoribbon 4. Band structure calculations use the unit cells of Figure 1. The zero of energy is set to the Fermi level, chosen as the midpoint of valence and conduction band edges. The polyacene band structure is consistent with ref 46, subject to the doubled unit cell. The band structure of 2a is qualitatively similar to the B3LYP band structure of 6 presented in ref 28. Figure 3 shows that graphane FLP nanoribbon 1a has narrow valence and conduction bands, consistent with a relatively low conductivity governed by hopping between donor and acceptor groups. The conduction band of 1a is broader than the valence band, suggesting that this graphane nanoribbon could act as a relatively large-gap n-type semiconductor. 2a has wider bands, implying somewhat higher conductivity.
Table 2. H2 Binding Energy ΔEbind (kcal/mol H2) and Band Gap Change upon H2 Binding ΔGap (eV) species
ΔEbind
ΔGap
2a 2b 7 1a 1b
15.8 6.3 −3.4 −25.8 −38.7
4.4 4.8 0.8 1.7 2.7
31G(d) H2 binding energies ΔEbind and band gap change upon H2 binding ΔGap for molecular and nanoribbon FLPs. Figure 4 illustrates the geometry and frontier orbitals of hydrogen-bound 1b and 2b. Calculations treat the limit of H2 saturation with all boron, nitrogen, and phosphorus atoms tetracoordinate. ΔGap of molecular FLP 7 is the change in HOMO−LUMO gap. The small, negative ΔEbind calculated for
Figure 4. Lowest unoccupied (top) and highest occupied (bottom) orbitals of H2-bound 1b (left) and 2b (right). Details as in Figure 2. 16469
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Figure 5. (Left) Unit cell of substituted graphane nanoribbon 8. Arrows denote the periodic direction. (Right) Calculated band gaps and H2 binding ΔEbind of each nanoribbon. Dotted line shows the calculated ΔEbind of molecular FLP 7. Solid line is a linear regression.
The pattern of Lewis-acid and -base substitution also creates a preferred binding order in ribbons that are partially filled with H2. Binding H2 to every other six-membered ring of 1a gives a large overall dipole moment and a small H2 binding energy of −5.4 kcal/(mol H2). Saturating one edge of 1a with H2 gives a binding energy of −25.5 kcal/(mol H2), comparable to the limit of edge saturation in Table 2. As for molecular FLPs,1 band gaps and H2 activation can also be modulated by changing the substituents on the Lewis acid and base groups. Figure 5 shows how changing the Lewis-acid and -base substituents on graphane FLP nanoribbon 8 tunes its band gap and H2 binding affinity. The Figure shows HSE06/631G(d) band gaps and H2 binding energies ΔEbind for substituted nanoribbons 8a−8i. Adding electron-donating groups to N and electron-withdrawing groups to B tends to decrease the band gap and increase the H2 affinity. For this architecture, relatively modest substituent strengths (8b, 8d, 8e) suffice to give calculated H2 affinities approaching the −2.2 kcal/(mol H2) HSE06/6-31G(d) ΔEbind of molecular FLP 7, known experimentally to bind reversibly H2.2 Wider Nanoribbons. Figure 6 shows calculated geometries and frontier orbitals of wider versions of 1a and 2a. Making graphene FLP nanoribbon 2a wider increases N−B hybridization, decreases the HSE06/6-31G(d) band gap to 0.11 eV, and makes the HSE06/6-31G(d) H2 binding energy 23.3 kcal/ (mol H2) less negative than that of 2a itself. In contrast, making graphane nanoribbon 1a wider gives an HSE06/6-31G(d) band
7 is consistent with its experimentally demonstrated reversible H2 activation.2 The H2 binding affinities of graphene and graphane FLP nanoribbons correlate with their relative energetic stabilities. Graphane FLP nanoribbon 1a binds H2 essentially irreversibly, whereas graphene FLP nanoribbon 2a has negligible H2 binding. Like band gaps, H2 binding is modulated by edge substituents. 1b and 2b have more negative ΔEbind than the unsubstituted variants 1a and 2a. Further optimization of H2 activation by chemical substitution is explored below. A notable result in Table 2 is coupling between H2 activation and electronic properties. Such coupling is known for molecular FLPs, where colored 7 becomes colorless upon binding H2.2 ΔGap is large for graphane FLP nanoribbons, consistent with the above observation that these species’ conductivity arises from the Lewis acid and base rather than the underlying nanoribbon backbone. Comparing Figure 2 with Figure 4 shows that the frontier crystal orbitals of 1b and 2b significantly change upon H2 edge saturation, consistent with the large changes in band gap. This coupling may be useful for sensing applications.50 Architecture and Substituent Effects on Band Gap and H2 Binding. The pattern of Lewis-acid and -base substitution plays an important role in H2 activation by FLP nanoribbons. In the limit of complete H2 saturation, each Lewis pair has a dipole moment pointing along the R2BH2−-R2NH2+ axis. Placing all of the Lewis acids on one edge, as in the conjugated ladder polymer 6 of ref 28, makes all of these dipoles point in the same direction and result in weak H2 binding. M06/6-311G(2d,2p)//HSE06/6-31G(d) calculations on 6 give a H2 binding energy ΔEbind of +27.1 kcal/(mol H2). Alternating Lewis-acid and Lewis-base substituents on each edge, as in conjugated FLP 2a, give a much more negative ΔEbind (Table 2). However, this pattern of alternating Lewisacid and Lewis-base substituents gives nearly irreversible H2 binding for nonconjugated FLP 1a. The alternate nonconjugated FLP nanoribbon 8a (Figure 5) strikes a balance between these extremes, with a M06/6-311G(2d,2p)//HSE06/ 6-31G(d) ΔEbind of only +1.5 kcal/(mol H2). This is consistent with the enhanced energetic stability of 8a relative to 1a. Figure 5 shows that each unit cell of graphane FLP nanoribbon 8a has molecular formula C12B2N2H20. One such unit cell may be formed from 3 and 5 as 3 1 C8H12 + B4 N4H4 → C12B2N2H 20 2 2
(4)
M06/6-311G(2d,2p)//HSE06/6-31G(d) calculations predict that this isodesmic reaction has an energy change of ΔE = +161.0 kcal/mol per unit cell (+0.75 kcal/g), more negative than the +1.55 kcal/g ΔE calculated for 1a.
Figure 6. Calculated geometry lowest unoccupied (top) and highest occupied (bottom) orbitals of wider versions of 1a (left) and 2a (right). Details as in Figure 2. 16470
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gap of 4.33 eV and a HSE06/6-31G(d) H2 binding energy within 1.5 kcal/(mol H2) of 1a itself. This suggests that graphene FLP nanoribbons’ properties may be tuned by changing the nanoribbon width, whereas graphane FLP nanoribbons are more robust to width. The H2 splitting activity of graphene FLP nanoribbons can be further tuned by simultaneously changing the nanoribbon width, Lewis acid and base substitution pattern, and edge substitutents. To illustrate, Figure 7 shows the calculated
Figure 7. Calculated geometry of 9 with and without bound H2.
geometry of graphene FLP nanoribbon 9. This system combines a (2,0) zigzag graphene nanoribbon, alternating B and N substituents on every other edge site, CH3 and CF3 substituents on N and B, and fluorines on the edge carbon atoms. This architecture gives an HSE06/6-31G(d) band gap of 1.06 eV and an HSE06/6-31G(d) H2 binding energy ΔEbind of +3.6 kcal/mol, improving upon the graphene FLP nanoribbons 2a and 2b and approaching molecular FLP 7 (Table 2). Inter-Ribbon Lewis Pairing. The rigid architecture of FLP nanoribbons prevents Lewis pairing of adjacent N−B pairs. However, just as for molecular FLPs, maintaining the frustrated Lewis pair character also requires preventing intermolecular Lewis pairing between adjacent nanoribbons (or distant regions of a single ribbon). Figure 8 presents calculations on nanoribbon dimerization. Unlike all other nanoribbon calculations in this work, these M06-2X//6-311G(2d,2p)//M062X/6-31G(d) calculations treat finite nanoribbon oligomers with hydrogen termination of dangling bonds. Figure 8 shows that oligomers of graphene FLP nanoribbons 2a and 2b have a somewhat larger dimerization energy than graphene itself.55 However, the graphene FLP nanoribbons’ dimerization is not driven by intermolecular Lewis pairing. The boron and nitrogen atoms in dimers of 2a and 2b are still nearly trigonal planar, and the HOMO−LUMO gaps are largely unchanged upon dimerization. In contrast, graphane FLP nanoribbon 1a has very strong intramolecular Lewis pairing consistent with its strong H2 binding (Table 2). The 1a dimer in Figure 8 has a relatively small band gap change due to donor−acceptor transitions from the remaining unpaired Lewis acids and bases. However, aggregation of multiple ribbons of 1a would be expected to give nearly complete inter-ribbon Lewis pairing and loss of H2 splitting activity. (Such architectures, with nanomaterial components linked by strong noncovalent interactions, might have other applications in adaptive or selfhealing materials.56) The bulky −CH3 and −CF3 edge substituents on 8i and the increased separation between B and N groups appear to help prevent inter-ribbon Lewis pairing.
Figure 8. Dimer geometries, dimerization energies ΔEdim (kcal/mol), and dimerization band gap changes ΔGap (eV) for graphane and graphene FLP nanoribbon oligomers.
Lewis-acid and Lewis-base edge substituents into “frustrated Lewis pairs”. Calculations suggest that graphane and graphene FLP nanoribbons show promise for H2 activation, with binding affinities approaching that of molecular FLPs. Both graphane and graphene nanoribbons have rather small band gaps, with the former providing a rare example of a nonconjugated organic semiconductor. The chemical reactivity and electronic properties of FLP nanoribbons may be tuned by changing the Lewisacid and -base substitution pattern of the nanoribbon edges and the steric bulk and electronic properties of Lewis-acid and -base edge substituents. Graphene FLP nanoribbons can also be tuned by changing nanoribbon width, whereas the properties of graphane FLP nanoribbons are more robust to width. Energy calculations and connections to the literature on boroncontaining polymers suggest that these materials may be synthetically accessible. More broadly, the results suggest that
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CONCLUSIONS This work proposes that the well-defined architectures of zigzag graphene and graphane nanoribbons can be used to separate 16471
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functionalized graphane and graphene nanoribbons can provide nanostructured architectures for rationally designed catalysts.
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ASSOCIATED CONTENT
S Supporting Information *
Calculated geometries, total energies, and band gaps of all species and full citation for Gaussian 09. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by startup funds from Texas Christian University. REFERENCES
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