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Electronic Structure and Potential Reactivity of Silaaromatic Molecules Published as part of The Journal of Physical Chemistry virtual special issue “Mark S. Gordon Festschrift”. Yang Yang,† Martín A. Mosquera,† Kwan Skinner,‡ Andres E. Becerra,‡ Vasgen Shamamian,‡ George C. Schatz,† Mark A. Ratner,*,† and Tobin J. Marks*,† †

Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Dow Corning Corporation, Midland, Michigan 48686, United States S Supporting Information *

ABSTRACT: Silicon-based materials are crucial for conventional electronics. The fascinating properties of the new two-dimensional material silicene, the silicon analogue of graphene (one atom-thick silicon sheets), offer a potential bridge between conventional and molecular electronics. The ground-state configuration of silicene is buckled, which compromises optimal constructive overlap of p orbitals. Because silicene is not planar like graphene, it has a lower intrinsic electron/hole mobility than graphene. This motivates a search for improved, alternative, planar materials. Miniaturization of silicene/graphene hybrid monolayers affords diverse siliconorganic and -inorganic molecules, whose potential as building blocks for molecular electronics is unexplored. Additionally, hybridization of pure silicon rings (or sheets) is a versatile way to control the geometrical and electronic characteristics of the aromatic ring. In this work we systematically investigate, computationally, architectures and electronic structures of a series of hybrid silaaromatic monomers and fused-ring oligomers. This includes the thermochemistry of representative reactions: hydrogenation and oxidation. The effect of various skeletal substituents of interest is elucidated as well. We show that the specific location of carbon and silicon atoms, and their relative populations in the rings are crucial factors controlling the molecular geometry and the quasi-particle gap. Furthermore, we suggest that electron-withdrawing substituents such as CN, F, and CF3 are promising candidates to promote the air-stability of silaaromatics. Finally, on the basis of the analysis of benzene-like silaaromatic molecules, we discuss a set of alternative, prototype ring molecules that feature planarity and delocalized π bonds. These motifs may be useful for designing new extended materials.

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INTRODUCTION Nanoelectronics is a rapidly growing field, mainly reflecting the demand for miniaturization of the basic units of electronic circuits. The information processing industry using electronic devices requires units capable of operating at ever higher frequencies. Graphene is a remarkable candidate to fit this need at the nanoscale because of its delocalized π electron clouds and very high electron/hole mobility. Nevertheless, graphene has a negligible band gap,1 implying that significant modification, either physical or chemical, must be performed to transform graphene into a p/n, and a nonzero gap semiconductor. Graphene is, however, a chemically stable material, making the efficient control of its semiconductivity an open challenge. This stability, however, is desirable for applications in medicine,2 biofuels,3 and chemical sensors.4 The versatility and abundance of silicon as a semiconducting material make it the dominant ingredient in the production of all modern circuitry. Research on the miniaturization of transistors and other components offers the opportunity to © XXXX American Chemical Society

explore new types of molecule-based materials that hold promise for fabricating new electronic circuitry building blocks. Thus, two-dimensional sheets,5 single molecules,6 and molecular assemblies,7 nanoribbons,8 and quantum dots,9 are among the currently most studied of such systems. Furthermore, organic electronic materials containing heteroatoms such as silicon, sulfur, nitrogen, and oxygen, are currently active research fields worldwide. Inspired by the remarkable properties of graphene, scientists are actively synthesizing and studying analogous but very different one-atom thick sheet-like materials, for example, borophene,10 silicene,11 germanene,12 phosphorene,13,14 etc. Silicene has been grown on silver surfaces15,16 and displays a honeycomb structure that appears to have extended conjugation. In contrast to graphene having entirely planar sheets, Received: September 20, 2016 Revised: November 1, 2016 Published: November 2, 2016 A

DOI: 10.1021/acs.jpca.6b09526 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A silicene is corrugated, or buckled,17,18 and rows of zigzag alternation can be seen along adjacent atoms. This structural corrugation of silicene causes out-of-plane vibrations that increase the resistivity.19,20 However, in the presence of electric fields, this buckling also allows tuning of the quasi-particle gap,21,22 which should be ideal for fabricating one-atom thick transistors.23−26 Takeda and Shiraishi,21 who reported one of the first theoretical studies of silicene electronic structure, suggest that the distortion in silicene (reduction of symmetry from planar to buckled) is caused by a balance of forces that reduces electron− electron repulsion. In the buckled conformation, the repulsion between the cores is diminished, thereby lowering the energy. Alteration of the chemical composition of silicene is an alternative means of controlling the quasi-particle gap,27−29 with examples including hydrogenation,30 fluorination,31 and lithiation.32 In addition, Şahin et al.33 showed by DFT calculations that a graphene/silicene hybrid, where carbon and silicon occupy alternating positions, has, in contrast, a planar structure and a gap around 4 eV, estimated using the GW method. Further studies exploring properties and/or potential applications of this system are available.34−39 The theoretical40−59 and (in)organic chemistry of silicon is vast,60−63 and diverse. Silaaromatic molecules of interest have been synthesized and characterized, for example, monosilabenzene,64 1,2-disilabenzene,65 1,4-disilabenzene,66 1-silanaphthalene,67 2-silanaphthalene,68,69 9-silaanthracene,70 Si6R6 (R = 2,4,6-triisopropylphenyl), and pentasilaproellane71,72 (Figure 1). Additionally, experimental evidence for 1,3,5-trisilabenzene

Hexasilabenzene is a simple, slightly buckled fragment of silicene. However, the heat of formation of hexasilabenzene is less than that of many of its isomers. For example, synthetically accessible hexasilaprismane82 (Figure 1f) is found to be more energetically stable.83,84 In addition, high-level ab initio calculations performed by Moteki et al.85 and Ivanov and Boldyrev86 reveal several cluster-like hexasilabenzene isomers that are more stable than the prismane or hexagon forms, including the global-minimum isomer with C2v point symmetry. In contrast, silaaromatics with lower silicon content tend to have stable planar structures.87 King et al.88 suggested, on the basis of CCSD(T) calculations, that 1,3,5-trisilabenzene is a stable aromatic molecule, in agreement with the calculations of Drissi et al.89 In contrast, lithiation of the hexagonal Si6 ring can lead to a planar structure.90 However, DFT calculations also indicate that there are Si6Li6 clusters with reduced symmetry that are lower in energy91 with respect to the planar geometry. Although there are already theoretical studies on the silabenzene series, there are few analyses of the corresponding fused-ring oligomers. Motivated by the potential applications in surface science and the engineering of silicene and its molecular fragments, here we analyze the following: First, the planar and corrugated states, and the reaction thermodynamics of a series of silaaromatic monomers and oligomers are analyzed. We find that the silicon-to-carbon ratio and the spatial distribution determine the planarity of the silaaromatic compounds in this series. A homogeneous distribution of silicon and carbon favors planar states and deviations from a homogeneous distribution favors corrugation, especially when silicon is the dominant species. We also argue that electron-withdrawing substituents such as CN, F, and CF3 could be used to moderate the reactivity of silaaromatic rings in general. In the second part of this work we study the electronic structure of small heteroarenes (three-, four-, and five-membered rings) containing mostly Si but also combined with Al, B, Mg, and Be, and with hydrogen, lithium, or sodium on the periphery. The equilibrium geometries of these heteroarenes are flat and show a strong delocalization of the silicon double bond, which are crucial features for the design of new 2-D materials.

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METHODOLOGY The molecular structures studied in the first part of this work were optimized using the B3LYP hybrid functional and the 631+G(d,p) basis set as implemented in Q-Chem92 (version 4.1). The nature of the optimized stationary point was checked by a frequency analysis to confirm that the calculation converged to an energy minimum. Note that some very small imaginary frequencies around 10−50i cm−1 located at terminating H or F atoms of the CH3, C6H5, or CF3 groups are ignored. The planar conformation of a hydrogen-terminated silaaromatic molecule, which could be either an energy minimum or a saddle point, was ensured by imposing symmetry whose subgroups include the Cs point group. For substituted silabenzenes, the planar structures were calculated by forcing ring atoms to relax only in two dimensions while the peripheral substituent atoms were allowed to relax freely. The grid used for DFT calculations was constructed from 120 radial points and 194 Lebedev angular points. This grid is recommended for high accuracy. For the second part of this study, we performed ground-state optimizations using the PBE0 exchange−correlation densityfunctional, and the 6-31G* basis set. PBE0 is a combination of 1/4 Hartree−Fock exchange, 3/4 PBE exchange, and pure PBE

Figure 1. Representative π-conjugated and Si−Si bonded organosilane molecules: (a) 1,4-disilabenzene, (b) 1-silanaphthalene, (c) 9silaanthracene, (d) a hexasilabenzene isomer, (e) pentasilapropellane, and (f) hexasilaprismane. TMS: trimethylsilyl. Tbt: 2,4,6-tris[bis(TMS)methyl]phenyl. Tip: 2,4,6-triisopropylphenyl. Mes: 2,4,6trimethylphenyl.

is available.73 Transition metal complexes with silabenzene have also been synthesized and characterized.74,75 Finally, although silicon prefers tetrahedral coordination,76 higher coordination numbers are known (for example, as shown by Vach in refs.77−79), due predominantly to the available d-shells, giving rise to diverse inorganic and metal−organic materials having a wide variety of structural motifs. Moreover, silaaromatic compounds are very reactive, as shown here, and strategies to stabilize these structures against destructive reactions would be highly desirable.80,81 B


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tetrasilabenzenes, planar 1,2,4,5-tetrasilabenzene (Figure 2g) is the only one with no more than two connected silicon atoms. Indeed, all nonplanar silabenzenes have more than two connected silicon atoms. Let us define the buckling energy as the energy difference between the energy minimum and the planar structure. The buckling energies of trisilabenzenes and tetrasilabenzenes are typically less than 1 kcal/mol (Table S1). Pentasilabenzene and hexasilabenzene have slightly larger, yet small, buckling energies. Thus, at moderate temperatures, some planar silabenzenes can be populated. We remark, however, as shown by Ivanov and Boldyrev,86 silabenzenes with formula Si6−nCnH6 (n < 5), except SiC5H6, are local-minimum geometries in vacuum. These observations suggest that global-minimum isomers with nonplanar structures can be found for hybrid silaaromatic molecules with moderate/high silicon content considered in this work. Oligomers/Fused Polyaromatics. Consider the siliconrich building blocks 1,3,5-trisilabenzene and 1,2,4,5-tetrasilabenzene. The corresponding oligomers are named by the combination of a Roman numeral corresponding to the number of fused rings and a set of numbers indicating the numbers of silicon atoms in each ring. When these two terms are identical, a suffixal small letter is added to distinguish two oligomers. All oligomers based on the 1,3,5-trisilabenzene building block (upper panel of Figure 3) are computed to be exactly planar.

correlation. We employ this functional to maintain consistency with potential solid-state calculations, for which the PBE (purely semilocal) functional is the first choice. The PBE0 functional performs similarly as the widely used B3LYP functional does.93 Some molecular structures were further validated with Møller−Plesset perturbation theory of second order (MP2), with correlation-consistent polarized quadruple-ζ basis sets (cc-pVQZ). MP2/cc-pVQZ has been shown to be a good point of reference for the search of energy-minimum ground states within the Born−Oppenheimer picture.94 The Supporting Information includes a summary of all the optimized structures, Tables S21−23.

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MOLECULAR STRUCTURES Monomers/Small Aromatic Molecules. We begin discussing the molecules shown in Figure 2, which were

Figure 2. Planar cyclic molecules: (a) benzene, (b) silabenzene, (c) 1,2-disilabenzene, (d) 1,3-disilabenzene, (e) 1,4-disilabenzene, (f) 1,3,5-trisilabenzene, and (g) 1,2,4,5-tetrasilabenzene. Distorted cyclic molecules: (h) 1,2,3-trisilabenzene, (i) 1,2,4-trisilabenzene, (j) 1,2,3,4tetrasilabenzene, (k) 1,2,3,5-tetrasilabenze, (l) 1,2,3,4,5-pentasilabenzene, and (m) hexasilabenzene. The solid circles refer to silicon atoms. C−H and Si−H bonds are implicit.

previously studied in ref 86. We use these monomers as building blocks for the polyaromatics considered in the following sections. Carbon-rich aromatic molecules, benzene, monosilabenzene, and the three disilabenzenes (Figure 2a−e) are computed to be exactly planar at their energy minima. Silicon-rich molecules such as pentasilabenzene and hexasilabenzene (Figure 2l,m), however, are buckled and their planar structures have normal modes, normal to the molecular planes, with imaginary frequencies. Displacement of the atoms along these vibrational modes is necessary to begin the search for the energy minima (Table S1). The geometrical pattern for the intermediate silabenzenes (i.e., trisilabenzenes and tetrasilabenzenes) is less straightforward. Although 1,3,5-trisilabenzene (Figure 2f) and 1,2,4,5-tetrasilabenzene (Figure 2g) are planar, the remaining four are slightly buckled. Hence, the distortion of the silaaromatic molecules is affected by the specific arrangement of the silicon and carbon atoms: Symmetric molecules with delocalized Si/C arrangements tend to have planar ground states. For example, planar 1,3,5-trisilabenzene (Figure 2f), which is between benzene and hexasilabenzene, has an alternating Si/C arrangement where all six Si−C bonds are equivalent. Note that 1,3,5-trisilabenzene is the only arrangement with six equivalent aromatic bonds. For the three

Figure 3. Structures of some oligomers built from two different building blocks: 1,3,5-trisilabenzene (upper panel) and 1,2,4,5tetrasilabenze (lower panel).

Some are acene-like and have linearly fused rings; others are expanded in two dimensions. There is only a single way to assemble two neighboring 1,3,5-trisilabenzene rings through the Si−C edge. Regarding the 1,2,4,5-tetrasilabenzene building block, two rings can be fused by sharing either a Si−Si edge or Si−C edge. The former leads to a planar dimer (II-44-a in C


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The Journal of Physical Chemistry A Figure 3), whereas the latter leads to a buckled dimer (II-44-b, Figure 3). Likewise, acene-like trimers and tetramers sharing Si−Si edges are planar and the linearly fused ones sharing Si−C edges are buckled. To design 2-D oligomers from the 1,2,4,5-tetrasilabenzene building block, the Si−C edges must be shared along one direction. The resulting oligomers (shown inside the green subpanel of Figure 3) are all computed to be buckled and have at least three connected silicon atoms. This agrees with the result that nonplanar cyclic molecules have more than two connected silicon atoms. On the basis of the previous results, we can predict the structures of some heteroatomic oligomers constructed from more than one building block (Figure 4). For example,

Figure 5. Coronene-like structures.

silicon analogue, Figure 5, is moderately buckled and its buckling energy is 10 kcal/mol. Coronene-like molecules can be far more distorted when only a fraction of the core or edge carbons are replaced by silicon atoms. Such molecules are found to have a buckybowl shape when only some core carbons are replaced by silicon atoms (molecules VII-3 and VII-4-a, Figure 5). In addition, the central rings (1,3,5-trisilabenzene, 1,2,4,5-tetrasilabenzene, and hexasilabenzene), in VII-3, VII-4-a, and VII-6-a (Figure 5) are only moderately distorted from their free-standing shapes, as are the peripheral rings. The buckling energies are all significant here: 45 kcal/mol for molecule VII-3, 115 kcal/mol for molecule VII-4-a, and 325 kcal/mol for molecule VII-6-a, Table S4, and increase with increasing numbers of core silicon atoms. In molecules VII-4-b, VII-6-b, and VII-12-b (bottom of Figure 5), there exist two-, three-, and six-edge 1,2disilabenzene rings, respectively. With respect to the planar configuration, the 1,2-disilabenzene edge rings initiate the distortion whereas the benzene edge rings still roughly retain their free-standing shapes. Hence, the buckling energy is greater when there are more edge silicon atoms: 4 kcal/mol for VII-4b, 14 kcal/mol for VII-6-b, and 123 kcal/mol for VII-12-b, Table S4. However, the buckling energies are typically smaller than those with core silicon atoms (i.e., molecules VII-3, VII-4a, and VII-6-a). In both cases, buckling appears to arise largely from the inability of the rings to remain coplanar by accommodating the larger silicon atoms and the accompanying greater Si−Si (≈2.2 Å) and Si−C (≈1.8 Å) bond distances.

Figure 4. Predicted structures of some silaaromatic oligomers. Upper panel: planar structures. Lower panel: slightly buckled oligomers.

molecules II-31 and II-42-a are predicted to be planar whereas molecule II-41 is buckled. The 1,2,4,5-tetrasilabenzene ring in II-41 shares a Si−C edge with the neighboring ring. This is reminiscent of the buckling in molecule II-44-b. Similarly, II-43a is computed to be planar whereas II-42-b, II-42-c, and II-43-b are buckled. The trimers III-131 and III-242 are computed to be planar, but the trimer III-141 is nonplanar. For these buckled molecules containing the 1,2,4,5-tetrasilabenzene ring, a plausible explanation for the buckling is that one edge atom is different from the other and this interrupts symmetry and delocalization. As with the monomers, the buckling energies of the nonplanar silicon−carbon hybrid oligomers are in general negligible (Tables S2 and S3); planarizing them is not energetically costly. Coronene is the smallest planar aromatic with rings resembling both the inner and edge aromatic rings in graphene. We examined eight silacoronene-like molecules (Figure 5). These oligomers are named by the combination of a Roman numeral corresponding to the number of rings and then a number indicating the total number of silicon atoms in each molecule. The coronene-like molecules formed by 1,3,5hexasilabenzene monomers are essentially planar. The allD


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SUBSTITUENT EFFECTS Per-Substituted Silabenzenes. A systematic study of benzene and all 12 possible silabenzenes was performed, and the effects of electron-withdrawing and -donating substituents were surveyed: H, F, CH3, CF3, CN, OH, OCH3, and Ph. The structural and electronic effects are similar for different silabenzenes (Tables S5−S17). For example, F, OH, and OCH3 substituents distort the aromatic ring of molecules such as 1,2-disilabenzene. The distortion usually localizes at the Si sites and elongates the participating bonds. The four carbon atoms in the F-, OH-, and OCH3-substituted 1,2-disilabenzenes are nearly coplanar, and the two silicon atoms are buckled out of the plane (Figure 6). As a result, the Si−Si bond lengths are

Table 2. Relative Stability, with Respect to 1,3,5-Si3C3R6, of Two Substituted Trisilabenzene Isomers Si3C3R6 (kcal/mol)

R

1,2,3-

1,2,4-

H CH3 CN Ph CF3 F OCH3 OH

5.2 3.9 14.8 12.8 25.2 −24.3 −10.4 −27.1

11.7 7.0 16.5 13.1 19.2 −13.2 −5.0 −14.2

are raised by CH3, Ph, OH, and OCH3 substituents. Moreover, the LUMO levels are lowered significantly by electronwithdrawing CN, CF3, and F groups. Because increased air stability is frequently observed for compounds with lowered HOMO and LUMO levels,96−99 CN, CF3, and F, especially CN, could afford such stability for the present molecules. Recent theoretical studies have emphasized the potential stability induced by peripheral substitutions with cyano groups.100−103 For example, Zhao et al.104 reported that the CN-shielded boron dodecahedron [B12(CN)12]2− features a remarkable second electron binding energy of 5.3 eV, quite large for a dianion. This molecule could be used for energy storage because of the low binding energies related to salts with cations such as Li+ and Mg+.104 For di-, tri-, and tetrasilabenzenes, each molecule has three possible isomers. Some substituents can alter the relative molecular stability, and the changes in orbital energetics are identical for the di-, tri-, and tetrasilabenzenes (Tables 2, S18, and S19). For the CF3 substituent, the orbital ordering of the two higher lying isomers inverts. For OH, OCH3, and F substituents, the isomer with sequentially more neighboring silicon atoms is stabilized. The Ph, CH3, and CN substituted silabenzenes exhibit the same isomer stability trends as the hydrogen-terminated analogues.

Figure 6. Structures of distorted 1,2-Si2C4R6, R = F (a), OH (b), and OCH3 (c).

considerably longer than the corresponding ones in the H-, CH3-, CF3-, CN-, and Ph-substituted disilabenzenes (Table 1). Note that F, OH, and OCH3 are all formally lone pair containing substituents, and the buckling energies show a significant correlation with the Si−Si bond distances: the longer the Si−Si distance, the higher the buckling energy. It was previously proposed that Si−Si multiple bonds have a pronounced nonbonding electron density character.95 Hence, the present distortions around the silicon atoms are plausibly related to electronic repulsion between the substituent lone pairs and the silicon nonbonding electron densities. In contrast, the Ph and CH3 substituents appear to slightly decrease the buckling of hexasilabenzene (Table S5). For the other nonplanar silabenzenes (Tables S12−S13, S15, and S16), the buckling energies are too small to be distinguishable for H, CH3, and Ph as substituents. The substituent effects on the HOMO and LUMO levels are similar for benzene and all the present silabenzenes. Thus, the HOMO levels are lowered by F, CF3, and CN substituents and

Table 1. Computed Structural and Electronic Properties of Substituted 1,2-Disilabenzenes, (1,2-Si2C4R6) (ΔE = Buckling Energy)

orbital levels (eV)

bond length (Å)

R

ΔE (kcal/mol)

HOMO

LUMO

gap (eV)

Si−Si

Si−C

C−C

C−C

C−C

C−Si

H CF3 CH3 CN F OCH3 OH Ph

n/a 4.0 0.3 0.1 10.7 7.9 20.8 0.2

−5.7 −7.7 −4.9 −8.4 −7.2 −5.4 −5.5 −5.2

−1.4 −4.1 −0.7 −5.0 −3.8 −1.9 −2.4 −1.7

4.3 3.6 4.2 3.4 3.4 3.5 3.1 3.5

2.18 2.18 2.16 2.18 2.31 2.26 2.37 2.17

1.80 1.84 1.81 1.81 1.87 1.86 1.86 1.81

1.39 1.39 1.41 1.42 1.37 1.39 1.39 1.41

1.42 1.47 1.45 1.44 1.43 1.45 1.43 1.45

1.39 1.39 1.41 1.42 1.38 1.39 1.38 1.41

1.8 1.84 1.81 1.81 1.87 1.85 1.86 1.81

E


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electron-withdrawing CN group. In the first case, three terminal hydrogens (those either at the C site or at the Si site, Table 3)

Substituted Double Bonds. In the ground state, and contrary to ethylene, disilene H2SiSiH2 is not planar but has a trans bent geometry,105 shown in Figure 7a. The bend angle,

Table 3. Substituent Effects on the HOMO and LUMO Energies of 1,3,5-Trisilabenzene (eV)

Si site (R1)

C site (R2)

HOMO

LUMO

gap

CH3 CN H H CH3 CN H CH3 CN

H H CH3 CN CN CH3 H CH3 CN

−5.7 −7.6 −5.7 −7.6 −7.1 −6.9 −6.2 −5.2 −8.5

−0.7 −3.1 −1.3 −3.4 −2.6 −2.9 −1.4 −0.6 −4.6

5.0 4.4 4.4 4.2 4.5 4.0 4.8 4.6 3.9

Figure 7. Ground-state geometries of (a) disilene, and definition of the bend angle, θ, (b) F2CSiF2, and (c) (OH)2CSi(OH)2.

θ, depicted in Figure 7a, is 33°, close to the bend angle in hexasilabenzene, 32° (Figures S1 and S2). We focus on the following systems: R2CCR2, R2SiCR2, and R2SiSiR2 (R = H, F, CH3, CF3, CN, OH, and Ph). The OCH3 substituent was not included here because it gives almost the same result as OH. For R2CCR2, the CF3 and Ph substituents are slightly displaced from the molecular plane to release steric repulsions engendered by their bulk, so that the two carbon atoms of the double bond and the four connected atoms are no longer coplanar. For the other small substituents (H, F, CH3, CN, and OH), all six atoms are coplanar. All CC bond lengths are similar, regardless of the substituent (Table S20). For R2Si CR2, the six atoms are coplanar for the H, CH3, and CN substituents. However, for the F and OH substituents, the resulting molecules display trans bent geometries, and the bending angle is larger at the silicon side in R2CSiR2 (Table S20 and Figure 7b,c). The computed CSi bond lengths in the molecules with F and OH substituents (F2CSiF2 and (OH)2CSi(OH)2) are ≈0.2 Å longer than those in molecules with the other substituents, whereas for the R2SiSiR2 molecules, the Si Si bond lengths with F and OH substituents are ≈0.4 Å longer versus the other substituents. All R2SiSiR2 molecules are trans bent, but the bending angles with F and OH substituents are significantly larger than with the other substituents. In contrast, the Ph and CH3 substituents induce less out-of-plane displacements (Table S20). This result parallels the finding above that these substituents tend to decrease the buckling of hexasilabenzene. Indeed, the bend angles of R2SiSiR2 agree very well (within 5°) with those of related Si6R6 molecules (Table S20). Furthermore, the buckling angles of the Si6R6 molecules are compressed by Ph and CH3 substitution, somewhat increased by CF3 and CN substitution, and significantly expanded by F and OH substitution, similar to the bending angles of the R2SiSiR2 molecules. Among all substituents, CN, CF3, and F are the most effective in lowering the LUMO energies of the R2CCR2, R2SiCR2, and R2Si SiR2 molecules. Heteroatomic Substitution. The 1,3,5-trisilabenzene skeleton is an ideal model to investigate the interplay of substituent effects because of its alternating C−Si arrangement. Initially, we consider the electron-donating CH3 group and the

are replaced by substituents. As expected, the effect with three substituents is weaker than that with six substituents, and the HOMO and LUMO energy displacements are in the same direction regardless of the number and location of the substituentsthe effect at the Si site is similar to that at the C site. In the second case, the C and Si sites are substituted by different groups (Table 3). The combination of two substituents with opposite electronic effects leads to mutual weakening, and finally the stronger electron-withdrawing effect from CN dominates. This combined effect is also weaker than that with only three CN substituents.

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THERMOCHEMISTRY AND REACTION PATTERNS Hydrogenation. Initially the series benzene, monosilabenzene, and the three disilabenzenes (1,3,5-trisilabenzene, 1,2,4,5tetrasilabenzene, and hexasilabenzene) was selected to investigate the thermochemistry of selected silaaromatic reaction patterns. The three disilabenzenes allow systematic comparison of a large number of hydrogenated or oxidized isomers. Most hydrogenated products are found to be significantly buckled. There are two exceptions: the 1,2Si2C4H8 molecule (Figure 8a), the Si−Si bond of which is

Figure 8. Planar structures of the two hydrogenated silabenzenzes.

hydrogenated, and a 1,2,4,5-Si4C2H8 molecule, in which one of the Si−Si bonds is hydrogenated, are both nearly planar (Figure 8b). Compared to the corresponding silabenzene, SinC6−nH6, the first hydrogenation product SinC6−nH8 has a compressed HOMO−LUMO gap, and compared to SinC6−nH6, the second hydrogenation product, SinC6−nH10, has the HOMO and LUMO slightly raised to similar extents and thus has a comparable HOMO−LUMO gap. The third hydrogenation F


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silicon atoms, a hydrogenated SiSi bond (Figure S4). Among the intermediate stability isomers, each has a SiC bond hydrogenated and among the least stable isomers, each has a hydrogenated CC bond. Likewise, for the Si2C4H10 isomers, the one with a SiSi bond remaining is the most unstable isomer (Figure S4). These each have a CC bond remaining and are the most stable ones, with the ones each having a Si C bond are of intermediate stability. The first hydrogenation reaction of benzene is endothermic, and this is classically taken as a metric of aromaticity.106 For the first hydrogenation reactions of monosilabenzene and disilabenzenes, products having CC bonds hydrogenated have slightly negative or even positive reaction enthalpies. All the other reactions are exothermic. For all silabenzenes, the second and third reaction enthalpies are far more negative than the first. This trend, again, is attributable to the difficulty in breaking the six-π-electron conjugation. As noted above, aromaticity can be quantified by hydrogenation reaction energetics.106 If the total enthalpy change of the three hydrogenation reactions is ΔHtotal and the three contributing reaction enthalpies are ΔH1, ΔH2, and ΔH3, then the resonance energy equals 3 × ΔH3 − ΔHtotal (Scheme S6). To estimate the resonance energy of the present silaaromatics, we assume that the probability for the reaction to occur at each aromatic bond is 1/6. Benzene has the highest resonance energy, 40 kcal/mol (Table 4), whereas the resonance energy of hexasilabenzene is 21 kcal/mol, which is roughly half of that of benzene.

product, SinC6−nH12, has a lowered HOMO and raised LUMO and thus a larger HOMO−LUMO gap. The trends highlighted above are valid for the other silabenzenes if the conjugated Si/C ratio is altered as little as possible by hydrogenation. Nevertheless, for a particular silabenzene, hydrogenation and oxidation occurring at different sites may lead to isomers with different conjugated Si/C ratios (although the total Si/C ratio is invariant) and very different properties. It is thus first essential to examine the electronic effects of the conjugated Si/C ratio. This effect can be elucidated via the series SinC6−nH6, in which the conjugated Si/ C ratios are the same as the total Si/C ratio. Here the HOMOs are raised, and the LUMOs are lowered; hence the HOMO− LUMO gaps are compressed with increasing Si’s in conjugation (Scheme 1, and S1−S5). For the hydrogenated series Scheme 1. Hydrogenation Reaction Enthalpies (kcal/mol) of Benzene, 1,3,5-Trisilabenzene, and Hexasilabenzene and Orbital Energies (eV) of Their Productsa

Table 4. Hydrogenation Enthalpy-Derived Aromatic Resonance Energies of Benzene and Some Silabenzenes (kcal/mol)

a

There are three hydrogenation steps, and the energy for each step is colored in red. The HOMO, LUMO, and HOMO−LUMO gap of the reactants and products are colored in green, blue, and magenta, respectively.

molecule

ERes

C6H6 SiC5H6 1,2-Si2C4H6 1,3-Si2C4H6 1,4-Si2C4H6 1,3,5-Si3C3H6 1,2,4,5-Si4C2H6 Si6H6

39.5 35.8 30.7 32.8 31.0 31.8 22.0 20.9

Oxidation. Oxidation reactions can generate more products than hydrogenation, yielding products such as epoxides and peroxides. Oxidation may also expand, or rearrange, cyclic molecules by oxygen insertion. Thus, such ring expansion reactions of benzene produces oxepin, 1,4-dioxocin, and 1,4,7trioxonin, which have been experimentally and theoretically investigated and are tautomers of benzene epoxides.107−113 On the basis of the computed reaction energetics, epoxidations are always more exothermic than the corresponding peroxidation reaction (Schemes 2 and S7), even though peroxidation requires two oxygen atoms and epoxidation needs only one for each event. Furthermore, typical epoxidation reactions are usually more exothermic than the ring expansion reactions with three exceptions: benzene monoxide, benzene dioxide, and hexasilabenzene monoxide (Scheme 2). The reaction inserting an oxygen atom into the hexasilabenzene ring is computed to be highly exothermic. Thus, products can be formed containing a three-membered Si−Si−Si ring and a six-membered ring with five silicon atoms and one O (Figure S5). The calculations searching minima for seven-membered rings were found to be unstable; the calculations converge either to

SinC6−nH8 and SinC6−nH10, isomers with greater numbers of hydrogenated Si’s have lower conjugated Si/C ratios and larger HOMO−LUMO gaps (Schemes 1, and S1−S5). Thus, the first hydrogenation reaction of monosilabenzene gives three SiC5H8 products (Scheme S1). Two of them have 25% silicon in their conjugation and thus reduced HOMO−LUMO gaps versus monosilabenzene, which has 17% silicon in its conjugation. This change is consistent with that in benzene, 1,3,5trisilabenzene, and hexasilabenzene (Scheme 1). The product with no silicon in its conjugation has a much larger HOMO− LUMO gap than its isomers. The relative stability of a hydrogenated product in this study is found to depend greatly on its conjugated Si/C ratio. For SinC6−nH8 and SinC6−nH10, the isomer with more hydrogenated Si’s (or less conjugated Si’s) is far more stable and the hydrogenation reaction enthalpy is greater (more exothermic). This is understandable because silicon generally disfavors multiple bonds and stabilizes “dangling” bonds via hydrogen saturation.60 The most stable Si2C4H8 isomer has two ortho G


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Scheme 2. Epoxidation Reaction Energies (kcal/mol) of Benzene (Upper Left), 1,3,5-Trisilabenzene (Upper Right), and Hexasilabenzene (below the Center), and Orbital Energies (eV) of Their Products

Figure 9. Approximately planar central ring structures of some hexasilabenzene epoxides: (a) Si6H6O, (b) cis-Si6H6O2, and (c) cis-Si6H6O3.

The first epoxidation reactions of monosilabenzene and disilabenzenes have slightly negative, or even positive, reaction enthalpies for oxidation of the CC bonds. In silabenzene isomers with both silicon and carbon sites, silicon sites are energetically much more favored for epoxidation, reflecting among factors, the far greater Si−O bond enthalpy. Stereoisomerism, however, does not induce large variations, and no significant energetic differences are found for the orbital energies between cis- and trans-epoxides. The energetic differences between spatial isomers are also small. The relative stabilities of epoxidized disilabenzene isomers are shown in Figures S7−S9. Note that the energy difference between two monoxide or dioxide isomers can be as large as 70−80 kcal/ mol. In contrast, the energy difference between two trioxide isomers is no more than 14 kcal/mol. This is not surprising because monoxides and dioxides may have different Si/C conjugated ratios, whereas the trioxides are fully saturated. For all disilabenzene monoxides, the isomer with two ortho- silicon atoms attached to the epoxy bridge is the most stable. Monoxide isomers with Si−C bonds oxidized have intermediate stability (∼40 kcal/mol less than most stable isomer) and those with oxidized C−C bonds are the least stable (70 kcal/mol less stable). Likewise, for all dioxide isomers, the ones with no conjugated silicon atoms remaining are the most stable and the

the isomeric epoxide or to a transition state, which is 12.5 kcal/ mol less stable and has a substantially distorted sevenmembered ring. Regarding other cyclic products, incorporation of three oxygen atoms (C6H6O3, C3Si3H6O3, and Si6H6O3) leads to completely distorted rings that can barely be regarded as nine-membered rings. In contrast, there are also three rigorously planar structures (with all atoms lying in the same plane) (Figure S6): C6H6O2 (i.e., 1,4-dioxocin), C3Si3H6O, and C3Si3H6O2. As with hydrogenation, the electronic effects of epoxidation can be clearly revealed by comparing benzene, 1,3,5trisilabenzene, and hexasilabenzene. The first epoxidation lowers the LUMO and thus reduces the HOMO−LUMO gap compared to the case for SinC6−nH6 molecules. The second epoxidation lowers both the HOMO and LUMO energies by similar extents. Thus, the gap is not significantly affected by this process. The third epoxidation lowers the HOMO energy and increases the HOMO−LUMO gap. For the other silabenzenes (Schemes S9−S13), the oxidized isomers may have different Si/C ratios, and the above conclusions are still valid if the oxidation minimally interrupts the conjugated Si/C ratio. However, again, products with more oxidized silicon sites have lower conjugated Si/C ratios and thus a larger HOMO− LUMO gap. H


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all the atoms in the ring. Then, the silicon atoms also accept a small charge from the X atom (X = Al, B, Mg, Be). The net positive charge of the X atom generates a weak electric field that delocalizes the electrons of the SiSi π bond. Note that 1magnesium-2,3-bisila-cyclopropene (MgSi2H2), a planar, triangular (isosceles) molecule, is one of the molecules calculated with C2v symmetry. The PBE0-derived HOMO (whose orbital energy is −5.5 eV) of this molecule is strongly delocalized, it extends over the Mg and Si atoms, as shown in Figure 11; the PBE0-derived HOMO−LUMO gap is 2.7 eV (in the blue). The Si−Mg−Si angle is ≈54°, and the Mg−Si−Si angles are each ≈63°. The MP2/cc-pVQZ calculation, as well as PBE0, indicates that the SiSi and Mg−Si bond lengths are approximately 2.23 and 2.45 Å, respectively. In silicene, the Si−Si π-like bond length is16 ∼2.32 Å. In the triangular molecules, however, the stabilized π bond lengths tend to be less than 2.2 Å. For example, the shortest Si−Si distance is found in the molecule BSi2H3, 2.12 Å. Zubarev et al.114 have synthesized and analyzed by several accurate ab initio methods such as RCCSD(T), several triangular molecules, among them, AlSi2− and Si3, using laser vaporization techniques. The ab initio calculations reveal that both AlSi2− and Si3 have a mixed aromatic/antiaromatic character (more types of metallic rings featuring this mixed character can be found in refs115−117). For AlSi2− the authors found that the HOMO has two nodal planes and the HOMO− 1 is a delocalized π cloud. This latter cloud, which resembles the one in Figure 11, is also featured in Si3, but in the HOMO− 2 level. In the triangular molecules studied here, addition of substituents such as hydrogen or alkali metal atoms eliminates the extra electron in the structures studied in ref 114. In these hydrogenated molecules, nonetheless, the triplet state can be close in energy to the singlet configuration. For example, at the CCSD/cc-pVTZ level, the triangle structure with magnesium (MgSi2H2) features an optimized (minimum energy geometry) triplet that is only 0.89 eV above the optimized singlet. In contrast, in the PBE0 equilibrium geometries, the triplet energy of AlSi2H3, at ROHF theory, is 2.0 eV above the singlet energy. The borinated triangle has a higher triplet energy, 2.7 eV above the singlet; this is due to the smaller size of the B atom. Four-Membered Rings. A resonance effect similar to that discussed in the previous subsection is also operative in rings with four members. In Figure 12a we show the HOMO surface of Al2Si2H2. Here there is also slight charge transfer from the silicon atoms to the aluminum atoms, which causes delocalization of the HOMO. For other combinations of atoms, the optimized geometries do not minimize to a planar state. Instead, the configurations tend to form either clusters or

ones with two conjugated silicon atoms (i.e., a SiSi bond) remaining are the least stable. Unlike the buckled hydrogenation products, oxidized silaaromatic products are usually planar. Hexasilabenzene epoxides are also nearly planar, even if hexasilabenzene is moderately buckled (Figure 9). Although hexasilabenzene trioxide is fully saturated, its monoxide and dioxide are still partly unsaturated and the monoxide is even locally conjugated. Hence, the related materials such as silicenes or carbosilicenes may exhibit interesting electronic properties because they maintain part of the conjugation.

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ALTERNATIVE MOLECULAR RINGS To search for alternatives to the set of silaaromatic compounds discussed above with honeycomb shapes, let us now consider small silicon-containing heteroarenes. Figure 10 shows the new

Figure 10. Target molecular models. X = Al, B, Be, Mg, S; Y = H, Li, Na; Z = H, Li, Na or none if X = Mg, S.

target molecular models. Replacing X, Y, and Z by atomic species affords a series of potential molecules to be explored. The optimizations were performed as specified in the Methodology section. Three-Membered Rings. Among the possible molecules with triangular arrangement, we found that all the combinations probed lead to planar structures, except those with X replaced by sulfur, which lead to molecules with pronounced antibonding character, i.e., distorted rings with localized HOMOs. The hydrogenated molecules AlSi2H3, BSi2H3, MgSi2H2, and BeSi2H2 minimize to triangular structures, as depicted in Figure 10. For the triangular forms with aluminum and boron, we find that the triangular shape is stable if the hydrogen atom bonded to aluminum or boron is replaced by sodium or lithium. For fully lithiated forms, the inner triangle is preserved for X = Al, B, whereas for BSi2Li3, two lithium atoms bind to the boron site, and a single lithium atom is shared by the two silicon atoms. All the calculated HOMO−LUMO gaps of the structures studied here are in the UV and blue regions of the electromagnetic spectrum. The computed triangular rings show a stabilization of the π bond between the silicon atoms. In general, the hydrogen atoms act as hydrides and accept a small negative charge from

Figure 11. HOMO surfaces of (a) MgSi2H2, (b) AlSiH3, and (c) BeSiH2 at the minimum-energy ground-state configurations. Here, the additional atom colors are green, cobalt, and orange for Mg, Al, and Be, respectively. I


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Figure 13. HOMO and LUMO of MgSi4H4.

earth atoms in silicene-based structures could improve the electronic conductivity.

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CONCLUSIONS For the carbon/silicon, aromatic, hybrid molecules and materials studied in this work, corrugation is determined by the Si/C ratios and the distribution of these two atoms. Thus, high Si/C ratios correlate with high buckling propensities. Symmetric rings such as 1,3,5-trisilabenzene and 1,2,4,5tetrasilabenzene have greater propensities to maintain exact planarity. Coronene-like silaaromatic molecules with some core or edge carbon atoms replaced by silicon atoms are significantly buckled. Unlike the buckling in hexasilabenzene, this buckling is due to the difficulty in accommodating the larger silicon atoms and greater Si−Si and Si−C bond distances within the cyclic structures. We have also investigated substituent effects on the structural and electronic properties of various silabenzenes. It is found that F, OH, and OCH3 substituents often distort the molecular structures and elongate related bonds at the silicon site. A similar effect is also observed in R2CSiR2 and R2SiSiR2 molecules. This is caused by the electronic repulsion between the lone pairs of the substituents and the nonbonding electron densities of the silicon atoms. The calculations also show that CN, F, and CF3 substituents are promising candidates to promote the air stability of silabenzenes and, by extension, of other silaaromatics and silicene-like materials. For many silabenzenes, H2 and O2 addition reactions can occur at different sites and generate variety of isomers. Reactions occurring at silicon sites are more energetically favorable than at carbon sites. Hydrogenated or oxidized isomers with fewer conjugated silicon atoms usually have lowered HOMOs, raised LUMOs, and consequently enhanced HOMO−LUMO gaps. Hydrogenation usually buckles the six-membered rings, but epoxides are nearly planar. Even for moderately buckled hexasilabenzene, the corresponding epoxides are also planar. Therefore, materials based on the monoxide and dioxide may exhibit some interesting features because they are still partly unsaturated and perhaps locally conjugated. Finally, we discussed a set of conjugated heteroarene building block molecules based on silicon. These units are triangles, trapezoids, and pentagons, where the usual weak sp2 character of pure, homogeneous, structures of silicon is modified by adding heteroatoms such as Al, B, Mg, Be, and S. These modifications lead to stronger delocalized bonds, often with planar molecular structures. The majority of the molecules studied in this work show HOMO/LUMO gaps in the UV region. On the basis of the observations made in previous work and in this study, we believe future directions, especially for

Figure 12. (a) HOMO surface of Al2Si2H4. (b) Cluster form of Al2Si2Li4. (c) and (d), HOMO and LUMO surfaces of LiS3H3.

structures with lone pairs. An example of an optimized cluster structure is shown in Figure 12b: Al2Si2Li4, a counterintuitive molecule in which three lithium atoms are coordinated to two other atoms, and one of the aluminum atoms has four neighbors. The ab initio calculations on the borinated form of the four-ring molecule, B2Si2H4, however, indicate there is a small puckering angle118,119 in this molecule. The B−Si distance in this molecule is short, and the amount of charge transferred is larger than in Al2Si2H4, inducing an attractive force between the nuclei that bends the molecule. Rings similar to those discussed here are being synthesized and characterized in several laboratories. Suzuki et al.120 synthesized a tetrasilabicylobutane. The ring is rhombic and distorted, a conformation identified as a Jahn−Teller polar distortion.120 Other rings can be designed and synthesized; for an updated compendium on experimentally available cyclic disilenes (and other disilenes) and their chemistry, see ref 63. Another special ring we find is XSi3H3, where X is an alkali metal atom. Parts c and d of Figure 12 show the HOMO and LUMO orbital surfaces of LiSi3H3. This molecule can be represented by a resonance structure; the double bond can be drawn on either the left or right of the second Si atom. The calculated binding energy of the lithium atom is ∼190 kJ/mol, close to that of the single bond in the Br−Br molecule.121 Five-Membered Rings. Molecules with five-membered rings obtained following the formula shown in Figure 10 are different. First, the double bonds are most stable between the silicon atoms 2,3 and 4,5. Second, regardless of the X substituent, the pentagon buckles because the amount of charge shared between the atoms is insufficient to stabilize planarity. In Figure 13 the HOMO and LUMO of MgSi4H4 are shown. Note that, due to buckling, the overlap between the silicon atom pz orbitals cannot be optimal. Nonetheless, the available 3p orbitals of the magnesium atom contribute to a noticeable delocalization of the HOMO. In contrast, a large part of the MgSi4H4 LUMO orbital is delocalized over all the silicon atoms, whereas a smaller portion displays a local character. The length of the SiSi double bond in the pentagon is 2.22 Å, and the angle between silicon atoms 2,3,4 (or 3,4,5) is 115°; this angle is close to the silicon bond angle in silicene 116.5°.16 The computed orbital delocalization in the molecule MgSi4H4 suggests that insertion of alkali and alkaline J


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applications in nanoelectronics, include examining the impact of peripheral substitutions, surface functionalization, and their associated HOMO/LUMO energetics, on coherent electronic transport (molecular junctions) and the optical properties of silaaromatic structures, as well as the benefits of silicon−silicon π-bond delocalization. For silicene molecular devices, substitutions with halides are likely to protect the edges against damaging reactions, but inner silicon atoms remain exposed, potentially leading to undesired surface defects. Possible solutions to this issue might involve the use of coating molecular layers, or taking advantage of axial, out-of-plane, electronic transport, as proposed for graphene layers in ref 122, where all the silicon atoms interface either with the electrodes or with silicon/carbon atoms from other layers. These concepts offer an opportunity to investigate alternative mechanisms, based on mechanical stress, for example, to exploit HOMO/ LUMO asymmetry in transport junctions.

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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.jpca.6b09526. Bond distances and angles, HOMO/LUMO energies and gaps, additional molecular drawings, and buckling and reaction energies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*M. A. Ratner. E-mail: [email protected]. Phone: +1847-491-5371. *T. J. Marks. E-mail: [email protected]. Phone: +1847-491-5658. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.Y. and G.C.S. were supported as part of the ANSER center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001059. Y.Y. and M.A.M. were also supported by a grant from Dow Corning Corp.

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