Structures and Electronic Properties of Heavier Congeners of Disk

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Structures and Electronic Properties of Heavier Congeners of DiskLike Molecules: (Si, Ge) Sulflower and (Si, Ge) Olympicene Tarun K. Mandal,*,† Deepthi Jose,‡ A. Nijamudheen,‡ and Ayan Datta*,‡ †

Department of Biotechnology, Haldia Institute of Technology, Haldia − 721657, West Bengal, India Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur − 700032, West Bengal, India

‡

S Supporting Information *

ABSTRACT: The ground-state structures, HOMO−LUMO gaps, singlet− triplet splitting, and the UV−vis absorption spectra for Si, Ge-substituted analogues of the recently synthesized disk-like π-conjugated molecules, like octathio[8]circulene, popularly termed “sulflower”, and 2H-benzo[cd]pyrene, popularly termed “olympicene”, are studied using density functional theory. Unlike their pure organic counterparts, these molecules are found to be nonplanar with substantial puckering from the high symmetric structures. The origin of puckering is traced to pseudo Jahn−Teller (PJT) distortions due to favorable mixing of the occupied molecular orbitals (OMO) and unoccupied molecular orbitals (UMO). Even though the HOMO−LUMO and singlet−triplet gaps for these molecules are smaller than their organic counterparts, the reorganization energies (both hole, λh, and electron, λe) for the Si, Ge analogues of sulflower and olympicene are much higher. Therefore, these molecules are expected to be rather inefficient for field effect transistor (FET) device fabrication.

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candidates for band-gap tuning via external electric fields.14 Also, being heavier than pure carbon-based systems, Si and Ge based 2D systems are expected to have substantial spin−orbit (SO) interactions.15 Due to partial sp3 mode of hybridization in these heavier buckled 2D materials, hydrogenation of silicene and germanene to form silicane and germanane is particularly favorable.16,17 Hence, hydrogenation of silicene and germanene immediately opens up their band gap, thereby providing the material scientists a tool to tune and control the band gap in these systems. Apart from technical importance, understanding the structure, bonding, and reactivity of heavier congeners of organic molecules is of fundamental importance in structural chemistry. Recently, a tricyclic analogue of the silicon benzene, hexasilabenzene (Si6H6) has been synthesized and its structure and electronic properties are substantially different from benzene.18 In the context of new Ge-based molecules, Tamao and co-workers have recently synthesized the geramanium analogue of ketone, namely, the germanone.19 Unlike their organic analogue, a GeO bond is much weaker, which allows addition reactions to the GeO bond. Hence, while gem− diols are unknown for CO, stable gem−diols for germanones have been isolated. The barrier heights for such addition reactions to germanones have been calculated to be about 20− 30 kcal/mol lower than that for the organic ketones.20 In a remarkable experimental accomplishment, Weinert and coworkers recently synthesized the linear-chain oligomeric Ge

INTRODUCTION Polyaromatic hydrocarbons (PAH) are molecules of interest ever since their abundance in carbon soot produced under extreme conditions like those found in the interstellar environment were discovered.1,2 The chemical and physical processes involved in the coalescence of PAH to form interesting nanomaterials like fullerenes, nanotubes, and graphene-like systems have been a subject of sustained research.3,4 PAH being molecules with high symmetry have also been fertile grounds for benchmarking new electronic structure theories to account for their ground-state and excitedstate spectra (including their fine structure due to vibronic splitting).5,6 Also, as a consequence of their high symmetry, many of PAH show sharp electronic transitions at a few particular regions in the spectra depending on the number and arrangement of the aromatic rings constituting them. Such sharp and well-resolved electronic spectra of PAH are now routinely used to quantitatively estimate their concentration in various samples including natural water sources as PAH are known to carcinogenic pollutants.7,8 From a technological viewpoint, PAH-like coronenes and perylene diimides (PDI) are being increasing utilized to fabricate FET-based devices for fast electronic and optical responses.9,10 Recently, the Si and Ge analogues of graphene-like silicene and germanene have been fabricated over surfaces like Ag(111) and Ir(111).11,12 Apart from their linear energy dispersions leading to a Dirac cone just like graphene, these materials should integrate easily with the existing silicon-based MOS FET technology.13 However, unlike graphene, silicene and germanene have a buckled structure that makes them ideal © XXXX American Chemical Society

Received: February 25, 2014 Revised: May 22, 2014

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Figure 1. Optimized structures for (a) C-sulflower, (b) Si-sulflower, (c) Ge-sulflower, (d) C-olympicene, (e) Si-olymicene, and (f) Ge-olymicene.

density over the five fused six-membered σ-framework.27 Quantum chemical calculations have shown that the groundstate structure of olympicene has a C2v symmetry and is 5.7 kcal/mol lower in energy than other low energy isomers.28 Isomerization of olympicene was unfavorable as the barrier, for isomerization was calculated to be as high as 70 kcal/mol. The closed-shell aromatic isomers were minima in the potential energy surfaces for isomerization, while the diradical forms were found to be the transition states. In this article, we have theoretically studied the structures and electronic properties of the Si and Ge analogues sulflower and olympicene. Our calculations show that these compounds are puckered due to pseudo Jahn−Teller distortion from the high symmetric geometries. Key electronic properties like the HOMO−LUMO gaps, reorganization energies, and optical absorption energies for these heavier congeners are compared with their organic counterparts. In the following sections we describe our computational scheme, the results, and discussions and conclusions.

analogue of n-hexane, namely, hexagermane (Ge6).21 Bulky ligands like the isopropyl and the phenyl groups along the linear Ge6 chain protected the linear chain from self-cyclization as well as atmospheric oxidation. Such catenation properties in Ge compounds opens up the possibility of designed polymeric materials from the heavier elements of group IV. Therefore, in our opinion, a molecular understanding of the structure and reactivity of heavier congeners of organic molecules can boost rational design of materials from these systems. In 2006, Nenajdenko and coworkers synthesized a highly symmetric (D8h) circulene, namely, the octathio[8]circulene, which was popularly termed as “sulflower” due to its resemblance with an eight-pointed star (or like a bloom of a sunflower, for the highly imaginative !).22 Soon afterward, one of us showed that planarity of sulflower arises due to the relatively longer C−S bond length in comparison to that of the C−C bond.23 As a consequence, while octathio[8]circulene (sulflower) is planar but its pure C-analogue, octa[8]circulene has a tub-shaped geometry. Planarity of sulflower leads to its ability to bind up to 10 hydrogen molecules via physisorption, thereby making it an attractive material for hydrogen storage. The 2D electron delocalization in sulflower also makes it an ideal candidate for designing organic molecular conductors for FET applications.24 In a similar context of curiosity-driven synthesis of beautiful molecules, a fused penta-aromatic ring structure, namely, 2H-benzo[cd]pyrene (C19H12, ChemSpider ID: 19896409) was synthesized in 2012.25 Due to its close resemblance with the Olympic ring (three linearly fused sixmembered rings on the top + two linearly fused six-membered rings on the bottom), it was popularly called olympicene.26 High resolution scanning tunneling microscopy (STM) imaging of olympicene indeed shows the delocalized π-electron

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COMPUTATIONAL METHODS All the calculations were performed within the hybrid metaGGA DFT functional, namely, the M06-2X functional.29 The Truhlar’s M06-2X functional has been shown to be capable of describing middle-range dispersion interactions that are important for quantitative molecular predictions.30,31 The added advantage of this functional is that it is computationally tractable, and the computed vibrational frequencies are comparable to experimental IR and Raman active stretching modes. The Pople’s 6-31+G (d,p) split basis was utilized for the organic molecules,32 while the TZVP basis set was used for their Si and Ge analogue. The Gaussian 09 suite of programs B

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were utilized.33 Once the minimal energy structures were located on the potential energy surfaces, harmonic frequencies were computed to ensure the absence of saddle-points. Based on the frequency calculations, the vibronic modes that lead distortion from the planar to the nonplanar structures were determined. The sufficiently strong coupled occupied molecular orbitals (OMO) and unoccupied molecular orbitals (UMO) having the correct symmetry to couple with the symmetry of vibronic modes (as available from the character table of the symmetry of the planar molecule) to lead to pseudo Jahn− Teller distortions (PJT) were determined from the molecular orbital eigenvectors and eigenvalues. The singlet−triplet gaps for all the molecules were calculated through the unrestricted open shell version of DFT for the triplet states. Additional broken-symmetry (BS) calculations were performed to verify that the singlet states for the molecules were indeed closed shell rather than open shell.

Table 1. HOMO−LUMO Gaps (in Electron Volts), Singlet− Triplet Gaps (eV), Singlet−Singlet Absorbtion Wavelengths (in nm) and Their Oscillator Strengths ( f), Hole Reorganization Energy, and Electron Reorganization Energies (in eV)

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RESULTS AND DISCUSSION Figure 1a−f show the optimized structures of C-sulflower, Sisulflower, Ge-sulflower, C-olympicene, Si-olympicene, and Geolympicene. As observed, the structures for carbo-sulflower, carbo-olympicene, and silicon-olympicene are planar, while the rest have puckered structures. For sulflower, the extent of puckering increases as one goes down the group IV to such a degree that, while Si-sulflower is bowl-shaped, Ge-sulflower has a puckered bowl-shaped structure. For Ge-sulflower, one of the Ge−S bond connecting the peripheral circumference of the eight sulfur atoms to the inner circumference of the eight Ge atoms is cleaved. For olympicene, substitution of C by Si to form Si−olympicene leads to buckling, while substitituting by the heavier congener, Ge makes Ge−olympicene transform to a bowl-shaped structure. This we believe arises due to the larger van der Waals volume of the S atoms compared to that of the H atoms, which occupy the peripheral positions in sulflower and olymicene, respectively. The important bond lengths, angles, and dihedral angles are reported in the Supporting Information, Table S1. To understand the performance of the heavier analogues of sulflower and olymicene toward electronic and charge-transfer properties, the HOMO−LUMO gaps, singlet−triplet (S−T) gaps, and the electron (λe) and hole (λh) reorganization energies were calculated. For all the systems, the singlet states were well described within a closed shell single configuration. Calculations for open shell singlet state (multiconfiguration, broken symmetry) converged to same structures (and energies) as restricted closed shell geometries. The spin contaminations for the triplet structures were less than 2% from the analytical ⟨S2⟩ 0.75. Table 1 lists the HOMO−LUMO gaps, singlet− triplet gaps, singlet−singlet absorption wavelengths, and hole and electron reorganization energies. The HOMO−LUMO gaps for both the systems decrease significantly due to substitution by heavier atoms like Si and Ge compared to their parent organic systems. For Si (Ge) olympicene and sulflower, the HOMO−LUMO gaps reduce by 45.7% (41.9%) and 46.1% (46.6%), respectively. The S−T gaps decrease with an increase in heavier substitution in the rings. Our calculated S−T gap for C-olympicene (2.89 eV at M06-2X/6-31G(d,p)) is in excellent agreement with the highlevel CR-CC(2,3)/DH value of 2.95 eV.28 Therefore, one might expect that Si and Ge analogues for the bowl shaped molecules are easily excitable under irradiation. This indeed is the case, as the calculated UV−vis spectra are

HOMO− LUMO gap (eV)

S−T gap (eV)

5.97

2.89

3.24

1.15

Colympicene Siolympicene Geolympicene C-sulflower

3.47

0.63

6.57

4.29

Si-sulflower

3.54

0.93

Ge-sulflower

3.51

0.22

λmax (f)

λh

λe

267.38 (0.7) 578.15 (0.4665) 586.97 (0.335) 225.54 (0.9622) 295.7 (0.053) 526.9 (0.038)

0.186

0.181

0.146

0.228

0.507

0.836

0.08

0.059

0.82

0.45

1.18

0.97

red-shifted by as much as 200 nm for the Ge systems. However, it is important to note that the oscillator strength (f) and, therefore, the intensity for these transitions are reduced significantly due to the creation of many metal-like band states with weak absorption intensities. The hole and the electron reorganization energies for the heavier analogues of olympicene and sulflower are substantially higher. Hence, the performance of device for field-effect transistors (FET) based on these heavier analogues are predicted to be rather poor. To understand the origin of the higher reorganization energies in these systems, we investigated the structures for the optimized structures for the cationic and anion forms of these molecules. For the organic systems, the cationic and anionic forms remain planar while for the heavier analogues, the structures get further distorted on ionization or attachment of electrons (see Supporting Information file for structures of the cationic and the anionic forms of olympicene and sulflower). Since the reorganization energy for a molecule is defined as the energy difference between the vertical and adiabatic ionization energy (electron affinity),34 a large distortion in the geometry of the molecule on ionization or electron attachment, stabilizes the adiabatic ionization energy (electron affinity) term. This, therefore, increases the difference between the vertical and adiabatic ionization energy (electron affinity), thereby eventually leading to higher hole (electron) reorganization energy. Clearly, distortion from planarity becomes an important parameter that determines the overall performance of devices while designing new materials from heavier congeners of carbon. Therefore, to understand the origin of the distorted/ buckled structures for the systems, we investigated the possibility of vibronic mixing of the filled states (occupied molecular orbitals, OMO) with empty states (unoccupied molecular orbitals, UMO) under the influence of the distortion modes. The fact that such vibronic mixing leads to stabilization of the distorted/buckled structures in comparison to the undistorted structures via pseudo-Jahn−Teller distortion (or second order Jahn−Teller) is rather well-established in the literature and PJT effects have been extensively discussed.35−37 Recently, there have efforts to rationalize molecular distortions in unusual molecules based on PJT effects.38,39 Based on nondegenerate second order perturbation theory,40 the stabilization of the distorted structure with respect to the C

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Table 2. Symmetry of the Puckering Normal Mode, Ground State Structure, Symmetry of the Electronic States that Couple with the Puckering Mode, and the OMO−UMO Energy Gap (in eV)a symmetry of the puckering mode

a

structure (optimized)

C-olympicene

B1

unpuckered (planar)

Si-olympicene

B1

puckered

Ge-olympicene

B1

puckered

C-sulflower

A2U

unpuckered (planar)

Si-sulflower

A2U

puckered

Ge-sulflower

B2U

puckered

PJT active orbitals A1 × B1 B1 × A1 A2 × B2 B2 × A2 A1 × B1 B1 × A1 A2 × B2 B2 × A2 A1 × B1 B1 × A1 A2 × B2 B2 × A2 E3g × E3u E2u × E2g B2u × B1g A2u × A1g E1g × E1u E2u × E2g E1u × E1g A1g × A2u E3u × E3g E3g × E3u E2u × E2g E1u × E3g A1g × B2u E1g × E3u A2u × B1g

HOMO-8 HOMO HOMO-1 HOMO-6 HOMO-10 HOMO HOMO-1 HOMO-8 HOMO-11 HOMO HOMO-1 HOMO-8 HOMO HOMO-2 HOMO-4 HOMO-8 HOMO-6 HOMO-2 HOMO HOMO-4 HOMO-19 HOMO-13 HOMO HOMO-2 HOMO-6 HOMO-4 HOMO-9

OMO−UMO gap (eV) LUMO LUMO+4 LUMO+5 LUMO+1 LUMO LUMO+6 LUMO+7 LUMO+1 LUMO LUMO+5 LUMO+7 LUMO+1 LUMO+9 LUMO+6 LUMO+8 LUMO+3 LUMO+11 LUMO+3 LUMO+6 LUMO+5 LUMO LUMO+8 LUMO LUMO+2 LUMO+4 LUMO+5 LUMO+7

14.54 (11.03) 11.39 12.27 14.91 9.34 (7.58) 7.88 8.18 9.35 8.98 7.12 7.45 8.98 11.74 (11.63) 12.15 12.71 14.75 14.85 5.48 6.7 8.1 8.6 11.21 3.32 4.95 6.77 8.3 9.74

The OMO−UMO gaps within brackets are at the QCISD/TZVP level.

for C-olympicene, Si-olympicene, and Ge-olympicene are 7.22 eV at M06-2X/6-31+G(d,p), 4.73 eV at M06-2X/TZVP, and 4.57 eV at M06-2X/TZVP, respectively. It is important to note, however, that to quantitatively determine the PJT energy requires accurate calculations for (i) energy gap between the states, (ii) vibronic coupling constants, and (iii) force constants of the two interacting diabatic surfaces. Calculations for these three quantities are nontrivial for a molecule of our size. Previously, MRCI+Q calculations have been performed on molecules like C4F4, C4H4, and Si4H4 to estimate these three quantities to explain their ground state conformational preferences.42 Since our systems have a nondegenerate ground state with large occupied−unoccupied MO gaps, we believe that our single reference calculations (substantiated by QCISD/TZVP level calculations for the computationally tractable systems, see Table 2) are able to capture the energy difference between the PJT active electronic states. This is also supported by the fact that ongoing from Cto Si/Ge-based systems, the relative energy difference between the lowest electronic states (occupied and unoccupied) decreases by as much as 3 eV, thereby facilitating PJT via vibronic coupling.

undistorted structure is proportional to the inverse of the energy gap between states which get coupled under the distortion, that is, ΔEstab ∼ 1/(EUMO − EOMO). So, closer are the energies of the two electronic states that couple, larger is the gain in stabilization due to distortions.41 In Table 2, the symmetry of the puckering mode, ground state geometry, electronic states of the OMO and UMO alongwith the product of their symmetries and the energy difference between the OMO and UMO states (at HF/6-31+G (d,p) level for the organic molecules and HF/TZVP for their Si and Ge analogues) are reported. For C-olympicene, the puckering mode (B1) can couple with the any of the two states (OMO and UMO) whose product will be B1. As shown in Table 2, the lowest energy difference between such set of orbitals is 11.39 eV (HOMO and LUMO +4 states). In cases of Si-olympicene, this gap (between the HOMO and the LUMO+6 states) shrinks to 7.88 eV. For, Geolympicene, the gap (between the HOMO and the LUMO+5 states) is only 7.12 eV. Similarly for C-sulflower, the A2u can couple with the HOMO and LUMO+9 states, which have a gap of 11.74 eV. However, in the case of Si-sulflower, the gap between the HOMO-2 and the LUMO+3 states reduces to 5.48 eV. Further going down the periodic table in the case of Ge-sulflower, the distorting mode (B2u), which can undergo vibronic coupling with the E2u and the E2g states, are very closed spaced with a OMO−UMO gap of only 3.32 eV. Calculations of the OMO−UMO gaps at the hybrid M06-2X level are also in agreement with the HF calculations (see Supporting Information for OMO−UMO gaps at the M06-2X level for the systems). For example, the PJT relevant OMO−UMO gaps

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CONCLUSION Based on a systematic computational study on the heavier analogues of olympicene and sulflower, it is predicted that these molecules, unlike their organic counterparts, are substantially buckled. Si and Ge substitution in these molecules lead to the appearance of a large number of metallic-like states with weak optical absorption. The presence of a large number of D

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(7) Yang, L.; Chen, B.; Luo, S.; Li, J.; Liu, R.; Cai, Q. Sensitive Detection of Polycyclic Aromatic Hydrocarbons Using CdTe Quantum Dot-Modified TiO2 Nanotube Array through Fluorescence Resonance Energy Transfer. Environ. Sci. Technol. 2010, 44, 7884− 7889. (8) Lopez-Tocon, I.; Otero, J. C.; Arenes, J. F.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. Multicomponent Direct Detection of Polycyclic Aromatic Hydrocarbons by Surface-Enhanced Raman Spectroscopy Using Silver Nanoparticles Functionalized with the Viologen Host Lucigenin. Anal. Chem. 2011, 83, 2518−2525. (9) Xiao, J.; Yang, H.; Yin, Z.; Guo, J.; Boey, F.; Zhang, H.; Zhang, Q. Preparation, Characterization, and Photoswitching/Light-Emitting Behaviors of Coronene Nanowires. J. Mater. Chem. 2011, 21, 1423− 1427. (10) Sagade, A. A.; Rao, K. V.; George, S. J.; Datta, A.; Kulkarni, G. U. A Charge Transfer Single Crystal Field Effect Transistor Operating at Low Voltages. Chem. Commun. 2013, 49, 5847−5849. (11) Vogt, P.; De Padova, P.; Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M. C.; Resta, A.; Ealet, B.; Le Lay, G. Silicene: Compelling Experimental Evidence for Graphene-Like Two-Dimensional Silicon. Phys. Rev. Lett. 2012, 108, 155501. (12) Meng, L.; Wang, Y.; Zhang, L.; Du, S.; Wu, R.; Li, L.; Zhang, Y.; Li, G.; Zhou, H.; Hofer, W. A.; Gao, H.-J. Buckled Silicene Formation on Ir(111). Nano Lett. 2013, 13, 685−690. (13) Jose, D.; Datta, A. Structures and Electronic Properties of Silicene Clusters: A Promising Material for FET and Hydrogen Storage. Phys. Chem. Chem. Phys. 2011, 13, 7304−7584. (14) Ni, Z.; Liu, Q.; Tang, K.; Zheng, J.; Zhou, J.; Qin, R.; Gao, Z.; Yu, D.; Lu, J. Tunable Bandgap in Silicene and Germanene. Nano Lett. 2012, 12, 113−118. (15) Liu, C.-C.; Feng, W.; Yao, Y. Quantum Spin Hall Effect in Silicene and Two-Dimensional Germanium. Phys. Rev. Lett. 2011, 107, 076802. (16) Osborn, T. H.; Farajian, A. A.; Pupysheva, O. V.; Aga, R. S.; Lew Yan Voon, L. C. Ab Initio Simulations of Silicene Hydrogenation. Chem. Phys. Lett. 2011, 511, 101−105. (17) Bianco, E.; Butler, S.; Jiang, S.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. Stability and Exfoliation of Germanane: A Germanium Graphane Analogue. ACS Nano 2013, 7, 4414−4421. (18) Abersfelder, K.; White, A. J. P.; Rzepa, H. S.; Scheschkewitz, D. A Tricyclic Aromatic Isomer of Hexasilabenzene. Science 2010, 327, 564−566. (19) Li, L.; Fukawa, T.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. A Stable Germanone as the First Isolated Heavy Ketone with a Terminal Oxygen Atom. Nat. Chem. 2012, 4, 361−365. (20) Jissy, A. K.; Meena, S. K.; Datta, A. Reactivity of Germanones: Far Removed from KetonesA Computational Study. RSC Adv. 2013, 3, 24321−24327. (21) Roewe, K. D.; Rheingold, A. L.; Weinert, C. S. A Luminescent and Dichroic Hexagermane. Chem. Commun. 2013, 49, 8380−8382. (22) Chernichenko, K. Y.; Sumerin, V. V.; Shpanchenko, R. V.; Balenkova, E. S.; Nenajdenko, V. G. “Sulflower”: A New Form of Carbon Sulfide. Angew. Chem., Int. Ed. 2006, 45, 7367−7370. (23) Datta, A.; Pati, S. K. Computational Design of High Hydrogen Adsorption Efficiency in Molecular “Sulflower”. J. Phys. Chem. C 2007, 111, 4487−4490. (24) Mohakud, S.; Pati, S. K. Large Carrier Mobilities in Octathio[8]circulene Crystals: A Theoretical Study. J. Mater. Chem. 2009, 19, 4356−4361. (25) Mistry, A.; Fox, D. unpublished work. For information, see http://www.chemconnector.com/2012/05/27/the-story-ofolympicene-from-concept-to-completion/. (26) http://www.bbc.co.uk/news/science-environment-18206015. (27) Youtube commentary of the methodology of STM imaging: https://www.youtube.com/watch?v=dFp8Eoh_Vqo&feature=player_ embedded.

electronic states though reducing the HOMO−LUMO gaps for the heavier systems also lead to stronger vibronic mixing between the occupied and unoccupied molecular orbitals. As a consequence, the buckled/bowl shaped structures are stabilized due to pseudo Jahn−Teller distortions. The hole and electron reorganization energies for these buckled structures for the heavier congeners are found to be substantially higher than the organic molecules. Therefore, FET devices based on these materials are predicted to have much less performance and utility. This we believe shall serve as an important guideline while designing new materials based on heavier elements of the periodic table. Though an understanding of the structure and properties of new molecules and materials is of fundamental importance from the viewpoint of basic chemistry, one needs to exert caution43 before claiming that many of these novel molecules can find immediate applications for designing devices. With clever methods of synthesis and fabrication, it should be possible to circumvent many of the drawbacks of using heavier congeners of organic systems in devices. Nevertheless, it is important to note that the electronic structure and, hence, the chemical and physical properties of the heavier elements are distinctly different from their organic counterparts.

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ASSOCIATED CONTENT

S Supporting Information *

Cartesian coordinates, electronic energies, harmonic frequencies, and important structural parameters, plots of important PJT active OMO and UMO orbitals, their energy differences, and the complete Gaussian 09 reference. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +91-33-24734971. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.K.M. thank CSIR India for financial assistance. A.D. thanks DST, CSIR, and INSA for partial funding.

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp501954m | J. Phys. Chem. C XXXX, XXX, XXX−XXX