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Density functional theory study on the [5,6]-diaryl-methano fulleroids of C70 with different functional groups

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We investigated the functionalization of [5,6] bonds of a C70 fullerene by diphenyl-methano (DPM) derivatives using density functional theory calculations. It was found that the stability of [5,6]-DPM-fulleroids (products) displays the same trend (α
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  Density functional theory study on the [5,6]-diaryl-methano fulleroidsof C 70  with different functional groups Zahra Rostami  a ,  * , Maziar Noei  b a Department of Chemistry, Payame Noor University (PNU), P.O. Box, 19395-3697, Tehran, Iran b Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran a r t i c l e i n f o  Article history: Received 23 August 2016Received in revised form11 September 2016Accepted 12 September 2016Available online 28 September 2016 Keywords: C 70 FunctionalizationDFTElectronic properties a b s t r a c t We investigated the functionalization of [5,6] bonds of a C 70  fullerene by diphenyl-methano (DPM)derivatives using density functional theory calculations. It was found that the stability of [5,6]-DPM-fulleroids (products) displays the same trend ( a  >  b  >  g  >  d )  to that detected experimentally. The re-action energy is calculated to be in the range of   10.1 to  20.5 kcal/mol in the toluene solvent. We alsoinvestigated the effect of different  para -substituent groups on the reaction and electronic properties.Amino group (especially e N(CH 2 CH 3 ) 2 ) in the  para  position of the phenyl groups of the DMP (comparedtothe epoxy functionality used in the experiment) may make more easier the synthesis of the [5,6]- DMP[70]  󿬂 uorides, releasing much more energy. Also, amino groups signi 󿬁 cantly increase the electricalconductivity and electron emission, and make the fullerene more suitable acceptor for solar cells. Wefound a linear relationship between the LUMO and reaction energy, and the  para -Hammett constant of the substitutes. Theoretical orbital and NMR analyses explain the experimentally observed UV  e visiblespectrums and NMR data, con 󿬁 rming the [5,6]-fulleroids production rather than [6,6]-methanofullerenes. ©  2016 Published by Elsevier Ltd. 1. Introduction Carbonaceous fullerenes, graphene, and carbon nanotubes(CNTs) have become the center of considerable attention in nano-science [1 e 6]. These nanostructures have been of special interestand recognized as key materials for use in advanced nanotech-nology, and intensive researches have been focused on exploringthe reactivity and preparing new derivatives [7 e 10]. Fullerenes arethe most interesting compounds for the solar cell devices due totheir exceptional optoelectronic properties [11 e 13]. Compared tothecommon [60] fullerene, abetterlight absorption isobservedforthe higher fullerenes such as [70] fullerene in the visible region[14]. Therefore, a higher photocurrent values and better powerconversion are expected. For instance, the popular phenyl C 71 butyric acid methyl ester (PCBM) mixed with  p -conjugated poly-mers displays a rise of over 50% in the photocurrent valuescompared to the corresponding C 60  [15]. The PCBM derivativesynthesis for [70] fullerene is more dif  󿬁 cult because of its lowersymmetry in comparison to the C 60 .Previously, two types of PCBM derivatives have been synthe-sized namely [6,6]-methanofullerenes and [5,6]-fulleroid whichhave employed in the solar cell (organic photovoltaic devices)[16,17]. The [5,6]-fulleroids are less stable and their synthesis ismore dif  󿬁 cult than the [6,6]-methanofullerenes [14]. One kind of PCBMs is diphenyl-methano (DPM) bridged fulleroid in which asymmetricmoleculereactswiththefullerenes[14].The[5,6]-open-DPM fulleroids are less known because of synthetic dif  󿬁 culties dueto their less stability. Generally, the PCBM derivatives are synthe-sized by two cycloaddition mechanisms [18 e 20]:  i ) 1,3-dipolarcycloaddition of diazocompound to the fullerene which produces a[5,6]-fulleroid and releases an N 2  molecule, and  ii ) thermolysis of diazocompound forms carbenes which can be added to the [6,6]bonds and produces methanofullerenes.Different properties of [5,6] DPM-fulleroids remained approxi-mately unexplored and scare investigations exist on these de-rivatives because of synthetic problems. Recently, the synthesis of [5,6] DPM-[70] fulleroids has been reported by Vidal et al. [14].They have characterized these derivatives of C 70  fullerene bydifferentmethodssuchasmass,andUV  e visiblespectroscopiesandnuclear magnetic resonance (NMR) [14]. Besides, several theoret-ical studies have scrutinized different aspects of the *  Corresponding author. E-mail address:  zahrarostami.pnu@gmail.com (Z. Rostami). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum http://dx.doi.org/10.1016/j.vacuum.2016.09.0240042-207X/ ©  2016 Published by Elsevier Ltd. Vacuum 134 (2016) 48 e 53  functionalization of different nanostructures, helping further un-derstanding the reaction mechanisms, energetic and structuralparameters, thermodynamic a kinetic aspects,  etc.  [21 e 24]. Herein,we explore the different isomers, energetic, electronic, and struc-tural properties of [5,6] DPM-[70] fulleroids using density func-tional theory (DFT) calculations, comparing the results with thoseof the experiment [14]. 2. Computational details We investigate the below reaction following the experimentalwork [14]:C 70 þ DPM ((Ph-C 3 H 6 COOCH 3 ) 2 CN 2 ) / [5,6]-DPM-[70]fulleroid þ N 2 þ E r  (1)The reaction energy (E r ) is predicted as follows:E r ¼ E ([5,6]-DPM-[70] fulleroid) þ E(N 2 )  E(DPM)  E(C 70 ) (2)where E ([5,6]-DPM-[70] fulleroid), E(N 2 ), E(DPM), E(C 70 ) are thetotal energies of the optimized [5,6]-DPM-[70]fulleroid, N 2  mole-cule, aryl-aryl diazocompound (DPM), and C 70  fullerene, respec-tively.NMR,densityofstates(DOS)andnaturalbondorbitals(NBO)analyses, the structural optimization, and energy calculations wereperformed by using B3LYP density functional jointed with 6 e 31G*basis set. The vibrational frequency analysis was performed at thesame level of theory to prove that all the structures belong to thetrue local minima. The B3LYP is a dependable and commonly usedapproach which has been exposed that predicts nearly satisfactoryenergy and electronic properties for carbon nanostructures[25 e 29]. For example, Zandler et al. have shown that the B3LYPgives reliable geometry and electronic properties for porphyr-in e fullerene complexes [25]. The results of Matsuda et al. havealsoindicated that the B3LYP leads to very accurate band gaps for CNTsin comparison with the experimental data, suggesting its use indesigning these materials [26].Chloroform was used as a solvent (like the experimental work[14]) and the tetramethylsilane as a reference for the NMR calcu-lations. The solvent in the other calculations is toluene. The polar-izable continuum model (PCM) was used as to model the solvents[30]. GAMESS program was employed to execute the all calcula-tions [31]. The HOMO-LUMO gap (E g ) is de 󿬁 ned as E g  ¼  E LUMO -E HOMO,  where E LUMO  and E HOMO  are energy of HOMO and LUMO.The change of E g  as a parameter of the electronic sensitivity of theC 70  toward the DPM is calculated by D E g ¼ [(E g2  E g1 )/E g1 ]  100;where E g1  and E g2  are the E g  values of the pristine and function-alized C 70,  respectively. 3. Results and discussion The optimized structure of a C 70  fullerene and its DOS plot areshown in Fig.1. The C 70  nearly has a spheroidal shape (like a rugbyball) with 25 hexagonal and 12 pentagonal rings. It differs from the[60] fullerene in an aromatic belt, containing  󿬁 ve hexagonal rings.The symmetry is D 5h  and there exist four [6,6] ( a e d ) and four [5,6]( 1 e 4 , Fig. 1) different bonds (numbers 6 and 5 designate the hex-agonal and pentagonal rings, respectively). The bond  1  is theshortest one with the length of 1.39 Å and the bond  4  is the largestone with the length of about 1.47 Å. The DOS plot indicates that theHOMO and LUMO levels lie at   5.89 and   3.17 eV, respectively,generating an E g  of 2.72 eV (Table 1).Two types of bonds of C 70 , namely [6,6] and [5,6], can be func-tionalized by the DPM, affording [6,6]-methanofullerenes or [5,6]-fulleroids. In this work, we will follow the experimental work inwhich the synthesis of different isomers of DPM-[70] fulleroids hasbeen reported [14]. As the DPM compound is symmetric and thereexistfourdifferent[5,6]-bonds,fourdifferentDPM-[5,6]-fulleroidscan be formed as shown in Fig. 2. The reaction energy is a properparameter for quantitatively considering the site-selectivity of theDPM addition to the C 70  molecule. Table 2 indicates that the moststable complex is  a  with the reaction energy of    20.5 kcal/molwhich is followed by  b ,  g , and  d  complexes. After the functionali-zation of C 70  the length of bond  a  in complex  a  is increased from1.39 to 2.11 Å, representing a bond cleavage. Based on the NBOanalysis, the hybridization of two carbon atoms of C 70  which Fig. 1.  Optimized structure of a C 70  fullerene and its density of states (DOS) plot. En-ergy in eV.  Table 1 The reaction energy of different [5,6] bonds (a, b, c, and d, Figs.1 and 2) of C 70  withdiphenyl-methano (DPM) derivatives in kcal/mol. Energy of HOMO, LUMO and theHOMO-LUMO gap (E g ) of C 70  and its complexes (eV). The  D E g  indicates the per-centage of change of the E g  of the C 70  after its reaction with DPM derivatives.Compound E r  E HOMO  E LUMO  E g  % D E g C 70  e   5.89   3.17 2.72  e a   20.5   5.58   3.07 2.51   7.79 b   18.3   5.64   3.04 2.60   4.52 g   13.7   5.57   3.04 2.52   7.19 d   10.1   5.59   3.07 2.52   7.39  Z. Rostami, M. Noei / Vacuum 134 (2016) 48 e 53  49  contributed in the reaction is changed from sp 2.16 to sp 2.98 , and theWiberg index of the [5,6]-bond is decreased from 1.06 to 0.18,approving the bond cleavage.The bond  a  (Fig.1) is the more strained bond because it is in thepole of the apex of C 70 . This comparatively high strain makes thebond a morereactivefortheDPMaddition.Theequatorialregionof the C 70  has lower bond strain because of the smaller curvature.Thus, the equatorial bonds such as  d  are less reactive and there ishigher energy barrier to be overcome. The HPLC data from Ref. [14](Fig. 3) indicate that the complex  a  ( a - [5,6] fulleroid in theexperiment) has the highest peak among the all isomers. Also, itshows that the formation of complex  d  is negligible and a peak forthiscomplexisnotappearedintheHPLC.Thisisingoodagreementwith our  󿬁 ndings which demonstrates that the complex  d  with thesmallest stability (E r  ¼  10.1 kcal/mol) may not be producedcompared to the other isomers.Our calculated  1 H NMR of the most stable complex ( a ) in Fig. 4shows that the most shielded hydrogens are three hydrogens of  e CH 3  groups which appear at 0.75 e 0.99 ppm. The experimentalvalue is about 0.99 ppm [14] being in good agreement with ourresults.Goingfromhydrogensof the e CH 3 towardtheoxygenatomthroughthealkylchainthehydrogensshift todown 󿬁 eldbecauseof inductive electron withdrawing effect of the oxygen atom. TheelectronegativeoxygenatomdrawselectronsawayfromadjacentHatoms on the alkyl chain, thus, these hydrogens display lower  󿬁 eldsignals. Two hydrogens of the  e CH 2 e  (methylene) group attachedto the oxygen atom appear signals at 3.57 e 3.80 ppm Fig. 2.  The complexes of   a ,  b ,  g , and  d - DPM - [5,6]-[70] fulleroids (see Table 1).  Table 2 The reaction energy of a- [5,6] bond of C 70  with diphenyl-methano (DPM) which isfunctionalized with different groups (kcal/mol). Energy of HOMO, LUMO and theHOMO-LUMOgap (E g ) of these complexes in eV. The D E g  indicatesthepercentage of change of the E g  of the C 70  after the reaction.  s p  is the Hammett para constant of different functional groups.Functional group E r  E HOMO  E LUMO  E g  % D E g  s p NEt 2   26.9   4.75   2.96 1.78   34.38   0.87NMe 2   26.6   4.82   2.97 1.85   32.06   0.83NH 2   25.5   4.98   2.97 2.01   26.19   0.66OCH 3   20.3   5.59   3.05 2.53   6.93   0.27H   17.8   5.63   3.08 2.55   6.35 0F   17.4   5.70   3.14 2.56   5.93 0.06COOH   16.2   5.77   3.14 2.57   5.48 0.45CF 3   15.9   5.47   3.17 2.57   5.66 0.54CN   14.4   5.85   3.27 2.58   5.66 0.66NO 2   13.2   5.89   3.31 2.58   5.14 0.78  Z. Rostami, M. Noei / Vacuum 134 (2016) 48 e 53 50  (Exp.~3.91 e 4.12ppm[14]).Thehydrogensonthephenylgrouparedivided to two categories including  meta  and  ortho  ones. Substit-uent  e OC 4 H 9  can act both as an electron withdrawing agent(because of high electronegativity of oxygen atom, thus causing ashift to down 󿬁 elds) and as a lone pair donor to the aromatic ring,thus causing a shift to up 󿬁 elds). The  meta  and  ortho  hydrogenscause signals at 6.66 (exp. ~ 7.19) and 8.05 (exp. ~ 8.47) ppm,respectively. The mesomeric effect of   e OC 4 H 9  can be used toelucidatethepredictedshieldingsinthearomaticring,inwhichtheprotons in the  ortho  sites are more powerfully shielded than in the meta  site.Our NMR results are in goodagreement with those of theexperiment.It is shown in Table 1 that upon the C 70  functionalization byDPM, the LUMO level is not changed signi 󿬁 cantly. The change of LUMO energy in the DPM-[70] fulleroids compared to the C 70  isabout 0.10 eV. In consistence with this result, the experimentalcyclic voltammetry (CV) the  󿬁 rst reduction potential (E 11/2 : ~   1.03 V) of these complexes is nearly equal to that of C 70  ( E 11/2 : ~  1.02 V) [14]. It is well known that the  󿬁 rst reduction potentialin is related to the LUMO energy and the higher LUMO levelscorrespond smaller reduction energies [32]. Table 1 demonstrates that after the functionalization, the E g  is also slightly narrowed.These results are in good agreement with the experimentalUV  e visible spectrums which show that the change of absorptionmodes are not signi 󿬁 cant [14]. The negligible change in the E g  andUV  e visible indicates that the  p -conjugated system (double [6,6]bonds) of C 70  is not affected upon the functionalization and theproducts are [5,6]-fulleroids rather than [6,6]-methanofullerenes.The  p -homoconjugation is not disturbed and the energy of HOMO and LUMO levels is not considerably changed (Table 1).The experimental UV  e visible spectrum for the most stablecomplex  a  in the toluene solvent (Fig. 3) demonstrates threedistinct peaks at 476, 394, and 338 nm which correspond toapproximately 2.61, 3.14, and 3.67 eV, respectively [14]. Theseexperimental energies are proportional to the electron transferfromthe HOMOof complex a tothe LUMO,LUMO þ 1, andLUMO þ 2which are calculated to be about 2.51, 3.15, and 3.48 eV, respec-tively. Upon absorption process in a compound, the lowest opticaltransition speci 󿬁 es the optical gap [33]. The experimental optical gap is about 2.61 eV corresponding to the theoretical E g  which isabout 2.51 eV. This shows a good agreement between the experi-mental and theoretical calculations.As experimentally the synthesis of [5,6]-fulleroids has beenreported to be a dif  󿬁 cult task, we investigate the effect of differentfunctional groups on the reaction and different other parameters.To this aim, we replaced the e OC 4 H 9  groups on the phenyls of thecomplex  a  by different groups as listed in Table 2. The results Fig. 3.  The experimental HPLC and UV  e visible plots for the complex  a  which werereproduced from Ref. 14 with permission from The Royal Society of Chemistry. Fig. 4.  Calculated H NMR for the complex  a  (Fig. 2).  Z. Rostami, M. Noei / Vacuum 134 (2016) 48 e 53  51  indicate that the electron withdrawing groups especially the e NO 2 and e CNones signi 󿬁 cantly makethe reaction energy morepositiveand are not favorable for the addition reaction. While electrondonating groups (compared to  e H group) present a stronger re-action and more negative reaction energy. Interestingly, we foundthatamino groupsare more favorable for [5,6]-fulleroids formationcompared to the epoxy ones which have been used in the experi-mental work [14]. Especially, the  e N(CH 2 CH 3 ) 2  group gives thelargest negative reaction energy (about  26.9 kcal/mol).The results in Table 2 indicate that the reaction energies withdifferent functional groups are proportional with their  para  Ham-mett constant ( s p ). As shown in Fig. 5, this relationship is nearlylinear with R  2 ¼  0.9494 and by decreasing the value of   s p  the re-action energy becomes more negative. When amino groups aresubstituted in the  para -position of the phenyl groups, the reactionis stronger and the hexagonal rings nearest to the binding sites areperturbed so that the length of the nearest C e C bonds ( 1 , Fig.1) tothe binding carbons is increased from1.39 to1.49 Å, decreasing the p  orbitals overlap. Thus, this breaks the  p -conjugated system of these rings and changes the E g  from 2.72 eV in the pristine C 70  to2.01,1.85, and 1.78 eV in the complexes inwhich the  para  positionsof phenyl groups are substituted by  e NH 2 ,  e N(CH 3 ) 2 , and e N(CH 2 CH 3 ) 2 , respectively. Overall, the major result of amino-substitution is a substantial increasing the HOMO level with asmaller change in the LUMO. However, this change of LUMO tohigherenergiesis ofgreatimportancefor thesolarcell applicationsof the fullerenes [34]. We  󿬁 nd approximately a linear relationshipbetween the  para  Hammett constant ( s p ) of the substituent groupswith the LUMO energy as shown in Fig. 6. It indicates that thefunctional groups with more negative  s p  more increase the LUMOlevel of the fullerene system; thereby, increasing the performanceof a solar cell.It has been previously shown that the functionalized [70] ful-lerenes are interesting materials as an acceptor for the solar cellsystems [34]. In the solar cell systems improving the open-circuit voltage (V  OC ) will signi 󿬁 cantly rise the performance of cell [34].The V  OC  and performance of a solar cell system is depended on thecharge transfer between the LUMO of the acceptor and the HOMOof the donor. Thus, the smaller gap between these orbitals giveseasier charge transfer and higher V  OC  which is possible byincreasing the LUMO level of the acceptor. We concluded that thenitro groups in the  para  position will increase the performance of [70] fullerene derivative as a solar cell acceptor (Fig. 6).The HOMO level is much more affected by the substitutinggroups especially bytheamino groups (Table 2). InFig.6,weshows the change of HOMO by the  para -Hammett constant of the func-tionalgroups.Asitcanbeseentherelationshipisnotlinearasgoodis it is for the LUMO level. However, the electron donating andwithdrawing groups shift the HOMO level to higher and lowerenergies. This trend can help the development of the electronemitters which use the fullerene derivatives. Nanostructures suchas fullerenes, nanotubes, graphene, and so on extensively havebeen studied as electron emitters [35 e 38]. For easier electronemission from the surface of a material, it is needed that the workfunctiontobereduced.ItiswellknownthatthehigherHOMOlevelthe smaller work function [39]. Thus, electron donating functionalgroups, especially  e N(CH 3 ) 2  and  e N(CH 2 CH 3 ), can improve the 󿬁 eld emission properties of the [70] DPM-fulleroids. Finally, it canbe deduced that the electronic properties of [70] DPM-fulleroidscan be tailored (depending on their applications) by usingdifferent functional groups on the  para  position of phenyl groupswith different Hammett constants. 4. Conclusions The functionalization a C 70  fullerene DPM derivatives wasinvestigated using DFTcalculations in toluene solvent. The stabilityof [5,6]-DPM-fulleroids demonstrates the same trend( a > b > g > d )  to that detected experimentally. We investigate theeffect of different functional groups on the reaction and different Fig. 5.  The para-position Hammett constant of different functional groups against thereaction energy of Eq. (1). Fig. 6.  The para-position Hammett constant of different functional groups against theHOMO and LUMO of different DPM - [5,6]-[70] fulleroids.  Z. Rostami, M. Noei / Vacuum 134 (2016) 48 e 53 52
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