Superatoms-Induced Effects of Phenalenyl Ï-dimer on NICS and NLO

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Superatoms-Induced Effects of Phenalenyl #-dimer on NICS and NLO Property: Not Always Enhancement Feng-Yi Zhang, Hong-Liang Xu, and Zhongmin Su J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05234 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017

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The Journal of Physical Chemistry

Superatoms-Induced Effects of Phenalenyl π-dimer on NICS and NLO Property: Not Always Enhancement Feng-Yi Zhang, Hong-Liang Xu*, and Zhong-Min Su* Institute of Functional Material Chemistry, National & Local United Engineering Laboratory for Power Batteries, Department of Chemistry, Northeast Normal University, Changchun, 130024, China.

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Abstract The introduction of the Li3O and BeF3 can exhibit an interlayer charge-transfer transition character for pure phenalenyl (PLY) dimer. A considerable question emerged: Can we modulate the interlayer charge-transfer transition character with different superalkalies and superhalogens? In the present work, nine complexes were designed with three superalkalis (Li3O, Li3S and Na3O) and three superhalogens (BeF3, BeCl3 and MgF3) introduced into the PLY dimers respectively. Results showed that the geometric structure, interlayer charge-transfer and electron transition character of PLY dimer were alternated by the superalkalis and superhalogens, especially for superalkali, with the increasing of atomic number for alkali metals, interlayer distance and interaction energy between PLY monomers increased, while first hyperpolarizability (βtot) decreased. The results of TD-DFT calculation indicated that the variation of βtot was mainly caused by the alteration of the difference in the dipole moments between the ground state and the crucial excited state ∆µ. Otherwise, due to the introduction of superatoms, the distribution of HOMO and LUMO was changed greatly compared with pure PLY dimer. In addition, aromaticity was also analyzed, and it was intriguing to find that introduction of superatoms caused asymmetric distribution of aromaticity along the central-carbon axis. All above consequences suggested that the introduction of superatoms was really an effective strategy to alter the structure and enhanced the nonlinear optical property of the system.


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Introduction Phenalenyl (PLY), a polycyclic odd-alternant hydrocarbon with a planar D3h symmetric πelectronic system, is one of the fundamental delocalized neutral radicals. Since PLY radical was first claimed by Reid in 1958 1, it has attracted many scientists due to its radical characters 2-9. For instance, some researchers aimed at aggregating diphenalenyl diradicals to get larger nonlinear optical (NLO) properties

7, 10-14

. Prof. Nakano et al. (2009) found that increasing number of

monomer could bring increased longitudinal second hyperpolarizability (γ) 10. The computation of Salustro et al. (2016) suggested that infinite polymer and intermediate diradical character was conducive to producing a high γ value

14

. Some researchers tried introducing some other groups

into the system to promote charge transfer from donor to accepter or modifying it into a perfect nonlinear optical material

13, 15-21

. Zhong et al. tried B/N to substitute the central carbon atoms of

the biphenalenyl diradical π dimer 16, or N to substitute C atoms at α- or β-sites, to get large first hyperpolarizability 19. A superatom is an atomic cluster with suitable size and composition being able to be designed to mimic the chemistry of atoms in the Periodic Table. As quasi-atomic systems consisting of a spherical core, modulation-doped with donors and surrounding impurity-free matrix with larger electron affinity, they can form the building blocks for a class of solids with unique structural, electronic, optical, magnetic, and thermodynamic properties introduced superatoms into the molecules

26-32

22-25

. Some researchers tried to

. On the one hand, introduction of superalkali was

able to enhance the hyperpolarizability. The work of Wang et al. (2012) showed introduction of superalkali atoms (Na3O and K3O) led to large β0 value of C20F20 26. Sun et al. (2016) introduced superalkali cluster into the system and enhanced hyperpolarizabilities (β0) compared with alkalimetal-based and previous superalkali-based clusters

27

. On the other hand, introduction of

superhalogen was also an effective strategy. Wang et al. (2013) and Srivastava et al. (2016) introduced BF6 and PF6 into Li@C60 respectively, and endohedral metallofullerene-superhalogen complex possessed significant nonlinear optical response 31-32. Chen et al. (2014) tried to induced superatoms (such as Li3O and BeF3) into PLY dimer, and got larger first hyperpolarizability than that of Boron/Nitrogen substitution because of the lower HOMO-LUMO gap 29. Based on this work, a considerable question emerged that if it was able to modulate the interlayer charge-transfer transition character with different superalkalis and superhalogens? Considering different induction effects of superalkali and superhalogen clusters, there were nine molecules designed, i. e. X-PLY2-Y; X = Li3O, Li3S and Na3O; Y = BeF3, BeCl3 and MgF3 (see Scheme. 1). Via analyzing the structures and properties of these molecules, we aimed to answer the question that i) how superalkali and superhalogen respectively influences the NLO properties, and ii) if superalkalis and superhalogens cause equal effects on the NLO properties of PLY dimer.



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Scheme 1. The designing regime of the molecules. There were three superalkalis (i. e. Li3O, Li3S and Na3O) and three superhalogens (BeF3, BeCl3 and MgF3) introduced into the upper and nether PLY dimer respectively and nine molecules were designed. Computational method The hybrid meta exchange-correlation functional (M05-2X and M06-2X) density function theory (DFT) methods were generally used for system with π-π stacking systems 16-17, 19, 29-30, 33-36. In this work, the stable geometric structures with real frequencies were obtained at M06-2X/6-31G(d) level. In order to understand the effects of superatoms on the interaction of π-π stacking better in the system, interaction energy between the two PLY monomers in the system were computed in this work. To correct the basis superposition error (BSSE), the counterpoise (CP) procedure was employed

37-38

in the computations at M06-2X/6-31+G(d) level. The interaction energy (Eint) was

calculated by the following equation:

Eint ( AB) = E ( AB) AB - [ E ( A) AB + E ( B) AB ] (1) In addition, the natural bond orbital (NBO) charge and Wiberg bond indices (WBI) of molecules were calculated at M06-2X/6-31G(d) level. Considering precision and cost and better connecting NLO properties and structures of molecules, the M06-2X method was proposed to be the most suitable method to calculate first-order hyperpolarizabilities (βtot) with 6-31+G(d) basis set39-46. For more details of the basis set effects on the first hyperpolarizability, readers are referred to Table S3 of the Supporting Information. The static first hyperpolarizability is expressed as:

b tot = ( b x 2 + b y 2 + b z 2 )1/ 2 (2) where b i = b iii + b ijj + b ikk

(i, j, k = x, y, z) (3)

All above calculations were operated by the Gaussian 09 package 47.


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Results and discussions Geometric structures In this work, the distance between the central carbon atoms (noted as Cc-Cc, Å) of PLY dimer which was used to approximately illustrate the interlayer distance of PLY dimer, and distances of α-carbon sites (noted as Cα-Cα, Å) in the PLY dimer where the unpaired electron delocalized in the singly occupied molecular orbital (SOMO) of optimized structures at M06-2X/6-31G(d) level, were listed respectively in Table 1. More details were showed in Supporting Information. Table 1. The distances of interlayer (Cc-Cc, Å) and α-carbon sites (Cα-Cα, Å) in the systems whose geometric structures were optimized at M06-2X/6-31G(d) level Cc-Cc

Cα-Cα

Li3O

Li3S

Na3O

BeF3

3.038

3.052

3.104

BeCl3

3.058

3.078

3.117

MgF3

3.054

3.054

3.107

BeF3

3.133

3.140

3.109

BeCl3

3.103

3.113

3.090

MgF3 3.141 3.137 3.104 By comparing the distance of central carbon (Cc-Cc, Å) which could present interlayer distance and α-carbon sites (Cα-Cα, Å) where SOMO of PLY monomer mainly located, it could be concluded that introduction of superatoms altered the structure of PLY dimer. According to Table 1, with the atomic number of alkali metals or halogens increasing, Cc-Cc distance increased from 3.038 Å to 3.117 Å, while Cα-Cα distance decreased from 3.141 Å to 3.090 Å. In detail, the Cc-Cc distances of the systems introduced Li3O and Li3S were significantly larger than that of Na3O. Additionally, the influences of Li3O and Li3S were comparable and they were both significantly larger than that of Na3O, which indicated that the alkali metals (i. e. Li and Na) contributed more to the induced effects than the central atoms (i. e. O and S) for superalkali. The induced effects of BeF3, BeCl3 and MgF3 on the structures were also showed similar variation trends, although the effects were not as obvious as that of superalkali. According to Table 1, CcCc distances of PLY dimer with BeCl3 induced were little larger than that of BeF3 and MgF3 which were nearly comparable. For the average distance of α-carbon atoms (Cα-Cα, Å), different superalkali or superhalogen atoms also showed different induced effects. With the atomic number of alkali metals or halogens, Cα-Cα distances became shorter. According to Table 1, Cα-Cα distances of Li3O- and Li3S-induced molecules were shorter than that of Na3O, and distances of molecule with BeF3 and MgF3 were larger than that of BeCl3. This phenomenon emphasized the influence of alkali atoms or halogen of the superatom clusters.



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Bonding characteristics The unpaired electrons delocalized at α-carbon (Cα) in the PLY dimer and thus 2e/12c bond was formed

19

. Therefore the induced effect of superatoms on Cα-Cα distance might lead to the

variation of interaction between two layers. To understand the π-π stacking of the system better while the superatom clusters were introduced, the natural bond orbital (NBO) charge was calculated in M06-2X method with 6-31G(d) basis set, and the result was shown in Table 2. More details were showed in Table S2 in Supporting Information. Table 2. NBO charge of all molecules at M06-2X/6-31G(d) level. Herein the parameter NBO referred to the sum of NBO charge of superatom and the PLY fragment close to superatom NBO

Li3O

Li3S

Na3O

BeF3

0.286

0.242

0.336

BeCl3

0.345

0.296

0.400

MgF3

0.302

0.248

0.345

The increased atomic number of alkali metals and halogens both led to the increase of NBO charge, with the sequence of NBO charge were Na3O > Li3O > Li3S for superalkali and BeCl3 > MgF3 > BeF3 for superhalogen. In addition, for all molecules, the values of Na or Cl were larger than Li or F, which might indicate that alkali metals and halogens had greater influence than alkali earth metals and chalcogens. Furthermore, the interaction energy between two PLY monomers and Wiberg bond indices (WBI) of molecules were also computed and the result was listed in Table 3. Table 3. The interaction energy (kcal/mol) between the two PLY dimer and the sum of six Cα-Cα Wiberg bond indices (WBI) of all molecules. Interaction energy was calculated at M06-2X/631+G(d) level. WBIs were computed at M06-2X/6-31G(d) level. WBI

Eint

Li3O

Li3S

Na3O

BeF3

0.1596

0.1453

0.1293

BeCl3

0.1815

0.1655

0.2023

MgF3

0.1639

0.1470

0.1834

BeF3

-20.42

-17.85

-22.60

BeCl3

-23.56

-20.19

-26.51

MgF3

-21.05

-18.25

-23.50

Similarly with the variation trend of NBO charge, the increased atomic number of alkali metals and halogens both led to the strengthening of interaction between the fragments on general. Concretely, the Cα-Cα WBIs of BeCl3-induced molecules were obviously larger than those of BeF3- and MgF3-induced systems generally. For superalkali atoms, WBIs of Na3O-induced molecules were larger than Li3O and Li3S on general and the fluctuation of Na3O-induced molecules was also very obvious, although for the molecules induced BeF3, the WBI value of


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Na3O was smallest. Interaction energy showed the strength of 2e/12c bond in PLY dimer. According to the results in Table 3, the order of Eint values were Na3O > Li3O > Li3S for superalkali and BeCl3 > MgF3 > BeF3 for superhalogen. Considering the effect of alkali metals or halogens, Na3O- or BeCl3introduced molecules were obviously larger than that of Li3O or BeF3, while for chalcogens or alkali earth metals, Eint values of molecules with Li3O or MgF3 were larger than those of Li3S or BeF3. Additionally, according to the above sequence, the alkali metals and halogens contributed more to the interaction, for Eint values of molecules with Li3O (or BeCl3 for superhalogen) were larger than those of Li3O and Li3S (or MgF3 and BeF3 for superhalogen). In conclusion, the variation trends of NBO charge, WBIs and interaction energy were connected with Cα-Cα distance. For the molecules induced superatom clusters, with the increasing of proton number of alkali metals or halogens, in spite of farther interlayer distance, α-site carbon atoms were closer and thus brought relatively stronger stacking interaction. Due to weaker induced strength of Na3O and BeCl3, the NBO charge distribution between two fragments was not separated as greatly as other superalkali or superhalogen clusters, and WBI and Eint values became larger, which might indicate π-π stacking showed more covalent character. Nonlinear optical property To compare the effects of superatoms on the NLO response of these systems, the static first hyperpolarizabilities (βtot) of molecules were obtained in M06-2X method at 6-31+G(d) level and the result was shown in Figure 1. Accordingly, superatoms caused great influences on the βtot values of molecules. On the one hand for superalkali clusters, the order of βtot was Li3S > Li3O > Na3O, which indicated that the increased βtot value was observed in the molecules with decreased proton number of alkali metals or increased chalcogens. On the other hand for superhalogen clusters, the βtot sequence was BeF3 ≈ MgF3 > BeCl3, which also suggested that the increased βtot value was related with decreased proton number of halogens while increased alkali earth metals had much less impact.



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Figure 1. The comparison of βtot with different superatoms. a) The gray column in the back presented the mean of βtot values of superhalogen-induced molecules. The red, yellow and blue columns respectively presented the βtot values of molecules induced by BeF3, MgF3, BeCl3 and corresponding superalkalis. b) The gray column in the back presented the mean of βtot values of superalkali-introduced molecules with error caused by the introduction of superalkali. The red, yellow and blue columns respectively presented the βtot values of molecules induced by Li3O, Li3S, Na3O and corresponding superhalogens. In order to further understand the origin of βtot, the two-level formula was used as following expression:

β tot ∝

∆µ ⋅ f 0 ∆E 3

(4)

where βtot was the static first hyperpolarizabilities, ∆E was transition energy, f0 was oscillator strength and ∆µ was difference in the dipole moments between the ground state and the crucial excited state 48-49. Therefore TD-DFT computation was operated in the present work and the results were summed in Table 4. As a result, ∆µ could qualitatively explain the change of βtot values on the basis of the two-level formula. Table 4. The first hyperpolarizability (βtot, au) at M06-2X/6-31+G(d) level, transition energy ∆E (eV), oscillator strength f0 and difference in the dipole moments between the ground state and the crucial excited state ∆µ (Debye) at TD-M06-2X/6-31+G(d) level Li3O βtot

∆E

Li3S 3

Na3O 3

BeF3

3.4×10

4.3×10

2.6×103

BeCl3

2.3×103

2.9×103

1.3×103

MgF3

3.6×103

4.3×103

2.5×103

BeF3

2.2578

2.2967

2.2814

BeCl3

2.2448

2.2634

2.2693


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f0

∆µ

MgF3

2.2556

2.2961

2.2780

BeF3

0.2487

0.2464

0.2758

BeCl3

0.2982

0.3010

0.3246

MgF3

0.2550

0.2526

0.2823

BeF3

2.9974

3.6435

2.2088

BeCl3

1.8196

2.2106

1.1631

MgF3

2.8035

3.4547

1.9967

Accordingly, the crucial transition was occurred between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for all molecules, and the distinction of ∆E among all systems was small. The values of oscillator strength f0 were also similar, with the range of 0.2464 to 0.3246. Therefore the impact of ∆E and f0 was weak. However for ∆µ, great change occurred when different superatoms were introduced. By comparing the ∆µ values of all molecules, the sequence occurred that Li3S > Li3O > Na3O and BeF3 > MgF3 > BeCl3, which was consistent with βtot values and stressed the effect of alkali metals and halogens on ∆µ. In addition, order of ∆µ was consistent with NBO charge distribution and interaction strength. By comparing the NBO charge distribution in Table 2 and ∆µ of all molecules in Table 4, it was amazing that larger ∆µ appeared with smaller NBO charge value for both superalkali and superhalogen. Actually the NBO charge distribution was related with ground-state dipole moment µg, which would influence the difference in the dipole moments between the ground state and the crucial excited state ∆µ. Thus by analyzing NBO charge, it was reasonable that the orders of βtot and ∆µ was Li3S > Li3O > Na3O for superalkali and BeF3 > MgF3 > BeCl3 for superhalogen. Electron transition characters In this work, the critical electron transition was from HOMO to LUMO for all molecules with changed transition energy ∆E. Compared with pure PLY dimer, superatom cluster induced the

asymmetric contribution of each fragment on HOMO and LUMO generally. As Figure 2 described, when superatoms were introduced into the system, HOMO distributed more on the upper layer close to superalkali and LUMO distributed more on the lower layer close to superalkali.



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Figure 2. the distribution change of a) HOMO and b) LUMO when superatoms were introduced into pure PLY dimer. In order to quantitatively describe the distribution change of HOMO and LUMO, the compositions of molecular orbitals were calculated in AOMix program50. The result was showed in Table 5. Table 5. Compositions of molecular orbitals (gross contributions, %). Herein the upper PLY monomer was close to the superalkali and the lower layer close to superalkali. HOMO

LUMO

upper

lower

upper

lower

Li3O/BeF3

84.75

15.25

15.58

84.42

Li3O/BeCl3

81.02

18.98

18.48

81.52

Li3O/MgF3

84.40

15.60

15.92

84.08

Li3S/BeF3

86.85

13.15

13.49

86.51

Li3S/BeCl3

82.81

17.19

16.09

83.91

Li3S/MgF3

86.57

13.43

13.75

86.25

Na3O/BeF3

82.41

17.59

17.98

82.02

Na3O/BeCl3

78.73

21.17

21.08

78.92

Na3O/MgF3

81.96

18.04

18.41

81.59

Above results indicated that compared with pure PLY dimer, superatom cluster induced the asymmetric contribution of each fragment on HOMO and LUMO significantly. Superatoms induced HOMO to transfer to the fragment closed to the superalkali clusters, and the situation was adverse for LUMO. The increased atomic number of both alkali metals and halogens was able to slightly reduce this induced effect on HOMO and LUMO with the sequence of Li3S > Li3O > Na3O for superalkali and BeF3 > MgF3 > BeCl3 for superhalogen, while the effect of

chalcogens and alkali earth metals was very tiny, with the comparable value of Li3S and Li3O for superalkali and BeF3 and MgF3 for superhalogen. For different superalkalis, ∆E of Na3O- and Li3S-induced molecules were comparable and both little larger than Li3O. For different superhalogens, introduction of BeF3 and MgF3 made the PLY dimer possessed little larger ∆E value. Considering the distribution of NBO charge, it was


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intriguing that with the NBO charge increasing, ∆E value was decreased. This variation tendency caused by superalkali and superhalogen clusters was analogous with the effect of external electric field

51-52

although the effects were not great enough to change the bond property of the PLY

53

dimer , and could not produce great change of ∆E that could further determine the variation of βtot

54

. However, when perfect non-linear optical property of a material arises with small ∆E value,

the transparency of the material is always unsatisfactory. Thus this result was also worthy of dealing with the conflict between the transparency of the materials and non-linear optical property. Aromaticity The nucleus-independent chemical shift (NICS) is a computational method that calculates the absolute magnetic shielding at the center of a ring. In this method, negative NICS values indicate aromaticity and positive values antiaromaticity

55

. In the present work, NICS along the central-

carbon axis was calculated to analyze the induced effects of superatoms on the aromaticity. NICS values were computed in GIAO-B3LYP/6-31G(d) method and the results were shown in Figure 3. More details were showed in Figure S1 in Supporting Information.

Figure 3. NICS values of pure PLY dimer and molecules with superatoms along the axis of central carbon atoms. The region with distance of negative values was close to the superhalogens,



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and superalkalis for positive values. a) Li3O-PLY2-BeF3, Li3O-PLY2-BeCl3 and Li3O-PLY2-MgF3 were taken as examples. The black curve presented the NICS variation along the central-carbon axis of pure PLY dimer. The green, pink and gray lines respectively presented the NICS values of molecules with BeF3, BeCl3 and MgF3. b) Li3O-PLY2-BeF3, Li3S-PLY2-BeF3 and Na3O-PLY2BeF3 were taken as examples. The black curve presented the NICS variation along the centralcarbon axis of pure PLY dimer. The green, orange and blue lines respectively presented the NICS values of molecules with Li3O, Li3S and Na3O. By comparing the curve of NICS in Figure 3, it could be concluded that the introduction of superatoms led to asymmetric decrease of aromaticity compared with pure PLY dimer. Accordingly, it was intriguing that the curve of NICS values for pure PLY dimer was symmetric. But when the superatoms were introduced, the curves became asymmetric. In the area close to the superhalogen, the variation of NICS value was tiny for whether different superalkalis (Li3O, Li3S and Na3O) or superhalogens (BeF3, BeCl3 and MgF3) compared with pure PLY dimer, while in the area close to the superalkali, the decrease of aromaticity became clearer compared with pure PLY dimer, although there were little difference by comparing the molecules with various superalkalis or superhalogens. Furthermore, it was worth noting that the shapes of curves were very similar in whether Figure 3a) or 3b), which suggested that although the superalkali and superhalogen both led to the asymmetric distribution of NICS values, their magnitude of impact was not be able to distinguish. Otherwise, the decreasing of aromaticity was clear when superatoms were introduced especially in the region close to superalkali. In the pure PLY dimer, the electron distribution showed great delocalized characteristics and its perfect configuration brought perfect aromaticity to the pure PLY dimer. However for molecules with superalkalis or superhalogens, the introduction of superatoms led to more positive or negative charge on the PLY dimer in the system, which would reduce the conjugation of PLY dimer and made the interaction have ionic characteristics. Conclusions Overall, the structure and properties of PLY dimer were changed when superatom clusters were introduced into the system, especially the distance of interlayer and α-site carbon atoms. For alkali metals or halogens, with the atomic number increasing, Cc-Cc distance increased and Cα-Cα distance decreased. In PLY dimer, 2e/12c bond was formed at α site, thus the decreased Cα-Cα distance led to relatively stronger stacking interaction and WBI values became larger as well when proton number of alkali metals or halogens increased. By calculating the first static hyperpolarizability βtot, the impact of superatoms on NLO property was obvious. Especially for superalkali clusters, the result indicated that increased proton number of chalcogens generated little larger βtot values, while the increased proton number of alkali metals


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led to obviously decreased βtot values. In conclusion, it was found that introduction of superatoms was an effective strategy to tune the NLO response. TD-DFT computation showed that the main impact on βtot was ∆µ. For ∆µ, decreased atomic number of alkali metals or halogens brought larger ∆µ valules, with the sequence of Li3S > Li3O > Na3O for superalkalis and BeF3 > MgF3 > BeCl3 for superhalogens. However for all molecules, ∆E was very similar since the critical electron transition was from HOMO to LUMO for all molecules, which was significantly asymmetric contributed by each fragment. This result was also worthy of dealing with the conflict between the transparency of the materials and non-linear optical property. Additionally, NICS values of all molecules along the central-carbon axis were calculated. The

asymmetric distribution of all molecules stressed the induced impact of superatoms compared with pure PLY dimer. All above results suggested that introduction of superatoms was really an effective strategy to tune the structure, interaction strength, NLO property and aromaticity of the system. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The detailed data are provided, including the distribution of NICS values for all optimized molecules and pure PLY dimer, atom coordinates and absolute energies of the optimized structures of studied complexes. Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Acknowledgment The authors gratefully acknowledge financial support from the National Science Foundation of China (NSFC) (21473026), the Science and Technology Development Planning of Jilin Province (20140101046JC), and H.-L. Xu appreciates support from Project funded by the China Postdoctoral Science Foundation (2014M560227). Thirteen Five-Year Sci-tech Research Guideline of the Education Department of Jilin Prov, China. References

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Superatoms-Induced Effects of Phenalenyl Ï-dimer on NICS and NLO

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