New AcceptorâBridgeâDonor Strategy for Enhancing NLO Response

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A New Acceptor-Bridge-Donor (A-B-D) Strategy for Enhancing NLO Response with Long Range Excess Electron Transfer From the NH…M/MO Donor (M = Li, Na, K) to Inside the Electron Hole Cage C F Acceptor Through the Unusual # Chain Bridge (CH) 2

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Yang Bai, Zhong-Jun Zhou, Jia-Jun Wang, Ying Li, Di Wu, Wei Chen, Zhi-Ru Li, and Chia-chung Sun J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp3120594 • Publication Date (Web): 14 Mar 2013 Downloaded from http://pubs.acs.org on March 15, 2013

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The Journal of Physical Chemistry

A New Acceptor-Bridge-Donor (A-B-D) Strategy for Enhancing NLO Response with Long Range Excess Electron Transfer from the NH2…M/M3O Donor (M = Li, Na, K) to Inside the Electron Hole Cage C20F19 Acceptor through the Unusual σ Chain Bridge (CH2)4 Yang Bai,[a] Zhong-Jun Zhou,*[a] Jia-Jun Wang,[ab] Ying Li,[a] Di Wu,[a] Wei Chen,[a] Zhi-Ru Li*[a] and Chia-Chung Sun[a]

[a] Dr. Yang Bai, Dr. Zhong-Jun Zhou, Dr. Jia-Jun Wang, Prof. Zhi-Ru Li, Prof. Di Wu, Prof. Ying Li, Prof. Wei Chen and Prof. Chia-Chung Sun State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, 130023 (P.R. China) Fax: (+86) 431-88945942 E-mail: [email protected]; [email protected] [b] Dr. Jia-Jun Wang Key Laboratory of Preparation and Application of Environmental Friendly Materials (Jilin Normal University), Ministry of Education, Chemistry Department of Jilin Normal University, Siping 136000 (P.R. China)

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Abstract Using the strong electron hole cage C20F19 acceptor, the NH2…M/M3O (M = Li, Na, and K) complicated donors with excess electron and the unusual σ chain (CH2)4 -

bridge, we construct a new kind of electride molecular salts e @C20F19-(CH2)4-NH2… M+/M3O+ (M = Li, Na, and K) with excess electron anion inside the hole cage (to be encapsulated excess electron-hole pair) serving as a new A-B-D strategy for enhancing nonlinear optical (NLO) response. An interesting push-pull mechanism of excess electron generation and its long range transfer is exhibited. The excess electron is pushed out from the (super)alkali atom M/M3O by the lone pair of NH2 in the donor and further pulled to inside the hole cage C20F19 acceptor through the efficient long σ chain (CH2)4 bridge. Owing to the long range electron transfer, the new designed electride molecular salts with the excess electron-hole pair exhibit large NLO -

response. For the e @C20F19-(CH2)4-NH2…Na+, its large first hyperpolarizability (β0) reaches up to 9.5×106 a.u. which is about 2.4×104 times of 400 a.u. for the relative -

e @C20F20…Na+ without the extended chain (CH2)4-NH2.1 It is shown that the new strategy is considerably efficient in enhancing the NLO response for the salts. In addition, the effects of different bridges and alkali atomic number on β0 are also exhibited. Further, three modulating factors are found for enhancing NLO response. They are the σ chain bridge, bridge-end group with lone pair and (super)alkali atom. The new knowledge may be significant for designing new NLO materials and electronic devices with electrons inside the cages. They may also be the basis of establishing potential organic chemistry with electron-hole pair.


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Keywords NLO response, electride molecular salts, long range excess electron transfer, electron inside single molecular cage, new acceptor-σ bridge-donor strategy 1. Introduction The acceptor-conjugated bridge-donor frameworks (A-B-D) are widely researched and have been successful motif for organic nonlinear optical (NLO) materials design,2-9 and their molecular switch properties are also reported.10-11 Alkali metal atom doped organic and inorganic molecules, as NLO molecules, have been paid much attention in recent years. In metal doping researches, Champagne et al.12 reported the dramatic effects of charging on the second hyperpolarizability by doping alkali atoms. Our previous researches13-17 show that the alkali atom doped polar molecule model systems have large static first hyperpolarizabilities (β0) due to the excess electrons formation. However, this type of NLO model with excess electrons does not have a large stability because the excess electron is not protected inside a container. Fortunately, an excess electron can be trapped inside a single molecular cage, such as synthesized C60F60 cage18 and C20F20 cage19-20 with sufficient interior electron attractive potential, forming novel single molecular solvated electrons (electron-hole pairs) e@C60F60 and [email protected] It is shown that perfluorinated fullerenes23 are strong acceptor containers and can protect excess electron. -

In 2012, the organic single-cage electride molecules M+(e@C20F20) , as the new nonlinear optical (NLO) molecules with excess electron inside the single-cage C20F20, have been reported.1 The NLO molecules have improved stabilities due to the excess



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electron trapped inside the single-cage. The β0 values of the single-cage structures (without the bridge between the cage and the alkali metal atom) are 400 a.u. -

(Na+(e@C20F20) ). The superalkali is an interesting cluster with a smaller VDE than that of the normal alkali atom24 and also strong donor. Using superalkali Na3O25 -

instead of Na, larger β0 of 13000 a.u. (Na3O+(e@C20F20) ) occurs. How to further enhance the β0 value for the organic electride molecules with excess electron protected inside the cage is a new challenge. In this work, the electron hole cage C20F19 with sufficient interior electron attractive potential is a strong acceptor (A) and the complicated NH2…M/M3O with loose bound excess electron is a stronger donor (D). Linking the A and D through an unusual σ long chain (CH2)4 bridge (to enlarge the distance between donor and acceptor, and assist charge transfer) surpassing the relative π-conjugate bridge (C≡C)2 for the system with excess electron, we construct a new kind of electride molecular -

salts e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na and K) with excess electron anions protected inside the hole cages serving as a new A-B-D strategy. For reported A-B-D systems without the long range transfer of excess electron from (super)alkali atom to inside the cage, organic conjugated bridges have been widely used, new conjugated systems including nanotube and graphene as bridges have also been reported.26-30 In our strategy with excess electron, the new σ chain (CH2)4 bridge is employed to assist the long range transfer of excess electron from (super)alkali atom to inside the cage. In this paper, our investigation aims at using unusual A, B and D to perform new


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NLO

strategy,

obtaining

a

new

kind

of

NLO

molecular

electrides,

-

e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na and K) with excess electron anions inside

the hole cages, exhibiting their structures and large

static first

hyperpolarizibilities, showing the obvious effects of the bridge, (super)alkali atom and bridge-end group on NLO response, and providing new design idea on high-performance NLO molecules. 2. Computational Details Because of our research systems including long range interaction and charge transfer, a new density functional, coulomb-attenuated hybrid exchange-correlation functional (CAM-B3LYP)31 is used. CAM-B3LYP provides molecular geometries close to experimentally observed structures is reported.32 In this work, the optimized -

geometric structures of the e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na and K) with all real frequencies in the ground state are obtained at CAM-B3LYP/6-31G(d) levels. For the calculations of hyperpolarizabilities, Nakano, and co-workers pointed out that for a medium-size system, the BHandHLYP method can also reproduce the (hyper)polarizability values from the more sophisticated CCSD(T).33-34 For many NLO systems, CAM-B3LYP has been also proved to be proper in calculating the hyperpolarizabilities.35-41 Recently, CAM-B3LYP method has been used satisfactorily in NLO researches for alkali metal doped systems.1, CAM-B3LYP/6-311+G(d)

method

is chosen

for the

42-44

In this work, the

calculations

of first

hyperpolarizability and the vertical detachment energy VDE (representing the redox



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properties of excess electron) to show some physical properties, especially the order of the β0, and further get the relationship between structure and β0. For precise calculations on molecular (hyper)polarizabilities of no large systems, systematical researches have been reported by G. Maroulis.45-49. For electron transition properties, the transition energy ∆E (eV), the oscillator strength f0, and the difference of dipole moment ∆µ (D) between the ground and the excited state are also calculated at TD-CAM-B3LYP/6-311+G(d) level. The reasonable MK (Merz–Kollmann) charges50 are calculated at the CAM-B3LYP/6-311+G(d) level, as NBO charge (2.994) of K in -

e @C20F19-(CH2)4-NH2…K+ is too large. Recently, some NLO methods achieved new progress including full configuration interaction51 and assessment of DFT exchange-correlation functional52 for organic and metal molecules including singlet radical systems.51 It is expected that new progress is used for large-size systems. For theoretical method, the energy of a molecular system in the weak and homogeneous electric field can be written as:

⋯

(1)

where E0 is the molecular energy without the applied electrostatic field and Fα is a component of the strength on the α direction of applied electrostatic field; µα, ααβ, βαβγ may be called a component of dipole, polarizability, and first hyperpolarizability tensor, respectively. The static first hyperpolarizability β0 and the vertical detachment energy VDE is noted as: ⁄


(2)

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Where

For electride molecule M, the vertical detachment energy is !"

(3)

The spin contamination is negligible, as the expected value of spin eigenvalue for each molecule is 0.75 in the calculations. All of calculations performed in this work are carried out using the GAUSSIAN 09 program package. The dimensional plots of molecular orbitals were generated with the GaussView program.53 3. Results and Discussion 3.1 Geometrical characteristics The optimized geometric structures of doped and undoped molecules (in Fig. 1) with all real frequencies were obtained at the CAM-B3LYP/6-31G(d) level. The geometrical parameters are listed in Table 1. Table 1: The cage height (H), the chain length (L(C1-N)), and distances (Å) and θ(°) for CR… M/M3O (CR=C20F19-(CH2)4-NH2; M = Li, Na and K) H CR CR…M/M3O Li Na K Li3O Na3O K3O

O…N

N…M1

N…M2

N…M3

3.453 3.453 3.453 3.451 3.421 3.453 3.448

3.713 2.756 2.810

2.035 2.454 2.884 2.056 2.362 2.750

4.531 4.219 3.950

5.157 4.245 4.415

L(C1-N)

θ

4.983

177.26

5.005 5.003 5.009 4.958 5.008 4.830

178.05 178.45 179.13 161.49 177.78 137.56

For the undoped C20F19-(CH2)4-NH2 complexant, the C20F19 cage height (H) is



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3.453 Å, the extended chain (R= (CH2)4-NH2) length L taking from C1 to N is 4.983 Å, and C1-C3-N angle θ (177.26º) is closing to 180º. For alkali doped systems C20F19-(CH2)4-NH2…M (M = Li, Na and K), M is doped on the end (NH2) of the extended chain (CH2)4-NH2 (see Fig. 1) due to alkali favoring N atom, the cage height almost remains unchanged (3.451-3.453 Å) due to the cage far away from the M. In these structures, the θ almost remains unchanged (178.05-179.13º), the extended chain almost still straight. Meanwhile, L also remains unchanged (5.003-5.009Å). The order of N…M distances is 2.035 (Li) < 2.454 (Na) < 2.884 Å (K), which comes from the order of alkali atom radii to be 1.586 (Li) < 1.713 (Na) < 2.162 Å (K).

Figure 1: The structures and geometric (CR=C20F19-(CH2)4-NH2; M = Li, Na and K).

parameters

for

CR

and

CR … M/M3O

For superalkali doped systems C20F19-(CH2)4-NH2…M3O (M = Li, Na and K),


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the molecular shapes are different (see Fig. 1). In Li3O doped molecule, the superalkali Li3O is doped on the side of the chain (CH2)4-NH2, one Li atom (Li…N of 2.056 Å) pulling the N atom of end-group NH2 and the other Li atom near the cage pulling F atom of the cage form the bent chain with the smaller θ of 161.49º, slightly shorter L of 4.958 Å and H of 3.421 Å. In the superalkali Na3O doped molecule, Na3O is doped on the end of the chain and Na atoms are far away from the cage, the chain structure of C20F19-(CH2)4-NH2 is still straight, which is similar to those in alkali doped systems. For geometric parameters, the H, L and θ (3.453, 5.008 Å and 177.78º) are similar to those in alkali doped systems, respectively. For Na…N distances, one length is shorter and the other two are longer in Na3O doped molecule. As Na3O has large size, only one Na atom can strongly dope on the N atom forming one short Na… N distance. Interestingly, the shorter Na…N in Na3O doped molecule is shorter than the Na…N in smaller Na doped molecule, because the Na atom (with shorter Na…N) in superalkali Na3O is easily polarized forming the shorter Na…N. In big-size superalkali K3O doped molecule, although K3O is also doped on the end of the chain, two K atoms are close to the cage and the O atom is also close to H atom of NH2, which lead to that the chain is strongly bent and the height H is decreased slightly. The superalkali K3O doped molecule with the bent chain has small θ, L and H value (137.56º, 4.830 and 3.448 Å). For superalkali doped systems, the relationship between structural parameters and physical properties, especially, the hyperpolarizability (β0) will be exhibited. Large θ relates to large β0 value (see Static First Hyperpolarizabilities).



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-

3.2 New electride molecule salts: e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na and K) For our acceptor, Wu, Jiao, and their coworker reported that synthesized perfluorinated fullerene C20F20 cage can trap an electron inside the cage.22 As exo polar C-F bonds and the dipoles of the C-F bonds are directed to the center of the cage forming spherical double electric layer structure, the cage inner generates sufficient electronic attractive potential. The C20F20 cage has large electron affinity EA of 2.63 eV54 which is between those of F (3.400 eV) and O atom (1.478 eV)，so the C20F19 cage is electron hole container and a strong acceptor. For the complicated donor NH2…M/M3O, the valent electron of (super)alkali metal is pushed by the lone pair of N atom forming the loosely bound excess electron with strong donor ability, so the NH2…M/M3O is a stronger donor with excess electron than M/M3O with valent electron. In order to obtain better physical properties, we add an σ chain (CH2)4 bridge between the acceptor and donor to enlarge the molecular size. For the C20F19-(CH2)4-NH2 …M/M3O (M = Li, Na and K) molecules, the HOMOs are shown in Fig. 2. From Fig. 2, the HOMO of the C20F19-(CH2)4-NH2 shows that the electron cloud locates almost in the end of the chain. When the C20F19-(CH2)4-NH2 are doped by (super)alkali atoms M/M3O (M = Li, Na and K), the s-style excess electron cloud locates almost inside the C20F19 cage, which indicates that long range electron transfer from M/M3O (M = Li, Na and K) to inside the C20F19 and electron hole cage pair (the excess electron inside the hole cage) occur. For M3O (M = Li and K) doped systems with bent chain, the electron cloud inside the cage


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deviates from the center of the cage due to additional attractive interaction between the excess electron inside the cage and positive charged M3O closing to the cage.(see Fig. 1) The Merz-Kollmann charges are listed in Table 2. From Table 2, we can see that the MK charges are 0.91 ~ 0.93 for M and 0.65 ~ 0.87 for M3O, which exhibits that M/M3O loses almost one electron. However the charges of cages are -0.73 ~ -0.99 closing to one electron, which exhibits that the cage gets almost one electron. Table 2: The Merz-Kollmann charge at the CAM-B3LYP/6-311+G(d) level for CR…M/M3O (CR=C20F19-(CH2)4-NH2; M = Li, Na and K) CR M

O M3O CR cage

0.0 -0.14

CR…Li

CR…Na

CR…K

CR…Li3O

CR…Na3O

CR…K3O

0.92

0.93

0.91

-0.92 -0.96

-0.93 -0.97

-0.91 -0.99

0.84 0.61 0.70 -1.50 0.65 -0.65 -0.73

0.97 0.90 0.92 -1.92 0.87 -0.87 -0.99

0.80 0.76 0.79 -1.55 0.80 -0.80 -0.96

Figure 2: The highest occupied molecular orbitals (HOMOs) for CR and CR … M/M3O (CR=C20F19-(CH2)4-NH2; M = Li, Na and K). The excess electron inside cage for doped CR… M/M3O systems.

Considering electron clouds and charges, we may conclude that a long range electron transfer (through a long σ chain bridge) from (super)alkali atom to inside the cage occurs and the electride molecule salts with excess electron inside the cage are



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-

formed. We may name these electride molecules as salts e @C20F19-(CH2)4-NH2… M+/M3O+ (M = Li, Na, and K).

3.3 Vertical detachment energy and interaction energy For those new electride molecule salts with excess electron, the excess electron occupies HOMO, the detachment energy of the excess electron (VDE) represents the stability of the excess electron (reducibility). Large VDE value corresponds to the large stability of excess electron (small reducibility). For those new electride molecule salts, the calculated interaction energy EInt is ion pair decomposition energy between -

e @C20F19-(CH2)4-NH2 and M+/M3O+. The VDEs and EInt values for the -

e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na and K) are listed in Table 3. Table 3: First Hyperpolarizability β0 (a.u.), Transition Energy ∆E (eV), Oscillator Strength f0, Difference of Dipole Moment ∆µ (D) between the Ground and the Excited State, Interaction Energy EInt (kcal/mol) between the doped M/M3O and the CR, the vertical detachment energy value VDE (eV) for CR…M/M3O (CR = C20F19-(CH2)4-NH2; M = Li, Na, and K). CR CR…M/M3O Li Na K Li3O Na3O K3 O a

β0

∆E

f0

∆µ

Transition

EInt

61.8

5.91

0.013

0.31

0.62(H→L+4)

3.4×104 9.5×106 3.1×104 4.9×102 6.9×106 3.6×102

2.35 2.38 2.38 4.11 2.13 2.21

0.131 0.124 0.111 0.060 0.107 0.125

1.29 8.98 5.81 3.47 6.73 2.88

0.82(H→L+12) 0.77(H→L+12) 0.71(H→L+14) 0.81(H→L+2) 0.73(H→L+24) 0.89(H→L)

VDE 9.68

-22.7 -12.9 -25.4 -138.5 -30.7 -92.9

4.81 4.76 4.74 7.85(3.49)a 4.35(3.19)a 6.23(2.75)a

the values in parentheses for the isolated superalkali M3O (M = Li, Na and K)

From

Table

3,

the

VDE

values

are

4.76

and

4.74

eV

for

-

e @C20F19-(CH2)4-NH2…M+ (M = Na, K), respectively, which are smaller than 7.38 -

and 7.09 for e @C20F20…M+ (M = Na, K).1 It means that the chain R = (CH2)4-NH2 decreases the VDE values for these electride salts with excess electron inside the cage. Why? When (CH2)4-NH2 substitutes a F atom of the cage C20F20, the negative charged


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F atom linked with the C atom of the cage is replaced by a positive charged C atom of the (CH2)4-NH2, which brings an effect of pulling out excess electron from the cage. This may be a main reason for (CH2)4-NH2 decreasing the VDE value. -

In contrast, an electride salt Li+@calix[4]pyrrole…e without excess electron inside the cage has a more smaller VDE value of 4.16 eV17 than those of the electride molecular salts with excess electron inside the cage, which shows that the cage -

trapping enhances the stability of excess electron. The e @C20F19-(CH2)4-NH2…M+ -

(M = Li, Na and K) have larger stabilities than Li+@calix[4]pyrrole…e . In atom cluster research, the superalkali is an interesting concept. A cluster with a smaller VDE than that of the normal alkali atom is superalkali. Because of 3.494 ( Li3O ) < 5.390 eV ( Li ), 3.185 ( Na3O ) < 5.183 eV ( Na ) and 2.746 ( K3O ) < 4.339 eV ( K ) at the MP2/6-311+G(3df) level for the VDE values,1 the M3O ( M = Li, Na and K ) are superalkali atoms. To understand the complexant effect on the stability of excess electron, we compare superalkali doped systems CR … M3O (CR= C20F19-(CH2)4-NH2) with superalkali M3O for VDE values. The VDE values at CAM-B3LYP/6-311+G(d) level are 7.85, 4.35 and 6.23 eV in CR…M3O and 3.494, 3.185 and 2.746 eV in M3O for M = Li, Na and K, respectively. The VDE values of the doped systems are larger than those of superalkalis. It is shown that CR increases VDE values and doped systems have larger stabilities towards oxidation due to the electron tripped inside the C20F19 cage. It is worth noting that the VDE values have nonmonotonic dependence of the M



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atomic number for CR…M3O systems M = Li, Na, and K. The CR…Na3O structure with a straight chain has small VDE (4.35 eV), while the CR…M3O (M = Li and K) with bend chain have larger VDEs (7.85 and 6.23 eV). From Fig. 1 and Fig. 2, the chain bending leads to M3O+ closing to the cage. The shorter distance between M3O+ and the cage brings a new attraction of M3O+ for the excess electron, which is a main reason of increasing VDE value. It is shown that the chain bending increases VDE value and stability of the excess electron. In order to research the stability between the doped M+/M3O+ atoms and the -

-

complexant e @C20F19-(CH2)4-NH2 (CR ), the interaction energy EInt (kcal/mol) are calculated at the CAM-B3LYP/6-311+G(d) level and listed in Table 3. From Table 3, -

-

M doped systems have small EInt to be -12.9 (CR …Na+), -22.7 (CR …Li+) and -25.4 -

-

kcal/mol (CR …K+) due to negative ion e inside the cage far away from positive ion M+. -

-

For M3O doped systems, -30.7 (CR …Na3O+) is smaller than -138.5 (CR … -

Li3O+) and -92.9 kcal/mol (CR …K3O+). The EInt values for M doped systems are smaller than that of the M3O doped systems, as some additional interactions of M…F and O…H occur in large M3O doped systems and increase the ion pair decomposition -

energy. In CR … M3O+ electride salts, different structures bring new different -

interactions, the different interaction values occur. CR …Li3O+ with bent chain has -

the largest EInt of -138.5 due to the stronger interaction of Li…F. CR …K3O+ with bent chain has larger EInt of -92.9 due to the interaction of stronger K…F and weak -

O…H. While, CR …Na3O+ with straight chain has the smallest EInt of -30.7 kcal/mol


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-

due to weak O…H interaction. It is indicated that the CR …M3O+ electride salts have -

larger stabilities than CR …M+ electride salts for ion pair decomposition. And the -

-

-

order of stabilities is CR …Li3O+ > CR …K3O+ > CR …Na3O+. Ionic liquids are not simple fluids: their ions are generally asymmetric, flexible, with delocalized electrostatic charges, and available in a wide variety. Ion liquid is an interesting research area.55-56 In general, enlarging the sizes of ions to reduce the interaction energy of ion pair may perform the evolution from solid salt to ion liquid. -

In the CR …M+/M3O+ electride salts, some small Eint occur, which may indicate a new form of ion inside single-cage for ion liquid.

3.4 Static First Hyperpolarizabilities -

The electronic properties of the e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na, and K) are listed in Table 3. From Table 3, we can see that the first hyperpolarizability

β0 of the C20F19-(CH2)4-NH2 without doped atom is only 61.8 a.u., but for alkali atom -

doped systems e @C20F19-(CH2)4-NH2 … M+ (M = Li, Na and K), the first hyperpolarizability β0 increases greatly up to 3.1×104 - 9.5×106 a.u. which shows that the alkali doping effect on the β0 value is dramatic and the β0 value may increase by about 100000 times. As an example, Na doping on the CR causes the long range electron transfer and dramatic increase of β0 value, which is illustrated in interesting Fig. 3. In order to exhibit the effect of the new strategy including the electron hole cage C20F19 acceptor, the strong NH2…M/M3O complicated donor with excess electron and the unusual σ chain (CH2)4 bridge, we compare the β0 values between the new and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-

corresponding (without bridge) strategy. The β0 of e @C20F19-(CH2)4-NH2…Na+ with σ chain (CH2)4 bridge is 9.5×106 which is much larger than the β0 of 4.0×102 a.u. of -

e @C20F20…Na+ (without both σ chain (CH2)4 bridge and excess electron in donor Na atom) by about 24000 times.1 Obviously, the new strategy is considerably efficient in enhancing the NLO response for electride molecular salts.

Figure 3: The valent electron of doped Na atom is pushed out by the lone pair of NH2 forming an excess electron which is trapped inside the cage (C20F19) through the bridge of σ chain ((CH2)4). The electride molecule e-@C20F19-(CH2)4-NH2…Na+ with excess electron inside cage is formed and its considerable NLO response occurs.

The structural reason of the new strategy having great effectiveness is the introductions of the NH2 group in donor and the long σ chain bridge between the donor and acceptor. For the introduced NH2 group, its lone pair pushes a valent electron of (super)alkali atom M/M3O to form an excess electron enhancing the capability of donor, which is beneficial for charge transfer from donor to acceptor. While the introduced long σ chain bridge enlarges the distance between donor and acceptor (providing the geometric condition for long range charge transfer) and assists


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the long range transfer of excess electron from (super)alkali atom to inside the cage. To reveal the effects of different alkali atoms and corresponding superalkali -

atoms on β0 values, at first, we compare β0 values for e @C20F19-(CH2)4-NH2…M+ (M = Li, Na, and K) (see Table 3). The dependence of alkali atomic number on β0 is shown

that

Na

(9.5×106)

>>

Li

(3.4×104)

>

K

(3.1×104

a.u.).

For

-

e @C20F19-(CH2)4-NH2…M3O+ (M = Li, Na and K), the dependence of superalkali atomic number on β0 is also found that Na3O (6.9×106) >> Li3O (4.9×102) > K3O (3.6×102 a.u.). For both CR-…M+ and CR-…M3O+ (see Table 3). Based on strong nonmonotonic dependences of alkali atomic number on β0, alkali atom as a modulating factor to enhance NLO response for (super)alkali doped electride salts is also exhibited in Fig. 4 and Fig. 5.

Figure 4: The dependence of alkali atoms on β0 value.



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Figure 5: The dependence of doping super-alkali atoms on β0 value. The structure with straight chain and doped on the chain end has larger β0 value.

To understand the change rules of the β0 values, we may find some clues in the two-level model which includes some possible controlling factors:57-58 ∝

∆ ∙ & ∆

where ∆E, f0, and ∆µ are the transition energy, oscillator strength, and the difference of the dipole moment between the ground state and the crucial excited state, respectively. -

From Table 3 we can see that the doped systems e @C20F19-(CH2)4-NH2… M+/M3O+ (M = Li, Na, and K) have larger ∆µ, f0, and smaller ∆E value for electron transition properties, which are beneficial for large β0 value (NLO response) due to the excess electron long range transfer, but the values of the undoped C20F19-(CH2)4-NH2 are not beneficial for large β0 value. From this, it is easily understood that M/M3O doped systems have large β0 values.


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From Table 3, in alkali doped systems, Na doped system has the largest β0 value and the largest ∆µ. And in superalkali doped systems, Na3O doped system also has the largest β0 value and the largest ∆µ. It is shown that the largest ∆µ is a main electronic factor for Na/Na3O doped system having the largest β0 values. Thus, the ∆µ value may be

the

main

factor

of

modulating

β0

value

for

the

electride

salts

-

e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na and K). -

For superalkali systems e @C20F19-(CH2)4-NH2…M3O+ (M = Li, Na and K), an interesting relevance is exhibited that the orders of the angle of chain bending θ, ∆µ and β0 values are monotonous decreasing with the order of doping atom Na3O, Li3O and K3O (see Table 1 and 3). 177.78, 6.73, 6.9×106 for Na3O > 161.49, 3.47, 4.9×102 for Li3O > 137.56 °, 2.88 D, 3.6×102 a.u. for K3O. It indicates the chain bending is a main geometric factor for modulating β0 value. The θ affects β0 value mainly through influencing ∆µ value in the superalkali doped systems. In published papers on A-B-D strategy, the utilized bridges (B) are conjugate bridges. Why a non-conjugate bridge of long σ chain is chosen in this paper? Next, we discuss the properties of different bridges. Keeping the same acceptor C20F19 cage and the same complicated donor NH2…Na, the different bridges (CH2)4, (C≡C)2, and CH2 are taken to research the effect of different bridges. The N…Na distance (Å), and the MK charge Q values for the molecules with the different bridges are listed in Table 4. Table 4: The N…Na distances (Å), the MK charge Q values and the First Hyperpolarizability β0 values (a.u.) for the molecules with different bridges including (CH2)4, (C≡C)2 and CH2. Cage-Bridge-NH2…Na

N…Na

Q(Na)

Q(Cage)

Q(Bridge)

Q(NH2)

β0

Cage-(CH2)4-NH2…Na Cage-(C≡C)2-NH2…Na Cage-CH2-NH2…Na

2.454 2.594 2.346

0.93 0.26 0.92

-0.97 0.27 -0.82

0.43 -0.72 0.18

-0.39 0.19 -0.28

9.5×106 1.6×106 2.3×104



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Considering the relative non-conjugated and conjugated bridge, for the N…Na distances relating to the interaction between the lone pair of N atom and Na atom, 2.454 ((CH2)4) < 2.594 Å ((C≡C)2) exhibits that the σ chain bridge (CH2)4 system is better than the conjugated bridge (C≡C)2 for the lone pair of N atom pushing valent electron of Na to form excess electron. Namely, the long σ chain bridge (CH2)4 obviously increases the effect of the donor D. It is supported by the Q(Na) values, 0.93 ((CH2)4) >> 0.26 ((C≡C)2). The charge Q(Cage) of -0.97 shows that one excess electron is located inside the cage for the molecule with the (CH2)4 bridge (see HOMO in Fig. 6). The charge Q(Cage) of +0.27 and Q(Bridge) of -0.72 show that one excess electron is not located inside the cage but located on the bridge for the molecule with the (C≡C)2 bridge. The HOMO in Fig. 6 shows that major part of excess electron cloud is located nearby Na for the molecule with the (C≡C)2 bridge, which indicates that the conjugate bridge trapping large negative charge (-0.72) is not a good bridge for the long range electron transfer from Na to inside the cage for the electride salt with excess electron. For the influence on the σ chain length, the molecule with the long bridge (CH2)4 has larger charge Q(Cage) of -0.97, while the molecule with the short bridge CH2 has the smaller charge Q(Cage) of -0.82 (see Table 4). It is exhibited that the long σ chain bridge is beneficial for the long range electron transfer. Turn to the bridge effect on the first hyperpolarizability, the β0 values are listed in Table 4. The order of β0 values is 9.5×106 (long σ chain bridge (CH2)4) > 1.6×106 (long conjugate bridge (C≡C)2) > 2.3×104 (short σ chain bridge CH2). Therefore a


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long σ chain bridge bringing large β0 value is chosen. It is also exhibited that a long σ chain bridge is an important modulating factor for enhancing β0 value in the electride -

molecular salts e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na and K) with excess electron inside the cage due to long range electron transfer from D to inside the cage.

Figure 6: The frontier molecular orbitals for A-B-D (A=C20F19; D = NH2…M, M=Li, Na and K) with different bridges, which are σ- (CH2)4, CH2 and π- (C≡C)2 respectively. The HOMOs shows that the σ chain (CH2)4 and CH2 are better than the conjugate π chain (C≡C)2 in the A-B-D model for long range electron transfer from D to inside the cage A.

4. Conclusion A new NLO strategy has been performed successfully. In the new model, using the interesting electron hole cage C20F19 acceptor which can trap excess electron inside the cage to increase the stability of the excess electron, the NH2…M/M3O complicated donor with excess electron as a more stronger donor and the unusual σ chain (CH2)4 bridge having stronger ability to assist excess electron transfer. A new -

kind of electride molecular salts e @C20F19-(CH2)4-NH2…M+/M3O+ (M = Li, Na, and K) with excess electron anion inside the hole cage is achieved. Owing to the long range excess electron transfer, the new designed electride



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molecules with excess electron-hole pair exhibit large nonlinear optical (NLO) -

response. For the e @C20F19-(CH2)4-NH2…Na+, its large first hyperpolarizability (β0) reaches up to 9.5×106 a.u. which is about 2.4×104 times of 400 a.u. for -

Na+(e@C20F20) with the similar C20F20 cage and alkali atom Na.1 Thus, the new strategy is considerably efficient for enhancing the NLO response of electride molecules, which attributes to the strong effectiveness introductions of the long σ chain bridge and the NH2 group in the donor. The introduction of the long σ chain bridge provides the geometric condition and assistance for long range charge transfer. The introduction of NH2 group in the donor, the lone pair of N atom pushes the valent electron of M/M3O forming the loosely bound excess electron and generating stronger donor NH2…M/M3O. Otherwise, the nonmonotonic alkali atomic number dependence on β0 in (super)alkali doped systems is also exhibited. Based on above conclusions, three modulating factors are found for enhancing NLO response. They are the σ chain bridge, bridge-end group with lone pair to form excess electron of donor and (super)alkali atom for the new electride molecular salts. The new knowledge may be significant for designing new NLO materials and electronic devices with electrons inside the cages. They may also be the basis of establishing potential organic chemistry with electron-hole pair. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 21173098, 21173095, 21103065 and 21043003). Supporting Information


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