1

Tuning electronic structure and optical properties of

Li@cyclo[18]carbon complex via switching doping

position of lithium atom

Zeyu Liu a,

, Xia Wang a, Tian Lu

b,, Aihua Yuan

a, Xiufen Yan

a

aSchool of Environmental and Chemical Engineering, Jiangsu University of Science and

Technology, Zhenjiang 212100, People’s Republic of China

bBeijing Kein Research Center for Natural Sciences, Beijing 100022, People’s Republic of China

Corresponding author. E-mail: liuzy@just.edu.cn (Zeyu Liu); sobereva@sina.com (Tian Lu)

2

Abstract

Doping alkali metal atoms, especially lithium (Li), in nanocarbon materials has

always been considered as one of the most effective methods to improve the optical

properties of the system. In this theoretical work, we doped a Li atom into the recently

observed all-carboatomic molecule, cyclo[18]carbon (C18), and finally obtained two

stable configurations with Li inside and outside the ring. The calculation results show

that the energy barrier of transition between the two Li@C18 complexes is quite low,

and thus the conversion is easy to occur at ambient temperature. Importantly, the

electronic structure, absorption spectrum, and optical nonlinearity of the two

configurations are found to be significantly different, which indicates that the

electronic structure and optical properties of the Li@C18 complex can be effectively

regulated by switching the location of the doped Li atom between inside and outside

the carbon ring. With the help of a variety of wave function analysis techniques, the

nature of the discrepancies in the properties of the Li@C18 complex with different

configurations has been revealed in depth. The relevant results of this work are

expected to provide theoretical guidance for the future development of

cyclocarbon-based optical molecular switches.

Graphical abstract:

3

1. Introduction

The optical switch is a special molecular device that can transform between various

molecular configurations and simultaneously plays a regulatory role on the optical

characteristics of the system [1]. For a long time, alkali metal-doped nanocomposite

has been considered as an ideal molecular switch whose optical nonlinearity can be

effectively controlled [2-11]. Such complexes have been extensively predicted and

analyzed in the nature of the optical regulation theoretically. Typically, the product

complexes of the interaction between alkali metals and nanomaterials can be classed

into two categories according to their electronic structures: salt- and electride-like

configurations. These two configurations for some species of complex can be

converted to each other by electric field induction or heating, as described by Wang et

al. on the Li@AR (AR = benzene and naphthalene) systems, and alkali metal salt

generally exhibits much higher hyperpolarizability than its electride-like counterpart

[12].

In alkali-based salt-like systems, the charge on alkali metal is always a large

positive value, even close to +1.0 e [12], which implies a strong ability of charge

transfer within the complexes, so excellent optical properties can be expected. Due to

its simple electronic structure, lithium (Li) atom is the most studied alkali metal in

coordination doping to enhance the optical nonlinearity of the molecules. For example,

doping Li atom(s) into nanomaterials has been proved to have a positive effect on

improving the first-order hyperpolarizability of systems such as acenes [2], supershort

single-carbon nanotubes [3], and carbon-boron-nitride heteronanotubes [4]. Li et al.

studied the first-order hyperpolarizability of Li-doped short aza-Möbius graphene

ribbon and revealed that the influence of Li atom on the response property of the

graphene ribbon is significant [5].

Cyclo[18]carbon (C18) is a novel all-carboatomic ring that has been successfully

generated and observed in experiments recently [13,14]. Several theoretical studies

have shown that C18 ring, as a prototype of nanocarbon system, possesses special

electronic structure and many unusual features [15-26], making it has great potential

applications in optoelectronic devices [19,20]. We have demonstrated that the C18

4

displays extraordinarily strong absorption in ultraviolet band and striking optical

nonlinearity, and it is expected to be utilized as ultraviolet filter and nonlinear optical

(NLO) material with excellent performance [17]. Our other research on the adsorption

of the C18 ring on some compounds revealed that the C18 exhibits the ability to bind

small molecules both inside and outside the ring, and its adsorption nature differs

depending on the property of the adsorbed compounds [21].

Inspired by the previous researches on Li-based salt-like NLO complexes and the

adsorption characteristics of the C18 ring, a question naturally arises: Can the optical

properties of the C18 ring be tuned by doping Li atom? To the best of our knowledge,

no investigations have been conducted on the optical properties of the Li@C18

complex so far, although it is a very typical system of the alkali metal-doped

nanocarbon. It is an important and practical topic to deeply understand the electronic

structure and optical properties of the Li-doped C18 complexes as well as the

regulatory effect of Li atom on the related properties, which is the purpose of this

work.

2. Computational details

Geometry optimizations of the Li@C18 complexes were realized by density

functional theory (DFT) using B97XD exchange-correlation functional [27] in

conjunction with ma-TZVP basis set [28,29] in the gas phase. Vibrational frequency

analyses were carried out on the optimized structures to determine whether they are at

minimum or saddle point on the potential energy surface. The intrinsic reaction

coordinate (IRC) was traced from transition state toward both forward and reverse

directions along the imaginary vibrational mode. The electronic energies were

obtained via the high-level DLPNO-CCSD(T) [30]/cc-pVTZ [31,32] calculations

based on the optimized structures. Free energies were evaluated by summing up the

electronic energies and thermal corrections to free energy obtained by frequency

analysis at B97XD/ma-TZVP level, where the scale factor for zero-point energy of

0.975 was employed [33]. Natural population analyses (NPA) were performed using

B97XD/ma-TZVP wave functions.

5

Electron excitations were investigated by means of the time-dependent DFT

(TD-DFT) method with B97XD/ma-TZVP level at optimized ground-state

geometries. Charge-transfer spectra (CTS) were plotted based on the TD-DFT

calculation data [34]. The B97XD functional combining with very large LPol-ds (for

C atoms) [35] and aug-cc-pVTZ (for Li atom) [32] basis sets were employed for

reliably estimating optical nonlinearities of the Li@C18 complexes. The LPol-ds is a

basis set specifically developed for accurate calculation of response properties to

electric field. Detailed formulas for calculating the CTS and response properties of the

Li@C18 are described in Supplementary Material.

All quantum chemistry calculations were carried out by Gaussian 16 (A.03)

program package [36]. Free energies were obtained by Shermo code [37]. The

analyses of electronic wave function and plotting CTS were finished via Multiwfn

3.8(dev) code [38]. The isosurface maps of electron density difference (EDD),

(hyper)polarizability density, and unit sphere representation of (hyper)polarizability

were all rendered by means of Visual Molecular Dynamics (VMD) software [39]

based on the analysis results exported by Multiwfn.

3. Results and discussion

3.1. Geometry and energy aspects of transformation between Li@C18in

and Li@C18out

configurations

Our theoretical calculations identified two stable configurations of Li doped C18

complexes, Li@C18in

and Li@C18out

, respectively corresponding to the two situations

when Li atom is bound inside and outside the ring. The transition state linking the two

configurations, denoted by Li@C18TS

, was also located. The optimized geometries of

Li@C18in

, Li@C18TS

, and Li@C18out

are shown in Fig. 1. The minimum structures of

Li@C18in

and Li@C18out

are observed to be exactly planar, in which the doped Li atom

is coplanar with the C18 ring, while the Li atom in the transition state Li@C18TS

is

binded above the plane of the carbon ring with distance about 1.79 Å. The Cartesian

coordinates of these complexes are provided in Table S1. Note that all following

discussions involving molecular orientation adopt the coordinate system shown in Fig.

1.

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Fig. 1. Free energy diagram along the configuration transformation between Li@C18in

and Li@C18out

. Also shown is the Cartesian axis. The inset displays the relative free

energy of each species at different temperatures.

Relative free energy (∆G) of different configurations are depicted in the inset of Fig.

1. It can be seen that the values of ∆G are not affected by temperature notably. The

small free energy barrier of transformation (3.82-8.61 kcal · mol-1

) implies that the

Li@C18in

and Li@C18out

can easily interconvert to each other at ambient temperature.

At 300 K, the rate constant of Li@C18in

Li@C18out

and Li@C18in

Li@C18out

transformations estimated by transition state theory is as high as 3.4 106 and 3.7

109 s

-1, respectively. This observation indicates that it possible to tune the electronic

structure of the Li@C18 complex by switching the Li atom between the inside and

outside the C18 ring.

3.2. Electronic structure analysis of Li@C18 complexes

Some characteristic parameters related to electronic structure for various Li@C18

configurations are listed in Table 1. It can be seen that regardless of the configuration

of the complex, the atomic charge of Li atom [q(Li)] derived from the natural

population analysis (NPA) is very close to +1.0 e, so the Li atom is essentially Li+

cation in all cases. Furthermore, spin population of Li [ps(Li)] in each species

calculated by Hirshfeld method is almost zero, indicating that the 2s unpaired valence

7

electron of Li atom has fully transferred to the C18 moiety. Therefore, the Li@C18

complexes are considered as salt-like structures with Li+@C18

form, and it can be

inferred that the interaction between Li and C18 in the complexes is dominated by

electrostatic attractive effect. Similar result has been observed in the electronic

structure analysis of Li@aphthalene complex by Wang et al [12]. The fairly small

Wiberg bond order between Li and all atoms in the C18 moiety [BO(LiC18)] reveals

that the contribution of electron-sharing effect to LiC18 binding can be neglected,

which further confirms the correctness of the above conclusion.

Table 1. Atomic charge of Li [q(Li), in e], spin population of Li [ps(Li)], Wiberg

bond order of LiC18 [BO(LiC18)], dipole moment of Li@C18 (μ, in Debye), and

binding energy of Li@C18 (Eb, in kcal · mol-1

) at various configurations

Li@C18

in Li@C18

TS Li@C18

out

q(Li)a 0.977 0.979 0.951

ps(Li)

b 0.004 0.006 0.007

BO(LiC18)c 0.049 0.043 0.096

μd 0.53 6.45 16.43

Ebe -36.0 -26.3 -29.4

aObtained based on NPA.

bObtained based on Hirshfeld population analysis.

cCalculated based on natural atomic orbital (NAO).

dCalculated at B97XD/ma-TZVP level.

eCalculated at DLPNO-CCSD(T)/cc-pVTZ level.

The electron density difference (EDD) map can vividly display the electron transfer

and reorganization in a chemical system caused by interaction between structural

units. The EDD maps of Li@C18 complexes in Fig. 2 clearly show that there is a large

area of spherical blue isosurface around Li atom in every configuration, which

indicates that the 2s electron of Li has lost when it is combined with C18. As shown by

the green isosurface, the regions where the electron density increases are mainly

distributed on the plane of the C18 moiety, reflecting that the 2s electron of Li has

mostly moved to the unoccupied in-plane orbital of C18. This strategy of doping Li

atom to induce intramolecular charge transfer is expected to improve the optical

8

properties of the system, and it should even be possible to regulate its optical

properties by switching the binding location of the Li atom in the complex.

Fig. 2. Electron density difference map of various configurations of Li@C18 complex.

Green and blue isosurfaces represent the positive and negative parts (isovalue = 0.002

au), respectively.

The dipole moment (μ) of the Li@C18 complex is markedly affected by the binding

position of Li with C18. The calculated values of μ at the stationary points of Li@C18in

,

Li@C18TS

, and Li@C18out

are 0.53, 6.45, and 16.43 Debye, respectively. The variation

of dipole moment along the IRC corresponding to switching the Li atom between the

inside and outside the ring is shown in Fig. S1. It is observed that as Li atom

gradually moves out of the C18 ring, the dipole moment of the system steadily

increases. This happens because the Li atom is essentially a cation carrying nearly a

unit of positive charge, and its movement from inside to outside the ring expands the

positive-negative charge separation of the complex.

Binding energy (Eb) represents the variation of electronic energy in the course of

formation of Li@C18 from infinitely separated Li atom and C18 molecule, and it is

calculated as

iso

b 18 18(Li@C ) (Li) (C ) E E E E

where iso

18C is the C18 in its isolated geometry. The large magnitude of Eb in Table 1

shows that the binding strength between Li and C18 is strong irrespective of the

position of doping, and the difference of Eb of Li@C18in

and Li@C18out

indicates that

Li atom is more inclined to bind to C18 on the inside.

3.3. Electronic absorption spectrum of Li@C18 complexes

Electronic absorption spectrum is a conventional but important technique for

material analysis, which is sensitive to status of electronic structure of molecules.

9

From Fig. 3, it can be seen that the optical absorption range of the Li@C18in

is wider

than that of the Li@C18out

, but the absorption intensity of the latter is much stronger

than that of the former. Two obvious absorptions of Li@C18in

configuration are

located at about 394 and 311 nm, while they are found to be blue-shifted to 350 and

279 nm at the geometry of Li@C18out

. Since Li@C18in

has a certain absorption in the

visible region but Li@C18out

does not, the switch between these two configurations

may bring change in the system color.

Fig. 3. Electronic absorption spectrum and charge-transfer spectrum of (a) Li@C18in

and (b) Li@C18out

. The Gaussian function with full width at half-maximum of 0.333

eV was employed for broadening the theoretical data as spectrum curves.

Inspired by the definition of molecular excitation spectrum, here we propose a new

concept called charge-transfer spectrum (CTS), as described in the Supplementary

Material, aiming to understand the nature of electron excitation from the perspective

of intrafragment charge redistribution and interfragment charge transfer. By using this

approach, the contributions of intrafragment electron redistributions and interfragment

electron transfers to the absorption spectra of Li@C18in

and Li@C18out

are analyzed

and also plotted in Fig. 3. As can be seen, except for the excitation at about 258 nm

that shows obvious characteristics of electron transfer from C18 to Li, almost the entire

10

optical absorption of Li@C18in

is assigned to be electron transition within the C18

moiety, as the brown curve representing the electron redistribution in the C18 fragment

is very close to the absorption spectrum (black curve) of the complex. In contrast, the

nature of optical absorption of Li@C18out

is different from that of the Li@C18in

,

because many excitations of the former in the range of 300-500 nm show strong

electron transfer from C18 to Li. It is worth to note that the S46 for Li@C18in

as well as

S20 and S35 for Li@C18out

, whose excitation energy/oscillator strength are 258

nm/0.02, 376 nm/0.06, and 277 nm/0.04, respectively, exhibit electron transfer

character of C18Li as large as 33.6 %, 82.7%, and 47.7 %. As an example, the

corresponding isosurface map of hole and electron distributions [17,40] of the S20 for

Li@C18out

is given in Fig. S2, from which one can see that the excited electron goes

almost exclusively from in-plane orbital of C18 to Li atom.

From the analysis results, it is found that switching the doped Li atom between the

inside and outside the ring can not only regulate the wavelength and intensity of the

absorption spectrum of Li@C18 complex, but also change the nature of its electronic

excitation.

3.4. (Hyper)Polarizabilities of Li@C18 complexes

The (hyper)polarizabilities of Li@C18in

and Li@C18out

, such as the isotropic average

polarizability (αiso), the projection of first-order hyperpolarizability on moleculer

dipole (βvec), and the average of second-order hyperpolarizability (γ||), which can be

compared with the results from experimental observation or inference, are plotted in

Fig. 4. The values of them as well as their axial components are given in Table S2.

There is no significant difference between the αiso of Li@C18in

and Li@C18out

, which

is similar to the law of polarizability between the analogues of many other types of

compounds we have studied [41-44]. However, the change in the way of binding of Li

atom not only causes the magnitude of βvec to vary, but also alters the sign of it. The

second-order hyperpolarizabilities of Li@C18in

and Li@C18out

exhibit considerable

discrepancy as well, more specifically, the migration of Li atom from the inside to the

outside the carbon ring makes the γ|| value enlarged by 1.51 times from 194117 to

292959 au. It is worth mentioning that the γ|| of the Li@C18in

and Li@C18out

are 1.38

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and 2.08 times, respectively, that of free C18 ring (140909 au) [17]. These

comparisons demonstrate that the Li@C18in

and Li@C18out

are good candidates for

high-performance nonlinear optical (NLO) materials, and more importantly, the

binding location of Li atom in these complexes plays a crucial role in determining

their hyperpolarizabilities.

Fig. 4. Isotropic average polarizability (αiso), projection of first-order

hyperpolarizability on molecular dipole (βvec), and average of second-order

hyperpolarizability (γ||) of Li@C18in

and Li@C18out

.

Since the direct experimental observation is usually the frequency-dependent

(hyper)polarizability measured in external electric field at a certain frequency, we

therefore calculated the dynamic (hyper)polarizability [corresponding to α(-ω;ω),

β(-2ω;ω,ω), and γ(-2ω;ω,ω,0)] under the external field of 1340 nm, which is one of

the most commonly used incident light adopted in experiments, and listed the values

also in Table S2. We can see that both the first- and the second-order

hyperpolarizability (βvec and γ||) of the Li@C18 exhibit strong polarization resonance

effect under the dynamic external field, since the dynamic hyperpolarizabilities are

significantly higher than the static cases. Especially, the βvec and γ|| values are

increased by more than 15 and 54-fold, respectively, for Li@C18in

after being induced

by external electric field of 1340 nm.

12

The unit sphere representation method of (hyper)polarizabilities proposed by Tuer

et al. is a very powerful technique to comprehensively characterize the

(hyper)polarizability tensor [45], which can intuitively reflect the global and local

characteristics of molecular response properties. The unit sphere representation of the

(hyper)polarizability for Li@C18in

and Li@C18out

in static electric field are displayed

in Fig. 5. One can conclude from the color and length of vector arrows that the

(hyper)polarizabilities of the Li@C18in

and Li@C18out

exhibit obvious anisotropy, and

their components on molecular plane (x- and y-components) are more dominant than

that in the vertical direction (z-component). To be more specific, for polarizability and

second-order hyperpolarizability, their tensors [Figs. 5(a) and (d) as well as Figs. 5(c)

and (f)] display some vector distributions, albeit short and blue, perpendicular to the

molecular plane, indicating that the response component of them in this direction

cannot be ignored; however, the z-component of the first-order hyperpolarizability is

exactly zero, because the arrows showing first-order hyperpolarizability tensor [Figs.

5(b) and (e)] fully vanish in this direction. The planar two-dimensional configuration

of the complexes studied is the main cause of the anisotropy of the

(hyper)polarizability.

13

Fig. 5. Unit sphere representation of (hyper)polarizability for Li@C18in

and Li@C18out

in static electric field. Longer and redder arrow indicates a larger tensor value in

corresponding direction.

As also can be seen, Li@C18in

and Li@C18out

show very similar characteristics of

the polarizability tensor in Figs. 5(a) and (d), while the vectors of their first-order

hyperpolarizability tensors [Figs. 5(b) and (e)] are oriented in the opposite direction

and the arrow lengths of the tensors of Li@C18out

are generally shorter than that of

Li@C18in

. Conversely, however, the second-order hyperpolarizability tensor exhibited

by the Li@C18out

complex [Fig. 5(f)] is significantly larger than that of Li@C18in

[Fig.

5(c)], and the directions of them are the same. These observations about

(hyper)polarizability tensors of the two complexes are completely consistent with the

trend of the adjustment of (hyper)polarizability value by switching Li atom inside and

outside the carbon ring, moreover, the unit sphere representation graphically conveys

more information about the (hyper)polarizability characteristics. For example, the

significantly induced second-harmonic-generation (SHG) dipole is expected to

generate if two incident electric fields are imposed along the direction of one of the

purple arrows in Figs. 5(b) and (e), while an induced dipole in opposite direction may

be observed when the two external fields are simultaneously exerted along one of the

cyan arrows.

As shown in Table S2, the components along x-axis occupy the dominant

contributions to the overall (hyper)polarizabilities, therefore, we will focus on the

x-component of all kinds of response properties in the following discussions.

The axial components of the (hyper)polarizabilities estimated by the finite field (FF)

method are listed in Table S3. By comparing it with Table S2, one can see that the

overall diagonal elements of (hyper)polarizabilities of the two Li@C18 complexes

calculated by the FF method with numerical difference technique are close to the

results obtained by the coupled perturbed Kohn-Sham (CPKS) method [46], which

solves the (hyper)polarizabilities in an analytical way. Therefore, the reliability of the

(hyper)polarizability density analysis and contribution decomposition of molecular

14

units to (hyper)polarizability that we will carry out next based on the FF method are

sufficiently ensured.

The (hyper)polarizability density analysis, which graphically exhibits the spatial

contribution of electrons in a molecule to electric response properties, is helpful to

gain a deep insight into the physical nature of molecular (hyper)polarizability

[17,41-44]. The (hyper)polarizability densities, namely the local contributions to the

(hyper)polarizabilities, in x direction [ (1)( )xx r , (2)( )xxx r , and (3)( )xxxx r ] of the

Li@C18in

and Li@C18out

in static electric field, are rendered in Fig. 6. The (1)( )xx r

of Figs. 6(a) and (d) are almost completely presented by blue isosurfaces, whose sizes

are basically indistinguishable for the two complexes Li@C18in

and Li@C18out

,

elucidating the reason why they have similar positive polarizability. Although the

distribution of the (2)( )xxx r isosurfaces are visually scattered, the Li@C18out

shows

overwhelmingly larger area of red isosurfaces compared to the blue parts, explaining

why Li@C18out

shows negative first-order hyperpolarizability. The blue (3)( )xxxx r

isosurfaces of Li@C18in

and Li@C18out

clearly occupy larger spatial areas than the red

ones, resulting in the observation that both complexes have a large positive

x-component of second-order hyperpolarizability. In addition, the proportion of the

blue area in (3)( )xxxx r isosurfaces is increased in the Li@C18out

[Fig. 6(f)] compared

to that in the Li@C18in

[Fig. 6(c)], which corresponds to the fact that Li@C18out

has a

remarkably higher second-order hyperpolarizability than Li@C18in

. The color-filled

contour maps of the (hyper)polarizability density isosurfaces on molecular plane (xy

plane) provide supplementary information on the internal details of the isosurfaces, as

shown in Fig. S3.

15

Fig. 6. Local contribution map of (hyper)polarizabilities of Li@C18 complexes in

static electric field: (a) and (d) (1)( )xx r for the polarizability (isovalue = 0.5 au), (b)

and (e) (2)( )xxx r for the first-order hyperpolarizability (isovalue = 10.0 au), and (c)

and (f) (3)( )xxxx r for the second-order hyperpolarizability (isovalue = 200.0 au). Blue

and red isosurfaces represent positive and negative electron contributions,

respectively.

We also studied the local contributions of the (hyper)polarizability in y and z

directions [ (1)( ) yy r , (2)( ) yyy r , and (3) ( ) yyyy r as well as (1)( ) zz r , (2)( ) zzz r , and

(3)( ) zzzz r ] of the Li@C18 complexes in static electric field, see Figs. S4 and S5

respectively, for comparison. The isosurface distribution of y-component with the

same isovalue as the x-component (Fig. 6) is obviously narrower, especially for the

first- and second-order hyperpolarizabilities. The isosurface of z-component is even

invisible at all under the same isovalue setting. It is found that there is a good

correspondence between the isosurface characteristics and response components in

the y and z axes, as the case of the x-axis component discussed above.

The contribution of molecular units to the axial components of (hyper)polarizability

is further quantitatively analyzed by using the multicenter numerical integration

strategy [47]. A single Li@C18 complex is divided into two substructures: Li atom

and C18 ring, and the (hyper)polarizability densities are numerically integrated within

16

the region of the two fragments. All results obtained are summarized in Table S3 and

the decomposition of the x-component of (hyper)polarizability of Li@C18in

and

Li@C18out

are depicted in Fig. 7.

Fig. 7. Decomposition of x-component of (hyper)polarizabilities of two Li@C18

complexes into contributions from Li atom and C18 ring.

According to Fig. 7, from the perspective of the spatial location, the contribution

from Li atom to the x-component of (hyper)polarizability in Li@C18in

can be

completely ignored, that is to say, its x-component of (hyper)polarizability almost

solely comes from the C18 fragment. However, Li atom has a clear contribution to the

x-component of the (hyper)polarizability in Li@C18out

, and more interestingly, with

the increase of response order to electric field, the contribution of Li atom increases

rapidly. The contribution from Li atom to second-order hyperpolarizability of

Li@C18out

is even dominant, indicating the unique role of introducing Li atom at

proper location in optical application of C18.

17

The analyses of the optical response characteristics of Li@C18 complex with

different configurations show that the (hyper)polarizability of the system can be

controlled by regulating the binding of Li atom to the C18 in different ways, and at the

meantime, the essential source of the (hyper)polarizability can also be adjusted.

4. Conclusions

Based on previous theoretical predictions on Li-doped nanocarbon materials and

our research interests in C18 ring, we theoretically constructed two typical

monolithiated-C18 complexes, Li@C18in

and Li@C18out

, which differ in the doping

position of Li atom. High-precision calculations show that interconversion can readily

occur between the two configurations at ambient condition. The electronic structure,

absorption spectrum, and optical nonlinearity of Li@C18in

and Li@C18out

are discussed

and compared. The results exhibit that switching the doped Li atom between the

inside and outside the carbon ring causes greatly different electronic structures of the

Li@C18 complex and hence considerable discrepancies in optical properties. The

range of electronic absorption of Li@C18in

is much wider than that of the Li@C18out

,

but the absorption intensity of the latter is more stronger. The novel charge-transfer

spectrum analysis shows that broad range of optical absorption of the Li@C18out

exhibits charge-transfer character from C18 to Li atom, but this effect is evidently less

prominent in Li@C18in

. By regulating the binding of Li atom to C18 in different ways

of inside and outside the ring, the value and the source of the first- and second-order

hyperpolarizability of the Li@C18 complex can be tuned, but insignificant change was

observed for molecular polarizability. The essences of discrepancy in electronic

structure, absorption spectrum, and optical nonlinearity of Li@C18 complex with

different configurations have also been analyzed through electron density difference,

hole-electron analysis, (hyper)polarizability density, and so on.

Overall, the excellent but considerable discrepancy between the electronic structure

and optical properties of the Li@C18 complex with different configurations implies

that they can serve as good candidates for high-performance nonlinear optical

meterials, and more importantly, they are expected to have potential applications in

18

optical molecular switches. Therefore, we hope that the present work will inspire

experimental chemists to design and synthesize novel optical switching devices based

on alkali-metallized cyclocarbons.

Author contributions

Zeyu Liu: Supervision, Formal analysis, Visualization, Funding acquisition,

Writing-original draft. Xia Wang: Visualization. Tian Lu: Methodology, Resources,

Software, Writing-original draft. Aihua Yuan: Writing-review & polishing. Xiufen

Yan: Investigation.

Acknowledgments

This work was partially supported by The Natural Science Foundation of the Jiangsu

Higher Education Institutions of China (Grant No. 18KJA180005).

Supporting Information Available

Detailed formulas for calculating the charge-transfer spectrum and molecular

(hyper)polarizabilities; optimized Cartesian coordinates for Li@C18 complexes;

variation of energy and dipole moment of Li@C18 complex along intrinsic reaction

coordinate path of switching Li atom between the inside and outside the ring;

isosurfaces of hole and electron distribution of S0S20 excitation of Li@C18out

;

axial components and overall values of the (hyper)polarizabilities of the Li@C18in

and

Li@C18out

; axial components of the (hyper)polarizabilities of Li@C18 estimated by the

finite field (FF) method in static electric field; color-filled contour map of

x-component of the (hyper)polarizabilities of Li@C18 complexes in static electric field;

local contribution functions of y- and z-components of the (hyper)polarizabilities of

Li@C18 complexes in static electric field.

References

19

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