Molecular Spin Switch and Conversions of Localized Excess Electrons under External Electric Field: Solvated Dielectron C20F20@K@C20F20@K@C20F20
Jia-Min Tang,[a] Yin-Feng Wang,*[a] Qin Tian,[a]Xue-Xia Liu,[a] Zhijun Wang,[a] Jiangen Huang,[a]Hua-Rong Zhang,*[b] Kai Yang,[a] and Zhi-Ru Li*[c]
[a] Dr. Y.-F. Wang, Bs. Q .Tian, Dr. X.-X. Liu, Dr. Z. Wang. Jiangxi Province Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Jinggangshan University Ji’an, Jiangxi 343009 (P.R. China) E-mail: cyclont@yeah.net
[b] Dr. H.-R. Zhang Key Laboratory of Organosilicon Chemistry and Material Technology, Hangzhou Normal University, Hangzhou, Zhejiang 311121 (P.R. China). E-mail: zhanghuaxy99@hznu.edu.cn
[c] Prof. Z.-R. Li Institute of Theoretical Chemistry, Laboratory of Theoretical Computational Chemistry, Jilin University Changchun 130023 (P.R. China) E-mail: lzr@jlu.edu.cn
Abstract: By doping two potassium atoms among three C20F20 cages, peanut-shaped single molecular solvated dielectron C20F20@K@C20F20@K@C20F20as new type of spin molecular switches was theoretically presented. The triplet structure with two single-excess-electrons individually inside left and middle cages is thermodynamically more stable than the singlet one with lone pair of excess electrons inside middle cage. It is found that applying an oriented external electric field (OEEF) of 111 × 10-4 au (0.5705 V/Å) or -120 × 10-4au (-0.6168 V/Å) in the x-axis direction firstly and then releasing it, the field-free triplet C20F20@K@C20F20@K@C20F20with two single-excess-electrons can change into singlet one with lone pair of excess electrons through a singlet one with lone pair of excess electrons inside the end cage. Different spin states can bring significantly different dipole moment component values and considerable different intensities of maxumum wavelengths in intense absorption band. Therefore, C20F20@K@C20F20@K@C20F20is a good candidate for spin molecular switching materials.
Introduction
Recently, theoretical design of new high-performance molecular switching materials driven by oriented external electric field (OEEF) have witnessed large developments in the field of molecular electronics.1-5
As the simplest form of an extremely reactive intermediate, the investigation of the solvated electron plays a prominent role in physics, chemistry, and biochemistry.6-14 For example, Adhikari et al have reported efficient capture of presolvated electrons by DNA base.10 Also, for the charge- and electron-transfer and the transitions, manipulation of one or several electrons at the molecular level remains a highly challenging task15 considering the utmost importance in the related fields of molecular electronics,16 artificial photosynthesis,15,17 molecular switches,15,18 quantum-dot cellular automata (QCA),19-21 solar cells,22 and supramolecular assemblies.23 As solvated electron systems (excess electron inside molecular cluster or cage) have not enough stabilities,14 improving their stabilities is an important task.
To improve stabilities of general solvated electrons, in our recent series of papers,24-29 basing on the synthesized C20F20 and potential C24F24 and C60F60 cage,30,31 we have constructed a series of interesting cage-like single molecular solvated (di)electron systems with large stabilities due to the using covalent cages: e@C60F60, e2@CnFn (n=20, 28, 36, 50, 60, and 80), e@C20F18(NH)2C20F18, and e@C24F22(NH)2C20F18, which shows these perfluorinated cages are efficient containers of excess electrons. For these cages, the dipole moments of all the exo polarized Cδ+-Fδ- bonds of each cage are directed toward the center of the cage to form an interior electronic attractive potential (IEAP) which can help to trap excess electron(s) inside these cages. These single molecular solvated (di)electron systems can provide a base of molecular electronics device. Basing on our e@C20F18(NH)2C20F18, Ma et al., have successfully suggested a new type of molecular quantum-dot cellular automata (MQCA) candidate.20,21Recently, Li et al.,32 have constructed new kind of electride molecular salts e@C20F19(CH2)4NH2…Na+as novel, potential high-performance NLO materials. Also, basing on triple-cage-like electride salt K+[e@3C8(O)]-, Li et al., also suggested33 that multicage strategy is effective to enhance nonlinear optical (NLO) response. For the double-cage-like single molecular solvated single electron systems e-@C20F18(NH)2C20F18,26-28the excess electron can be trapped inside different cages to form interesting inter-cage electron transfer isomers.26 In this case, besides the double-cage-like single molecular solvated electron systems,20,21,26-28 We are interesting in the influence of the number of cage units on the localizations and spin states of two excess electrons. Especially, can it switch between different spin states by external stimulus? Obviously, exploring new molecular spin switching material is necessary for promoting its application in the field of molecular electronics, which is the target of our work.
Recently, Sadlej-Sosnowska has reported that, in an uniform OEEF, a reversible switching between the two configurations of Li-benzene complex with significant dipole moments took place.34Straka et al demonstrated for MX@C70 (M: metal, X: nonmetal) systems that the relative orientation of enclosed MX with respect to a set of electrodes connected to the system can be controlled by application of OEEF(s).2 For the double-cage-like single molecular solvated electron systems of e@C20F18(NH)2C20F18, and e@C24F22(NH)2C20F18,26-27we have also used an external electric field to realize the inter-cage excess electron transfer and isomerization among three inter-cage electron transfer isomers. These will provide an approach for the manipulating localization(s) and spin states of the excess electron(s) in multi-cage-shaped solvated dielectron systems by using external electric field.
In this paper, by doping two potassium atoms among three C20F20 cages, our investigation aims at obtaining the structures, excess electron localizations, and electronic absorption spectra of the peanut-shaped single molecular solvated dielectron systems of C20F20@K@C20F20@K@C20F20, revealing the influence of the number of cage units on the localizations and spin states of two excess electrons, exhibiting excess electron transition under the external electric field, and suggesting the possible candidate for the new kind of molecular spin switching metherial.
Results and Discussion
Geometrical characteristics and excess electron localizations
The optimized geometries with all real frequencies of C20F20@K@C20F20@K@C20F20were shown in Figure 1. The selected structural parameters in both singlet and triplet states were listed in Table 1. For both singlet (S) and triplet structures (T), the C20F20@K@C20F20@K@C20F20represents face-to-face stacked C20F20cages connected by two doped K atoms. From Figure 1, for the plane where five-membered ring is located, the plane α of cage 1 and plane β of cage 2 are staggered with very small dihedral angle (0.55 (S) or 0.26°(T)). The similar situation happens between the five-membered plane γ (cage 2 ) and δ (cage3 ) but some large dihedral angle between them (15.08 (S) or 15.55°(T)).
The different sizes of three cages (1 , 2 , and3 ) are closely related to excess electron localization(s) in both states (S and T). Results in Table 1 exhibit that cage 2are the smallest one among three cages (h 2(2 ) < h 1 (1 ) ≈h 3 (3 )) in singlet structure, and both cage 2 (h 2) and 1(h 1) are smaller in size than cage 3(h 3) in triplet one.
Results in Table 2 show that the natural popular analysis (NPA) charges of K1 and K2 atoms in field-free C20F20@K@C20F20@K@C20F20are larger than 0.66 |e| for both singlet and triplet states, which indicates the valences of both K atoms are +1 for both states. At the same time, the NPA charge of cage 2 in singlet structure and that of cage 1 and 2 in triplet one suggests that the valence of cage 2 in singlet structure is -2 and that of both cage 1 and 2 are -1. Therefore, the 4s electrons of both K atoms are pulled out and trapped inside the fluorinated C20 cage(s) to form localized excess electrons due to the IEAPs.