Figure 5. Excess electronic absorption spectra at CIS/6-31G (d) level, a) singlet state and b) triplet state.
The electronic absorption spectra of the C20F20@K@C20F20@K@C20F20with and without OEEFs are depicted in Figure 5. The diagram of these selected electronic excitations and the contour plots for their corresponding molecular orbitals (MOs) are also presented in Figure 5.
From Figure 5a, it shows an intense absorption band with two maximum absorption (wavelengths of 534 and 460 nm) and a weaker absorption band at 183 nm for the field-free singlet C20F20@K@C20F20@K@C20F20with the lone pair of excess electrons inside middle C20F20 cage (2 ). When going from field-free C20F20@K@C20F20@K@C20F2with lone pair of excess electrons inside middle C20F20 cage (2 ) to the electrified ones with that inside the left or right C20F20 cage (1 or 3 ), it is found that the absorption in long wavelength appears blue-shift (from 534 to 511 nm) and that on short wavelengths appear red-shift (from 460 to 481 or 484 nm) for the intense absorption band. The two absorption bands of C20F20@K@C20F20@K@C20F20under F x = -120 × 10-4 au with lone pair of excess electrons inside the right C20F20 cage (3 ) are almost same to that under F x = 110 × 10-4au with that inside the left C20F20 cage (1 ). Results in Fig. 5a also shows that the electronic excitation of each above absorption bands corresponds to intra-cage s → p transition for all three C20F20@K@C20F20@K@C20F20under F x = -120 × 10-4, 0 and 120 × 10-4 au with lone pair of excess electrons inside right, middle and left cage, respectively.
From Figure 5b, it shows an intense absorption band with three maximum absorptions (wavelengths of 555, 474, and 467 nm) and a much weaker absorption band at 212 nm for field-free triplet C20F20@K@C20F20@K@C20F20with two single-excess-electrons, respectively, inside the left and middle C20F20 cages (1 and2 ). The electronic excitation of the strongest absorption (555 nm) corresponds to intra- and inter-cage s → p transitions (HOMO-1 → LUMO+3), while that of the second and third ones (474 and 467 nm) corresponds to intra-cage s → p transitions (HOMO → LUMO+11 and LUMO+8). Obviously, the intense absorption bands with absorption wavelengths of 555 and 474 nm for field-free triplet C20F20@K@C20F20@K@C20F20with two single-excess-electrons inside two cages (1 and2 ) just corresponds to that of 534 and 460 nm for field-free singlet C20F20@K@C20F20@K@C20F20with the lone pair of excess electrons inside one cage (2 ), respectively. From field-free triplet to field-free singlet C20F20@K@C20F20@K@C20F20, the wavelengths of both absorptions appear blue-shift and intensity of both absorptions considerably strengthen. When going from field-free triplet C20F20@K@C20F20@K@C20F2with two single-excess-electrons inside left and middle C20F20 cage (1 and 2 ) to the electrified ones with that inside the middle or right C20F20 cage (2 and 3 ), it is found that the absorption in long wavelengths appears both red-shifts for the intense absorption band.
Conclusion
In this paper, we have presented a new kind of spin molecular switches by doping two potassium atoms among three C20F20 cages to form peanut-shaped single molecular solvated dielectron C20F20@K@C20F20@K@C20F20.
The triplet structure with two single-excess-electrons inside two cages (1 and 2 ) is thermodynamically more stable than the singlet one with lone pair of excess electrons inside one cage (2 ) for field-free C20F20@K@C20F20@K@C20F20.
For the localizations of excess electrons, it is found that the lone pair of excess electrons moves from middle C20F20 cage to right or left one by applying an OEEF of 111 × 10-4 au (0.5705 V/Å) or -120 × 10-4 au (-0.6168 V/Å) in the x-axis direction of singlet C20F20@K@C20F20@K@C20F20. Applying an OEEF of -20 × 10-4 au (-0.1018 V/Å) in the x-axis direction of triplet C20F20@K@C20F20@K@C20F20can result in both left-to-right transfers of the two single-excess-electrons.
For the interconversion between spin states, it is found that applying an OEEF of 111 × 10-4 or -120 × 10-4au in the x-axis direction of triplet C20F20@K@C20F20@K@C20F20firstly and then releasing it can reduce two single-excess-electron (triplet state) changing into lone pair of excess electrons (singlet state) and triplet C20F20@K@C20F20@K@C20F20changing into singlet one. the field-free triplet C20F20@K@C20F20@K@C20F20with two 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 material.
Computational Details
Because of our research systems including long-range interaction and charge transfer, the density functional, coulomb-attenuated hybrid exchange-correlation functional (CAM-B3LYP),37 is used. It was reported that CAM-B3LYP provides molecular geometries close to experimentally observed structures.38 In this work, the optimized geometric structures of the C20F20@K@C20F20@K@C20F20with all real frequencies in both singlet and triplet states are obtained at CAM-B3LYP/6-31G(d) level. The spin density distribution and natural popular analysis (NPA)39 of the structures were also obtained at CAM-B3LYP/6-31G(d) level.
Recently, Population and novel hybrid meta exchange-correlation functional M06-2X40 were successfully used to calculate the vertical electron detachment energy (VDE) of series of excess electron systems.4,5, 41,42 Therefore, the VDEs of our structures were calculated at M06-2X/6-31G(2d) level, as the following formulas (Cage = C20F20@K@C20F20@K@C20F20):
VDE (I) = E [Cage]+opt -E [Cage]opt (1)
VDE (II) = E [Cage]2+opt -E [Cage]+opt (2)
The spin contamination is negligible. In the calculations, the expected values of spin eigenvalue 〈S 2〉 are 0.0 for [Cage] (singlet) and 2.0 for M2+ (triplet), 0.75 for [Cage]+, and 0.0 for [Cage]2+ species.
In our previous work,28 it is found that the first transition energy of CIS method is more close to the higher SAC-CI results than the TD-HF, TD-B3LYP, TD-CAM-B3LYP and TD-LC-BLYP results for molecular cluster anion (FH)2{e-}(FH) and neutral (HCN)···Li with excess electron. Therefore, The CIS/6-31G(d) calculations were performed to obtain the excess electronic absorption spectrum of the C20F20@K@C20F20@K@C20F20.
The calculations were performed with the GAUSSIAN program package (GAUSSIAN 09 A02).43
Acknowledgements
We acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21662018, and 21764007). This work was also supported by the Science and Technology Project of Jiangxi Provincial Department of Science &Technology (No. 20192BAB203005), and the Natural Science Foundation of Zhejiang Province (No. LQ17E030001).
Keywords: Spin switch • solvated electron• external electric field • electron localization • DFT
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