Figure 3. a) Evolution of NPA charges in singlet state, b)
evolution of NPA charges in triplet state, c) evolution of spin density
in triplet state, and d) evolution of energy under external electric
fields (EEFs). molecular orbitals at the isovalue of 0.004 au.
For triplet
C20F20@K@C20F20@K@C20F20,
from Figure 3b and Table 2 as well as Figure 3c, it can be found that
both spin density and the absolute values of NPA charges of cage1 decrease but that of cage 3 increase with increasing
the intensity of OEEF in negative directions of x-axis (fromF x = 0 to -40 × 10-4 au). The
NPA charge of cage 1 under F x = 0 au and
that of cage 3 under F x = -20 ×
10-4 au (-0.1018 V/Å) in triplet
C20F20@K@C20F20@K@C20F20are, respectively, -0.816 |e| and -0.842
|e|. The SOMOs show that an excess electron is
confined inside the left C20F20 cage
(1 ) for
C20F20@K@C20F20@K@C20F20under F x = 0 au but that is confined inside the
right C20F20 cage (3 ) for
C20F20@K@C20F20@K@C20F20under F x = -20 × 10-4 au
(-0.1018 V/Å).
Therefore, applying an OEEF of -20 × 10-4 au (-0.1018
V/Å) in the x-axis direction of triplet
C20F20@K@C20F20@K@C20F20results in both left-to-right transfers of the two
single-excess-electrons. The transfers of the excess electrons are
exhibited that a single-excess-electron transfers from middle
C20F20 cage (2 ) to right
C20F20 cage (3 ) and the other
one transfers from right cage (1 ) to middle
C20F20 cage (2 ). Theμx value also increases enormously when going
from field-free triplet
C20F20@K@C20F20@K@C20F2to electrified ones.
Molecular spin switch and stabilities
By comparing the total energies for field-free
C20F20@K@C20F20@K@C20F20in different spin states (see Table 3), it is found that the triplet
structure is 8.72 kcal/mol in energy lower than the corresponding
singlet one. Therefore, owing to different localizations of the excess
electrons, 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.
So, the triplet structure is the ground state for field-free
C20F20@K@C20F20@K@C20F20.
In addition, the small energy difference between singlet and triplet
structures may be beneficial to the interconversion between them.
It is worth noticing that, results in Figure 3d and Table 3 show that
the energies of triplet
C20F20@K@C20F20@K@C20F20under both F x < 111 ×
10-4 and F x > -120
× 10-4 au are lower than that of singlet one, while
the singlet
C20F20@K@C20F20@K@C20F20under both F x ≥ 111 × 10-4 andF x ≤ -120 × 10-4 au are lower
in energy than the triplet one. That is to say, the triplet
C20F20@K@C20F20@K@C20F20with the range of -120 × 10-4 <F x < 111 × 10-4 au
but singlet one with the range of F x ≥ 111 ×
10-4 or ≤ -120 × 10-4 au are ground
state for
C20F20@K@C20F20@K@C20F20.
Considering the electronic stabilities of
C20F20@K@C20F20@K@C20F20,
we focus on the first and second vertical electron detachment energies
(VDE (I) and (II)) in ground state. From Table 3, for the triplet
C20F20@K@C20F20@K@C20F20,
the VDE (I) and VDE(II) values are 6.61 and 9.34 eV, respectively. These
VDEs are far larger than that of the reported solvated dielectron
e2@C60F60 structures
((VDEs (I) of 1.720 ~ 2.283 eV, and VDEs (II) of
3.959–5.288 eV)),25e2@(LiF)n (n = 3 ~ 5,
-0.021 ~ 1.254 eV (VDE(1))),35 and
dipole bound dianions of
[(PF5)3・・・dropentalene →
Ca].36 Obviously, the triplet
C20F20@K@C20F20@K@C20F20are stable solvated dielectrons. Changing from triplet to singlet
C20F20@K@C20F20@K@C20F20,
both VDE (I) and VDE(II) values considerably decrease to 4.42 and 8.33
eV as applying the EEF of -120 × 10-4 au in the
x-axis direction but significantly increase to 11.56 and 13.14 eV as
applying the EEF of 110 × 10-4 au in the x-axis
direction. Still, the VDE (I) and VDE(II) values singlet
C20F20@K@C20F20@K@C20F20under F x = -120 × 10-4 au are
far larger than the corresponding ones of reported
e2@C60F60 and
e2@(LiF)n, which indicates that the
singlet
C20F20@K@C20F20@K@C20F20under F x = 111 × 10-4 or -120 ×
10-4 au may be still stable.
Table 3. Total energies (E tot, au),
relative energies (E rel, kcal/mol), interaction
energies (E int), and vertical detachment energies
(VDE(I&II), eV).