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(a)
(b)
(c)
Figure 3.19
(a) Pictures of
TBDAP
single crystal showing a continuous color change from colorless to dark
green depending on the temperature. (b) Polarized absorption spectra of a
TBDAP
single crystal with polarized
light parallelto the c axis (solid line) and perpendicular to the c axis (dashed line) on the crystal plane (010) at
300K. (c) Temperature-dependent polarized absorption spectra of a
TBDAP
single crystal with polarized light
paralleltothe c axisat160-360K.(ReprintedbypermissionfromMacmillanPublishersLtd,
29
copyright2008.)
Afull-colourversionofpart(a)ofthisfigureappearsintheColourPlatesectionof this topic.
Figure 3.20
Crystal structures of the
dimer of
TBDAP
and the experimentally obtained energy gap
betweentwostates.tert-Butylgroupsareomittedforclarity.
σ
and
π
properties exhibiting the color change only for light from a specific direction. The energy gap between
the two dimer states is experimentally determined to be 1
10
3
K (2.6 kcal/mol) by the temperature-
dependent changes of the oscillator strength in visible area, being in a good agreement with the difference
in the total energy of 3.0 kcal/mol between the optimized
.
3
×
dimers calculated by the B3LYP/6-31G*
method. This value is as same as the rotation barrier energy in the C-C bond of ethane (2.9 kcal/mol). By
the EPR observation of the thermally accessible triplet state, a complete energy diagram of
TBDAP
in the
crystalline state is unequivocally determined to have three states: two singlet states (corresponding to the
σ
σ
and
π
10
3
K.
Note that this is the first example where the energy levels of open shell organic molecules with three
10
3
K and 4
and
π
dimers) and an excited
π
dimer triplet state with the energy gap of 1
.
3
×
.
2
×
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