Chemistry Reference
In-Depth Information
Fig. 6.7
Allowed intra-ligand transitions from
χ
-to
ψ
-type ligand orbitals for tris-chelate com-
plexes with
D
3
symmetry. The circular dichroism has a lower right-circularly polarized (rcp) band
and an upper left-circularly polarized (lcp) band. This gives the CD spectrum the appearance of the
first derivative of a Gaussian curve, with a negative part at longer wavelength and a positive part at
shorter wavelength
We designate these dipole moments as
μ
A
,
μ
B
,
μ
C
. These vectors can be ex-
pressed in a row notation for the primed
x
,y
,z
coordinate system as follows:
√
3
,
√
2
μ
A
=
μ
0
,
1
√
3
2
√
3
,
√
2
μ
1
2
,
1
μ
B
=
(6.109)
−
−
√
3
2
√
3
,
√
2
μ
1
1
μ
C
=
2
,
−
√
3
The scalar products between these orientations are equal to 1
/
2, which corresponds
to angles of 60
◦
. Each of the three transitions gives rise to an excited state. In
D
3
symmetry these states transform as
A
2
+
E
. The composition of these
exciton
states
8
is as follows:
1
A
2
=
√
3
(χ
A
→
ψ
C
)
1
1
ψ
A
)
1
ψ
B
)
1
+
(χ
B
→
+
(χ
C
→
1
E
θ
=
√
2
−
ψ
C
)
1
1
ψ
B
)
1
(χ
B
→
+
(χ
C
→
(6.110)
1
E
=
√
6
2
(χ
A
→
ψ
A
)
1
−
(χ
C
→
ψ
C
)
1
1
−
(χ
B
→
ψ
B
)
1
8
The excitation creates an electron-hole pair, which can move from one ligand to another. This is
called an exciton.
