Chemistry Reference
In-Depth Information
that the
σ
∗
(
b
2u
) feature exhibits a significant
d
z
2
contribution, an orbital pointing
perpendicularly to the TTF molecular plane. Since the normal of this molecular
plane forms an angle of about 60
◦
with the direction perpendicular to the
ab
-plane
(
c
∗
-direction), the
d
z
2
contribution to the NEXAFS spectrum would be more evident
for lower
θ
E
values and should be strongly reduced for large
θ
E
values, as experimen-
σ
∗
(
b
2u
) feature,
π
∗
(
b
3g
tally observed. Similarly to the
,
a
u
,
b
2g
) should decrease at
larger values of
E
because of the orbital distribution. This behaviour is not evident
from the spectra because of the overlap with
θ
σ
∗
(
b
2u
) but the intensity ratio between
π
∗
(
b
3g
,
,
σ
∗
(
a
g
) decreases for increasing
θ
a
u
b
2g
) and
E
values, as expected.
TCNQ-derived spectral features
Let us now consider the case of neutral and charged TCNQ. Both the C1
s
and
N1
s
spectra are quite well resolved (see Figs. 6.14(b) and (c)). The PDOS of both
nitrogen and carbon in the region of interest are almost completely dominated
by the
p
-type contributions. Looking at the TCNQ DOS in Fig. 6.5(d) we can
see four clear peaks above that of the LUMO band. The first peak, appearing at
around 2.8 eV above the LUMO minimum, originates from the lower
a
u
and
b
1u
orbitals of TCNQ, schematized in Fig. 6.17, which for the isolated molecule lie at
2.55 and 2.65 eV from the LUMO. These
π
∗
(
a
u
b
1u
), are quite
strongly concentrated on the benzenic region (small contributions are not shown
in the schematic representations of Fig. 6.17). These orbitals originate from the
degenerate pair of lowest empty
π
-type orbitals,
,
π
orbitals (
e
2u
)ofC
6
H
6
, which only slightly
delocalize towards the substituent in TCNQ.
The next two peaks are associated to the CN groups. The first one contains the
contribution of the bands based on the
a
g
and
b
3u
orbitals of TCNQ lying at 3.51
and 3.63 eV from the LUMO, and will be labelled
σ
∗
(
b
3u
)), while the second
peak contains the contribution of bands based on four orbitals of TCNQ lying at
4.55 (
b
3g
) and 4.66 eV (
a
u
), labelled as
π
(
a
g
,
π
∗
(
π
(
b
3g
,
a
u
)), and 4.77 (
b
1g
) and 4.86 eV
σ
∗
(
(
b
2u
), labelled as
π
(
b
1g
,
b
2u
)), from the LUMO (see Fig. 6.17).
π
∗
(
a
g
,
π
∗
(
b
1g
,
Essentially, the
b
3u
) and
b
2u
) orbitals are the four symmetry-
π
∗
orbitals of the CN groups. It is impor-
adapted combinations of the in-plane
π
∗
orthogonal orbitals of the CN group. They
are degenerate for CN itself because of the cylindrical symmetry but become non-
degenerate in TCNQ. Because of the symmetry plane of the molecule, the two
formally degenerate orbitals lead to one in-plane
tant to distinguish between the two
π
∗
orbital, denoted as
σ
∗
(
),
which is symmetrical with respect to the molecular plane even if locally it is a
π
π
π
∗
orbital, denoted as
π
∗
(
-type orbital, and one out-of-plane
π
), which is anti-
symmetrical with respect to the molecular plane and is locally a
π
-type orbital.
σ
∗
(
) orbitals lead to four symmetry-adapted combinations which are
the main components of the four molecular orbitals. Two of them are lower in
The four
π
