Chemistry Reference
In-Depth Information
also been found to exhibit interesting optical, magnetic, catalytic, semiconductive
and photoconductive properties (Law, 1993;McKeown, 1998). PTCDA is an electri-
cally neutral pigment and consists of a perylene core with a delocalized
-electron
system and two anhydride endgroups. The endgroups give rise to a permanent
quadrupole moment with the positive charge situated around the centre of the
molecule, and the negative charge around the endgroups.
Let us now briefly examine some aspects of their crystal structures. The crystal
structure of pentacene grown from solution is shown in Figs. 1.13(a) and (b). This
structure is triclinic and the projections along the
c
- and
b
-axes reveal the modified
herringbone structure and the 2Dmolecular plane distribution, respectively. Vapour-
grown pentacene exhibits a slightly different crystal structure and higher mobility
values have been reported for this polymorph (Siegrist
et al.
, 2001).
Projections of the monoclinic
π
β
-phase of
p
-6P are shown in Figs. 1.13(c) and
(d). As depicted in Fig. 1.13(c), the molecules are organized forming a herringbone
pattern while the layered arrangement is shown in Fig. 1.13(d). The long axes of
the planar molecules are aligned parallel to each other and tilted about 17 degrees
relative to the layer normal.
The monoclinic arrangement of
α
-6T is very similar to that of
p
-6P, as observed
from Figs. 1.13(e) and (f). In
-6T the molecules are also planar and the molecular
long axis is at an angle of 23.5
◦
with the
a
-axis. The most salient difference is the
more pronounced dimerized character in
α
-6T.
C
60
, originally termed buckminsterfullerene and nowadays called colloquially
buckyballs or even footballene, is an extremely appealing and interesting molecule
with truncated icosahedral structure (
I
h
point group), readily viewed as a football
or a soccerball by replacing each vertex of such a ball by a carbon atom (Kroto
et al.
, 1985). Solid C
60
is a semiconductor with a face-centred-cubic structure (space
group
Fm
3
m
) at RT, in which the molecules exhibit dynamic orientational disorder.
C
60
opened two main avenues of research soon after its discovery: doping-induced
superconductivity and carbon nanotubes. Superconductivity will be discussed later
and carbon nanotubes are a matter of intense research because of their extraor-
dinary properties: metallic/semiconductor depending on helicity, larger stiffness,
flexibility, chemical inertness, etc. (Dresselhaus
et al.
, 1996). Such nanotubes have
allowed the preparation of single-molecule transistors, since nanotubes are very
long 1D molecules (Postma
et al.
, 2001).
Our last example is Mo
3
S
7
[(dmit)
2
]
3
, a single-component material based on
the trinuclear Mo
3
S
7
cluster (with idealized
C
3
v
symmetry) coordinated to three
dithiolate ligands dmit (see Fig. 1.14(a)) (Llusar
et al.
, 2004). This solid ex-
hibits sizeable intermolecular magnetic interactions together with a significant elec-
tron delocalization and contains, in addition, large open channels. Single crystals
and powder samples of Mo
3
S
7
[(dmit)
2
]
3
show
α
−
1
cm
−
1
,
σ
RT
∼
20 and
σ
RT
∼
1
