Chemistry Reference
In-Depth Information
Figure 1.25. Projection onto the (102) plane of
-PTCDA. C and O atoms are
represented by black and medium grey balls, respectively. H atoms are omitted for
clarity. Crystallographic data from Ogawa
et al.
, 1999.
α
molecules. This angle is of 42 and 38 degrees for the
-phases, respec-
tively. Note that the molecules are slightly inclined and do not lie flat on the (102)
plane.
Polymorphism is also a common phenomenon in Pcs. Among them, CuPc has
been the most studied compound because of its stability and its extensive use in
the dye industry. Five polymorphs, namely the
α
- and
β
α
-,
β
-,
γ
-,
δ
- and
ε
-forms of CuPc
β
α
γ
are known.
-CuPc is the thermodynamically stable polymorph while the
-,
-,
δ
ε
β
- and
-CuPc modifications are metastable and can be converted to
-CuPc upon
heating or solvent recrystallization.
The stacking arrangements of CuPc molecules in the
-polymorphs are
depicted in Fig. 1.26. Two major differences distinguish both phases. First of all
β
α
- and
β
-CuPc exhibits the
γ
- or L-shaped herringbone structure in clear opposition to the
α
-phase. On the other hand the inclination of the plane of the molecules is larger
for the
-CuPc are not
available. In Section 3.2 we shall see the case of the M-CuPc polymorph, obtained
as a thin film grown under microgravity.
β
-phase. Details of the stacking arrangements of
δ
-,
γ
-, and
ε
1.7 Orbitals, bonds and bands
When studying the electronic structure of crystalline solids, physicists tend to think
in terms of the mathematical concept of electronic bands, the so-called dispersion
