Growth and physical properties of crystalline rubrene - BOA Bicocca ...

2.3 Crystalline rubrene 13
Figure 2.2: (a) Three-dimensional sketch of a rubrene endoperoxide molecule. The
oxygen atoms are coloured in red. (b) Imaginary part of the dielectric function of
an amorphous rubrene film measured during its exposition to oxygen and light.
From [29]
2.3 Crystalline rubrene
Rubrene crystallization can be achieved by following several different
routes, obtaining different polymorphs with their characteristic different
molecular arrangments[46,47]. The molecular arrangment in the three main
known rubrene polymorphs is shown in figure 2.3. The three structures are
all observed from a direction normal to the long axis of the molecules, in or-
der to show the amount of π − π stacking between the conjugate backbones
of adjacent molecules. The first two crystalline structures have a monoclinic
and a triclinic unit cell, respectively, and can be obtained by solution crys-
tallization methods[47]; the third structure has an orthorhombic symmetry
and can be obtained by physical vapour growth[46].
The analysis of the three crystalline structures reported in figure 2.3
shows that:
• In the monoclinic polymorph (figure 2.3a) the molecular planes of two
adjacent molecules are almost normal to each other, thus yielding no
π − π stacking at all;
• in the triclinic polymorph (figure 2.3b) the molecular planes of adjacent
molecules are all parallel but also laterally shifted with respect to each
other, leading only to a partial superposition of their π orbitals in the
stacking direction;
• in the orthorhombic polymorph (figure 2.3c) the molecules are arranged
in the so-called herringbone packing, with an almost complete π − π