Chemistry Reference
In-Depth Information
Scheme 6 Structures of rylenes and synthesis of tetra-
tert
-butyl rylenes [
66
,
67
]
3.1 Rylenes
Rylenes are large PAHs in which two or more naphthalene units are
peri
-fused
together by single bonds. Only one aromatic sextet benzenoid ring can be drawn for
each naphthalene unit and two zig-zag edges exist at the terminal naphthalene units.
On the basis of numbers of fused naphthalenes, rylenes can be termed as perylene,
terrylene, and so on (Scheme
6
). In pursuit of stable dyes with high extinction
coefficients and long-wavelength absorption/emission, rylenes have received a
great deal of attention. Among them, perylene 70a and its imide derivatives have
shown obvious advantages including outstanding chemical, thermal, and photo-
chemical inertness, and are already well investigated and documented. So in this
part, discussion will be focused on higher order rylenes with lower bandgap and
longer wavelength absorption and emission, as well as their applications as
materials.
The preparation of more highly ordered rylenes generally suffered from two
main obstacles. First of all, high order rylenes are electron-rich
-systems and are
relatively unstable upon exposure to air, so mild conditions are needed to construct
these systems. Moreover, due to the rigid molecular backbone and high tendency to
aggregate in solution, poor solubility is expected and it is essential to introduce
substituents to suppress the aggregation and increase solubility. In search of mild
conditions to synthesize higher rylenes, a variety of intramolecular cyclodehydro-
genation methods have been developed. These include oxidative cyclodehydro-
genation using FeCl
3
, or a combination of CuCl
2
-AlCl
3
as Lewis acid as well as
oxidant, and reductive cyclodehydrogenation via anion radical mechanism pro-
moted by base. In 1990, four
tert
-butyl groups were designed in M¨ llen's group to
resolve the solubility problem of rylenes, and the enhanced solubility allowed the
investigation of physical properties in solution for the first time for terrylene 72a,
quaterrylene 72b, and pentarylene 72c (Scheme
6
)[
66
,
67
]. The synthesis of
terrylene derivative 72a was achieved by treatment of the precursor 71a with
potassium followed by oxidation with anhydrous cadmium chloride, while the
preparation of 72b and 72c took a stepwise manner by an initial reductive
cyclodehydrogenation to generate partially cyclized intermediate followed by
p
