Chemistry Reference
In-Depth Information
Fig. 21 Structures and absorption spectra of the tetraphenoxy-substituted rylenediimides 73a-e in
CHCl
3
[
68
]. Reprinted with permission from [
68
]. Copyright 2010 John Wiley and Sons
oxidative cyclodehydrogenation to give the final product. A significant
bathochromic shift in absorption spectrum was observed from 72a to 72c due to
the elongation of the conjugation.
Although the
tert
-butyl group plays an important role in solubilizing rylenes, the
scope is limited to quaterrylene. The solubility of the substituted pentarylene 72c is
strongly decreased which prevents the measurement of its
1
H NMR spectrum.
Alternatively, a breakthrough of making stable and soluble high order rylene
derivatives was achieved by introducing dicarboxylic imide functional groups.
The electron-withdrawing nature of imide stabilizes the electron-rich rylene core
by reducing the electron density, and the bulky groups such as the diisopro-
pylphenyl group can be introduced on the imide end to prevent dye aggregation
(Fig.
21
). Further substitution can be realized in the bay region of the rylene core
with the development of new synthetic strategies, which further increases the
solubility. The improved stability and solubility make the preparation of higher
rylene bisimides, namely pentarylene bisimide, hexarylene bisimide, heptarylene
bisimide, and octarylene bisimide, practically possible. Moreover, additional
bathochromic shift in absorption was observed for rylene bisimides compared to
parent rylenes due to intramolecular donor-acceptor interaction. The extension of
conjugation along the long molecular axis of the rylene bisimides generally induces
a bathochromic shift of about 100 nm per additional naphthalene unit, along with a
nearly linear increase in extinction coefficient (Fig.
21
)[
68
].
Two approaches were available for the preparation of quaterrylene bisimides,
taking advantage of either intramolecular cyclization or intermolecular dimeriza-
tion reactions. These two methods were developed independently in M¨ llen's and
Langhals' groups. As shown in Scheme
7
,M¨ llen's intramolecular approach started
from bromination of perylene monoimide 74, which was subjected to Yamamoto
coupling reaction to produce precursor 76, and the quaterrylene bisimide 77 was
