Chemistry Reference
In-Depth Information
tetraisopropoxide. After sol gel synthesis of TiO
2
in the outer shell and removal of the
organic dendritic template by calcination at 500
C hollow nanospheres of TiO
2
could
be obtained. The controllable TiO
2
morphology provides tremendous applications in
many different fields such as photocatalysis, gas sensors, lithium-ion batteries, and
dye-sensitized solar cells [81-83].
At the end of this chapter on surface functionalization, we should mention that
combinations of functions at different locations have also been considered, for
example, from the surface, through the scaffold to the core as shown by some
multichromophoric dendrimers (Scheme 5.23) [53,54,84]. For example, the dendri-
mer
74
consists of a single terrylenetetracarboxydiimide (
TDI
) core and multiple
PMI
groups in the periphery, while on the surface naphthalenedicarboxymonoimids
(
)s are located [83]. This is a nice model for light-harvesting systems, in which
energy transfer processes from
NMI
as an acceptor were studied.
It was further shown that the number of perylenes can easily be increased by using a
CP-building block with two PMI units yielding
NMI
as donors to
TDI
75
or the perylendiimide
44
mentioned earlier for further extension.
5.7 DESYMMETRIZATION APPROACHES
The cyclopentadienone chemistry also represented a powerful method for the
synthesis of dendrimers bearing two different types of functional groups at the
surface of a single dendrimer. In the beginning, the only known synthetic route to
desymmetrized PPDs proceeded via several protection and deprotection steps of
the ethynyl groups of tetrakis(4-ethynylphenyl)methane and subsequent stepwise
Diels-Alder cycloaddition as schematically depicted in Scheme 5.24 [56]. This
synthesis started from a partially protected tetrahedral core (
76
, Scheme 5.24) and a
cyclopentadienone building block containing a functional group of type “A”
.
Deprotection of the acetylene units of the core and Diels-Alder reaction with a
cyclopentadienone coated with a functionality of type “B”
77
79
led to a first-generation
dendrimer
with two kinds of functionalities (type A and type B) in a well-defined
spatial arrangement at the rim. This pathway enabled the synthesis of a second-
generation bifunctional dendrimer
80
using the convergent synthetic approach. It had
the disadvantages of low yields and time-consuming purifications.
Thus, a new synthetic approach for the desymmetrization of PPDs was
developed and described. Tetrakis(4-ethynylphenyl)methane
81
undergoes facile
Diels-Alder cycloaddition with substoichiometric quantities of tetraphenylcyclo-
pentadienones bearing one polar functional group
14
(Scheme 5.25). This method
allowed the preparation of dendrimers possessing two types of functionalities in
only two steps. A single ethynyl group was thereby converted to a rigid, selectively
functionalized polyphenylene moiety, which serves as a focal point for further
transformations [28,85-87].
By using the above-described core molecule
83
82a
, it was possible to build up the
first-generation dendrimer
, bearing six PDI chromophores [51] and one aliphatic
ester group for anchoring (Scheme 5.26).
84
