Chemistry Reference
In-Depth Information
emission can be tuned by changing the luminescent guest hosted in the same
dendrimer.
An example of this behavior is exhibited by dendrimer 10 (Fig.
12
, top).
It consists of a hexaamine core with appended four branches, which carry a great
number of units capable of absorbing and emitting light. More specifically, this
dendrimer contains eight dansyl-, 24 dimethoxybenzene-, and 32 naphthalene-type
units [
63
]. Upon light absorption, energy transfer from the peripheral dimethox-
ybenzene and naphthalene units to the light-emitting dansyl unit occurs with high
efficiency (
90%). When the dendrimer hosts a molecule of the eosin dye (Fig.
12
,
bottom), the dansyl light-emission is no longer observed and the characteristic
emission of the eosin guest takes place instead. The encapsulated eosin molecule
collects electronic energy from all the 64 light-absorbing units of the dendrimer
(antenna effect), so that UV input signals are converted into visible output signals.
By using different dyes, a fine tuning of the visible output signal can be achieved.
>
7.3.2 Molecular Batteries
A dendrimer consisting of multiple identical and non-interacting redox units, able
to reversibly exchange electrons with another molecular substrate or an electrode,
can perform as a molecular battery [
64
,
65
]. The redox-active units should exhibit
chemically reversible and fast electron transfer processes at easily accessible
potential difference and chemical robustness under the working conditions.
Because of their reversible electrochemical properties, ferrocene and its methyl
derivatives are the most common electroactive units used to functionalize
dendrimers. A recently reported example of this class of dendrimers is constituted
by giant redox dendrimers (see e.g., the 81-Fc second generation compound 11
shown in Fig.
13
) with ferrocene and pentamethylferrocene termini up to a theoret-
ical number of 3
9
tethers (seventh generation), evidencing that lengthening of the
tethers is a reliable strategy to overcome the bulk constraint at the dendrimer
periphery [
66
].
These redox metallodendrimers were investigated with a variety of techniques:
(1) Cyclic voltammetry has revealed a full chemical and electrochemical revers-
ibility up to the seventh generation with a single redox wave corresponding to the
oxidation of all the ferrocene units at the same potential. (2) Coulometry has
evidenced that the number of exchanged electrons is equal to the number of
peripheral ferrocene units (the difference of 17% between the theoretical and
experimental numbers found for the largest dendrimer was attributed to structural
defects). (3) Chemical oxidation was used to isolate and characterize the blue
17-electron ferrocenium and deep-green mixed-valence Fe(III)/Fe(II) dendritic
complexes. (4) Atomic force microscopy, employed to study the behavior of the
dendrimers on a mica surface, enabled a comparison of the size of the oxidized
cationic form of the dendrimers with that of their neutral form. For the fifth
generation dendrimer it was found that the average height of the oxidized species
(6.5
0.6 nm) is much larger than that of its neutral form (4.5
0.4 nm).
