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the nanocrystals are randomly oriented. The I-V plot is nonlinear
and symmetric with a zero-current region between -2 V and +2 V.
At higher bias, the current increase at sub-
A levels is typical of a
metal-semiconductor-metal device. An interesting property of this
120 nm device is its high photoresponsivity, as the current increases
from sub-
µ
A level in the dark to sub-mA levels under illumination of
a few mW of power [28c].
As discussed in the previous sections, guanosine derivatives
allow both the control and diversification of self-assembled
nanoarchitectures. This opens the possibility of a systematic
fabrication of different architectures and material morphologies
starting from one single molecule, with a minimum of synthetic
efforts. The idea of using self-assembled guanines as scaffold to
organize functional residues was originally proposed by us some
years ago [29].
The inspiration came from the structure of the natural
photosynthetic light harvesting system LH2 [30], a supramolecular
complex where two distinct sets of 18 and 9 bacteriochlorophylls-a,
respectively, are arranged in a circular disposition. Preparing a
molecule that can mimic such a system by covalent synthesis is a
true challenge. Alternatively, one can copy from nature and form
a mimicking system by supramolecular synthesis. Provided that,
upon functionalization with porphyrin moieties, the self-assembly
behavior of guanosine is preserved and it is not overruled by
porphyrins stacking, but eventually it is enhanced by the latter, the
functionalization of guanosine with one to three porphyrin residues
and subsequent cation-templated self-assembly could lead to an
octameric complex where up to 24 porphyrins were organized in a
circular array. So far, mono- [29] and bi-functionalized [31] derivatives
have been prepared and showed the expected supramolecular
behavior: the addition of K
µ
+
ions produces the octameric complex
and the process can be followed by both NMR and CD spectroscopy.
As shown in Fig. 4.15, which refers to the mono-functionalised
porphyrin derivative
(Chart 4.3), CD spectra show very clearly two
exciton couplings that take place in the complex: in addition to the
couplet at around 290 nm due to
5
-symmetric octamers of guanine
chromophores (see above), also a couplet corresponding to the
Soret band of the porphyrin chromophore is present, indicating the
chiral arrangement between chromophores belonging to different
molecules.
D
4
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