Angewandte
Chemie
from the red-shifted broad Soret band. The addition of
methanol leads to complete disassembly, and the intense CD
signals vanish, which indicates that the observed chirality is
not because of the chirality of the monomeric porphyrins.
However, the different configurations (R or S) of the
hydroxethyl groups induce the different chirality in the self-
assembled species. From the intense CD signals this chirality
appears to be helical in origin, and opposite helicities (M or P)
are generated from the two enantiomers. Giant CD, some-
times referred as Polymer and Salt Induced (PSI) CD may
appear if the chiral objects (absorbing light at around 500 nm)
are greater than 50 nm.[23,24] This also affects the overall shape
of the CD curves, which can vary widely because of the
different sizes of self-assembled species and the scattering
contribution to the CD. However, the application of the
exciton chirality method[25] leads the firstly eluted enantiomer
to a self-assembled species in which the chirality of the
transition dipole moments must be counterclockwise (defined
as negative chirality). Whereas, the second eluted enantiomer
leads to an aggregate with positive chirality (i.e. a clockwise
arrangement of the transition dipole moments).
The very broad absorption of a film cast from self-
assembling 5-Zn is spectacular. If light harvesting is to be
harnessed by artificial devices, a broad absorption with high
extinction coefficients over the entire spectral range is
beneficial. This is certainly the case with the aggregates of
5-Zn and 2-Zn. The fluorescence is not quenched and intense
signals are observed as well in solution as from cast films (see
Supporting Information), which is untypical for aggregated
chromophores, and in our case arises from the well defined
supramolecular architecture. This should allow photosensiti-
zation of a wide band semiconductor, such as nanocrystalline
titania. Until now, photosensitization has been realized only
with ruthenium polypyridine complexes in the most efficient
artificial solar energy conversion devices, which were pio-
neered by Grätzel and coworkers[26,27] or very recently with
phthalocyanine dyes.[28]
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[20] A kind referee indicated the similar behavior between cyanine
dyes, which form J aggregates in water and are disassembled by
methanol, and our porphyrin assemblies which, however, are
formed only in nonpolar solvents. Furthermore, similar effects
such as red-shifted and broadened absorption bands, as well as
the precipitation of fluffs, which are disrupted upon shaking,
were observed with a water insoluble porphyrin, which was
slightly soluble in methanol. These latter aggregates could be
seen with TEM and had dimensions between 50 and 110 nm. It
appears that this same mesoscopic behavior can thus be
engineered with different supramolecular interactions. In our
case, because the same functional groups are present as those in
the natural self-assembling BChls, we are surely mimicking these
Confocal fluorescence microsocopy images of self-assem-
bled 5-Zn onto TiO2 nanoparticles with average dimensions
of 5 nm show bright spots proving that intense fluorescence is
coming out from the porphyrin aggregates that have sizes of
several hundred nm. This high fluorescence could be used in a
solid device similar to the dry Grätzel solar cell[29] but with
appreciably lower production costs and environmentally
more friendly materials than the currently used ruthenium
complexes. In a fully operational device in which a hole
transporter is also present, part of this fluorescence should be
quenched because of the competing electron injection into
the conduction band of the TiO2 semiconductor. Efforts
employing our self-assembling porphyrins as photosensitizers
in such photovoltaic devices are currently under way.
Received: November 4, 2002
Revised: February 12, 2003 [Z50465]
Keywords: chirality · light harvesting · photosynthesis ·
.
porphyrinoids · self-assembly
Angew. Chem. Int. Ed. 2003, 42, 2140 – 2144
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2143