Light harvesting and energy transfer in a ruthenium-coumarin-2 copolymer

Article abstract of DOI:10.1039/b103792g

Copolymers containing Ru(bpy)₃ and coumarin-2 chromophores prepared using both grafting and copolymerization approaches exhibit Foerster-type energy-transfer efficiencies ranging from 70% to above 98%.

Full text of DOI:10.1039/b103792g

Light harvesting and energy transfer in a ruthenium–coumarin-2
copolymer†
Xavier Schultze, Jason Serin, Alex Adronov and Jean M. J. Fréchet*
University of California, Berkeley, CA 94720-1460, USA. E-mail: frechet@cchem.berkeley.edu;
Fax: +1(510) 643-3079; Tel: +1(510) 643-3077
Received (in Cambridge, UK) 26th April 2001, Accepted 15th May 2001
First published as an Advance Article on the web 7th June 2001
Copolymers containing Ru(bpy)3 and coumarin-2 chromo-
phores prepared using both grafting and copolymerization
approaches exhibit Förster-type energy-transfer efficiencies
ranging from 70% to above 98%.
dichloride 2 were refluxed in MeOH in the presence of an
excess of AgPF6 for 12 h and the solution was then added to a
suspension of copolymer 3 in dimethoxyethane. After 48 h at
reflux, the reaction mixture was filtered, concentrated, re-
dissolved in dichloromethane and extensively washed with
water to remove any unreacted ruthenium complex. A dark red
polymer was obtained after precipitation in Et2O.
The UV spectra of the polymer before and after the grafting
reaction with ruthenium complex 2 are given in Fig. 1. The
starting polymer displays two strong absorption bands at 290
and 350 nm, which are characteristic of the coumarin-2 and the
bipyridine units. After reaction with the ruthenium complex a
new absorption band was observed at 465 nm, which indicates
successful grafting of the ruthenium bis(bipyridine) onto the
Molecular assemblies capable of harvesting light and trans-
forming the absorbed energy have attracted great interest in
recent years because of their applicability in such domains as
light emitting diodes, fluorescent labeling of biological mole-
cules, and photonic devices.1 We have recently described
several dendritic systems containing light-harvesting dyes at the
periphery that efficiently absorb and channel light to a different
dye at the focal point by a Förster-type mechanism.2 Although
these systems exhibit extremely high energy-transfer effi-
ciencies, the demanding multi-step synthesis required for their
preparation may limit their use to speciality applications. To
circumvent this limitation, we have explored analogous light-
harvesting linear polymers in which the donor–acceptor ratios
were optimized by mimicking the dendrimer models and found
that their antenna properties,3 although less impressive than
those of the corresponding dendrimers, are nevertheless very
good.
Ruthenium-containing macromolecules are attracting wide-
spread interest due to their photochemical and electrochemical
properties.4,5 We now report our results concerning the
synthesis and characterization of a linear Ru(bpy)3–coumarin-2
copolymer (1) that exhibits the energy transfer efficiency
generally observed with light harvesting dendrimers,5 without
requiring the multi-step synthesis involved in the design of
these molecules. Two strategies were used (Scheme 1): (i)
grafting the ruthenium complex 2 on a bipyridine–coumarin-2
functionalized copolymer (3), and (ii) copolymerization of a
ruthenium functionalized monomer (4) with a coumarin-2
functionalized monomer (5). The first route enabled us to obtain
a copolymer that exhibits an energy transfer efficiency of 70%
between the coumarin-2 and the Ru(bpy)3 units. More inter-
estingly, the copolymerization route afforded a better perform-
ing copolymer that displays quantitative energy transfer
efficiency.
The styrene functionalized coumarin-2 monomer (5) was
synthesized by coupling coumarin-2 with vinyl benzyl chloride
as described elsewhere.3 The 4-vinyl-4A-methylbipyridine mon-
omer (6) was obtained using a modified literature procedure.6
Copolymerization of these two monomers using a 3 to 1 feed
ratio (mol fraction of 5 = 0.75) was carried out at 90 °C with
AIBN as the initiator in a minimum amount of dichlorobenzene.
The polymer was precipitated twice into Et2O to remove the
unreacted monomers. Molecular weights ranging from 5000 to
15 000 Daltons were obtained along with a polydispersity of
1.6, typical of AIBN initiated polymerizations. Elemental
analysis of copolymer 3 showed that the resulting polymer was
composed of ca. 25% of the bipyridine monomer and 75% of
the coumarin-2 monomer, as expected from the feed ratio.
To perform grafting of the ruthenium complex, we used the
method reported by Fraser and co-workers.7 Two equiv. of
Scheme 1
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b1/b103792g/
Fig. 1 Normalized UV-Vis spectra of copolymer 3 before and after
grafting.
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Chem. Commun., 2001, 1160–1161
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103792g

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Fig. 2 Fluorescence spectrum of copolymer 3, in the region of coumarin-2
emission, before and after grafting. Inset shows the spectrum of the same
copolymer in the region where the Ru(bpy)3 acceptor complex emits.
Fig. 3 UV spectrum of polymer 1, obtained by the copolymerization
route.
polymer. The absorption band at 290 nm is further increased due
to the two additional bipyridine ligands introduced by the
reaction. Fluorescence resonance energy transfer studies were
carried out to determine the efficiency of energy transfer from
the coumarin-2 chromophore to the ruthenium complex. The
efficiency was calculated by comparing the coumarin-2 emis-
sion in the starting polymer 3 (a donor model compound) to the
emission of this dye in the polymer obtained after grafting of
ruthenium (Fig. 2). Based on the quenching of this emission, an
energy transfer efficiency of 70% was determined for the
grafted copolymer. Additionally, a 5-fold increase of the
ruthenium emission was observed upon excitation of the
coumarin-2 units (lex = 350 nm) over excitation of the
ruthenium complex directly (lex = 465 nm), this type of
amplification is the direct result of energy transfer (Fig. 2,
inset).
Fig. 4 Fluorescence spectrum of polymer 1, obtained by the copoly-
merization route.
Although the grafting route enabled us to obtain a polymer
displaying good energy transfer characteristics, the functional-
ization of the polymer, estimated from the relative UV-Vis
absorbance of the coumarin-2 and Ru(bpy)3 units, was not
quantitative (ca. 30%) and some insoluble material was also
formed, decreasing the polymer yield. Therefore the copoly-
merization of monomers 4 and 5 was studied. The ruthenium-
containing monomer 4 was prepared using a modified literature
procedure.8 Polymerization of the monomers was then per-
formed at 90 °C with AIBN in DMF (mol fraction of 5 = 0.75).
The resulting polymer was purified by precipitation in Et2O and
extensive washing with MeOH. The UV-visible spectrum of the
polymer showed that incorporation of the ruthenium monomer
4 was as expected from the feed ratio, ca. 25%, with the
remainder of the polymer repeat units consisting of the
coumarin-containing monomer 5, therefore the copolymeriza-
tion route is much more effective for the introduction of the
Ru(bpy)3 complex than the grafting reaction.
The absorption spectrum of the copolymer (Fig. 3) is similar
to that of the material obtained by the grafting route except that
it displays stronger bipyridine and ruthenium metal to ligand
charge transfer bands at 290 and 465 nm. Excitation of this
polymer at 350 nm results in an emission spectrum (Fig. 4) that
is totally quenched at 440 nm (the coumarin emission),
indicating that quantitative energy transfer between the cou-
marin-2 and the Ru(bpy)3 units occurs in this copolymer.
Furthermore, the intensity of the ruthenium emission at 630 nm
is increased by a factor of 2.7 when the polymer is illuminated
at 350 nm vs. 465 nm. This increase is lower than that observed
for the grafted copolymer, again indicating that, as a result of
incomplete grafting, a higher concentration of donors relative to
acceptors was present in that case.
In conclusion, readily accessible linear copolymers contain-
ing coumarin-2 and Ru(bpy)3 units can provide quantitative
energy transfer from the coumarin-2 units to the ruthenium
centers. This finding opens interesting perspectives in the field
of solar energy conversion.9 The application of such a system to
photovoltaic devices is currently under investigation.
Financial support of this research by the AFOSR (F-
49620-01) and the U. S. Department of Energy (# DE-AC03-
76SF00098). Fellowship support from the Eastman Kodak
Company is also gratefully acknowledged (A. A).
Notes and references
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