Synthesis of light harvesting polymers by RAFT methods

Article abstract of DOI:10.1039/b206166j

Polymers prepared by RAFT polymerisation containing acenaphthyl energy donors and a terminal anthryl energy acceptor have a narrow molecular weight distribution and exhibit excitation energy transfer efficiencies up to 70%.

Full text of DOI:10.1039/b206166j

Synthesis of light harvesting polymers by RAFT methods
Ming Chen,a Kenneth P. Ghiggino,*a Albert W. H. Mau,b Ezio Rizzardo,b San H. Thang*b and Gerard
J. Wilsonb
a School of Chemistry, The University of Melbourne, Victoria 3010, Australia.
E-mail: ghiggino@unimelb.edu.au; Fax: +61(3)93475180; Tel: +61(3)83447137
b CSIRO Molecular Science, Bag 10, Clayton South, Victoria 3169, Australia. E-mail: San.Thang@csiro.au;
Fax: +61(3)95452446; Tel: +61(3)95452490
Received (in Cambridge, UK) 27th June 2002, Accepted 22nd August 2002
First published as an Advance Article on the web 11th September 2002
Polymers prepared by RAFT polymerisation containing
acenaphthyl energy donors and a terminal anthryl energy
acceptor have a narrow molecular weight distribution and
exhibit excitation energy transfer efficiencies up to 70%.
The development of molecules capable of mimicking the light
harvesting process of natural photosynthesis has been a long-
standing goal of synthetic chemists.1 The use of polymeric
structures for the assembly of arrays of chromophores within a
single macromolecule containing relatively few energy-ac-
ceptor units was pioneered by Fox et al.2 and Guillet et al.3 and
was further explored by other researchers.4 These polymers are
known as ‘light-harvesting’ or ‘antenna’ polymers. Many of the
light harvesting polymers reported to date are synthesized by
conventional free radical polymerisation. Unfortunately, due to
the inevitable, near diffusion-controlled bimolecular radical
coupling and disproportionation reactions, the polymers made
by conventional radical polymerisation usually have a broad
molecular weight distribution and lack molecular weight and
architecture control.
Scheme 1
Living radical polymerisation has recently emerged as one of
the most effective synthetic routes to well-defined polymers.
Several approaches have been reported: nitroxide mediated
polymerisation5 (NMP), atom transfer radical polymerisation6
(ATRP) and reversible addition fragmentation chain transfer
polymerisation7 (RAFT). However, the use of NMP and ATRP
in the synthesis of block copolymers and other structures of
complex architecture has been limited by the fact that the
processes are not compatible with certain monomers or reaction
conditions, or both. For example, although NMP can be
successfully used for making block copolymers based on
styrene and its derivatives, it appears to have less utility for
other systems. ATRP has limitations with monomers or
initiators containing acid functionality and generates products
contaminated with metal ions. However, in principle, all classic
radical systems can be converted to the RAFT process in the
presence of efficient transfer reagents.
Energy acceptors can be incorporated into the RAFT agents
by reaction of appropriate species with functional dithio-
benzoates such as 4-cyano-4-((thiobenzoyl)sulfanyl)pentanoic
acid (RAFT-Acid).8 These RAFT agents can be used to control
the free radical polymerisation of the monomer containing
energy donor chromophores. Acenaphthyl and 9-anthryl chro-
mophores were selected as the energy donor and acceptor
respectively as they fulfil the spectral overlap requirements for
radiationless energy transfer and acenaphthyl can be preferen-
tially excited with minimal direct excitation of the anthracene
from 290 to 320 nm.4
Scheme 2
overlaps with the fluorescence emission of acenaphthyl chro-
mophores, dithiobenzoyl groups can also acts as traps of the
energy in photoexcited poly(acenaphthylene). To avoid this
effect of dithiobenzoyl groups on the energy transfer efficiency
from acenaphthyl to anthryl groups, two strategies were used
(Scheme 3): (i) treating the polymers with amine to convert the
dithiobenzoyl group to a thiol (SH-P(AcN)-AN), and (ii)
inserting a photoinactive ‘spacer’ between the dithiobenzoyl
The anthryl acceptor was introduced into the RAFT agent to
produce RAFT-AN by the coupling of RAFT-Acid and
9-anthracenemethanol in the presence of dicyclohexylcarbodii-
mide (DCC) as coupling agent and 4-dimethylaminopyridine
(DMAP) as catalyst in dichloromethane (Scheme 1).
Polymers were synthesized according to RAFT methods as
described previously7 using the RAFT-AN (see Scheme 2) and
were precipitated twice into MeOH to remove the unreacted
monomers. As the absorption spectrum of dithiobenzoyl groups
Scheme 3
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group and acenaphthyl sequences by block copolymerisation
with methyl acrylate (P(MA)-b-P(AcN)-AN).
The polydispersity of the P(AcN)-AN sample before precip-
itation was determined to be 1.08 by gel permeation chromatog-
raphy (GPC). Matrix assisted laser desorption/ionization time-
of-flight mass spectrometry (MALDI-TOF/MS) was used to
determine the actual molecular weights of the polymers (see
Fig. 1). The polymer mass peaks observed are consistent with
the predicted structure. This also serves to confirm the
mechanism of RAFT polymerisation.
Fig. 2 Fluorescence spectra of acenaphthylene polymers in dilute, degassed
dichloromethane solution excited at 295 nm. All solutions have the same
absorbance at 295 nm.
A comparison of the fluorescence spectra of the different
polymers is presented in Fig. 2. Photoexcitation of P(AcN)-AN,
SH-P(AcN)-AN and P(MA)-b-P(AcN)-AN at 295 nm, where
absorption is almost exclusively due to acenaphthyl groups,
results in fluorescence being predominantly emitted from the
anthryl end (Fig. 2). Furthermore, the fluorescence excitation
spectrum of the 9-anthryl emission contained a contribution
attributable to acenaphthyl absorption. These observations
confirm that energy transfer to the anthryl end group is
occurring. At the solution concentrations used (polymer
concentrations < 1025 M) only intrachain energy transfer
processes are possible during the excited state lifetime.
The efficiency of excitation energy transfer in the polymers
can be determined by comparison of the fluorescence excitation
spectrum and the absorption spectrum as described pre-
viously.9,10 The efficiency of acenaphthyl to anthryl energy
transfer in poly(AcN)-AN is 15%. This low value can be
attributed to the presence of a competing non-fluorescent
energy trap (i.e. the dithiobenzoyl group). After removing the
dithiobenzoyl group, the energy transfer efficiency increased to
70% in SH-P(AcN)-AN confirming this hypothesis. Introduc-
ing ‘spacer’ monomer units between the dithiobenzoyl group
and the acenaphthyl donors as in P(MA)-b-P(AcN)-AN also
increased the energy transfer efficiency to 56%. The smaller
effect on energy transfer efficiency by synthesising the block
copolymer is most likely due to the ability of the dithiobenzoyl
group to fold back and quench the excited acenaphthyl groups
due to the flexibility of the poly(methyl acrylate) chain.
In summary, we have shown that well defined light
harvesting polymers can be synthesised using RAFT methods.
The mechanisms of excitation energy transfer operating in such
polymers are currently under investigation.
Fig. 1 Top: MALDI-TOF mass spectrum of poly(AcN)-AN. Matrix:
dithranol, cationizing agent: silver trifluoroacetate, solvent: chloroform.
Middle: Expanded region of top spectrum. The peaks are labelled with their
measured mass and the corresponding number of repeat units. The interpeak
distances reflect the mass of the constituent repeating units. Bottom:
Demonstration of end group analysis by calculation of a representative peak
of the polymer distribution.
Notes and references
The molecular weight of the polymer can also be determined
by comparing the UV absorption spectrum of a polymer
solution to solutions of known concentration of RAFT-AN and
acenaphthene. The molecular weights of the polymer obtained
by the different techniques are shown in Table 1. The Mn values
determined by UV and MALDI-TOF mass spectrometry are
quite close to the calculated Mn, which indicates that most
poly(acenaphthylene) chains have one dithiobenzoyl and one
anthryl group as chain ends.
1 A. Adronov and J. M. J. Fréchet, Chem. Commun., 2000, 1701.
2 R. F. Cozzens and R. B. Fox, J. Chem. Phys., 1969, 50, 1532; R. B. Fox
and R. F. Cozzens, Macromolecules, 1969, 2, 181.
3 D. Ng and J. E. Guillet, Macromolecules, 1982, 15, 724.
4 S. E. Webber, Chem. Rev., 1990, 90, 1469; M. A. Fox, Acc. Chem. Res.,
1992, 25, 569; K. P. Ghiggino and T. A. Smith, Prog. React. Kinet.,
1993, 18, 375.
5 D. H. Solomon, E. Rizzardo and P. Cacioli, US Patent 4581429; Chem.
Abstr., 1985, 102, 221335.
6 J. S. Wang and K. Matyjaszewski, Macromolecules, 1995, 28, 7901; M.
Sawamoto and M. Kamigaito, Trends Polym. Sci., 1996, 4, 371.
7 J. Chiefari, Y. K. Chong, F. Ercole, J. Krstina, J. Jeffery, T. P. T. Le, R.
T. A. Mayadunne, G. F. Meijs, C. L. Moad, G. Moad, E. Rizzardo and
S. H. Thang, Macromolecules, 1998, 31, 5559; Y. K. Chong, P. T. Le,
G. Moad, E. Rizzardo and S. H. Thang, Macromolecules, 1999, 32,
2071.
Table 1 Comparison of molecular weights obtained by various methods
Mn
by UV
Mn by
MALDI-TOF/MS
Mn by
calculationa
Poly(AcN)-AN
1975
1791
1875
8 S. H. Thang, Y. K. Chong, R. T. A. Mayadunne, G. Moad and E.
Rizzardo, Tetrahedron Lett., 1999, 40, 2435.
9 F. Bai, C. H. Chang and S. E. Webber, Macromolecules, 1986, 19,
2484.
10 K. P. Ghiggino, D. J. Haines, T. A. Smith and G. J. Wilson, Can. J.
Chem., 1995, 73, 2015.
a Theoretical molecular weights were calculated using the expression
Mn(calc) = ([monomer]/[RAFT agent]) 3 conversion 3 M.W. of monomer
+ M.W. of RAFT agent. The above expression does not include the small
number of chains formed from the initiator. It also assumes complete
consumption of RAFT agent.
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