Controlled synthesis of luminescent polymers using a bis-dithiobenzoate RAFT agent

Article abstract of DOI:10.1039/b716471h

A higher efficiency of excitation energy transfer occurs to a luminescent diphenylanthracenyl acceptor incorporated at the centre, rather than the end, of an acenaphthylene polymer chain. The Royal Society of Chemistry.

Full text of DOI:10.1039/b716471h

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Controlled synthesis of luminescent polymers using a bis-dithiobenzoate
RAFT agent
Ming Chen,a Kenneth P. Ghiggino,*a Ezio Rizzardo,b San H. Thangb and
Gerard J. Wilsonb
Received (in Cambridge, UK) 25th October 2007, Accepted 7th December 2007
First published as an Advance Article on the web 3rd January 2008
DOI: 10.1039/b716471h
A higher efficiency of excitation energy transfer occurs to a
luminescent diphenylanthracenyl acceptor incorporated at the
centre, rather than the end, of an acenaphthylene polymer chain.
linkage and can be used to control the polymerisation of
suitable vinyl monomers containing energy donor chromo-
phores.
The method described in Scheme 2 can be extended to
synthesize a bis-RAFT agent by using a divinyl monomer as
the reactant and, for the application described here, we have
used 9,10-(p,p0-divinyl)-diphenylanthracene.w The synthesis is
summarized in Scheme 3. The resulting bis-RAFT agent has
been used to control the polymerisation of the monomer
Luminescent polymers are finding increasing applications in
display devices, as sensors and probes of molecular environ-
ments and as light harvesting systems that mimic the light
collection processes of natural photosynthetic pigment ar-
rays.1 The synthesis of luminescent polymers with well defined
architectures is a requirement for the development of opti-
mized structures for these, and new, applications. In particular
the ability to place luminescent groups at particular locations
along a polymer chain can potentially provide control of the
process of excitation energy transfer.2 We have previously
described the application of controlled living radical polymer-
ization using reversible-addition fragmentation chain transfer
(RAFT) to produce linear and star-shaped light harvesting
homopolymers and block copolymers of defined structure and
low polydispersity.3 We report here a further refinement of the
RAFT method to place a highly luminescent group (dipheny-
lanthracenyl, DPAn) at the centre position of a linear aromatic
polymer chain (poly(acenaphthylene), PAcN) and describe the
effects on energy transfer processes in the resulting polymer.
The RAFT method has proved one of the most versatile
living free radical polymerisation techniques for synthesizing
polymers and, in principle, can be applied to all classical
radical polymerisation systems.4 The RAFT process uses
particular added chain transfer agents, often of the thiocarbo-
nylthio type, to control reversible addition–fragmentation
steps during the free radical polymerisation of a monomer.
The overall process is given in Scheme 1.
acenaphthylene to produce poly(acenaphthylene) with
a
9,10-diphenylanthracenyl chromophore centrally placed in
the polymer chain. For comparative purposes, poly(ace-
naphthylene) with DPAn as an end group was also prepared
by similar means but using the monofunctional RAFT agent
prepared from 9-(p-vinylphenyl)-10-phenylanthracene (see
Scheme 4).z The terminal dithiobenzoyl end groups, that have
been shown by us previously to quench fluorescence in poly-
mers,3 were desulfurated by tributyltin hydride to obtain the
required polymers P(AcN)–DPAn–P(AcN) and P(AcN)–
DPAn.
The molecular weights of the final polymers determined by
calculation, UV-vis spectroscopy and gel permeation chroma-
tography (GPC) are provided in Table 1. The narrow mole-
cular weight distribution (Mw/Mn o1.3) combined with the
close agreement between Mn values determined by the spectro-
photometric method and those calculated assuming a con-
trolled RAFT mechanism, confirms that the polymer chains
have the structure predicted. The Mn values determined by
GPC are substantially lower reflecting the different elution
behaviour of PAcN compared to the polystyrene GPC
calibration standards.
Acenaphthyl and 9,10-diphenylanthracenyl were selected as
the chromophores for the present work since they form an
energy donor–acceptor pair that fulfils the spectral overlap
requirements for radiationless electronic energy transfer (in
It is apparent that RAFT polymerisation provides a means
of introducing specifically placed luminescent groups into the
polymer since a component of the functionalised RAFT agent
(R) will be incorporated at the end of each polymer chain.3 We
have also described a method to readily synthesize appropri-
ately functionalised RAFT agents via a reversible radical
reaction between a vinyl monomer and 2-cyanoprop-2-yl
dithiobenzoate (RAFT-IBN) (Scheme 2).5
dichloromethane, the Forster6 critical transfer distance for this
¨
donor–acceptor pair is 28 A). Furthermore, acenaphthyl can
be preferentially excited between 290 nm and 320 nm with
minimal direct excitation of DPAn.
A comparison of the fluorescence spectra observed from
dilute (o10ꢀ5 M) degassed dichloromethane (DCM) solutions
The resulting RAFT agent incorporates the required energy
accepting luminescent group (A) via a stable carbon–carbon
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 and Health Technologies, Bag 10, Clayton South,
Victoria 3169, Australia
Scheme 1 Overall process for RAFT polymerization.
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Scheme 2 General procedure for the synthesis of functionalized
RAFT agents.
of the two polymers is presented in Fig. 1. The fluorescence
quantum yield of the DPAn fluorophore in the polymers in
deoxygenated DCM solutions using an excitation wavelength
of 355 nm (where only DPAn asborbs) was determined to be
0.70 and 0.76 for P(AcN)–DPAn–P(AcN) and P(AcN)–
DPAn, respectively by comparison with the fluorescence of
anthracene in ethanol (fluorescence quantum yield of 0.27)7 as
a reference compound.
Photoexcitation of both polymers at 295 nm (see Fig. 1),
where absorption is almost exclusively due to acenaphthyl
groups, results in fluorescence being predominantly emitted
Scheme 4 Synthesis of P(AcN)–DPAn.
from DPAn (emission maximum at 420 nm). Furthermore, the
fluorescence excitation spectrum of the DPAn emission con-
tained a major contribution attributable to acenaphthyl ab-
sorption demonstrating that energy transfer to the DPAn
moiety from acenaphthyl groups in the polymer is occurring.
The efficiencies of energy transfer from acenaphthyl to DPAn
in the polymers were determined by the method described
previously3 and are reported in Table 2.
The luminescence spectrum in Fig.
1 for P(AcN)–
DPAn–P(AcN) exhibits a more strongly quenched emission
from acenaphthyl (at 328 nm) and enhanced emission from
DPAn (at 420 nm) compared to P(AcN)–DPAn. The calcu-
lated excitation energy transfer (EET) efficiencies confirm that
energy transfer in the polymer with the centrally located
DPAn trap is significantly higher (80%) than that for the
end-located trap (61%), even though both polymers have
almost identical donor : acceptor ratios.
Table 1 Comparison of polymer molecular weights determined by
various methods
Mn by GPCa
Mn by UVb Calculated Mn
c
Mn Mw/Mn
P(AcN)–DPAn–P(AcN) 5000 1.26
P(AcN)–DPAn
10 200
9500
9900
9200
3500 1.21
a
GPC was performed in THF using a refractive index detector, and
b
molecular weights are reported as linear polystyrene equivalents. In
the UV method, the absorbance of the polymer solution is compared
to the absorbance of solutions of known concentrations of ace-
naphthylene homopolymer and the incorporated RAFT agents at
c
297 and 347 nm, respectively. Theoretical molecular weights were
calculated using the expression Mn(calc) = ([monomer]/[RAFT
agent]) ꢂ fractional conversion ꢂ MW of monomer + MW of RAFT
agent.
Scheme 3 Synthesis of P(AcN)–DPAn–P(AcN).
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The authors acknowledge financial support of this research
from the Australian Research Council.
References
w A mixture of 2-cyanoprop-2-yl dithiobenzoate (2.0 molar equiv.),
AIBN, 9,10-(p,p0-divinyl)-diphenylanthracene,9 and chlorobenzene
was degassed through three freeze–pump–thaw cycles, sealed under
vacuum, and heated in a 70 1C oil bath for 64 h. After purification by
column chromatography, an orange solid product was obtained (47%
yield). 1H NMR (500 MHz, DCCl3-d3, ppm): d 1.33 (t, 6H, 2 ꢂ
–CH3), 1.53 (s, 6H, 2 ꢂ –CH3), 2.30–2.45 (m, 4H, 2 ꢂ –CH2–), 5.32
(q, 2H, 2 ꢂ –S–CH(Ar)–), 6.88–7.94 ppm (m, 13H, ar.).
z A mixture of 2-cyanoprop-2-yl dithiobenzoate (1.0 molar equiv.),
AIBN, 9-(p-vinylphenyl)-10-phenylanthracene,9 and chlorobenzene
was degassed through three freeze–pump–thaw cycles, sealed under
vacuum, and heated in a 70 1C oil bath for 20 h. After purification by
column chromatography, an orange solid was obtained (75% yield).
1H NMR (400 MHz, DCCl3-d3, ppm): d 1.33 (t, 3H, –CH3), 1.53 (s,
3H, –CH3), 2.30–2.45 (m, 2H, –CH2–), 5.32 (q, 1H, –S–CH(Ar)–),
6.88–7.94 ppm (m, 21H, ar.).
Fig. 1 The fluorescence spectra of P(AcN)–DPAn–P(AcN) (—) and
P(AcN)–DPAn (- - -) in degassed DCM solutions excited at 295 nm.
The polymer solutions have the same absorbance at the excitation
wavelength.
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Table 2 Donor/acceptor ratios and excitation energy transfer (EET)
efficiencies for the polymers
Donor/acceptor
EET efficiency (%)
P(AcN)–DPAn–P(AcN)
P(AcN)–DPAN
60
57
80
61
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rather than an end-located, DPAn trap. The energy migration
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to significantly higher energy transfer efficiencies.
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