Amplified quenching in metal-organic conjugated polymers

Article abstract of DOI:10.1039/b211575a

The luminescence from conjugated polyelectrolytes that contain pendant metal complex units is quenched very efficiently by oppositely charged electron acceptors.

Full text of DOI:10.1039/b211575a

Amplified quenching in metal–organic conjugated polymers†
Yao Liu, Shujun Jiang and Kirk S. Schanze*
Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, FL, USA.
E-mail: kschanze@chem.ufl.edu
Received (in Purdue, IN, USA) 22nd November 2002, Accepted 6th January 2003
First published as an Advance Article on the web 11th February 2003
The luminescence from conjugated polyelectrolytes that
contain pendant metal complex units is quenched very
efficiently by oppositely charged electron acceptors.
The polymers and model compounds were prepared by
7
Sonogashira coupling reactions and the new materials were
fully characterized by NMR and mass spectrometry (for the
models).‡ Photophysical data for the metal–organic materials
are provided in Table 1. Polymers P-Ru and P-Os display
intense absorption bands at 425 nm due to the long-axis
polarized p,p* transition of the PPE backbone. Weaker
absorptions in the 450–500 nm region are due to M?2,2A-
bipyridine (bpy) metal-to-ligand charge transfer (MLCT). P-Os
also features a weak ‘tailing’ band (530–700 nm) due to the
spin-forbidden S?T MLCT transition. The absorption of the
model compounds is very similar, Fig. 1, except that the p,p*
bands arising from the oligo(phenylene ethynylene) (OPEs) are
slightly blue shifted relative to the polymers. The MLCT bands
The fluorescence of conjugated polyelectrolytes (CPEs) is
quenched by charged electron and energy acceptors with
1–3
remarkable efficiency. Stern–Volmer (SV) constants as large
8
21
as 10 M have been reported for CPE quenching, and studies
of corresponding monomer model compounds show that
4
amplification factors of 10 are typical. The amplified quench-
ing effect has been attributed to two factors.1 First, the
charged quenchers are believed to complex to the CPE chains
by ion-pair interactions, effectively increasing the local concen-
tration of the quencher. Second, the singlet exciton is believed
to diffuse extremely rapidly along the CPE chain, an effect
which increases the ‘quenching radius’ of the quencher.3
We have an interest in amplified quenching of CPEs in which
the luminescent exciton is a triplet spin-state. Theoretical and
experimental studies indicate that the triplet exciton in conju-
gated polymers is more strongly confined than the singlet.5,6
Owing to this difference, it is possible that the triplet exciton
will diffuse more slowly than the singlet, and this may be
reflected by attenuation of the amplified quenching.
–4
2
+
of the M(bpy)
3
chromophores are clearly visible in Fig. 1
(marked by arrows).
The Ru-containing materials feature a broad luminescence
with lmax = 642 nm. This emission is believed to arise from the
Ru?bpy MLCT excited state. The MLCT assignment is
a
Table 1 Photophysical properties and Stern–Volmer constants
UV-vis lmax/nm
K
M
SVc/106
21
(log e)
l
maxem/nm
fem (%)
tem/ms
In the present communication we report the results of
photophysical and excited-state quenching studies of metal–
organic polymers P-Ru and P-Os along with the corresponding
monomeric model compounds M-Ru and M-Os. The metal–
organic polymers are cationic polyelectrolytes because of the
dipositive charge on the metal complex units. The polymers
display a long-lived luminescent excited state that is quenched
very efficiently by anionic electron transfer quenchers. Compar-
ison of the Stern–Volmer (SV) quenching of the polymers with
the corresponding monomers reveals that the polymers are
quenched 75 and 20 times more efficiently than the models, for
M = Ru and Os, respectively.
P-Ru
M-Ru
P-Os
425 (4.65)
80 (4.10)
390 (4.91)
57 (4.20)
427 (4.85)
487 (4.11)
382 (4.76)
487 (4.04)
642
643
771
766
1.0
1.2b
1.6
4
6.0
1.3
0.021
0.37
0.018
4
b
0.23
0.28
0.031
0.033
M-Os
a
b
All data for argon purged DMF solutions.
Multiexponential decay,
median lifetime is reported (ref. 10b). c Stern–Volmer quenching by NDI.
†
Electronic supplementary information (ESI) available: complete details
concerning the synthesis and characterization of the new materials, NMR
and electrospray mass spectra, absorption and emission spectra of P-Ru and
P-Os, and Stern–Volmer plots. See http://www.rsc.org/suppdata/cc/b2/
b211575a/
Fig. 1 Ground state absorption (solid lines, scale at left) and transient
absorption (symbols, scale at right) spectra. TA spectra obtained im-
mediately following 10 ns, 355 nm excitation pulse. (a) M-Ru; (b) M-
Os.
6
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CHEM. COMMUN., 2003, 650–651
This journal is © The Royal Society of Chemistry 2003

supported by the fact that the emission quantum yield (fem) and
lifetime (tem) in M-Ru and P-Ru are similar to that of other Ru–
assumed that the anionic quencher (Q) is ion-paired to a metal
complex unit (M). Quenching involves diffusion of the exciton
to a quencher ‘trap’ site (see below) and/or diffusion of the
quencher along the chain (path e) to the exciton. We cannot rule
out the latter pathway, and in any event it is likely to play a role,
since earlier work has shown that polyelectrolytes accelerate
reactions by allowing oppositely charged reactants to diffuse to
8
polypyridyl systems. A red emission is also observed from P-
Os and M-Os, but in this case it is shifted to considerably lower
energy ( ≈ 770 nm). This shift is consistent with the emission
emanating from an Os?bpy MLCT excited state, and this
assignment is supported by the considerably reduced fem and
9
11
t
em values which are typical for Os–polypyridyl systems.
a reaction site by ‘directed diffusion’.
While the UV absorption and luminescence properties of the
Two possible mechanisms for exciton diffusion along the
polymer chains can be envisioned for P-Ru. In the first (path
metal–organic materials suggest that the lowest excited state in
all cases arises from a M?bpy MLCT state, transient
absorption (TA) spectroscopy provides clear evidence that the
2
+
a?b?c), the triplet exciton transfers from a Ru(bpy)
3
unit to
the PPE chain (by Dexter exchange transfer), the triplet exciton
diffuses along the backbone, and then undergoes exchange
3
p,p state from the PPE segment is also involved in the
2+
photophysics of P-Ru and M-Ru. Fig. 1 compares the TA
difference spectra acquired on M-Ru and M-Os immediately
following the 10 ns, 355 nm excitation pulse. (The TA spectra
of the corresponding polymers are similar.) As shown in Fig.
transfer to another Ru(bpy)
3
. The second mechanism involves
‘self-exchange hopping’ of the MLCT exciton between adjacent
2+
Ru(bpy)
non-conjugated polymers that contain pendant Ru(bpy)
3
chromophores (path d). Indeed, in recent work on
2
+
3
and
moieties, it was shown that such MLCT state self-
2+
1
(a) the difference spectrum of M-Ru is characterized by a
Os(bpy)
3
9
21 12
broad and intense absorption with lmax ≈ 680 nm, along with
bleaching of the ground state absorption bands in the 400–500
nm region. The difference absorption features decay uniformly
with t = 1.2 ms, in agreement with the emission lifetime. The
strong TA absorption band at 680 nm and bleach at 400 nm are
exchange hopping occurs, k ≈ 10 s .
The exciton diffusion mechanism that involves the PPE
3
p,p* state (a?b?c) is infeasible in P-Os, because in this case
the MLCT state is at too low an energy to equilibrate with the
PPE-based triplet state. Because amplified quenching also
occurs in P-Os, we conclude that the most likely mechanism for
exciton diffusion in both polymers involves self-exchange
3
clearly due to the p,p* state which is localized on the OPE
segment. However, there is also bleaching in the ground-state
Ru?bpy MLCT absorption band (marked by arrow) and
excited state absorption in the 350–370 nm region. The latter
2+
hopping between the M(bpy)
3
chromophores.
In summary, the results demonstrate that amplified quench-
ing occurs in metal–organic polymers where the lowest excited
state has triplet spin character. Analysis of the quenching data
suggests that diffusion of the 3p,p* state along the PPE
backbone is not kinetically competitive with alternate pathways
for quenching, including self-exchange exciton hopping and/or
directed diffusion of the quencher along the polyelectrolyte
chain. Comparison of these results with those obtained on
fluorescent CPEs, where amplified quenching involves a singlet
8
features are hallmarks of the MLCT state. The MLCT TA
features, coupled with the observation of MLCT emission,
strongly suggest that in M-Ru (and by inference also in P-Ru),
3
the MLCT and p,p* states are in equilibrium. This is consistent
3
with previous studies which indicate that the p,p* state of
OPEs and PPEs is 1.90 eV (650 nm), which is very close to the
energy of the MLCT state.10
The TA difference spectrum of M-Os shown in Fig. 1(b) is
2+
1–3
essentially identical to that of the parent complex, Os(bpy)
3
exciton, hints that diffusion of the triplet exciton is slow.
which indicates that for the Os-containing materials the MLCT
state is the only state populated at long times following
excitation. The MLCT assignment for the TA spectrum is
supported by the fact that the transient decays with t = 27 ns.
The OPE triplet state is not populated because in the Os-systems
the MLCT state is too low in energy.
Steady-state emission quenching was carried out on the
polymers and model complexes (c = 1 mM) using the anionic
electron acceptor NDI, and the KSV values are listed in Table 1.
Interestingly, NDI quenches the polymers much more effi-
ciently than the corresponding monomers. In addition, P-Ru is
quenched approximately 4-fold more efficiently than P-Os,
which reflects the fact that the excited state lifetime of the
former is longer. Quenching studies carried out using time
resolved emission demonstrate that for both polymers dynamic
quenching is significant (see ESI†). The fact that ion-pairing
between the polymers and NDI plays an important role in the
quenching process is illustrated by the fact that P-Ru is
We acknowledge the US National Science Foundation for
support of this work (grant No. CHE-0211252).
Notes and references
‡
The polymers were soluble only in DMF and DMSO solution, and due to
the limited solubility molecular weight determination by GPC was not
possible. GPC analysis of model PPE polymers synthesized under identical
conditions as P-Ru and P-Os reproducibly showed M
ca. 20 metal units and 40 phenylene rings).
n
≈ 20 kD (X ≈ 20,
n
1
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6
21
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.4 3 10
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4
21
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quenching. Fig. 2 shows a cartoon in which several possible
mechanisms are considered. In all of the mechanisms it is
2
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1
Fig. 2 Schematic diagram of exciton diffusion. See text for details.
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