Towards chemosensing phosphorescent conjugated polymers: Cyclometalated platinum(II) poly(phenylene)s

Article abstract of DOI:10.1039/b503539b

The synthesis and optical properties of several phosphorescent conjugated poly(phenylene)s containing cyclometalated square-planar platinum (II) complexes are reported. These electronic polymers were synthesized via Suzuki cross-coupling of a dibromopheny

Full text of DOI:10.1039/b503539b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Towards chemosensing phosphorescent conjugated polymers:
cyclometalated platinum(II) poly(phenylene)s{
Samuel W. Thomas III, Shigeyuki Yagi and Timothy M. Swager*
Received 9th March 2005, Accepted 13th April 2005
First published as an Advance Article on the web 3rd May 2005
DOI: 10.1039/b503539b
The synthesis and optical properties of several phosphorescent conjugated poly(phenylene)s
containing cyclometalated square-planar platinum (II) complexes are reported. These electronic
polymers were synthesized via Suzuki cross-coupling of a dibromophenylpyridine-ligated Pt(II)
complex with a fluorene diboronic ester. Their optical properties are characterized by relatively
strong orange room-temperature phosphorescence with well-resolved vibronic structure in both
frozen 2-methyltetrahydrofuran glass and room-temperature fluid solution. Time-resolved
phosphorescence spectroscopy has shown that the polymers have excited state lifetimes of
approximately 14 ms. These optical properties of the oligomers and polymers are contrasted with
those of small model complexes, the optical properties of which have a strong dependence on the
identity of the b-diketonate ligand used. The potential utility of phosphorescent conjugated
polymers is illustrated by examination of the diffusive quenching due to oxygen as a function of
molecular structure.
phosphorescent materials in increasing the efficiency of
Introduction
organic light-emitting diodes by harnessing the large percen-
tage of triplet excitons created upon electron-hole recombina-
tion.8 Principal amongst these are cyclometalated complexes of
iridium (III)9 and platinum (II),10 which are attractive due to
their modular synthesis and high luminescence efficiencies.
Related work in the field of phosphorescent materials has
recently included the use of these phosphors as pendant
groups on both traditional vinyl and conjugated polymer
backbones,11 the preparation and study of an electrophos-
phorescent fluorene-based conjugated polymer with a small
percentage of cyclometalated Ir complex in the backbone,12 as
well as the detailed photophysical study of several different
p-conjugated oligo(arylene ethynylene)s containing transition
metal complexes.13
The use of fluorescent conjugated polymers is an established
method for achieving a large degree of amplification for the
chemosensing of analytes in both solution and solid states.1
This approach has been successfully applied to biological
binding events via Fo¨rster energy transfer,2 as well as nitro-
aromatics and quinones, including 2,4,6-trinitrotoluene (TNT),
via photoinduced electron transfer from the excited polymer to
the bound analyte.3 We have sought to investigate the poten-
tial of phosphorescent conjugated polymers as chemosensing
materials. The longer lifetimes of triplet excitons in conjugated
materials may lead to increased sensitivity to trace analytes.
This is an important goal for the purposes of detecting
dangerous analytes with much smaller vapor pressures than
TNT, such as the high explosives RDX and PETN.4
We have directed our attention and efforts towards
conjugated polymers based on cyclometalated transition metal
complexes for their modularity in both the choice of central
metal atom as well as ligand environment around the metal
center. We report herein the synthesis and optical character-
istics of a poly(phenylene) type of conjugated polymer in
which the phenylpyridine ligand of a square planar Pt(II)
complex constitutes one of the two repeat units. The stark
contrast between the optical properties of the polymeric
systems and several model complexes are highlighted and
rationalized. The increased sensitivity of the polymeric system
to dissolved oxygen demonstrates the conjugated nature of the
system as well as the potential for using phosphorescent
conjugated polymers as chemosensing materials.
There are numerous examples of the incorporation of
transition metal complexes into conjugated systems.5 Research
into highly phosphorescent conjugated polymers with high
loading of transition-metal complexes has focused on the plati-
num acetylide class of materials.6 These polymers incorporate
the phosphine-substituted platinum atoms as part of the
conjugated backbone, the significant spin–orbit coupling of
which partially allows the intrinsically spin-forbidden pro-
cesses of intersystem crossing and phosphorescence.7 However,
one of the major advantages of working with organic con-
jugated materials is the modularity of material design, and this
feature is limited in the platinum acetylide polymers.
Highly phosphorescent late-transition metal complexes and
materials incorporating these moieties have received increasing
attention of late. This is principally due to the utility of
Experimental
{ Electronic supplementary information (ESI) available: Synthetic
procedures for P1 and associated model complexes. See http://
www.rsc.org/suppdata/jm/b5/b503539b/
All synthetic manipulations were performed under an argon
atmosphere using standard Schlenk techniques unless other-
wise noted. NMR (1H and 13C) spectra were recorded on a
*tswager@mit.edu
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Varian 300 MHz spectrometer. NMR chemical shifts are
referenced to residual protonated solvent. Polymer molecular
weights were determined by gel-permeation chromatography
(GPC) using an HP series 1100 GPC system running at
1.0 mL min21 in THF equipped with a diode array detector
(254 and 450 nm) and a refractive index detector. Molecular
weights are reported relative to poly(styrene) standards.
Preparative GPC was performed in THF using a 10 mm
PLgel preparative GPC column. The flow was supplied by a
synthesized in one step via a regiospecific palladium-mediated
Suzuki cross-coupling reaction between 2,5-dibromopyridine
and 4-bromophenylboronic acid in good yield following chro-
matography and recrystallization from hexanes. The higher
reactivity of the 2-position of the 2,5-dibromopyridine has
been identified previously to make regiospecific poly(pyridyl
vinylene)s.14 The insoluble dichloride-bridged platinum dimer
was isolated from the reaction between the phenylpyridine
ligand and K2PtCl4 in good yield as a yellow powder after
Soxhlet extraction with dichloromethane.
Rainin HPXL solvent delivery system at 10 mL min21
.
Absorbance was monitored with a Dynamax UV-1 absorbance
detector. MALDI-TOF data was collected using a Bruker
Omniflex instrument with 2,5-dihydroxybenzoic acid as the
matrix. The instrument was calibrated with the Sequazyme
Peptide Mass Standards Kit (Applied Biosystems).
The b-diketonate ligand 2 was obtained in good yield
following a procedure that involved alkylation of the triol
derivative followed by a Claisen-type condensation in the
presence of sodium hydride in THF.15 The methyl ketone was
prepared by methyllithium monoreduction of the carboxylic
acid. This particular ligand was chosen to help improve the
solubility of any polymers made from this material, especially
given the square planar geometry of Pt(II) complexes. The
monomeric complex 4 could be readily obtained by fragment-
ing the chloride-bridged platinum dimer in the presence of the
dihalophenylpyridine and Ag2O. Column chromatography
allowed for the isolation of pure platinum monomer. This
known procedure is highly suitable for making these halogen-
containing monomers,10 in contrast to that for homoleptic
biscyclometalated complexes which require the prior forma-
tion of organolithium phenylpyridines and intermediates.16
In addition to monomer 4, several different model com-
plexes were synthesized for comparison with the platinated
conjugated polymer. These are illustrated in Chart 1.
Dialkylated phenylpyridine (Scheme 2) could be prepared
from the dibromo derivative in the two-step sequence of
Sonogashira coupling with the corresponding terminal acetyl-
ene, followed by catalytic hydrogenation in ethyl acetate at
45 psi H2 in the presence of palladium on carbon. All
cyclometalated Pt(II) complexes were made following the
same procedure described above. Polymerization of 4 was
accomplished by standard Suzuki polymerization with the
commercially available 2,7-diboronic ester of fluorene as a
comonomer. The number average molecular weight of P1, as
determined by gel-permeation chromatography (versus poly-
styrene standards), was 12000 g mol21 with a polydispersity
index of approximately 2.5. In addition, model complex 5, a
short oligomeric model of the platinum-containing polymer,
was synthesized as shown in Scheme 3 by dialkylation of
2-bromofluorene, followed by formation of the boronic acid
and Suzuki coupling with 4.
UV-vis spectra were recorded on an Agilent 8453 diode-
array spectrophotometer and corrected for background signal
with a solvent-filled cuvette. Emission spectra were acquired
on a SPEX Fluorolog-t3 fluorometer (model FL-321, 450 W
Xenon lamp) using right angle detection. All samples were
degassed by at least three freeze–pump–thaw cycles in an
anaerobic cuvette and were repressurized with Ar following
each cycle. The absorbance of all samples was kept below
0.1 OD in order to minimize reabsorption artifacts. Quantum
yields of emission were determined by comparison to appro-
priate standards and are corrected for solvent refractive
index and absorption differences at the excitation wavelength.
Emission spectra at 77 K were acquired in frozen 2-methyl-
pentane glass. All solvents used in photophysical experiments
were of spectral grade or better.
Phosphorescence lifetimes were determined by time-resolved
phosphorescence spectroscopy. The irradiation source was
an Oriel nitrogen laser (Model 79111) with a 5 ns pulsewidth
operating at approximately 25 Hz. The emitted light was
dispersed in an Oriel MS-260i spectrograph with
a
300 lines mm21 grating. The detector was an Andor
Technologies Intensified CCD camera (1024 6 128 pixels)
with an onboard delay generator and a minimum gate width of
5 ns operating in full vertical binning mode and triggered by a
TTL prepulse from the nitrogen laser. Data taken of 77 K
glasses were subjected to no horizontal binning, while solution
data was acquired with a horizontal binning of 2 or 3. 10–15
spectra at different delay times after the laser pulse were taken
per lifetime measurement, the integrated intensities of which
were fit to a single-exponential function. The detector was
calibrated with a Hg(Ar)pencil-style calibration lamp.
Gas mixing was performed by flowing air and nitrogen
through two separate Sierra mass flow controllers that were
coordinated and controlled with a Sable Systems MFC-4 Gas
Mixer. The controlled gas flows were led into the bottom of a
round bottom flask equipped with a magnetic stirring bar. The
gas mixture was led out of the top of the flask and introduced
to the vacuum-degassed sample.
Solution and frozen glass optical properties
Table 1 displays some of the critical optical properties of the
polymers and model complexes synthesized in both deoxy-
genated tetrahydrofuran (THF) or cyclohexane (CHX) fluid
solution, as well as in frozen 2-methyltetrahydrofuran glass at
77 K. The absorbance and emission spectra of P1 at room
temperature, as well as the 77 K emission spectrum are shown
in Fig. 1. The absorbance spectrum of this material reflects the
conjugated nature of the backbone. As the length of the
phenylene backbone increases from two phenylene units, (6) to
six phenylenes (5), and ultimately to the polymeric species (P1)
Results and discussion
Synthesis of Pt(II) polymer and model complexes
The synthesis of the Pt(II) polymerization monomer used is
illustrated in Scheme 1. The dihalophenylpyridine ligand was
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Scheme 1 Synthesis of P1.
Chart 1 Other model complexes studied.
there is a corresponding extension of the absorbance spectrum
to the red. For P1 onset of absorption begins at 480 nm and
there is a local maximum at 450 nm. Intensity-normalized
absorbance spectra of these three species are illustrated in
Fig. 2.
the well-resolved vibronic structure observable even in the
room temperature sample. The energy difference between the
vibronic bands is approximately 1350 wavenumbers, a value
typically observed in the fluorescence spectra of phenylene-
based conjugated polymers. In addition, the pattern of relative
Frank–Condon factors of this material, with the 0,0 band
being the most intense and smaller intensities with decreasing
energy, is also a common feature in the emission spectra of
conjugated polymers.18
The emission spectrum is dominated by a strong phos-
phorescence peak at about 585 nm, with a Stokes shift of more
than 200 nm. This large difference between the absorbance and
emission energies is indicative of phosphorescence, due to the
approximately 0.7 eV singlet–triplet energy gap that is curiously
characteristic of all phosphorescent conjugated polymers.17
Another noteworthy feature of the phosphorescence spectra is
There is little contrast between the 77 K spectrum of P1 and
the spectrum at ambient temperature, save for the typical small
rigidochromic blue shift and more highly resolved vibronic
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Scheme 2 Synthesis of alkyl substituted ppy.
Fig. 1 P1 room temperature absorbance and emission spectra in
THF (solid lines) and 77 K emission spectrum (dotted line) in 2-methyl
THF frozen glass.
Scheme 3 Synthesis of model complex 5.
structure in the glass. Metal-to-ligand charge-transfer (MLCT)
based transitions often show a substantial solvent dependence
because of the potentially large solvent reorganization energies
in polar solvents as opposed to non-polar or rigid media.13a
The lack of a large blue shift in the frozen glass is consistent
with the transition being primarily a triplet p–p* ligand
centered (3LC) transition with mixing from an MLCT
transition to decrease the forbidden character of the inter-
system crossing and phosphorescence. The basic spectral
Fig. 2 Height-normalized absorption spectra of P1 (solid line), 5
(dotted line) and 6 (dot–dash) in THF at room temperature.
characteristics discussed above also support this conclusion.
This is very consistent with structurally related square planar
platinum complexes previously studied.10
Table 1 Optical properties of Pt(II) complexesa
Emission lmax/nm
tp/ms
Complex
Absorbance lmax/nm (rt)
rt
77 K
Wp (rt)
rt
77 K
P1
5
6
6 (CHX)
7
7 (CHX)
8
374
336
361
359, 381
361
359
585
575
531
490, 526
533
584, 634
560, 606
501, 522
—
531
—
0.05
0.19
,0.01
0.24
,0.01
0.30
0.16
14.4
13.3
0.035
2.2
0.037
2.4
1.4
22.3
19.2
5.3
—
7.2
496, 530
485, 519
—
281, 317, 373
476, 494, 513
8.910
a
Room temperature data in THF unless otherwise noted as ‘‘CHX’’ (cyclohexane). 77 K data in 2-methyl THF glass.
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Another noteworthy feature of this material is its long room
temperature excited state lifetime of approximately 14 ms. This
is about an order of magnitude longer than the simple model
complex 8 that is very similar to the those studied by
Thompson.10 This effect can be understood in terms of the
LC state with
a
non-emissive CT state involving the
b-diketonate ligand accounts for this observation as well. In
addition, phenyl substituted b-diketonate ligands have been
shown to dramatically reduce the phosphorescence efficiency
of cyclometalated Ir(III) complexes in 2-methyl THF.9b
3
This CT model is further supported by the very close
similarities of the optical properties of P1 and P2, which are
identical polymer structures except for the nature of the
diketonate ligand. The room temperature absorption and
emission spectra of P1 and P2 are overlaid in Fig. 4. Extending
the conjugation beyond the immediate phenylpyridine ligand
relative energy levels of the LC and MLCT states. Extending
the conjugation length causes a lowering of the energy of the
3LC state as is evident from the absorption and emission
spectra. This leads to a decreased amount of mixing between
the two states and hence the observed longer phosphorescence
lifetime.
3
shifts the emission of the based LC transition by over 50 nm
The contrasts between P1 and other, more structurally
related model compounds become even more dramatic upon
changing the b-diketonate ligand from dipivaloylmethane to
phenyl-substituted ligands such as 6. Fig. 3 illustrates the
emission spectra of 6 in several different solvents. Although
this changing of the non-cyclometalating ligand does not result
in a significant change in the color of emission versus those
such as compound 8, it has a large effect on both the quantum
yield of phosphorescence and the phosphorescence lifetime in
THF. Those complexes with phenyl-substituted b-diketonate
ligands show much lower emission efficiencies and excited
state lifetimes as low as 35 ns in THF, which are almost three
orders of magnitude less than P1. The lifetime of complexes 6
and 7 at 77 K is about 6–7 ms, similar in magnitude to 8.
A reasonable model that is consistent with these observa-
tions is the presence of a very weakly or non-emissive state that
is in thermal equilibrium with the phenylpyridine-based 3LC at
room temperature in THF. The substitution of aromatic rings
on the diketonate ligand extends its conjugation and lowers
its oxidation potential (raising the local HOMO), thereby
lowering the energy of a charge transfer transition from this
ligand to the central platinum atom. This is consistent with
previous photophysical data of fluorine-substituted Pt(II)
to longer wavelength. This lower energy nature of the polymer’s
emissive state prevents thermal equilibrium with the non-
emissive Pt-diketonate CT state, regardless of that diketonate’s
chemical structure. As a result, the spectral positions, shapes,
and most importantly excited state lifetimes of P1 (14.4 ms) and
P2 (14.2 ms) are all very similar, in stark contrast to those of
the corresponding model compounds 8 and 6.
The nature of the very weakly emissive state has been
probed with a solvent-dependence study. When one of the
highly soluble complexes 6 or 7 is dissolved in cyclohexane
(CHX) as opposed to THF, the excited state lifetime changes
from approximately 35 ns to over 2 ms. In addition, the
quantum yield of emission is raised from less than one percent
to approximately 25–30%. Therefore, the emission from the
lowest energy 3LC state remains undisturbed in CHX, whereas
the polar CT Pt-diketonate states are destabilized by the non-
polar nature of CHX relative to the more polar THF. Fig. 5
summarizes the model deduced from the experimental data
with a qualitative energy level diagram.
These comparisons emphasize that the design of materials
containing organometallic complexes for optical and optoelec-
tronic applications is still a relatively unexplored field com-
pared to purely organic materials. The presence of multiple
transitions such as metal centered, MLCT and LC transitions
3
phenylpyridine complexes with high-energy LC transitions.10
These complexes also have very short lifetimes and low
emission quantum yields. Equilibration of the high-energy
Fig. 4 Height-normalized absorption and emission spectra of P1
(solid line) and P2 (dotted line). These two polymers also have very
similar excited state lifetimes (14.4 and 14.2 ms, respectively).
Fig. 3 Height-normalized emission spectra of 6 in room temperature
THF (dot–dashed line), 77 K 2-methyl THF glass (dotted line) and
room temperature cyclohexane (solid line).
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proof of principle for this type of approach, the sensitivity of
P1 towards dissolved oxygen was compared to the fluorene-
containing polymer mimic 5.
The oxygen sensitivity experiments were conducted in
THF solvent. All samples were degassed by at least three
freeze–pump–thaw cycles and then repressurized with the
appropriate mixture of nitrogen and air. In addition, the
solutions were shaken for approximately 2 min before data
collection to ensure equilibration between the gas and the
solution. Phosphorescence lifetimes as a function of the
percentage of air were acquired a minimum of two times to
ensure reproducibility.
Fig. 6 shows the dependence of the excited state lifetimes of
P1 and 5 on the volume fraction of air in solution. As can be
inferred from this information, the polymeric species demon-
strates a noticeable increase in oxygen sensitivity via diffu-
sional quenching of approximately 30–40%. The nature of the
Stern–Volmer plot serves to ‘‘normalize out’’ any difference in
lifetime since the operable quantity is t0/t This result is
indicative of a somewhat larger exciton present in the excited
state of P1 as compared to 5.
Fig. 5 Qualitative energy diagram of Pt(II) complexes, displaying the
emissive 3LC state and the non-emissive charge-transfer state involving
the diketonate ligand. Lowering of the CT state energy by incorpora-
tion of phenyl groups on the diketonate group allows the LC state to
be in thermal equilibrium with it (6). Dramatic lowering of the 3LC
energy by incorporation into a conjugated polymer (P1) restores the
energy difference between the two states.
This experiment demonstrates that extension of the excited
state of organometallic complexes into conjugated polymers is
a potentially viable means towards improving oxygen sensing
materials. Though the effect of increased conjugation length is
modest in solution, it could be greatly enhanced in conjugated
polymers that are highly phosphorescent in the solid state,
which may allow for more sensitive amplified detection of
oxygen due to intramolecular exciton migration. The current
technology of dispersing small molecule phosphors in sol–gel
or inert polymer hosts precludes this type of amplification.
This enhancement due to the extension of the exciton length is
coupled with the increase of the intrinsic excited state lifetime
resulting from the lower energy of the 3LC state relative to the
MLCT (vide supra). The longer excited state lifetime offered by
adds a significant amount of information to be deconvoluted
for each individual metal-containing system. The wide choices
of available metal atoms and ligands will not only increase the
modularity of potential phosphorescent materials, but also the
complexity of data and performance analysis. This being said,
these features of organometallic complexes offer additional
opportunities for the design and application of new sensory
transduction schemes. Manipulation of excited state energy
levels in these systems with environmental and chemical struc-
ture changes such as this study demonstrates can be envisioned
to be critical elements of novel chemosensing mechanisms.
Oxygen induced quenching
The field of optical oxygen sensing has been dominated by
phosphorescent transition metal complexes because of their
reactivity towards molecular oxygen, through a combination
of triplet–triplet annihilation and photoinduced electron
transfer, to form singlet oxygen and their long excited state
lifetime that allows for competitive diffusion of oxygen with
their radiative rates.19 Oxygen detection is often performed by
analyzing the reductions of the excited state lifetimes due to
purely diffusive phosphorescence quenching. An advantage
of this method is that it does not depend on absolute phos-
phorescence intensity, which can vary due to partial decom-
position of the complex. Hence, lifetime analysis lowers
inaccuracies in oxygen concentration measurements brought
about by artifacts such as photobleaching.20
Many current research efforts into oxygen detection involve
the encapsulation or dispersal of monomeric transition metal
complexes into an inert matrix such as poly(dimethylsiloxane)
or a sol–gel.21 The potential for higher sensitivity brought
about by conjugated polymer sensors along with their material
properties and processability may make them viable candi-
dates for the next generation in oxygen sensing materials. As a
Fig. 6 Lifetime Stern–Volmer Plots for P1 and 5 as a function of
percent air in nitrogen. P1 shows a 30–40% increased sensitivity to
dissolved O2. The maximum error in lifetime measurements was 8%.
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phosphorescent conjugated polymers should have the practical
effect of increasing the dynamic range and resolution of optical
oxygen sensing since a wider range of lifetimes would be
available for a given range of oxygen concentrations.
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Our future efforts will be focused in several different
directions. These include the development of phosphorescent
conjugated polymers that are highly emissive in the solid
state or that are emissive under ambient conditions. In
addition, we are currently investigating the unique chemistry
and excited state features of organometallic complexes, such
as oxidative addition, as novel transduction mechanisms
for the trace detection of potentially dangerous vapor phase
analytes.
Acknowledgements
The authors thank Sandia National Laboratories, The
Technical Support Working Group, and The Transportation
Security Administration for support of this research, as well as
Dr S. Kooi of the MIT Institute for Soldier Nanotechnologies
for assistance with time-resolved spectroscopy.
Samuel W. Thomas III, Shigeyuki Yagi and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology,
77 Massachusetts Avenue, Cambridge, MA, USA.
E-mail: tswager@mit.edu; Fax: 617-253-7929; Tel: 617-253-4423
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 2829–2835 | 2835