A platinum acetylide polymer with sterically demanding substituents: Effect of aggregation on the triplet excited state

Article abstract of DOI:10.1021/ic048961s

The effect of interchain interaction on the triplet excited state is explored in two Pt-acetylide polymers of the type [-trans-Pt(PBu ₃)₂-C≡C-Ar-C≡C-]_n, where Ar is either 1,4-phenylene or is based on the pentiptycene unit (polymers 2 and 3, respectively). To explore the effect of interchain interaction in Pt-acetylide materials, the optical properties of parent polymer 2 are compared with those of polymer 3 in which interchain interaction is precluded by the sterically bulky pentiptycene moiety. Insight into the effect of the pentiptycene unit on packing in the solid state comes from the X-ray structure of monomer 1b, Ph-C≡C-[trans-Pt(PBu₃)₂]-C≡C-Ar-C≡C- [trans-Pt(PBu₃)₂]-C≡C-Ph. Spectroscopic studies indicate that weak phosphorescence emission from an interchain aggregate is observed from parent polymer 2, both in solution and in the solid state. By contrast, the photophysics of 3 is dominated by the intrachain triplet exciton. Interestingly, the phosphorescence emission of polymer 3 in the solid state is nearly superimposable with that of a single crystal of monomer 1b, suggesting that the solid polymer experiences an environment that is similar to that of the monomer in the crystal.

Full text of DOI:10.1021/ic048961s

Inorg. Chem. 2005, 44, 2619−2627
A Platinum Acetylide Polymer with Sterically Demanding Substituents:
Effect of Aggregation on the Triplet Excited State
Xiaoming Zhao, Thomas Cardolaccia, Richard T. Farley, Khalil A. Abboud, and Kirk S. Schanze*
Department of Chemistry, UniVersity of Florida, P.O. Box 117200,
GainesVille, Florida 32601-7200
Received July 30, 2004
The effect of interchain interaction on the triplet excited state is explored in two Pt
trans-Pt(PBu3)2 Ar ]n, where Ar is either 1,4-phenylene or is based on the pentiptycene unit
(polymers 2 and 3, respectively). To explore the effect of interchain interaction in Pt acetylide materials, the optical
−acetylide polymers of the type
[
−
−CtC− −CtC−
−
properties of parent polymer 2 are compared with those of polymer 3 in which interchain interaction is precluded
by the sterically bulky pentiptycene moiety. Insight into the effect of the pentiptycene unit on packing in the solid
state comes from the X-ray structure of monomer 1b, Ph−CtC−[trans-Pt(PBu3)2]−CtC−Ar−CtC−[trans-Pt(PBu3)2]−
CtC−Ph. Spectroscopic studies indicate that weak phosphorescence emission from an interchain aggregate is
observed from parent polymer 2, both in solution and in the solid state. By contrast, the photophysics of 3 is
dominated by the intrachain triplet exciton. Interestingly, the phosphorescence emission of polymer 3 in the solid
state is nearly superimposable with that of a single crystal of monomer 1b, suggesting that the solid polymer
experiences an environment that is similar to that of the monomer in the crystal.
Introduction
been investigated thoroughly for polymers such as poly-
(phenylene vinylene) and poly(phenylene ethynylene) (PPV
and PPE, respectively), which exhibit fluorescence from the
singlet exciton. In particular, the fluorescence of thin films
of PPV- and PPE-type polymers is red-shifted and broadened
relative to the fluorescence from the same materials in dilute
solution.8,11-15 The red-shifted fluorescence is attributed to
an excimer-like state that results from π-stacking of aromatic
residues in the solid. This model for aggregate formation in
the solid materials is supported by the fact that the optical
properties of conjugated polymers that are decorated with
sterically demanding groups such as dendrimers16,17 or
pentiptycene12,13,18 are nearly the same in solution and in the
solid state.
Conjugated polymers are promising candidates for a
variety of applications including chemo- and biosensing,1
electroluminescence,2-4 and optical limiting.5,6 Most applica-
tions of conjugated polymers require that the materials be
used in a solid-state format such as a thin film. It is well-
known that the optical and electronic properties of conjugated
molecules in the solid state differ significantly from those
observed for the same materials in dilute solution or dispersed
at low concentration into a nonconjugated host polymer.7,8
For example, in the solid state, aggregates dominate the
photophysics of most conjugated polymers.9,10 The topic has
* Author to whom correspondence should be addressed. E-mail:
kschanze@chem.ufl.edu. Tel: 352-392-9133. Fax: 352-392-2395.
(1) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100,
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10.1021/ic048961s CCC: $30.25
Published on Web 03/11/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 8, 2005 2619

Zhao et al.
Although a great deal is known about the properties of
the singlet state in conjugated polymers and how its
properties are influenced by aggregation, comparatively less
is known about the lowest triplet state.19-22 This is clearly
an important issue, since the triplet exciton plays an important
role in applications such as electroluminescence,23,24 optical
limiting,25 and luminescent sensors.26,27 A factor that limits
the ability to investigate the properties of the triplet state in
all-organic conjugated materials is the fact that phosphores-
cence is rarely observed.28 As a consequence, emission
spectroscopy cannot be used to examine how the properties
of the triplet state are influenced by factors such as chain
structure and medium. A number of recent investigations
have demonstrated that platinum(II)-acetylide-based poly-
mers, which are structurally analogous to poly(phenylene
ethynylene)s, have high intersystem crossing efficiencies and
display phosphorescence from the triplet exciton at room
temperature, both in solution and in the solid state.29-31
Because of this property, Pt-acetylide polymers and oligo-
mers provide a unique opportunity to use spectroscopy to
explore factors that influence the triplet exciton in π-con-
jugated systems. Specifically, work to date on this class of
materials has shown conclusively that the triplet exciton is
less delocalized than the singlet,28,30,32 that the energy gap
law for nonradiative decay holds for a series of Pt-acetylide-
based polymers and oligomers,33 and that the singlet-triplet
branching ratio in electroluminescent devices may be influ-
enced by the degree of conjugation in the material.34
In the present investigation, we have characterized the
optical properties of a series of Pt-acetylide-based oligomers
and polymers that contain the bulky pentiptycene unit to
disrupt interchain interactions in the solid-state materials. As
noted above, Swager and co-workers developed the iptycene
moiety as a sterically demanding unit to disrupt interchain
aggregates in PPE-type polymers.12,13,18 In the present
investigation we have applied their strategy to explore the
influence of aggregates on the triplet state in π-conjugated
polymers. The Pt-acetylide materials that are the focus of
this study display strong phosphorescence from the triplet
exciton in the solid state, affording the opportunity to explore
how aggregation influences the properties of the triplet state.
Comparison of the PL spectra of an iptycene-containing
monomer and polymer with those of a corresponding
phenylene-based polymer shows that aggregation in the
phenylene system clearly affects the triplet state. The effects
of aggregation are discussed in terms of possible interaction
mechanisms, including π-π stacking of the phenylene units
and the introduction of low-lying excitations derived from
dσ* f pσ/π* MMCT (metal-metal to ligand charge
transfer)35,36 configurations which arise from interchain
Pt-Pt interactions.
Experimental Section
General Synthetic Procedures and Source of Starting Materi-
als. All chemicals used for synthesis were of reagent grade and
used without purification unless noted. Reactions were carried out
under an argon atmosphere with freshly distilled solvents, unless
1
otherwise noted. H, 13C, and 31P NMR spectra were recorded on
(17) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mu¨llen, K.;
Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001,
123, 946-953.
either a Varian Gemini 300 or Mercury 300 spectrometer, and
chemical shifts are reported in ppm relative to TMS. cis-Dichlo-
robis(tri-n-butylphosphine)platinum(II),37 1,4-diethynylbenzene,32
and polymer 231 were prepared by literature methods. (GPC analysis
(18) Zhu, Z.; Swager, T. M. Org. Lett. 2001, 3, 3471-3474.
(19) Walters, K. A.; Ley, K. D.; Schanze, K. S. Chem. Commun. 1998,
10, 1115-1116.
of polymer 2 gave Mn ) 11 400 g mol-1, Mw ) 32 100 g-mol-1
,
(20) Burrows, H. D.; Seixas de Melo, J.; Serpa, C.; Arnaut, L. G.;
Monkman, A. P.; Hamblett, I.; Navarathnam, S. J. Chem. Phys. 2001,
115, 9601-9606.
and PDI ) 2.81. The 1H NMR spectrum of the polymer is provided
as Supporting Information.) The diethynyl iptycene compound
(6,13-diethynyl-5,7,12,14-tetrahydro-5,14[1′,2′]:7,12[1′′,2′′]diben-
zopentacene, CAS Registry No. 214461-10-6, herein referred to
as compound 4) was prepared as described by Yang and Swager.13
The molecular weights of the polymers were determined on a GPC
column with a Beckman Instruments Spectroflow 757 absorbance
detector at 270 nm and a flow rate of 1.0 mL/min, using THF as
the eluent and two PLgel 5 µm MIXED-D columns (Polymer
Laboratories). Molecular weights are reported relative to polystyrene
standards.
(21) Monkman, A. P.; Burrows, H. D.; Hamblett, I.; Navarathnam, S.;
Svensson, M.; Andersson, M. R. J. Chem. Phys. 2001, 115, 9046-
9049.
(22) Monkman, A. P.; Burrows, H. D.; Hartwell, L. J.; Horsburgh, L. E.;
Hamblett, I.; Navaratnam, S. Phys. ReV. Lett. 2001, 86, 1358-1361.
(23) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-153.
(24) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby,
C. E.; Ko¨hler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc.
2004, 126, 7041-7048.
(25) Perry, J. W. In Nonlinear Optics of Organic Molecules and Polymers;
Nalwa, H. S., Miyata, S., Eds.; CRC Press: Boca Raton, FL, 1997;
pp 813-840.
(26) Carraway, E. R.; Demas, J. N.; DeGraff, B. A.; Bacon, J. R. Anal.
Chem. 1991, 63, 337-342.
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Gerhard, A.; Ba¨ssler, H. J. Chem. Phys. 2002, 116, 9457-9463.
(29) Wittmann, H. F.; Friend, R. H.; Khan, M. S.; Lewis, J. J. Chem. Phys.
1994, 101, 2693-2698.
(30) Beljonne, D.; Wittmann, H. F.; Ko¨hler, A.; Graham, S.; Younus, M.;
Lewis, J.; Raithby, P. R.; Khan, M. S.; Friend, R. H.; Bre´das, J. L. J.
Chem. Phys. 1996, 105, 3868-3877.
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M. R. A.; Khan, M. S.; Raithby, P. R. J. Chem. Phys. 2000, 113,
7627-7634.
Complex 1a. Diethynyl iptycene 4 (47.6 mg, 0.1 mmol) was
dissolved in toluene/piperidine (4:1, 10 mL), and CuI (1 mg) was
added. The mixture was degassed for 20 min, whereupon cis-
dichlorobis(tri-n-butylphosphine)platinum(II) (200 mg, 0.3 mmol)
was added quickly and the reaction mixture was stirred over-
night at 70 °C. The solvents were removed under reduced pres-
sure, and the residue was purified by column chromatography on
1
silica. A white solid (140 mg, 80%) was collected. H NMR (300
MHz, CDCl3): δ 0.89 (t, J ) 7.5 Hz, 36H), 1.39-1.52 (m, 24H),
(34) Wilson, J. S.; Dhoot, A. S.; Seeley, A. J. A. B.; Khan, M. S.; Ko¨hler,
A.; Friend, R. H. Nature 2001, 413, 828-831.
(32) Liu, Y.; Jiang, S.; Glusac, K.; Powell, D. H.; Anderson, D. F.; Schanze,
K. S. J. Am. Chem. Soc. 2002, 124, 12412-12413.
(33) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus, M.;
Khan, M. S.; Raithby, P. R.; Ko¨hler, A.; Friend, R. H. J. Am. Chem.
Soc. 2001, 123, 9412-9417.
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2620 Inorganic Chemistry, Vol. 44, No. 8, 2005

A Platinum Acetylide Polymer
Table 1. Crystallographic Parameters for 1b
1.63-1.78 (m, 24H), 2.08-2.20 (m, 24H), 5.93 (s, 4H), 6.82-
6.89 (dd, J1 ) 3.0 Hz, J2 ) 3.3 Hz, 8H), 7.18-7.26 (dd, J1 ) 3.3
Hz, J2 ) 3.6 Hz, 8H). 13C NMR (75 MHz, CDCl3): δ 13.83, 21.92,
22.15, 22.36, 24.15, 24.24, 24.32, 26.34, 52.15, 89.27, 96.52,
117.11, 123.19, 124.51, 141.70, 146.08. 31P NMR (CDCl3, 121
MHz): δ 8.25 (J ) 2375.8 Hz). Anal. Calcd for C86H128Cl2P4Pt2:
C, 59.13; H, 7.39. Found: C, 58.92; H, 7.49.
empirical formula
temp
C102H138P2Pt
173(2) K
cryst system
space group
unit cell dimens
monoclinic
C2/c
a ) 35.608(2) Å, R ) 90°
b ) 13.7399(8) Å, â ) 124.806(2)°
c ) 23.5629(14) Å, γ ) 90°
9465.6(10) Å3
V
Complex 1b. Complex 1a (106 mg, 0.061 mmol) was dissolved
in toluene/piperidine (2:1, 3 mL), and CuI (1 mg) was added. The
mixture was degassed for 20 min with argon, whereupon phenyl-
acetylene (12.44 mg, 0.122 mmol) was added quickly and the
reaction mixture was stirred overnight at room temperature. The
solvents were removed under reduced pressure, and the residue was
purified by column chromatography on silica. A yellow solid (95
Z
4
cryst size
0.24 × 0.20 × 0.19 mm3
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]a
R indices (all data)a
10 650/0/469
1.031
R1 ) 0.0521, wR2 ) 0.1053 [6994]
R1 ) 0.0914, wR2 ) 0.1218
a R1 ) Σ(||Fo| - |Fc||)/Σ|Fo|. wR2 ) [Σ[w(Fo2 - Fc )2]/Σ[w(Fo )2]]1/2
.
2
2
1
S ) [Σ[w(Fo - Fc )2]/(n - p)]1/2. w ) 1/[σ2(Fo ) + (mp)2 + np], p )
2
2
2
mg, 83%) was collected. H NMR (300 MHz, CDCl3, spectrum
2 2
[max(Fo ,0) + 2Fc ]/3, m and n are constants.
shown in Supporting Information): δ 0.91 (t, J ) 7.2 Hz, 36 H),
1.41-1.55 (m, 24 H), 1.65-1.81 (m, 24H), 2.19-2.37 (m, 24H),
5.98 (s, 4H), 6.81-6.85 (m, 8H), 7.16-7.19 (m, 2H), 7.20-7.24
(m, 12H), 7.32-7.38 (m, 4H). 31P NMR (121 MHz, CDCl3): δ
4.63 (J ) 2362.0 Hz). Anal. Calcd for C102H138Cl2P4Pt2: C, 65.23;
H, 7.41. Found: C, 64.58; H, 7.58.
X-ray Structure Determination of 1b. Data were collected at
173 K on a Siemens SMART PLATFORM equipped with A CCD
area detector and a graphite monochromator utilizing Mo KR
radiation (λ ) 0.710 73 Å). Cell parameters were refined using up
to 8192 reflections. A full sphere of data (1850 frames) was
collected using the ω-scan method (0.3° frame width). The first 50
frames were remeasured at the end of data collection to monitor
instrument and crystal stability (maximum correction on I was
<1%). Absorption corrections by integration were applied on the
basis of the measured indexed crystal faces.
The structure was solved by the direct methods in SHELXTL640
and refined using full-matrix least squares. The non-H atoms were
treated anisotropically, whereas the hydrogen atoms were calculated
in ideal positions and were riding on their respective carbon atoms.
The complexes are located on centers of inversion; thus, the
asymmetric unit consists of a half complex. All of three butyl groups
around the P atom are disordered, and each was refined in two
parts with their site occupation factors dependently refined.
Crystallographic parameters are listed in Table 1.The atomic
numbering is shown in Chart 1.
Polymer 3. To a mixture of cis-dichlorobis(tri-n-butylphosphine)-
platinum(II) (210 mg, 0.31 mmol) and diethynyl iptycene 4 (210
mg, 0.31 mmol) in toluene/i-Pr2NH (3:1, 10 mL), CuI (10 mg, 0.05
mmol) was added quickly, and the resulting solution was degassed
for 20 min with argon. The yellow solution was stirred at 65 °C
overnight. All volatile components were removed under reduced
pressure, and the residue was dissolved in dichloromethane and
filtered through a short alumina column. After removal of solvent
under reduced pressure, a yellow solid was collected. Further
purification was accomplished by precipitating the polymer solution
in dichloromethane from methanol. Polymer 3 (260 mg, 77%) was
obtained as a yellow solid. 1H NMR (300 MHz, CDCl3, spectrum
shown in Supporting Information): δ 0.79-0.94 (br m, 18 H),
1.36-1.58 (br m, 12H), 1.60-1.94 (br m, 12H), 2.21-2.49 (br m,
12H), 6.00-6.19 (br m, 4H), 6.82-7.03 (br m, 8H), 7.59-7.82
(br m, 8H). 31P NMR (121 MHz, CDCl3): δ 8.28 (J ) 2382.8
Hz). GPC (polystyrene standards): Mn ) 4350 g mol-1; Mw
)
Results and Discussion
5850 g mol-1; PDI ) 1.34.
Photophysical Measurements. Corrected steady-state emission
measurements were conducted on a SPEX F-112 fluorescence
spectrophotometer. Sample concentrations were adjusted to produce
optically dilute solutions (Amax < 0.20, c ≈ 1 µM) and optically
thick films (Amax ≈ 1.0). Fluorescence quantum yields were
determined according to the optically dilute method described by
Demas and Crosby, with the quantum yield being computed
according to eq 14 in their paper.38 Solutions of perylene and
9,10-dicyanoanthracene in ethanol were used in parallel as acti-
nometers (ΦF ) 0.89 for both in ethanol). Steady-state absorption
spectra were recorded on a Varian Cary 100 dual-beam spectro-
photometer. Transient absorption measurements were conducted on
an apparatus which has been described elsewhere.39 Excitation was
effected with the 3rd harmonic of a Nd:YAG laser (λ ) 355 nm,
10 ns fwhm, 10 mJ pulse-1). Sample concentrations were adjusted
so that A ≈ 0.8 at 355 nm. Time-resolved emission measurements
were recorded on an apparatus consisting of a Quanta Ray GCR
Series Nd:YAG laser as a source (3rd harmonic, λ ) 355 nm, 10
ns fwhm) with time-resolved detection provided by an intensified
CCD detector (Princeton Instruments, PI-MAX iCCD) coupled to
an Acton SpectraPro 150 spectrograph.
Polymer and Monomer Structures. The focus of this
investigation is to determine the effect of interchain interac-
tion on the spectroscopic properties of the triplet excited state
in Pt-acetylide-based π-conjugated polymers. To address
this problem, we sought to compare the optical properties
of the “parent” polymer, 2, with those of the iptycene-
substituted polymer 3 (see Chart 2 for structures). Polymer
2 features a repeat consisting of the trans-Pt(PBu3)2 unit
alternating with 1,4-diethynylbenzene. Due to the square-
planar geometry around the Pt(II) center, in the solid-state
polymer 2 can adopt an “all planar” conformation in which
strong interchain interactions might be expected. However,
due to the significant steric constraints imposed by the
iptycene unit in polymer 3, strong interchain interaction is
not expected for this material in the solid state.12,13 To a first-
order approximation, the electronic structure with respect to
the π-conjugated electronic system is the same in the two
polymers. However, as discussed in more detail below, due
to the bulky iptycene unit, there are likely to be differences
in conformation of the monomer units with respect to the
(38) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.
(39) Wang, Y.; Schanze, K. S. Chem. Phys. 1993, 176, 305.
(40) Sheldrick, G. M. SHELXTL6; Bruker-AXS: Madison, WI, 2000.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2621

Zhao et al.
Chart 1. Atomic Numbering for 1b
Chart 2
polymer backbone in 2 and 3, and these differences may
result in measurable differences in the optical properties of
the two polymers.
Å, crystal structure reported by Yang and Swager).13 Thus,
the difference in CtC bond lengths in 1b may be dictated
by the electronic properties of the iptycene unit, which is
likely to be more electron rich than a phenyl group lacking
alkyl substituents.
To provide more information concerning the structure of
the iptycene unit on the Pt-acetylides, we also prepared two
model monomers, 1a,b. Fortunately, these materials were
crystalline solids, and as a result it was possible to deduce
their structures by X-ray crystallography (see below).
X-ray Crystallography. The X-ray crystal structures of
compounds 1a,b were elucidated. The thermal ellipsoid plot
for 1b is shown in Figure 1 (top), while the corresponding
plot for 1a is included as Supporting Information. Detailed
crystal structure data for 1a,b is provided as CIF files in
Supporting Information, and selected structural parameters
for 1b are listed in Table 2. Compound 1b crystallizes in a
monoclinic crystal system and belongs to the C2/c space
group. As expected, the PtP2C2 unit is approximately square
planar, with a P-Pt-C bond angle of 89°.
Figure 1 (bottom) illustrates an end-on view of 1b in the
crystal, with the hydrogens removed and the butyl chains
Close inspection of the data in Table 2 reveals that there
is asymmetry with respect to the Pt-acetylide bonding.
Specifically, the bond lengths on the terminal phenyl
acetylene groups alternate strongly in distance, i.e., Pt-C1
is long (2.10 Å), C1-C2 is short (1.02 Å), and C2-C3 is
long (1.52 Å). By contrast, there is less bond length
alternation in the central acetylides: Pt-C9 (1.98 Å);
C9-C10 (1.22 Å); C10-C11 (1.42 Å). While this effect
could signal stronger d f π* back-bonding from Pt to the
central acetylide ligand, the C9-C10 bond is only slightly
longer than the CtC bond in the free diethynyl iptycene
ligand (as the -Si(CH3)3 derivative, the bond length is 1.215
Figure 1. Top: ORTEP diagram of 1b. Bottom: X-ray ORTEP diagram
of 1b viewed along the Pt-acetylide bond axis (crystallographic a-axis).
Only the first carbon atoms of the butyl groups on the phosphine ligands
are shown for clarity. (Note that angles θ1 and θ2 are defined in the diagram;
see text for a discussion.)
2622 Inorganic Chemistry, Vol. 44, No. 8, 2005

A Platinum Acetylide Polymer
Table 2. Selected Bond Lengths and Angles for Complex 1ba
bond
length/Å
bond angle
angle/deg
Pt-P1
2.291(2)
2.296(2)
2.104(7)
1.982(5)
1.025(9)
1.221(7)
1.527(10)
1.427(7)
C1-Pt-C9
P1-Pt-P2
C1-Pt-P1
C1-Pt-P2
C1-C2-C3
C9-C10-C11
179.3(3)
178.06(7)
89.6(2)
88.7(2)
176.8(10)
178.8(7)
Pt-P2
Pt-C1
Pt-C9
C1-C2
C9-C10
C2-C3
C10-C11
a Atomic numbering system is shown in Chart 1 (see Experimental
Section).
on the phosphine ligands truncated to improve clarity. This
view allows one to visualize the orientation of the planes
defined by the PtP2C2 units relative to the planes defined by
the iptycene phenylene unit and the terminal phenylene rings
(θ1 and θ2, respectively; see Figure 1, bottom). Interestingly,
the two terminal phenylenes lie in the same plane and the
two PtP2C2 units are in the same plane. However, each of
these sets of planar units is twisted relative to one another.
Specifically, the angle between the plane defined by the
iptycene phenylene and the PtP2C2 units is θ1 ) 60 ( 1°,
whereas the angle between the PtP2C2 units and the terminal
phenylenes is θ2 ) 77 ( 1°. The fact that these planes are
twisted relative to one another is consistent with observations
on other Pt-acetylide complexes which have been studied
by X-ray crystallography, where the planes are twisted by
≈60°.41,42 It is likely that steric interactions between the PBu3
ligands and the iptycene unit have an influence on the relative
orientation of the PtP2C2 units and the iptycene phenylene
ring. Computer-aided molecular modeling (Chem 3D, version
7.0) suggests that rotation of the units with respect to the
Pt-acetylide chain axis is limited by these steric interactions
to a range such that the planes defined by the PtP2C2 and
iptycene phenylene units remain twisted by >60°. It is
important to note that, despite the twisting, the π-electron
systems of the individual units of the Pt-acetylide chain are
conjugated via interactions with the (filled) dxy and/or dxz
orbitals on the Pt centers.43
Figure 2. Unit cell packing diagram for compound 1b. Only the first
carbon atom of each butyl chain on the phosphine ligands is shown for
clarity.
A view of the unit cell packing diagram for 1b is shown
in Figure 2. It is quite evident from this diagram that, due to
the large steric constraints imposed by the PBu3 groups (not
shown) and the iptycene units, the Pt-acetylide chains are
well-separated in the crystal. The separation distance between
the chains is exactly half of the c-axis length, or 11.782(1)
Å. This distance is sufficiently large so as to preclude any
significant π-π or d-π interactions between adjacent
monomers in the crystal. Thus, it is expected that the optical
properties of the molecules in the crystal should be very
similar to those for the isolated molecules in solution.
Photophysics in Solution. The left panel of Figure 3
compares absorption spectra for compound 1b and polymers
2 and 3 in THF solution. The spectra of all three materials
Figure 3. Normalized absorption (a) and photoluminescence (b) spectra
of polymer 2 (- - -), polymer 3 (s), and monomer 1b (‚‚‚) in THF solution.
Photoluminescence measurements were carried out on argon-deoxygenated
solutions. Concentrations: 2, 0.5 µM; 3, 1.0 µM; 1b, 5 µM.
are dominated by a strong and relatively broad band in the
near-UV region (≈350 nm), which is due to the long-axis
polarized π,π* transition of the Pt-acetylide chromophore.
Although the spectra are similar, close inspection reveals
quantitative differences among them. In particular, monomer
1b and polymer 3 feature an additional absorption band in
the 270-280 nm region arising from the nonconjugated aryl
(41) Davy, J.; Gunter, M. E.; Tiekink, E. R. T. Z. Kristallogr. 1998, 213,
483-486.
(42) Haskins-Glusac, K.; Ghiviriga, I.; Abboud, K. A.; Schanze, K. S. J.
Phys. Chem. B 2004, 108, 4969-4978.
(43) Emmert, L. A.; Choi, W.; Marshall, J. A.; Yang, J.; Meyer, L. A.;
Brozik, J. A. J. Phys. Chem. A 2003, 107, 11340-11346.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2623

Zhao et al.
Table 3. Photophysical Data for Monomer 1b and Polymers 2 and 3
THF soln
τp
solid state
PL λmax/nm
material
UV-vis λmax/nm
PL λmax/nm
Φp/%
PL/µsa
TA/µsb
UV-vis λmax/nm
τp PL/µs
1b
2
341
335
491
509
0.66
0.45
0.6
0.7
0.9
499
516
0.9 (90%)c
150 (10%)c
160
335
345
0.9
1.0
3
342
562
0.11
63
492
a Photoluminescence decay lifetime. b Biphasic emission decay; number in parentheses indicates relative amplitude of component. c Transient absorption
decay lifetime.
groups in the iptycene moiety. (Since there are 4 noncon-
jugated aryl units/molecular unit, this band is relatively
strong.) In addition, it is evident that the near-UV band in
polymer 2 is broader than in 1b or 3. The broadening of the
band in polymer 2 is believed to arise due to several factors.
First, since rotation of the phenylene units with respect to
the Pt-acetylide chain axis is relatively unrestricted in 2,
heterogeneity with respect to conjugation length along the
chain is expected to arise. By contrast, in polymer 3, steric
interactions between the PBu3 ligands and the iptycene
moiety constrain rotation around the chain axis. This gives
rise to less heterogeneity with respect to conjugation length.
A second factor which could contribute to broadening of the
absorption of polymer 2 may be the presence of interchain
aggregates. As indicated below, the existence of polymer
aggregates is suggested by the photoluminescence spectrum
of 2 in solution.
Another noteworthy feature in the absorption spectrum of
3 is the presence of a weak but well-defined band extending
from 375 to 430 nm. This band is believed to be due to the
spin-forbidden S0 f T1 transition. This assignment is
supported by the photoluminescence excitation spectrum of
the polymer obtained while monitoring the phosphorescence
emission (see Supporting Information). The excitation spec-
trum is similar to the absorption spectrum of the polymer,
and it clearly shows that the long wavelength band gives
rise to phosphorescence emission. Although the S0 f T1
transition is weak in the spectrum of monomer 1b, it is also
clear in its phosphorescence excitation spectrum (see Sup-
porting Information). The S0 f T1 assignment for the long
wavelength absorption in 3 is also supported by the fact that
the spin-forbidden S0 f T1 absorption is observed in
structurally similar Pt-acetylide systems.44,45 The reason that
the spin-forbidden absorption is more pronounced in the
iptycene-substituted polymer 3 compared to 1b is unclear.
Photoluminescence spectra of 1b, 2, and 3 at ambient
temperature in THF solution are illustrated in the right panel
of Figure 3, and emission band maxima, quantum yields,
and lifetimes are listed in Table 3. The emission of the three
materials is dominated by a phosphorescence band between
500 and 600 nm, but all three materials also show weak
fluorescence emission in the 400-500 nm region. The
fluorescence assignment for the blue emission is supported
by the fact that it decays very rapidly (τ < 200 ps). Although
intersystem crossing is likely to be very rapid in these
materials, fluorescence is still observed because the radiative
rate is high.32,45 The assignment of the longer wavelength
emission to phosphorescence is supported by time-resolved
emission spectroscopy (see below), which indicates its decay
lifetime is >0.5 µs in each material. The phosphorescence
emission maxima vary in the sequence 1b < 2 < 3, with
the latter polymer featuring the largest Stokes shift (λem
)
562 nm). The phosphorescence maxima of 1b and 2 are
similar to those observed for structurally related Pt-acetylide
oligomers and polymers,28,29,32,45 and therefore, the significant
Stokes shift observed for 3 appears to be unusual in this
respect. The phosphorescence band shape for 1b and 3 is
dominated by a single high-energy band (the 0-0 vibronic
band), with weaker bands at lower energy. The weaker bands
are believed to arise from a vibronic progression due to
coupling of the excitation to high-frequency modes in the
Pt-acetylide backbone and the aryl rings.29,30 By con-
trast, while the emission spectrum of 2 is also dominated
by the 0-0 band at 509 nm, there is a second peak with
λmax ≈ 570 nm, which is stronger than expected for a vi-
bronic band. We believe that this transition may be due to
emission from an interchain aggregate (see below). This
assignment is supported by the observation that the relative
intensity of the long-wavelength band increases with increas-
ing polymer concentration in THF solution (see Supporting
Information).
Time-resolved emission experiments carried out on poly-
mers 2 and 3 in THF solution provide more insight regarding
the origin of the emission (Figure 4a,b). At very early times
after laser excitation (0.5 µs), the emission of 2 is dominated
by a band with λmax ) 510 nm, which corresponds to the
high-energy band seen in the steady-state emission (Figure
3). However, this emission decays very rapidly (τ ≈ 0.9 µs),
and at later times a considerably weaker but much longer-
lived emission is observed (λmax ≈ 573 nm, τ ≈ 150 µs).
Note that the weak long-lived emission corresponds to the
low-energy band observed in the steady-state emission. We
3
attribute the short-lived emission to the π,π* exciton and
the red-shifted, long lifetime emission to a triplet aggregate
arising from interchain interactions. The emission of polymer
3 decays more uniformly and on a much longer time scale
compared to 2 (τ ≈ 160 µs). At all times following excitation
the emission is dominated by the band with λmax ≈ 562 nm,
which corresponds to the band maximum in the steady-state
emission spectrum.
(44) Staromlynska, J.; McKay, T. J.; Bolger, J. A.; Davy, J. R. J. Opt.
Soc. Am. B 1998, 15, 1731-1736.
(45) Rogers, J. E.; Cooper, T. M.; Fleitz, P. A.; Glass, D. J.; McLean, D.
G. J. Phys. Chem. A 2002, 106, 10108-10115.
2624 Inorganic Chemistry, Vol. 44, No. 8, 2005

A Platinum Acetylide Polymer
Figure 4. Time-resolved emission spectra obtained following 355 nm laser excitation: (a) polymer 2 in THF solution, c ) 0.5 µM (delay times, 0.5-400
µs; delay increment, 10 µs); (b) polymer 3 in THF solution, c ) 1 µM (delay times, 0.5-400 µs; delay increment, 10 µs); (c) polymer 2 spin-coated film
(delay times, 0.5-40 µs; delay increment, 1 µs); (d) polymer 3 spin-coated film (delay times, 0.5-40 µs; delay increment, 1 µs).
The important point with respect to the time-resolved data
is that it shows that the majority of the emission from
phenylene polymer 2 decays very quickly, leaving behind a
weak, long-lived emission, which we attribute to an inter-
chain aggregate. The rapid decay is believed to occur due
to quenching of the 3π,π* exciton by the interchain aggregate
state. In contrast, the emission from iptycene substituted
polymer 3 decays uniformly across the spectrum and con-
siderably more slowly than 2. We believe that the iptycene-
based polymer behaves differently because the steric con-
straints preclude the formation of interchain aggregates which
quench the intrachain exciton in 2.
The effect of the iptycene unit on the properties of the
triplet state is also manifest in the transient absorption of
the polymers. Transient absorption spectra of 1b and
polymers 2 and 3 were obtained in THF solution following
355 nm excitation (Figure 5). The spectra are qualitatively
similar, and they are characterized by strong bleaching of
the ground-state absorption in the near-UV and transient
absorption in the visible region. These spectra are similar to
those observed previously for Pt-acetylide complexes which
3
feature lowest π,π* excited states, and on this basis the
visible transient absorption is assigned to T1 f Tn transi-
tions.45,46 The transient absorptions decay with lifetimes that
are in reasonable agreement with those determined by time-
resolved photoluminescence (Table 3). Note that the transient
decay lifetime of 3 is considerably longer than that of 2 (63
vs 0.9 µs) which reinforces the notion that the iptycene unit
prevents triplet quenching due to interchain interactions.46
Figure 5. Transient absorption spectra obtained for samples in degassed
THF solution following 355 nm excitation: (a) polymer 2, 100 ns delay;
(b) polymer 3, 1 µs delay; (c) monomer 1b, 100 ns delay.
(46) Li, Y.; Whittle, C. E.; Walters, K. A.; Ley, K. D.; Schanze, K. S. In
Electronic, Optical and Optoelectronic Polymers and Oligomers;
Jabbour, G. E., Meijer, E. W., Sariciftci, N. S., Swager, T. M., Eds.;
Materials Research Society: Warrendale, PA, 2002; Vol. 665, pp 61-
72.
Another noteworthy feature is that the triplet-triplet absorp-
tion band for 2 is considerably broader compared to that of
Inorganic Chemistry, Vol. 44, No. 8, 2005 2625

Zhao et al.
Figure 6. Absorption (left) and photoluminescence (right) spectra of spin-cast films of polymer 2 (- - -) and 3 (s) and photoluminescence of a single
crystal of compound 1b (‚‚‚). The intensity of 1b is normalized to the maximum of emission of polymer 3. The spectra of 2 and 3 are unnormalized, and
therefore, their intensities reflect the relative quantum yields for the two polymers (the spectra were obtained with films having matched absorbance).
3. This feature may also arise because there is greater con-
formational heterogeneity within the conjugated backbone
of 2.
More information concerning the photoluminescence
properties of the polymer films comes from the time-resolved
emission (Figure 4c,d). At early times following excitation,
the emission of 2 features two bands: one at λmax ≈ 515 nm
and a second at lower-energy with λmax ≈ 575 nm. The high-
energy band decays quickly (τ , 1 µs) while the low-energy
band persists to longer times. Nevertheless, the overall
emission from the film decays relatively rapidly with τ ≈ 1
µs. Interestingly, the time-resolved emission from 3 is
considerably simpler. At all delay times the spectra appear
much the same as the steady-state emission (λmax ≈ 492 nm),
and the emission decays uniformly at all wavelengths with
τ ≈ 1 µs.
Photophysics in the Solid State. The absorption and
photoluminescence spectra of polymers 2 and 3 were
compared as spin-coated films, while the photoluminescence
of a single crystal of complex 1b was examined. Figure 6
shows the normalized absorption spectra of the films of
polymers 2 and 3. There is no significant change in the
absorption maxima compared to the solution absorbance;
however, the absorption bands of both polymer films are
broader compared to the solution spectra. The broadening
could arise due to conformational heterogeneity in the
polymer chains. Another point of note is the fact that the
low-energy tail which is assigned to the S0 f T1 transition
is also evident in the solid-state absorption of 3.
The right-hand side of Figure 6 compares the photolumi-
nescence spectra of films of 2 and 3 and the single crystal
of 1b. The emission from the polymers was measured using
films that had matched optical density at the excitation
wavelength (λex ) 340 nm), and the spectra in Figure 6 are
not normalized; therefore, they reflect the relative emission
quantum efficiencies from the two films. Several points are
of interest with respect to the solid-state emission spectra.
First, the phosphorescence emission from the single crystal
of 1b and the film of 3 are very similar in energy and band
shape. This feature strongly suggests that the triplet exciton
in the solid state of polymer 3 experiences an environment
that is very similar to that of the single crystal of 1b. Second,
the phosphorescence from 3 is considerably stronger com-
pared to that of 2. In addition, while the phosphorescence
from 3 displays a well-defined structure, the emission from
2 is broad, and the intensities of the high- and low-energy
bands (λ ) 516 and 575 nm, respectively) are almost the
same. Finally, it is clear that the fluorescence emission from
both polymer films (λmax ≈ 410 nm) is nearly as efficient as
the phosphorescence. This contrasts to the observations on
the polymers in solution, where the phosphorescence emis-
sion was considerably more intense compared to the fluo-
rescence. It is believed that the latter effect is likely due to
the phosphorescence efficiency being lower (relative to the
fluorescence) in the solid state materials, possibly due to
triplet-triplet annihilation (see below).
In general, the photoluminescence decay lifetimes of the
solid polymers are shorter than when the materials are
dissolved in dilute solution. While triplet quenching by
interchain aggregates may play a role in quenching of
polymer 2, given the significant reduction in lifetime
observed for 3, it is possible that triplet-triplet annihilation
also plays a role in reducing the lifetime of the triplet in the
films.
Aggregate Effects on the Triplet State in Pt-Acetylides.
The photophysical studies presented herein indicate that
interchain interactions have an influence on the properties
of the triplet exciton in Pt-acetylides. In particular, the
steady-state and time-resolved photoluminescence studies of
2 provide evidence for emission from an interchain aggregate,
both in solution and in the solid state. The photoluminescence
data on polymer 3 indicate that interchain interactions are
effectively disrupted by the bulky iptycene unit. The most
dramatic evidence that interchain interaction is precluded in
the iptycene-substituted polymer comes from the fact that
the phosphorescence of solid state 3 can almost be super-
imposed onto that of the single crystal of monomer 1b.
An important question concerns the structure of the
interchain aggregate(s) which give rise to the red-shifted
emission observed from polymer 2 at long delay times
following excitation in solution and in the solid state. It seems
likely that within the aggregate the Pt-acetylide chains adopt
a conformation such that the planes defined by the Pt(PBu3)2-
(CtC-Ar)2 and phenylene units are coplanar. Two or more
chains in this conformation can come into close approach
allowing direct π-π interactions between the phenylene
2626 Inorganic Chemistry, Vol. 44, No. 8, 2005

A Platinum Acetylide Polymer
residues. Although there is little in the chemical literature
concerning the photophysics of triplet excimers of organic
aromatic hydrocarbons,47-50 the available evidence47,48,50
suggests that π-π interactions lead to a decrease in the
T1-S0 energy gap as reflected by a decrease in the excimer
phosphorescence energy relative to the monomer phospho-
rescence energy. (Note that this decrease in the phospho-
rescence energy could arise due stabilization of T1 or
destabilization S0.) Thus, the red-shifted emission observed
from polymer 2 in solution and in the solid could arise due
to π-π interactions which either lead to stabilization of T1
or destabilization of S0.
An alternate possibility is that interchain interaction in
polymer 2 leads to a low-energy excited state arising from
(metal-metal) d-p interactions. Specifically, it is well-
established that in face-to-face dimers of d8-d8 metal
complexes there exists a manifold of energetically low-lying
excited states arising from a dσ* f pσ transition.35 The
prototypical example of this interaction is provided by the
complex [Pt2(µ-P2O5H2)4]4- (Pt-pop), which is a d8-d8
complex with a Pt-Pt separation distance of 2.95 Å.35
Pt-pop exhibits phosphorescence at 514 nm from a triplet
excited-state arising from the dσ* f pσ configuration. Yam
and co-workers reported a low-energy phosphorescence (λmax
≈ 600 nm, τ ≈ 20 µs) from a Pt-acetylide dimer in which
two trans-Pt(PR3)2(CtCAr)2 units are held in a face-to-face
orientation with a Pt-Pt separation distance of ≈3.4 Å.51
They ascribe the luminescence to a triplet state arising from
a dσ* f pσ/π* configuration (where pσ/π* is an orbital
having mixed Pt p and acetylide π* character). The mixed
pσ/π* character of the acceptor ligand in the Pt-acetylide
dimer lowers the energy of the transition relative to its
position in Pt-pop, despite the fact that the Pt-Pt separation
distance is longer in the acetylide dimer.
Summary and Conclusion
This investigation has examined the effect of aggregation
on the properties of the triplet exciton in two Pt-acetylide
polymers, 2 and 3. Parent polymer 2 features a π-conjugated
backbone in which the organic repeat unit is 1,4-diethynyl-
benzene. The photoluminescence of this polymer in solution
and in the solid state consists of two bands, and the lower
energy band is believed to arise from an interchain aggregate.
The photoluminescence of polymer 3, which is structurally
similar to 2 but contains a sterically demanding iptycene unit,
is dominated by a single emission band, both in solution and
in the solid state. The emission from 3 is believed to arise
from the triplet exciton. No evidence for interchain interac-
tion is present in the photoluminescence spectrum of 3,
presumably because the sterically demanding iptycene units
prevent the Pt-acetylide chains from approaching suf-
ficiently to allow strong interchain interaction. Interestingly,
the phosphorescence of 3 in the solid state is virtually
superimposable with the emission of a single crystal of
iptycene-substituted monomer 1b. The X-ray crystal structure
packing diagram of 1b shows clearly that the Pt-acetylide
chains are well-separated (≈12 Å) due to the sterically
demanding iptycene units. Although it is likely that the
packing in the solid state of the iptycene-substituted polymer
3 is less ordered than in the crystal of 1b, the average
interchain separation distance in the solid polymer is probably
similar to that of the single crystal of the monomer.
Two possible mechanisms are considered to explain the
origin of the aggregate emission in 2. The first involves
stabilization of T1 (or destabilization of S0) due to interchain
π-π interactions. The second mechanism involves interchain
Pt-Pt interaction leading to a manifold of low-energy excited
states based on a dσ* f pσ/ π* transition. Studies in progress
on structurally well-defined Pt-acetylide complexes in which
a 1,8-naphthalene unit is used as a scaffold to bring two
Pt-acetylide chains into close proximity will hopefully
provide more insight concerning the dominant mechanism
for interchain interactions in this class of materials.
On the basis of this work, we suggest that the low-energy
aggregate emission observed from 2 may arise from a dσ*
f pσ/π* state that is produced by face-to-face Pt-Pt
interaction in the interchain aggregate. Although the PBu3
groups may hinder close approach of two Pt centers, packing
forces in the solid may be sufficient to overcome the steric
hindrance. Importantly, the low-energy emission observed
from 2 occurs at a slightly higher energy compared to the
dσ* f pσ/π* emission in the face-to-face Pt-acetylide dimers
studied by Yam and co-workers. This suggests that if the
dσ* f pσ/π* assignment for the low-energy emission of 2
is correct, then the Pt-Pt separation distance in the ag-
gregates is likely >3.4 Å.
Acknowledgment. We gratefully acknowledge the Air
Force Office of Scientific Research (Grant No. F49620-03-
1-0127) for support of this work. K.A.A. wishes to acknowl-
edge the National Science Foundation and the University of
Florida for funding of the purchase of the X-ray equipment.
We also express our appreciation to Prof. Timothy Swager
for a sample of the diethynyl iptycene (compound 4) which
was used in preliminary synthetic work.
Supporting Information Available: X-ray crystal structure data
and thermal ellipsoid plot for compound 1a, H NMR spectra of
1b, 2, and 3, emission spectra of 2 in THF at various polymer
concentrations, emission excitation spectra of 1b and 3, and
crystallographic data for 1a,b in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(47) Langelaar, J.; Rettschnick, R. P. H.; Lambooy, A. M. F.; Hoytink, G.
J. Chem. Phys. Lett. 1968, 1, 563-565.
1
(48) Gijzeman, O. L. J.; Langelaar, J.; Van Voorst, J. D. W. Chem. Phys.
Lett. 1970, 5, 269-272.
(49) Lim, E. C. Acc. Chem. Res. 1987, 20, 8-17.
(50) Yamaji, M.; Tsukada, H.; Nishimura, J.; Shizuka, H.; Tobita, S. Chem.
Phys. Lett. 2002, 357, 137-142.
(51) Yam, V. W.-W.; Yu, K.-L.; Wong, M.-C.; Cheung, K.-K. Organo-
metallics 2001, 20, 721-726.
IC048961S
Inorganic Chemistry, Vol. 44, No. 8, 2005 2627