Phosphorescent platinum acetylide organogelators

Article abstract of DOI:10.1021/ja0765316

The series of platinum acetylide oligomers (PAOs) with the general structure trans,trans-[(RO)₃Ph-C≡C-Pt(PMe₃) ₂-C≡C-(Ar)-C≡C-Pt(PMe₃)₂-C≡C- Ph(OR)₃], where Ar = 1,4-phenylene, 2,5-thienyle

Full text of DOI:10.1021/ja0765316

Published on Web 01/31/2008
Phosphorescent Platinum Acetylide Organogelators
Thomas Cardolaccia, Yongjun Li, and Kirk S. Schanze*
Department of Chemistry, UniVersity of Florida, P.O. Box 117200, GainesVille, Florida 32611
Received August 29, 2007; Revised Manuscript Received December 13, 2007; E-mail: kschanze@chem.ufl.edu
Abstract: The series of platinum acetylide oligomers (PAOs) with the general structure trans,trans-
[(RO)3PhsCtCsPt(PMe3)2sCtCs(Ar)sCtCsPt(PMe3)2sCtCsPh(OR)3], where Ar ) 1,4-phenylene,
2,5-thienylene, or bis-2,5-(S-2-methylbutoxy)-1,4-phenylene and R ) n-C12H25 gel hydrocarbon solvents
at concentrations above 1 mM. Gelation is thermally reversible (Tgel-sol ≈ 40-50 °C), and it occurs due to
aggregation of the PAOs resulting in the formation of a fibrous network that is observed for dried gels
imaged by TEM. The influence of aggregation/gelation on the photophysical properties of the PAOs is
explored in detail. Aggregation induces a significant blue shift in the oligomers’ absorption spectra, and the
shift is attributed to exciton interactions arising from H-aggregation of the chromophores. Strong circular
dichroism (CD) is observed for gelled solutions of a PAO substituted with homochiral S-2-methylbutoxy
side chains on the central phenylene unit. The CD is attributed to formation of a chiral supramolecular
aggregate structure. The PAOs are phosphorescent at ambient temperature in solution and in the aggregate/
gel state. The phosphorescence band is blue-shifted ca. 20 nm in the aggregate/gel, and the shift is assigned
to emission from an unrelaxed conformation of the triplet excited state. Phosphorescence spectroscopy of
mixed aggregate/gels consisting of a triplet donor/host oligomer (Ar ) 1,4-phenylene) doped with low
concentrations of an acceptor/trap oligomer (Ar ) 2,5-thienylene) indicates that energy transfer occurs
efficiently in the aggregates. Triplet energy transfer involves exciton diffusion among the host chromophores
followed by Dexter exchange energy transfer to the trap chromophore.
Introduction
van der Waals, hydrogen-bond, and solvophobic interactions
among the constituent units.3
The performance of organic electronic devices such as light
emitting diodes, photovoltaic cells, and field effect transistors
that rely on π-conjugated oligomers or polymers as active
materials depends on the intrinsic optical and electronic proper-
ties of the constituent molecules as well as intermolecular
interactions and morphology of the solid-state material. While
molecular optical and electronic properties have received the
most attention, supramolecular interactions and material mor-
phology are arguably more important in determining the
performance of an organic material in device applications.1,2
Controlling self-assembly of π-conjugated molecular systems
is important as it provides an avenue to organic materials having
desired optical, electronic, and optoelectronic properties.3 Recent
investigations of self-assembly of π-conjugated polymers and
oligomers into supramolecular aggregates and gels have pro-
vided considerable insight into the structural factors that are
important in producing assemblies with long range order.4-42
Self-assembly of π-conjugated systems occurs mainly due to
Suitably functionalized derivatives of oligo(phenylene
vinylene)4,6-8,10,11 and oligo(phenylene ethynylene)14,15 self-
assemble in solution, producing helical or lamellar supramo-
lecular structures that sometimes induce gelation. Many of these
supramolecular structures are chiral, as evidenced by the
observation of strong ellipticity in circular dichroism spectra.3,6,20
The supramolecular aggregates and gels consisting of π-con-
jugated oligomers provide ideal platforms for the study of
phenomena that are usually only encountered in solid-state
materials. For example charge carrier mobility,25,43,44 exciton
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2535

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Cardolaccia et al.
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Phosphorescent Platinum Acetylide Organogelators
A R T I C L E S
and exciton transport in aggregates and films have been
reported.65 The PAOs that are the subject of this investigation
provide an ideal platform for the study of triplet exciton transport
and energy transfer. In particular, as described in this article,
supramolecular aggregates consisting of Pt2M exhibit moder-
ately intense phosphorescence from the platinum acetylide
chromophore at ca. 500 nm. Interestingly, mixed aggregates
consisting of Pt2M as the host with small amounts of Pt2MT
exhibit phosphorescence emission predominantly from the latter
chromophore, which emits at ca. 600 nm. Efficient sensitization
of the phosphorescence from the low energy Pt2MT chro-
mophores occurs via a mechanism involving triplet exciton
diffusion among the aggregated Pt2M chromophores followed
by energy transfer to the low energy Pt2MT acceptors. This
study is the first to explore the properties of the triplet state
and exchange energy transfer in supramolecular aggregates
consisting of π-conjugated chromophores.
Figure 1. (a) Photograph taken of vial containing a gelled solution of Pt2M
in dodecane (c ) 1 mM). The photograph was taken with near-UV
illumination, and a UV-blocking filter was placed in front of the camera
lens. (b) Transmission electron microscope image of xerogel of Pt2M
obtained by evaporation of dodecane solvent under vacuum, white scale
bar is 2 µm in length. Inset shows the same sample under increased
magnification; the white scale bar is 500 nm in length.
Experimental Section
Materials. Complete details concerning the synthesis and spectro-
scopic characterization of the platinum-acetylide oligomers are provided
as Supporting Information. Solvents used for the spectroscopic
measurements were spectral quality and were used without further
purification.
gel concentration is approximately 1 × 10-3 M in dodecane
and hexane for Pt2M and Pt2MT (∼2 wt %), while it is 4 ×
10-3 M for Pt2MC. For Pt2M and Pt2MT the gels formed
quickly upon cooling a hot concentrated solution; however,
gelation took approximately 3 days for a solution of Pt2MC in
dodecane. Upon heating to ca. 50 °C, the gels melt to a clear
nonviscous solution that could be reversibly brought back to
the gel state upon cooling.
As shown in Figure 1, the gelated dodecane solution of Pt2M
is rigid enough such that its flow is suppressed when the
container is inverted.66 The gelated material exhibits strong
photoluminescence, and the spectroscopic properties of the
emission are discussed below. In order to provide more insight
into the morphology of the gelated Pt2M solution, TEM images
were obtained on a zerogel sample that was prepared by drying
of a film of a gel prepared in hexane. As can be seen in Figure
1b, the xerogel features a complex network of fibers consisting
of the aggregated Pt2M molecules. The high-resolution image
(inset, Figure 1b) shows that on average the widths of the
individual fibers range from 50 to 200 nm. This suggests that
the individual fibers are likely composed of many individual
subfibers that consist of arrays of stacked Pt2M molecules.
Close inspection of the enlarged TEM image suggests that the
fibers may have a ribbon structure, which is consistent with
the oligomers packing in a lamellar aggregate structure (Vide
infra).67
Photophysical Measurements. Steady-state absorption spectra were
recorded on a Varian Cary 100 dual-beam spectrophotometer. Samples
were placed in a cell with a path length sufficiently short so as to
maintain the absorption below 1.0 (1 or 0.1 mm path length). Corrected
steady-state photoluminescence measurements were conducted on a
SPEX F-112 fluorescence spectrometer. Samples were degassed by
argon purging for 30 min and heated to a solution state regularly during
this time for adequate degassing of gel-forming solutions. Samples were
placed in a triangular-shaped cell that was positioned in the spectrometer
so that the incident beam was at 45° from the face of the cell and
emission detected at 45° (front-face geometry to minimize the effects
of self-absorption). Time-resolved emission measurements were re-
corded on a home-built apparatus consisting of a nitrogen laser as a
source (λ ) 337 nm, 5 ns fwhm), with time-resolved detection provided
by an intensified CCD detector (Princeton Instruments, PI-MAX iCCD)
coupled to an Acton SpectraPro 150 spectrograph. Time-resolved
spectra were analyzed by Global Kinetic Analysis using the Specfit/
32 software package (Biologic SAS, www.bio-logic.info/rapid-kinetics/
specfit.html). Circular dichroism (CD) spectroscopy was carried out
on an Aviv-202 circular dichroism spectrometer with a cell path length
of 0.3 mm.
Results
Chemical Structures, Gel Formation, and Gel Structure.
The three platinum acetylide oligomers (PAOs) shown in Chart
1 are the focus of the work described herein. Each complex
features an aromatic core moiety sandwiched between two trans-
Pt(P2)(C2) units, and the oligomer ends are capped with tri-
(dodecyloxy)-substituted benzenes. Each of the PAOs induces
gelation of hydrocarbon solvents such as n-dodecane, hexane,
and cyclohexane.
Gelation was accomplished by mixing the PAO and solvent
at ambient temperature, followed by warming the mixture to
ca. 60 °C to produce a clear solution, followed by cooling to
ambient temperature. Gelation typically occurred while the
solution cooled (except for Pt2MT, see below). The critical
UV-visible Absorption Spectroscopy. Absorption spectra
of the PAOs were measured in dodecane solution, and in each
case the spectra were temperature- and concentration-dependent.
For solutions with concentrations above 0.1 mM, the absorbance
was maintained below a value of 1.0 by using short path length
(1 mm or 0.1 mm) cells as necessary.
Figure 2a illustrates normalized absorption spectra of solutions
of Pt2M in dodecane at three concentrations (1 × 10-4, 5 ×
10-4, and 1 × 10-3 M). The solutions were subjected to a
(66) The Pt2M/dodecane gel has the consistency of “jello”. By comparison,
the dodecane gels produced by Pt2MT and Pt2MC were somewhat less
stiff, and with agitation they would “run”. All of the gels exibited a slight
opacity due to light scattering.
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A R T I C L E S
Cardolaccia et al.
(λmax ) 358 nm). (Over the same temperature range the sample
undergoes a gel-sol transition.) It is also significant that there
is an approximate isosbestic point maintained during the
thermally induced transition, suggesting that the spectra are
dominated by two principal forms of the oligomer, i.e., monomer
at high temperature and H-aggregate at low temperature (gel).
Note that the most significant changes in the absorption spectrum
occur over the 40-60 °C temperature range, which brackets
the temperature range of the sol-gel transition for Pt2M in
dodecane. Similar concentration- and temperature-dependent
behavior was observed for the absorption spectrum of Pt2MT
sol
gel
in dodecane (λmax ) 378 nm and λmax ) 338 nm, see
Supporting Information Figure S-1).
While Pt2MC also has the ability to gel dodecane, gelation
occurs considerably more slowly than that for Pt2M and
Pt2MT. The slow dynamics of aggregation/gelation are also
apparent by the fact that the absorption spectral changes
characteristic of the aggregate take several days to develop. As
shown in the Supporting Information (Figure S-2), freshly
prepared solutions with c ) 1 × 10-4, 1 × 10-3, and 4 × 10-3
M exhibit nearly identical absorption spectra, with λmax ) 368
nm (each solution was subjected to a heating-cooling cycle
immediately prior to measuring the absorption spectrum).
However, after standing for a period of several days the more
concentrated solution gels, accompanied by a blue-shift of the
UV absorption band to λmax ) 345 nm. The appearance of the
gel/aggregate spectrum for Pt2MC is qualitatively similar to
that of Pt2M and Pt2MT, suggesting that this complex also
forms an H-aggregate. However, it is interesting that the spectral
changes associated with H-aggregation of Pt2MC are less
pronounced compared to the other two oligomers, suggesting
that the exciton interactions are weaker. This is likely due to
steric factors arising from the presence of the branched (and
chiral) side groups present in Pt2MC.
Figure 2. Absorption spectra of Pt2M in dodecane solution (or gel). (a)
Room temperature. (b) c ) 10-3 M. Arrow indicates change in spectra
with increasing temperature from 21 to 66 °C. The heating rate was
approximately 1 °C min-1, and spectra were acquired within 1 min after
the temperature stabilized.
heating/cooling cycle before the spectra were obtained, but only
the 1 × 10-3 M solution is in the gel state. The absorption
spectrum of Pt2M in the dilute solution at 1 × 10-4 M (and
below, data not shown) is dominated by a strong band with
λmax ) 358 nm, with weaker transitions at higher energy (Figure
2a). The dominant absorption band is assigned to the long-axis
polarized π,π* transition of the molecularly dissolved (mono-
meric) chromophore.61,62 Interestingly, when the concentration
of Pt2M is increased above 10-4 M, the spectrum broadens
and the primary oscillator strength shifts considerably to the
blue. The spectrum of the solution with c ) 5 × 10-4 M exhibits
residual absorption in the region where the monomeric chro-
mophore absorbs (shoulder at λ ≈ 365 nm). By contrast, the
spectrum of the gel of Pt2M with c ) 1 × 10-3 M exhibits
λmax ≈ 304 nm and there is very little residual absorption in
the region where the monomer absorbs (λ > 350 nm). It is
important that the spectrum observed for the Pt2M/dodecane
gel is very similar to the spectra of H-aggregates of linear
π-conjugated chromophores.40,41,68
Circular Dichroism Spectroscopy. Following the initial
observation that Pt2M aggregates hydrocarbon solvents, pre-
sumably due to aggregation of the oligomers, we designed the
structurally analogous oligomer Pt2MC with the objective of
using CD to provide evidence for the existence of the supramo-
lecular aggregates as well as to provide insight into structure
of the aggregate.3,4,12-14,20 Pt2MC has the same basic structure
as Pt2M, with the exception that the central phenylene unit is
substituted with optically active S-2-methylbutoxy side chains.
We reasoned that by analogy to the chiral oligo(phenylene
vinylene) aggregates3,4,12-14,20 the supramolecular aggregates of
Pt2MC would exhibit strong CD signals.
In initial experiments with Pt2MC it was observed that a
freshly prepared, thermally cycled solution of the oligomer (c
) 4 × 10-3 M) exhibits only a very weak CD signal (see Figure
3). However, as noted above, a freshly prepared solution of
Pt2MC is not a gel, and the solution does not exhibit the
absorption spectrum characteristic of the H-aggregate. However,
if the thermally cycled solution is allowed to stand for ca. 3
days, the solution gels, and remarkably the gelled solution
exhibits the strong CD spectrum shown in Figure 3. Several
features are significant with this experiment. First, it clearly
demonstrates that the CD signal is not a property of the
monomeric chromophoressthe signal is only observed when
the sample is gelled and when the H-aggregate absorption
spectrum is present. Second, the observation of the CD signal
Figure 2b illustrates the temperature dependence of the
absorption spectrum of the Pt2M gel (solution) with c ) 10-3
M in dodecane. Upon heating the sample from 21 to 66 °C the
absorption spectrum changes from that characteristic of the gel
(λmax ≈ 304 nm) to that of the monomeric form of the complex
(68) Schenning, A. P. H. J.; Kilbinger, A. F. M.; Biscarini, F.; Cavallini, M.;
Cooper, H. J.; Derrick, P. J.; Feast, W. J.; Lazzaroni, R.; Leclere, P.;
McDonell, L. A.; Meijer, E. W.; Meskers, S. C. J. J. Am. Chem. Soc. 2002,
124, 1269-1275.
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Figure 3. Circular dichroism spectra of Pt2MC in dodecane. Freshly
prepared solution (s); gelled solution, 3 days after preparation
(- - -).
for the Pt2MC aggregate signals that there is strong exciton
coupling among the oligomer chromophores within a chiral,
supramolecular environment provided by the aggregate.
The CD spectrum for the Pt2MC aggregate/gel primarily
exhibits negative ellipticity throughout the near-UV spectral
region where the aggregate absorbs. In particular, the CD
features negative band maxima at 352, 295, and 272 nm, and
each of these bands corresponds closely to the maxima of
specific bands in the absorption spectrum of the Pt2MC
H-aggregate (see Figure S-2). The observation of primarily
negative ellipticity in the aggregate spectrum of Pt2MC
contrasts with the CD spectra typically observed for the helical
aggregates formed by chiral oligo(phenylene vinylene)s that
have been previously studied.3,4,12-14,20 In the latter case the
CD spectra exhibit the bisignate CD signature typical for exciton
coupling among a helical array of π-conjugated chromophores.
The unusual CD spectrum observed for the Pt2MC aggregate/
gel likely signals that the aggregate structure is not a helix.
Although sufficient evidence does not exist to pinpoint the
aggregate structure, it is possible that the negative ellipticity
signals that the oligomers pack into a lamellar structure in the
aggregate. The environment within the aggregate may be
provided by a supramolecular twist in the lamellar structure or
alternatively due to packing of the oligomers into a chiral subunit
within the aggregate structure.40,41
Steady-State Photoluminescence Spectroscopy. Photolu-
minescence spectra of the platinum acetylide oligomers were
measured in argon outgassed solutions (or gels). For the gels,
the samples were warmed while degassing to prevent gel
formation. Following degassing the solutions were cooled to
produce degassed gels.
Figure 4 provides photoluminescence spectra of Pt2M
obtained in dodecane gel (or solution) as a function of excitation
wavelength, concentration, and temperature.69 First, in all cases
the Pt2M photoluminescence is dominated by a moderately
efficient emission that is Stokes shifted significantly from the
π,π* absorption band. The emission has a lifetime in the
microsecond time domain (Vide infra), and on this basis it is
Figure 4. Photoluminescence spectra of Pt2M in deoxygenated dodecane.
(a) c ) 1 × 10-3 M. (b) λex ) 326 nm. (c) λex ) 326 nm and c ) 1 × 10-3
M, from 23 to 61 °C. The arrow indicates the spectral change with increasing
temperature. The heating rate was approximately 1 °C min-1, and spectra
were acquired within 1 min after the temperature stabilized.
assigned to phosphorescence from the 3π,π* state of the
oligomer. The Pt2M phosphorescence is very similar in energy
and band shape to the emission of structurally similar PAOs
previously reported.61,62 As shown in Figure 4a, the phospho-
rescence spectrum of Pt2M in the gel (1 × 10-3 M) depends
on the excitation wavelength. In particular, with an excitation
wavelength that corresponds to the absorption maximum for
the gel (λ ) 326 nm, cf. Figure 2a), the 0-0 band maximum
of the phosphorescence is at 495 nm. However, with an
excitation wavelength of 366 nm which corresponds to the
absorption maximum for the molecularly dissolved state of the
oligomer (cf. Figure 2a), the 0-0 band red-shifts to 516 nm.
With an intermediate excitation wavelength λ ) 346 nm, the
phosphorescence consists of a broad manifold with two maxima
at λ ) 495 and 516 nm.
Several additional photoluminescence experiments provide
insight concerning the origin of the excitation wavelength
dependence seen for the Pt2M gel phosphorescence spectrum.
First, as shown in Figure 4b, with excitation at 326 nm
(corresponding to the absorption maximum of the aggregate)
the 0-0 band of the phosphorescence appears at 516 nm for
the most dilute solution (c ) 1 × 10-5 M), whereas it blue-
shifts to 495 nm for the two more concentrated solutions (1 ×
10-4 and 1 × 10-3 M). Second, as shown in Figure 4c, warming
a gelled solution of Pt2M (1 × 10-3 M) induces the 0-0
phosphorescence band to shift from 495 to 516 nm. The
thermally induced red-shift in the phosphorescence is clearly
(69) Measurement of quantum yields and excitation spectra was precluded
because the samples had a very high optical density and the emission
experiments were carried out in a front-face geometry.
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A R T I C L E S
Cardolaccia et al.
associated with the gel-sol transition. On the basis of these
observations we attribute the “blue” phosphorescence (λmax
)
495 nm) to the Pt2M aggregates present in the gel and the “red”
phosphorescence (λmax ) 516 nm) to the oligomer in the
unaggregated (molecularly dissolved) state.
Several points are of interest with respect to the phospho-
rescence properties of Pt2M. First, the phosphorescence of both
the aggregated and molecularly dissolved oligomers has a similar
band shape, appearing as a distinct 0-0 transition with a
shoulder on the low-energy side due to vibronic coupling. The
important point is that the phosphorescence band shape is not
strongly perturbed by close packing of the chromophores in the
aggregates. This is in distinct contrast with the behavior typically
seen when fluorescent chromophores are packed into an
aggregate. In this case, the fluorescence spectra of the chro-
mophores are strongly perturbed by exciton and eximer interac-
tions, which signals that there is strong interaction between
aggregated chromophores in the singlet excited state.4,10 The
fact that the aggregated state of Pt2M exhibits only a small
shift in the phosphorescence energy is consistent with the notion
that in the triplet excited state the oligomers interact only weakly
with other chromophores in the aggregate. Importantly, there
is no evidence for the existence of a triplet excimer in the Pt2M
aggregates. This finding is consistent with previous studies
which suggest that triplet excimer formation is not energetically
favorable for aromatic chromophores.70-72
A second feature of interest is the fact that the changes in
the phosphorescence spectrum that accompany the thermally
induced gel-sol transition strongly suggest that there are two
distinct “states” for the excited-state oligomer that exist in the
gel and sol environments. In particular, the series of spectra
shown in Figure 4c feature an isoemissive point, suggesting that
as temperature increases (and the system undergoes the gel-
sol conversion), the oligomers exist in two distinct states which
give rise to either the 495 or 516 nm phosphorescence. On the
basis of previous studies by our group on the temperature
dependence of the phosphorescence emission of PAOs, we
believe that the two states that give rise to the different
phosphorescence emissions correspond to intramolecular con-
formations that differ with respect to the orientation of the
phenylene and square planar Pt(P2)(C2) units.62 This issue will
be considered more fully below in a later section.
Although the photoluminescence of Pt2MT was not studied
extensively, qualitatively its emission responds to aggregation
in a similar manner as that observed for Pt2M. First, in degassed
solution Pt2MT (c ) 5 × 10-5 M) exhibits phosphorescence
that appears as a broad band with λmax ) 602 nm (see Figure
S-3 in the Supporting Information). In the dodecane gel (c ) 1
× 10-3 M) the phosphorescence band shape remains the same,
but the band maximum shifts slightly to the blue (λmax ) 595
nm).
Figure 5. Time-resolved emission spectra of Pt2M in deoxygenated
dodecane following pulsed excitation at λex ) 337 nm. (a) c ) 1 × 10-5
M, 100 ns camera delay, 5.2 µs delay increment; (b) c ) 1 × 10-3 M, 100
ns camera delay, 10.5 µs delay increment; (c) Principal components of
emission decay of Pt2M at 1 × 10-3 M in dodecane for fast component τ
) 9 µs (- -) and slow component τ ) 59 µs (s).
excitation wavelength corresponds closely to the absorption of
the Pt2M aggregate), the emission was monitored using an
intensified CCD camera/spectrograph system, and the emission
decay kinetics were analyzed by global analysis73 of the time-
resolved spectra.
Figure 5 illustrates the time-resolved emission data, and the
decay parameters recovered from global analysis of the data
are listed in Table 1. First, Figure 5a illustrates the time-resolved
emission of Pt2M in a dilute (1 × 10-5 M) dodecane solution.
The spectra were obtained with a delay increment of ca. 5 µs
and the series of spectra shown in the figure were obtained over
a time scale of ca. 50 µs. The emission spectrum does not change
with time, and the λmax (517 nm) and band shape are identical
to those observed in the steady-state spectrum obtained for the
dilute Pt2M solution where the emission occurs from the
molecularly dissolved state of the oligomer. The emission decays
as a single exponential with a lifetime of 10 µs, which is
comparable to the lifetime of structurally similar PAOs.61
Figure 5b illustrates time-resolved spectra for Pt2M in a
degassed dodecane gel (c ) 1 × 10-3 M). These spectra were
Time-Resolved Emission Spectroscopy. In order to provide
further insight concerning the effects of aggregation on the triplet
state of Pt2M, time-resolved photoluminescence experiments
were carried out. In these studies the excitation source was a
nitrogen laser (λ ) 337 nm, 5 ns pulse width; note that this
(70) Lim, E. C. Acc. Chem. Res. 1987, 20, 8-17.
(71) Yanagidate, M.; Takayama, K.; Takeuchi, M.; Nishimura, J.; Shizuka, H.
J. Phys. Chem. 1993, 97, 8881-8888.
(72) Yamaji, M.; Tsukada, H.; Nishimura, J.; Shizuka, H.; Tobita, S. Chem.
Phys. Lett. 2002, 357, 137-142.
(73) Stultz, L. K.; Binstead, R. A.; Reynolds, M. S.; Meyer, T. J. J. Am. Chem.
Soc. 1995, 117, 2520-2532.
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Phosphorescent Platinum Acetylide Organogelators
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Table 1. Phosphorescence Decay Lifetimes in Dodecane Solution or Gela
complex
Pt2M
Pt2MT
Pt2M/Pt2MT
concn/M
lifetime/µs
(amplitude%)
1 × 10-5
1 × 10-4
1 × 10-3 b
59 (22%)
9 (78%)
1 × 10-5
1 × 10-3 b
1 × 10-3/5 × 10-5 b
10
11
5.2
23
5 (71%)
41 (29%)
b
a Lifetimes in microseconds. Values in parentheses indicate relative amplitude of component. Dodecane gel.
obtained with a delay increment of ca. 10 µs, and the series of
spectra shown were obtained over a time scale of ca. 120 µs.
Interestingly, it can be seen that the shape of the spectrum
evolves with timesthe first few spectra (for t < 30 µs) are
dominated by a band with λmax ) 517 nm, whereas at later delay
times the spectra are dominated by the blue-shifted band with
λmax ) 495 nm. Note that the longer-lived, blue-shifted emission
band corresponds to the aggregated oligomer, while the red-
shifted emission corresponds to the monomeric state. Global
analysis of the time-resolved data shows that the emission decays
with two exponential components (τ ) 9 and 59 µs), and the
spectra of the two principal emission decay components
recovered from the global analysis are shown in Figure 5c. The
spectrum corresponding to the “fast” decay component (9 µs)
has λmax ≈ 517 nm, and the spectrum corresponds closely to
that of the monomeric Pt2M. Note that the lifetime of this
component is very close to the lifetime of Pt2M in dilute
solution (10 µs) supporting assignment of the “fast” decay
component to molecularly dissolved Pt2M. The spectrum
corresponding to the “slow” decay component (59 µs) is blue-
shifted and has λmax ≈ 495 nm. This spectral-decay component
clearly arises from the Pt2M aggregate. The considerably slower
decay of the triplet state in the aggregate may arise because the
microenvironment slows nonradiative decay (e.g., enhanced
microviscosity, etc.). Alternatively, the lifetime may be increased
because quenching by residual oxygen present in the solutions
is slowed for the aggregated oligomers.
While the time-resolved emission spectrum of Pt2MT in a
dodecane gel (c ) 1 × 10-3 M, data not shown) does not display
two emission bands as Pt2M, the lifetime is also much longer
than is observed for the complex in dilute solution (τ ) 23 µs
in gel versus 5.2 µs in 10-5 M solution). This observation is
consistent with that for Pt2M in the gel, supporting the notion
that the microenvironment present in the aggregates reduces the
rate of nonradiative decay.
Energy Transfer in Mixed Gels Containing Pt2M and
Pt2MT. A key objective of this study is to explore triplet-
triplet energy transfer within the molecular aggregates consisting
of the phosphorescent platinum-acetylide oligomers. The energy
transfer experiments were designed by analogy to previous
studies which indicate that singlet-singlet energy transfer is
very efficient in molecular aggregates consisting of fluorescent,
π-conjugated organic oligomers.11,18,45,48,74 In the energy transfer
experiments described below, Pt2M serves as the “host” and
the energy donor, while Pt2MT serves as the energy acceptor
(i.e., the energy “trap”). Energy transfer from Pt2M to Pt2MT
is moderately exothermic, since the triplet energy of Pt2M is
ca. 0.45 eV above that of Pt2MT.
Figure 6. Photoluminescence spectra of Pt2M-Pt2MT mixed-oligomer
gel in deoxygenated dodecane. [Pt2M] ) 1 × 10-3 M and different mol %
of Pt2MT, λex ) 300 nm. Spectra are normalized at 495 nm.
summarizes the results of a series of steady-state emission
experiments carried out on dodecane gels containing Pt2M at
a fixed concentration (1 × 10-3 M) with varying amounts of
Pt2MT added (0-5 mol %, concentration range (0-5) × 10-5
M).75 The excitation wavelength (300 nm) was carefully selected
such that in each case the light is absorbed almost exclusively
(g95%) by the Pt2M donor chromophore.76 As can be seen in
the spectra, as the amount of Pt2MT added to the gel increases,
the red phosphorescence (λmax ≈ 580 nm) from Pt2MT rapidly
increases in intensity, to the point where it dominates the spectra
for g4 mol % added. A control experiment using the same
concentration of Pt2MT alone in dodecane (c ) 5 × 10-5 M)
with 300 nm excitation gave virtually no phosphorescence
signal. A second control experiment was carried out in a
chloroform solution containing Pt2M and Pt2MT (c ) 1 ×
10-3 and 5 × 10-5 M, respectively). In chloroform Pt2M is
molecularly dissolved even at high concentration, and regardless
of the excitation wavelength only emission from Pt2M is
observed.
Taken together, the results on the mixed dodecane gels (and
chloroform solution) clearly indicate that the Pt2MT phospho-
rescence emission arises via a mechanism involving triplet
energy transfer from Pt2M which acts as a “sensitizer”. In
addition, the process only occurs when the PAOs are aggregated,
suggesting that exciton diffusion among the Pt2M units is
involved. Note that the emission spectra in Figure 6 are
normalized at 495 nm (the wavelength corresponding to the
Pt2M aggregate emission). With this normalization in mind,
comparison of the spectra with increasing Pt2MT concentration
Emission spectroscopy was used to monitor the energy
transfer process in the mixed gel solutions. First, Figure 6
(75) TEM images of a xerogel produced from the Pt2M/Pt2MT (5 mol%)
mixture are provided in the Supporting Information.
(76) At 300 nm, the molar absorptivity of Pt2M and Pt2MT is 50 000 and
35 000 M-1 cm-1, respectively. Thus, at 300 nm for an equimolar solution
of Pt2M and Pt2MT the ratio of light absorption by the two chromophores
is ≈1.4:1. For the mixed gel containing the highest mole fraction of Pt2MT
used (5 mol%), the Pt2M/Pt2MT light absorption ratio is 29:1 at 300 nm.
(74) Daniel, C.; Herz, L. M.; Beljonne, D.; Hoeben, F. J. M.; Jonkheijm, P.;
Schenning, A. P. H. J.; Meijer, E. W.; Phillips, R. T.; Silva, C. Synth. Met.
2004, 147, 29-35.
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A R T I C L E S
Cardolaccia et al.
components are shown in Figure 7b. The spectrum of the fast
lifetime component is dominated by the phosphorescence from
the Pt2M (donor) chromophores, whereas the spectrum corre-
sponding to the slow component is dominated by phosphores-
cence from the Pt2MT (acceptor), with a small contribution
from the aggregate band (λ ) 495 nm) of Pt2M. Several points
are significant with respect to the time-resolved emission data.
First, it is evident that the Pt2M phosphorescence decays more
rapidly in the presence of Pt2MT; virtually all of the Pt2M
emission decays with τ ≈ 5 µs. This is much faster than the
natural decay of the Pt2M aggregate state (τ ≈ 59 µs),
suggesting that the Pt2M to Pt2MT energy transfer occurs
rapidly and efficiently in the mixed aggregate system. The
second important point is that the lifetime of the Pt2MT
phosphorescence (the slow component) in the mixed gel is
longer than that observed in the pure Pt2MT aggregates (41
µs versus 23 µs). The observation further supports the notion
that the phosphorescence from Pt2MT is being sensitized by
the Pt2M donor chromophores in the aggregates.
Discussion
Aggregates of π-Conjugated Oligomers. Before considering
the aggregation and photophysical properties of the platinum
acetylide oligomers (PAOs), it is important to briefly summarize
relevant prior work on aggregates and organogels consisting of
supramolecular assemblies of organic π-conjugated oligomers.
This review is significant because it provides insight relevant
to the results obtained on the PAOs that are the focus of the
present study.
Figure 7. Time-resolved emission spectra of Pt2M with 5 mol % Pt2MT
in deoxygenated dodecane gel following pulsed excitation at λex ) 337
nm. (a) Initial spectrum obtained at 1 µs delay and succeeding spectra
obtained with 10.5 µs delay increments. (b) Principal components of
emission decay of Pt2M at 1 × 10-3 M with 5 mol % Pt2MT in dodecane
for fast component τ ) 5 µs (- -) and slow component τ ) 41 µs (s).
As noted above, Meijer investigated the solution and optical
properties of a series of oligo(phenylene vinylene)s (OPVs) end-
capped with trialkoxybenzene units. The OPVs self-assemble
into supramolecular assemblies that are proposed to consist of
helical arrays of stacked π-conjugated chromophores.4 The
arrays are stabilized by solvophobic and π-π interactions
between the stacked aromatic chromophores.3 Aggregation of
the OPVs is accompanied by a slight red-shift in their π,π*
absorption and a more pronounced red-shift and broadening of
the fluorescence. The change in fluorescence is attributed to
emission from an “excimer like” state arising from π-stacking
of the chromophores in the aggregates. The OPVs are substituted
with chiral aliphatic side chains, and the supramolecular
aggregates exhibit a strong, bisignate CD spectrum arising from
exciton coupling of the chromophores within the chiral ag-
gregate. The bisignate CD is typical of π-conjugated chro-
mophores that are in a helical environment. More recently,
Ajayghosh studied a series of hydroxymethyl-substituted OPVs
that produce stable organogels in hydrocarbon solvents.10,20
Gelation occurs due to the formation of nanofibers consisting
of supramolecular assemblies of the OPVs stabilized by
hydrogen bonding and π-π stacking interactions between the
π-conjugated chromophores. The hydroxymethyl-substituted
OPVs undergo very similar changes in absorption and fluores-
cence concomitant with aggregation and gelation as described
by Meijer for his supramolecular OPV aggregates, and they also
exhibit strong bisignate CD spectra signaling that the aggregate
provides a chiral environment.
reveals that the Pt2M monomer emission intensity (516 nm)
increases relative to that of the Pt2M aggregate (495 nm). This
suggests that the Pt2MT acceptor quenches the phosphorescence
from the Pt2M aggregate more efficiently than the Pt2M
aggregate which is in free solution. As discussed below, this
notion is at least qualitatively consistent with a model in which
triplet exciton transport and energy transfer occur within
aggregates consisting of Pt2M as the host and Pt2MT as the
energy trap.
The phosphorescence dynamics of the Pt2M/Pt2MT mixed
oligomer system were explored using time-resolved emission
(with 337 nm excitation) in order to gain insight concerning
the dynamics of energy transfer. Figure 7a shows time-resolved
emission spectra of a dodecane gel containing of Pt2M (c ) 1
× 10-3 M) and 5 mol % Pt2MT (c ) 5 × 10-5 M). The series
was obtained with a delay increment of ca. 10 µs, and the spectra
shown were obtained over a time scale of ca. 100 µs.
Interestingly, the first spectrum (obtained with a 1 µs delay from
laser pulse) is dominated by the Pt2M phosphorescence;
however, succeeding spectra (delay g10 µs) are dominated by
the phosphorescence of Pt2MT. The results provide very clear
qualitative support for the notion that the triplet state of Pt2M
is quenched efficiently by energy transfer to Pt2MT.
Global analysis of the time-resolved emission data provides
insight into the energy transfer dynamics in the mixed ag-
gregates. The analysis affords two principal decay compo-
nents: “fast” with τ ≈ 5 µs and “slow” with τ ≈ 41 µs. The
principal component spectra corresponding to the two decay
Another series of investigations relevant to the results reported
here is work by Whitten and co-workers on aggregates consist-
ing of stilbene-functionalized phospholipids.39-41 The absorption
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Phosphorescent Platinum Acetylide Organogelators
A R T I C L E S
Scheme 1
and fluorescence properties of the stilbene-phospholipids were
studied in Langmuir monolayer films and in bilayer vesicles.
In these lipid assemblies the stilbene chromophores exist in a
chiral H-aggregate structure characterized by a strongly blue-
shifted absorption and a slightly red-shifted fluorescence. Crystal
structures and molecular dynamics simulations suggest that the
stilbene aggregates consist of a glide layer structure that features
chiral “pinwheel” or “herringbone” units consisting of four
chromophore assemblies that interact predominantly via non-
covalent edge-to-face interactions of the stilbenes.40
Structure of Platinum Acetylide Aggregates. The optical
and materials data show that in hydrocarbon solvents at
concentrations above 1 × 10-4 M Pt2M, Pt2MT, and Pt2MC
form supramolecular aggregates, and when the concentration
is above 1 × 10-3 M aggregation leads to gelation of the
solutions. Since the PAO structures are analogous to the
aggregating OPVs previously studied (i.e., they consist of rigid,
π-conjugated cores capped on each end by trialkoxybenzene
units),4 it is compelling to suggest that the structure of the PAO
aggregates is similar to those produced by the OPVs. Specifi-
cally, the PAOs take on an overall planar conformation, packing
into a supramolecular “card stack” that has a helical twist giving
rise to the chiral environment that is signaled by the observed
strong CD spectrum.
triplet excited state in the aggregate relative to that in the
molecularly dissolved state. This conclusion is based on previous
studies where we examined the temperature dependence of the
phosphorescence for a series of PAOs that are structurally
similar to Pt2M.62 Of particular significance to the present work
is the temperature dependence of the phosphorescence for the
complex trans,trans-[PhsCtCsPt(PBu3)2sCtCs(1,4-Ph)s
CtCsPt(PBu3)2sCtCsPh ] (Pt2, where Ph is phenyl and
1,4-Ph is 1,4-phenylene).62 Note that Pt2 has the same chro-
mophore as Pt2M but it lacks the trialkoxy groups on the
terminal phenyl groups. The phosphorescence of Pt2 was
investigated in dilute 2-methyltetrahydrofuran (MTHF) solution
over the temperature range from 300 to 80 K. Above the glass
point of MTHF (ca. 120 K) the emission of Pt2 is very similar
to that of Pt2M in dilute solution with a single 0-0 band at
λmax ) 519 nm. However, at temperatures below 120 K where
the MTHF is a rigid glass, the emission of Pt2 exhibits an
additional 0-0 band that is blue-shifted to λmax ) 497 nm.
Excitation wavelength studies demonstrate that the two emission
0-0 bands arise from two distinct triplet states of Pt2 that do
not interconvert in the glass. On the basis of density functional
theory (DFT) calculations these two emitting states were
identified as differing in the conformation of the central
phenylene unit. The low-energy emission (λmax ) 519 nm) arises
from the conformation of the triplet state in which the central
phenylene ring is oriented coplanar relative to the planes defined
by the square planar PtP2C2 units. The higher energy emission
(λmax ) 497 nm) arises from the conformer in which the central
phenylene ring is perpendicular to the planes defined by the
square planar PtP2C2 units. These two conformers are illustrated
in Scheme 1 and are labeled p and t, respectively. DFT
calculations indicate that for Pt2 the p conformer is the most
stable conformer in the triplet state, whereas t is the most stable
in the ground state.
However, there are important differences in the spectroscopic
properties when comparing the OPV and the PAO aggregates,
and these differences suggest that the manner in which the
oligomers pack in the aggregates is different, leading to a
different supramolecular structure. First, the OPV aggregates
exhibit a red-shifted absorption and broad, red-shifted “excimer-
like” fluorescence.4,10 Thus, the spectroscopy of the OPV
aggregates is apparently dominated by the π-π interchro-
mophore interactions that arise when the relatively planar OPV
chromophores π-stack in the card pack aggregate structure. By
contrast, Pt2M, Pt2MT, and to a lesser extent Pt2MC exhibit
a significantly blue-shifted absorption with little perturbation
of the phosphorescence emission. These features suggest that
the spectroscopy of the aggregated PAOs is dominated by
relatively weaker excitonic interactions between the chro-
mophores. An important point is that the absorption spectrum
of the Pt2M aggregate, which has been attributed to an
H-aggregate, is remarkably similar to that of the stilbene
H-aggregates reported by Whitten.40 In the stilbene aggregates
the absorption and emission spectra are also dominated by
exciton coupling among the chromophores, as there is little
evidence for the existence of “excimer like” fluorescence.40 As
noted above, in the stilbene aggregates the interchromophore
interactions are dominated by edge-to-face interactions between
the aryl rings, as opposed to π-π stacking. The similarity of
the spectroscopy of the stilbene and PAO aggregates suggests
that in the latter the chromophores pack in a way such that π-π
interactions are not significant. This is consistent with the fact
that the PAOs are less likely to adopt an all-planar structure
(see below) and with the presence of the comparatively bulky
PMe3 ligands on the platinum centers.
The implications of the observations on Pt2 in the MTHF
solvent glass are obvioussthe high-energy phosphorescence
observed from Pt2M in the aggregates (λmax ) 495 nm) must
arise from the t conformer, whereas the low-energy emission
observed from the molecularly dissolved complex (λmax ) 516
nm) arises from the p conformer. When Pt2M is molecularly
dissolved, following initial excitation and intersystem crossing
to the triplet state, the oligomer is able to rapidly relax into the
energetically preferred p conformer prior to emission. By
contrast, in the aggregate Pt2M is sterically constrained so that
conformational relaxation cannot occur in the triplet excited
A second important feature concerns the origin of the ca. 20
nm blue-shift in the phosphorescence that is seen for the Pt2M
aggregate relative to the molecularly dissolved state (λmax shifts
from 516 to 495 nm). As noted above, the spectral shift is
believed to arise due to a difference in the conformation of the
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J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2543

A R T I C L E S
Cardolaccia et al.
Figure 8. Schematic illustrating possible arrangement of Pt2M units within supramolecular aggregate: (a) Sheetlike packing of individual monomers in
t-conformation, (b) stacking of sheets, (c) lamellar packing of stacked sheets, and (d) ribbon with supramolecular twist.
Scheme 2
state. Consequently for the Pt2M aggregates the emission occurs
from the unrelaxed t conformation. It follows from this that in
the aggregate the Pt2M oligomers are present in the t
conformation.
Although it is only possible to speculate as to how the PAOs
pack in the aggregates, on the basis of the spectroscopic data
and its relationship to the other π-conjugated oligomer ag-
gregates, we suggest the model shown in Figure 8 as one
possible motif. In this model the individual PAOs are in the t
conformation, packing into a slipped stack arrangement (Figure
8a). The individual sheets may stack and assemble into a
lamellar sheet structure (Figure 8b and c). Finally, on a longer
length scale the lamellar sheet may produce a ribbon structure
which is twisted (in a helical sense), giving rise to the optical
does not occur in a chloroform solution that contains the same
activity that is observed for the Pt2MC aggregates. This packing
Pt2M and Pt2MT concentrations (the oligomers do not ag-
model is consistent with the observation that H-aggregate
gregate in CHCl3). Second, the photophysical results indicate
exciton coupling is the dominant interchromophore interac-
that triplet energy transfer is relatively efficient, even when the
acceptor is present at a low concentration (i.e., 1 - 2 mol %)
in the aggregates.
tion.77,78 It also would explain the observation that in the triplet
excited state the conformation of the oligomers is constrained
from undergoing the t f p relaxation, as the slipped stack
While the structure of the mixed aggregates at the molecular
structure (Figure 8a) is sterically constrained such that rotation
level is not known, it is likely that even when the Pt2MT trap
of the individual phenylene rings is precluded.
is present at the 5 mol % level (i.e., the ratio of Pt2M/Pt2MT
Triplet Energy Transfer in the Pt2M/Pt2MT Mixed Gels.
is 20:1) the median distance between the traps is greater than
The study of the mixed aggregates provides clear evidence for
25 Å. In view of this fact, given that the rate of triplet (exchange)
the occurrence of triplet energy transfer from Pt2M (donor/
energy transfer falls exponentially with distance,79 we conclude
host) to Pt2MT (acceptor/trap). First, energy transfer is only
that energy transfer within the mixed aggregates occurs via a
observed when the mixed oligomers are in the aggregates. This
mechanism involving diffusion of the exciton within the Pt2M
conclusion is based on the fact that while energy transfer occurs
host aggregate until it encounters a Pt2MT acceptor site where
for the mixed aggregates in dodecane solution (Figure 6), it
it becomes trapped (Scheme 2). Exciton diffusion within the
aggregate must occur via a random walk wherein the triplet
excitation hops between adjacent Pt2M monomers via a Dexter
(77) McRae, E. G.; Kasha, M. In Physical Processes in Radiation Biology;
Augenstein, L., Mason, R., Rosenberg, B., Eds.; Academic Press: New
York, 1964; pp 23-42.
(78) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Pure Appl. Chem. 1965,
11, 371.
(79) Dexter, D. L. J. Chem. Phys. 1953, 21, 836-850.
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2544 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008

Phosphorescent Platinum Acetylide Organogelators
A R T I C L E S
exchange triplet energy transfer mechanism.79 While the dynam-
ics of the triplet hopping and trapping steps are not known with
certainty, the time-resolved emission studies indicate that the
overall exciton diffusion/trapping process occurs on the 1-10
µs time scale. This suggests that the time scale for triplet exciton
hopping between the individual Pt2M oligomers in the ag-
gregate occurs on the nanosecond time scale (e.g., site-to-site
hopping rate on the order of 107-108 s-1). This hopping rate is
significantly less than the rate of singlet exciton migration in
square planar platinum terpyridine chromophores, where Pt-
Pt interactions are believed to play a significant role in the
aggregation (and gelation) behavior of the complexes.52,53 In
the platinum terypyridine systems, aggregation is accompanied
by development of new low-energy absorption and emission
bands. These bands are attributed to a charge-transfer excited
state based on promotion of an electron from a metal-metal
bonding orbital into the terpyridine ligand (metal-metal to
ligand charge transfer, MMLCT). The lack of significant Pt-
Pt interactions in the Pt2M aggregates is consistent with the
fact that the platinum centers are sterically congested by the
PMe3 ligands.
Efficient triplet-triplet energy transfer is observed in mixed
aggregates consisting of Pt2M as the donor/host and Pt2MT
as the acceptor/trap. Energy transfer is only observed when the
mixed oligomer system is in the aggregate/gel state, indicating
that exciton diffusion within the Pt2M host is important in
transporting the exciton to a Pt2MT trap. This work provides
the first example of the observation of triplet exciton diffusion
and trapping in a molecular aggregate system. Ongoing experi-
ments in our laboratory seek to provide quantitative insight into
the efficiency and dynamics of triplet exciton diffusion and
trapping in phosphorescent aggregates consisting of platinum
acetylide chromophores.
aggregates which occurs on a time scale of 1010-1011 s-1 48
.
The much slower rate of triplet exciton hopping is consistent
with a mechanism involving Dexter exchange energy transfer
between weakly coupled monomers.79
Summary and Conclusions
A series of platinum acetylide oligomers designed to self-
assemble in hydrocarbon solvents was synthesized with the
primary objective of studying the consequences of aggregation
on the triplet excited state. The PAOs feature a rigid-rod linear
platinum acetylide chromophore end-capped with tridodecy-
loxyphenyl units. Each of the oligomers aggregates in hydro-
carbon solvents, and at sufficiently high concentration (ca. 2
wt. %) aggregation leads to gelation.
Comparison of the photophysics of the PAOs in solution and
in the aggregates provides insight into the molecular and
supramolecular structure of the aggregates. The absorption
spectra of the aggregated oligomers are blue-shifted significantly
compared to the absorption when they are molecularly dissolved.
The blue shift is attributed to exciton coupling within a
supramolecular structure in which the chromophores are packed
into a H-aggregate array. The phosphorescence of the aggregated
PAOs is blue-shifted only slightly compared to the molecularly
dissolved state, indicating a triplet excimer is not produced in
the aggregates. The phosphorescence blue shift is attributed to
emission from a high energy (molecular) conformation that is
unable to undergo geometric relaxation in the constrained
environment presented by the aggregate structure.
Acknowledgment. We thank the National Science Foundation
for support of this research (Grant No. CHE-0515066). We
acknowledge Dr. Kye-Young Kim for assistance with the
graphics.
Supporting Information Available: Full experimental section,
including description of synthesis of Pt2M, Pt2MT, and Pt2MC
with complete spectral characterization data (1H, 13C, and 31P
NMR spectra), temperature dependent absorption spectra of
Pt2MT, concentration dependent absorption spectra of Pt2MC,
concentration dependent phosphorescence spectra for Pt2MT,
and TEM images of xerogels of Pt2M/Pt2MT mixture (2
Schemes, 13 Figures). This material is available free of charge
via the Internet at http://pubs.acs.org.
The spectroscopic studies indicate that Pt-Pt interactions are
not important in the aggregate structures. This finding contrasts
with recent work on supramolecular aggregates consisting of
JA0765316
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