Photophysics and Photochemistry of Stilbene-Containing Platinum Acetylides

Article abstract of DOI:10.1021/jp0496811

The photophysics and photochemistry of four platinum acetylide complexes containing 4-ethynylstilbene (4-ES) ligands were investigated. As the excited-state properties of the stilbene chromophore were well understood, the 4-ES ligand was chosen. The compl

Full text of DOI:10.1021/jp0496811

J. Phys. Chem. B 2004, 108, 4969-4978
4969
Photophysics and Photochemistry of Stilbene-Containing Platinum Acetylides
Ksenija Haskins-Glusac, Ion Ghiviriga, Khalil A. Abboud, and Kirk S. Schanze*
Department of Chemistry, UniVersity of Florida, P.O. Box, 117200, GainesVille, Florida 32611-7200
ReceiVed: January 22, 2004
A series of four platinum acetylide complexes that contain 4-ethynylstilbene (4-ES) ligands have been subjected
to a detailed photochemical and photophysical investigation. The objective of the work is to understand the
structure and reactivity of the lowest excited states in π-conjugated Pt-acetylides. The 4-ES ligand was chosen
for this work because the excited-state properties of the stilbene chromophore are well understood. In particular,
stilbene features fluorescence and phosphorescence, and also undergoes trans-cis photoisomerization from
the triplet excited state. Two of the complexes investigated contain phosphine ligands and have the structures
trans-Pt(PBu3)2(4-ES)2 and cis-Pt(dppe)(4-ES)2 (2 and 3, respectively, where PBu3 ) tributylphosphine and
dppe ) bis-1,2-diphosphinoethane). The other two complexes contain substituted 2,2′-bipyridine ligands and
have the structures Pt(t-Bu-bpy)(4-ES)2 and Pt(4,4′-CO2Et-bpy)(4-ES)2 (3 and 4, respectively, where t-Bu-
bpy ) 4,4′-di-tert-butyl-2,2′-bipyridine and 4,4′-CO2Et-bpy ) 4,4′-bis(carboethoxy)-2,2′-bipyridine). The crystal
structure of 2 is reported. The complex features a square planar PtP2C2 geometry and the planes of the phenyl
groups in the 4-ES ligands are twisted approximately 60° relative to the plane defined by the PtP2C2 core.
The series of complexes was examined by using absorption, variable-temperature photoluminescence, and
transient absorption spectroscopy. In addition, the photochemical reactivity of the complexes was explored
by UV-visible absorption and NMR spectroscopy. The available experimental data indicate that in all of the
3
complexes excitation leads to high-yield production of a π,π* excited state that is localized on one of the
4-ES ligands. At low temperatures, the 3π,π* state exhibits strong phosphorescence that is very similar to the
phosphorescence of trans-stilbene. At temperatures above the glass-to-fluid temperature of the solvent medium,
3
the π,π* state decays rapidly (τ ≈ 40 ns). The decay pathway is believed to involve rotation around the
CdC bond of one of the 4-ES moieties. This model is supported by the photochemical results, which show
that steady-state photolysis leads to trans-cis isomerization of one of the 4-ES ligands with a quantum efficiency
of 0.4.
Introduction
sponding triplets.9-15 One reason for the lack of information
regarding the triplet state is that the singlet-triplet optical
π-Conjugated materials exhibit a variety of useful optical
properties for applications such as organic light-emitting diodes
(OLEDs), photovoltaic devices, and nonlinear optical effects.1-5
Although the focus of most work in the field has been on all-
organic materials, there is increasing interest in the properties
and applications of organometallic systems that feature transition
metals which interact strongly with the π-conjugated electronic
system.6 Metals such as Re, Ru, Os, Ir, Pt, and Au exert a strong
influence on the optical properties of a conjugated system via
several mechanisms, including spin-orbit coupling induced
singlet-triplet mixing and introduction of low-energy charge
transfer states into the excited-state manifold. One area that has
received considerable recent attention is the use of organome-
tallic materials in OLEDs to increase the electron-to-photon
quantum efficiency of the devices.7,8 The heavy metals are
thought to increase the quantum efficiency by increasing the
efficiency of light emission from triplet excitons created by
electron-hole pair recombination. The effect of the heavy metal
is believed to result from its large spin-orbit coupling constant
which increases the rates of intersystem crossing and triplet
radiative decay (phosphorescence).
transitions (absorption and light emission) are spin-forbidden,
which makes it difficult to apply optical spectroscopy to probe
the triplet states. Issues that are of interest with respect to the
triplet manifold include the effect of conjugation length on triplet
energy and electron-phonon coupling, and the dependence of
the singlet-triplet splitting on molecular structure and conjuga-
tion length.
We have an ongoing interest in the photophysical properties
of π-conjugated systems that contain transition metals which
interact strongly with the π-electron system.16-18 One area of
particular interest in our work involves platinum acetylides.19,20
This class of materials exhibits optical properties that are similar
to oligo- and poly(phenylene ethynylene)s,21 which can be
thought of as the all-organic analogues of platinum acetylide
oligomers and polymers. Because of the heavy-atom effect
arising from the platinum center, the excited-state properties of
Pt-acetylides are dominated by the triplet manifold because the
materials feature high singlet-triplet intersystem crossing yields
and efficient radiative decay from the triplet state.12,15,22
The present investigation examines a series of π-conjugated
Pt-acetylide oligomers toward the objective of understanding
the photophysics and photochemistry of the triplet state in this
class of materials. Some important questions that are addressed
by this work include the following. (1) What is the extent of
Although much is known about the properties of singlet states
in π-conjugated systems, less is known regarding the corre-
* Author to whom correspondence should be addressed. Tel: 352-392-
9133. Fax: 352-392-2395. E-mail: kschanze@chem.ufl.edu.
10.1021/jp0496811 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/30/2004

4970 J. Phys. Chem. B, Vol. 108, No. 16, 2004
Haskins-Glusac et al.
TABLE 1: Selected Bond Lengths (Å) and Angles (deg) for
Complex 2
Pt-C1
Pt-P
P-C21
C1-C2
2.002(3)
2.3099(7)
1.821(3)
1.201(5)
C2-C3
1.436(4)
1.396(5)
1.335(5)
1.466(5)
C7-C8
C9-C10
C10-C11
C1-Pt-C1
C1-Pt-P
C2-C3-C4
C5-C6-C9
180.00(15)
86.66(9)
120.5(3)
118.6(3)
C10-C9-C6
127.2(3)
C9-C10-C11
C16-C11-C10
C12-C11-C10
124.9(3)
118.6(3)
123.4(3)
expected that in 4 the energy of the MLCT state would be higher
3
than that of the 4-ES localized π,π* state, while in 5 the
converse would be true. Thus, we anticipated that the photo-
physics and photochemistry of these two complexes would
differ.
Although Pt-acetylide oligomers and polymers have been the
focus of a number of previous photophysical investiga-
tions,10,12,15,22,27,28,31-34 the present work is unique in that it uses
a combined photochemical and photophysical approach to
provide clear insight into the structure, properties, and reactivity
of the triplet excited-state manifold in a set of π-conjugated
Pt-acetylides.
Figure 1. Structures of the Pt-acetylide complexes.
π-delocalization in the singlet and triplet manifolds of Pt-
acetylides? (2) How does the coordination geometry at the
platinum center influence π-delocalization? (3) How do low-
energy charge-transfer excited states influence the photophysics
and photochemistry of the systems? To meet the project
objectives, we selected stilbene as a model π-conjugated
“ligand” at the platinum center. Stilbene was selected for this
investigation because it has been studied for more than 50 years
and consequently a great deal is known regarding its excited-
state properties and those of its substituted derivatives.23-26
Stilbene is of interest not only because it features interesting
singlet- and triplet-state photophysics, but also because it
undergoes efficient trans-cis photoisomerization.23 This rich
excited-state behavior affords the opportunity to use both
photophysical and photochemical methods to characterize the
excited states of π-conjugated Pt-acetylides.
Results and Discussion
Crystal Structure of Complex 2. The molecular structure
of 2 is shown in Figure 2a, while the important bond lengths
and angles are shown in Table 1. The geometry of this complex
is slightly distorted from square planar, with a P-Pt-C angle
of 86.6°. The Pt-P and Pt-C bond lengths are 2.31 Å and
2.00 Å, respectively, in good agreement with the lengths in
analogous Pt-acetylide systems.35,36 The phenyl rings are rotated
at an angle of 62.7° with respect to the P2PtC2 plane, even
though the conformation in which the aromatic rings lie in the
same plane as the P2PtC2 plane is expected to be more stable.10
This geometric distortion is believed to be due to the bulky
butyl groups on phosphine ligands forcing the phenyl ligands
to twist. Similar out-of-plane phenyl orientations were observed
in the crystal structures of Pt(bpy)(-CtC-C6H4-CH3)2 and
1.37-39
In the present contribution we probe the photophysics and
trans-cis olefin photoisomerization of platinum acetylide
complexes 2-5, each of which contains two 4-ethynylstilbene
(4-ES) chromophores attached to the platinum center via
acetylide linkages (Figure 1). In phosphine complexes 2 and 3,
the coordination geometry is varied with the goal to probe how
geometry influences π-conjugation through the metal center.
In complex 1, the stilbene units are replaced by diphenylacety-
lene ligands.27,28 Due to the presence of the triple bond, complex
1 is unable to undergo twisting in the excited state leading to
photoisomerization, and thus it serves as a “model compound”
for comparison with the 4-ES complexes. The 2,2′-bipyridine
ligand in complexes 4 and 5 introduces an energetically low-
lying dπ (Pt) f π* (bpy) metal-to-ligand charge transfer
(MLCT) excited state. Previous work by Eisenberg and co-
workers29 and our group30 on complexes of the type (di-
imine)Pt(-CtC-Ar)2 demonstrates that the energy of the
MLCT state can be tuned by varying the substituents on the
diimine ligand. Electron donor substitutents, such as the tert-
butyl group used in compound 4, increase the energy of the
MLCT state, while electron-withdrawing substituents, such as
the carboxy ester groups in compound 5, reduce the energy of
MLCT. In the present investigation, we sought to determine
how the MLCT manifold influences the photophysics and
photochemistry of the stilbene complexes. In particular, we
The crystal packing diagram for 2 is shown in Figure 2b.
The organization of the molecules into the crystal appears to
be driven by the bulky PBu3 groups and has little similarity to
the crystal packing of oligo(p-phenylene vinylene)s, which have
similar long axis dimensions compared to complex 2.40 The
molecules are aligned in a zigzag configuration, and there is
no evidence for π-stacking between aromatic rings on adjacent
complexes. The shortest distance between adjacent Pt atoms in
the crystal is 12.8 Å. This is significant because it indicates
that there is no interaction between Pt-centers in the solid.
UV-Visible Absorption Spectroscopy. The absorption
spectra of complexes 2-5 are shown in Figure 3a along with
the spectrum of (E)-4-ethynylstilbene (the free ligand, 4-ES).
The absorption of 4-ES appears as a broad band with three well-
resolved vibronic bands that are due to CdC stretching modes
of the ethylene unit and the aromatic rings. The absorption
maximum of 4-ES (λ ) 324 nm) is red-shifted in comparison
with the absorption of trans-stilbene (λ ) 293 nm),41 but the
spectra of the two compounds have a similar band shape and
vibronic structure. The red-shifted absorption of 4-ES is due to
conjugation of the ethynyl group with the stilbene chromophore.
The spectra of the platinum complexes are dominated by an
intense band in the near-UV region that is clearly derived from
the stilbene chromophore. First, in complex 2, where the 4-ES
ligands are in a trans geometry, the band is relatively featureless

Stilbene-Containing Platinum Acetylides
J. Phys. Chem. B, Vol. 108, No. 16, 2004 4971
Figure 2. (a) Molecular structure of 2 with atomic numbering scheme, and (b) crystal packing diagram for 2.
tions imply that there is weaker interaction between the 4-ES
ligands in the complexes 3-5 and as a result the singlet excited
state is more localized on a single stilbene ligand.
The absorption spectra of 4 and 5 exhibit an additional low-
energy absorption band (λ ≈ 400-450 nm). This band is similar
to that observed in other complexes of the type (diimine)PtIIL2,
and it is clearly due to the dπ (Pt) f π* (bpy) MLCT transition.
Photoluminescence Spectroscopy. The emission spectra of
complexes 2-5 are presented in Figure 3b. At room temperature,
all of the complexes exhibit a short-lived, weak emission (τ <
300 ps, Φ < 0.01) with λmax ≈ 410 nm (Figure 3b, inset). At
low temperature in a solvent glass (T < 120 K), an additional
structured emission band is observed with λmax ≈ 630 nm
(Figure 3b, main panel). By analogy with trans-stilbene, we
Figure 3. (a) UV-visible absorption spectra of 4-ethynylstilbene and
complexes 2-5 in THF solution: (-) 4-ethynylstilbene; (---) 2;
(‚‚‚) 3; (-‚-) 4; (-‚‚-) 5. (b) Fluorescence (inset, F) and phospho-
rescence (P) spectra of complexes 2-5: (---) 2; (‚‚‚) 3; (-‚-) 4;
(s) 5. Fluorescence measurements were conducted at room temperature
in THF solution, while the phosphorescence spectra were recorded at
80 K in 2-MTHF glass.
1
attribute the 410 nm emission to fluorescence from the π,π*
excited state,26 and the 630 nm emission to phosphorescence
3
from the π,π* state.25,42 These assignments are supported by
the emission lifetimes, as the fluorescence has τ < 1 ns and the
low-temperature phosphorescence lifetimes are greater than 100
µs. An interesting feature is that the phosphorescence band
maxima are nearly identical in the four complexes. This
observation implies that the 3π,π* state is localized on a single
4-ES ligand, regardless of the coordination geometry (i.e, cis
or trans). This finding is consistent with a number of recent
studies of π-conjugated systems which show that the triplet
excited state is localized.10,12,15,20 Another interesting finding
is that we observe no evidence for MLCT emission in complexes
4 and 5 (MLCT emission would appear as a broad, structureless
emission band).29,30 This finding contrasts with studies of related
complexes of the type (diimine)Pt(CtC-Ar)2,which feature
moderately efficient MLCT emission in fluid solution at ambient
with λmax ) 370 nm. Note that the absorption is red-shifted
significantly from that of 4-ES, a fact which implies that there
is delocalization of the stilbene localized molecular orbitals
mediated by the Pt dπ orbitals. In addition, the lack of vibronic
structure in the absorption band indicates that electron-vibronic
coupling is weak, which is also consistent with a delocalized
excited state.
The 4-ES ligands in 3-5 are in a cis geometry at the Pt center.
In these complexes, the absorption band is blue-shifted relative
to its position in 2. In addition, the bands display a more
pronounced vibrational structure, which indicates that electron-
vibrational coupling is stronger. Taken together, these observa-

4972 J. Phys. Chem. B, Vol. 108, No. 16, 2004
Haskins-Glusac et al.
Figure 5. Transient absorption difference spectrum of 2 in argon-
deoxygenated THF solution at 298 K following 355 nm pulsed
excitation. Inset shows transient absorption decay at 420 nm.
rate of CdC bond rotation in the triplet excited state above the
glass to fluid transition. In effect, triplet state nonradiative decay
via CdC bond rotation is prohibited in the frozen matrix, and
consequently phosphorescence is observed.
To confirm that the decrease in the phosphorescence intensity
of complexes 2-5 above the solvent glass to fluid transition
originates from twisting of the olefinic bond in the stilbene
moiety, the phosphorescence intensities and lifetimes of com-
plexes 1 and 2 as a function of temperature were compared. In
contrast to stilbene chromophore in complex 2, the acetylene
linkage in complex 1 does not undergo twisting in the excited
state, thereby enhancing the probability for radiative decay.
Figures 4b and 4c show the variable temperature emission of
complex 1 and the corresponding changes in the phosphores-
cence decay lifetime. In contrast to the properties of complex
2, the emission intensity and lifetime of 1 varies weakly over
the 80-140 K range. Indeed, even at ambient temperature 1
exhibits efficient phosphorescence from the diphenylacetylene
Figure 4. Variable temperature emission spectra, 2-MTHF solvent:
(a) complex 2; (b) complex 1. In order of decreasing intensity: 80,
120, 130, and 140 K. (c) Phosphorescence decay lifetimes as a function
of temperature, 2-MTHF solvent. (9): complex 1; (2): complex 2.
3
localized π,π* state (Φ ) 0.15, τ ) 42 µs).28 These results
temperatures and in low-temperature glasses.29,30 We elaborate
on the significance of this finding in the discussion section.
Figure 4a illustrates the variable temperature emission spectra
(range 80 f 140 K) for complex 2, while Figure 4c shows the
dependence of the phosphorescence lifetime on temperature. As
the temperature is raised, both the phosphorescence intensity
and lifetime of complex 2 decrease dramatically to the point
where the phosphorescence can barely be observed at 140 K.
In the temperature range from 80 to 130 K, the solvent (2-Me-
THF) undergoes a glass-to-fluid transition, suggesting that the
change in phosphorescence intensity of 2 is correlated with the
change in the solvent viscosity. Similar behavior was also
observed for complexes 3-5.
Extensive work on the stilbene triplet excited state has
demonstrated that the phosphorescence quantum efficiency and
lifetime increase with increased solvent viscosity.24,25 This effect
has been attributed to the influence of viscosity on the rate of
triplet state nonradiative decay via twisting around the CdC
bond. In effect, the rate of twisting is slowed in viscous or glassy
solvents, and consequently the triplet state radiative decay
(phosphorescence) efficiency increases. In accord with this, it
has also been shown that the quantum efficiency for trans f
cis photoisomerization of stilbene via the triplet state decreases
with increasing solvent viscosity.24 By analogy to the observa-
tions on stilbene,25 we believe that the precipitous decline of
the phosphorescence intensity at the 2-Me-THF glass to fluid
transition for complexes 2-5 arises due to an increase in the
support the hypothesis that the rapid nonradiative decay pathway
operative in 2-4 above the solvent glass-to-fluid temperature
is rotation of the 4-ES CdC bond.
Transient Absorption Spectroscopy. Laser flash photolysis
investigations have been carried out on a variety of Pt-acetylide
complexes and polymers, both in solution and as solid
films.22,27,28,43 These systems typically exhibit excited-state
3
absorption in the visible region due to the π,π* excited state.
For example, the transient absorption spectrum of 1 features a
broad excited-state absorption with λmax ≈ 580 nm and a lifetime
of 42 µs.28 The transient absorption is believed to arise from
the 3π,π* excited-state localized on the diphenylacetylene ligand.
In an attempt to observe excited states present following
nanosecond excitation of the Pt-stilbene complexes, laser flash
photolysis experiments were carried out on 2-5. In each case,
degassed solutions of the complexes were excited by using 10
ns, 355 nm pulses from a Nd:YAG laser. Relatively short-lived
(i.e., sub-µs lifetime) transient absorptions were observed
following pulsed excitation of 2-4. The transient absorption
spectrum of complex 2 is illustrated in Figure 5. The spectrum
is characterized by bleaching of the ground-state absorption in
the 300-400 nm region, combined with a broad excited-state
absorption feature extending from 400 to 600 nm. The transient
which gives rise to the absorption decays with a lifetime of 40
ns. Virtually identical transient absorption spectra and decay
lifetimes were observed for complexes 3 and 4, so these data
are not presented herein. An interesting feature is that no

Stilbene-Containing Platinum Acetylides
J. Phys. Chem. B, Vol. 108, No. 16, 2004 4973
transient absorption could be observed for complex 5, even at
very small delay times after the laser pulse. This result indicates
that for this complex the excited state decays too rapidly to be
detected in the nanosecond time regime.
Go¨rner and Schulte-Frohlinde (G/SF) published a detailed
study of the transient absorption spectroscopy of stilbene triplet
excited states.44 These authors reported that in solution at
ambient temperature the triplet excited state of trans-stilbene
features a broad excited state absorption that extends from below
350 nm to approximately 400 nm. The transient decays with a
lifetime of ≈60 ns at ambient temperature. A detailed analysis
of the solvent, sensitizer, and quenching effects on the transient
led G/SF to assign the transient absorption to a mixture of the
trans and twisted triplet excited states (3t* and 3p*, respectively)
that are in equilibrium.
A comparison of our observations with the results reported
by G/SF on trans-stilbene lead us to assign the short-lived
3
transient absorption observed for 2-4 to a π,π* state that is
localized on a single 4-ES ligand. In particular, we believe that
the transient is likely a stilbene localized triplet, in which the
olefin is in the trans geometry (i.e., 3t*). The short lifetime likely
arises due to rapid nonradiative decay of the triplet via twisting
around the CdC bond in the 4-ES moiety. Importantly, the
behavior of the stilbene acetylide complexes 2-5 in solution
differs significantly from that of other Pt-acetylide oligomers
Figure 6. The changes in the absorption spectrum of 2 upon irradiation
at 366 nm in 10 min intervals. Top: absorption spectra at different
photolysis times; bottom: absorption difference spectra relative to the
spectrum at t ) 0.
3
and polymers, where long-lived π,π* states are observed by
transient absorption and phosphorescence.28 We attribute the
different properties of the 4-ES complexes to rapid excited-
state decay via CdC bond rotation in the stilbene chromophore.
The premise that twisting around the olefinic bond is an
important nonradiative decay channel in 2-5 is supported by
the results of photochemical studies outlined in the following
section. The significance of the lack of a transient absorption
for 5 will be discussed below.
Photochemical Measurements. In the photophysical study
described above, it was found that the triplet excited state in
complexes 2-5 decays rapidly via a nonradiative pathway. To
confirm that this rapid nonradiative decay pathway involves
twisting of the stilbene CdC bond in the triplet excited state,
steady-state photochemical investigations were carried out to
produce evidence for trans f cis photoisomerization. In initial
experiments, solutions of the complexes were irradiated with
366 nm light and the changes in the absorption spectra as a
function of irradiation time were recorded. The changes in the
absorption spectrum induced by photolysis of 2 are shown in
Figure 6, and similar spectra were observed for 3-5. In all cases,
irradiation induces bleaching of the π,π* absorption band in
the 300-400 nm region, while there is a concomitant appearance
of a weaker absorption band in the 250-300 nm region. The
changes observed in the UV absorption spectra are very similar
to those that accompany the trans f cis photoisomerization of
stilbene.45,46 This finding strongly suggests that UV photolysis
of 2-5 leads to trans f cis photoisomerization of one (or both)
of the 4-ES ligands. Another interesting finding is that photolysis
of 4 and 5 with 436 nm light induces the same changes in the
UV absorption as observed when the compounds are subjected
to UV photolysis. This result shows that excitation into the dπ
(Pt) f π* (bpy) MLCT manifold leads to trans f cis
isomerization of the 4-ES ligand. This photochemical result is
consistent with the transient absorption studies of 4 and 5, which
suggest that the MLCT state is very short-lived.
Figure 7. (a) Aromatic region 1H NMR spectrum of 2 in CDCl3/C6D6
1
(2:1 mixture); (b) Aromatic region H NMR of the same sample after
it was irradiated at 366 nm for 10 min. Insets: 1H NMR spectrum of
olefinic hydrogen atoms (spectrum obtained with sample in pure C6D6
as a solvent).
1
illustrates the aromatic region of the H NMR spectrum of
complex 2 (CDCl3/C6D6 mixture solvent) prior to photolysis.
1
A complete assignment of the H resonances was achieved by
2D NMR techniques (see Supporting Information). The reso-
nances in the region between 6.9 and 7.30 ppm correspond to
protons on the 4-ES phenyl rings. The two alkene protons appear
in the spectrum as a closely spaced pair of doublets at δ )
6.79 and 6.83 ppm with coupling constant J ) 16.4 Hz. Figure
1
7b shows the H NMR spectrum after the sample of 2 was
irradiated at 366 nm for several minutes. There are clearly a
number of new peaks that arise in the aromatic region as a result
of the photolysis, but of most significance is the new set of
resonances that appear at δ ≈ 6.3 ppm. As can be seen in the
Figure 7b inset (right side), this resonance is due to a closely
spaced pair of doublets at δ ) 6.27 and 6.30 ppm with coupling
constant J ) 12.5 Hz. On the basis of the chemical shift and
To fully characterize the products formed by steady-state
photolysis of the Pt-stilbene complexes, 1H NMR spectroscopy
was used to monitor the photochemical reaction. Figure 7a

4974 J. Phys. Chem. B, Vol. 108, No. 16, 2004
Haskins-Glusac et al.
Figure 8. Jablonski diagrams for (a) complexes 2 and 3, (b) complex 4, and (c) complex 5; solid arrows represent radiative processes, and dashed
arrows represent nonradiative decays. Horizontal dashed lines in Figures 8b and 8c represent the 3MLCT state at 80 K. Note that in all figures, the
energy of 1π,π* is taken from the fluorescence maximum (i.e., the relaxed state energy) while that of 1MLCT is taken from the absorption maximum
(i.e., an unrelaxed state energy).
coupling constant, this signal is attributed to the olefinic protons
on a cis-stilbene unit produced by photoisomerization. The left
inset in Figure 7b shows an expansion of the resonances due
the olefinic protons in the trans-stilbene unit(s) in the photolyzed
sample (this expansion is for a sample that was run in pure
C6D6 as a solvent). Interestingly, it is evident in this expansion
that there is a set of two chemically distinct pairs of olefinic
protons that appear to belong to trans-stilbene (i.e., they have
similar chemical shifts and coupling constants). One pair is due
to the olefinic hydrogens in the starting complex, 2. We attribute
the extra set of resonances (δ ) 6.78 and 6.82 ppm, J ) 16.4
Hz) to the olefinic protons in the trans-stilbene moiety of the
photoproduct. This result strongly suggests that the primary
photoproduct arises from isomerization of one stilbene moiety,
producing a complex that has a trans- and a cis-stilbene unit,
i.e.,
Such delocalization is consistent with the results of molecular
orbital calculations on square planar, bis-acetylide complexes
of d8 metals, which imply that mixing of the metal-centered dπ
(and p) orbitals with the π and π* MOs of the acetylide ligands
leads to π-type conjugation through the metal center.10 Thus,
in effect, the lowest energy absorption in 2 and 3 arises from a
transition between HOMO and LUMO levels that are substan-
tially delocalized over the entire molecules. The fact that the
π,π* transition is blue-shifted in 3, in which the two 4-ES units
are coordinated cis at the Pt center implies that the degree of
interaction between the two 4-ES ligands is lower.47 This finding
is in agreement with the results of an investigation by Ziessel
and co-workers which demonstrated that the rate of triplet energy
transfer between chromophores linked via a cis-Pt acetylide is
lower than in the corresponding trans-linked system.48 These
authors concluded that the difference in energy transfer rate
results from a lower electronic interaction between chro-
mophores that are linked by the cis-Pt acetylide unit.
(Bu3P)2Pt(-CtC-t-St)2 + hν f
(Bu3P)2Pt(-CtC-t-St)(-CtC-c-St) (1)
Once the 1π,π* excited state is produced by light absorption,
it relaxes predominantly via intersystem crossing (path 3, Figure
3
Similar 1H NMR spectra were observed as a result of photolysis
of complexes 3-5, suggesting that the same process occurs in
all of the complexes. The quantum efficiency for photoisomer-
ization of complex 2 via eq 1 was determined by integrating
the NMR peak that corresponds to cis-stilbene olefinic protons
and the value was found to be φt-c ) 0.42.
Photophysical Model: Pt(P)2(-CtC-St)2 Complexes. In
view of their photophysical properties, complexes 2-5 can be
classified into two subgroups, and their behavior will be
discussed separately in the sections that follow. The first group
consists of phosphine-containing complexes 2 and 3, in which
the absorption, emission, and transient absorption arise solely
from π,π* transitions. The second group consists of the 2,2′-
bipyridine-containing complexes which feature a manifold of
energetically low-lying MLCT states in addition to the π,π*
states.
Figure 8a presents a Jablonski diagram that is consistent with
the photophysical behavior of phosphine complexes 2 and 3.
Light absorption gives rise to the 1π,π* excited state (path 1 in
Figure 8a), with energy of 3.3 eV in complex 2 and 3.5 eV in
complex 3. For both complexes the π,π* absorption is red-
shifted relative to its position in the 4-ES free ligand which
suggests that the Pt center promotes delocalization of the 4-ES
based π and π* orbitals that are involved in the optical transition.
8b) to produce the π,π* state (although some fluorescence is
observed, it has a low quantum efficiency). Time-resolved
photoacoustic calorimetry indicates that the intersystem crossing
quantum efficiency (φisc) in 2 is 0.75 ( 0.05.49 Phosphorescence
from the 3π,π* excited state (path 4) occurs only at low
temperatures, where the solvent is a rigid glass. The phospho-
rescence energy in complexes 2 and 3 is 1.95 eV, which is very
similar to the phosphorescence energy of trans-stilbene (2.10
eV),25,42 implying that the triplet excited state is “confined” on
a single 4-ES moiety. It is interesting that the phosphorescence
spectra of complexes 2 and 3 are almost identical, even though
their molecular structures and absorption spectra are different.
A similar trend was observed in the transient absorption
measurements, where spectra of complexes 2 and 3 are almost
identical. In addition, the phosphorescence band shape and
vibronic structure observed for 2 and 3 are very similar to those
of trans-stilbene.25,42 Taken together, these results strongly
3
suggest that the π,π* state in 2 and 3 is localized on a single
4-ES chromophore. Interestingly, the presence of the Pt center
appears to have little effect on the electronic structure of the
triplet; the metal simply acts as a “heavy-atom substitutent”
which increases the spin-orbit coupling which enhances
intersystem crossing and radiative decay. The finding that the
triplet is be localized on a single 4-ES unit in 2 and 3 (rather

Stilbene-Containing Platinum Acetylides
J. Phys. Chem. B, Vol. 108, No. 16, 2004 4975
than delocalized over the entire complex) is consistent with
theory and recent experimental work which demonstrates that
triplet excited states are less spatially delocalized compared to
the singlet states of the same orbital parentage.10,12,15,20,50
Although the triplet state appears to be localized on a single
4-ES unit, during the lifetime of the triplet it seems likely the
that the triplet transfers from one chromophore to another via
an energetically degenerate triplet energy transfer process.
The observations on 4 and 5 in the frozen glass show that
the expectations based on this analysis are borne out by
experiment. Thus, both complexes feature strong phosphores-
cence from the 4-ES 3π,π* state, as expected if the state ordering
3
3
is MLCT > π,π*. Moreover, in fluid solution complex 4
3
3
behaves as expected if the state ordering is MLCT > π,π*:
trans-cis photoisomerization is observed, even for irradiation
into the MLCT absorption, and 3MLCT emission is not
observed. On the other hand, the properties of 5 in fluid solu-
tion are opposite to what is expected if the state ordering is
Above the glass-to-fluid temperature of the solvent, the
phosphorescence of 2 and 3 disappears, signaling that the triplet
excited state decays rapidly via twisting around the CdC bond
(path 6). By analogy with the photophysics and photochemistry
of other substituted stilbenes,23-26 it is believed that this process
leads to the twisted triplet state (3p) in which the dihedral angle
between the phenyl rings is ≈90°. The rate of the CdC bond
rotation corresponds to the rate of triplet excited-state decay,
which was found from transient absorption measurements to
be k ) 1.6 × 107 s-1. This value is in excellent agreement with
the triplet decay rate of trans-stilbene, which is also believed
to decay via CdC bond rotation.44 Furthermore, there is good
correspondence between the photoisomerization quantum yield
of complex 2 and that for trans-stilbene under triplet sensitized
conditions. In particular, the overall yield for photoisomerization
of complex 2 is φt-c 0.42, while φisc ) 0.75 (photoacoustic
calorimetry, see above). The product, φt-c x φisc affords the
efficiency of isomerization from the triplet state, φT*,t-c ) 0.53.
By comparison, under triplet sensitized conditions the photo-
isomerization yield of trans-stilbene is φΤ*,t-c ) 0.55.51 These
3
3MLCT < π,π*sthis complex behaves the same as 4!
Jablonski diagrams that are used to explain the observed
properties of 4 and 5 are shown in Figures 8b and 8c,
respectively. The energies for 1π,π* and 1MLCT transition are
estimated from the fluorescence and absorption spectra, respec-
3
tively, while the energies of the MLCT states are taken to be
the same as in the corresponding (4,4′-X2-bpy)Pt(CtC-tol)2
complexes.30 (In these diagrams the 3MLCT levels in fluid and
glassy media are shown as solid and dashed lines, respectively.)
Since the energetic ordering is 3MLCT > 3π,π* in solution and
in the glass for 4, the photophysics of this complex are very
similar to that of 2 and 3. Thus, excitation of 4 in the near-UV
(path 1) produces the 1π,π* state, which undergoes rapid
3
intersystem crossing (path 3) to π,π*. Alternatively, visible
1
excitation (path 7) affords MLCT which rapidly intersystem
3
crosses to MLCT (path 8).52 Since no emission is observed
from 3MLCT, we conclude that intramolecular energy transfer
3
3
from MLCT to π,π* (path 9) is very rapid, i.e., k g 109 s-1
.
Rapid energy transfer is consistent with the fact that the process
3
results confirm the hypothesis outlined above that the π,π*
is exothermic (∆G e -0.1 eV), and there is strong coupling
state is localized on a 4-ES chromophore.
3
3
between the MLCT and π,π* manifolds. Finally, once it is
Photophysical Model: Pt(bpy)(-CtC-Ar)2 Complexes.
Complexes 4 and 5 each contain a 2,2′-bipyridine ligand which
introduces a manifold of low-lying MLCT states based on the
dπ (Pt) f π* (bpy) transition. In a previous study,30 we analyzed
the photophysical properties of a series of complexes of the
type (4,4′-X2-bpy)Pt(CtC-tol)2 (where 4,4′-X2-bpy is a 2,2-
bipyridine that is substituted in the 4,4′-positions, and CtC-
tol is p-ethynyltoluene). In this series the dπ (Pt) f π* (bpy)
MLCT state is the lowest excited state, and MLCT photolumi-
nescence is observed at 298 and 80 K.29,30 The emission energies
of the tert-butyl- and diester-substituted bpy complexes afford
values for the energy of the relaxed MLCT state at 298 K in
fluid solution, EMLCT ) 2.12 and 1.84 eV, respectively.30 Due
to the rigidochromic effect, the energy of the MLCT state is
increased in a glass at 80 K, and for the same two complexes
in the glass EMLCT ) 2.42 and 2.14 eV, respectively.30
3
formed π,π* state relaxes either by CdC bond twisting (path
6, fluid solution) or by phosphorescence (path 4, solvent glass).
In a solvent glass, the photophysics of 5 are identical to those
of 4. Excitation into either the π,π* or MLCT manifolds (paths
3
1 and 7, Figure 8c) is followed by relaxation into the π,π*
state (via path 3 or path 8 followed by 9), which then relaxes
by phosphoresecence or nonradiative decay. The unique feature
3
with respect to 5 is that in fluid solution MLCT is more than
3
0.1 eV below the spectroscopic energy of the π,π* state. In
view of this, one would expect that: (1) 3MLCT to 3π,π* energy
transfer is slow (if it occurs at all), (2) the 3MLCT state is “long-
lived” (i.e., τ > 20 ns),30 and (3) 3MLCT emission is observed.
A possible explanation for the anomalous behavior of 5 is that
3MLCT is rapidly deactivated by intramolecular “nonvertical”
energy transfer53-55 to directly afford the 4-ES localized twisted
3
3
triplet state, p (path 9a, Figure 8c). Because the energy of p
lies below that of the (spectroscopic) trans-triplet, nonvertical
energy transfer is exothermic and consequently quite rapid. In
In the analysis of the photophysics and photochemistry of 4
and 5, we assume that EMLCT is the same as in the corresponding
(4,4′-X2-bpy)Pt(CtC-tol)2 complexes. Thus, given that the
3
addition, p is likely to be very short-lived, and thus, the two-
3
step process (3MLCT f p f ground state) provides a rapid
3
energy of the 4-ES localized π,π* state is ≈1.97 eV, it was
anticipated that for 4 in fluid solution the energetic ordering of
the excited states would be 3MLCT > 3π,π*, and for 5 in fluid
solution the ordering would be reversed (i.e., 3MLCT < 3π,π*).
On the other hand, due to the rigidochromic effect, it was
anticipated that in a low-temperature glass the state ordering
pathway for excited-state decay. This explains why no transient
is observed by laser flash photolysis for 5 and that trans-cis
photoisomerization is observed even for irradiation into the low-
energy MLCT absorption band. Note that nonvertical energy
transfer has been the subject of extensive investigation in
bimolecular systems, and it is well-established that photo-
isomerization of trans-stilbene is efficiently sensitized by triplet
energy donors with energies 0.3-0.4 eV below the spectroscopic
energy of trans-stilbene.53,54,56 The present investigation may
provide the first example of a system in which intramolecular
nonvertical energy transfer plays an important role in determin-
ing the photochemical and photophysical properties of a
molecular system.
3
3
would be MLCT > π,π* for 4 and 5. Consequently, in fluid
solution it was expected that the properties of 4 and 5 would
be different. Specifically, we anticipated that MLCT emission
would be observed from 5, and that that trans-cis photoisomer-
ization would be observed in 4, but not in 5. By contrast, in a
low-temperature glass, the photophysics of both complexes was
expected to be the same, with phosphorescence from the 4-ES
3
based π,π* being observed.

4976 J. Phys. Chem. B, Vol. 108, No. 16, 2004
Haskins-Glusac et al.
Summary and Conclusions
(CDCl3, 300 MHz) δ 7.50 (d, 6H), 7.33 (t, 2H), 7.22 (t, 1H),
7.08 (dd, 2H), 0.12 (s, 9H) ppm.
This study was carried out with the objective of providing
experimental information concerning the properties of singlet
and triplet excited states in π-conjugated Pt acetylide systems.
To address these objectives, a family of Pt-acetylides that contain
the trans-stilbene chromophore were synthesized and fully
characterized. Stilbene was chosen for the investigation because
it contains an extended π-conjugated electronic system, and
because its photochemical and photophysical properties have
been thoroughly investigated.
(E)-4-Ethynylstilbene. A solution of 100 mg (3.6 × 10-4
mol) of (E)-4-(trimethylsilylethynyl)stilbene in 4 mL of metha-
nol was placed in a round-bottomed flask. Then a solution of
200 mg of KOH in 2 mL of water was added to the flask. The
reaction mixture was stirred at room temperature for 3 h. After
extraction with CH2Cl2 and removal of the solvent, the crude
product was purified by silica column chromatography by using
hexanes/methylene-chloride mixture in a 3:1 ratio as eluant. The
product was obtained in 95% yield (70 mg). 1H NMR (CDCl3,
300 MHz) δ 7.55 (m, 6H), 7.38 (t, 2H), 7.25 (t, 1H), 7.10 (dd,
2H), 3.15 (s, 1H) ppm. HRMS: Calcd for C16H12, 204.0971;
Found, 204.0943.
The absorption spectra of the family of complexes reveals
that the singlet excited state is delocalized over the entire
π-conjugated system. The extent of delocalization appears to
be greatest in complex 2, in which the two 4-ES ligands are
trans at the Pt center. This result indicates that π-conjugation
through Pt is most efficient when the acetylide ligands are
coordinated trans. The photophysical and photochemical in-
vestigations designed to probe the long-lived excited states
demonstrate that in all of the complexes the properties are
Syntheses of Complexes 2-5. Complexes 2-5 were syn-
thesized by reaction of the corresponding L2PtCl2 complex with
(E)-4-ethynylstilbene, according to the method described by
James and co-workers.32 The crude complexes were purified
by chromatography on alumina using CH2Cl2 as the eluant.
Further purification was achieved by crystallization from acetone
(compound 2) or a CH2Cl2/hexanes mixture (compounds 3-5).
3
dominated by the π,π* excited-state localized in the 4-ES
chromophore. Interestingly, the properties of the 3π,π* state in
the Pt-acetylide complexes are remarkably similar to those of
trans-stilbene. From this we conclude that in the Pt-acetylide
1
The purified complexes were fully characterized by H NMR
and high-resolution mass spectrometry. Spectral data for the
new compounds are listed below.
3
complexes the π,π* state is localized on a single 4-ES unit
Complex 2. 1H NMR (CDCl3/C6D6 ) 3:1, 500 MHz) δ 7.50
(d, 4H), 7.36 (t, 4H), 7.34 (d, 4H), 7.20 (d, 4H), 7.10 (t, 2H),
7.01 (d, J ) 16.4 Hz, 2H), 6.97 (d, J ) 16.4 Hz, 2H), 2.18 (m,
12H), 1.70 (m, 12H), 1.46 (sextet, 12H), 0.96 (t, 18H) ppm.
HRMS (LSIMS): Calcd for C56H76P2Pt, 1005.512; Found,
1005.512.
(i.e., in the triplet, there is not significant delocalization of the
excitation energy through Pt into the other 4-ES ligand). This
finding is consistent with a mounting body of evidence which
demonstrates that in organic and organometallic π-conjugated
systems singlet excitations are delocalized, while the triplet states
of the same electronic configuration are spatially confined.12,15,20
Another component of the project examined how introduction
of a manifold of MLCT states influences the photophysical
properties of Pt-acetylide complexes. Two molecular systems
were examined that contained 2,2-bipyridine ligands having
different electron-acceptor properties. The bpy-substituted com-
plexes exhibit low-energy absorption bands arising from the dπ
(Pt) f π* (bpy) transition. However, the remarkable finding is
that the photophysics and photochemistry of the complexes are
hardly affected by the MLCT state. This finding was unexpected
1
Complex 3. H NMR (CDCl3, 300 MHz) δ 7.94 (t, 4H),
7.18-7.51 (m, 30H), 7.11 (d, 4H), 7.02 (d, J ) 18 Hz, 2H),
6.96 (d, J ) 18 Hz, 2H), 2.41 (t, 4H) ppm. HRMS (LSIMS):
Calcd for C58H46P2Pt, 1000.280; Found, 1000.281.
1
Complex 4. H NMR (CDCl3, 300 MHz) δ 7.99 (s, 2H),
7.73 (d, 4H), 7.60 (d, 4H), 7.50 (d, 4H), 7.44 (d, 4H), 7.35
(t, 4H), 7.20 (t, 2H), 7.09 (d, J ) 14.1 Hz, 2H), 7.04 (d, J )
14.1 Hz, 2H), 1.41 (s, 18H) ppm. HRMS (LSIMS): Calcd for
C50H46N2Pt, 870.339; Found, 870.338.
3
1
in complex 5, where the spectroscopic energy of MLCT is
Complex 5. H NMR (CDCl3, 300 MHz) δ 9.95 (d, 2H),
below that of the 4-ES localized 3π,π* state. It is believed that
8.60 (s, 2H), 8.05 (d, 2H), 7.53 (d, 4H), 7.32-7.48 (m, 12H),
7.25 (t, 2H), 7.09 (d, J ) 15 Hz, 2H), 7.06 (d, J ) 15 Hz, 2H),
4.40 (q, 4H), 1.42 (t, 6H) ppm. HRMS (LSIMS): Calcd for
C48H38N2O4Pt, 902.256; Found, 902.260.
the unusual properties of this complex arise as a result of rapid
nonvertical energy transfer from MLCT to π,π*.
3
3
Experimental Section
X-ray Diffraction Analysis. 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 ) 0.71073 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 based on measured indexed crystal faces.
The structure was solved by the Direct Methods in SHELX-
TL5, 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. Crystal, data collection, and refinement
parameters are summarized in Table 2. The asymmetric unit
consists of a half Pt complex. A total of 268 parameters were
refined in the final cycle of refinement using 8529 reflections
with I > 2σ(I) to yield R1 and wR2 of 3.27% and 8.52%,
respectively. Refinement was done using F2.
Synthesis. (E)-4-Bromostilbene and the substituted 2,2′-
bipyridine ligands were prepared according to literature proce-
dures.57,58 The (2,2′-bipyridine)PtCl2 complexes were prepared
by reaction of K2PtCl4 with the bipyridine ligand in refluxing
aqueous HCl solution.29 trans-Pt(PBu3)2Cl2 and Pt(dppe)Cl2
were prepared as reported in the literature.59,60 A sample of 1
was provided by Dr. Thomas Cooper of the Air Force Research
Laboratory.
(E)-4-(Trimethylsilylethynyl)stilbene. A mixture of 100 mg
(3.8 × 10-4 mol) of (E)-4-bromostilbene, 16 mg (2.28 × 10-5
mol) of Pt(PPh3)2Cl2, 8 mg (4.56 × 10-5 mol) of CuI, 3 mL of
diisopropylamine, and 5 mL of THF was prepared in a round-
bottomed flask. The solution was degassed for 15 min under
argon, whereupon 90 mg (9.6 × 10-4 mol) of trimethylsilyl-
acetylene was added and the solution was heated overnight at
80-90 °C. The solvent was evaporated under reduced pressure
and the crude product was purified by silica column chroma-
tography, using a 1:1 mixture of hexanes/CH2Cl2 as an eluant.
1
The product was obtained in 85% yield (90 mg). H NMR

Stilbene-Containing Platinum Acetylides
J. Phys. Chem. B, Vol. 108, No. 16, 2004 4977
0.21 × 0.17 × 0.11 mm3
TABLE 2: Crystallographic Data for Complex 2
empirical formula
formula weight
temperature
wavelength
crystal system
space group
C56H76P2Pt
1006.20
173(2) K
0.71073 Å
Triclinic
P1h
crystal size
θ range for data collection
index ranges
1.97 to 28.91°
-13 e h e 13
-14 e k e 12
-12 e l e 17
reflections collected
8529
unit cell dimensions
a ) 10.0075(5) Å
b ) 10.7197(6) Å
c ) 12.8122(7) Å
R ) 101.670(1)°
â ) 109.882(1)°
γ ) 95.533(1)°
1245.3(1) Å3
1
independent reflections
completeness to θ ) 28.91°
absorption correction
max and min transmission
refinement method
data/restraints/parameters
Goodness-of-fit on F2
final R indices [I > 2θ(I)]
R indices (all data)
5704 [R(int) ) 0.0661]
86.8%
integration
0.7729 and 0.5162
Full-matrix least squares on F2
5704/0/268
volume
Z
density (calcd)
absorption coefficient
F(000)
1.082
R1 ) 0.0327, wR2 ) 0.0852 [5704]
R1 ) 0.0327, wR2 ) 0.0852
1.383 and -0.731e‚Å-3
1.342 Mg/m3
2.916 mm-1
520
largest diff. peak and hole
Photophysical Measurements. Spectroscopic experiments
were carried out either in CH2Cl2 or 2-methyltetrahydrofuran
(2-MTHF), and these solvents were purified by distillation from
CaH2 and Na/benzophenone, respectively. Spectroscopy carried
out at room temperature was performed using samples that were
degassed by argon purging for 20 min. Low-temperature
spectroscopic experiments were carried out on samples that were
degassed by four repeated freeze-pump-thaw cycles on a high
vacuum line. UV-visible absorption spectra were obtained on
a Cary 100 instrument. All photophysical experiments were
carried out with sample concentrations in the range of 2-20
µM. At these low concentrations, the lifetimes were not
concentration dependent.
UG-11 filter which allows for light to reach the sample). The
sample was irradiated by a 450 W medium-pressure mercury
lamp, which was placed ∼15 in. from the housing. The lamp
was held in a quartz immersion well which allowed for water
cooling. The sample was irradiated in 1 min intervals, and the
changes were monitored by NMR spectroscopy (300 MHz
Gemini). The concentration of the cis isomer produced after
photolysis was determined by integrating the olefinic peaks in
the NMR spectra.
The photoisomerization quantum yield for compound 2 was
determined using Aberchrome 540 as an actinometer.62 Aber-
chrome is known to undergo a photocyclization process with a
quantum efficiency of 0.20. The solution was prepared by
dissolving 11.92 mg of Aberchrome in 10 mL of toluene. A
volume of 1 mL of the actinometer solution was placed in an
NMR tube and photolyzed for 1 min, upon which time the
sample was diluted to 5 mL and the absorbance at 494 nm was
measured. The procedure was repeated for different photolysis
times (2, 3, 4 and 5 min). The results were plotted as the change
of absorption at 494 nm per time, which gave a slope S ) A/t.
The intensity of the incident light was calculated from the slope
according to following equation:
Steady-state photoluminescence spectroscopy was carried out
on a SPEX Fluorolog instrument. Emission spectra were
corrected by using correction factors generated in-house with a
standard calibration lamp. Sample concentrations were suf-
ficiently low such that the absorbance at all wavelengths was
<0.2 (typical concentration ) 5 × 10-6 M). Low-temperature
photoluminescence experiments were carried out with samples
contained in an LN2 cooled optical cryostat (Oxford Instruments,
DN-1704) connected to an Omega CYC3200 temperature
controller.
S × V × N
Transient absorption spectroscopy was carried out on an
instrument that has been described previously.61 Samples were
contained in a cell that holds a total volume of 10 mL, and the
contents were continuously recirculated through the pump-
probe region of the cell. Samples were degassed by argon
purging for 30 min. Excitation was provided by the 3rd harmonic
output of a Nd:YAG laser (355 nm, Spectra Physics, GCR-
14). Typical pulse energies were 5 mJ/pulse which corresponded
to an irradiance in the pump-probe region of 20 mJ/cm.
Photoisomerization Studies. Steady-state photochemical
experiments were carried out by using the 375 nm (compound
2) and 350 nm (compounds 3-5) output from a 75 W high-
pressure Xe lamp. The photolysis beam was passed through a
grating monochromator and focused onto a 1 × 1 cm quartz
cell that contained 3 mL of an argon degassed sample solution.
Samples were exposed to light in 10 min intervals and the
change in the UV-visible absorption spectrum was monitored.
The trans-cis photoisomerization reaction was monitored by
1H NMR spectroscopy. Compound 2 was dissolved in CDCl3
and placed in an NMR tube with a vacuum valve. The solution
was degassed by four repeated freeze-pump-thaw cycles on
a high vacuum line. The NMR tube was placed in a housing
(an aluminum box with an opening on the top to place the NMR
tube and another 2 in. × 2 in. opening with a LG350 and a
I )
(2)
Φ × ꢀ
where V is the volume of the sample, Φ is the quantum yield
for the Aberchrome cyclization (Φ ) 0.20), N is Avogadro’s
number, and ꢀ is the molar extinction coefficient of the cyclic
product at 494 nm (ꢀ ) 8200 M-1 cm-1).
Acknowledgment. We gratefully acknowledge the National
Science Foundation (CHE-0211252) for support of this work.
Supporting Information Available: Two-dimensional NMR
1
spectra and detailed H and 13C assignments for complex 2.
Crystallographic data for complex 2 in CIF format. This
information is available free of charge via the Internet at http://
pubs.acs.org.
References and Notes
(1) Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A.,
Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York,
1998.
(2) McGehee, M. D.; Miller, E. K.; Moses, D.; Heeger, A. J. In
AdVances in Synthetic Metals. Twenty Years of Progress in Science and
Technology; Bernier, P., Lefrant, S., Bidan, G., Eds.; Elsevier: Amsterdam,
1999; pp 98-205.

4978 J. Phys. Chem. B, Vol. 108, No. 16, 2004
Haskins-Glusac et al.
(3) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R.
W.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990,
347, 539-541.
(33) Chawdhury, N.; Ko¨hler, A.; Friend, R. H.; Wong, W.-Y.; Lewis,
J.; Younus, M.; Raithby, P. R.; Corcoran, T. C.; Al-Mandhary, M. R. A.;
Khan, M. S. J. Chem. Phys. 1999, 110, 4963-4970.
(34) 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.
(4) Gunder, P. Nonlinear Optical Effects and Materials; Springer: New
York, 2000.
(5) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater.
2001, 11, 15-26.
(35) Baraban, J. M.; McGinnety, J. A. Inorg. Chem. 1974, 13, 2864-
(6) Kingsborough, R. P.; Swager, T. M. Prog. Inorg. Chem. 1999, 48,
123-231.
2869.
(36) Mayor, M.; von Hanisch, C.; Weber, H. B.; Reichert, J.; Beckmann,
D. Angew. Chem.-Int. Ed. 2002, 41, 1183-1186.
(37) Davy, J.; Gunter, M. E.; Tiekink, E. R. T. Zeitschrift Fur
Kristallographie 1998, 213, 483-486.
(38) Lang, H.; del Villar, A.; Rheinwald, G. J. Organomet. Chem. 1999,
587, 284-289.
(39) Whittle, C. E. Ph.D. Dissertation, University of Florida, Gainesville,
2001.
(7) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-154.
(8) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. App.
Phys. 2001, 90, 5048-5051.
(9) Scurlock, R. D.; Wang, B. J.; Ogilby, P. R.; Sheats, J. R.; Clough,
R. L. J. Am. Chem. Soc. 1995, 117, 10194-10202.
(10) Beljonne, D.; Wittmann, H. F.; Ko¨hler, A.; Graham, S.; Younus,
M.; Lewis, J.; Raithby, P. R.; Khan, M. S.; Friend, R. H.; Bredas, J. L. J.
Chem. Phys. 1996, 105, 3868-3877.
(40) Claes, L.; Francois, J. P.; Deleuze, M. S. Chem. Phys. Lett. 2001,
339, 216-222.
(11) Walters, K. A.; Ley, K. D.; Schanze, K. S. J. Chem. Soc., Chem.
Commun. 1998, 10, 1115-1116.
(41) Albert, I. D. L.; Ramasesha, S. J. Phys. Chem. 1990, 94, 6540-
6543.
(12) Wilson, J. S.; Ko¨hler, A.; Friend, R. H.; Al-Suti, M. K.; Al-
Mandhary, M. R. A.; Khan, M. S.; Raithby, P. R. J. Chem. Phys. 2000,
113, 7627-7634.
(42) Saltiel, J.; Khalil, G. E.; Schanze, K. Chem. Phys. Lett. 1980, 70,
233-235.
(43) Wittmann, H. F.; Friend, R. H.; Khan, M. S.; Lewis, J. J. Chem.
Phys. 1994, 101, 2693-2698.
(13) Monkman, A. P.; Burrows, H. D.; Hamblett, I.; Navarathnam, S.;
Svensson, M.; Andersson, M. R. J. Chem. Phys. 2001, 115, 9046-9049.
(14) Monkman, A. P.; Burrows, H. D.; Hartwell, L. J.; Horsburgh, L.
E.; Hamblett, I.; Navaratnam, S. Phys. ReV. Lett. 2001, 86, 1358-1361.
(15) Ko¨hler, A.; Wilson, J. S.; Friend, R. H.; Al-Suti, M. K.; Khan, M.
S.; Gerhard, A.; Bassler, H. J. Chem. Phys. 2002, 116, 9457-9463.
(16) Walters, K. A.; Ley, K. D.; Cavalaheiro, C. S. P.; Miller, S. E.;
Gosztola, D.; Wasielewski, M. R.; Bussandri, A. P.; van Willigen, H.;
Schanze, K. S. J. Am. Chem. Soc. 2001, 123, 8329-8342.
(17) Wang, Y.; Liu, S.; Pinto, M. R.; Dattelbaum, D. M.; Schoonover,
J. R.; Schanze, K. S. J. Phys. Chem. A 2001, 105, 11118-11127.
(18) Liu, Y.; Li, Y.; Schanze, K. S. J. Photochem. Photobiol. C:
Photochem. ReV. 2002, 3, 1-23.
(44) Go¨rner, H.; Schulte-Fro¨hlinde, D. J. Phys. Chem. 1981, 85, 1835-
1841.
(45) Go¨rner, H.; Kuhn, H. AdV. Photochem. 1995, 19, 1.
(46) Schanze, K. S.; Lucia, L. A.; Cooper, M.; Walters, K. A.; Ji, H.-
F.; Sabina, O. J. Phys. Chem. A 1998, 102, 5577-5584.
(47) We note that the fluorescence of 3 is red-shifted relative to its
position in the other complexes. The red-shift may have its origin in a steric
or electronic interaction between the ethynylstilbene and dppe ligands.
(48) Grosshenny, V.; Harriman, A.; Hissler, M.; Ziessel, R. J. Chem.
Soc., Faraday Trans. 1996, 92, 2223.
(49) Walters, K. A.; Schanze, K. S. The Spectrum (Newsletter of the
Center for Photochemical Sciences, Bowling Green State University, http://
www.bgsu.edu/departments/photochem/Summer1998Spectrum.pdf) 1998,
11, 1-4.
(50) McGlynn, S. P.; Smith, F. J.; Cilento, G. Photochem. Photobiol.
1964, 3, 269-294.
(51) Hammond, H. A.; DeMeyer, D. E.; Williams, J. L. R. J. Am. Chem.
Soc. 1969, 91, 5180-5181.
(52) Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C.
V.; McCusker, J. K. Science 1997, 275, 54-57.
(53) Hammond, G. S.; Saltiel, J. J. Am. Chem. Soc. 1963, 85, 2516-
2517.
(19) 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.
(20) Liu, Y.; Jiang, S. J.; Glusac, K.; Powell, D. H.; Anderson, D. F.;
Schanze, K. S. J. Am. Chem. Soc. 2002, 124, 12412-12413.
(21) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605-1644.
(22) Chawdhury, N.; Ko¨hler, A.; Friend, R. H.; Younus, M.; Long, N.
J.; Raithby, P. R.; Lewis, J. Macromolecules 1998, 31, 722-727.
(23) Saltiel, J.; D’Agostino, J.; Megarity, E. D.; Metts, L.; Neuberger,
K. R.; Wrighton, M.; Zafiriou, O. C. In Organic Photochemistry; Chapman,
O. L., Ed.; Marcel Dekker: New York, 1973; Vol. 3, p 1.
(24) Go¨rner, H.; Schulte-Frohlinde, D. J. Phys. Chem. 1979, 83, 3107-
3118.
(25) Go¨rner, H. J. Phys. Chem. 1989, 93, 1826-1832.
(26) Waldeck, D. H. Chem. ReV. 1991, 91, 415-436.
(27) Cooper, T. M.; McLean, D. G.; Rogers, J. E. Chem. Phys. Lett.
2001, 349, 31-36.
(28) Rogers, J. E.; Cooper, T. M.; Fleitz, P. A.; Glass, D. J.; McLean,
D. G. J. Phys. Chem. A 2002, 106, 10108-10115.
(29) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-457.
(30) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S.
Inorg. Chem. 2001, 40, 4053-4062.
(31) Khan, M. S.; Kakkar, A. K.; Long, N. J.; Lewis, J.; Raithby, P.;
Nguyen, P.; Marder, T. B.; Wittmann, F.; Friend, R. H. J. Mater. Chem.
1994, 4, 1227-1232.
(32) James, S. L.; Younus, M.; Raithby, P. R.; Lewis, J. J. Organomet.
Chem. 1997, 543, 233-235.
(54) Saltiel, J.; Hammond, G. S. J. Am. Chem. Soc. 1963, 85, 2515-
1516.
(55) Saltiel, J.; Mace, J. E.; Watkins, L. P.; Gormin, D. A.; Clark, R.
A.; Dmitrenko, O. J. Am. Chem. Soc. 2003, 125, 16158-16159.
(56) Saltiel, J.; Marchand, G. R.; Kirkorkaminska, E.; Smothers, W. K.;
Mueller, W. B.; Charlton, J. L. J. Am. Chem. Soc. 1984, 106, 3144-3151.
(57) Plevyak, J. E.; Dickerson, J. E.; Heck, R. F. J. Org. Chem. 1979,
44, 4078-4080.
(58) Cook, M. J.; Lewis, A. P.; McAuliffe, G. S. G.; Skarda, V.;
Thomson, A. J.; Glasper, J. L.; Robbins, D. J. J. Chem. Soc.-Perkin Trans.
2 1984, 1293-1301.
(59) Chock, P. B.; Halpern, J.; Paulik, F. E. Inorg. Synth. 1990, 28,
349-351.
(60) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411-
428.
(61) Wang, Y. S.; Schanze, K. S. Chem. Phys. 1993, 176, 305-319.
(62) Heller, H. G.; Langan, J. R. J. Chem. Soc.-Perkin Trans. 2 1981,
341-343.