Spectroscopic characterization of a series of platinum acetylide complexes having a localized triplet exciton

Article abstract of DOI:10.1021/jp056663q

In this work, we describe the spectroscopic properties of a series of platinum complexes containing one acetylide ligand per platinum, having the chemical formula trans-Pt(PBu₃)₂((C≡CC ₆H₄)_n-H)Cl, n = 1-3 (designated as half-PEn-Pt) and compare their spectroscopic behavior with the well-characterized series trans-Pt(PBu₃)₂((C≡CC₆H₄) _n-H)₂, n = 1-3 (designated as PEn-Pt). This comparison aims to determine if the triplet state of PEn-Pt is confined to one ligand or delocalized across the central platinum atom. We measured ground-state absorption spectra, fluorescence spectra, phosphorescence spectra, and triplet-state absorption spectra. The ground-state absorption spectra and fluorescence spectra both showed a blue shift when comparing half-PEn-Pt with PEn-Pt, showing the Si state is delocalized across the platinum. In contrast, the phosphorescence spectra of the two types of compounds had the same 0-0 band energy, showing the T₁ state was confined to one ligand in PEn-Pt. The triplet state absorption spectra blue shifted when comparing half-PEn-Pt with PEn-Pt, showing the T_n state was delocalized across the central platinum. This comparison supports recently published work that suggested this confinement effect

Full text of DOI:10.1021/jp056663q

J. Phys. Chem. A 2006, 110, 4369-4375
4369
Spectroscopic Characterization of a Series of Platinum Acetylide Complexes Having a
Localized Triplet Exciton
Thomas M. Cooper,*,† Douglas M. Krein,†,‡ Aaron R. Burke,†,‡ Daniel G. McLean,†,§
Joy E. Rogers,†,| Jonathan E. Slagle,†, and Paul A. Fleitz†
Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base,
Ohio 45433, Anteon Corporation, Dayton, Ohio 45431, SAIC, Dayton, Ohio 45434, UES, Inc., Dayton,
Ohio 45432, and AT&T Corporation, Dayton, Ohio 45434
ReceiVed: NoVember 17, 2005; In Final Form: February 13, 2006
In this work, we describe the spectroscopic properties of a series of platinum complexes containing one
acetylide ligand per platinum, having the chemical formula trans-Pt(PBu3)2((CtCC6H4)nsH)Cl, n ) 1-3
(designated as half-PEn-Pt) and compare their spectroscopic behavior with the well-characterized series trans-
Pt(PBu3)2((CtCC6H4)nsH)2, n ) 1-3 (designated as PEn-Pt). This comparison aims to determine if the
triplet state of PEn-Pt is confined to one ligand or delocalized across the central platinum atom. We measured
ground-state absorption spectra, fluorescence spectra, phosphorescence spectra, and triplet-state absorption
spectra. The ground-state absorption spectra and fluorescence spectra both showed a blue shift when comparing
half-PEn-Pt with PEn-Pt, showing the S1 state is delocalized across the platinum. In contrast, the
phosphorescence spectra of the two types of compounds had the same 0-0 band energy, showing the T1
state was confined to one ligand in PEn-Pt. The triplet state absorption spectra blue shifted when comparing
half-PEn-Pt with PEn-Pt, showing the Tn state was delocalized across the central platinum. This comparison
supports recently published work that suggested this confinement effect (Rogers, J. E et al. J. Chem. Phys.
2005, 122, 214701).
Introduction
on one ligand.8 In our group, we measured time-resolved
infrared spectra of the triplet state trans-Pt(PBu3)2((CtCs
Platinum acetylide complexes are good systems to probe
triplet-state phenomena like ground-state absorption to the triplet
state (S0fT1), intersystem crossing (S1fT1), triplet-state ab-
sorption spectrum (T1fTn), and phosphorescence (T1fS0).1,2
In our group, we have been investigating the relation between
chemical structure and spectroscopic properties in platinum
acetylide complexes such as trans-Pt(II)(PBu3)2((CtCC6H4)ns
H)2, where n ) 1-3.3-5 An issue that has been investigated by
our group as well as other groups is the relation between
molecular structure and the delocalization of the singlet and
triplet excitons. A theoretical and experimental investigation of
the polymer poly(Pt(PBu3)2(CtCC6H4)) finds the triplet exciton
T1 is localized on a single phenylene ring while the S1 and Tn
excitons are delocalized over several monomer units.6 More
recently, a series of oligomers having the formula C6H5(Ct
CsPt(PBu3)2sCtCC6H4)nsH, n ) 1-5, 7 has been prepared.7
The ground-state absorption and fluorescence spectra show
systematic red shifts as the oligomer length increases. In
contrast, the phosphorescence spectra show only small red shifts
with increasing oligomer length. The results support the idea
that the triplet state is more localized than the singlet state. An
investigation of the photophysics and photochemistry of plati-
num acetylide stilbenes gives evidence that triplet state resides
C6H4sCtCC6H5)2.9 We also did density functional theory
calculations of the S0 and T1 state and found the S0 state contains
contributions from Pt 5d and ligand π orbitals. The T1 state
contains only contributions from ligand π* orbitals. From
symmetry considerations, the calculations predict the triplet state
will reside on both ligands. A related time-resolved infrared
spectroscopy and theoretical study suggests the triplet exciton
is confined to one ligand.10 In another study, the intervalence
charge-transfer bands of a monocationic diphenylamino-
substituted platinum acetylide complex is compared with a
model compound.11 They conclude the platinum atom has only
a small effect on the electronic delocalization of the molecule.
To help resolve these issues, we recently published a detailed
investigation of the photophysical properties of a series of
butadiynes having the formula Hs(C6H4sCtC)ns(CtCs
C6H4)nsH, n ) 1-3 and ligands Hs(C6H4sCtC)nsH, n )
1-3 and compared these to previous work done on a compli-
mentary series of platinum-containing complexes having the
formula trans-Pt((P(C4H9)3)2((CtCsC6H4)nsH)2, n ) 1-3.5,12
By analysis of the behavior of the singlet and triplet state with
increasing molecular size, we found evidence that the singlet
state is delocalized throughout the molecule but that the triplet
state is confined to one ligand in the platinum complexes.
Recently, another theoretical study also suggests the triplet state
is confined to one ligand.13
* To whom correspondence should be addressed. E-mail:
Thomas.Cooper@wpafb.af.mil.
† Materials and Manufacturing Directorate, Air Force Research Labora-
tory, Wright-Patterson Air Force Base, OH 45433.
‡ Anteon Corporation, Dayton, OH 45431.
§ SAIC, Dayton, OH 45434.
In this work, we describe the synthesis and spectroscopic
properties of a series of platinum complexes containing one
acetylide ligand per platinum, having the chemical formula
trans-Pt((PC4H9)3)2((CtCsC6H4)nsH)Cl, n ) 1-3 (designated
as half-PEn-Pt) and compare their spectroscopic behavior with
| UES, Inc., Dayton, OH 45432.
AT&T Corporation, Dayton, OH 45434.
10.1021/jp056663q CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/15/2006

4370 J. Phys. Chem. A, Vol. 110, No. 13, 2006
Cooper et al.
Mw ) 836.41. IR: (KBr, thin film) 2114 cm-1 ν(PtsCtC).
1H NMR (CDCl3): δ 0.95 (m, 18H, CH3), 1.46 (m, 12H, CH2),
1.59 (m, 12H, CH2), 2.02 (m, 12H, CH2), 7.21-7.54 (m, 9H,
ArH) ppm. 13C NMR (CDCl3): δ 14.07 (s, CH3), 22.29 (t, J(CP)
) 17 Hz, CH2), 24.58 (t, J(CP) ) 7 Hz, CH2), 26.35 (s, CH2),
87.32 (t, J(CPt) ) 14.3 Hz, Pt-CtC), 101.61 (t, J(CPt) ) 2.5
Hz, CtC), 90.27 (s, CtC), 90.19 (s, CtC), 119.8, 123.8, 128.3,
128.6, 129.3, 130.9, 131.5, 131.7(Ar) ppm. 31P NMR (CDCl3):
δ 8.14 (s and d centered at δ8.14 J(PPt) ) 2366 Hz, PBu3)
ppm. EIMS: (m/z) 836.
Figure 1. List of compounds discussed in this study.
PtCl(CtCsC6H4sCtCsC6H4sCtCsC6H5)(PBu3)2 (half-
PE3-Pt). A solution of PtCl2(PBu3)2 (755 mg, 1.1 mmol) and
C6H5CtCsC6H4CtCsC6H4CtCH (300 mg, 1.0 mmol) in
NHEt2 (25 mL) and CuI (20 mg, 0.17 mmol) was heated in a
microwave for 30 min at 115 °C. Solvent was removed and the
yellow residue dissolved in CH2Cl2 and filtered through an
alumina plug. The residue was purified with reverse-phase
chromatography, forming a yellow solid, identified as PtCl(Ct
CsC6H4CtCsC6H4sCtCsC6H5)(PBu3)2 (365 mg, 39%).
MA: found C, 61.28; H, 7.11%. C48H67ClP2Pt requires C, 61.56;
H, 7.21%. Mw ) 936.52. IR: (KBr, thin film) 2113 cm-1 ν-
the well-characterized series trans-Pt((PC4H9)3)2((CtCsC6H4)ns
H)2, n ) 1-3 (designated as PEn-Pt)(Figure 1) The previous
investigations5,12 provide a baseline study with which it is
possible to compare their spectroscopic behavior with the
asymmetric complexes. This study is a direct measurement of
the delocalization of the singlet and triplet excitons in platinum
acetylide complexes. The results from this work show the singlet
exciton of PEn-Pt complexes is delocalized across the platinum
with contributions from both ligands, while the triplet exciton
is confined to one ligand.
1
(PtsCtC). H NMR (CDCl3): δ 0.96 (m, 18H, CH3), 1.47
General Synthesis Techniques
(m, 12H, CH2), 1.61 (m, 12H, CH2), 2.04 (m, 12H, CH2), 7.22-
7.56 (m, 13H, ArH) ppm. 13C NMR (CDCl3): δ 13.93 (s, CH3),
22.45 (t, J(CP) ) 17 Hz, CH2), 24.58 (t, J(CP) ) 7 Hz, CH2),
26.37 (s, CH2), 87.93 (t, J(CPt) ) 14.3 Hz, Pt-CtC), 101.72
(t, J(CPt) ) 2.5 Hz, CtC), 89.47 (s, CtC), 90.01 (s, CtC),
90.49 (s, CtC), 92.18 (s, CtC), 119.6, 123.2, 123.4, 123.7,
128.6, 128.8 129.6, 130.9, 131.5, 131.7, 134.0(Ar) ppm. 31P
NMR (CDCl3): δ 8.26 (s and d centered at δ 8.26, J(PPt) )
2374 Hz, PBu3) ppm. EIMS: (m/z) 936.
All reactions were carried out using dry, distilled solvents
and under dry, high-purity nitrogen. All reagents were purchased
from Aldrich Chemical Co. and used without further purifica-
tion. Alumina column refers to a support of Al2O3 (Activated,
neutral, Brockman grade I, standard grade, ∼150 mesh, 58 Å).
Reverse-phase column refers to Alltech Extract-Clean C18.
Microwave refers to CEM Inc. Discover System. The com-
pounds Pt(CtCsC6H5)2(PBu3)2 (PE1-Pt), Pt(CtCsC6H4sCt
CsC6H5)2(PBu3)2 (PE2-Pt) and Pt(CtCsC6H4sCtCsC6H4s
CtCsC6H5)2(PBu3)2 (PE3-Pt) were synthesized as described
previously.5
General Spectroscopy Techniques
All absorption and fluorescence spectra were obtained from
benzene solutions. Ground-state UV/Vis absorption spectra were
measured on a temperature-controlled Cary 500 spectropho-
tometer. Emission spectra at 5-nm slit width were measured
using a Perkin-Elmer model LS 50B fluorometer. Low-
temperature phosphorescence was done in methyltetrahydrofuran
as a frozen glass at 77 K and exciting at 350 nm except half-
PE1-Pt and PE1-Pt, which were excited at 325 nm. Nanosecond
transient absorption measurements were carried out using the
third and fourth harmonics (355 and 266 nm) of a Q-switched
Nd:YAG laser (Quantel Brilliant, pulse width ca. 5 ns). All
samples were deoxygenated with three freeze-pump-thaw
cycles. Pulse fluences of up to 8 mJ cm-2 are typically used at
the excitation wavelength. Ground-state absorption spectra were
obtained before and after the flash photolysis experiment. Most
samples showed less than 10% degradation. If necessary, spectra
were collected from photosensitive samples by collecting the
spectrum in a 100-nm increment and then putting a fresh sample
into the instrument. A detailed description of the laser flash
photolysis apparatus has been published.5
Synthesis
PtCl(CtCsC6H5)(PBu3)2 (half-PE1-Pt). A solution of
PtCl2(PBu3)2 (739 mg, 1.1 mmol) and C6H5CtCH (102 mg,
1.0 mmol) in NHEt2 (25 mL) and CuI (33 mg, 0.17 mmol) was
heated in a microwave for 30 min at 115 °C. Solvent was
removed and the yellow residue dissolved in CH2Cl2 and filtered
through an alumina plug. The residue was purified with reverse-
phase chromatography, forming a yellow oil, identified as PtCl-
(CtCsC6H5)(PBu3)2 (407 mg, 55%). MA: found C, 52.09;
H, 7.66%. C32H59ClP2Pt requires C, 52.20; H, 8.08%. Mw
)
736.29. IR: (KBr, thin film) 2119 cm-1 ν(PtsCtC). 1H NMR
(CDCl3): δ 0.93 (m, 18H, CH3), 1.45 (m, 12H, CH2), 1.59 (m,
12H, CH2), 2.02 (m, 12H, CH2), 7.08-7.28 (m, 5H, ArH) ppm.
13C NMR (CDCl3): δ 14.03 (s, CH3), 22.26 (t, J(CP) ) 17 Hz,
CH2), 24.55 (t, J(CP) ) 7 Hz, CH2), 26.35 (s, CH2), 83.28 (t,
J(CPt) ) 14.6 Hz, Pt-CtC), 101.28 (t, J(CPt) ) 2.5 Hz, Ct
C), 125.31, 128.10, 129.17, 130.9(Ar) ppm. 31P NMR
(CDCl3): δ 8.07 (s and d centered at δ8.07, J(PPt) ) 2380 Hz,
PBu3) ppm. EIMS: (m/z) 736.
Fluorescence quantum yields were determined using the
actinometry method previously described.5 Quinine sulfate was
used as an actinometer with a known fluorescence quantum yield
of 0.55 in 1.0 N H2SO4.14 To minimize the contribution of
phosphorescence to the emission spectrum, all samples were
measured under air-saturated conditions and excited at 350 nm-
(325 nm for half-PE1-Pt) with a matched optical density of
0.1.
PtCl(CtCsC6H4sCtCsC6H5)(PBu3)2 (half-PE2-Pt). A
solution of PtCl2(PBu3)2 (748 mg, 1.1 mmol) and C6H5CtCs
C6H4CtCH (203 mg, 1.0 mmol) in NHEt2 (25 mL) and CuI
(33 mg, 0.17 mmol) was heated in a microwave for 30 min at
115 °C. Solvent was removed and the yellow residue dissolved
in CH2Cl2 and filtered through an alumina plug. The residue
was purified with reverse-phase chromatography, forming a
yellow oil that slowly solidified, identified as PtCl(CtCs
C6H4sCtCsC6H5)(PBu3)2 (329 mg, 39%). MA: found C,
57.28; H, 7.50%. C40H63ClP2Pt requires C, 57.44; H, 7.59%,
Results and Discussion
Figure 2 summarizes IR and 13C NMR data from half-PEn-
Pt and PEn-Pt. The acetylenic PtsC1tC2 stretch frequency

Series of Platinum Acetylide Complexes
J. Phys. Chem. A, Vol. 110, No. 13, 2006 4371
Figure 2. Summary of 13C NMR and IR data for the acetylenic carbons.
Figure 3. Ground-state absorption spectra of samples in benzene.
of PEn-Pt ranges from 2092 to 2102 cm-1, while that of half-
PEn-Pt ranges from 2113 to 2119 cm-1. The lower frequency
in PEn-Pt reflects back-bonding from Pt 5d orbital to the
ligand’s lowest-unoccupied molecular orbital (LUMO), lowering
the bond order.15 The vibration frequency observed in half-
PEn-Pt is in the range of monosubstituted acetylenes.16 The
13C NMR chemical shifts of PEn-Pt of C1 and C2 range from
108.3 to 112.4 ppm and from 109.1 to 109.6 ppm, respectively.
In contrast, the shifts of half-PEn-Pt in C1 and C2 range from
83.3 to 87.9 ppm and from 101.3 to 101.7 ppm. The large
downfield shift in half-PEn-Pt results from lower electron
density on the terminal acetylene, especially in C1. These
changes in the ground-state charge distribution result from
the electron-withdrawing effect of the chlorine atom in
half-PEn-Pt.
TABLE 1: Summary of Ground State Absorption Spectra
Data
extinction
absorbance
(λmax, nm)
coefficient
oscillator
strengthb
compound
(M-1 cm-1
)
half-PE1-Pt
half-PE2-Pt
half-PE3-Pt
PE1-Pt
PE2-Pt
PE3-Pt
309(sh)
344
350
324
355
12 170 (700)a
53 600 (6600)
60 800 (2500)
24 700 (500)
89 000 (1200)
85 200 (1800)
0.31c
1.33
1.83
0.63
1.95
2.25
377
a Number in parentheses is the standard deviation. Spectra were
measured in benzene. b Oscillator strength was determined by fitting
the absorption spectrum to three Gaussians and calculating the area.25
c The oscillator strength for half-PE1-Pt was estimated from the ratio
of its extinction coefficient to that of PE1-Pt.
The ground-state absorption spectra of platinum acetylide
complexes are ππ* transitions with metal-to-ligand charge
transfer (MLCT) character.12 The highest-occupied molecular
orbital (HOMO) consists of π orbitals on the acetylene and
aromatic groups with contribution from the 5dxy orbital on the
platinum, while the LUMO consists of π* orbitals with no
contribution from the platinum.10,17 Both the HOMO and LUMO
are delocalized across the platinum. We previously found in
the PEn-Pt complexes that, as the size of the ligand increased,
the ππ* character of the optical transitions increased.5 By
assumption of D2h symmetry, the most intense optical transition
is a HOMO to LUMO (b2g configuration f b3u configuration)
transition to the B1u state polarized parallel to the molecular
axis.18 Similarly, the half-PEn-Pt complexes have C2V sym-
metry. In this case, the HOMO to LUMO (b1 configuration f
b1 configuration) transition to the A1 state will also be polarized
along the molecular axis. The symmetry argument predicts the
ground-state absorption spectra of the half-PEn-Pt complexes
will be blue-shifted from their PEn-Pt counterparts. Figure 3
and Table 1 give ground-state absorption spectra data of half-
PEn-Pt in comparison with those of PEn-Pt in benzene. For
all three ligands, the ground-state absorption spectrum of half-
PEn-Pt is blue-shifted from that of PEn-Pt. These results give
evidence the S0 and S1 states are delocalized across the central
platinum atom.
difference between asymmetric and symmetric complexes. As
the ground-state absorption spectra have MLCT character, the
S1 state will be similar to the LUMO of the ligand. Published
investigations of the excited states of phenyl acetylene and
diphenyl butadiyne provide insight into the ligand excited state.
The spectroscopic properties of the asymmetric complexes are
similar to phenylacetylene, where the aromatic rings of the
LUMO have a quinone-like structure and the acetylene π*
orbitals are antibonding.19,20 The ground-state spectrum band
shape results from molecular vibrations in the LUMO, including
CtC and CdC stretch modes. Similarly, the ground-state
absorption spectrum of diphenyl butadiyne is a model of
symmetric complexes.21,22 As in phenyl acetylene, the aromatic
rings become quinone-like and the terminal acetylene π* orbitals
become antibonding. The excited-state vibrations are similar to
those seen in diphenyl butadiyne, where CtC and CdC stretch
modes have been reported. Finally, The S1 state of PE1-Pt has
been shown to be ligand-centered with quinone-like aromatic
rings and antibonding acetylene π* orbitals.13 On the basis of
these results, the overlapping bands have vibronic contributions.
The relative intensity of the bands reverses upon going from
PEn-Pt to half-PEn-Pt. For example, in PE2-Pt I(361 nm)/
I(352 nm) ) 1.20, while in half-PE2-Pt I(345 nm)/I(330 nm)
)1.03. Similarly, in PE3-Pt I(377)/I(360) ) 1.09, while in half-
PE3-Pt I (371 nm)/ I(350 nm) ) 0.80. The changes in band
shape suggest a decrease in 0-0 band intensity and increase in
0-1, 0-2, etc. intensity in the half-PEn-Pt complexes. The S1
The spectra of all the complexes in Figure 3 contain at least
two overlapping bands. Comparison of the band shapes for half-
PE2-Pt vs PE2-Pt and half-PE3-Pt vs PE3-Pt show a

4372 J. Phys. Chem. A, Vol. 110, No. 13, 2006
Cooper et al.
Figure 4. Fluorescence spectra. All samples are in benzene and excited
at 290 nm (half-PE1-Pt), 325 nm (PE1-Pt and half-PE2-Pt), or 350
nm (PE2-Pt, half-PE3-Pt and PE3-Pt). Emission marked with asterisk
is room-temperature phosphorescence of PE1-Pt.
Figure 5. Phosphorescence spectra of complexes dissolved in a Me-
THF glass at 77 K.
TABLE 3: Summary of Triplet State Absorption Spectra
Data
TABLE 2: Summary of Fluorescence and Phosphorescence
Data
compound
T1-Tnλmax (nm)
ETTmax (eV)
Flλ
ES
Phλ
ET
∆EST
N/Aa
570
620
630b
586
640
N/A
2.18
2.00
1.97
2.12
1.94
max
(nm)a
max
half-PE1-Pt
half-PE2-Pt
half-PE3-Pt
PE1-Pt
PE2-Pt
PE3-Pt
compound
φFl
(eV) (nm)b (eV)
(eV)
half-PE1-Pt
half-PE2-Pt
half-PE3-Pt
PE1-Pt
PE2-Pt
PE3-Pt
349
367
391
364
385
400
0.004
0.009
0.088
<0.0006
0.001
3.91
3.56
3.30
3.58
3.29
3.21
435
524
552
435
527
557
2.82
2.39
2.28
2.82
2.38
2.26
1.09
1.16
1.02
0.76
0.91
0.95
a Unable to excite sample at 355 nm. b Data taken from ref 5.
0.016
a Fluorescence spectra collected in benzene. b Phosphorescence
spectra collected in Me-THF at 77 °K.
For both half-PEn-Pt and PEn-Pt, the fluorescence quantum
yield increases with ligand length. For a given ligand length,
the quantum yield is 5-10× larger in half-PEn-Pt. The increase
in quantum yield with ligand length has been attributed to the
increasing ππ* character of the excited state with ligand length.5
One factor influencing fluorescence quantum yield is the rigidity
of the chromophore. The PEn-Pt chromophores have two
ligands and the S1 state is delocalized across Pt atom. In contrast,
the half-PEn-Pt chromophores have the S1 excited-state
confined to one ligand. Twisting around the various bonds brings
the S1 state to a geometry that allows for radiationless conversion
to the ground state. There is more conformation flexibility(two
acetylide ligands) in the PEn-Pt chromophores than in the half-
PEn-Pt chromophores(one acetylide ligand). Rotation about the
bonds promotes radiationless conversion to the ground state by
providing a pathway for dissipation of excess thermal energy
from the excited state, promoting increased radiationless decay
and a lower fluorescence quantum yield.25
Figure 5 gives phosphorescence spectra of the half-PEn-Pt
complexes and the PEn-Pt complexes dissolved in a glass. Table
2 gives additional phosphorescence data. If the T1 state is
delocalized across the platinum, we would expect the phospho-
rescence spectra of the half-PEn-Pt complexes to be blue-
shifted from those of the PEn-Pt complexes. In contrast to the
ground-state absorption and fluorescence spectra described
above, conversion of PEn-Pt to half-PEn-Pt results in virtually
no difference in the emission spectra. Especially in PE1-Pt vs
half-PEn-Pt, the 0-0 bands have nearly the same energy. There
is a slight blue shift in half-PE2-Pt and half-PE3-Pt, suggesting
a small amount of delocalization across the platinum. The
phosphorescence spectra give evidence that the T1 state is nearly
state is delocalized across the Pt in PEn-Pt but is confined to
one ligand in PEn-Pt. The changes in band shape give evidence
for a more distorted S1 state in half-PEn-Pt. As discussed below,
similar changes in band shape occur in the phosphorescence
spectrum.6,7 Although the intensity ratio effects probably result
from excited state geometry differences, the oscillator strengths
approaching 2 in the larger complexes suggest the presence of
more than one strongly allowed electronic transition in the
ground-state absorption spectrum.
For a series of chromophores which consist of a number of
repeat units, the oscillator strength and extinction coefficient
of the ground-state absorption spectrum increases with chro-
mophore length23 but will be limited by conformation twisting
effects.24 The trend in the oscillator strength ratios is: f(PEn-
Pt)/f(half-PEn-Pt) ) 2.0, 1.5 and 1.2 for n ) 1, 2, and 3. If
the S1 state were ligand-centered, the ratio would be 2.0 for all
three types of chromophores. If the S1 state were delocalized
across the platinum, the oscillator strength would also increase
with length, but the ratio would not necessarily be 2.0. We
observe the latter trend, so the oscillator strength data is con-
sistent with the S1 state being delocalized across the platinum.
Figure 4 and Table 2 give fluorescence spectra data of half-
PEn-Pt and PEn-Pt. Emission from the S1 state occurs after
solvent and molecular relaxation. As with the ground-state
absorption spectra, the emission spectra of all the half-PEn-Pt
complexes are blue-shifted from those of the PEn-Pt counter-
parts. From these results, we conclude the fluorescing S1 state
is delocalized across the platinum atom.

Series of Platinum Acetylide Complexes
J. Phys. Chem. A, Vol. 110, No. 13, 2006 4373
entirely confined to one of the ligands. Similar behavior has
been seen comparing the phosphorescence of ClPt(PBu3)2Ct
CC6H4CtCPt(PBu3)2Cl and the polymer (Pt(PBu3)2CtCC6H4Ct
C)n,6 as well as oligomers of increasing length.7 The spectral
shift of the triplet state 0-0 band upon conversion from
monomer to polymer is very small, showing evidence of strong
confinement of the triplet exciton. These results support
published results suggesting the intersystem crossing process
in PEn-Pt consists of the S1 state, having D2h symmetry, being
delocalized across the platinum atom, followed by movement
of the T1 exciton to one ligand, having C2V symmetry.10 The
intersystem crossing process has been proposed to occur by
distortion along the B3u antisymmetric stretch vibration mode,
where the CtC triple bond attached to the platinum stretches
on one ligand and shortens on the other. The stretching stabilizes
the π* antibonding orbitals on one ligand and the shortening
stabilizes the π bonding orbitals on the other ligand. The conical
intersection between the singlet D2h potential energy surface
and the lower-energy triplet C2V surface resides where one of
the triple bonds is shortened and the other is lengthened. Upon
spin flip, the triplet exciton resides on one ligand.
Figure 6. Triplet state absorption spectra in benzene for half-PEn-Pt
and PEn-Pt, n ) 2 and 3.
Additional evidence for symmetry changes during intersystem
crossing comes from measurements of phosphorescence polar-
ization in platinum acetylide polymers.18 This study shows that
the S1 state is polarized parallel to the molecular axis while the
T1 state is polarized perpendicular to the molecular axis. The
change in polarization results from spin-orbit-coupling-induced
state mixing. The vibronic sidebands of the emission spectrum
of half-PE1-Pt are more intense than those of PE1-Pt. This
behavior is similar to a comparison of platinum acetylide
monomer and polymer6 and oligomers of increasing length7
where a Huang-Rhys analysis of the vibronic sidebands shows
more distortion of the T1 state vs the S1 state than the polymer.
Even though the T1 state energy of half-PE1-Pt is the same as
that of PE1-Pt, intersystem crossing from the half-PE1-Pt S1
state to the T1 state results in a more distorted T1 state. Because
of the large fluorescent background in the phosphorescence
spectra of half-PE2-Pt and half-PE3-Pt, we could not
make band shape comparisons with the spectra of PE2-Pt and
PE3-Pt.
The average singlet-triplet splitting ∆EST for half-PEn-Pt
is 1.09 ( 0.07 eV, while it is 0.87 ( 0.10 eV in PEn-Pt. This
can be easily explained as the singlet-triplet splitting is
proportional to the overlap integral between the two singly
occupied molecular orbitals (SOMOs) of the triplet state. Both
the SOMO1 and SOMO2 of the half-PEn-Pt chromophores are
located on the half-chromophore. In contrast, SOMO1 and
SOMO2 of PEn-Pt probably have some delocalization through
the platinum atom into the other ligand. As a result, the overlap
integral is larger in the half-PEn-Pt complexes, resulting in a
larger ∆EST.
Figure 6 and data from Table 3 show triplet state absorption
spectra for half-PEn-Pt and PEn-Pt, n ) 2 or 3. Unlike the
phosphorescence results described above, the triplet-state ab-
sorption spectra suggest that the Tn state is delocalized across
the platinum center. The triplet state absorption spectrum of
PE2-Pt is considerably broader than that of half-PE2-Pt. The
phosphorescence data show that the T1 state is confined to one
ligand of PE2-Pt. Clearly the Tn state of PE2-Pt is delocalized
across the central platinum and well into the other ligand. The
band shapes for half-PE3-Pt and PE3-Pt are similar. In our
earlier work we showed the excited states of PEn-Pt have more
ππ* character and platinum influence as n increases to 3.5 The
similarity in shape may say the Tn states of half-PE3-Pt and
Figure 7. Plot of the state energy difference E(half-PEn-Pt) - E(PEn-
Pt) as a function of the number of phenyl ethynyl groups per ligand.
Energies plotted are ES, ET, and ETT
.
PE3-Pt have mostly ππ* character. For both n ) 2 and 3, the
triplet state absorption spectrum of half-PEn-Pt is blue shifted
compared to that of PEn-Pt. Theoretical calculations of the
spatial extent of singlet and triplet excitons in platinum acetylide
polymers have estimated the Tn exciton to be at least three
repeating units in size.6 The authors of that paper compare triplet
state absorption spectra of the model compound ClPt(PBu3)2Ct
CC6H4CtCPt(PBu3)2Cl and the polymer (Pt(PBu3)2CtCC6H4Ct
C)n. They observe a large red shift upon conversion of the
monomer to the polymer, again giving evidence the Tn state is
delocalized across the platinum center. Polarized photoinduced
absorption experiments have shown the T1-Tn transition is
polarized parallel to the molecular axis.18 As a result, the Tn
state has ligand-to-metal charge-transfer character, with a
contribution from the 5dxz orbital residing on the platinum center.
Figure 7 gives a plot of the state energy difference ∆E )
E(half-PEn-Pt) - E(PEn-Pt) as a function of n. Energies
plotted are ES, ET, and ETT. The largest effects appear for n )
1, with a substantial blue shift in ∆ES. The magnitude of the
shift becomes smaller for n ) 2 and 3. This supports the idea

4374 J. Phys. Chem. A, Vol. 110, No. 13, 2006
Cooper et al.
Figure 8. Plot of ES and ET vs the reciprocal of chromophore length
(Å-1). Also included for comparison are fitted lines for butadiynes and
ligands.12
Figure 9. Plot of transition length (e Å) as a function of the reciprocal
of chromophore length (Å-1).
and ligands, showing a stronger sensitivity of ET to chromophore
length. The metal effects are largest in half-PE1-Pt and PE1-
Pt, with small effects in the other platinum-containing chro-
mophores. This result reflects the increased ππ* character of
the excited states with larger ligands.
To find further evidence for delocalization of the singlet state
through the platinum atom, we calculated the transition length
from the oscillator strength25
that the S1 state is delocalized across the platinum center in
PEn-Pt. In comparison, ∆ET ) 0 for n ) 1 and is slightly
positive for n ) 2 and 3. The T1 state is definitely confined to
one ligand in PE1-Pt. In the other two types of chromophores
there is slight delocalization across the platinum center. In our
previous work we have shown the optical transitions of the PEn-
Pt chromophores have mixed MLCT and ππ* character.5 As
the ligand size increases, the ππ* character increases. A possible
explanation for this is spin-orbit-coupling-induced mixing of
the T1 state with higher energy Sn states that are delocalized
over the entire chromophore.18 The energy difference ∆ETT does
not change as n increases from 2 to 3, although, as shown in
Figure 6, there are major bandwidth differences in the triplet
state spectrum of half-PE2-Pt vs PE2-Pt not seen in half-PE3-
Pt vs PE3-Pt. Because of the bandwidth changes, ∆ETT is not
an accurate descriptor of the effect of n on the triplet state
spectrum. An alternative descriptor is the energy shift at half-
maximum. In this case, the energy shift is 0.23 eV for n ) 2
and 0.07 eV for n ) 3, showing the Tn state is delocalized across
the platinum atom.
In a previous publication we have shown a plot of ES and ET
vs the reciprocal of end-to-end chromophore length gives a
straight line.12 The maximum conjugation length can be
estimated from the intercept. Figure 8 shows a plot of ES and
ET vs 1/L. L is defined as the end-to-end chromophore length,
calculated with Cambridgesoft Inc. Chem3D software. Because
of the electron-withdrawing and possible conjugation effects
of chlorine shown in Figure 2, the chromophore length of
half-PEn-Pt included the default Pt-Cl bond length of 2.2905
Å. Also included in this plot is the regression line obtained
previously from butadiynes and ligands.12 Optical transitions
in the butadiynes and ligands have ππ* character and can be
used for comparison with the analogous platinum acetylides.
The slope of the line measures the sensitivity of the state energy
to increasing chromophore length, while the intercept estimates
the state energy at large chromophore length. The ES plots for
half-PEn-Pt and PEn-Pt are very similar to that obtained from
butadiynes and ligands. This suggests the S1 state of both half-
PEn-Pt and PEn-Pt has the same dependence on length as the
butadiynes and ligands, showing it is delocalized throughout
the entire chromophore. The plots for the platinum complex’s
triplet state have larger slopes than found for the butadiynes
8π2me
3he2
f )
νM2
The quantity ν is the state energy in cm-1 and M is the transition
length in e Å. Figure 9 shows a plot of transition length as a
function of 1/L in the two types of chromophores. The linear
behavior of the transition length parallels the trend in singlet
state energy shown in Figure 8, giving further evidence the
singlet state is delocalized across the platinum center.
In this article, we have compared the spectroscopic behavior
of a series of PEn-Pt and half-PEn-Pt chromophores and have
given evidence that the S1 and Tn states of the PEn-Pt
chromophores are delocalized through the central platinum,
while the T1 state is confined to one ligand. Published studies
of the effect of conjugation length on S1 and T1 energies show
variable results. Poly(thiophenes)26 and carotenoids27 with
variable chromophore length have T1 states less sensitive to
chromophore length than the S1 states. In contrast the T1 states
in monodisperse R-oligothiophenes28 carbazole-spirobifluorene
copolymers29 and a homologous series of fluorene oligomers30
have the same conjugation length dependence as the S1 states.
Our previous paper12 compares the chromophore length depen-
dence for a series of platinum acetylide chromophores PEn-
Pt, the corresponding ligands PEn-H and the butadiynes PEn-
BD. The clearest trend appears for n )1, where ES(PE1-H) >
ES(half-PE1-Pt) > ES(PE1-BD) > ES(PE1-Pt), while ET(PE1-
H) > ET(half-PE1-Pt) ) ET(PE1-Pt) > ET(PE1-BD). The trend
for ES shows conjugation length increases with chromophore
length. In contrast, the trend for ET shows the platinum atom
causes a break in the chromophore, confining the triplet exciton
to one ligand. The trends for n ) 2 and 3 are less clear because
the chromophore behavior becomes more strongly influenced
by ligand length. Despite the ligand length effect, ET(half-PEn-

Series of Platinum Acetylide Complexes
J. Phys. Chem. A, Vol. 110, No. 13, 2006 4375
(9) Cooper, T. M.; Blaudeau, J.; Hall, B. C.; Rogers, J. E.; McLean,
D. G.; Liu, Y.; Toscano, J. P. Chem. Phys. Lett. 2005, 400, 239-244.
(10) Emmert, L. A.; Choi, W.; Marshall, J. A.; Yang, J.; Meyer, L. A.;
Pt) ≈ ET(PEn-Pt) for n ) 2 and 3, showing the central platinum
atom confines the triplet exciton to one ligand.
What causes the confinement of the triplet state in these
systems? In general, the two electrons in the triplet state are
not near each other as much as in the singlet state, resulting in
less Coulombic repulsion.31 An investigation of exciton mobility
in platinum acetylide polymers had shown the rate of triplet
exciton migration along the polymer chain is less than the rate
of singlet exciton migration, suggesting the exciton migration
time is longer than the triplet state lifetime.32 The observation
that during intersystem crossing the triplet state symmetry
converts from D2h to C2V suggests the delocalized D2h potential
energy surface to be considerably higher in energy than the
ligand-confined C2V surface. After relaxation, the triplet exciton
is confined in a C2V symmetry potential energy well.10 A recently
published theoretical paper on PE1-Pt sheds more light on these
issues.13 They find the ground state to have D2h symmetry, but
the triplet state exciton has ππ* nature with the unpaired electron
and hole delocalized over one phenylethynyl ligand. They
calculate the barrier for the triplet exciton to hop to the other
ligand to be 0.61 eV. This high-energy barrier will confine the
exciton to one ligand.
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McLean, D. G.; Fleitz, P. A.; Cooper, T. M. J. Chem. Phys. 2005, 122,
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