Asymmetry in platinum acetylide complexes: Confinement of the triplet exciton to the lowest energy ligand

Article abstract of DOI:10.1021/jp0654516

To determine structure-optical property relationships in asymmetric platinum acetylide complexes, we synthesized the compounds trans-Pt(PBu ₃)₂(C≡CC₆H₅)(C≡C-C ₆H₄-C≡CC₆H₅

Full text of DOI:10.1021/jp0654516

13370
J. Phys. Chem. A 2006, 110, 13370-13378
Asymmetry in Platinum Acetylide Complexes: Confinement of the Triplet Exciton to the
Lowest Energy Ligand
Thomas M. Cooper,*,† Douglas M. Krein,†,‡ Aaron R. Burke,†,‡ Daniel G. McLean,†,§
Joy E. Rogers,†,# and Jonathan E. Slagle†,+
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: August 23, 2006; In Final Form: October 11, 2006
To determine structure-optical property relationships in asymmetric platinum acetylide complexes, we
synthesized the compounds trans-Pt(PBu3)2(CtCC6H5)(CtC-C6H4-CtCC6H5) (PE1-2), trans-Pt(PBu3)2-
(CtCC6H5)(CtC-C6H4-CtC-C6H4-CtCC6H5) (PE1-3) and trans-Pt(PBu3)2(CtC-C6H4-CtCC6H5)-
(CtC-C6H4-CtC-C6H4-CtCC6H5) (PE2-3) that have different ligands on either side of the platinum
and compared their spectroscopic properties to the symmetrical compounds PE1, PE2 and PE3. We measured
ground state absorption, fluorescence, phosphorescence and triplet state absorption spectra and performed
density functional theory (DFT) calculations of frontier orbitals, lowest lying singlet states, triplet state
geometries and energies. The absorption and emission spectra give evidence the singlet exciton is delocalized
across the central platinum atom. The phosphorescence from the asymmetric complexes comes from the
largest ligand. Time-dependent (TD) DFT calculations show the S1 state has mostly highest occupied molecular
orbital (HOMO) f lowest unoccupied molecular orbital (LUMO) character, with the LUMO delocalized
over the chromophore. In the asymmetric chromophores, the LUMO resides on the larger ligand, suggesting
the S1 state has interligand charge transfer character. The triplet state geometries obtained from the DFT
calculations show distortion on the lowest energy ligand, whereas the other ligand has the ground state geometry.
The calculated trend in the triplet state energies agrees very well with the experimental trend. Calculations of
triplet state spin density also show the triplet exciton is confined to one ligand. In the asymmetric complexes
the spin density is confined to the largest ligand. The results show Kasha’s rule applies to these complexes,
where the triplet exciton moves to the lowest energy ligand.
Introduction
that the singlet exciton is delocalized through the central
platinum, whereas the triplet exciton is confined to one ligand.
There has been considerable interest in the synthesis,
spectroscopy, nonlinear optics and structure-property relation-
ships of platinum acetylides.1-7 They are exceptional systems
for investigating triplet state phenomena like ground state
absorption to the triplet state, intersystem crossing, and the triplet
state absorption spectrum and phosphorescence.8 In our labora-
tory, we have been investigating the relation between chemical
structure and spectroscopic properties in platinum acetylide
complexes.1,9,10 We recently published a detailed investigation
of the photophysical properties of a series of butadiynes having
the formula H-(C6H4-CtC)n-(CtC-C6H4)n-H, n ) 1-3,
and ligands H-(C6H4-CtC)n-H, n ) 1-3, and compared
these to previous work done on a complimentary series of
platinum-containing complexes having the formula trans-
Pt(P(C4H9)3)2((CtC-C6H4)n-H)2, n ) 1-3.1,11 More recently,
we synthesized a series of half-platinum acetylide complexes
having the formula trans-PtII(PBu3)2((CtCC6H4)n-H)Cl, n )
1-3.12 In all these articles, analysis of the dependence of singlet
and triplet state energies on chromophore length gives evidence
Time-resolved infrared spectra of the triplet state trans-
Pt(PBu3)2((CtC-C6H4-CtCC6H5)213 give evidence the triplet
state geometry has cumulene and quinone character. A related
time-resolved infrared spectroscopy and theoretical study of
trans-Pt(PBu3)2(CtCC6H5)2 gives evidence the triplet exciton
is confined to one ligand.6 Another theoretical study of the same
compound also suggests the triplet state is confined to one
ligand.14 A suggested intersystem crossing mechanism in
platinum acetylides involves an initial delocalized singlet state
exciton converting to a triplet exciton confined to one ligand.
In this paper we describe the synthesis and characterization
of three platinum acetylides trans-Pt(PBu3)2(CtCC6H5)(CtC-
C6H4-CtCC6H5) (PE1-2), trans-Pt(PBu3)2(CtCC6H5)(CtC-
C6H4-CtC-C6H4-CtCC6H5) (PE1-3) and trans-Pt(PBu3)2-
(CtC-C6H4-CtCC6H5)(CtC-C6H4-CtC-C6H4-Ct
CC6H5) (PE2-3) that have different ligands on either side of
the central platinum. These compounds have Cs symmetry and
we compare their spectroscopic properties to the C2h symmetry
compounds PE1, PE2 and PE3. The nomenclature is described
in Figure 1. With the lower symmetry we can distinguish
between two possible intersystem crossing mechanisms where
the triplet exciton goes to either ligand or preferentially goes
to one ligand. We found the singlet exciton to be delocalized
throughout the molecule, and the triplet exciton to be confined
* To whom correspondence should be addressed. E-mail:
Thomas.Cooper@wpafb.af.mil.
† Wright-Patterson Air Force Base.
‡ Anteon Corp.
§ SAIC.
# UES, Inc.
+ AT&T Corp.
10.1021/jp0654516 CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/30/2006

Asymmetry in Platinum Acetylide Complexes
J. Phys. Chem. A, Vol. 110, No. 50, 2006 13371
mmol) of half-PE1 was dissolved in 40 mL of diethylamine,
followed by addition of 12.5 mg of CuI and 173 mg (0.5721
mmol) of PE3-H. The solution was heated to reflux and stirred
overnight. The solvent was removed on a rotovap, and the
remaining solid was then dissolved in DCM and adsorbed to a
small amount of silica gel. This was layered over a 4 in. column
of silica gel and eluted with first hexane and then varying
percentages of a hexane/DCM mixture. Two fractions of the
hexane/DCM eluents showed a mixture of product and PE1 by
NMR. These mixtures were combined and put through a 1 in.
reversed phase column using acetonitrile and DCM as eluents.
Approximately 130 mg of a waxy light yellow solid was isolated
in an acetonitrile/DCM fraction. MA Found: C, 67.68; H, 6.90.
C56H72P2Pt Requires: C, 67.11; H, 7.24. MW ) 1001. IR (KBr,
thin film): 2096 cm-1 [ν(Pt-CtC)]. 1H NMR (CDCl3): δ 0.99
(m, 18H, CH3), 1.49 (m, 12H, CH2), 1.66 (m, 12H, CH2), 2.19
(m, 12H, CH2), 7.29-7.55 ppm (m, 18H, ArH). 13C NMR
(CDCl3): δ 14.15 (s, CH3), 24.19 (t, J(CP) ) 17 Hz, CH2),
24.64 (t, J(CP) ) 7 Hz, CH2), 24.82 (s, CH2), 26.66, (s, CH3),
107.96 (t, J(CP) ) 14 Hz, Pt-CC), 112.89 (t, J(CP) ) 14 Hz,
Pt-CtC), 89.5 (s, CtC), 89.9 (s, CtC), 91.5 (s, CtC), 92.4
(s, CtC), 109.4 (s, CtC), 109.5 (s, CtC), 119.2, 123.1, 123.4,
123.7, 125.2, 128.2, 128.7 (br), 129.3, 129.7, 131.1 (br), 131.6,
131.7, 131.8, 131.9 ppm (Ar). 31P NMR (CDCl3): s and d
centered at δ 4.26 ppm (J(PPt) ) 2351 Hz, PBu3). EIMS: m/z
1001.
Figure 1. Chemical formulas and nomenclature. Arrows show mo-
lecular axes. Nomenclature for the asymmetric compound a-Pt(PBu3)2-
b, designated as PEa-b, labels two ligands of the compound. In
discussions below, ligand a is designated as the “left ligand” and ligand
b is the “right ligand”. When a ) Cl, the chromophore is designated
as “half-PEb”. The butadiyne PEn-CtC-CtC-PEn is labeled as
“PEn-BD”.
to the lowest energy ligand. We also did density functional
theory (DFT) calculations on the ground and lowest triplet state
of PE1, PE2, PE3, PE1-2, PE1-3 and PE2-3. The experimental
trends are reproduced very well by the DFT calculations.
Pt(CtCC6H4CtCC6H5)(CtCC6H4CtCC6H4CtCC6H5)-
(PBu3)2 (PE2-3) In a 100 mL three-neck round-bottom flask
546 mg (0.653 mmol) of half-PE2 was dissolved in 50 mL of
diethylamine, followed by addition of 12.5 mg of CuI and 157.5
mg (0.521 mmol) of PE3-H, and the mixture stirred for 4 days
at room temperature. The solvent was removed on a Rotovap,
and the remaining solid was then dissolved in DCM and
occluded to a small amount of silica gel. This was layered over
a 4 in. column of silica gel and eluted with first hexane and
then varying percentages of a hexane/DCM mixture. The
fraction eluted with a hexane/25% DCM mixture contained the
product with a small amount of half-PE2. This fraction was
put through a 1 in. reversed phase using methanol and varying
MeOH/DCM mixtures as the eluent. Approximately 280 mg of
a waxy light yellow solid was isolated from the MeOH/25%
DCM fraction. MA Found: C, 69.71; H, 6.69. C64H76P2Pt
Requires: C, 69.73; H, 6.95. MW ) 1101. IR (KBr, thin film):
2095 cm-1 [ν(Pt-CtC)]. 1H NMR (CDCl3): δ 0.97 (m, 18H,
CH3), 1.49 (m, 12H, CH2), 1.66 (m, 12H, CH2), 2.19 (m, 12H,
CH2), 7.26-7.60 (m, 22H, ArH) ppm. 13C NMR (CDCl3): δ
14.10 (s, CH3), 24.47 (t, J(CP) ) 17 Hz, CH2), 24.69 (t, J(CP)
) 17 Hz, CH2), 26.65 (s, CH3), 111.9 (t, J(CP) ) 14 Hz,
Pt-CtC), 112.5 (t, J(CP) ) 14 Hz, Pt-CtC), 109.7 (s, br,
CtC), 89.5 (s, CtC), 89.9 (s, CtC), 90.1 (s, CtC), 90.3 (s,
CtC), 91.4 (s, CtC), 92.3 (s, CtC), 119.2, 119.5, 123.1, 123.4,
123.7, 123.8, 128.3, 128.6, 128.79 (br), 129.4, 129.6, 131.0 (br),
131.50, 131.54, 131.65, 131.74, 131.8, 131.9 (Ar) ppm. 31P
NMR (CDCl3): s and d centered at δ 4.34 (J(PPt) ) 2351 Hz,
PBu3) ppm. EIMS: m/z 1101.
General Synthesis Methods
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. Reverse phase column refers to Alltech Extract-Clean C18.
The ligands PE2-H and PE3-H and compounds Pt(CtC-
C6H5)Cl(PBu3)2 (half-PE1), Pt(CtC-C6H4-CtC-C6H5)Cl-
(PBu3)2 (half-PE2), Pt(CtCC6H4CtCC6H5)2(PBu3)2 (PE2) and
Pt(CtCC6H4CtCC6H4CtCC6H5)2(PBu3)2 (PE3) were syn-
thesized as described previously.1,12
Synthesis
Pt(CtCC6H5)(CtCC6H4CtCC6H5)(PBu3)2 (PE1-2) In a 100
mL three-neck round-bottom flask 480 mg (0.6535 mmol) of
half-PE1 was dissolved in 35 mL of diethylamine, followed
by addition of 12.5 mg of CuI and 133 mg (0.6535 mmol) of
PE2-H. The solution was heated to reflux and stirred overnight.
The solvent was removed on a rotovap, and the remaining solid
was then dissolved in dichloromethane (DCM) and adsorbed
to a small amount of silica gel. This was layered over a 4 in.
column of silica gel and eluted with first hexane and then
varying percentages of a hexane/DCM mixture. Approximately
200 mg of the light yellow solid product was isolated. MA
Found: C, 64.18; H, 7.36. C48H68P2Pt Requires: C, 63.91; H,
7.60. MW ) 901. IR (KBr, thin film): 2099 cm-1 [ν(Pt-Ct
1
C)]. H NMR (CDCl3): δ 0.96 (m, 18H, CH3), 1.47 (m, 12H,
CH2), 1.60 (m, 12H, CH2), 2.17 (m, 12H, CH2), 7.25-7.40 ppm
(m, 14H, ArH). 13C NMR (CDCl3): δ 14.09 (s, CH3), 23.96 (t,
J(CP) ) 17 Hz, CH2), 24.19 (t, J(CP) ) 7 Hz, CH2), 24.68 (t,
J(CP) ) 7 Hz, CH2), 26.64 (s, CH3), 108.0 (t, J(CP) ) 14 Hz,
Pt-CtC), 112.4(t, J(CP) ) 14 Hz. Pt-CtC), 90.0 (s, CtC),
90.4 (s, CtC), 109.3 (s, CtC), 109.4 (s, CtC), 119.4, 123.9,
125.1, 128.1, 128.3, 128.6, 129.3, 129.5, 130.9, 131.0, 131.5,
131.7 ppm (Ar). 31P NMR (CDCl3): s and d centered at δ 4.25
ppm (J(PPt) ) 2351 Hz, PBu3). EIMS: m/z 901.
Computational Methods
Calculations were done using Gaussian 03W, revision D.01.15
The presence of the heavy platinum center required a basis set
that includes relativistic effects through an effective core
potential. We used DFT with the B3LYP functional and the
relativistic LANL2DZ basis set.16,17 To save computer time, the
phosphine portion of the molecule was converted from tribu-
tylphosphine to trimethylphosphine. We performed geometry
Pt(CtCC6H5)(CtCC6H4CtCC6H4CtCC6H5)(PBu3)2 (PE1-
3) In a 100 mL three-neck round-bottom flask 418 mg (0.5721

13372 J. Phys. Chem. A, Vol. 110, No. 50, 2006
Cooper et al.
TABLE 1: Summary of 13C NMR Data
dipole momentd
b
compd
δaa
∆
ab
δb
∆
ba
PE1-2
PE1-3
PE2-3
PE1c
PE2
108.0
108.0
111.9
108.3
112.0
112.4
-0.3
-0.3
-0.1
112.4
112.9
112.5
+0.4
+0.5
+0.1
1.153
1.627
0.458
PE3
a In the asymmetric PEa-b complexes, ligand a is the left ligand
and ligand b is the right ligand. Chemical shifts are in ppm. b Change
in chemical shift from reference symmetric complex. c Chemical shift
values (δaa) for symmetric complexes obtained from the literature.19
d Dipole moment (debye) calculated as described in the methods.
optimizations for the ground and T1 states. For the ground state,
energy minimizations were performed with the symmetrical
complexes PEn having the ligand plane perpendicular to the
P-Pt-P axis, constraining the symmetry to C2h and the
asymmetric complexes PEa-b to Cs. We used DFT to calculate
the T1 state geometry of the triplet state. DFT is known as a
ground state theory only rigorously valid for the ground state
of a given symmetry (including spin symmetry).18 In this
instance the T1 state is the “ground state”. Our starting geometry
for these minimizations was that previously found for PE1,14
where the ligand plane was parallel to the P-Pt-P axis and
the symmetry allowed to be C1. We used the ∆SCF method to
estimate ET as given by the expression
Figure 2. Ground state absorption and emission spectra of the
complexes dissolved in benzene at room temperature. For the emission
spectra, the excitation wavelength was 350 nm.
TABLE 2: Summary of Spectroscopic Data
a
compd λmax(GS)a ꢀmax
fc
λ
max(fl)d
λ
max(ph)e max(TT)f τ(TT)g
λ
PE1-2
PE1-3
PE2-3
349
357
359
57 665 1.34
69 414 1.97
101 815 2.74
377
403
401
524
555
553
590
650
650
93
144
16a
ET ) E(triplet state relaxed geometry) - E
a Maximum of ground state absorption spectrum in benzene (nm).
b Extinction coefficient (M-1cm-1). c Oscillator strength was obtained
by fitting absorption spectrum to three Gaussians. d Maximum of
fluorescence spectrum in benzene (nm). Excitation wavelength 355 nm.
e Peak of 0-0 band of phosphorescence of complex in methyl-THF
glass at 77 K. f Peak of triplet state absorption spectrum obtained from
flash photolysis experiment. g Triplet state lifetime in µs.
(ground state relaxed geometry)
The singlet excited states were investigated by density functional
response theory (TDDFT), where the 6 lowest singlet roots were
obtained.
General Spectroscopy Techniques
PEab, the interaction between ligands a and b is determined
All absorption and fluorescence spectra were obtained in
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. 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.1
by calculating the difference
∆ab ) δa - δaa
and
∆
ba ) δb - δbb
where δa is the chemical shift of the Pt-C carbon of ligand a
of the complex and δaa is the corresponding chemical shift for
the complex PEa. ∆ab is the influence of ligand b on ligand a.
Similarly, the value ∆ba is calculated from values for ligand b
of the complex and the complex PEb. For a complex PEa-b,
the interaction effect caused a small upfield shift in the left
ligand a and a corresponding small downfield shift in the right
ligand b. The magnitude of ∆ behaves as PE1-3 ∼ PE1-2 >
PE2-3. The dipole moment calculated by DFT follows a similar
ordering: µ(PE1-3) > µ(PE1-2) > µ(PE2-3).
In Figure 2, the absorption spectrum of PE1-2 has an
absorption maximum of 349 nm, and that for PE1-3 red-shifts
to 357 nm. The band shape and absorption maximum of PE2-3
are identical to those of PE1-3, although its spectrum has an
increased extinction coefficient, as given in Table 2. All three
absorption spectra have at least two closely spaced bands near
the absorption maximum. A Gaussian fit of the spectra shows
the spacing between the two bands is ∼0.2 eV. The oscillator
strengths increase according to the order f(PE1-2) < f(PE1-3)
< f(PE2-3). The behavior of the fluorescence spectra mirrors
that of the absorption spectra, with PE1-2 blue-shifting from
Results
Table 1 lists selected 13C NMR data for the mixed com-
pounds. The table lists the chemical shift for the Pt-C carbon.
Also included are corresponding chemical shifts for the PEn
complexes that have been previously published.19 The magnitude
of the shifts were similar to those seen in the symmetric
complexes. For example, δ(PE1-2, ligand 1) ∼ δ(PE1) and
δ(PE1-2, ligand 2) ∼ δ(PE2). The chemical shifts for the
asymmetric complexes split into two values. For a complex

Asymmetry in Platinum Acetylide Complexes
J. Phys. Chem. A, Vol. 110, No. 50, 2006 13373
TABLE 3: Summary of State Energies
a,c
compd
ES
ET
∆EST
ETT
PE1-2
PE1-3
PE2-3
PE1b
PE2
3.44
3.21
3.21
3.58
3.29
3.21
2.38
2.26
2.26
2.82
2.38
2.26
1.06
0.95
0.95
0.76
0.91
0.95
2.10
1.91
1.91
1.97
2.14
1.94
PE3
a eV. b State energies for PE1, PE2 and PE3 were previously
published. c ES was measured from the intersection of the ground state
absorption and fluorescence spectra and ET from the blue edge of the
phosphorescence spectrum.
TABLE 4: Various DFT Calculation Results
a
b
c
e
compd
EGS
ETS
ET
µd ES
ff
CI result
PE1
PE2
PE3
-987.2223 -987.1177 2.85 1.49 4.25 0.10 0.48(H f L)7
-1601.5540 -1601.4691 2.31 3.17 3.37 2.49 0.67(H f L)
-2215.8844 -2215.8046 2.17 5.05 2.96 3.88 0.65(H f L)
PE1-2 -1294.3882 -1294.3032 2.31 4.55 3.48 1.52 0.66(H f L)
PE1-3 -1601.5534 -1601.4737 2.17 6.86 3.03 2.08 0.67(H f L)
PE2-3 -1908.7192 -1908.6394 2.17 5.56 3.02 2.40 0.66(H f L)
Figure 3. Triplet state absorption spectra obtained from excitation at
355 nm of a degassed benzene solution at room temperature.
a Ground state energy (H) calculated from optimized singlet state
geometry. PE1, PE2 and PE3 are assumed to have C2h symmetry.
PE1-2, PE1-3, and PE2-3 are assumed to have Cs symmetry. b Triplet
state energy (H) calculated from optimized triplet state geometry. All
chromophores are assumed to have C1 symmetry. c ET (eV) is the
difference between triplet state energy and ground state energy. d Triplet
state dipole moment (D). e Transition energy (eV) calculated from a
TDDFT calculation. In PE1, the state shown is state 5. For all others,
f
the state shown is state1. Oscillator strength. g PE1 also had 0.48 (H-1
f L+1).
In the PE2-3 emission spectrum, we also observe a weak
emission attributable to the 0-0 emission band from the PE2
ligand. Table 2 summarizes data obtained from ground state
absorption and emission spectra.
Table 3 lists all the state energies plus previously published
state energy data for PE1, PE2 and PE3.1,11 The trend in state
energies for ES is PE1 > PE1-2 > PE2 > PE1-3 ∼ PE2-3 ∼
PE3. The trend for the ET state energy is PE1 > PE1-2 ) PE2
> PE1-3 ) PE2-3 ) PE3. With the exception of PE1, all ∆EST
values are around 1 eV. The ETT values follow the trend PE1-2
∼ PE2 > PE1-3 ∼ PE2-3 ∼ PE3.
Table 4 lists results of various calculations. Listed is the
ground state energy after geometry optimization and the
corresponding energy of the triplet-state-optimized geometry.
As a test of our method, we did similar ∆SCF calculations for
the monosubstituted half-PEn chromophores and compared
them to experimental ET values.12 For the half-PEn compounds,
we found the calculated (experimental) ET values to be half-
PE1: 2.88 (2.92); half-PE2: 2.32 (2.39); half-PE3: 2.17 (2.28).
By calculating the difference between the two energies, we
determine the ET trend to be half-PE1 ∼ PE1 > half-PE2 )
PE2 ) PE1-2 > half-PE3 ) PE1-3 ) PE2-3 ) PE3. The
calculated triplet state dipole moments follow the trend PE1 <
PE2 < PE1-2 < PE3 < PE2-3 < PE1-3. ES is estimated from
the lowest-lying allowed transition having oscillator strength
greater than 0.1. The Supporting Information lists all the
electronic states obtained from the TDDFT calculation. The next
most intense state is about 0.3-0.4 eV higher in energy with
70-80% lower oscillator strength. The ES values follow the
trend PE1 > PE1-2 > PE2 >PE1-3 ∼ PE2-3 ∼ PE3. The
oscillator strengths order according to PE1 < PE1-2 < PE1-3
< PE2-3 < PE2 < PE3. Examination of the configuration
interaction (CI) coefficients show the largest configuration of
this state is a highest occupied molecular orbital (HOMO) f
lowest unoccupied molecular orbital (LUMO) transition.
Figure 4. Phosphorescence spectra of chromophores dissolved in
methyltetrahydrofuran glass at 77 K resulting from excitation at 350
nm. The peak marked with an asterisk is the weak emission from the
PE2 ligand of PE2-3.
PE1-3 and PE2-3, both of which have similar emission spectra.
Unlike the absorption spectra, each of the emission spectra have
only one band and do not show distinct vibronic structure.
The triplet state absorption spectra (Figure 3) are similar to
ground state absorption and emission spectra. PE1-2’s spectrum
is blue-shifted from those of PE1-3 and PE2-3, which are nearly
identical. All three spectra have a major band: 590 nm for
PE1-2 and 650 nm for PE1-3 and PE2-3 as well as a minor
band: ∼485 nm for PE1-2 and 525 nm for PE1-3 and PE2-3.
The bleaching region shows PE1-2 bleaching at ∼350 nm,
whereas PE1-3 and PE2-3 bleach at ∼370 nm. The average
energy difference between the two bands is ∼0.4 eV. The triplet
state lifetimes (Table 2) follow the trend τ(PE1-2) < τ(PE1-3)
∼ τ(PE2-3).
We collected phosphorescence spectra of PE1-2, PE1-3 and
PE2-3 and compared these spectra to phosphorescence from
PE2 and PE3 (Figure 4). We find that the phosphorescence
spectrum of PE1-2 to be identical to that of PE2, and the
spectrum of PE1-3 and PE2-3 to be identical to that of PE3.

13374 J. Phys. Chem. A, Vol. 110, No. 50, 2006
Cooper et al.
“g” symmetry. In PE1-3 and PE2-3 there is less electron density
on the outer phenyl of the PE3 ligand. The phenylacetylene
node pattern is the same as seen in the PEn complexes. The
LUMO consists of p orbitals on the phenylacetylene units and
smaller contribution from the d orbital on the platinum atom
and has b1 symmetry. The “inversion” symmetry through the
platinum atom shows the p orbitals of corresponding atoms on
opposite sides of the ligand have opposite signs, giving them
“u” symmetry. Because the d orbital has “g” symmetry, there
is bonding between the platinum d orbital and one of the
acetylene p orbitals. As in PEn, each phenylacetylene unit has
three nodes. An allowed HOMO f LUMO transition is a
z-polarized A1 state. In PE1-2 the HOMO has 10 nodes, and
the LUMO has 11 nodes, showing a ∆q ) (1 selection rule.
Analysis of the p orbital signs shows “g” f “u” character in
the HOMO f LUMO transition. Similar analyses hold true in
PE1-3 and PE2-3.
Table 5 summarizes ground and triplet state geometry data
for PE2. Complete geometry data for all the other chromophores
are listed in Table S1 of the Supporting Information. In Table
S1, a comparison of two chromophores PEa-b and PEc-d,
shows that when a ) c, the ground and triplet state geometries
of the two left ligands are identical. When b ) d, the ground
and triplet state geometries of the two right ligands are identical.
Table 5 shows ground and triplet state bond lengths for the right
PE2 ligand. The bond lengths for the left ligand triplet state
are the same as the ground state. For a given number of
phenylacetylene units in a ligand, the ground and triplet state
geometries in both the left and right ligands are the same. The
average ground state acetylene bond length, 1.23 Å, is close to
the standard bond length of 1.20 Å. The bonds R3, R7 and R9
have an average length of 1.43 Å, making them intermediate
between a single bond (1.54 Å) and a double bond (1.36 Å).
The average bond length in the phenyl rings is 1.41 Å, close to
the standard length for a benzene carbon-carbon bond (1.39
Å). In the geometry optimizations, the PE ligand is constrained
to be planar. A published X-ray structure of PE2 gives R2 )
1.214 Å and R8 ) 1.199 Å4 vs our calculated values of 1.243
and 1.229 Å. Our calculated bond lengths are 0.03 Å larger
than the experimental values, but there is good agreement for
the difference between R2 and R8, 0.015 Å. In the triplet state
there are only small geometry changes in the left ligand. In the
right ligand, the acetylene bond lengths increase, whereas the
adjoining carbon-carbon bonds decrease in length, giving the
linkage more allene character. There are also decreases in the
2′-3′ bond lengths, while the 1′-2′ and 3′-4′ lengths increase.
The net result of these changes give the ligand more quinone
character. Table S1 also lists root mean square (rms) bond length
changes in each of the phenylethynyl units of the right ligand
as a function of position from the central platinum. The largest
rms bond length changes occur in PE1, with smaller changes
in larger ligands. In PE3, the largest geometry distortion occurs
in the central phenylethynyl unit. To estimate ligand flexibility,
we calculated the barrier to rotation about the phenylacetylene
linkage in the PE2-H ligand to be 1.6 kcal/mol. The rotation
barrier of the PE2-H ligand in the triplet state is estimated to
be 9.4 kcal/mol. For each of the ligands, the electron affinity
(-ELUMO in eV from optimized geometry) is calculated to be
PE1-H: 0.78; PE2-H, 1.61; PE3-H, 1.92.
Figure 5. Comparison of calculated and experimental state energies.
The triplet state energy ET was calculated by the ∆SCF DFT method.
The singlet state energy was calculated by the TDDFT method.
Figure 5 shows a plot of the calculated vs measured ES and
ET values. A plot of ET(experimental) vs ET(calculated) have
an excellent linear correlation. The linear fit gives the expression
(in eV),
ET(expt) (eV) ) 0.4758 + 0.8228ET(calc) r ) 1
A similar plot for ES gives the expression
ES(expt) (eV) ) 2.3014 + 0.3048ES(calc) r ) 0.9649
Although there is a lower correlation, the calculated ES predicts
the correct ordering of state energies with chromophore size.
Figure 6 shows images of the HOMO and LUMO for the
chromophores. Neglecting the phosphines, PE1, PE2 and PE3
have D2h symmetry. The HOMO consists of p orbitals on each
of the phenylacetylene units and a d orbital on the platinum
and has b2g symmetry. The nodes cut through the bonds. Each
phenylacetylene unit has two nodes, one through the phenyl
group and one through the phenylacetylene carbon-carbon
bond. There is another node between the platinum-carbon bond
and a node between the lobes of the d orbital on the platinum
atom. The LUMO consists of p orbitals on the phenylacetylene
units and an empty d orbital on the platinum and has b3u
symmetry. The nodes cut through the bonds. Each phenylacety-
lene unit has three nodes, two in the phenyl group and one
bisecting the acetylene group. There is another node bisecting
the Pt-C bond and the Pt d orbital. An allowed HOMO f
LUMO transition is the z-polarized B1u state and is assigned as
an 1La state.20-22 In PE1, there are 7 nodes in the HOMO, and
6 in the LUMO, showing the allowed optical transition follows
the ∆q ) (1 selection rule, as well as the g f u rule. The
same analysis holds true for PE2 and PE3.
If the phosphines in PE1-2, PE1-3 and PE2-3 are neglected,
the chromophores have C2V symmetry. Both B2g and B3u
representations in D2h transform as B1 in C2V. Similarly, both
Ag and B1u representations in D2h transform as A1 in C2V. Like
the PEn complexes, the HOMO consists of p orbitals on the
phenylacetylene groups and a d orbital on the platinum and had
b1 symmetry. By looking at the “inversion” symmetry through
the platinum atom, the p orbitals of corresponding atoms on
opposite sides of the ligand have the same sign, giving them
Figure 7 shows plots of the spin density of the triplet state.
The chromophores are divided into the individual phenylacety-
lene units of either the left or right ligand, and the central
platinum/phosphine units. With the exception of the platinum/
phosphine unit of PE1, having a spin density of 0.20, there is

Asymmetry in Platinum Acetylide Complexes
J. Phys. Chem. A, Vol. 110, No. 50, 2006 13375
Figure 6. Frontier molecular orbitals of the various compounds obtained from DFT calculations of the geometry optimized structure.
TABLE 5: Bond Length Data for Ground State and Triplet
State of PE2
b
bond
PE2(S0)a
PE2(T1)
∆
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
2.014
1.243
1.434
1.423
1.399
1.421
1.431
1.229
1.433
1.420
1.404
1.409
1.993
1.264
1.392
1.464
1.370
1.470
1.377
1.255
1.405
1.435
1.399
1.415
-0.021
0.021
-0.042
0.041
-0.029
0.049
-0.054
0.026
-0.028
0.015
-0.005
0.006
no significant spin density on the platinum/phosphine unit. For
all the compounds, the left ligand has virtually no spin density.
Instead, most of the spin density is confined to the right ligand.
In the asymmetric complexes, the triplet exciton is confined to
the larger, lower energy ligand. The magnitude of the spin
density is only a function of the right ligand size. The right
ligand spin densities are identical in PE1-2 and PE2. The spin
densities of the right ligands of PE1-3, PE2-3 and PE3 are also
identical. As the length of the right ligand increases from one
to three phenylacetylene units, the spin density of the phenyl-
acetylene nearest the platinum atom decreases from 1.70 to
0.25. The spin density of the second phenylacetylene unit
increases from 0.59 to 0.95. The spin density on the third
phenylacetylene unit is 0.73. The average spin density per
Figure 7. Spin densities obtained from DFT calculations of triplet
state optimized geometry. The labels on the abscissa refer to monomer
units. “Pt” refers to the central Pt(PBu3)2 group. Labels “Pt+1”, “Pt+2”
and “Pt+3” refer to successive phenylethynyl monomer units on the
right ligand, with “Pt+1” being the phenylethynyl group bound to
platinum. Similarly, “Pt-1”, “Pt-2” and “Pt-3” refer to the left ligand,
with “Pt-1” being the phenylethynyl group bound to the platinum. The
ordinate refers the sum of atomic spin density of each phenylethynyl
or Pt(PBu3)2 group.
functional group follows the trend Pt/phosphine, 0.086; ethynyl,
0.694 and phenyl, 1.22. As a comparison, a geometry optimiza-
tion for the T1 state of the butadiyne PE1-BD (C6H5-CtC-
CtC-C6H5) shows the spin densities are symmetrically placed
throughout the molecule, with the acetylene carbons having a

13376 J. Phys. Chem. A, Vol. 110, No. 50, 2006
Cooper et al.
spin density of 1.064 and the phenyl groups having a spin
density of 0.936.
experimental ET values. The accuracy of our calculations agrees
with previous theoretical work,26 where DFT successfully
calculates ET within 0.1 eV of experiment.
Discussion
A simple picture of the triplet state is a linear combination
of localized triplet excitons.
What 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 T1 exciton is localized on a single
phenylene ring and the S1 and Tn excitons are delocalized over
several monomer units.23 The spectra of a series of oligomers
having the formula C6H5(CtC-Pt(PBu3)2-CtCC6H4)n-H, n
) 1-5, 7, show similar trends.2 The ground state absorption
and fluorescence spectra have systematic red shifts as the
oligomer length increases. In contrast, the phosphorescence
spectra have only small red shifts with increasing oligomer
length. An investigation of the photophysics and photochemistry
of platinum acetylide stilbenes also gives evidence that triplet
state resides on one ligand.24 Our group has done several studies
on the relationship between platinum acetylide length and singlet
state energy, including PEn,2 their analogous butadiynes,11 the
half-PEn complexes12 and sydnone-containing complexes.19 In
all these studies the singlet state energy ES decreases as the
molecular length increases, whereas the triplet state energy ET
has less dependence on molecular length, supporting the idea
that the triplet state is more localized than the singlet state.
The HOMO consists of π orbitals residing on the phenyl-
acetylene units and 5d orbitals on the platinum. As shown in
Table 1, the Pt-C carbon chemical shifts of the asymmetric
complexes split into two values. The result suggests the Pt-C
chemical shift is more strongly influenced by the carbon’s own
ligand environment rather than by the ligand across the plati-
num center. Examining Figure 6, the antibonding nature of
the Pt-C bond increases the influence of the ligand on the
Pt-C chemical shift and decreases the influence of the other
ligand. The 13C NMR data and DFT calculations on the
asymmetric complexes give evidence of a ground state di-
pole moment. Polar asymmetric complexes have been described
in the literature, where the asymmetrical complexes Pt(Ct
CC6H4OCH3)(CtCC6H4NO2)(PBu3)2 and (Pt(CtCC6H4N-
(CH3)2)(CtCC6H4NO2)(PBu3)2 have been shown to have dipole
moments of 5 D.25
3Ψ* ) c1 3φ/a 1φb + c2 1φ a3φ/b
(2)
The triplet exciton is confined to one ligand, and the other ligand
is in the ground state. The triplet exciton migrates along a
reaction coordinate describing ligand distortion, with the exciton
migration potential energy surface having a double minimum.14
During a phosphorescence experiment, emission of a photon
occurs from the ligand carrying the exciton, and the spectrum
gives no information about exciton migration, so the data are
interpreted as describing “confinement”. When the triplet energy
difference between a and b is small, the chromophore behaves
more like the symmetric PEn chromohores, with the exciton
migrating between the two ligands. In symmetric PEn chro-
mophores, the exciton has an equal probability of residing in
either ligand, so c1 ) c2 ) 2-1/2. In the asymmetric PEa-b
chromophores, our experimental and computational results show
the triplet exciton resides in the more conjugated, lower energy
ligand, so c2 > c1. When the difference between the ligand
energies is large, the exciton migration rate becomes small and
the triplet exciton becomes confined to the lower energy ligand
and c2 ∼ 1.
Does the distance between the ligands affect the localization
of the triplet state? When electron exchange is small, energy
transfer between the two ligands will occur by Forster transfer.27
As electron exchange increases, energy transfer occurs by the
Dexter mechanism. Very strong electron exchange causes the
two ligands to behave as a single chromophore and our picture
of localized triplet excitons migrating between the two ligands
breaks down. As an example of this behavior, we did DFT
calculations on PE1-BD and find the triplet state is sym-
metrically delocalized throughout the chromophore. The triplet
state is then described by the following expression.
3Ψ* ) c1 3φ/a + c2 3φ/b
(3)
In the platinum complexes, the greater distance between the
two ligands and the presence of the platinum center decreases
their coupling. The triplet state can potentially reside on either
ligand, having energies ET and ET . Intramolecular triplet energy
Our TDDFT calculations of the ES values correlate well with
the experimental values. There does appear to be a limit to the
conjugation length, as the ES values of PE1-3, PE2-3 and PE3
are about the same, a trend also seen in the TDDFT calculations.
The CI coefficients depict the S1 state as having predominantly
HOMO f LUMO character. The LUMO consists of π* orbitals
on the ligands with the central d orbital empty. Inspection of
the PEa-b LUMOs (Figure 6) reveals transfer of electron density
from the left to the right ligand, giving them charge transfer
(CT) character. From these results we conclude the S1 state in
PEa-b is a metal to ligand charge transfer (MLCT) with ligand-
to-ligand CT character. A simple picture of the LUMO is a linear
combination of excited ligand orbitals situated between an empty
platinum d orbital.
a
b
transfer occurs between ligand a and ligand b via Dexter
coupling, resulting in phosphorescence only from ligand b.28
The average singlet-triplet splitting is 0.93 ( 0.10 eV in our
compounds. Our calculated ∆EST, 1.02 ( 0.23 eV, is within
0.1 eV of the measured value but has a larger standard deviation.
Our measured splitting contrasts to a ∆EST of 0.7 eV in
polymeric platinum acetylide complexes.3 The splitting energy
is proportional to the overlap integral between the singlet and
triplet excitons. Because the singlet state exciton resides in a
chromophore than in the polymeric systems, it results in a larger
energy gap.
Recent published data support our picture of the triplet state.
A recently synthesized PE1-hexabenzocoronene shows a
phosphorescence band at 578 nm from the hexabenzocoronene
ligand and no emission from the PE1 ligand.29 Similar evidence
of confinement of the triplet exciton to the core of a series of
branched platinum acetylide complexes rather than the outer
PE1-like ligands has been described.30 All these results derive
from Kasha’s rule, where emission occurs from the lowest
energy electronically excited-state of the molecule. This behavior
1Ψ* ) c1 1φ/a + c2 1φ/b
(1)
In the symmetric PEn complexes, c1 ) c2 ) 2-1/2. The LUMO
of the asymmetric PEab chromophores has CT character, so c2
> c1. When the LUMO is only located on ligand b, c2 ∼ 1.
The phosphorescence spectra of the asymmetric complexes
PEa-b are nearly identical to those of the symmetric complexes
PEb, where b is the more conjugated ligand. Our DFT
calculations give a strong correlation between calculated and

Asymmetry in Platinum Acetylide Complexes
J. Phys. Chem. A, Vol. 110, No. 50, 2006 13377
is observed in PE1-2 and PE1-3, where the energy of ligand 1
is considerably higher than that of the other ligand. However,
in PE2-3, we observe a small emission from ligand 2. The
LUMOs depicted in Figure 6 give evidence for the CT character
of the excited states in the asymmetric complexes. In PE1-2
and PE1-3, the difference in ligand electron affinity between
ligand 1 and ligands 2 and 3, calculated as 0.83 and 1.14 eV,
respectively, is large enough that the triplet exciton resides only
on ligands 2 or 3 (c2 ∼ 1). However, the difference in ligand
electron affinity in PE2-3, 0.31 eV, is small enough that some
measurable triplet exciton population of ligand 2 is possible
(c1 > 0). The measured energy difference between the 0-0
bands of the phosphorescence of ligands 2 and 3 is small enough
(0.12 eV) that, during intersystem crossing, some of the
excitation goes to ligand 2. A similar result from the literature
describes photoluminescence from a platinum acetylide polymer
containing both phenyl and thiophene monomer units. Most of
the emission comes from the lower energy thiophene units, but
some emission is also observed from the higher energy phenyl
units.31 The study describes similar excited-state dynamics,
where intersystem crossing results in the T1 exciton residing
on the phenyl unit with some phosphorescence from the phenyl,
followed by energy transfer to the thiophene unit and subsequent
phosphorescence from the thiophene unit.
avoided crossing minimum or a conical intersection between
the S1 and T1 surfaces where a nonadiabatic transition will
occur.33-37
A possible critical geometry is described by theoretical
calculations of excited-state dynamics in phenylacetylene38 and
diphenylacetylene,39,40 which suggest the formation of a stilbene-
like biradicaloid S1 state prior to the nonadiabatic jump to the
T1 state. This type of mechanism may occur in platinum
acetylide complexes, where the initial D2h symmetry Franck-
Condon S1 state relaxes to a biradicaloid state stilbene-like
geometry, followed by conversion to the triplet state. Evidence
for symmetry breaking during intersystem crossing has been
obtained from a time-resolved infrared spectroscopy study of
PE1, which shows a splitting of the Pt-CtC stretch vibration
into two peaks, suggesting the intersystem crossing mechanism,
which includes symmetry breaking from the S1 state having D2h
symmetry to the T1 state having C2V symmetry.6 The authors
of this paper propose the intersystem crossing process occurs
by coupling of S1 state B3u symmetry PtCtC antisymmetric
stretch vibration to two uncoupled A1 modes in the T1 state.
Similar spectroscopic behavior is observed in PE2, where a
cumulated CdCdC stretch vibration appears in the T1 state
vibration spectrum.13 Both of these experimental studies suggest,
during intersystem crossing, conversion from aromatic ethynyl
linkages to allene linkages occurs. The calculated bond length
data given in Table 5 and Table S1 support this mechanism.
The bonds undergoing greater than rms length change between
the ground and triplet state in the right ligand are R2 to R4 in
PE1, R3 to R7 in PE1-2 and PE2 and R6, R7, R9 and R10 in
PE1-3, PE2-3 and PE3. All of these geometry changes involve
the bonds connecting the phenyl groups. There are smaller
distortions with the phenyl groups involving conversion from
aromatic to quinone character. During intersystem crossing, the
reaction coordinate for distortion of the molecule moves toward
the critical geometry involved in the nonadiabatic transition
between the singlet and triplet potential energy surfaces.32 Does
the symmetry-breaking process begin while the chromophore
is still in the singlet state, or during the spin flip to the triplet
state? Perhaps the relaxed singlet state is delocalized throughout
the molecule, resulting from a Jahn-Teller distortion mecha-
nism, leading to a geometry similar to the triplet state, promoting
a nonadiabatic crossing to the triplet state. To understand these
processes, it is necessary to monitor the rates of Franck-Condon
S1 state conversion to the relaxed S1 state, followed by
conversion to the triplet state, including possible possible
population of both ligands, formation of stilbenoid intermediates
and energy transfer to the lowest energy ligand. Future work
will focus on the structure changes occurring on a subpicosecond
time scale following excitation.41
In contrast to the behavior of the phosphorescence spectra,
the T1 f Tn spectrum red-shifts with increasing chromophore
size.1,11 A comparison of the T1 f Tn transition energy of the
PEn chromophores vs the monosubstituted half-PEn chro-
mophores gives evidence this is an LMCT transition delocalized
across the platinum center.12 In the current work, the T1 f Tn
transition shows increased conjugation when PE1-2 and PE1-3
are compared, but the Tn state conjugation length of PE2-3 is
the same as that of PE1-3. The trend in triplet state lifetimes
supports this idea, with the lifetimes of PE1-3 and PE2-3 being
nearly equal. The conjugation length trends in the T1 f Tn
spectra mirror those seen in the ground state absorption and
fluorescence spectra, showing the conjugation length of the Tn
state changes in a similar manner as the S1 and T1 states.
The expression for the intersystem crossing rate constant can
be used to analyze the factors underlying the conversion from
the singlet to triplet state.32,33
2
2
S1|He|T1 S1|Hso|T1
2
kisc ) kmax
øS |øT
∆E2
∆E2
1
1
This equation describes the interactions between S1 and T1
contributing to the rate of intersystem crossing. The first term
describes electronic interactions. The second term describes
spin-orbit coupling and the third term describes Franck-
Condon factors.
Conclusions
We have synthesized the asymmetric complexes PE1-2,
PE1-3 and PE2-3 and have determined various spectroscopic
trends. The singlet state energy ES decreases with increasing
chromophore length, giving evidence that the singlet state is
delocalized through the platinum. The DFT calculations suggest
the S1 state contains both intraligand CT and MLCT character.
The triplet state energy ET is a function of the longest ligand.
The ET trends suggest that the intersystem crossing mechanism
involves movement of the triplet exciton to the lowest energy
ligand. Our calculated ET energies correlate well with the
experimental values. The calculated triplet state geometry shows
the geometry changes occur only on the lowest energy ligand.
Spin density calculations also show the triplet state is confined
to one ligand.
Our experiments involve excitation of these chromophores
initially in the ground state and observing the steady state and
time-resolved behavior of the triplet state. We have previously
shown the singlet state lifetime of these compounds is less than
30 ps and the intersystem crossing quantum yield is nearly
unity.1 Our calculations give good information about the S0 state,
the Franck-Condon S1 state, and the T1 state. We currently
have no information about the S1 and T1 potential energy
surfaces and the dynamics underlying the intersystem crossing
process. Intersystem crossing occurs at geometries where the
energy difference between the surfaces is small, and there is a
strong electronic and vibrational interaction between the S1 and
T1 states. These critical geometries may correspond to an

13378 J. Phys. Chem. A, Vol. 110, No. 50, 2006
Cooper et al.
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Acknowledgment. We thank Jean-Philippe Blaudeau, Gary
Kedziora, and Kiet Nguyen for very helpful guidance in DFT
calculations and Edward Lim for insightful discussions on the
triplet state potential energy surface.
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Supporting Information Available: Complete results of the
TDDFT calculations, complete geometry and spin density data
are available free of charge via the Internet at http://pubs.acs.org.
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