Photophysics of diimine platinum(II) bis-acetylide complexes

Article abstract of DOI:10.1021/ic0102182

A comprehensive photophysical investigation has been carried out on a series of eight complexes of the type (diimine)Pt(-C≡C-Ar)₂, where diimine is a series of 2,2′-bipyridine (bpy) ligands and -C≡C-Ar is a series of substituted aryl acetylide ligands. In one series of complexes, the energy of the Pt → bpy metal-to-ligand charge transfer (MLCT) excited state is varied by changing the substituents on the 4,4′- and/or the 5,5′-positions of the bpy ligand. In a second series of complexes the electronic demand of the aryl acetylide ligand is varied by changing the para substituent (X) on the aryl ring (X = -CF₃, -CH₃, -OCH₃, and -N(CH₃)₂). The effect of variation of the substituents on the excited states of the complexes has been assessed by examining their UV-visible absorption, variable-temperature photoluminescence, transient absorption, and time-resolved infrared spectroscopy. In addition, the nonradiative decay rates of the series of complexes are subjected to a quantitative energy gap law analysis. The results of this study reveal that in most cases the photophysics of the complexes is dominated by the energetically low lying Pt ← bpy ³MLCT state. Some of the complexes also feature a low-lying intraligand (IL) ³π,π* excited state that is derived from transitions between π- and π*-type orbitals localized largely on the aryl acetylide ligands. The involvement of the IL ³π,π* state in the photophysics of some of the complexes is signaled by unusual features in the transient absorption, time-resolved infrared, and photoluminescence spectra and in the excited-state decay kinetics. The time-resolved infrared difference spectroscopy indicates that Pt ← bpy MLCT excitation induces a +25 to + 35 cm^-1 shift in the frequency of the C≡C stretching band. This is the first study to report the effect of MLCT excitation on the vibrational frequency of an acetylide ligand.

Full text of DOI:10.1021/ic0102182

Inorg. Chem. 2001, 40, 4053-4062
4053
Photophysics of Diimine Platinum(II) Bis-Acetylide Complexes
C. Ed Whittle,† Julia A. Weinstein,‡ Michael W. George,‡ and Kirk S. Schanze*,†
Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, and
School of Chemistry, University of Nottingham, University Park, Nottingham, U.K. NG7 2RD
ReceiVed February 23, 2001
A comprehensive photophysical investigation has been carried out on a series of eight complexes of the type
(diimine)Pt(-CtC-Ar)2, where diimine is a series of 2,2′-bipyridine (bpy) ligands and -CtC-Ar is a series
of substituted aryl acetylide ligands. In one series of complexes, the energy of the Pt f bpy metal-to-ligand
charge transfer (MLCT) excited state is varied by changing the substituents on the 4,4′- and/or the 5,5′-positions
of the bpy ligand. In a second series of complexes the electronic demand of the aryl acetylide ligand is varied by
changing the para substituent (X) on the aryl ring (X ) -CF3, -CH3, -OCH3, and -N(CH3)2). The effect of
variation of the substituents on the excited states of the complexes has been assessed by examining their UV-
visible absorption, variable-temperature photoluminescence, transient absorption, and time-resolved infrared
spectroscopy. In addition, the nonradiative decay rates of the series of complexes are subjected to a quantitative
energy gap law analysis. The results of this study reveal that in most cases the photophysics of the complexes is
dominated by the energetically low lying Pt f bpy 3MLCT state. Some of the complexes also feature a low-lying
3
intraligand (IL) π,π* excited state that is derived from transitions between π- and π*-type orbitals localized
3
largely on the aryl acetylide ligands. The involvement of the IL π,π* state in the photophysics of some of the
complexes is signaled by unusual features in the transient absorption, time-resolved infrared, and photoluminescence
spectra and in the excited-state decay kinetics. The time-resolved infrared difference spectroscopy indicates that
Pt f bpy MLCT excitation induces a +25 to + 35 cm-1 shift in the frequency of the CtC stretching band. This
is the first study to report the effect of MLCT excitation on the vibrational frequency of an acetylide ligand.
Introduction
technological standpoint, finding application as sensitizers in
dye-sensitized photovoltaic cells19-21 and as luminescent report-
Transition metal complexes that feature metal-to-ligand
charge transfer (MLCT) excited states have fascinated photo-
chemists for four decades.1-12 This fascination derives from a
combination of features exhibited by MLCT excited states,
which include visible light absorption, photoluminescence, long
lifetimes, and relative inertness toward unimolecular photo-
chemistry.9 From the fundamental standpoint, MLCT excited
states have provided considerable insight into nonradiative
decay,8,13,14 electron transfer processes,7,15 solvent effects,16,17
and small molecule-biopolymer interactions.18 Moreover,
recently MLCT excited states have become useful from a
ers in optical gas sensors.22-25
By far the most work that has been carried out on MLCT
states has focused on d6 metal polypyridine complexes of Ru(II),
Os(II), and Re(I).8,9,13,14 However, recently there has been
increasing interest in the properties of d8 Pt(II) complexes of
the type (diimine)PtL2, where L ) halide, nitrile, thiolate,
isonitrile, and acetylide.26-38 These Pt(II)-diimine complexes
feature long-lived excited states that are believed to be based
(15) Schanze, K. S.; Walters, K. A. In Organic and Inorganic Photochem-
istry; Ramamurthy, V., Schanze, K. S., Eds.; Molecular and Supra-
molecular Photochemistry, 2; Marcel-Dekker: New York, 1998; pp
75-128.
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(17) Chen, P. Y.; Meyer, T. J. Chem. ReV. 1998, 98, 1439-1477.
(18) Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. ReV. 1999, 99, 2777-
2795.
* Corresponding author. E-mail: kschanze@chem.ufl.edu.
† University of Florida.
‡ University of Nottingham.
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10.1021/ic0102182 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/04/2001

4054 Inorganic Chemistry, Vol. 40, No. 16, 2001
Whittle et al.
Chart 1
tive energy gap law correlation and compare it to correlations
observed for MLCT excited states in other families of com-
plexes.8,13 Second, the work described herein also characterizes
the UV-visible excited-state absorption of the MLCT excited
states. Most of the complexes feature excited-state absorptions
that are consistent with the MLCT assignment. However, the
difference absorption spectra of complexes 2a and 2b are distinct
from those of the other members of the series, suggesting that
in these complexes the lowest excited state is not MLCT. Third,
this paper reports the infrared difference spectrum in the region
of the CtC stretch for the excited-state complexes 1a, 1b, and
2b. This the first study to examine the influence of MLCT (or
π,π*) excitation on the vibrational spectrum of a metal-
acetylide complex.
Experimental Section
on dπ Pt f π* diimine MLCT. In particular, in a recent study
Eisenberg and co-workers examined the properties of a series
of complexes of the type (diimine)Pt(-CtC-Ar)2, where
diimine is a substituted 1,10-phenanthroline or a 2,2′-bipyridine
ligand and -CtC-Ar is an aryl-substituted acetylide ligand.34,36
This group demonstrated that the (diimine)Pt(-CtC-Ar)2
complexes feature a moderately intense absorption band in the
blue and a strongly Stokes shifted emission arising from the Pt
f diimine MLCT state. Consistent with the MLCT assignment,
the absorption and emission energies decreased by substitution
of electron-withdrawing groups on the diimine acceptor ligand.36
Concomitant with the Eisenberg group’s work, we were
engaged in a photophysical study of the series of (diimine)Pt-
(-CtC-Ar)2 complexes 1a-d and 2a-d (Chart 1). The
objective of this effort was to systematically examine the
influence of the electronic demand of the diimine and aryl
acetylide ligands on the properties of the MLCT excited state.
Specifically, we wished to investigate whether a correlation
exists between the energy of the MLCT excited state and the
nonradiative decay rate (knr). In addition, we sought to charac-
terize the spectroscopy of the excited state by using variable-
temperature luminescence, transient absorption, and time-
resolved infrared spectroscopy. The present paper comprises a
report of our photophysical investigation on this series of
complexes. Although three of the eight complexes that we
examined were characterized in the earlier study,36 the present
investigation extends the previous work in several ways. First,
by using the series of substituted 2,2′-bipyridine ligands, we
are able to vary the energy of the Pt f diimine MLCT state by
approximately 4000 cm-1 (0.5 eV). This energy range is
considerably larger than that afforded by the series of substituted
phenanthroline complexes examined previously.36 Given this
larger energy gap, it becomes possible to examine the quantita-
Photophysical Measurements. Spectroscopic experiments were
carried out either in CH2Cl2 or in 2-methyltetrahydrofuran (2-MTHF),
and these solvents were purified by distillation from CaH2 and Na/
benzophenone, respectively. Spectroscopy carried out at room temper-
ature 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
(except for time-resolved infrared specroscopy, see below) were carried
out with sample concentrations in the range 2-20 µM. At these low
concentrations, the lifetimes were not concentration dependent (i.e.,
self-quenching is not a significant decay pathway).34 For example,
within experimental error the emission lifetime of 1a was invariant
over the concentration range 2-50 µM.
Steady-state photoluminescence spectroscopy was carried out on a
SPEX Fluorolog 2 instrument. Emission spectra were corrected by using
correction factors generated in-house with a standard calibration lamp.
Sample concentrations were sufficiently low such that the absorbance
at all wavelengths was <0.2 (typical concentration ) 5 × 10-6 M).
Photoluminescence quantum yields were determined relative to
Ru(bpy)32+ in water (Φem ) 0.055),39 and appropriate corrections were
made for the difference in refractive index of the sample and reference
solutions. Low-temperature photoluminescence experiments were car-
ried out with samples contained in an LN2 cooled optical cryostat
(Oxford Instrument, DN-1704) connected to an Omega CYC3200
temperature controller.
Photoluminescence decay lifetimes were determined by time-
correlated single photon counting on an instrument that was constructed
in-house. Excitation was provided either by a blue diode laser (λ )
405 nm, nanoLED-07, IBH, Glasgow, U.K.) or by a near-UV light-
emitting diode (λ ) 370 nm, nanoLED-03, IBH, Glasgow, U.K.)
operating at a 100 kHz repetition rate. Emission light was filtered by
using interference filters (10 nm bandpass, λem ) 500, 550, 600, or
650 nm, as appropriate). Photoluminescence decay times were deter-
mined by using the DECAN fluorescence deconvolution software.40
Transient absorption spectroscopy was carried out on an instrument
that has been described previously.41 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
third harmonic output of a Nd:YAG laser (355 nm, Spectra Physics,
GCR-14). Typical pulse energies were 5 mJ‚pulse-1, which cor-
(30) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949-
1960.
(31) Zhang, Y.; Ley, K. D.; Schanze, K. S. Inorg. Chem. 1996, 35, 7102-
7110.
(32) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 119, 11620-
11627.
(33) Paw, W.; Cummings, S. D.; Mansour, M. A.; Connick, W. B.; Geiger,
D. K.; Eisenberg, R. Coord. Chem. ReV. 1998, 171, 125-150.
(34) Connick, W. B.; Geiger, D.; Eisenberg, R. Inorg. Chem. 1999, 38,
3264-3265.
(35) Connick, W. B.; Miskowski, V. M.; Houlding, V. H.; Gray, H. B.
Inorg. Chem. 2000, 39, 2585-2592.
(36) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachiocotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-
457.
responded to an irradiance in the pump-probe region of 20 mJ‚cm-2
.
Artifacts caused by photoluminescence were corrected by using a data
collection cycle that subtracts the transient signal obtained with the
probe light blocked from the signal obtained with the probe light
present. Transient absorption decay lifetimes were determined from
(37) Hissler, M.; McGarrah, J. E.; Connick, W. B.; Geiger, D. K.;
Cummings, S. D.; Eisenberg, R. Coord. Chem. ReV. 2000, 208, 115-
137.
(38) Adams, C. J.; James, S. L.; Liu, X. M.; Raithby, P. R.; Yellowlees,
L. J. J. Chem. Soc., Dalton Trans. 2000, 63-67.
(39) Harriman, A. J. Chem. Soc., Chem. Commun. 1977, 777-778.
(40) Boens, N.; De Roeck, T.; Dockx, J.; DeSchryver, F. C. DECAN, 1.0
ed.; Leuven, BE, 1991.
(41) Wang, Y. S.; Schanze, K. S. Chem. Phys. 1993, 176, 305-319.

Diimine Platinum(II) Bis-Acetylide Complexes
Inorganic Chemistry, Vol. 40, No. 16, 2001 4055
Table 1. Photophysical Parameters for Platinum Acetylide Complexesa
ꢀmax/M-1 cm-1
λ
em/nm
553
∆Es/cm-1 b
Φem
τ
em/nsd
kr/105 s-1 e
knr/106 s-1 f
τ
TA/nsg
c
complex
λ
abs/nm
1a
294
379
286
386
294
424
268
291 (sh)
309 (sh)
445
43200
9000
45200
8500
57300
4500
28800
21000
16500
5500
1150
0.27
1300
800
103
20
2.08
0.562
1285
793
100
22
1b
1c
1d
570
642
670
2400
2480
2350
0.113
0.007
0.0046
1.41
0.68
2.30
1.11
9.64
49.8
2a
2b
294
389
288
305 (sh)
323
371
300
454
292
67200
12400
22600
18700
27000
42000
49000
6100
538
561
1950
650
0.207
607
3.41
0.25
1.31
600
0.0899
3600
0.253
3720
2c
586
623
-600
0.0075
0.019
250
60
0.30
3.17
3.97
16.4
246
51
2d
41800
7500
2900
401
a Argon-degassed CH2Cl2 solutions. b Thermally induced Stokes shift (∆Es ) E00(77 K) - E00(298 K)). c Emission quantum yield relative to
Ru(bpy)32+ actinometer (Φem(H2O) ) 0.057).39 d Emission decay lifetime. e Radiative decay rate, computed by kr ) Φem/τem f Nonradiative decay
rate, computed by knr ) (1 - Φem)/τem
g Decay lifetime of transient absorption.
.
.
the multiwavelength difference-absorption data by using the SPECFIT/
32 factor analysis software.42
James and co-workers.50 The crude complexes were purified by
chromatography on alumina using CH2Cl2 as the eluant. The purified
complexes were fully characterized by 1H NMR, 13C NMR, and high-
resolution mass spectroscopy. Complexes 1b, 2b, and 2d were reported
previously, and our spectroscopic data match those reported in the
literatrure.36 Spectral data for the new (diimine)Pt(CtC-Ar)2 com-
plexes is listed below.
Time-resolved infrared measurements were performed using a step-
scan FTIR interferometer. A detailed description of the Nottingham
time-resolved step-scan FTIR (s2-FTIR) apparatus is given elsewhere.43
Briefly, the apparatus consists of a commercially available step-scan
FTIR spectrometer (Nicolet Magna 860) equipped with a 100 MHz
12-bit digitizer and a 50 MHz MCT detector interfaced to a Nd:YAG
laser (Spectra Physics GCR 12). Synchronization of the Nd:YAG laser
with data collection was achieved by means of a pulse generator
(Stanford DG535). In some experiments the signal was passed through
a low-noise preamplifier (Stanford Research System, model SR560)
which has been set up at 1 MHz for these experiments. Experiments at
77 K were performed in a home-built two-window cryogenic IR cell,
with CaF2 windows and Teflon spacers regulating the path length.44
Sample concentrations were ≈1 mM for the s2-FTIR experiments.
Under these conditions the excited-state lifetime of 1a was shorter than
observed by emission decay kinetics, presumably due to concentration
quenching.34
Synthesis. 4-Ethynyltoluene was purchased (Aldrich Chemical) and
was used as received. 4-Nitrophenylacetylene,45 4-methoxyphenylacety-
lene,46,47 4-(trifluoromethyl)phenylacetylene47 and 4-(N,N-dimethyl-
amino)phenylacetylene48 were prepared according to literature methods.
The substituted 2,2′-bipyridine ligands were also prepared according
to literature procedures.49 The necessary (diimine)PtCl2 complexes were
prepared by reaction of K2PtCl4 with the substituted bipyridine in
refluxing aqueous HCl solution.36
1
1a. H NMR (CDCl3, TMS): δ 9.41 (s, 2H), 7.75 (s, 2H), 7.42 (d,
4H), 7.07 (d, 4H), 2.39 (s, 6H), 2.37 (s, 6H), 2.36 (s, 6H). 13C NMR
(CD2Cl2): δ 150.85, 139.96, 136.92, 132.31, 128.96, 124.10, 123.95,
120.82, 112.24, 102.03, 90.48, 21.83, 20.53, 17.43 ppm. HRMS: calcd
for C32H30N2Pt 637.206, found 637.205 [M + H].
1c. 1H NMR (CDCl3, TMS): δ 9.63 (d, 2H), 7.95 (s, 2H), 7.41 (d,
2H), 7.38 (d, 4H), 7.06 (d, 4H), 3.54 (q, 4H), 3.33 (q, 4H), 2.35 (s,
6H), 1.24 (t, 6H), 1.09 (t, 6H). 13C NMR (CD2Cl2): δ 166.91, 156.73,
151.35, 146.56, 135.25, 131.62, 128.68, 125.15, 124.24, 121.06, 101.94,
85.68, 43.38, 39.51, 21.31, 14.32, 12.67. HRMS: calcd for C38H40N4O2-
Pt 779.280, found 780.288 [M + H].
1d. 1H NMR (CDCl3, TMS): δ 9.98 (d, 2H), 8.62 (s, 2H), 8.07 (d,
2H), 7.36 (d, 4H), 7.07 (d, 4H), 4.47 (q, 4H), 2.35 (s, 6H), 1.43 (q,
6H). 13C NMR (CD2Cl2, TMS): δ 163.88, 156.52, 152.55, 139.76,
135.56, 132.21, 128.72, 127.38, 125.13, 122.63, 85.85, 81.31, 63.21,
21.59, 14.40 ppm. HRMS: calcd for C34H30N2O4Pt 725.185, found
726.193 [M + H]
1
2a. H NMR (CDCl3): δ 9.62 (d, 2H), 7.97 (s, 2H), 7.62 (m, 6H),
7.43 (d, 4H), 1.42 (s, 18H). (This compound was not sufficiently soluble
to allow acquisition of 13C NMR.) HRMS: calcd for C36H32F6N2Pt
801.212, found 802.2191 [M + H].
All of the (diimine)Pt(CtC-Ar)2 complexes were synthesized by
reaction of the corresponding (diimine)PtCl2 with the suitably substi-
tuted acetylenes, Ar-CtC-H, according to the method described by
1
2c. H NMR (CDCl3, TMS): δ 9.78 (broad s, 2H), 7.92 (s, 2H),
7.51 (d, 2H), 7.43 (d, 4H), 6.64 (d, 4H), 2.93 (s, 12H), 1.43 (s, 18H).
13C NMR (CD2Cl2): δ 162.80, 156.23, 151.20, 148.42, 132.90, 124.42,
118.50, 116.99, 112.13, 102.23, 82.39, 40.69, 35.70, 30.23 ppm.
HRMS: calcd for C38H44N4Pt 751.3214, found 751.3227.
(42) Binstead, R. A. SPECFIT/32, 1.0 ed.; Spectrum Software Associates:
Chapel Hill, NC, 2000.
(43) Sun, X.-Z.; Colley, C. S.; Nikiforov, S. N.; Yang, J.; George, M. W.
Appl. Spectrosc., in press.
(44) George, M. W. Ph.D. Dissertation, University of Nottingham, Not-
tingham, U.K., 1990.
Results and Discussion
Structures. This study examines the photophysical properties
of the two series of (diimine)Pt(-CtC-Ar)2 complexes shown
in Chart 1. In the first series, 1a-d, the substituents on the
diimine acceptor ligand are varied while the structure of the
two acetylene ligands is held constant. Variation of these
substituents has a strong effect on the energy of the LUMO of
(45) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627-630.
(46) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org. Lett.
2000, 2, 1729-1731.
(47) Crisp, G. T.; Flynn, B. L. J. Org. Chem. 1993, 58, 6614-6619.
(48) Leonard, K. A.; Nelen, M. I.; Anderson, L. T.; Gibson, S. L.; Hilf,
R.; Detty, M. R. J. Med. Chem. 1999, 42, 3942-3952.
(49) 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.
(50) James, S. L.; Younus, M.; Raithby, P. R.; Lewis, J. J. Organomet.
Chem. 1997, 543, 233-235.

4056 Inorganic Chemistry, Vol. 40, No. 16, 2001
Whittle et al.
Table 2. Emission Spectral Fitting Parametersa
complex
E00/cm-1
pωm/cm-1
∆νj0,1/2/cm-1
Sm
1a
1b
1c
1d
2a
2b
2c
2d
18750
17150
15970
14850
19400
18350
17700
16800
1300
1300
1300
1300
1300
1300
1300
1300
1900
1850
2400
2400
1890
1800
3200
2460
1.45
1.25
1.2
0.85
1.4
1.25
1.12
1.6
a
Estimated errors: ∆νj0,1/2 and Sm, (10%; Eoo and pωm, (5%.
There are some unique features that emerge from examination
of the absorption spectra of the new complexes examined in
this study. First, as shown in Figure 1, the MLCT absorption
band red-shifts by nearly 3000 cm-1 over the series 1a f 1d.
This red-shift is considerably larger than observed for a series
of substituted (1,10-phenanthroline)Pt(-CtC-Ar)2 com-
plexes36 and is due to a very large difference in LUMO energies
for the substituted 2,2′-bipyridine ligands in 1a-d. Another
point of interest is that in complexes 1c and 1d the low-energy
MLCT feature clearly consists of two overlapping bands. This
splitting suggests the possibility that there may be two orbitally
distinct MLCT transitions, e.g., dxz(Pt) f π* (diimine) and
dyz(Pt) f π* (diimine).
Figure 1. UV-visible absorption spectra of 1a-d in CH2Cl2 solution.
(s) 1a; (- -) 1b; (- - -) 1c; (-‚‚-) 1d.
Photoluminescence. All of the (diimine)Pt(-CtC-Ar)2
complexes feature moderately intense photoluminescence (PL)
at room temperature in fluid solution and at 80 K in a 2-MTHF
glass. Example spectra are illustrated for complexes 1a and 2b
in Figure 2. Table 1 lists the emission wavelength maxima at
298 K, as well as the thermally induced Stokes shifts (∆Es),
which correspond to the difference in the 80 and 298 K emission
energies. The room temperature emission spectra were fitted
using a single-mode Franck-Condon expression (eq 1),
E00 - νmpωm 3 (Sm)νm
5
I(νj) )
∑
{
(
)
E00
νm!
νm)0
2
νj - E00 + νmpωm
∆νj0,1/2
Figure 2. Photoluminescence spectra of 1b (top) and 2b (bottom) in
2-MTHF solution. In order of increasing intensity: 298, 140, and 80
K. The intensity of spectrum 2b at 298 K is increased 5-fold to make
it visible on the same scale.
exp -4 ln 2
(1)
}
]
(
)
[
where I(νj) is the relative emission intensity at energy νj, E00 is
the energy of the zero-zero transition, νm is the quantum
number of the average medium-frequency vibrational mode,
pωm is the average of medium-frequency acceptor modes
coupled to the MLCT transition (1300 cm-1 was assumed
throughout the series), Sm is the Huang-Rhys factor (i.e., the
electron-vibration coupling constant), and ∆νj0,1/2 is the half-
width of the individual vibronic bands. The parameters obtained
by application of eq 1 for the complexes 1a-d and 2a-d are
listed in Table 2, and fitted emission spectra are attached as
Supporting Information. The parameters recovered from these
spectral fits are used in a later section where a quantitative
energy gap law analysis of the photophysical data for the
complexes is considered.
the diimine acceptor ligand, which in turn is expected to exert
a profound effect on the energy of the metal f diimine MLCT
state as demonstrated in previous studies of Ru(II) and Re(I)
complexes.9,14 In the second series, 2a-d, the diimine ligand
is held constant (4,4′-bis(tert-butyl)-2,2′-bipyridine) while the
para substituent on the arylacetylene ligand is varied. Variation
of this substituent is anticipated to exert an effect on both the
σ- and π-donor (or acceptor) properties of the acetylide ligands.
UV-Visible Absorption Spectroscopy. The (diimine)-
Pt(-CtC-Ar)2 complexes feature an array of absorptions in
the UV-visible region that are due to a combination of
intraligand (IL) π,π* and MLCT transitions. Table 1 summarizes
the band maxima and molar absorptivities for the transitions,
and Figure 1 shows the long-wavelength region of the spectra
for complexes 1a-d. In general, all of the complexes feature
moderately intense absorptions (ꢀ < 104 M-1 cm-1) in the 350-
500 nm region that arise from Pt f diimine MLCT. In addition,
almost all of the complexes feature an intense band in the 290-
310 nm region (ꢀ ≈ 5 × 104 M-1 cm-1) which is clearly due
to the π,π* IL transitions. In general, the absorption features
observed for the new complexes are in accord with those
reported previously.36
Several features are worthy of note with respect to the PL
spectra. First, the emission energy decreases substantially across
series 1a-d. At 298 K the emission energy of 1a is 3900 cm-1
greater than that of 1d. This large red shift is consistent with
the PL emanating from a Pt f diimine MLCT state, since the
emission energy decreases with the energy of the LUMO which
is localized on the diimine acceptor ligand. Interestingly, the
difference in E00 for 1b and 1d (∆E00 ) 2300 cm-1) is almost
identical to that for (dcebpy)Re(CO)3Cl and (dmb)Re(CO)3Cl

Diimine Platinum(II) Bis-Acetylide Complexes
Inorganic Chemistry, Vol. 40, No. 16, 2001 4057
(∆E00 ) 2210 cm-1) (where dcebpy ) 4,4′-dicarboethoxy-2,2′-
bipyridine and dmb ) 4,4′-dimethyl-2,2′-bipyridine).14 This
correlation points to the similarity in the nature of the emitting
MLCT states for the d8 Pt(II) and d6 Re(I) systems. A second
point of note is that all of the complexes 1a-d feature large
∆Es values. It has been shown that, for MLCT excited states,
∆Es ) 2λs, where λs is the outer-sphere reorganization energy
for the MLCT excited state.16,17 Thus, we estimate that λs varies
from 0.075 eV (1a) to 0.15 eV (1b-d). These comparatively
large λs values indicate that there is a large difference between
the dipole moments of the ground and MLCT excited states in
the (diimine)Pt(-CtC-Ar)2 complexes. This is consistent with
the findings of a transient DC photoconductivity study of the
structurally related complex (Ph2phen)Pt(S2) (where Ph2phen
) 4,7-diphenylphenanthroline and S2 is a dithiolate) which
suggest that for this complex the ground-state dipole moment
is ≈10 D and the dipole moment of the MLCT state is ≈0 D
(i.e., ∆µ ) -10 D).51 In addition, the variable-temperature
emission experiments on 1a-d were carried out in 2-MTHF.
This solvent is comparatively nonpolar, and consequently the
λs values will be lower than if the studies were carried out in a
more polar solvent such as EtOH/MeOH (this solvent mixture
is commonly used for low-temperature spectroscopy of metal
complexes).16,17 By comparison, Ru(bpy)32+ features a λs value
of ≈ 0.05 eV in EtOH/MeOH solution.52 This comparison
suggests that the difference in dipole moments of the ground
In contrast to the behavior of 1a-d, there are no clear trends
in the φem and τem (or kr and knr) values for 2a-d, despite the
fact that E00 varies by ≈2500 cm-1 across the series. The lack
of a systematic trend in the photophysical parameters for the
series points to the possibility that more than one excited state
contributes to the photophysics of these complexes. The most
obvious exception among the series 2a-d is the -NO2-
substituted complex 2b, which features a lifetime that is nearly
10-fold larger compared to the lifetimes of the other complexes.
The large τem for 2b arises because both kr and knr are 10-fold
lower compared to the values for the other complexes. Dramati-
cally lower kr and knr values have been reported for Ru(II) and
3
Re(I) complexes where IL π,π* excited states are in close
energetic proximity to the MLCT manifold,54-56 and we suspect
that a similar effect is manifest in complex 2b. A more subtle,
yet significant, deviation is seen in the photophysical parameters
of the -CF3-substituted complex 2a. This complex features an
unusually large knr value compared to the complexes in series
1a-d, despite the fact that it has the highest emission energy
of any of the (diimine)Pt(-CtC-Ar)2 complexes.
Transient Absorption Spectroscopy. Pulsed photoexcitation
of the (diimine)Pt(-CtC-Ar)2 complexes with the 355 nm
output of a Q-switched Nd:YAG laser affords transients that
absorb relatively strongly throughout the near UV-visible
region. Time-resolved absorption-difference (TA) spectra for
all of the complexes (except 2c, which features only a Very weak
TA spectrum) are illustrated in Figure 3, and the TA decay
lifetimes (τΤΑ) are listed in Table 1. For all of the complexes
τTA and τem values are in excellent agreement, which implies
that in each case the transient observed by absorption is the
excited-state complex. (Another possibility is that the absorbing
transient is in equilibrium with the emitting excited state.) The
TA spectra of 1a-d and 2d are similar, with each spectrum
and MLCT excited states in 1a-d is considerably larger than
2+
that of Ru(bpy)3
.
The acetylide-substituted complexes 2a and 2d also feature
comparatively large ∆Es values (and by inference large λs),
suggesting that the PL from these complexes also emanates from
the Pt f diimine MLCT manifold. However, the situation is
more complicated for 2b and 2c. Specifically, as shown in
Figure 2, the emission energy for 2b decreases with temperature
down to 140 K, but below this temperature the emission energy
increases with decreasing temperature. The net effect is that
the emission energy of 2b is virtually the same at 80 and 298
K. A qualitatively similar pattern is seen for 2c. The lack of
significant ∆Es values for 2b and 2c clearly indicates that for
these complexes the emission does not emanate from a MLCT
manifold. Rather, we believe that for these two complexes the
featuring (1) a strong absorption band in the near UV (λmax
≈
360-370 nm); (2) bleaching or a “dip” in the 400-450 nm
region; and (3) broad, moderately intense absorption through-
out the visible region. These TA spectral features are hallmarks
of metal f bipyridine MLCT excited states.57-61 The strong
near-UV and moderate visible absorptions arise from the
bipyridine anion radical that is present in the MLCT state, i.e.,
(diimine-• )PtIII(-CtC-Ar)2, and the “dip” that occurs in the
400-450 nm region is due to bleaching of the ground-state
MLCT absorption band.61 The MLCT assignment is further
supported by the fact that the TA spectra of 1a-d and 2d are
remarkably similar to those of the analogous (diimine)Re-
(CO)3Cl complexes, which are known to feature lowest energy
excited states based on the Re f diimine MLCT transition.59,62
A striking feature is that the TA spectra of complexes 2a
and 2b are distinct from those of the remaining complexes. The
TA spectrum of -CF3-substituted complex 2a features only a
3
PL arises from a state having IL π,π* character. Additional
evidence supporting this hypothesis is presented below.
The PL lifetimes and quantum yields (τem and φem, respec-
tively) for all of the (diimine)Pt(-CtC-Ar)2 complexes in
CH2Cl2 solution are listed in Table 1. The τem and φem values
have also been used to compute radiative and nonradiative decay
rate constants (kr and knr, respectively), and these parameters
are also compiled in the table. Several clear trends emerge from
this data for complexes 1a-d. Specifically, τem and φem decrease
systematically with the emission energy. Inspection of the kr
and knr values for this series reveals that the energy gap
dependence of τem and φem arises almost exclusively from
variation in the nonradiative decay rate, i.e., knr increases almost
100-fold over the series 1a f 1d, while kr varies randomly and
by less than a factor of 4. The observed trend in knr is in accord
with the energy gap law,8,53 and a more thorough analysis of
this trend is presented below.
(54) Shaw, J. R.; Webb, R. T.; Schmehl, R. H. J. Am. Chem. Soc. 1990,
112, 1117-1123.
(55) Shaw, J. R.; Schmehl, R. H. J. Am. Chem. Soc. 1991, 113, 389-394.
(56) Baba, A. I.; Shaw, J. R.; Simon, J. A.; Thummel, R. P.; Schmehl, R.
H. Coord. Chem. ReV. 1998, 171, 43-59.
(57) Schanze, K. S.; Neyhart, G. A.; Meyer, T. J. J. Phys. Chem. 1986,
90, 2182-2193.
(58) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S.; Anthon,
D.; Neveaux, P. E., Jr.; Meyer, T. J. Inorg. Chem. 1987, 26, 1116-
1126.
(59) MacQueen, D. B.; Schanze, K. S. J. Am. Chem. Soc. 1991, 113, 7470-
(51) Vanhelmont, F. W. M.; Johnson, R. C.; Hupp, J. T. Inorg. Chem.
2000, 39, 1814-1816.
7479.
(60) Sun, H.; Hoffman, M. Z.; Mulazzani, Q. G. Res. Chem. Intermed.
1994, 20, 735-754.
(61) Stufkens, D. J.; Vlcek, A. Coord. Chem. ReV. 1998, 177, 127-179.
(62) Schanze, K. S.; Macqueen, D. B.; Perkins, T. A.; Cabana, L. A. Coord.
Chem. ReV. 1993, 122, 63-89.
(52) Kitamura, N.; Kim, H.-B.; Kawanishi, Y.; Obata, R.; Tazuke, S. J.
Phys. Chem. 1986, 90, 1488-1491.
(53) Caspar, J. V. Ph.D. Dissertation, University of North Carolina, Chapel
Hill, NC, 1982.

4058 Inorganic Chemistry, Vol. 40, No. 16, 2001
Whittle et al.
bleach at 375 nm along with a broad absorption that extends
throughout the visible (see Supporting Information).64 Although
assignment of the TA spectrum of -CF3-substituted complex
2a is less clear, the presence of the strong mid-visible transient
absorption band suggests that the lowest excited state in this
3
complex may also have IL π,π* character.
Time-Resolved Infrared Spectroscopy. The method of fast
time-resolved infrared spectroscopy is invaluable for probing
the vibrational and electronic structure of excited states and
reactive intermediates formed by photoexcitation of transition
metal organometallic complexes.66-69 The technique is particu-
larly useful when applied to complexes featuring ligands that
absorb strongly in the IR (e.g., CO and CN) and when the
excited states are MLCT in nature.66,70,71 For example, in
complexes of the type (diimine)ReI(CO)3(L), (diimine)2OsII-
(CO)(L), and W(CO)5(pyridine-X), the CO vibrational absorp-
tion band(s) undergo(es) a significant shift to higher frequency
in the excited state.66,70,71 The shifts to higher frequency are
attributed to the fact that MLCT excitation effectively decreases
the electron density at the metal center, resulting in decreased
M f CO back-bonding. By contrast, complexes that feature
metal-centered or intraligand excited states typically exhibit very
little shift in the frequency of the CO bands.69
In order to gain further insight into the electronic structure
of the excited states of the (diimine)Pt(-CtC-Ar)2 complexes,
time-resolved step-scan FTIR spectroscopy (s2-FTIR) was used
to probe the shifts in the ν(CtC) IR bands upon formation of
the excited state. In the MLCT excited state the decrease in
electron density that occurs at the Pt center due to Pt f diimine
MLCT should be reflected by an increase in the frequency of
the acetylide stretching band(s).
Figure 4a shows the ground-state FTIR spectrum of 1a in
CH2Cl2. There are two ν(CtC) bands at 2115 and 2124 (sh)
cm-1. Figure 4b shows the s2-FTIR difference spectrum of this
solution obtained 50 ns after irradiation (355 nm). The parent
IR bands are bleached, and a new excited-state band is produced
at 2142 cm-1. Under the higher concentration used in the s2-
FTIR experiments, the excited-state lifetime is 420 ns. In
principle there should be two excited-state bands produced.
However, only one strong absorption is resolved. This may be
due to an accidental overlap of the two excited-state bands, or
the second band may not be resolved due to the signal-to-noise
of the data.
The s2-FTIR spectra of 1b in CH2Cl2 at 298 K and in a PrCN/
BuCN glass at 77 K were also characterized, and these spectra
are shown in Figure 4c. The room temperature s2-FTIR of 1b
shows features that are similar to those of 1a. In both 1a and
1b, the excited-state ν(CtC) IR band is shifted ca. 30-35 cm-1
relative to the mean band position of the ground-state absorp-
tions.
Figure 3. Transient absorption difference spectra for CH2Cl2 solutions
at ambient temperature following 355 nm excitation. Spectra acquired
at the following time increments (beginning immediately following the
laser pulse): 1a, 320 ns; 1b, 320 ns; 1c, 80 ns; 1d, 80 ns; 2a, 320 ns;
2b, 4 µs; 2d, 320 ns.
weak near-UV absorption, and the spectrum is dominated by a
strong visible absorption band with λmax ≈ 510 nm. The
spectrum of -NO2-substituted complex 2b is even more
unique: it features strong bleaching in the region of the ground-
state IL π,π* absorption band (λ ≈ 370 nm) and broad
absorption throughout the visible and extending into the near-
IR region. Although the assignment of excited states based on
TA spectra is not without pitfalls, in the case of -NO2 complex
2b the shape of the TA spectrum clearly signals that the excited
state involves the nitro-substituted aryl acetylide ligands (-Ct
C-Ar-NO2). There are two possibilities for the lowest excited
3
state that is observed by TA spectroscopy: (1) a π,π* state
that is localized on the -CtC-Ar-NO2 ligands; (2) a Pt f
CtC-Ar-NO2 MLCT excited state. We favor the former
assignment based on the fact that the TA spectrum of 2b is
very similar to that of conjugated phenylene ethynylenes
(i.e., [-CtC-Ar-]x) and complexes of the type trans-
Pt(PR3)2(-CtC-Ar)2, which are known to feature lowest 3π,π*
excited states.63-65 For example, the transient absorption of the
3π,π* state of the complex trans-(Bu3P)2Pt(-CtC-t-St)2
(where t-St is trans-stilbene) features a strong ground-state
The shift of the ν(CtC) bands to higher frequency is
consistent with the MLCT character of the excited states of 1a
and 1b, i.e., the decreased (dπ) Pt f (π*) -CtC-Ar back-
(66) Turner, J. J.; George, M. W.; Johnson, R. C.; Westwell, J. R. Coord.
Chem. ReV. 1993, 125, 101-114.
(67) Gamelin, D. R.; George, M. W.; Glyn, P.; Grevels, F.-W.; Johnson,
F. P. A.; Klotzbucher, W.; Morrison, S. L.; Russell, G.; Schaffner,
K.; Turner, J. J. Inorg. Chem. 1994, 33, 3246-3250.
(68) Clark, I. P.; George, M. W.; Johnson, F. P. A.; Turner, J. J. Chem.
Commun. 1996, 1587-1588.
(63) Walters, K. A.; Ley, K. D.; Schanze, K. S. J. Chem. Soc., Chem.
Commun. 1998, 10, 1115-1116.
(64) Glusac, K.; Schanze, K. S. Unpublished.
(65) Cooper, T. M.; McLean, D. G.; Rogers, J. E. MRS Symp. Proc., in
press.
(69) Schoonover, J. R.; Strouse, G. F.; Dyer, R. B.; Bates, W. D.; Chen,
P.; Meyer, T. J. Inorg. Chem. 1996, 35, 273-274.
(70) Schoonover, J. R.; Bignozzi, C. A.; Meyer, T. J. Coord. Chem. ReV.
1997, 165, 239-266.
(71) Schoonover, J. R.; Strouse, G. F. Chem. ReV. 1998, 98, 1335-1355.

Diimine Platinum(II) Bis-Acetylide Complexes
Inorganic Chemistry, Vol. 40, No. 16, 2001 4059
diimine MLCT state.67,68,70 Studies have also examined the shift
in the MLCT excited state of complexes of the type (diimine)2OsII-
(CO)(L), and in this case ∆νjCtO ≈ +70 cm-1 is typical.70,71 It
is clear from this comparison that ∆νjCtC in 1a and 1b is
substantially less than is typical for ∆νjCtO in M f diimine
MLCT states. Given that the extent of π-back-bonding in
metal-acetylide complexes is considerably less than in CO
complexes, a priori we anticipate that the CtC bond will be
less sensitive to the change in electron density that is induced
by MLCT excitation.72 This is likely the primary reason that
∆νjCtC is less than ∆νjCtO for MLCT excited states. An
additional contributing factor leading to the lower ∆νjCtC values
observed for the bis-acetylide complexes is that the acetylide-
based π* orbital(s) are more highly delocalized than the π* CO
orbitals in the carbonyl complexes (e.g., the acetylide-based π*
orbitals are delocalized into the aromatic rings). This de-
localization may attenuate the effect of decreased (dπ) Pt f
(π*) -CtC-Ar back-bonding on the CtC stretching fre-
quency.
In distinct contrast to the s2-FTIR spectra of 1a and 1b, the
spectrum of 2b (5:4 BuCN/PrCN, glass at 77 K) features only
depletion of the parent absorption without formation of any
strong new bands (Figure 4e, solid line). The two ground-state
IR bands of 2b are well separated (Figure 4d, 2115 and 2130
cm-1), and careful inspection of the parent depletion shows that
the intensity ratio of the bleaches is different from their ratio in
the ground-state FTIR spectrum. Subtraction of the ground-state
spectrum of 2b from its s2-FTIR spectrum affords the excited-
state absorption spectrum (Figure 4e, dashed line) that features
a single band at 2128 cm-1. This excited-state spectrum implies
that there is a very small ∆νjCtC in 2b, which in turn suggests
that there is significantly less charge-transfer character in the
complex’s excited state. This is clearly consistent with an IL
3π,π* assignment for the excited state of 2b.69 It has been
previously shown that in metal-carbonyl complexes which
Figure 4. Ground- and excited-state infrared spectra for complexes
1a, 1b, and 2b. (a) Ground-state FTIR spectrum of 1a in CH2Cl2
solution at 298 K. (b) s2-FTIR difference spectrum for 1a in CH2Cl2
solution at 298 K 50 ns after excitation. (c) s2-FTIR difference spectra
of 1b. Solid line: 298 K in CH2Cl2 solvent. Dashed line: 77 K in
BuCN/PrCN (5:4 v:v) solvent glass. (d) Ground-state FTIR spectrum
of 2b in BuCN/PrCN (5:4 v:v) solvent glass at 77 K. (e) s2-FTIR spectra
for 2b in BuCN/PrCN (5:4 v:v) solvent glass 77 K 50 ns after excitation.
Solid line: difference spectrum. Dashed line: calculated excited-state
absorption spectrum.
3
feature an IL π,π* excited state νjCtO shifts slightly to lower
frequency relative to the ground state.69 The small increase in
νjCtC seen in the exited state of 2b may arise because in the
3π,π* excited state the CtC bond order is slightly diminished
due to increased π-conjugation between the CtC bonds and
the phenyl rings.
Quantitative Analysis of Nonradiative Decay. Extensive
studies have been carried out to demonstrate that the nonradia-
tive decay rates of MLCT excited states in transition metal
complexes can be accurately modeled using nonradiative decay
theory.8,13,73-77 Specifically, it has been shown that for series
of structurally related polypyridine complexes of Os(II),13,77
Re(I),14 and Ru(II)78 the nonradiative decay rate decreases
exponentially as the energy of the MLCT states increases. This
effect is termed the energy gap law, and the most often used
bonding that is expected concomitant with Pt f diimine charge
transfer. The s2-FTIR spectrum of complex 1b was also obtained
in a BuCN/PrCN (5:4 v:v) glass at 77 K (dashed line in Figure
4c). Interestingly, ∆νjCtC is considerably less in the glass (+24
cm-1) compared to the value in solution. A similar effect was
previously reported for the complex (bpy)ReI(CO)3Cl, and in
this system ∆νjCtO was significantly lower for the each of the
three CtO bands.68 The decreased ∆νj seen in the glass has
been termed “infrared rigidochromism” and is clearly associated
with the rigidity of medium (as opposed to being simply due to
the low temperature), but the mechanism for effect is not fully
understood.68
(72) The fact that back-bonding is not particularly strong is evidenced by
the fact that the ground-state νjCtC is nearly the same for the (diimine)-
Pt(-CtC-Ar)2 complexes and free phenylacetylene (νjCtC ) 2124
cm-1).
(73) Robinson, G. W.; Frosch, R. P. J. Chem. Phys. 1963, 38, 1187-1203.
(74) Siebrand, W. J. Chem. Phys. 1967, 46, 440-447.
(75) Siebrand, W. J. Chem. Phys. 1966, 44, 4055-4057.
(76) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. ReV. 1977, 77,
793-834.
(77) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.
Chem. 1986, 90, 3722-3734.
(78) Barqawi, K. R.; Murtaza, Z.; Meyer, T. J. J. Phys. Chem. 1991, 95,
47-50.
It is also of interest to compare the room temperature ∆νjCtC
values for 1a and 1b with shifts seen for CO ligands in MLCT
excited-state complexes. The most extensive work in this area
has centered on complexes of the type (diimine)ReI(CO)3(L),
and in these systems ∆νjCtO is typically +60 cm-1 in the Re f

4060 Inorganic Chemistry, Vol. 40, No. 16, 2001
Whittle et al.
expression to analyze nonradiative decay rate data is77
γoE00
pωm
(∆νj0,1/2/pωm)2
16 ln 2
ln knr ) ln âo -
- Sm + (γo + 1)2
-
pωmE00
1000 cm-1
0.5 ln
(2a)
(
)
x
π/2
1000 cm-1
2
âo ) |Ck| ωk
(2b)
(2c)
(
)
E00
γo ) ln
- 1
(
)
pωmSm
where Sm, E00, ∆νj0,1/2, and pωm are the same terms defined in
eq 1, âo is the vibronically induced electronic coupling term,
Ck is the vibronic coupling matrix element, and ωk is the
frequency of the promoting vibrational mode (i.e., the mode
that mixes the ground and excited electronic states). Two
simplifications of eqs 2a-c are possible which facilitate anal-
ysis of experimental data.77 First, under the assumption that all
but the second term on the right-hand side (rhs) of eq 2a are
nearly invariant with E00 leads to eq 3a, which predicts that the
log of the nonradiative decay rate will vary linearly with E00.
This equation is the mathematical statement of the energy gap
law. Alternatively, noting that all but the first term on the rhs
in eq 2a can be calculated by fitting the photoluminescence
spectra by using eq 1 suggests that eq 2a can be recast as eq 3b
where the last four terms are represented by a single term,
ln[FCF(calcd)] (where FCF(calcd) is an abbreviation for “cal-
culated Franck-Condon factors”).
Figure 5. Energy gap law analysis for the (diimine)Pt(-CtC-Ar)2
complexes in CH2Cl2 solution at 298 K. (a) Plot of ln knr versus E00.
(b) Plot of ln[FCF(calcd)] versus ln knr. Closed triangles: data for series
1a-d. Open triangles: data for series 2a-d.
states14,77 and has been attributed to the fact that the extent of
charge transfer in the MLCT state increases with E00.
Figure 5a illustrates the correlation of ln knr vs E00 for the
series of complexes 1a-d and 2a-d. As can be seen from this
plot, the data for the series in which only the bipyridine ligand
is substituted (1a-d) follow a reasonable linear correlation. The
dashed line is a linear least-squares fit of the data for complexes
1a-d with slope, δ(ln knr)/δ(E00) ) -1.18 × 10-3 cm-1. By
contrast, the data for the series in which the aryl acetylide ligand
is varied clearly does not follow the same energy gap correlation.
In addition, the point for -NO2-substituted complex 2b is
markedly skewed from any correlation. (Note that this complex
has already been identified as the most likely to have a lowest
3π,π* excited state.) The slope of the energy gap law correlation
for 1a-d is consistent with the premise that the dominant
acceptor mode has pωm ≈ 1300 cm-1, since by using this value
for pωm we compute a slope that is in good agreement with the
least-squares slope (i.e., γo/pωm ≈ 1.16 × 10-3 cm-1).79 This
is significant, because it implies that the CtC stretch does not
play a significant role in accepting the energy that is released
concomitant to nonradiative decay of the MLCT state (i.e., a
larger δ(ln knr)/δ(E00) would be expected if the CtC stretch
operates as an acceptor mode). Interestingly, δ(ln knr)/δ(E00) is
in reasonable agreement with the value obtained from an energy
gap law analysis of nonradiative decay rates for the series of
complexes (X2-bpy)ReI(CO)3Cl, where (δ(ln knr)/δ(E00) ) 0.91
× 10-3 cm-1. This correspondence underscores the similarity
in the structure of the MLCT states in the (diimine)ReI(CO)3Cl
and (diimine)Pt(-CtC-Ar)2 systems, and also suggests that
the carbonyl and acetylide ligands play similar (and marginal)
roles in the MLCT photophysics of the chromophores. Also of
γo
ln knr ) a -
E00
(3a)
(3b)
(
)
pωm
ln[FCF(calcd)] ) ln knr - ln âo
Equation 3a implies that a plot of ln knr vs E00 will be linear
with slope ) γo/pωm, while eq 3b suggests that a plot of
ln[FCF(calcd)] vs ln knr will be linear with a slope ) 1.0 and
an intercept of -ln âo.
Eisenberg and co-workers noted that the nonradiative decay
rates for their series of (diimine)Pt(-CtC-Ar)2 complexes
obeyed the energy gap law,36 but due to the relatively small
range of E00 values accessed by their series it is not possible to
apply quantitative energy gap law models to analyze the rate
data. As noted above, the E00’s for complexes 1a-d and 2a-d
span a wide energy range, making it possible to apply eqs 3a
and 3b, along with the parameters recovered from the photo-
luminescence spectral fitting procedure, to assess how well the
nonradiative decay rates for the series are modeled by theory.
As noted above, the room temperature photoluminescence
spectra of 1a-d and 2a-d were fitted using eq 1, and the
parameters recovered from those fits are reported in Table 2.
All of the spectra were fitted assuming a dominant acceptor
mode of 1300 cm-1. (This choice is based on the idea that C-C
diimine skeletal vibrations are the dominant acceptor modes,
and is supported by the observation of a resolved vibronic
progression in the low-temperature emission spectra, e.g., Figure
2, top.) The other parameters were varied to optimize the fits
(see Supporting Information for the fitted spectra). One point
of note is that, in general, Sm increases with E00; correlation
between Sm and E00 has been observed in other series of MCLT
(79) γ0 is computed by using eq 2c, assuming pωm ) 1300 cm-1 and Sm
) 1.0.

Diimine Platinum(II) Bis-Acetylide Complexes
Inorganic Chemistry, Vol. 40, No. 16, 2001 4061
interest is the fact that there is good agreement in the energy
gap law correlation for the (diimine)Pt(-CtC-Ar)2 complexes
and a series of (diimine)Pt(tdt) complexes, where tdt ) 2,3-
toluenedithiolate.30 This similarity further underscores the small
role played by the acetylide ligands in the MLCT photophysics
of the complexes.
Finally Figure 5b presents a correlation of ln[FCF(calcd)] vs
ln knr for complexes 1a-d and 2a-d. The dashed line has slope
) 1.0 and intercept ) -35 (note eq 3b). While the correlation
is not outstanding, there is at least a qualitative agreement for
the series of complexes. The intercept of the correlation suggests
a value of â0 ) 5.8 × 1014 cm-1. This vibronically induced
electronic coupling term that is estimated for the (diimine)Pt-
(-CtC-Ar)2 complexes is in excellent agreement with values
of âo recovered from energy gap analysis of several series of
Ru(II), Os(II), and Re(I) polypyridyl complexes, where values
of âo ranging from 1.3 × 1014 to 5.8 × 1014 cm-1 have been
estimated.
temperature (i.e., the rigidochromic effect, which is characteristic
of charge-transfer emission, is absent).
The 3π,π*CtCAr state is more important than the 1π,π*CtCAr
state with respect to the excited-state spectroscopy and decay
kinetics of the complexes. However, in order for the 3π,π*CtCAr
state to have a significant influence, it must be no greater than
a few hundred millielectronvolts higher in energy than the
3MLCT state. On the basis of singlet-triplet splittings typical
of organic aromatic hydrocarbons (1.2-1.5 eV),81,82 we predict
that for -NO2-substituted complex 2b, the 3π,π*CtCAr state lies
in the energy range 1.8-2.1 eV, and for the remaining
3
complexes the π,π*CtCAr state lies between 2.5 and 2.7 eV.
The existence of this low-lying 3π,π*CtCAr state has important
consequences for the photophysics of the (diimine)Pt(-CtC-
Ar)2 complexes. First, for complexes 1a-d it is clear that
3MLCT is the lowest excited state. In each case the available
3
photophysical data are in accord with a MLCT assignment.
3
Second, for 2b, it is evident that π,π*CtCAr is lower energy
than 3MLCT. The transient absorption, photoluminescence, and
s2-FTIR spectra of this complex are distinctly different from
those of 1a-d, and its excited-state decay rate parameters (i.e.,
kr and knr) are considerably less than those for the other
complexes. The situation regarding the nature of the lowest
excited state is less clear for complexes 2a, 2c, and 2d.
Specifically, the transient absorption of complex 2a is not typical
Excited-State Scheme for the (diimine)Pt(-C≡C-Ar)2
Complexes. Consideration of the spectroscopic data presented
above for the diimine platinum acetylide complexes makes it
evident that there are two manifolds of excited states that play
a role in their photophysics. First, there is a set of 1MLCT and
3MLCT states which derive from the dπ (Pt) f π* (diimine)
transition. The existence of the MLCT manifold is clear: in
most of the complexes the low-energy absorption is dominated
by the allowed transition to the 1MLCT state, while the
photoluminescence, transient absorption, and s2-FTIR spectros-
3
for a MLCT state, but its excited-state decay rate parameters
3
are “normal”. We conclude that in this complex the MLCT
state is pushed to a relatively high energy (because of the
3
copy are dominated by the long-lived MLCT state. For the
electron-withdrawing -CF3 groups) bringing it close in energy
series of complexes the energies of the 1MLCT states lie in the
range 2.8-3.3 eV (as estimated from the position of the allowed
MCLT absorption band). More importantly, the long-lived
3MLCT state falls in the energy range 1.85-2.4 eV, depending
upon the substituents on the diimine and aryl acetylide ligands.
In addition to the IL π,π* states that are based on the diimine
ligand, each complex also features a manifold of IL π,π* states
based on transitions between π and π* orbitals localized largely
upon the aryl acetylide ligands (i.e., 1π,π*CtCAr and 3π,π*CtCAr).
In most of the complexes the optical transition to the 1π,π*CtCAr
state is obvious: there is an intense band (ꢀ ≈ 5 × 104 M-1
cm-1) in the absorption spectra in the 290-300 nm region. This
transition places the energy of the 1π,π*CtCAr state in the region
of 3.8-4.0 eV. Several points are of interest with respect to
this absorption. First, the aryl acetylide based π,π* absorption
occurs at a longer wavelength (lower energy) compared to the
absorption bands of the corresponding free aryl acetylenes.80
This shift to lower energy implies that there is an interaction
between the π electron systems of the two aryl acetylide ligands,
and this interaction leads to a decrease in the energy of the
π,π*CtCAr manifold. Second, it is obvious that the aryl acetylide
based π,π* absorption band is shifted to considerably lower
energy in complex 2b. On the basis of the position of this band,
the energy of the 1π,π*CtCAr state in this complex is ≈3.3 eV.
This considerable red shift of the band may be due to mixing
of some Pt f CtC-Ar-NO2 charge-transfer character into
the transition. However, we believe that, despite the existence
of some charge-transfer character, the excited state that derives
from this transition retains the characteristics of a π,π* excited
state. This conjecture is based on the observation that the
3
to π,π*CtCAr. In this case, the two excited states may be in
equilibrium, which would explain the discontinuity between the
transient absorption spectrum (which suggests 3π,π*CtCAr) and
the decay rate parameters (which are indicative of 3MLCT).56,83,84
The photophysics of -OCH3-substituted complex 2d appears
to be rather typical for an unperturbed 3MLCT state. By contrast,
the -N(CH3)2-substituted complex is unusual, since it features
a negatiVe ∆Es (Table 1), very weak emission at room
temperature, and no easily observable transient absorption. The
excited-state scheme for this complex may be the most
complicated of the entire series, since, in addition to the MLCT
and π,π*CtCAr manifolds, this complex likely also features a
low-energy ligand-to-ligand charge-transfer (LLCT) state based
on charge transfer from the -N(CH3)2-substituted acetylide
ligand to the diimine acceptor.85,86
Conclusion
A comprehensive photophysical investigation has been carried
out on a series of eight (diimine)Pt(-CtC-Ar)2 complexes.
Series 1a-d, which feature a fixed aryl acetylide (p-tolylacety-
lene) and a series of diimine ligands with varying electron
demand, have a low-lying 3MLCT excited state which dictates
the complexes’ photophysics. The 3MLCT assignment is
(81) Go¨rner, H. J. Phys. Chem. 1989, 93, 1826-1832.
(82) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993.
(83) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak,
P.; Jin, X.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012-
11022.
(84) Schmehl, R. H. The Spectrum (Newsletter of the Center for Photo-
chemical Sciences, Bowling Green State University, http://www.
bgsu.edu/departments/photochem/) 2000, 13, 17-21.
(85) Perkins, T. A.; Humer, W.; Netzel, T. L.; Schanze, K. S. J. Phys.
Chem. 1990, 94, 2229-2232.
(86) Perkins, T. A.; Pourreau, D. B.; Netzel, T. L.; Schanze, K. S. J. Phys.
Chem. 1989, 93, 4511-4522.
emission from this complex (which arises from the 3π,π*
CtCAr
state, see below) does not vary in energy significantly with
(80) Younis, M.; Long, N. J.; Raithby, P. R.; Lewis, J.; Page, N. A.; White,
A. J. P.; Williams, D. J.; Colbert, M. C. B.; Hodge, A. J.; Khan, M.
S.; Parker, D. G. J. Organomet. Chem. 1999, 578, 198-209.

4062 Inorganic Chemistry, Vol. 40, No. 16, 2001
Whittle et al.
supported by optical transient absorption and s2-FTIR spectros-
copy. In addition, the nonradiative decay rates for series 1a-d
adhere quantitatively to an energy gap law correlation. Analysis
of the correlation indicates that the electronic and vibronic
characteristics of the Pt f diimine MLCT state are remarkably
similar to that of the Re f diimine MLCT state in complexes
of the type (diimine)ReI(CO)3(L). The photophysical properties
of series 2a-d, which feature a fixed diimine acceptor ligand
(4,4′-bis(tert-butyl)-2,2′-bipyridine) along with a series of aryl
acetylide ligands with varying electron demand, are more
complicated. The photophysical rate parameters for these
complexes fit an energy gap law correlation only qualitatively;
in addition, several of the complexes feature unusual transient
absorption and photoluminescence spectral characteristics. This
leads to the conclusion that for at least some of these complexes
the 3π,π*CtCAr state is close in energy to the 3MLCT state and
plays a major role in determining the photophysical properties
of the complexes.
Acknowledgment. This work was supported by grants from
the National Science Foundation (CHE-9901861) and the Royal
Society (J.A.W.). We also acknowledge Dr. Yao Liu for
obtaining optical absorption spectra. We also acknowledge
helpful comments from Prof. R. Eisenberg and the reviewers
regarding interpretation of the experimental data.
Supporting Information Available: A figure showing the room
temperature emission spectra of all of the complexes in CH2Cl2 solution
along with the fitted spectra computed using eq 1 and the parameters
shown in Table 2. A figure showing the transient absorption spectrum
of trans-(Bu3P)2Pt(-CtC-t-St)2. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC0102182