PtII Diimine Chromophores with Perfluorinated Thiolate Ligands: Nature and Dynamics of the Charge-Transfer-to-Diimine Lowest Excited State

Article abstract of DOI:10.1021/ic026286l

The synthesis of new Pt^II diimine complexes bearing perfluorinated thiolate ligands, Pt^II(NN)(4-X-C₆F ₄-S)₂, where NN = 2,2′-bipyridine or 1,10-phenanthroline and X = F or CN, is reported, together with an investigation of the nature and dynamics of their lowest excited states. A combined UV-vis, (spectro)electrochemical, resonance Raman, and time-resolved infrared (TRIR) study has suggested that the HOMO is mainly composed of thiolate(φ)/S(p)/Pt(d) orbitals and that the LUMO is largely localized on the φ* (diimine) orbital, thus revealing the {charge-transfer-to-diimine} nature of the lowest excited state. An enhancement of the thiolate ring vibrations, C-F vibrations, and the vibration of the CN-substituent on the thiolate moiety was observed in the resonance Raman spectra, whereas no such enhancement was seen for the nonfluorinated analogues. Thus, the introduction of fluorine substituents on the thiolate moiety probably leads to a more pronounced contribution of the intrathiolate modes to the HOMO compared to the analogous complexes with nonfluorinated thiolates. Furthermore, the introduction of the p-CN group into the thiolate moiety has allowed the dynamics of the lowest excited state of Pt(bpy)(4-CN-C₆F ₄-S)₂ to be monitored by picosecond TRIR spectroscopy. The dynamics of the lowest {charge-transfer-to-diimine} excited state are governed by ca. 2-ps vibrational cooling and 35-ps back electron transfer.

Full text of DOI:10.1021/ic026286l

Inorg. Chem. 2003, 42, 7077−7085
PtII Diimine Chromophores with Perfluorinated Thiolate Ligands: Nature
and Dynamics of the Charge-Transfer-to-Diimine Lowest Excited State
Julia A. Weinstein,*,†,‡ Alexander J. Blake,† E. Stephen Davies,† Adrienne L. Davis,†
,†
Michael W. George,* David C. Grills,† Igor V. Lileev,‡ Alexander M. Maksimov,§ Pavel Matousek,|
Mikhail Ya. Mel’nikov,‡ Anthony W. Parker,| Vyacheslav E. Platonov,§ Michael Towrie,|
Claire Wilson,† and Natalia N. Zheligovskaya‡
School of Chemistry, UniVersity of Nottingham, Nottingham NG7 2RD, United Kingdom,
Department of Chemistry, Moscow LomonosoV State UniVersity, Moscow 119899, Russia,
NoVosibirsk Institute of Organic Chemistry, 630090, NoVosibirsk, Russia, and CCLRC, Rutherford
Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom
Received December 19, 2002
The synthesis of new PtII diimine complexes bearing perfluorinated thiolate ligands, PtII(NN)(4-X−C F −S)2, where
6
4
NN ) 2,2′-bipyridine or 1,10-phenanthroline and X ) F or CN, is reported, together with an investigation of the
nature and dynamics of their lowest excited states. A combined UV−vis, (spectro)electrochemical, resonance Raman,
and time-resolved infrared (TRIR) study has suggested that the HOMO is mainly composed of thiolate(π)/S(p)/
Pt(d) orbitals and that the LUMO is largely localized on the π* (diimine) orbital, thus revealing the {charge-transfer-
to-diimine} nature of the lowest excited state. An enhancement of the thiolate ring vibrations, C−F vibrations, and
the vibration of the CN-substituent on the thiolate moiety was observed in the resonance Raman spectra, whereas
no such enhancement was seen for the nonfluorinated analogues. Thus, the introduction of fluorine substituents on
the thiolate moiety probably leads to a more pronounced contribution of the intrathiolate modes to the HOMO
compared to the analogous complexes with nonfluorinated thiolates. Furthermore, the introduction of the p-CN
group into the thiolate moiety has allowed the dynamics of the lowest excited state of Pt(bpy)(4-CN−C F −S)2 to
6
4
be monitored by picosecond TRIR spectroscopy. The dynamics of the lowest {charge-transfer-to-diimine} excited
state are governed by ca. 2-ps vibrational cooling and 35-ps back electron transfer.
Introduction
detail elsewhere.1-3 Coordinatively unsaturated PtII d8 diimine
chromophores have received much less attention,4-9 partly
due to the low solubility many of these complexes exhibit.
However, PtII chromophores have shown significant potential
Charge-separated excited states are key intermediates in
a wide variety of processes including bimolecular photo-
catalysis and light-chemical energy conversion.1 An un-
derstanding of the primary photophysical processes in such
systems is a prerequisite for the rational design of charge-
separated excited states with specific properties. Research
into metal-chromophore-based systems has focused mainly
on d6 RuII, OsII, and ReI diimines, as has been reviewed in
(2) Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.; Venturi, M.
Acc. Chem. Res. 2001, 34, 445.
(3) Chen, P. Y.; Meyer, T. J. Chem. ReV. 1998, 98, 1439.
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Chem. 1992, 436, 367. (b) Braterman, P. S.; Song, J. I.; Vogler, C.;
Kaim, W. Inorg. Chem. 1992, 31, 222. (c) Connick, W. B.; Gray, H.
B. J. Am. Chem. Soc. 1997, 119, 11620. (d) Kunkely, H.; Vogler, A.
Inorg. Chim. Acta 1997, 264, 305. (e) Base, K.; Tierney, M. T.; Fort,
A.; Muller, J.; Grinstaff, M. W. Inorg. Chem. 1999, 38, 287. (f) Klein,
A.; Scheiring, T.; Kaim, W. Z. Anorg. Allg. Chem. 1999, 625, 1177.
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1984, 23, 506. (b) Tzeng, B.-C.; Fu, W.-F.; Che, C.-M.; Chao, H.-Y.;
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Chem. 2001, 40, 4053.
* Authors to whom correspondence should be addressed. E-mail:
julia.weinstein@nottingham.ac.uk (J.A.W.); mike.george@nottingham.ac.uk
(M.W.G.).
† University of Nottingham.
‡ Moscow Lomonosov State University.
§ Novosibirsk Institute of Organic Chemistry.
| Rutherford Appleton Laboratory.
(1) (a) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, Chichester, 2001. (b) Organic and Inorganic Photochem-
istry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New
York, 1998.
10.1021/ic026286l CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/30/2003
Inorganic Chemistry, Vol. 42, No. 22, 2003 7077

Weinstein et al.
as DNA intercalators,10 as building blocks in polynuclear
transition metal systems designed for electron/energy trans-
fer,8,11 and as sensing devices.12
reporter groups such as carboxylic acids and esters on
substituted diimine ligands have also been used24-26 to
indirectly probe charge transfer-processes using both nano-
second- and picosecond-TRIR. However, there have been
relatively few TRIR investigations of PtII diimine chromo-
phores.9,26,27
Herein, we present an investigation into the photophysics
of PtII diimines bearing perfluorinated28 thiolate ligands. We
have used UV-vis absorption spectroscopy, resonance
Raman spectroscopy, and (spectro)electrochemistry to probe
the origin of the lowest excited state in these new, highly
soluble fluorinated compounds. Our results are consistent
with previously reported data on the related non-fluorinated
Pt(diimine)(bis)thiolates.
We have introduced conjugated perfluorinated thiolate
ligands to decrease the donor capacity of the thiolate ligand
moiety and to produce a more charge-delocalized excited
state. Furthermore, the introduction of the para-CN group
into the thiolate moiety has allowed the dynamics of the
lowest excited state of Pt(bpy)(4-CN-C6F4S)2 to be moni-
tored in solution at room temperature by picosecond TRIR
spectroscopy.
The nature and dynamics of the lowest excited state is an
issue of particular importance for metal chromophores since
these possess a manifold of low-lying excited states of
different origin.13 The nature of the lowest excited state in
PtII diimine dithiolates, where diimine is a derivative of 2,2′-
bipyridine (bpy) or 1,10-phenanthroline (phen), has previ-
ously been assigned as either {charge-transfer-to-diimine}8
or ligand-ligand charge transfer (LLCT).5 The lowest excited
state for flexible bis-thiolate systems, Pt(bpy)(4-X-C6H4S)2,
has also been shown to be {charge-transfer-to-diimine} with
the highest occupied molecular orbital (HOMO) being mainly
of S(lone pair)/Pt(d) origin.7,14,15 The desired properties of
excited states such as their lifetime or degree of charge
separation can be achieved by the careful design and
modification of the ligands.
PtII diimine complexes that show emission in fluid solution
on the nanosecond or longer time scale include some
dithiolates,8,16-18 acetylides,8,9,19 and cyanides.20 However,
the majority of PtII excited states are short lived and the
elucidation of their dynamics requires fast spectroscopic
methods.
Experimental Section
Materials. All solvents were distilled over CaH2 under a dry
nitrogen atmosphere prior to use. 2,2′-Bipyridine, 1,10-phenan-
throline, ferrocene (Aldrich), potassium tert-butoxide, and hexa-
methylenetetraamine (hmt) (Sigma) were used as received. For
electrochemical experiments, anhydrous dimethylformamide (DMF)
(Fluka) was used as received. The supporting electrolyte, [NBu4]-
[BF4], was prepared from [NBu4]Cl and Na[BF4] (Aldrich) and
recrystallized twice from dichloromethane.29
A modified literature procedure was employed to synthesize Pt-
(diimine)Cl2 from K2[PtCl4] and the corresponding diimine.30 The
thiolate ligands C6F5SH and 4-CN-C6F4SH were synthesized as
described elsewhere.31
Time-resolved infrared (TRIR) spectroscopy has proved
to be a powerful technique for probing the electronic
redistribution which occurs upon formation of excited states
of coordination compounds, particularly those bearing IR
reporter ligands such as CO or CN.21-23 Peripheral IR
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1,10-Phenanthroline, X ) F or CN. Pt(bpy)(C6F5S)2, 1. To a
mixture of Pt(bpy)Cl2 (422 mg, 1 mmol) and hexamethylenetet-
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Chem. Soc. 1993, 115, 10996.
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(b) Khan, S. I.; Beilstein, A. E.; Smith, G. D.; Sykora, M.; Grinstaff,
M. W. Inorg. Chem. 1999, 38, 2411. (c) Smith, G. D.; Maxwell, K.
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K. M.; Smith, G. D.; Kavaliunas, D. A.; Chen, P. Y.; Treadway, J.
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(26) Weinstein, J. A.; Grills, D. C.; Towrie, M.; Matousek, P.; Parker, A.
W.; George, M. W. Chem. Commun. 2002, 382.
(14) Weinstein, J. A.; Zheligovskaya, N. N.; Mel’nikov, M. Y.; Hartl, F.
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(15) Weinstein, J. A.; Zheligovskaya, N. N.; Lileev, I. V.; Galin, A. M.;
Mel’nikov, M. Y. Russ. J. Inorg. Chem., Engl. Transl. 1998, 43, 1361.
(16) (a) Huertas, S.; Hissler, M.; McGarrah, J. E.; Lachicotte, R. J.;
Eisenberg, R. Inorg. Chem. 2001, 40, 1183. (b) Paw, W.; Lachicotte,
R. J.; Eisenberg, R. Inorg. Chem. 1998, 37, 4139. (c) Bevilacqua, J.
M.; Eisenberg, R. Inorg. Chem. 1994, 33, 2913.
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Eisenberg, R. Inorg. Chem. 1992, 31, 2396.
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39, 447. (c) Chan, S. C.; Chan, M. C. W.; Wang, Y.; Che, C. M.;
Cheung, K. K.; Zhu, N. Y. Chem.-Eur. J. 2001, 7, 4180. (d) Fleeman,
W. L.; Connick, W. B. Comments Inorg. Chem. 2002, 23, 205. (e)
Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano,
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J.; Eisenbeg, R. Inorg. Chem. 2003, 42, 3772.
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7078 Inorganic Chemistry, Vol. 42, No. 22, 2003

Excited States of Pt(II) Diimine (Bis)Thiolates
referenced according to the 2001 IUPAC recommendations33 with
¥ ) 21.496784.
Raman data were collected on a Nicolet-Almega instrument using
785 nm excitation; the band positions are determined by means of
internal calibration. Resonance Raman data were collected from
spinning KNO3 pellets (approximately 200 mg of KNO3 and 15
mg of platinum complex) at room temperature upon excitation with
the 457.9 and 488.1 nm lines of a Spectra Physics model 2016
Ar+ laser. The incident laser power was varied from 90 to 100
mW. The data were collected using a Dilor Modular spectrometer
with backscattering geometry and a charge-coupled device (CCD)
detection system. The positions of the bands are reported relative
raamine (280 mg, 2 mmol) in degassed EtOH (100 cm3), a degassed
solution of C6F5SH (2.4 mmol) containing KOtBu (269 mg, 2.4
mmol) in 20 cm3 of EtOH was added dropwise. The reaction
mixture was heated on a water bath under a nitrogen atmosphere
with continuous stirring for ca. 4 h. The resulting orange precipitate
was filtered, washed with hot water (3 × 20 cm3) and ethanol (2
× 20 cm3), and dried under vacuum overnight. Yield 78%. 1H NMR
[(CD3)2SO]: δ ) 9.62 (dd, 2H), 8.75 (dd, 2H), 8.47 (t, 2H), 7.92
(t, 2H). 19F NMR: δ ) -130.52 (2F), -160.15 (1F), -163.04
(2F). 195Pt NMR: δ ) -3245. Anal. Calcd for C22H8F10N2PtS2:
C, 35.25; H, 1.07; N, 3.74. Found: C, 35.02; H, 0.96; N, 3.40.
Pt(bpy)(4-CN-C6F4S)2, 2. The dark-yellow complex was
prepared using the same procedure as for Pt(bpy)(C6F5S)2. Yield
82%. 1H NMR [(CD3)2SO]: δ ) 9.49 (dd, 2H), 8.74 (d, 2H), 8.48
(t, 2H), 7.89 (t, 2H). 19F NMR: δ ) -131.71 (2F), -138.25 (2F).
195Pt NMR: δ ) -3178. Anal. Calcd for C24H8F8N4PtS2: C, 37.77;
H, 1.05; N, 7.34. Found: C, 37.58; H, 0.89; N, 6.78.
to the KNO3 bands at 1051.3 and 716.4 cm-1
.
Crystals of 2 suitable for X-ray diffraction analysis were obtained
by slow diffusion of pentane into a 2-Me-THF solution at room
temperature. A yellow, prismatic crystal, 0.30 × 0.09 × 0.04 mm,
was mounted in perfluoropolyether oil. Data were collected at 150
K on a Bruker SMART1000 CCD area detector diffractometer
equipped with an Oxford Cryosystems open-flow nitrogen cryo-
stat.34 Data were corrected for Lorentz and polarization effects and
for absorption, using a semiempirical method35 (T range 0.483-
0.774). The structure was solved by direct methods,36 and the
structure was refined using full-matrix least squares refinement
against F2. All non-H atoms were refined with anisotropic displace-
ment parameters and H atoms placed in geometrically calculated
positions and refined as part of a riding model, with U(H)iso
)
1.2Ueq(C).37 C24H8F8N4PtS2, M ) 763.55, triclinic, a ) 8.768(2)
Å, b ) 11.084(3) Å, c ) 13.007(3) Å, R ) 111.553(3)°, â )
95.589(3)°, γ ) 98.220(3)°, U ) 1148.1(5) Å3, T ) 150 K, space
group P1h (no. 2), Z ) 2, µ (Mo KR) ) 6.382 mm-1, 7283
reflections measured, 5135 unique (Rint ) 0.029), which were used
in all calculations. The final wR2(F2) was 0.0474 for all data, R1
(F) was 0.0333 for 4074 observed data where I > 2σ(I). CCDC
reference number: 217858.
Pt(phen)(C6F5S)2, 3. The orange complex was prepared using
1
the same procedure as for Pt(bpy)(C6F5S)2. Yield 80%. H NMR
[(CD3)2SO]: δ ) 9.70 (dd, 2H), 9.10 (dd, 2H), 8.25 (t, 2H), 8.35
(s, 2H). 19F NMR: δ ) -130.31 (2F), -160.03 (1F), -163.01 (2F).
195Pt NMR: δ ) -3275. Anal. Calcd for C24H8F10N2PtS2: C, 37.26;
H, 1,03; N, 3.62. Found: C, 37.06; H, 0.98; N, 3.76.
The electrochemical and (spectro)electrochemical samples were
prepared in DMF under an inert atmosphere using Schlenk
techniques.
Pt(phen)(4-CN-C6F4S)2, 4. To a mixture of solid Pt(phen)Cl2
(170 mg, 0.38 mmol) and hmt (106 mg, 0.76 mmol) in degassed
THF (100 cm3), a degassed solution of 4-CN-C6F4SH (174 mg,
0.84 mmol) and KOtBu (94 mg, 0.84 mmol) was added dropwise.
The reaction mixture was stirred at reflux for ca. 7 h, with a gradual
change of color from yellow to orange. The volume of THF was
then reduced, and the resulting orange precipitate was filtered off,
washed with hot water (3 × 30 cm3) and ethanol (2 × 20 cm3),
and dried under vacuum overnight. The product was then purified
by column chromatography on silica gel with CH2Cl2/CH3CN (10:
1) as an eluent to yield a bright-orange solid. Yield 70%. 1H NMR
[(CD3)2SO]: δ ) 9.75 (dd, 2H), 9.12 (dd, 2H), 8.37 (s), 8.25 (dd,
2H). 19F NMR: δ ) -131.4 (2F), -138.2 (2F). 195Pt NMR: δ )
-3215. The mass calculated for C24H8F10N2PtS2 based on 195Pt,
Standard cyclic voltammetry was carried out under an atmosphere
of argon using a three-electrode arrangement in a single compart-
ment cell. A glassy carbon working electrode, a Pt wire secondary
electrode, and a saturated calomel electrode (S.C.E.) were chemi-
cally isolated from the test solution via a bridge tube containing
electrolyte solution with a porous Vycor frit. The solutions were
10-3 M in the test compound and 0.2 M in [NBu4][BF4] as
supporting electrolyte. The redox potentials are quoted versus the
E
ferrocenium-ferrocene couple;38 1/2 ([Cp2Fe]+/[Cp2Fe]) was 0.493
V vs S.C.E. under these conditions.
The UV-vis spectroelectrochemical experiments were carried
out with an optically transparent thin-layer electrochemical (OT-
TLE) cell39 (modified quartz cuvette, with 0.5 mm optical path
length). The cell comprised a three-electrode configuration, consist-
ing of a Pt/Rh gauze working electrode, a Pt wire secondary
1
12C, H, 14N, 19F, 32S ) 787. ES MS: found m/z+ ) 787 as the
most intense peak.
Compounds 1-4 are stable as solids and in deaerated solutions
of organic solvents. The 195Pt-{1H} NMR spectra exhibit the singlet
resonance in the region expected for PtII imine thiolate compounds.32
Molar conductivity measurements revealed the compounds studied
to be nonelectrolytes, stable as 1 mM ethanolic solutions for at
least 24 h.
(33) IUPAC-Guidlines Pure Appl. Chem. 2001, 73, 1795.
(34) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105.
(35) Area Detector Absorption Correction SADABS. In Version 6.01 ed.;
Bruker AXS Inc.: Madison, WI, 1996.
Methods and Instrumentation. 1H, 19F, and 195Pt-{1H} NMR
spectra were recorded on a Bruker 300 MHz spectrometer at 298
K in DMSO-d6 unless otherwise stated. 19F chemical shifts are
reported vs CFCl3 (δ ) 0). 195Pt NMR shifts were measured at
ambient temperature (1 and 2) or 298 K (3 and 4) and were
(36) Sheldrick, G. M. SHELXS-97, Acta Crystallogr., Sect. A 1990, 46,
467.
(37) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(38) Gagne´, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19,
2854.
(39) (a) Krejcˇik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem., Interfacial
Electrochem. 1991, 313, 243. (b) Murray, R. M.; Heineman, W. R.;
O’Dom, G. W. Anal. Chem. 1967, 39, 1666.
(32) Fazlur-Rahman, A. K.; Verkade, J. G. Inorg. Chem. 1992, 31, 2064.
Inorganic Chemistry, Vol. 42, No. 22, 2003 7079

Weinstein et al.
Table 1. Crystal Data for Pt(bpy)(4-CN-C6F4S)2, 2
formula
C24H8F8N4PtS2 V (Å3)
1148.1(5)
2
6.382
7283
molecular weight 763.55
Z
crystal system
space group
T (K)
triclinic
P1h
150(2)
µ (Mo KR) (mm-1
reflns collected
unique reflns
)
5135
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
8.768(2)
11.084(3)
13.007(3)
111.553(3)
95.589(3),
98.220(3)
data with I > 2σ(I)
4074
0.029
0.0333
R(int)
R1 (F) for I > 2σ(I)
wR2 (F2) for all data 0.0474
∆Fmax (e/Å3)
∆Fmin (e/Å3)
1.08
-1.63
The angle between the planes of these two rings is 1.49(9)°,
and they lie approximately normal to the plane of the
bipyridyl rings [dihedral angles 82.26(5) and 81.72(5)°]. It
is noteworthy that the fluorinated thiolate rings are π stacked
in the solid state, as the X-ray structure of neither the related
PtII disulfide complex with perfluorinated thiophenols, Pt-
(CH3SCH(CH3)CH(CH3)SCH3)(C6F5S)2,28b nor of the related
PtII diimine complexes with non-perfluorinated thiolates show
such interaction.5b,16b
Figure 1. The molecular structure of Pt(bpy)(4-CN-C6F4S)2 showing the
atom numbering scheme. Anisotropic displacement parameters are shown
at 50% probability level and hydrogen atoms are omitted for clarity. Selected
bond lengths and angles (Å, deg): Pt1-N1P, 2.049(4); Pt1-N12P, 2.054-
(4); Pt1-S1, 2.2823(13); Pt1-S2, 2.2963(13); N1P-Pt1-N12P, 79.77-
(16); N1P-Pt1-S1, 92.58(12); N12P-Pt1-S1, 170.82(11); N1P-Pt1-
S2, 173.56(12); N12P-Pt1-S2, 95.15(12); S1-Pt1-S2, 92.10(5).
electrode (in a fritted PTFE sleeve), and a SCE, chemically isolated
from the test solution via a bridge tube containing electrolyte
solution and terminated in a porous frit. The potential at the working
electrode was controlled by a Sycopel Scientific Ltd. DD10M
potentiostat. The (spectro)electrochemical UV-vis data were
recorded on a Perkin-Elmer Lambda 16 spectrophotometer. The
cavity was purged with dinitrogen, and temperature control at the
sample was achieved by flowing cooled dinitrogen across the
surface of the cell. Sample solutions were prepared under an
atmosphere of argon using Schlenk line techniques and contained
10-3 M test compound and 0.2 M [NBu4][BF4] as supporting
electrolyte. The test species in solution was electrolyzed at constant
potential, typically 100 mV more negative than Ep,c for a reduction.
The redox process was considered complete when consecutive
spectra were identical.
The picosecond TRIR studies were performed on the PIRATE
setup in the Rutherford Appleton Laboratory, details of which are
described elsewhere.40 Briefly, part of the output from a 1 kHz,
800 nm, 150 fs, 2 mJ Ti/Sapphire oscillator/regenerative amplifier
was used to pump a white light continuum seeded BBO optical
parametric amplifier (OPA). The signal and idler produced by this
OPA were difference frequency mixed in a type I AgGaS2 crystal
to generate tunable mid-IR pulses (ca. 150 cm-1 full width at half
maximum, 1 µJ). Second harmonic generation of the residual 800-
nm light provided 400-nm pulses, which were used to excite the
sample. Changes in IR absorption were recorded by normalizing
the outputs from a pair of 64-element HgCdTe linear-IR array
detectors on a shot-by-shot basis. 300 lines/mm gratings were used
in the spectrographs to achieve a high spectral resolution (approxi-
mately 4 cm-1 in the 2200 cm-1 region).
The closest intermolecular Pt‚‚‚Pt distance is 4.62 Å, the
corresponding interplanar distance between the bipyridine
moieties is 3.492(4) Å, and the offset is 2.089 Å. The two
aromatic rings are offset by 1.02 Å and twisted by 28.6° in
such a way as to favor π-π interactions. They are almost
coplanar, the dihedral angle being only 2.9°. The centroid-
centroid distance is 3.34 Å, and the perpendicular separation
of the rings is 3.18 Å. The crystal data are listed in Table 1.
Electronic Absorption Spectra. The electronic absorption
spectra of the Pt(NN)(4-X-C6F4-S)2 compounds in room-
temperature solutions exhibit a broad asymmetric absorption
band of medium intensity in the visible region (Table 2,
Figure 2). The energy of this lowest absorption is sensitive
to the nature of the ligands and to the polarity of the solvent.
In all of these complexes, the lowest absorption band shows
a negative solvatochromic behavior (Figure 3a), i.e., a
decrease in energy with a decrease of solvent polarity. There
is a linear correlation between this energy and the solvent
polarity parameter value based on the energy of either the
metal-ligand charge transfer (MLCT) transition of W(CO)4-
(bpy)41 or the {charge-transfer-to-diimine} transition in Pt-
(dbbpy)(tdt) (dbbpy ) 4,4′-di-tert-butyl-2,2′-bipyridine, tdt
) toluene-3,4-dithiolate)18 (see Figure 3b). The slope of the
linear correlation based on the latter scale gives the solva-
tochromic shift; the values obtained for Pt(phen)(C6F5S)2
(0.56 eV), Pt(bpy)(C6F5S)2 (0.44 eV), Pt(bpy)(4-CN-C6F4S)2
(0.41 eV), and Pt(phen)(4-CN-C6F4S)2 (0.38 eV) are typical
values of those reported for charge-transfer transitions in
other PtII diimine thiolates.18
Results and Discussion
Crystal Structure. The crystal structure of Pt(bpy)(4-
CN-C6F4S)2 is illustrated in Figure 1. It shows that the Pt
coordination geometry is approximately square planar, with
one 2,2′-bipyridyl and two 4-cyano-perfluorothiophenolate
ligands coordinated to the Pt center. The two phenyl rings
of the 4-cyano-perfluorothiophenolates are almost coplanar.
The asymmetry of the lowest absorption manifold becomes
more pronounced with decreasing solvent polarity, with the
lowest-energy shoulder finally resolving as an independent
band in CCl4 (Figure 3a). Similar behavior has previously
been observed for other Pt(diimine)thiolates.6,14,15,17,42 The
different solvatochromic behavior exhibited by the compo-
(40) Towrie, M.; Grills, D. C.; Dyer, J.; Weinstein, J. A.; Matousek, P.;
Barton, R.; Bailey, P. D.; Subramaniam, N.; Kwok, W. M.; Ma, C.;
Phillips, D.; Parker, A. W.; George, M. W. Appl. Spectrosc. 2002,
57, 367.
(41) Manuta, D. M.; Lees, A. J. Inorg. Chem. 1983, 22, 3825.
(42) Larionov, O.; Weinstein, J. A.; Zheligovskaya, N. N.; Mel’nikov, M.
Y. Russ. J. Coord. Chem., Engl. Transl. 1999, 45.
7080 Inorganic Chemistry, Vol. 42, No. 22, 2003

Excited States of Pt(II) Diimine (Bis)Thiolates
Table 2. The Position (λmax), Extinction Coefficient (ꢀ), and the Solvatochromic Shift (∆ν) for the Lowest Absorption Band in
PtII(diimine)(bis)thiolates 1-4 and Related Compounds at 298 K and Absorption Maxima for Their Electrochemically Generated Reduction Products
(at 273 K in DMF unless stated otherwise)
compound
λ
max/nm (ꢀ/103 M-1 cm-1
)
∆ν/eV
UV-vis absorption maxima of the reduced products/nm
a
Pt(bpy)(C6H5S)2
498 (2.14b)
457 (3.20)
444 (2.96)
471 (3.18)
452 (2.20)
390 (3.56)
271, 364, 454, 484, 706, 787
Pt(bpy)(C6F5S)2, 1
0.44
0.37
0.56
0.38
281, 314, 339, 384, 412, 533, 577, 622 very broad, 753
367, 411, 460, 495, from 570 development to the NIR region
280, 312, 383, 410, 534, 577, 631 tailing to 800
366, 410, 526, 571, broad manifold centered at 632 tailing to 800
310, 335, 377, 437, 530, 572, 634 (very broad), 759
Pt(bpy)(4-CN-C6F4S)2, 2
Pt(phen)(C6F5S)2, 3
Pt(phen)(4-CN-C6F4S)2, 4
Pt(phen)Cl2
a From ref 14. b At 512 nm in CH2Cl2.
Figure 2. Electronic absorption spectra of 1-4 in DMF at room
temperature.
Figure 4. Raman spectra of polycrystalline samples obtained using 785
nm excitation: (a) Pt(phen)Cl2; (b) Pt(phen)(4-CN-C6F4S)2; (c) 4-CN-
C6F4-SH; (d) Pt(bpy)(4-CN-C6F4S)2; (e) Pt(bpy)Cl2. The bands that are
resonantly enhanced upon excitation of Pt(bpy)(4-CN-C6F4S)2 at 457.9
and 488 nm (see text for details) are denoted with an asterisk.
which provides a better orbital overlap between the thiolate
and the diimine moieties.
Resonance Raman Spectroscopy. To understand further
the nature of the excited state, we have undertaken a
resonance Raman investigation. The Raman spectra of the
polycrystalline samples of compounds 2 and 4, the precursor
complexes Pt(bpy)Cl2 and Pt(phen)Cl2, and the free ligand
4-CN-C6F4SH collected under 785 nm excitation at room
temperature are presented in Figure 4.
It is clear from Figure 4 that the Raman spectra of the
mixed-ligand complexes are a superposition of the vibrations
observed for the Pt(NN)Cl2 complex and the thiolate ligand.
An additional vibration was observed for the mixed-ligand
complexes at 405 cm-1 (2) and 414 cm-1 (4), and we
tentatively assign this band to ν(Pt-S), which was reported
at 384 cm-1 for Pt(2-thpy)2 (2-thpy ) 2-thienyl-pyrdine).43
Most of the vibrations of Pt(bpy)Cl2 that are resonantly
enhanced under 457.9 nm excitation belong to the coordi-
nated bpy.44 In the 300-400 cm-1 region, Pt-Cl and Pt-N
vibrations might also be anticipated.45
Figure 3. (a) Absorption spectra of Pt(phen)(C6F5S)2, 3, as a function of
solvent polarity. In the order of decreasing energy of the lowest absorption
band the solvents are DMSO (dashed line), C2H4Cl2 (solid line), CHCl3
(dotted line), C6H6 (solid line), and CCl4 (solid line). (b) A plot of energy
of the lowest absorption band of Pt(phen)(C6F5S)2, 3, as a function of solvent
parameter18 for DMSO, CH3CN, DMF, C2H4Cl2, CH2Cl2, CHCl3, C6H6,
C7H8, and CCl4.
nents of the lowest energy absorption manifold observed
suggests that more than one electronic transition is respon-
sible for this band envelope.
Two possible assignments for the lowest absorption band
in this absorption manifold can be envisaged, HOMO-
LUMO transition or a singlet-triplet satellite to singlet-
singlet HOMO-LUMO transition, which would be of
considerable oscillator strength due to the presence of a heavy
PtII center. We tentatively prefer to assign this to the latter
by comparison with the previous work on Pt(diimine)-
(dithiolates), where dithiolate ) 1,2-ethane-dithiolate or
meso-1,2-diphenyl-1,2-ethanedithiolate.6
The electronic absorption spectra suggest involvement of
orbitals centered at both diimine and thiolate ligands in the
lowest electronic transition. The value of the extinction
coefficient (Table 2) for the lowest absorption band suggests
involvement of Pt d orbitals in this electronic transition,
Pt(bpy)(4-CN-C6F4S)2, 2. The resonance Raman spectra
of Pt(bpy)(4-CN-C6F4S)2 in KNO3 pellets were obtained
under 457.9 and 488 nm excitation. These spectra exhibit
bands corresponding to the vibrational modes of the coor-
dinated bpy-ligand, as has been shown previously by
resonance Raman spectroscopy and normal coordinate
(43) Yersin, H.; Donges, D. Top. Curr. Chem. 2001, 214, 81.
(44) Hartl, F.; Snoeck, T. L.; Stufkens, D. J.; Lever, A. B. P. Inorg. Chem.
1995, 34, 3887 and references therein.
(45) Alkins, J. R.; Hendra, P. J. J. Chem. Soc. A 1967, 1325.
Inorganic Chemistry, Vol. 42, No. 22, 2003 7081

Weinstein et al.
0/-
-/2-
Table 3. First E1/2 and Second E1/2
Reduction Potentials and Anodic Half-Wave Potential Ep,a Obtained for 1 mM Solutions of
PtII(diimine)(bis)thiolates 1-4 in DMF at 298 K in the Presence of 0.2 M [Bu4N][BF4] (under 100 mV/s scan rate unless stated otherwise)
thiolate
C6F5S-
diimine
E1/20/- (∆E)a/V
∆E for Fc/Fc+/mV
E1/2-/2-/V
Ep,a/V
1
Bpy
Bpy
Bpy
Bpy
Bpy
-1.60 r (70)
-1.54 r (70)
-1.57 r (80)
-1.72
-1.78
-1.64
-1.61 qr (80)
-1.54 qr (70)
-1.61 (80)
70
70
90
-2.29 (82)
+0.77, +0.99 sh
+0.95 sh, +1.02 sh
2
4-CN-C6F4S-
4-CN-C6F4S-e
C6H5S-b
-2.14 (136) qrc
2e
-2.37
+0.22
4-Me2N-C6H4S-b
-2.41
-0.23
Pt(bpy)Cl2
-2.30f
+0.67
b
3
4
C6F5S-
4-CN-C6F4S-
Pt(phen)Cl2
phen
phen
70
70
70
-2.35 (90)c
-2.32d
+0.78 and +0.99
+0.99 and +1.07
+0.82
a Potentials are measured vs SCE, quoted vs Fc/Fc+ ; anodic/cathodic peak separation in mV is given in parentheses. b From ref 14. c Has a wave at
reversed scan at scan rates >100 mV. d Multireduction process. e In CH2Cl2, 0.4 M [Bu4N][BF4]. f Chemically irreversible.
analysis.14,44,46 There are also bands at 1631, 1394, 1379,
and 1040 cm-1 in the resonance Raman spectrum of Pt(bpy)-
(4-CN-C6F4S)2, which were not seen for either Pt(bpy)Cl2
or Pt(bpy)(PhS)2.14 All of these vibrations are seen in the
Raman spectrum of the corresponding perfluorothiolate
ligand (Figure 4). The 1394 and/or 1379 cm-1 bands are
tentatively assigned to a mode with a large contribution from
the C-F stretch. (The C-F vibration frequency was reported
as 562 and 1491 cm-1 for C6F6,47 586 and 1410 cm-1 for
C6Cl3F3,47 580 and 1354 cm-1 for C6H3F3,47 and 1323 cm-1
for 3-F-(CF3)-C6H4.)48 The 1040-cm-1 band, which is also
Figure 5. Cyclic voltammograms of Pt(bpy)(C6F5S)2 (a) and Pt(bpy)(4-
observed in the resonance Raman spectrum of [Pt(mnt)2]2-
CN-C6F4S)2 (b) obtained in 1 mM solutions in DMF, containing 0.2 M of
[nBu4N][BF4], at 298 K (scan rate 100 mV/s).
(mnt ) 1,2-maleonitrile-dithiolate),49 is likely to be due to
a mode with a significant amount of the CdC stretch of
perfluorinated thiophenol.
(baba)(mnt)] (baba ) biacetylbisaniline). Thus, resonance
Raman data show the involvement of both diimine and
thiolate ligand vibrational modes in the lowest electronic
transition. The involvement of the thiolate ligand is clearly
seen by the strong ν(CN) resonance at 2234 cm-1, the
intensity of which increases on changing from 457.9 to 488
nm excitation, corresponding to an increase in absorbance
along the lowest absorption band. This demonstrates a
difference between the localization of electronic density in
the excited state of PtII diimines with perfluorinated thiolates
compared to Pt(diimine)(bis)thiolates, since the latter do not
show any considerable enhancement of thiolate vibrations
upon excitation along the lowest absorption band.
A strong enhancement of the 2239-cm-1 band, which is
assigned to the vibration of the CN group of the thiolate
ligand, is noteworthy since this indicates the involvement
of the thiolate ligand modes in the electronic transition
studied.
The resonance Raman spectrum of Pt(bpy)(4-CN-C6F4S)2
also displays a number of weak bands, which are not present
in the Pt(bpy)Cl2 spectrum. We assigned these to the
thiophenol ring vibrations (892 and 1005 cm-1), to C-F
vibrations (531 and 556 cm-1), and to the C-S vibration
(871 cm-1). The 892- and 1005-cm-1 bands were detected
in the resonance Raman spectrum of [Pt(bpy)(PhS)2]14 and
are present as medium intensity bands in Raman spectra of
C6F5SH49 and 4-CN-C6F4SH. C6F5SH displays intense
Raman bands at 583 and 510 cm-1;50 liquid C6F6 at 542,
562, and 745 cm-1;51 and 4-CN-C6F4SH at 521 and 552
cm-1. The ν(CS) assignment has been made for the 866-
cm-1 band of medium intensity observed in the Raman
spectra of C6F5SH50 and was also observed for 4-CN-C6F4-
SH and for the 880 cm-1 band of [Ni(baba)(mnt)] and [Pd-
(Spectro)electrochemistry. To probe the nature of the
frontier orbitals and elucidate the electrochemical properties
of 1-4, cyclic voltammetry and UV-vis spectroelectro-
chemical studies have been performed. The redox potentials
are listed in Table 3.
Reduction. The cyclic voltammograms of compounds 1
and 2 in DMF are shown in Figure 5. The first reduction is
a one-electron process, electrochemically reversible for the
bpy complexes 1 and 2 and quasi-reversible for the phen
complexes 3 and 4 under the experimental conditions applied.
The value of the first reduction potential is largely unaffected
by the nature of the thiolate or diimine and is close to the
value of the first reduction potential for the corresponding
Pt(diimine)Cl2 (Table 3). This suggests that the LUMO is
mainly localized on the diimine ligand. Substitution of
-C6F5S- with 4-CN-C6F4-S- introduces a small (ca. 60
mV) positive shift in the first reduction potential of the
phenanthroline and bipyridine complexes due to the intro-
duction of the more-electron-withdrawing thiolate.
(46) (a) Forster, M.; Hester, R. E. Chem. Phys. Lett. 1981, 81, 42. (b)
Mallick, P. K.; Danzer, G. D.; Strommen, D. P.; Kincaid, J. R. J.
Phys. Chem. 1988, 92, 5620.
(47) Yumura, T.; Koga, M.; Hoshikawa, H.; Nibu, Y.; Shimada, R.;
Shimada, H. Bull. Chem. Soc. Jpn. 1998, 71, 349.
(48) Yadav, R. A.; Singh, I. S. Indian J. Pure Appl. Phys. 1982, 20, 677.
(49) Clark, R. J. H.; Tortule, P. C. J. Chem. Soc., Dalton Trans. 1977,
2142.
(50) Green, J. H. S.; Harrison, D. J.; Stockley, C. P. Spectrochim. Acta,
Part A 1977, 33A, 423.
(51) Abramowitz, S.; Levin, I. W. Spectrochim. Acta, Part A 1970, 26A,
2261.
7082 Inorganic Chemistry, Vol. 42, No. 22, 2003

Excited States of Pt(II) Diimine (Bis)Thiolates
process. However, introduction of the strongly-electron-
withdrawing CN-substituent to the thiolate ligand of Pt(bpy)-
(4-CN-C6F5S)2 leads to a less defined character of reduction
of [Pt(bpy•-)(4-CN-C6F4S)2], with at least two overlapping
processes.
A spectroelectrochemical investigation into the nature of
the second reduction process of Pt(bpy)(4-CN-C6F4S)2 and
Pt(bpy)(C6F5S)2 has been performed. At 273 K, the products
of double reduction of 1 and 2 were unstable on the time
scale of the experiment. This observation is consistent with
the previously reported data on the instability of products
of double reduction of Pt(4,4′-X2-bpy)Cl2 in the temperature
range 273-243 K, unless the substituent X is strongly
electron withdrawing.53a Cooling the solution of 1 to 223 K
allowed the UV-vis spectrum of the doubly reduced
[Pt(bpy)(C6F5S)2]2- (Figure 6b) to be obtained (with ca. 95%
regeneration of the initial spectrum after reoxidation). The
spectrum of the doubly reduced 1 displays bands at 382, 592,
645, and 706 nm and gradually increasing absorption toward
the NIR region, indicating that the second reduction in Pt-
(bpy)(C6F5S)2 is likely to be directed to the bpy-ligand.
However, the second reduction of Pt(bpy)(4-CN-C6F4S)2
remains chemically irreversible under the same conditions
(227 K). This may be due to a close overlap between second
and third reductions, one of which might be centered on the
electron-withdrawing thiolate ligand.
Oxidation. The compounds studied show an irreversible
oxidation (Figure 5) at scan rates of e300 mV/s at room
temperature. Similar behavior has been reported for Pt(bpy)-
(4-X-C6H4S)2 complexes with X ) H, MeO, or NO2.14 The
diimine ligands have essentially no effect on the values of
the oxidation potentials Ep,a. However, the Ep,a values are
strongly affected by the donor capability of the thiolate
ligand; substitution of -C6F5S- with 4-CN-C6F4-S- shifts
the oxidation potentials ca. 200 mV more positive (Table
3). The previously reported Pt(bpy)(C6H5S)2 and Pt(bpy)-
(Me2N-C6H4-S)214 complexes contain more electron-donat-
ing thiolate ligands than those used in the present study and
hence display less positive Ep,a values for the first irreversible
oxidation process (Table 3). This indicates that the HOMO
in 1-4 is mostly centered on the thiolate ligand.
Figure 6. (a) UV-vis spectral changes accompanying the first reduction
of a 1 mM solution of Pt(bpy)(4-CN-C6F4-S)2 in DMF, containing 0.2
M of [nBu4N][BF4], 273 K. Applied potential -1.20 V vs SCE. (b) UV-
vis spectral changes accompanying the second reduction of a 1 mM solution
of Pt(bpy)(C6F5-S)2 in DMF, containing 0.2 M of [nBu4N][BF4], 223 K.
Applied potential -1.98 V vs S.C.E.
UV-vis (spectro)electrochemical investigations into the
nature of the first reduction process have been performed in
order to obtain further information about localization of the
LUMO in 1-4. The absorption maxima of the electrochemi-
cally generated radical anions of 1-4 and Pt(phen)Cl2 in
DMF at 273 K are listed in Table 2. As a typical example,
the spectral changes which accompany the first reduction
process of Pt(bpy)(4-CN-C6F4S)2 are shown in Figure 6a.
The UV-vis spectra of the [Pt(bpy)(4-CN-C6F4S)2]•- and
[Pt(bpy)(C6F5S)2]•- radical anions possess spectral features
characteristic52 of an intraligand electronic transition in a
coordinated bpy•- radical anion. Similarly, the UV-vis
spectra of [Pt(phen)(4-CN-C6F4S)2]•- and [Pt(phen)-
(C6F5S)2]•- show spectral profiles comparable to that of
[Pt(phen)Cl2]•- (Table 2). Thus, (spectro)electrochemical data
support the conclusion that the LUMO in 1-4 is mainly
centered on the π1* orbital of the corresponding diimine
ligand.
Time-Resolved Infrared Spectroscopy. The ground-state
IR spectrum of 2 in CH2Cl2 at room temperature exhibits a
single ν(CN) band at 2240 cm-1 (Figure 7a). 400-nm (150-
fs) excitation leads to an instantaneous bleaching of this
parent absorption together with the formation of a new band
at ca. 2232 cm-1. Picosecond TRIR spectra recorded at
several pump-probe delays between 1 and 500 ps are shown
in Figure 7b.
The values of the second reduction potentials of 1-4
(Table 3) are very similar to each other. The second reduction
process shows no return wave for either of the phen
compounds, 3 and 4, under scan rates of e300 mV/s. For
Pt(bpy)(C6F5S)2, the second reduction is a quasi-reversible
The small negative shift of the CN vibration observed in
the TRIR spectra (ca. 8 cm-1) (Figure 7b) is consistent with
a depopulation of the bonding π-orbital of the CN group
upon promotion to the excited state. This therefore supports
(53) (a) McInnes, E. J. L.; Farley, R. D.; Rowlands, C. C.; Welch, A. J.;
Rovatti, L.; Yellowlees, L. J. J. Chem. Soc., Dalton Trans. 1999, 4203.
(b) Kaim, W.; Dogan, A.; Wanner, M.; Klein, A.; Tiritiris, I.; Schleid,
T.; Stufkens, D. J.; Snoeck, T. L.; McInnes, E. J. L.; Fiedler, J.; Zalis,
S. Inorg. Chem. 2002, 41, 4139.
(52) (a) Krejcˇik, M.; Vlcˇek, A. A. J. Electroanal. Chem., Interfacial
Electrochem. 1991, 313, 243. (b) Vichova, J.; Hartl, F.; Vlcˇek, A. J.
Am. Chem. Soc. 1992, 114, 10903. (c) Noble, B. C.; Peacock, R. D.
Spectrochim. Acta, Part A 1990, 46, 407.
Inorganic Chemistry, Vol. 42, No. 22, 2003 7083

Weinstein et al.
Figure 8. Kinetic traces obtained from the peak heights of the parent
band at 2240 cm-1 (a) and transient decay at 2232 cm-1 (b) in picosecond
TRIR experiment: b, experimental data; solid lines, monoexponential and
biexponential fits to the data, respectively.
Figure 7. (a) Ground-state infrared spectrum of a 1 mM CH2Cl2 solution
of Pt(bpy)(4-CN-C6F4-S)2 (2) at 298 K. (b) Time-resolved infrared spectra
obtained from this solution at 1-, 2-, 5-, 15-, 30-, 60-, and 500-ps time
delays after 150-fs 400-nm pulse excitation. Solid lines represent Lorentzian
fits to the data.
carbonyls exhibit a 50-60 cm-1 positive shift of the ν(CO)
stretch21 upon promotion to the MLCT state. In contrast,
organic carbonyl reporters in RuII,25 ReI ,3 or PtII 26 complexes
bearing 2,2′-bpy-4,4′-(C(O)OR)2 ligands show only a -25
to -30 cm-1 shift upon promotion to the MLCT excited state.
The TRIR kinetic traces for parent recovery (Figure 8a)
and transient decay (Figure 8b) have been plotted as a height
of the corresponding band versus pump-probe delays.
Within 200 ps, the bleach and transients have completely
recovered back to the baseline. The parent absorption
recovers monoexponentially, with a rate constant 3.0 ((0.3)
× 1010 s-1, as is represented in Figure 8a. However, the
transient absorption decays in a biexponential fashion (Figure
8b), with the rate constants of 4.7 ((1.0) × 1011 and 2.9
((0.4) × 1010 s-1. The slower component agrees well with
the rate constant for the bleach recovery. We therefore
tentatively attribute these two decay constants to vibrational
cooling 2.1 ((0.5 ps) and back-electron-transfer processes
in the {charge-transfer-to-diimine} excited state of Pt(bpy)-
(4-CN-C6F4-S)2.
the assignment of the HOMO as being mostly thiolate
centered and the lowest electronic transition in Pt(bpy)(4-
CN-C6F4-S)2 being from the thiolate moiety to the R-di-
imine.
The changes in the energy of the cyanide stretching
frequencies upon promotion to the excited state for various
transition-metal complexes containing a CN group directly
bound to the metal center have been discussed elsewhere.23,54
An alternative interpretation could have been to attribute
this small negative shift of ν(CN) to a population of the CN
π* antibonding orbital. This is unlikely since it requires a
thiolate-centered LUMO. As discussed above, the absorption
spectrum of the electrochemically reduced species corre-
sponds to a coordinated bpy radical anion, indicating that
the LUMO is centered on the bpy ligand in Pt(bpy)(4-CN-
C6F4-S)2. This is also supported by the electrochemical data
on reduction potentials and absorption spectra.
The spectra were fitted, assuming a Lorentzian band shape
(solid lines in Figure 7b), allowing the determination of the
areas of the bleach and transient at each time delay. A small
narrowing of the transient band, observed at early time delays
(Figure 7b), is assigned to early relaxation processes,
associated with the decay of “hot” vibrational modes initially
formed upon excitation. The production of “hot” excited
states and their effect on the IR spectrum is well established
and has been discussed in detail.55
The lifetime of vibrational cooling has been reported56 to
be ca. 5 ps for [RuIII(4,4′-dimethyl-2,2′-bpy•-)3]2+* and ca.
2 ps for [RuIII(4,4′-diphenyl-2,2′-bpy•-)3]2+* in CD3CN.
Recently, we have reported the lifetime of vibrational cooling
of Pt(4,4′-(C(O)OEt)2-2,2′-bpy)Cl2 as 2.3 ((0.3) ps in the
MLCT excited state in CH2Cl2 solution at room tempera-
ture.26 The 2-ps lifetime obtained in the present study is in
reasonable agreement with the values for the RuII and PtII
complexes mentioned above.
The Nature of the Lowest Excited State in the Pt-
(diimine)(bis)perfluorothiolate Complexes 1-4. There are
four main orbital manifolds to consider in respect to
localization of HOMO and LUMO in the complexes under
study: π-system of the diimine ligand, Pt atom d-orbitals, S
atom lone pairs, and the π-system of the perfluorinated
thiolate rings.
The lowest UV-vis absorption band of the studied
compounds displays negative solvatochromic behavior,
characteristic of the charge-transfer nature of the correspond-
The very small negative shift observed for the ν(CN)
stretch is consistent with organic peripheral IR reporters
generally exhibiting smaller shifts in the excited state than
metal carbonyls or cyanides, possibly due to a larger degree
of delocalization of electronic density. Typically, metal
(54) (a) Bignozzi, C. A.; Argazzi, R.; Schoonover, J. R.; Gordon, K. C.;
Dyer, B. R.; Scandola, F. Inorg. Chem. 1992, 31, 5260. (b) Roncaroli,
F.; Baraldo, L. M.; Slep, L. D.; Olabe, J. A. Inorg. Chem. 2002, 41,
1930. (c) Turner, J. J. Coord. Chem. ReV. 2002, 230, 213. (d)
Macatangay, A. V.; Mazzetto, S. E.; Endicott, J. F. Inorg. Chem. 1999,
38, 5091. (e) Schoonover, J. R.; Bignozzi, C. A.; Meyer, T. J. Coord.
Chem. ReV. 1997, 165, 239.
(55) (a) Dougherty, T. P.; Heilweil, E. J. Chem. Phys. Lett. 1994, 227, 19.
(b) Hamm, P.; Ohline, S. M.; Zinth, W. J. Chem. Phys. 1997, 106,
519. (c) Marks, S.; Cornelius, P. A.; Harris, C. B. J. Chem. Phys.
1980, 73, 3069.
(56) (a) Damrauer, N. H.; McCusker, J. K. J. Phys. Chem. A 1999, 103,
8440. (b) Yeh, A. T.; Shank, C. V.; McCusker, J. K. Science 2000,
289, 935. (c) Monat, J. E.; McCusker, J. K. J. Am. Chem. Soc. 2000,
122, 4092.
7084 Inorganic Chemistry, Vol. 42, No. 22, 2003

Excited States of Pt(II) Diimine (Bis)Thiolates
ing transition. The influence of both ligands on the absorption
energy suggests participation of both ligands in this electronic
transition. This is supported by resonance Raman enhance-
ment of both diimine and thiolate intraligand vibrations upon
excitation along the lowest absorption band.
Thus, we assign the lowest electronic transition in Pt-
(diimine)(4-X-C6F4-S)2 to a charge-transfer transition from
the mixed platinum/thiolate moiety to the R-diimine. The
dynamics of this {charge-transfer-to-diimine} lowest excited
state are governed by vibrational cooling (ca. 2 ps) and back-
electron-transfer (35 ps) processes.
Absorption spectra of the electrochemically generated
monoreduced species correspond to the coordinated diimine
radical anions, indicating that the LUMO is predominantly
centered on the diimine ligand in Pt(diimine)(4-X-C6F4-
S)2 1-4. The first reduction potentials of 1-4 are only
slightly affected by the changes in the thiolate ligands and
are close to the value of the reduction potential of the
corresponding Pt(diimine)Cl2. This supports the assignment
of the LUMO as mostly localized on the diimine ligand. A
considerable contribution of Pt d orbitals in the LUMO has
been proposed for the Pt(4,4′-X2-2,2′-bpy)Cl2 complexes on
the basis of an extensive EPR study.53a A similar contribution
has been reported for the singly-occupied MO of the
electrochemically generated radical anion of Pt(2,2′-bi-
pyrimidine)Cl2.53b A small negative shift of the CN vibration
compared to the ground state, observed in the TRIR spectrum
of Pt(bpy)(4-CN-C6F4S)2, is consistent with the bpy-centered
assignment of the LUMO since it reflects a depopulation of
the bonding π-orbital of the CN group upon promotion to
the excited state.
Conclusions
A combined (spectro)electrochemical, resonance Raman,
and TRIR study of a series of PtII diimine complexes with
perfluorinated thiolate ligands has suggested that the HOMO
is mainly composed of thiolate(π)/S(p)/Pt(d) orbitals and the
LUMO is predominantly localized on the π* (diimine)
orbital. The introduction of fluorine substituents on the
thiolate moiety possibly leads to a more pronounced con-
tribution of the intrathiolate modes to the HOMO as
manifested by the enhancement of the thiolate ring vibrations,
C-F vibrations, and the vibration of the CN-substituent on
the thiolate moiety in the resonance Raman spectra. This
contrasts with the observation for the nonfluorinated thiolate
analogue where no such enhancement was seen.14 The
dynamics of the lowest {charge-transfer-to-diimine} excited
state on the picosecond time scale are governed by ca. 2-ps
vibrational cooling and 35-ps back electron transfer.
The energy of the lowest absorption in Pt(diimine)(bis)-
thiolate and the value of the oxidation potential, Ep,a, both
decrease with an increase of the thiolate ligand donor ability
in the order 4-CN-C6F4-S < C6F5S- < C6H5S- <
4-Me2N-C6H4S-. This indicates that the HOMO is mainly
thiolate in character. The same conclusion has been drawn
previously for, e.g., Pt(bpy)L, where L ) toluene dithiolate
or catecholate.57 An alternative term, {charge-transfer-to-
diimine}, was introduced by Eisenberg8,17,18,58 while discuss-
ing the nature of the lowest excited state in a large library
of Pt(diimine)(dithiolates). This definition assumes a sub-
stantial contribution of Pt d orbitals in the HOMO and was
later accepted for some Pt(diimine)(bis)thiolates. For the
complexes in this study, the Pt d orbitals contribution in the
HOMO is supported by the significant value of the extinction
coefficient for the lowest electronic transition.
Acknowledgment. We thank the Royal Society, Russian
Fund of Basic Research (03-03-32300), and the EPSRC (GR/
M40486) for financial support, and also the EPSRC for the
award of a diffractometer. We wish to gratefully acknowl-
edge Mr. T. L. Snoeck (University of Amsterdam) for
experimental assistance, Academician A. L. Buchachenko,
Professors J. J. Turner, M. Poliakoff, J. K. McCusker, and
A. B. P. Lever, and Dr. E. S. Dodsworth for fruitful
discussions, and Professor D. J. Stufkens for helpful discus-
sions and for kindly allowing access to the resonance Raman
instrumentation.
Supporting Information Available: The table listing Raman
data for Pt(diimine)(bis)thiolates 2 and 4 and related compounds,
obtained under 785 nm excitation, and a CIF file for 2 are also
available. This material is available free of charge via the Internet
at http://pubs.acs.org.
(57) Kumar, L.; Puthraya, K. H.; Srivastava, T. S. Inorg. Chim. Acta 1984,
86, 173.
(58) Cummings, S. D.; Eisenberg, R. Inorg. Chim. Acta 1996, 242, 225.
IC026286L
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