Synthesis, electrochemistry, photophysics, and solvatochromism in new cyclometalated 6-phenyl-4-(p-r-phenyl)-2,2′-bipyridyl (R = Me, COOMe, P(O)(OET)2) (CNN) platinum(II) thiophenolate chromophores

Article abstract of DOI:10.1021/ic801767q

Three new cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2′-bipyridyl (CNN) Pt(ll) thiophenolate complexes (R = Me (2a), COOMe (2b), and P(0)(OEt)₂ (2c)) have been synthesized and studied. The new CNN ligands L2 (R = COOMe) and L3 (R = P(0)(OEt)₂) undergo cyclometalation with a Pt(ll) source to give the Pt(ll) chloro complexes 1b and 1c, respectively, which are luminescent in fluid solution with λ_max ～ 575 nm, assigned to a metal-to-ligand charge-transfer (³MLCT) emissive state. Reaction of the chloro complexes 1a (R = Me), 1b, and 1c with sodium thiophenolate gives 2a-2c, respectively, in good yields. The novel thiophenolate complexes have two interesting absorption bands in their electronic spectra tentatively assigned to a charge-transfer to CNN (¹CT) (λ_abs ～ 415 nm) transition and a mixed metal/ligand-to-ligand' charge-transfer (MMLL'CT, λ_abs ～ 555 nm) transition, respectively. The MMLL'CT band is solvatochromic with absorption maxima in the range of 496 nm in MeOH to 590 nm in toluene (ε ～ 4000 dm³ mor^-1 cm^-1), which correlate well with an empirical charge-transfer-based solvent scale. Excitation of 2a-2c into the MMLL'CT band gives emission maxima around 680 nm in frozen CH₂CI₂ solution, and no emission in fluid solution. Ligand L2 and complexes 1aMeCN, 1b, and 2bCH ₂CI₂ have been characterized by single crystal X-ray crystallography. The electrochemical properties of ligands L1 (R = Me) and L2 and complexes 1a-1c and 2a-2c have been examined by cyclic voltammetry and are shown to exhibit reversible and quasi-reversible reductions and irreversible oxidations.

Full text of DOI:10.1021/ic801767q

Inorg. Chem. 2009, 48, 1498-1506
Synthesis, Electrochemistry, Photophysics, and Solvatochromism in
New Cyclometalated 6-Phenyl-4-(p-R-phenyl)-2,2′-bipyridyl (R ) Me,
COOMe, P(O)(OEt)2) (C∧N∧N) Platinum(II) Thiophenolate Chromophores
Jacob Schneider, Pingwu Du, Xiaoyong Wang, William W. Brennessel, and Richard Eisenberg*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received September 15, 2008
Three new cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2′-bipyridyl (C∧N∧N) Pt(II) thiophenolate complexes (R ) Me
(2a), COOMe (2b), and P(O)(OEt)2 (2c)) have been synthesized and studied. The new C∧N∧N ligands L2 (R )
COOMe) and L3 (R ) P(O)(OEt)2) undergo cyclometalation with a Pt(II) source to give the Pt(II) chloro complexes
1b and 1c, respectively, which are luminescent in fluid solution with λmax ∼ 575 nm, assigned to a metal-to-ligand
charge-transfer (3MLCT) emissive state. Reaction of the chloro complexes 1a (R ) Me), 1b, and 1c with sodium
thiophenolate gives 2a-2c, respectively, in good yields. The novel thiophenolate complexes have two interesting
absorption bands in their electronic spectra tentatively assigned to a charge-transfer to C∧N∧N (1CT) (λabs ∼415
nm) transition and a mixed metal/ligand-to-ligand′ charge-transfer (MMLL′CT, λabs ∼ 555 nm) transition, respectively.
The MMLL′CT band is solvatochromic with absorption maxima in the range of 496 nm in MeOH to 590 nm in
toluene (ε ∼ 4000 dm3 mol-1 cm-1), which correlate well with an empirical charge-transfer-based solvent scale.
Excitation of 2a-2c into the MMLL′CT band gives emission maxima around 680 nm in frozen CH2Cl2 solution, and
no emission in fluid solution. Ligand L2 and complexes 1a· MeCN, 1b, and 2b·CH2Cl2 have been characterized
by single crystal X-ray crystallography. The electrochemical properties of ligands L1 (R ) Me) and L2 and complexes
1a-1c and 2a-2c have been examined by cyclic voltammetry and are shown to exhibit reversible and quasi-
reversible reductions and irreversible oxidations.
Introduction
DNA,8-10 electroluminescent materials in light-emitting
devices (LEDs),11,12 chromophores for photoinduced charge
separation and energy storage,13-16 and sensitizers in the
Square planar platinum(II) complexes are known to have
a rich history of photochemical and photophysical proper-
ties,1 due in part to their high quantum efficiencies and the
nature of their excited states.2-4 One set of these complexes
consists of platinum(II) bi- and terpyridyl systems that have
recently received attention for possible use in a range of
applications including biosensing,5-7 intercalation into
(7) Yu, C.; Chan, K. H.-Y.; Wong, K. M.-C.; Yam, V. W.-W. Proc. Natl.
Acad. Sci. U.S.A. 2006, 103, 19652–19657.
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S.; Higgins, V. J.; Aldrich-Wright, J. R. Dalton Trans. 2007, 5055–
5064.
*
To whom correspondence should be addressed. E-mail:
(10) Lu, W.; Vicic, D. A.; Barton, J. K. Inorg. Chem. 2005, 44, 7970–
7980.
eisenberg@chem.rochester.edu.
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1498 Inorganic Chemistry, Vol. 48, No. 4, 2009
10.1021/ic801767q CCC: $40.75  2009 American Chemical Society
Published on Web 01/13/2009

Platinum(II) Thiophenolate Chromophores
photocatalytic generation of hydrogen from water.17-19
Interestingly, many of these di- and triimine square-planar
complexes exhibit solvatochromism in which the energies
and intensities of charge-transfer excited state transitions are
influenced by solvent polarity.20 For Pt(diimine)(dithiolate)
complexes, the solvatochromic absorption band is a low
energy transition corresponding to an excited state that
involves a highest occupied molecular orbital (HOMO) of
both metal and dithiolate character and a lowest unoccupied
molecular orbital (LUMO) that is a π* function of the
diimine ligand.21,22 More recent studies on bis(phenolate)
and bis(thiolate) Pt(II) diimine complexes exhibit similar
behavior with likewise HOMO assignments of mixed Pt/O
or Pt/S character.23 In a study of the terpyridyl complex,
[Pt(tpy)(C≡C-C≡CH)]+ (tpy ) 2,2′-6′,2′′-terpyridine),
Yam and co-workers attributed the appreciable color change
and emission enhancement in different solvent mixtures to
a solvatochromic effect modified by metal-metal and π-π
stacking interactions.24
A separate set of Pt(II) cyclometalated complexes that
include C∧N,25-27 C∧N∧N,28-30 C∧N∧C,31-34 and N∧C∧N
derivatives35,36 are analogous to the aforementioned bi- and
terpyridyl complexes and possess desirable photophysical
properties as well.37 In fact cyclometalated Pt(II)(C∧N∧N)
complexes have been of interest for many of the same
applications, including luminescent molecular sensors,38
metallointercalators,39 and dopant emitters in LEDs.40,41
In this article, we report three novel cyclometalated
Pt(C∧N∧N) thiophenolate complexes (2a-2c) having a
solvatochromic lowest energy absorption band. The synthesis
of two new 6-phenyl-4-(p-R-phenyl)-2,2′-bipyridyl C∧N∧N
ligands is described for R ) COOMe (L2) and P(O)(OEt)2
(L3), and cyclometalation of these ligands to a Pt(II) chloride
source yields neutral luminescent Pt(C∧N∧N)Cl complexes.
Treatment of these chloride complexes with sodium thiophe-
nolate gives dark purple solids that possess lowest energy
absorption maxima assignable to mixed metal/ligand-to-
ligand′ charge-transfer (MMLL′CT) transitions. The new
complexes are characterized electrochemically and spectro-
scopically and in several cases crystallographically.
Experimental Section
Characterization and Instrumentation. All NMR spectra were
recorded using either Bruker Avance 400 or 500 MHz spectrom-
1
eters. H NMR chemical shifts (in ppm) are referenced using the
chemical shift of CDCl3 or DMSO-d6. 31P NMR chemical shifts
(in ppm) are relative to an external 85% solution of H3PO4 in the
appropriate solvent. Mass determinations were accomplished by
atmospheric pressure chemical ionization (APCI) mass spectrometry
using a Hewlett-Packard Series 1100 mass spectrometer (model
A) equipped with a quadrupole mass filter. Elemental analyses were
performed by Quantitative Technologies Inc. (QTI). Absorption
spectra were recorded using a Hitachi U2000 scanning spectro-
photometer (200-1100 nm). Steady-state emission spectra were
obtained using a Spex Fluoromax-P fluorimeter corrected for
instrument response with monochromators positioned with a 2 nm
band-pass. The metal complex concentration of solution samples
was 1.0 × 10-5 M in CH2Cl2 for complexes 1a-1c and 2.1 ×
10-5 M in CH2Cl2 for complexes 2a-2c. Each sample was degassed
by at least two freeze-pump-thaw cycles. The room temperature
emission quantum yields (φem) are measured relative to Ru(bpy)3Cl2
in degassed H2O as the reference solution (φem ) 0.042).42 They
were calculated using eq 1
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Soc. 2007, 129, 7726–7727.
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φs ) φr(Is ⁄ Ir)(Ar ⁄ As)(n2s ⁄ nr2)
(1)
where the subscripts s and r, respectively, denote sample and
reference; I is the integrated emission intensity; A is the absorbance
in a dilute solution (∼10-5 M, A < 0.5) at the wavelength of
excitation; and n is the refractive index that corrects for differences
between the reference and the sample solutions.43
Excited state lifetime measurements of deoxygenated CH2Cl2
solution samples (∼10-4 M) of complexes 1a-1c and 2a-2c were
obtained by frequency doubling and pulse picking a femtosecond
Ti-Sapphire laser at 450 nm (complexes 1a-1c) and 403 nm
(complexes 2a-2c) with a 3.8 MHz repetition rate. The laser beam
was focused on the samples in a quartz cuvette by a microscope
objective (50 ×, NA 0.55) with a power density of ∼1 kW/cm-1
.
The photoluminescence (PL) from the samples was collected by
the same objective and sent through a 0.5 m spectrometer to a
charge coupled device (CCD) camera or a time-correlated single
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Schneider et al.
photon counting system (∼200 ps resolution) for the time-integrated
or time-resolved measurements at room temperature. For the time-
resolved measurements, the collected PL was spectrally selected
by the spectrometer within a narrow bandwidth (∼3 nm) so that
only the PL decay at the peaks of maximum emission was
monitored. The PL decay was measured at 605 nm for complex
1a, 600 nm for complexes 1b and 1c, 675 and 600 nm for complex
2a (two maximum PL peaks), and 585 nm for complexes 2b and
2c.
Cyclic voltammetry experiments were conducted on an EG&G
PAR 263A potentiostat/galvanostat using a three-electrode single
compartment cell. A glassy carbon working electrode, a Pt wire
auxiliary electrode, and a Ag wire reference electrode were used.
For all measurements, samples were dissolved in dimethylforma-
mide (DMF) and degassed with argon. Tetrabutylammonium
hexafluorophosphate (Fluka) was used as the supporting electrolyte
(0.1 M), and ferrocene (ROC/RIC) was employed as an internal
redox reference. All redox potentials are reported relatiVe to the
ferrocenium/ferrocene (Fc+/0) couple (0.45 V vs SCE44 and SCE
vs NHE ) 0.24 V). All scans were done at 100 millivolts per second
unless otherwise noted.
Crystallographic Structure Determinations. Single crystals
were attached to glass fibers and mounted under a stream of N2,
maintained at 100.0(1) K, on a Bruker SMART platform diffrac-
tometer equipped with an APEX II CCD detector, positioned at
-33° in 2θ and 5.0 cm from the samples. The X-ray source,
powered at 50 kV and 30 mA, provided Mo KR radiation (λ )
0.71073 Å, graphite monochromator). Preliminary random sam-
plings of reflections were used to determine unit cells and orientation
matrices.45 Complete data collections were obtained by ω scans of
183° (0.5° steps) at four different ꢀ settings. Final cell constants
were calculated from the xyz centroids of approximately 4000 strong
reflections from the actual data collection after integration.46 The
data were additionally scaled and corrected for absorption.47 Space
groups were determined based on systematic absences, intensity
statistics, and space group frequencies. Structures were solved by
direct or Patterson methods.48,49 Non-hydrogen atoms were located
by difference Fourier syntheses and their positions were refined
with anisotropic displacement parameters, unless noted otherwise,
by full-matrix least-squares cycles on F2.48 In a similar fashion all
hydrogen atoms were placed geometrically and refined as riding
atoms with relative isotropic displacement parameters. The final
set of refinement cycles, run to convergence, provided the reported
quality assessment parameters.
SO)2Cl2,53 and [Pt(L1)Cl] (1a)29 were prepared as reported
previously. All other solvents and reagents were purchased com-
mercially and used as received unless otherwise noted.
Synthesis and Characterization of New Ligands and Pt(II)
Complexes. 1-Phenyl-3-(4-methylbenzoate)-1-oxoprop-2-ene.
This substrate was made by a previously reported method54 from
acetophenone (3.76 mmol, 452 mg) and methyl 4-formylbenzoate
(3.76 mmol, 617 mg). It was used as isolated. Yield: 392 mg (39%).
6-Phenyl-4-(p-methylbenzoate)-2,2′-bipyridine (L2). To a flask
charged with (2-pyridacyl)pyridinium iodide (1.37 mmol, 446 mg),
1-phenyl-3-(4-methylbenzoate)-1-oxoprop-2-ene (1.37 mmol, 364
mg), and anhydrous ammonium acetate (13.7 mmol, 1.05 g) was
added anhydrous MeOH (8 mL) and the mixture was heated to
reflux under nitrogen. After ∼24 h the suspension was allowed to
cool to room temperature (r.t.) and the precipitate was collected
by filtration, rinsing with cold MeOH. Yield: 230 mg (45%). MS
(APCI): m/z 367.2 (M+). 1H NMR (CDCl3, 400 MHz): δ 8.70 (m,
3H), 8.20 (m, 4H), 8.00 (s, 1H), 7.88 (m, 3H), 7.51 (m, 3H), 7.36
(m, 1H), 3.97 (s, 3H, C∧N∧N-Ph-COOCH3). FT-IR (neat): cm-1
1709 (νCdO), 1290 (νC-O).
6-Phenyl-4-(phenyl-p-ethylphosphonate)-2,2′-bipyridine (L3).
This ligand was prepared by a literature method,55 refluxing L4
(0.41 mmol, 159 mg), diethyl phosphite (0.94 mmol, 0.12 mL),
Pd(PPh3)4 (0.04 mmol, 47 mg), PPh3 (4.1 mmol, 1.07 g), and
triethylamine (∼1 mL) in toluene for 8 h under nitrogen. The
product was purified by column chromatography (Silica) eluting
with CH2Cl2 to remove excess PPh3, followed by elution with
CH2Cl2:MeOH (99:1, v/v). In all attempts to isolate pure product,
small amounts of triphenylphosphine oxide remained. Even so, the
isolated product was used to coordinate to platinum. Yield: 150
1
mg of 77% pure L3 (based on 31P{1H} NMR) (63%). H NMR
(DMSO-d6, 500 MHz): δ 8.76 (d, 1H, J ) 3.9 Hz), 8.65 (m, 2H),
8.38 (s, 1H), 8.37 (s, 2H), 8.16 (m, 2H), 8.03 (t, 1H, J ) 7.2 Hz),
7.91 (m, 2H), 7.57 (m, 4H, overlaps with (O)PPh3 resonances),
4.07 (m, 4H, C∧N∧N-Ph-P(O)(OCH2CH3)2), 1.27 (t, 6H, J ) 7.0
Hz, C∧N∧N-Ph-P(O)(OCH2CH3)2). 31P{1H} NMR (DMSO-d6, 202
MHz): δ 17.34 (s).
[Pt(L2)Cl] (1b). This complex was prepared by two procedures.
The first was a modification of reported methods.28,29,56,57 L2 (0.82
mmol, 300 mg) was suspended in MeCN:MeOH (20 mL, 4:3, v/v)
and a solution of K2PtCl4 (0.82 mmol, 340 mg) in H2O (5 mL)
was added. The mixture was refluxed under N2 for ∼20 h and
allowed to cool to r.t. The orange precipitate was collected by
filtration, rinsing with MeOH and Et2O. Yield: 208 mg (43%).
For the second method of preparation, L2 (0.14 mmol, 50 mg)
was dissolved in a small amount of CHCl3 (∼5 mL) and cis-
Pt(DMSO)2Cl2 (0.17 mmol, 70 mg) was added, and the mixture
was heated to reflux for ∼24 h under nitrogen. The orange
precipitate was collected by filtration, rinsing with cold CHCl3 and
Et2O. Yield: 68 mg (82%). MS (APCI): m/z 596.1 (M+). 1H NMR
(DMSO-d6, 500 MHz): δ 8.96 (d, 1H, J ) 4.7 Hz), 8.80 (d, 1H, J
) 8.0 Hz), 8.61 (s, 1H), 8.41 (m, 2H), 8.29 (d, 2H, J ) 8.1 Hz),
Materials. K2PtCl4 (VWR), methyl 4-formylbenzoate (Fluka),
acetophenone (Aldrich), Pd(PPh3)4 (Strem), ammonium acetate
(EMD), diethyl phosphite (Aldrich), sodium thiophenolate (Fluka),
triethylamine (Aldrich), CDCl3 (Cambridge Isotope Laboratories),
and DMSO-d6 (Cambridge Isotope Laboratories) were purchased
commercially and used without further purification. 6-Phenyl-4-
(4-bromophenyl)-2,2′-bipyridine (L4),50 6-phenyl-4-(p-tolyl)-2,2′-
bipyridine (L1),29,51 (2-pyridacyl)pyridinium iodide,52 cis-Pt(DM-
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1500 Inorganic Chemistry, Vol. 48, No. 4, 2009

Platinum(II) Thiophenolate Chromophores
8.16 (d, 2H, J ) 8.3 Hz), 7.97 (t, 1H, J ) 6.7 Hz), 7.87 (d, 1H, J
) 7.0 Hz), 7.54 (d, 1H, J ) 7.9 Hz), 7.19 (t, 1H, J ) 7.3 Hz), 7.12
(t, 1H, J ) 6.7 Hz), 3.93 (s, 3H, C∧N∧N-Ph-COOCH3). FT-IR
(neat): cm-1 1730 (νCdO), 1279 (νC-O). Anal. Calcd for
C24H17ClN2O2Pt: C, 48.37; H, 2.88; N, 4.70. Found: C, 47.78; H,
2.72; N, 4.62.
FT-IR (neat): cm-1 1713 (νCdO), 1273 (νC-O). Anal. Calcd for
C30H22N2O2SPt: C, 53.80; H, 3.31; N, 4.18. Found: C, 53.75; H,
3.52; N, 3.86.
[Pt(L3)(SPh)] (2c). This compound was prepared by the same
method as for 2a, using 1c (0.074 mmol, 50 mg) in place of 1a,
with sodium thiophenolate (0.087 mmol, 12 mg). Final recovered
product stored under nitrogen. Yield: 42 mg (76%). MS (APCI):
m/z 748.2 (M+). 1H NMR (DMSO-d6, 500 MHz): δ 8.76 (d, 1H, J
) 7.8 Hz), 8.66 (s, 1H), 8.51 (d, 1H, J ) 4.9 Hz), 8.44 (s, 1H),
8.31 (m, 3H), 7.93 (m, 3H), 7.67 (t, 1H, J ) 6.2 Hz), 7.55 (d, 3H),
7.09 (m, 2H), 7.00 (t, 2H, J ) 7.5 Hz), 6.89 (t, 1H, 7.3 Hz), 4.09
(m, 4H, C∧N∧N-Ph-P(O)(CH2CH3)2), 1.28 (t, 6H, J ) 7.0 Hz,
C∧N∧N-Ph-P(O)(CH2CH3)2). 31P{1H} NMR (DMSO-d6, 202 MHz):
δ 17.12 (s). FT-IR (neat): cm-1 1250 (νP)O). Anal. Calcd for
C32H29N2O3PSPt: C, 51.40; H, 3.91; N, 3.75. Found: C, 51.11; H,
3.81; N, 3.74.
[Pt(L3)Cl] (1c). This compound was prepared in the same
manner as the first method for 1b, using L3 (0.33 mmol, 150 mg)
in place of L2, with K2PtCl4 (0.33 mmol, 140 mg). After refluxing
for ∼20 h, the solvent was removed under vacuum. To the orange
residue was added a small amount of MeOH (2 mL), and after a
few minutes the orange precipitate was collected by filtration,
rinsing with Et2O. Yield: 81 mg (46%). MS (APCI): m/z 674.1
1
(M+). H NMR (DMSO-d6, 500 MHz): δ 8.95 (d, 1H, J ) 4.3
Hz), 8.77 (d, 1H, J ) 7.8 Hz), 8.59 (s, 1H), 8.41 (t, 1H, J ) 7.4
Hz), 8.36 (s, 1H), 8.27 (d, 2H, J ) 4.8 Hz), 7.93 (m, 3H), 7.85 (d,
1H, J ) 7.3 Hz), 7.53 (d, 1H, J ) 7.4 Hz), 7.19 (t, 1H, J ) 7.3
Hz), 7.12 (t, 1H, J ) 7.2 Hz), 4.08 (m, 4H, C∧N∧N-Ph-
P(O)(OCH2CH3)2), 1.28 (t, 6H, J ) 7.0 Hz, C∧N∧N-Ph-
P(O)(OCH2CH3)2). 31P{1H} NMR (DMSO-d6, 202 MHz): δ 17.07
(s). FT-IR (neat): cm-1 1240 (νP)O). Anal. Calcd for
C26H24ClN2O3PPt: C, 46.33; H, 3.59; N, 4.16. Found: C, 41.52; H,
3.48; N, 4.46.
Results and Discussion
Synthesis and Characterization. The substituted 6-phen-
yl-4-(p-R-phenyl)-2,2′-bipyridine ligands (R ) Me (L1),
COOMe (L2), and Br (L4)) were made from a modification
of the Kro¨hnke pyridine synthesis that involves heating a
methanol mixture of the appropriate (E)-propenone and
pyridinium iodide substrates in the presence of ammonium
acetate (Scheme 1).54,58 For R ) P(O)(OEt)2 (L3), the aryl
phosphonate was made by a Pd catalyzed phosphonation of
L4 with diethylphosphite, using Pd(PPh3)4 as the Pd source
and ∼10 fold excess PPh3.55 The resulting ligands L1-L3
were then allowed to react with K2PtCl4 or Pt(DMSO)2Cl2
using a combination of known procedures to give the neutral
cyclometalated Pt C∧N∧N chloride complexes 1a, 1b, and
1c, respectively (Scheme 1).28,29,56,57 It was found that using
Pt(DMSO)2Cl2 as the Pt(II) source nearly doubled the yields
of 1a-1c. Facile displacement of the chloride by thiophe-
nolate occurred from stirring a MeOH mixture of 1a-1c with
an aqueous solution of sodium thiophenolate under nitrogen,
giving complexes 2a, 2b, and 2c, respectively, in high yields
(Scheme 1). The new ligands and complexes have been
[Pt(L1)(SPh)] (2a). To a suspension of 1a (0.072 mmol, 41 mg)
in MeOH (10 mL) was added a solution of sodium thiophenolate
(0.083 mmol, 12 mg) in H2O:MeOH (2:1, v/v) (10 mL). The
mixture was purged with N2 for several minutes and allowed to
stir at r.t. for ∼3 days, or until the mixture became dark blue/green.
Volatile solvent was removed under vacuum, and the dark solid
was isolated by centrifuge, rinsing with H2O and a small amount
of MeOH, until thiolate smell had dissipated. The dark colored
product was dried under vacuum and was recrystallized by slow
diffusion of Et2O into a CH2Cl2 solution of the product. The
recrystallized product was collected by filtration and stored under
1
nitrogen. Yield: 37 mg (82%). MS (APCI): m/z 626.1 (M+). H
NMR (DMSO-d6, 500 MHz): δ 8.78 (d, 1H, J ) 7.9 Hz), 8.61 (s,
1H), 8.52 (d, 1H, J ) 6.1 Hz), 8.38 (s, 1H), 8.32 (t, 1H, J ) 7.5
Hz), 8.09 (d, 2H, J ) 7.9 Hz), 7.89 (d, 1H, 6.4 Hz), 7.66 (t, 1H, J
) 6.6 Hz), 7.54 (m, 3H), 7.44 (d, 2H, J ) 7.7 Hz), 7.07 (m, 2H),
6.99 (t, 2H, J ) 7.4 Hz), 6.88 (t, 1H, J ) 7.1 Hz), 2.42 (s, 3H,
C∧N∧N-Ph-CH3). Anal. Calcd for C29H22N2SPt: C, 55.67; H, 3.54;
N, 4.48. Found: C, 54.50; H, 3.36; N, 4.50.
1
characterized by various methods that include H and 31P
NMR spectroscopies, mass spectrometry, FT-IR spectros-
copy, elemental analysis, and single crystal X-ray diffraction.
The elemental analysis for complex 1c is not satisfactory;
the low percent carbon is due in part to difficulties getting
complete combustion of the complex under experimental
conditions.
Crystal Structure Determinations. Crystals suitable for
single crystal X-ray diffraction were obtained for ligand L2
and metal complexes 1a, 1b, and 2b. The unit cell, data
collection and refinement parameters are located in Table 1,
with selected bond lengths and angles summarized in Table
2. Crystals of 2b were obtained by the slow diffusion of
diethyl ether into a CH2Cl2 solution of the complex (Figure
1). There are two independent Pt molecules in the asymmetric
unit, with two co-crystallized CH2Cl2 solvent molecules per
Pt complex, one of which is disordered over a crystal-
lographic inversion center (50:50). The molecule containing
Pt(1) has a C:N disorder (52:48) at the Pt(C∧N∧N) core; the
[Pt(L2)(SPh)] (2b). To a solution of 1b (0.047 mmol, 28 mg)
in CH2Cl2 (25 mL) was added a suspension of sodium thiophenolate
(0.052 mmol, 7 mg) in MeOH (5 mL). The dark purple mixture
was purged with N2 and allowed to stir at r.t. for ∼2 days after
which the solvent was removed under vacuum, and the residue was
resuspended in MeOH:H2O (1:5, v/v), isolated by centrifuge, rinsing
with H2O and MeOH until thiolate smell had dissipated. The dark
purple product was purified by column chromatograpy (neutral
Alumina) eluting with CH2Cl2 followed by MeCN. The recovered
product was redissolved in CH2Cl2 and addition of Et2O caused a
dark purple precipitate to crash out. The product was collected by
filtration and stored under nitrogen. Yield: 14 mg (45%). MS
1
(APCI): m/z 670.2 (M+). H NMR (DMSO-d6, 400 MHz): δ 8.78
(d, 1H, J ) 7.6 Hz), 8.68 (s, 1H), 8.49 (d, 1H, J ) 4.5 Hz), 8.46
(s, 1H), 8.31 (d, 3H, J ) 7.7 Hz), 8.16 (d, 2H, J ) 7.8 Hz), 7.91
(m, 1H), 7.67 (t, 1H), 7.55 (d, 3H, J ) 7.7 Hz), 7.08 (m, 2H), 7.00
(t, 2H, J ) 7.4 Hz), 6.90 (t, 1H), 3.93 (s, 3H, C∧N∧N-Ph-COOCH3).
(58) Kro¨hnke, F. Synthesis 1976, 1–24.
Inorganic Chemistry, Vol. 48, No. 4, 2009 1501

Schneider et al.
Scheme 1
Table 1. Crystallographic Data and Structure Refinement for Ligand L2 and Complexes 1a·MeCN, 1b, and 2b·CH2Cl2
L2
1a ·MeCN
1b
2b ·CH2Cl2
formula
Fw
C24H18N2O2
366.40
C25H20Cl0.89I0.11N3Pt
602.58
C24H17ClN2O2Pt
595.94
C32H24Cl2N2O2PtS
754.57
T (K)
λ (Å)
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
(deg)
100.0(1)
0.71073
orthorhombic
Pbcn
28.8215(18)
6.9172(4)
18.2398(11)
90
90
90
3636.4(4)
8
1.339
100.0(1)
0.71073
orthorhombic
Pnma
13.315(3)
6.6392(17)
23.221(6)
90
90
90
2052.7(9)
4
1.950
100.0(1)
0.71073
monoclinic
P21/n
7.209(3)
17.849(8)
30.341(14)
90
93.407(6)
90
3897(3)
8
2.031
7.363
orange needle
0.22 × 0.03 × 0.01
1.76-25.02
6897
100.0(1)
0.71073
triclinic
j
P1
7.5080(9)
16.2851(19)
23.239(3)
81.577(2)
89.130(2)
89.437(2)
2810.3(6)
4
1.783
5.290
red-black plate
0.24 × 0.22 × 0.04
1.43-32.03
19323
γ (deg)
V (Å3)
Z
Fcald (Mg/m3)
µ (mm-1
)
0.086
7.128
crystal color
colorless plate
0.50 × 0.36 × 0.08
2.23-32.56
6608
orange needle
0.45 × 0.12 × 0.06
1.75-32.56
3979
crystal size (mm3)
θ range (deg)
no. of data
no. of parameters
325
1.044
0.0427, 0.1103
0.0513, 0.1173
184
1.083
0.0222, 0.0450
0.0295, 0.0470
261
1.046
0.0970, 0.1805
0.2141, 0.2243
b
716
1.036
0.0466, 0.1133
0.0727, 0.1234
GOFa
R1, wR2 (F2, I > 2σ(I))b
R1, wR2 (F2, all data)b
2
a
GOF ) S ) ∑w(Fo - Fc2)2/(m - n)]1/2, where m ) number of reflections and n ) number of parameters. R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) [∑w(Fo
2
2
2
2/3
- Fc2)2/∑w(Fo )2]1/2, where w ) 1/[σ2(Fo)2 + (aP) + bP] and P ) ∑ max(0, Fo ) + Fc2.
molecule containing Pt(2) has its thiophenolate modeled as
disordered over two positions (50:50). Pi-stacking of the
central C∧N∧N coordination sphere is observed between pairs
of molecules (∼3.3 Å). To the best of our knowledge there
are no literature reports for the structural characterization of
Pt(II) (C∧N∧N) thiolate complexes, and there are few
accounts of Pt(II) complexes with a coordination sphere that
contains an anionic carbon, a thiolate, and two neutral
N-donors. Ma et al.59 and Koshiyama et al.60 have structur-
ally characterized cyclometalated dinuclear Pt(II) complexes
of the general formula, [Pt2(ppy)2(pyt)2] (ppy ) 2-phenylpy-
ridine, pyt ) pyridine-2-thiol), that are bridged by pyridine-
2-thiolate. Moreover, there is only one reported structure of
a platinum thiophenolate complex, the cationic Pt(IV)
octahedral species [PtMe2(SPh)(PEt3)(dbbpy)]+ (dbbpy )
4,4′-di-tert-butyl-2,2′-bipyridine).61
Slow evaporation of a CH2Cl2/MeCN solution of 1a
yielded long orange needles suitable for single crystal X-ray
diffraction. There is a halogen (chloride:iodide) site oc-
(61) Janzen, M. C.; Jennings, M. C.; Puddephatt, R. J. Can. J. Chem. 2002,
(59) Ma, B.; Djurovich, P. I.; Garon, S.; Alleyne, B.; Thompson, M. E.
AdV. Funct. Mater. 2006, 16, 2438–2446.
(60) Koshiyama, T.; Ai, O.; Kato, M. Chem. Lett. 2004, 33, 1386–1387.
80, 41–45.
(62) It is presumed that the iodide present remains from the synthesis of
the L1 ligand.
1502 Inorganic Chemistry, Vol. 48, No. 4, 2009

Platinum(II) Thiophenolate Chromophores
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Ligand L2, and Complexes 1a·MeCN, 1b, and 2b·CH2Cl2
Ligand L2
C(4)sC(5)sC(6)sN(2)
12.00(12)
22.65(11)
C(7)sC(8)sC(17)sC(22)
21.14(12)
N(2)sC(10)sC(11)sC(12)
Complex 1a·MeCN
Pt(1)sN(1)
Pt(1)sN(2)
1.954(3)
2.109(3)
Pt(1)sC(1)
1.993(3)
2.3657(7)
Pt(1)sCl(1)a
N(1)sPt(1)sN(2)
N(1)sPt(1)sC(1)
79.31(11)
82.08(12)
C(1)sPt(1)sN(2)
N(1)sPt(1)sCl(1)a
161.38(12)
179.25(8)
C(10)sC(9)sC(17)sC(22)
Pt(1)sCl(1)
0.000(2)
2.323(7)
Complex 1bb
Pt(2)sCl(2)
2.307(7)
c
Complex 2b·CH2Cl2
Pt(1)sN(2)
Pt(1)sN(1)
Pt(1)sC(7)
Pt(1)sS(1)
1.984(4)
2.062(5)
2.061(4)
2.994(13)
Pt(2)sN(4)
Pt(2)sN(3)
Pt(2)sC(37)
Pt(2)sS(2)d
1.977(4)
2.104(5)
1.998(6)
2.298
N(2)sPt(1)sN(1)
N(2)sPt(1)sC(7)
C(7)sPt(1)sN(1)
N(2)sPt(1)sS(1)
80.09(17)
80.17(17)
160.22(18)
178.45(11)
N(4)sPt(2)sN(3)
N(4)sPt(2)sC(37)
C(37)sPt(2)sN(3)
N(4)sPt(2)sS(2)d
79.25(18)
82.1(2)
161.4(2)
174.1
C(7)sPt(1)sS(1)sC(1)
C(14)sC(15)sC(23)sC(24)
53.3(2)
39.7(7)
C(37)sPt(2) sS(2)sC(31)d
C(46)sC(45)sC(53)sC(54)
73.1
37.0(9)
a There is a halogen (chloride:iodide) site occupancy disorder of 89:11 (Cl:I).62 b There are two unique molecules per asymmetric unit, but only the Pt
and Cl atoms were refined with anisotropic displacement parameters. c There are two unique molecules per asymmetric unit. d Thiophenolate modeled as
disordered over two positions (50:50), values reported are an average of the two.
Figure 1. ORTEP drawing of one of the unique molecules of complex
2b·CH2Cl2.
cupancy disorder of 89:11 (Cl:I),62 and since the disordered
atoms were refined isopositionally, the bond length is based
on the percentage of each component. Though it is roughly
10% iodide in character, the Pt(1)sCl(1) bond distance of
2.3657(7) Å is only slightly elongated (∼0.05 Å) compared
to PtsCl distances reported for similar [Pt(C∧N∧N)Cl]
complexes: 2.312(2), 2.307(2), and 2.316(2) Å for [Pt(pb-
py)Cl] where pbpy ) 6-phenyl-2,2′-bipyridine;63 2.319(4)
Å for [Pt(4-(aza-15-crown-5)-pbpy)Cl];64 and 2.302(2) Å for
[Pt(4′-(4-dodecyloxy)phenyl)-pbpy)Cl].57 Complex 1a and
one MeCN solvent molecule lie on crystallographic mirror
planes, shown in Figure 2 without the MeCN. There are no
observed PtsPt interactions since the nearest Pt · · ·Pt distance
is 5.693 Å; however, there is π-stacking (∼3.33 Å) between
the bipyridyl parts of C∧N∧N ligands of adjacent molecules.
Figure 2. ORTEP drawings of complex 1a·MeCN. Looking down the
ClsPt bond, the bottom drawing shows the planarity of the pendant tolyl
group with the Pt(C∧N∧N) core.
They are aligned in such a manner that the cyclometalated
phenyl ring is not participating in π-stacking interactions.
The orientation of the pendant phenyl ring is also noteworthy,
as it is planar with the cyclometalated portion of the C∧N∧N
ligand (Figure 2, bottom). This observation is atypical for
these types of ligands and is not the case for the pendant
ring in crystal structures of L2, 1b, and 2b.
Crystals of 1b were grown by the slow evaporation of a
concentrated CH2Cl2 solution. There are two unique mol-
ecules per asymmetric unit, but only the Pt and Cl atoms
were refined with anisotropic displacement parameters; all
other atoms could not be refined anisotropically. This
structure is informative for connectivity only, and the bond
lengths and angles reported, other than the PtsCl distances,
(63) Hofmann, A.; Dahlenburg, L.; van Eldik, R. Inorg. Chem. 2003, 42,
6528–6538.
(64) Lu, W.; Chan, M. C. W.; Zhu, N.; Che, C.-M.; Li, C.; Hui, Z. J. Am.
Chem. Soc. 2004, 126, 7639–7651.
Inorganic Chemistry, Vol. 48, No. 4, 2009 1503

Schneider et al.
Figure 3. Steady-state room temperature absorption and emission (λex
435 nm) spectra of complexes 1a-1c in CH2Cl2 solution.
)
Figure 4. Room temperature absorption and 77 K emission spectra of
complexes 2a-2c in CH2Cl2 solution.
Table 3. Electrochemical Potentials for Ligands L1 and L2 and
are intended as mere guidelines. The PtsCl distances in 1b
were found to be 2.323(7) Å for Pt(1)sCl(1) and 2.307(7)
Å for Pt(2)sCl(2); these values compare well with PtsCl
distances reported for analogous [Pt(C∧N∧N)Cl] struc-
tures.57,63,64
Electronic Absorption Spectroscopy. The electronic
spectra of 1b and 1c in CH2Cl2 fluid solution are very similar
to that of 1a, which has been discussed elsewhere.65 Figure
3 shows that the UV region of the spectrum (λ < 370 nm)
is dominated by intraligand (IL) transitions with molar
absorptivities (ε) that range from ∼8500-24,000 dm3 mol-1
cm-1. There is a maximum at about 285 nm, and there are
two additional shoulders near 335 and 370 nm. Another
lower energy maximum centered around 440 nm (ε ∼4800
dm3 mol-1 cm-1) and ending just beyond 500 nm has been
assigned to a metal-to-ligand charge-transfer (1MLCT)
transition.29,57
Like their precursors in CH2Cl2 fluid solution, complexes
2a, 2b, and 2c exhibit IL transitions in the UV region of
their electronic spectra with maxima at about 285 nm and
shoulders around 330 and 370 nm with molar absorptivities
ranging between ∼10,500-33,500 dm3 mol-1 cm-1. In the
visible region, the absorption bands at 411, 418, and 415
nm for 2a, 2b, and 2c, respectively, are tentatively assigned
to a dπ (Pt)/ p (thiophenolate) f π* (C∧N∧N ligand) charge-
transfer (1CT) transition, while a second absorption band of
slightly greater magnitude (ε ∼4000 dm3 mol-1 cm-1) is
observed at 551, 560, and 560 nm, respectively (Figure 4).
While the 1CT transition at 411-418 nm varies only slightly
(∼5 nm) with solvent, the lower energy bands observed for
complexes 2a-2c at 551-560 nm are solvent dependent and
are likely charge-transfer (CT) in nature,22 an assignment
discussed further in the solvatochromism section of the paper.
Electrochemistry. The electrochemical behavior of ligands
L1 and L2 and complexes 1a-1c and 2a-2c has been
examined by cyclic voltammetry, and the data are sum-
marized in Table 3, with cyclic voltammograms for all
complexes available in the Supporting Information. All of
the redox potentials are reported relative to NHE with the
Complexes 1a-1c and 2a-2c, As Determined by Cyclic Voltammetrya
complex/ligand
reduction E1/2/V
oxidationb E/V
L1
L2
1a
1b
1c
2a
2b
2c
-1.81
-1.46
-1.04, -1.66
-0.98, -1.48
-0.98, -1.54
-1.06, -1.67
-0.98, -1.46
-1.00, -1.51
+1.10
+1.10
+1.14
+1.29c, +1.51c
+1.29d, +1.53d
+1.34e
a All redox potentials are reported versus NHE relatiVe to the Fc+/0 couple
(0.45 V vs SCE44 and SCE vs NHE ) 0.24 V). b All oxidations are
irreversible under experimental conditions. c 500 mV/sec scan rate. d 300
mV/sec scan rate. e 500 mV/sec scan rate; only one observed oxidation.
ferrocenium/ferrocene (Fc+/0) couple used as an internal
redox standard and a value of 0.45 V versus SCE taken for
the Fc+/0 couple,44 and a value of 0.24 V taken for SCE
versus NHE. The ligands show one quasi-reversible reduction
at -1.81 and -1.46 V (vs NHE) for L1 and L2, respectively.
Ligand L2 is easier to reduce than L1 because of the electron
withdrawing nature of the pendant R ) COOMe group. For
1a-1c there are two reductions that reside on the coordinated
C∧N∧N ligand for each complex with potential values of
-1.04, -0.98, and -0.98 V for the first reduction and -1.66,
-1.48, and -1.54 V for the second reduction for 1a, 1b,
and 1c, respectively. That complexes 1b and 1c are only
slightly easier to reduce than 1a suggests the electron
withdrawing nature of the pendant groups in 1b and 1c
relative to that of 1a are exerting only a minimal electronic
effect on the C∧N∧N ligand, possibly because of the
intervening p-phenylene group which is twisted relative to
the central pyriding ring of the ligand. Complexes 2a-2c
also show two reductions with potentials of -1.06, -0.98,
and -1.00 V for the first reduction and -1.67, -1.46, and
-1.51 V for the second reduction for 2a, 2b, and 2c,
respectively. Like their chloride precursors, the thiophenolate
complexes with electron withdrawing ester groups on the
C∧N∧N ligands are only slightly easier to reduce than the R
) Me analogue. A previous study for a series of
[(C∧N∧N)PtC≡CR] complexes reported only one observed
reduction in CH2Cl2 solution,28 possibly because of a
different solvent window than for the current study, which
was done in DMF.
(65) Cheung, T.-C.; Cheung, K.-K.; Peng, S.-M.; Che, C.-M. J. Chem. Soc.,
Dalton Trans. 1996, 1645–1651.
1504 Inorganic Chemistry, Vol. 48, No. 4, 2009

Platinum(II) Thiophenolate Chromophores
Table 4. Spectroscopic and Photophysical Data for Complexes 1a-1c in 1.0 × 10-5 M CH2Cl2 Solution at Room Temperature
a c
,
λ
abs/nm (ε/dm3 mol-1 cm-1
)
solution λemmax/nma
τ/nsb
φem
1a
1b
1c
291 (20 040), 336 (14 580), 368 (8460), 436 (4440)
288 (24 160), 336 (15 410), 369 (9220), 440 (4940)
285 (19 560), 336 (14 770), 367 (8880), 439 (4640)
b
577, 640 (sh)
574, 610 (sh), 640 (sh)
573, 610 (sh), 640 (sh)
580
390
385
0.066
0.038
0.035
a λex ) 435 nm; sh ) shoulder. λex ) 450 nm. c Emission quantum yields (φem) measured relative to Ru(bpy)3Cl2 in H2O (φem ) 0.042).42
Table 5. Spectroscopic and Photophysical Data for Complexes 2a-2c in 2.1 × 10-5 M CH2Cl2
77 K soln
c
λ
abs/nm (ε/dm3 mol-1 cm-1
)
soln λemmax/nma
τ/nsb
φem
λemmax/nm
2a 286 (30 770), ∼330 (17 770), ∼370 (10 650), 411 (3220), 551 (3440) 570; 566; 564
135, 235 0.016; 0.049
672d
694e
692e
2b 288 (33 560), ∼330 (15 220), ∼370 (11 350), 418 (3190), 560 (4690) 544, 570 (sh); 545, 570 (sh); 562 230
0.003; 0.011
0.002; 0.002
2c 286 (29 730), ∼330 (13 960), ∼370 (10 570), 415 (2950), 560 (4130) 566; 559; 563
305
b
a λex ) 410 nm; 370 nm; 335 nm; sh ) shoulder. λex ) 403 nm; for complex 2a, the emission lifetime was measured for two different maxima.
c Emission quantum yields (λex ) 335 nm; 410 nm) measured relative to Ru(bpy)3Cl2 in H2O (φem ) 0.042).42 d λex ) 550 nm. λex ) 560 nm.
e
Neither L1 or L2 display an oxidation, but for complexes
1a-1c there is an irreversible oxidation observed at +1.10,
+1.10, and +1.14 V (vs NHE), respectively, that has
tentatively been assigned to a PtII/III oxidation process.13 In
addition, the oxidation potentials for 2a-2c at +1.29, +1.29,
and +1.34 V, respectively, are also speculated to be a PtII/III
oxidation process. Though they are difficult to discern,
complexes 2a and 2b display an additional irreversible
oxidation wave at +1.51 and +1.53 V, respectively, which
has tentatively been assigned to an oxidation of the thiophe-
nolate ligand. This second oxidation was not observed for
complex 2c.
Steady-State and Frozen Emission Spectra. The absorp-
tion and emission data for complexes 1a-1c and 2a-2c are
summarized in Tables 4 and 5. The chloro complexes 1a,
1b, and 1c are emissive in fluid solution with λmax at 577,
574, and 573 nm, respectively, when λex ) 435 nm (Figure
Figure 5. Steady-state room temperature absorption spectra of complex
2a in different solvents. Insets: Plot of the maximum absorption energy of
the MMLL′CT transition (eV) versus empirical Solvent Parameter values;
and a photograph of solutions of 2a in diethylether, methylene chloride,
and methanol.
3), and have quantum yields (φem) of 0.066, 0.038, and 0.035,
respectively, measured relative to Ru(bpy)3Cl2 in H2O.42 The
like that of 2a and 2c, with an emission maximum centered
at 563 nm. There are only subtle differences in the emission
maxima between complexes. The luminescence lifetimes for
exciting complexes 2b and 2c at 403 nm are 230 and 305
ns, respectively. When measuring the photoluminescent
decay of complex 2a at 403 nm excitation, there were two
observed emission maxima, one having a lifetime of 135 ns
and the other a slightly longer lifetime of 235 ns. On the
basis of the lifetime measurements, the observed emission
for complexes 2a-2c excited at 403 nm are assigned to a
charge-transfer to C∧N∧N (3CT) emissive state, which we
think has the same LUMO as suggested for 1a-1c but
possesses a different HOMO.
Solvatochromism. Further investigation of the lower
energy CT absorption maxima at 551, 560, and 560 nm for
complexes 2a, 2b, and 2c, respectively, shows evidence that
the CT bands are solvatochromic. For complex 2a the lower
energy CT absorption ranges between 496 nm in MeOH to
578 nm in toluene (see Figure 5). The photograph in Figure
5 shows the distinct color change of 2a from pink in MeOH
to purple in CH2Cl2 to blue in diethyl ether. Complexes 2b
and 2c display similar solvatochromic behavior with absorp-
tion maxima that range between 507 nm in MeOH and 590
nm in toluene (Table 6). This type of behavior has been
observed with a number of Pt(diimine)(dithiolate) complexes
and has previously been assigned as a mixed metal/ligand-
luminescence lifetimes range between 385-580 ns, leading
29
3
to assignment of the excited state as a MLCT transition.
Similar to what was observed electrochemically for com-
plexes 1a-1c, the electron withdrawing nature of the ester
groups in 1b and 1c exert little effect on the lowest
unoccupied π* orbital localized on the C∧N∧N ligands, as
the difference in maximum emission between complexes is
not significant.
When complexes 2a-2c are excited into the lower energy
CT band (λex ) 550 nm for 2a, λex ) 560 nm for 2b and
2c) there is no emission in fluid solution; however, they are
emissive in frozen CH2Cl2 solution with λmax of 672, 694,
and 692 nm, respectively (Figure 4). The electron withdraw-
ing ester groups in 2b and 2c appear to have a similar effect,
slightly shifting the CT emission by ∼20 nm compared to
the emission of 2a.
Complexes 2a-2c are emissive in fluid CH2Cl2 when
excited at λ e 410 nm, and the respective emission maxima
are nearly the same regardless of the excitation wavelength
(Table 5, Supporting Information, Figure S4). Complexes
2a and 2c show broad structureless emissions with λmax that
range between 559 and 570 nm whereas complex 2b, when
excited at 410 or 370 nm, exhibits narrower emission maxima
at about 545 nm with shoulders at ∼570 nm. When excited
at 335 nm, complex 2b exhibits a broader emission profile
Inorganic Chemistry, Vol. 48, No. 4, 2009 1505

Schneider et al.
Table 6. Charge-Transfer Absorption Maxima for Complexes 2a-2c in
Conclusions
Different Solventsa
Three cyclometalated Pt(C∧N∧N) thiophenolate complexes
have been synthesized and characterized from newly de-
signed 6-phenyl-2,2′-bipyridine ligands L2 and L3 and their
Pt(C∧N∧N) chloride precursors, 1a-1c. Complexes 1a-1c
solvent
2a
2b
2c
pentane
toluene
Et2O
CH2Cl2
THF
DMSO
MeCN
MeOH
574
578
572
551
567
544
539
496
586
590
583
560
585
589
582
560
578
max
are luminescent in fluid CH2Cl2 solution with λem ∼575
nm, assigned to a 3MLCT emissive state. The thiophenolate
complexes 2a-2c exhibit two charge-transfer bands in the
electronic absorption spectrum tentatively assigned to a 1CT
(λabs ∼415 nm) transition and a MMLL′CT (λabs ∼560 nm
in CH2Cl2) transition, respectively. The MMLL′CT band
possesses interesting solvatochromic properties that are
characteristic of a polar ground state and a charge-transfer
excited state in which the dipole is greatly diminished. At
room temperature and at 77 K, complexes 2a-2c are not
emissive and weakly emissive, respectively, when excited
into the MMLL′CT absorption band in CH2Cl2 solution.
However, 2a-2c are emissive in CH2Cl2 solution when
555
548
508
547
507
Solvatochromic Shiftb
0.18
0.18
0.20
2a
2b
2c
a 2.1 × 10-5 M in CH2Cl2:Solvent (1:4, v/v) at room temperature. λabs/nm.
b Unitless slope from the plot of charge-transfer absorption maxima (eV)
vs solvent parameter.21
to-ligand′ charge-transfer (MMLL′CT) transition.19,21 Specif-
ically, while the LUMO in these complexes is localized on
a diimine π* orbital, the HOMO has mixed Pt(d), S(p), and
dithiolate ligand character. In the context of complexes
2a-2c, a similar assignment involving a charge-transfer
transition from an orbital of mixed metal-and-thiophenolate
character to a π* orbital localized on the C∧N∧N ligand can
be made. For the solvents examined in this study, the lower
energy CT maxima, going from lowest energy to highest
energy, are in the order toluene < pentane < Et2O < THF
< CH2Cl2 < DMSO < MeCN < MeOH. This order
correlates well when comparing the energy of the CT band
to the solvent polarity parameter value (Figure 5, inset)
derived from the solvent dependent CT band in Pt(dbbpy)(t-
dt) (dbbpy ) 4,4′-di-tertbutyl-2,2′-bipyridine, tdt ) toluene-
3,4-dithiolate).21,23 The values for the solvatochromic shift
for 2a, 2b, and 2c are 0.18, 0.18, and 0.20, respectively,
and are all lower in magnitude than those for reported
Pt(diimine)(dithiolate)21 and Pt(diimine)bis(thiolate)23 com-
plexes, but are well within the range for a CT transition.21,23
The negative solvatochromism seen here and for the related
Pt(diimine)(dithiolate) and bis(thiolate) complexes in which
charge-transfer transition increases in energy with solvent
polarity indicates that like those diimine systems, the
Pt(C∧N∧N) thiolate complexes possess a very polar ground
state and a CT excited state in which the dipole is greatly
diminished or even reversed.66-68
1
excited into the higher energy CT band, with lifetimes
between 135-305 ns (λex ) 403 nm), and the observed
emission at ∼550 nm is assigned to a charge-transfer to
C∧N∧N (3CT) emissive state, which we think has the same
LUMO as suggested for 1a-1c but possesses a different
HOMO.
Acknowledgment. This work was supported by the
Department of Energy, Division of Basic Sciences (DE-
FG02-90ER14125). The authors also wish to acknowledge
the following support from the University of Rochester: Elon
Huntington Hooker Fellowship (P.D.), Graduate Assistance
in Areas of National Need (GAANN) Fellowship (J.S.), and
Weissberger Memorial Fellowship (J.S.). The authors also
thank Prof. Todd Krauss for help in making possible the
excited state lifetime measurements.
Supporting Information Available: Further details are given
in Figures S1-S16; crystallographic data is given in CIF file format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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1506 Inorganic Chemistry, Vol. 48, No. 4, 2009