Platinum(II) complexes of NCN- coordinating 1,3-bis(2-pyridyl)benzene ligands: Thiolate coligands lead to strong red luminescence from charge-transfer states

Article abstract of DOI:10.1021/ic500555w

A new family of platinum(II) complexes of the form PtLⁿSR have been prepared, where Lⁿ represents a cyclometalating, N CN-bound tridentate ligand and SR is a monodentate thiolate ligand. The complexes fall into two groups, those of PtL¹SR where HL¹ = 1,3-bis(2-pyridyl)benzene, and those of PtL²SR, where HL² = methyl 3,5-bis(2-pyridyl)benzoate. Each group consists of five complexes, where R = CH₃, C ₆H₅, p-C₆H₄-CH₃, p-C ₆H₄-OMe, p-C₆H₄-NO₂. These compounds, which are bright red, orange, or yellow solids, are formed readily upon treatment of PtLⁿCl with the corresponding potassium thiolate KSR in solution at room temperature. The replacement of the chloride by the thiolate ligand is accompanied by profound changes in the photophysical properties. A broad, structureless, low-energy band appears in the absorption spectra, not present in the spectra of PtLⁿCl. In the photoluminescence spectra, the characteristic, highly structured phosphorescence bands of PtLⁿCl in the green region are replaced by a broad, structureless emission band in the red region. These new bands are assigned to a φ_S/d_Pt → φ*N charge-transfer transition from the thiolate/platinum to the NN ligand. This assignment is supported by electrochemical data and TD-DFT calculations and by the observation that the decreasing energies of the bands correlate with the electron-donating ability of the substituent, as do the increasing nonradiative decay rate constants, in line with the energy-gap law. However, the pair of nitro-substituted complexes do not fit the trends. Their properties, including much longer luminescence lifetimes, indicate that the lowest-energy excited state is localized predominantly on the arenethiolate ligand for these two complexes. Red-emitting thiolate adducts may be relevant to the use of PtL ⁿCl complexes in bioimaging, as revealed by the different distributions of emission intensity within live fibroplast cells doped with the parent complex, according to the region of the spectrum examined.

Full text of DOI:10.1021/ic500555w

Article
pubs.acs.org/IC
Platinum(II) Complexes of N∧C∧N‑Coordinating 1,3-Bis(2-
pyridyl)benzene Ligands: Thiolate Coligands Lead to Strong Red
Luminescence from Charge-Transfer States
William A. Tarran,† Gemma R. Freeman,† Lisa Murphy,† Adam M. Benham,‡ Ritu Kataky,†
and J. A. Gareth Williams*,†
†Department of Chemistry, Durham University, Durham DH1 3LE, U.K.
‡School of Biological and Biomedical Sciences, Durham University, Durham DH1 3LE, U.K.
S
* Supporting Information
ABSTRACT: A new family of platinum(II) complexes of the
form PtLnSR have been prepared, where Ln represents a
cyclometalating, N∧C∧N-bound tridentate ligand and SR is a
monodentate thiolate ligand. The complexes fall into two
groups, those of PtL1SR where HL1 = 1,3-bis(2-pyridyl)-
benzene, and those of PtL2SR, where HL2 = methyl 3,5-bis(2-
pyridyl)benzoate. Each group consists of five complexes, where
R = CH3, C6H5, p-C6H4-CH3, p-C6H4-OMe, p-C6H4-NO2.
These compounds, which are bright red, orange, or yellow
solids, are formed readily upon treatment of PtLnCl with the
corresponding potassium thiolate KSR in solution at room
temperature. The replacement of the chloride by the thiolate
ligand is accompanied by profound changes in the photophysical properties. A broad, structureless, low-energy band appears in
the absorption spectra, not present in the spectra of PtLnCl. In the photoluminescence spectra, the characteristic, highly
structured phosphorescence bands of PtLnCl in the green region are replaced by a broad, structureless emission band in the red
region. These new bands are assigned to a πS/dPt → π*N C N charge-transfer transition from the thiolate/platinum to the N∧C∧N
ligand. This assignment is supported by electrochemical data and TD-DFT calculations and by the observation that the
decreasing energies of the bands correlate with the electron-donating ability of the substituent, as do the increasing nonradiative
decay rate constants, in line with the energy-gap law. However, the pair of nitro-substituted complexes do not fit the trends. Their
properties, including much longer luminescence lifetimes, indicate that the lowest-energy excited state is localized predominantly
on the arenethiolate ligand for these two complexes. Red-emitting thiolate adducts may be relevant to the use of PtLnCl
complexes in bioimaging, as revealed by the different distributions of emission intensity within live fibroplast cells doped with the
parent complex, according to the region of the spectrum examined.
∧
∧
energy states (e.g., those of metal-to-ligand charge-transfer or
ligand-centered character) can emit efficiently.6
INTRODUCTION
■
The development of brightly photoluminescent platinum(II)
complexes has been driven over the past 15 years by a number
of emerging applications, such as triplet-harvesting phosphors
for light-emitting devices,1 emissive units for chemosensing,2
probes for biological molecules, including nucleic acids and
proteins,3 and bioimaging agents.4 Cyclometalated complexes
of ligands featuring a combination of aryl and heterocyclic rings
that bind through a partnership of C and N atoms have been
featured in many studies.5 These ligands are typically both good
σ-donors, by virtue of the metalated carbon atom in particular,
and good π acceptors through the coordinated heterocycle.
Consequently, they are strong-field ligands that lead to large
separations between the highest occupied and vacant d orbitals.
This feature ensures that metal-centered excited stateswhich
are highly antibonding and therefore strongly deactivatingare
displaced to higher energy, making it more likely that lower-
Most cyclometalated platinum(II) complexes studied to date
have employed bidentate ligands such as 2-phenylpyridine
(ppy) and derivatives thereof, and indeed, such complexes are
frequently quite strongly luminescent.5 Nevertheless, bis-
bidentate complexes are potentially susceptible to a distortion
away from square planarity toward a D2d geometry in the
excited state, and such distortions may facilitate nonradiative
decay, imposing a limit on the luminescence efficiency.7 The
use of tridentate and tetradentate analogues can lead to more
rigid complexes and hence to brighter emission. In particular,
the platinum(II) complex of N∧C∧N-coordinated 1,3-bis(2-
pyridyl)benzene (PtL1Cl; Scheme 1) has a luminescence
quantum yield of 0.6 in solution at room temperature,8
1
Received: March 11, 2014
© XXXX American Chemical Society
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calculations, which reveal that the lowest-energy triplet excited
state on PtL1Cl is largely localized on the dipyridylbenzene,
albeit with sufficient metal contribution to promote the
formally forbidden radiative process.23 The coligand apparently
makes only a relatively minor contribution to the frontier
orbitals involved in the excitation. Here, we show that the
introduction of methanethiolate (MeS−) or a range of
arenethiolates (ArS−) into the coordination sphere of the
platinum ion has a profound influence on the excited-state
energy, strongly shifting the emission to the red region. A
combination of electrochemical measurements and TD-DFT
calculations allows the origins of the effect to be traced to a
fundamental change in the nature of the excited state when the
chloride ligand is replaced by a more electron rich sulfur atom.
Scheme 1. Synthesis of the Two Series of Thiolate
Complexes PtLnSAr from the Parent Chloro Complexes
a
PtLnCl (n = 1, 2)
a
The methanethiolate complexes were prepared similarly, but using an
aqueous solution of NaSMe.
RESULTS
■
Synthesis of Complexes. The 10 new complexes
produced and studied in this work are shown in Figure 1.
order of magnitude higher than that of its bidentate cousin
Pt(N∧C-ppy)(N-ppyH)Cl9 or its N∧N∧C-coordinated iso-
mer,10 despite the individual ligating units being identical.11
Moreover, the excited-state energy and hence color of such
tridentate complexes can be tuned over a wide range through
the introduction of substituents into the aryl or pyridyl rings,
without substantially compromising the quantum yields.12 The
low molecular weights and charge neutrality of these complexes
render them attractive for incorporation as triplet-harvesting
phosphors into vacuum-sublimed or spin-coated OLEDs, and
devices with impressive performance characteristics have been
obtained.13
There remain rather few platinum(II) complexes that emit
efficiently in the red region of the spectrum.1e Typically, the
quantum yields of red-emitting complexes tend to be limited
not by the energetic proximity of d−d states (problematic for
blue emitters) but by increased nonradiative deactivation
through intramolecular transfer of electronic energy into
vibrational energy. The so-called “energy-gap law”which
describes the increase in nonradiative decay with decreasing
excited-state energy14has been found to hold quite well for
many series of metal complexes,15 including some of platinum-
(II).16,17
One approach to obtaining red platinum(II)-based emitters
relies on the formation of excimers or aggregates, either
intermolecular or intramolecular, where two Pt units are
brought in close proximity and are able to interact, normally in
a face-to-face manner in such a way as to stabilize the excited
state.18 More recently, it has been shown that the presence of a
second metal ion in dinuclear Pt(II) complexes with bridging
cyclometalating ligands (e.g., those based on 4,6-diphenylpyr-
imidine and 2,3- and 2,5-diphenylpyrazine) also leads to
significant red shifts while retaining high quantum yields.19 In
both of these strategies for obtaining red emission, however,
two or more metal centers are evidently required.
The present work deals with the synthesis and the
photophysical and electrochemical characterization of two
new series of mononuclear N∧C∧N-coordinated platinum(II)
complexes, related to PtL1Cl and its 4-methyl ester analogue
PtL2Cl (Scheme 1), but in which the monodentate chloride
coligand is replaced by a thiolate. Previously, we have found
that the replacement of chloride in PtL1Cl by ligands such as
pyridines, methoxide, arylphosphines, and isothiocyanate has
relatively little effect on the excited-state energy.20,21 Other
researchers have likewise found that phenols and acetylides do
not have large effects on the emission energies.13a,12d,22 This
can be rationalized quite readily with the aid of TD-DFT
Figure 1. Structural formulas of the 10 new thiolate complexes
prepared in this work.
The arenethiolate complexes PtLnSAr (n = 1, 2; Ar = C6H5, p-
C6H4CH3, p-C6H4OMe, p-C6H4NO2) were readily prepared
from the parent chloro complexes PtL1Cl and PtL2Cl by
addition of a solution of the respective potassium thiolate
ArS−K+ in methanol at room temperature, itself prepared
immediately before use from the corresponding thiol ArSH by
treatment with potassium tert-butoxide (Scheme 1). The
thiolate complexes precipitated from solution as yellow, orange,
or red solids. Successive washing of the solids with water,
methanol, and diethyl ether led to analytically pure samples of
the desired complexes. Even upon treatment with the thiol
itself, without prior addition of base, partial conversion of the
starting chloro complexes to the thiolate derivatives was
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Table 1. Ground-State UV−Visible Absorption and Electrochemical Data of the Platinum(II) Thiolate Complexes in CH2Cl2
Solution at 298 K
ox
Ep
−
a
b
b
complex
absorption λmax/nm (ε/M−1 cm−1
)
E1/2red/V (Δ/mV)
Epox/V
E1/2red/V
PtL1SMe 234 (33700), 261 (31000), 290 (21300), 307 (13700), 332 (7490), 365 (5660), 377 (5980),
465 sh (2210)
−1.36
−0.36
−0.03
−0.10
−0.12
+0.26
1.00
PtL1SPh
236 (36400), 261 (35100), 282 (31300), 289 (31800), 306 sh (18500), 332 sh (8300),
363 (6420), 376 (6510), 452 sh (2040)
−1.39 (240)
−1.36 (220)
c
1.36
1.26
PtL1STol 236 (30100), 261 (29300), 280 (25700), 288 (25600), 306 sh (14700), 330 sh (7500),
363 (5700), 377 (5990), 460 sh (1820)
PtL1SAni 237 (41600), 262 (41400), 290 (32700), 306 (19400), 330 sh (10600), 365 (7490),
377 (7580), 463 sh (2450)
PtL1SNit 237 (35200), 258 (33000), 288 (24300), 305 (15500), 335 (8950), 376 sh (11000),
415 (17100), 455 sh (13600)
−1.36, −1.73 (160)
1.62
c
PtL2SMe 264 (54600), 285 (34000), 307 (19800), 330 sh (11600), 369 (6690), 468 (3250)
−0.35
−0.07
−0.11
−0.22
+0.13
PtL2SPh
266 (61600), 285 (45700), 305 (23600), 330 sh (12200), 360 (7350), 373 (7780), 465 (2890)
−1.44 (290)
−1.38 (280)
−1.43 (250)
−1.45 (200), −1.87 (90)
1.37
1.27
1.21
1.58
PtL2STol 266 (58400), 306 (21300), 332 sh (11100), 362 (7380), 375 (7820), 469 (2720)
PtL2SAni 265 (60100), 307 (21000), 332 sh (11600), 362 (7490), 376 (7660), 472 (3150)
PtL2SNit 262 (31500), 305 (17000), 333 sh (8030), 378 (11500), 415 (17800), 455 sh (13800)
a
Corresponding data for PtL1Cl: 238 (33000), 256 (25800), 277 (20400), 290 (21100), 332 (6510), 380 (8690), 401 (7010), 454 (270), 485 (240).
b
For PtL2Cl: 329 (7560), 380 (9990), 397 (7880), 446 (180), 478 (200). Using Bu4NPF6 (0.1 M) as the supporting electrolyte. Peak potentials are
given for the oxidations, all of which were irreversible, and for those reductions where the return wave was ill-defined. For reductions showing return
waves, the quoted values refer to E1/2 and the peak-to-peak separation is given in parentheses. Values refer to a scan rate of 100 mV s−1 and are
c
quoted relative to Fc+|Fc. The reduction wave was poorly defined for this complex.
observed. However, a substantial quantity of PtLnCl starting
material remained in this case, which proved to be difficult to
separate from the products. The methanethiolate complexes
PtLnSMe were similarly prepared by addition of aqueous
sodium methanethiolate to PtLnCl suspended in methanol.
bridged by SMe units. Also present in some spectra were more
conventional clusters such as [2M + Na]+ and [3M + Na]+.
In the solid state, the thiolate complexes are indefinitely
stable: no decomposition has been observed over a period of 9
years. In solution, however, the compounds degrade at a rate
that varies significantly with the solvent. In d3-acetonitrile, for
1
The identity of the new complexes was confirmed by H
1
example, new sets of peaks begin to appear in the H NMR
NMR spectroscopy, mass spectrometry, and elemental analysis.
1
spectra over a period of several minutes. This degradation also
occurs in degassed solutions. The process is slower in
dichloromethane, allowing some physicochemical character-
ization to be achieved in this solvent (see below). Of the
common solvents tested, the stability appears to be highest in
d6-dimethyl sulfoxide. The identity of the decomposition
products remains somewhat elusive, but the process probably
involves decoordination of the thiolate facilitated by the well-
known high trans effect of cyclometalated carbon atoms. van
Eldik and co-workers have previously studied the rate of
displacement of chloride from Pt(N∧C∧N)Cl by nucleophiles
and found that the reactivity may be 103−104 times higher than
that of isomeric Pt(N∧N∧C)Cl complexes.25
The atom-numbering scheme for the H NMR assignments is
1
shown in Figure 1. The H NMR spectra of the arenethiolate
complexes displayed a pattern of resonances similar to that of
the parent chloro complexes, with the additional set of expected
aromatic signals due to the aryl ring of the thiolate. In the case
of PtLnSTol, a weak coupling in the COSY spectrum between
the tolyl methyl group and H3″ allowed unambiguous
assignment of the two pairs of doublets in the thiolate aryl
ring, that nearest the sulfur atom resonating at higher
frequency. The use of conventional electron beam ionization
(EI) mass spectrometry proved to be surprisingly successful at
identifying the desired products, revealing the molecular ion for
each arenethiolate complex, together with some fragmentation
peaks (e.g., loss of thiolate, loss of CO2Me in the L2 series,
tropylium ion in the tolyl complexes). In contrast, electrospray
(ES) mass spectrometry generally failed to detect the molecular
ion, M+, although a number of cluster species were observed.
Mass and isotope patterns indicated various dimetallic clusters
incorporating complete or partially fragmented thiols. A feature
common to several of the ES mass spectra recorded was the
appearance of a pair of cluster ions comprising two Pt(N∧C∧N)
units and one intact thiolate (RS) either with or without an
additional sulfur atom. It is likely that these species involve
bridging sulfurs between two Pt centers, and indeed, in the
chemistry of related platinum terpyridyl complexes, sulfur-
bridged species such as {[Pt(tpy)]3S} have been unambigu-
ously characterized by X-ray diffraction.24 The ES mass
spectrum of PtL1SMe showed a series of peaks representing
[3M]+, [3M − Me]+, [3M − 2Me]+, and [3M − 3Me]+. A
likely structure of the gas-phase species formed that would lead
to such a pattern is a ring comprising three Pt(N∧C∧N) units
Electrochemistry of the Complexes and Frontier
Orbital Description. The ground-state redox potentials of
the complexes were measured by cyclic voltammetry in
dichloromethane solution, in the presence of Bu4NPF6 (0.1
M) as the supporting electrolyte. Data are reported in Table 1
relative to the ferrocene|ferrocenium couple (Fc|Fc+) under the
same conditions. All of the complexes display an irreversible but
well-defined oxidation wave, in the region −0.4 to +0.3 V. In all
cases, the oxidation is cathodically shiftedoccurring at less
positive/more negative potentialsin comparison to the
ox
parent chloro complexes (Ep = +0.35 and +0.39 V for
PtL1Cl and PtL2Cl, respectively). Among the four arenethiolate
complexes of a given Ln, the trend in oxidation potentials (i.e.,
R = OMe < Me < H < NO2) qualitatively reflects the electron-
donating character of the thiolate substituent (i.e., R = OMe >
Me > H > NO2). The two methanethiolate complexes,
ox
PtLnSMe, are the most readily oxidized complexes, with Ep
values around −0.35 V. For the two series PtL1SAr and
PtL2SAr, it is seen that those of L2 are slightly shifted
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cathodically in comparison to their L1 counterparts, although
the differences are small.
The UV−visible spectra of all five PtL2SR complexes are
shown in Figure 3a, and numerical data are compiled in Table
All of the arenethiolate complexes display a first reduction
wave at around −1.40 V (the potentials are within 50 mV of
this value in each case; Table 1). This process is partially
reversible at higher scan rates, with a peak-to-peak separation of
around 200−300 mV. At lower scan rates, typically <100 mV
s−1, the return wave becomes poorly defined.
Pioneering studies by Eisenberg and co-workers26 and by
Weinstein and colleagues27 have previously examined com-
plexes of the form Pt(N∧N)(S∧S), where N∧N represents a
diimine ligand such as bipyridine and S∧S a chelating dithiolate
or a pair of monodentate thiolate ligands. These provide an
interesting point of comparison with the present systems. They
too typically display an irreversible oxidation process and one
or two reversible or partially reversible reduction processes.
Similar results have been reported for terpyridyl analogues of
the form [Pt(N∧N∧N)SR]+.28 Detailed studies involving
techniques such as EPR and resonance Raman spectroscopy,
in conjunction with TD-DFT calculations, have indicated that
the reduction is based primarily on the diimine (or triimine)
ligand, while the oxidation involves the metal and the thiolate
ligands.29,27 Similar conclusions are made in the present case, as
discussed in the sections that follow.
UV−Visible Absorption Spectra. The replacement of the
chloride ligand in PtLnCl by the thiolates has a striking effect on
the absorption, as is immediately evident during the syntheses:
the products have intense red, orange, and yellow colors, in
contrast to the pale yellow of the starting materials. The
spectral origin of the observed change is highlighted in Figure
2, where the absorption spectrum of PtL2SPh (used as a
Figure 3. (a) Absorption spectra of PtL2SR complexes in CH2Cl2 at
298 K (the inset shows an expansion of the region of the low-energy
bands around 465 nm) and (b) the corresponding spectra of PtL1SR
complexes.
1. It can be seen that the spectra of PtL2STol and PtL2Ani are
very similar to that of PtL2SPh, with a small shift of the long-
wavelength band toward the red, in the λmax order phenyl <
tolyl < anisyl. The spectrum of PtL2SMe is also very similar. In
contrast, the spectrum of PtL2SNit is strikingly different in the
region λ >350 nm, displaying a very intense band at 415 nm (ε
= 17800 M−1 cm−1), on which are superimposed bands at 378
and 455 nm which may correspond to those seen at such
wavelengths in the other four thiolate complexes.
An essentially identical picture emerges from the spectra of
the corresponding complexes of L1 (Figure 3b), the spectrum
of the nitro complex again being very different from those of
the other four complexes. In this case, the long-wavelength
band in each of the phenyl, tolyl, anisyl, and methyl complexes
is slightly blue-shifted in comparison to its L2 counterpart, and
the overlap with the higher-energy bands renders it more of a
shoulder than a clear-cut band.
The appearance of the long-wavelength band in the region
452−472 nm that accompanies the change from chloride to
thiolate can be rationalized in terms of a new, low-energy
charge-transfer state, as observed in the related diimine
complexes referred to above.26−29 The electron-rich sulfur
atoms are expected to lead to a HOMO that will span the
thiolate and the metal atom, in contrast to the parent chloride
Figure 2. Absorption and emission spectra of PtL2Cl (green lines) and
PtL2SPh (red lines) in CH2Cl2 at 298 K.
representative model for the thiolate complexes) is shown
together with that of its parent chloro complex PtL2Cl. The two
complexes have in common a set of intense bands in the far-UV
(λ <300 nm). They also both feature a band at around 375 nm,
of fairly similar intensity. However, while the longest-wave-
length, spin-allowed band in PtL2Cl is a shoulder at 400 nm,
PtL2SPh displays a well-defined, intense, broad band centered
at around 465 nm, ε ≈ 3000 M−1 cm−1, for which there is no
counterpart in PtL2Cl (note that the very weak, sharp bands at
446 and 478 nm in PtL2Cl are spin-forbidden excitations to
triplet ligand-centered states8).
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complex, where the most electron rich part of the molecule
comprises the metal and cyclometalating aryl ring.23 Mean-
while, the LUMO can be expected to remain localized largely
on the pyridyl rings of the N∧C∧N ligand. A low-energy excited
state of dPt/πRS → π*NCN character may thus be anticipated,
and indeed, such a conclusion is supported by time-dependent
density functional theory calculations (vide infra).
Further support for such an assignment comes from the
variation in λmax with substituent, either in the aryl ring of the
thiolate or in the N∧C∧N ligand. Thus, the red shift in the L2
complexes versus those with the corresponding thiolate in the
L1 series can be understood in terms of the electron-
withdrawing influence of the ester group slightly stabilizing
the N∧C∧N-based LUMO. Meanwhile, the order of λmax phenyl
< tolyl < anisyl is attributed to the effect of the increasingly
electron-donating substituent in destabilizing the HOMO.
Indeed, although the differences between the λmax values are
small, there appears to be a correlation of the absorption energy
with |Eox − Ered| (Figure 4), consistent with the notion that the
that responsible for the well-known color of the 4-nitro-
phenolate anion. Similarly intense bands in this region have
been observed previously in complexes incorporating this
ligand both with Pt(II)27a and with other metal ions.30 Notably,
this band shows little solvatochromism (as exemplified by the
spectra of PtL2SNit in different solvents, Figure S2 (Supporting
Information)), in line with a more limited and localized
redistribution of the electron density in the molecule. TD-DFT
calculations support this assignment (see Time-Dependent
Density Functional Theory (TD-DFT) and Interpretation of
Emission Data below).
Photoluminescence. Experimental Observations. All of
the new complexes are luminescent in solution upon UV/blue-
light excitation, emitting in the red region of the spectrum.
Reference to Figure 2 once again makes clear the profound
effect of the substitution of chloride by thiolate on the emission
spectrum. Whereas the chloro complex PtL2Cl displays
vibronically structured bands in the green region (λ0−0 481
nm) with a luminescence lifetime of 8 μs, the emission of
PtL2SPh is shifted far to the red (λmax 643 nm) and comprises a
broad, unstructured band with a lifetime that is 1 order of
magnitude shorter (τ = 880 ns). The luminescence quantum
yield is 0.16. Although it is lower than that of the parent chloro
complex, it should be noted that this value is exceptionally high
for such a deeply red emitting mononuclear Pt(II) complex.
Quantum yields of red phosphors are invariably compromised
by increased nonradiative decay processes, a point returned to
in more detail below. The complex PtL1SPh displays similar
emission characteristics (λmax 634 nm; τ = 770 ns; Φ = 0.17).
The intensity-normalized emission spectra of the family of
five PtL2SR complexes are shown in Figure 5a and those of
PtL1SR in Figure 5b. The emission maxima, luminescence
lifetimes, and quantum yields are compiled in Table 2. A
number of trends emerge from these data.
−
(i) For a given thiolate SR, the emission maximum is red-
shifted for PtL2SR in comparison to PtL1SR, with the exception
of the pair of nitro-substituted complexes, where the order is
reversed.
Figure 4. Energy of absorption (top) and emission (bottom) versus
the difference between the oxidation and reduction potentials of
PtL2SR. The blue dotted line through the absorption data points
represents the linear best fit for all four complexes (gradient 2100
cm−1/V). The red dashed line for the emission data represents the best
fit for the phenyl, tolyl, and anisyl complexes, excluding the nitro
analogue (gradient 7800 cm−1/V).
(ii) For both series of arenethiolate complexes (n = 1, 2), the
emission maxima are increasingly red-shifted in the λmax order
PtLnSPh < PtLnSTol < PtLnSAni < PtLnSNit. For the first three
complexes in the PtL2SAr series, there is a convincing linear
correlation between the emission energy and |Eox − Ered|, but
the nitro-substituted complex clearly does not follow the trend
(Figure 4). The best fit line through the data points for the
complexes excluding the nitro-substituted compound gives a
gradient of 7800 cm−1/V.
orbitals involved in the excitation are similar to those ionized
and populated through electrochemical oxidation and reduc-
tion, respectively.
(iii) For both series of arenethiolate complexes, the
luminescence lifetime of the nitro-substituted complex is
much longer than those of the other three complexes. The
lifetimes of PtL1SNit and PtL2SNit are 7.4 and 11.3 μs,
respectively, whereas all the other complexes have lifetimes <1
μs. Among the other complexes, the order of luminescence
lifetimes for both series (n = 1, 2) is PtLnSPh > PtLnSTol >
PtLnSAni.
Charge-transfer excited states frequently give rise to
significant solvatochromism, if the redistribution of electron
density in the excited state is noncentrosymmetric, as in this
instance. The limited solubility of the compounds and the
propensity to degradation in some solvents hampers a detailed
investigation, but using as an example PtL2SPh in four solvents
of differing polarity, it can be seen that there is significant
negative solvatochromism in the low-energy absorption band
(see Figure S1 and Table S1 in the Supporting Information).
The λmax value shifts from 494 nm in toluene to ∼453 nm in
acetonitrile, a destabilization of around 1800 cm−1.
Finally, the much more intense band that appears around
415 nm in the complexes containing the nitro-substituted
thiolate ligand can be attributed to a strongly allowed
intraligand charge-transfer (ILCT) transition within the 4-
nitrobenzenethiolate ligand, i.e., π(SC6H4) → π*(NO2), akin to
(iv) The luminescence quantum yields in both series also
follow the order PtLnSPh > PtLnSTol > PtLnAni, as observed
for the lifetimes. The nitro-substituted complexes have small
quantum yields, similar to those for the anisyl-subtituted
analogues, despite their long lifetimes.
(v) The spectrum of PtL2SMe is essentially identical with
that of PtL2SPh, and likewise the spectrum of PtL1SMe is
similar to that of PtL1SPh. However, the methanethiolate
E
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Figure 6. Emission spectra of PtL2SR complexes at 77 K in EPA, λex
=
460 nm (EPA = diethyl ether/isopentane/ethanol, 2:2:1 v/v).
complexes (3400 cm−1), most of the others having values of
around 2500 cm−1. The lifetimes of the nitro-substituted
complexes at 77 K are conspicuously very long, around 2 orders
of magnitude longer than those of the other complexes.
Assuming that the excited state is formed with unitary
efficiency, we can estimate the radiative, kr, and nonradiative,
∑knr, decay rate constants from the experimentally determined
quantum yields and lifetimes: kr = Φ/τ and ∑knr = τ−1 − kr.
The data (Table 2) reveal that the radiative rate constants for
the nitro complexes are 2−3 orders of magnitude smaller than
for the other arenethiolate and methanethiolate complexes.
Time-Dependent Density Functional Theory (TD-DFT) and
Interpretation of Emission Data. These observations can be
rationalized successfully with the aid of time-dependent density
functional theory (TD-DFT) calculations, to evaluate the
nature of the lowest-energy triplet state, T1, from which
emission occurs. The triplet state geometry was first optimized
by direct calculation (i.e., by minimization of the geometry with
a triplet spin), and then frequency calculations were utilized to
confirm that the geometries obtained were true minima. TD-
DFT calculations were performed on the optimized geometries
to give the frontier orbitals and the relative contributions of
transitions between filled and virtual orbitals to the lowest
Figure 5. (a) Emission spectra of PtL2SR complexes in degassed
CH2Cl2 at 298 K and λex = 460 nm and (b) the corresponding spectra
of PtL1SR complexes.
complexes are weaker emitters than their benzenethiolate
analogues, with lower quantum yields and shorter lifetimes.
(vi) All spectra are strongly blue-shifted in a glass at 77 K in
comparison to the spectra at room temperature (Table 2;
spectra for PtL2SAr series are shown in Figure 6 and for
PtL1SAr in Figure S3 (Supporting Information)). The temper-
ature-induced shifts are largest for the two nitro-substituted
a
Table 2. Luminescence Data for the Platinum Complexes in CH2Cl2 Solution at 298 K and in EPA at 77 K
emission 77 K
b
c
c
d
complex
PtL1Cl
emission λmax/nm
Φlum × 102
τ/ns
kr/103 s−1
∑knr/104 s−1
λmax/nm
ΔE77K−298 K/cm−1
τ/ns
6400
491, 524, 562
60
7200
430
83
60
5.5
230
110
180
2100
13
486, 516, 548
538
210
2860
2780
3120
3070
3420
130
PtL1SMe
PtL1SPh
PtL1STol
PtL1SAni
PtL1SNit
PtL2Cl
PtL2SMe
PtL2SPh
PtL2STol
PtL2SAni
PtL2SNit
636
2.6
17
6600
6800
7400
8200
634
770
220
140
290
0.28
72
539
661
6.9
1.4
0.21
58
510
548
691
48
570
714
7400
8000
510
574, 610
478, 510, 547
558
7.6 × 105
481, 513, 550
642
5.3
7000
6600
7800
8900
9500
3.8
16
75
190
95
2340
2630
2240
2490
3410
643
880
180
160
120
0.57
550
668
6.0
0.35
0.64
370
250
3400
8.8
581
701
29
597
703
11300
567, 603
4.7 × 105
a
b
c
EPA = diethyl ether/isopentane/ethanol, 2/2/1 v/v. Measured using [Ru(bpy)3]Cl2 as the standard. kr and ∑knr are the radiative and
d
nonradiative rate constants estimated from the quantum yield and lifetime at 298 K. Difference in energy of emission at 77 K in comparison to 298
K estimated using the λmax values.
F
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Figure 7. Density difference plots for the triplet states of PtL1SAr, obtained by TD-DFT using PBE0 and the PCM model for dichloromethane as the
solvent. Purple and yellow represent zones of augmentation and depletion of electron density, respectively, in the excited state relative to the ground
state.
triplet excited states. Frontier orbitals and tables of contribu-
tions for the PtL1SR series of complexes are shown in Figure S4
and Table S2 in the Supporting Information. The correspond-
ing density difference plotswhich take into account all of the
contributions and hence provide a more informative picture of
the rearrangement of electron density in the exctied stateare
shown in Figure 7. They show that the excited state has dPt/πRS
→ π*NCN character for the phenyl, tolyl, and anisyl complexes.
A directional charge-transfer excited state of this type readily
accounts for several of the experimental trends in emission data
for the complexes with these substituents listed above. (i) The
lower energy of each PtL2SR complex in comparison to its
PtL1SR analogue (except R = Nit; see below) can be
rationalized in terms of the electron-withdrawing ester group
stabilizing the NCN-based LUMO. The effect is, however,
rather small, since the ester lies in the central aryl group rather
than the pyridyl rings which dominate the LUMO. (ii) The
trend of decreasing emission energy in the order SPh > STol >
SAni is consistent with the increasing electron-donating power
of the substituent, which will favor charge transfer from the
thiolate. The gradient of 7800 cm−1/V from the plot of Eem
versus |Eox − Ered| mentioned above is quite large, suggesting a
significant charge-transfer character. Indeed, the value is
remarkably similar to that of 7300 cm−1/V found for a
selection of 13 complexes of the form Pt(N∧N)(S∧S) studied
by Cummings and Eisenberg.26d In that case too, all the
evidence pointed toward a charge-transfer-to-diimine assign-
ment to the lowest-energy singlet and triplet excited states.
Finally, observation vi, namely the large blue shift of the
emission maxima observed on cooling to 77 K, is consistent
with an excited state in which there is a high degree of charge
transfer in a well-defined direction. The substantial rearrange-
ment of electron density that is associated with the formation of
such an excited state is accompanied by significant solvent
reorganization to stabilize the excited state (large Stokes shift).
Such stabilization is inhibited in a frozen glass, leading to a large
temperature-induced shift, as observed here. The contrast with
PtL1Cl and PtL2Cl is particularly dramatic in this respect: they
show only very small shifts on cooling to 77 K (Table 2),
π*NCN, is the relative contribution of the metal and ligand: i.e.,
the Pt and RS− units. In other words, is assignment as MLCT
or LLCT more appropriate? The Mulliken charges on the metal
and the two different ligands in each of the PtL1SAr complexes
are given in Table S3 of the Supporting Information. They
provide a crude, albeit convenient, means of quantifying the
relative importance of metal and thiolate orbitals. It can be seen
that the relative contribution of the thiolate increases, and that
of the metal decreases, on going from phenyl through tolyl to
anisyl. One could thus describe a trend toward increasing
LLCT character at the expense of MLCT character as the
electron-donating power of the thiolate increases.
The TD-DFT results for the nitro-substituted complex give a
strikingly different picture; in this case, the excited state is seen
to be localized on the arenethiolate, with essentially no
involvement of either the metal or the N∧C∧N ligand. The
most fitting description is that of a π(SC6H4) → π*(NO2)
transition, in line with that observed in the previously reported
complexes of this ligand mentioned above.27a,30 The Mulliken
charges are again useful, in this case in highlighting the lack of
involvement of the N∧C∧N ligand.
The differing nature of the excited state in PtLnSNit accounts
for the observed differences in the photoluminescence
properties for the nitro complexes in comparison to the others
listed above. The deviation from the Eem versus |Eox − Ered| plot
(Figure 4, bottom) is accounted for. The slightly higher energy
of the emission of PtL2SNit compared to PtL1SNit can be
explained by a small influence of the ester in stabilizing the Pt/S
filled orbitals (HOMO), but with no effect on the nitrophenyl-
based LUMO. Moreover, the much longer luminescence
lifetimes observed for the nitro complexes (both at room
temperature and at 77 K) can be explained, in part, by the more
limited participation of the metal in the excited state. Significant
involvement of the metal is required to successfully promote
the formally forbidden T1 → S0 radiative transition,31 whereas
the TD-DFT (Figure 7) indicates an essentially ligand-centered
(ILCT) transition. As a result, the kr values for these two
complexes are much lower than for all the others.
It is notable, however, that this complex seems to “fit” well
with the Eabs versus |Eox − Ered| plot (Figure 4, top). Indeed, in
UV−Visible Absorption Spectra, we interpreted the absorption
spectrum in terms of a lowest-energy transition to a singlet
excited state having the same charge transfer to N∧C∧N
3
consistent with the largely N∧C∧N-based π−π* assignment of
the excited state in these parent chloro complexes.23
A point to consider in excited states with heavily mixed
contributions of metal and ligand, as in this case of dPt/πRS
→
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character as that of the other complexes, superimposed on a
more intense but higher-energy transition more localized on
the nitrobenzenethiolate ligand. Such a difference between
absorption and emission (i.e., differing orbital parentage of the
S1 and T1 excited states) is quite plausible. Excited states with a
quenching observed in solution at room temperature over a
20-fold concentration range (up to 4 × 10−4 M), nor was there
was any evidence for excimer emission.33 The lack of effect
could be associated with the greater steric hindrance
engendered by the arenethiolate to the face-to-face approach
of pairs of molecules, which is necessary for excimers to form
(the optimum distance is typically around 3.5 Å). It should also
be noted that the excited state lifetime in the thiolate complexes
is 1 order of magnitude shorter than in PtL1Cl, and the
propensity to excimer formation will necessarily decrease for
that reason as well. Steric hindrance to interfacial interactions
should be minimal in the case of the methanethiolate
complexes, but again, no significant concentration-induced
quenching was observed. However, for this pair of complexes it
was noted that, at 77 K, elevated concentrations led to the
appearance of an additional low-energy emission band (Figure
S5). The excitation spectra registered for the two emission
bands are slightly different from one another around the low-
energy tail, suggesting that the additional band may be due to
the formation of a ground-state aggregate. No such behavior
was observed for the arenethiolate complexes.
high degree of charge transfer have smaller S−T energy gaps
3
than those which are more localized, and hence [dPt/πRS
→
π*NCN] may be higher in energy than 3[π(SC6H4) →
π*(NO2)] even though 1[dPt/πRS → π*NCN] is lower in energy
1
than [π(SC6H4) → π*(NO2)].
Nonradiative Decay Trends. It is instructive to inspect in
more detail the nonradiative decay rate constants. Typically, for
compounds with a mutually common type of excited state, the
nonradiative decay constant should increase as the excited state
energy decreases, owing to the increased probability of
intramolecular energy transfer into high-energy vibrational
modes within the molecule. Detailed quantitative treatments
indicate that a logarithmic dependence may be anticipated,14 as
observed in some classic studies with ruthenium and with
platinum complexes, for example.15−17 Figure 8 illustrates a plot
Relevance to Biological Thiols and Bioimaging
Applications. PtL1Cl and its derivatives have been inves-
tigated as cell imaging agents.4a,e The studies to date reveal that
they localize in the nuclei of a variety of cell lines, from which
intense green emission with the characteristic spectrum of the
Pt(N∧C∧N) unit is observed, with a lifetime of around 1 μs.
The fate of the rather labile Pt−Cl bond is, however, rather
uncertain, and it is possible that the observed nuclear
localization is due to platination of nucleobases or histidine
residues through their nitrogen atoms. In the event that the
Pt−Cl bond is displaced by a thiolate group of a cysteine
residue within a protein, however, green emission might no
longer be expected, but rather a shift to red emission, on the
basis of the results described in Photoluminescence and
illustrated in Figure 2.
In order to explore this possibility, NIH 3T3 cells incubated
with PtL1Cl were imaged using fluorescence microscopy in two
different regions of the spectrum. When the emission was
monitored in the green region, the typical nuclear staining seen
previously4a was observed (Figure 9a). However, when the
emission was monitored using a longer-wavelength emission
filter (660−710 nm) that completely eliminates green light but
transmits only red light, a distinctly different localization
pattern was revealed (Figure 9b). Emission in that case was
observed mainly within the cytoplasmic region. The cytoplas-
mic emission is not homogeneous in the image: emission
emanates from discrete areas within a cytoplasm-based
organelle. Costaining of CHO cells with PtL1Cl and
MitoTracker Red (MTR) suggests that the organelles in
question are the mitochondria (Figure S6 (Supporting
Information)).
It is notable that the mitochondria typically contain an
abundance of thiols.34 Indeed, recent work indicates that the
labeling of protein targets by MTR involves covalent bond
formation and is related to the availability of cysteine residues.35
Meanwhile, Coogan and co-workers have developed rhenium
complexes to stain mitochondria in which protein thiols react
with a 3-chloromethylpyridyl group on the complex.36 Thus, it
seems likely that the emission detected in Figure 9b is due to
thiolate adducts of the form PtL1SR. We note that Chan and
co-workers have observed a diminution of the green
luminescence from Pt(N∧C∧N)Cl units upon treatment with
Figure 8. Plot of ln knr versus the emission energy. The complexes of
L1 are represented by red circles and those of L2 by blue squares. The
dashed black line is the least-squares linear fit through all data points
except for the two nitro-substituted complexes (which appear in the
bottom left of the graph).
of {ln knr} versus the excited state energy, estimated from the
emission maxima,32 for the eight aryl-substituted complexes. It
can be seen that there is a convincing linear relationship, but
only if the nitro-substituted compounds are excluded. They are
seen to have much lower nonradiative decay constants than
would be anticipated for their emission energies, if they had
excited states similar to those of the other aryl-substituted
complexes. Clearly, this observation provides further evidence
in support of a very different nature of the excited state in these
two complexes.
Intermolecular Interactions. The parent complex PtL1Cl is
known to display a quite sensitive dependence of the emission
spectrum upon concentration. The lifetime decreases with
concentration at values greater than around 10−5 M,
accompanied by the appearance of an excimer emission band
centered at about 700 nm.8 Derivatives incorporating
substituents in the tridentate ring behave similarly, albeit
typically with smaller Stern−Volmer quenching constants and,
in some cases, significantly shifted excimer bands.12 For the
present set of complexes, there was no significant self-
H
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Figure 9. Fluorescence microscopy images of NIH 3T3 cells incubated with PtL1Cl (50 μM, 5 min): (left) image acquired using a G365ex/FITCem
filter set (green emission region only); (right) image acquired using a G365ex/660−710em filter set (red emission only).
with full assignment only in the cases indicated. For the other
complexes (for which either the solubility is lower or the stability
inferior), impurities begin to form during the time necessary to acquire
the 13C spectrum, hampering the assignment. Chemical shifts (δ) are
in ppm, referenced to residual protio-solvent resonances, and coupling
constants are in hertz. Electron ionization mass spectra were acquired
at the EPSRC National Mass Spectrometry Service Centre, Swansea,
U.K., using perfluorotributylamine as the standard for accurate mass
measurements. All solvents used in preparative work were at least
Analar grade, and water was purified using the Purite system. Solvents
used for optical spectroscopy were HPLC grade. In all but three cases,
elemental analyses gave percent C, H, N data to within the normally
accepted limit of 0.4% of the theoretical values. The exceptions were
PtL1SNit and the complexes of methanethiolate, PtL1SMe and
PtL2SMe, where percent C deviations were slightly larger. Repeated
attempts to obtain improved quality samples failed to give better data,
mostly likely due to traces of the decomposition product forming in
solution, as discussed in Synthesis of Complexes.
cysteine, interpreted in terms of displacement of the chloride
ligand by a cysteine sulfur atom.37 In their case, no red emission
appeared, but this may be due to efficient quenching of the
emission under the polar aqueous conditions employed.
Concluding Discussion. PtL1Cl and derivatives featuring
modified tridentate ligands have emerged over the past decade
as a family of complexes that offer unusually high
phosphorescence quantum yields. However, the emission is
typically limited to the green or yellow region, and the
replacement of the chloride ligand by other anionic and neutral
ligands, such as phenolates and pyridine derivatives, respec-
tively, has little effect, since the excited state is based primarily
on the Pt(N∧C∧N) unit. The present work has revealed that
the use of electron-rich thiolates as monodentate coligands
leads to a profound change in the nature of the lowest-energy
singlet and triplet excited states, to ones which feature charge
transfer from the thiolate (mixed with some metal character) to
the tridentate ligand. This leads to a red shift in the absorption
and phosphorescence. Although the luminesence quantum
yields are reduced in comparison to those of the parent
complexes, the values of around 0.16 for the benzenethiolate
complexes are unusually high for platinum(II) complexes that
emit at such low energy, deep into the red region of the
spectrum. The ability to further tune the properties of this
interesting class of complex may open up new potential, for
example, in the fields of solar energy conversion and
bioimaging, where lower-energy absorption and emission,
respectively, are important. Although the stability of the
complexes in solution is rather low, it may be noted that the
previously reported instability of the corresponding phenolate
complexes has been successfully overcome by covalent linkage
of the phenolate to the N∧C∧N unit, to generate a tetradentate
ligand.38 A similar strategy involving arenethiolates looks set to
offer a promising approach to stable, red-emitting platinum(II)
complexes.
PtL1SMe. Sodium methanethiolate (50 mg, 0.15 mmol, 21% w/w
in H2O) was dissolved in methanol (5 mL) and the solution degassed
via four freeze−pump−thaw cycles prior to back-filling the reaction
vessel with nitrogen gas. PtL1Cl (63 mg, 0.l4 mmol) was added under
a flow of nitrogen, and the suspension was stirred at room temperature
for 18 h. The solid was separated using a centrifuge, washed
successively with methanol (5 mL), water (3 × 5 mL), and diethyl
ether (2 × 5 mL), and finally dried under reduced pressure, giving the
desired product as an orange solid (26 mg, 40%). 1H NMR (400 MHz,
d6-DMSO): δ 9.40 (2H, d, 3J = 4.8, 3J(195Pt) = 42, H6), 8.19 (2H, t, 3J
= 8.0, H4), 8.11 (2H, d, 3J = 6.4, H3), 7.81 (2H, d, 3J = 7.6, H3′), 7.53
(2H, t, 3J = 6.4, H5), 7.29 (1H, t, 3J = 7.6, H4′), 2.23 (3H, s, CH3). 13
C
NMR (125.7 MHz, d6-DMSO): δ 167.9, 152.2, 140.9, 140.2, 124.8,
124.1, 123.4, 120.5, 12.6. Anal. Calcd for C17H14N2PtS: C, 43.13; H,
2.98; N, 5.92%. Found: C, 42.57; H, 2.90; N, 5.75%.
PtL1SPh. Thiophenol (47 mg, 0.42 mmol) was dissolved in
methanol (6 mL) and the solution degassed via four freeze−pump−
thaw cycles before being placed under an atmosphere of nitrogen.
Potassium tert-butoxide (48 mg, 0.43 mmol) was added under a flow
of nitrogen gas, and the mixture was stirred for 5 min followed by
addition of PtL1Cl (129 mg, 0.28 mmol). The suspension was stirred
at room temperature for 16 h under nitrogen. The solid was then
separated by centrifugation, washed successively with methanol and
diethyl ether (3 × 5 mL of each), and dried under vacuum, to give the
EXPERIMENTAL SECTION
■
The complexes PtL1Cl and PtL2Cl were prepared as described
previously.39,8 1H and 13C NMR spectra, including NOESY and
COSY, were recorded on Varian or Bruker spectrometers operating at
the frequencies indicated below. It was possible to obtain 13C spectra
1
product as a yellow solid (120 mg, 80%). H NMR (500 MHz, d6-
DMSO): δ 9.19 (2H, d, 3J = 5.5, 3J(195Pt) = 38, H6), 8.14 (4H,
overlapping m, H3 and H4), 7.81 (2H, d, 3J = 7.5, H3′), 7.51 (2H, d, 3J
I
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= 7.5, H2″), 7.40 (2H, m, H5), 7.31 (1H, t, 3J = 7.5, H4′), 6.94 (2H, t,
PtL2STol. This complex was prepared similarly, from PtL2Cl (98
mg, 0.19 mmol), 4-methylthiophenol (50 mg, 0.40 mmol), and KOtBu
(50 mg, 0.045 mmol), leading to the product as a yellow-orange solid
3
3J = 7.5, H3″), 6.79 (1H, t, J = 7.5, H4″). 13C NMR (HSQC observe
1
1H 500 MHz, decouple 13C 125.7 MHz and HMBC observe H 500
1
3
MHz, d6-DMSO): δ 169.1 (C1′), 167.8 (C2), 152.7 (C6), 148.8 (C1″),
141.2 (C2′), 140.3 (C4), 132.2 (C2″), 127.6 (C3″), 124.8 (C3′), 124.1
(C5), 123.8 (C4′), 121.1 (C4″), 120.5 (C3). MS (EI): m/z 535 M+, 426
[M − SPh]+. HRMS (EI): m/z 535.0675, calcd for C22H16N2SPt m/z
535.0676. Anal. Calcd for C22H16N2PtS: C, 49.34; H, 3.01; N, 5.23.
Found: C, 48.96; H, 2.95; N, 5.14.
(78 mg, 68%). H NMR (400 MHz, d6-DMSO): δ 9.22 (2H, d, J =
3
3
4.0, J(195Pt) = 28, H6), 8.40 (2H, s, H3′), 8.32 (2H, d, J = 8.5, H3),
8.20 (2H, t, J = 8.5, H4), 7.48 (2H, t, J = 6.0, H5), 7.38 (2H, d, J =
3
3
3
8.5, H2″), 6.79 (2H, d, 3J = 8.5, H3″), 3.92 (3H, s, OCH3), 2.13 (3H, s,
CH3). MS (EI): m/z 607 (M+), 484 (M+ − thiolate), 425 (M+
−
+
thiolate − CO2Me), 124 (thiol+), 91 (C7H7 ). HRMS (EI): m/z
607.0889 (M+), calcd for C25H20N2O2PtS 607.0888. Anal. Calcd for
C25H20N2O2PtS: C, 49.42; H, 3.32; N, 4.61. Found: C, 49.20; H, 3.29;
N, 4.42.
PtL1STol. This complex was prepared in a manner similar to that
for PtL1SPh, starting from PtL1Cl (128 mg, 0.28 mmol), 4-
methylthiophenol (55 mg, 0.44 mmol), and KOtBu (60 mg, 0.54
PtL2SAni. The title complex was prepared in the same way, starting
from PtL2Cl (50 mg, 0.096 mmol), 4-methoxythiophenol (28 mg, 0.20
mmol), and KOtBu (24 mg, 0.21 mmol), leading to the product as a
red solid (43 mg, 71%). 1H NMR (400 MHz, d6-DMSO): δ 9.19 (2H,
d, 3J = 5.6, 3J(195Pt) = 38, H6), 8.38 (2H, s, H3′), 8.31 (2H, d, 3J = 8.4,
H3), 8.19 (2H, t, 3J = 7.2, H4), 7.47 (2H, t, 3J = 6.8, H5), 7.38 (2H, d,
1
mmol), leading to the product as an orange solid (118 mg, 78%). H
3
3
NMR (500 MHz, d6-DMSO): δ 9.19 (2H, d, J = 5.5, J(195Pt) = 30,
H6), 8.14 (4H, m, H3 and H4), 7.82 (2H, d, 3J = 8.0, H3′), 7.41 (2H, t,
3J = 7.5, H5), 7.39 (2H, d, J = 8.0, H2″), 7.32 (1H, t, J = 8.0, H4′),
3
3
6.77 (2H, d, J = 8.0, H3″) 2.12 (3H, s, CH3). 13C NMR (HSQC
3
observe H 500 MHz, decouple 13C 125.7 MHz and HMBC observe
1
3J = 8.4, H2″), 6.61 (2H, d, J = 8.4, H3″), 3.92 (3H, s, COOCH3),
3
1H 500 MHz, d6-DMSO): δ 170.0 (C1′), 168.5 (C2), 153.3 (C6), 145.4
(C1″), 141.8 (C2′), 141.0 (C3), 132.8 (C2″), 130.5 (C4″), 129.1 (C3″),
125.5 (C3′), 124.8 (C5), 124.4 (C4′), 121.2 (C4), 20.44 (CH3). Anal.
Calcd for C23H18N2PtS: C, 50.27; H, 3.30; N, 5.10. Found: C, 49.89;
H, 3.19; N, 4.92.
3.62 (3H, s, OCH3). Anal. Calcd for C25H20N2O3PtS: C, 48.15; H,
3.32; N, 4.49. Found: C, 48.01; H, 3.17; N, 4.46.
PtL2SNit. This complex was also prepared using the same general
procedure, from PtL2Cl (96 mg, 0.19 mmol), 4-nitrothiophenol (59
mg, 0.38 mmol), and KOtBu (44 mg, 0.39 mmol), giving the product
PtL1SAni. This complex was prepared in a manner similar to that
for PtL1SPh, starting from PtL1Cl (30 mg, 0.065 mmol), 4-
methoxythiophenol (23 mg, 0.16 mmol), and KOtBu (21 mg, 0.19
mmol), giving the product as a red solid (20 mg, 55%). 1H NMR (400
1
as a yellow solid (109 mg, 92%). H NMR (300 MHz, d6-DMSO): δ
9.03 (2H d, J = 5.0, J(195Pt) = 34, H6), 8.91 (4H, m, H3 and H4),
3
3
8.26 (2H, s, H3′), 7.80 (2H, d, 3J = 8.0, H2″ or H3″), 7.66 (2H, d, 3J =
8.0, H3″ or H2″) 7.47 (2H, t, J = 5.5, H5), 3.92 (3H, s, OCH3). MS
3
MHz, d6-DMSO): δ 9.18 (2H, d, J = 4.8, J(195Pt) = 18, H6), 8.13
3
3
(EI): m/z 640 (M+), 484 (M+ − thiolate), 425 (M+ − thiolate −
CO2Me), 155 (thiol+). HRMS (EI): m/z 638.0579 (M+), calcd for
C24H17N3O4PtS 638.0582. Anal. Calcd for C24H17N3O4PtS: C, 45.14;
H, 2.63; N, 6.58. Found: C, 44.96; H, 2.67; N, 6.43.
(4H, m, H3 and H4), 7.82 (2H, d, J = 7.5, H3′), 7.40 (2H, m, H5),
3
7.39 (2H, d, 3J = 8.5, H2″), 7.32 (1H, t, 3J = 7.5, H4′), 6.59 (2H, d, 3J =
8.5, H3″), 3.61 (3H, s, OCH3). 13C NMR (HSQC, observe H 500
1
MHz, decouple 13C 125.7 MHz, d6-DMSO): δ 167.9 (quat), 152.5
(C6), 141.1 (quat), 140.3 (C3 or C4), 133.1 (C5 or C2″ or C3″), 124.8
(C3′), 124.0 (C5 or C2″ or C3″), 123.6 (C4′), 120.5 (C3 or C4), 113.6
(C2″ or C3″), 54.9 (OCH3); remaining quaternaries were not
detected. Anal. Calcd for C23H18N2OPtS: C, 48.85; H, 3.21; N, 4.95.
Found: C, 48.54; H, 3.16; N, 4.92.
Electrochemistry. Cyclic voltammetry was carried out using a
μAutolab Type III potentiostat with computer control and data
storage via GPES Manager software. Solutions of concentration ca. 1
mM in CH2Cl2 were used, containing [Bu4N][PF6] as the supporting
inert electrolyte. A three-electrode assembly was employed, consisting
of a glassy-carbon working electrode and platinum-wire counter and
reference electrodes. Solutions were purged for 5 min with solvent-
saturated nitrogen gas with stirring, prior to measurements being taken
without stirring. The voltammograms were referenced to the
ferrocene|ferrocenium couple (E1/2 = 0.42 V versus SCE).
Density Functional Theory Calculations. The Gaussian 09
program was used for all calculations.40 DFT calculations for
geometries and excitation energies were performed with the PBE0
hybrid exchange-correlation functional.41,42 The all-electron cc-pVDZ
basis set was used on the main-group atoms. For the platinum ion, the
Los Alamos LANL2DZ effective core potentials were used to treat the
core electrons, in combination with the LANL2DZ basis set for the
5s2, 5p6, and 5d8 valence electrons. The geometries were fully
optimized without symmetry constraints. The polarizable continuum
model (PCM) was used to take into account the solvent (dichloro-
methane). Frequency calculations were performed at the same level of
theory as the geometry optimizations to ensure that the identified
geometries do indeed correspond to minima on the potential energy
surface. Density difference plots were produced with Gaussview using
the default contour value of 0.02 au.
Photophysical Measurements. Absorption spectra were meas-
ured on a Biotek Instruments XS spectrometer, using quartz cuvettes
of 1 cm path length. Steady-state luminescence spectra were measured
using a Jobin Yvon FluoroMax-2 spectrofluorimeter, fitted with a red-
sensitive Hamamatsu R928 photomultiplier tube; the spectra shown
are corrected for the wavelength dependence of the detector, and the
quoted emission maxima refer to the values after correction. Samples
for emission measurements were contained within quartz cuvettes of 1
cm path length modified with appropriate glassware to allow
connection to a high-vacuum line. Degassing was achieved via a
minimum of three freeze−pump−thaw cycles while the cuvette was
connected to the vacuum manifold; the final vapor pressure at 77 K
was <5 × 10−2 mbar, as monitored using a Pirani gauge. Luminescence
PtL1SNit. This compound was prepared from PtL1Cl (52 mg, 0.13
mmol), 4-nitrothiophenol (36 mg, 0.23 mmol), and KOtBu (25 mg,
0.23 mmol), using the same procedure as for PtL1SPh, giving the
1
product as a red-orange solid (52 mg, 72%). H NMR (400 MHz, d6-
DMSO): δ 9.08 (2H, d, 3J = 6.0, 3J(195Pt) 30, H6) 8.17 (4H, m, H3 and
H4), 7.84 (2H, d, J = 7.5, H3′), 7.82 (2H, d, J = 8.5, H2″ or H3″),
3
3
7.71 (2H, d, 3J = 8.5, H2″ or H3″), 7.46 (2H, t, 3J = 6.5, H5), 7.35 (1H,
t, J = 7.5, H4′). MS (EI): m/z 581 M+, 426 [M − SC6H4NO2]+.
3
HRMS (EI): m/z 580.0529, calcd for C22H15N3O2SPt m/z 580.0527.
Anal. Calcd for C22H15N3O2PtS: C, 45.52; H, 2.60; N, 7.24. Found: C,
44.57; H, 2.58; N, 6.51.
PtL2SMe. This complex was prepared using the method described
for PtL1SMe, starting from PtL2Cl (45.8 mg, 0.088 mmol) and sodium
methanethiolate (31 mg, 0.092 mmol, 21% w/w in H2O) in methanol
(5 mL), leading to the product as an orange-red solid (24 mg, 51%).
1H NMR (400 MHz, d6-DMSO): δ 9.39 (2H, d, J = 5.0, J(195Pt) =
3
3
42, H6), 8.73 (2H, d, 3J = 7.0, H3), 8.33 (2H, s, H3′), 8.21 (2H, t, 3J =
7.5, H4), 7.58 (2H, t, J = 7.0, H5), 3.90 (3H, s, OCH3), 2.26 (3H, s,
3
SCH3). Anal. Calcd for C19H16N2O2PtS: C, 42.94; H, 3.03; N, 5.27.
Found: C, 42.50; H, 2.98; N, 4.92.
PtL2SPh. The title compound was prepared using the same
procedure as that described for PtL1SPh, starting from PtL2Cl (93 mg,
0.18 mmol), thiophenol (41 mg, 0.37 mmol), and KOtBu (46 mg, 0.41
1
mmol), leading to the product as a yellow solid (86 mg, 81%). H
3
3
NMR (400 MHz, d6-DMSO): δ 9.19 (2H, d, J = 5.0, J(195Pt) = 38,
H6), 8.35 (2H, s, H3′), 8.29 (2H, d, 3J = 8.0, H3), 8.18 (2H, t, 3J = 7.5,
H4), 7.51 (2H, d, 3J = 7.5, H2″), 7.45 (2H, t, 3J = 7.0, H5), 6.97 (2H, t,
3
3J = 7.5, H3″), 6.82 (1H, t, J = 7.5, H4″), 3.92 (3H, s, OCH3). MS
(EI): m/z 593 (M+), 484 (M+ − thiolate), 425 (M+ − thiolate −
CO2Me), 110 (thiol+). HRMS (EI): m/z 593.0726 (M+), calcd for
C24H18N2O2PtS 593.0731. Anal. Calcd for C24H18N2O2PtS: C, 48.56;
H, 3.06; N, 4.72. Found: C, 48.34; H, 3.03; N, 4.59.
J
dx.doi.org/10.1021/ic500555w | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
quantum yields were determined using [Ru(bpy)3]Cl2 in degassed
aqueous solution as the standard (ϕ = 0.04243); the estimated
uncertainty in Φ is 20% or better.
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The luminescence lifetimes of the complexes were measured by
time-correlated single-photon counting, following excitation at 374.0
nm with an EPL-375 pulsed-diode laser. The emitted light was
detected at 90° using a Peltier-cooled R928 PMT after passage
through a monochromator. The estimated uncertainty in the quoted
lifetimes is 10% or better. Lifetimes at 77 K in excess of 10 μs were
measured by multichannel scaling following excitation with a
microsecond-pulsed xenon lamp; an excitation wavelength of 374
nm (band pass 5 nm) was selected with a monochromator.
Cell Imaging. The instrumentation and procedures for imaging of
NIH 3T3 or CHO cells incubated with the complex were as described
in previous work.4a,44
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures and tables giving absorption spectra and λmax values of
the complexes in a selection of different solvents, emission
spectra of the PtL1SR complexes at 77 K, emission spectrum of
a concentrated solution of PtL2SMe at 77 K, fluorescence
microscopy images of CHO cells costained with PtL1Cl and
MitoTracker Red, and further TD DFT data, including frontier
orbital plots and Mulliken charges. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.A.G.W.: j.a.g.williams@durham.ac.uk.
Notes
́
Marder, S. R.; Bredas, J. L.; Weck, M. Macromolecules 2009, 42, 6855.
■
(k) Zhou, G. J.; Wang, Q.; Wang, X. Z.; Ho, C. L.; Wong, W. Y.; Ma,
D. G.; Wang, L. X.; Lin, Z. Y. J. Mater. Chem. 2010, 20, 7472.
(l) Vezzu, D. A. K.; Deaton, J. C.; Jones, J. S.; Bartolotti, L. C.; Harris,
F.; Marchetti, A. P.; Kondakova, M.; Pike, R. D.; Huo, S. Inorg. Chem.
2010, 49, 5107. (m) Wu, W. H.; Wu, W. T.; Ji, S. M.; Guo, H. M.;
Song, P.; Han, K. L.; Chi, L. N.; Shao, J. Y.; Zhao, J. Z. J. Mater. Chem.
2010, 20, 9775. (n) Chan, J. C.-H.; Lam, W. H.; Wong, H.-L.; Zhu, N.
Y.; Wong, W.-T.; Yam, V. W.-W. J. Am. Chem. Soc. 2011, 133, 12690.
(o) Turner, E.; Bakken, N.; Li, J. Inorg. Chem. 2013, 52, 7344.
(p) Harris, C. F.; Vezzu, D. A. P.; Bartolotti, L.; Boyle, P. D.; Huo, S.
Inorg. Chem. 2013, 52, 11711.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the EPSRC (studentship to G.R.F., grant ref EP/
G06928X/1), ONE North East, and Durham University
(studentship for W.A.T.). We thank the EPSRC National
Mass Spectrometry Service Centre, Swansea, U.K., for
recording EI mass spectra.
(6) (a) Williams, J. A. G. Top. Curr. Chem. 2007, 281, 205.
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dx.doi.org/10.1021/ic500555w | Inorg. Chem. XXXX, XXX, XXX−XXX