Pt(II) mono-carbonyl complexes of a cyclometallating 2-(2'-thienyl)- (pyridinato-C,3N') ligand: Nature and dynamics of the lowest excited state of the chloro- And thiolato-complexes

Article abstract of DOI:10.1039/b502375k

The synthesis of a cyclometallated Pt(II) thiolate carbonyl complex Pt(thpy)(CO)(mts), (thpy = 2-(2'-thienyl)-pyridinate, mts = methylthiosalicylate) is reported. A combination of emission and time-resolved infrared (TRIR) techniques revealed for both Pt(thpy)(CO)(mts) and its chloride analogue Pt(thpy)(CO)Cl the predominant intra 2-(2'-thienyl)-pyridinate ³ππ* character of the lowest electronic excited state. The unusually short lifetime (780 ps) of the intraligand ³ππ * lowest excited state of Pt(thpy)(CO)(mts) indicates that this electronic state is influenced by another close-lying excited state, probably charge-transfer in origin. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502375k

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F U L L P A P E R
Pt(II) mono-carbonyl complexes of a cyclometallating
2-(2ꢀ-thienyl)-(pyridinato-C,3Nꢀ) ligand: nature and dynamics
of the lowest excited state of the chloro- and thiolato-complexes†
Yurii A. Kovelenov,a,b Alexander J. Blake,a Michael W. George,*a Pavel Matousek,c
Mikhail Ya. Mel’nikov,b Anthony W. Parker,c Xue-Zhong Sun,a Michael Towriec and
Julia A. Weinstein*‡a,b
a School of Chemistry, University of Nottingham, Nottingham, UK NG7 2RD.
E-mail: Julia.Weinstein@sheffield.ac.uk, Mike.George@nottingham.ac.uk
b Department of Chemistry, Moscow Lomonosov State University, Moscow, 119899, Russia
c Central Laser Facility, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon,
UK OX11 0QX
Received 16th February 2005, Accepted 20th April 2005
First published as an Advance Article on the web 11th May 2005
The synthesis of a cyclometallated Pt(II) thiolate carbonyl complex Pt(thpy)(CO)(mts), (thpy = 2-(2ꢀ-thienyl)-
pyridinate, mts = methylthiosalicylate) is reported. A combination of emission and time-resolved infrared (TRIR)
techniques revealed for both Pt(thpy)(CO)(mts) and its chloride analogue Pt(thpy)(CO)Cl the predominant intra
2-(2ꢀ-thienyl)-pyridinate 3pp* character of the lowest electronic excited state. The unusually short lifetime (780 ps) of
the intraligand 3pp* lowest excited state of Pt(thpy)(CO)(mts) indicates that this electronic state is influenced by
another close-lying excited state, probably charge-transfer in origin.
and the lowest excited state in Pt(thpy)(CO)Cl has been at-
Introduction
tributed to both a predominantly MLCT,7 and to a predomi-
The square-planar Pt(II) complexes with bidentate pyridine-
nantly 3pp* state.2 Clearly, the presence of a manifold of close in
based cyclometallating ligands, N∧C have attracted much at-
energy excited states of different origin can lead to ambiguities
tention recently. Their rich photophysics has been investigated
while spectroscopically assigning the lowest excited states in
from a fundamental point of view1,2 and has provided the basis
these complexes.
for light-induced applications of these compounds.3,4
In the Pt(II) diimine mixed-ligand complexes, the LUMO12–14
The light-induced properties of these chromophores are
is mainly residing on the diimine ligand with some Pt(II) par-
affected by variations in both cyclometallating N∧C, and the
ticipation, while the ancillary anionic ligands (such as chloride
non-cyclometallating ancillary ligands. It is characteristic of
or thiolate) have a small contribution. However, the nature of
transition metal compounds to possess a manifold of close in
the HOMO is significantly changed from being mainly metal
energy excited states, such as charge-transfer, intra-ligand p–p*,
based in Pt(II) diimine chloride complexes to thiolate(p)/Pt(d)
or metal-centred ones, which differ in their orbital parentage,
based in Pt(II) diimine thiolate complexes. This leads to a change
localisation within the molecule, and consequently lifetime
in the nature of the lowest excited state from mainly MLCT in
and reactivity. The lowest excited state in many (N∧C)Pt(II)
the former to the {charge-transfer-to-diimine} in the latter.12–14
compounds has been characterised as a mixed intra-N∧C ligand
This stimulated our interest in identifying whether changing
3pp/MLCT excited state.5–7 It has been shown that the degree of
from a chloride to a thiolate ligand would change the nature of
the MLCT contribution in the lowest excited state and hence
the lowest excited state in (N∧C)Pt(II) carbonyl complexes. The
emission energy, quantum yield and lifetime are affected by
work reported here investigates the nature and dynamics of the
both N∧C and ancillary ligands.1,2,4,7,8 The metal-centred states
lowest excited state of two Pt(II) carbonyl compounds containing
in these chromophores normally lie at much higher energies
cyclometallated ligand 2-(2ꢀ-thienyl)-pyridinate, Pt(thpy)(CO)Cl
due to the strong ligand field of cyclometallating carbon, and
are unlikely to have a significant impact on the photophysical
properties.
and Pt(thpy)(CO)(mts), where mts = methylthiosalicylate.
Time-resolved infrared spectroscopy (TRIR) has been devel-
oped over recent years as a powerful tool for characterising the
Although there is a large amount of research in the area
nature of excited states of coordination compounds, particularly
of (N∧C)Pt(II) compounds, studies on the excited states of
those bearing IR reporter ligands such as CO or CN.15,16 The
Pt(II) N∧C cyclometallated complexes with carbonyl ligands are
vibrational spectra afforded by TRIR allows one to follow
limited and include Pt(ppy)(CO)Cl (ppy = 2-phenylpyridine),9
directly subtle bond changes that occur within electronic excited
Pt(ptpy)(CO)Cl (ptpy
=
tolyl-pyridine),9 Pt(thpy)(CO)Cl
states in response to redistribution of electron density using the
IR CO modes as reporters of the molecular dynamics. Thus
the application of TRIR is particularly useful in distinguishing
between excited states of different origin, such as an intraligand
(pp*) excited state and one such as MLCT in which the metal
centre is directly affected.
(thpy = 2-thienylpyridine)6,7,10 and Pt(biphenyl)(CO)2.11 While
the nature of the lowest excited state was assigned to the
MLCT for the first two compounds,9 the emissive state of
Pt(biphenyl)(CO)2 has been described as mixed 3pp*/MLCT,11
We have used a combination of emission spectroscopy and
nanosecond and picosecond TRIR to probe the nature of the
excited state in the cyclometallated Pt(II) carbonyl complexes
and to investigate the effect of substitution of the chloride by
a thiolate ligand on the localisation of the frontier orbitals in
these chromophores.
† Electronic supplementary information (ESI) available: Quenching of
emission of Pt(thpy)(CO)Cl by benzyl viologen tetrafluoroborate in
deoxygenated CH3CN at room temperature. See http://www.rsc.org/
suppdata/dt/b5/b502375k/
‡ Permanent address: Department of Chemistry, The University of
Sheffield, Sheffield, UK S3 7HF.
2 0 9 2
D a l t o n T r a n s . , 2 0 0 5 , 2 0 9 2 – 2 0 9 7
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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examples including dinuclear,20 trinuclear,21 and tetranuclear22
compounds.
Experimental
Materials
Reaction of Pt(thpy)(CO)Cl in the presence of one equivalent
t
of BuOK with one equivalent of methythiosalicilate in dry
All solvents were distilled over CaH2 under a dry argon
atmosphere prior to use. 2-Thienylpyridine, methylthiosalicylate
(Aldrich) and potassium tetrachloroplatinate (Acros), were used
as received.
CH3CN yielded a yellow-orange solid whose 1H NMR spectrum
is identical to the one described above, indicating a formation
of a mixture of cis- and trans-isomers of [Pt(thpy)(l-mts)]2.
Treatment of (nBu4N)[Pt(thpy)Cl2] in dry CH3CN with one
equivalent of AgPF6, followed by treatment with a solution of
one equivalent of methylthiosalicylate/tBuOK, led to the same
dimeric product. Anal. Calc. for C34H26N2O4Pt2S4: C, 39.10; H,
2.48; N, 2.68. Found: C, 38.37; H, 2.70; N, 2.26%. MS (ES):
m/z 1044 with the correct isotopic distribution; calculated for
C34H26N2O4Pt2S4 based on Pt 195, S 32: 1044.
Synthesis of Pt(thpy)(CO)Cl (1) and Pt(thpy)(CO)(mts) (2);
thpy = 2-thienylpyridine; mts = methylthiosalicylate
The synthesis of (1) and (2) was performed according to Fig. 1.
(nBu4N)[Pt(thpy)Cl2]. This compound was prepared accord-
ing to the literature procedure.17,18 All operations were performed
under Ar. 1H NMR (CDCl3): d = 9.65 (H6, d, 1H, JPt 42 Hz), 7.60
(H4, t, 1H), 7.40 (H5ꢀ, d, 1H), 7.35 (H4ꢀ, d, 1H), 7.20 (H3, d, 1H),
6.80 (H5, t, 1H), 3.24 (m, 8H), 1.60 (m, 8H), 1.35 (m, 8H), 0.88
(t, 12H). These NMR data are consistent with the literature data
reported17,18 for the structure of formula (nBu4N)[Pt(thpy)Cl2].
Pt(thpy)(CO)(mts) (2). [Pt(thpy)(l-mts)]2 (42 mg, 0.04 mmol)
was dissolved in a mixture of dry CH3CN (20 ml) and CH2Cl2
(20 ml) and flushed with CO for 48 h at ambient temperature.
The orange precipitate formed was filtered off, washed with
Et2O, and dried in vacuum. Yield 60%. 1H NMR (CDCl3): d =
9.30 (H6, d, 1H, JPt 36 Hz), 7.85 (H4 and one of the Hs of the Ph
ring, m, 2H), 7.62 (H5ꢀ, d, 1H), 7.48 (H3 and one of the protons
of the Ph-ring, m, 2H), 7.15 (H5, H4ꢀ, and two protons of the
Ph-ring, m, 4H), 4.00 (H7, s, 3H). MS (ES): m/z (MH+) = 551;
calculated for C18H13NO3PtS2 based on Pt 195, S 32: 550. IR
(CH2Cl2): –C(O)OCH3, 1716 cm−1 and m(CO), 2095 cm−1.
Pt(thpy)(CO)Cl (1)18. (nBu4N)[Pt(thpy)Cl2] (288 mg,
0.43 mmol) was dissolved in a mixture of dry CH2Cl2 (30 ml)
and dry CH3OH (20 ml) and flushed with CO for 48 h.
The solution was then cooled to 0 ◦C, the resulting orange
precipitate filtered off and washed with cold CH2Cl2 and Et2O
to yield a yellow-orange crystalline powder. Yield 61%. 1H
NMR (CDCl3): d = 9.30 (H6, d, 1H, JPt 36 Hz), 7.88 (H4, t,
1H), 7.48 (H3, H5ꢀ, m, 2H), 7.23 (H5, t, 1H), 7.06 (H4ꢀ, t, 1H,
JPt 32 Hz) are consistent with Pt(thpy)(CO)Cl structure. Anal.
Calc. for C10H6NPtClO: C, 28.68; H, 1.44; N, 3.34. Found: C,
28.62; H, 1.32; N, 3.15%. IR (CH2Cl2): m(CO) 2105 cm−1.
Methods and instrumentation
1H NMR spectra were recorded on a Bruker 300 MHz spec-
trometer at 298 K in CDCl3 unless otherwise stated.
[Pt(thpy)(l-mts)]2. To a solution of (nBu4N)[Pt(thpy)Cl2]
(96 mg, 0.14 mmol) in dry CH3CN (20 ml), containing suspended
tBuOK (18 mg, 0.16 mmol), a solution of methylthiosalicylate
(0.045 ml, 0.32 mmol) in dry CH3CN (10 ml) was added dropwise
under intense stirring. The reaction mixture was then heated to
55 ◦C for 6 h. The resulting yellow-orange precipitate was filtered
off, washed with diethyl ether and dried in vacuum. Yield 67%.
1H NMR (CDCl3) consists of two similar patterns: d = 8.85 (d,
Pt satellites); 8.75 (t); 7.62 (m); 7.42 (d), 7.1–7.3 (m), 6.94 (t), 6.36
(d, Pt broadening) and 8.62 (d), 8.56 (t), 7.48 (d), 6.79 (t), 6.72 (d,
Pt broadening) with intensity ratio ca. 2 : 3. The signals at 4.01
(s) and 3.95 (s) with intensity ratio ca. 2 : 3 arise from the CH3
group of the methylthiosalicylate ligand being in two different
environments. A CDCl3 solution of the single crystals used for
X-ray analysis has an identical 1H NMR spectrum pattern.
Thus, 1H NMR data suggest the presence of two isomers (cis
and trans with respect to N∧C/N∧C and N∧C/C∧N coordina-
tion of two 2-(2ꢀ-thienyl)-pyridinate moieties), in an approximate
3 : 2 ratio. The disorder components in the ratio 59 : 41 observed
in the crystal structure are consistent with the co-crystallization
of two such isomers. The formation of thiolato-bridged ho-
mometallic Pt(II) complexes has been reported previously, with
Emission measurements were performed on an Edinburgh
Instrument F900 spectrometer. Steady state emission spectra
were obtained with a Xe lamp as the excitation source. Lifetimes
were obtained in the single photon counting mode, with
a nanosecond flash lamp (repetition rate 10–40 kHz, 1 ns
FWHM), or a microsecond flash lamp (repetition rate 40 Hz,
ca. 2 ls FWHM) as excitation sources. The relative emission
quantum yield φ was determined using a solution of quinine
sulfate in 0.1 M H2SO4 (φ 0.546) and a solution of [Ru(bpy)3]Cl2
in CH3OH at 77 K as emission standards (φ 0.376).23
The set-up for nanosecond step-scan FTIR time-resolved
infrared studies has been described elsewhere.24 Briefly, the
apparatus comprises a commercially available step-scan FTIR
spectrometer (Nicolet Magna 860) equipped with a 100 MHz
12-bit digitiser and a 50 MHz MCT detector interfaced to a
Nd:YAG laser (Spectra Physics GCR-12). Synchronization of
the Nd:YAG laser and data collection was achieved using a
pulse generator (Stanford DG535).
The picosecond time-resolved infrared studies were per-
formed on the PIRATE set-up in the Rutherford Appleton
Laboratory, details of which are described elsewhere.25 Briefly,
part of the output from a 1 kHz, 800 nm, 150 fs, 2 mJ Ti:Sapphire
Fig. 1 Synthesis of Pt(thpy)(CO)Cl and Pt(thpy)(CO)(mts), and the numbering scheme used in the 1H NMR assignment.
D a l t o n T r a n s . , 2 0 0 5 , 2 0 9 2 – 2 0 9 7
2 0 9 3

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oscillator/regenerative amplifier was used to pump a white light
continuum seeded BBO optical parametric amplifier (OPA). The
signal and idler produced by this OPA were difference frequency
mixed in a type I AgGaS2 crystal to generate tuneable mid-
infrared pulses (ca. 150 cm−1 FWHM, 1 lJ). Second harmonic
generation of the residual 800 nm light provided 400 nm pulses,
which were used to excite the sample. Changes in infrared
absorption were recorded by normalizing the outputs from a
pair of 64-element HgCdTe linear infrared array detectors on
a shot-by-shot basis. Gratings of 150 lines per mm were used
in the spectrographs to achieve a spectral resolution of approx.
6 cm−1 in the 2000 cm−1 region.
due to intra-ligand transitions of the thpy anion and by a band
at 404 nm in room temperature CH2Cl2 (Fig. 3, thick solid line).
As outlined in the Introduction, the nature of this low-energy
transition has been debated in the literature and both intra-
ligand pp* and MLCT assignments have been proposed.2,7
Crystallography
Crystals of [Pt(thpy)(l-mts)]2 suitable for X-ray diffraction
analysis were obtained by slow diffusion of Et2O into a CH2Cl2
solution at room temperature. The X-ray structure of product 2
shows that the complex is the thiolate-bridged dinuclear species
[Pt(thpy)(l-mts)]2 which lies across a crystallographic inversion
centre (Fig. 1). The core of the complex is a planar four-
˚
membered Pt2S2 ring with Pt–S 2.332(3) and 2.354(3) A, S–Pt–S
85.28(12)◦ and Pt–S–Pt 94.72(12)◦. The bridging methylthios-
alicylate is ordered but there is complete disorder of the five-
and six-membered rings of the 2-(2ꢀ-thienyl)-pyridine ligand.
The modelling of the disorder in this structure19 suggested the
presence of a mixture of cis and trans isomers in approximately
Fig. 3 Absorption spectra of (nBu4N)[Pt(thpy)Cl2] (dashed line, e at
445 nm is 1790 dm3 mol−1 cm−1), Pt(thpy)(CO)Cl (thick solid line, e at
406 nm in DMF6 is 4170 dm3 mol−1 cm−1) and Pt(thpy)(CO)(mts) (thin
solid line, e at 445 nm is 2300 dm3 mol−1 cm−1) in CH2Cl2 solution at room
temperature. The emission excitation spectrum of Pt(thpy)(CO)(mts) in
2-Me-THF glass at 77 K registered at 610 nm is shown as a dotted line;
the insert shows the low energy part of this spectrum ×100.
1
the same proportion as evidenced by the H NMR spectrum
(note that Fig. 2 arbitrarily shows only the trans isomer).
The electronic absorption spectrum of Pt(thpy)(CO)(mts) (2)
in room temperature CH2Cl2 (Fig. 3, thin solid line) resembles
that of Pt(thpy)(CO)Cl, and also exhibits a number of bands in
the region below 380 nm, which may be attributed to the intrali-
gand transitions of the coordinated thpy anion. The substitution
of the chloride by thiolate leads to the disappearance of the
absorption band at 404 nm, while a new broad band of moderate
intensity emerges at 445 nm (e = 2.3 × 103 dm3 mol−1 cm−1).
The lack of vibrational progression, and the absence of such
a band in the spectrum of Pt(thpy)(CO)Cl is consistent with a
considerable ligand-to-ligand charge transfer character of the
electronic transition(s) being responsible for this absorption
band.
The lowest energy absorption bands in both Pt(thpy)(CO)Cl
and Pt(thpy)(CO)(mts) show negatively solvatochromic be-
haviour, as the band position shifts to lower energies by 250 cm−1
and 410 cm−1, respectively, when the solvent is changed from
CH3CN to CH2Cl2. A linear correlation has been observed be-
tween the energy of this lowest absorption band and the solvent
polarity parameter value based on the energy of the {charge-
transfer-to-diimine} transition in Pt(dbbpy)(tdt) (dbbpy = 4,4ꢀ-
di-tert-butyl-2,2ꢀ-bipyridine, tdt = toluene-3,4-dithiolate).13 The
slope of the linear correlation based on this scale gives a
solvatochromic shift of 0.13 and 0.22 eV for Pt(thpy)(CO)Cl
and Pt(thpy)(CO)(mts), respectively. These values are lower than
those usually associated with charge-transfer transitions in Pt(II)
chromophores (0.3–0.6 eV),13 and this observation indicates
either that there is only partial charge-transfer character of the
transition(s) responsible for this absorption envelope, or that
there is an overlap of a solvatochromic charge-transfer transition
with a non-solvatochromic transition, e.g. an intraligand pp*
one. The solvent effect is also more pronounced for the thiolate
complex Pt(thpy)(CO)(mts) than for its chloride counterpart,
indicating a larger contribution of the charge-transfer transition
in the latter. In transition metal complexes, the singlet–triplet
transitions can be of considerable intensity due to the presence
of the heavy atom effect induced by the Pt(II) centre, as has been
shown for other Pt(II) compounds.26 In light of this fact, the very
weak structureless band (ca. 45 times lower in intensity than the
band at 438 nm) observed in the spectrum of Pt(thpy)(CO)(mts)
Fig. 2 X-Ray structure of the dimeric complex [Pt(thpy)(l-mts)]2. Only
the major disorder component of the thpy ligand is shown. Ellipsoids
are drawn at the 50% probability level.
Crystal data. C34H26N2O4Pt2S4, M = 1044.99, triclinic, a =
˚
7.531(3), b = 9.359(4), c = 11.903(5) A, a = 90.683(5), b =
◦
3
˚
100.588(5), c = 99.488(5) , U = 812.5(10) A , T = 150 K,
−1
¯
space group P1 (no. 2), Z = 1, l(Mo-Ka) = 8.899 mm , 7285
reflections measured, 3519 unique (Rint = 0.079) which were used
in all calculations. The thiophene and pyridine rings of the 2-(2ꢀ-
thienyl)-pyridinate ligand are disordered such that the five- and
six-membered rings overlap. This was modelled by two disorder
components in each of which all atoms were refined isotropically.
There is pseudo-merohedral twinning by 180◦ rotation about
(100). The final R1 [2479 F > 4r(F)] = 0.0617, wR(F2) [all data]
was 0.160.
CCDC reference number 266311.
See http://www.rsc.org/suppdata/dt/b5/b502375k/ for cry-
stallographic data in CIF or other electronic format.
Results and discussion
Electronic absorption spectra
The electronic absorption spectrum of Pt(thpy)(CO)Cl is
characterised7 by a series of bands in the region below 390 nm
2 0 9 4
D a l t o n T r a n s . , 2 0 0 5 , 2 0 9 2 – 2 0 9 7

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at 550 nm is tentatively attributed to a singlet–triplet satellite to
singlet–singlet HOMO–LUMO transition.
In contrast to (nBu4N)[Pt(thpy)Cl2] and 1, no emission was
detected on the nanosecond time scale for the room temperature
solutions of the thiolate complex 2 in CH3CN, CH2Cl2, 2-Me-
THF and toluene. However, 2 emits very intensely at 77 K
in a toluene (φ 0.74 ( 0.08)) or 2-Me-THF glass. The low-
temperature emission spectrum of 2 appears in the same spectral
region as (nBu4N)[Pt(thpy)Cl2] and 1, exhibits pronounced
vibrational progression (Fig. 4), and has identical shape in
2-Me-THF and toluene. The decay profile of the emission
remains mono-exponential along the spectrum profile, with a
corresponding lifetime of 64 ( 5) ls in 2-Me-THF and this
is constant across the concentration range from 0.5 to 5 ×
10−5mol dm−3.*
The excitation spectrum of 2 at 77 K displays a band at ca.
440 nm with pronounced vibrational structure. This band is only
slightly shifted in energy if compared to the absorption spectrum
in fluid solution (Fig. 3, thin solid line). The low intensity band
observed at 550 nm in the absorption spectrum of a fluid solution
of 2 is also present in the excitation spectrum at 77 K. However,
in the frozen solution this band exhibits pronounced vibrational
structure, and is a mirror image of the emission spectrum. The
lowest energy component (557 nm) of the low intensity band
virtually overlaps with the highest energy component of the
emission spectrum of 2 (559 nm).
Emission properties
The emission properties of Pt(thpy)(CO)(mts) have been
studied and compared to those of (nBu4N)[Pt(thpy)Cl2] and
Pt(thpy)(CO)Cl.7 (nBu4N)[Pt(thpy)Cl2] and Pt(thpy)(CO)Cl
both show emission (ca. 600 nm) in deoxygenated CH3CN
solution at room temperature (Fig. 4). Their emission profiles
are virtually identical, and exhibit a pronounced vibrational
structure, which matches the vibrational frequencies observed
in the infrared spectrum of the 2-(2ꢀ-thienyl)-pyridine ligand.
For example, the difference of 1460 cm−1 between the energies
of the two most pronounced bands in the emission spectra cor-
responds to a ring stretch vibration of 2-(2ꢀ-thienyl)-pyridinate.
The position of the highest energy feature of the emission
spectrum in CH3CN differs only slightly from those reported
previously in DMF (555 vs. 565 nm, and 561 vs. 565 nm for
(nBu4N)[Pt(thpy)Cl2] and Pt(thpy)(CO)Cl, respectively).7 The
emission of (nBu4N)[Pt(thpy)Cl2] and Pt(thpy)(CO)Cl decays
monoexponentially in room temperature CH3CN solutions,
with lifetimes of 15 ( 1) and 24 ( 2) ls, respectively. The
structured emission spectral profile detected, and the relatively
long emission lifetimes suggest that the emission observed for
(nBu4N)[Pt(thpy)Cl2] and Pt(thpy)(CO)Cl originates from an
intra-thpy 3pp* state.
In the excitation spectrum, the band at 440 nm can thus be
1
assigned to the transition of predominantly pp* nature of the
2-(2ꢀ-thienyl)-pyridinate ligand, and the band at 550 nm to the
3
corresponding pp* transition. This assignment implies a very
small emission Stokes shift, which in a combination with the
pronounced vibrational structure of the emission spectrum, and
the relatively long emission lifetime suggests a predominant 3pp*
origin of the lowest excited state of 2 at 77 K.
Time-resolved infrared spectroscopy
The ground state infrared spectrum of 1 in CH2Cl2 at room
temperature exhibits a single m(CO) band at 2106 cm−1 (Fig. 5a),
very similar to the 2105 cm−1 value reported for Pt(ppy)(CO)Cl.9
Excitation at 355 nm (7 ns) leads to an instantaneous bleaching
of this ground state absorption together with the formation of
a new band at 2094 cm−1 (Fig. 5b). The small negative shift of
the CO vibration observed in the TRIR spectra (ca. 12 cm−1)
suggests an increased back-donation from the Pt(II) center in
the excited state. Such an effect can be anticipated upon the
formation of a thienyl-pyridinate based pp* excited state due
to a decrease in the electron accepting properties of the thpy
ligand upon promotion to such an excited state. Similar effects
have been observed for the excited states of metal carbonyls with
diimine ligands.15,30 This should be contrasted to the Pt → thpy
charge transfer excited state, where the metal centre is partly
oxidized compared to the ground state, and this should lead to
an increase in the energy of the m(CO) vibration. Typically, metal
carbonyls, such as Re(I) and Os(II), exhibit a 50–60 cm−1 positive
shift of the m(CO) stretch upon promotion to a MLCT state.15,16
The TRIR data therefore support the assignment of the lowest
excited state as a 2-(2ꢀ-thienyl)-pyridine localised pp* state for
Pt(thpy)(CO)Cl. The recovery of the bleach of the parent band
observed at 2106 cm−1 is commensurate with the decay of the
transient band at 2094 cm−1 (Fig. 5c) within experimental error,
and yields a value of 15 ( 1) ls for the lifetime of the excited
state.
Fig.
4
Emission spectra of (nBu4N)[Pt(thpy)Cl2] (dashed line),
Pt(thpy)(CO)Cl (thick solid line) in room temperature degassed CH3CN
solution, and of Pt(thpy)(mts)(CO) (solid line) in a 2-MeTHF glass at
77 K (excitation at 400 nm).
The emission of Pt(thpy)(CO)Cl in deoxygenated CH3CN
solution was quenched by benzyl viologen (4,4ꢀ-dibenzyl-4,4ꢀ-
bipyridine) tetrafluoroborate.§ The bimolecular quenching rate
constant (7 × 109 M−1s−1) was determined using Stern–Volmer
plots (see ESI†). Prolonged irradiation of Pt(thpy)(CO)Cl in the
presence of benzyl viologen in deoxygenated CH3CN generated
a new absorption band (ca. 608 nm), which can be assigned to
the benzyl viologen radical cation.27 This observation confirms
that quenching of the excited state of Pt(thpy)(CO)Cl occurs via
an electron transfer process.¶
§ No change of the absorption spectra of Pt(thpy)(CO)Cl was observed
upon addition of the quencher in concentrations from 1 lmol dm−3 to
1 mmol dm−3. This is strong evidence that no complexation was taking
place between the luminophore and the quencher in the ground state.
¶ This fact is also supported by the thermodynamic considerations.
The free energy change of the electron transfer process, DGet, can be
estimated from DGet = Eox − Ered − E00.28 The oxidation potential for
1 is Eox > +1.70 V,7 and the reduction potential of the benzyl viologen
dication is Ered = −0.37 V vs. NHE.29 The highest energy emission band
(17860 cm−1 = 2.21 eV if 1 eV = 8064 cm−1) was used to estimate
the energy of the zero–zero transition, E00. This gives DGet > −0.1
eV. Therefore, the rate constant of bimolecular quenching should be less
than the rate constant of the diffusion-controlled process,28 which is 2 ×
1010 dm3 mol−1 s−1 in CH3CN at 298 K. Thus the experimental value
of 7 × 109 dm3 mol−1 s−1 obtained for the bimolecular quenching rate
constant is in agreement with the proposed electron transfer mechanism
of emission quenching.
* Differently from the observation made for Pt(ppy)(CO)Cl (ppy = 2-
phenylpyridine),9 no changes in the shape of the emission spectrum
with an increase in concentration has been registered for 2, possibly due
to the bulky nature of the thiolate ligand preventing the formation of
aggregates.
D a l t o n T r a n s . , 2 0 0 5 , 2 0 9 2 – 2 0 9 7
2 0 9 5

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at each time delay. A small narrowing and shift of the transient
band, observed at early time delays (Fig. 6a), is assigned to
early relaxation processes associated with the decay of “hot”
vibrational modes of the initially formed electronic excited state.
The production of “hot” excited states and their effect on the
infrared spectrum are well established and have been discussed
in detail elsewhere.31
The TRIR kinetic traces for parent recovery (Fig. 6b) and
transient decay have been plotted as an area of the corresponding
band versus pump–probe delays. The parent absorption recovers
mono-exponentially, with a rate constant of 1.2 ( 0.2) × 109 s−1.
However, the transient absorption exhibits a bi-exponential
decay (Fig. 6b), with rate constants of 1.25 ( 0.40) × 1011 and
1.28 ( 0.20) × 109 s−1. The slower component agrees well with
the rate constant for the bleach recovery, and gives a value of the
excited state lifetime of 780 ( 60) ps. We therefore tentatively
attribute the faster decay (8.0 2.0 ps) as evidence for relaxation
from a vibrationally hot electronic excited state and the decay
at the longer timescale as the decay of the “relaxed” electronic
excited state of Pt(thpy)(mts)(CO).
Thus, both the nanosecond TRIR data for Pt(thpy)(CO)Cl
and the picosecond TRIR data for Pt(thpy)(mts)CO are indica-
tive of the intra-thienyl-pyridine pp* nature of the lowest excited
state in these chromophores.
Fig. 5 (a) Ground state FTIR spectrum of Pt(thpy)(CO)Cl (1) in
CH2Cl2 solution; (b) TRIR spectrum of the solution (a) acquired at
2 ls delay after the 355 nm laser pulse; (c) the kinetic traces of the parent
bleach recovery (᭹, 2106 cm−1) and transient decay (᭺, 2094 cm−1).
Solid lines represent mono-exponential fit to the data.
Conclusions
The nature and dynamics of the lowest excited state
of a new cyclometallated Pt(II) thiolate carbonyl complex
Pt(thpy)(CO)(mts) and its chloride analogue Pt(thpy)(CO)Cl
have been studied by a combination of nanosecond and pi-
cosecond TRIR, and emission techniques. The energy of the
m(CO) vibration of the carbonyl ligand in the excited state of
Pt(thpy)(CO)Cl and Pt(thpy)(CO)(mts) at room temperature
in fluid solution is lower by 12 and 21 cm−1, respectively than
observed in the ground state. These features are characteristic of
the pp* excited states, and we thus assign the lowest excited state
The ground state infrared spectrum of 2 in CH2Cl2 at room
temperature exhibits a single m(CO) band at 2095 cm−1. The
somewhat lower energy of this vibration compared to that for
Pt(thpy)(CO)Cl indicates the stronger donating effect of the
thiolate moiety to the metal center, leading to an increase in
the back-donation to the p* orbitals of the CO ligand.
On the picosecond timescale, excitation of 2 at 400 nm (ca.
150 fs, 2 lJ, 100 × 150 lm beam size) leads to an instantaneous
bleaching of the parent absorption at 2095 cm−1 and the
formation of a new band at 2076 cm−1 (Fig. 6a). Picosecond
TRIR spectra recorded at several pump–probe delays between
5 ps and 2000 ps are shown in Fig. 6a. The observed negative
shift in the m(CO) vibration upon promotion to the excited state
also suggests the pp* nature of this excited state, as concluded
from the above discussion for the Pt(thpy)(CO)Cl compound.
3
in both chloro (1) and thiolato (2) complexes to the pp* state
centred on the cyclometallating ligand 2-(2ꢀ-thienyl)-pyridine.
The pronounced vibrational structure of the emission spectrum,
and long (24 ls) emission lifetime exhibited by Pt(thpy)(CO)Cl
in a room-temperature solution, which is typical for emission
from a 3pp* excited state, also supports this assignment. In the
case of thiolato-complex 2 Pt(thpy)(CO)(mts), the pronounced
vibrational structure of the emission spectrum similar to that
of Pt(thpy)(CO)Cl, suggests the 3pp* nature of the emissive
state in a toluene or 2-Me-THF glass at 77 K. The excited
state lifetime exhibited by 2 (ca. 780 ps) in fluid solution is
very short compared to the typical values of the lifetimes for
3pp* states. This suggests that the lowest 3pp* excited state
of 2 is perturbed by a charge-transfer excited state (probably
of predominantly ligand-to-ligand charge transfer character)
lying in close proximity. Thus, in contrast to the Pt(II) diimine
complexes, substitution of chloride by a thiolate (2 vs. 1) in the
cyclometallated complexes studied does not alter the nature of
the HOMO from being mainly Pt-centred to the mainly thiolate-
centred,13,14 and the lowest excited state retains its 3pp* nature.
The ps-TRIR spectral bands were fitted assuming
a
Lorentzian bandshape (solid lines in Fig. 6a), allowing for the
determination of the areas of the bleached and transient bands
3
The close proximity of the intra-ligand pp* excited state and
a charge-transfer state in the thiolate complex 2 clearly offers
an opportunity to tune both the lifetime and the nature of the
lowest excited state in this class of chromophores through the
variation of the donor/acceptor properties of the ligands.
Acknowledgements
Fig. 6 (a) TRIR spectra of Pt(thpy)(mts)(CO) (2) in CH2Cl2 obtained
at (᭹) 5 ps, (᭺) 200 ps, (ꢀ) 500 ps, (ꢁ) 1000 ps and (ꢂ) 2000 ps following
400 nm (ca. 150 fs FWHM, 14 lJ per pulse) photolysis. Solid lines
represent curve-fitted spectra. (b) Kinetic traces of the parent bleach
recovery (᭹, 2095 cm−1) and transient decay (᭺, 2074 cm−1) following
400 nm photolysis. Solid lines represent mono-exponential (parent) and
bi-exponential (transient) fit to the data.
The financial support of the University of Nottingham, EPSRC
(GR-R02863/01), and Russian Fund of Basic Research (03-03-
32300) is acknowledged. We thank CCLRC for supporting the
access to the picosecond TRIR facility and Dr P. Portius for
experimental assistance and valuable discussions.
2 0 9 6
D a l t o n T r a n s . , 2 0 0 5 , 2 0 9 2 – 2 0 9 7

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J. A. Weinstein, N. N. Zheligovskaya, M. Ya. Mel’nikov and F. Hartl,
J. Chem. Soc., Dalton Trans., 1998, 2459.
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D a l t o n T r a n s . , 2 0 0 5 , 2 0 9 2 – 2 0 9 7
2 0 9 7