Coumarin phosphorescence observed with NN Pt(ii) bisacetylide complex and its applications for luminescent oxygen sensing and triplet-triplet- annihilation based upconversion

Article abstract of DOI:10.1039/c1dt10490j

A dbbpy platinum(ii) bis(coumarin acetylide) complex (Pt-1, dbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) was prepared. Pt-1 shows intense absorption in the visible region (λ_abs = 412 nm, ε = 3.23 × 10⁴ M^-1 cm^-1) compared to the model complex dbbpy Pt(ii) bis(phenylacetylide) (Pt-2, λ_abs = 424 nm, ε = 8.8 × 10³ M^-1 cm^-1). Room temperature phosphorescence was observed for Pt-1 (³IL, τ_P = 2.52 μs, λ_em = 624 nm, Φ_P = 2.6%) and the emissive triplet excited state was assigned as mainly intraligand triplet excited state (³IL), proved by 77 K steady state emission, nanosecond time-resolved transient absorption spectroscopy and DFT calculations. Complex Pt-1 was used for phosphorescent oxygen sensing and the sensitivity (Stern-Volmer quenching constant K _SV = 0.012 Torr^-1) is 12-fold of the model complex Pt-2 (K_SV = 0.001 Torr^-1). Pt-1 was also used as triplet sensitizer for triplet-triplet-annihilation based upconversion, upconversion quantum yield Φ_UC up to 14.1% was observed, vs. 8.9% for the model complex Pt-2.

Full text of DOI:10.1039/c1dt10490j

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An international journal of inorganic chemistry
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Volume 40 | Number 31 | 21 August 2011 | Pages 7793–8036
ISSN 1477-9226
COVER ARTICLE
Guo, Zhao et al.
Coumarin phosphorescence observed with N^N Pt(II) bisacetylide complex and its applications
for luminescent oxygen sensing and triplet–triplet-annihilation based upconversion

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PAPER
Coumarin phosphorescence observed with NŸN Pt(II) bisacetylide complex
and its applications for luminescent oxygen sensing and
triplet–triplet-annihilation based upconversion†
Haiyang Sun, Huimin Guo,* Wenting Wu, Xin Liu and Jianzhang Zhao*
Received 23rd March 2011, Accepted 26th April 2011
DOI: 10.1039/c1dt10490j
A dbbpy platinum(II) bis(coumarin acetylide) complex (Pt-1, dbbpy = 4,4¢-di-tert-butyl-2,2¢-bipyridine)
was prepared. Pt-1 shows intense absorption in the visible region (labs = 412 nm, e = 3.23 ¥ 104 M-1
cm-1) compared to the model complex dbbpy Pt(II) bis(phenylacetylide) (Pt-2, labs = 424 nm, e = 8.8 ¥
103 M-1 cm-1). Room temperature phosphorescence was observed for Pt-1 (3IL, tP = 2.52 ms, lem
=
624 nm, UP = 2.6%) and the emissive triplet excited state was assigned as mainly intraligand triplet
excited state (3IL), proved by 77 K steady state emission, nanosecond time-resolved transient
absorption spectroscopy and DFT calculations. Complex Pt-1 was used for phosphorescent oxygen
sensing and the sensitivity (Stern–Volmer quenching constant KSV = 0.012 Torr-1) is 12-fold of the
model complex Pt-2 (KSV = 0.001 Torr-1). Pt-1 was also used as triplet sensitizer for
triplet–triplet-annihilation based upconversion, upconversion quantum yield UUC up to 14.1% was
observed, vs. 8.9% for the model complex Pt-2.
well as long-lived triplet excited state are rarely reported.9 (2) The
Introduction
structural diversity of the acetylide ligands is limited to the typical
aromatic moieties, such as substituted phenyl, naphthyl, pyrenyl,
etc., and very few organic fluorophores have been attached to the
Pt(II) center to investigate the phosphorescence of the acetylide
ligand, i.e. emission of the intraligand triplet excited state (3IL).1
Diimine (NŸN) Pt(II) bisacetylide complexes are of particu-
lar interesting due to their applications in electroluminescence,
photovoltaics, photocatalysis and molecular probes, etc.1–4 The
photon emission of Pt(II) bis(acetylide) complexes are attributed
to triplet excited state with a 3MLCT/3LLCT mixed fea-
ture (metal-to-ligand charge transfer and ligand-to-ligand-charge
transfer).1 The photophysics of the complexes, such as absorp-
tion/phosphorescence wavelength and phosphorescence quantum
yields, are largely dependent on the acetylide ligands, thus the
photophysical properties of these complexes can be readily tuned
by variation of the acetylide ligands, which is feasible from a
synthetic perspective.1,2,5–8
However, much room is left for the current development of the
NŸN Pt(II) bis(acetylide) complexes to optimize their photophys-
ical properties, such as (1) the UV-vis absorption and emission
properties. For example, the absorption of the typical diimine
Pt(II) bisacetylide complex is usually weak in visible region (the
e is less than 5000 M-1 cm-1),6 and the phosphorescence lifetime
(tP), or more generally, the triplet excited state lifetime (tT), is
usually short (ca. 1.0 ms). Complexes with intense absorption as
3
The IL excited state is particularly interesting, because longer
lifetimes or red-shifted emission were usually observed with the
3
3IL excited state, vs. the normal MLCT excited state. We have
shown that the 3IL excited state is important for applications
such as oxygen sensing and upconversion.10–15 Previously we
prepared an NŸN Pt(II) bis(acetylide) complex containing the
naphthalimide (NI) subunit,14,15 which shows intense long-lived
room temperature NI localized 3IL phosphorescence (tP = 118 ms,
UP = 17.5%). To the best of our knowledge, however, only
the ethynylated pyrene and NI that show intense absorption
were attached to Pt(II) to investigate the emissive 3IL excited
states.1,2,14–20
In order to tackle the above limitations, herein we designed
a NŸN Pt(II) bis(acetylide) complex containing coumarin (Pt-
1, Scheme 1). Enhanced absorption in the visible range, as well
as prolonged phosphoresence lifetime, were observed for Pt-1
compared to the model complex Pt-2 (Scheme 1). With steady
state and time-resolved spectroscopy and DFT calculations, we
proved that the intraligand triplet excited state (3IL) is the main
component of the triplet excited state of Pt-1. Pt-1 was used as
energy donor in two triplet–triplet-energy transfer processes, i.e.
luminescent oxygen sensing and triplet–triplet-annihilation (TTA)
based upconversion.
State Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, Dalian, 116024, P. R. China. E-mail:
zhaojzh@dlut.edu.cn, guohm@dlut.edu.cn; Fax: + 86 (0)411 8498 6236;
Tel: + 86 (0)411 8498 6236
† Electronic supplementary information (ESI) available: structural char-
acterization spectra, determination of the upconversion quantum yields
and NBO analysis. See DOI: 10.1039/c1dt10490j
7834 | Dalton Trans., 2011, 40, 7834–7841
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The Royal Society of Chemistry 2011
©

Scheme 1 Molecular structures of Pt-1 and the model complex Pt-2. The coumarin acetylide (L-1) and the triplet acceptor 9,10-diphenylabthracene
(DPA) used in TTA upconversion are also shown. The model complex used as a triplet sensitizer in the upconversion, Ru-1, is also shown.
solution was cooled to room temperature and poured into ice
water (100 mL). NaOH solution (40%) was added dropwise to
Experimental
Materials and reagents
adjust pH of the solution to 5.0, and a pale precipitate formed
immediately. After stirring for 30 min, the mixture was filtered,
washed with water, dried, then the residue was purified with
column chromatography (silica gel, dichloromethane) to give 1.05
4,4¢-Di-tert-butyl-2,2¢-bipyridine (dbbpy) is a product of Aldrich.
Other chemicals are analytical pure and were used as received
without further purification. Solvents were dried or distilled before
used for synthesis or spectroscopic studies.
1
g product 5 (yield: 48%) as a yellow solid. H NMR (400 MHz,
CDCl3) d 7.52 (d, 1H, J = 8.0 Hz, ArH), 7.23 (d, 1H, J = 8.0 Hz,
Ar-CH), 6.57 (d, 1H, J = 12.0 Hz, ArH), 6.48 (s, 1H, ArH), 6.04
(d, 1H, J = 12.0 Hz, Ar-CH), 3.42 (m, 4H, CH2), 1.20 (t, 6H, CH3).
Apparatus
NMR spectra were recorded on a 400 MHz Varian Unity Inova
spectrophotometer. Mass spectra were recorded with Q-TOF
Micro spectrometer and MALDI micro MX. UV-vis absorption
spectra were recorded on a HP8453 UVvisible spectrophotometer.
Fluorescence spectra were recorded on a JASCO FP-6500 or a
Sanco 970 CRT spectrofluorometer.
Fluorescence or phosphorescence lifetimes were measured on
Horiba Jobin Yvon Fluoro Max-4 (TCSPC). Nanosecond time-
resolved transient absorption spectroscopy was obtained with
LP-920 pump–probe spectrometer (Edinburgh Instruments). The
emission at 77 K was measured with a Oxford Optistat DNTM
cryostat (with liquid nitrogen filling) and FS920 fluorospectrom-
eter (Edinburgh Instruments). Flow cell coupled to a fluorospec-
trometer was used in the luminescent O2 sensing.
Synthesis of compound 4
Br2 (0.23 mL, 4.5 mmol) was added dropwise to a solution of 7-
diethylaminocoumarin (1.0 g, 4.5 mmol) in acetic acid (23 mL),
and the mixture was stirred at rt for 2–3 h. Resulting white
solid was filtered and washed with acetic acid, and dried. The
solid was recrystallized in acetonitrile (white solid, 1.1 g, 80%
yield). 1H-NMR (400 MHz, CDCl3): d 8.37 (s, 1H), 7.45 (d,
J = 8.6 Hz, 1H), 6.72 (dd, J = 8.8 Hz, J = 2.4 Hz, 1H), 6.56
(d, J = 2.4 Hz, 1H), 3.48 (q, J = 7.2 Hz, 4H), 1.21 (t, 6H,
CH3).
Synthesis of compound 3
PdCl2(PPh3)2 (24.0 mg, 0.03 mmol, 5.0 mol%), CuI (6.5 mg,
0.03 mmol, 5.0 mol%), TEA (190 mL, 1.4 mmol), and (trimethylsi-
lyl)acetylene (192 mL, 2.0 mmol) were added to a solution of
4 (200.0 mg, 0.67 mmol) in anhydrous DMF (7 mL) under
Ar. The mixture was stirred at 60 ◦C for 6 h. The resulting
solution was cooled to rt, diluted with water, and extracted with
CH2Cl2. The organic layer was dried over Na2SO4. The solvent was
removed under reduced pressure and the compound was purified
by column chromatography (silica gel, petroleumether–EA 4 : 1,
Synthesis of compound 5
4-Diethylaminosalicylaldehyde (1.93 g, 10 mmol), diethyl-
malonate (3.2 g, 20 mmol) and piperidine (1 mL) were combined
in absolute ethanol (30 mL) and stirred for 6 h under reflux
conditions. Ethanol was evaporated under reduced pressure, then
concentrated HCl (20 mL) and glacial acetic acid (20 mL) were
added to hydrolyze the reaction with stirring for another 6 h. The
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7834–7841 | 7835
©

1
v/v) to afford product 3. (Yellow solid, 190 mg, 90% yield). H-
5 digital camera. The exposure times are the default values of the
camera.
NMR (400 MHz, CDCl3): d 7.93 (s, 1H), 7.44 (d, J = 8.9 Hz, 1H),
6.78 (dd, J = 8.9 Hz, J = 2.4 Hz, 1H), 6.51 (d, J = 2.4 Hz, 1H),
3.58 (q, J = 7.0 Hz, 4H), 1.26 (t, J = 7.0 Hz, 6H), 0.24 (s, 9H).
2
⎛
⎜
⎜
⎞⎛
⎞⎛
⎜
⎟
⎜
h
⎠⎝
⎞
⎟
⎟
⎟
⎟
⎠
Astd Iunk ⎜hunk
⎟
⎟
⎜
⎟
⎟
⎜
(1)
Φ
= 2Φstd
⎜
⎟
⎟
unk
⎜
⎜
⎝
⎜
⎟
⎜
A
I
unk ⎠⎝
std
std
Synthesis of compound 2
A 1.0 M solution of tetrabutylammoniumfluoride (TBAF) in THF
(0.78 mL, 0.78 mmol) was added to a solution of 3 (163 mg,
0.52 mmol) in 10 mL of THF and MeOH (4/1, v/v). The mixture
was stirred at rt for 8 h. The solvent was removed under reduced
pressure and the product was purified by column chromatography
on silica gel with PE–EA (7 : 2, v/v) as the eluent, affording the
product 2. (Yellow solid, 76 mg, 60% yield). 1H-NMR (400 MHz,
CDCl3): d 7.77 (s, 1H), 7.24 (d, J = 8.9 Hz, 1H), 6.59 (dd, J = 8.9 Hz,
J = 2.4 Hz, 1H), 6.46 (d, J = 2.4 Hz, 1H), 3.43 (q, J = 7.0 Hz, 4H),
3.26 (s, 1H), 1.22 (s, J = 7.0 Hz, 6H). 13C NMR (100 MHz, CDCl3):
d 161.16, 156.45, 151.39, 146.96, 129.06, 109.27, 108.01, 103.41,
97.29, 81.14, 78.57, 77.22, 44.95, 12.43. TOF–MS: C15H15NO2,
calculated m/z = 241.1103, found m/z = 241.1112.
Results and discussion
Solvents-dependent absorption and emission
Coumarins are well-known fluorophores that show intense ab-
sorption and strong fluorescence, and were widely used as
fluorophores for molecular probes or lanthanide luminescence.22a
Previously coumarin was used as a light-harvesting antenna for
lanthanide luminescence,22b or a CŸN ligand for cyclometalated
Ir(III) complexes,23,24 or Pt(II)/Pd(II) complexes,25,26 but its room
temperature phosphorescence was never reported. Inspired by the
3
related work that IL emission can be observed by connection
of arylacetylide to a Pt(II) center, and our own work with
naphthalimide,15 herein we designed a NŸN Pt(II) bisacetylide
complex with the coumarin acetylide ligand (Pt-1, Scheme 1).
The p-conjugation framework of coumarin is directly attached to
Pt(II) in Pt-1 thus a significant heavy atom effect is expected, by
which the intersystem crossing (ISC) from the singlet excited state
to the triplet excited state can be facilitated, thus IL emission is
possible.1 We used a model complex Pt-2 in our photophysical and
photophysical application studies (Scheme 1).
Synthesis of Pt-1
Pt(dbbpy)Cl2 (50.0 mg, 0.09 mmol), CuI (5.0 mg, 0.09 mmol) and
diisopropylamine (1.0 mL) were dissolved in 5 mL of CH2Cl2
and DMF (2/3, v/v), the mixture was stirred for 10 min. The
mixture was purged with Ar, then product 2 (70.0 mg, 0.27 mmol)
was added and the mixture was stirred at room temperature for
24 h. The resulting white solid was filtered and washed with
CH2Cl2. The residue was purified by column chromatography
(silica gel, CH2Cl2–CH3OH = 80 : 1, v/v) 23.3 mg, yield: 85%.
1H NMR (400 MHz, CDCl3): d 10.40(d, J = 4.0 Hz 2H),
7.92 (s, 2H), 7.79 (s, 2H), 7.73 (d, J = 4.0 Hz, 2H), 7.26
(d, J = 8.0 Hz, 2H), 6.52 (m, 4H), 1.44 (m, 18H), 1.25 (m,
12H). 13C NMR (100 MHz, CDCl3): d 163.57, 163.07, 156.16,
155.08, 152.88, 149.60, 141.91, 128.13, 125.01, 118.25, 109.47,
108.82, 97.49, 96.61, 44.78, 35.74, 29.70, 12.53. HR-MALDI–
MS: [C48H52N4O4Pt+H]+, calculated m/z = 944.3715, found m/z =
944.3776. Anal. calcd for C48H52N4O4Pt(·0.5CH2Cl2): C, 59.05; H,
5.42; N, 5.68; Found: C, 59.26; H, 5.32; N, 5.62.
3
Firstly the UV-vis absorptions of the complexes were studied
(Fig. 1). Pt-2 shows maximal absorption at 424 nm with a
moderate molar extinction coefficient (e = 8800 M-1 cm-1). For
Pt-1, however, the absorption at 424 nm is greatly enhanced (e =
24903 M-1 cm-1). Furthermore, an intense absorption band at
414 nm was observed (e = 32300 M-1 cm-1). By comparison with
that of L-1, the absorption of Pt-1 at 414 nm is clearly due to the
1
coumarin ligand, i.e. S0→ IL transition.
Computational methods. All calculations were performed us-
ing the Gaussian 09 W software (Gaussian, Inc.).21 The gas phase
geometry optimizations were carried out using B3LYP functional
with the 6-31G(d) basis set. The vertical excitation energy was
calculated with the Time-Dependent DFT (TDDFT) method
based on the singlet ground state geometry. The spin-density of
the triplet state was calculated with the energy minimized triplet
state geometries.
Upconversions. A diode pumped laser was used for the up-
conversions. The solution samples were purged with N2 or Ar
for 15 min before measurement. The upconversion quantum
yields were determined with a laser dye 4-dicyanomethylene-6-
(p-dimethylaminostyryl)-2-methyl-4H-pyran (DCM) as the quan-
tum yield standards (U = 0.10 in CH2Cl2) and the quantum yields
were calculated with eqn (1),11 where Uunk, Aunk, Iunk and hunk
represents the quantum yield, absorbance, integrated photolumi-
nescence intensity and the refractive index of the samples. The
photography of the upconversion were taken with Sumsang NV
Fig. 1 UV-vis absorption of L-1, Pt-1 and Pt-2, and the normalized
emission of Pt-1 and Pt-2. c = 1.0 ¥ 10-5 M in toluene, 20 ◦C.
The absorption feature of L-1 is recognizable in Pt-1. This is
different from the normal Pt(II) bisacetylide complexes, which
usually show red-shifted absorption compared to the free acetylide
ligand.1,6,7,14 Furthermore, the UV-vis absorption of Pt-1 is not
sensitive to the polarity of the solvents (Fig. 2a). On the contrary,
Pt-2 shows negative solvatochromism (Fig. 2b), indicating that
7836 | Dalton Trans., 2011, 40, 7834–7841
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Fig. 2 UV-Vis absorption spectra of (a) Pt-1 and (b) Pt-2 in different
solutions. c = 1.0 ¥ 10-5 M, 20 ◦C.
Fig. 3 Emission spectra of (a) Pt-1 (lex = 474 nm) and (b) Pt-2 (lex = 403
nm) in different solutions. Under N2 atmosphere. c = 1.0 ¥ 10-5 M, 20 ◦C.
area in Pt-2 defines a region of high electron density at the metal
center, while the corresponding green area depicts a diminished
electronic density. For Pt-1, a similar profile was observed, i.e. the
Pt(II) center has higher electron density than the other areas of the
complex (Fig. 4), thus the Lewis base quenching mechanism is not
applicable to Pt-1.32
CT absorption is the major component of the UV-vis absorption
bands of Pt-2.27 All these results infer that the absorption of Pt-1
is IL in character, not the typical MLCT for Pt(II) bisacetylide
complexes.1
An emission band at 624 nm with vibrational progression
was found for Pt-1 (Fig. 1). A structured band is characteristic
3
for IL emission.1,10,12,14,20 On the contrary, structureless emission
was observed for Pt-2 (567 nm). Thus the emission of Pt-1 was
attributed to the 3IL excited state localized on the coumarin
(tT = 2.44 ms). A Pt(II) acetylide complex with an ethynyl–flavone
ligand was reported to show emission at 570 nm (t = 21 ms).28
The phosphorescence quantum yield of Pt-1 is low (UP = 2.6%),
3
probably due to the equilibrium of the MLCT state (herein it is
3
non-emissive) with the IL excited state.29 It is known that Pt(II)
bisacetylide complexes with an electron-donating acetylide ligand
are non-phosphorescent.30,31 Pt-1 is phosphorescent in the solid
state (ESI†).
The emission of Pt-1 is strongly solvent polarity dependent
(Fig. 3a). It is highly emissive in toluene but the phosphorescence
is significantly quenched in polar solvents, such as in ethyl acetate
or in methylene dichloride, and it becomes non-phosphorescent in
acetonitrile. We propose that the non-luminescent 3MLCT excited
state will be more significantly stabilized in polar solvents than 3IL
Fig. 4 Potential surfaces of electron density for the T1 excited state of
Pt-1 (left) and Pt-2 (right). The red color represents areas of high electron
density and the green color indicates areas with diminished electron density
(isovalue = 0.0004). Calculated at B3LYP/3-21G level with the T1 state
geometry with Gaussian 09W.
3
states (3MLCT becomes lower in energy than IL excited state),
thus Pt-1 is non-phosphorescent in CH3CN.16 The emission of
Pt-2 is relatively less solvent polarity-dependent (Fig. 3b).
77 K emission, nanosecond time-resolved transient absorption and
spin density of the complexes: assignment of the 3IL features of the
T1 state
Previously it was found that the phosphorescence of the 2-(4¢-
nitrophenyl)pyridine Pt(II) (acac) complex can be quenched by a
weak Lewis base, such as toluene, dichloromethane, etc., due to the
strong positively charged Pt(II) center of the complex.32 In order
to exclude the exciplex quenching of Pt-1 by the polar solvents
(Lewis bases), the electrostatic potential surfaces of the complexes
Pt-1 and Pt-2 at the T1 state were studied (Fig. 4). The red-colored
The emission of Pt-1 at 77 K shows more significant vibration
progression than the emission spectrum at RT. Notably the
thermally induced Stokes shift is very small (DEs = 182 cm-1) (Fig.
3
5), a small DEs is an established evidence for the IL emission.
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7834–7841 | 7837
©

Fig. 5 Photoluminescence spectra of Pt-1 at RT (in toluene) and 77 K (in
EtOH–MeOH, 4 : 1, v/v). lex = 414 nm.
Fig. 7 Spin density surfaces for the T1 excited state of Pt-1 and Pt-2.
Calculated at B3LYP/6-31g(d)/LanL2DZ level based on the optimized
T1 state geometry (isovalue = 0.0004), with Gaussian 09W.
In contrast, substantial a DEs value of 2470 cm-1 (0.31 eV) was
reported for the model complex Pt-2, for which the emissive T1
state is MLCT.8b
Application of Pt-1 as triplet sensitizer for
triplet–triplet-annihilation upconversion
3
In order to prove the IL feature of the triplet excited state of
Next we set out to utilize Pt-1 for triplet–triplet-annihilation
(TTA) based upconversion.11,34 TTA upconversion requires a
triplet sensitizer and an acceptor to accomplish the cascade pro-
cesses of light-harvesting, triplet–triplet-energy-transfer (TTET),
TTA and the upconverted fluorescence emission.35 This upcon-
version scheme is particularly promising for application due to
its low excitation power (even lower than the terrestrial solar
radiation level, 0.1 W cm-2, AM 1.5G).36,37 Furthermore, the exci-
tation/emission wavelength of TTA upconversion can be readily
optimized by independent selection of the triplet sensitizers and
acceptors. Currently the triplet sensitizers of TTA upconversion
are limited to porphyrin Pt(II)/Pd(II) complexes.35–37 However, it
Pt-1, nanosecond time-resolved transient absorption spectroscopy
was studied (Fig. 6). Significant bleaching at 400 nm was observed
for Pt-1 upon pulsed laser excitation, which unambiguously
proved the coumarin localized 3IL excited state. Minor bleaching
was observed at 470 nm. Transient absorption was observed
in the range of 500 nm–700 nm. The transients of Pt-1 are
drastically different from that of the model complex Pt-2.33
Pt-2 did not show any transient absorption in the range of
500 nm–700 nm.17,20,34
Fig. 6 Transient absorption spectra of Pt-1 after pulsed laser excitation
(lex = 532 nm). c = 1.5 ¥ 10-5 M in toluene, 20 ◦C.
The isosurfaces of the spin density of T1 state of Pt-1 support the
IL feature of the T1 state (Fig. 7). For Pt-2, the spin density surface
is localized on the phenyl acetylide ligands, the Pt(II) center and
the dbbpy ligand. This spin density distribution is in line with the
known 3MLCT/3LLCT feature of dbbpy Pt(II) bisphenylacetylide.
For Pt-1, the coumarin ligand contributes more significantly to
the spin density. We proved that the energy level of the triplet
excited state of the coumarin acetylide (T1 = 576 nm, by DFT
calculation) is close to the 3MLCT state of the complex (see
Supporting Information†), thus the 3IL excited state is the T1 state
of Pt-1. This assignment was supported by the NBO analysis of
the T1 excited state (see ESI for detailed analysis†).
Fig. 8 Triplet–triplet-annihilation upconversion with Pt-1 and Pt-2 as
triplet sensitizers. (a) Emission of the sensitizers Pt-1 and Pt-2 alone. (b)
Emission of Pt-1 and Pt-2 in the presence of DPA. Excited with 473 nm
laser (5 mW). csensitizers = 1.0 ¥ 10-5 M, cDPA = 1.32 ¥ 10-4 M. 20 ◦C. The
asterisks indicate the scattered 473 nm laser.
7838 | Dalton Trans., 2011, 40, 7834–7841
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©

Table 1 Photophysical parameters of L-1, Pt-1 and Pt-2
is difficult to modify the porphyrin complexes to optimize the
excitation/emission wavelength. Thus new sensitizers have to be
developed. Herein we used Pt-1 as the triplet sensitizer for TTA
upconversion.
a
c
e
labs
eb
lem
Ud
tL
tT (ms)f
L-1
Pt-1
Pt-2
397
414
424
3.66
3.23
0.88
443
624
567
84.2%
2.6%
42.2%
2.8 ns
2.52 ms
1.27 ms
—
2.44
—
The emission of Pt-1 is much weaker than Pt-2 with 473 nm
laser excitation (Fig. 8a). DPA was selected as the triplet
acceptor.11,38 In the presence of DPA, upconverted fluorescence
(400 nm–550 nm) was observed upon selective excitation of the
sensitizers at 473 nm. Irradiation of DPA alone at 473 nm did
not produce any emission, and thus verified the upconverted
emission with Pt-1 or Pt-2 as the triplet sensitizers. Upconversion
quantum yields (UUC) of 14.1% and 8.9% were determined for
Pt-1 and Pt-2 under our experimental conditions, respectively
(Table 2).
a In toluene (1.0 ¥ 10-5 mol dm-3). b Molar extinction coefficient at the
absorption maxima. e: 104/cm-1 mol-1 dm3. c In toluene. d In toluene, with
[Ru(bpy)2(phen)][PF6]2 as standard (U = 0.06 in deaerated acetonitrile).
e Luminescence lifetime. In toluene. f Triplet state lifetime, measured by
time-resolved transient absorption. 1.5 ¥ 10-5 mol dm-3 in toluene.
Table 2 Stern–Volmer quenching constant (KSV
quenching constants (kq) of Pt-1 and Pt-2 for TTET with DPA as acceptor
and the upconversion quantum yields (UUC). 20 ◦
) and bimolecular
C
Notably, the peak area of the quenched phosphorescence of Pt-1
by DPA is not significant, although the upconverted fluorescence
is significant. For Pt-2, the upconversion is accompanied with
quenching of the phosphorescence. We propose a non-emissive
portion of the Pt-1 sensitizers that in the triplet excited state is
Ksv/(103 M-1)
kq/(109 M-1s-1)
UUC
Pt-1
Pt-2
5.56
3.77
2.21
2.97
14.1%
8.9%
3
involved in TTET. It should be pointed out that IL is weakly
Application of the complexes for luminescent oxygen sensing
phosphorescent (UP = 2.6%). This is the first time that sensitizer
molecules at excited states that are otherwise non-emissive were
proposed to be involved in the TTA upconversion. The efficiency
of the TTET process of the TTA upconversion was studied.
The Stern–Volmer quenching constants of 5.56 ¥ 103 M-1 and
3.77 ¥ 103 M-1 were determined for Pt-1 and Pt-2, respectively
(Fig. 9 and Table 2).
Firstly the sensitivity of the complex to oxygen in toluene
solution was studied (Fig. 10). Pt-1 is strongly phosphorescent in
nitrogen saturated solution (Fig. 10a). The emission is quenched
under air atmosphere or in oxygen saturated solution. Thus the
phosphorescence of Pt-1 is sensitive to O2. For Pt-2, however,
the emission under air atmosphere is still significant compared
to that under N2 atmosphere. These results indicate that the
phosphorescence lifetime of Pt-1 is longer than that of Pt-2
(Table 1).
Fig. 9 Stern–Volmer plots generated from the phosphorescence intensity
quenching of complex Ru-1 (lex = 474 nm), Pt-1 (lex = 474 nm), Pt-2 (lex
=
403 nm) as a function of DPA concentration. In toluene, c = 1.0 ¥ 10-5 M,
20 ◦C.
The upconversion quantum yield of Pt-1 is among the highest
values ever reported.35 Considering the large molar extinction co-
efficient, the absolute upconverted fluorescence emission of Pt-1 is
ca. 5-fold of Pt-2. Recently [Pt(ttpy)(C CPh)]ClO4 (where ttpy =
4¢-p-tolylterpyridine) was reported for TTA based upconversion.39
However, the UUC is only 1.1%, despite its long phosphorescence
lifetime (4.6 ms).39 Using Pt(II) acetylide complexes as triplet
sensitizers for TTA upconversion is attractive because the pho-
tophysical properties of the Pt(II) complex sensitizers, such as the
excitation wavelength, can be readily optimized by using different
acetylide ligands.
Fig. 10 Emission spectra of (a) Pt-1 (lex = 474 nm) and (b) Pt-2 (lex
=
403 nm) in toluene saturated with air, oxygen and nitrogen. c = 1.0 ¥
10-5 M, 20 ◦C.
For practical applications the O2 sensing will be performed
with the complexes embedded in solid supporting matrix, such
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7834–7841 | 7839
©

as in polymer films.40 We selected a polymer IMPEK-C as the
supporting matrix and the oxygen sensing with the films was
studied with a flow-cell, which is coupled to a spectrometer with an
optical fiber bundle.12 Two kinds of experiment were performed.
The first one is to test the response of the films to the saturation
switch between the O2 and N2 (Fig. 11). Fast response and recovery
was observed with the films. Both t↓95 and t↑95 are 4 s.
Fig. 12 Phosphorescence intensity response of the complexes to O2/N2
switches. (a) Pt-1, lex = 412 nm, lem = 624.3 nm. (b) Pt-2, lex = 424 nm,
lem = 567 nm. 20 ◦C.
Fig. 11 Phosphorescence intensity response of the oxygen sensing films
toward O2/N2 switches. (a) Pt-1, lex = 412 nm, lem = 624.3 nm. (b) Pt-2,
lex = 424 nm, lem = 567 nm. 20 ◦C.
Good photostability were observed for the complexes, i.e. the
phosphorescence of the complexes is fully recovered after exposure
to continuous irradiation during the O2 sensing experiments, which
is ideal for practical applications. Previously a cyclometalated
Ir(III) complex were used for O2 sensing but it was unstable toward
photoexcitation.23
In order to quantitatively evaluate the O2 sensitivity of the
complexes, the response of the phosphorescence of the sensing
films toward small steps of variation of O2 partial pressure were
studied (Fig. 12). Pt-1 is more significantly quenched than Pt-2.
The oxygen sensing data of Pt-1 and Pt-2 were fitted with the
two-sites model (Fig. 13 and ESI†).10 The Stern–Volmer quenching
constants of Pt-1 and Pt-2 were determined as 0.012 and 0.001
Torr-1, respectively. Thus the oxygen sensitivity of Pt-1 is 12 -fold
of Pt-2.
Fig. 13 Two-sites model plots for sensing film of Pt-1 and Pt-2 in
IMPEK-C. Intensity ratios I0/I vs. O2 partial pressure (Torr). 20 ◦C.
connected to a Pt(II) center. The complex shows enhanced UV-vis
absorption (e = 32 300 M-1 cm-1 at 414 nm), red-emission at 625 nm,
phosphorescence lifetime (tP) of 2.44 ms and quantum yield
(UP) of 2.6%. The phosphorescence of the complex is dependent
on the polarity of the solvents. Coumarin-localized intraligand
emissive triplet excited (3IL) was proposed to be responsible for
the emission of the complex. To the best of our knowledge,
this is the first time that the room temperature phosphorescence
of coumarin was observed. The complex was used as triplet
sensitizer for triplet–triplet-annihilation based upconversion and
upconversion quantum yield (UUC) up to 14.1% was observed. We
propose that sensitizer molecules at the triplet excited state that
are otherwise non-emissive were involved in the triplet–triplet-
energy-transfer (TTET) process. The complexes were also used
for another TTET process, i.e. luminescent oxygen sensing. The
Conclusions
In conclusion, a diimine Pt(II) bis(coumarin acetylide) complex
was prepared, in which the fluorophore of coumarin was directly
7840 | Dalton Trans., 2011, 40, 7834–7841
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The Royal Society of Chemistry 2011
©

Stern–Volmer quenching constant of the coumarin-containing
complex is 12-fold of the model complex. Good photostability was
observed for the complexes during the luminescent oxygen sensing
experiments. Our strategy of directly attaching a fluorophore to
Pt(II) atom via an acetylide linker to access Pt(II) complexes with
intense absorption in the visible region and long-lived emissive 3IL
excited state (ligand phosphorescence) will be useful for design of
light-harvesting transition metal complexes and for applications
in photovoltaics, in photocatalysis and in upconversions materials,
etc.
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Acknowledgements
We thank the NSFC (20972024 and 21073028), the Fundamental
Research Funds for the Central Universities (DUT10ZD212 and
DUT11LK19), Ministry of Education (SRFDP-200801410004
and NCET-08-0077), the Royal Society (UK) and NSFC (China-
UK Cost-Share Science Networks, 21011130154), the Education
Department of Liaoning Province (2009T015) and State Key
Laboratory of Fine Chemicals (KF0802) for financial support.
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©