Visible light-harvesting trans bis(alkylphosphine) platinum(ii)-alkynyl complexes showing long-lived triplet excited states as triplet photosensitizers for triplet-triplet annihilation upconversion

Article abstract of DOI:10.1039/c3dt50496d

Symmetric and asymmetric linear trans-bis(tributylphosphine) Pt(ii) bis(acetylide) complexes with functionalized aryl alkynyl ligands (coumarin, naphthalimide and phenyl acetylides) were prepared, which show enhanced absorption in the visible region (mola

Full text of DOI:10.1039/c3dt50496d

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Visible light-harvesting trans bis(alkylphosphine)
platinum(II)-alkynyl complexes showing long-lived
triplet excited states as triplet photosensitizers for
triplet–triplet annihilation upconversion†
Cite this: DOI: 10.1039/c3dt50496d
Lianlian Liu,a Dandan Huang,a Sylvia M. Draper,b Xiuyu Yi,a Wanhua Wua and
Jianzhang Zhao*a
Symmetric and asymmetric linear trans-bis(tributylphosphine) Pt(II) bis(acetylide) complexes with functio-
nalized aryl alkynyl ligands (coumarin, naphthalimide and phenyl acetylides) were prepared, which show
enhanced absorption in the visible region (molar absorption coefficients up to 76 800 M−1 cm−1 at
459 nm) and long-lived triplet excited states (up to 139.9 μs). At room temperature, the naphthalimide
acetylide–phenyl acetylide complex (Pt-4) shows dual emission (fluorescence–phosphorescence), whereas
other complexes show only fluorescence emission. The triplet excited states of the complexes were
studied with nanosecond time-resolved transient difference absorption spectroscopy and DFT calculations
on the spin density surface. The complexes were used as triplet photosensitizers for ratiometric O2
sensing, as well as triplet–triplet annihilation (TTA) upconversion (upconversion quantum yield up to
27.2%). The TTA upconversion of the complexes requires triplet acceptors with different T1 state energy
levels and was studied with nanosecond time-resolved emission spectroscopy. Our results are useful for
designing new Pt(II) complexes that show strong absorption of visible light and long-lived triplet excited
states, as well as for the application of these complexes as triplet photosensitizers for O2 sensing, photo-
catalysis and TTA upconversion.
Received 23rd February 2013,
Accepted 3rd April 2013
DOI: 10.1039/c3dt50496d
www.rsc.org/dalton
been used for non-linear optics and photovoltaics.3–8 However,
most of the reported linear phosphine Pt(II) acetylide com-
Introduction
Pt(II) complexes have attracted much attention due to their plexes show weak absorption in the visible region and short-
applications in photocatalysis,1,2 photovoltaics,3 non-linear lived triplet excited states (at RT),3,4,19–21 which are detrimental
optics and molecular probes,4–8 and more recently in triplet– for the application of these complexes as triplet photosensiti-
triplet annihilation upconversions.9–12 In most of these appli- zers. To the best of our knowledge, no linear bis(alkylphos-
cations, the absorption of visible light and long-lived triplet phine) Pt(II) acetylide complexes have been used as triplet
excited states is crucial. However, conventional Pt(II) complexes photosensitizers for luminescent oxygen (O2) sensing and very
usually show weak absorption in the visible region and short few were used for TTA upconversion.9–12,21c,22
T1 state lifetimes (less than 10 μs).13–18
In order to address these challenges, herein we used aryl
Linear bis(alkylphosphine) Pt(II) acetylide complexes are in acetylides to synthesis linear bis(alkylphosphine) Pt(II) bis-
particular interesting because their molecular structure and (acetylide) complexes, i.e. naphthalimide and coumarin acety-
their photophysical properties are readily tunable by using lides, to induce strong absorption of visible light and long-
different acetylide ligands.3,4,14,19–21 These complexes have lived triplet excited states. The photophysical properties of the
complexes were studied with UV-vis absorption and lumines-
cence spectra. The triplet excited states of the complexes were
aState Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, E 208 Western Compus, 2 Ling-Gong Road, Dalian 116012,
P. R. China. E-mail: zhaojzh@dlut.edu.cn; http://finechem.dlut.edu.cn/photochem
bSchool of Chemistry, University of Dublin, Trinity College, Dublin 2, Ireland
†Electronic supplementary information (ESI) available: The synthesis and the
structure characterization data of the Pt(II) complexes; the details of TTA upcon-
version and DFT calculation information. See DOI: 10.1039/c3dt50496d
studied with nanosecond time-resolved transient difference
absorption spectroscopy and DFT calculations on the spin
density surfaces. These studies demonstrate that the com-
plexes show strong absorption of visible light and long-lived
triplet excited states such that the complexes could be used as
triplet photosensitizers for luminescent oxygen (O2) sensing
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and TTA upconversion. The performance of the triplet photo- 1.19 (t, 6H, J = 8.0 Hz). ESI-HRMS: calcd ([C13H14BrNO2 + H]+)
sensitizers was much improved compared to the conventional m/z = 296.0286, found m/z = 296.0270.
Pt(II) complexes that show weak absorption of visible light and
Compound 3
short-lived triplet excited states.
Compound 2 (300.0 mg, 1.013 mmol), 2-thienylboronic acid
(281.8 mg, 2.2 mmol) and K2CO3 (608.2 mg, 4.4 mmol) were
dissolved in a mixed solvent MeOH–PhCH3 (3 : 4, v/v, 20 mL),
and then Pd(PPh3)4 (60.2 mg, 0.052 mmol) was added. The
flask was vacuumed and back-filled with Ar several times. The
Experimental
Materials and reagents
All the chemicals were analytically pure and were used as reaction mixture was heated at 75 °C for 30 min in a microwave
received. Solvents were dried and distilled before use.
reactor. Water was added, then the mixture was extracted with
CH2Cl2 (3 × 50 mL). The organic phase was dried with Na2SO4,
filtered and the solvent was removed under reduced pressure.
The crude product was further purified using column chrom-
atography (silica gel, CH2Cl2–petroleum ether = 1 : 1, v/v) to
Analytical measurements
NMR spectra were recorded on a 400 MHz Varian Unity Inova
spectrophotometer (with TMS as the standard). Mass spectra
were recorded with a Q-TOF Micro MS spectrometer. UV-vis
absorption spectra were measured with an Agilent 8453 UV-vis
spectrophotometer. Fluorescence spectra were recorded on a
Shimadzu RF-5301PC spectrofluorometer. Upconversion was
carried out on a modified Sanco 970 CRT spectrofluorometer.
Phosphorescence quantum yields were measured with [Ru-
(dmb)3(PF6)2] as the standard (Φ = 0.073 in acetonitrile. dmb =
4,4′-dimethyl-2,2′-bipyridine). Luminescence lifetimes were
measured on an OB920 luminescence lifetime spectrometer
(Edinburgh Instruments, UK) and an FLS920 spectrofluoro-
meter (Edinburgh Instruments, UK). Complex 3 was syn-
thesised with a microwave reaction (MCR-3 microwave reactor).
The temperature of the reaction mixture was kept at 75 °C.
1
give 3 as a light yellow solid (250.4 mg), yield: 82.4%. H NMR
(400 MHz, CDCl3): 7.88 (s, 1H), 7.66 (d, 1H, J = 4.0 Hz),
7.33–7.31 (m, 2H), 7.08 (d, 1H, J = 4.0 Hz), 6.61 (d, 1H, J =
8.0 Hz), 6.54 (s, 1H), 3.44–3.42 (m, 4H), 1.21 (t, 6H, J = 8.0 Hz).
ESI-HRMS: calcd ([C17H17SNO2 + H]+) m/z = 300.1058, found
m/z = 300.1044.
Compound 4
Compound 3 (62.0 mg, 0.21 mmol) was dissolved in a mixed
solvent CHCl3–AcOH (5 mL/5 mL), then NBS (44.2 mg,
0.249 mmol) was added at 0 °C, in three portions at 20 min
intervals. The mixture was stirred at RT for 4 h. After addition
of water, the mixture was extracted with CH2Cl2 (3 × 50 mL).
The organic phase was dried with Na2SO4, filtered and the
solvent was removed under reduced pressure. The crude
product was further purified using column chromatography
(silica gel, CH2Cl2–petroleum ether = 2 : 3, v/v) to give 4 as a
Complex 13,8,14
Under an Ar atmosphere, phenylacetylene (13.8 mg,
0.135 mmol) was added to the solution of cis-Pt[P(n-Bu)3]2Cl2
(100.0 mg, 0.149 mmol) in diethylamine (NHEt2) (6 mL), the
flask was evacuated and back-filled with Ar several times. The
mixture was heated at 45 °C and stirred for 6 h. After complete
consumption of the starting material, the solvents were evap-
orated under reduced pressure, and the crude product was
1
light yellow solid (62.1 mg), yield: 78.0%. H NMR (400 MHz,
CDCl3): 7.83 (s, 1H), 7.34–7.33 (m, 2H), 7.03 (s, 1H), 6.62 (d,
1H, J = 8.0 Hz), 6.53 (s, 1H), 3.46–3.41 (m, 4H), 1.21 (t, 6H, J =
8.0 Hz). ESI-HRMS: calcd ([C17H16SNO2Br + Na]+) m/z =
399.9983, found m/z = 399.9975.
further purified using column chromatography (silica gel, Compound 5
CH2Cl2–hexane = 2 : 3, v/v) to give 1 as a light yellow solid
Under an Ar atmosphere, Pd(PPh3)2Cl2 (9.3 mg, 0.0132 mmol),
1
(75.2 mg), yield: 75.7%. H NMR (400 MHz, CDCl3): 7.25–7.18
PPh3 (6.9 mg, 0.0264 mmol) and CuI (5.0 mg, 0.0264 mmol)
were added to a solution of 4 (50.0 mg, 0.132 mmol) in a
mixed solvent of THF–Et3N (6 mL/2 mL). Trimethylsilylacety-
lene (64.8 mg, 0.66 mmol) was added via a syringe. The
mixture was stirred at 70 °C for 8 h. After the addition of
(m, 4H), 7.14–7.11 (m, 1H), 2.03–1.99 (m, 12H), 1.57–1.54
(m, 12H), 1.49–1.40 (m, 12H), 0.92 (t, 18H, J = 7.3 Hz).
Compound 2
Bromine (367.6 mg, 2.3 mmol) was added via a syringe to a water, the mixture was extracted with CH2Cl2 (3 × 50 mL). The
solution of coumarin (500.0 mg, 2.3 mmol) in acetic acid organic phase was dried with Na2SO4, filtered and the solvent
(13 mL) in 40 min. The mixture was stirred at RT for 2 h. Then was removed under reduced pressure. The crude product was
Na2S2O3 (570.8 mg, 2.3 mmol) was added and the mixture was further purified using column chromatography (silica gel,
stirred at RT for 10 min. The solid was collected with filtration. CH2Cl2–petroleum ether = 1 : 1, v/v) to give 5 as a dark yellow
1
After washing with acetic acid and water, the residue was dried solid (23.5 mg), yield: 44.7%. H NMR (400 MHz, CDCl3): 7.86
under vacuum. The crude product was further purified using (s, 1H), 7.47 (d, 1H, J = 4.0 Hz), 7.30 (d, 1H, J = 12.0 Hz), 7.18
column chromatography (silica gel, CH2Cl2–petroleum ether = (d, 1H, J = 4.0 Hz), 6.60 (d, 1H, J = 12.0 Hz), 6.50 (s, 1H),
3 : 2, v/v) to give 2 as a light yellow solid (613.1 mg), yield: 3.44–3.41 (m, 4H), 1.21 (t, 6H, J = 8.0 Hz), 0.25 (s, 9H).
90.0%. 1H NMR (400 MHz, CDCl3): 7.88 (s, 1H), 7.20 (d, 1H, J = ESI-HRMS: calcd ([C22H25SNO2Si + H]+) m/z = 396.1454, found
8.0 Hz), 6.58 (d, 1H, J = 8.0 Hz), 6.48 (s, 1H), 3.44–3.39 (m, 4H), m/z = 396.1446.
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Ligand L-1
calcd ([C51H75NO2P2SPt]+) m/z = 1022.4642, found m/z =
1022.4685.
K2CO3 (207.0 mg, 1.5 mmol) was added to a solution of com-
pound 5 (300.2 mg, 0.77 mmol) in a mixed solvent THF–
MeOH (8 mL/4 mL). The mixture was stirred at room tempera-
ture for 1.5 h. After the addition of water, CH2Cl2 was added to
extract the product (3 × 50 mL). The organic phase was dried
with Na2SO4, filtered and the solvent was evaporated under
reduced pressure. The crude product was further purified
using column chromatography (silica gel, CH2Cl2–petroleum
ether = 1 : 1, v/v) to give L-1 as a light red solid (35.6 mg), yield:
87.3%. 1H NMR (400 MHz, CDCl3): 7.86 (s, 1H), 7.46 (d, 1H, J =
4.0 Hz), 7.30 (d, 1H, J = 8.0 Hz), 7.22 (d, 1H, J = 4.0 Hz), 6.59
(d, 1H, J = 12.0 Hz), 6.50 (s, 1H), 3.45–3.40 (m, 5H), 1.20 (t, 6H,
J = 8.0 Hz). 13C NMR (100 MHz, CDCl3): 160.6, 155.8, 151.0,
139.3, 137.3, 133.4, 129.3, 124.0, 121.9, 114.0, 109.6, 108.7,
Compound Pt-2
The synthetic procedure is similar to that of Pt-1, except that
Pt-0 (65.7 mg, 0.068 mmol) was used. The crude product was
further purified using column chromatography (silica gel,
CH2Cl2–petroleum ether = 1 : 1, v/v) to give Pt-2 as a dark
yellow solid (43.9 mg), yield: 51.5%. 1H NMR (400 MHz,
CDCl3): 8.77 (d, 1H, J = 8.0 Hz), 8.57 (d, 1H, J = 4.0 Hz), 8.44 (d,
1H, J = 8.0 Hz), 7.78 (s, 1H), 7.69–7.66 (m, 1H), 7.6 (d, 1H, J =
8.0 Hz), 7.5 (d, 1H, J = 4.0 Hz), 7.3 (d, 1H, J = 12.0 Hz), 6.85 (d,
1H, J = 4.0 Hz), 6.64–6.55 (m, 2H), 4.08–4.14 (m, 2H), 3.44–3.40
(m, 4H), 2.09 (t, 12H, J = 8.0 Hz), 1.64 (s, 12H), 1.48–1.23 (m,
27H), 0.95–0.88 (m, 24H). 13C NMR (100 MHz, CDCl3): 165.1,
164.8, 160.6, 155.4, 150.3, 135.7, 134.4, 134.0, 133.5, 132.2,
131.2, 131.0, 128.7, 126.0, 125.0, 122.9, 118.5, 115.2, 109.3,
109.1, 97.3, 94.5, 70.6, 46.0, 45.0, 44.2, 38.1, 32.0, 30.9, 29.8,
28.9, 26.6, 24.5, 24.2, 23.2, 22.8, 14.2, 14.0, 12.6, 10.8, 8.7 ppm.
MALDI-HRMS: calcd ([C65H92N2O4P2SPt]+) m/z = 1253.5901,
found m/z = 1253.5989.
97.2, 82.2, 45.1, 12.7 ppm. ESI-HRMS: calcd ([C19H17SNO2
+
H]+) m/z = 324.1058, found m/z = 324.1056.
Complex Pt-0
The synthetic procedure is similar to that of 1, except that
naphthalimide acetylene (41.3 mg, 0.124 mmol) was used. The
crude product was further purified using column chromato- Compound Pt-3
graphy (silica gel, CH2Cl2–petroleum ether = 1 : 2, v/v) to give
The synthetic procedure is similar to that of 1, except that L-1
Pt-0 as a light yellow oil (63.2 mg), yield: 52.2%. 1H NMR
(400 MHz, CDCl3): 8.68 (d, 1H, J = 8.0 Hz), 8.53 (d, 1H, J =
4.0 Hz), 8.40 (d, 1H, J = 8.0 Hz), 7.64–7.62 (m, 1H), 7.54 (d, 1H,
J = 8.0 Hz), 4.12–4.03 (m, 2H), 1.97 (t, 12H, J = 4.0 Hz),
1.57–1.56 (m, 12H), 1.21–1.41 (m, 21H), 0.83–0.89 (m, 24H).
13C NMR (100 MHz, CDCl3): 165.0, 164.7, 133.8, 133.3, 131.3,
131.2, 128.9, 128.7, 126.2, 123.1, 118.8, 102.2, 100.6, 44.2, 38.1,
30.9, 29.8, 28.9, 26.3, 24.5, 24.4, 24.2, 23.2, 22.3, 22.2, 14.2,
14.0, 10.8 ppm. MALDI-HRMS: calcd ([C46H76NO2P2PtCl + H]+)
m/z = 967.4766, found m/z = 967.4691.
(40.0 mg, 0.008 mmol) was used. The crude product was
further purified by column chromatography (silica gel,
CH2Cl2–petroleum ether = 2 : 1, v/v) to give Pt-3 as a dark
yellow solid (59.8 mg), yield: 38.7%. 1H NMR (400 MHz,
CDCl3): 7.75 (s, 2H), 7.5 (d, 2H, J = 4.0 Hz), 7.28 (d, 2H, J = 12.0
Hz), 6.81 (d, 2H, J = 4.0 Hz), 6.58 (d, 2H, J = 4.0 Hz), 6.52 (s,
2H), 3.43–3.4 (m, 8H), 2.11 (t, 12H, J = 4.0 Hz), 1.72–1.6 (m,
12H), 1.51–1.48 (m, 12H), 1.2 (t, 12H, J = 8.0 Hz), 0.93 (t, 18H,
J = 8.0 Hz). MALDI-HRMS: calcd ([C62H86N2O4P2S2Pt + H]+)
m/z = 1244.5230, found m/z = 1244.5228.
Compound Pt-1
Compound Pt-4
Under an Argon atmosphere, compound
1 (50.1 mg, The synthetic procedure is similar to that of Pt-1, except that
0.068 mmol) and L-1 (22.0 mg, 0.068 mmol) were dissolved in Pt-0 (20.0 mg, 0.0207 mmol) and phenylacetylene (2.5 mg,
a mixed solvent THF–Et2NH (3 mL/3 mL). The flask was evacu- 0.025 mmol) were used. The crude product was further puri-
ated and back-filled with Ar several times, and then CuI fied using column chromatography (silica gel, CH2Cl2–pet-
(2.9 mg, 0.015 mmol) was added, the mixture was stirred at RT roleum ether = 1 : 2, v/v) to give Pt-4 as a light yellow oil
for 30 min. Deionized water was used to quench the reaction (15.0 mg), yield: 70.1%. 1H NMR (400 MHz, CDCl3): 7.78 (d,
mixture, CH2Cl2 was added to extract the product (3 × 50 mL). 1H, J = 8.0 Hz), 8.56 (d, 1H, J = 8.0 Hz), 8.44 (d, 1H, J = 8.0 Hz),
The organic phase was dried with Na2SO4, filtered and the 7.69–7.65 (m, 1H), 7.6 (d, 1H, J = 8.0 Hz), 7.30–7.22 (m, 4H),
solvent was removed under reduced pressure. The crude 7.15–7.12 (m, 1H), 4.17–4.06 (m, 2H), 2.13 (t, 12H, J = 4.0 Hz),
product was further purified using column chromatography 1.64–1.63 (m, 12H), 1.48–1.30 (m, 21H), 0.89 (t, 24H, J =
(silica gel, CH2Cl2–petroleum ether = 1 : 2, v/v) to give Pt-1 as a 8.0 Hz). 13C NMR (100 MHz, CDCl3): 165.2, 164.9, 134.2, 133.7,
1
dark yellow solid (29.8 mg), yield: 42.9%. H NMR (400 MHz, 132.3, 131.3, 131.0, 128.7, 128.1, 126.1, 125.3, 118.5, 110.0,
CDCl3): 7.75 (s, 1H), 7.49 (s, 1H), 7.30–7.11 (m, 6H), 6.81 (d, 108.1, 106.6, 44.2, 38.2, 31.0, 29.9, 28.9, 26.6, 24.6, 24.3, 24.1,
1H, J = 4.0 Hz), 6.58 (d, 1H, J = 8.0 Hz), 6.51 (s, 1H), 3.44–3.39 23.3, 14.3, 14.0, 10.9 ppm. MALDI-HRMS: calcd
(m, 4H), 2.11 (t, 12H, J = 8.0 Hz), 1.6 (s, 12H), 1.49–1.46 (m, ([C54H81NO2P2Pt
12H), 1.2 (t, 6H, J = 8.0 Hz), 0.92 (t, 18H, J = 8.0 Hz). 13C NMR 1033.5457.
(100 MHz, CDCl3): 160.6, 155.4, 150.3, 135.5, 134.1, 130.9,
+ = 1033.5469, found m/z =
H]+) m/z
Compound Pt-5
130.3, 129.2, 128.7, 128.0, 125.3, 125.0, 117.0, 115.4, 109.3,
109.2, 107.8, 101.6, 97.3, 65.7, 45.0, 32.1, 30.7, 29.9, 29.5, 26.5, The synthetic procedure is similar to that of 1, except that
24.6, 24.1, 23.9, 22.9, 19.4, 14.0, 12.7 ppm. MALDI-HRMS: phenylacetylene (20.0 mg, 0.2 mmol) was used. The crude
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product was further purified using column chromatography absorption in the UV/blue region, and their molar extinction
(silica gel, CH2Cl2–petroleum ether = 2 : 3, v/v) to give Pt-5 as a coefficient is not large. Thiophene units are often used to
white solid (120.3 mg), yield: 75.0%. 1H NMR (400 MHz, extend π-conjugation.29,30 Herein we attached the thiophene
CDCl3): 7.26 (d, 4H, J = 4.0 Hz), 7.17 (t, 4H, J = 8.0 Hz), moiety to a coumarin unit to enhance the UV-Vis absorption.
7.12–7.10 (m, 2H), 2.13 (t, 12H, J = 4.0 Hz), 1.61–1.60 (m, 12H), The ethenyl bond is attached on the thiophene unit therefore
1.47–1.41 (m, 12H), 0.90 (t, 18H, J = 8.0 Hz). TOF MS: calcd we envisaged an electronic communication between the Pt(II)
([C40H64P2Pt]+) m/z = 801.4131, found m/z = 801.5756.
center and the coumarin unit (Scheme 1). Furthermore, we
also used naphthalimide (NI) for accessing the long-lived 3IL
excited state.31–33 The two acetylide ligands in the complex
molecules are the same (Pt-3) or different (Pt-1, Pt-2 and Pt-4).
Pt-0 was prepared as a model complex.
The preparation of the coumarin ligands is based on the
Pd(0) catalyzed Suzuki and Sonogashira coupling reactions.
The preparation of the NI acetylide ligand was based on the
reported method. We observed a unique fluorescence–phos-
phorescence dual emission for Pt-4. In order to rule out any
impurity, Pt-4 was prepared using two different procedures
(Scheme 1). All the complexes were obtained with moderate to
satisfactory yields.
Triplet–triplet annihilation upconversion
A diode pumped solid state (DPSS) continuous laser (445 nm)
was used as an excitation source for the upconversion. The dia-
meter of the 445 nm laser spot is ca. 3 mm. The power of the
laser beam was measured with a VLP-2000 pyroelectric laser
power meter. For the upconversion experiments, the mixed
solution of the complexes (triplet photosensitizers) and 9,10-
diphenylanthracene (DPA) or perylene was degassed with N2
for at least 15 min. The upconverted fluorescence of DPA was
observed with a CRT 920 spectrofluorometer. In order to
repress the scattered laser, a small black box was put behind
the fluorescent cuvette to trap the laser beam.
Absorption and emission spectra
The upconversion quantum yields (ΦUC) were determined
with the prompt fluorescence of coumarin-6 as the lumines-
cence quantum yield standard (ΦF = 78% in ethanol). The
upconversion quantum yields were calculated using eqn
(1),9,23–26 where ΦUC, Asam, Isam and ηsam represent the
quantum yield, absorbance, integrated photoluminescence
intensity and the refractive index of the solvents (where the
subscript “std” stands for the standard used in the measure-
ment of the quantum yield and “sam” for the samples to be
measured). The equation is multiplied by a factor of 2 in order
to set the maximum quantum yield to unity. All these data
were independently measured three times (with different
sample solutions).
The UV-vis absorption of the complexes and the ligands was
studied (Fig. 1). The acetylide ligand L-1 gives absorption
maxima in the visible region (436 nm with 19 100 M−1 cm−1).
On metalation of the acetylide ligand (Pt-1), the absorption
band moves bathochromically to 449 nm (33 700 M−1 cm−1).
For the Pt(II) complex that contains two thienyl coumarin
units, the absorption is very strong at 459 nm (76 800 M−1
cm−1). Complexes with NI acetylide show similar absorption
maxima.
The luminescence properties of the complexes were studied
(Fig. 2). All the complexes give emission at ca. 500 nm, except
Pt-5, which was non-luminescent (see ESI† for details). The
Stokes shifts of the complexes were small compared to typical
phosphorescent transition metal complexes.16,34–37 Further-
more, we found that the luminescence intensity of Pt-1, Pt-2
and Pt-3 was not sensitive to oxygen (O2). Usually the phos-
phorescence of transition metal complexes can be substan-
tially quenched by O2. The luminescence lifetime of the
emission at ca. 500 nm was determined to be in the range of a
few nanoseconds (Table 1). Based on these results, we con-
clude that the luminescence bands at 500 nm for complexes
Pt-1–Pt-3 are due to fluorescence, not phosphorescence, which
is different from the N^N Pt(II) bis(acetylide) complexes which
are usually phosphorescent.18,38–41
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Astd
Asam
Isam
Istd
ηsam
ηstd
ΦUC ¼ 2Φstd
ð1Þ
The CIE coordinates (x, y) of the emission of the sensitizers
alone and the emission of the upconversion were derived from
the emission spectra with the software of CIE color Matching
Linear Algebra.
DFT calculations
Density functional theory (DFT) calculations were used for
optimization of both singlet states and triplet states. The
energy level of the T1 state (energy gap between S0 state and T1
state) was calculated with time-dependent DFT (TDDFT),
based on the optimized singlet ground state geometries
(S0 state). All the calculations were performed with Gaussian
09W.27
Two emission bands were observed for Pt-4 (Fig. 2d), one
fluorescence emission band at 469 nm and the other emission
band at 630 nm. The band at 630 nm is highly sensitive to the
presence of O2, and the luminescence lifetime was determined
as 278.6 μs (in toluene). These data and the large Stokes shift
(49 020 cm−1) indicated that the emission at 630 nm can be
attributed to phosphorescence. The lifetime of the emission
band at 469 nm is 1.0 ns. Thus the emission band can be
assigned as fluorescence. In order to validate that these dual
Results and discussion
Design and synthesis of complexes
We have previously used coumarin for preparation of Pt(II) emissive features of Pt-4 were not due to fluorescent impuri-
complexes.28 However, coumarin–Pt(II) complexes show ties, the complex was prepared with two different procedures,
Dalton Trans.
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Scheme 1 Synthesis of the complexes Pt-0, Pt-1, Pt-2, Pt-3, Pt-4 and Pt-5. Please note Pt-4 was prepared by two different methods to rule out the presence of
impurities in the product. (i) NHEt2, 45 °C, Ar, 8 h; (ii) Br2, AcOH, r.t., 3 h; (iii) 2-thienylboric acid, K2CO3, Pd(PPh3)4, MeOH–PhCH3, Ar, 75 °C, 30 min, microwave
reactor; (iv) NBS, CHCl3–AcOH, r.t., 4 h; (v) trimethylsilylacetylene, Pd(PPh3)2Cl2, PPh3, CuI, NEt3, Ar, reflux, 8 h; (vi) K2CO3, THF–MeOH, r.t., 1.5 h; (vii) THF–HNEt2, CuI,
Ar, r.t., 30 min.
but the same emission bands were observed. The lumines- fluorescence. It should be noted that fluorescence–phosphor-
cence lifetimes of the precursors of Pt-4 also support this con- escence dual emissive transition metal complexes are
clusion (Table 1).
rare.22,42–44 Such complexes are interesting for photophysical
Dual emission was also observed for Pt-0 (458 nm and studies, as well as for applications in ratiometric oxygen (O2)
628 nm) but with much weaker phosphorescence than the sensing.
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Fig. 1 UV-Vis absorption spectra of the Pt(II) complexes and ligand L-1. c =
1.0 × 10−5 M in toluene, 25 °C.
77 K emission spectra
The photoluminescence of the complexes at 77 K was studied
(Fig. 3). The emission bands of Pt-1, Pt-2 and Pt-3 become
structured, no phosphorescence bands were observed (see ESI†
for details). The luminescence lifetimes of Pt-1, Pt-2 and Pt-3
are close to that at RT (1–2 ns), therefore the fluorescence
feature of the luminescence of these complexes is proved
unambiguously. For Pt-0 and Pt-4, the luminescence intensity
at 77 K generally increased compared to that at RT. The life-
time of the emission at the higher energy side of the dual
emission bands is a few ns, thus these emission bands can be
assigned as fluorescence. For the emission at a lower energy
region, the luminescence lifetime increased by ca. 2-fold at
77 K. The lifetimes of 424.4 μs and 623.3 μs were observed for
Pt-0 and Pt-4, respectively.
Fig. 2 The emission spectra of the Pt(II) complexes under different atmos-
pheres. (a) Pt-1 (λex = 449 nm), (b) Pt-2 (λex = 448 nm), (c) Pt-3 (λex = 459 nm),
(d) Pt-4 (λex = 420 nm). In toluene, c = 1.0 × 10−5 M, 25 °C.
spectra of the complexes were studied (Fig. 4).45 Upon pulsed
laser excitation, bleaching bands at the steady state UV-Vis
absorption band were observed. Moreover, similar transient
absorption features in the range 500–700 nm were observed.
Since Pt-1, Pt-2 and Pt-3 show similar positive transient
absorption bands and lifetimes, we propose that the triplet
excited state is localized on the coumarin unit. As a support
for this postulation, Pt-4 and Pt-0 show drastically different
transient absorption features, e.g. their triplet state lifetimes
are much longer.
Nanosecond time-resolved transient difference absorption
spectroscopy
In order to study the efficiency of the triplet excited state
production of the complexes, the photosensitization of singlet
oxygen (1O2) with the complexes upon photoexcitation was
In order to study the triplet excited states of the complexes,
the nanosecond time-resolved transient difference absorption
Table 1 Photophysical parameters of Pt-0–Pt-5, L-1 and L-2
τF f (ns)
τPg (μs)
εb
λem
ΦF
ΦP
298 K
77 K
298 K
77 K
τTh/μs
ΦΔ
a
c
d
e
i
λabs
Pt-0
Pt-1
Pt-2
Pt-3
Pt-4
Pt-5j
L-1
421
449
448
459
426
323
436
350
2.09
3.37
4.81
7.68
2.39
2.45
1.91
1.78
458/628
521
514
520
469/630
—
13.4%
66.4%
0.95%
3.8%
1.1%
—
1.05%
—
—
—
2.4%
—
0.5
2.8
1.5
1.1
1.0
—
1.2
2.8
1.1
1.3
1.0
—
232.7
—
—
—
278.6
—
424.4
—
—
—
623.3
—
139.9
16.0
20.3
17.2
100.7
—
0.86
0.21
0.99
0.86
0.82
0.07
0.12
—
456
401
81.9%
—
—
—
2.4
0.2
—
—
—
—
—
—
—
—
L-2
a In toluene (1.0 × 10−5 M). b Molar extinction coefficient at the absorption maxima ε: 104 cm−1 M−1 c In toluene. d In toluene, with quinine
.
sulfate (Φ = 0.546 in 0.5 M H2SO4) as the standards. e In toluene, with [Ru(dmb)3(PF6)2] (Φ = 0.073 in acetonitrile) as the standards.
f Luminescence lifetime, λex = 443 nm, at RT (Pt-0–Pt-4 in deaerated ethanol–methanol, 4 : 1, v/v; L-1 and L-2 in deaerated toluene) and 77 K (in
ethanol–methanol, 4 : 1, v/v). g Phosphorescence lifetime, Pt-0 (λex = 420 nm), Pt-4 (λex = 420 nm), at RT (in deaerated ethanol–methanol, 4 : 1, v/v)
and 77 K (in ethanol–methanol, 4 : 1, v/v). h Triplet state lifetime, determined with nanosecond time-resolved transient difference absorption
spectroscopy, λex = 355 nm, c = 1.0 × 10−5 M in aerated and deaerated toluene. i Quantum yield of singlet oxygen (1O2), Pt-0–Pt-4 with TPP as
standard (ΦΔ = 0.62 in CH2Cl2), Pt-5 with Rose Bengal as standard (ΦΔ = 0.8 in methanol), c = 1.0 × 10−5 M in CH2Cl2 and methanol. j Pt-5 is non-
luminescent, therefore some photophysical parameters were not measured.
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Fig. 3 Photoluminescence spectra of the complexes at RT and 77 K. (a) Pt-1
(λex = 449 nm) and (b) Pt-4 (λex = 420 nm). c = 6.0 × 10−5 M in C2H5OH–CH3OH
(4 : 1, v/v).
Fig. 5 Spin density of the complexes Pt-0, Pt-1, Pt-2, Pt-3, Pt-4 and Pt-5. Cal-
culated based on the optimized triplet state by DFT at the B3LYP/6-31G(d)/
LanL2DZ level using Gaussian 09W.
of Pt-1 was calculated based on the optimized ground state
geometry.
The absorption band in the visible region was predicted at
491 nm, which is close to the experimental result of 449 nm
(Fig. 1). It is known that the DFT methods usually underesti-
mate the excitation energy for charge transfer. The electronic
components of the transition are H→L, which can be assigned
as the IL and MLCT mixed transition, with IL as the major con-
tributor. The singlet–triplet state energy gaps were also calcu-
lated (see ESI† for details). The energy level of the T1 state was
estimated to be 1.65 eV (735 nm). H→L is involved in the T1
3
state, thus the T1 state can be assigned as the IL state, which
is in agreement with the spin density analysis and the long-
lived triplet excited state observed for Pt-1.
Fig. 4 Nanosecond time-resolved transient difference absorption spectra of (a)
Pt-2 and (c) Pt-4 after pulsed excitation (λex = 355 nm). Decay traces of (b) Pt-2
at 530 nm and (d) Pt-4 at 430 nm. c = 1.0 × 10−5 M in deaerated toluene, 25 °C.
The geometry of Pt-3 was also optimized. The calculated
absorption is close to the experimental observations. The
energy gap of S1/T1 was calculated as 774 nm (see ESI† for
details). T1 state is with a significant intraligand state feature.
studied and the 1O2 quantum yields (ΦΔ) were determined
(Table 1 and ESI†). The ΦΔ values of Pt-2, Pt-3 and Pt-4 are
large (0.82–0.99). For Pt-1 and Pt-5, however, it is much
smaller (0.21 and 0.07).
Ratiometric O2 sensing
Since Pt-4 shows fluorescence–phosphorescence dual emis-
sion, fluorescence is insensitive to O2, but the phosphor-
escence is highly sensitive to O2, thus Pt-4 was used for
ratiometric luminescent O2 sensing.22,44,46 Luminescent O2
DFT calculations on the complexes: spin density surfaces and
electronic transitions
In order to study the triplet states of the complexes, the spin sensing is important for chemical, biological and environ-
density surfaces were calculated using DFT methods. Generally mental science. Ratiometric O2 sensing is more favorable
the triplet excited states are localized on the organic ligands, under some circumstances than O2 sensing based on lumines-
instead of the Pt(II) coordination centers (Fig. 5). This result is cence intensity.46–49 However, reports on ratiometric O2
in agreement with the long-lived triplet excited states of the sensing with uni-chromophore complexes are rare.22,46 Usually
complexes.
two methods were used for ratiometric O2 sensing. One is to
The ground state geometries of the complexes were opti- use two luminophores, for which one is fluorescent, and
mized using DFT methods. For Pt-1, the dihedral angle another is phosphorescent. But the different photostability
between the coumarin and the thienyl part is 1.89 degree, indi- may make the sensor less durable. The second method is to
cating a good π-conjugation. The two arylacetylides take a non- use a luminophore with fluorescence–phosphorescence dual
coplanar geometry (see ESI† for details). The UV-vis absorption emission.22,42,44,46 However, it is still a substantial challenge to
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develop such fluorescence–phosphorescence dual emissive
luminophores.
With increasing O2 partial pressure, the phosphorescence
at 630 nm was drastically quenched, but the fluorescence at
Previously Pt(II) and Ir(III) complexes have been reported to 469 nm was intact. Thus ratiometric O2 sensing using emis-
show the fluorescence–phosphorescence dual emission, but sion intensity can be established (Fig. 6a and 6b). Further-
the ratiometric O2 sensitivity was not studied.42 Recently we more, the phosphorescence lifetimes were monitored with
studied the ratiometric O2 sensing with cyclometalated Pt(II) increasing O2 partial pressure (Fig. 6c and 6d). The O2 sensing
complexes.44 It is clear that the diversity of such luminophores was also monitored with lifetime changes (Fig. 6 and Table 2).
needs to be increased.46 Pt-4 shows the dual emission, thus With increasing O2 partial pressure, the fluorescence lifetime
the ratiometric O2 sensing using Pt-4 was studied (Fig. 6).
hardly changed (Table 2, the fluctuations of the lifetime values
are within experimental errors), but the phosphorescence life-
time decreased sharply from 169.4 μs to 27.2 μs.
The balanced fluorescence–phosphorescence emission is
beneficial for ratiometric O2 sensing. To the best of our knowl-
edge, ratiometric O2 sensing based on uni-chromophore
sensing material is rarely reported.
Triplet–triplet annihilation upconversion
TTA upconversion has attracted much attention, due to its
potential applications in photovoltaics,9–11,50–52 photocataly-
sis,53 luminescent bioimaging,54 and O2 sensing.55 Triplet
photosensitizers are crucial for TTA upconversion.9–11 Conven-
tionally the triplet photosensitizers are limited to the Pt(II) or
Pd(II) porphyrin complexes.9,10 However, the absorption wave-
length and the T1 excited state energy levels of these complexes
cannot be readily optimized. Ru(II) and Ir(III) complexes and
some organic compounds were used as triplet photosensitizers
for TTA upconversion.56,57 However, these compounds usually
show weak absorption of visible light and short-lived triplet
excited states. Recently we developed a series of Ru(II), Pt(II),
Ir(III) and Re(I) complexes as triplet photosensitizers for TTA
upconversion.10,58–65 These complexes show strong absorption
in the visible region and long-lived triplet excited states.
However, to the best of our knowledge, very few linear bis-
(alkylphosphine) Pt(II) bisacetylide complexes have been used
as triplet photosensitizers for TTA upconversion.10,21c Herein
the Pt(II) complexes were used for TTA upconversion.
A triplet acceptor is an essential component for TTA
upconversion.9–11 One of the prerequisites for a useful triplet
photosensitizer is that the energy level of the T1 state of a
triplet acceptor must be lower than the energy level of the T1
state of the photosensitizer. Pt-1–Pt-3 are not phosphorescent
and the triplet excited state energy level cannot be accessed
easily, therefore, 9,10-diphenylanthracene (DPA) was used as
the triplet acceptor for the preliminary tests. The T1 state
energy level of DPA is 1.77 eV, which is lower than the T1 state
energy level of Pt-4 (approximated as 1.97 eV based on the RT
phosphorescence at 630 nm). The complexes show their
Fig. 6 Ratiometric luminescence response of Pt-4 to variation of the O2 con-
centration. (a) Emission spectra of Pt-4 in the presence of 0%, 0.01%, 0.03%,
0.04%, 0.06%, 0.1%, 0.16%, 0.2% O2. (b) The linear correlation between the
ratiometric photoluminescence response toward the O2 partial pressure. (c)
Phosphorescence lifetime (λex = 420 nm) variation of Pt-4 in the presence of
0%, 0.01%, 0.02%, 0.03%, 0.04%, 0.06%, 0.1%, 0.15%, 0.2% O2. (d) The linear
correlation between the phosphorescence lifetime ratios τ0/τ toward the O2
partial pressure. (e) Emission images of Pt-4 under different O2 partial pressure.
c [Pt-4] = 1.0 × 10−5 M in toluene, 25 °C.
Table 2 The variation of the luminescence lifetime of Pt-4 against the O2 concentrations (v/v% in O2–N2 mixture)a
[O2]b
0.0
0.01
0.02
0.03
0.04
0.06
0.1
0.15
0.2
τ468 (ns)c
τ630 (μs)d
0.34
169.4
0.40
160.0
0.46
130.6
0.41
105.7
0.46
87.0
0.48
62.9
0.55
50.2
0.55
37.7
0.65
27.2
a c [Pt-4] = 1.0 × 10−5 M in toluene, 25 °C. b O2 concentrations, v/v × 100. c Luminescence lifetime, excited with pulsed laser (λex = 405 nm), 298 K
(in deaerated toluene). d Phosphorescence lifetime, excited with pulsed laser (λex = 420 nm), 298 K (in deaerated toluene).
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Fig. 7 Emission and upconversion of the complexes with 445 nm (5 mW) laser
excitation. (a) Emission of the complexes alone. (b) The up-converted DPA fluor-
escence and the residual fluorescence and phosphorescence of the complexes.
c [DPA] = 6.0 × 10−5 M; c [sensitizers] = 1.0 × 10−5 M in deaerated toluene. The
asterisks in (a) and (b) indicate the scattered 445 nm excitation laser (5 mW),
25 °C.
Fig. 9 (a) Photographs of the emission of sensitizers alone and (b) the upcon-
version. (c) CIE diagram of the emission of sensitizers alone and (d) in the pres-
ence of DPA (upconversion). λex = 445 nm (laser power: 5 mW). In deareated
toluene, c[sensitizer] = 1.0 × 10−5 M, c[DPA] = 6.0 × 10−5 M, 25 °C.
upconversion for Pt-2 and Pt-3 (with triplet acceptor DPA,
Fig. 9).
Fig. 8 Delayed fluorescence observed in the TTA upconversion with (a) Pt-4 as
the triplet photosensitizer and DPA as the triplet acceptor. Excited at 445 nm
and monitored at 410 nm. (b) The prompt fluorescence decay of DPA deter-
mined in a different experiment (excited with picosecond 405 nm laser, the
decay of the emission was monitored at 410 nm). c [DPA] = 6.0 × 10−5 M;
c [sensitizers] = 1.0 × 10−5 M in deaerated toluene, 25 °C.
In order to study the efficiency of the triplet–triplet-energy-
transfer (TTET) process, which is crucial for the TTA upconver-
sion, the triplet state quenching of the triplet photosensitizer
was measured using nanosecond transient absorption (see
ESI† for details). Significant quenching was observed for Pt-4,
with quenching constants of 5.38 × 104 M−1 (Table 3). For Pt-2
and Pt-3, however, the quenching of the triplet state lifetime is
not significant, which may be responsible for the lack of TTA
upconversion for Pt-2 and Pt-3.
Pt-2 and Pt-3 show high singlet O2 sensitizing quantum
yields (Table 1), but no upconversion was observed for these
complexes with DPA as the triplet acceptor. DFT calculations
indicated that the T1 state energy levels of these complexes are
lower than that of Pt-4 (see ESI† for details). It is known that
for TTA upconversion, the T1 state energy level of the triplet
characteristic emission upon 445 nm laser excitation. For
example, Pt-4 shows fluorescence–phosphorescence dual emis-
sion, whereas complexes Pt-2 and Pt-3 show only the fluo-
rescence. Blue emission was observed for Pt-4 upon the
addition of DPA to the solution, but no blue emission was
observed for Pt-2 and Pt-3 (Fig. 7b). Pt-5 cannot be photo-
excited at 445 nm.
In order to unambiguously confirm that the blue emission
is due to the upconverted emission of DPA, the luminescence
lifetime of the blue emission band in Fig. 7b was measured
(Fig. 8).66–68 An exceptionally long-lived lifetime of 167.0 μs
was observed. This delayed luminescence lifetime is a typical
feature of the TTA upconversion. In comparison, the prompt
fluorescence lifetime of DPA was determined as 5.5 ns
(Fig. 8b).
The upconversion is clearly visible to the eyes (Fig. 9). For
Pt-2 and Pt-3, green emission was observed, which is due to
the residual fluorescence of the photosensitizers. For Pt-4, a
purple emission was observed, which is due to the simul-
taneous blue/red emission. In the presence of the triplet
photosensitizer DPA, a blue emission band was observed for
Pt-4, but the emission wavelength of the Pt-2 and Pt-3 solu-
tions did not change, in agreement with the lack of TTA
Table 3 Triplet excited state lifetimes (ττ), Stern–Volmer quenching constant
(KSV) and bimolecular quenching constants (kq) of the Pt(II) sensitizers. In de-
aerated toluene solution, 20 °C
a
KSV/103 M−1
kq/109 M−1 s−1
ΦUC
τT /μs
Py
DPA
Py
DPA
Py
DPA
b
b
Pt-0
Pt-1
Pt-2
Pt-3
Pt-4
139.9
16.0
20.3
17.2
100.7
1.0
196.0
0.17
0.80
1.3
537.8
0.007
0.53
2.11
5.44
0.09
1.40
0.01
0.039
0.076
5.34
—
—
—
b
b
8.4
42.8
93.5
9.5
—
b
17.1%
13.6%
—
b
—
b
—
27.2%
a Upconversion quantum yields, Pt-4 with coumarin-6 as the standard
(ΦF = 78% in ethanol), Pt-2 and Pt-3 with diiodo-BODIPY (ΦF = 2.7% in
acetonitrile) as the standard. b No results were obtained.
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Fig. 10 Emission and upconversion of the complexes with 473 nm (5 mW)
laser excitation. (a) Emission of the complexes alone. (b) The upconverted pery-
lene (Py) fluorescence and the residual fluorescence of the triplet photosensiti-
zers. c [Py] = 4.0 × 10−5 M; c [sensitizers] = 1.0 × 10−5 M in deaerated toluene.
The asterisks in (a) and (b) indicate the scattered 473 nm excitation laser
(5 mW), 25 °C.
Fig. 11 Stern–Volmer plots for lifetime quenching of Pt-0, Pt-1, Pt-2, Pt-3 and
Pt-4 with increasing the concentration of perylene (Py) after pulsed excitation
(λex = 355 nm). c [sensitizers] = 1.0 × 10−5 M in deaerated toluene, 25 °C.
acceptor must be lower than that of the triplet photosensitizer.
Therefore, we studied the TTA upconversion with a different
triplet acceptor that shows a lower T1 state energy level, i.e.
perylene (ET1 = 1.53 eV).
With perylene as the triplet photosensitizer, TTA upconver-
sion was observed for Pt-2 and Pt-3 (Fig. 10). Pt-4 was not
studied under these conditions because this complex cannot
be excited by the 473 nm laser.
The delayed fluorescence lifetimes of the TTA upconversion
for Pt-2 and Pt-3 with perylene as the triplet acceptor were also
studied (see ESI† for details). The long-lived luminescence life-
times are in agreement with the TTA upconversion feature.
The upconversion with perylene as the triplet acceptor is
also visible to eyes (see ESI† for details). For Pt-2 and Pt-3
alone, the emission color is green upon 473 nm laser exci-
tation. In the presence of perylene, the emission colour of the
solution turns blue. For Pt-4, however, no upconversion was
observed because Pt-4 cannot be photoexcited at 473 nm, due
to its absorption in this blue-shifted region.
The TTET with perylene as the triplet acceptor were also
studied with the triplet state lifetime quenching experiments
(Fig. 11). Pt-2 and Pt-3 give much larger quenching constants
than the other complexes. The photophysical properties
related to the upconversion are summarized in Table 3.
Fig. 12 Time-resolved emission spectra (TRES) of Pt-4 alone and the TTA
upconversion with the DPA as the triplet acceptor. Pt-4 alone: (a) the phosphor-
escence region was measured (570 nm–800 nm, τ = 201.2 μs) excited with
nanosecond pulsed laser (445 nm) and (b) the fluorescence region was
measured (430 nm–570 nm, τ = 0.5 ns) excited with picosecond pulsed laser
(443 nm). TRES of Pt-4 in the presence of DPA: (c) upconverted emission in the
range of 400 nm–500 nm was observed (τ = 179.1 μs) with nanosecond pulsed
laser (445 nm) and (d) fluorescence range 430 nm–520 nm was observed
(τ = 0.6 ns) excited with pico-second pulsed laser (443 nm). c [DPA] = 6.0 × 10−5
M; c [photosensitizers] = 1.0 × 10−5 M in deaerated toluene, 25 °C.
Time-resolved emission spectroscopy
(Table 1) is due to the fact that the background dynamics of
The emission of Pt-4 alone and the emission of TTA upconver- the pulsed laser cannot be calibrated in the TRES mode.
sion with Pt-4 as triplet photosensitizers were studied by using
The TRES of Pt-4 in the presence of DPA (TTA upconver-
nanosecond and microsecond time-resolved emission spectra sion) was also studied (Fig. 12c and d). In the presence of DPA,
(TRES, Fig. 12). For Pt-4 alone, the TRES of the phosphor- the fluorescence band shows no changes (Fig. 12d, 0.6 ns). A
escence and the fluorescence bands were recorded separately long-lived emission band in the 400–500 nm region was
(note that there is substantial difference between the lifetimes observed (Fig. 12c, τ = 179.1 μs), which is due to the upcon-
of the two emission bands, it is technically difficult to record verted emission. The fluorescence emission band in the region
the two emission bands simultaneously with one pulsed of 430 nm–520 nm was also measured (Fig. 12d), the decay of
laser). The results confirmed that the phosphorescence decay the fluorescence band did not change compared to that of Pt-4
gives much longer lifetime (Fig. 12a, 201.2 μs) than the fluo- alone (lifetime = 0.5 ns). This is a reasonable result since the
rescence band (Fig. 12b, 0.5 ns). The difference between the singlet excited state of the triplet photosensitizer is not
lifetime values and that determined in the previous section involved in the TTET process, therefore the singlet excited
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Acknowledgements
We thank the NSFC (20972024, 21073028 and 21273028), the
Science Foundation Ireland (SFI E.T.S. Walton Program 11/
W.1/E2061), the Royal Society (UK) and NSFC (China-UK
Cost-Share
Science
Networks,
21011130154),
the
Fundamental Research Funds for the Central Universities
(DUT10ZD212), Ministry of Education (NCET-08-0077 and
SRFDP-20120041130005) for financial support.
Scheme 2 Jablonski diagram of triplet–triplet-annihilation (TTA) upconversion
with Pt-4 as a triplet sensitizer (the triplet states of sensitizers are emissive) and
9,10-diphenylanthracene (DPA) was the triplet acceptor. TTET stands for triplet–
triplet-energy-transfer.
Notes and references
1 K. Mori, K. Watanabe, K. Fuku and H. Yamashita,
Chem.–Eur. J., 2012, 18, 415–418.
2 W.-G. Wang, F. Wang, H.-Y. Wang, C.-H. Tung and
L.-Z. Wu, Dalton Trans., 2012, 41, 2420–2426.
state of the photosensitizer will not be alternated. These
results confirmed the triplet–triplet annihilation upconver-
sion. Similar TRES spectra were observed for Pt-2 (see ESI† for
details).
The TTA upconversion process can be summarized in
Scheme 2. The light-harvesting ability and the lifetimes of the
triplet excited states of the triplet photosensitizers are crucial
for the TTA upconversion. The TTET process can be enhanced
with the long-lived triplet excited states of the triplet
photosensitizer.
3 F.-R. Dai, H.-M. Zhan, Q. Liu, Y.-Y. Fu, J.-H. Li, Q.-W. Wang,
Z. Xie, L. Wang, F. Yan and W.-Y. Wong, Chem.–Eur. J.,
2012, 18, 1502–1511.
4 G. Zhou, W.-Y. Wong, S.-Y. Poon, C. Ye and Z. Lin, Adv.
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5 S.-C. Chan, M. C. W. Chan, Y. Wang, C.-M. Che,
K.-K. Cheung and N. Zhu, Chem.–Eur. J., 2001, 7, 4180–
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6 C. K. M. Chan, C.-H. Tao, K.-F. Li, K. M.-C. Wong, N. Zhu,
K.-W. Cheah and V. W.-W. Yam, Dalton Trans., 2011, 40,
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7 (a) R. Liu, A. Azenkeng, Y. Li and W. Sun, Dalton Trans.,
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Conclusions
In conclusion, a series of symmetric and unsymmetric trans-
bis(alkylphosphine) Pt(II) bisacetylide complexes were pre-
pared, and their photophysical properties were studied with
steady state and time-resolved spectroscopies, as well as DFT
calculations. Thienyl coumarin and naphthalimide acetylides
were used as ligands. To the best of our knowledge, this is the
first time that coumarin and naphthalimide acetylides have
been used for the preparation of trans-bis(alkylphosphine)
Pt(II) bis(acetylide) complexes. These complexes show strong
absorption of visible light and long-lived triplet excited states
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