Tuning the photophysical properties of N∧N Pt(II) bisacetylide complexes with fluorene moiety and its applications for triplet-triplet- annihilation based upconversion

Article abstract of DOI:10.1039/c2jm15678d

Fluorene-containing aryl acetylide ligands were used to prepare N∧N Pt(ii) bisacetylide complexes, where aryl substituents on the fluorene are phenyl (Pt-1), naphthal (Pt-2), anthranyl (Pt-3), pyrenyl (Pt-4), 4-diphenylaminophenyl (Pt-5) and 9,9-di-n-octylfluorene (Pt-6) (where N∧N ligand = 4,4′-di-tert-butyl-2,2′-bipyridine, dbbpy). All the complexes show room temperature (RT) phosphorescence. The emissive T₁ excited states of Pt-1, Pt-5 and Pt-6 were assigned as metal-to-ligand-charge- transfer state (³MLCT), whereas for Pt-2, Pt-3 and Pt-4, the emissive T₁ excited states were identified as the intraligand state ( ³IL), based on steady state emission spectra, the lifetime of the T₁ state, emission spectra at 77 K, spin density analysis and the time-resolved transient absorption spectroscopy. Exceptionally long lived T ₁ excited state was observed for Pt-3 (τ = 66.7 μs) and Pt-4 (τ = 54.7 μs), compared to a model complex dbbpy Pt(ii) Bisphenylacetylide (τ = 1.25 μs). RT phosphorescence of anthracene was observed at 780 nm with Pt-3. The critical role of the fluorene is to move the absorption of the complexes to the red-end of the spectra, but at the same time, without compromising the energy level of the T₁ state of the complexes. The advantage of this unique spectral tuning effect and the long-lived T₁ excited states of Pt-4 was demonstrated by the enhanced performance of the complexes as triplet sensitizers for triplet-triplet annihilation (TTA) based upconversion; an upconversion quantum yield (Φ_UC) up to 22.4% was observed with Pt-4 as the sensitizer. Other complexes described herein show negligible upconversion. The high upconversion quantum yield of Pt-4 is attributed to its intense absorption of visible light and long-lived T₁ excited state. Based on the result of Pt-4, we propose that weakly phosphorescent, or non-phosphorescent transition metal complexes can be used as triplet sensitizers for TTA upconversion, compared to the phosphorescent triplet sensitizers currently used for TTA upconversion. Our results will be useful for the design of transition metal complexes to enhance the light-absorption and thereafter the cascade photophysical processes, without decreasing the T₁ excited state energy levels, which are important for the application of the complexes as triplet sensitizers in various photophysical processes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm15678d

As featured in:
Highlighting research results from the lab of
Molecular Photochemistry & Photophysics,
State Key Laboratory of Fine Chemicals, Dalian
University of Technology, China.
Title: Tuning the photophysical properties of N^N Pt(II)
bisacetylide complexes with fluorene moiety and its applications
for triplet–triplet-annihilation based upconversion
A series of N^N Pt(II) bisacetylide complexes, with
fluorene-containing aryl acetylide ligands, were prepared
and used for triplet–triplet annihilation (TTA) upconversion.
We proposed that non-phosphorescent transition metal
complexes can also be used as the triplet sensitizers.
See Q. Li et al.,
J. Mater. Chem., 2012, 22, 5319.
www.rsc.org/materials
Registered Charity Number 207890

Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 5319
www.rsc.org/materials
PAPER
Tuning the photophysical properties of N^N Pt(II) bisacetylide complexes
with fluorene moiety and its applications for triplet–triplet-annihilation based
upconversion†
*
Qiuting Li, Huimin Guo, Lihua Ma, Wanhua Wu, Yifan Liu and Jianzhang Zhao
*
Received 5th November 2011, Accepted 24th November 2011
DOI: 10.1039/c2jm15678d
Fluorene-containing aryl acetylide ligands were used to prepare N^N Pt(II) bisacetylide complexes,
where aryl substituents on the fluorene are phenyl (Pt-1), naphthal (Pt-2), anthranyl (Pt-3), pyrenyl (Pt-
4), 4-diphenylaminophenyl (Pt-5) and 9,9-di-n-octylfluorene (Pt-6) (where N^N ligand ¼ 4,40-di-tert-
butyl-2,20-bipyridine, dbbpy). All the complexes show room temperature (RT) phosphorescence. The
emissive T1 excited states of Pt-1, Pt-5 and Pt-6 were assigned as metal-to-ligand-charge-transfer state
(3MLCT), whereas for Pt-2, Pt-3 and Pt-4, the emissive T1 excited states were identified as the
intraligand state (3IL), based on steady state emission spectra, the lifetime of the T1 state, emission
spectra at 77 K, spin density analysis and the time-resolved transient absorption spectroscopy.
Exceptionally long lived T1 excited state was observed for Pt-3 (s ¼ 66.7 ms) and Pt-4 (s ¼ 54.7 ms),
compared to a model complex dbbpy Pt(II) Bisphenylacetylide (s ¼ 1.25 ms). RT phosphorescence of
anthracene was observed at 780 nm with Pt-3. The critical role of the fluorene is to move the absorption
of the complexes to the red-end of the spectra, but at the same time, without compromising the energy
level of the T1 state of the complexes. The advantage of this unique spectral tuning effect and the long-
lived T1 excited states of Pt-4 was demonstrated by the enhanced performance of the complexes as
triplet sensitizers for triplet–triplet annihilation (TTA) based upconversion; an upconversion quantum
yield (FUC) up to 22.4% was observed with Pt-4 as the sensitizer. Other complexes described herein
show negligible upconversion. The high upconversion quantum yield of Pt-4 is attributed to its intense
absorption of visible light and long-lived T1 excited state. Based on the result of Pt-4, we propose that
weakly phosphorescent, or non-phosphorescent transition metal complexes can be used as triplet
sensitizers for TTA upconversion, compared to the phosphorescent triplet sensitizers currently used for
TTA upconversion. Our results will be useful for the design of transition metal complexes to enhance
the light-absorption and thereafter the cascade photophysical processes, without decreasing the T1
excited state energy levels, which are important for the application of the complexes as triplet sensitizers
in various photophysical processes.
was usually observed upon photoexcitation.1,9,10 The photo-
Introduction
physical properties of the N^N Pt(II) bisacetylide complexes,
such as the UV-vis absorption, the emission wavelength and the
lifetime of the T1 excited state, are dependent on the structure
of the acetylide ligand which is attached to the Pt(II) center.1,9–11
Thus the photophysical properties of Pt(II) acetylide complexes
can be tuned by changing the acetylides. To date phenyl-
acetylide or substituted phenylacetylides, naphthal and pyrenyl
were attached to the Pt(II) center. Recently, we attached
naphthalimide (NI),12,13 coumarin,14 and naphthaldiimide
(NDI) acetylide to the Pt(II) center of N^N Pt(II) bisacetylide
complexes,15 and we observed exceptionally long triplet excited
state lifetimes and room temperature near-IR emission of the
Recently Pt(II) acetylide complexes have attracted much atten-
tion,1 due to their applications in photocatalysis,2 photoinduced
charge separation,3 optical limiting,4 molecular probes,5–8 etc.
Representative complexes are N^N Pt(II) bisacetylide
complexes, for which room temperature (RT) phosphorescence
State Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, E-208 Western Campus, 2 Ling-Gong
Road, Dalian, 116024, P. R. China. E-mail: guohm@dlut.edu.cn;
zhaojzh@dlut.edu.cn; Web: http://finechem.dlut.edu.cn/photochem; Fax:
+86 0411 84986236; Tel: +86 0411 84986236
† Electronic supplementary information (ESI) available: 1H and 13C
emissive IL excited state of the complexes.12,15
3
NMR, and MS spectra. Absorption and emission spectra,
DOI:
However, much room is left for fine tuning the photophysical
properties of the Pt(II) complexes. For example, (1) the UV-vis
upconversion and
10.1039/c2jm15678d
DFT
calculation
details.
See
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 5319–5329 | 5319

absorption of the Pt(II) acetylide is usually weak in the visible
region;1,9,10 (2) the T1 excited state lifetime is short (less than
5 ms);1,9,10 (3) the difficulty to move the absorption to the red-end
of the spectrum with ligand modification, but without any
significant decrease of the T1 excited state energy level. Recently
we proved that with ligand modification, the UV-vis absorption
band of the Pt(II) acetylide complexes can be drastically red-
shifted, but the T1 energy level decreased sharply.15 This decrease
is detrimental to the application of the transition metal
complexes because the matched triplet energy acceptor will be
limited. Thus, it is desired to develop a strategy to move the UV-
vis absorption of the complexes to the red-end, but at the same
time, without compromising the high T1 state energy levels. We
propose that ligand modification with a specific chromophore is
one of the promising approaches to address this challenge.
On the other hand, fluorene is a robust chromophore, due to
its emission and charge carrier property, and has been extensively
used in luminescent materials,16–19 electroluminescent mate-
rials,20 optical limiting,21,22 and molecular probes.23 To the best of
our knowledge, however, the acetylide fluorene moiety has never
been attached to the Pt(II) center of N^N Pt(II) bisacetylide
complexes.1 Herein for the first time we attached the fluorene
chromophore to the Pt(II) center of the N^N Pt(II) bisacetylide
complexes and importantly, we found that the aforementioned
challenge, that is, to move the absorption to the red-end of the
spectra and maintain the T1 excited state energy level, can be
addressed to some extent. Prolonged triplet excited state lifetimes
were observed, especially for the complexes containing pyrenyl
and anthranyl fluorene ligands (sT is up to 66.7 ms, compared to
1.36 ms of a parent complex). The photophysical properties of the
complexes were studied with steady state, time-resolved spec-
78.0% in CH3CH2OH) and the quantum yields were calculated
with eqn (1), where Funk, Aunk, Iunk and hunk represent the
quantum yield, absorbance, integrated photoluminescence
intensity and the refractive index of the samples. The photo-
graphs of the upconversion were taken with a Samsang NV 5
digital camera.
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Astd
Iunk
Istd
hunk
hstd
Funk ¼ 2Fstd
(1)
Aunk
The DFT calculations were used for optimization of the singlet
states and triplet states. The spin density surfaces of the
complexes were calculated based on the optimized triplet state
geometries. All the calculations were performed with Gaussian
09W.24
2,7-Dibromofluorene (1)
A 100 mL two-necked flask equipped with a magnetic stirrer bar
and a dropping funnel was loaded with fluorene (10.0 g, 60.2
mmol) and CHCl3 (32 mL). The solution was stirred and
bromine (7.2 mL, 140.4 mmol) in CHCl3 (8.0 mL) was added
dropwise. The solution was stirred at room temperature for 20 h
in the dark. The solid material was filtered and washed with
CHCl3 (70 mL). The product was re-crystallized from CHCl3. A
1
white solid was obtained, 17.8 g, yield 92.2%. H NMR (400
MHz, CDCl3): d 7.62 (s, 2H), d 7.55 (d, 2H, J ¼ 8.0 Hz), d 7.48
(d, 2H, J ¼ 8.0 Hz), d 3.80 (s, 2H). ESI-MS C13H8Br2 calculated
m/z ¼ 321.8993, found m/z ¼321.9002.
9,9-Dioctyl-2,7-dibromofluorene (2)
3
troscopy and density-functional theory (DFT) calculations. IL
2,7-Dibromofluorene (9.0 g, 27.8 mmol), tetrabutyl ammonium
bromide (0.9 g, 2.70 mmol), 50% aqueous NaOH (45 mL), and
toluene (60 mL) were mixed. The mixture was stirred under N2
and then octylbromide (13.4 g, 69.3 mmol) was added. The
mixture was stirred and heated at 60 ꢀC for 4 h. The reaction
mixture was extracted with ethyl acetate, then washed with
water, and dried over Na2SO4. The solvent was evaporated and
a clear yellow viscous product was obtained. The crude product
was crystallized from absolute ethanol to give 2,7-dibromo-9,9-
dioctylfluorene, 11.4 g, yield 75.0%. 1H NMR (400 MHz,
CDCl3): d 7.52 (d, 2H, J ¼ 12.0 Hz), d 7.46 (d, 2H, J ¼ 4.0 Hz),
d 7.44 (s, 2H), d 1.93–1.88 (m, 4H), d 1.26–0.81 (m, 20H), d 0.6–
0.56 (m, 4H). ESI-MS C29H40Br2 calculated m/z ¼ 546.1503,
found m/z ¼ 546.1497.
emissive triplet excited states were proposed to be responsible for
the anthracene and pyrenyl-containing complexes. The
complexes were used as triplet sensitizers for triplet–triplet
annihilation (TTA) based upconversion and an upconversion
quantum yield (FUC) up to 22.4% was observed. We propose that
weakly phosphorescent or non-phosphorescent transition metal
complexes can be used as triplet sensitizers for the TTA
upconversion.
Experimental
General information
NMR spectra were taken on a 400 MHz Varian Unity Inova
spectrophotometer. Mass spectra were recorded with a Q-TOF
Micro MS spectrometer. UV-Vis spectra were taken on
a HP8453 UV-visible spectrophotometer. Fluorescence spectra
were recorded on a Shimadzu RF5301 fluorospectrometer and
a Sanco 970 CRT spectrofluorometer. Luminescence quantum
yields were measured with quinine sulfate as the standard (F ¼
54.6% in 0.05 M H2SO4). Luminescence lifetimes were measured
on a Horiba Jobin Yvon Fluoro Max-4 and OB 920 lumines-
cence lifetime spectrometer (Edinburgh, UK).
1-(2-(2-Ethynyl-9,9-dioctyl-9H-fluoren-7-yl)ethynyl)-benzene
(3a)
Phenylacetylene (0.10 g, 1.00 mmol), 2 (2.73 g, 5.00 mmol), CuI
(10.0 mg, 0.05 mmol), Pd(PPh3)2Cl2 (3.5 mg, 0.005 mmol), tri-
phenylphosphine (5.0 mg, 0.02 mmol), and dry triethylamine
(150 mL) were placed in a 250 mL round bottom flask. After the
solution was purged with N2 for 30 min, the mixture was stirred
and refluxed under N2 for 4 h. Then the reaction mixture was
filtered and the filtrate was evaporated under reduced pressure.
The residue was purified by column chromatography (silica gel,
hexane). A white solid was obtained, 360.0 mg, yield: 63.4%. 1H
NMR (400 MHz, CDCl3): d 7.65 (d, 1H, J ¼ 8.0 Hz), d 7.57–7.47
Diode pumped solid state laser (CW) was used for the
upconversions. The samples were purged with N2 or Ar for 15
min before measurement. The upconversion quantum yields were
determined with coumarin-6 as the quantum yield standard (F ¼
5320 | J. Mater. Chem., 2012, 22, 5319–5329
This journal is ª The Royal Society of Chemistry 2012

(m, 3H), d 7.38 (d, 4H, J ¼ 8.0 Hz), d 1.96–1.93 (m, 4H), d 1.22–
1.01 (m, 20H), d 0.87–0.81 (m, 6H), d 0.59–0.56 (m, 4H). ESI-MS
C37H45Br calculated m/z ¼ 568.2705, found m/z ¼ 568.2705.
NMR (100 MHz, CDCl3): d 163.5, 156.4, 151.3, 150.7, 142.1,
138.0, 133.4, 131.4, 130.9, 130.4, 128.7, 127.8, 126.6, 125.5, 124.9,
121.4, 121.0, 119.6, 119.4, 119.9, 103.5, 96.0, 87.5, 55.3, 40.8,
36.0, 32.0, 30.4, 29.5, 24.0, 22.8, 14.3. HR-MALDI-MS:
C104H118N2Pt calculated m/z
¼ 1590.8943, found m/z ¼
1-(7-Ethynyl-9,9-dioctyl-9H-fluoren-2-yl)ethynylbenzene (L-1)
1589.8973.
3a (284.0 mg, 0.50 mmol), 3-methyl-1-butyn-3-ol (63.0 mg,
0.75 mmol), CuI (5.0 mg, 0.025 mmol), Pd(PPh3)2Cl2 (1.8 mg,
0.0025 mmol), triphenylphosphine (2.5 mg, 0.01 mmol), and dry
triethylamine (50 mL) were placed in a 100 mL round bottom
flask. After the solution was purged with N2 for 30 min, it was
stirred and refluxed for 8 h. The reaction mixture was filtered,
and the filtrate was evaporated under reduced pressure. The
residue was purified by column chromatography (silica gel,
hexane : dichloromethane ¼ 1 : 10, v/v) to give P-1. Then P-1,
KOH (250.0 mg), and 2-propanol (50 mL) were added in a 100
mL round bottom flask. After the solution was purged with N2
for 30 min, it was stirred and refluxed for 4 h. The solution was
then washed with water and the aqueous phase was extracted
with CH2Cl2 (3 ꢁ 30 mL). The solvent was evaporated under
reduced pressure and the crude product was purified by column
chromatography (silica gel, hexane) to give yellow oily L-1, 238.0
mg, yield 78.0%. 1H NMR (400 MHz, CDCl3): d 7.67 (t, 3H, J ¼
8.0 Hz), d 7.58 (d, 1H), d 7.56 (d, 1H), d 7.54 (s, 1H), d 7.51 (d, 1H,
J ¼ 4.0 Hz), d 7.48 (d, 1H, J ¼ 4.0 Hz), d 7.38–7.35 (m, 3H), d 3.16
(s, 1H), d 1.98–1.94 (m, 4H), d 1.26–1.04 (m, 20H), d 0.86–0.80
(m, 6H), d 0.59–0.56 (m, 4H). 13C NMR (100 MHz, CDCl3),
d 151.3, 141.4, 140.7, 131.8, 131.4, 130.9, 128.6, 126.7, 126.2,
123.4, 122.3, 120.8, 120.1, 94.6, 90.5, 89.9, 84.8, 55.4, 40.5, 32.0,
30.1, 29.4, 23.9, 22.8, 14.3. HR-MALDI-MS: C39H46 calculated
m/z ¼ 514.3600, found m/z ¼ 514.3605.
Pt-3
Pt-3 was prepared with similar method of Pt-1. Yellow solid,
1
90.0 mg, yield: 59.2%. H NMR (400 MHz, CDCl3): d 9.85 (d,
2H, J ¼ 8.0 Hz), d 8.75 (d, 4H, J ¼ 8.0 Hz), d 8.44 (s, 2H), d 8.05
(d, 4H, J ¼ 8.0 Hz), d 7.99 (s, 2H), d 7.75–7.73 (m, 4H), d 7.68–
7.59 (m, 14H), d 7.56 (t, 4H, J ¼ 8.0 Hz), d 2.07–2.04 (m, 8H),
d 1.48 (s, 18H), d 1.20–1.11 (m, 40H), d 0.74–0.67 (m, 12H),
d 0.62–0.60 (m, 8H). 13C NMR (100 MHz, CDCl3): d 163.6,
156.4, 151.5, 150.8, 142.2, 138.0, 132.7, 131.5, 130.9, 128.9, 127.8,
127.1, 126.7, 125.9, 124.9, 121.3, 119.7, 119.5, 118.9, 117.9, 102.6,
87.2, 86.3, 55.4, 40.9, 36.0, 30.5, 29.6, 24.0, 22.8, 14.3. HR-
MALDI-MS: C112H122N2Pt calculated, m/z ¼ 1689.9256, found
m/z ¼ 1689.9415.
Pt-4
Pt-4 was prepared with a method similar to that used for Pt-1
and obtained as a yellow solid, 115.0 mg, yield: 78.2%. 1H NMR
(400 MHz, CDCl3): d 9.85 (d, 2H, J ¼ 4.0 Hz), d 8.77 (d, 2H, J ¼
8.0 Hz), d 8.27–8.21 (m, 8H), d 8.17 (d, 2H, J ¼ 8.0 Hz), d 8.10–
8.04 (m, 6H), d 7.99 (s, 2H), d 7.70 (s, 4H), d 7.65–7.60 (m, 8H),
d 1.46 (s, 18H), d 1.19–1.09 (m, 40H), d 0.82–0.79 (m, 12H),
d 0.71–0.65 (m, 8H). 13C NMR (100 MHz, CDCl3): d 163.4,
156.3, 151.2, 150.6, 142.0, 137.9, 131.8, 131.2, 131.1, 130.8, 129.6,
128.3, 128.0, 127.7, 127.3, 126.9, 125.8, 125.6, 124.7, 124.6, 124.4,
121.0, 119.3, 118.7, 118.3, 55.2, 40.7, 35.8, 31.9, 31.6, 30.3, 29.7,
29.4, 29.4, 23.9, 22.7, 14.1. HR-MALDI-MS: C116H122N2Pt
calculated m/z ¼ 1737.9256, found m/z ¼ 1737.9130.
Pt-1
Pt(dbbpy)Cl2 (50.0 mg, 0.09 mmol), CuI (5.0 mg, 0.019 mmol)
and diisopropylamine (1.0 mL) were dissolved in 5 mL CH2Cl2,
the mixture was stirred for 10 min. The mixture was purged with
Ar, L-1 (138.0 mg, 0.27 mmol) was added and the mixture was
stirred at r.t. for 24 h. The mixture was evaporated to dryness, the
residue was purified by column chromatography (silica gel,
CH2Cl2 : hexane ¼ 1 : 1, v/v). The product was collected as the
third fraction. 102.0 mg of yellow powder was obtained, yield:
76.0%. 1H NMR (400 MHz, CDCl3): d 9.82 (d, 2H, J ¼ 4.0 Hz),
d 7.99 (s, 2H), d 7.63 (d, 6H, J ¼ 8.0 Hz), d 7.58–7.54 (m, 8H),
d 7.51–7.48 (m, 4H), d 7.36 (m, 6H), 1.98–1.95 (m, 8H), d 1.47 (s,
18H), d 1.21–1.05 (m, 40H), d 0.82–0.79 (m, 12H), d 0.62–0.58 (m,
8H). 13C NMR (100 MHz, CDCl3), d 163.5, 156.4, 151.2, 150.7,
141.9, 138.0, 131.7, 130.7, 128.5, 128.2, 127.0, 126.0, 120.8, 119.5,
119.4, 119.0, 103.4, 91.1, 89.3, 87.3, 55.2, 40.8, 36.0, 32.0, 30.4,
29.5, 24.0, 22.8, 14.3. HR-MALDI-MS: C96H114N2Pt calculated
m/z ¼ 1489.8630, found, m/z ¼ 1489.8531.
Pt-5
Pt-5 was prepared with a method similar to that used for Pt-1
1
and obtained as a yellow solid, 30.0 mg, yield: 60.9%. H NMR
(400 MHz, CDCl3): d 9.82 (d, 2H, J ¼ 4.0 Hz), d 7.97 (s, 2H),
d 7.62–7.53 (m, 10H), d 7.47 (d, 4H, J ¼ 8.0 Hz), d 7.42 (d, 4H,
J ¼ 8.0 Hz), d 7.30 (m, 6H), d 7.13 (d, 8H, J ¼ 8.0 Hz), d 7.08–7.01
(m, 8H), d 1.19–1.94 (m, 8H), d 1.46 (s, 18H), d 1.21–1.05 (m,
40H), d 0.82–0.79 (m, 12H), d 0.65–0.59 (m, 8H). 13C NMR (100
MHz, CDCl3): d 163.4, 156.3, 151.3, 151.0, 150.5, 147.7, 147.3,
141.5, 137.9, 132.5, 131.1, 130.4, 129.4, 127.4, 126.9, 125.7, 124.9,
123.5, 122.5, 121.0, 119.2, 118.7, 116.5, 90.1, 89.4, 86.6, 55.0,
40.6, 35.8, 31.8, 30.3, 29.7, 29.4, 23.8, 22.6, 14.1. HR-MALDI-
MS: C120H132N4Pt calculated m/z ¼ 1824.0100, found m/z ¼
1824.0171.
Pt-2
Complex Pt-6
Pt-2 was prepared with a method similar to that used for Pt-1
and obtained as a yellow solid, 105.0 mg, yield: 73.4%. 1H NMR
(400 MHz, CDCl3): d 9.84 (d, 2H, J ¼ 8.0 Hz), d 8.54 (d, 2H, J ¼
8.0 Hz), d 7.89 (s, 2H), d 7.90–7.80 (m, 6H), d 7.67–7.58 (m, 18H),
d 7.50–7.46 (m, 2H), d 2.03–1.99 (m, 8H), d 1.47 (s, 18H), d 1.22–
1.08 (m, 40H), d 0.82–0.79 (m, 12H), d 0.69–0.63 (m, 8H). 13C
Pt-6 was prepared with a method similar to that used for Pt-1.
Yellow solid 40.5 mg, yield: 52.6%. 1H NMR (400 MHz, CDCl3):
d 9.82 (d, 2H, J ¼ 8.0 Hz), d 7.98 (s, 1H), d 7.70–7.68 (m, 5H),
d 7.65–7.59 (m, 8H), d 7.57–7.52 (m, 9H), d 7.33–7.32 (m, 6H),
d 2.00–1.96 (m, 16H), d 1.46 (s, 18H), d 1.20–1.05 (m, 80H),
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 5319–5329 | 5321

d 0.83–0.80 (m, 24H), d 0.66–0.60 (m, 16H). 13C NMR (100 MHz,
CDCl3): d 163.4, 156.3, 151.3, 151.1, 150.8, 150.5, 141.7, 141.2,
140.5, 137.9, 131.2, 130.6, 127.4, 126.9, 125.9, 124.7, 122.9, 121.7,
120.9, 120.0, 119.6, 119.4, 119.2, 103.4, 90.8, 90.1, 86.8, 55.1,
40.7, 40.4, 35.8, 31.9, 30.8, 30.3, 30.1, 29.7, 29.4, 29.3, 23.8, 23.7,
22.6, 14.1. HR-MALDI-MS: C142H182N2Pt calculated m/z ¼
2114.4264, found m/z ¼ 2114.4287.
(Table 1). Thus we propose that the emissive triplet excited states
of Pt-3 and Pt-4 are drastically different from those of other
complexes. Based on the steady state spectroscopy and DFT
3
calculations, we proposed a IL emissive excited state for Pt-3
and Pt-4. This assignment was supported by the exceptionally
prolonged T1 state lifetime of Pt-3 and Pt-4, 66.7 ms and 54.7 ms,
respectively, compared to the other complexes (Table 1). Such an
exceptionally long-lived T1 state is usually the feature of the 3IL
excited state.12,25,30–33 Furthermore, the fluorescence of the ligand
was completely quenched, which is an indication of the efficient
intersystem crossing (ISC). It should be pointed out that with
large ligands, such as for the complexes studied in this manu-
script, it is very likely that the fluorescence of the ligands cannot
be quenched completely.34
Results and discussion
Design and synthesis of complexes
The design rationales of the fluorene-containing complexes
(Scheme 1) are (1) different aryl acetylide ligands were attached
to the fluorene moiety to tune the UV-vis absorption and to
investigate the T1 state energy levels; (2) alkyl chains were
introduced to the fluorene moieties to improve the solubility of
the complexes; (3) pyrene and anthracene moieties were intro-
To the best of our knowledge, this is the first time that the
anthracene-related RT phosphorescence was observed.
Previously anthracene was incorporated into Ru(II) bispyrene
complexes, but very often the phosphorescence of the Ru(II)
complexes was quenched and no emissions due to the anthracene
were observed.28 We attribute the RT emission of the anthracene-
3
duced with the intention to observe the long-lived IL triplet
excited state.15,25–28 Fluorene was used as the starting material
(Scheme 1). After bromination and introduction of the alkyl
chain, acetylide moieties were attached by using the Sonogashira
cross-coupling reactions. All the complexes were obtained in
excellent to moderate yields.
3
related IL excited state to the enhanced heavy atom effect on
anthracene in Pt-3, in which the p-conjugation core of anthra-
cene is directly connected to the Pt(II) center. Previously the
anthracene moiety was dangled on the periphery of Ru(II)
complexes,28,35 thus we anticipated that the heavy atom effect of
Ru(II) on anthracene in those complexes is weak, as a result no
emission from the anthracene moiety can be observed even
Absorption and emission spectra
The UV-vis absorption of the ligands was studied (Fig. 1a).
Ligands L-1, L-2 and L-6 give absorption in the UV region.
Ligands with anthracene, pyrene and triphenylamine moieties
show absorption extended to 440 nm.
3
though the anthracene-localized IL state was populated upon
1
3
3
photoexcitation, via the MLCT / MLCT / IL ISC and
internal conversion (IC). This observation of the organic chro-
3
mophore-localized IL emission is in line with our recent obser-
The UV-vis absorption of the complexes was also studied
(Fig. 1b). Intense absorption in the visible region was observed
for Pt-3 and Pt-4. The molar extinction coefficient is up to 8.54 ꢁ
104 Mꢂ1 cmꢂ1 for Pt-3 at 445 nm. For Pt-4, 3 is 9.70 ꢁ 104 Mꢂ1
cmꢂ1 at 420 nm. It should be pointed out that the normal Pt(II)
bisacetylide complexes show weak absorption in the visible
region.9–11,29 Interestingly, we found that the fluorene-containing
complexes show red-shifted absorption compared to that of the
analogues without the fluorene moiety. For example, Pt-4 shows
absorption at 420 nm. By comparison, the dbbpy Pt(II) bispyr-
enylacetylide complex shows weak and blue-shifted absorption.25
Pt-1 also shows enhanced and red-shifted absorption than the
model complex dbbpy Pt(II) bisphenylacetylide.10 The enhanced
absorption of the complexes in the visible region will be beneficial
for the applications.
vation of similar ligand localized emission in Pt(II)
complexes.12,14,15
Interestingly, we found the energy levels of the T1 excited state
of these fluorene-containing complexes do not decrease
compared to the analogue complexes without the fluorene
moieties, despite the red-shifted UV-vis absorption of the new
complexes. For example, the emission band of Pt-4 is at 663 nm.
By comparison, the analogue dbbpy Pt(II) bispyrenylacetylide
gives emission at 655 nm.25 Furthermore, Pt-1 emits at 592 nm,
by comparison dbbpy Pt(II) bisphenylacetylide emits at 570 nm.10
This is interesting because the energy levels of the T1 excited
states of the N^N Pt(II) bisacetylide complexes usually decrease
sharply with red-shift of the UV-vis absorption.15 Keeping the T1
at a high energy level is important for applications because
a higher excited state energy level is crucial for many photo-
physical processes, such as triplet energy transfer, photovoltaics,
sensitizers, etc.
The emissions of the ligands and the complexes were studied
(Fig. 2). The ligands show intense fluorescence emission in the
range of 350–500 nm (Fig. 2a). Intense room temperature (RT)
emissions were observed for Pt-1, Pt-2, Pt-5 and Pt-6. The
emission maximum of these complexes is similar, located at 585
nm. Furthermore, these complexes show similar phosphorescent
quantum yields (Table 1). Thus we propose that these complexes
share the similar emissive triplet excited states (T1 excited state).
For complexes Pt-3 and Pt-4, however, much weaker emission
was observed (F ¼ 0.0% and 0.3%, respectively). Furthermore,
the emission band of Pt-4 is more structured than the emission
bands of Pt-1, Pt-2, Pt-5 and Pt-6, which is a clear indication of
The emission of the complexes can be significantly quenched
by O2, this is a clear indication of the phosphorescence feature of
the emissions (Fig. 3).13,14,30,31,36,37 The emission of Pt-4 is more
sensitive to oxygen than the other complexes, indicating longer
emission lifetime for Pt-4.13,30,31,33 This is in agreement with the
photophysical data of the complexes (Table 1).
Emission spectra at 77 K
3
an emissive state with significant IL feature, not the normal
3MLCT.1,9,10 For Pt-3, near-IR emission at 780 nm was observed
5322 | J. Mater. Chem., 2012, 22, 5319–5329
In order to clarify the feature of the emissive triplet excited
state of the complexes, the emissions at 77 K were also
This journal is ª The Royal Society of Chemistry 2012

Scheme 1 Synthesis of the Pt(II) complexes Pt-1, Pt-2, Pt-3, Pt-4, Pt-5 and Pt-6. (a) Br2, CHCl3, r.t., 12 h. (b) Octyl bromide, TBAB, aqueous NaOH
(w/w ¼ 50%), toluene, reflux; (c) Pd(PPh3)2Cl2, CuI, PPh3, NEt3, reflux, 8 h; (d) 3-methyl-1-butyn-3-ol, Pd(PPh3)2Cl2, PPh3, CuI, NEt3, reflux, 12 h; (e)
2-propanol, KOH, reflux, 4 h; (f) CH2Cl2, CuI, (i-Pr)2NH, r.t., 12 h; (g) Pd(PPh3)4, CuI, trimethylsilylacetylene, NEt3, reflux, 8 h; (h) K2CO3, CH3OH,
CH2Cl2, 3 h. The triplet acceptor 9,10-diphenylanthracene (DPA) and the quantum yield standard coumarin-6 used in the TTA upconversion are also
shown.
studied and were compared with those at RT (Fig. 4).25 For
all the complexes the emissions at 77 K become more struc-
tured than those at RT. For Pt-1, the emission band gives
a blue shift of 43 nm at 77 K, the large thermally induced
Stokes shift (DEs) of 132 cmꢂ1 indicates that the 3MLCT
component of Pt-1 is significant. For Pt-2 and Pt-4, however,
the DEs values are very small (9 cmꢂ1 and 5 cmꢂ1, respec-
tively). Thus we propose that the emissions of Pt-2 and Pt-4
3
are due to the emissive triplet excited state with significant IL
feature.25
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 5319–5329 | 5323

Spin density surface of the complexes
In order to study the T1 state of the complexes from a theoretical
perspective, the spin densities of the triplet states of the
complexes were studied (Fig. 6).38 For complexes Pt-1, Pt-2, Pt-5
and Pt-6, the spin density isosurfaces are distributed on the
dbbpy ligand, Pt(II) center and the acetylide ligands, thus the
triplet state of these complexes can be assigned as the typical
3MLCT for the N^N Pt(II) bisacetylide complexes. For the
complexes Pt-3 and Pt-4, however, the spin density isosurfaces
are localized on the anthracene–fluorene or the pyrene–fluorene
ligands, the dbbpy ligand and the Pt(II) center do not contribute
to the spin density isosurfaces, thus the triplet state of these
complexes can be assigned as the ligand-localized 3IL state.38 This
conclusion is in line with the steady state and the time-resolved
spectra of the complexes.
Application for triplet–triplet annihilation upconversion
Since we have shown that the fluorene-containing Pt(II) bisace-
tylide complexes show enhanced absorption in the visible region
and long-lived triplet excited states, especially for Pt-3 and Pt-4,
these complexes are used as triplet sensitizers for TTA based
upconversion.39–49 TTA upconversion is in particular interesting
among the upconversion approaches, such as the two-photon
absorption dyes (TPA) or the rare earth materials, for which
either a very high excitation power density has to be used (106 W
cmꢂ2),50 or very often a low upconversion quantum yield was
observed. Furthermore, it is difficult to tune the excitation/
emission wavelength of these upconversion schemes. For the
TTA upconversion, however, low excitation power is sufficient,
the excitation power density can be down to mW cmꢂ2, on the
same order of the terrestrial solar light.51 Furthermore, the
excitation/emission wavelength of the TTA upconversion scheme
can be readily tuned, simply by independent selection of the
triplet sensitizers and triplet acceptors (but the energy levels of
the sensitizer and the acceptor must be matched).42
Fig. 1 UV-vis absorption spectra of (a) the ligands L-1, L-2, L-3, L-4, L-
5 and L-6, c ¼ 1.0 ꢁ 10ꢂ5 M and (b) the complexes Pt-1, Pt-2, Pt-3, Pt-4,
Pt-5 and Pt-6, c ¼ 5.0 ꢁ 10ꢂ6 M. In toluene, 25 ꢀC.
Nanosecond time-resolved difference absorption spectra
The nanosecond time-resolved transient difference absorption
spectra of the complexes were studied (Fig. 5). Bleaching of the
ground state absorption was observed for the complexes. The
transient profile of the anthracene-containing Pt-3 is similar to
that of the anthracene-containing Ru(II) complexes, thus the T1
state of Pt-3 can be assigned as the anthracene–fluorene localized
3IL excited state. Furthermore, the transient absorption of Pt-4 is
similar to a N^N Pt(II) bis(pyrenylacetylide) complex, which was
proven to show the pyrene localized 3IL triplet excited state, thus
the T1 state of the Pt-4 can be assigned as the pyrene–fluorene
However, we noticed that the development of the TTA
upconversion is facing some challenges, for example, the triplet
sensitizers are very limited, currently the typical sensitizers are
limited to the Pt(II)/Pd(II) porphyrin complexes.52 Unfortunately,
3
localized IL excited state.25
Fig. 2 (a) The normalized emission spectra of ligands L-1, lex ¼ 335 nm, L-2, lex ¼ 345 nm, L-3, lex ¼ 410 nm, L-4, lex ¼ 385 nm, L-5, lex ¼ 370 nm and
L-6, lex ¼ 355 nm, c[ligands] ¼ 1.0 ꢁ 10ꢂ5 M in toluene, 25 ꢀC. (b) The phosphorescence emission and (c) the normalized emission spectra of complexes
Pt-1, lex ¼ 380 nm, Pt-2, lex ¼ 390 nm, Pt-3, lex ¼ 445 nm, Pt-4, lex ¼ 420 nm, Pt-5, lex ¼ 390 nm and Pt-6, lex ¼ 390 nm. c[complexes] ¼1.0 ꢁ 10ꢂ5 M in
toluene, 25 ꢀC.
5324 | J. Mater. Chem., 2012, 22, 5319–5329
This journal is ª The Royal Society of Chemistry 2012

Table 1 Photophysical parameters of the ligands and the platinum complexes
lem/nm
f
labsa/nm
3/10ꢂ4 Mꢂ1 cmꢂ1
298 K
77 K
F
sL
c
L-1
L-2
L-3
L-4
L-5
L-6
Pt-1
Pt-2
Pt-3
Pt-4
Pt-5
Pt-6
334/351
350/370
341/410/436
329/385/410
371/386
5.59/4.56
5.00/4.44
2.05/1.32/1.11
3.81/4.58/4.80
2.61/2.49
357/367a
377/397a
445/472a
416/441a
413a
—
—
0.843d
0.764d
0.687d
0.612d
0.858d
0.723d
0.179e
0.118e
0.000e
0.000e
0.113e
0.156e
3.86 ns
0.95 ns
2.44 ns
1.18 ns
1.02 ns
0.73 ns
0.51 ms
1.73 ms
66.7 msg
54.7 msg
0.56 ms
0.72 ms
c
c
—
—
c
c
—
c
—
348/373
3.67/3.45
379/400a
592b
358/374/432
366/389/431
358/420/445
344/396/420
371/393/460
366/389/470
13.36/13.70/1.38
11.80/9.92/1.26
9.84/9.64/8.54
6.46/10.40/9.70
13.10/12.44/1.02
12.36/10.86/0.86
546/587/598b
571/613/627/65b
777/800b
574/610b
780b
665/680/723/744b
562b
663/682/700/722/744b
562/610b
564b
569/645b
a
b
c
Recorded in toluene at 298 K. Recorded in deaerated EtOH : MeOH ¼ 4 : 1 (v/v). Not determined. d Result in deaerated toluene solution, with
e
quinine sulfate (F ¼ 0.546 in 0.05 M sulfuric acid) as the standard. Result in deaerated toluene solution, with the complex [Ru(dmb)3]2+ (F ¼ 0.073
f
g
in MeCN) as the standard (the solution is purged with Ar for about 15 min before measurement). Luminescence lifetimes. Too weak to be
determined accurately with the luminescence method. Determined with the nanosecond time-resolved transient difference absorption spectroscopy.
Fig. 3 Emission spectra of (a) Pt-1, lex ¼ 380 nm, (b) Pt-4, lex ¼ 420 nm,
Fig.
5 Nanosecond time-resolved transient absorption difference
c ¼ 1.0 ꢁ 10ꢂ5 M under different atmospheres of nitrogen, air and oxygen
(the solution is purged with Ar or O2 for about 15 min before measure-
ment) in toluene, 25 ꢀC.
spectra of (a) Pt-3 and (b) Pt-4 excited with 355 nm pulsed-laser. c ¼ 1.0
ꢁ 10ꢂ5 M in toluene, 25 ꢀC.
Fig. 4 Emission spectra of the complexes at 77 K and room temperature
(25 ꢀC). (a) Pt-1, lex ¼ 380 nm, (b) Pt-2, lex ¼ 390 nm. c[complexes] ¼ 1.0
ꢁ 10ꢂ5 M in EtOH : MeOH ¼ 4 : 1 (v/v).
it is difficult to tune the T1 state energy level for these complexes
by modification of the molecular structures. Furthermore, the
absorptions of these complexes in the visible region are usually
weak. Thus, it is highly desired to develop new triplet sensitizers
with readily tunable T1 state energy levels, as well as intense
absorption in the visible region.
Firstly the emissions of the complexes upon 445 nm laser
excitation were studied (Fig. 7). Pt-1, Pt-2, Pt-5 and Pt-6 give
strong emission at ca. 600 nm. Notably Pt-4 is weakly lumines-
cent and Pt-3 is nonluminescent (Fig. 7a). In the presence of the
Fig. 6 The spin density of Pt-1, Pt-2, Pt-3, Pt-4, Pt-5 and Pt-6 based on
the optimized triplet state geometry. Calculation is based on DFT at the
B3LYP/6-31G(d)/LanL2DZ level using Gaussian 09W.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 5319–5329 | 5325

Fig. 8 The upconversion of (a) Pt-2 and (b) Pt-4, c ¼ 1.0 ꢁ 10ꢂ5 M with
increasing DPA concentration. With 445 nm (5 mW) laser excitation. In
toluene, 25 ꢀC.
radiative decay of the T1 state, is actually competitive to the
TTET process. Thus the phosphorescence is detrimental to the
TTA upconversion. Therefore, we propose that weakly phos-
phorescent, or non-phosphorescent transition metal complexes
can be used as the triplet sensitizers for the TTA upconversion, as
long as the triplet excited state of the complexes can be efficiently
populated upon photoexcitation.15,32,54
The dark excited state of Pt-4 that is involved in the TTET,
thus the TTA upconversion, is clearly demonstrated in Fig. 8.
For Pt-2, with an increase in the concentration of DPA, the
upconverted fluorescence emission of DPA increased with the
significant quenching of the phosphorescence in the range of
550–750 nm. This quenching of the phosphorescence by DPA is
reasonable since the TTET process is enhanced with more
concentrated DPA and the T1 excited state, which is responsible
for the phosphorescence of Pt-4, is quenched. For the triplet
sensitizer Pt-4, however, the upconverted fluorescence emission
is greatly enhanced without any phosphorescence to be
Fig. 7 Emission and upconversion with the complexes as triplet sensi-
tizers upon 445 nm laser excitation. (a) Emission of the Pt(II) complexes
without DPA. (b) Upconverted emission in the presence of DPA, lex
445 nm, 5 mW, c[sensitizer] ¼ 1.0 ꢁ 10ꢂ5 M, c[DPA] ¼ 4.3 ꢁ 10ꢂ5 M in
¼
toluene, 25 ꢀC.
triplet acceptor DPA, intense blue emissions in the range of 400–
550 nm were found (Fig. 7a). Irradiation of DPA alone with
a 445 nm laser does not produce blue emission. Thus the blue
emission with Pt-4 can be assigned as the upconverted fluores-
cence emission with Pt-4 as the triplet sensitizer. Minor upcon-
version was observed with Pt-2 as the triplet sensitizer. For other
complexes, however, the upconversion is negligible. Although
the emissions of these complexes are much stronger than that of
Pt-4, we attribute the lack of upconversion with Pt-1 to the short
T1 state lifetime and the weak absorption at the excitation
wavelength,49 thus the population of the triplet excited sensitizers
is under the threshold for TTA upconversion.
The role of the dark excited states in TTA upconversion
Interestingly, the upconverted fluorescence peak area with Pt-4 is
much higher than the quenched phosphorescent peak areas. This
is abnormal because the conversionally phosphorescent triplet
sensitizers are used and usually the upconverted fluorescence
emission band is accompanied by the quenching of the phos-
phorescence.42,53 According to the photophysics of the TTA
upconversion, the quenched phosphorescence emission band
area should be at least two fold of that of the upconverted
fluorescence emission band.42,53
Fig. 9 Phosphorescence emission spectra of complex (a) Pt-1, lex ¼ 380
nm, (b) Pt-2, lex ¼ 390 nm, and (c) Pt-4, lex ¼ 420 nm with increasing
DPA concentration in toluene. (d) Stern–Volmer plots for phosphores-
cence quenching of Pt-1, lex ¼ 380 nm, Pt-2, lex ¼ 390 nm, Pt-4, lex ¼ 420
nm, Pt-5, lex ¼ 385 nm and Pt-6, lex ¼ 385 nm, c ¼ 1.0 ꢁ 10ꢂ5 M in
toluene at 25 ꢀC.
For Pt-4, we propose that the triplet sensitizers at the triplet
excited state that are otherwise non-emissive were involved in the
key photophysical process of the TTET.15,32,54 Actually it is
unnecessary for the triplet sensitizer to be phosphorescent to
sensitize the TTET process, and the phosphorescence, i.e. the
5326 | J. Mater. Chem., 2012, 22, 5319–5329
This journal is ª The Royal Society of Chemistry 2012

Table
2 Upconversion related photophysical properties of Pt(II)
quenching curves following the Stern–Volmer equation (Fig. 9d).
A quenching constant of 1.19 ꢁ 104 Mꢂ1 was determined for Pt-
2. For Pt-4, however, a much higher quenching constant of 1.21
ꢁ 106 Mꢂ1 was observed. Thus the significant upconversion with
Pt-4 as the triplet sensitizer can be attributed to its strong
absorption and the efficient TTET process (Table 2).
complexesa
c
d
sT/msb
Ksv/103 Mꢂ1
FUC
Pt-1
Pt-2
Pt-3
Pt-4
Pt-5
Pt-6
[Ru(dmb)3]2+
0.74
2.02
66.7
54.7
0.59
0.88
—
1.98
11.89
—
1209
2.07
1.83
—
0.0
3.0 ꢃ 1.0%
0.0
Recently a N^N^N Pt(II) acetylide complex was reported as
a triplet sensitizer for TTA upconversion, but the upconversion
quantum yield is much lower (less than 2%).55 This low upcon-
version quantum yield is due to the weak absorption of the
complex at the excitation wavelength, and the short T1 excited
state lifetime (4.8 ms).
22.4 ꢃ 3%
0.0
0.0
0.5 ꢃ 0.3%
a
b
Recorded in deaerated toluene at 298 K. Triplet state lifetime.
Stern–Volmer quenching constants for phosphorescence quenching.
Upconversion quantum yields, with coumarin 6 as the standard (F ¼
c
d
The upconversion with the complexes as the triplet sensitizer is
visible with unaided eyes (Fig. 10). For complexes Pt-1, Pt-2, Pt-
5 and Pt-6, yellow emission was observed for the sensitizer
solution upon 445 nm laser excitation. For the complexes Pt-3
and Pt-4, however, blue beam paths were observed, which is due
to the weak emission of the two complexes and the scattered laser
light. In the presence of the triplet acceptor DPA, the emission
color of the beam path for Pt-1, Pt-5 and Pt-6 is yellow, due to
the lack of the upconversion with these complexes. For Pt-2, the
emission colour is close to white, due to the upconverted fluo-
rescence emission and the residual phosphorescence emission.
Intense blue emission was observed for Pt-4, due to its intense
upconversion.
0.78 in CH3CH2OH).
quenched. Thus the gain of the upconversion can only be ratio-
nalized by the involvement of the dark excited state, i.e. those
non-radiative T1 excited states of Pt-4, in the TTET process. This
new concept of using non-phosphorescent triplet sensitizers for
TTA upconversion will greatly broaden the availability of the
triplet sensitizers for TTA upconversion, and other photo-
physical processes that require a triplet excited state to initiate.
The efficiency of the TTET process
The upconversion with the Pt(II) bisacetylide complex as the
triplet sensitizer can be summarized in Scheme 2. Photoexcita-
1
tion of Pt-4 produces the sensitizers at the IL singlet excited
The TTET process was studied by the quenching of the phos-
phorescence of sensitizers with DPA (Fig. 9).14,15,49,54 For Pt-1,
the phosphorescence peak was quenched slightly. For Pt-2, the
quenching by increasing the DPA concentration is significant.
The most significant quenching was observed for Pt-4. With 5 eq.
of DPA, the phosphorescence peak is almost completely
quenched. The quenching effect on the phosphorescence of the
triplet sensitizers can be quantified by the construction of the
state. Then with ISC, the triplet excited state 3IL, most probably
3
via MLCT, was populated. Then the TTET occurred between
the 3IL excited state of the sensitizer and the DPA triplet
acceptor. The light-harvesting ability of the sensitizer will
produce more concentrated sensitizer molecules at the triplet
Fig. 10 Photographs of the upconversion with the Pt(II) complexes as triplet sensitizers. (a) Images before and after adding DPA of Pt-1, Pt-2, Pt-3, Pt-
4, Pt-5 and Pt-6 in toluene. (b) The CIE of Pt-1, Pt-2, Pt-3, Pt-4, Pt-5 and Pt-6 emission and (c) the CIE of the upconversion. c[complexes] ¼ 1.0 ꢁ 10ꢂ5
M, c[DPA] ¼ 4.33 ꢁ 10ꢂ5 M, 25 ꢀC.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 5319–5329 | 5327

Scheme 2 Qualitative Jablonski diagram illustrating the sensitized TTA upconversion process between Pt(II) complexes (exemplified by Pt-4) and DPA.
The effect of the light-harvesting ability and the luminescence lifetime of the Pt(II) sensitizer on the efficiency of the TTA upconversion is also shown.
Please note that Pt-4 shows absorption at 420 nm, but it is excitable at 445 nm (laser was used for upconversion). E is energy. GS is ground state (S0).
1IL* is intraligand singlet excited state (fluorene–pyrenyl localized). IC is internal conversion. ISC is intersystem crossing. 3MLCT* is the Pt(II) based
metal-to-ligand-charge-transfer triplet excited state. 3IL* is intraligand triplet excited state (fluorene–pyrenyl localized). TTET is triplet–triplet energy
transfer. 3DPA* is the triplet excited state of DPA. TTA is triplet–triplet annihilation. 1DPA* is the singlet excited state of DPA. The emission bands
3
3
observed for the sensitizers alone are the IL emissive excited state. The emission bands observed in the TTA experiment are the simultaneous IL*
emission (phosphorescence) and the 1DPA* emission (fluorescence).
excited state, whereas the long-lifetime of the T1 excited state of
the sensitizer can enhance the TTET process. Thus the light-
harvesting ability of the complexes and the lifetime are important
for the TTA upconversion. The collision of the DPA molecules
will produce the singlet excited state, in a probability of 11.1%,
determined by the spin statistic rule.43,54 Radiative decay of the
singlet excited state of the DPA molecules will produce the
fluorescence emission, for which the wavelength is shorter than
the excitation wavelength.
the enhanced performance as triplet sensitizer for triplet–triplet-
annihilation based upconversion; an upconversion quantum
yield of 22.4% was observed, in stark contrast to the previously
reported upconversion quantum yields for the transition metal
complexes of less than 20%. Other complexes described herein
show negligible upconversion. The high upconversion quantum
yield of Pt-4 is attributed to its intense absorption of visible light,
long-lived T1 excited state and appropriate T1 state energy level.
Based on the result of Pt-4, we propose that weakly phospho-
rescent, or non-phosphorescent transition metal complexes can
be used as triplet sensitizers for TTA upconversion, vs. the
typical phosphorescent triplet sensitizers. Our results will be
useful for the rational design of transition metal complexes to
enhance the light-absorption, without decreasing the T1 excited
state energy levels, which is important for the application of the
complexes as triplet sensitizers in various photophysical
processes.
Conclusions
Fluorene-containing aryl acetylides were used to prepare N^N
Pt(II) bisacetylide complexes, where aryl substituents on the flu-
orene moiety are phenyl, naphthal, anthranyl, pyrenyl, 4-
diphenylaminophenyl and 9,9-di-n-octylfluorene (where N^N
ligand ¼ 4,40-di-tert-butyl-2,20-bipyridine). All the complexes
show room temperature (RT) phosphorescence. The emissive T1
excited state of the complexes Pt-1, Pt-5 and Pt-6 was assigned as
Acknowledgements
3
the MLCT state, whereas for Pt-2, Pt-3 and Pt-4, the emissive
T1 excited states were identified with the 3IL as the major
components, based on the steady state emission spectra, the
lifetime of the T1 state, the emission spectra at 77 K, the spin
density analysis and the time-resolved transient absorption
spectroscopy. Exceptionally long-lived T1 excited states were
observed for complexes containing anthranyl (s ¼ 66.7 ms) or
pyrenyl acetylide (s ¼ 54.7 ms), compared to the other complexes
(less than 2.0 ms). RT phosphorescence of anthracene was
observed at 780 nm with complex Pt-3. We found that with
fluorene substitution, the absorption of the complexes is red-
shifted compared to the analogues, without compromising the T1
energy levels. The advantage of this unique spectral tuning effect
and the long-lived T1 excited states of Pt-4 were demonstrated by
We thank the NSFC (20972024, 21073028 and 21103015), 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 Dalian University of Technology for financial
support.
References
1 J. A. G. Williams, Top. Curr. Chem., 2007, 281, 205.
2 J. Zhang, P. Du, J. Schneider, P. Jarosz and R. Eisenberg, J. Am.
Chem. Soc., 2007, 129, 7726.
5328 | J. Mater. Chem., 2012, 22, 5319–5329
This journal is ª The Royal Society of Chemistry 2012

3 J. E. McGarrah, Y.-J. Kim, M. Hissler and R. Eisenberg, Inorg.
Chem., 2001, 40, 4510.
31 W. Wu, W. Wu, S. Ji, H. Guo, P. Song, K. Han, L. Chi, J. Shao and
J. Zhao, J. Mater. Chem., 2010, 20, 9775.
4 W.-Y. Wong and C.-L. Ho, Acc. Chem. Res., 2010, 43, 1246.
5 Q. Zhao, F. Li and C. Huang, Chem. Soc. Rev., 2010, 39, 3007.
6 Y. Chen, K. Li, W. Lu, S. S.-Y. Chui, C.-W. Ma and C. Che, Angew.
Chem., Int. Ed., 2009, 48, 9909.
7 R. Zhang, Z. Ye, G. Wang, W. Zhang and J. Yuan, Chem.–Eur. J.,
2010, 16, 6884.
32 J. Sun, W. Wu, H. Guo and J. Zhao, Eur. J. Inorg. Chem., 2011, 3165.
33 W. Wu, W. Wu, S. Ji, H. Guo and J. Zhao, Dalton Trans., 2011, 40,
5953.
34 D. N. Kozhevnikov, V. N. Kozhevnikov, M. Z. Shafikov,
A. M. Prokhorov, D. W. Bruce and J. A. G. Williams, Inorg.
Chem., 2011, 50, 3804.
8 S. Ji, H. Guo, X. Yuan, X. Li, H. Ding, P. Gao, C. Zhao, W. Wu,
W. Wu and J. Zhao, Org. Lett., 2010, 12, 2876.
9 C. E. Whittle, J. A. Weinstein, M. W. George and K. S. Schanze,
Inorg. Chem., 2001, 40, 4053.
35 A. F. Morales, G. Accorsi, N. Armaroli, F. Barigelletti, S. J. A. Pope
and M. D. Ward, Inorg. Chem., 2002, 41, 6711.
36 W. Wu, W. Wu, S. Ji, H. Guo and J. Zhao, J. Organomet. Chem.,
2011, 696, 2388.
10 M. Hissler, W. B. Connick, D. K. Geiger, J. E. McGarrah, D. Lipa,
R. J. Lachicotte and R. Eisenberg, Inorg. Chem., 2000, 39, 447.
11 F. N. Castellano, I. E. Pomestchenko, E. Shikhova, F. Hua,
M. L. Muro and N. Rajapakse, Coord. Chem. Rev., 2006, 250, 1819.
12 H. Guo, M. L. Muro-Small, S. Ji, J. Zhao and F. N. Castellano, Inorg.
Chem., 2010, 49, 6802.
37 W. Wu, S. Ji, W. Wu, H. Guo, X. Wang, J. Zhao and Z. Wang, Sens.
Actuators, B, 2010, 149, 395.
38 K. Hanson, A. Tamayo, V. V. Diev, M. T. Whited, P. I. Djurovich
and M. E. Thompson, Inorg. Chem., 2010, 49, 6077.
39 L. Huang, L. Zeng, H. Guo, W. Wu, W. Wu, S. Ji and J. Zhao, Eur. J.
Inorg. Chem., 2011, 4527.
13 H. Guo, S. Ji, W. Wu, W. Wu, J. Shao and J. Zhao, Analyst, 2010,
135, 2832.
40 W. Wu, J. Sun, S. Ji, W. Wu, J. Zhao and H. Guo, Dalton Trans.,
2011, 40, 11550.
14 H. Sun, H. Guo, W. Wu, X. Liu and J. Zhao, Dalton Trans., 2011, 40,
7834.
15 Y. Liu, W. Wu, J. Zhao, X. Zhang and H. Guo, Dalton Trans., 2011,
40, 9085.
16 G. Zhou, W.-Y. Wong, S.-Y. Poon, C. Ye and Z. Lin, Adv. Funct.
Mater., 2009, 19, 531.
17 J. B. Seneclauze, P. Retailleau and R. Ziessel, New J. Chem., 2007, 31,
1412.
41 J. Zhao, S. Ji and H. Guo, RSC Adv., 2011, 1, 937.
42 T. N. Singh-Rachford and F. N. Castellano, Coord. Chem. Rev., 2010,
254, 2560.
43 A. Monguzzi, J. Mezyk, F. Scotognella, R. Tubino and F. Meinardi,
Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 195112.
44 Y. Y. Cheng, T. Khoury, R. G. C. R. Clady, M. J. Y. Tayebjee,
N. J. Ekins-Daukes, M. J. Crossley and T. W. Schmidt, Phys.
Chem. Chem. Phys., 2010, 12, 66.
18 Z. Ji, S. Li, Y. Li and W. Sun, Inorg. Chem., 2010, 49, 1337.
19 Z. Zhao, X. Xu, Z. Jiang, P. Lu, G. Yu and Y. Liu, J. Org. Chem.,
2007, 72, 8345.
20 S. Tao, Y. Zhou, C.-S. Lee, X. Zhang and S.-T. Lee, Chem. Mater.,
2010, 22, 2138.
45 R. R. Islangulov, D. V. Kozlov and F. N. Castellano, Chem.
Commun., 2005, 3776.
46 H.-C. Chen, C.-Y. Hung, K.-H. Wang, H.-L. Chen, W. S. Fann,
F.-C. Chien, P. Chen, T. J. Chow, C.-P. Hsu and S.-S. Sun, Chem.
Commun., 2009, 4064.
21 J. E. Haley, D. M. Krein, J. L. Monahan, A. R. Burke,
D. G. McLean, J. E. Slagle, A. Fratini and T. M. Cooper, J. Phys.
Chem. A, 2011, 115, 265.
22 G.-J. Zhou and W.-Y. Wong, Chem. Soc. Rev., 2011, 40, 2541.
23 Y. Qu, J. Hua and H. Tian, Org. Lett., 2010, 12, 3320.
24 M. G. Frisch, G. W. Truchs and H. B. Schlegel, et al., Gaussian 09W
Revision A.1, Gaussian Inc, Wallingford, CT, 2009.
25 I. E. Pomestchenko, C. R. Luman, M. Hissler, R. Ziessel and
F. N. Castellano, Inorg. Chem., 2003, 42, 1394.
47 T. N. Singh-Rachford, R. R. Islangulov and F. N. Castellano, J.
Phys. Chem. A, 2008, 112, 3906.
48 K. Tanaka, K. Inafuku and Y. Chujo, Chem. Commun., 2010, 46,
4378.
49 S. Ji, W. Wu, W. Wu, H. Guo and J. Zhao, Angew. Chem., Int. Ed.,
2011, 50, 1626.
50 H. M. Kim and B. R. Cho, Acc. Chem. Res., 2009, 42, 863.
51 S. Baluschev, V. Yakutkin, T. Miteva, Y. Avlasevich, S. Chernov,
€
S. Aleshchenkov, G. Nelles, A. Cheprakov, A. Yasuda, K. Mullen
26 N. Armaroli, ChemPhysChem, 2008, 9, 371.
27 D. V. Kozlov, D. S. Tyson, C. Goze, R. Ziessel and F. N. Castellano,
Inorg. Chem., 2004, 43, 6083.
and G. Wegner, Angew. Chem., Int. Ed., 2007, 46, 7693.
52 T. N. Singh-Rachford, A. Haefele, R. Ziessel and F. N. Castellano, J.
Am. Chem. Soc., 2008, 130, 16164.
28 G. J. Wilson, A. Launikonis, W. H. F. Sasse and A. W.-H. Mau, J.
Phys. Chem. A, 1997, 101, 4860.
29 X. Han, L.-Z. Wu, G. Si, J. Pan, Q.-Z. Yang, L.-P. Zhang and
C.-H. Tung, Chem.–Eur. J., 2007, 13, 1231.
30 S. Ji, W. Wu, W. Wu, P. Song, K. Han, Z. Wang, S. Liu, H. Guo and
J. Zhao, J. Mater. Chem., 2010, 20, 1953.
53 Y. Y. Cheng, B. Fckel, T. Khoury, R. G. C. R. Clady,
M. J. Y. Tayebjee, N. J. Ekins-Daukes, M. J. Crossley and
T. W. Schmidt, J. Phys. Chem. Lett., 2010, 1, 1795.
54 S. Ji, H. Guo, W. Wu, W. Wu and J. Zhao, Angew. Chem., Int. Ed.,
2011, 50, 8283.
55 P. Du and R. Eisenberg, Chem. Sci., 2010, 1, 502.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 5319–5329 | 5329