Tuning the emissive triplet excited states of platinum(ii) Schiff base complexes with pyrene, and application for luminescent oxygen sensing and triplet-triplet-annihilation based upconversions

Article abstract of DOI:10.1039/c1dt11001b

Pt(ii) Schiff base complexes containing pyrene subunits were prepared using the chemistry-on-complex approach. This is the first time that supramolecular photochemical approach has been used to tune the photophysical properties of Schiff base Pt(ii) complexes, such as emission wavelength and lifetimes. The complexes show intense absorption in the visible region (ε = 13100 M ^-1 cm^-1 at 534 nm) and red phosphorescence at room temperature. Notably, much longer triplet excited state lifetimes (τ = 21.0 μs) were observed, compared to the model complexes (τ = 4.4 μs). The extension of triplet excited state lifetimes is attributed to the establishment of equilibrium between the metal-to-ligand charge-transfer (³MLCT) state (coordination centre localized) and the intraligand (³IL) state (pyrene localized), or population of the long-lived ³IL triplet excited state. These assignments were fully rationalized by nanosecond time-resolved difference absorption spectra, 77 K emission spectra and density functional theory calculations. The complexes were used as triplet sensitizers for triplet-triplet-energy-tranfer (TTET) processes, i.e. luminescent O ₂ sensing and triplet-triplet annihilation (TTA) based upconversion. The O₂ sensitivity (Stern-Volmer quenching constant) of the complexes was quantitatively evaluated in polymer films. The results show that the O ₂ sensing sensitivity of the pyrene containing complex (K _SV = 0.04623 Torr^-1) is 15-fold of the model complex (K_SV = 0.00313 Torr^-1). Furthermore, significant TTA upconversion (upconversion quantum yield Φ_UC = 17.7% and the anti-Stokes shift is 0.77 eV) was observed with pyrene containing complexes being used as triplet sensitizers. Our approach to tune the triplet excited states of Pt(ii) Schiff base complexes will be useful for the design of phosphorescent transition metal complexes and their applications in light-harvesting, photovoltaics, luminescent O₂ sensing and upconversion, etc.

Full text of DOI:10.1039/c1dt11001b

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PAPER
Tuning the emissive triplet excited states of platinum(II) Schiff base complexes
with pyrene, and application for luminescent oxygen sensing and
triplet–triplet-annihilation based upconversions†
Wanhua Wu, Jifu Sun, Shaomin Ji, Wenting Wu, Jianzhang Zhao* and Huimin Guo*
Received 29th May 2011, Accepted 4th August 2011
DOI: 10.1039/c1dt11001b
Pt(II) Schiff base complexes containing pyrene subunits were prepared using the chemistry-on-complex
approach. This is the first time that supramolecular photochemical approach has been used to tune the
photophysical properties of Schiff base Pt(II) complexes, such as emission wavelength and lifetimes. The
complexes show intense absorption in the visible region (e = 13100 M-1 cm-1 at 534 nm) and red
phosphorescence at room temperature. Notably, much longer triplet excited state lifetimes (t = 21.0 ms)
were observed, compared to the model complexes (t = 4.4 ms). The extension of triplet excited state
lifetimes is attributed to the establishment of equilibrium between the metal-to-ligand charge-transfer
(3MLCT) state (coordination centre localized) and the intraligand (3IL) state (pyrene localized), or
population of the long-lived 3IL triplet excited state. These assignments were fully rationalized by
nanosecond time-resolved difference absorption spectra, 77 K emission spectra and density functional
theory calculations. The complexes were used as triplet sensitizers for triplet–triplet-energy-tranfer
(TTET) processes, i.e. luminescent O2 sensing and triplet–triplet annihilation (TTA) based
upconversion. The O2 sensitivity (Stern–Volmer quenching constant) of the complexes was
quantitatively evaluated in polymer films. The results show that the O2 sensing sensitivity of the pyrene
containing complex (KSV = 0.04623 Torr-1) is 15-fold of the model complex (KSV = 0.00313 Torr-1).
Furthermore, significant TTA upconversion (upconversion quantum yield UUC = 17.7% and the
anti-Stokes shift is 0.77 eV) was observed with pyrene containing complexes being used as triplet
sensitizers. Our approach to tune the triplet excited states of Pt(II) Schiff base complexes will be useful
for the design of phosphorescent transition metal complexes and their applications in light-harvesting,
photovoltaics, luminescent O2 sensing and upconversion, etc.
applications of these complexes in luminescent oxygen sensing and
Introduction
triplet–triplet annihilation based upconversions.5,6,28,29
Pt(II)/Ir(III) complexes have attracted much attention due to their
However, the development of transition metal complexes is
applications in electroluminescence, photovoltaics, optical limit-
ing, photocatlysis and molecular probes, etc.1–14 These complexes
facing several challanges, for example, the absorption of the
complexes in visible region is weak (the molar extinction coefficient
show 3MLCT/3IL mixed triplet excited state, thus the emission is
assigned as phosphorescence, featuring large Stoke shifts and long
excited state lifetimes.1,15–26 We have been interested in luminescent
transition metal complexes for a while and found that the
emission properties of the Pt(II) complexes, such as the emission
wavelength and the luminescent lifetimes, can be significantly
tuned by introducing organic chromophores.4–6,27 We explored the
is less than 5000 M-1 cm-1), and the triplet excited state lifetimes are
short (usually < 5 ms).1,2 For the applications of phosphorescent
transition metal complexes, intense absorption in visible region
and long-lived triplet excited state are highly desired.30,31 One
straightforward approach to acquire intense visible light absorp-
tion and long-lived triplet excited states is to attach an organic
chromophore to the metal centre. However, it has been shown
that the photophysical properties of these dyads are elusive, for
example, very often the photophysics of the components collapsed
in the dyad, i.e. quenching of the phosphorescence, shortening of
the triplet excited state lifetime, etc, were observed.31b,31c
State Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, E 208 Western Campus, 2 Ling-Gong
Road, Dalian, 116012, P.R. China. E-mail: zhaojzh@dlut.edu.cn, guohm@
dlut.edu.cn; Web: http://finechem.dlut.edu.cn/photochem
† Electronic supplementary information (ESI) available: the synthesis
and structure characterization data for the Pt(II) complexes; the de-
tailed description of preparation of the sensing films; the details of
upconversion, oxygen sensing and calculation information. See DOI:
10.1039/c1dt11001b
Recently, Pt(II) Schiff base complexes were synthesized and their
photophysical properties were studied.32 The red-shifted UV-vis
absorption of these complexes (labs = 520 nm) compared to the
normal cyclometalated Pt(II) complexes with CŸN ligand (below
11550 | Dalton Trans., 2011, 40, 11550–11561
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450 nm) is particularly interesting.1 Red-shifted absorption will
be beneficial for applications, such as photovoltaics, molecular
probes and photocatlysis.33 Furthermore, these complexes are
easy to synthesise and have high phosphorescent quantum yields.
However, no applications of these complexes in photophysical
processes have been reported. Furthermore, we noticed that the
luminescent lifetimes of the typical Pt(II) Schiff base complexes are
less than 5 ms.1,34,35 Longer luminescent lifetime is desired for ap-
plications, such as photo-induced charge separation, luminescent
oxygen sensing, etc.4,5,36,37 On the other hand, while the tuning of
the triplet excited lifetime of the Ru(II) polyimine complexes has
been reported,36,38–40 to the best of our knowledge, however, no
study has been carrried out to prolong the triplet excited lifetime
of the Pt(II) Schiff base complexes. Thus, much room is left for the
study of this kind of novel phosphorescent materials.
TDS 3012B oscilloscope. The lifetime values (by monitoring the
decay trace of the transients) were obtained with the LP920
software. Luminescence quantum yields of the complexes were
measured with Ru(phen)(bpy)2[PF6]2 as the standard (U = 0.06
in MeCN). The emission spectra at 77 K was measured with
a Oxford Optistat DNTM cryostat (with liquid nitrogen filling)
and FS920 fluorospectrometer (Edinburgh Instruments Ltd.,
U.K.).
Synthesis of Pt-5
Complex Pt-5 were prepared using chemistry-on-complex ap-
proach. 1-Pyrenylboronic acid (50.0 mg, 0.20 mmol), complex
2 (119.3 mg, 0.20 mmol), Pd(PPh3)4 (11.6 mg, 0.01 mmol) and
K2CO3 (110.4 mg, 0.81 mmol) was charged in a two-necked flask
under nitrogen. Degassed DMF (10 mL) was added to the mixture,
then the mixture was stirred at 75 ◦C for 48 h. The majority of
solvent was removed and the residue was poured into ice water, the
precipitation was collected and washed with water (3 ¥ 30 mL). The
crude product was further purified by column chromatography
(dichloromethane). Evaporation of solvent from the solution gave
Previously we studied Ru(II) polyimine complexes and found
that the luminescent lifetime can be extended by 130-fold with the
introduction of pyrene subunits to the imine ligand.36,41 Herein
we extend this supramolecular photophysical strategy to tune the
excited state lifetimes of Pt(II) Schiff base complexes. We attached
pyrene subunits to the Schiff base complexes by C–C bond and
C
C triple bonds, via Suzuki cross coupling or Sonogashira
1
a red solid (65.0 mg, 45.8%). H NMR (400 MHz, DMSO-d6):
coupling reactions. We found that the emissive triplet excited state
of the Pt(II) Schiff base complexes can be significantly tuned by
pyrene subunits. For example, the triplet excited state lifetime is
9.67 (s, 2H), 8.77 (s, 1H), 8.68 (d, 1H, J = 8.7 Hz), 8.47 (d, 1H, J =
8.1 Hz), 8.39–8.29 (m, 4H), 8.26–8.12 (m, 4H), 7.95 (d, 1H, J =
8.1 Hz), 7.84 (d, 1H, J = 6.7 Hz), 7.75 (d, 1H, J = 8.5 Hz), 7.64–7.56
(m, 2H), 7.16 (t, 2H, J = 9.5 Hz), 6.84 (t, 1H, J = 7.5 Hz), 6.76 (t,
1H, J = 7.6 Hz). 13C NMR (100 MHz, d6-DMSO): 165.1, 152.3,
151.9, 145.4, 140.8, 136.2, 136.0, 131.5, 131.1, 130.3, 128.6, 128.4,
127.9, 127.1, 126.1, 125.7, 125.3, 125.1, 124.6, 124.4, 122.6, 121.7,
118.9, 117.1, 116.8. MALDI-MS: calcd ([C36H22N2O2Pt]+), m/z =
709.1329, found, m/z = 709.1324. Elemental analysis calculated
(%) for C36H22N2O2Pt: C 60.93, H 3.12, N 3.95; found: C 61.02, H
3.52, N 3.92.
3
extended to 21.0 ms. We proposed IL triplet excited state was
populated in Pt-6 (Scheme 1). To the best of our knowledge, this is
the first time that the excited states of Schiff base Pt(II) complexes
are tuned by introducing organic chromophores.
Furthermore, it is for the first time that the long-lived triplet
excited states of the Schiff base Pt(II) complexes were used
for triplet–triplet-energy-transfer, i.e. luminescent oxygen sensing
and triplet–triplet annihilation based upconversion. Enhanced
luminescence oxygen sensing property (e.g. KSV = 0.04623 Torr-1)
and enhanced upconversion (e.g. UUC = 17.7%) were observed for
Pt-5 and Pt-6. Our results will be helpful for further exploration
of the supramolecular photophysics of the Pt(II) Schiff base
complexes and their applications.
Synthesis of Pt-6
Complex Pt-6 were prepared using chemistry-on-complex ap-
proach. Under nitrogen atmosphere, complex 2 (70.0 mg,
0.12 mmol), 1-ethynylpyrene (27.0, 0.12 mmol), Pd(PPh3)2Cl2
(8.5 mg, 0.01 mmol), PPh3 (21.2 mg, 0.08 mmol), and CuI (8.5 mg,
0.05 mmol) were dissolved in DMF (10 mL). The flask was
vacuumed and back-filled with argon several times. The mixture
was heated to 90 ◦C for 8 h. The majority of solvent was removed
and the residue was poured into ice water, the precipitation was
collected and washed with water (3 ¥ 30 mL). The crude product
was further purified by washing with CH2Cl2 several times, dark
red solid was obtained (40 mg, 45.5%). 1H NMR (400 MHz,
DMSO-d6): 9.72 (s, 1H), 9.61 (s, 1H), 8.90 (s, 1H), 8.76 (d, 1H, J =
8.7 Hz), 8.60 (d, 1H, J = 8.1 Hz), 8.46–8.25 (m, 7H), 8.17 (t, 1H,
J = 8.1 Hz), 7.97–7.88 (m, 3H), 7.62 (t, 2H, J = 6.7 Hz), 7.17 (d, 2H,
J = 8.5 Hz), 6.86–6.81 (m, 2H). 13C NMR (100 MHz, d6-DMSO):
164.7, 152.0, 151.6, 144.8, 135.8, 135.7, 131.3, 130.8, 129.7, 129.1,
128.7, 127.2, 126.9, 126.1, 125.0, 121.3, 119.6, 117.1, 116.5, 116.5,
94.3, 90.1. MALDI-MS: calcd ([C38H22N2O2Pt]+), m/z = 733.1329,
found, m/z = 733.1336. Elemental analysis calculated (%) for
C38H22N2O2Pt · 2.5 (DMSO): C 55.59, H 4.01, N 3.02; found:
C 55.41, H 3.41, N 3.05 (the sample was recovered from the NMR
measurement).
Experimental
Materials and reagents
K2PtCl4 was purchased from Shandong Boyuan Chemical Co.
Ltd. (China), and 1-pyrenylboronic acid is a product of Sigma–
Aldrich Co. All the chemicals are analytical purity and were
used as received. Solvents were dried and distilled before used
for synthesis.
Analytical Measurements
NMR spectra were recorded on a Bruker 400 MHz spectrometer
(CDCl3 or DMSO-d6 as solvents, TMS as standard, d = 0.00
ppm). High resolution mass spectra (HRMS) were determined on
a MALDI micro MX. Fluorescence spectra were measured on a
RF5301 fluorospectrometer (Shimadzu) or a CRT 970 spectrofluo-
rometer. The nanosecond time-resolved transient absorption spec-
tra were detected by laser flash photolysis spectrometer (LP920,
Edinburgh Instruments, U.K.) and recorded on a Tektronix
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Scheme 1 Synthesis of the complexes Pt-5 and Pt-6. The known complexes Pt-1, Pt-2, Pt-3 and Pt-4 were used in photophysical studies as model
complexes. i) Ethanol, r.t.; ii) K2CO3, K2PtCl4, DMSO; iii) 1-Pyrenylboronic acid, Pd(PPh3)4, K2CO3, iv) Trimethylsilylacetylene, Pd(PPh3)4, CuI, TEA,
reflux, 8 h; v) K2CO3, MeOH; vi) 2, Pd(PPh3)4, CuI, TEA, DMF.
Computational Methods
fractive index of the samples, respectively (unk means unknown).43
The photography of the upconversion were taken with Samsung
NV 5 digital camera. The exposure times are the default values of
the camera.
The geometry optimizations were calculated using B3LYP func-
tional with the 6-31G(d)/LanL2DZ basis set with density
functional theory (DFT). The excitation energy was calculated
using the time-dependent DFT (TDDFT) method based on the
optimized singlet ground state geometry. The spin-density of the
triplet state was calculated with the energy minimized triplet state
geometries. All calculations were performed using Gaussian 09 W
(Gaussian, Inc.).42
Results and Discussions
Design and Synthesis of the Complexes
The reported complexes Pt-1–Pt-4 were prepared for comparison
in the photophysical studies (Scheme 1). Complexes Pt-2 and Pt-4
have significant intramolecular charge transfer (ICT) effects due
to the diethylamino group on the salicaldehyde moiety. In order to
introduce the pyrene units, we used 4-bromo-1,2-benzenediamine
as the precursor. Then chemistry-on-complex approach was used
to attach pyrene or pyreneethynylene subunit on the complexes
by Sonogashira or Suzuki coupling reaction (Scheme 1).36,41
To the best of our knowledge, chemistry-on-complexes synthetic
methodology was rarely used for preparation of the Schiff base
Pt(II) complexes. All the complexes were obtained with good yields.
Upconversions
Diode pumped solid state laser was used as the excitation source
for the upconversions. The samples were purged with N2 or Ar for
15 min before measurement. The upconversion quantum yields
were determined with the phosphorescence of Ru(dmb)3[PF6]2
as the quantum yield standard (U = 0.074 in MeCN) and the
quantum yields were calculated using eqn (1),
2
⎛
⎜
⎜
⎞⎛
⎞⎛
⎜
⎟
⎜
h
⎠⎝
⎞
⎟
⎟
⎟
⎟
⎠
Astd Iunk ⎜hunk
⎟
⎟
⎜
⎟
⎟
⎜
(1)
Φ
= 2Φ
std
⎜
⎟
⎟
unk
⎜
⎜
⎝
⎜
⎟
⎜
A
I
unk ⎠⎝
std
std
UV-vis absorption spectra of the complexes
where Uunk, Aunk, Iunk and hunk represent the quantum yield,
absorbance, integrated photoluminescence intensity and the re-
The UV-vis absorptions of the complexes were studied (Fig. 1). Pt-
3 shows more red-shifted absorption (521 nm) than Pt-1 (419 nm).
11552 | Dalton Trans., 2011, 40, 11550–11561
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emissive state of Pt-6 is different from Pt-3, most probably the 3IL
triplet excited state (localized on the ethynylate pyrene subunit) is
populated. Previously we observed similar 3IL emission for Ru(II)
complex containing ethynylated pyrene ligand.36
Emission spectra at 77 K
In order to study the emissive triplet excited states of the
complexes, the emission spectra at 77 K were studied (Fig. 3). It is
known that thermally induced Stokes shift (DES) is an indication
of the 3IL or 3MLCT feature of the emissive triplet excited state.44,45
For Pt-3, the emission at 77 K is more structured than that at RT.
The thermally induced Stokes shift is 584 cm-1. Thus we propose
that 3MLCT is the major component of the emissive triplet excited
state of Pt-3. For Pt-5 and Pt-6, however, much smaller thermally
induced Stokes shifts were observed, DES = 211 cm-1 and 90 cm-1
Fig. 1 UV-vis absorption spectra of Pt-3, Pt-5 and Pt-6 in MeCN (1.0
¥ 10-5 M), (b) UV-vis absorption spectra of Pt-1, Pt-2, Pt-3 and Pt-4 in
MeCN (1.0 ¥ 10-5 M). 20 ◦C.
Interestingly, the UV-vis absorption of Pt-2 and Pt-4 show blue-
shifting with the introduction of electron-donating diethylamino
group, compared to Pt-1 and Pt-3. This is probably due to the
attachment of electron-donating moiety on the moiety where
LUMO is localized (see ESI†).1,32 The intense absorption band
at the red end of the spectra is due to the 1MLCT/1IL transitions,
usually, the absorption of 1MLCT of the Pt(II) complexes is very
weak and at much shorter wavelength. Previously, we increased
the absorption wavelength by introducing organic chromophores
with intense absorption in visible range.5,6 The absorption in the
blue range is due to the p–p* transition of the ligands. With
introduction of pyrene, the absorptions of Pt-5 and Pt-6 show
red-shifted absorption bands.
3
for Pt-5 and Pt-6, respectively. Thus we propose that IL is the
major component of the emissive triplet excited states of Pt-5 and
Pt-6. This conclusion is supported by the time-resolved transient
difference absorption spectra and the DFT calculations.
The UV-vis absorption of Pt-6 in different solvents was studied
(see ESI†) and the results show that blue-shifted absorption was
observed in polar solvents relative to that in non-polar solvents.
This result indicates that the dipole moment of the Frank Condon
excited state is smaller than that of ground state.32 Same results
were found for other complexes (see ESI†).
Fig. 3 Emission spectra of Pt-3, and Pt-6 in ethanol-methanol (4 : 1, v/v)
glass at 77 K and solution at RT (298 K). (a) Pt-3, lex = 520 nm; (b) Pt-6,
lex = 530 nm.
Emission spectra of the complexes at room temperature
Nanosecond time-resolved transient difference absorption
The emission of the complexes was studied and room temperature
(RT) phosphorescence was observed (Fig. 2). With extension of
the p-conjugation (Pt-1 vs. Pt-3), the emission is red-shifted by
60 nm. Interestingly, with introduction of the electron-donating
diethylamino group, a hypsochromic shift was observed for the
emission of the complexes. For example, emission at 600 nm was
observed for Pt-4, vs. the emission at 614 nm for Pt-3. Pt-5 shows
an emission band which is similar to Pt-3. For Pt-6, however,
red-shifted emission at 667 nm was observed. Furthermore, the
vibrational structure of the emission band of Pt-6 is different
from those of the other complexes. Thus we propose that the
The nanosecond time-resolved difference absorption spectra were
studied (Fig. 4). For the parent complex Pt-3, a significant
bleaching band at 450 nm was observed upon pulsed laser
excitation, which is due to the depletion of the ground state of Pt-3
upon photoexcitation. Transient absorption bands in the 500 nm–
600 nm range were observed. The triplet excited state lifetime is
4.4 ms, which is in good agreement with the result obtained by the
luminescence method. Thus we conclude that the triplet excited
state we observed is the emissive excited state, which leads to
the RT phosphorescence of Pt-3. For Pt-5, bleaching at 450 nm
Fig. 2 Normalized emission of Pt-3, Pt-5 and Pt-6 in MeCN solution
(1.0 ¥ 10-5 M). (b) Normalized emission of Pt-1, Pt-2, Pt-3 and Pt-4 in
MeCN solution (1.0 ¥ 10-5 M). 20 ◦C.
Fig. 4 Nanosecond time-resolved transient difference absorption spectra
of (a) Pt-3 and (b) Pt-6 after pulsed excitation (lex = 532 nm). In deaerated
MeCN. 20 ◦C.
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Table 1 Photophysical parameters of the Pt(II) Schiff base complexes
Emission Properties
labs (nm)a
eb
lem (RT)a
lem (77 K)c
Ud
I0/I100
t (10-5mol L-1) (ms)f
e
Compounds
Pt-1
Pt-2
Pt-3
Pt-4
Pt-5
Pt-6
336, 419
363, 402
358, 376, 450, 521
454, 482
363, 378, 459, 527
384, 462, 534
1.31, 0.487
4.61, 2.2
3.90, 4.01,0.85, 0.89
5.41, 6.5
5.43, 5.37, 1.03, 1.2
6.01, 1.35, 1.31
554
535
614
600
621
632, 667
520, 564, 612
504, 547, 598
589, 646, 716
589, 648, 716
612, 675, 753
593, 663, 742
0.19
0.13
0.23
0.22
0.14
0.034
212.5
301.5
194
649
1256
423
3.5 (lit. 3.4)
3.1
4.4
6.1
13.4g
21.0g
a Result of compounds in MeCN solution (1.0 ¥ 10-5 mol dm-3). b Molar extinction coefficient at the absorption maxima. e:104 M-1 cm-1. c Result in
ethanol-methanol (4 : 1, v/v) glass at 77 K. d Result of deaerated solution, with complex Ru(phen)(bpy)2 (U = 0.06 in MeCN) as the standard. e The
emission intensity ratio of the complexes under nitrogen and oxygen atmosphere, I0 stands for the emission intensity in N2 and I100 stands for the emission
in O2 atmosphere. f Luminescent lifetimes. g Lifetimes measured by nanosecond time-resolved transient absorptions, 1.0 ¥ 10-5 M in MeCN. In deaerated
solution.
was observed (see ESI†), which is due to the depletion of the
ground state of the complex upon photoexcitation. Furthermore,
significant transient absorption in the 600 nm–800 nm region was
found. Similar transients were observed for Pt-6 (Fig. 4(b)). The
significant absorption profile in the 600 nm–800 nm region is
similar to that for pyrene-containing Pt(II) complexes, reported
3
by us previously.27–37 Thus we conclude that IL component is
significant for the emissive excited state of Pt-5 and Pt-6.
The photophysical parameters of the complexes are compiled in
Table 1. The highly phosphorescent quantum yield is still present
in Pt-5 (U = 14%). For Pt-6, however, the quantum yield is much
lower. Furthermore, we observed that the luminescent lifetimes of
Pt-5 and Pt-6 (t = 21.0 ms) are much longer than that of the model
complex Pt-3 (t = 4.4 ms), and much longer than the dbbpy Pt(II)
Fig.
HOMO and
B3LYP/6-31G((d)/LanL2DZ level using Gaussian 09W.42
5
Frontier molecular orbitals of Pt-3.
H
stands for
complexes, e.g. 8-fold longer than a dbbpy Pt(II) complexes we
recently reported.5 We propose that the extended lifetime of Pt-5
is due to the equilibrium between the 3MLCT state (coordination
centre localized) and the 3IL state (pyrene localized).36,38,39,40
L
stands for LUMO. Calculated by DFT at the
good agreement with the experimentally observed absorption at
521 nm (Fig. 1 and Table 1). HOMO→LUMO is involved in the
S0→S1 transition. Thus the absorption band of Pt-3 at 521 nm
(Fig. 1) can be assigned as 1MLCT/1IL transition.
3
For Pt-6, the long lifetime is due to the IL state.36 The high
phosphorescence quantum yield and the long triplet excited state
lifetime are ideal for applications such as photo-induced charge
separation, triplet–triplet-energy-transfer (luminescent oxygen
sensing and TTA upconversion), etc.
In order to study the emission of Pt-3, the energy gap between
the S0 state and the T1 excited states were calculated by TDDFT
(Table 2). The calculated emission band at 660 nm is in good
agreement with the experimental results (Fig. 2). Based on the
distributions of the frontier MOs of the transition, the emissive
excited state of Pt-3 can be assigned as 3MLCT/3IL mixed feature.
For the dyad Pt-5, the pyrene moiety is not in full p-conjugation
with the Schiff base-Pt(II) coordination center. The optimized
geometry shows that the pyrene moiety tilted by 53.9◦ to the
Schiff base-Pt(II) coordination plane (Fig. 6). The calculated
UV-vis absorption at 533 nm is in good agreement with the
experimentally observed absorption band at 527 nm (Fig. 1). Based
on the TDDFT calculations, HOMO→LUMO is involved in the
S0→S1 transition (See ESI†). The distributions of HOMO and
DFT calculations
DFT calculations are important for study of fluorescent molecular
probes, with which luminophores can be prepared with predeter-
mined photophysical properties.4,17–19,21,46,47 Recently Zn2+ probes
were studied by DFT calculations.48 Previously we studied the
fluorescence response of thiol probes, boronic acid molecular
probes, and predicted the photophysics of phosphorescent transi-
tion metal complexes with DFT calculations.3–6,36,46,47,49 Herein we
carried out DFT and time-dependent DFT (TDDFT) calculations
to rationalize the photophysical properties of the Pt(II) Schiff base
complexes.
In order to study the UV-vis absorption and the phospho-
rescence of the complexes, density functional theory (DFT)
calculations were carried out. For Pt-3, the optimized ground
state geometry is coplanar. The frontier molecular orbitals are
presented in Fig. 5. HOMO is distributed on the Pt(II) metal and
the p-conjugation framework of the complex. Pt(II) is not involved
in the LUMO. The predicted UV-vis absorption at 526 nm is in
LUMO demonstrated that the S0→S1 transition is MLCT/1IL
1
(Fig. 6). Pyrene moiety does not contribute significnatly to the
S0→S1 transition. The phosphorescence of Pt-5 was studied by
TDDFT calculations (See ESI† for the details of the transitions).
The calculated S0→T1 energy gap (i.e. the emission wavelength) is
in good agreement with the experimental results.
The geometry of Pt-6 was also optimized by DFT calculations.
The pyrene moiety takes a coplanar geometry toward the Pt(II)
11554 | Dalton Trans., 2011, 40, 11550–11561
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Table 2 Electronic excitation energies (eV) and corresponding oscillator Strengths (f ), main configurations and CI coefficients of the low-lying
electronically excited states of Pt-3, calculated by TDDFT//B3LYP/6-31G(d)/lanl2dz, based on the optimized ground state geometries
TDDFT//B3LYP/6-31G(d)
Electronic transition
Energya
f b
Compositionc
CId
character
Singlet
Triplet
S0→S1
S0→S2
S0→S5
S0→S6
2.36 eV 526 nm
2.78 eV 446 nm
3.41 eV 364 nm
3.43 eV 362 nm
0.0554
0.0320
0.4982
0.1728
HOMO → LUMO
HOMO-1→LUMO
HOMO-2→LUMO
HOMO-4→LUMO
HOMO-1→LUMO+1
HOMO-1→ LUMO+1
HOMO → LUMO
HOMO-1→ LUMO
HOMO→ LUMO+1
0.6708
0.6710
0.6519
0.1604
0.6467
0.2348
0.7130
0.5907
0.4735
MLCT
ILCT
MLCT
MLCT
ILCT
ILCT
MLCT
ILCT
S0→T1
S0→T2
1.88 eV 660 nm
2.06 eV 601 nm
0.0000e
0.0000e
MLCT
a Only the selected low-lying excited states are presented. b oscillator strength. c Only the main configurations are presented. d The CI (configuration
interaction) coefficients are in absolute values. e No spin-orbital coupling effect was considered, thus the f values are zero
Fig.
HOMO and
B3LYP/6-31G((d)/LanL2DZ level using Gaussian 09W.42
6
Frontier molecular orbitals of Pt-5.
H
stands for
L
stands for LUMO. Calculated by DFT at the
coordination plane of the complex. Thus the pyrene moiety is
in full p-conjugation with the coordination center. We found the
frontier MOs are distributed on the coordination and the pyrene
moiety, which is different from that of Pt-5 (Fig. 7). The calculated
UV-vis absorption maxima of Pt-6 are located at 548 nm, 483 nm
and 423 nm, which are in good agreement with the experimental
results of 534 nm, 462 nm and 384 nm (Fig. 1).
Fig. 7 Frontier molecular orbitals of Pt-6. H stands for HOMO
and
L stands for LUMO. Calculated by DFT at the B3LYP/
6-31G((d)/LanL2DZ level using Gaussian 09W.42
For the T1 excited state (Table 3), HOMO →LUMO and
HOMO-1→LUMO+1 are the main components of the S0→T1
3
transition, thus the major feature of the T1 state is IL, which is
localized on the pyrene-acetylide conjugated ligand.
The calculated S0→T1 energy gap (714 nm) is in good agreement
with the experimentally observed 667 nm (note there is an emission
shoulder at 720 nm, Fig. 2).
Fig. 8 Spin density surface of Pt-3, Pt-5 and Pt-6 at the optimized
triplet state geometry. Calculated at B3LYP/6-31G/LANL2DZ level with
Gaussian 09W.42
The involvement of the pyrene moiety in the triplet excited state
manifold of Pt-5 and Pt-6 were also indicated by the spin density
analysis of the triplet state of the complexes (Fig. 8). For the model
complex Pt-3, the spin density is distributed on the Schiff base
ligand, as well as on the Pt(II) center. For Pt-5 and Pt-6, however,
the contributions of pyrene moiety to spin density are significant,
which rationalize the long-lived T1 excited state of the complexes.
Based on the steady state spectroscopy, we propose that for Pt-
Application of the Efficient Triplet–triplet energy tranfer:
Enhanced Luminescent O2 sensing
Previously we proposed that the long-lived 3IL excited state can be
used for efficient triplet–triplet energy transfer, such as luminescent
oxygen sensing,36 because the phosphorescence of transition metal
complexes can be significantly quenched by O2.50 Since the current
Pt(II) Schiff base complexes show absorption in visible region and
3
5, the MLCT/3IL equilibrium and for Pt-6, the population of
the 3IL state, are responsible for the prolonged T1 state lifetimes,
respectively.
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Table 3 Electronic excitation energies (eV) and corresponding oscillator Strengths (f ), main configurations and CI coefficients of the low-lying
electronically excited states of complex Pt-6, calculated by TDDFT//B3LYP/6-31G(d)/lanl2dz, based on the optimized ground state geometries
TDDFT//B3LYP/6-31G(d)
Electronic transition
Energy (eV)a
f b
Compositionc
CId
character
Singlet
S0→S1
2.26 eV 548 nm
0.2694
HOMO-1 → LUMO
HOMO→ LUMO
0.2303
0.6171
0.1276
0.2021
0.6024
0.2039
0.6147
0.1307
0.2029
0.6163
0.2368
0.1066
0.1333
0.1734
0.2248
0.2711
0.1822
0.5874
0.2240
0.1157
0.1919
0.4777
0.1665
0.3414
0.3497
0.1415
MLCT, ILCT
MLCT
MLCT, ILCT
ILCT
MLCT, ILCT
MLCT
MLCT, ILCT
ILCT
MLCT, ILCT
MLCT
ILCT
ILCT
ILCT
HOMO→ LUMO+1
HOMO-2→LUMO
HOMO-1→LUMO
HOMO→LUMO
S0→S2
S0→S5
S0→S9
S0→T1
2.57 eV 483 nm
2.93 eV 423 nm
3.30 eV 376 nm
1.74 eV 714 nm
0.2923
0.7691
0.2919
0.0000e
HOMO-1→LUMO+1
HOMO-1→LUMO+2
HOMO→LUMO+1
HOMO-3→LUMO
HOMO-2→LUMO+1
HOMO-1→LUMO+2
HOMO-2 → LUMO+1
HOMO-2 → LUMO+2
HOMO-1 → LUMO
HOMO-1 → LUMO+1
HOMO-1 → LUMO+2
HOMO → LUMO
Triplet
ILCT
MLCT, ILCT
MLCT, ILCT
ILCT
MLCT
MLCT, ILCT
ILCT
ILCT
MLCT, ILCT
ILCT
MLCT
MLCT, ILCT
MLCT
HOMO → LUMO+1
HOMO-2→LUMO
HOMO-2→LUMO+1
HOMO-1→LUMO
HOMO-1→LUMO+2
HOMO→LUMO
S0→T2
1.87 eV 665 nm
0.0000e
HOMO→LUMO+1
HOMO→LUMO+2
a Only the selected low-lying excited states are presented. b Oscillator strength. c Only the main configurations are presented. d The CI coefficients are in
absolute values. e No spin-orbital coupling effect was considered, thus the f values are zero.
long lived triplet excited state, thus these complexes are used for
luminescent O2 sensing.
The emission of the complexes was significantly quenched in
air relative to that under inert atmosphere. Thus, we anticipate
efficient triplet–triplet energy transfer between the triplet excited
states of the complexes and the O2 molecule (which is at triplet
state).50,51 Furthermore, we found that the emission of the pyrene-
containing Pt-5 and Pt-6 was completely quenched in air, but
the emission of the model complex Pt-3 is detectable in air,
thus we anticipate longer luminescent lifetimes for Pt-5 and Pt-6
compared to Pt-3. In later sections we explore the application of
the complexes in triplet–triplet-energy-transfer processes, such as
the luminescent oxygen sensing and the triplet–triplet annihilation
based upconversion.
In order to quantitatively evaluate the O2 sensing properties,
sensing films, prepared by distribution of the complexes in polymer
film, were used together with flow cell which is coupled to
spectrofluorometer via optical fibers.4,36,37,41,43,52–55 Thus we can
continuously monitor the change of phosphorescence intensity
Fig. 9 Phosphorescence intensity response of the complexes to step
variations of O2 concentration levels. (a) and (c) emission change of Pt-3
and Pt-6 film vs. O2/N2 saturation switch, (b) and (d) dynamic response
of the Pt-3 and Pt-6 film vs. small steps of variation of O2 partial pressure.
Pt-3: lex = 526 nm, lem = 609 nm; Pt-6: lex = 530 nm, lem = 667 nm. 20 ◦C.
of the sensing films vs. the variation of O2 partial pressure in the
gas sample (Fig. 9).
Two types of sensing experiments were carried out, one is to
monitor the phosphorescence response of the sensing films by
switching between N2 and O2. Thus the response time (t95↓) and
the recovery time (t95↑) of the films can be studied. Moreover,
the phosphorescence response of the films toward small steps of
O2 partial pressure variation were also monitored, the data can be
used to derive the Stern–Volmer quenching constant of the sensing
film.
Fast response time and recovery were found for the sensing films.
For example, the response time of the Pt-6 is 6.4 s and the recovery
time is 14.6 s, similar kinetics were found for the other complexes.
Fast response and recovery were usually found for the O2 sensing
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materials with porous materials, such as molecular sieves. The
preparation of our O2 sensing films is dissolution-casting, which
is straightforward for practical applications.
Application of the complexes for efficient triplet–triplet-energy-
transfer: enhanced triplet–triplet annihilation based low-power
upconversion
The phosphorescence emission of Pt-3 is sensitive to O2. For
example, by increasing the O2 partial pressure from 0.0% to
3.5%, the phosphorescence emission was quenched by 18.3%.
The dynamic range is up to 100% O2 (mixed gas with N2, v/v).
For Pt-5, higher O2 sensitivity was observed. For example, the
phosphorescence is more significantly quenched with 3.5% O2.
Higher O2 sensitivity was observed for Pt-6 (Fig. 9 and ESI† for
other complexes). It should be pointed out that the photostability
of the sensing films is excellent, the emission intensity of the sensing
films are fully reversible after long time of continuous irradiation,
which is better than the PtOEP O2 sensing materials.52,56
For the heterogeneous O2 sensing, usually two-site model was
used.50 In this model, the dye molecules are considered as two
different portions, which are defined as f 1 and f 2, respectively
(f 1 + f 2 = 1). Each portion show different quenching constants
(KSV1 and KSV2, eqn (2)).
Transition metal complexes with intense absorption in the visible
region and long-lived triplet excited states can be used for
applications other than luminescent O2 sensing. Herein, we set
out to employ the long-lived triplet excited states of Pt-5 and Pt-6
in the newly developed triplet–triplet annihilation (TTA) based
upconversion.5,6,28,29,58–65
TTA upconversion is a promising upconversion scheme due to
it requiring low excitation power (sunlight is sufficient), and the
readily changeable excitation/emission wavelength. However, the
current development of TTA based upconversion is facing some
challenges. For example, the triplet sensitizers are limited to Ru(II)
and Pt(II) porphyrin complexs.28,43,58–60 It is difficult to tune the T1
energy levels of these complexes.
Similar to the luminescent O2 sensing studies, we propose that
complexes with longer triplet excited state lifetimes will be more
efficient to sensitize a TTA based upconversion.5,6,28,29 However,
no studies have been carried out to investigate the relationship
between the lifetime and the efficiency of the upconversion.
Previously we have demonstrated that Ru(II) polyimine com-
plexes with 3IL state are much more efficient to sensitize TTA based
upconversion.28 However, the Ru(II) complexes we used previously
still suffer from poor absorption in the visible range. The Pt(II)
Schiff base complexes (Scheme 1) show more intense absorption
in the visible range. For example, the molar extinction coefficient of
Pt-6 is 1.3 ¥ 104 M-1cm-1 at 532 nm, vs. 1.0 ¥ 103 for [Ru(dmb)3]2+ or
Ru(Phen-pyrene)(bpy)2.36 Furthermore, the lifetime of the typical
3MLCT state of the normal Ru(II) polyimine complexes is less than
1 ms (for [Ru(dmb)3]2+, t = 0.84 ms in CH3CN and 0.33 ms in H2O).
Therefore, we expected more efficient upconversion with Pt-5 and
Pt-6. The long lifetimes of the T1 states of Pt-5 and Pt-6 will also
improve the upconversion efficiency.
I0
I
1
=
f1
f2
(2)
+
1+ KSV 1pO2 1+ KSV 2pO2
The fitting result is presented in Fig. 10. Pt-5 and Pt-6 gave
faster responses than the other complexes. Numeric fitting give
the Stern–Volmer quenching constants (Table 4). We can see
that the oxygen sensitivity of Pt-6 is 9.1-fold of that of the
model complex Pt-3, and 4-fold of the complexes reported by
us recently.5 Furthermore, the O2 sensitivity of Pt-6 is 13.2-fold of
a recently reported cyclometallated Ir(III) complex (t = 4.3 ms in
inert atmosphere) for O2 sensing purpose.57
Since the complexes show intense absorption in the visible
range, which is better than the normal Ru(II) polyimine complexes
and the CŸN cyclometalated Pt(II) complexes,1,5 thus we used a
532 nm laser to excite the complexes (Fig. 11). The complexes
demonstrated different emission intensities, which are roughly in
line with the luminescent quantum yields (Table 1). Notably Pt-
6 emits weakly with 532 nm excitation. Pt-3 gives the strongest
emission (ca. 4 times that of Pt-6). Pt-5 gives emission similar
to Pt-3. With addition of 9,10-diphenylanthracene (DPA) into
the complex solution, the phosphorescence of the complexes was
Fig. 10 Fitting of the O2 sensing property of the IMPEK-C films of the
complexes based on the two site model (eqn (2)).
Table 4 Parameters of O2 sensing films of complexes with IMPEK-C cardo poly(aryl ether ketone)4 as supporting matrix (fitting result of the two site
model)
a
a
b
b
appd
f 1
f 2
KSV1
KSV2
r2c
KSV
pO2 e/Torr
Pt-1
Pt-2
Pt-3
Pt-4
Pt-5
Pt-6
0.3328
0.5926
0.7381
0.9337
0.6451
0.6336
0.6672
0.4074
0.2619
0.0663
0.3549
0.3664
0.0094
0.0188
0.0069
0.0202
0.0179
0.0729
0.0000
0.0000
0.0000
0.0001
0.0000
0.0001
0.9998
0.9993
0.9996
0.9999
0.9996
0.9993
0.00313
0.0111
0.00509
0.01887
0.01155
0.04623
106.4
53.2
144.9
49.5
55.9
13.7
a The ratio of the two portions of the dyes. b The quenching constants of the two portions. c The determination coefficients. d Weighted quenching constant,
= f 1¥KSV1+f 2¥KSV2.
e The oxygen partial pressure at which the initial emission intensity of film is quenched by 50%, and can be calculated as
app
KSV
1/KSV
.
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Table 5 Lifetimes (t), Stern–Volmer quenching constant (KSV) and
bimolecular quenching constants (kq) of the Pt(II) complexes with 3MLCT
excited states and 3IL excited states. In deaerated CH3CN solution. 25 ◦C
t/ms
UUC
Ksv/M-1(¥103) kq/M-1s-1 (¥109) r2
Pt-1
Pt-2
Pt-3
Pt-4
Pt-5
Pt-6
3.5
3.1
4.4
6.1
13.4
21.0
0.0%
0.0%
1.4%
0.0%
9.9%
17.7% 394.7
0.0% 3.08
30.6
83.1
31.5
136.4
145.7
8.74
26.80
7.88
22.36
10.87
18.80
3.54
0.998
0.989
0.991
0.995
0.990
0.998
0.975
Ru(dmb)3 0.87
Fig. 11 Upconversion with the complexes as sensitizers and DPA as
acceptor. (a) Upconverted DPA emission intensity and residual photolu-
minescence of the sensitizers following excitation at 532 nm. In deaerated
CH3CN. c [complexes] = 2.0 ¥ 10-6 M, c [DPA] = 1.1 ¥ 10-5 M. (b)
Phosphorescence intensity profiles of complexes without DPA under the
same conditions. 20 ◦C.
quenched to different extents. For example, the emission of Pt-3
was quenched slightly (by 32%). For Pt-5, however, the quenching
is more significant (by 61%). At the same time as the quenching of
phosphorescence by DPA, upconverted blue fluorescence of DPA
at 400 nm–500 nm was observed. Excitation of the complexes or
DPA alone by 532 nm laser did not produce the blue fluoresecence.
This control experiment verified the upconverted fluorescence of
DPA.
Interestingly, we observed more efficient upconversion for Pt-
5 and Pt-6 compared to Pt-3. The Ru(II) polyimine complex,
[Ru(dmb)3][PF6]2, was also studied and no upconversion was
observed under the same experimental conditions (such as the
concentration of DPA, please note that the concentration of DPA
here is much lower than the literature value and the values we have
reported),5,6,28,29,58–60 indicates that the upconversion threshold is
much higher for [Ru(dmb)3][PF6]2. We attribute the more efficient
upconversion of Pt-5 and Pt-6 to the long-lived triplet excited
states, especially 3IL state of Pt-6.
Fig. 12 Photographs of the emission of (a) sensitizers alone and (b) the
upconversion. (c) CIE diagram of the emission of sensitizers alone and (d)
in the presence of DPA (upconversion). lex = 532 nm (17 mW). In deareated
MeCN, c[sensitizer] = 2.0 ¥ 10-6 M, c[DPA] = 1.1 ¥ 10-5 M. 20 ◦C.
The upconversion is visible with unaided eye (Fig. 12). For
Ru(dmb)3[PF6]2 and Pt-3, orange and red emission were observed,
respectively. The emission hardly changed in the presence of
DPA, due to the lack of TTA upconversion (Fig. 12). For Pt-5
and Pt-6, however, the emission color changed drastically in the
presence of DPA, due to the significant TTA upconversion with
the two sensitizers (Fig. 12). The color changes of the solutions
were quantified using the CIE coordinates of the emission of the
solutions (Fig. 12c and 12d). For example, the emission of Pt-3 is
(0.63, 0.37). In the presence of DPA, the CIE coordinates changed
a little to (0.58, 0.34). For Pt-5 and Pt-6, however, significant
changes in the CIE coordinates were observed in the presence of
DPA. For Pt-5, the CIE changed from (0.63, 0.37) to (0.30, 0.14)
in the presence of DPA. Similar results were observed for the CIE
change of Pt-6, for which the CIE coordinates (0.64, 0.36) moved
into the deep blue region (0.17, 0.05) in the presence of DPA.
The quenched phosphorescence peak area is much smaller than
the observed upconversion peak area (Fig. 11). Thus we propose
that dark excited state will also sensitize the TTET. TTET does
not required an emissive triplet excited state to initiate the process.
In order to quantitatively study the efficiency of the triplet–
triplet energy transfer from the sensitizers to DPA, the quenching
effect of DPA on the phosphorescence emission of the complexes
was studied (Fig. 13). Based on the quenching constants (Table 5),
the TTET between Pt-6 and DPA (KSV = 3.95 ¥ 105 M-1) is 12.5
Fig. 13 (a) Stern–Volmer plots generated from the intensity quenching
of complexes photoluminescence measured as a function of DPA concen-
tration in CH3CN. The concentrations of the complexes are 2.0 ¥ 10-6 mol
dm-3. lex = 530 nm. (b) Phosphorescence emission spectra of complex
Pt-5 with increasing DPA concentration in deaerated CH3CN. 20 ◦C.
times that of Pt-3 (KSV = 3.15 ¥ 104 M-1). Noteably, the Stern–
Volmer quenching constant is 128 times that of the triplet sensitizer
[Ru(dmb)3][PF6]2 (KSV = 3.08 ¥ 103 M-1).59 We attribute this large
quenching constant to the long-lived triplet excited state of Pt-6.
Similar efficient quenching was observed for Pt-5 (KSV = 1.46 ¥
105 M-1).
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Scheme 2 Jablonski Diagram illustrating the sensitized TTA upconversion process between Schiff base Pt(II) arrays and DPA, exemplified by Pt-6.
The effect of the light-harvesting ability and the phosphorescence lifetimes of the Pt(II) sensitizers on the efficiency of TTA upconversion is also shown
(please note that the vibration energy levels of each electronic state are omitted for clarity). E is energy. GS is ground state (S0). 1IL* is intraligand singlet
excited state (pyrene localized). IC is inner conversion. 1MLCT* is the Pt(II) based metal-to-ligand charge transfer singlet excited state. ISC is intersystem
crossing. 3MLCT* is the Pt(II) based metal-to-ligand-charge-transfer triplet excited state. 3IL* is intraligand triplet excited state (pyrene localized). We
propose that the 3IL* and 3MLCT* of Pt-5 and Pt-6 is in equilibrium, but the 3IL feature is more significant for the T1 state of Pt-6. TTET is triplet–triplet
3
1
energy transfer. DPA* is the triplet excited state of DPA. TTA is triplet–triplet annihilation. DPA* is the singlet excited state of DPA. The emission
bands observed in the TTA experiment are the simultaneous 3MLCT* emission (phosphorescence) and 1DPA* emission (fluorescence). The typical power
density of the laser used in the upconversion is 240 mW cm-2, too low to observe simultaneous two-photon absorption.
Thus our results demonstrated the potential for tuning the
triplet excited states of Pt(II) Schiff base complexes and the
application of the new materials for triplet–triplet-energy tranfer,
e.g. for luminescent O2 sensing and triplet–triplet-annihilation
based upconversion.
For the complexes with ethynylated pyrene subunit, we propose
that the phosphorescence is due to the long-lived 3IL excited
state. For the complex with C–C linkage between the pyrene
3
and the Schiff base ligand, an equilibrium between the MLCT
and 3IL state is proposed. These assignments are supported
by steady state and nanosecond time-resolved spectra, emission
spectra at 77 K and DFT calculations. The visible-light harvesting
and the long-lived triplet excited states of the pyrene containing
complexes were employed for triplet–triplet-energy-tranfer, i.e.
luminescent O2 sensing and triplet–triplet annihilation (TTA)
based upconversion. The O2 sensitivity (Stern–Volmer quenching
constant) of the complexes was quantitatively evaluated with
polymer films. The results show that the O2 sensitivity of the pyrene
containing complex is 9.0 times that of the model complex Pt-3
and 13.0 times that of a recently reported cyclometallated Ir(III)
complex. Furthermore, significant upconversion (upconversion
quantum yield UUC = 17.7%) were observed with the pyrene
containing complexes as the triplet sensitizers for the TTA based
upconversion. Our approach to tune the triplet excited states of
the Pt(II) Schiff base complexes with organic chromophores that
show appropriate energy levels and the application of the long-
lived triplet excited states of the dyads will be useful for design of
phosphorescent transition metal complexes and their applications
in upconversion, luminescent O2 sensing, photovoltaics, photo-
induced charge seperation, etc.
The effect of the intense absorption and long-lived triplet excited
states of the Pt(II) Schiff base complexes, especially Pt-5 and Pt-6,
on the TTA upconversion, as well as the photophysics of TTA
upconversion, are summarized in Scheme 2. Intense absorption is
a prerequisite for efficient TTA upconversion, for which more T1
states of the sensitizers can be populated with photoexcitation.
The crucial step for TTA upconversion is the TTET between
the triplet sensitizer and the triplet acceptor (DPA), this energy
transfer process can be enhanced by the long lifetime of the triplet
excited state of the sensitizer. As we have demonstrated that the
lifetime of T1 states of Pt-5 and Pt-6 are much longer than those
of the model complexes, thus significant TTA upconversion can
be expected for Pt-5 and Pt-6 compared to other Pt(II) Schiff base
complexes described herein.
Conclusions
We have synthesized new Pt(II) Schiff base complexes containing
pyrene or ethynylated pyrene subunits by using the chemistry-
on-complex approach. The complexes show intense absorption in
the visible region (e = 1.31 ¥ 104 M-1 cm-1 at 530 nm) and room
temperature phosphorescence (550–670 nm). The complexes show
much longer phosphorescence lifetimes (t = 21.0 ms) than the
model complex (t = 4.4 ms). The extension of the triplet excited
Acknowledgements
We thank the NSFC (20972024 and 21073028), the Fundamental
Research Funds for the Central Universities (DUT10ZD212 and
DUT11LK19), Ministry of Education (SRFDP-200801410004
3
state lifetimes are attributed to the perturbation of the MLCT
3
(coordination core localized) by the IL state (pyrene localized).
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and NCET-08-0077), the Royal Society (UK) and NSFC (China-
UK Cost-Share Science Networks, 21011130154), State Key
Laboratory of Fine Chemicals (KF0802), State Key Laboratory
of Chemo/Biosensing and Chemometrics (2008009), Education
Department of Liaoning Province (2009T015) for financial sup-
port.
Retailleau and F. N. Castellano, Inorg. Chem., 2010, 49, 3730; (c) M.
Galletta, S. Campagna, M. Quesada, G. Ulrich and R. Ziessel, Chem.
Commun., 2005, 4222.
32 (a) C.-M. Che, C.-C. Kwok, S.-W. Lai, A. F. Rausch, W. J. Finkenzeller,
N. Zhu and H. Yersin, Chem.–Eur. J., 2010, 16, 233; (b) Z. Guo, W.-L.
Tong and M. C. W. Chan, Chem. Commun., 2009, 6189.
33 L. Xiong, Q. Zhao, H. Chen, Y. Wu, Z. Dong, Z. Zhou and F. Li, Inorg.
Chem., 2010, 49, 6402.
34 J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau
and M. E. Thompson, Inorg. Chem., 2002, 41, 3055.
35 G.-J. Zhou, W.-Y. Wong, B. Yao, Z. Xie and L. Wang, J. Mater. Chem.,
2008, 18, 1799.
36 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.
Notes and references
1 J. A. G. Williams, Top. Curr. Chem., 2007, 281, 205.
2 (a) L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura and F. Barigelletti,
Top. Curr. Chem., 2007, 281, 143; (b) S. Develay and J. A. Gareth
Williams, Dalton Trans., 2008, 4562–4564; (c) P.-H. Lanoe¨, H. L. Bozec,
J. A. Gareth Williams, J.-L. Fillaut and V. Guerchais, Dalton Trans.,
2010, 39, 707.
3 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.
4 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.
37 H. Guo, S. Ji, W. Wu, W. Wu, J. Shao and J. Zhao, Analyst, 2010, 135,
2832.
38 X. Wang, A. D. Guerzo and R. H. Schmehl, J. Photochem. Photobiol.,
C, 2004, 5, 55.
39 N. D. McClenaghan, Y. Leydet, B. Maubert, M. T. Indelli and S.
Campagnab, Coord. Chem. Rev., 2005, 249, 1336.
40 A. I. Baba, J. R. Shaw, J. A. Simon, R. P. Thummel and R. H. Schmehl,
Coord. Chem. Rev., 1998, 171, 43.
41 W. Wu, S. Ji, W. Wu, H. Guo, X. Wang, J. Zhao and Z. Wang, Sens.
Actuators, B, 2010, 149, 395.
5 W. Wu, W. Wu, S. Ji, H. Guo and J. Zhao, Dalton Trans., 2011, 40,
5953–5963.
6 H. Sun, H. Guo, W. Wu, X. Liu and J. Zhao, Dalton Trans., 2011, 40,
7834–7841.
42 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J.
R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson,
H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J.
Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov,
R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J.
E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, GAUSSIAN 09
(Revision A.1), Gaussian, Inc., Wallingford, CT, 2009.
43 T. N. Singh-Rachford and F. N. Castellano, Coord. Chem. Rev., 2010,
254, 2560.
7 (a) N. Chawdhury, A. Ko¨hler, R. H. Friend, W.-Y. Wong, J. Lewis, M.
Younus, P. R. Raithby, T. C. Corcoran, M. R. A. Al-Mandhary and M.
S. Khan, J. Chem. Phys., 1999, 110, 4963; (b) J. Ni, L.-Y. Zhang, H.-M.
Wen and Z.-N. Chen, Chem. Commun., 2009, 3801.
8 W.-Y. Wong, Dalton Trans., 2007, 36, 4495.
9 W.-Y. Wong and C.-L. Ho, Coord. Chem. Rev., 2006, 250, 2627.
10 W.-Y. Wong and C.-L. Ho, Coord. Chem. Rev., 2009, 253, 1709.
11 W.-Y. Wong and C.-L. Ho, J. Mater. Chem., 2009, 19, 4457.
12 G.-J. Zhou and W.-Y. Wong, Chem. Soc. Rev., 2011, 40, 2541.
13 G.-J. Zhou, W.-Y. Wong, Z. Lin and C. Ye, Angew. Chem., Int. Ed.,
2006, 45, 6189.
14 G. Zhou, W.-Y. Wong, S.-Y. Poon, C. Ye and Z. Lin, Adv. Funct. Mater.,
2009, 19, 531.
15 M. Hissler, A. Harriman, A. Khatyr and R. Ziessel, Chem.–Eur. J.,
1999, 5, 3366.
16 P.-H. Lanoe¨, J.-L. Fillaut, L. Toupet, J. A. G. Williams, H. Le Bozeca
and V. Guerchais, Chem. Commun., 2008, 4333.
17 G.-J. Zhou, Q. Wang, W.-Y. Wong, D. Ma, L. Wang and Z. Lin, J.
Mater. Chem., 2009, 19, 1872.
44 I. E. Pomestchenko and F. N. Castellano, J. Phys. Chem. A, 2004, 108,
3485.
45 D. N. Kozhevnikov, V. N. Kozhevnikov, M. M. Ustinova, A. Santoro,
D. W. Bruce, B. Koenig, R. Czerwieniec, T. Fischer, M. Zabel and H.
Yersin, Inorg. Chem., 2009, 48, 4179.
46 X. Zhang, L. Chi, S. Ji, Y. Wu, P. Song, K. Han, H. Guo, T. D. James
and J. Zhao, J. Am. Chem. Soc., 2009, 131, 17452.
18 Z. He, W.-Y. Wong, X. Yu, H.-S. Kwok and Z. Lin, Inorg. Chem., 2006,
45, 10922.
19 B. Yin, F. Niemeyer, J. A. G. Williams, J. Jiang, A. Boucekkine, L.
Toupet, H. L. Bozec and V. Guerchais, Inorg. Chem., 2006, 45, 8584.
20 W.-Y. Wong and C.-L. Ho, Acc. Chem. Res., 2010, 43, 1246.
21 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.
22 I. V. Sazanovich, M. A. H. Alamiry, J. Best, R. D. Bennett, O. V.
Bouganov, E. S. Davies, V. P. Grivin, A. J. H. M. Meijer, V. F. Plyusnin,
K. L. Ronayne, A. H. Shelton, S. A. Tikhomirov, M. Towrie and J. A.
Weinstein, Inorg. Chem., 2008, 47, 10432.
23 F. N. Castellano, I. E. Pomestchenko, E. Shikhova, F. Hua, M. L. Muro
and N. Rajapakse, Coord. Chem. Rev., 2006, 250, 1819.
24 C. Ed Whittle, J. A. Weinstein, M. W. George and K. S. Schanze, Inorg.
Chem., 2001, 40, 4053.
25 M. Hissler, W. B. Connick, D. K. Geiger, J. E. McGarrah, D. Lipa, R.
J. Lachicotte and R. Eisenberg, Inorg. Chem., 2000, 39, 447.
26 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.
27 H. Guo, M. L. Muro-Small, S. Ji, J. Zhao and F. N. Castellano, Inorg.
Chem., 2010, 49, 6802.
28 (a) S. Ji, W. Wu, W. Wu, H. Guo and J. Zhao, Angew. Chem., Int. Ed.,
2011, 50, 1626; (b) S. Ji, H. Guo, W. Wu, W. Wu and J. Zhao, Angew.
Chem., Int. Ed., 2011, 50, 8283.
29 Y. Liu, W. Wu, X. Zhang, H. Guo and J. Zhao, Dalton Trans., 2011,
40, 9085.
30 J. I. Goldsmith, W. R. Hudson, M. S. Lowry, T. H. Anderson and S.
Bernhard, J. Am. Chem. Soc., 2005, 127, 7502.
47 S. Ji, J. Yang, Q. Yang, S. Liu, M. Chen and J. Zhao, J. Org. Chem.,
2009, 74, 4855.
48 T. Kowalczyk, Z. Lin and T. V. Voorhis, J. Phys. Chem. A, 2010, 114,
10427.
49 X. Zhang, Y. Wu, S. Ji, H. Guo, P. Song, K. Han, W. Wu, W. Wu, T. D.
James and J. Zhao, J. Org. Chem., 2010, 75, 2578.
50 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 2nd ed.,
Kluwer Academic/Plenum Publishers, New York, 1999.
51 B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley-
VCH, Verlag GmbH, New York, 2001.
52 S. Ji, W. Wu, Y. Wu, T. Zhao, F. Zhou, Y. Yang, X. Zhang, X. Liang,
W. Wu, L. Chi, Z. Wang and J. Zhao, Analyst, 2009, 134, 958.
53 W. Wu, W. Wu, S. Ji, H. Guo and J. Zhao, J. Organomet. Chem., 2011,
696, 2388.
54 W. Wu, C. Cheng, W. Wu, H. Guo, S. Ji, P. Song, K. Han, J. Zhao, X.
Zhang, Y. Wu and G. Du, Eur. J. Inorg. Chem., 2010, 29, 4683.
55 W. Wu, W. Wu, S. Ji, H. Guo and J. Zhao, Eur. J. Inorg. Chem., 2010,
29, 4470.
56 S. M. Borisov and I. Klimant, Anal. Chem., 2007, 79, 7501.
57 C. S. K. Mak, D. Pentlehner, M. Stich, O. S. Wolfbeis, W. K. Chan and
H. Yersin, Chem. Mater., 2009, 21, 2173.
58 D. V. Kozlov and F. N. Castellano, Chem. Commun., 2004, 2860.
59 R. R. Islangulov, D. V. Kozlov and F. N. Castellano, Chem. Commun.,
2005, 3776.
60 P. Du and R. Eisenberg, Chem. Sci., 2010, 1, 502.
61 S. Baluschev, V. Yakutkin, T. Miteva, Y. Avlasevich, S.
Chernov, S. Aleshchenkov, G. Nelles, A. Cheprakov, A. Yasuda,
31 (a) X. Wang, S. Goeb, Z. Ji, N. A. Pogulaichenko and F. N. Castellano,
Inorg. Chem., 2011, 50, 705; (b) A. A. Rachford, R. Ziessel, T. Bura, P.
11560 | Dalton Trans., 2011, 40, 11550–11561
This journal is
The Royal Society of Chemistry 2011
©

View Online
K. Mu¨llen and G. Wegner, Angew. Chem., Int. Ed., 2007, 46,
7693.
62 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.
63 Y. Y. Cheng, T. Khoury, R. G. C. R. Clady, M. J. Y. Tayebjee, N. J.
Ekins-Daukes, M. J. Crossleya and T. W. Schmidt, Phys. Chem. Chem.
Phys., 2010, 12, 66.
64 A. Monguzzi, J. Mezyk, F. Scotognella, R. Tubino and F. Meinardi,
Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 195112–
1.
65 (a) A. Monguzzi, R. Tubino and F. Meinardi, Phys. Rev. B: Condens.
Matter Mater. Phys., 2008, 77, 155122–1; (b) J. Sun, W. Wu, H. Guo
and J. Zhao, Eur. J. Inorg. Chem., 2011, 3165–3173; (c) L. Huang, L.
Zeng, H. Guo, W. Wu, W. Wu, S. Ji and J. Zhao, Eur. J. Inorg. Chem.,
2011, DOI: 10.1002/ejic.201100777.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 11550–11561 | 11561
©