Tridentate cyclometalated platinum(II) complexes with strong absorption of visible light and long-lived triplet excited states as photosensitizers for triplet-triplet annihilation upconversion

Article abstract of DOI:10.1016/j.dyepig.2012.07.021

C^N*N cyclometalated platinum(II)L complexes with functionalized acetylide ligands were prepared (L = phenyl, pyrenyl or naphthalimide acetylides). The absorption of the pyrene and naphthalimide derived Pt(II) complexes in the visible region of the spectr

Full text of DOI:10.1016/j.dyepig.2012.07.021

Dyes and Pigments 96 (2013) 220e231
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Tridentate cyclometalated platinum(II) complexes with strong absorption
of visible light and long-lived triplet excited states as photosensitizers for
tripletetriplet annihilation upconversion
,
,
,
Wenting Wu a b, Dandan Huang a, Xiuyu Yi a b, Jianzhang Zhao a
*
a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 Western Campus, 2 Ling-Gong Road, Dalian 116024, China
b State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Qingdao 266555, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
C^N*N cyclometalated platinum(II)L complexes with functionalized acetylide ligands were prepared
(L ¼ phenyl, pyrenyl or naphthalimide acetylides). The absorption of the pyrene and naphthalimide
derived Pt(II) complexes in the visible region of the spectrum was enhanced relative to the phenylethenyl
Received 17 May 2012
Received in revised form
25 July 2012
Accepted 31 July 2012
Available online 14 August 2012
substituted model complex. Long-lived deep-red emissions were observed for the pyrene (83.7
naphthalimide (135.7 s) derived Pt(II) complexes and compared to the phenylethenyl substituted model
complex (9.2 s). Room temperature and 77 K phosphorescence, time-resolved transient absorption and
ms) and
m
m
theoretical calculations indicated intraligand triplet excited states for the complexes. The photophysical
properties of the complexes were fully rationalized by density functional theory calculations. The
complexes were used as triplet photosensitizers for tripletetriplet annihilation based upconversion.
Upconversion quantum yields up to 19.5% were observed. These results are useful for design of visible
light-harvesting transition metal complexes that shows long-lived triplet excited states.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Naphthalimide
Photochemistry
Platinum
Pyrene
Tripletetriplet annihilation
Upconversion
1. Introduction
interesting approach to improve the emission properties of the
transition metal complexes.
Transition metal complexes, such as either cyclometalated Pt(II)
or Pt(II) acetylide complexes, have attracted much attention [1e16],
owing to their applications in molecular probes and photocatalysis
[15b,17,18], electroluminescence [19], optical power limiting [14],
and more recently for tripletetriplet annihilation (TTA) upconver-
sion [20e22]. The Pt(II) acetylide complexes are particularly inter-
esting because the photophysical properties of the complexes can be
readily tuned by changing the acetylide ligands [1,23e26]. The tri-
dentate Pt(II) acetylide complexes have been known for a while, such
as N^N^N Pt(II) acetylide complexes [5,27e30]. However, due to the
non-planar coordination center (N^N^N-Pt), the photophysical
properties are very often not ideal. For example, a low phosphores-
cence quantum yield of 4.9% was observed for [Pt(ttpy)(C^CPh)]
[ClO4] (where ttpy ¼ 40-p-tolylterpyridine) [31]. It has been shown
that the phosphorescence properties of the transition metal
complexes can be improved by optimization of the coordination
geometry of the ligand [32]. Thus ligand optimization is an
Recently tridentate cyclometalated platinum (II) complexes
(C^N*N) PtL (L ¼ acetylide) featuring a fused 5e6 membered met-
allacycle were reported [26]. However, no studies have been carried
out to enhance the visible absorption of the complexes and to
prolong the triplet excited state lifetimes by modification of the
acetylide ligand L. T1 state lifetimes of the (C^N*N)Pt^L complexes
are less than 10 ms [26]. On the other hand, transition metal
complexes with strong absorption of visible light and long-lived
triplet excited state are important for applications such as photo-
catalysis [33]. According to our previous studies on Ru(II) and
cyclometalated Pt (II) complexes with C^N ligand, the triplet
excited state lifetime can be substantially prolonged by accessing
the intraligand (3IL) excited state [33].
On the other hand, long-lived T1 excited states of transition
metal complexes are significant in many aspects, for example the
TTA upconversion [20e22,31,33e35]. TTA upconversion is inter-
esting, it requires only low-power non-coherent excitation, and it
shows intense absorption of the excitation light and high upcon-
version quantum yields [20e22]. One of the crucial steps of the TTA
upconversion, the tripletetriplet energy transfer (TTET) process,
can be enhanced by the long-lived triplet excited states (Scheme 1).
* Corresponding author. Tel./fax: þ86 411 3960 8007.
E-mail address: zhaojzh@dlut.edu.cn (J. Zhao).
URL: http://finechem.dlut.edu.cn/photochem
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.07.021

W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
221
Scheme 1. Qualitative Jablonski Diagram illustrating the sensitized TTA upconversion process between Pt(II) complexes (exemplified by Pt-3) and triplet acceptor 9,10-
diphenylanthracene (DPA). The effects of the light-harvesting ability and the luminescence lifetime of the Pt (II) sensitizer on the efficiency of the TTA upconversion are also
shown. E is energy. GS is ground state (S0). 1IL* is naphthalimide (NI) intraligand singlet excited state (NI localized). 3MLCT* is the Pt (II) based metal-to-ligand-charge-transfer triplet
excited state. 3IL* is intraligand triplet excited state (NI localized). TTET is tripletetriplet energy transfer. 3DPA* is the triplet excited state of DPA. TTA is tripletetriplet annihilation.
1DPA* is the singlet excited state of DPA. The emission bands observed for the sensitizers alone is the 3IL emissive excited state. The emission bands observed in the TTA experiment
is the simultaneous 3IL* emission (phosphorescence) and the 1DPA* emission (fluorescence). The 3IL state phosphorescence of Pt-3 is quenched.
Previously the triplet photosensitizers for TTA upconversion
were limited to the Pt(II)/Pd(II) porphyrin complexes [21]. A tri-
dentate Pt(II) acetylide complex was used as triplet photosensitizer
for TTA upconversion [31], but the complex shows weak absorption
in visible region and short-lived T1 excited states. Such properties
are not ideal for TTA upconversion (Scheme 1) [31]. Thus Pt(II)
acetylide complexes with intense absorption of visible light and
long-lived triplet excited state are desired for TTA upconversion
[21,33]. Recently we developed a series of Ru(II), Ir(III) and Pt(II)
complexes as triplet photosensitizer for TTA upconversion, these
complexes share the same feature of strong absorption of visible
light and long-lived triplet excited states [21,33].
Herein, we used the novel coordination motif of (C^N*N)PtL
(L ¼ acetylide) and functionalized acetylide ligands to prepare the
Pt(II) complexes that show strong absorption in visible range and
long-lived T1 excited states, for which L ¼ pyrenyl acetylide (Pt-2)
and L ¼ naphthalimide (NI) acetylide (Pt-3) (Scheme 2). The pho-
tophysical properties of the complexes were thoroughly studied
with steady state and time-resolved spectroscopy, as well as DFT
computations (Scheme 1). The new complexes show enhanced
absorption in the visible range. Long-lived 3IL excited states
localized on the arylacetylide ligands were observed for Pt-2
sT ¼ 84 s) and Pt-3 (sT ¼ 136 s), compared to the model complex
Pt-1 (sT ¼ 9.2
(
m
m
m
s, L ¼ phenylacetylide), for which the triplet excited
state is localized on the tridentate ligand. The complexes were used
as triplet sensitizers for TTA upconversion and upconversion
quantum yield up to 19.5% was observed.
2. Experimental section
2.1. General information
Ethynylpyrene, N-(2-ethylhexyl)-4-ethynylnaphthalimide, Pt-
0 and Pt-1, were prepared according to literature procedures [26].
NMR spectra were recorded on a 400 MHz Varian Unity Inova NMR
spectrophotometer. Mass spectra were recorded with Q-TOF Micro
MS spectrometer and MALDI micro MS. UVevis absorption spectra
were measured with a HP8453 UVevisible spectrophotometer.
Fluorescence spectra were recorded on a Shimadzu RF5301PC
spectrofluorometer or a Sanco 970 CRT spectrofluorometer. Lumi-
nescence lifetimes were measured on an OB920 phosphorescence/
fluorescence lifetime spectrometer (Edinburgh Instruments, U.K.).
R =
N
N
a
N
b
N
N
N
N
N
N
L-1
L-2
Pt
Pt
O
N
O
Cl
Pt-0
R
DPA
L-3
N
N
N
N
N
N
N
Pt
N
N
NC CN
Pt
Pt
N
N
N
Ru
N
O
O
N
N
N
Pt-1
N
Pt-2
Pt-3
O
[Ru(dmb)3]2+
Ru-1
DCM
Scheme 2. Synthesis of complexes Pt-1, Pt-2, and Pt-3. The molecular structures of the triplet acceptor/annihilator 9,10-diphenylanthracene (DPA), Ru-1 ([Ru(dmb)3][PF6]2) and
compound DCM (quantum yield standard) used in the upconversion were also shown. Reagents and conditions: (a) K2PtCl4, and glacial acetic acid reflux, 21 h. (b) Aryl acetylides,
Et3N, CuI, and CH2Cl2, Ar, r.t. 4e12 h.

222
W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
2.2. Synthesis
DFT (TDDFT) method based on the optimized singlet ground state
geometry. The spin density was calculated with the energy mini-
mized triplet state geometries. The transient absorption spectra
were calculated by the TDDFT methods based on the optimized
triplet state geometry at the B3LYP/6-31G(d)/LanL2DZ. The calcu-
lated transient absorption (TA) was constructed by the Gaussview
(data was exported into Origin 5.0). The bleaching bands in the
transient absorption cannot be predicted by the TDDFT calculations
(stimulated emission was not considered). One should be careful in
using this theoretical method for study the transient absorption
spectra, because the triplet state is not always the transient species
observed in the TA spectra, in some cases it can be other species,
such as the charge-separated state. All calculations were performed
using Gaussian 09W (Gaussian, Inc.) [36].
2.2.1. (2-pyrenylethynyl)[2-[6-[phenyl(2-pyridinyl-kN)amino]-2-
pyridinyl-kN]phenyl-kC]-Platinum (Pt-2)
To a 25 mL dry, N2 flushed flask was charged Pt-0 (30.0 mg,
0.054 mmol), ethynylpyrene (40.7 mg, 0.18 mmol), Et3N (1 mL), CuI
(2.6 mg) and dichloromethane (10 mL). The mixture was stirred at
room temperature in the absence of light for 4 h. The solvents were
evaporated and the crude product was purified by column chroma-
tography (silica gel, hexane/CH2Cl2 ¼1:2, v/v). A yellow solid was ob-
tained, 29.0 mg, 72.4%. M.p. >250 ꢀC. 1H NMR (400 MHz, CDCl3):
d
10.46 (d, J ¼ 5.6 Hz,1H), 9.06 (d, J ¼ 9.6 Hz,1H), 8.71 (d, J ¼ 7.6 Hz, 1H),
8.29 (d, J ¼ 8.0 Hz, 1H), 8.15e7.94 (m, 8H), 7.71e7.61 (m, 6H), 7.51 (d,
J ¼ 8.0 Hz, 1H), 7.46 (d, J ¼ 7.6 Hz, 2H), 7.14 (t, J ¼ 8.0 Hz, 1H), 6.97 (d,
J ¼ 6.4 Hz, 1H), 6.75 (d, J ¼ 8.4 Hz, 1H), 6.55 (d, J ¼ 8.4 Hz, 1H). 13C NMR
(100 MHz, CDCl3)d165.49,154.02,150.72,150.46,144.73,143.11,142.20,
2.5. TTA upconversions
139.43, 138.00, 137.82, 131.79, 130.83, 130.36, 130.21, 129.99, 129.16,
127.69, 127.60, 127.04, 126.54, 125.91, 125.05, 124.71, 124.59, 123.76,
A diode pumped solid-state laser was used as the excitation
source for the upconversion. The samples were purged with either
N2 or Ar for 15 min before measurement. A black box was put behind
the cuvette to trap the laser, after the laser beam passed through the
cuvette. The laser scattering was greatly repressed with this method
and no notch filter is required. The upconversion was carried out by
irradiation of the mixed solution of the photosensitizer and the
triplet acceptor (9,10-diphenyanthraene, DPA). The emission spectra
of the upconversion were recorded on a spectrofluorometer (the
excitation source of the spectrometer is completely blocked and in
the upconversion the excitationwas carried out with the lasers). The
upconversion quantum yields were determined with the fluores-
cence of 4-dicyanomethylene-2-methyl-6-(p(dimethylamino)
styryl)-4H-pyran (DCM) (Scheme 2) as the quantum yield standard
123.62, 118.76, 117.55, 114.49, 112.81, 101.53. IR (KBr):
n
¼ 706, 754, 838,
845, 1180, 1231, 1313, 1351, 1425, 1434, 1498, 1576, 1597, 2092,
3042 cmꢁ1. MALDI-HRMS:calcdforC40H25N3Pt ([M]þ)m/z ¼ 742.1696,
found m/z ¼ 742.1681. Anal. Calcd for C40H25N3Pt þ 0.25CH2Cl2: C,
63.28; H, 3.36; N, 5.50; Found C, 63.36; H, 3.44; N, 5.19.
2.2.2. [2-[N-(2-ethylhexyl)-4-ethynylnaphthalimide]][2-[6-[phenyl(2-
pyridinyl-kN)amino]-2-pyridinyl-kN]phenyl-kC]platinum (Pt-3)
Pt-0 (30.0 mg, 0.054 mmol), N-(2-ethylhexyl)-4-ethynylnaph-
thalimide (60.0 mg, 0.18 mmol), Et3N (1 mL), CuI (0.84 mg), and
dichloromethane (10 mL) were mixed together. The mixture was
stirred at room temperature in the absence of light for 4 h. The
solvent was evaporated and the crude product was purified by
column chromatography (silica gel, hexane/CH2Cl2 ¼ 1:4, v/v). An
orange solid was obtained, 28.8 mg, 62.8%. M. p. 234.2e237.0 ꢀC. 1H
(
F
¼ 0.10 in CH2Cl2). The upconversion quantum yields were calcu-
lated with Eq. (1), where Uunk, Aunk, and Iunk represent the quantum
yield, absorbance, integrated photoluminescence, intensity of the
unknown sample and hunk represents the refractive index of the
solvents, respectively [20]. The photography of the upconversion
was takenwith Samsung NV5 digital camera. The exposure times are
the default values of the camera.
NMR (400 MHz, CDCl3):
d
10.10 (d, J ¼ 5.6 Hz, 1H), 9.09 (d, J ¼ 7.6 Hz,
1H), 8.59e8.45 (m, 3H), 7.91 (d, J ¼ 7.6 Hz, 1H), 7.72e7.57 (m, 7H),
7.49e7.43 (m, 3H), 7.21 (t, J ¼ 7.8 Hz, 1H), 7.12 (t, J ¼ 6.0 Hz, 1H), 6.90
(t, J ¼ 8.8 Hz,1H), 6.73 (d, J ¼ 8.8 Hz,1H), 6.52 (d, J ¼ 8.8 Hz,1H), 4.19e
4.08 (m, 2H),1.97e1.95 (m,1H),1.41e1.25 (m, 8H), 0.96e0.86 (m, 6H).
13C NMR (100 MHz, CDCl3)
d 165.18, 164.92, 153.44, 150.56, 150.30,
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
144.67, 142.86, 141.65, 138.84, 138.26, 137.93, 134.33, 134.21, 132.98,
131.79, 131.35, 130.74, 130.21, 130.04, 129.88, 129.75, 128.75, 126.50,
126.37, 123.93, 122.92, 121.84, 118.85, 117.83, 117.71, 114.84, 114.65,
112.84,100.71, 44.26, 38.15, 31.00, 28.97, 24.32, 23.33,14.33,10.93. IR
Astd
Aunk
Iunk
Istd
hunk
hstd
VUC ¼ 2Vstd
(1)
3. Results and discussions
3.1. Design and synthesis of the complexes
(KBr):
n
¼ 705, 755, 783, 1024, 1094, 1180, 1235, 1352, 1380, 1434,
1462, 1552, 1580, 1653, 1692, 2089, 2856, 2926, 2955, 3049 cmꢁ1
.
MALDI-HRMS calcd for C44H39N4O2Pt ([M]þ) m/z ¼ 850.2721, found
m/z ¼ 850.2671. Anal. Calcd for C43H36N4O2Pt þ 0.2CH2Cl2
þ 0.3C6H14: C, 61.89; H, 4.81; N, 6.28; Found C, 61. 97; H, 3.49; N, 6.26.
Pt-1 was reported to show high phosphorescence quantum
yields due to its planar coordination arrangement [26]. Previously
we have shown that the acetylide ligand L, such as pyrene acetylide
and naphthalimide acetylide, gives a long-lived T1 state. Thus,
pyrene acetylide (Pt-2) and naphthalimide acetylide (Pt-3) were
used to access the possible 3IL emission. The (C^N*N)PtL
(L ¼ acetylide) complexes were prepared following the synthetic
procedures (Scheme 2) and were obtained in moderate to satisfying
yields. We proved that for Pt-2 and Pt-3, the triplet excited states
are localized on either the pyrenyl or the naphthalimide acetylide
ligands (3IL state), respectively. For Pt-1, the triplet excited state is
mainly localized on the tridentate ligand.
2.3. Nanosecond time-resolved transient difference absorption
spectroscopy
The nanosecond time-resolved transient difference absorption
spectra were recorded on LP 920 laser flash photolysis spectrom-
eter (Edinburgh Instruments, UK). The samples were purged with
N2 or Ar for 15 min before measurements. The samples were
excited with 355 nm pulsed laser and the transient signals were
recorded on a Tektronix TDS 3012B oscilloscope. The data were
analyzed by LP900 software.
2.4. Computational methods
3.2. Absorption and emission spectra
The geometry optimizations were calculated using B3LYP
functional at 6-31G(d)/LanL2DZ basis set. The energy of the triplet
excited state of the ligand was calculated with the time-dependent
The UVevis absorption of the ligands and the complexes were
studied (Fig. 1). L-1 shows absorption in the UV range (247 nm).
Ligands with larger
p-conjugation framework show red-shifted

W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
223
a
b
Fig. 1. UVeVis absorption spectra of (a) ligands and (b) the Pt(II) complexes. c ¼ 1.0 ꢂ 10ꢁ5 mol dmꢁ3 in CH2Cl2. 25 ꢀC.
absorption. For example, L-2 shows intense absorption at 327 nm,
342 nm and 359 nm. Naphthalimide acetylide L-3 shows intense
absorption at 350 nm and 367 nm. Interestingly, intense absorption
in the visible range were observed for Pt-2 and Pt-3 when
compared to Pt-1, as well as other known tridentate cyclometalated
Pt(II) complexes [26]. The molar extinction coefficient of Pt-3 is up
to 33,800 Mꢁ1 cmꢁ1 at 430 nm. Our DFT/TDDFT calculations clearly
predicted that the naphthalimide (NI) moiety contributes signifi-
cantly to this absorption band (see later section).
intensity was quenched by 84.0% in air-saturated solution
compared to that in N2 saturated solution. The phosphorescence
intensities of Pt-2 and Pt-3 are more drastically quenched in
aerated solution compared to that of Pt-1, thus the phosphorescent
lifetimes of Pt-2 and Pt-3 are longer than that of Pt-1, which are in
agreement with the experiment results (Fig. 3d and Table 1). The
phosphorescence quantum yield of Pt-1 (56%) is higher than that of
Pt-2 (0.41%) and Pt-3 (4.83%) [26].
The emission of the ligands and complexes was studied (Fig. 2).
The ligands are fluorescent and the emission wavelengths are
3.3. Nanosecond time-resolved transient difference absorption
spectra
dependent on the
p-conjugation of the chromophores. For
example, L-2 and L-3 show red-shifted emission (425 nm) relative
to L-1 (296 nm). For Pt-2, structured phosphorescence in the red
The transient absorption spectra of the complexes were studied
(Fig. 4). For Pt-1, transient absorption bands at 350e500 nm range
were observed upon pulsed laser excitation. Bleaching was observed
at 350 nm, which is in agreement with the steady state absorption of
Pt-1. It should be pointed out that the bleaching is diminished by the
strong transient absorption (Fig. 4a). For Pt-2, bleaching of the ground
state absorption at 350e420 nm was found upon pulsed laser exci-
tation, while transient absorption bands in 420e700 nm range were
observed, which is different from that of Pt-1. This transient absorp-
tion profile is similar to the pyrene localized 3IL excited state [3].
Significant bleaching was found for Pt-3 in the range 380e460 nm
upon pulsed laser excitation. This bleaching is due to the depletion of
the ground state of the NI ligand, which shows strong steady state
absorption at 430 nm (Fig. 1). Furthermore, transient absorption in
the range of 460e750 nm was also observed which shows the char-
acteristic triplet absorption of NI moiety [37]. By following the decay
and near-IR range was found
(
lem ¼ 659, 719 nm) at room
temperature. The emission band was greatly red-shifted when
compared to Pt-1 (lem ¼ 495 nm). The Stokes shift is up to 273 nm
for Pt-2. Pt-3 has similar photophysical properties, which shows an
emission band at 636 nm and the Stokes shift is up to 206 nm.
For the normal cyclometalated platinum (II) complexes, the
phosphorescence lifetime is usually less than 10.0
the Pt-1 was reported to show a lifetime of 9.2 s [26]. For Pt-2 and
Pt-3, however, the phosphorescence lifetimes were prolonged to
83.7 s and 135.7 s, respectively (Fig. 3 and Table 1). All these
ms. For example,
m
m
m
photophysical properties indicate that the emission of Pt-2 and Pt-
3 are due to the 3IL emissive states, not the normal 3MLCT excited
state [5,26].
The sensitivity of the emission of the Pt(II) complexes towards
O2 was investigated (Fig. 3). For Pt-1, the phosphorescence
traces of the transient at 520 nm and 650 nm, lifetimes of 84 ms and
a
b
1.2
Pt-2
Pt-3
Pt-1
L-2
L-1
L-3
1.00
0.75
0.50
0.25
0.00
1.0
0.8
0.6
0.4
0.2
0.0
I
I
400 500 600 700 800 900
Wavelength / nm
300
400
500
Wavelength / nm
Fig. 2. Normalized emission spectra of (a) the ligands and (b) the Pt(II) complexes. c ¼ 1.0 ꢂ 10ꢁ5 mol dmꢁ3, in CH2Cl2, 25 ꢀC.

224
W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
compared (Fig. 5). Generally 3IL emissive excited state gives small
thermally induced Stokes shift ( ES, i.e. the energy gap between the
Table 1
Photophysical parameters of platinum complexes Pt-1, Pt-2 and Pt-3.
D
a
b
labs (nm)
3
lem (nm)
Fp (%)
sP
(
m
s)c
9.2
emission of the complex at RT and low temperature). For the Pt-1
and Pt-2, small thermally induced Stokes shifts of 166 cmꢁ1 and
70 cmꢁ1 were observed, respectively. Interestingly, Pt-3 shows
Pt-1
Pt-2
Pt-3
378
397
430
6800
27,600
33,800
495, 532
659, 719
636
56
0.4
4.8
83.7
135.7
a red-shift emission (
emission at RT (Fig. 5c).
D
ES ¼ ꢁ262 cmꢁ1) at 77 K compared to the
a
Extinction coefficients. Mꢁ1 cmꢁ1
Phosphorescence quantum yield determined with [Ru(dmb)3][PF6]2 as the
.
b
standard (FP ¼ 7.3% in MeCN). In CH2Cl2.
c
Phosphorescence lifetimes of the Pt(II) complexes.
3.5. DFT calculations: rationalization of the electronic spectra and
the 3IL excited states
136 ms were determined for Pt-2 and Pt-3, respectively, which coin-
cide with the phosphorescence lifetime (Fig. 3 and Table 1).
These transient absorptions can be rationalized by TDDFT
calculations on the transitions T1 / Tn (n > 1). The calculations
Application of DFT calculations in luminescence studies has
attracted much attention [39e41]. Previously DFT calculations have
been used for the study of phosphorescent transition metal
complexes [42e55]. Herein theoretical calculations were per-
formed on Pt-1, Pt-2 and Pt-3 to investigate the electronic spectra
and the 3IL excited states.
Based on the DFT optimization of the ground state geometry, the
CePteN coordination angle of Pt-1 is 170.02ꢀ, which is close to the
reported value of 173.18ꢀ determined by X-ray diffraction of a single
crystal [26]. Pt-1 has a planar square coordination geometry.
However, with L as NI or pyrene ethynyl moieties, there are small
distortions on the CePteN angle for Pt-2 (169.35ꢀ) and Pt-3
(169.49ꢀ).
Previously the HOMO and LUMO of Pt-1 were studied [26].
However, the excited state of the complex was not studied in detail
by DFT calculations. More complete information can be obtained by
studying the excited states of the compounds, instead of the
molecular orbitals. Herein we carried out TDDFT calculations to
study the photophysical properties of the complex, such as the UVe
vis absorption and the phosphorescence (Table 2 and Fig. 6).
The calculated UVevis absorption bands of Pt-1 are at 461 nm
and 379 nm (Table 2). The molecular orbitals involved in the
predict absorption bands at 497 nm for Pt-2 (f ¼ 0.3313, T1 / T15
)
and 531 nm (f ¼ 0.0533, T1 / T13) (Fig. 4). For Pt-3, the calculations
predict absorption bands of the T1 state at 415 nm (f ¼ 0.1605,
T1/T22), 570 nm (f ¼ 0.2138, T1/T16), 575 nm (f ¼ 0.0327,
T1/T15), 608 nm (f ¼ 0.0241, T1 / T13), 638 nm (f ¼ 0.0374,
T1 / T10). The calculated transient absorption spectra are in
general agreement with the experimental results (Fig. 4). Note that
the transient absorption bands may be distorted by the bleaching
bands, which is clear for Pt-1 (the weak positive transient
absorption of Pt-1 in the range of 480e650 nm is due to the
bleaching caused by the strong emission of the complex). Similarly,
the failure to observe the calculated transient absorption at 415 nm
in the experimental spectra of Pt-3 may be due to the intense
bleaching at 430 nm.
3.4. 77 K emission spectra
In order to confirm the intraligand (3IL) phosphorescence of Pt-
1, Pt-2 and Pt-3, the emission spectra at 77 K and RT were
b
a
500
600
400
N2
450
N2
300
300
I
I
200
air
air
O2
150
0
100
O2
0
500 600 700 800 900
Wavelength / nm
400
500
600
700
Wavelength / nm
d
c
300
200
100
0
10000
N2
1000
I
air
O2
Pt-3
Pt-2
Pt-1
100
0
200
Time /
400
µs
600
600
700
800
900
Wavelength / nm
Fig. 3. Phosphorescence emission of (a) Pt-1, (b) Pt-2 and (c) Pt-3 in solutions saturated with N2, air and O2. (d) Phosphorescence decays of Pt-1, Pt-2 and Pt-3. The scattered
symbols represent experimental data, and the solid lines are monoexponential fitting results. c ¼ 1.0 ꢂ 10ꢁ5 mol/L, in CH2Cl2, 25 ꢀC.

W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
225
c
a
e
b
d
f
Fig. 4. Nanosecond time-resolved transient absorption difference spectra of (a) Pt-1, (c) Pt-2 and (e) Pt-3. In deaerated toluene after pulsed excitation (lex ¼ 355 nm), 20 ꢀC. (b) (d)
(f) are the calculated transient absorption spectra of Pt-1, Pt-2 and Pt-3, respectively (please note that the information of bleaching is not included in the calculated transient
absorption spectra). TDDFT calculation is based on the optimized triplet state geometry at the B3LYP/6-31G(d)/LanL2DZ level using Gaussian 09W.
transitions indicate that these transitions are featured with metal-
to-ligand-charge-transfer (MLCT) and ligand-to-ligand-charge-
transfer (LLCT) character (Fig. 6). The calculated absorption band
at 461 nm was not found in the experimental UVevis absorption
spectrum, and it may be related to the observed absorption at
378 nm (Fig. 1b). This is within expectations because it was known
that the TDDFT calculations usually underestimate the excitation
energy for charge transfer transitions [41,56].
The phosphorescence of Pt-1 was studied by calculation of the
triplet excited state. The phosphorescence emission wavelength,
approximated as the energy gap between the S0 and T1 state, was
calculated as 505 nm, which is in good agreement with the
experimental result of 495 nm (Fig. 2b). The main transition
involved in the T1 excited state is HOMO / LUMO. HOMO ꢁ 1 and
HOMO ꢁ 2 are also involved in the transitions, thus the tridentate
ligand-localized 3IL state is also indentified.
(Fig. 1b) can be assigned as mixed IL/MLCT transitions. This result is
different from Pt-1, for which no IL state can be observed.
The emission of Pt-2 was assigned as phosphorescence, so the
triplet excited state was also studied with the TDDFT calculations.
HOMO / LUMO þ 2 is the main transition of the T1 state and
pyrene contributes exclusively to both orbitals (Table 3 and Fig. 7).
The calculated S0/T1 energy gap (662 nm, 1.87 eV) is in good
agreement with the experimental results (659 nm). Thus the DFT
calculations support the assignment of the phosphorescence of Pt-
2 to the 3IL excited state.
For Pt-3, the calculation on the singlet excited state predicted
a maximum absorption at 435 nm (S0 / S1 transition). The calcu-
lated absorption wavelength is in accordance with experimental
results (430 nm, Fig. 1b). HOMO / LUMO transition is the main
component (Table S1), thus the excitation can be assigned as IL
transition (Fig. S11).
Similar calculations were carried out for Pt-2 (Table 3 and Fig. 7).
The calculations predicted a strong UVevis absorption band at
395 nm (S0 / S5 transition, f ¼ 0.5585), as well as a weaker
absorption band at 393 nm (S0 / S6 transition, f ¼ 0.1000) (Table 3).
Experimentally UVevis absorption bands at 386 nm and 397 nm
were observed (Fig. 1b). It is clear that the absorption at 395 nm
(HOMO / LUMO þ 2 and HOMO ꢁ 1 / LUMO þ 1) can be attrib-
uted to the intraligand (IL) and MLCT transitions. For the absorption
at 393 nm, the transitions can be mainly assigned as MLCT transi-
tion. Thus, the absorption bands in the range of 330e450 nm
An absorption band at 410 nm was also observed in the calcu-
lations (Table S10). The main components of the transition are
HOMO / LUMO þ 1 and HOMO / LUMO þ 2 (Table S1), which can
be assigned as IL and LLCT transitions.
In order to study the phosphorescence of Pt-3, the triplet
excited state was also calculated (Table S1 and Fig. S11). The
phosphorescence wavelength, approximated as the S0eT1 state
energy gap, was calculated as 634 nm, which is in good agreement
with the experimental result of 636 nm (Fig. 2b). The HOMO
/ LUMO transition is the main component of S0 / T1. The
b 1.0
a
c 1.0
1.0
0.8
0.6
0.4
0.2
0.0
RT
RT
0.8
0.6
0.4
0.2
0.0
0.8
77 K
77 K
77 K
0.6
I
I
I
0.4
0.2
0.0
RT
500
400
600
Wavelength / nm
700
600
700
800
600
700
800
Wavelength / nm
Wavelength / nm
Fig. 5. Photoluminescence spectra of (a) Pt-1, (b) Pt-2 and (c) Pt-3 at RT and 77 K (EtOH:MeOH, 4:1, v/v), lex ¼ 450 nm.

226
W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
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-1,
calculated by TDDFT//B3LYP/6-31G(d)/LanL2DZ, based on the optimized ground state geometries.
Electronic transition
TDDFT//B3LYP/6-31G(d)
Energya
fb
Compositionc
CId
Charactere
Singlet
S
S
0 / S1
0 / S6
2.69 eV/461 nm
0.0555
H ꢁ 2 / L
H / L
H ꢁ 2 / L þ 1
H ꢁ 1 / L þ 1
H / L þ 2
0.1127
0.6892
0.5584
0.1717
0.3581
IL
LLCT/MLCT
LLCT/MLCT
LLCT/MLCT
LLCT
3.27 eV/379 nm
2.45 eV/505 nm
0.0232
0.0000
f
Triplet
S
0 / T1
H ꢁ 4 / L
H ꢁ 2 / L
H ꢁ 1 / L
H / L
0.1095
0.2617
0.1115
0.5812
e
IL
LLCT/MLCT
LLCT/MLCT
a
Only the selected low-lying excited states are presented.
Oscillator strength.
H stands for HOMO and L stands for LUMO. Only the main configurations are presented.
The CI coefficients are in absolute values.
IL: intraligand, LLCT: ligand-to-ligand-charge-transfer, and MLCT: metal-to-ligand-charge-transfer.
No spin-orbital coupling effect was considered, thus the f values are zero. Calculation is based on the S0 state geometry.
b
c
d
e
f
naphthalimide subunit contributes significantly to both the HOMO
and LUMO orbitals, but the Pt(II) atom contributes slightly to these
HOMO and LUMO orbitals. Therefore, the TDDFT calculations
support the assignment of 3IL state for the emissive triplet excited
state of Pt-3.
The distribution of the triplet excited states of the complexes
were also evaluated by the spin density surfaces (Fig. 8) [38,48,52e
55,57e60]. The T1 excited state for Pt-1 was localized on phenyl-
pyridine, and only slightly on the Pt (II) atom and phenylacetylene
ligand (Fig. 8). The emissions of Pt-1 at 77 K and RT give a small
3.6. Application of the complexes as triplet sensitizer for triplete
triplet annihilation based upconversions
Upconversion has attracted much attention due to the potential
applications in photovoltaics [61], and luminescent bioimaging,
molecular probes and photocatalysis [62]. Among the upconversion
methods, such as with rare earth materials [61,62], and two-photon
absorption dyes [63], tripletetriplet annihilation (TTA) based
upconversion is a new upconversion scheme and has attracted
much attention due to its advantages over the other upconversion
methods [20e22,64,65]. TTA upconversion has been used for
photovoltaics [66], photocatalysis [67], and luminescent bio-
imaging [68], and oxygen sensors [69]. We developed Ru(II)
[21,48,49,70], Pt(II) [50e55,60,71], Ir(III) [72], Re(I) complexes [73],
and organic triplet photosensitizers for TTA upconversion [58].
Recently a N^N^N Pt(II) acetylide complex was used as triplet
sensitizer for TTAupconversion, the onlyone example of using trident
Pt(II) acetylide complex as triplet sensitizer for TTA upconversion, but
theupconversionquantumyieldisnotsatisfying(FUC ¼ 1.1%)[31]. We
reported a few N^N Pt(II) bisacetylide complexes as triplet sensitizer
for TTA upconversion [38,52,54,60,71], but very few Pt(II) acetylide
thermally induced Stokes shift (DES) (Fig. 5a). All of these properties
established that the emission of Pt-1 may originate from C^N*N
ligand-localized excited state (3IL) [26]. However, for the function-
alized acetylides of pyrene and naphthalimide connected to the
platinum coordination centre (Pt-2 and Pt-3, respectively), the spin
density surfaces are localized mainly on the arylacetylide ligands,
the contribution of the Pt(II) atom and the C^N*N trident ligand is
very small. Thus we conclude that the phosphorescence of the Pt (II)
complexes in which the functional chromophores connected to Pt-2
and Pt-3 are due to the 3IL excited states. This spin density analysis is
in good agreement with the experimental results.
Fig. 6. Electron density maps of the frontier molecular orbitals of Pt-1. Based on ground state optimized geometry. Calculated at B3LYP/6-31G/LANL2DZ level with Gaussian 09W.

W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
227
Table 3
Electronic excitation energies and the corresponding oscillator strengths (f), the main configurations and CI coefficients of the low-lying electronically excited states of the
complex Pt-2 calculated by TDDFT//B3LYP/6-31G(d)/LanL2DZ based on the optimized ground state geometries.
Electronic transition
TDDFT//B3LYP/6-31G(d)
Energya
fb
Compositionc
CId
Charactere
Singlet
S
S
0 / S1
0 / S5
2.44 eV/508 nm
0.0302
H / L
0.6762
0.1837
0.2772
0.1501
0.3220
0.4841
0.2030
0.1656
0.1617
0.6076
0.2178
LLCT
LLCT
IL
MLCT
MLCT
IL
MLCT
IL
MLCT
MLCT
IL
H / L þ 1
H ꢁ 2 / L
H ꢁ 1 / L
H ꢁ 1 / L þ 1
H / L þ 2
H ꢁ 1 / L þ 3
H ꢁ 2 / L
H ꢁ 1 / L
H ꢁ / L þ 1
H / L þ 2
3.14 eV/395 nm
0.5585
S
S
0 / S6
3.15 eV/393 nm
1.87 eV/662 nm
0.1000
0.0000f
Triplet
0 / T1
H / L
H / L þ 2
0.1358
0.6619
LLCT
IL
a
Only the selected low-lying excited states are presented.
Oscillator strength.
H stands for HOMO and L stands for LUMO. Only the main configurations are presented.
The CI coefficients are in absolute values.
IL: intraligand, LLCT: ligand-to-ligand-charge-transfer, and MLCT: metal-to-ligand-charge-transfer.
No spin-orbital coupling effect was considered, thus the f values are zero. Calculation is based on the S0 state geometry.
b
c
d
e
f
Fig. 7. Electron density maps of the frontier molecular orbitals of the complex Pt-2. Based on optimized ground state geometry. Calculated at B3LYP/6-31G/LANL2DZ level with
Gaussian 09W.
complexes with very long-lived T1 excited state have been used for
TTA upconversion [60,71].
The emission spectra of the complexes with 445 nm laser exci-
tation were studied (Fig. 9a). The known complex [Ru(dmb)3][PF6]2
(Ru-1) was used as model complex. Previously it was used as triplet
sensitizers for TTA upconversion [74]. Pt-3 and Ru-1 give either red
or near-IR emission. But no emission was observed for Pt-1 and Pt-2,
due to poor absorption of these complexes at 445 nm.
9,10-Diphenylanthracene (DPA) was used as the triplet acceptor
[74], and the emission of the mixture by 445 nm laser excitation was
studied (Fig. 9b). The phosphorescence of Pt-3 was quenched
completelyinthepresenceofDPA (molarratioDPA/Pt-3is4.3:1), due
totheTTET betweenPt-3andDPA. Atthesametime, thefluorescence
emission ofDPAwas observed (Fig. 9b), which is the TTAupconverted
emission of the energy acceptor (DPA). Irradiation of the DPA alone
with 445 nm laser did not produce any emission at 410 nm, thus the
upconversion of the Pt-3/DPA mixture can be verified [21]. The DPA
concentration used in the TTA upconversion was optimized, that is,
the relationship between the upconversion intensity- and the DPA
concentration was investigated, and we found that the 4.3 ꢂ 10ꢁ5 M
DPA is a optimal concentration (Fig. S12).
Pt/Pt intermolecular interactions are possible for these planar
Pt(II) complexes. Therefore, the concentration-dependency of the
UVevis absorption and the emission spectra of the complexes was
also studied (Figs. S13 and S16). No significant intermolecular
Fig. 8. Spin density surfaces for the lowest-lying triplet state of Pt-1, Pt-2 and Pt-3.
The calculations are based at the optimized triplet state geometry (isovalue: ꢃ0.0004).
Calculated at B3LYP/6-31G/LANL2DZ level with Gaussian 09W.

228
W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
b 800
a 400
300
600
400
Pt-3
200
I
Ru-1
Pt-3
I
Pt-1
Pt-2
Ru-1
100
0
200
*
Pt-1
Pt-2
*
0
400 500 600 700 800
Wavelength / nm
400
500
600
700
Wavelength / nm
Fig. 9. (a) Emission profile of Pt-1, Pt-2, Pt-3 and Ru-1 (1.0 ꢂ 10ꢁ5 M). Excitation by blue laser (lex ¼ 445 nm, 5 mW). (b) Emission of the upconverted fluorescence of 9,10-
diphenylanthracene (DPA, 4.3 ꢂ 10ꢁ5 M) and the residual phosphorescence of Pt-1, Pt-2, Pt-3, and Ru-1 in the upconversion experiments. In deaerated CH2Cl2, 25 ꢀC. The aster-
isks in (a) and (b) denote the scattered laser at 445 nm.
interactions were observed for the complexes Pt-1 e Pt-3. There-
fore the blue emission observed in the upconversion experiments is
the result of TTA upconversion.
relationship was desired because it is more energy-efficient than
the normally observed quadratic relationship for the TTA upcon-
versions [64b].
In order to further prove the TTA upconversion, the delayed
fluorescence of the upconverted emission was studied. The lifetime
We propose the overall upconversion capability of the sensi-
tizers that can be described by
molar extinction coefficient of the sensitizer at the excitation
wavelength and FUC is the upconversion quantum yield [21].
h
¼
3
ꢂ
FUC [21], where
3
is the
of the upconverted emission was determined as 167.4
ms (see
Supporting Information, Fig. S14) [35], which is 25,000-fold longer
than the prompt fluorescent lifetime of DPA (6.7 ns), thus the
upconversion was confirmed. The long-lived delayed fluorescence
of the TTA upconversion may find applications in lifetime based
luminescent bioimaging. It should be pointed out that this overlap
between the upconverted fluorescence emission and the absorp-
tion of the triplet photosensitizers may deminish the observed
upconversion efficiency.
The upconversion quantum yields were also determined. For
Pt-3, the upconversion quantum yield is 19.5%, and no upconver-
sion was observed for other complexes under the same experi-
mental conditions. We found that the triplet excited state lifetime
has a profound effect on the upconversion. Pt-3 shows long T1 state
For Pt-3,
h
¼ 2470 Mꢁ1 cmꢁ1
, but for [Pt(ttpy)(C^CPh)]ClO4,
h
¼ 62 Mꢁ1 cmꢁ1. Thus the upconversion capability of Pt-3 is 39
times higher than that of [Pt(ttpy)(C^CPh)]ClO4. But it should be
pointed out that the upconversion is dependent upon factors
including the concentration of the photosensitizer/acceptor and
the excitation power.
It should be pointed out that re-absorption of the upconverted
emission of DPA by the photosensitizers are inevitable, due to the
overlap of the emission of the DPA and the UVevis absorption of
the photosensitizers (Fig. 1). Thus the upconversion quantum yield
we observed is underestimated.
Longer triplet excited state lifetime will produce efficient
TTET process. In order to study the efficiency of the TTET
process, the SterneVolmer quenching curves were constructed.
Significant quenching effect was observed for Pt-3 in the pres-
ence of DPA (Fig. 10). The SterneVolmer quenching constants
were calculated as KSV ¼ 5.09 ꢂ 105 Mꢁ1 for Pt-3, which is 135
times higher than that of Ru-1 (KSV ¼ 3.77 ꢂ 103 Mꢁ1) (Table 4).
Thus it is proved that the long triplet excited state lifetime is
beneficial for a significant quenching effect, thus efficient TTET
can result.
lifetime (
s
¼ 136
ms), thus the critical TTET process of the TTA
upconversion can be enhanced, indicated by the significant
upconversion with Pt-3 as the sensitizer (Fig. 9b).
The upconversioneexcitation power relationship was also
studied and linear relationship was found for the upconversion
with excitation power higher than 5 mW (Fig. S13). A linear
30
The upconversion is significant for unaided eye (Fig. 11). For the
photosensitizers alone, Pt-3 gives deep-red phosphorescence with
445 nm laser excitation and the emission of Ru-1 was red. When
Pt-3
20
Table 4
Luminescence lifetimes (s), SterneVolmer quenching constant (KSV), bimolecular
quenching constants (kq) and upconversion quantum yield (FUC) of the Pt-1, Pt-2,
Pt-3 and Ru-1. In deaerated CH2Cl2 solution, 25 ꢀC.
Pt-2
10
c
Complex
s
(
m
s)a
9.2
83.7
Ksv (Mꢁ1 (103))b
kq (109 Mꢁ1 sꢁ1
)
FUCd (%)
Ru-1
Pt-1
e
Pt-1
Pt-2
Pt-3
Ru-1
30.8
223.4
508.6
3.8
3.35
2.67
5.11
4.75
ꢁ
e
ꢁ
0
135.7
0.8
19.5
0.0
0
1
2
3
4
5
6
c [DPA] / 10-5M
a
Luminescence lifetimes (
s
).
b
SterneVolmer quenching constant (KSV) of the quenching of the phosphores-
Fig. 10. SterneVolmer plots generated from the phosphorescence intensity quenching
of complex Pt-1 (lex ¼ 378 nm), Pt-2 (lex ¼ 387 nm), Pt-3 (lex ¼ 430 nm) and Ru-1
cence of the complexes by the triplet acceptor DPA.
c
Bimolecular quenching constants (kq).
Upconversion quantum yield.
d
(
lex ¼ 445 nm) as a function of DPA concentration. 1.0 ꢂ 10ꢁ5 M in deaerated CH2Cl2,
e
25 ꢀC.
No upconversions were observed.

W. Wu et al. / Dyes and Pigments 96 (2013) 220e231
229
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We thank the NSFC (20972024 and 21073028), Fundamental
Research Funds for the Central Universities (DUT10ZD212), the
Royal Society (UK) and NSFC (ChinaeUK Cost-Share program,
21011130154), Ministry of Education (NCET-08-0077) and Dalian
University of Technology for financial support.
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