Room temperature long-lived triplet excited state of fluorescein in N^N Pt(II) bisacetylide complex and its applications for triplet-triplet annihilation based upconversions

Article abstract of DOI:10.1016/j.jorganchem.2012.05.010

The room temperature (RT) long-lived triplet excited state of fluorescein was observed in a dbbpy Pt(II) bisfluorescein acetylide complex (Pt-1, where dbbpy = 4,4′-di(tert-butyl)-2,2′-bipyridine). The complex shows strong absorption of visible light (ε = 24,600 M^-1 cm ^-1 at 457 nm). Interestingly, the phosphorescence of the coordination centre is completely quenched and only the fluorescence of ligand was observed for Pt-1 at RT. At 77 K, a phosphorescence band at 667 nm was observed. Spin density analysis and nanosecond time-resolved transient difference absorption spectroscopy proved that the lowest-lying triplet excited state of the complex Pt-1 is localized on the fluorescein acetylide ligand (³IL state), for which the lifetime was determined as 16.4 μs (RT, by the time-resolved transient absorption spectra). The quenching of the ³MLCT phosphorescence in Pt-1 is due to the intramolecular energy transfer of ³MLCT → ³IL. The complex Pt-1 was used as the triplet sensitizer for the triplet-triplet annihilation (TTA) based upconversion. Our results will be useful for preparation of transition metal complexes that show intense absorption of visible light and long-lived triplet excited states, these photophysical properties are useful for applications in photovoltaics, photocatalysis or upconversions, etc.

Full text of DOI:10.1016/j.jorganchem.2012.05.010

Journal of Organometallic Chemistry 713 (2012) 189e196
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Room temperature long-lived triplet excited state of fluorescein
in N^N Pt(II) bisacetylide complex and its applications for
tripletetriplet annihilation based upconversions
*
Wenting Wu, Jianzhang Zhao , Wanhua Wu, Yinghui Chen
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 Western Campus, 2 Ling-Gong Road, Dalian 116024, China1
a r t i c l e i n f o
a b s t r a c t
Article history:
The room temperature (RT) long-lived triplet excited state of fluorescein was observed in a dbbpy Pt(II)
bisfluorescein acetylide complex (Pt-1, where dbbpy ¼ 4,40-di(tert-butyl)-2,20-bipyridine). The complex
shows strong absorption of visible light (ε ¼ 24,600 Mꢀ1 cmꢀ1 at 457 nm). Interestingly, the phospho-
rescence of the coordination centre is completely quenched and only the fluorescence of ligand was
observed for Pt-1 at RT. At 77 K, a phosphorescence band at 667 nm was observed. Spin density analysis
and nanosecond time-resolved transient difference absorption spectroscopy proved that the lowest-lying
triplet excited state of the complex Pt-1 is localized on the fluorescein acetylide ligand (3IL state), for
Received 21 March 2012
Received in revised form
11 May 2012
Accepted 15 May 2012
Keywords:
Photochemistry
Fluorescein
Platinum
Upconversions
DFT
which the lifetime was determined as 16.4 ms (RT, by the time-resolved transient absorption spectra). The
quenching of the 3MLCT phosphorescence in Pt-1 is due to the intramolecular energy transfer of
3MLCT / 3IL. The complex Pt-1 was used as the triplet sensitizer for the tripletetriplet annihilation
(TTA) based upconversion. Our results will be useful for preparation of transition metal complexes that
show intense absorption of visible light and long-lived triplet excited states, these photophysical
properties are useful for applications in photovoltaics, photocatalysis or upconversions, etc.
Ó 2012 Elsevier B.V. All rights reserved.
Triplet state
1. Introduction
(where N^N is bidentate ligand, such as dbbpy ¼ 4,40-di(tert-butyl)-
2,20-bipyridine) [23,27]. The heavy atom effect of Pt(II) facilitates
Fluorescein has attracted much attention due to its fluorescence
properties, and has been extensively used in fluorescent molecular
probes [1ꢀ4]. However, in stark contrast to the studies of its fluo-
rescence (i.e. radiative S1 / S0 transition), the phosphorescence or
the triplet excited state of fluorescein has never been explored.
Different from the manifold of singlet excited states, chromophores
with triplet excited states populated upon photoexcitation are crucial
for applications such as photodynamic therapy (PDT) [5], photo-
catalysis [6], and more recently the tripletetriplet annihilation (TTA)
based upconversions [7ꢀ20]. However, it is not easy to access the
triplet excited states of the organic chromophores, usually heavy
atom effect is required for the inter-system crossing (ISC) [21,22].
On the other hand, phosphorescent transition metal complexes
usually show efficient ISC, by which the triplet excited state is
produced [21,23ꢀ26]. For example, N^N Pt(II) bisacetylide are
representative for their room temperature (RT) phosphorescence
the ISC, thus the fluorescence (S1 / S0) of the acetylide ligands is
quenched. Previously perylenediimide acetylide has been attached
to Pt(II) centre [28], but no phosphorescence has been observed,
which is in contrast to the model complex dbbpy Pt(II) bispheny-
lacetylide gives emission at 560 nm (
F
¼ 51%.
s
¼ 1.36
ms) [27,29].
However, usually transition metal complexes show weak absorp-
tion in the visible region [23]. For example, dbbpy Pt(II) bispheny-
lacetylide shows ε ¼ 9300 Mꢀ1 cmꢀ1 at 397 nm [23,27,28].
Furthermore, the lifetime of the T1 excited state of these complexes
are usually less than 5.0
ms [23,30]. Complexes with strong
absorption of visible light and long-lived triplet excited state are
useful for applications such as photocatalysis [6], photovoltaics
[31,32], and TTA upconversions, etc [8ꢀ20].
Recently we proposed a straight forward approach to prepare
transition metal complexes that show intense absorption of visible
light and long-lived triplet excited states, that is, to attach organic
chromophore to the Pt(II) centre via acetylide bonds [16,17,33]. With
this approach, chromophores of naphthalimide (NI) [33], coumarin
[16], naphthalenediimide (NDI) [17], and rhodamine have been
attached to the Pt(II) centre [20]. Based on the photophysical
properties of these complexes, we found that direct attachment of
* Corresponding author. Tel./fax: þ86 (0) 411 8498 6236.
E-mail address: zhaojzh@dlut.edu.cn (J. Zhao).
1
http://finechem.dlut.edu.cn/photochem.
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.05.010

190
W. Wu et al. / Journal of Organometallic Chemistry 713 (2012) 189e196
the
p
-core of organic chromophore to the metal centre usually gives
carboxylic moiety (L-1, Scheme 1). 8-Bromobenzenealdehyde and
m-dihydroxylbenzene were used as the starting materials. Acid
catalyzed condensation of the two compounds produces the
intermediate 9-(4-bormo)-6-hydroxy-3-fluorone. Then the acetylide
bond was introduced by the Pd(0) catalyzed Sonogashira coupling
reaction. We found that protection of the free hydroxyl moiety is
necessary, otherwise the later metallation of the acetylide ligands
will become troublesome. Thus the butylether L-1 was prepared
and was used for preparation of the complex Pt-1. Pt-1 was
obtained in satisfying yield.
RT phosphorescence of the organic chromophore [17,33].
Conversely, for those complexes with the organic chromophore
dangled at the peripheral moiety (not the p-core) of the coordina-
tion centre, no RT phosphorescence of the ligand was observed,
instead, the fluorescence of ligand was observed [20]. However, we
also proved that evenwithout the RT phosphorescence, the transient
metal complexes are still useful for applications such as TTA based
upconversions, given that the non-emissive intraligand triplet
excited state (3IL) was populated upon photoexcitation of the
complex [20]. The lifetime of the triplet excited state of these organic
chromophore-containing Pt(II) complexes are usually long. For
2.2. Steady state photophysical properties
example, T1 state lifetime up to 83.0
ms was observed with rhoda-
mine attached to the Pt(II) centre [20].
The UVevis absorption of the complexes were studied (Fig. 1).
The model complex Pt-2 gives weak absorption in the visible region,
ε ¼ 8800 Mꢀ1 cmꢀ1 at 424 nm. In comparison, Pt-1 shows much
stronger absorption, with ε ¼ 24,600 Mꢀ1 cmꢀ1 at 457 nm. Enhanced
absorption in the visible region is beneficial for applications such as
photocatalysis, photovoltaics, etc. Furthermore, we found that
UVevis absorption of Pt-1 is roughly the sum of the ligand L-1 and
the model complex Pt-2, thus we propose that there is no strong
electronic communication between the Pt(II) centre and the fluo-
rescein chromophore core in Pt-1. This observation is weaker than
the rhodamine-containing Pt(II) acetylide complex [20].
The emission of the ligands and the complexes were also
studied (Fig. 2). Pt-1 gives slightly red-shifted emission than the
model complex Pt-2 and the ligands (with toluene as solvent). All
the emission bands are broad and structureless. We noted that the
emission of Pt-1 in ethanol is blue-shifted compared to that in
toluene (see Supplementary data). The emission of Pt-1 in ethanol
is close to the emission of the ligand L-1.
In order to study the RT triplet excited state of fluorescein,
herein we devised a fluorescein-containing Pt(II) acetylide complex
Pt-1 (Scheme 1). Model complex Pt-2 with phenylacetylide ligands
was used in the photophysical studies. Pt-1 shows intense
absorption of visible light (ε ¼ 24,600 Mꢀ1 cmꢀ1 at 457 nm).
Interestingly, only fluorescence of the fluorescein ligand was
observed for Pt-1 at RT. Nanosecond time-resolved transient
absorption spectra and spin density analysis proved that the 3IL
state localized on the fluorescein moiety was populated. Phos-
phorescence of the fluorescein ligand was observed for Pt-1 at 77 K.
Pt-1 was used as efficient triplet sensitizer for TTA upconversion
and upconversion quantum yield of 10.7% was observed.
2. Results and discussion
2.1. Design and synthesis of the complexes
In order to exclude the undesired effect of the lactum/lactone
structure of the fluorescein on the photophysics of the complex Pt-
1 (Scheme 1) [3,4], we used an acetylide ligand that without the
The luminescence lifetimes
(sL) of the compounds were
measured. Pt-1 gives L value of 0.45 ns, which is much shorter than
s
that of Pt-2 (1.36
m
s). Pt-2 is known to show phosphorescence at RT.
Br
Si
Br
c
b
HO
OH
a
+
d
CHO
HO
O
O
Br
HO
O
L-2
O
HO
O
O
O
e
O
O
O
Cl
Cl
N
N
Pt
N
O
O
L-1
O
Pt
N
O
Pt-1
O
2+
2PF6
-
N
N
N
N
N
Pt
N
N
Ru
N
Pt-2
DPA
[Ru(dmb)3] [PF6]2
Scheme 1. Synthesis and molecular structures of L-1 and Pt-1. The triplet acceptor 9,10-diphenylanthracene (DPA) and the model complex [Ru(dmb)3][PF6]2 were also shown. (a)
Methanesulfonic acid, 80 ꢁC, 24 h. (b) (Trimethylsilyl) acetylene, CuI, PdCl2(PPh3)2, PPh3, Et3N/THF, Ar, 70 ꢁC, 8 h. (c) K2CO3, MeOH/CH2Cl2, r.t. 3 h. (d) K2CO3, DMF, Ar, 60 ꢁC, 8 h. (e)
Diethylamine/CH2Cl2, CuI, Ar, r.t., 3 h.

W. Wu et al. / Journal of Organometallic Chemistry 713 (2012) 189e196
191
250
200
150
100
50
300
a
b
in N2
in air
in O2
in N2
200
in air
in O2
100
0
0
500
600
700
800
500
600
700
Wavelength / nm
Wavelength / nm
Fig. 3. Emission spectra of (a) Pt-1 in EtOH and (b) Pt-2 in toluene saturated with air,
O2 and N2. c ¼ 1.0 ꢂ 10ꢀ5 M, 25 ꢁC.
1 at 77 K shows new bands at 518 nm and 667 nm. This emission
band at 667 nm was assigned to the phosphorescence of the fluo-
rescein ligand, which is supported by the calculated T1 state energy
level of the ligand (1.67 eV, 742 nm). Observation of the phospho-
rescence at 77 K is reasonable since most of the non-radiative decay
channel of the 3IL excited state will be inhibited at 77 K, in solid
matrix [34]. Previously rhodamine acetylide was attached to the
Pt(II) centre, but the phosphorescence of rhodamine was not
observed at 77 K [20].
The emission of Pt-2 at 77 K was also measured (Fig. 4b).
Compared to the RT emission, the emission at 77 K was blue-shifted
to 499 nm and the emission band becomes more structured. The
thermally induce Stokes shift is 2887 cmꢀ1. This large value is an
indication of 3MLCT emission for Pt-2 [15e17,19,23,35].
It should be pointed out that the coordination centre of Pt-1 is
similar to that of Pt-2. In Pt-1 the fluorescein core is electronically
isolated from the Pt(II) centre by the phenyl moieties, which is
confirmed by the UVevis absorption of the complex (Fig. 1).
However, the emission of Pt-1 at 77 K (667 nm and 732 nm) is
drastically different from that of Pt-2, thus, this result indicates that
the near-IR emission of Pt-1 is due to the 3IL state (localized on the
fluorescein moiety), not the 3MLCT state. Some organic
chromophore-containing complexes have been reported
Fig. 1. UVevis absorption of Pt-1, Pt-2, L-1 and L-2, c ¼ 1.0 ꢂ 10ꢀ5 M in toluene, 20 ꢁC.
We tentatively attribute the emission of Pt-1 to the fluorescence of
the ligand, based on the emission wavelength and the lifetime.
The luminescence lifetime of Pt-1 is shorter than the ligand L-1
(s
¼ 1.87 ns). This is due to the intramolecular energy transfer of
1IL / 1MLCT state, or the intersystem crossing (ISC) 1IL / 3IL.
However, these spectroscopic data are insufficient to be used for
elucidation of the feature of emissive excited state of Pt-1 as fluo-
rescence or phosphorescence.
In order to assign the emission of Pt-1, the emission under
different atmosphere of N2, air and O2 were studied (Fig. 3a). We
found that the emission intensity is independent on O2, i.e. the
emission was not quenched by O2. This is an unambiguous indi-
cation of fluorescence for the emission of Pt-1 [21]. By comparison
with the emission of the ligands, the RT emission of Pt-1 (Fig. 2) can
be assigned as fluorescence of the fluorescein ligand. On the
contrary, the emission of Pt-2 is sensitive to O2, i.e. the emission
was remarkably quenched by O2, which is an indication of phos-
phorescence [21].
[8b,20,28], with the
p-core of the chromophore not be directly
2.3. 77 K Emission spectra
connected to the metal centre. These complexes usually do not
show the RT phosphorescence of the organic chromophore, due to
the relatively weak ISC effect [20,36,37] Table 1.
In order to study the possible intraligand 3IL phosphorescence of
the fluorescein, the emission spectra of Pt-1 and Pt-2 at 77 K were
studied (Fig. 4). Compared to the emission at RT, the emission of Pt-
2.4. Spin density analysis of the complex: assignment of the 3IL
excited state
1.0
Our anticipation of the population of the fluorescein-localized
3IL excited state for Pt-1 was supported by spin density analysis
of the triplet state of the complex (Fig. 5) [14e20,38]. For the model
Pt-1
0.8
0.6
0.4
0.2
0.0
L-1
Pt-2
1.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
77 K
RT
77 K
RT
a
b
1.0
0.8
0.6
0.4
0.2
0.0
L-2
500
600
700
800
500 600 700 800
Wavelength / nm
500
600
700
800
Wavelength / nm
Wavelength / nm
Fig. 2. The normalized emission of Pt-1, Pt-2, L-1 and L-2. c ¼ 1.0 ꢂ 10ꢀ5 M in toluene,
Fig. 4. Photoluminescence spectra of (a) Pt-1 (lex ¼ 442 nm), (b) Pt-2 (lex ¼ 442 nm)
20 ꢁC.
at RT and 77 K (EtOH:MeOH, 4:1, v/v).

192
W. Wu et al. / Journal of Organometallic Chemistry 713 (2012) 189e196
Table 1
The calculations predict transient absorptions at 500e700 nm
(Fig. 6b), which is in good agreement with the experimental
results (Fig. 6a). Transient absorption at 530 nm, 620 nm and
660 nm were observed. These transient absorptions can be fully
rationalized by TDDFT calculations on the transitions between the
triplet excited states, such as T1 / T2, etc. The calculations predict
absorption bands of the T1 state for Pt-1 at 468 nm (f ¼ 0.2941,
T1 / T24), 530 nm (f ¼ 0.0552, T1 / T18), 629 nm (f ¼ 0.0473,
T1 / T12) and 662 nm (f ¼ 0.0332, T1 / T10). The intensive
absorption of the T1 state at 468 nm may be responsible for the
decreased bleaching of ground state at ca. 460 nm.
Photophysical parameters of Pt(II) complexes and the ligands.
labs (nm)a
lem (nm) FF
sc
kr/sꢀ1d
knr/sꢀ1e
b
L-1
L-2
440 (1.00), 534
460(1.14)
437 (1.00), 532
353 (1.07)
0.86% 1.87 ns 4.60 ꢂ 106 5.30 ꢂ 108
1.14% 1.25 ns 9.12 ꢂ 108 6.33 ꢂ 108
0.66% 0.45 ns 1.47 ꢂ 107 2.21 ꢂ 109
Pt-1 435 (2.48), 570
457 (2.46)
Pt-2 424 (0.88)
567
42.2%
1.27
ms
3.32 ꢂ 105 4.55 ꢂ 105
Extinction coefficients (104 Mꢀ1 cmꢀ1) are shown in parentheses.
Fluorescence quantum yield with quinine sulphate as the standard (FF ¼ 0.547
a
b
in 0.05 M sulphuric acid) in toluene.
Thus the transient difference absorption spectra supports the
assignment of the triplet excited state of Pt-1 as fluorescein-
localized 3IL excited state [20]. By following the decay of the tran-
c
Fluorescence lifetimes. Measured with luminescence method.
Radiative deactivation rate constant (kr).
Non-radiative deactivation rate constant (knr). kr
d
e
¼
Fem
/s
em; knr ¼ (1/sem
)
sients at 530 nm, a lifetime of 16.4
state lifetime is shorter than the rhodamine acetylide-containing
Pt(II) complex (83.0 s) [20]. However, the triplet excited state
lifetime of Pt-1 is still much longer than the usual N^N Pt(II)
bisacetylide complexes, usually shorter than 5 s [23]. The T1 state
lifetime of Pt-1 is longer than the PDI acetylide containing Pt(II)
complex (0.246 s) [28]. Transient metal complexes with long-lived
ms was observed. This T1 excited
(1ꢀFem).
m
complex Pt-2, it is known that the lowest-lying triplet excited state
is in 3MLCT/3LLCT (acetylide / dbbpy) mixed feature [23]. The
phenylacetylide, Pt(II) centre and the dbbpy moieties are all
involved in the T1 excited state of Pt-2. The spin density surface of
Pt-2 is distributed over the entire molecular framework, which is in
full agreement with the 3MLCT/3LLCT excited state of Pt-2 (Fig. 5).
For Pt-1, however, the spin density surface of the triplet state is
located on the fluorescein moiety, the contribution of the Pt(II)
atom is very small and the dbbpy ligand does not contribute to the
spin density (Fig. 5). Thus, the lowest-lying triplet excited state of
Pt-1 can be described as a fluorescein-localized 3IL excited state
[14e20,38]. Furthermore, the minor involvement of the Pt(II) in the
3IL state may be responsible for the lack of phosphorescence of Pt-1
at RT, due to the weak heavy atom effect. Recently we observed
similar results with the Pt(II) complexes that contain rhodamine
moiety [20].
m
m
T1 excited states are beneficial for applications of these complexes
in luminescent O2 sensing [15,39e42], photocatalysis [43], and
more recently TTA upconversions [8,13,15,19,20].
2.6. Energy level diagram of Pt-1
The photophysics of Pt-1 can be summarized in Scheme 2. With
excitation into the 1IL state of Pt-1, i.e. the singlet excited state
localized on the fluorescein ligand, the non-efficient ISC to 3IL will
lead to fluorescence of the ligand (1IL* / S0) (Fig. 2). 3IL is non-
emissive due to the minor involvement of the Pt(II) centre,
demonstrated by the molecular structure (the p-conjugation core is
not directly attached to the Pt(II) centre) and the spin density
analysis (Fig. 5). However, the long-lived 3IL* excited state can be
detected with the time-resolved transient difference absorption
spectroscopy (Fig. 6).
2.5. Nanosecond time-resolved transient difference absorption
spectra
In order to prove the fluorescein-localized 3IL excited state for
Pt-1, the nanosecond time-resolved transient difference absorption
spectra of Pt-1 were studied (Fig. 6). Significant bleaching was
found in the region 400e500 nm upon pulsed laser excitation. This
bleaching is due to the depletion of the ground state of the fluo-
rescein ligand, which shows strong steady state absorption at
450 nm (Fig. 1). Furthermore, transient absorption in the region of
500e800 nm was observed for Pt-1 upon pulsed laser excitation.
These features are drastically different from the time-resolved
transient difference absorption spectra of Pt-2 [37].
The Pt-1 can also be excited with excitation into the MLCT band
of the UVevis absorption 380e450 nm (Fig. 1). In this case a fast ISC
will lead to the efficient population of the 3MLCT excited state, very
often the efficiency is believed to be close to unity [8], although an
energy transfer to the 1IL can’t be completely excluded. However, in
both pathways the 3IL* excited state will be populated, for example,
via internal conversion (IC) 3MLCT / 3IL, because the energy level
of the 3IL state is lower than the 3MLCT state. Thus the typical
phosphorescence of the Pt(II) bisacetylide coordination centre at
499 nm was completely quenched in Pt-1.
We believe that Pt-1 can be used for sensitizing some photo-
physical processes even it is non-phosphorescent [12,18,20,44].
One of such applications is TTA based upconversion [7,8].
2.7. Application of the intense visible light absorption and long-
lived T1 excited state of Pt-1 for TTA upconversion
Upconversion is important for photovoltaics, bioimaging and
optics, etc [45,46]. Recently a new upconversion scheme has been
developed, i.e. the TTA upconversion, which shows various
advantages compared to the other upconversion techniques, such
as two-photon absorption fluorescent dyes [25], or upconversion
with rare earth metal materials [46], inorganic crystals (KDP crys-
tals, etc) [47]. These tradition upconversions suffer from some
disadvantages, such as requirement of coherent excitation light
with high excitation power density (MW cmꢀ2, thus only pulsed
laser is sufficient), low upconversion quantum yield, weak
absorption of the excitation light, etc. With the TTA upconversion,
Fig. 5. Spin density surfaces of Pt-1 and Pt-2. Solvent EtOH was considered in the
calculation (PCM model). The calculations are based at the optimized triplet state
geometry (isovalue: ꢃ0.0004). Calculated at B3LYP/6-31G/LANL2DZ level with
Gaussian 09 W.

W. Wu et al. / Journal of Organometallic Chemistry 713 (2012) 189e196
193
Fig. 6. (a) Transient absorption difference spectra of Pt-1 in deaerated toluene after pulsed excitation (lex ¼ 355 nm). Inset: decay trace of Pt-1 at 530 nm c[Pt-1] ¼ 2.0 ꢂ 10ꢀ5 M in
toluene. 25 ꢁC. (b) The calculated transient absorption spectra of the T1 state of Pt-1. Please note that the information of bleaching is not included in the calculated transient
absorption spectra. Based on the optimized triplet state geometry. EtOH was used as solvent in the calculations (PCM model). Calculated at B3LYP/6-31G(d)/LANL2DZ level with
Gaussian 09 W.
however, all these disadvantages can be addressed [7ꢀ10]. For
example, the TTA upconversion shows intense absorption of the
excitation light, non-coherent excitation light with low excitation
power density is sufficient for the upconversion, and the upcon-
version quantum yield is high [7,8,9,48].
quantum yield of Pt-1 is comparable to that with the rhodamine
acetylide Pt(II) complex (11.2%) [20]. For the visible-light harvesting
PDI-acetylide complex, however, we propose the TTA upconversion
will be not significant, due to the short T1 state lifetime of that
complex [28].
TTA upconversion requires triplet sensitizer and triplet acceptor,
and the compounds are mixed together for the upconversion [7,8].
With selective excitation of the triplet sensitizer, the energy is
transferred to the triplet acceptor via tripletetriplet-energy-
transfer (TTET), then two acceptor molecules at the triplet state
will annihilate to produce an acceptor molecule at the singlet
excited state. According to the spin statistical rule, the probability of
producing of the singlet excited states in the annihilation product is
11.1% [7,8,49].
DPA was selected as the triplet acceptor for the TTA upconver-
sion [7,8]. Pt-2 was used as the model complex. Upon 473 nm CW
laser irradiation, the sensitizers Pt-1 or Pt-2 give emission bands at
580 nm, and the emission of Pt-2 is stronger than that of Pt-1
(Fig. 7). Note the emission of Pt-2 is phosphorescence but the
emission of Pt-1 is fluorescence. Pt-1 is non-phosphorescent at RT.
In the presence of DPA, the phosphorescence of Pt-2 was quenched
and weak upconverted emission at 415 nm was observed. For Pt-1,
much more intense upconverted fluorescence of DPA was observed.
It was known that DPA can not be excited with 473 nm laser, thus
the emission for the Pt-1/DPA mixtures in the range of
400e500 nm is upconverted fluorescence of DPA. The upconver-
sion quantum yield of the complexes Pt-1 and Pt-2 were deter-
mined as 10.7% and 4.4%, respectively. The TTA upconversion
Recently we propose that the upconversion capability of TTA
upconversion can be described by
extinction coefficient of the sensitizer at the excitation wavelength
h
¼ ε ꢂ FUC, where ε is the molar
and FUC is the upconversion quantum yield [8a]. For Pt-1,
h
¼ 2018
M
ꢀ1 cmꢀ1, but for Pt-2,
h
¼ 180 Mꢀ1 cmꢀ1. Thus the upconversion
capability of Pt-1 is ca. 11-fold of that of Pt-2.
The upconversion with Pt-1 as the triplet sensitizer is demon-
strated by the visibility of the upconversion with un-aided eye
(Fig. 8a). Blue emission was observed for the upconversion with Pt-
1. For Pt-2, however, much weaker upconverted blue emission was
observed. The emission colour changes of the solution were char-
acterized by the CIE colour coordinates (Fig. 8b). The CIE coordi-
nates of the emission of Pt-1 and Pt-2 alone are (0.43, 0.42) and
(0.45, 0.46), respectively. In the presence of DPA, the CIE coordi-
nates changed to (0.21, 0.13) and (0.43, 0.42), respectively. Thus the
upconversion with Pt-1 is more significant than that with Pt-2, and
h
value is a better criteria than FUC value for the evaluation of the
upconversion capability of a sensitizer, especially for applications
such as photovoltaics and photocatalysis, etc.
600
Pt-2
Pt-1+DPA
450
300
150
0
Pt-2+DPA
Pt-1
*
400
500
600
700
Wavelength / nm
Scheme 2. Quantitative Jablonski diagram for the photophysics of Pt-1. The excitation
and the radiative decay from the excited state to the ground state are depicted by solid
lines; whereas the non-radiative processes are illustrated by dotted lines. The energy
levels of the 3MLCT* state and the 3IL* state are approximated as the emission of Pt-2
and Pt-1 at 77 K, respectively.
Fig. 7. Upconversion with Pt-1 and Pt-2 (1.0 ꢂ 10ꢀ5 M) as the triplet sensitizers. DPA
(2.0 ꢂ 10ꢀ4 M) was used as the triplet acceptor. The emission of the sensitizers alone
were also presented. Excited with 473 nm laser (5 mW). In toluene, 25 ꢁC. The asterisks
indicate the scattered 473 nm laser.

194
W. Wu et al. / Journal of Organometallic Chemistry 713 (2012) 189e196
Fig. 8. (a) Photographs of upconversions with Pt-1 and Pt-2 as the triplet sensitizers and 9,10-diphenylanthracene (DPA) as the triplet acceptor. Photographs of the phospho-
rescence and the upconversion of the complexes. Excitation by blue laser (lex ¼ 473 nm, 5 mW). 20 ꢁC. (b) The CIE coordinate changes of triplet sensitizers before and after adding
9,10-diphenylanthracene (DPA). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The process of TTA upconversion can be summarized in Scheme
3 [7,8]. Based on the Jablonski diagram, the TTET process is
important for the TTA upconversion. Accordingly, two parameters
of the triplet sensitizer are crucial for the TTA upconversion. One is
the absorption of the excitation light and another is the lifetime of
the T1 excited state of the triplet sensitizer. Either high concentra-
tion of the triplet sensitizer (as a result of the intense absorption of
the excitation light by the sensitizer) at the T1 excited state or the
long-lived T1 excited state will enhance the TTET process, with
which the TTA upconversion can be improved. The intense
absorption of visible light and the long-lived T1 excited state of Pt-1
make it an ideal candidate as the triplet sensitizer for TTA upcon-
version. Furthermore, it should be pointed out that the triplet state
involved in the TTET process is non-emissive. Most of the triplet
sensitizers for TTA upconversion are phosphorescent [7,8], except
very few examples [12,14,18]. Application of non-phosphorescent
transition metal complexes as triplet sensitizers for TTA upcon-
version will greatly increase the availability of the triplet sensitizers
for TTA upconversion (please note that the triplet excited state of
the sensitizer must be populated upon photoexcitation), and other
appropriate photophysical processes.
3. Conclusions
Fluorescein-containing N^N Pt(II) bisacetylide complex (Pt-1)
was prepared (where N^N
¼
3,30-ditertbutyl-2,2-bipyridine,
dbbpy). The fluorescein moiety was connected to the Pt(II) centre
via acetylide bond at the 4-position of the phenyl group of the
fluorescein. Pt-1 shows strong absorption of visible light
(ε ¼ 24,600 Mꢀ1 cmꢀ1 at 457 nm). Fluorescence of the ligand was
observed for Pt-1 at room temperature (
s
¼ 0.45 ns). At 77 K,
a phosphorescence band at 667 nm was observed. Spin density
analysis of the triplet state and the nanosecond time-resolved
transient difference absorption spectroscopy indicated that the
lowest-lying triplet excited state of the complex is the fluorescein-
localized intraligand triplet excited (3IL state), for which the life-
time was determined as 16.4
transient difference absorption spectroscopy. This lifetime is
much longer than the lifetime of the T1 excited state of ordinary
Pt(II) bisacetylide complexes (less than 5 ms). The lack of the normal
MLCT phosphorescence of Pt-1 is attributed to the internal
conversion from the 3MLCT excited state to the 3IL excited state, due
to the much lower energy level of the 3IL state than the 3MLCT state.
The complex Pt-1 was used as the triplet sensitizer for
tripletetriplet annihilation upconversion, for which the quantum
yield is 10.7%. For the model complex (Pt-2) without the fluorescein
moiety, the upconversion quantum yield is 4.4%. The overall
ms by nanosecond time-resolved
upconversion performance of Pt-1 (h) is 11-fold of Pt-2. We proved
that the complex with intense absorption of visible light and long-
lived triplet excited state can be accessed with the Pt(II) complex in
which an organic chromophore is attached to the transition metal
atom. These transition metal complexes are beneficial for the
applications in the tripletetriplet-energy-transfer, which is crucial
for a variety of photophysical processes, such as photodynamic
therapy (PDT) or TTA upconversions, etc.
4. Experimental section
Scheme 3. Qualitative Jablonski Diagram illustrating the sensitized TTA up conversion
process between Pt(II) complexes (exemplified by Pt-1) and DPA. The effect of the
light-harvesting ability and the luminescence lifetime of the Pt(II) sensitizer on the
efficiency of the TTA upconversion is also shown. E is energy. GS is ground state (S0).
1IL* is intraligand singlet excited state (fluorescein localized). IC is internal conversion.
ISC is intersystem crossing. 3MLCT* is the Pt(II) based metal-to-ligand-charge-transfer
triplet excited state. 3IL* is intraligand triplet excited state (fluorescein localized). TTET
is tripletetriplet energy transfer. 3DPA* is the triplet excited state of DPA. TTA is tri-
pletetriplet annihilation. 1DPA* is the singlet excited state of DPA. The emission bands
observed for the sensitizers alone is the 1IL* emissive excited state. The emission bands
observed in the TTA upconversion experiment is the simultaneous 1IL* emission
(fluorescence) and the 1DPA* emission (fluorescence).
4.1. General information
NMR spectra were recorded on a 400 MHz Varian Unity Inova
NMR spectrophotometer. Mass spectra were recorded with Q-TOF
Micro MS spectrometer. UVevis absorption spectra were measured
with
a HP8453 UVevisible spectrophotometer. Fluorescence
spectra were recorded on a Shimadzu RF5301PC spectrofluorom-
eter or a Sanco 970 CRT spectrofluorometer. Fluorescence quantum
yields were measured with reference quinine sulphate (
F
¼ 54.7%

W. Wu et al. / Journal of Organometallic Chemistry 713 (2012) 189e196
195
in 0.05 M sulphuric acid). Luminescence lifetimes were measured
on an OB920 phosphorescence/fluorescence lifetime spectrometer
(Time resolution is 100 ps. Edinburgh Instruments, U.K.).
mixture was purged with Ar for 20 min, then CuI (2.5 mg,
0.05 mmol) was added under Ar, and the mixture was stirred at
room temperature for 3 h. The reaction mixture was purified by
column chromatography (silica gel, CH2Cl2/CH3OH ¼ 30:1, v/v). A
red solid was obtained (22.1 mg), yield 92.2%. 1H NMR (400 MHz,
4.2. Synthesis
CDCl3):
d
9.71 (d, J ¼ 8.0 Hz, 2H), 8.01 (s, 2H), 7.72 (d, J ¼ 8.0 Hz,
4.2.1. 9-(4-Bormophenyl)-6-hydroxy-3-fluorone
4H), 7.65 (d, J ¼ 4.0 Hz, 2H), 7.23e7.29 (m, 8H), 6.93 (s, 2H), 6.78
(d, J ¼ 12.0 Hz, 2H), 6.61 (d, J ¼ 8.0 Hz, 2H), 6.47 (s, 2H), 4.09
(t, J ¼ 8.0 Hz, 4H), 1.81e1.85 (m, 4H), 1.45e1.57 (m, 22H), 1.00
4-Bromobenzaldehyde (1.85 g, 10 mmol) and m-dihydrox-
ybenzene (2.20 g, 20 mmol) was dissolved in methanesulfonic acid
(25 mL). The solution was stirred at 80 ꢁC for 24 h. After cooled to
r.t., the reaction mixture was poured into 200 mL NaOAC solution
(3 M). Black precipitate was collected and washed with water
(3 ꢂ 50 mL). After drying in vacuum, the precipitate was purified by
column chromatography (silica gel, CH2Cl2/MeOH ¼ 10:1, v/v) for
twice. 0.53 g dark red solid was obtained. Yield: 14.4%. 1H NMR
(t, J ¼ 8.0 Hz, 6H). 13C NMR (100 M Hz, CDCl3)
d 163.8, 156.1, 154.7,
151.1, 132.1, 131.0, 129.9, 129.5, 129.0, 118.8, 117.6, 114.3, 113.6,
105.6, 100.6, 68.6, 35.8, 30.9, 30.2, 19.1, 13.7. MALDI-HRMS:
calcd for C68H63N2O6Pt ([M þ H]þ) m/z ¼ 1198.4334, found
m/z ¼ 1198.4250.
(400 MHz, CDCl3/CD3OD):
d
7.77 (d, J ¼ 8.8 Hz, 2H), 7.29 (d,
4.3. Nanosecond time-resolved transient difference absorption
spectroscopy
J ¼ 8.0 Hz, 2H), 7.22 (d, J ¼ 9.2 Hz, 2H), 6.78e7.76 (m, 4H). TOF MS
EI: calcd for C19H11O3Br [M]þ m/z
z ¼ 365.9904.
¼
365.9892, found m/
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.
4.2.2. L-2
(Trimethylsilyl) acetylene (0.33 mL, 2.32 mmol) and CuI (5.7 mg,
0.03 mmol) were added to degassed solution of 9-(4-
a
bromophenyl)-6-hydroxy-3-fluorone (0.35 g, 1.00 mmol),
PdCl2(PPh3)2 (21.2 mg, 0.03 mmol), PPh3 (15.7 mg, 0.06 mmol) in
triethylamine (15 mL) and THF (20 mL). The reaction mixture was
heated at 70 ꢁC for 8 h. Then the mixture was evaporated to dryness
and the crude product was purified by column chromatography
(silica gel, CH2Cl2/MeOH ¼ 10:1, v/v) twice. A red solid was
obtained (the trimethylsilyl protected acetylide). K2CO3 (414.0 mg,
3.00 mmol) was added to a methanol (5 mL) and CH2Cl2 solution
(5 mL) of the red solid obtained above and the mixture was stirred
at room temperature for 3 h. The solvent was evaporated and water
(10 mL) was added. Then the mixture was extracted with CH2Cl2.
The organic layer was dried and then the crude product was puri-
fied by column chromatography (silica gel, CH2Cl2/MeOH ¼ 10:1, v/
v) twice. A red solid was obtained; yield 170.0 mg, 0.54 mmol,
4.4. Computational methods
The geometry optimizations were calculated using B3LYP
functional at 6-31G(d)/LanL2DZ level. The energy of the triplet
excited state of the ligand was calculated with the time-dependent
DFT (TDDFT) method based on the optimized singlet ground state
geometry. The spin-density was calculated with the energy mini-
mized triplet state geometries. All calculations were performed
using Gaussian 09 W (Gaussian, Inc.) [50].
54.5%. 1H NMR (400 MHz, CDCl3/CD3OD):
d
7.74 (d, J ¼ 7.2 Hz, 2H),
7.39 (d, J ¼ 7.2 Hz, 2H), 7.27 (d, J ¼ 8.8 Hz, 2H), 6.81e6.90 (m, 4H),
3.41 (s,1H). 13C NMR (100 M Hz, CDCl3 þ CD3OD) 158.0,155.0,132.3,
131.4,129.4, 124.3, 121.5, 115.1, 103.5, 82.2, 79.7. TOF MS ES: calcd for
C21H11O3 ([M ꢀ H]ꢀ) m/z ¼ 311.0708, found m/z ¼ 310.9245.
4.5. Upconversions
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
4.2.3. L-1
L-2 (150.0 mg, 0.48 mmol) and K2CO3 (198.9 mg, 1.44 mmol)
was dissolved in 2 mL DMF under nitrogen and heated to 60 ꢁC.
Then, 1-bromobutane (198.0 mg, 1.44 mmol) was added to the
mixture dropwise, and the mixture was heated at 60 ꢁC for 8 h. The
reaction mixture was poured into ice water (100 mL). Red precip-
itate was collected and washed with water (3 ꢂ 50 mL), After drying
under vacuum, the precipitate was purified by column chroma-
tography (silica gel, CH2Cl2/MeOH ¼ 30:1, v/v) twice. A dark red
solid was obtained; yield: 60.0 mg (0.16 mmol), 33.3%. 1H NMR
the quantum yield standard (
F
¼ 0.074 in MeCN) and the quantum
yields were calculated with Eq. (1), where Uunk, Aunk, Iunk and hunk
represent the quantum yield, absorbance, integrated photo-
luminescence Intensity, and the refractive index of the solvents,
respectively. The photography of the upconversion were taken with
Samsung NV5 digital camera. The exposure times are the default
values of the camera.
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Astd
Aunk
Iunk
Istd
hunk
hstd
(400 MHz, CDCl3):
d
7.69 (d, J ¼ 8.0 Hz, 2H), 7.32 (d, J ¼ 8.0 Hz, 2H),
VUC ¼ 2Vstd
(1)
7.06e7.12 (m, 2H), 6.93 (s, 1H), 6.76 (d, J ¼ 12.0 Hz, 1H), 6.58 (d,
J ¼ 8.0 Hz, 1H), 6.46 (s, 1H), 4.09 (t, J ¼ 8.0 Hz, 2H), 3.22 (s, 1H),
1.81e1.84 (m, 2H), 1.49e1.54 (2H), 1.00 (t, J ¼ 8.0 Hz, 3H). 13C NMR
(100 MHz, CDCl3)
d 185.5, 164.0, 158.8, 154.7, 148.4, 133.4, 132.4,
130.5, 130.1, 129.5, 129.4, 123.6, 117.9, 114.0, 113.8, 106.0, 100.8, 82.6,
79.1, 68.7, 30.9, 19.1, 13.7. TOF MS ES: calcd for C25H20O3Na
([M þ Na]þ) m/z ¼ 391.1310, found m/z ¼ 391.1887.
Acknowledgements
We thank the NSFC (20972024 and 21073028), the Fundamental
Research Funds for the Central Universities (DUT10ZD212),
Ministry of Education (SRFDP-200801410004 and NCET-08-0077),
the Royal Society (UK) and NSFC (China-UK Cost-Share Science
Networks, 21011130154) for financial support.
4.2.4. Pt-1
Pt(dbbpy)Cl2 (12.8 mg, 0.02 mmol), L-2 (22.0 mg, 0.06 mmol),
and diethylamine (0.2 mL) were dissolved in CH2Cl2 (3 mL), The

196
W. Wu et al. / Journal of Organometallic Chemistry 713 (2012) 189e196
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