Room-temperature long-lived 3IL excited state of rhodamine in an NN PtII bis(acetylide) complex with intense visible-light absorption

Article abstract of DOI:10.1002/ejic.201100777

An NN Pt^II bis(acetylide) complex was prepared with the rhodamine fluorophore [Pt-Rho, in which NN is dbbpy = 4,4′-bis(tert-butyl- 2,2′-bipyridine)]. Complex Pt-Rho shows intense absorption in the visible region (Iμ = 185800 M^-1cm

Full text of DOI:10.1002/ejic.201100777

FULL PAPER
DOI: 10.1002/ejic.201100777
3
∧
Room-Temperature Long-Lived IL Excited State of Rhodamine in an N N
PtII Bis(acetylide) Complex with Intense Visible-Light Absorption
Ling Huang,[a] Le Zeng,[a] Huimin Guo,[a] Wanhua Wu,[a] Wenting Wu,[a] Shaomin Ji,[a] and
Jianzhang Zhao*[a]
Keywords: Photochemistry / Rhodamine / Platinum / Triplet–triplet annihilation / Upconversion / Chromophores
∧
An N N PtII bis(acetylide) complex was prepared with the
parison, the model complex (dbbpy)PtII bis(phenylacetylide)
(Pt–Ph) shows weak absorption in the visible region (ε =
14700 M
–1 cm–1 at 424 nm) and a short-lived metal-to-ligand
∧
rhodamine fluorophore [Pt–Rho, in which N N is dbbpy =
4,4Ј-bis(tert-butyl-2,2Ј-bipyridine)]. Complex Pt–Rho shows
intense absorption in the visible region (ε = 185800
at 556 nm) and a fluorescence emission of the ligand (λem
580 nm, ΦL = 41.1%, τL = 2.50 ns). The long-lived rhodamine-
localised intraligand triplet excited state (3IL, τT = 83.0 μs)
was proposed to be populated upon excitation of the rhoda-
mine ligand, proved by nanosecond time-resolved transient
absorption and spin density analysis; the triplet excited state
was studied by time-dependent DFT calculations. In com-
M
–1 cm–1
charge-transfer excited state (3MLCT) (τT = 1.36 μs). Com-
plex Pt–Rho was used as triplet sensitiser for upconversion
based on triplet–triplet annihilation. An upconversion quan-
tum yield of 11.2% was observed. Our strategy to access the
long-lived 3IL excited state of the organic chromophore by
metallation with PtII will be useful for the preparation of tran-
sition metal complexes that show intense absorption of vis-
ible light and long-lived triplet excited states.
=
properties; tunability can be achieved simply by changing
the acetylide ligands attached to the PtII centre, which is
feasible from a synthetic perspective.[18,22]
Introduction
The population of long-lived triplet excited states upon
visible-light excitation is significant due to their potential
applications in areas such as photovoltaics,[1] photoinduced
charge separation,[2,3] photocatalysis,[4] optical limiting[5,6]
and molecular probes.[7] For example, it has been demon-
strated that a long-lived triplet excited state can improve the
oxygen-sensing properties of RuII and PtII complexes.[8–11]
Recently, we proved that upconversion based on triplet–
triplet annihilation (TTA) can be significantly enhanced by
sensitisers with long-lived triplet excited states.[12–15]
The heavy-atom effect or, more generally, spin-orbital
coupling is required for population of the triplet excited
state of a chromophore, through the S0ǞS1ǞT1 process.
Transition-metal complexes are representative chromo-
phores for which the triplet excited state can be populated
upon photoexcitation, such as in RuII–diimine com-
plexes,[16–18] PtII/PdII–porphyrin complexes[19] and more re-
cently the PtII/IrIII complexes.[1,20–23] Concerning this as-
pect, the (diimine)PtII bis(acetylide) complexes are particu-
larly interesting due to their readily tuneable photophysical
The typical PtII acetylide complexes, such as (dbbpy)PtII
bis(phenylacetylide) (Pt–Ph; dbbpy = 4,4Ј-bis(tert-butyl)-
2,2Ј-bipyridine; Scheme 1), however, show very poor ab-
sorption in the visible region (λmax = 398 nm with ε =
9300 m–1 cm–1), and the lifetime of the T1 excited state is
short (τ = l.36 μs).[22] On the other hand, to establish an
intraligand triplet excited state (3IL) is fascinating, because
3
the IL state usually shows a prolonged lifetime compared
with the 3MLCT state (metal-to-ligand charge trans-
fer).[16,17,24,25] We noticed that some attempts have been
made to prepare light-harvesting molecular arrays by at-
taching organic chromophores, such as benzyl, naphthyl,
pyrenyl, perylene or boron-dipyrromethene (BODIPY)
moieties, to N N (or N N N) PtII acetylide scaf-
folds.[20,22,26,27] The UV/Vis absorption of these dyads is still
limited to the UV/blue region; however, in some cases the
photophysics of the respective components was found to
collapse. For example, perylenediimide (PDI) was attached
to PtII through the acetylide bond (Pt–PDI);[28] the lifetime
∧
∧ ∧
3
of its non-emissive IL excited state (τT = 0.246 μs)[28] was
found to be much shorter than that of the model complex
[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
Pt–Ph (τT = 1.36 μs).[22]
Previously, we have attached naphthalimide (NI), couma-
rin and naphthalenediimide (NDI) moieties to the PtII cen-
tre in PtII bis(acetylide) complexes, and in some cases long-
lived 3IL triplet excited state were observed.[10,14,15,29] These
complexes, however, suffer from either short absorption
Fax: +86-411-8498-6236
E-mail: zhaojzh@dlut.edu.cn
http://finechem.dlut.edu.cn/zhaojianzhang/guohm/index.htm
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100777.
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FULL PAPER
Scheme 1. Synthesis of complex Pt–Rho. (i) CH3COOH, p-TsOH, chloranil 45%; (ii) Pd(PPh3)2Cl2, CuI, NEt3, ethynyltrimethylsilane,
Ar, 70 °C, then CH2Cl2, methanol, K2CO3, r.t., 5 h, 40%; (iii) CH2Cl2, CuI, iPr2NH, r.t., 28 h. The model complex Pt–Ph and the triplet
acceptor perylene are shown for comparison.
wavelengths or short T1 lifetimes.[15] Considering the large difference absorption spectroscopy, analysis of the spin den-
numbers of organic chromophores, much room is left for sity of the complex and the electronic component of the
exploring different organic chromophores to prepare PtII triplet excited state (T1) of Pt–Rho. The complex was used
bis(acetylide) complexes that show intense absorption in the as a triplet sensitiser for TTA-based upconversion; an up-
visible region and to access the long-lived 3IL triplet excited conversion quantum yield (ΦUC) of 11.2% was observed.
state.
We have proposed a new concept of using transition-metal
Rhodamine is a versatile fluorophore that has been used complexes with a non-emissive triplet excited state as triplet
extensively for fluorescent molecular probes and fluorescent sensitiser for TTA upconversion. The availability of triplet
bioimaging due to its intense visible-light absorption and sensitisers will increase with this new concept.
the high fluorescence quantum yields.[30–38] To the best of
our knowledge, the studies of its triplet excited state and its
application as a light-harvesting moiety in transition-metal
Results and Discussion
complexes have never been reported. In our continuous
attempts to access the long-lived 3IL excited state of organic
Design of a Rhodamine-Containing PtII Complex
chromophores in transition-metal complexes,[8–10,13–15,29]
we herein report on the incorporation of the rhodamine
We employed a new strategy for the preparation of the
chromophore into a transition-metal complex to access the chromophoric rhodamine. In the conventional preparation
3
rhodamine-localised long-lived IL excited state (Pt–Rho, of rhodamine, harsh conditions, such as high temperature
Scheme 1). Our strategy was to use the aforementioned ver- (150 °C) and strong acid H2SO4, are required, regioisomers
II
satile N N Pt bis(acetylide) scaffold to attach the acetylide are obtained and purification is difficult.[39,40] In our strat-
∧
rhodamine chromophore. Intense absorption in the visible egy, we used the p-TsOH-catalysed condensation of 4-bro-
region (λabs = 556 nm, ε = 185800 m–1 cm–1 in CH2Cl2) and mobenzaldehyde and 3-(diethylamino)phenol under mild
3
an exceptionally long-lived IL excited state (τT = 83.0 μs) conditions (70 °C). Importantly, chloranil was used as an
were observed for the Pt–Rho complex. The fluorescence oxidant in the final step of the preparation. The product
due to the rhodamine–acetylide ligand was observed for Pt– was readily purified and the yield satisfactory (see the Ex-
Rho (Scheme 1), which is not phosphorescent at either perimental Section). The acetylide moiety (–CϵC–) was in-
room temperature or 77 K. We proved, however, that the troduced onto the rhodamine moiety by using the Sonoga-
3
rhodamine-localised IL excited state was populated upon shira cross-coupling reaction. Complex Pt–Rho was ob-
photoexcitation with nanosecond time-resolved transient tained in a satisfying yield (31%; Scheme 1).
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∧
3IL Excited State of Rhodamine in an N N PtII Bis(acetylide) Complex
Intense absorption at 556 nm (ε = 185800 m–1 cm–1) was triplet energy transfer from the MLCT state to the rhod-
3
3
observed for Pt–Rho, which is due to the π-π* transition of amine-localised IL state. Indeed our time-dependent DFT
the rhodamine core (Figure 1). Ligand Rho–CϵCH shows (TDDFT) calculations showed that the energy level of the
weaker absorption in the 500–600 nm region. For the model T1 state of the rhodamine ligand Rho–CϵCH is 709 nm
complex Pt–Ph, however, the absorption was observed at a (see the Supporting Information), which is significantly
blueshifted wavelength (400 nm). The absorption of Pt–Rho lower than the T1 excited state of Pt–Ph (ca. 580 nm) ap-
at 556 nm was not redshifted compared to that of the free proximated by the emission of Pt–Ph in EtOH/MeOH (4:1,
ligand Rho–CϵCH, which is in contrast to the normal PtII v/v) at 77 K (see the Supporting Information). The emission
bis(acetylide) complexes.[28] This is also different from our profiles of Pt–Rho at 77 K and room temperature were
previous PtII bis(acetylide) complexes with NI and NDI compared (see the Supporting Information); the emission
chromophores.[10,15,29] We therefore propose that the π-con- of Pt–Rho at 77 K is more structured compared with that
jugated core of Rho–CϵCH is not directly coupled to the at room temperature.
PtII centre. Thus, interaction of the excited states is possible
between the PtII coordination centre and the Rho–CϵCH
ligand.
Nanosecond Time-Resolved Transient Difference Absorption
The lack of phosphorescence of Pt–Rho does not neces-
sarily mean that the triplet excited state was not populated
upon photoexcitation. To investigate the triplet exited state
of Pt–Rho, nanosecond time-resolved transient difference
spectra of Pt–Rho were studied (Figure 3). Upon pulsed la-
ser excitation at 532 nm, significant bleaching at 600 nm
was observed (Figure 3a). This is due to depletion of the
ground state of the rhodamine–acetylide ligand. Transient
absorption at 620–800 nm was observed. Thus, the T1 ex-
cited state of Pt–Rho can be assigned as the rhodamine-
localised 3IL excited state. The lifetime of the triplet excited
state was determined as τT = 83.0 μs. To the best of our
Figure 1. UV/Vis absorption spectra of the ligand Rho–CϵCH and
the complexes Pt–Rho and Pt–Ph in CH2Cl2 (5.0ϫ10–6 m; 25 °C). knowledge, this is the first time that the triplet excited state
of rhodamine was studied in transition-metal complexes.
The intense UV/Vis absorption of Pt–Rho and its long-
3
lived IL triplet excited state will be useful for applications
such as photovoltaics and photocatalysis.
Steady-State Spectroscopic Properties
Intense emission at 580 nm was observed for Pt–Rho
(Figure 2) in comparison with the emission of the acetylide
ligand Rho–CϵCH (see the Supporting Information); the
emission of Pt–Rho is attributed to the fluorescence of
Rho–CϵCH.[41–43] This is supported by the observation of
the small Stokes shift (24 nm) and the fact that the emission
of Pt–Rho could not be quenched by O2 (Figure 2b). On
the contrary, the emission of Pt–Ph was quenched by O2
(Figure 2c), and a large Stokes shift was observed (131 nm).
No phosphorescence was observed for Pt–Rho (Figure 2b).
Figure 3. Nanosecond time-resolved transient difference absorp-
Furthermore, the intrinsic phosphorescence of the PtII bis-
tion spectra of Pt–Rho: (a) transient absorption difference spectra
(phenylacetylide) coordination moiety (Pt–Ph) at 555 nm
was completely quenched in Pt–Rho.[42] This is due to the
and (b) decay trace at 560 nm. The spectra were recorded in deaer-
ated toluene; 2.0ϫ10–5 m, λex = 532 nm, 25 °C.
The transient absorption feature of Pt–Rho is drastically
different from that of the model complex Pt–Ph.[22c–22e] For
example, the model complex does not show any bleaching
at 600 nm, instead Pt–Ph is bleached at 380 nm.[22c]
The photophysical properties of Pt–Rho, the acetylide
ligand and the model complex Pt–Ph are summarised in
Table 1. The fluorescence quantum yield of the free ligand
Rho–CϵCH is decreased in the complex Pt–Rho, as well
as the fluorescence lifetime (τ) (Table 1). The quench of
the phosphorescence of the PtII coordination moiety in Pt–
Figure 2. (a) Excitation and emission spectra of Pt–Rho and Rho–
CϵCH. The O2 dependency of the emission of (b) Pt–Rho and (c)
Rho can be rationalised by the internal conversion of
3
Pt–Ph. The spectra were recorded in CH2Cl2 (5.0ϫ10–6 m), 25 °C. 3MLCTǞ3IL, with IL as a dark state.
Eur. J. Inorg. Chem. 2011, 4527–4533
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L. Huang, L. Z., H. Guo, W. Wu, W. Wu, S. Ji, J. Zhao
FULL PAPER
Table 1. Photophysical parameters of the PtII complexes and the This result is in agreement with the spin-density analysis,
ligand.[a]
the long-lived T1 excited state observed with the time-re-
solved transient absorption, and the lack of MLCT phos-
phorescence of Pt–Rho.
3
[b]
[e]
[f]
λabs
ε[c]
λem Φ[d]
τT
τL
Pt–Rho
Pt–Ph[g]
Rho–CϵCH 561
556
424
18.58 580 41.1% 83.0 μs 2.50 ns
[h]
1.47
7.96
555 31.3%
–
–
1.36 μs[i]
3.81 ns
[k]
581 67.1%[j]
[a] In CH2Cl2, 25 °C. [b] 5.0ϫ10–6 m. [c] Molar extinction coeffi-
cient at the absorption maxima ε: 104 m–1 cm–1. [d] [Ru(dmb)3]-
[PF6]2 as standard (Φ = 0.073 in CH3CN). [e] Triplet-state lifetimes,
measured by transient absorption 2.0ϫ10–5 m in toluene. [f] Lumi-
nescence lifetime. [g] In CH3CN. [h] Not determined. [i] Literature
value.[22] [j] Rhodamine-B as standard (Φ = 0.65 in EtOH). [k] Not
applicable.
3
Assignment of the IL Excited State by DFT Calculations
To study the low-lying triplet excited state of Pt–Rho, the
spin densities of Pt–Rho and Pt–Ph were compared (Fig-
ure 4). The optimised T1 geometry showed that the two
phenyl groups on the rhodamine moiety differ in orienta-
tion with respect to the PtII coordination plane; one is al-
most coplanar with the PtII coordination plane (dihedral
angle of 0.2°), whereas the other is tilted about 28° towards
the PtII coordination plane. On the other hand, the two
phenyl rings have a similar orientation toward the xanthene
moiety of the rhodamine (the dihedral angles are 56° and
59°). For Pt–Ph, however, the two phenyl rings of the acet-
ylide moiety are coplanar with the PtII coordination plane.
Figure 5. Frontier molecular orbitals involved in the T1 excited
state of Pt–Rho. The calculated S0–T1 energy gap is 1.69 eV
(735 nm). The T1 state is localised on the rhodamine moiety (for
which the phenyl ring is tilted by 28° toward the PtII coordination
plane), this is in line with the spin density analysis (see the main
text). The T1-state geometry was optimised with the TDDFT
method, toluene was used as the solvent in the calculation (CPCM
model). The calculation was carried out at the B3LYP/6-31G(d)/
LanL2DZ level with Gaussian 09W.
II
∧
To the best of our knowledge, N N Pt bis(acetylide)
3
complexes with intense absorption and long-lived IL ex-
cited states have rarely been reported. For example, a PDI-
containing PtII bis(acetylide) complex was reported with in-
tense absorption at 573 nm (ε = 68000 m–1 cm–1).[28] The life-
3
time of the PDI-localised IL excited state, however, was
short (τ = 0.246 μs)[28] compared with that of Pt–Ph (τ =
1.36 μs).[22] Recently, we prepared an NDI-containing PtII
3
complex[15] for which the IL triplet excited state lifetime
was found to be τ = 22.3 μs. We have also reported the PtII
bis(acetylide) complex containing NI, which showed a long-
lived emissive 3IL state (τ = 118 μs), but very weak absorp-
tion in the visible region.[10,29] The dipyrrin moiety has been
3
coordinated to iridium and the emissive IL excited state
accessed; however, the T1 state lifetime was found to be
Figure 4. Isosurfaces of the spin density of Pt–Rho and Pt–Ph at
the optimised triplet state. The calculations were performed at the Ͻ15 μs.[44] The BODIPY chromophore has been connected
to the bpy ligand of a cyclometalated IrIII complex; in this
B3LYP/6-31G/LANL2DZ level with Gaussian 09W.
case the lifetime of the T1 excited state was 25 μs.[42] All
For Pt–Rho, the spin density of the triplet state is local-
ised on the rhodamine (the chromophore for which the
these examples demonstrate the elusive photophysics of the
PtII-chromophoric dyads, and Pt–Rho represents the first
phenyl ring is tilted by 28° toward the PtII coordination
example that shows intense absorption in the region beyond
plane; Figure 4). For Pt–Ph, however, the spin density of
500 nm and, at the same time, shows a significantly pro-
the triplet state is distributed over the phenylacetylide moi-
longed T1 excited-state lifetime (τ = 83.0 μs at r.t. for Pt–
ety, the PtII centre and the dbbpy ligand (Figure 4), which
Rho; Table 1).
3
are in line with the known MLCT/3LLCT feature of the
T1 state of Pt–Ph. The isosurface distributions clearly indi-
cated that the T1 state of Pt–Rho is a rhodamine-localised
Triplet–Triplet Annihilation Upconversion
3IL excited state.[13–15,44]
The triplet excited states of Pt–Rho were also studied
As an application of the intense absorption and long-
with TDDFT calculations. The T1 state has an energy level lived T1 state of Pt–Rho, the complex was used as a triplet
of 1.69 eV (735 nm). The transition involved in the T1 state sensitiser for TTA upconversion.[45–47] TTA upconversion is
is the HOMO-3ǞLUMO; both orbitals are localised on the a particularly interesting upconversion scheme in that the
rhodamine moiety (Figure 5). This means that the T1ǞS0 excitation/emission wavelength can be readily tuned by in-
transition is localised on the rhodamine moiety, that is, the dependent selection of the triplet sensitiser and the acceptor
3
T1 excited state can be recognised as a IL excited state. (these should have matched T1-state energy levels). Further-
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∧
3IL Excited State of Rhodamine in an N N PtII Bis(acetylide) Complex
more, the excitation-power density for TTA upconversion 3MLCT state was completely quenched in Pt–Rho, thus in-
3
can be very low, in the same order of terrestrial sunlight dicating a IL excited state with a lower energy level than
(0.1 Wcm–2; AM1.5G).[48,49]
that of the 3MLCT state. The upconversion with Pt–Rho as
Currently, the major challenge for the development of the triplet sensitiser proved the new concept that the TTA
TTA upconversion is the poor availability of triplet sensi- upconversion can be sensitised with a non-emissive triplet
3
tisers; these are limited to the PtII/PdII–porphyrin com- excited state, that is, the non-emissive IL excited state was
plexes for which the T1 energy level is difficult to tune.[45] involved in the triplet–triplet energy transfer (TTET) be-
This challenge can be addressed by using PtII bis(acetylide) tween the sensitiser and the acceptor perylene (Scheme 2),
complexes for which the photophysical properties, including and eventually leads to the upconverted emission of per-
the energy level of the T1 excited state, can be readily tuned ylene. This new concept of TTA upconversion with a non-
II
∧ ∧
by using different arylacetylides. Previously, an N N N Pt
phosphorescent triplet excited state in transition-metal
acetylide complex was used as a triplet sensitiser, but with complexes will greatly increase the availability of triplet sen-
weak absorption in visible region and a short T1 lifetime sitisers for TTA upconversion, which are limited to the
(4.8 μs), the upconversion quantum yield was less than phosphorescent PtII, PdII or IrIII complexes.
1.5%.[50] Recently, we used coumarin- or NDI-containing
II
∧
N N Pt bis(acetylide) complexes as the triplet sensitisers
for TTA upconversion and achieved quantum yields up to
9.5%.[13–15]
In the presence of perylene as the triplet acceptor (T1 =
1.53 eV; 810 nm), upconverted blue emission was observed
with Pt–Rho upon excitation at 532 nm (Figure 6). Irradia-
tion of perylene with a 532 nm laser did not produce any
emission, thus verifying the upconversion feature of the
blue emission for the mixed solution of Pt–Rho and per-
ylene. The upconversion quantum yield was determined as
ΦUC = 11.2%. Considering the intense absorption of Pt–
Rho, this upconversion quantum yield is underestimated,
Scheme 2. Qualitative Jablonski diagram illustrating the sensitised
because the emission of the perylene will be reabsorbed by
Pt–Rho (the emission of Pt–Rho was enhanced in the pres-
ence of perylene; Figure 6b). Thus, an upconversion quan-
tum yield higher than 11.2% is expected for Pt–Rho. The
significant TTA upconversion for Pt–Rho is due to its in-
tense absorption of the excited light and the long-lived T1
excited state. No upconversion was observed for Pt–Ph un-
der the same experimental conditions.
TTA upconversion process between Pt–Rho and perylene (Py). The
effect of the light-harvesting ability and the luminescence lifetime
of the PtII sensitiser on the efficiency of TTA upconversion is also
shown. E = energy, GS = ground state (S0), 1IL* = intraligand
singlet excited state (rhodamine-localised), IC = internal conver-
3
sion, ISC = intersystem crossing, MLCT* = PtII-based metal-to-
3
ligand charge-transfer triplet excited state, IL* = intraligand trip-
3
let excited state (rhodamine-localised), Py* = triplet excited state
of perylene, 1Py* = singlet excited state of Py. Note that direct
1
1ILǞ3IL is also possible. The MLCT state is not presented in the
scheme. The emission bands observed for the sensitisers alone is
3
the IL emissive excited state. The emission bands observed in the
1
TTA experiment are attributed to the simultaneous IL* emission
1
(rhodamine fluorescence) and the Py* emission (fluorescence).
Conclusion
Rhodamine was incorporated into a PtII acetylide com-
plex (Pt–Rho), and intense UV/Vis absorption at 556 nm
(ε = 185800 m–1 cm–1) and an extensively prolonged triplet
excited state lifetime (τT = 83.0 μs) were observed. The T1
excited state of Pt–Rho was assigned as the rhodamine-
localised IL state by using steady and nanosecond time-
resolved spectroscopy, as well as DFT calculations. Pt–Rho
was used as the sensitiser for TTA upconversion. We proved
Figure 6. (a) Upconversion with Pt–Rho and Pt–Ph (1.0ϫ10–5 m)
as the triplet sensitisers. Perylene (Py; 3.0ϫ10–5 m) was used as the
triplet acceptor; λex = 532 nm (5 mW laser). The residual lumines-
cence of the sensitisers are not shown. (b) Enhancement of the
emission of Pt–Rho in the presence of perylene; c(complex) =
1.0ϫ10–5 m, c(perylene) = 3.0ϫ10–5 m. The measurements were re-
corded in deaerated CH2Cl2, 25 °C.
3
The photophysics of Pt–Rho and the TTA upconversion that the non-emissive 3IL excited state can be used for TTA
with Pt–Rho as the triplet sensitiser are summarised in upconversion, and an upconversion quantum yield of
Scheme 2. Excitation of either the rhodamine ligand or the 11.2% was observed. Using transition metal complexes
MLCT state lead to the prompt fluorescence emission of with non-emissive triplet excited states as the triplet sensi-
the rhodamine ligand. Our studies, however, show that the tisers for TTA upconversion will greatly increase the avail-
3
long-lived non-emissive IL excited state was finally popu- ability of triplet sensitisers. Our strategy paved the way for
lated. It should be pointed out that the emission from the the preparation of transition-metal complexes that show in-
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recorded with a Shimadzu RF 5301PC spectrofluorometer, or a
Sanco 970 CRT spectrofluorometer. Fluorescence/phosphorescence
lifetimes were measured with an OB920 fluorescence/phosphores-
cence lifetime spectrometer (Edinburgh, U.K.). The nanosecond
time-resolved transient-difference absorption spectra were detected
by using Edinburgh LP920 instruments (Edinburgh Instruments,
U.K.). The signal was buffered with a Tektronix TDS 3012B os-
cilloscope and analysed by the LP900 software. All samples in
flash-photolysis experiments were deaerated with argon for ca.
15 min before measurement, and the gas flow was maintained dur-
ing the measurement. A diode-pumped solid-state (DPSS) laser was
used for the upconversions (532 nm). The samples were purged
with N2 or Ar for 30 min before measuring. The upconversion
quantum yields (ΦUC) were determined with [Ru(dmb)3][PF6]2 as
the quantum-yield standard (ΦF = 0.073 in deaerated CH3CN)
[Equation (1)],[45] in which ΦUC, Aunk, Iunk and ηunk represents the
quantum yield, absorbance, integrated photoluminescence intensity
and the refractive index of the samples and solvents, respectively.[45]
tense visible-light absorption and long-lived triplet excited
states, and for their applications in areas such as photovol-
taics, photocatalysis and upconversions.
Experimental Section
Rhodamine-Br:
A mixture of 4-bromobenzaldehyde (1.85 g,
10 mmol), 3-(diethylamino)phenol (3.4 g, 20 mmol), p-TsOH
(0.258 g, 1.5 mmol) and acetic acid (100 mL) was heated to 70 °C
and stirred for 7 h. The reaction mixture was cooled to r.t., and
the pH was adjusted to above 7 with a 10% NaOH solution. The
precipitate was filtered and washed with water (100 mL). The solid
was dissolved in CH2Cl2 (100 mL), to which chloranil (1.23 g,
5 mmol) was added. The mixture was stirred for 2 h. After removal
of the solvent, the residue was purified by column chromatography
(silica gel; CH2Cl2/methanol, 10:1, v/v) to give a purple solid; yield
1
2.15 g (45.0%). H NMR (400 MHz, CDCl3): δ = 7.79–7.77 (d, J
= 8.0 Hz, 2 H), 7.34–7.28 (m, 4 H), 7.00–6.98 (m, 2 H), 6.85 (s, 2
H), 3.72–3.66 (t, J = 7.2 Hz, 8 H), 1.37–1.33 (m, 12 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 12.8, 46.4, 96.8, 113.3, 114.5, 125.2,
130.9, 131.2, 131.9, 132.5, 155.8, 158.1 ppm. ESI-HRMS
([C27H30N2OBr]+): calcd. 477.1541; found 477.1545.
(1)
Rhodamine-Ethynyl:
A mixture of rhodamine-Br (956.0 mg,
DFT calculations were used for the optimisation of the ground-
state geometries, for both singlet states and triplet states. The en-
ergy levels of the T1 states (energy gap between S0 and T1) were
calculated with time-dependent DFT (TDDFT) on the basis of op-
timised triplet-state geometries. All calculations were carried out
with Gaussian 09W.[51]
2 mmol), PPh3 (26.2 mg, 0.1 mmol) and Pd(PPh3)2Cl2 (70.2 mg,
0.1 mmol) was dissolved in a solvent mixture of triethylamine
(10 mL) and THF (10 mL) under Ar. Ethynyltrimethylsilane
(1.0 mL) and CuI (19.1 mg, 0.1 mmol) were sequentially added to
the above solution. The mixture was stirred at 70 °C for 7 h. After
removal of triethylamine and THF in vacuo, the residue was puri-
fied by column chromatography (silica gel; CH2Cl2/methanol, 10:1,
v/v) to give a purple solid. The solid was dissolved in a solvent
mixture (30 mL; CH2Cl2/methanol, 2:1, v/v); then K2CO3
(840.0 mg, 6 mmol) was added, and the mixture was stirred at r.t.
for 5 h. The precipitate was removed by filtration, and the organic
phase was washed with water (30 mL), and the residue was purified
by column chromatography (silica gel; CH2Cl2/methanol, 10:1, v/v)
Supporting Information (see footnote on the first page of this arti-
cle): Additional characterisation and photophysical data.
Acknowledgments
We thank the National Natural Science Foundation of China
(NSFC) (20972024 and 21073028), the Fundamental Research
Funds for the Central Universities (DUT10ZD212 and
DUT11LK19), The Royal Society (U.K.)–NSFC (China) China–
U.K. Cost-Share Program (21011130154), the State Key Labora-
tory of Fine Chemicals (KF0802) and the Ministry of Education
of China (SRFDP-200801410004 and NCET-08-0077) for financial
support.
1
to give a purple solid; yield 336.0 mg (40%). H NMR (400 MHz,
CDCl3): δ = 7.78–7.72 (m, 2 H), 7.38–7.30 (m, 4 H), 6.98–6.93 (m,
2 H), 6.89–6.88 (d, J = 4.0 Hz, 2 H), 3.70–3.65 (m, 8 H), 3.27 (s, 1
H), 1.36–1.32 (t, J = 8.0 Hz, 12 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 12.6, 46.5, 96.9, 113.3, 114.5, 125.0, 129.7, 131.2,
131.9, 132.5, 132.8, 155.7, 158.1 ppm. ESI-HRMS ([C29H31N2O]
+): calcd. 423.2436; found 423.2432.
Synthesis of Pt–Rho: Rhodamine–ethynyl (30.0 mg, 0.0709 mmol)
and Pt(dbbpy)Cl2 (24.7 mg, 0.0473 mmol) were dissolved in a solu-
tion of CH2Cl2 (10 mL) under Ar. Diisopropylamine (2 mL) and
CuI (5.0 mg, 0.0024 mmol) were sequentially added to the above
solution. The mixture was stirred at 25 °C for 28 h. After removal
of the solvent in vacuo, the residue was purified by column
chromatography (silica gel; CH2Cl2/methanol, 10:1, v/v), to give a
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Received: July 26, 2011
Published Online: September 1, 2011
Eur. J. Inorg. Chem. 2011, 4527–4533
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4533