A highly luminescent europium complex showing visible-light-sensitized red emission: Direct observation of the singlet pathway

Article abstract of DOI:10.1002/anie.200454141

Red glow in daylight ... chemist's delight! An efficient transfer of excitation energy from the ligand to the luminescent states of the coordinated Eu^III ion in 1 occurs from the singlet excited state of the ligand. The complex shows characteri

Full text of DOI:10.1002/anie.200454141

Angewandte
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Visible-light excitation of the europium complex shown leads to
characteristic red emission from the EuIII center. A spectroscopic study
shows the nature of the sensitization process and accounts for why the
complex can be excited at these longer wavelengths. For more
information see the Communication by Y. Wang, J.-P. Zhang, W.-T.
Wong et al. on the following pages.
Angew. Chem. Int. Ed. 2004, 43, 5009
DOI: 10.1002/anie.200454141
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Communications
Luminescence
observed sensitization mechanism for luminescent europium
complexes involves a triplet pathway, in which the transfer of
the energy absorbed by the ligand to the EuIII ion takes place
from the ligand-centered triplet excited state (T1). In this case,
the optical excitation window for luminescent Eu complexes
appears to be limited to < 385 nm owing to the energetic
constraints highlighted by Reinhoudt and co-workers.[2] With
the use of ꢀantennaꢁ chromophoric groups, which have a
smaller energy gap between the lowest singlet excited state
(S1) and the T1 state (e.g. acridone, diaryl ketones), it has been
demonstrated by several research groups that the excitation
wavelength for Eu complexes can be extended into the visible
region through the usual triplet pathway.[3–4] Another promis-
ing means of longer-wavelength sensitization of EuIII emission
is through the singlet pathway, in which the excited-state
energy of a chromophore is directly transferred from its S1
state to the luminescent states of the EuIII center. In this way
the energetic constraints from the T1 state of the ligand can be
avoided.
A Highly Luminescent Europium Complex
Showing Visible-Light-Sensitized Red Emission:
Direct Observation of the Singlet Pathway**
Chi Yang, Li-Min Fu, Yuan Wang,* Jian-Ping Zhang,*
Wing-Tak Wong,* Xi-Cheng Ai, Yi-Fang Qiao,
Bing-Suo Zou, and Lin-Lin Gui
A challenge in the chemistry of the lanthanide ions is to
develop luminescent Eu complexes that can be sensitized by
visible light and to determine the energy-transfer mechanisms
in these systems. Recently, this field has become much more
important because of the demand for less-harmful labeling
reagents in the life sciences and low-voltage-driven pure-red
emitters in optoelectronic technology.[1] The commonly
The singlet pathway for the sensitization of lanthanide
luminescence was first proposed by Kleinerman in 1969.[5]
Horrocks and co-workers later examined the energy-transfer
processes in lanthanide-ion-binding proteins and assumed
that it was the singlet excited state of the chromophore that
sensitized the lanthanide emission.[6] However, owing to the
lack of information regarding the emission from the excited
states of the coordinated ligand and the difficulties in the
determination of the ligand-localized triplet–triplet absorp-
tion spectra for lanthanide chelates, it has been very difficult
to prove for certain which state is responsible for the energy-
transfer process.[7] All the experimental work conducted on
the sensitization of lanthanide–chelate luminescence seems to
support the triplet pathway, whereas the singlet pathway for
the sensitization of the Eu complex has not been observed
experimentally.[7] Herein, we report the successful extension
of the excitation window to the visible region for a Eu
complex, which subsequently exhibits highly efficient EuIII-
centered luminescence. The sensitization mechanism is shown
to take place through the singlet pathway by means of time-
resolved luminescence spectroscopic studies.
[*] Prof. Dr. Y. Wang, Prof. L.-L. Gui
State Key Laboratory for Structural Chemistry of Unstable and
Stable Species
College of Chemistry and Molecular Engineering, Peking University
Beijing 100871 (China)
Fax: (+86)10-6276-5769
E-mail: wangy@pku.edu.cn
Prof. Dr. J.-P. Zhang, Prof. Dr. X.-C. Ai
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100080 (China)
Fax: (+86)10-8261-6163
E-mail: jpzhang@iccas.ac.cn
Prof. Dr. W.-T. Wong
Department of Chemistry, The University of Hong Kong
Hong Kong (China)
Fax: (+85)2-2547-2933
E-mail: wtwong@hkucc.hku.hk
Dr. C. Yang+
State Key Laboratory for Structural Chemistry of Unstable and
Stable Species
College of Chemistry and Molecular Engineering, Peking University
Beijing 100871 (China)
and
Faculty of Chemical Engineering and Light Industry
We have previously demonstrated that dipyrazolyltriazine
derivatives are good chromophores for the efficient sensiti-
zation of lanthanide ion luminescence.[8] To achieve longer-
wavelength sensitization of the EuIII emission, we explored
the effect of incorporating N,N-dialkyl aniline moieties into
the dipyrazolyltriazine systems. The new ligand 3 was
prepared by heating 2 and the potassium salt of 3,5-
dimethylpyrazole in dry THF at reflux for 12 h (Scheme 1).
The reaction of 3 and europium thenoyltrifluoroacetonate
[Eu(tta)3·3H2O]in THF yielded a yellow solution of the
complex 4, which showed a bright red emission under daylight
illumination. After evaporation of the solvent, the residue
was taken up in dry diethyl ether, and the solution was
triturated with n-hexane; the complex Eu(tta)3L (4) was
obtained as a bright orange powder (Scheme 1).
Guangdong University of Technology, Guangzhou 510090 (China)
L.-M. Fu+
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100080 (China)
and
Institute of Physics, Chinese Academy of Sciences
Beijing 100080 (China)
Y.-F. Qiao
Faculty of Chemical Engineering and Light Industry
Guangdong University of Technology
Guangzhou 510090 (China)
Prof. Dr. B.-S. Zou
Institute of Physics, Chinese Academy of Sciences
Beijing 100080 (China)
[+] These authors contributed equally to this work.
[**] We are grateful for the grants-in-aid from the NSFC (29925308,
20105003, 90206011, 20273077, 90101010), the Major State
Basic Research Development Program (G2000077503), the
Research Grant Council of Hong Kong, and the NSF of Guangdong
(980128).
The photophysical properties of a solution of complex 4 in
toluene were investigated systematically. Upon the coordina-
tion of 3 to EuIII, the ligand charge-transfer (CT) absorption
band is shifted from 387 nm (e = 38000 cmmꢀ1) to 406 nm
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DOI: 10.1002/anie.200454141
Angew. Chem. Int. Ed. 2004, 43, 5010 –5013

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emission band at 430 nm vanished
completely to leave only the intense
7
5D0! F0–4 emission of EuIII. This
indicates that the Ligand-to-EuIII
energy transfer is more efficient at
lower temperatures.
The kinetics and the time-
resolved spectra of the lumines-
cence from 4 were recorded on
different timescales as shown in
Figures 2a–d. On the nanosecond
timescale (Figure 2a) and immedi-
ately following the pulsed excitation
at 400 nm, broad spectra which
extend up to 620 nm were observed
(inset, 1–4 ns). These broad signals
arise from the red edge of the S1!S0
fluorescent emission of the coordi-
Scheme 1. Synthesis of the complex 4. a) N,N-diethylaniline; b) potassium 3,5-dimethylpyrazolate;
c) ([Eu(tta)3]·3H2O).
(e = 55000 cmmꢀ1) owing to the stabilization of the very polar
excited states (Figure 1). The excitation spectrum of complex
4 (emission monitored at 614 nm) is in agreement with its
nated ligand that is centered at ꢁ 430 nm. Simultaneously, the
7
emission from the 5D1! F3 transition of the EuIII ion at
585 nm is also clearly seen. The intensities of these signals
vary upon increasing the delay time. The decay-time constant
of the ligand S1-state emission exhibits a weak wavelength
dependence, that is, 1.3 ns at 430 nm and 1.8 ns at 605 nm. On
the other hand, the rise-time constant of the 5D1 luminescence
at 585 nm is found to be 1.8 ns. This decay-to-rise correlation
strongly suggests direct energy transfer from the S1 state of
the coordinated ligand 3 to the 5D1 state of EuIII.
On the sub-microsecond timescale (Figure 2b), a tight
7
correlation between the decay of the 5D1! F1–3 luminescence
signals at 535, 555, and 585 nm (decay constant, 387 ns) and
7
the rise of the 5D0! F2 luminescence at 614 nm (rise constant,
392 ns) can be established; this indicates the transfer of the
excited-state population from the 5D1 state to the 5D0 state of
the EuIII ion. The kinetics of the D0! F2 luminescence of
EuIII at 614 nm was examined on the millisecond timescale
(Figure 2c). The lifetimes of the excited state are 0.48 and
0.65 ms at room temperature and 77 K, respectively.
5
7
Figure 1. The room temperature UV/Vis absorption spectra of the Eu
complex 4 (dashed line) and the ligand 3 (dotted line) assolutionsin
toluene, and the fluorescence emission spectrum of complex 4 (solid
line, lex =402 nm) at ~1.0ꢀ10ꢀ5 m. Luminescence quantum yields (F)
for 4 are given in parentheses.
To determine the energy of the T1 state of the coordinated
ligand 3, we performed time-resolved phosphorescence
measurements at 77 K (Figure 2d). The emission spectrum
reveals an asymmetric broad phosphorescence band centered
5
7
ground-state absorption spectrum. The excitation window for
complex 4 can be extended beyond 460 nm when the
concentration is increased up to 1 ” 10ꢀ2 m. Upon selective
excitation at the ligand CT band at room temperature, the
emission spectrum of complex 4 displays a broad band
centered at 430 nm, derived from the coordinated ligand, and
at ꢁ 525 nm (19048 cmꢀ1) with a weak reminiscent D0! F2
emission band at 614 nm. Most importantly, the lifetime of the
T1-state of the coordinated ligand is determined to be 3.9 s,
which is significantly longer than the lifetime of the 5D0 state
of the EuIII metal center (0.65 ms). Therefore the excitation-
energy transfer from the triplet state of the coordinated ligand
5
7
5
the characteristic sharp peaks associated with the D0! FJ
3 to the emissive states of the EuIII ion (5D1 and D0), if any,
transitions of the EuIII ion (Figure 1). The five expected
would have a very low probability (< 10ꢀ3).[9–10] On the basis
of the above observations, we conclude that the sensitization
of the EuIII emission in complex 4 proceeds through the
singlet pathway, whereas the triplet pathway is essentially
inactive (Figure 3).
5
7
components of the D0! F0–4 transitions are well resolved
7
and the hypersensitive 5D0! F2 transition is very intense,
which reflects a low symmetry of the EuIII site. The overall
luminescence quantum yields for the emissions from the EuIII
ion and the coordinated ligand in 4 in toluene are 0.52 and
0.27, respectively (lex = 402 nm, room temperature). The
observed quantum yield for the EuIII emission is the highest
reported value for such visible-light-sensitized Eu complexes.
When the temperature was reduced to 77 K, the ligand
In conclusion, we have demonstrated the first observable
case of excitation-energy transfer from the ligand to the
luminescent states of EuIII ion through the singlet pathway in
a visible-light-sensitized europium complex. The excitation
window for this complex has been extended up to 460 nm.
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Figure 3. Energy-level diagram showing the energy-transfer pathways in
complex 4; isc denotes intersystem crossing.
Experimental Section
3: Potassium (0.08 g, 2.07 mmol) was added to a stirred solution of
3,5-dimethylpyrazole (0.288 g, 3.0 mmol) in dry THF (30 mL) under
N2 at 708C. After the metal dissolved, the colorless solution was
cooled to ꢁ 108C and 2 (0.297 g, 1.0 mmol) was added. The reaction
mixture was stirred at room temperature for 0.5 h and then heated at
reflux for 5 h. The solution was concentrated under reduced pressure,
and the residue was purified by column chromatography on silica gel
with a mixture of petroleum ether and benzene as eluent to afford 3
(0.36 g, 86%). M.p. 235–2368C; 1H NMR (CDCl3, 300 MHz, 258C,
3
3
TMS): d = 8.39 (d, J(H,H) = 9.2 Hz, 2H; Ph-H), 6.72 (d, J(H,H) =
9.1 Hz, 2H; Ph-H), 6.08 (s, 2H; Pz-H), 3.46 (q, 3J(H,H) = 7.0 Hz, 4H;
NCH2CH3), 2.85 (s, 6H; Pz-CH3), 2.35 (s, 6H; Pz-CH3), 1.23 ppm (t,
3J(H,H) = 6.9 Hz, 6H; NCH2CH3); 13C NMR (300 MHz, CDCl3,
258C, TMS): d = 173.37, 163.98, 152.48, 151.59, 143.95, 131.57,
121.23, 111.35, 110.68, 44.64, 16.06, 14.06, 12.60 ppm; IR (KBr): n˜ =
=
=
2983, 2955, 2893, 2863 (C-H), 1587 (C N), 1560 (C N), 1537, 1509
=
=
(C N), 1409 (C N), 1389, 1355, 1338, 1268, 1184, 1067, 970, 803,
747 cmꢀ1; FAB MS (nba): m/z : 417[M + 1]; elemental analysis: calcd
for C23H28N8: C 66.32, H 6.78, N 26.90; found: C 66.28, H 6.78, N
26.81%.
4: A solution of Eu(tta)3·3H2O (87 mg, 0.1 mmol) in THF
(10 mL) was added to a solution of 3 (41.6 mg, 0.1 mmol) in THF
(10 mL) to give instantaneously a yellow solution with very bright red
luminescence in daylight. After evaporation of the solvent, the
residue was redissolved in a small amount of diethyl ether. Addition
of n-hexane to the solution led to the precipitation of the ternary
complex of [Eu(tta)3(3)]( 4) as an orange powder (ꢁ 86%). M.p. 138–
1398C; 1H NMR (CDCl3, 300 MHz, 258C, TMS): d = 25.54 (s, 3H;
CH), 12.22 (s, 2H; Pz-H), 7.43 (d, 3J(H,H) = 8.2 Hz, 2H; Ph-H), 6.91
(s, 3H; Th-H), 6.05 (s, 3H; Th-H), 5.90 (d, 3J(H,H) = 8.2 Hz, 2H; Ph-
H), 5.03 (s, 3H; Th-H), 4.82 (s, 6H; Pz-CH3), 3.31 (q, 3J(H,H) =
Figure 2. The time-resolved luminescence spectra of complex 4 in tolu-
ene on different timescales recorded at room temperature (a, b) and
*
*
77 K (c, d). a) Kineticscurvesat 430 ( ) and 585 nm ( ) with an exci-
3
6.9 Hz, 4H; NCH2CH3), 0.95 (t, J(H,H) = 6.6 Hz, 6H; NCH2CH3),
tation pulse at 400 nm (130 fs); the inset shows the luminescence
*
spectra at different delay times (td). b) Kineticscurvesat 585 ( ) and
ꢀ0.06 ppm (s, 6H; Pz-CH3); IR (KBr): n˜ = 2978, 2931 (C-H), 1612,
=
=
614 nm (&) with an excitation pulse at 417 nm (5 ns); the inset shows
the luminescence spectra at different delay times. c) Kinetics curves at
614 nm with an excitation pulse at 417 nm (5 ns). d) Kinetics curves at
1591, 1559 (C N), 1538, 1502, 1415 (C N), 1393, 1353, 1304, 1185,
1140, 1037, 808 (C N), 783, 751, 716, 692, 641, 580 cmꢀ1; FAB MS:
=
m/z: 1232 [M+]; elemental analysis: calcd for EuC47H40N8F9O6S3: C
45.82, H 3.27, N 9.10, S 7.81; found: C 45.35, H 3.28, N 9.03, S 7.59%.
Steady-state UV/Vis absorption and photoluminescence (PL)
measurements were carried out on an absorption spectrometer (U-
*
525 ( ) and 614 nm (&); the inset shows the phosphorescence spec-
trum of complex 4 after a delay time of 1 s.
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3310, Hitachi) and
a
fluorescence spectrophotometer (F-2500,
[10]As the T level of the coordinated 3 (19048 cmꢀ1) in 1 seems
1
Hitachi), respectively. Luminescence quantum yields were deter-
mined by the method described by Demas and Grosby,[11] with 4-
dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran
(F = 0.71) as the reference.[12]
suitably located for energy transfer to the 5D1 (19000 cmꢀ1) and
5
the D0 (17500 cmꢀ1) states of the EuIII ion, the unusually low
probability of the ligand(T1)-to-EuIII(5D0–1) energy transfer is
interesting and the reasons behind this are under current
investigation.
Time-resolved PL spectroscopy on the nanosecond timescale was
performed on
Hamamatsu) combined with
a
photon-counting type streak camera (C2909,
polychromator (Chromex 250is,
[11]J. N. Demas, G. A. Crosby, J. Phys. Chem. 1971, 75, 991 – 1024.
[12]P. R. Hammond, Opt. Commun. 1979, 29, 331 – 334.
a
Chromex); the excitation pulses were provided by a regenerative
amplifier (Spitfire, Spectra Physics), whose output wavelength was
converted to 400 nm with the second-harmonic generation (ꢁ 200 nj,
120 fs, 1 KHz). The time-resolution was 30 ps. For the sub-micro-
second PL measurements, a gated linear photodiode-array detector
(1420, EG&G; gate width 5 ns) attached to a polychromator was
employed; the excitation pulses (417 nm, 1 mj, 5 ns) were obtained
with a hydrogen-charged Raman shifter (2 MPa) which was pumped
by the third harmonics of a Nd:YAG laser (355 nm, 1 Hz, 40 mJ/
pulse). The PL kinetics on a millisecond timescale were recorded on a
PMT (R298, Hamamatsu) detector attached to a monochromator.
The time-resolved phosphorescence measurements were carried out
at 77 K. Excitation was realized with the aforementioned 400 nm
laser beam, which was chopped by an electric shutter; phosphor-
escence emissions were sent to a polychromator (Spectropro 550i,
Acton) equipped with a liquid-nitrogen cooled CCD detector (SPEC-
10-400B/LN, Roper Scientific). A delay-pulse generator (DG535,
Stanford Research System) was used to trigger the CCD detection
(exposure time 0.1 s) at various delay times with intervals of 0.5 s.
Received: February 28, 2004
Revised: May 25, 2004 [Z54141]
Keywords: energy-transfer mechanisms · europium ·
.
luminescence · sensitizers · time-resolved spectroscopy
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