Aggregation-induced white-light emission from the triple-stranded dinuclear Sm(iii) complex

Article abstract of DOI:10.1039/c4dt00820k

A novel bis-β-diketone ligand, 4,4′-bis(4,4,4-trifluoro-1,3- dioxobutyl)(phenoxy)-1,1′-binaphthalene (BTPB), is designed for synthesis of a white light emissive lanthanide complex. The ligand bears two benzoyl β-diketonate sites linked by a 1,1′-binaphthoxy spacer. Reaction of the doubly negatively charged bis-bidentate ligand with lanthanide ions forms triple-stranded dinuclear complexes Sm₂(BTPB)₃(H ₂O)₄ (1) and Gd₂(BTPB)₃(H ₂O)₄ (2), which have been fully characterized by various spectroscopic techniques. UV-Vis absorption and emission spectroscopic techniques are used to investigate photophysical properties of the ligand and its complexes in THF and CHCl₃. In some cases aggregation of the ligand results in the appearance of a new luminescence band at about 510 nm in addition to the monomer fluorescence. In complex 1, partial energy transfer from BTPB results in Sm(iii)-based red light emission in addition to the BTPB-based blue/green emission. With the variation of the excited wavelength and concentration of the solution, complex 1 shows a tunable white light emission with the balance of three primary colors. This is an unusual case of observation of white light emission from a single molecule Sm(iii) complex.

Full text of DOI:10.1039/c4dt00820k

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Aggregation-induced white-light emission from
the triple-stranded dinuclear Sm(III) complex†
Cite this: Dalton Trans., 2014, 43,
12228
Jiaqi Leng, Hongfeng Li,* Peng Chen, Wenbin Sun, Ting Gao and Pengfei Yan*
A
novel bis-β-diketone ligand, 4,4’-bis(4,4,4-trifluoro-1,3-dioxobutyl)(phenoxy)-1,1’-binaphthalene
(BTPB), is designed for synthesis of a white light emissive lanthanide complex. The ligand bears two
benzoyl β-diketonate sites linked by a 1,1’-binaphthoxy spacer. Reaction of the doubly negatively charged
bis-bidentate ligand with lanthanide ions forms triple-stranded dinuclear complexes Sm2(BTPB)3(H2O)4 (1)
and Gd2(BTPB)3(H2O)4 (2), which have been fully characterized by various spectroscopic techniques.
UV-Vis absorption and emission spectroscopic techniques are used to investigate photophysical pro-
perties of the ligand and its complexes in THF and CHCl3. In some cases aggregation of the ligand results
in the appearance of a new luminescence band at about 510 nm in addition to the monomer fluor-
escence. In complex 1, partial energy transfer from BTPB results in Sm(III)-based red light emission in
addition to the BTPB-based blue/green emission. With the variation of the excited wavelength and con-
centration of the solution, complex 1 shows a tunable white light emission with the balance of three
primary colors. This is an unusual case of observation of white light emission from a single molecule
Sm(III) complex.
Received 19th March 2014,
Accepted 25th April 2014
DOI: 10.1039/c4dt00820k
www.rsc.org/dalton
Tb(III) ions can emit intense red and green light, respectively.
Thus, the lanthanide complexes, which combine red- (EuIII,
Introduction
White-light-emitting materials have attracted much attention SmIII) and green-emissive (TbIII) ions and blue/green emitting
because of their broad applications in lighting, backlights, and organic chromophores, should be perfect candidates for
full-colour displays.1 Generally, the realization of white light designing the white-light-emitting materials.
emission requires the generation and intensity control of the
Presently, there mainly exist two types of lanthanide com-
three fundamental red, green and blue (RGB) light emissions plexes for constructing the white-light emitters. One is the
in bulk materials. The trichromatic approach is mostly lanthanide metal–organic frameworks (LnMOFs)11 and the
employed for producing white-light materials.2 In addition, other is single molecule Eu(III) complexes.12 In LnMOFs, white
monochromatic, dichromatic and tetrachromatic approaches light emission is generally achieved by co-doping Eu(III) and/or
are often used for achieving this goal.3 So far, reported white- Tb(III) ions into the corresponding isostructural LnMOFs (Ln =
light-emitting materials contain small organic molecules,4 La and Gd) through adjusting the ratio of the lanthanide ions.
polymers,5 metal complexes,6 lanthanide-doped inorganic For example, Zang et al. used a combination of blue-emitting
materials,7 nanocrystals,8 quantum dots,9 and hybrid ligand/La(III), green-emitting Tb(III) and red-emitting Eu(III)
materials based on rare earth complexes.10 Benefitting from units to generate white light from La(III)/Tb(III)/Eu(III)-MOFs.13
unique optical properties of lanthanide ions, the luminescence This is known as a three-component approach. Another
from Ln(III) ions is featured by high colour purity and long- strategy for generating white light emission is to dope Eu(III)
lived excited lifetimes and the emission covers the whole ions into Gd(III) frameworks. It is named the two-component
visible range from 400 to 700 nm. Specifically, Eu(III) and approach.14
In single molecule Eu(III) complexes, the white light
emission is achieved by incorporating the Eu(III) ion red light
emission and ligand-based blue/green light emission. In com-
Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang
parison with the LnMOFs, the single molecules have advan-
tages such as facile preparation, high emission quantum
yields and simpler device fabrication.15 To obtain effective
white light emission, the complexes are generally composed of
two different kinds of ligands simultaneously, one as a sensi-
tizer of the Eu(III) ion luminescence and the other as blue/
University), Ministry of Education, School of Chemistry and Materials Science,
Heilongjiang University, Harbin 150080, PR China. E-mail: lhf4612@163.com,
yanpf@vip.sina.com; Fax: +86-451-86608042
†Electronic supplementary information (ESI) available: The 1H NMR spectrum
of 4,4′-bis-(acetyl)phenoxy-1,1′-binaphthalene (BAPB), ESI-MS for the ligand, the
phosphorescence spectrum of complex 2 at 77 K and the CIE coordinates of 1 at
excitation wavelengths 350–390 nm. See DOI: 10.1039/c4dt00820k
12228 | Dalton Trans., 2014, 43, 12228–12235
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green light emissive chromophores to balance the red-emis- sion band.21 In most cases, emission bands of both the
sion bands. For example, Duan reported a single molecule monomer and aggregate can be observed simultaneously.
Eu(III) complex, in which the Eu(III) moiety acted as the source Therefore, the organic molecule that combines the monomer
of red light, while a covalently linked coumarin–rhodamine blue- and aggregate blue/green-luminescence is suitable for
ligand acted as the blue- and green-emitting source.16 De Cola the construction of single molecule white light materials.
et al. reported an Ir(III)/Eu(III) trinuclear complex that exhibited Among the luminescent lanthanide ions, the Sm(III) ion gener-
white light emission by combining a blue-emitting Ir(III) and a ally shows three characterized emission bands in the visible
red-emitting Eu(III) moiety.17
region, green (558 nm), orange (597 nm) and red (640 nm).
Recently, a new approach to obtain white light emission Consequently, the Sm(III) complex that combines the ligand-
from the single molecule Eu(III) complex was reported in based blue/green luminescence and the Sm(III) red lumine-
which only one kind of ligand is used to tune the luminescent scence has potential as a single molecule white-light emitter.
colors.18 We name it the one-ligand approach. In the complex, In this paper, we utilized BTPB and Sm(III) ions successfully
partial energy transfer from the ligand results in sensitized to prepare
a
new triple-stranded dinuclear complex
Eu(III)-based emission in addition to the ligand-based blue or Sm2(BTPB)3(H2O)4, in which the white light emission has been
blue/green luminescence. The one-ligand approach will con- observed by incorporating the ligand-based blue-green emis-
siderably simplify the preparation of the white-light-emitting sion and Sm(III) ion red emission. To the best of our knowl-
complexes. To the best of our knowledge, this is the only edge, this is the first example to observe white light emission
example to achieve white light emission from a single mole- from a single molecule Sm(III) complex.
cule lanthanide complex where only one type of ligand is
present.
We are interested in exploring new white-light-emitting
materials of the single molecule lanthanide complex. Herein, a
novel bis-β-diketone, 4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)-
(phenoxy)-1,1′-binaphthalene (BTPB) (see Scheme 1), is
Results and discussion
Characterization of the ligand and complexes
The 1H NMR spectrum of BTPB obtained at 400 MHz in CDCl3
is shown in Fig. 1. The β-diketones generally exhibit keto–enol
tautomerism. The amounts of keto and enol forms can be
determined by integration of the keto and the enol resonance
peaks in the 1H NMR spectrum. The observed broad single
peak, 2H at δ 15.20, shows the characteristic Henol protons
(Hj), and the singlet, 2H at δ 6.35, is assigned to methine Hketo
protons (Hi). By integrating the areas corresponding to both
species, it is found that the ligand exists completely in the
enolic form in CDCl3. The observed two double peaks at
δ 6.74–6.76 and δ 7.61–7.63 are attributed to the Hg, Hh in
phenylene, with the areas that integrate to a relative value of
4H atoms, respectively. The double peaks are the results of the
spin–spin coupling of the adjacent protons, and being close to
the withdrawing carboxyl group makes the Hh present at
downfield. A doublet peak, 2H observed at δ 7.96–7.98, is
assigned to the protons Hd due to coupling of Hc protons.
designed for syntheses of white-light-emitting lanthanide com-
plexes. It is known that the β-diketones are perfect sensitizers
for Ln(III) ion luminescence due to their effective sensitization
ability to the metal.19 Recently, we and others developed
several bis-β-diketone ligands, which coordinate to lanthanide
ions to form triple-stranded helicates and effectively sensitize
lanthanide ion visible or NIR luminescence.20 In this ligand,
binaphthol is used as a spacer to link the two β-diketone units.
Binaphthol has a free rotation C2 axis, so the ligand is prone
to twist and wrap about the metal ions to form the triple-
stranded dinuclear structure. Meanwhile, the large amounts of
aromatic ring structures make the ligand to be more prone to
accumulate and form aggregates. Compared to the monomer,
the aggregate often provides a red-shifted and enhanced emis-
Scheme 1 Syntheses of the BTPB and complexes Ln2(BTPB)3(H2O)4 (Ln
= Eu, Gd).
Fig. 1 400 MHz 1H NMR spectrum of BTPB in CDCl3.
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nation is further supported by the appearance of bands in the
range of 461–488 cm−1 due to Ln–O stretching vibrations. The
ligand is indicated to be in the enolic form by the presence of
a band at 1430 cm−1 of a CvC–O stretching vibration, which
is shifted to 1361–1398 cm−1 in complexes 1 and 2. In
addition, the absorbance frequency due to CvC of benzene
rings at 1502 and 1470 cm−1 of the ligands is split into two
peaks at 1529–1595 cm−1 and 1463–1497 cm−1, respectively, in
the complexes. To examine the thermal stability and water
content of the complexes, thermogravimetric analysis is
carried out for 1 (Fig. S3†). It is clear from the TG curve that
complex 1 undergoes a mass loss of about 2.9% (calcd 3.0%)
in the first step, which corresponds to the loss of four co-
ordinated water molecules. Then, a long plateau is observed
until a full decomposition at ca. 310 °C.
Fig. 2 Expanded regions of the ESI-TOF-MS of Sm2(BTPB)3 in acetone.
UV-Vis absorption spectra
A doublet, 2H, Ha is observed at δ 7.92–7.94 due to the coup-
ling of Hb protons. Protons Hb, Hc appear as two triplet peaks
at δ 7.31–7.49 and correctly integrate for four protons. Multiple
signals in the range of δ 7.24–7.28 are attributed to the
methine protons Hf, He, integrating for four protons.
In view of the soft ionization of the sample provided by
ESI-TOF mass spectrometry, which avoids destructive fragmen-
tation of the complexes, we have resorted to this technique to
ascertain the formation of the dinuclear complexes. The mass
spectrum of complex 1 is shown in Fig. 2. The patterns of
mass spectra show characteristic mass distribution of the com-
plexes Sm2(BTPB)3. The peak centred at m/z 2461 is attributed
to [Sm2(BTPB)3 + Na]+ for the dinuclear complex.
The FT-IR spectra of the complexes (1 and 2) show a broad
absorption in the 3657–3220 cm−1 region, thereby indicating
the presence of solvent molecules in the coordination sphere
of the Ln(III) ions. The carbonyl stretching frequency of BTPB
(1625 cm−1) has been shifted to lower wavenumbers in the
complexes (1, 1615 cm−1; 2, 1617 cm−1), indicating the coordi-
nation of the oxygen atoms to the lanthanide ions. This coordi-
The UV-Vis absorption spectra of the ligand BTPB in THF at
10−6–10−4 M are shown in Fig. 3 (a, top). Clearly, the absorp-
tion of a ligand in solution is dependent on its concentration.
They all show a typical binaphthol absorption band at 279 nm,
and an absorption maxima at about 329 nm corresponding to
the singlet–singlet π–π* electronic transition of the ligand. At a
concentration above 10−6 M, the absorption maximum red-
shift from 329 to 341 nm and the intensity of the shoulder at
369 nm increase significantly. These changes can be explained
by the aggregation behaviors of the aromatic units. Binaphthol
possesses a C2-axis of free rotation and a large conjugate
plane, which make it prone to form aggregates. In addition,
the clear red shift of the absorption bands suggests that the
molecules self-assembled into J-aggregates in solution at
higher concentrations.22 In CHCl3, the absorption spectra of
the ligand are about the same over the 10−6–10−4 M concen-
tration range, and are very similar to that observed in THF at
high concentrations. It means that the aggregate of the ligand
is more easily formed in CHCl3 than in THF, even in highly
dilute solution (10−6 M).
Fig. 3 Normalized UV-Visible absorption spectra of BTPB (a) and 1 (b) in CHCl3 and THF at different concentrations.
12230 | Dalton Trans., 2014, 43, 12228–12235
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The absorption spectra of complex 1 at 10−6–10−4 M are band and the appearance of a new luminescence band at
shown in Fig. 3(b), which is very similar to that observed in the 510 nm. According to the results observed in UV-Vis absorp-
free ligand BTPB at 10−6 M in THF. It is noted that the relative tion spectra, we can confirm that the new bands should orig-
intensity of the low-energy bands at 369 nm decreases signifi- inate from the emission of a preformed aggregate. The
cantly in complexes compared to that observed in ligands. It emission spectra of BTPB in CHCl3 also show aggregates’
means that the intermolecular aggregation begins to reduce or emission, which is similar to that observed in THF solution
disappear in complexes. In addition, the absorption maxima (Fig. 4(b)). Notably, we simultaneously observed the emission
of complexes are blue-shifted by about 8 nm compared to the of the monomers and aggregates, even at 10−6 M concen-
ligand. To investigate the aggregation behaviors of the complex tration. With the increase of the concentration, the ligand
in both solvents, the absorption spectra of 1 at 10−5 M are com- shows similar behaviors to that in THF.
pared in Fig. 3 (b, middle). In CHCl3, the low-energy tail red-
The emission spectra of complex 1 in THF at different con-
shifts by 10 nm in comparison with that observed in THF, and centrations are shown in Fig. 4(c). Upon excitation at 360 nm,
extends to 395 nm. This result means that the complexes still 1 displays a series of characteristic narrow band emissions of
exhibit aggregation behaviors in CHCl3.
Sm(III) ions at 565 nm, 602 nm, and 645 nm, corresponding to
4
the G5/2→6HJ (J = 5/2, 7/2, 9/2) transitions. Among them, the
Luminescence properties
most intense emission peak at 645 nm belongs to the hyper-
sensitive 4G5/2→6H9/2 transition. It is worth noting that the
emission of the ligand is hardly observed, indicating efficient
energy transfer from the ligand to Sm(III) ions. In CHCl3, the
emission spectra of 1 (Fig. 4(d)) likewise are dominated by the
characteristic emission bands of Sm(III) ions. In addition, we
also observed the emission bands of the ligand again, which
The emission spectra of BTPB in THF at various con-
centrations are shown in Fig. 4(a). At low concentrations
(≈10−6 M), the ligand shows characteristic emission of the
β-diketonate monomer (BTPB) with the emission maximum at
430 nm [see Fig. 4(a) (inset)]. An increase in the concentrations
leads to a relative decrease in the intensity of this emission
Fig. 4 Emission spectra of (a) BTPB in THF (inset: enlarged figure of the green line); (b) BTPB in CHCl3; (c) complex 1 in THF; (d) complex 1 in CHCl3
[the inset shows how the CIE coordinates vary: a, 1.0 × 10−4 M (0.498, 0.373); b, 1.0 × 10−5 M (0.447, 0.356); c, 5.0 × 10−6 M (0.381, 0.321); d,
1.0 × 10−6 M (0.293, 0.269)] (λEx = 360 nm).
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indicates that the energy transfer from the ligand to the metal
center is not complete. Upon excitation at 360 nm, the relative
intensity of the Sm(III) ion emission increases and that of the
ligand emission decreases as the 1 concentration increases.
The decrease in the relative intensity of ligand emission is
mainly due to the red-shift of the excitation bands with the
concentration increase, which result in the ligand having a
lower molar extinction coefficient at 360 nm. As shown in
Fig. S4,† the high energy band of the excitation spectra for
complex 1 shows obvious red-shift following the increase of
solution concentration. It indicates that the complex will
display relatively lower emissive intensity in the region of
400–550 nm with the increase of solution concentration, as
the excitation wavelength is located at high energy bands
(360 nm). Notably, the emission spectra cover the whole visible
range from 390 to 680 nm. Therefore, through changing the
concentration of 1 in CHCl3 achievement of white light emis-
sion is possible. As expected, the CIE chromaticity diagram
with the change of concentration exhibited a color shift from
yellow to white with the excitation at 360 nm. As shown in
Fig. 4(d) (inset), the c point (5 × 10−6 M) is the closest to the
white light emission to the eye with CIE coordinates of (0.381,
0.320), which fall well within the white region of the 1931 CIE
diagram (for pure white x = 0.33, y = 0.33).
Furthermore, we also investigated the variation trends of
the emission spectra for 1 with the change of excitation wave-
lengths. Fig. 5 shows the emission spectra of 1 in CHCl3 (1.0 ×
10−5 M) with the excitation wavelength varying from 355 to
390 nm. For comparison of the relative intensity of the ligand
and Sm(III) ion emission, the 4G5/2→6H9/2 transition is normal-
ized. It is found that the relative emission intensity of the
ligand increases gradually as excitation wavelengths vary from
355 nm, every 5 nm, to 390 nm. The corresponding emission
colors are illustrated in the CIE chromaticity diagram,
while the corresponding CIE color coordinates are listed in
Table S1.† It can be seen that most of the colours of emissions
fall within the white region of the 1931 CIE chromaticity
diagram. At excitation at 385 nm, 1 exhibited a nearly white
light emission with CIE coordinates of (0.335, 0.318), which
are very close to that for pure white light (x = 0.33, y = 0.33)
according to the 1931 CIE diagram.
In addition to the steady-state emission, we also carried out
the luminescence quantum yields and the time-resolved PL
decay dynamics study. The calculation results are listed in
Table 1. The luminescence quantum yields of the BTPB in
THF and CHCl3 are measured to be in the range of 0.08–0.74%
and 0.53–0.90%, respectively. An increase in the concentration
leads to the decrease of the luminescence quantum yields,
indicating the presence of the concentration quenching effect
in solution. The luminescence quantum yields of 1 in THF are
measured to be 0.19–0.27%, in which only the Sm(III) ion
luminescence is observed. The luminescence quantum yields
of 1 are measured to be 0.48–1.43% in CHCl3. However, the
values are comprised of two part emissions of the ligand and
Sm3+ ions. At a concentration of 1.0 × 10−5 M, complex 1
shows the maximum luminescence quantum yield of 1.43%.
Fig. 5 Emission spectra (top) and CIE chromaticity diagram (bottom) of
1 at different excitation wavelengths in CHCl3 (1.0 × 10−5 M).
The luminescence lifetimes of the BTPB are determined by
monitoring the emission decay curves of the ligand at 510 nm
(at 430 nm, 1.0 × 10−6 M, THF). In THF, the decay curves can
be fitted by single-exponential functions at 1.0 × 10−6 M,
whereas the kinetic behavior at higher concentrations (10−5
–
10−4 M) requires two exponential decay parameters to fit the
data. Similar kinetic behaviors for ligand emission bands are
also observed in 1, and the lifetimes used for the two-com-
ponent decay model for these are included in Table 1. The life-
time of 1.49 ns for the free ligand calculated according to the
single-exponential function can be attributed to the monomer
luminescence. The aggregate luminescence decay measured
at a higher concentration has two components, one at
1.10–1.59 ns and a longer-lived one at 2.9–3.96 ns. The life-
times of the shorter lived components are very close to that
observed in monomers (1.49 ns), so it is likely to arise from
the contribution of monomers. The luminescence lifetimes of
the Sm(III) ions are determined by monitoring the emission
4
decay curves within the G5/2→6H9/2 transition at 645 nm. All
the decay curves give satisfactory fits to the double-exponential
lifetimes, which suggests that there exist two species in the
excited state for 1 in both solvents. It is noted that the life-
times of Sm(III) ions in CHCl3 are obviously longer than that
observed in THF. The relatively shorter lifetimes observed for
Sm(III) ions in THF may be caused by the vibronic coupling of
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Table 1 Summary of fluorescence decay kinetics and quantum yields of BTPB and 1 at different concentrations in THF and CHCl3 with 360 nm
excitation
BTPB (THF)
τobs (ns)
1.49
BTPB (CHCl3)
1 (THF)
1 (CHCl3)
Conc. (mol L−1
1.0 × 10−6
)
QY (%)
0.74
τobs (ns)
QY (%)
Sm3+ τobs (μs)
QY (%)
0.25
BTPB τobs (ns)
Sm3+ τobs (μs)
QY (%)
0.48
1.59
2.98
1.47
3.22
1.19
2.90
1.10
3.64
0.90
0.86
0.63
0.53
7.43
26.22
5.90
27.92
8.39
28.88
6.15
29.00
1.06
3.67
1.07
3.59
1.28
3.32
1.49
3.12
13.93
46.26
12.24
45.75
14.57
46.25
15.07
47.79
1.0 × 10−5
1.52
3.11
1.52
3.45
0.92
3.96
0.56
0.24
1.43
5.0 × 10−5
0.40
0.27
0.71
1.0 × 10−4
0.08
0.19
0.60
the C–H oscillators in THF with the metal centers, which
increases the nonradiative decay rates.
Energy transfer between the ligand and Sm(III) ions
In general, the widely accepted energy transfer mechanism for
the sensitization of Ln(III) ion luminescence with a ligand is
proposed by Crosby.23 In order to make energy transfer
effective, the energy-level match between the triple states of
the ligand and the Ln(III) ion becomes one of the most impor-
tant factors dominating the luminescence properties of the
complexes. To elucidate the energy transfer process of the
samarium complexes, the energy levels of the relevant elec-
tronic states should be estimated. The singlet and triplet
energy levels of BTPB were estimated by referring to wave-
lengths of UV-Vis absorbance edges and the lower wavelength
emission peaks of the corresponding phosphorescence
spectra.
Fig. 6 Schematic energy level diagram and energy transfer process for
complex 1. (S1BTPB: first excited singlet; T1BTPB: first excited triplet state).
Conclusions
On account of the difficulty in observing the phosphor-
escence spectra of the ligand, the emission spectrum of
complex 2 at 77 K can be used to estimate the triplet state
energy level. Because the lowest excited energy level of the
Gd3+ ion (6P7/2) is too high to accept energy from the ligand,
the triplet state energy level of the ligand is not significantly
affected. As shown in Fig. S5,† the triplet energy level of 2 is
20 243 cm−1, which corresponds to their lower emission peak
wavelength at 494 nm. Obviously, the triplet levels of the BTPB
In summary, we have successfully designed and synthesized a
new single molecule Sm(III) complex Sm2(BTPB)3(H2O)4 to
achieve white light emission. The complex displays different
luminescence behaviors in THF and CHCl3. In CHCl3, a
tunable blue/green emission from the ligand according to its
degree of aggregation, and a sensitized red emission from the
Sm(III) center produce a concentration and excitation wave-
length dependent white light emission from complex 1. This
work realized the idea of only one type of ligand participation
in the construction of single molecule white-light-emitting
materials. Our future research will focus on exploiting Eu(III)-
based white light systems with high quantum yield based on
similar components.
4
is higher than the G5/2 level (17 900 cm−1) of Sm3+ ions, and
their energy gap is 2343 cm−1, which is large enough to allow
back energy transfer. Therefore, the ligand can transfer energy
effectively to the emitting states of the Sm(III) ion. The single
state energy (1ππ*) level of the ligand is estimated by referring
to its absorbance edge, which is 25 641 cm−1 (390 nm). It is
noted that the energy gap between the 3ππ* and 1ππ* levels
is 5398 cm−1 for the ligand BTPB. According to Reinhoudt’s
empirical rule,24 the intersystem crossing process becomes
effective when ΔE (1ππ*–3ππ*) is at least 5000 cm−1, and hence
the intersystem crossing process is effective for this ligand.
Experimental
Materials and instruments
According to the above experimental results, the schematic The commercially available chemicals were of analytical
energy level diagram and the energy transfer process of 1 are reagent grade and used without further purification. 1,1-Bi-2-
shown in Fig. 6. In conclusion, the effective intersystem cross- naphthol (BINOL) (99%, A. R.) was purchased from Shanghai
ing and ligand-to-metal energy transfer make the ligand suit- Darui Finechemical Co. (Shanghai, China); LnCl3·6H2O was
able for sensitizing the Sm(III) luminescence.
prepared according to the literature by dissolving 99.99%
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oxide in a slight excess of hydrochloric acid. The solution was
evaporated and the precipitate was recrystallized from water.
Syntheses
of
4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)-
(phenoxy)-1,1′-binaphthalene (BTPB). A mixture of sodium
FT-IR spectra were obtained on a Perkin-Elmer Spectrum methoxide (2.16 g, 0.04 mol) and ethyl trifluoroacetate
One spectrophotometer using KBr disks in the range of (7.14 mL, 0.06 mol) in 120 mL dry DME (DME = dimethoxy-
4000–370 cm−1. UV spectra were recorded on a Perkin-Elmer ethane) was stirred for 10 min, followed by the addition of
Lambda 25 spectrometer. The absorbance was recorded using BAPB (10.45 g, 0.02 mol) in portions, which was further stirred
quartz cells with an optical path length of 1.0 mm for 1.0 × at room temperature for 24 h (Scheme 1).The resulting solu-
10−4 M, and 10 mm for the rest of solutions. The 1H NMR tion was quenched with water and was acidified to pH 2–3
spectra were recorded on a Bruker Avance III 400 MHz spectro- using hydrochloric acid (2 M solution). The resulting yellow
meter in CDCl3 solution. Electron ionization (EI) and Electro- precipitate was filtered and dried at 40 °C under reduced
spray TOF (ESI-TOF) mass spectra were recorded on Agilent pressure. Recrystallization from acetone gave yellow flake crys-
5973 N and Bruker maXis mass spectrometers, respectively. tals (13.20 g, 92%). Anal. Calc. for C40H24F6O6: C, 67.23; H,
Excitation and emission spectra were measured with an Edin- 3.39; O, 13.43. Found: C, 67.29; H, 3.48; O, 13.47. IR (KBr,
burgh FLS 920 fluorescence spectrophotometer. Luminescence cm−1): 3060, 1594, 1504, 1243, 1167, 1108, 797. 1H NMR
lifetimes were recorded on a single photon counting spectro- (CDCl3, 400 MHz): 15.20 (s, 2H, Hj), 7.96–7.98 (d, 2H, Hd),
meter with an Edinburgh FLS 920 fluorescence spectro- 7.92–7.94 (d, 2H, He), 7.61–7.63 (d, 4H, Hh), 7.31–7.49 (m, 4H,
photometer with a microsecond pulse lamp and a picosecond Hb, Ha), 7.24–7.28 (m, 4H, Hc, Hf), 6.74–6.76 (d, 4H, Hg), 6.35
laser as the excitation sources. The data were analyzed using (s, 2H, Hi). ESI-MS m/z 737 [M + Na]+.
the software supplied by Edinburgh Instruments. Lumines-
Syntheses of the complexes Ln2(BTPB)3(H2O)4 [Ln = Sm (1),
cence quantum yields for the ligand and Sm(III) complex were Gd (2)]. To a 100 mL methanol solution of BTPB (1.0 g,
measured by an optically dilute relative method25 using 1.40 mmol), NEt3 (0.40 mL, 2.80 mmol) was added, and the
Ru(bpy)3Cl2·6H2O (0.028 in aerated H2O)26 as a standard. The mixture was allowed to stir for 5 min. To this solution,
calculated process is given by the well-known equation:
LnCl3·6H2O (0.34 g, 0.93 mmol) in methanol (50 mL) was
added dropwise and stirred overnight at room temperature.
The product was filtered and washed with H2O (2 × 10 mL)
and CH3OH (2 × 10 mL) and dried under vacuum to give the
Arefn2I
φoverall
¼
φref
2
Anref Iref
where n, I, and A denote the refractive index of the solvent, the desired product Ln2(BTPB)3(H2O)4 (Ln = Sm, Gd).
Sm2(BTPB)3(H2O)4 (1). Yield: 84%. Anal. Calc. for
area of the emission spectrum, and the absorbance at the exci-
tation wavelength, respectively, and φref represents the
quantum yield of the standard. The subscript ref denotes the
reference, and the absence of a subscript implies an unknown
sample.
C
120H74F18O22Sm2: C, 57.41; H, 2.97; O, 14.02. Found: C, 57.29;
H, 2.88; O, 13.97. IR (KBr, cm−1): 3060, 1594, 1504, 1243, 1167,
1108, 797. ESI-MS m/z 2461 [Sm2(BTPB)3 + Na]+.
Gd2(BTPB)3(H2O)4 (2). Yield: 80%. Anal. Calc. for
C
120H74F18O22Gd2: C, 57.10; H, 2.95; O, 13.94. Found: C, 57.27;
Syntheses of 4,4′-bis-(acetyl)phenoxy-1,1′-binaphthalene
(BAPB). The reaction (Scheme 1) was conducted in a 1000 mL
three-necked round-bottom flask that was equipped with a
mechanical stirrer, a nitrogen inlet and a condenser. A mixture
of DMF (400 mL), potassium carbonate anhydrous (13.86 g,
0.10 mol), and 4′-fluoroacetophenone (104.60 mL, 1.0 mol)
was added into the reaction vessel and heated to reflux under
pure nitrogen with stirring. Then a DMF solution (100 mL) of
1,1-bi-2-naphthol (28.63 g, 0.10 mol) was added over a period
of 1 h. The solution was kept under reflux for another 12 h to
ensure the completion of the reaction, and then it was cooled
and filtered to remove the salt. The solution was poured to
1000 mL distilled water to precipitate the product. The precipi-
tate was collected by filtration and washed with water three
times, and then it was washed with ethanol three times. The
solid was dried by drawing air through the filter cake for 1 h.
The crude product was purified by crystallization from acetone
to give white crystals (37.53 g, 72%). Anal. Calc. for C36H26O4:
C, 82.74; H, 5.01; O, 12.25. Found: C, 82.79; H, 4.98; O, 12.30.
IR (KBr, cm−1): 3053, 1675, 1583, 1501, 1245, 810. 1H NMR
(DMSO, 400 MHz): 8.13–8.11 (d, 2H, Hd), 8.05–8.03 (d, 2H,
He), 7.75–7.73 (d, 4H, Hh), 7.51–7.47 (d, 2H, Hb), 7.39–7.34
(m, 4H, Hc, Ha), 7.17–7.15 (d, 2H, Hf), 6.83–6.81 (d, 2H, Hg),
2.44 (s, 6H, Hi). ESI-MS m/z 545 [M + Na]+.
H, 2.90; O, 13.87. IR (KBr, cm–1): 3060, 1598, 1501, 1235, 1170,
1109, 795. ESI-MS m/z 2475 [Gd2(BTPB)3 + Na]+.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (no. 51302068 & 21272061 &
51102081 & 21102039 & 21302045).
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