OFF-ON-OFF Dual Emission at Visible and UV Wavelengths from Carbazole Functionalized β-Diketonate Europium(III) Complex

Article abstract of DOI:10.1021/acs.jpca.6b04414

This work demonstrates dual emission "OFF-ON-OFF" switching at visible and UV wavelengths of a carbazole functionalized β-diketone (LH) by a simple change of a europium(III) ion (Eu³⁺) concentration in the submicromolar concentration range. In

Full text of DOI:10.1021/acs.jpca.6b04414

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Article
OFF–ON–OFF Dual Emission at Visible and UV Wavelengths from
Carbazole Functionalized #-Diketonate Europium(III) Complex
Yuki Imai, Tsuyoshi Kawai, and Junpei Yuasa
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04414 • Publication Date (Web): 31 May 2016
Downloaded from http://pubs.acs.org on June 6, 2016
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OFF–ON–OFF Dual Emission at Visible and UV Wavelengths from
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Carbazole Functionalized β-Diketonate Europium(III) Complex
Yuki Imai , Tsuyoshi Kawai* , and Junpei Yuasa*
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††
†
†‡†
†
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192,
Japan
‡
Precursory Research for Embryonic Science and Technology (PRESTO), Science and Technology Agency (JST), 4-1-8 Honcho,
Kawaguchi, Saitama 332-0012, Japan
ABSTRACT: This work demonstrates dual emission “OFF–ON–OFF” switchꢀ
ing at visible and UV wavelengths of a carbazole functionalized
(
β
ꢀdiketone
LH) by a simple change of a europium(III) ion (Eu ) concentration in the
subꢀmicro molar concentration range. In the presence of 0.25 equivalent of
3+
3
+
3+
–
–
Eu (5
ꢀM), LH forms a luminescent 4:1 complex ([Eu (L ) ] ) exhibiting
4
dual emission at 357 nm and 613 nm resulting from the local excitation of the
3+
carbazole ring and ligandꢀsensitized luminescence from the Eu ꢀ
βꢀdiketonate
unit, respectively. The 4:1 complex begins to convert into a 2:1 complex
3
+
–
+
3+
–
(
(
[Eu (L ) ] ) via a 3:1 complex [Eu (L ) ] above the molar ratio
2 3
3+
[Eu ]/[LH]) of 0.25, which provides the opportunity for binding of solvent
3
+
3+
–
+
methanol molecules to the vacant site of the Eu ion in the complex ([Eu (L ) (MeOH) ] ). The OH oscillators of coordinated methanol
molecules facilitate the nonꢀradiative pathway of the Eu emission, hence the emission at 613 nm almost disappears above the 0.50 equivꢀ
alent of Eu (11 ꢀM), while the UV emission at 357 nm remains mostly constant over the whole concentration range.
2
n
3+
3
+
ꢀ
INTRODUCTION
tiple configurable dual emission mode will be of interest in the
future development of molecular technology.
In the fabrication of molecular information processing devices,
ranging from sensors and logic gate functions to molecularꢀlevel
machines, a key component in every facet of the process is
ꢀ
EXPERIMENTAL SECTION
1–6
switching.
A promising direction in switching is fabrication of
stimuliꢀresponsive luminescent materials possessing tunable emisꢀ
1
7
–15
General. H NMR spectra were measured with JEOL ECXꢀ
sions with nonꢀlinear switching behavior.
To this end, lumiꢀ
4
00P (400 MHz). Chemicals were purchased from Wako Pure
nescent systems are required such that single application of an
input can switch the system to the ON (emissive) state and be set
back to the OFF (nonꢀemissive) state by an equal input, affording
a bellꢀshaped emission profile with the maximum luminescent
output within a narrow input range. However, such “OFF–ON–
OFF” luminescence switchability has been only with fluorescent
materials showing a single emission output in a wide input range
Chemical Industries Ltd. and used as received without further
purification. Europium(III) trifluoromethanesulfonate was obꢀ
tained from Aldrich. The emission lifetimes were recorded using
FluoroCube (HORIBA, 3000UꢀYSP). Mass spectra were measꢀ
ured with mass spectrometers (JEOL AccuTOF JMSꢀT100LC for
ESI, JMSꢀ700 MStation for EI, JMSꢀQ1000TD for DART). DFT
calculations were performed with Gaussian 09. The emission
spectra were measured at room temperature using JASCO FPꢀ
6500. UVꢀvis absorption spectra of LH were recorded by JASCO
Vꢀ660 in the absence and presence of Eu(OSO CF ) in methanol
(e.g., wide pH range from acidic to basic conditions), and furꢀ
thermore, luminescent intensity dose not go return to zeroꢀlevel
even in saturation range.
We report herein “OFF–ON–OFF” switchable dual emisꢀ
sion at visible and UV wavelengths from the carbazole functionalꢀ
2
3 3
at room temperature.
Preparation of ethyl 2-(9H-carbazol-9-yl)acetate (1a): 1a
was prepared according to a procedure described previously,
where the process were inspired by the strategy published for the
synthesis of a similar molecule. To a 100 mL flask containing
9Hꢀcarbazol (3.0 g, 17.9 mmol) was added potassium carbonate
(3.7 g, 26.9 mmol) and dry DMF (25 mL). The reaction mixture
ized
βꢀdiketone (LH) via simple adjustments in the concentration
3
+
of europium(III) ion (Eu ).
diketonate Eu complexes were widely used as emitters in organꢀ
ic lightꢀemitting diodes.
put (sharp Eu emission line) can be switched ON and returned to
zeroꢀlevel (OFF) within a narrow input range (narrow Eu conꢀ
centration range) while maintaining a second emission output
Carbazole functionalized
βꢀ
3+
2
6
1
6–25
In this new system, an emission outꢀ
3+
3+
o
was stirred at 55 C for 2 h. After this time, ethyl bromoacetate
4.4 mL, 26.9 mmol) was added to the flask and the reaction mixꢀ
(
(
UV emission from the carbazole ring) in the ON state. This mulꢀ
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o
ture was stirred at room temperature for 2 h and 55 C for 1 h.
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Then, the mixture was poured into water and filtered. This comꢀ
pound was used for the next reaction without further purification.
1
The purity (higher than 95%) of 1a was checked by the H NMR
spectrum. Yield: 4.42 g (97%)
. MS (DART) m/z calcd for
+
1
C H NO [M+H] 254, found 254. H NMR (400 MHz, CD Cl )
1
6
15
2
2
2
δ 8.11 (d, J = 7.8 Hz, 2H), 7.45–7.49 (m, 2H), 7.35 (d, J = 8.2 Hz,
2
1
H), 7.25–7.29 (m, 2H), 5.02 (s, 2H), 4.20 (q, J = 7.0 Hz, 2H),
.25 (t, J = 7.1 Hz, 3H).
Preparation of 4-(9H-carbazol-9-yl)-1-phenylbutane-
,3-dione (LH): To a flameꢀdried 50 mL 2ꢀneck flask containing
0
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1
hexamethyldisilazane potassium salt: KHMDS (11.8 mL of a 0.5
M solution in toluene, 5.92 mmol), acetophenone (0.69 mL, 5.92
mmol) in THF (4 mL) was added dropwise at 0 °C. Subsequently,
the solution was stirred at room temperature for 1 h. After this
time, compound 1a (1 g, 3.95 mmol) in THF (4 mL) was added
dropwise, and the resulting reaction mixture was stirred at room
temperature for 12 h. Afterwards, the mixture was poured into
water and acidified with diluted hydrochloric acid. The organic
layer was extracted with chloroform, washed with water, and then
dried with anhydrous sodium sulfate. The solvent was removed by
reduced pressure. The crude product was purified by silica colꢀ
umn chromatography (dichloromethane) and recrystallized in
Figure 1. (a) Selected molecular orbitals (MO) of LH calculated by a
DFT method (CAM B3LYP/6ꢀ31G(d,p)). UVꢀvis absorption spectra of 2.0
–5
×
10 M of (b) LH, (c) DK (red line) and Cz (blue line) in methanol at
98 K. Dashed line shows the sum of the absorption spectra of DK and
Cz.
2
chloroform/methanol
to
provide
4ꢀ(9Hꢀcarbazolꢀ9ꢀyl)ꢀ1ꢀ
(
Figure 2a; for emission decay of LH at 613 nm in the presence of
3+
phenylbutaneꢀ1,3ꢀdione (LH) as a pale yellow crystal (520 mg,
Eu , see Figure S1). Of particular interest is that the UV emisꢀ
sion at 357 nm remains mostly constant over the whole concentraꢀ
tion range (Figure 2a), hence enabling “OFF–ON–OFF” switchꢀ
ing of dual emission at visible and UV wavelengths by a simple
change in Eu ion concentration. This “OFF–ON–OFF” switchꢀ
ing mechanism appears to be due to stepwise complex formation
1
40%). H NMR (400 MHz, CDCl ): δ 8.17–8.11 (m, 2H), 7.59–
3
7.52 (m, 2H), 7.52–7.25 (m, 9H), 5.80 (s, 2H), 5.08 (s, 2H).
+
HRMS (EI) m/z calcd for C H NO [M] 327.1259, found
2
2
17
2
3+
1
3
3
27.1267.
140.48, 133.71, 132.63, 128.34, 127.02, 126.16, 123.18, 120.53,
19.82, 108.48, 93.17, 48.92.
C NMR (100 MHz, CDCl ): δ 192.79, 183.32,
3
3+
34–41
3+
of LH with Eu (Scheme 1).
ty at 613 nm ( D → F ) was plotted versus the molar ratio
Then, the Eu emission intensiꢀ
1
5
7
0
2
3+
3+
([Eu ]/[LH] ) to estimate the binding stoichiometry of the Eu
0
complexes. The emission intensity (I613) reaches the maximum
ꢀ
RESULTS AND DISCUSSION
3
+
value at around [Eu ]/[LH] = 0.25 and then diminishes at around
0
3+
[
Eu ]/[LH] = 0.50 (inset of Figure 2a), indicating the 4:1 and 2:1
0
binding stoichiometry of LH:Eu . We tentatively conclude that
the 4:1 complex ([Eu (L ) ] ) converts to the 2:1 complex
OFF–ON–OFF Dual Emission at Visible and UV Wave-
3+
lengths. In the present work, a fluorescent carbazole ring was
functionalized into a  functionalized into a ꢀdiketonate ligand
with a methylene spacer electronically uncoupled the two units.
3+
–
–
4
3+
–
+
3+
–
β
(
[Eu (L ) ] ) via the 3:1 complex [Eu (L ) ], because it is diffiꢀ
2
3
16–
cult to assume that the 3:1 complex is missing between the 2:1
and 4:1 complexes. Nevertheless, the equilibrium between the 3:1
complex and 2:1 complex (Scheme 1b) should lie largely to forꢀ
mation of the 2:1 complex, and therefore there is no clear break at
23
A DFT calculation (CAMꢀB3LYP/6ꢀ31G(d,p)) gives an overꢀ
view of the molecular orbitals of LH, where LUMO belongs to
the phenyl βꢀdiketonate unit, and HOMO is essentially localized
3+
the molar ratio of [Eu ]/[LH] = 0.33 in the titration curves (inꢀ
0
on the carbazole ring (Figure 1a), suggesting that these two units
are electronically uncoupled. Actually, the UVꢀvis absorption
spectrum of LH (Figure 1b) is almost identical to the sum of the
absorption spectra of the corresponding chromophore units (DK +
Cz, Figure 1c dashed line), demonstrating that there is no signifiꢀ
cant ground state interaction between them.
sets of Figure 2a and 2f). Consequently, concentration of the 3:1
complex should be negligible as compared to that of the 2:1 comꢀ
plex or that of the 4:1 complex in solution.
3+
LH visible luminescence (20
ꢀM) in the Eu concentration
range of 0–12 M is shown to visualize the “OFF–ON–OFF”
ꢀ
switchability of LH on dual emission at visible and UV waveꢀ
With these results in hand, the luminescence properties of LH
lengths (Figure 2b). LH exhibits strong red luminescence correꢀ
3
+
were investigated in a Eu ion concentration range 0–50
ꢀM
5
7
3+
sponding to the D → F transition at 613 nm in a narrow Eu
–
27–29
3+
0
2
(
counter anion: CF OSO ).
In the absence of Eu , LH (20
3
2
concentration range (2–9
ꢀM), while there is no appreciable lumiꢀ
ꢀ
M) exhibits UV emission centered at 357 nm (Figure 2a red
nescence in the absence or in the presence of more than 11 ꢀM of
3
+
line). Upon addition of Eu (0–4.8 ꢀM), due to the fꢀf transition
of Eu ( D → F , J = 0, 1, and 2, respectively)
3+
Eu (Figure 2b). Under the same experimental conditions, the
3+
5
7
30–33
emission
0
J
emission intensities at visible (613 nm) and UV wavelengths (357
bands at 580, 591, and 613 nm appear and start to increase with
3+
nm) were recorded and plotted against the Eu concentration
Figure 2c and 2d, respectively). The visible emission intensity at
13 nm is observed in a certain and very narrow concentration
3+
increasing Eu concentration (Figure 2a, red–blue line). Conꢀ
(
6
3
+
3+
versely, the Eu luminescence begins to decrease above [Eu ] =
4
3
+
.8 ꢀM, and almost disappears at Eu concentration of 11 ꢀM
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Scheme 1. Schematic Illustration of “OFF–ON–OFF” Switching of
Dual Emission at UV and Visible Wavelengths in Multistep Equilibri-
um Between β-Diketone Ligands and Eu
1
2
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3+
0
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0
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0
1
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3
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9
0
Scheme 2. Energy Diagram for Stepwise Complex Formation Between
3+
LH and Eu
3
+
range (0 < [Eu ] < 10
ꢀ
M) and brought back to zero level at
3
+
[Eu ] = 11
sity at 357 nm is almost constant over the whole Eu concentraꢀ
tion range (0–50 M), where there is a slight break at around
ꢀM (Figure 2c). Conversely, the UV emission intenꢀ
3
+
ꢀ
3
+
3+
[Eu ] = 5 ꢀM corresponding to the molar ratio of [Eu ]/[LH] =
0
0.25 for the formation of the 4:1 complex (Figure 2d).
Eq 1 and 2 can fit such nonꢀlinear “OFF–ON–OFF” switching
of the visible emission (613 nm) as a function of concentration
3+
Eu concentration (Figure 2c; for derivation of eq 1 and 2, see
Supporting Information S2 and S3, respectively). Eq 1 represents
a fitting equation for conversion ratio (α) of LH to the 4:1
–5
Figure 2. (a) Emission spectra of LH (2.0 × 10 M) in the presence of
3
+
–6
–5
Eu [0 M (red line)–4.8 × 10 M (blue line)–1.6 × 10 M] in methanol at
98 K. Top panel shows the corresponding stacked plot of emission specꢀ
tra. Excitation wavelength: ex = 324 nm. (b) Visible luminescence of LH
3
4
3+
α
= K [LH] (1 –
α
) (4[Eu ] –
α
[LH]0
(1)
(2)
1
0
2
λ
2
3+
–5
3+
–5
4[LH] (1 –
β
) = K2β(4[Eu ] – 2[LH] + [LH]0β
)
(2.0 × 10 M) in the presence of Eu (0–1.2 × 10 M) in methanol. Plots
of emission intensity at (c)
0
0
3+
λ = 613 nm and (d) 357 nm vs [Eu ]. Fitted
20
–4
3+
–
–
lines calculated by using eq 1 with K
1
= 10
2
M and using eq 2 with K =
complex ([Eu (L ) ] ) [Scheme 2], where
α changes from 0 to 1
4
3
3
10 are shown (red and blue lines, respectively). (e) Excitation spectrum of
+
3+
with increasing Eu concentration below [Eu ]/[LH]0 = 0.25
–5
3+
–6
LH (2.0 × 10 M) in the presence of Eu (4.8 × 10 M) recorded at
λ =
3+
(
[Eu ] < 5
ꢀM). On the other hand, eq 2 is a fitting equation for
3
57 nm (solid line) and = 613 nm (dashed line). (f) UVꢀvis absorption
λ
3+
–
–
–5
3+
–6
conversion ratio (
β
) of the 4:1 complex ([Eu (L ) ] ) to the 2:1
spectra of LH (2.0 × 10 M) in the presence of Eu (0 M–4.8 × 10 M)
4
3+
–
+
–6 –5 –5
[
red lines], (4.8 × 10 M–1.1 × 10 M) [blue lines], and (1.1 × 10 M–
–5
complex ([Eu (L ) ] ) [Scheme 2], where
β decreases from 1 to 0
2
3
+
3+
2.8 × 10 M) [yellow lines] in methanol at 298 K. Insets: Plots of (a)
emission intensity at = 613 nm and (f) absorbance at = 320 nm vs
with increase in Eu concentration above [Eu ]/[LH] = 0.25
0
λ
λ
3+
(
[Eu ] > 5
ꢀ
M). The
α
and
β
values correspond to I613/Imax at
3+
0
[Eu ]/[LH] .
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3
+
below and above [Eu ]/[LH] = 0.25, respectively, where I
0
max
1
2
3
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5
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
denotes the emission intensity at
λ = 613 nm due to the 4:1 comꢀ
3+
–
–
plex ([Eu (L ) ] ). Since the titration solution contains three
4
species (free LH ligand, the 4:1 and 2:1 complexes) at around
3+
3+
[
Eu ]/[LH] = 0.25 ([Eu ] = 5 ꢀM), eq 1 and 2 could not fit well
0
the data around this concentration range (Figure 2c). However,
the best fit lines can be obtained by using eq 1 with the equilibriꢀ
2
0
–4
3+
um constants K = 1.0 × 10
M
below [Eu ]/[LH] = 0.25 (red
1
0
3
line in Figure 2c; Figure S4) and by using eq 2 with K = 1.0 × 10
2
3
+
above [Eu ]/[LH] = 0.25 (blue line in Figure 2c; Figure S4). The
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
0
–4
large K value (1.0 × 10 M ) corresponds to the binding energy
–ꢁG = 27.3 kcal mol ) between Eu and the four L ligands
1
0
–1
3+
–
(
1
3
(Scheme 2). In contrast, the small K value (1.0 × 10 ) indicates
2
that the 4:1 and 2:1 complexes are on a relatively flat potential
0
–1
energy surface (–ꢁG = 4.1 kcal mol ), enabling “OFF–ON–
2
OFF” switching based on stepwise complex formation between
3+
LH and Eu (Scheme 2).
In order to confirm the origin of dual emission at the visible
and UV wavelengths, the excitation spectra of LH were recorded
–5
Figure 3. Emission spectra of (a) LH (2.0 × 10 M) in the presence of
3+
–6
–5
–5
Eu (4.8 × 10 M) (b) Cz (2.0 × 10 M), and (c) DK (2.0 × 10 M) in
the presence of Eu (4.8 × 10 M) in methanol at 298 K. Excitation
wavelength: (a) λex = 324 nm, (b) λex = 292 nm, and (c) λex = 297 nm.
3
+
3+
–6
at
corresponding to the 4:1 complex formation (Figure 2e). The
excitation spectrum recorded at = 357 nm (Figure 2e solid line)
λ = 357 nm and 613 nm at the molar ratio of [Eu ]/[LH] = 0.25
λ
closely resembles the absorption spectrum of the unfunctionalized
carbazole ring (Cz, Figure 1c blue line), suggesting that the UV
emission centered at 357 nm can be assigned to the fluorescence
band arising from the local excitation of the carbazole ring. Conꢀ
versely, the excitation spectrum recorded at
2e dashed line) is identical to the absorption spectrum of LH in
the whole region including both absorption bands of the phenyl
λ = 613 nm (Figure
β
ꢀ
3+
diketonate unit and the carbazole ring (Figure 2f at [Eu ] = 4.8
3
+
ꢀ
M). Hence the Eu luminescence stems from the antenna efꢀ
fects of both chromophore units, where intramolecular energy
transfer (EN) should occur partially from the localꢀexcited state of
the carbazole ring to the phenyl
βꢀdiketonate unit (Scheme 1).
1
–3
Actually, introduction of the ꢀdiketonate unit into the carbazole
β
Figure 4. H NMR spectra (400 MHz) of LH (2.0 × 10 M) in (a) CDCl
3
OD at 298 K. Small peaks (black circles) indicate the minor
isomer in equilibrium. (c) H NMR spectrum (400 MHz) of LH (2.0 × 10
3
and (b) CD
ring (Cz → LH) results in an almost 30ꢀfold decrease in the carꢀ
bazole fluorescence, indicating the efficient energy transfer from
the carbazole ring to the βꢀdiketonate unit (Scheme 1a). In addiꢀ
tion, it should be noted that the emission bands at 357 nm and 613
1
–
3
3+
–3
3
M) in the presence of Eu (2.0 × 10 M) in CD OD at 298 K.
nm are essentially identical to those of the unfunctionalized carꢀ
3+
(Figure 2a). Although this work forces complex formation of LH
with Eu at subꢀ10 micromolar, H NMR of LH in the presence
of Eu was also monitored at millimolar, where NMR signals due
bazole ring (Cz), and Eu complex with phenyl βꢀdiketone ligand
3+
1
(DK), respectively (Figure 3), which supports the above assumpꢀ
3+
tion.
to aromatic protons of LH (typically –NCH – protons of the carꢀ
2
UVꢀVis titration experiments were also performed under the
same experimental conditions as employed for the emission titraꢀ
bazole ring) undergo upfield shift due to complex formation with
3+
1
3+
Eu (Figure 4c). Small peaks (black circles in Figure 4b) of H
tion. The spectral changes are triphasic with respect to the Eu
concentration of 0–4.8 M (firstꢀphase: red lines), 4.8 M–11
secondꢀphase: blue lines), and 11 M–28 M (thirdꢀphase: yelꢀ
3+
NMR spectrum of LH in the absence of Eu should be assigned
to the minor isomer (enol or keto form) of LH in equilibrium.
ꢀ
ꢀ
ꢀM
(
ꢀ
ꢀ
Those small peaks disappear in CDCl probably due to the equiꢀ
3
low lines), with distinct isosbestic points in each step (Figure 2f;
Figure S5). Consequently, the titration plot shows a unique Vꢀ
shape curve, with two breaks at around the molar ratio of
librium shift (Figure 4c).
Mechanism of OFF–ON–OFF Dual Emission at Visible
and UV Wavelengths. Based on these results, the “OFF–ON–
OFF” dual emission at the visible and UV wavelengths with reꢀ
spect to the Eu concentration can be reasonably explained by
stepwise complex formation between LH and Eu (Scheme 1).
The LH ligand itself exhibits UV emission from the localꢀexcited
state of the carbazole ring (Scheme 1a). In the presence of 0.25
equiv of Eu , LH forms the 4:1 complex with a structure of
3
+
[
Eu ]/[LH] = 0.25 and 0.50 (inset of Figure 2f). This also sugꢀ
0
3+
gests the 4:1 and 2:1 binding stoichiometry between LH and Eu .
In the third phase, the absorbance at 320 nm saturates at around
3+
3+
3
+
[
Eu ]/[LH] = 1–1.5 (inset of Figure 2f), indicating a further
0
3+
change in the Eu complex (e.g., 1:1 complex) at the higher moꢀ
lar ratio. This point is not further addressed in this study, because
its emission properties are essentially the same as the 2:1 complex
3
+
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3+
–
+
[
Eu (L ) (MeOH) ] in solution (Scheme 1d). Coordination of
2 n
1
2
3
4
5
6
7
8
9
MeOH molecules to the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
–
5
Figure 5. (a) Emission spectra of LH (2.0 × 10 M) in the presence of
Gd(OSO CF in deaerated methanol at 298 K (red line) and at 223 K
blue line). Excitation wavelength: ex = 324 nm. (b) The corresponding
excitation spectrum recorded at = 495 nm in deaerated methanol at 223
2
3 3
)
–
5
3+
–5
Figure 6. Emission spectra of DK (2.0 × 10 M) in the presence of Eu
(
λ
–6
–6
(
a) (0 M–4.8 × 10 M) [red–blue line] and (b) (4.8 × 10 M–1.0 × 10
M–4.3 × 10 M) [blue–yellow–green line] in methanol at 298 K. Excitaꢀ
λ
–5
K. (c) The corresponding UVꢀvis absorption spectrum in deaerated methꢀ
tion wavelength:
λ
ex = 297 nm. (c) Plot of emission intensity at 614 nm vs
anol at 298 K.
3+
[
0
Eu ]/[DK] .
3
+
–
–
[
Eu (L ) ] (Scheme 1b), where the europium coordination
4
3+
3+
Eu significantly quenches the Eu emission, since the OH oscilꢀ
lators of coordinated molecules facilitate the nonꢀradiative pathꢀ
way. When methanol is replaced by deuterated methanol
MeOD), detectable Eu emission can be observed even above
the molar ratio of [Eu ]/[LH] = 0.50 (Figure S6). The OD oscilꢀ
lators of coordinated molecules have been suggested to be quite
and therefore, this observation
supports the above assumption that coordination of MeOH moleꢀ
cules to the Eu ion triggers the turn off the Eu emission. On
the other hand, binding of Eu on the βꢀdiketonate unit should
not significantly affect on the electronic properties of the carbaꢀ
zole ring, since the carbazole ring is electronically uncoupled with
βꢀdiketonate unit by methylene spacer (vide supra).
Hence, the UV emission intensity is almost constant for whole
concentration range of Eu . The negative control experiments
were performed with the reference compound (DK). The DK
ligand also shows OFFꢀONꢀOFF switching for the Eu emission
in response to the Eu concentration (Scheme 1e–f), while there
is no UV emission over the whole concentration range (Figure 6
and Figure S7).
ESI Mass Detection of the Complexes in Solution. Finally,
the stepwise complex formation processes between LH and Eu
was identified by the negativeꢀion ESI mass of a methanol soluꢀ
tion of LH containing 0.2 and 0.5 equiv of Eu(OSO CF ) (Figure
a and 7b, respectively). The mass spectrum of LH with 0.2
equiv of Eu shows intense signals at m/z = 1101.1 and 1455.4 in
the mass range above m/z of 1000 (Figure 7a), where the mass
spectrum is normalized to their most intense fragment at m/z =
sphere is occupied by the four
vorable coordination number (n = 8) for Eu ions. Tetrakisꢀ
diketonate lanthanide complexes are known to be stable in both
βꢀdiketonate ligands with the faꢀ
3+
β
ꢀ
3
+
(
42,43
the solid state and in solution.
In addition, it should be noted
that the triplet excited state energy of LH was determined to be
0,100 cm from the phosphorescence spectrum of LH in the
presence of gadolinium(III) (vide infra). A deaerated methanol
solution of LH containing Gd(OSO CF ) shows a broad emission
3+
0
–1
2
49,50
inefficient at the deexcitation,
2
3 3
3+
3+
–
1
band centered at
blue line), whereas it disappears at 298 K (Figure 5a red line).
The excitation spectrum recorded at = 498 nm (Figure 5b)
agrees well with the corresponding absorption spectrum of LH
λ = 498 nm (20,100 cm ) at 223 K (Figure 5a
3+
λ
the the phenyl
3+
with Gd (Figure 5c), and therefore, the broad emission band
Figure 5a blue line) is assignable to the phosphorescence specꢀ
trum of LH. The triplet energy level (20,100 cm ) of LH is highꢀ
er than the D state of Eu (17,200 cm ) and the LH ligand acts
as an effective antenna molecule for Eu emission in [Eu (L ) ]
(
3+
–
1
5
3+
–1
0
3+
3
+
3+
–
–
4
3+
44–48
3+
–
–
.
Hence, the [Eu (L ) ] complex shows the dual emission at
4
visible and UV wavelengths, the UV emission from the local exꢀ
3
+
cited state of the carbazole ring and the Eu emission via photo
sensitization of the ꢀdiketonate ligands, respectively (Scheme
b). Conversely, when [Eu (L ) ] converts to the 2:1 complex
β
3+
3
+
–
–
1
(
4
3+
–
+
3+
–
[Eu (L ) ] ) via a 3:1 complex [Eu (L ) ] above the molar ratio
of [Eu ]/[LH] = 0.25 (Scheme 1c and 1d), the resulting [Eu (L
) ] complex shows the UV emission from the local excited state
of the carbazole ring, but does not exhibit Eu emission. The
vacant sites of Eu in these complexes should be occupied by
solvent methanol molecules (MeOH), meaning formation of
2
3
3
+
3+
–
2
3 3
0
7
+
2
3+
3
+
3+
3
1
26.1 due to the free ligand (Figure S8). The signals at m/z =
101.1 and 1455.4 correspond to {[Eu(L) ](OSO CF ) } and
–
2 2 3 2
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Page 6 of 9
–
[
Eu(L) ] , respectively, where those signals agree closely with
emission via simple adjustments of the ion in the subꢀmicro molar
concentration range.
4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
their calculated isotopic distributions (inset of Figure 7). The
ꢀ
ASSOCIATED CONTENT
Supporting Information. Experimental details and supporting
spectral data. This material is available free of charge via the Inꢀ
ternet at http://pubs.acs.org.
ꢀ
AUTHOR INFORMATION
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Corresponding Author
yuasaj@ms.naist.jp; tkawai@ms.naist.jp
Phone: +81-743-72-6170
–3
†Present address Department of Applied Chemistry, Tokyo Univer-
sity of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8061, Japan
Figure 7. Negativeꢀion ESI MS of a solution of LH (2.0 × 10 M) in
–4
2 3 3
methanol in the presence of Eu(OSO CF ) (a) (4.0 × 10 M) and (b) (1.0
× 10 M). Intensity is normalized by their most intense fragment at m/z =
326.1 due to the free ligand. Insets: isotopically resolved signals at (a) m/z
–
3
ꢀ
ACKNOWLEDGMENT
=
1455.4 and (b) 1101.1 with the calculated isotopic distributions for
–
–
[
4 2 2 3 2
Eu(L) ] (top) and {[Eu(L) ](OSO CF ) } (bottom), respectively.
This work was partly supported by JSTꢀPRESTO “Molecular
technology and creation of new functions”; a GrantꢀinꢀAid for
Scientific Research (C) (No. 21750147) and Scientific Research
(A) (No. 21107520) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
molecular mass due to the 2:1 complex (L M) was detected even
at the molar ratio of [Eu ]/[LH] = 0.2 (Figure 7a). This is not
surprising since the intense fragment at m/z = 1101.1 should come
from a dissociation of the 4:1 complex (L M). More importantly,
2
3
+
4
the mass peak at m/z = 1455.4 due to the 4:1 complex (L M) alꢀ
4
ꢀ
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(2)
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0
(3)
ꢀ
SUMMARY AND CONCLUSIONS
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