Efficient 4f-5d emission processes of Ce3+complexes with benzimidazole-based tetradentate ligands

Article abstract of DOI:10.1246/cl.140449

A Ce³⁺complex with tetradentate ligands based on benzimidazole moieties was synthesized. The photochemical properties of the Ce³⁺complex were studied via absorption and emission spectra and emission quantum yield measurements. The effective values of the molar absorption coefficient and the emission quantum yield were estimated to be 890 dm³mol^-1cm^-1and 0.58, respectively; these values are relatively large compared to those of common rare-earth metal complexes that exhibit 4f-4f emissions.

Full text of DOI:10.1246/cl.140449

CL-140449
Received: May 2, 2014 | Accepted: May 25, 2014 | Web Released: May 30, 2014
Efficient 4f­5d Emission Processes of Ce3 Complexes
+
with Benzimidazole-based Tetradentate Ligands
Takashi Harada, Ryo Hasegawa, and Katsura Nishiyama*
Faculty of Education, Shimane University, Matsue, Shimane 690-8504
(
E-mail: katsura_nishiyama@edu.shimane-u.ac.jp)
A Ce3 complex with tetradentate ligands based on
+
(a)
(b)
benzimidazole moieties was synthesized. The photochemical
3+
properties of the Ce complex were studied via absorption and
emission spectra and emission quantum yield measurements.
The effective values of the molar absorption coefficient and
the emission quantum yield were estimated to be 890
3
¹1
¹1
dm mol cm
and 0.58, respectively; these values are
relatively large compared to those of common rare-earth metal
complexes that exhibit 4f­4f emissions.
Figure 1. Chemical structures of (a) the ligand ntb and (b)
¹
Ce¢ntb synthesized in this work. Note that CF3SO3 is omitted
in (b).
Trivalent rare-earth complexes have been applied as
luminescent materials because of their 4f­4f transitions with
a sharp spectral band width and longer emission lifetimes
1
compared with organic molecules. Such rare-earth complexes
because of its high emission yield, as reported by Zheng and
1
4
can be applied to luminescent resources for biomarkers, optical
fibers, and chemical sensors.1 After the photoexcitation of the
organic ligands of rare-earth complexes, energy transfer to the
metal center gives rise to the excited state, which contributes
to luminescent processes. Thus far, the issue concerning the
luminescence of rare-earth complexes is that blue emission due
to the 4f­4f transition has been shown to be unavailable because
the energy differences employed for emission are very small to
co-workers. This ntb ligand has been shown to be applicable to
­3
3+
15
various compounds, including a Tb complex. In this study,
we took advantage of the nonsubstituted benzimidazole moiety
at the 5-nitrogen of benzimidazole to simplify the ligand
molecular architecture. Figure 1 illustrates the chemical struc-
3
+
¹
tures of ntb and the Ce
complex, [Ce(ntb) ](CF SO ) ,
2 3 3 3
3
+
synthesized in this work. We hereafter denote the Ce complex
as Ce¢ntb.
1
achieve the blue color. If rare-earth complexes emitting in the
The details of the synthetic procedure of Ce¢ntb are provided
in the Supporting Information. Upon irradiation of UV light
at 365 nm, Ce¢ntb clearly exhibited blue emission. Figure 2a
exhibits the absorption spectrum of Ce¢ntb in acetonitrile, as
measured using a Jasco V-650 spectrometer. Two noticeable
absorption bands are observed in the wavelength region below
300 nm, with maxima at 274 and 282 nm; these maxima are
blue region are further developed, they can be used to tune the
luminescent color, such as to generate white light.3
­5
As a strategy for obtaining blue emission, we can employ
a complex, the emission of which is based on transitions other
than the 4f­4f transition. Aull and Jenssen have proposed that
inorganic compounds with Eu2 , Sm , or Ce ions may
+
2+
3+
6
16
exhibit emissions based on their 4f­5d transitions. Among such
complexes, Ce compounds have been proven to exhibit 4f­5d
emission that covers a range from 400 to 600 nm.
attributed to the benzimidazole moiety. In contrast, Figure 2b
3+
shows an enlarged view of the absorption band with a maximum
at 378 nm, which is assigned to Ce¢ntb, not to the ntb free ligand.
This absorption band at 378 nm is assigned to the 4f­5d transition
7,8
In an organic environment with respect to Ce3+, Ce3+
complexes have been reported to exhibit 4f­5d transitions
ranging from the UV to the blue region.9 An advantage in
producing 4f­5d emission by using metal complexes is that
we can design organic ligands so as to obtain complexes with
physical properties that enable their use in additional applica-
tions. An appropriate ligand design can render the complex
sufficiently soluble in the material environment. Organic ligands
serve as light-harvesting antennas with photoexcitation when the
complexes are employed as emitting materials.
1
4
according to the literature. We, therefore, investigated the
Ce¢ntb concentration ([Ce¢ntb]) dependence of the absorbance
at 378 nm, A378, where the absorption bands of ntb and the 4f­
5d transition overlap with each other. Figure 2b indicates the
[Ce¢ntb]-dependent feature of the absorption spectra. We ob-
served a linear relationship between A378 and [Ce¢ntb], as shown
­13
¹
in Figure 2c. This linearity implies that the [Ce(ntb)2](CF3SO3 )3
structure that we denote as Ce¢ntb would be stable in solution
¹
6
over the investigated [Ce¢ntb] range: [Ce¢ntb] = 5.4 © 10 to
Under such a research environment, Ce3 complexes with
+
1.2 © 10 mol dm . On the basis of our detailed analysis of
the results in Figure 2, the molar absorption coefficient ¾ of
Ce¢ntb at 378 nm (corresponding to the 4f­5d transition) is
¹4
¹3
bright emission have been developed for use in organic LED
1
3,14
devices.
To our knowledge, however, the literature contains
3
¹1
¹1
few reports on the basic properties and mechanisms of emitting
estimated to be 890 dm mol cm , excluding the contribution
from ntb. The calculation details are given in the Supporting
Information. The ¾ value for Ce¢ntb is significantly larger than
those for other rare-earth complexes exhibiting 4f­4f transitions,
3
+
Ce complexes.
3+
In the present work, we synthesized a Ce complex that
provides blue emission under UV excitation. Tris(1H-benz[d]-
imidazol-2-ylmethyl)amine (ntb) was chosen as the ligand
3
+
3+
17,18
such as Eu and Yb complexes.
1496 | Chem. Lett. 2014, 43, 1496–1498 | doi:10.1246/cl.140449
© 2014 The Chemical Society of Japan

(a)
(b)
Figure 3. Emission and excitation spectra of Ce¢ntb in acetoni-
¹4
¹3
trile at [Ce¢ntb] = 1.0 © 10 mol dm . The black bold line
indicates the emission spectrum detected with ­ex = 270 nm,
whereas the white bold-dot line is the emission with ­ex = 365 nm.
The red line corresponds to the excitation spectrum monitored at
4
38 nm.
responsible for the emission, irrespective of the ligand or the
3+
direct excitation of the Ce absorption band. Such a ­ex-
independent feature of the emission spectra has also been
3+
14
reported for Ce
complexes with different ligands. In
Figure 3, the overlapping Ce¢ntb emission spectra have a
maximum at 438 nm along with a shoulder at 465 nm. The
emission for both the spectra is attributable to the transition from
the 5d orbital interacting with the ntb ligand to the 4f orbital.
With respect to the spectral structure, the transition at the
maximum and shoulder may be due to the two possible ground
(c)
3+
1
states of the Ce ion with 4f , which respectively correspond to
2
14
FJ (J = 5/2, 7/2). In particular, the absorption and emission
maxima of Ce¢ntb are observed at 378 and 438 nm, respectively,
as shown in Figures 2 and 3. Zheng and co-workers reported
¹
that the absorption and emission maxima of Ce(CF3SO3 )3
in ethanol are at 244 and 299 nm, and 350 nm, respectively.
Therefore, the spectra obtained in the present study can be
3
+
3+
attributed to the Ce complex, not to the free Ce ion.
In Figure 3, the excitation spectrum of Ce¢ntb monitored at
the emission maximum, 438 nm, is also provided. The structure
in the shorter wavelength region (¯300 nm) and the peak at
3
+
3
80 nm correspond to the ntb and Ce 4f­5d absorption bands,
Figure 2. (a) Absorption spectrum of Ce¢ntb in acetonitrile,
with a concentration [Ce¢ntb] = 7.6 © 10 mol dm . (b) The
¹6
¹3
respectively. Notably, the spectral band shapes for both the
structure and the peak are similar to those of the absorption
spectra. The intensity of the 380-nm peak observed in the
excitation spectra is 38% compared with that of the structure
in the region at ¯300 nm. In addition, as already observed in
[
[
Ce¢ntb] dependence of the absorption band near 378 nm with
¹6
Ce¢ntb] = 5.4, 7.6, 29, 55, 76, 97, 100, and 120 [©10
¹3
mol dm ], drawn from the bottom to top. (c) The plot of the
absorbance at 378 nm, A378, versus [Ce¢ntb]. The red line is a
linear regression with R = 0.999.
2
3+
Figure 2a, the relative absorbance of the Ce 4f­5d transition
at 380 nm is 1% of that of ntb. We, therefore, propose that the
3
+
Figure 3 shows the corrected emission and excitation
spectra of Ce¢ntb in acetonitrile, as measured using a Jasco
FP-6500 spectrofluorometer. We first compare the excitation
emission quantum yield Φ can be larger under Ce 4f­5d
excitation than under ntb excitation.
Table 1 summarizes ΦX, where X = “overall” or “4f­5d,”
wavelength (­ ) dependence of the emission spectra. The
depending on ­ , as determined using an integrating sphere
ex
ex
emission spectra observed under ­ex = 270 and 365 nm almost
overlap with each other. Notably, these ­ex correspond to the
Jasco ILF-533. In this notation, Φoverall represents the emission
quantum yield according to the photoexcitation of ntb, whereas
Φ4f­5d represents that caused by the direct excitation of the Ce
3
+
3+
ntb and Ce 4f­5d absorptions, respectively. We can, therefore,
assume that the unique Ce3 lowest excitation state is
+
absorption band. In Table 1, Φ4f­5d is at least equal to the
Chem. Lett. 2014, 43, 1496–1498 | doi:10.1246/cl.140449
© 2014 The Chemical Society of Japan | 1497

Ce3 4f­5d absorption to be 890 dm mol cm¹1. In addition,
+
3
¹1
Table 1. Summary of emission quantum yields Φ for Ce¢ntb
Φoveralla
Φ4f­5da
we estimated that the hypothetical emission quantum yield of the
­
ex/nm
Φ4f­5d,hyp
4
f­5d emission is 0.58. Such an attractive emission property of
295
78
0.11 « 0.01
®
®
0.51 « 0.02
®
0.58
Ce¢ntb would be useful in luminescent color tuning in
conjunction with the use of rare-earth ions that exhibit 4f­4f
3
aDetected with 1.0 © 10¹4 mol dm¹3 Ce¢ntb in acetonitrile
solution. The measurements were performed three times; the
average value and standard deviation are provided.
3+
3+ 15,16
transitions, such as Eu and Tb
,
which emit in the orange
and green regions, respectively. Such a color tuning process can
cover most of the visible spectrum and may be applicable to
various emitting resources.
literature value for a Ce3 complex with a similarity of ligand
+
architecture, where Φ4f­5d = 0.55.14 Such a high value of Φ4f­5d
KN thanks financial supports from JSPS KAKENHI (Grant
Number 25410211), JST A-STEP (No. AS242Z01279M), the
Mazda Foundation, and the Electric Technology Research
Foundation of Chugoku.
1
9
can be ascribed to the spin-allowed 4f­5d transition of Ce¢ntb.
Our results are also supported by the fact that Ce3 complexes
have emission lifetimes in the ns domain. Such lifetimes are
substantially shorter than those of conventional rare-earth 4f­4f
complexes, which are on the order of ¯s to ms.20 Meanwhile, we
+
Supporting Information is available electronically on J-STAGE.
can roughly estimate the radiative rate constant k based on the
r
oscillator strength f and the maximal absorption wavenumber
References
6
¹1
2
¯
m. We obtained kr = 6.3 © 10 s , where kr = ¯m f and f =
1
2
J.-C. G. Bünzli, C. Piguet, Chem. Soc. Rev. 2005, 34, 1048.
C. Chen, D. Zhang, T. Li, D. Zhang, L. Song, Z. Zhen, Appl.
Phys. Lett. 2009, 94, 041119.
¹
9
21
4.3 © 10 ∫ ¾d¯ were used. Zheng and co-workers have
3+
estimated the quantum yield and lifetime of a Ce complex
with another type of imidazole ligand, from which we obtained
14
3
4
5
K. Miyata, Y. Konno, T. Nakanishi, A. Kobayashi, M. Kato, K.
Fushimi, Y. Hasegawa, Angew. Chem., Int. Ed. 2013, 52, 6413.
P. Coppo, M. Duati, V. N. Kozhevnikov, J. W. Hofstraat, L.
De Cola, Angew. Chem., Int. Ed. 2005, 44, 1806.
L. Armelao, G. Bottaro, S. Quici, M. Cavazzini, M. C. Raffo,
F. Barigelletti, G. Accorsi, Chem. Commun. 2007, 2911.
B. F. Aull, H. P. Jenssen, Phys. Rev. B 1986, 34, 6640.
R. Le Toquin, A. K. Cheetham, Chem. Phys. Lett. 2006, 423,
352.
7
¹1
14
k = 1.1 © 10 s . Note that in the literature data, the overlap
r
3
+
between Ce 4f­5d and ligand absorptions was not decoupled,
which might result in overestimation of the kr value. We thus
confirm the applicability of our spectroscopic results, at least on
a qualitative level.
To be precise, the Φ4f­5d result presented in Table 1 contains
a component associated with the ntb excitation because of
the overlapping absorption bands of ntb and the Ce3 4f­5d
transition under ­ex = 378 nm, as shown in Figure 2b. There-
fore, the absorption spectra around 378 nm was decoupled to
estimate the contribution solely from Ce3 4f­5d absorption.
With this analysis, the hypothetical emission quantum yield
Φ4f­5d,hyp that could arise from the Ce3 4f­5d excitation was
also estimated. In addition, the energy-transter efficiency © from
excited ntb to the Ce3 5d state was calculated. Notably, the
initial state of the 4f­5d emission in the present context
corresponds to the Ce3 5d state. Consequently, © = 0.19 was
obtained; the calculation details are available in the Supporting
Information. We emphasize here that the value of Φ4f­5d,hyp is a
6
7
+
8
9
K. Klier, A. C. Miller, L. L. Zhang, M. K. Hatalis, Chem.
Mater. 2008, 20, 1359.
W. Li, T. Mishima, G.-Y. Adachi, J. Shiokawa, Inorg. Chim.
Acta 1986, 121, 93.
+
+
10 W. Li, T. Mishima, G.-Y. Adachi, J. Shiokawa, Inorg. Chim.
Acta 1987, 131, 287.
11 P. N. Hazin, J. W. Bruno, H. G. Brittain, Organometallics
1987, 6, 913.
12 H. Kunkely, A. Vogler, J. Photochem. Photobiol., A 2002, 151,
45.
13 T. Yu, W. Su, W. Li, R. Hua, B. Chu, B. Li, Solid-State
Electron. 2007, 51, 894.
14 X.-L. Zheng, Y. Liu, M. Pan, X.-Q. Lü, J.-Y. Zhang, C.-Y.
Zhao, Y.-X. Tong, C.-Y. Su, Angew. Chem., Int. Ed. 2007, 46,
7399.
15 R. Wietzke, M. Mazzanti, Chem. Commun. 1999, 209.
16 X.-P. Yang, C.-Y. Su, B.-S. Kang, X.-L. Feng, W.-L. Xiao,
H.-Q. Liu, J. Chem. Soc., Dalton Trans. 2000, 3253.
17 T. Harada, Y. Nakano, M. Fujiki, M. Naito, T. Kawai, Y.
Hasegawa, Inorg. Chem. 2009, 48, 11242.
+
+
3+
hypothetical value estimated under the assumption that the Ce
f­5d absorptions were solely excited. Hazin and co-workers
4
have suggested that if the radiationless transition is suppressed
for the 4f­5d transition, the value of Φ4f­5d,hyp may approach
2
2
unity. A solvation structure in the vicinity of the rare-earth ion
23
may give rise to a radiationless transition. On the basis of
emission lifetime measurements, Wietzke and co-workers have
3+
reported that a few solvent molecules may surround the Tb ion
in the case of the Tb¢ntb complex in methanol.15 We, therefore,
suggest that a similar solvation structure could be also applicable
in the case of Ce¢ntb because the tetradentate ntb ligands
properly encapsulate the rare-earth ion. Such a complex
architecture of Ce¢ntb may result in a relatively high value of
Φ4f­5d,hyp.
18 S. Kishimoto, T. Nakagawa, T. Kawai, Y. Hasegawa, Bull.
Chem. Soc. Jpn. 2011, 84, 148.
19 A. Vogler, H. Kunkely, Inorg. Chim. Acta 2006, 359, 4130.
20 J.-C. G. Bünzli, Acc. Chem. Res. 2006, 39, 53.
21 N. Mataga, T. Kubota, Molecular Interactions and Electronic
Spectra, Marcel Dekker, Inc., New York, 1970.
22 P. N. Hazin, C. Lakshminarayan, L. S. Brinen, J. L. Knee, J. W.
Bruno, W. E. Streib, K. Folting, Inorg. Chem. 1988, 27, 1393.
23 W. D. Horrocks, Jr., D. R. Sudnick, Acc. Chem. Res. 1981, 14,
384.
To conclude, we synthesized a blue-emitting Ce3+ complex
with tetradentate ntb ligands, Ce¢ntb. On the basis of its
absorption spectra, we suggest that Ce¢ntb may form a stable
¹
6
coordination structure at concentrations greater than 1 © 10
¹3
mol dm . We determined the molar absorption coefficient of
1498 | Chem. Lett. 2014, 43, 1496–1498 | doi:10.1246/cl.140449
© 2014 The Chemical Society of Japan