Remarkable chiral and luminescent properties of novel Yb(III) and Eu(III) complexes containing BINAPO ligand

Article abstract of DOI:10.1016/j.ica.2008.03.093

Lanthanide complexes are of great importance for their prospective applications in wide range of science and technology. Chiral lanthanide complexes can constitute stereo-discriminating probes in biological media, owing to the luminescent properties of th

Full text of DOI:10.1016/j.ica.2008.03.093

Inorganica Chimica Acta 362 (2009) 136–142
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Inorganica Chimica Acta
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Remarkable chiral and luminescent properties of novel Yb(III) and Eu(III)
complexes containing BINAPO ligand
b
c
c
b
Md. Abdus Subhan a, , Yasuchika Hasegawa , Takayoshi Suzuki , Sumio Kaizaki , Yanagida Shozo
*
a Department of Chemistry, Shah Jalal University of Science and Technology, Sylhet, Bangladesh
b Department of Material and Life Science, Faculty of Engineering, Osaka University, Osaka, Japan
c Department of Chemistry, Faculty of Science, Osaka University, Osaka, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Lanthanide complexes are of great importance for their prospective applications in wide range of science
and technology. Chiral lanthanide complexes can constitute stereo-discriminating probes in biological
media, owing to the luminescent properties of the rare-earth ions. Sensitized emission with narrow band-
width, having fast radiation rate and high emission quantum efficiency are the main perspective for syn-
thesizing the complexes. Attention has been given on remarkable chirality with high dissymmetry factors
Received 2 September 2007
Received in revised form 29 January 2008
Accepted 11 March 2008
Available online 23 March 2008
(g = Deext/emax
) of the complexes. For this purpose, beta-diketonato ligands with chiral BINAPO
Keywords:
(1,10-binapthyl phosphine oxide) ligand were chosen to achieve the goal. The complexes [Ln(TFN)3-
(S-BINAPO)](TFN = 4,4,4-trifluoro-1(2-napthyl)-1,3-butanedione), [Ln(HFT)3(S-BINAPO)] (HFT = 4,4,5,5,6,6,6-
heptafluoro-1-(2-thienyl)-1,3-hexanedione) and [Ln(HFA)3(S-BINAPO)](hfa = hexafluoroacetylacetonate)
(where Ln = Yb, Eu) were synthesized. The complex, [Eu(TFN)3(S-BINAPO)] gives strong red emission at
615 nm with narrow emission band (<10 nm) when excited by 465 nm light with quantum efficiency
Circular dichroism
Dissymmetry factors
Red emission
Lanthanide
Chirality
86%. The dissymmetry factors (g = Deext/emax) corresponding to the 7F1
?
5D0 transition at 590 nm is
0.091 for [Eu(TFN)3(S-BINAPO)] and for [Yb(hfa)3(S-BINAPO)](hfa = hexafluoroacetylacetonate) corre-
2
sponding to the F7/2
?
2F5/2 transitions is 0.12, are among the largest values for both Eu and Yb
complexes to date, respectively. The Eu complexes, [Eu(HFT)3(S-BINAPO)] and [Eu(TFN)3(S-BINAPO)]
are found to be spontaneously emissive, showing bright red emission, when placed in sunlight or even
in the laboratory when light is switched on.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
transitions with the exception of the hypersensitive transitions,
which due to the vibronic mixing, have the large intensity. Transi-
Chiral as well as luminescent lanthanide complexes are of value
for their applications in both material and life science. Chiral lan-
tion from the 4f inner shell of free Eu(III) is forbidden because it
does not associate with the change of parity. However, transitions
that are forbidden by odd parity become partially allowed by mix-
ing 4f and 5d states through ligand field effects of Eu(III) complexes
[5]. An important point of the studies is to determine how the elec-
tron transitions in Eu(III) can be controlled by the molecular design
of Eu(III) complexes. Population inversion in 4f orbitals in Ln(III)
complexes is a great benefit in the development of organic chelate
laser and plastic optical fiber applications [4,6]. Eu(III) complexes
with higher emission quantum yields and faster radiation rates
than conventional Eu(III) complexes have been designed [7]. Eu(III)
complexes that exhibit both high emission quantum yields and fast
radiation rates are desirable luminescent materials for laser and fi-
ber applications. Thus, Eu(III) complexes are designed to meet sev-
eral conditions: (1) higher emission quantum yields to increase qs
(energy density) values (2) faster radiation rates to produce large B
(Einstein coefficient) values and (3) large dissymmetry factors for
chiral discrimination. Eight coordinated Eu(III) complexes with
square anti prism (SAP) geometry may generate stronger electric
thanide complexes with large dissymmetry factors (g = Deext/emax
)
may represent stereo-discriminating probes in biological media,
due to the luminescent properties of the most rare-earth ions
[1a]. Highly emissive and enantiopure lanthanide complexes have
been used as cellular imaging and reactive probes by their interac-
tions with DNA [1b]. Deployment of chiral lanthanide complexes as
catalysts for various asymmetric synthesis has also been received
much awareness [2]. Europium(III) complexes have been regarded
as attractive for use as luminescent materials because of their red
emissions (615 nm) [3]. Characteristic red emissions of Eu(III)
complexes emerge from both the magnetic dipole transitions to
7F1 (orange-red) and the electric dipole transitions to 7F2 [4]. Mag-
netic dipole transitions are allowed, which makes them having
approximately the same intensity as the forbidden electric dipole
* Corresponding author. Tel.: +880 821 713491x151; fax: +880 821 715257.
E-mail address: subhan-che@sust.edu (M.A. Subhan).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.03.093

M.A. Subhan et al. / Inorganica Chimica Acta 362 (2009) 136–142
137
dipole radiation through suitable ligand field effects. SAP struc-
tured Eu(III) complexes are expected to have increased radiation
rates and quantum yields because of the increases in 5D0 ? 7F2
emissions (electronic dipole transition), related to odd parity.
Phosphine oxide ligands can produce antisymmetrical structures
that promote faster radiation rates. Furthermore, increased emis-
sion quantum efficiency of Eu(III) complexes can be expected, be-
cause the coordination of phosphine oxide moiety (1) prevents
the coordination of water or solvent molecules and (2) lowers
vibrations (P@O:1125 cmꢀ1) [7]. The BINAPO ligand is anticipated
to offer dissymmetric environment to the Eu(III) ion causing an in-
crease in the electron transition probability (5D0 ? 7F2 transition)
in the 4f orbitals due to odd parity. In the present study we de-
scribe the synthesis, characterization and chiral as well as lumines-
cent properties of lanthanide (Eu(III) and Yb(III)) complexes [8]
with a BINAPO ligand (see Scheme 1).
stirring in an ice bath to synthesise Ln(TFN)3. A representative syn-
thesis of Eu(HFT)3 is described here. Europium acetate monohy-
drate (5.0 g, 12.5 mmol) was dissolved in 20 mL ethanol in a
beaker by stirring at 0 °C. Then
a solution of HFT (10.8 g,
33.6 mmol) in 5 mL methanol was added dropwise to the above
solution with continuous stirring. A white yellow precipitate was
obtained after 2 h. The reaction mixture was filtered and air dried
(% yield, 70%).
2.3.2. Synthesis of [Eu(HFA)3(S-BINAPO)], [Eu(HFT)3(S-BINAPO)] and
[Eu(TFN)3(S-BINAPO)]
An acetone solution (200 mL) containing Eu(HFA)3, Eu(HFT)3 or
Eu(TFN)3 and BINAPO ligand (1:1 molar ratio) was refluxed at
ꢁ50 °C for 10 h with continuous stirring to obtain a clear solution
of desired complexes [Eu(HFA)3(S-BINAPO)], [Eu(HFT)3(S-BINAPO)]
or [Eu(TFN)3(S-BINAPO)], respectively. In each case, the resulting
mixture was concentrated and hexane was added to it, which gave
crystalline precipitate. For an example synthesis of [Eu(TFN)3-
(S-BINAPO)] is given here. [Eu(TFN)3] ꢂ 2H2O (5.35 g, 1 mmol) was
dissolved in 200 mL acetone in a round bottom flask. Then (3 g,
1 mmol) of S-BINAPO was added to the above solution. A turbid
mixture obtained was shaken and a clear solution was obtained
in a few minutes. This solution was refluxed for 10 h. The resulting
mixture was concentrated and hexane was added. A yellow crys-
talline solid obtained was filtered and dried in air (% yield, 60%).
The single crystals of these complexes were grown by recrystalliza-
tion from toluene–cyclohexane mixed solvent for a few days. The
needle like crystal was mounted on a capillary but decomposes
in the course of X-ray analysis. The complexes [Yb(HFA)3(S-BINAP-
O)], [Yb(HFT)3(S-BINAPO)] and [Yb(TFN)3(S-BINAPO)] were also
synthesized by the same procedure as described for the corre-
sponding Eu(III) complexes. Single crystals of [Yb(HFA)3(S-BINAP-
O)] were grown by recrystallization from toluene–cyclohexane
mixed solvent for several days. Elemental analysis for the following
complexes was performed, results are as given below: Anal. Calc.
for C90H56O8F9P2Eu [Eu(TFN)3 (S-BINAPO)]: H, 3.42; C, 65.50.
Found: H, 3.62; C, 65.49%. Anal. Calc. for C90H56O8F9P2Yb
[Yb(TFN)3(S-BINAPO)]: H, 3.38, C, 64.68. Found: H, 3.58; C,
64.65%. Anal. Calc. for C59H35O8F18P2Yb ꢂ C6H14 [Yb(HFA)3(S-BINAP-
O)] ꢂ C6H14: H, 3.22, C, 50.86. Found: H, 2.80; C, 50.70%. Anal. Calc.
for C70H48O10F21S3PEu [Eu(HFT)3(S-BINAPO)]: H, 2.75; C, 47.82.
Found: H, 2.41; C, 48.82%. The ESI-MS data were collected for
[Yb(HFA)3(S-BINAPO)] and [Yb(TFN)3(S-BINAPO)] complexes. For
[Yb(HFA)3(S-BINAPO)], m = 1449, m/z 1242, [1449ꢀhfa]+; m/z
1896, [1449ꢀhfa+BINAPO]+ and [Yb(TFN)3(S-BINAPO)], m = 1623,
m/z 655, BINAPO+H+; m/z 1358, [MꢀTFN]+; m/z 2012,
[MꢀTFN+BINAPO]+ were obtained.
2. Experimental
2.1. Chemicals and reagents
Europium and ytterbium acetate monohydrate (99.9%), 1,1,1,5,5,5-
hexafluoro-2,4-pentanedione (HFA), were purchased from Wako
Pure Chemical Industries Ltd., 4,4,4-trifluoro-1(2-napthyl)-1,3-
butanedione and 4,4,5,5,6,6,6-heptafluoro-1-(2-thienyl)-1, 3-hex-
anedione were purchased from Aldrich Chemical Co. and Acros
Organics Co., respectively. BINAP was purchased from Wako Pure
Chemical Industries Ltd. All chemicals and reagents were of analyt-
ical grade and used as received.
2.2. Synthesis and characterization of the complexes
The S-BINAPO ligand was prepared by the oxidation of BINAP
[9] with H2O2 in THF at 0 °C for 12 h. The ligand was characterized
by IR, NMR and elemental analysis. 1H NMR (acetone-d6, TMS)
peaks were obtained at d 6.63 (d, 2H), d 6.77 (t, 2H), d 7.30–7.36
(m, 8H), d 7.38–7.50 (m, 12H), d 7.79–7.87 (q, 4H), d 7.88–7.91
(d, 2H) and d 7.93–7.96 (d, 2H) ppm. The elemental analysis of S-
BINAPO ligand was performed, found: C, 80.72; H, 4.89 and Calc.
for C44H32O2P2: C, 80.75; H, 5.03%.
2.3. Synthesis of lanthanide complexes
2.3.1. Synthesis of Ln(HFA)3 and Ln(HFT)3 (Ln = Eu, Yb)
Ln(HFA)3 and Ln(HFT)3 (Ln = Eu, Yb) were synthesized by dis-
solving europium or ytterbium acetate monohydrate in ethanol
in a baker and then adding slowly the ethanol solution of HFA or
HFT with stirring in an ice bath (ꢁ1:3 molar ratio). Ligand TFN
was dissolved in a mixed solvent of methanol and ethanol and then
added to ethanol solution of Ln(acetate)3 (ꢁ1:3 molar ratio) with
Other complexes, [Ln(TFN)(HBpz3)2], [Ln(HFT)(HBpz3)2],
[Ln(HFA) (HBpz3)2] and [Ln(TFN)2(HBpz3)], [Ln(HFT)2(HBpz3)] (Ln =
ꢀ
Eu, Ce and HBpz3 ¼ Hydrotrisðpyrazole-1-ylÞborateÞ were synthe-
sized by dissolving respective hydrated salts of Ln(NO3)3 or LnCl3
into water and then adding simultaneously an aqueous solution
of HBpz3 and ethanol solution of TFN, HFT or HFA to the above
aqueous solution in a beaker, respectively. In each case, mixing
of solutions following stirring for 15 min produced precipitates of
Ln-complexes, which were subsequently filtered, air-dried and
characterized by IR.
2.4. Measurements
For measuring the spectra in solution lanthanide complexes
were dissolved in acetone. Absorption spectra were measured on
a Perkin–Elmer Lambda-19 spectrophotometer. CD data were col-
lected on a Jasco J-720W spectropolarimeter. Infrared spectra were
obtained with a Perkin–Elmer FT-IR 2000 spectrometer. Elemental
analyses were performed with a Perkin–Elmer 240C. Emission
Scheme 1. Schematic diagram of BINAPO ligand.

138
M.A. Subhan et al. / Inorganica Chimica Acta 362 (2009) 136–142
spectra were measured at room temperature using a HITACHI
F-4500 system. The spectra were corrected for detector sensitivity
and lamp intensity variations. Quantum efficiency was determined
using a standard integrating sphere (diameter 6 cm) [10].
3. Results and discussion
The complexes were synthesized and characterized by elemen-
tal analysis, FT-IR, ESI-MS and X-ray. The IR spectra for the com-
plex, [Eu(TFN)3(S-BINAPO)] (Fig. S1), show characteristic peaks. In
addition to the satisfactory results from the elemental analysis,
IR, ESI-MS and finally the structure was confirmed by X-ray analy-
sis. The complexes were found to be highly soluble in acetone and
moderately soluble in water.
2.5. Crystal structure determination
A colorless needle crystal of complex [Yb(HFA)3(S-BINAPO)]
was sealed in a glass capillary tube to prevent possible efflores-
cence. The X-ray intensities (2hmax = 60°) were measured on a Rig-
aku AFC-5R, and absorption corrections were made by either the
empirical W-scan method [11] or the numerical integration meth-
od [12]. The structure was solved by the direct method using SHELXS
86 program [13], and refined on F2 against all the reflections by the
full matrix least-squares technique using anisotropic thermal
parameters for all non-hydrogen atoms. Hydrogen atoms were
placed at the positions generated by theoretical calculations and
fixed during the structural refinement cycles. All the calculations
were carried out using a TEXSAN software package [14]. The ORTEP
[15] view of the complex is shown in Fig. 1.
3.1. X-ray analysis
The crystallographic data are shown in Table 1 and the selected
bond lengths and angles are summarized in Table 2. The complex
[Yb(HFA)3(S-BINAPO)] crystallizes in monoclinic crystal system
with P1211 space group. This complex indicates the effective eight
coordination with six oxygens from three HFA ligands and two
oxygen from BINAPO to the Yb(III) ions in [Yb(HFA)3 (S-BINAPO)],
forming the square antiprism as shown in Fig. 1. The Rw value was
found to be 0.2807 as shown in Table 1. Two crystallographically
Fig. 1. Ortep view of two crystallographically independent complexes of [Yb(HFA)3(S-BINAPO)] (phenyl rings are omitted for clarity).

M.A. Subhan et al. / Inorganica Chimica Acta 362 (2009) 136–142
139
Table 1
independent complexes, which corresponded to D-[Yb(HFA)3-
(S-BINAPO)] and K-[Yb(HFA)3(S-BINAPO)], in which Yb–O bond
lengths are between 2.23 and 2.4 Å in the Yb-moiety. These bond
lengths are very similar to that found for mononuclear
D-[Yb(HBpz3)2(S-pba)], K-[Yb(HBpz3)2(S-pba)] and dinuclear (K–D)-
Cr(ox)Yb [16]. This results indicate effective coordination of
oxygen ligands from either BINAPO or HFA to Yb(III). Bond angle,
O2 Yb1 O1 is 76.5(3)° and O9 Yb2 O10 is 75.8(4)° as shown in Table
2. These values are comparable to that observed for (K–D)-Cr(ox)-
Yb (70.9(5)°) but much smaller than that observed for
either D-[Yb(HBpz3)2(S-pba)] (55.5(1)°) or K-[Yb(HBpz3)2(S-pba)]
(55.6(1)°) [16]. The coordination environment around the Yb(III)
is seems to be similar to that of dinuclear, Cr(ox)Yb, which was
found to be configurationally chiral with the retention of chirality
[16]. Furthermore, the SAP structure of [Yb(HFA)3(S-BINAPO)] is
analogous to mononuclear (BIPHEPO)Eu(hfa)3 with more dissym-
metry to the Yb(III) exerted by BINAPO ligand [17].
Crystallographic data for [Yb(HFA)3(S-BINAPO)]
Formula
M
T (K)
k(Mo Ka)/Å
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
C59H35F18O8P2Yb
1448.85
293(2)
0.71069
monoclinic
P1211
13.9079(5)
18.1271(7)
26.9541(10)
90.00
93.6170(10)
90.00
6781.9(4)
4
1.419
1.522
0.1033
c (°)
V (Å3)
Z
qcalc. (g cmꢀ3
)
l(Mo Ka) (cmꢀ1
R1 (F2)
)
wR2 (F2)
0.2807
3.2. CD spectra analysis
3.2.1. CD spectra of [Eu(TFN)3(S-BINAPO)]
Table 2
Selected bond length (Å) and bond angles (°) of [Yb(HFA)3 (BINAPO)]
The Eu(III) complex gives sharp electronic transitions at
7F0 ? 5D0, 7F0 ? 5Dj and 7Fj ? 5D0 (J = 0, 1 or 2). We studied the
CD spectral features of the f–f transitions of [Eu(TFN)3(S-BINAPO)]
in order to reveal the nature of formally forbidden transition with
jDJj = 0, jDJj = 1, magnetic dipole transition and the electric dipole
transition, jDJj = 2, respectively. For a clear observation of the circu-
lar dichoric f–f transitions of Eu(III), optically active and sterically
bulky enantiopure BINAPO ligand was chosen to induce the config-
urational chirality around the Eu(III). For the complex, [Eu(TFN)3-
(S-BINAPO)], the CD bands corresponding to the electric dipole
transitions, 7F1 ? 5D1, 7F0 ? 5D1 and 7F0 ? 5D2 was observed at
around 540 nm (jgj = 0.0014), 525 nm (jgj = 0.04) and 465 nm
(jgj = 0.001), respectively, as shown in Fig. 2. Moreover, a promi-
nent CD signal corresponding to magnetic dipole transition,
Bond length
Yb1 O2
Yb1 O6
Yb1 O1
Yb1 O7
Yb1 O3
Yb1 O8
Yb1 O5
Yb1 O4
Yb1 C53
Yb2 O9
Yb2 O14
Yb2 O12
Yb2 O10
2.229(8)
2.261(13)
2.272(9)
2.294(12)
2.300(10)
2.330(9)
2.340(10)
2.339(13)
3.15(6)
2.235(9)
2.282(11)
2.288(9)
2.304(10)
Yb2 O15
Yb2 O16
Yb2 O13
Yb2 O11
Yb2 P3
P1 O1
P2 O2
P3 O9
P3 C72
P3 C60
P3 C66
P4 O10
2.299(10)
2.331(11)
2.351(10)
2.403(10)
3.534(3)
1.480(9)
1.481(8)
1.482(10)
1.786(14)
1.785(16)
1.783(15)
1.477(11)
Bond angle
O2 Yb1 O6
O2 Yb1 O1
O6 Yb1 O1
O2 Yb1 O7
O6 Yb1 O7
O1 Yb1 O7
O2 Yb1 O3
O6 Yb1 O3
O1 Yb1 O3
O7 Yb1 O3
O2 Yb1 O8
O6 Yb1 O8
O1 Yb1 O8
O7 Yb1 O8
O3 Yb1 O8
O2 Yb1 O5
O6 Yb1 O5
O1 Yb1 O5
O7 Yb1 O5
O3 Yb1 O5
O8 Yb1 O5
O2 Yb1 O4
O6 Yb1 O4
O1 Yb1 O4
O7 Yb1 O4
O3 Yb1 O4
O8 Yb1 O4
O5 Yb1 O4
O9 Yb2 O14
O9 Yb2 O12
O14 Yb2 O12
O9 Yb2 O10
O14 Yb2 O10
O12 Yb2 O10
O9 Yb2 O15
O14 Yb2 O15
O12 Yb2 O15
79.7(4)
76.5(3)
89.1(5)
123.1(4)
146.9(5)
75.5(4)
142.3(4)
101.9(5)
140.7(4)
75.1(5)
76.2(3)
140.7(5)
114.5(4)
72.1(4)
80.0(4)
141.4(4)
71.5(5)
77.9(4)
76.7(5)
70.4(5)
141.4(4)
74.6(4)
71.4(7)
147.5(4)
133.9(5)
70.5(4)
72.5(6)
117.6(6)
88.9(4)
141.2(4)
92.4(4)
75.8(4)
90.8(4)
142.8(4)
117.7(4)
145.8(4)
80.0(4)
O10 Yb2 O15
O9 Yb2 O16
O14 Yb2 O16
O12 Yb2 O16
O10 Yb2 O16
O15 Yb2 O16
O9 Yb2 O13
O14 Yb2 O13
O12 Yb2 O13
O10 Yb2 O13
O15 Yb2 O13
O16 Yb2 O13
O9 Yb2 O11
O14 Yb2 O11
O12 Yb2 O11
O10 Yb2 O11
O15 Yb2 O11
O16 Yb2 O11
O13 Yb2 O11
O9 Yb2 P3
O14 Yb2 P3
O12 Yb2 P3
O10 Yb2 P3
O15 Yb2 P3
O16 Yb2 P3
O13 Yb2 P3
O11 Yb2 P3
C112 Yb2 P3
P2 O2 Yb1
76.9(4)
76.2(4)
139.7(4)
78.1(5)
120.3(4)
71.5(4)
144.3(4)
73.4(4)
71.8(4)
73.7(4)
72.6(4)
136.3(4)
73.4(4)
69.0(4)
70.9(4)
143.2(4)
136.1(4)
70.9(4)
124.9(4)
14.5(3)
100.2(3)
148.0(3)
66.9(3)
103.7(3)
73.2(3)
140.0(3)
86.1(3)
115.7(8)
145.7(6)
143.2(6)
163.8(7)
P3 O9 Yb2
P4 O10 Yb2
Fig. 2. Absorption and circular dichroism (CD) spectra of complex [Eu(TFN)3(S-
BINAPO)].

140
M.A. Subhan et al. / Inorganica Chimica Acta 362 (2009) 136–142
7F1 ? 5D0 was observed at around 590 nm with a dissymmetry
factor of (jgj = 0.091). This is the largest value of the dissymmetry
factors for Eu(III) known to date (Fig. 2), which is close to that
observed for CPL spectra of a Eu(III) complex, for sensitive transi-
tions 5D0 ? 7F1 (gem = ꢀ 0.088) [18]. This large dissymmetry factor
[16,19] indicates that the configurational chirality is induced by
BINAPO around the Eu(III). CD signal corresponding to the
7F0 ? 5D0 was not clearly observed. This transition is formally for-
bidden but it is in fact allowed by J mixing with the neighboring
electronic transitions. The CD silent 7F0 ? 5D0 transitions may be
attributed to a lack of the magnetic transition moment upon tran-
sition, for which the much smaller J mixing of the magnetic and
electric dipole moments upon CD transition would be responsible.
This seems to be reasonable, since CD transition is affected in prin-
ciple by the angular momentum and the total angular momentum
of both the ground state (7F0) and excited state (5D0) is absolutely
zero for the particular transition, 7F0 ? 5D0. Furthermore, a CD
band corresponding to electric dipole transition was observed at
615 nm (jgj = 0.01). The CD bands corresponding to the 7F ? 5D
transitions, from the different levels of ground 7F0 to that of excited
5D within the f6 configurations of Eu(III) chromophore are sche-
matically presented in Fig. 3. The 7F ? 5D transitions of Eu(III)
may be classified into two categories in terms of angular momen-
tum change, the magnetic dipole allowed and electric dipole al-
lowed. It is noted that among these transitions 7F1 ? 5D0, which
involve the magnetic dipole change gives dissymmetry factor g,
much larger than that observed for the corresponding electric
dipole transitions, 7F2 ? 5D0(615 nm), 7F0 ? 5D2(465 nm), and the
CD silent transition 7F0 ? 5D0. These results indicate that the mag-
netic dipole, rather than the electric dipole transitions enhance the
J mixing and eventually giving rise to a stronger CD signal, for the
transitions accompanying an angular momentum change. The
small dissymmetry factor observed for transitions at around 530
and 540 nm might be due to the larger mixing between magnetic
dipole 7F0 ? 5D1 and allowed transition 7F1 ? 5D1, which finally
shows lessening of CD intensity due to the mutual cancellation.
The similar CD behaviors are observed for other configurationally
chiral lanthanide complexes [16,19].
Fig. 4. Absorption (top) and CD (middle) spectra of complex [Yb(TFN)3(S-BINAPO)]
and CD spectra (bottom) of [Yb(HFA)3(S-BINAPO)].
factors (jgj) for [Yb(TFN)3(S-BINAPO)] and [Yb(HFA)3(S-BINAPO)]
complexes corresponding to bands at 945 nm, 960 nm, 975 nm,
are found to be 0.036, 0.12; 0.02, 0.075 and 0.061, 0.023, respec-
tively. These large dissymmetry factors indicate configurational
chirality around the Yb(III) ion [16,21]. These multiple CD compo-
nents in both the complexes arise from the ligand field splitting
within the 2F7/2 ? 2F5/2 transitions of the Yb(III) chromophores.
These are expected to be few transitions from four Kramers dou-
3.2.2. CD spectra of [Yb(TFN)3(S-BINAPO)] and [Yb(HFA)3(S-BINAPO)]
2
The unique f–f transitions of Yb(III) from ground states F7/2 to
2
excited state F5/2 are split by the crystal field into four and three
doubly degenerate sublevels, respectively. This is magnetically al-
lowed (DJ = 1) and can be classified among the most CD sensitive of
the rare-earth series [20]. Due to the small energy difference be-
tween states of the lower terms (of the order of 102 cmꢀ1), a tem-
perature-reliant Boltzmann distribution of the populations of four
2
2
blets of the ground F7/2 states to three of the excited F5/2 states,
as schematically shown in Fig. 5, where positive and negative signs
refer to the CD sign.
Either [Eu(TFN)3(S-BINAPO)] or [Yb(TFN)3(S-BINAPO)] and
[Yb(HFA)3(S-BINAPO)] complexes are highly soluble in acetone
and moderately soluble in water. CD intensities as well as CD signs
2
states of ground F7/2 state is predictable. Accordingly, up to 12
transitions might be observable about the center of gravity. For
each of the Yb(III) complexes with in f13 electronic configuration,
five CD bands were merely observed at around 927 nm, 945 nm,
960 nm, 975 nm and 995 nm as shown in Fig. 4. The dissymmetry
Fig. 5. Schematic diagram of transitions from four Kramers doublets of the ground
2
2F7/2 states to three of the excited F5/2 states of Yb(III) ions and positive and neg-
Fig. 3. Schematic diagram of f–f transitions of Eu(III) in [Eu(TFN)3(S-BINAPO)].
ative signs from left to right refer to the CD signs from the longer wavelength side.

M.A. Subhan et al. / Inorganica Chimica Acta 362 (2009) 136–142
141
Fig. 6. Excitation and emission spectra of [Eu(TFN)3(S-BINAPO)].
were found to remain unchanged on standing for several days in
acetone solution. For these S–BINAPO complexes negative sign cor-
responding to major CD bands indicates preference of K isomer in
solution as shown in Figs. 2 and 4, respectively [19]. However, on
long standing change in CD intensities as well as CD signs may
be attributed to ligand scrambling in the solution and presence
of mixtures of fluxional complexes in equilibrium.
complexes to date, respectively. Further investigations on spectro-
scopic and photo-physical properties of complexes are a require-
ment for their potential applications in material as well as life
sciences.
Acknowledgements
Venture business laboratory (VBL) of Osaka University is grate-
fully acknowledged for providing a postdoctoral fellowship to
Dr. Md. Abdus Subhan for a year during this work. Thanks to
Dr. Nobuko Kanehisa, Department of Material and Life Science,
Division of Advanced Science and Biotechnology, Graduate School
of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka
565-0871, Japan, for her cooperation during X-ray measurements.
3.3. Luminescent properties
The Eu complexes were found to be naturally and spontane-
ously emissive, showing bright red emission (at 615 nm), when
placed in sunlight or even in the laboratory when light is switched
on. Emission bands were observed at around 580, 590, 615, 650,
and 700 nm, and are attributed to the f–f transitions 5D0 ? 7FJ
(J = 0, 1, 2, 3 and 4, respectively) when excited at 465 nm. The
strongest emission band at around 615 nm (5D0 ? 7F2) was due
to the electronic dipole transition as shown in Fig. 6 [7,17]. The
similar complexes without BINAPO ligand are not naturally emis-
sive but these show emissions, when irradiated by UV–vis light.
For example, we synthesized similar complexes of Eu(III) and
Ce(III); [Ln(TFN)3], [Ln(HFT)3], [Ln(HFA)3] and [Ln(TFN)(HBpz3)2],
[Ln(HFT)(HBpz3)2], [Ln(HFA)(HBpz3)2] and [Ln(TFN)2(HBpz3)],
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2008.03.093.
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142
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