Circularly polarized luminescence of Sm (III) and Eu (III) complexes with chiral ligand (R/S)-BINAPO

Article abstract of DOI:10.1002/chir.23056

Luminescent lanthanide (III) ions have been exploited for circularly polarized luminescence (CPL) for decades. However, very few of these studies have involved chiral samarium (III) complexes. Complexes are prepared by mixing axial chiral ligands (R/S))-2

Full text of DOI:10.1002/chir.23056

Received: 1 November 2018
DOI: 10.1002/chir.23056
Revised: 14 December 2018
Accepted: 18 December 2018
R E G U L A R A R T I C L E
Circularly polarized luminescence of Sm (III) and Eu (III)
complexes with chiral ligand (R/S)‐BINAPO
Daniel Cotter | Spencer Dodder | Valentine J. Klimkowski | Todd A. Hopkins
Department of Chemistry and
Abstract
Biochemistry, Butler University,
Indianapolis, Indiana
Luminescent lanthanide (III) ions have been exploited for circularly polarized
luminescence (CPL) for decades. However, very few of these studies have
involved chiral samarium (III) complexes. Complexes are prepared by
mixing axial chiral ligands (R/S))‐2,2’‐bis(diphenylphosphoryl)‐1,1′‐binaphthyl
Correspondence
Todd A. Hopkins, Department of
Chemistry and Biochemistry, Butler
University, 4600 Sunset Avenue,
Indianapolis, IN 46208.
(
(
BINAPO) with europium and samarium Tris (trifluoromethane sulfonate)
Eu (OTf) and Sm (OTf) ). Luminescence‐based titration shows that the com-
Email: tahopkin@butler.edu
3
3
plex formed is Ln((R/S)‐BINAPO) (OTf) , where Ln = Eu or Sm. The CPL spec-
2
3
Funding information
Butler Program for Undergraduate
Research
tra are reported for Eu((R/S)‐BINAPO) (OTf) and Sm((R/S)‐BINAPO) (OTf) .
2
3
2
3
The sign of the dissymmetry factors, gem, was dependent upon the chirality of
the BINAPO ligand, and the magnitudes were relatively large. Of all of the com-
plexes in this study, Sm((S)‐BINAPO) (OTf) has the largest g
= 0.272,
em
2
3
3+
which is one of the largest recorded for a chiral Sm complex. A theoretical
three‐dimensional structural model of the complex that is consistent with the
experimental observations is developed and refined. This report also shows that
3+
3+
(
R/S)‐BINAPO are the only reported ligands where gem (Sm ) > g (Eu ).
em
KEYWORDS
BINAPO, circularly polarized luminescence, europium, samarium
1
| INTRODUCTION
light (I ‐I ) and sum of the emission intensity of left vs
L
R
right circularly polarized light (I + I ), which is the total
L
R
Circularly polarized luminescent materials have potential
application in the development of three‐dimensional dis-
luminescence. To compare across systems with different
overall emission efficiencies, it is common to use the emis-
sion dissymmetry factor, gem(λ), shown in Equation 1.
1
2
plays, information storage, and as probes of biomolecu-
3,4
lar processes. This has led to the development of a
number of circularly polarized luminescent molecules
2ðILðλÞ − IRðλÞÞ
gemðλÞ ¼
;
(1)
5-10
7,11-14
including organic dyes,
helicenes,
However, chiral complexes of
luminescent lanthanide ions have been exploited for their
and transition
ðI ðλÞ þ I ðλÞÞ
L
R
15-17
metal complexes.
where I (λ) and I (λ) are the intensity of left‐ and right‐
L
R
18-20
circularly polarized luminescence properties for decades
and that continues with many recent studies.
circularly polarized light at wavelength, λ, respectively.
The value of gem(λ) can vary between +2 and −2 giving
information about both the sign and magnitude of the
polarization of light. Chiral lanthanide complexes typi-
cally have larger |gem| than chiral transition metal or
21-25
Circularly polarized luminescence (CPL) is the differ-
ential emission of left vs right circularly polarized light.
The observables in CPL spectroscopy are the difference
in emission intensity of left vs right circularly polarized
18,19
organic molecules.
In fact, a chiral europium complex
Chirality. 2019;1–11.
wileyonlinelibrary.com/journal/chir
© 2019 Wiley Periodicals, Inc.
1

2
COTTER ET AL.
demonstrates an unusually large gem = +1.38, with ~80%
of the emission as left circularly polarized.
Additionally, tetrakis((+)‐3‐heptafluorobutylyrylcamphorato)
samarium (III) shows |gem| = 1.15, among the highest
measured of all lanthanide complexes, for transitions in
26
The reason that luminescent lanthanides show large
CPL comes from the properties of the 4f‐4f transitions.
The emission dissymmetry factor between two states
4
6
4
6
30
both G5 → H and G → H7/2.
/2
5/2
5/2
One of the objectives of this study is to further demon-
strate that chiral luminescent samarium complexes can
give chiroptical properties worth exploiting in chiral
luminescence applications (eg, optical displays or biomo-
lecular probes). In order to achieve this objective, lantha-
nide complexes are prepared with chiral ligands, and the
2
0
(1 → 2) is represented by Riehl and Richardson:
4
R12
gem
¼
;
(2)
jD12j
where R12 is the rotatory strength and D12 is the dipole
strength of the transition. The dipole strength and rota-
tory strengths are represented by the following:
emission dissymmetry factors, gem, are measured.
Because direct excitation of 4f‐4f transitions is weak, the
ligand should be both chiral and act as a sensitizer for
19,37
the lanthanide emission.
The ligands with axial
2
2
2
2
D12 ¼ jh1jbμj2ij þ jh1jmbj2ij ¼ jμ j þ jm12j ; (3)
1
2
chirality used in this study, (R)‐ and (S)‐2,2’‐
bis(diphenylphosphoryl)‐1,1′‐binaphthyl (BINAPO), are
shown in Figure 1. The (R/S)‐BINAPO ligands are easy
R
¼ jm j·jμ j cosτ ;
(4)
12
12
12
12
29
to synthesize and have π → π* transitions that can sen-
sitize lanthanide luminescence. A chiroptical spectro-
scopic study of (R/S)‐BINAPO in solid state
demonstrates that the emission shows an observable
where μ12 and m12 are the electric and magnetic dipole
transition moment vectors, and τ12 is the angle between
the vectors. Since the 4f‐4f transitions are LaPorte forbid-
den, they acquire electric dipole transition strength
through both static and dynamic coupling mecha-
10
29,38
CPL spectrum at 355 nm. Harada et al
have studied
the CPL of BINAPO complexes with europium that dem-
onstrated that BINAPO can act as a sensitizer and gener-
ate chirality in the emission. However, the largest gem's
came from complexes with additional chiral ligands,
and the study did not extend to samarium complexes. In
this study, chiral complexes are formed by mixing Eu
(OTf) and Sm (OTf) (where OTf = trifluoromethane sul-
18,27
nisms.
The static coupling mechanism involves
interconfigurational mixing of opposite parity and 4f elec-
tron states of the metal through interaction with the
27
ligand field. Therefore, this mechanism depends on
the type and arrangement of the ligands about the metal
ion. The dynamic coupling mechanism involves coupling
a metal‐centered 4f‐4f electric multipole (eg, quadropole)
transition with ligand‐centered electric dipole moment
3
3
fonate) with the (R/S)‐BINAPO ligands. Because the
5
7
D → F luminescence spectrum of europium is easier
0
0–2
18,27
transitions (eg, π → π* transitions).
Since 4f‐4f transi-
to interpret, the ratio of the BINAPO:Ln (OTf) (where
3
tions are magnetic dipole allowed (as long as ΔJ = 0,±1),
the electric dipole transition mechanisms do not always
dominate and can lead to large rotatory strengths com-
pared with the dipole strength, resulting in a large gem
Ln = Eu or Sm) emitting complex is determined by titrat-
ing BINAPO with a constant concentration of Eu (OTf)3
to be 2:1. The CPL of the resulting samarium and euro-
pium complexes are presented and characterized. Ab
initio quantum chemical calculations offer a prediction
of the structure of the samarium and europium BINAPO
complexes that is consistent with experimental titration
and CPL spectroscopic data.
(
Equation 2).
5
7
6
The D → F transition within the 4f configuration
0
1
3+
of Eu is an example of a magnetic dipole allowed tran-
sition that typically leads to large emission dissymmetry
factors, gem. In fact, this is the transition that leads to
the unusually large gem = +1.38 in tetrakis((+)‐3‐
26
heptafluorobutylyrylcamphorato) europium (III). The
large gem and small number of Stark levels, leading to
simpler to interpret CPL spectra structure, are key
factors contributing to the large number of CPL studies
involving chiral europium complexes (most with
2 | MATERIALS AND METHODS
(R/S)‐2,2’‐bis(diphenylphosphino)‐1,1′‐binaphthyl ((R/S)‐
BINAP) was purchased from Strem chemicals and used
without further purification. Europium Tris (trifluoromethane
4,21,22,24,25,28,29
3+
|gem| > 0.1).
While Eu has been widely
exploited for CPL applications, there are very few studies
3+
30-36
involving the CPL of Sm complexes.
This is some-
what surprising given the fact that there are two
4
6
magnetic‐dipole allowed emissive transitions, G → H
5/2
5/2
3+
4
6
5
and G5 → H , within the 4f configuration of Sm .
FIGURE 1 Structures of chiral ligands, (R)‐ and (S)‐BINAPO
/2
7/2

COTTER ET AL.
3
sulfonate) (Eu (OTf) ), samarium Tris (trifluoromethane
aromatic), 749 (w, aromatic), 725 (m, aromatic), 703 (s,
aromatic) cm .
3
−
1
sulfonate) (Sm (OTf) ), and lutetium Tris (trifluoromethane
3
sulfonate) (Lu (OTf) ) were purchased from Sigma‐
3
Aldrich and used without further purification. Deuterated
solvents, chloroform and methanol, were purchased from
Cambridge Isotopes Laboratories.
2
.3 | Titration analysis
The complexation titration samples were created by
adding 0.000628‐0.0075 M BINAPO to 0.0025 M Eu
(
OTf) in methanol. A total of seven samples were created
3
2
.1 | Preparation of (R/S)‐2,2’‐
bis(diphenylphosphoryl)‐1,1′‐binaphthyl
R/S‐BINAPO)
with BINAPO:Eu ratio of 0.25:1‐3:1. The emission spec-
trum of each solution was measured at an excitation
wavelength (355 nm) corresponding to BINAPO excita-
tion. The emission intensities were determined by mea-
(
(
R/S)‐BINAPO was prepared using a literature proce-
5
7
29
suring the area under the D → F peak (605‐625 nm).
0
2
dure. (R/S)‐BINAPO was dissolved in dichloromethane,
stirred for ~1 hour at 0°C, and then an excess of 30%
hydrogen peroxide solution was added dropwise. The
resulting solution was stirred under nitrogen for
2
.4 | Spectroscopic analysis
~
16 hours and quenched with water. The solution was
Emission and excitation spectra of the samples were mea-
sured using a Perkin‐Elmer LS‐55 Luminescence Spec-
trometer. Circularly polarized luminescence spectra and
luminescence lifetimes were recorded on instrumentation
assembled in our lab and described previously. H‐NMR
spectra for BINAPO were recorded on a Bruker 400 MHz
extracted with dichloromethane (three times), and the
dichloromethane phase was dried with magnesium
sulfate. The resulting solution was filtered, the solvent
was removed with a rotary evaporator, and recrystalliza-
tion in methanol/water gave white powder/crystals. S‐
BINAPO mp 258°C, H NMR (CDCl , 400 MHz) δ
39 1
1
3
NMR. IR spectra for BINAPO and Eu (BINAPO) (OTf)
2
3
7
7
1
.85‐7.80 (m, 4H), 7.70‐7.65 (q, 4H), 7.45‐7.30 (m, 12H),
.25‐7.20 (m, 8H), 6.80 (d, 4H). FTIR (ATR) 1433 (m),
305 (w), 1199 (s, P¼O), 1116 (s), 1100 (m), 870
were recorded on a Thermo Scientific Smart OMNI‐
Transmission Nicolet iS10 with the Smart iTR accessory.
(
(
w, aromatic), 814 (m, aromatic), 746 (s, aromatic), 722
s, aromatic), 695 (s, aromatic) cm .
−
1
2.5 | Computational methods
Initial components of the Sm (BINAPO) (OTf) complex
2
3
40
2
.2 | Preparation of LN (OTf) ((R/S)‐
were built and assembled using Avogadro, version 1.2.
All electronic structure calculations were performed
3
BINAPO) (where LN = EU, LU, OR SM)
samples
X
41
using GAMESS version April 20, 2017, along with com-
42
panion program MacMolPlt, version 7.7. The images
43
Solutions were created by combining 0.010 M Eu (OTf)3,
presented were produced using VMD, version 1.9.2.
Without an available experimental three‐dimensional
structure for the complex, it was important to build and
refine a structure that matched experimental stoichiome-
try. In order to exhibit CPL, the structure of the samar-
ium complex must belong to a chiral point group with
only proper rotations. Therefore, these requirements were
imposed and monitored on the stepwise building and
refinement process. An initial conformation for BINAPO
was built in the S configuration with the two phosphoryl
groups roughly eclipsing each other. After initial optimi-
0
.010 M Lu (OTf) 0.010, or 0.0025 M Sm (OTf) with
3 3
(
R)‐ or (S)‐BINAPO in methanol to achieve 1:2 stoichio-
metric ratio of Ln:BINAPO. These solutions were used
for spectroscopic analysis. Ethyl acetate was added to
the solution to precipitate the Eu (BINAPO) (OTf) com-
plex as a white powder for FTIR analysis. Solutions made
from dissolving precipitated Eu (BINAPO) (OTf)3 in
methanol were also used for spectroscopic analysis.
Lu((S)‐BINAPO) (OTf)3 H‐NMR (CDCl , 400 MHz) δ
2
3
2
1
2
3
7
.70‐6.60 (m, aromatic). Eu((R)‐BINAPO) (OTf) FTIR
2
3
4
4,45
(
ATR) 3500‐3000 (br), 1589 (s), 1438 (s), 1272 (s, S¼O),
zation using the semiempirical PM3 method,
the
1
1
7
223 (w, C‐F), 1150‐1130 (br, P=O), 1115 (s), 1084 (m),
structure was reoriented so as to insure D point group
2
030 (w, S¼O), 875 (w, aromatic), 820 (m, aromatic),
symmetry. Subsequent ab initio RHF optimization was
−1
46 (s, aromatic), 722 (s, aromatic), 702 (s, aromatic) cm .
done imposing D symmetry, using the all electron 6‐
2
46-48
Sm((R)‐BINAPO) (OTf)3 FTIR (ATR) 3400 (br), 1660
31G basis set,
supplemented with appropriate polari-
2
(
m), 1636 (m), 1438 (w), 1241 (br, S¼O, C‐F), 1185 (s,
zation functions (d‐type Gaussians on C, O, P, and p‐type
Gaussians on H) on all atoms.
49
P¼O), 1117 (m), 1029 (s, S¼), 875 (w, aromatic), 816 (w,

4
COTTER ET AL.
5
7
In the next step, a Sm((S)‐BINAPO) complex was
The emission spectra for the D → F transitions
0 0–3
2
3+
4
6
constructed by placing the Sm (III) ion at the coordinate
origin, and two copies of the refined S‐BINAPO structure
positioned symmetrically along the X‐axis. This was done
so as to have the four phosphoryl oxygens, of two (S)‐
BINAPO groups, symmetrically positioned approximately
of Eu in Eu((R)‐BINAPO) (OTf) and G → H
5/2–9/2
2
3
5/2
3+
transitions of Sm
in Sm((R)‐BINAPO) (OTf) are
shown in Figure 2. The excitation wavelength of 355 nm
overlaps with a BINAPO electronic transition, which
shows that BINAPO sensitizes both Eu and Sm lumi-
nescence. Because these are room temperature solution
phase measurements, the individual Stark level to Stark
level transitions are not resolved enough to determine
the site symmetry at the Eu or Sm , but the spectra in
Figure 2 show 4f‐4f transition locations that are typical
2
3
1
0
3+
3+
3
.0 Angstroms from the Sm (III) ion, in a square planer
arrangement. The initial complex possessed D symmetry
2
that was imposed during the subsequent ab initio UHF
optimization. Again, the 6‐31G** basis set was used for
3+
3+
3+
all S‐BINAPO atoms. The lanthanide ions (Sm and
3+
3+
3+
Eu ) were treated using the SBKJC effective core poten-
for Eu and Sm . Samples created by dissolving the
recrystallized Eu((R)‐BINAPO) (OTf) in methanol gave
50
tials and associated valence basis set.
2
3
Starting from the optimized Sm((S)‐BINAPO) struc-
identical emission spectra as shown in Figure 2.
2
ture, it was determined that an initial arrangement
The luminescence lifetime (trace shown in Figure S1)
−
5
possessing C symmetry was possible if two OTf ligands
for the D0 state of Eu((R)‐BINAPO) (OTf) was
2
2
3
−
were symmetrically placed along the Y‐axis. The OTf
monexponential with a lifetime in methanol of 993 μs
ligands were initially arranged with the three sulfonate
oxygens complexing the Sm (III) at approximately 3.0
Angstroms distance. Although this arrangement gives
the complex the unusual coordination number of 10, it
was expected that optimization would likely reduce this.
Optimization of the complex used the same basis set
and 1.157 ms in deuterated methanol. Unfortunately,
4
the luminescence lifetime of the G5 state of Sm((R)‐
/2
BINAPO) (OTf) is too short (<100 μs) to reliably mea-
2
3
sure with our instrumentation. There is an accepted
empirical formula (ie, Horrocks equation) for determin-
3+
ing the number of coordinated waters to Eu by compar-
5
52
treatment for the Sm(S‐BINAPO) portion as previously
ing D0 luminescence lifetimes in H O vs D O.
2 2
2
−
described. However, each OTf was treated using the 6‐
Assuming that the O―H oscillator in methanol is as
effective at quenching as O―H in water, the equation
can be modified (see Figure S1) to estimate the number
of coordinating methanol molecules using lifetimes in
3
1G* basis set, along with the addition of a diffuse func-
tion on each of the O and S atoms of the sulfonate
51
groups. Ab initio UHF optimization proceeded to con-
vergence with C2 symmetry imposed to produce the
53
methanol and deuterated methanol. Based on the
Horrocks equation, these lifetimes predict that there are
Sm(S‐BINAPO) (OTf)2 complex with an overall +1
2
+
3+
charge. Finally, an equivalent Eu((S)‐BINAPO) (OTF)
0.3 ± 0.5 methanol molecules coordinating to Eu ,
2
2
complex was produced by replacing the Sm (III) atom
with Eu (III) in the final optimized Sm (III) structure.
The ab initio UHF optimization was reinitialized and
proceeded to convergence.
essentially no solvent coordination.
The luminescence lifetime, 993 μs, of Eu((R)‐
BINAPO) (OTf)3 can be used determine the intrinsic
2
3+
quantum yield of the Eu according to the following:
τobs
τrad
Q ¼
;
(5)
3
3
| RESULTS AND DISCUSSION
.1 | Spectroscopic results
where τobs is the experimental lifetime, and τrad is the
radiative lifetime. This can be estimated by Bünzli :
3
7
Samples were prepared by mixing molar ratios of the (R/
S)‐BINAPO with Eu (OTf) (or Sm (OTf) ) in methanol
3
3
ꢀ
ꢁ
and allowing the mixture to establish an equilibrium.
ATR‐FTIR spectra (shown in Figure S1) of (R)‐BINAPO
vs Eu((R)‐BINAPO) (OTf) and Sm((R)‐BINAPO) (OTf)
1
Itot
3
¼ Amdn
;
(6)
τrad
Imd
2
3
2
3
show almost identical BINAPO peak locations for all of
−
1
the peaks except for those assigned to the P¼O stretch.
where Amd = 14.65 s , n = 1.329 (refractive index of
−
1
5
7
In (R)‐BINAPO, the peak at 1200 cm is assigned to
methanol), Itot is the integrated intensity for D → F
0 0–6
is the integrated intensity for the
D → F transition. This leads to a τ
Inserting these values into Equation 5 gives an intrinsic
quantum yield of 0.17.
2
9
the P¼O stretch, but in both Eu((R)‐BINAPO) (OTf)3
transitions, and I
2
md
5
7
and Sm((R)‐BINAPO) (OTf) , this peak shifts to between
= 4.79 ms.
2
3
0
1
rad
−
1
1
185 and 1150 cm indicating that the P¼O bonds are
3+
3+
weakened by coordination to the Eu and Sm .

COTTER ET AL.
5
FIGURE 2 Emission spectra for the
5
7
D
0
→ F0–3 transitions of 0.010 M
Eu((R)‐BINAPO) (OTf) (red) and the
5/2 → H5/2–9/2 transitions of 0.010 M
Sm((R)‐BINAPO) (OTf) (blue) in
methanol. The excitation wavelength is
55 nm
2
3
4
6
G
2
3
3
4
| COMPLEXATION TITRATION
ratio changes. This indicates that the equilibrium in
methanol is a single‐step complexation equilibrium.
The titration of Eu (OTf) with increasing concentrations
3
of BINAPO resulted in an increase in emission intensity
from the 4f‐4f transitions of Eu . Figure 3 shows the
emission intensity vs BINAPO concentration for
(7)
3+
with equilibrium constant,
0
.0025M Eu (OTf) , where the BINAPO concentration
h
i
3
EuðBINAPOÞ ðOTf Þ
ranges from 0.000628 to 0.0075M (BINAPO:Eu ratio of
0
only Eu coordinated by BINAPO ligand(s), the emis-
sion intensity is a direct measure of the concentration of
the Eu (BINAPO) (OTf) complex. While the overall
x
y
β ¼ ꢂ
ꢃ
:
(8)
x
.25:1‐3:1). Because the excitation wavelength excites
EuðOTf Þ ½BINAPOꢀ
3
3+
The data in Figure 3 fit to a modified Hill equation shown
in Equation 9:
x
y
intensity increases with increased BINAPO concentra-
tion, the peak locations and relative intensities of the
peaks within a spectrum were unchanged. Because the
location and relative intensities of 4f‐4f transitions are
sensitive to the coordination environment of the Eu ,
the titration data is consistent with a single major Eu‐
BINAPO emitting species. This is further evidenced by
the fact that the observed CPL spectra (Figure 5 and Fig-
Imax − I0
I ¼ I0 þ
ꢄ
ꢅ
n;
(9)
½
BINAPOꢀ
1
=2
1 þ
½
BINAPOꢀ
0
3+
where I is emission intensity, I
is the emission intensity
0
with no BINAPO added, Imax is the maximum emission
intensity, [BINAPO]1/2 is the [BINAPO]
at 50% of the
0
ure S6) also does not change as the Eu (OTf) :BINAPO
3
5
7
FIGURE 3 Integrated emission intensity of the D
of Eu((R)‐BINAPO) (OTf) vs [BINAPO] added. The red line is a fit
of the Hill equation to the data. The excitation wavelength is
55 nm
0 2
→ F region
2
3
+
FIGURE 4 Calculated structure of [Sm((S)‐BINAPO)
shown along the C axis
2 2
(OTf) ]
3
2

6
COTTER ET AL.
5
7
FIGURE 5 Total luminescence (red) and CPL spectra (blue) of the
D
0
→
F
1,2 transitions for 0.010 M Eu((R)‐ (solid lines) vs
(
2 3
S)‐BINAPO) (OTf) (dotted lines). The excitation wavelength is 355 nm
maximum emission intensity, or the dissociation con-
stant, and n is the Hill coefficient. The fit of the
assumed that Sm (BINAPO) (OTf) is the primary emit-
2 y
ting species in the samarium samples. The experimental
3
+
titration data to Equation
6
gave [BINAPO]1/2
data shows four of the coordination sites to the Sm or
3+
−
=
0.00257 ± 0.00006 M, and n = 4.0 ± 0.4. The Hill coef-
Eu but cannot show how many (or if) OTf ligands
are also coordinated to the metal.
ficient, n > 1, indicates that there are multiple BINAPO
ligands binding (x > 1 in Equation 5) to the europium,
54
and that the binding is cooperative. Assuming that
BINAPO]1/2 (half the maximum emission intensity) rep-
[
5 | CALCULATIONS
resents the complexation of half of [Eu] , the ratio of Eu:
0
BINAPO is 1:2 (0.00125:0.00257 M) and therefore x = 2.
Calculations are used to predict the coordination struc-
ture, including the number and possible orientation of
The equilibrium constant, β, can be estimated by assum-
2
−
ing that K = ([BINAPO] ) , where K is the dissocia-
OTf ligands. Figure 4 shows the optimized structure cal-
d
1/2
d
+
tion constant, and β = 1/K . Using the values from a fit
culated, which is [Sm((S)‐BINAPO) (OTf) ] . As shown
d
2
2
of the titration data in Figure 3, logβ = 5.20 for the equi-
librium shown in Equation 7 in methanol at room tem-
perature (293 K).
in Figure 4, the two BINAPO ligands are bulky enough
to optimize to a six‐coordinate Sm structure with two
triflate oxygens (one each) coordinating along with the
3+
The titration and spectroscopic data suggests that the
emissive species in methanol is Eu (BINAPO) (OTf) ,
four phosphoryl oxygens. Attempts to construct structure
−
2
y
with multidentate OTf
ligands converged to
3+
3+
and the methanol vs deuterated methanol lifetime shows
monodentate. This six‐coordinate Sm
or Eu
The symmetry
of the [Sm((S)‐BINAPO) (OTf) ] (Figure 4), and site
is
3+
55-58
that there are no methanol molecules coordinating Eu .
unusual but not without precedent.
3+
+
Since the small ionic size difference between Sm and
2
2
3+
Eu does not typically result in large variations in chem-
ical behavior, it can be assumed that the structure at the
metal ion is very similar. Therefore, in this study, it is also
symmetry at the Sm (or Eu) is C , a chiral point group
2
−
The chiral arrangement of the BINAPO and OTf ligands
around the metal center satisfies the requirement for a
TABLE 1 Emission dissymmetry factors for europium BINAPO complexes
5
7
a
D
0
→ F
1
Eu((R)‐BINAPO)
2
(OTf)
3
Eu((S)‐BINAPO)
−0.110(8)
2
(OTf)
3
Eu((R)‐BINAPO)(D‐facam)
3
Eu((R)‐BINAPO)(hfa)
3
b
b
g
em(593)
0.120(8)
−0.44 (594 nm)
0.03
5
7
D
0
→ F
2
b
b
g
em(614)
em(620)
−0.026(1)
0.025(1)
0.029 (613 nm)
0.003
g
0.014(1)
−0.012(1)
a
g
3
em's are the same for 1:1, 1:2, and 1:3 Eu (OTf) :R‐ and S‐BINAPO samples. Uncertainties are in parentheses.
b
28
gem's are from Harada et al.

COTTER ET AL.
7
FIGURE 6 Total luminescence and
4
6
6
CPL spectra of the G5/2 → H5/2, H7/2
transition for 0.0025M Sm((R)‐
2 3
BINAPO) (OTf) . The excitation
wavelength is 355 nm
static coupling mechanism leading to dipole and rotatory
opposite in sign and nearly equal in magnitude for (R)‐
(positive CPL) vs (S)‐BINAPO (negative CPL). The CPL
spectra of the D → F transition (Figure 5) show two
18,27
strengths for magnetic‐dipole allowed CPL transitions
3+
3+
5
7
0 2
in Eu and Sm . Because the BINAPO ligands have
π → π* transitions, the dipole and rotatory strengths
could also derive from the dynamic coupling mechanism
peaks of opposite sign located at 614 and 620 nm. Neither
of these peaks matches the location of the maximum of
3
+
3+
5
7
between the Eu or Sm electric quadrupole (or larger
the D → F transition in the total luminescence, but
0 2
18,27
multipole) moment and ligand π → π* transitions.
there is evidence of two shoulders that match the loca-
tions of the two CPL peaks. The CPL peaks of the
Therefore, the structure predicted in Figure 4 is consis-
tent with CPL spectra and emission dissymmetry factors,
gem, derived from a combination of the static and
dynamic coupling mechanisms.
5
7
D → F transition are also opposite in sign and nearly
0
2
equal in magnitude for (R)‐ vs (S)‐BINAPO.
Table 1 shows that the gem(λ) determined for Eu((R)‐
vs (S)‐BINAPO) (OTf) complexes are opposite sign and
2
3
equal in magnitude. The magnitude of gem(λ) for the
5
7
6
| CPL SPECTROSCOPY
D → F transitions are five to eight times larger
0 1
5
7
than the gem(λ) for D → F transitions. For comparison
0
2
The CPL and total luminescence spectra of the
purposes, Table 1 includes g
values observed by
em
5
7
29,38
D → F transitions for Eu((R)‐ vs (S)‐BINAPO) (OTf)
Harada et al
for Eu((R)‐BINAPO)(D‐facam)3 and
0
1,2
2
3
1
5
7
are shown in Figure 5. The locations of the D → F
Eu((R)‐BINAPO)(hfa) (where facam = 3‐trifluoroacetyl‐
0
3
5
7
and D → F total luminescence transitions are identical
d‐camphor and hfa = 1,1,1,5,5,5‐hexafluoropentane‐2,4‐
dione). Eu((R)‐BINAPO)(D‐facam)3 has the largest
magnitude gem shown in the table, but the magnitude
and sign of gem are primarily a result of coordination by
0
2
for (R)‐ vs. (S)‐BINAPO. However, the CPL spectra of the
5
7
D → F transition (Figure 5) shows one broad peak
0
1
centered at 593 nm that corresponds to the maximum of
the peak in the total luminescence. These peaks are
29
the D‐facam ligands. Eu(R‐BINAPO)(hfa) derives its
3
TABLE 2 Emission dissymmetry factors for samarium BINAPO complexes
4
6
a
G
5/2 → H5/2
Sm((R)‐BINAPO)
2
(OTf)
3
2 3
Sm((S)‐BINAPO) (OTf)
g
em(559)
em(564)
−0.224(9)
0.036(6)
0.272(9)
−0.062(6)
0.064(6)
g
gem(569)
−0.054(6)
4
6
G
5/2 → H7/2
g
em(595)
em(602)
0.050(6)
−0.068(6)
g
−0.016(2)
0.017(2)
a
Uncertainties are shown in parentheses.

8
COTTER ET AL.
chirality and CPL (gem) from the R‐BINAPO ligand simi-
lar to Eu(R‐BINAPO) (OTf) , but the number of BINAPO
ligands coordinated to the europium ion is different.
7 | CONCLUSION
2
3
3
+
3+
This study reports CPL studies of both Eu and Sm
coordinated by chiral ligand, (R)‐ and (S)‐BINAPO. Titra-
tion data show that the coordination complex formed in
methanol is 2:1 BINAPO:Ln , and lifetime measure-
ments show that solvent (methanol) is not coordinating
the metal ions. The resulting Eu((R/S)‐BINAPO) (OTf)
While the signs of the gem for Eu(R‐BINAPO)(hfa) and
3
Eu(R‐BINAPO) (OTf)3
are
the
same,
Eu(R‐
2
3+
BINAPO) (OTf) has a much larger magnitude.
2
3
4
The CPL and total luminescence spectrum of the G
5/2
6
6
→
H5/2, H
transitions of Sm((R)‐Binapo) (OTf) is
7/2
2
3
2
3
4
shown in Figure 6. The CPL spectrum of the G5/2
and Sm((R/S)‐BINAPO)
(OTf) complexes show strong
2 3
6
→
H5/2 transition shows three distinct transitions (two
CPL signal with opposite sign for opposite BINAPO enan-
tiomers. The measured |gem| for the D → F regions
0 1,2
are large but not unusual for chiral Eu complexes.
However, the measured |gem| for the G5/2 → H5/2 region
is among the largest reported for a chiral Sm complex
in solution. Additionally, (R)/(S)‐BINAPO is the first chi-
ral ligand system that gives a larger gem for Sm vs Eu .
These promising CPL results for Sm((R/S)‐
5
7
negative and one positive) at 559, 565, and 569 nm. The
peak at 565 nm corresponds to the maximum and the
locations of the other two peaks are not as evident in
the total luminescence spectrum. If the emission origi-
3
+
4
6
3
+
4
nates from only one Stark level in G5/2, the CPL spec-
3+
3+
trum is showing the location of all three energy levels
6
(
Kramers doublets) in the H5 multiplet. Since all tran-
/2
sitions are symmetry allowed in C symmetry, the spec-
BINAPO) (OTf)3 are unlikely to be unique to the
2
2
trum (Figure 6) is consistent with the calculated structure
BINAPO ligand. The data in this study combined with
59
4
6
30,31,35,36
(
Figure 4). The CPL spectrum of the G5 → H tran-
those of some other recent studies
should
/2
7/2
3+
sition shows two peaks (negative and positive) at 595 and
02 nm. The peak at 602 nm is coincident with the peak
encourage more exploration of chiral Sm complexes
for CPL applications.
6
maximum in the total luminescence spectrum for the
4
6
G5/2 → H transition. The CPL spectrum for Sm((S)‐
7/2
ACKNOWLEDGEMENTS
BINAPO) (OTf) shows identical peak locations for the
2
3
total luminescence and CPL but opposite signs for the
CPL peaks (Figure S1).
The gem values determined at each of the CPL peak
transition are shown in Table 2. All of the |gem| shown
in Table 2 are larger in magnitude for the complex with
The authors would like to acknowledge Matt Goldey for
writing the CPL instrument software. The authors would
also like to thank the Butler Program for Undergraduate
Research for funding to support this work.
(
5
S)‐ vs (R)‐BINAPO, but the difference in |gem| at 559,
64, and 595 nm is outside of the uncertainty in the
ORCID
measurement. Unfortunately, the experimental and cal-
culated structural information in this study is not
detailed enough to rationalize these differences in rota-
tory strengths for 4f‐4f transitions. However, the average
Todd A. Hopkins https://orcid.org/0000-0001-9943-6047
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How to cite this article: Cotter D, Dodder S,
Klimkowski VJ, Hopkins TA. Circularly polarized
luminescence of Sm (III) and Eu (III) complexes
with chiral ligand (R/S)‐BINAPO. Chirality.
2019;1‐11. https://doi.org/10.1002/chir.23056
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