A highly luminescent chiral tetrahedral Eu4L4(L′)4 cage: Chirality induction, chirality memory, and circularly polarized luminescence

Article abstract of DOI:10.1021/jacs.9b07178

Chiral lanthanide cages with circularly polarized luminescence (CPL) properties have found potential application in enantioselective guest recognition and sensing. However, it still remains a big challenge to develop a simple and robust method for the dia

Full text of DOI:10.1021/jacs.9b07178

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Article
A Highly Luminescent Chiral Tetrahedral Eu4L4(L')4 Cage: Chirality
Induction, Chirality memory and Circularly Polarized Luminescence
Yanyan Zhou, Hongfeng Li, Tianyu Zhu, Ting Gao, and Pengfei Yan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07178 • Publication Date (Web): 20 Nov 2019
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Journal of the American Chemical Society
A Highly Luminescent Chiral Tetrahedral Eu4L4(L')4 Cage: Chirality
Induction, Chirality memory and Circularly Polarized Luminescence
Yanyan Zhou, Hongfeng Li,* Tianyu Zhu, Ting Gao and Pengfei Yan*
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Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education; School of Chemistry and
Materials Science, Heilongjiang University, Harbin 150080, P. R. China
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ABSTRACT: Chiral lanthanide cages with circularly polarized luminescence (CPL) properties have found potential application in
enantioselective guest recognition and sensing. However, it still remains a big challenge to develop simple and robust method for the
diastereoselective assembly of homochiral lanthanide cage in view of the large lability of the Ln(III) ions. Herein, we report the first
example of the formation of enantiopure lanthanide tetrahedral cage via chiral ancillary ligand induction strategy. One such
[(Eu4L4)(R/S-BINAPO)4] cage is assembled by four achiral C3-symmeric tris-β-diketones [4,4',4''-tri(4,4,4-trifluoro-1,3-
dioxobutyl)triphenylamine, L)] as faces, four Eu(III) ions as vertices, and four chiral R/S-bis(diphenylphosphoryl)-1,1′-binaphthyl
(R/S-BINAPO) as ancillary ligands. X-ray crystallography, NMR and CD spectra confirm the formation of a pair of enantiopure
chiral topological tetrahedral cages (Eu4L4)(R-BINAPO)4 and (Eu4L4)(S-BINAPO)4 (ΔΔΔΔ-1 and ΛΛΛΛ-1). As expected, the
tetrahedral cages present strong CPL with the |glum| up to 0.20, while unexpectedly give the ultrahigh luminescent quantum yields
(QYs) of up to 81%, a highest value reported in chiral Ln(III) complexes. More impressively, the chiral memory effect for lanthanide-
based assembly is observed for the first time. The chirality of the original cage-1 framework is retained after the R/S-BINAPO being
replaced by achiral bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO), and thus another pair of enantiopure Eu(III) tetrahedral
cages ΔΔΔΔ- and ΛΛΛΛ-[(Eu4L4)(DPEPO)4] (ΔΔΔΔ-2 and ΛΛΛΛ-2) are isolated. Encouragingly, cage-2 also presents impressive
luminescence quantum yield (QYs = 68%) and intense CPL (|glum| = 0.10). This study offers a simple and low-cost synthesis strategy
for the preparation of lanthanide cage with CPL properties.
geometries make the control of the stereochemistry during
Introduction
the self-assembly process is rather challenging.23 As
Chiral circularly polarized luminescence (CPL) materials
have been attracting increasing attention in recent years
for their potential applications in biological probes,1 three-
dimensional (3D) displays,2 optical storage,3 and chiral
photoelectric devices.4 Because the existence of magnetic
dipole transition, the lanthanide complexes show
obviously higher luminescence dissymmetry factor glum (10–
2–0.5) than those of fluorescence molecules,5
supramolecular aggregates,6 polymers,7 transition metal
complexes (10–5–10–3).8 Since 1970s, the simple chiral
mononuclear Ln(III) complexes are the most extensively
studied CPL materials,9 and some of which have been
developed as chiral probe10 and luminescent layers of
OLED.11 In contrast with mononuclear system,12 the chiral
dinuclear helicate13 and more sophistical polynuclear
architectures such as chiral tetranuclear tetrahedron14 are
much less explored for CPL investigation due to their
difficulties in controlling the stereochemistry of the
assemblies.
reported by Sun, and Law that the slight structure changes
on the spacers or the terminal chiral amines of the ditopic
ligands, either cause the structural transformation from
tetrahedron to helicate or cube, or the presence of
diastereoselective breaking.14,24
On the other hand, the large coordination numbers of
Ln(III) ions (commonly 8–10) also offers the chance for the
introduction of chiral ancillary ligand to regulate the
stereochemistry of assemblies, which is rarely realized in
3d transition metal cages due to their limited coordination
numbers (commonly 4–6).22,25 We infer that the
introduction of chiral ancillary ligand with large steric
hindrance
would
effectively
control
the
diastereoselectivity in self-assembly process. In this
context, a [(Eu4L4)(R/S-BINAPO)4] tetrahedron cage is
synthesized by employing four achiral C3-symmeric tris-β-
diketones
[4,4',4''-tri(4,4,4-trifluoro-1,3-
dioxobutyl)triphenylamine, L)] as faces, four Eu(III) ions as
vertices, and four chiral R/S-bis(diphenylphosphoryl)-1,1'-
binaphthyl (R/S-BINAPO) as ancillary ligands. Single
crystal X-ray crystallographic analyses show that two R/S-
Metal-organic supramolecular cages with chirotopic
cavities enabling encapsulation of guests have proved
useful in stereoselective guest recognition,15 sensing,16 and
as asymmetric reaction vessels.17 Supposing that the chiral
supramolecular cage is rendered with CPL characteristic, it
would undoubtedly extend their applications, especially in
chiral sensing area. A general strategy for controlling the
stereochemistry of the cage is the introduction of chiral
elements,18 such as using enantiopure building blocks,19
chiral counterions,20 chiral guests,21 or chiral ancillary
ligand.22 However, the Ln(III) ions are very rarely selected
as metal center of cage, because their variable coordination
numbers (6–12), complicated and undefined coordination
BINAPO
enantiomers
successfully
induce
diastereoselective self-assembly of a pair of enantiopure
Eu(III) tetrahedral cages, (Eu4L4)(R-BINAPO)4 and
(Eu4L4)(S-BINAPO)4 (ΔΔΔΔ-1 and ΛΛΛΛ-1). The
photophysical experiment results show that the cage-1
present strong circularly polarized luminescence with the
|glum| up to 0.20, and the highest luminescent quantum
yields (QYs = 81%) among the chiral lanthanide complexes
reported so far.
As chiral luminescence materials, expensive chiral
source is a key factor to hinder their widespread use in
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future. Inspired by chirality-memory principle,26 we
examine the chiral memory effect of the cage-1 by
employing an achiral bis[2-
(diphenylphosphino)phenyl]ether oxide (DPEPO) to
replace the chiral R/S-BINAPO. Encouragingly, the
configurations of the enantiopure (Eu4L4)(R-BINAPO)4
and (Eu4L4)(S-BINAPO)4 (ΔΔΔΔ-1 and ΛΛΛΛ-1) are
preserved during the substitution process, and thus
another pair of new enantiopure tetrahedral cages ΔΔΔΔ-
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and ΛΛΛΛ-[(Eu4L4)(DPEPO)4] (ΔΔΔΔ-2 and ΛΛΛΛ-2) are
isolated. CD and CPL experiments verify their optical
activity despite the absence of chiral elements in two
ligands. Two possible substitution pathways for the
stereochemical memory are proposed by means of CD and
a simple reaction kinetic experiments, which enable us to
well control the enantiomer purity.
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Scheme 1. Syntheses of the tetrahedral cages-1 and -2
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signals. Compared with the two free ligands, the most of
the signals from the tetrahedral cage undergo high-field
shifts. Especially, three groups of resonances from tris-β-
diketone are easier to be distinguished, which show limited
broadening after the coordination with the metal.
Moreover, there is no observation of new peaks attributed
to L indicating the high diastereopurity of the cages. 31P and
19F NMR spectra also indicate the formation of a single
species, with only one sharp singlet peak observed in cage-1
and cage-3 (Figure S11–S14). Furthermore, 1H NMR
diffusion ordered spectroscopy (DOSY) of cage-1 and cage-
3 show one diffusion band at a diffusion coefficient of D =
4.03 × 10–6 cm2 s–1 and 5.88 × 10–6 cm2 s–1, respectively, in line
with the formation of a single species (Figure 1c, Figure S15,
S16). The dynamic radii of the tetrahedron is calculated to
be 32 Å with the Stokes-Einstein equation, which is in good
agreement with that observed from the crystal structure of
cage-1 (vide infra for structure analysis). High-resolution
electrospray ionization mass spectrometry (ESI-TOF-MS)
analyses further confirm the formation of tetrahedral
supramolecular architecture with a formula of (Ln4L4)(R/S-
BINAPO)4 (Figure S17–S20). The spectra show the
Results and discussion
Synthesis and Characterization of Ligand and
Tetrahedral Cage. The C3-symmetric achiral tris-β-
diketone
dioxobutyl)triphenylamine, L)] is synthesized in two
steps via Friedel-Crafts acylation to afford the
ligand
[4,4',4''-tri(4,4,4-trifluoro-1,3-
a
intermediate 4,4',4''-triacetyl triphenylamine, followed by
a traditional Claisen condensation reaction to obtain the
desired molecule (see Supporting Information for details).
1H NMR and ESI-TOF-MS confirm the formation and
purity of the intermediate and ligand (Figures S1–S4).
The Ln(III) tetrahedral cages are obtained by stirring a
1:1:1 mixture of L, (R/S)-BINAPO and the corresponding
Ln(III) salts in methanol at room temperature. H, P, F
NMR and DOSY NMR suggest the formation of a single
species in solution. Due to the poor resolution of the H
NMR for Eu(III) complexes (Figure S5, S6), the
isostructural Y(III) complexes, [(Y4L4)(R/S-BINAPO)4,
cage-3] are chosen as substitute for H NMR experiments
(Figure S7–S10, COSY; NOESY). As shown in Figure 1b, the
cage-3 show a single set of broadened but distinguishing
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corresponding molecular ion peaks of the cages, and their S21). In (Eu4L4)(S-BINAPO)4, each three β-diketonate units
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isotopic patterns agreed well with the simulated isotopic
distributions.
helically wrap about an Eu(III) centre in a left-hand
propeller-like fashion, leading to a Λ configuration at each
of the four vertices. In contrast, a Δ configuration around
the metal center is observed in (Eu4L4)(R-BINAPO)4.
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We propose that the formation of homochiral ΔΔΔΔ-1
and ΛΛΛΛ-1 enantiomers should attribute to the chiral
induction of R/S-BINAPO and the mechanical coupling
through the rigid ligands. In ΛΛΛΛ-1, by viewing along the
pseudo-C3 symmetry axis direction of a tetrahedron
(Figure 2c and 2d), one can observe that the O, P and C
atoms in a S-BINAPO form a left-hand helical clamp, with
the configuration being consistent with that observed in
triple stranded helix around metal center. While the
opposite case can be observed in ΔΔΔΔ-1. From a
geometric point of view, as two helixes move closer
together by a head-to-head orientation, adopting the same
helical conformations is energy favorable. Additionally, a
plenty of intramolecular C-H···X interaction between the
F, O atoms of L and H atoms of BINAPO are formed at each
of its Eu(III) vertice (Figure S22). We suggest that these
weak interactions play vital roles for inducing and
stabilizing the resulting homochiral structure.
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1
Figure 1. (a) H NMR assignment of L and (R)-BINAPO.
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(b) H NMR (400 MHz) spectra of free ligands L, (R)-
BINAPO and their self-assembled yttrium complexes
(Y4L4)(R-BINAPO)4 and (Y4L4)(S-BINAPO)4 (ΔΔΔΔ-3
and ΛΛΛΛ-3) in CDCl3. (c) 1H DOSY spectrum of ΔΔΔΔ-
3 in CDCl3. Asterisk denotes the solvent NMR peak.
X-ray Crystal Structure of Eu(III) Tetrahedral
Cage. To better understand how chiral (R/S)-BINAPO
induce achiral L to form diastereopure tetrahedral cage,
the X-ray-quality crystals of (Eu4L4)(R-BINAPO)4 and
(Eu4L4)(S-BINAPO)4 (ΔΔΔΔ-1 and ΛΛΛΛ-1) are grown
from a THF-hexane solution. X-ray diffraction analyses
reveal that ΔΔΔΔ-1 and ΛΛΛΛ-1 are the expected M4L4
tetrahedral structure (Figure 2a and 2b), and both
crystallize in the chiral space group I222, suggesting that
chirality deriving from chiral R/S-BINAPO has been
transferred successfully into the resulting solid structures.
In cage-1, four europium ions site on the vertices, and four
ligands span the faces. Interestingly, the triangle formed by
its three Eu(III) centers is scalene, with the distances
between each two metals centres being 13.158 Å, 14.846 Å
and 15.394 Å, respectively. In one cage, each Eu centre is
chelated by three β-diketonate units from three ligands
and one BINAPO molecule, giving rise to a distorted
triangular dodecahedron coordination geometry (Figure
Figure 2. Crystallographic structures of (a) (Eu4L4)(R-
BINAPO)4 (ΔΔΔΔ-1) and (b) (Eu4L4)(S-BINAPO)4
(ΛΛΛΛ-1). (c) Side view and (d) top view of the helical
configuration of (S)-BINAPO.
On the other hand, in contrast to the most of homochiral
3d transition metal-based tetrahedra with well-defined T
point symmetry,27 ΔΔΔΔ-1 and ΛΛΛΛ-1 have highly twisted
tetrahedral backbone that obviously deviate from T-
symmetry. It is because of the large steric bulk of R/S-
BINAPO that enforces the L losing C3-symmetry. Notably,
the twisted structure as well as the propeller-like
conformation of the triphenylamine (TPA) units are
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expected to effectively prevent luminescence quenching show five characteristic emission bands of Eu(III) ion at
578, 592, 612, 650, and 700 nm, corresponding to 5D0 →7FJ
(J = 0–4) transitions, respectively. Under 365 nm UV light,
the cages emit very strong red luminescence in CHCl3
(Figure 3b, insert). The Eu(III) center luminescence
quantum yields Φoverall are measured to be 81% (Figure S26,
S27), which represent the highest value for chiral Ln-based
complexes in solution. With the broad excitation window
in the visible region up to 450 nm, the cage even emits
bright red light under natural illumination.
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caused by intermolecular aggregation.
Chiral optical properties. The UV-vis and CD spectra
of (Eu4L4)(R-BINAPO)4 and (Eu4L4)(S-BINAPO)4 (ΔΔΔΔ-1
and ΛΛΛΛ-1)in CHCl3 are shown in Figure 3a. As expected,
the enantiomers show the same absorbance curves and
mirror-image CD signals. In the range of 220–450 nm,
three well-resolved absorbance bands can be observed. The
high energy bands in 220–275 nm range are attributed to
the π–π* transition of chiral R/S-BINAPO, and show
mirror-image Cotton effects at 243 nm (negative signal for
ΛΛΛΛ-1 and positive signal for ΔΔΔΔ-1) and 261 nm
(positive signal for ΛΛΛΛ-1 and negative signal for ΔΔΔΔ-
1, respectively. The other two bands in the ranges of 280–
350 nm and 350–450 nm are attributed to the characteristic
absorbance of achiral tris-β-diketone ligand. The relatively
intense absorbance at lower energy band are attributed to
π–π* charge transfer (CT) transition from triphenylamine
core to withdrawing β-diketone units. Moreover, a
shoulder is observed at about 400 nm, an exciton coupling
characteristics arising from the spatial proximity of three
coordination β-diketone arms around one Eu(III) center.28
Compared with the absorbance of free L (Figure S23), the
band exhibits apparent hypsochromic shift with the
maxima from 430 nm to 383 nm. It is reasonable that the
higher degree twist of ligand wrapping the metal center
reduce the conjugation of the π-skeleton. While the higher
energy absorbance in 280–350 nm are complicated and
exhibit several vibrational progressions with a spacing of
(5.9–6.7) × 105 cm–1 (ring breathing feature), which should
be attributed to π–π* transitions of the triphenylamine
core and the intra-diketones.
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In CD spectra, the achiral tris-β-diketone shows two
large exciton couplets with the Davydov splitting of 40 nm
and 25 nm, respectively, representing two Cotton effect
splits of the 392 nm- and 316 nm-centered absorbance
bands. This result indicates the chirality of R/S-BINAPO
have been successfully transferred into the L, where the
helical chirality is formed. In (Eu4L4)(R-BINAPO)4, the two
bands both show negative exciton couplets, corresponding
to the Δ configuration around the Eu(III) ion; conversely,
a positive exciton couplet associated with Λ configuration
is observed for (Eu4L4)(S-BINAPO)4. These results accord
with the general rule used for relating the sign of the
exciton couplet with the absolute configuration of the
metal centre for the family of octahedral tris(bidentate)
complexes.29 Although the application of this empirical
rule to assign the absolute configuration of Ln(III)
complexes are rare, several examples reported still prove
its universality.30
Figure 3. (a) UV-visible absorption (lower curve, right axis)
and CD spectra (upper curves, left axis) of (Eu4L4)(R-
BINAPO)4 (ΔΔΔΔ-1) (blue lines) and (Eu4L4)(S-BINAPO)4
(ΛΛΛΛ-1) (red lines) in CHCl3 (2.5 × 10–6 M). (b) Total
luminescence (lower curve, right axis) and CPL spectra
(upper curves, left axis) of ΔΔΔΔ-1 (blue lines) and ΛΛΛΛ-
1 (red lines) in CHCl3 (λex = 395 nm, 2.5 × 10–6 M).
The CPL spectra show two relatively intense emitting
signals at 592 nm and 612 nm, which are attributed to the
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5D0 → F1 magnetic-dipole transition and 5D0 → F2
electronic-dipole transition, respectively. Generally, the
magnetic dipole transition often shows particularly large
circular polarization. The degree of the CPL is assessed in
terms of the luminescent dissymmetry factor, glum. Herein,
glum = 2(IL – IR)/(IL + IR), where IL and IR represent the left
and right polarized emission intensities, respectively (with
The CPL and PL spectra of (Eu4L4)(R-BINAPO)4 and
(Eu4L4)(S-BINAPO)4 in CHCl3 are shown in Figure 3b. The
enantiomers exhibit almost identical emission spectra, but
show mirror image CPL spectra. The excitation spectra are
obtained by monitoring the emission maxima of Eu(III) ion
at 612 nm (Figure S24), which well match their absorption
spectra (Figure S25). Upon excitation with the β-
diketonate antennae moieties at 395 nm, the emission
spectra of (Eu4L4)(R-BINAPO)4 and (Eu4L4)(S-BINAPO)4
–2 ≤ glum ≤ 2). The |glum| values for D0→7F1 transitions of
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(Eu4L4)(R-BINAPO)4 and (Eu4L4)(S-BINAPO)4 are found to
be 0.20, which are comparable to those of the reported
Eu(III) complexes with chiral 2-hydroxyisophthalamide-, 1-
hydroxy-2-pyridinone-, pyridyl diamide-, and DOTA-
based ligands.31 In (Eu4L4)(R-BINAPO)4, a negative CPL
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replace the chiral R/S-BINAPO for examining the
signal at 5D0→7F2 transition and a positive CPL signal at 5D0
→ F1 transition are observed, while the (Eu4L4)(S-
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stereochemistry memory effect of the Ln-cage. In view of
the similar bidentate structures, the coordination abilities
for two PO ancillary ligands are probably the same or differ
slightly with each other. Thus, the equivalent ratios of
DPEPO to BINAPO for achieving the entire displacement
is firstly studied. The progress of the reaction is monitored
by CD spectra. Upon the addition of 4 equivalent of
DPEPO to a solution of cage-1 (6 × 10–6 M in CHCl3), the CD
BINAPO)4 gives the mirror-image signals. The spectra
signatures are in analogy with that of the exciton coupling
in CD spectra. Therefore, it is suggested that the CPL
spectral information can relate with the absolute
configuration of lanthanide complexes, although the
corresponding reports are rare.32
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Stereochemical Memory of the Cage. In
consideration of the rigid of the ligand and the strong
mechanical coupling usually existing in multinuclear
metals framework,33 we infer that the cage Ln4L4 could
process and conserve the original information first
introduced by chiral R/S-BINAPO after its replacement.
0
intensity shows a 16% drop after 48 h of reaction at 25 C
(Figure 4a). With further addition of DPEPO by each 4
equivalent (time interval 48 h), the decreased tendency
gradually slow down, until the CD values tend to be
constant after 32 equivalent addition (Figure 4b),
suggesting the completion of the majority of the
substitution.
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Herein,
we
employ
the
achiral
bis[2-
(diphenylphosphino)phenyl]ether oxide (DPEPO) to
Figure 4. (a) CD spectra of cage-1 and (b) their intensities at 368 nm with the addition of different equiv of DPEPO in CHCl3
(cage-1, 6.0 × 10–6 M). (c) CD spectra of cage-1 and (d) their intensities at 368 nm with reaction time in the presence of 32
equiv of DPEPO in CHCl3 (cage-1, 6.0 × 10–6 M). (e) 1H NMR spectra of (Y4L4)(R-BINAPO)4 (ΔΔΔΔ-3) substituted by 32 equiv
of DPEPO at different reaction times. (f) 31P NMR spectra of the filtrate of (Y4L4)(DPEPO)4 (ΔΔΔΔ-4) prepared from ΔΔΔΔ-3
via the two (Sub.2) and three times (Sub.3) substitutions.
To monitor the substitution rate, the CD intensity
variations (at 368 nm) with reaction time are monitored in
the presence of 32 equivalent of DPEPO (Figure 4c). As
shown in Figure 4d, the Δε shows a gradual decrease with
the reaction time until reaching a plateau after stirring for
36 h, indicating the completion of substitution. The
luminescence intensities variations (at 612 nm) with the
time show a similar tendency as that observed in CD
measurement (Figure S28).
This process is also monitored by the employment of 1H
NMR spectroscopy. To eliminate the interference of the
excess DPEPO on NMR measurement, at the end of each
reaction time interval (12 h), the partially substituted
species is isolated as precipitate by the addition of excess
n-hexane to its CHCl3 solution. As shown in Figure 4e,
1
during the course of the reaction, H NMR spectra show
temporary desymmetrization and broaden, eventually
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and ΛΛΛΛ-2 (red lines) in CHCl3 (λex = 395 nm, 2.5 × 10–6
M).
converging to a spectrum consistent with the racemic
(Y4L4)(DPEPO)4 (cage-4) after 48 h of reaction (Figure S29).
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To insure the complete substitution for R/S-BINAPO,
the firstly substituted and isolated product is re-reacted
twice with 32 equivalent DPEPO. 31P NMR spectrum of the
filtrate of the second times substituted product shows a
trace of free R-BINAPO signal at 28.8 ppm (Figure 4f,
sub.2), indicating the firstly substituted product with a
trace of R-BINAPO ligating on the cage. However, the
signal of R-BINAPO disappears after the third times
repetition (Figure 4f, sub.3), which indicates the pure cage-
2, (Eu4L4)(DPEPO)4 are obtained. The purity of the cages-
2, (Eu4L4)(DPEPO)4 and cage-4, (Y4L4)(DPEPO)4 are
further confirmed by NMR and ESI-MS measurements
(Figure S30–S45). Notably, the CD intensities of three
times substitution products are almost the same (Figure
S46), which indicates the optical activity originating from
the tetrahedral backbone is well preserved, although a
large amount of achiral DPEPO exist in solution.
The CD spectra of the cage-2 prepared from ΔΔΔΔ-
/ΛΛΛΛ-1 show mirror-image CD signals and similar
spectral signatures as observed in cage-1. The same exciton
coupling signs suggest the configuration of cage-1 is
retained during the exchange process, and another pair of
enantiomeric tetrahedral cages ΔΔΔΔ- and ΛΛΛΛ-
[(Eu4L4)(DPEPO)4] (ΔΔΔΔ-2 and ΛΛΛΛ-2) are obtained.
Compared with the CD spectra of the cage-1, the
magnitude of molar ECD (Δε) of three exciton coupling
drop about 55%. We suggest that this drop should relate to
the changes of the coordination environment around
Ln(III) ion. From the optimized structure (Figure S48), by
calculations using LUMPAC with a Sparkle/PM6 model,34
we can observe that the relatively flexible structure of
DPEPO release the tension of the highly twisted
tetrahedral backbone caused by BINAPO. The distances
between two metal centers in a triangle plane change from
13.158 Å, 14.846 Å and 15.394 Å in cage-1 to 13.493 Å, 14.895
Å and 14.908 Å in cage-2, a less deviatation from T
symmetry. Notably, the CD signals arising from achiral
DPEPO are observed at low wavelength range (230–280
nm), which indicates the chirality from the cage has
transferred to the DPEPO. In other word, a chiral cage
comprised of achiral elements is successfully constructed
after a successive chiral induction and exchange.
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To rule out the possibility that the optical activity
observed in this system arising from the trace amount of
R/S-BINAPO in solution, we perform a control experiment.
The cage-2 prepared from the assemble of L and Ln(III)
ions in the presence of 32 equivalent of DPEPO and 4
equivalent of R-BINAPO or the stirring of a solution of
racemic cage-2 in the presence of 4 equivalent of R-
BINAPO and 28 equivalent DPEPO for 48 h, both show
negligible CD signals (Figure S47).
The cage-2 also display mirror-image CPL signals and
the similar spectral signatures as that in cage-1, in line with
the expected retention of the configuration around the
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Eu(III) ions. The |glum| values for D0 →7F1 transitions of
cage-2 are found to be 0.11, which is about the half of the
value of cage-1. The cage-2 also show bright red emission
under UV light and naked-eye visible luminescence under
room light. The luminescence quantum yields Φoverall are
measured to be 68% (Figure S49, 50).
Finally, the stability of the cage-1 and cage-2 in different
polar solvents and their abilities to resists hydrolysis are
investigated in view of their potential applications on bio-
analytical assays. In CHCl3, THF and methanol, the optical
activity of the cages did not degrade after staying for 7 days
at room temperature (Figure S51–S53). During this period,
there were no new sets of resonances relating to L and/or
free PO ligand present in 1H NMR of corresponding Y-cages
(Figure S54–S59). These observations prove that the
structures of the cages are stable in these solvents. While,
in DMSO, the optical activity disappeared (Figure S60),
and the signals from the free BINAPO and DPEPO can be
1
observed in H NMR (Figure S61, S62). This can be
explained from the strong coordination ability of DMSO,
where PO ligands are replaced. In methanol/H2O (v:v =
99:1), the cages are also relatively stable, as indicated by
their CD and 1H NMR spectra (Figure S63–S65). Upon the
addition of HCl and NaOH aqueous solution, the CD
intensities initially show slightly drop, then a gradual
decrease (Figure S66, S67), which are attributed to the part
disruption of the cages, as confirmed by the observation of
free L and PO ligands in 1H NMR spectra (Figure S68–S71).
Figure 5. (a) UV-visible absorption (lower curve, right axis)
and CD spectra (upper curves, left axis) of ΔΔΔΔ-
(Eu4L4)(DPEPO)4 (ΔΔΔΔ-2, blue lines) and ΛΛΛΛ-
(Eu4L4)(DPEPO)4 (ΛΛΛΛ-2, red lines) in CHCl3 (2.5 × 10–
6 M). (b) Total luminescence (lower curve, right axis) and
CPL spectra (upper curves, left axis) of ΔΔΔΔ-2 (blue lines)
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These results indicate that the two cages enable to resists The existence of dissociation pathway can be verified
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hydrolysis in relatively mild acidity and alkalinity
conditions. However, the excess addition of acid or base,
the irreversible hydrolysis of the free Ln(III) ions into
hydroxides and the protonation of the L2– will happen.
from the following concentration and temperature
dependence experiments of the CD intensities. In the
relatively higher concentration ranges (2.5 μM–1.0 mM),
the molar ECDs (Δε) of (Eu4L4)(S-BINAPO)4 remain
constant, but at much lower concentration, 0.6 μM and 1.0
μM, the Δε have a 16.3% and 9.8% drops, respectively
(Figure 6a, 6b). In the substitution experiment, a similar
tendency is also observed (Figure 6c, 6d). Moreover, the
substitution process also show temperature dependence.
Mechanism of Chirality Memory. The CD and CPL
measurement results reveal that the substitution process is
stereoselective. We infer the transformation from cage-1 to
cage-2 should proceed through two possible pathway: (1)
association pathway, where the formation of activated
complex that one metal centre is ligated with a BINAPO
and a DPEPO is reasonable by expanding the coordination
numbers of Ln ions. In this case, the dissociation of
BINAPO will not lead to the inversion of configuration on
the vertices; (2) dissociation pathway, where the
substitution proceed following the dissociation of BINAPO
from one and/or more vertices. The two proposed
pathways are shown in Scheme 2.
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o
In temperature range of 10–30 C, the Δε values tend to
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constant after 48 h of reaction (Figure S72–S74, Figure 6f).
In contrast, the CD intensities gradually decrease until
o
reaching to zero after reaction for 148 h at 40 C (Figure
6e). These results are consistent with the suggested
dissociation pathway, since the dissociative degree would
be enhanced following with the decrease of the
concentration and the increase of temperature. Notably,
the Δε curves with reaction times are well overlapped at 10–
30 oC ranges.
Scheme 2. Schematic representation of the two parallel pathways operating during the transformation of ΔΔΔΔ-
1 into ΔΔΔΔ-2.
In the proposed dissociation pathway, there are four
possible species in solution. We suggest the racemization
of the cage at relatively dilute solution and the higher
temperature should originate from the presence of a
completed dissociation species IV, “naked cake”, where the
stereochemistry control from chiral R/S-BINAPO will
disappear. For the other species I-III, the configuration of
the cage would be maintained during the substitution
process, because the partial chelation of chiral units will
induce all the vertices to adopt the same helical
handedness due to the strong mechanical coupling
between the metal centers.35 On the base of the
concentration and temperature dependence results, it can
be concluded that the high fidelity stereochemical memory
could be realized as the concentration of [cage-1] ≥ 0.25 μM
ΛΛΛΛ-2 by employing the chiral NMR chemical shift
agents and chiral HPLC technologies are unsuccessful.36
To gain further insight into the substitution mechanism,
we perform a simple kinetic analysis for substitution
process by monitoring the amounts of free BINAPO in
solution by HPLC (Figure S75). In the temperature range
of 10–40 oC, the fitting curves of the reaction time with the
concentration of BINAPO neither well match the first-
order kinetic nor the second-order reaction kinetic (R2 <
0.99, Figure S76–S79). It indicates that the substitution
process probably involve the both reaction kinetics
simultaneously.
In dissociation pathway, the substitution process should
obey first-order reaction kinetics as the dissociation of
BINAPO from the cage-1 should be the rate-determining
step. On the other hand, in association pathway the
formation of transition state should be the rate-
o
and reaction temperature T ≤ 30 C. However, it is a pity
that the efforts for obtaining the ee values of ΔΔΔΔ-2 and
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S4), the following thermodynamic parameters, ΔrG θm =
19.4 kJ mol–1, ΔrHθm = 11.9 kJ mol–1 and ΔrSθm = -22.6 J mol–1
K–1 relating to the substitution process at 20 oC are
estimated.
determining step, which can be described using second-
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order reaction kinetics. With these analyses, we suggest
that the two proposal pathways should coexist in the
substitution process, and the each probably represents two
poles of a continuum of possibilities in practice. With the
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calculated apparent equilibrium constant Kθ (Equation S1-
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Figure 6. (a) CD spectra of (Eu4L4)(S-BINAPO)4 (ΛΛΛΛ-1) and (b) their intensities at 368 nm in concentrations of 0.6 μM–
1.0 mM in CHCl3. (c) CD spectra of (Eu4L4)(S-BINAPO)4 (ΛΛΛΛ-1) and (b) their intensities at 368 nm in concentrations of
0.6 μM–1.0 mM in the presence of 32 equiv of DPEPO after stirring 48 h in CHCl3. (e) CD spectra of (Eu4L4)(S-BINAPO)4
(ΛΛΛΛ-1) with 32 equiv of DPEPO with different reaction time (T = 40 oC, cage-1, 2.5 μM). (f) The plots of the CD intensity
decay at 368 nm with reaction time under different reaction temperatures.
Ultrahigh Luminescence Quantum Yields Analyses.
As excellent CPL materials, the high luminescence
quantum yield (Φtot) is another important parameter for
estimating the performance of the materials. Absolute
luminescence quantum yields of the cage-1 and cage-2 in
CHCl3 are measured to be 81% and 68%, respectively.
These ultrahigh luminescence quantum yields are rare in
lanthanide materials.
We propose that the high luminescence quantum yields
of the complexes should benefit from (1) effective
sensitization ability of β-diketonate moieties on Eu(III)
ions luminescence; (2) effective suppression of the
nonradiative transition via vibrational relaxation in the
complexes due to the introduction of PO ancillary ligands,
which exclude the coordinated solvent molecules from the
primary coordination sphere of the metal ions and strength
the rigidity of the tetrahedral skeleton.
Table 1. Radiative (kr) and nonradiative (knr) decay rates, observed luminescence lifetime of Eu3+ (τobs), intrinsic
quantum yield (ΦLn), sensitization efficiency (ηsens), quantum yield of Eu3+ (ΦEu), quantum yield of ligands (ΦL)
and overall quantum yield (Φoverall). Error in τobs: ±0.05 ms; 10% relative error in the other values; λex = 395 nm. glum
values for 5D0 → 7FJ of Eu3+ ion.
glum 5D0 → 7FJ (J = 0, 1, 2, 3, 4)
Cages
kr (s‒1)
knr (s‒1)
τobs (μs)
ΦLn (%)
ηsens (%)
Φoverall (%)
J = 0
J = 1
J = 2
J = 3
J = 4
ΔΔΔΔ-1
ΛΛΛΛ-1
1506
1509
274
270
562
563
84.6
84.8
96.1
95.6
81.3
81.1
0.007
0.202
-0.009
0.008
0.021
0.050
-0.007
-0.204
-0.022
-0.049
ΔΔΔΔ-2
ΛΛΛΛ-2
1283
1282
509
511
558
558
71.6
71.5
95.3
95.2
68.2
68.1
0.009
0.113
-0.002
0.002
0.024
0.021
-0.009
-0.113
-0.024
-0.021
푘푟
휏obs
In order to clarify theses effects, we have estimated
sensitization efficiency (ηsens), and radiative (kr),
nonradiative (knr) rate constants of the complexes by using
the following equations:
훷퐿푛
=
=
(ퟐ)
(ퟑ)
푘푟 + 푘푛푟 휏푟푎푑
퐼푡표푡
1
푘푟 =
= 퐴푀퐷, 0푛3
(
)
휏푟푎푑
퐼푀퐷
훷표푣푒푟푎푙푙 = 휂푠푒푛훷퐿푛
(ퟏ)
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As shown in equation 1, the luminescence quantum yield
On the other hand, we also report the first example of
the chiral memory effect within the lanthanide assemblies.
The chirality of cage-1 deriving from chiral ancillary ligands
can be retained via the substitution with an achiral
analogue. The mechanism analyses reveal that the
stereoselective substitution process is high fidelity as the
concentration [cage-1] ≥ 0.25 μM and reaction temperature
T ≤ 30 oC, because the racemizing pathway arising from the
complete dissociation of four ancillary ligands can be
excluded or ignored on this condition. Encouragingly,
cage-2 also present impressive luminescence quantum
yield (QYs = 68%) and modest luminescence dissymmetry
factors (|glum| = 0.10). All in all, this work provides a simple
and low-cost synthesis strategy for fabricating chiral
lanthanide cage with excellent CPL properties.
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(Φoverall) is determined by the efficiency of the energy
transfer (ηsen) and by the intrinsic quantum yield (ΦLn) of
the lanthanide luminescence.
The intrinsic quantum yields of europium luminescence
could be estimated by using the equation 2, after the
calculation of the radiative lifetime (τrad) from equation 3.
Where AMD,0 = 14.65 s–1 is the spontaneous emission
probability of the magnetic dipole 5D0→7F1 transition, n is
the refractive index of the solution. Itot is the total
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integrated emission of the D0→7FJ transitions, and IMD is
the integrated emission of the D0 → 7F1 transition. The
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observed lifetimes (τobs) are determined by monitoring the
emission decay curves within the 5D0→7F2 transition at 612
nm (Figure S80–S83).With the calculated radiative
lifetimes (τrad) and observed lifetimes, the intrinsic
quantum yields (ΦLn) are found to be 84% and 71% for cage-
1 and cage-2.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures,
supporting figures and tables, and X-ray crystallographic data
for [(Eu4L4)(R/S-BINAPO)4]. Crystallographic data have been
deposited with the Cambridge Crystallographic Data Centre
as entries CCDC 1935644 and 1936609.
According to equation 2, the intrinsic quantum yield is
the result of radiative transition (kr) competes with non-
radiative transition (knr) processes. The corresponding
parameters are summarized in Table 2. Compared with the
cage-2, the cage-1 have the higher radiative rate constants
and the lower nonradiative rate constants. Generally, the
reduction of the geometrical symmetry of the coordination
structure will leads to a larger radiative rate constant (kr).37
Coordination geometry calculated by SHAPE 2.1 software
show that the cage-1 has the lower symmetrical triangular
dodecahedral structure (8-TDH, D2d) than the square
antiprismatic structure (8-SAP) for cage-2. Moreover, the
lower knr for cage-1 can be rationalized by their higher
degree twist backbone, the resulting rigidity well suppress
energy loss via vibrational relaxation. Additionally, the
efficiency of energy transfer for two complexes are about
the same, up to 95%. Therefore, the higher luminescence
quantum yields of the cages should ascribe to their
relatively high intrinsic quantum yield and energy transfer
efficiency.
AUTHOR INFORMATION
Corresponding Author
*E-mail: lihongfeng@hlju.edu.cn
*E-mail: Yanpf@vip.sina.com
ORCID
Hongfeng Li: 0000-0003-4646-0515
Pengfei Yan: 0000-0002-5124-1707
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work is financially supported by the National Natural
Science Foundation of China (Nos. 51872077 & 51773054). We
also thank the Key Laboratory of Functional Inorganic
Material Chemistry, Ministry of Education, P. R. China, for
supporting this work. The authors thanks Prof. Xin B.-F. for
helpful discussions on mechanism of chirality memory, and
thanks Prof. Mao, G.-J. for assistance with NMR
measurements.
Conclusion
In summary, a pair of enantiopure tetrahedral cage
[(Eu4L4)(R/S-BINAPO)4] with excellent circularly polarized
luminescence (CPL) properties are successfully
synthesized. We, herein, report the first example of
employing the chiral ancillary ligand to control the
stereochemistry of lanthanide tetrahedron. X-ray
crystallographic analysis reveals that the configuration of
R/S-BINAPO relate to the homochiral configuration of the
cage, and determining the exciton coupling signatures of
the CD spectra and the Eu(III) center’s CPL signs. The R-
BINAPO induces the formation of a right-handed P helical
homochiral cage, ΔΔΔΔ-1, and a negative exciton coupling
of CD signal and a positive CPL signal at 5D0→7F1 transition.
The luminescence measurements show that the cage-1
present high circularly polarized luminescence with the
|glum| up to 0.20, and a highest value of luminescent
quantum yields (QYs = 81%) that reported in chiral
lanthanide materials.
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