Chiral phosphorescent probes for amino acids: hybrids of iridium(iii) complexes with synthetic saponite

Article abstract of DOI:10.1039/c6nj03777a

An attempt of developing a chiral luminescent probe for amino acids was described. Two types of chiral iridium(iii) complexes were synthesized: [Ir(dfppy)₂(R-pep-bpy)]⁺ (dfppyH = 2-(2′,4′-difluorophenyl)pyridine); R-pep-bpy = 4,4′-bis((R-1,2-dimethylpropyl)aminocarbonyl-2,2′-bipyridine) and [Ir(piq)₂(R-pep-bpy)]⁺ (piqH = 1-phenyisoquinoline). In both complexes, amido groups (R-pep-) were attached to 2,2′-bipyridine with an intension of increasing the affinity for an amino acid. The complexes were optically resolved on a chiral column. When the complexes were adsorbed by the colloidal particles of synthetic saponite, they were highly emissive even in a medium containing water. The remarkable quenching of emission was observed for the combination of [Ir(dfppy)₂(R-pep-bpy)]⁺ and tryptophan methyl ester. By the use of the enantiomeric complexes, the possibility of enantioselective quenching was examined for various amino acid derivatives.

Full text of DOI:10.1039/c6nj03777a

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Chiral Phosphorescent Probes for Amino Acids:
Hybrids of Iridium(III) Complexes with Synthetic
Saponite
Received 00th January 20xx,
Accepted 00th January 20xx
Hisako Sato,a* Kenji Tamura,b Tomoko Yajima,c Fumi Sato,d and Akihiko Yamagishi d
DOI: 10.1039/x0xx00000x
www.rsc.org/
An attempt of developing a chiral luminescent probe for amino acids was described. Two types of chiral iridium(III)
complexes were synthesized: [Ir(dfppy)2(R-pep-bpy)]+ (dfppyH = 2-(2’,4’-difluorophenyl)pyridine; R-pep-bpy = 4,4’-bis((R-
1,2-dimethylpropyl)aminocarbonyl-2,2’-bipyridine) and [Ir(piq)2(R-pep-bpy)]+ (piqH
= 1-phenyisoquinoline) In both
complexes, amido groups (R-pep-) were attached to 2,2’-bipyridine with an intension of increasing affinity towards an
amino acid. The complexes were optically resolved on a chiral column. When the complexes were adsorbed by the
colloidal particles of synthetic saponite, they were highly emissive even in a medium containing water. The remarkable
quenching of emission was observed for the combination of [Ir(dfppy)2(R-pep-bpy)]+ and tryptophan methyl ester. By use
of the enantiomeric complexes, the possibility of enantioselective quenching was examined for various amino acid
derivatives.
iridium(III) complex reduces its emitting properties in
a
Introduction
medium containing water. It was found previously that
emission capability recovered when the complexes were
adsorbed by a colloidal particle of a clay mineral.25-29 Based on
this fact, we investigated energy transfer by co-adsorbing two
iridium(III) complexes with different emitting properties onto a
colloidal particle of synthetic saponite.25 Efficient transfer of
photon energy occurred in the concentration range as low as
10-6 M. The accumulation of a donor and an acceptor of light
energy on the same clay particles was considered to be a main
factor to achieve high efficiency.
Cyclometalated iridium(III) complexes have recently attracted
an extensive attention as a potentially useful photosensitizer
with high quantum yield and long lifetime.1-12 There have been
reported several attempts of using luminescent iridium(III)
complexes within biological systems.13-24 For example, the
patterning of cancer tissue in living cells was performed by use
of luminescent Ir(III) complexes.17 An iridium(III) complex
capable of coordinating histidine was used as a selective
sensor of histidine residues in proteins. It remains to be
clarified yet how the emission behaviours of iridium complexes
are affected by interacting with amino acids particularly as to
chirality effects.
The present work intended to develop a chiral photo-sensor of
amino acids by use of the hybrid of a clay and an iridium(III)
complex. Clay minerals are harmless for bioorganisms in spite
of their high capability in adsorption. Such a sensor would be
useful to detect a target amino acid in situ in living cells. Our
purpose at the present stage was to study the interaction of
iridium(III) complexes with amino acids as a benchmark for
their selective sensing.
The present work studied the interaction of chiral iridium(III)
complexes with amino acids or their derivatives. Generally an
Results and Discussion
Preparation and optical properties of Ir(III) complexes: Chart
1 shows the molecular structures of Ir(III) complexes used in
the present work. [Ir(dfppy)2(R-pep-bpy)]+ (dfppyH = 2-(2’,4’-
difluorophenyl)pyridine;
dimethylpropyl)aminocarbonyl-2,2’-bipyridine; denoted as Ir-
) and [Ir(piq)2(R-pep-bpy)]+ (piqH = 1-phenyisoquinoline;
R-pep-bpy
=
4,4’-bis((R-1,2-
1
denoted as Ir-2) were used as an emitting molecule. The chiral
amido group, R-pep-, was attached to 2, 2’-bipyridine in order
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to enhance affinity towards amino acids through the formation
of hydrogen bonding. The molecules were prepared by
reacting a dimer, [{Ir(dfppy)Cl}2] or [{Ir(piq)Cl}2], with R-pep-
bpy in glycerol. The products were identified from 1H-NMR and
mass spectroscopy (ESI†).
Figure 2. The CD spectra of the perchlorate salts of the diastereomers of (a) Ir-1
(left) and (b) Ir-2 (right) dissolved in methanol. The solid and dotted curves are
for the ꢀ- and Λ-isomers, respectively.
Emission properties in solutions: Figures 3(a) and (b) show the
emission spectra of Ir-1 and Ir-2 in various media, respectively.
Both complexes were emissive in chloroform with the
quantum yield (denoted as Q.Y.) of 0.35 and 0.02 as shown by
curves (I), respectively. The emission was assigned to the
transition from the mixed state of 3MLCT and 3LC.4 Their
The diastereomeric mixtures (
ꢀ
on chiral column (Experimental Section).
- and
Λ
-R-isomers) were eluted
typical
a
A
chromatogram is shown in ESI†. Figures 1 and 2 show the UV-
vis and circular dichroism (CD) spectra of the separated
diastereomers, respectively. Comparing the CD spectra of
analogous compounds ([Ir(ppy)3]; ppy = 2-phenylpyridine), the
emission ability decreased in
a mixture of 3:1 (v/v)
first and second fractions were concluded to contain
enantiomers, respectively.25,30
ꢀ- and Λ-
water/methanol to Q. Y. = 0.06 and 0.001 as shown by curves
(II), respectively. When synthetic saponite (denoted as SAP)
was present at the loading level of c.a. 7 % in the same
medium, the emission recovered remarkably up to Q.Y. = 0.21
and 0.01 as shown by curves (III), respectively. The observed
enhancement of emission was ascribed to the partial
elimination of water molecules around the iridium(III)
complexes adsorbed on the surface of synthetic saponite. The
displacement of the emission peak towards the shorter
wavelength might be caused by the electric field effect arising
from the negative charge of a synthetic saponite layer in
stabilizing the ground state of an adsorbed Ir(III) molecule.
Chart 1. The molecular structures of Ir(III) complexes used in the present work:
(upper) [Ir(dfppy)2(R-pep-bpy)]+ (denoted as Ir-1) and (lower) [Ir(piq)2(R-pep-
bpy)]+ (denoted as Ir-2).
Figure 1. The UV-vis spectra of the perchlorate salts of Ir-1 (solid) and Ir-2
(dotted) dissolved in methanol.
Figure 3. The emission spectra of the perchlorate salts of (a) Ir-1 (1.8
(left) and (b) Ir-2 (1.5
10-5 M) (right). The medium was (I) chloroform, (II) 3:1
(v/v) water/methanol and (III) 3:1 (v/v) water/methanol containing SAP (2.5
10-4
eqL-1). The excitation wavelength was 350 nm.
×
10-5 M)
×
×
2 | J. Name., 2012, 00, 1-3
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Effects of amino acids on stationary emission: The effects of In the above equations, KSV1 and KSV2 are two quenching
amino acids on emission were investigated. SAP was dispersed parameters and f1 and f2 their fractions, respectively. Ksvav is
in a mixture of 3:1 (v/v) water/methanol. The concentration of the average value of the quenching constants, which
SAP was adjusted at [SAP] = 2.5×
10-4 eqL-1 in terms of the represents the slope of the Stern-Volmer plot at [RS-tryp-Me]
cation-exchange capacity (CEC). Racemic Ir-1 or Ir-2 in a = 0 (Fig. 5). KSV1 and KSV2 were obtained to be 500 and 15000
methanol solution was added to the suspension under M-1, leading to 8400 M-1 for Ksvav. The origin for the presence of
rigorous stirring to a loading level of ca. 7 % of CEC. Thereafter two quenching paths might be sought in the difference in
a methanol solution of amino acid methyl ester (aa) was adsorption states. SAP was exfoliated mostly to a single layer.
added. The emission spectra were measured at various Some layers, however, might be double or triple layers before
amounts of aa
.
complete exfoliation. Accordingly iridium(III) complexes were
Among the investigated aa’s, tryptophan methyl ester adsorbed either on an external surface or between the
(denoted as tryp-Me) exhibited remarkable influence on the interlayer space of layers. It was expected that quenching
emission of Ir-1. Figures 4(a) and (b) show the change of the efficiency was more rapid for an iridium (III) complex adsorbed
emission spectra on adding various amounts of racemic on an external surface than for an iridium complex adsorbed
tryptophan methyl ester (denoted as RS-tryp-Me) to
a
between the interlayer spaces. These situations might result in
dispersion containing SAP/Ir-1 and SAP/Ir-2, respectively. In the appearance of two routes for quenching processes. For
case of SAP/Ir-1, the intensity of emission decreased with the comparison, the quenching of the same system was
increase of the added amount of RS-tryp-Me, while no change investigated in methanol. In such a homogeneous medium, the
of the emission spectrum was observed in case of SAP/Ir-2
.
Stern-Volmer plot was linear, giving a single value for KSV (ESI†).
The results implied that RS-tryp-Me acted as a quencher for Ir- Ksv in methanol was obtained to be 235 M-1 that was lower
1
, but not for Ir-2
.
than those of the colloidal clay systems. The above results
implied that the adsorption by synthetic saponite resulted in
enhancing both the emission intensity and the quenching rate.
Figure 4. The emission spectra of (a) SAP/Ir-1 (left) and (b) SAP/Ir-2 (right) (both
7
% loading) in 3:1 (v/v) water/methanol containing SAP (2.5×
10-4 eqL-1),
respectively. The concentration of added RS-tryp-Me is indicated in Figure 5.
The quenching effect in Figure 4 (a) was analysed by plotting
Io/I against [aa] (Stern-Volmer plots) (Figure 5). Here Io and I
are the heights of the emission peak in the absence and
presence of RS-tryp-Me, respectively. Since the plot was
curved, two quenching parameters were introduced in order
to reproduce the experimental results according to the
expressions below:
Figure 5. The Stern-Volmer plots for the quenching effect of RS-trp-Me on the
emission from SAP/Ir-1: [Ir-1] = 2.0× ×
10-5M and [SAP] = 2.5 10-4 eq L-1. The
solvent was 3:1 (v/v) water/methanol. The dotted curve is the simulated one
assuming two quenching parameters (see the text).
The effect on the emission from Ir-1 was investigated by using
the methyl esters of other amino acids. The results are
summarized in in ESI†. According to the results, tryp-Me was
most effective in quenching the emission from SAP/Ir-1. One
possibility for high quenching efficiency by tryp-Me was that
an indole moiety acted as an energy acceptor from excited Ir-1
The electronic absorption for exciting an indole group
extended to ca. 400 nm in a polar solvent, which was close to
.
(1)
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the edge of the emission band from Ir-1. Phenylalanine methyl
ester acted as a quencher but was less effective than tryp-Me
by more than 7 times. Little quenching was observed for
alanine, valine and isoleucine methyl esters. In case of
histidine methyl ester, emission increased slightly to give
negative KSV. The results suggested that this molecule affected
the adsorption state of Ir-1 in the ground states.
The effect was studied for Ir-1 on adding free amino acids. No
quenching occurred for alanine, histidine and tryptophan. It
was interpreted that a free amino acid was adsorbed by a clay
particle and fixed firmly so that it interacted with iridium(III)
complexes to a negligible extent. When N-3, 5-dinitrobenzoyl
amino alanine methyl ester (denoted as DNB-al-Me) was
added, efficient quenching occurred. The details are described
in the succeeding section.
Other emitting Ir(III) complexes were investigated by using RS-
tryp-Me as a quencher. As is summarized in Table 1, efficient
quenching was achieved only for the iridium(III) complexes
with fluorinated ligands. One reason was that the introduction
of fluorine atoms in ligands resulted in the elongation of the
life-time of an excited state. As described in the succeeding
section, the quenching by RS-tryp-Me occurred effectively only
when the lifetime of an excited Ir(III) complex was longer than
Figure 6. Decay curves of the emission at 550 nm for Ir-1 and 600 nm for Ir-2
when a SAP suspension containing (a) Ir-1 or (b) Ir-2 was irradiated with a light
pulse at 355 nm: [SAP] = 1.5×10-4 eq L-1, [Ir-1] = 2.0 10-5 M, [Ir-2] = 2.0 10-5
× × M
and the medium 3:1 (v/v) water/methanol. The concentration of RS-tryp-Me was
0 M (square) and 9.3
10-5 M (circle) for Ir-1 respectively. The concentration of
RS-tryp-Me was 3.7x10-4 M (square) and 9.3 10-4 M (circle) for Ir-2 respectively.
×
×
The grey lines indicate the fitting curves according to equation (2) in the text.
The decay curves were analysed in terms of the sum of two
exponential decays:
1 ꢁs.
Table 1. The quenching parameter (KSV) of iridium(III) complexes for tryptophan
methyl ester as an quencher.
y = F exp(−t /τ1) + F2 exp(−t /τ2 )
,
1
(2)
in which τ1 and τ2 are the fast and slow relaxation times and
F1 and F2 their fractions, respectively. The decay curve was
obtained by varying the added concentration of RS-tryp-Me. In
Complex
KSV (M-1
)
[Ir(ppy)2(dmbpy)]+
[Ir(ppy)2(C9-bpy)]+
[Ir(dfppy)2(C9-bpy)]+
[Ir(piq)2(C9-bpy)]+
Ir-1
200
150
case of Ir-1, both τ1 and τ2 decreased the increase of [RS-tryp-
Me]. For both relaxation paths, the following relation was
4500
20
assumed to hold:
8400
*ppy (2-phenylpyridine), dmbpy(4,4’-dimethylbipyridine), C9-bpy (4,4’-
dinonylbipyridine)
τ0/τ = 1 + τ0kq[tryp-Me],
(3)
*The slope of the Stern-Volmer plots for the quenching effect of trp-Me on the
emission from iridium(III) complexes: [iridium(III) complex] = 2.0×
10-5 M, [SAP] =
2.5×10-6 eq L-1, (tryp-Me)
water/methanol.
= 1 – 6 ×
10-3 M and the solvent was 3:1 (v/v)
in which τ0 is lifetime in the absence of RS-tryp-Me and kq the
bimolecular quenching rate constant, respectively. The plots
according to equation (3) are given in ESI†. From the results, kq
Transient emission properties: The decay rates of emission
from the excited iridium(III) complexes were measured on a
3:1 (v/v) water/methanol suspension containing SAP/Ir-1 or
SAP/Ir-2 (Figure 6).
and
τ
that the values of
0kq were obtained as given in Table 2. It was concluded
0kq for the fast and slow processes were of
τ
the same order in magnitude as KSV2 and KSV1 obtained from
the stationary measurements, respectively.
In case of Ir-2, the decay curve was unaffected on adding
3.7×
10-4 M of RS-tryp-Me. This was consistent with the results
of stationary measurements in which no quenching took place
for this complex. The above results lead to the following
conclusion as to the occurrence of quenching: in case of Ir-1
,
its lifetime was long enough for deactivation of photon energy
to occur, while in case of Ir-2, the lifetime was too short and
the collisional intermediate returned to the ground state
before the occurrence of quenching.
4 | J. Name., 2012, 00, 1-3
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Table 2. The kinetic results for the effects of RS-tryp-Me on the emission from SAP/Ir-1:
[SAP] = 1.5×10-6 eq L-1, [Ir-1] = 2×10-5 M and the medium 3:1 (v/v) water/methanol.
Table 3. The quenching parameter (KSV) of the emission from Ir-1 for R- or S-DNB-al-Me
as an quencher.
τo/τ vs. RS-tryp-
Me
F
kq
Ksvav
τ0 (ꢁs)
τ0kq (or KSV
[M-1]
)
(a)
(b)
(c)
(d)
[M-1 s-1]
[M-1]
concentration
Ksv1
Ksv2
f1
1325
22092
0.65
0.35
7836
979
167
0
Fast
0.22
0.75
0.30 1.60x1010
3650
15542
0.69
0.31
4895
5783
0.41
0.59
3409
6916
0.43
0.57
3940
lifetime(τ1)
1500
Slow
0.70 8.24x108
615
lifetime(τ2)
f2
Ksvav
Enantioselectivity: The possibility of enantioselectivity was
examined by using the pure enantiomer of Ir-1 as an emitter
and R- or S-tryp-Me as a quencher. The efficiency of quenching
was compared among the following four pairs: ꢀ-Ir-1/R-tryp-
Me, Ir-1/S-tryp-Me, Ir-1/S-tryp-Me and Ir-1/R-tryp-Me.
(*) The columns denoted in the first row corresponded to (a)
ꢀ-
Λ-Ir-1/S-DNB-al-Me,
(b) Ir-1/R-DNB-al-Me, (c) Ir-1/S-DNB-al-Me and (d) Ir-1/R-DNB-al-Me in
ꢀ
-
Λ
-
Λ-
Λ
-
ꢀ
-
Figure 7. The average Stern-Volmer constant (Ksvav) corresponds to the initial
slope at the zero concentration of a quencher.
As shown in ESI†, all of these pairs gave nearly the same Stern-
Volmer plots, leading to the conclusion that these pairs
exhibited no observable stereoselectivity. Probably the
quenching occurred even when a quencher was located too far
According to Table 3,
higher KSVav than Ir-1/R-DNB-al-Me, while there was no
significant difference between Ir-1/S-DNB-al-Me and Ir-
/R-DNB-al-Me. In other words, -R-Ir-1 is more effective than
-R-Ir-1. The results implied that the chiral discrimination was
Λ-Ir-1/S-DNB-al-Me gave 1.2 times
Λ
-
from the chiral part of Ir-1
.
ꢀ
-
ꢀ-
In order to pursue the possibility of enantioselectivity further,
DNB-al-Me was chosen as a quencher. The reason was that
DNB-al-Me was adsorbed stereoselectively by an ion-exchange
adduct of clay/ꢀ
-[Ru(phen)3]2+ (phen = 1,10-phenanthroline)
(experimental section) (ESI†). It was noted that two molecules,
[Ru(phen)3]2+ and Ir-1, both belonged to the tris(chelated)
complexes with polypyridyl ligands. The quenching of emission
1
Λ
ꢀ
achieved under the coupled contribution of ꢀΛ and RS
chiralities. Enantioselectivity for the above systems was
examined by dynamic measurements. The decay of Ir-1 was
compared among three cases of (1)
Me and (3) Ir-1/R-DNB-al-Me. As given in ESI†, the decay
rate from Ir-1/SAP increased on adding DNB-al-Me. The
Λ-Ir-1, (2) Λ-Ir-1/S-DNB-al-
Λ
-
from Ir-1 was investigated for the following four pairs:
/S-DNB-al-Me, Ir-1/R-DNB-al-Me, Ir-1/S-DNB-al-Me and
Ir-1/R-DNB-al-Me. As shown in Figure 7, they gave curved
Λ-Ir-
Λ
-
1
Λ
-
ꢀ-
effect was higher for S-DNB-al-Me than for R-DNB-al-Me. Thus
the same enantioselectivity was confirmed by both the
stationary and dynamic measurements.
ꢀ
-
Stern-Volmer plots so that the results were analysed in terms
of two quenching constants (eq. (1)).
The enantioselectivity observed for the cases of DNB-al-Me
suggested that the phenyl group in DNB-al-Me was attracted
by the phenylpyridyl groups in Ir-1 through π−π interaction
and that the stereoselectivity appeared through the
interaction at the chiral amido groups in Ir-1
.
Experimental Section
Materials:
[Ir(dfppy)2Cl]2
(dfppyH
=
2-(2’,4’-
1-
difluorophenyl)pyridine) and [Ir(piq)2Cl]2 (piqH
=
phenyisoquinoline) were purchased from Furuya Metal Co.,
Ltd., Japan. 4,4’-Bis((R-1,2-dimethylpropyl)aminocarbonyl-2,2’-
bipyridine (R-pep-bpy) was synthesized according to the
reported method.31 The compound was identified by 1H-NMR
spectroscopy. The cationic cyclometalated Ir(III) complex,
[Ir(dfppy)2(R-Pep-bpy)]ClO4, was prepared by refluxing
[Ir(dfppy)2Cl]2 with an equal amount of R-Pep-bpy in glycerol at
180 °C for 12 h. The compound was purified
chromatographically by elution on a high performance liquid
chromatography (HPLC) column (CAPCELL PAK C18, MG,
Shiseido Inc. Ltd., Japan) with 4:1 (v/v) methanol/chloroform
as the eluent. [Ir(piq)2(R-Pep-bpy)]ClO4 was synthesized in the
Figure 7. The Stern-Volmer plots for the quenching effect of DMB-al-Me on the
emission from Ir-1;(a)
Λ-Ir-1/S-DNB-al-Me,(b)Λ-Ir-1/R-DNB-al-Me,(c)ꢀ-Ir-1/S-
DNB-al-Me and (d)ꢀ
M. The solvent was 3:1 (v/v) water/methanol.
-
Ir-1/R-DNB-al-Me: [SAP]= 2.5×10-4 eq L-1 and [Ir-1] = 2.0×10-6
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same way by reacting [Ir(piq)2Cl]2 with R-Pep-bpy. The results promised a possibility of developing a photo-sensor of
compounds were identified using 1H-NMR in CDCl3 and mass amino acids in biological systems.
spectroscopy.32 Synthetic saponite (SAP; Kunimine Ind. Co.)
was used as the adsorbent, of which the elemental
composition and cation-exchange capacity (CEC) were stated
Acknowledgements
to be [(Na0.25Mg0.07)(Mg2.98Al0.01)(Si3.6Al0.4)O10(OH)2)] and 80
meq per 100 g, respectively.25 The average SAP particle size
was reported to be ca. 20 nm.
We thank for Ms. Narumi Hama (Toho Univ.) for the
preliminary measurements of emission spectra. Thanks are
also due to Prof. Takafumi Kitazawa (Toho Univ.) for the
permission of the measurements. This work has been
financially supported by the JSPS KAKENHI Grant-Aid-for
Scientific Research (B) Number JP26288039, Exploratory
Research JP16K15175, and Research on Innovative Areas
JP16H00840.
Amino acids were used as purchased from Wako Pure
Chemicals. Methyl esters were prepared by reacting amino
acid with thionyl chloride in methanol. DNB-al-Me was
prepared by reacting RS-alanine methyl ester with 3, 5-
dinitrobenzoyl chloride in dehydrated tetrahydrofuran. The
racemic mixture was optically resolved chromatographically.
The chromatogram and the CD spectra of the enantiomers are
given in ESI+.
Diastereomeric separation: The prepared Iridium(III)
compounds were eluted on a chiral column (CHIRALPACK IA,
Daicel, Japan) with acetonitrile containing 0.1% of
diethylamine and trifluoroacetic acid as an eluent at a flow
rate of 0.5 mL min-1. The elution was monitored at 430 nm.
Notes and references
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)
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6 | J. Name., 2012, 00, 1-3
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Journal Name
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New Journal of Chemistry
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DOI: 10.1039/C6NJ03777A
The effect of amino acids on the emission properties of iridium(III) complexes with a
colloidal particle of synthetic saponite was investigated with a purpose of developing a
photo-sensor of amino acids, leading to the conclusion that the interplay of chiral
coordination structure and asymmetric structure of a carbon atom achieved the
enantioselectivity.

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