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Double Action: Toward Phosphorescence Ratiometric
Sensing of Chromium Ion
Yejee Han, Youngmin You,* Yong-Min Lee, and Wonwoo Nam*
Photoluminescent sensors provide useful information about
identification, concentration, and spatiotemporal fluctuation of
analytes. Of particular importance is the development of photoluminescent sensors for oxophilic transition metal ions such as
Fe and Cu.[1,2] These d-block metal ions are critically involved in
many of biological and environmental processes, but the exact
chemical mechanisms of their actions are yet to be fully elucidated. Therefore, information concerning these metal ions is of
prime importance, and photoluminescent sensors are in great
demand for this purpose. Unfortunately, however, creation of
useful photoluminescence turn-on and ratiometric sensors is
difficult because low-lying d−d transition states and low redox
potentials of the paramagnetic transition metals efficiently
quench photoluminescence emission.[3] In addition, lack of
selective receptors for specific metal ions poses great challenges
to the development of sensors for these transition metal ions.
Chromium is widely used in a variety of industrial applications, such as electroplating and tanning. Chromium complexes have also been employed as colorants, catalysts, and
wood preservatives. This widespread consumption of chromium has resulted in significant bio- and environmental accumulation. It is considered that excessive Cr ions are toxic, and
particularly associated with genotoxicity.[4] High-valent Cr ions
such as Cr(V)[5,6] and Cr(VI)[7,8] are strongly implicated in the
toxic process, which is believed to involve production of reactive
oxygen species. Studies on Cr-oxo model compounds have provided mechanistic insight into the biomimetic Cr-induced oxidation reactions.[9–11] Information on identification and spatiotemporal fluctuation of chromium ion would be of enormous
value in relating the established oxidation chemistry of Cr with
chromium toxicity. Selective photoluminescent sensors for
chromium ion are, therefore, in high demand. In this context,
there have been many efforts to develop photoluminescence
turn-on[12–19] or ratiometric[20,21] sensors based on fluorescent
platforms such as conjugated imine,[12] rhodamine,[15,16,19–21]
phthalimide,[13,17] dansyl,[14] and BODIPY.[18] Despite these
advances, the sensors still suffer from inadequacies particularly with respect to reversibility, response times, and selectivity.
Most of all, identification of chromium ion with high fidelity
Y. Han, Dr. Y. You, Dr. Y.-M. Lee, Prof. W. Nam
Department of Bioinspired Science
Ewha Womans University
Daehyun-dong, Seodaemun-gu, Seoul 120-750, Korea
E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201104467
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in the presence of other divalent transition metal ions such as
Cu(II) remains a challenging task.
Herein, we report and demonstrate a novel strategy for
the double ratiometric detection of chromium(III) ion. The
first phosphorescent probe (YJ1) for chromium ion has
been developed based on a cyclometalated Ir(III) complex
([Ir(dfppy)2phen]+) bearing two 2-(2,4-difluorophenyl)pyridine
(dfppy) and 1,10-phenanthroline (phen) ligands. The cyclometalated Ir(III) complex was chosen as a signal transmitter because
of high efficiency room temperature phosphorescence and
marked robustness against photobleaching.[22,23] The Cr(III) ionresponsive bis(2-(2-(methylthio)ethylthio)ethyl)amino (BTTA)
receptor was introduced at the 4-position of the phen ligand.
The interaction between Cr(III) ion and the BTTA receptor produces a double-stage phosphorescence ratiometric response in
acetonitrile (Figure 1). The first stage is a prompt and reversible
green-to-orange phosphorescence change (Figures 1b,c), which
is due to perturbation of the excited states of [Ir(dfppy)2phen]+
that is weakly interacting with the BTTA receptor. The second
stage is a relatively slow conversion of the orange phosphorescence to green phosphorescence through a specific Cr-mediated
oxidative cleavage (Figures 1c,d). The overall observable are
thus sequential double phosphorescence ratiometric responses
(i.e., green → orange → green) that are specific to Cr(III) ion.
Therefore, this sensing strategy allows for highly selective and
accurate identification of Cr(III) ion in the presence of other
metal ions.
This phosphorescent sensor for Cr(III) ion has been prepared
in a nine step synthesis. Details on the synthetic procedures
are described in the Supporting Information (SI, Scheme S1).
Despite the fact that the sulfur-rich BTTA receptor is known to
bind soft metal ions such as Cu(I) ion in water,[24–26] in our case
we observed a strong response to Cr(III) ion instead of Cu(I)
in acetonitrile and no response in buffers (SI, Figure S1). In
fact, there was also negligible response to soft metals such as
Cu(I) and Ag(I) in CH2Cl2, THF, and CH3OH (SI, Figure S2).
The reason for the Cr(III) ion specificity of YJ1 is not clear but
may be due to the tighter Cr(III) binding of the tertiary amine
of BTTA in acetonitrile. The Cr(III) receptor was introduced to
the phen ligand through reductive amination between 4-formyl1,10-phenanthroline and bis(2-(2-(methylthio)ethylthio)ethyl)
amine. Substitution of the chlorides of the [Ir(dfppy)2(μ-Cl)]2
with the BTTA-functionalized phen ligand afforded YJ1.
The probe has been characterized by using spectroscopic
methods based on 1H, 13C, 19F, and {1H,1H} COSY NMR and
an electrospray ionization mass (ESI MS) spectrometry (SI,
Figures S19−S23), and is in agreement with the proposed structure. The positively charged complex YJ1 is highly soluble in a
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Figure 1. (a) Schematic representation of double phosphorescence ratiometric response of YJ1 for Cr(III) ion. Time-resolved photoluminescence
spectra of CH3CN solutions of YJ1 (10 μM) in the absence (b) and presence (c) of Cr(III) ion (1 equiv) and (d) after Cr(III)-induced oxidative cleavage.
Inset photos are phosphorescence emission of the YJ1 solutions illuminated under 365 nm excitation.
variety of common organic solvents such as acetonitrile up to a
concentration of 10 mM.
An air-equilibrated acetonitrile solution of YJ1 (10 μM) displays weak green phosphorescence emission with a peak wavelength at 515 nm (λex = 315 nm). Addition of Cr(ClO4)3 to the
YJ1 solution evokes a rapid phosphorescence ratiometric change
with a 8-fold turn-on and a 42 nm red-shift (Figure 2a).[27]
Corresponding photoluminescence quantum yield (PLQY;
determined by a relative method using fluorescein standard)
increases from 0.016 to 0.13 for Ar-saturated CH3CN solutions
(10 μM) at room temperature. In contrast, there is no distinct
change in the absorption bands of the singlet ligand-centered
π−π* (1LC; < 330 nm) transition and the metal-to-ligand chargetransfer (1MLCT; 330−450 nm) transition after the addition of
Cr(ClO4)3 because of low concentration (SI, Figures S3−S4).[28]
Cr titration has been carefully performed by measuring the
prompt increase in the phosphorescence intensity by injecting
a CH3CN solution of Cr(ClO4)3 (1 − 1.5 equiv). The titration isotherm plotting the phosphorescence intensity at 557 nm as a
function of concentration of added Cr(III) ion indicates a 1:1
binding stoichiometry (Figure 2a, inset). Binding of Cr(III) ion
is also supported by the appearance of a peak at m/z = 265.1 in
the ESI MS spectrum which corresponds to [K2CrIII(YJ1)(PF6)]5+
(calcd m/z = 265.0; SI, Figure S5). The ratiometric Cr(III)
response is fully reversible as demonstrated by restoration of
the original green phosphorescence after subsequent addition
of a strong metal chelator, N,N,N′,N′-tetrakis(2-picolyl)ethylenediamine (TPEN; Figure 2b). A reference Ir(III) complex (Ir5F;
Adv. Mater. 2012, 24, 2748–2754
i.e., [Ir(dfppy)2phen]PF6) lacking the BTTA receptor shows no
change in the phosphorescence spectrum upon the addition of
Cr ion (10 equiv; SI, Figure S6), supporting that the ratiometric
response of YJ1 is due to the Cr chelation of BTTA.[29]
The phosphorescence ratiometric response of YJ1 is a consequence of an excited-state interaction between [Ir(dfppy)2phen]+
and BTTA. Actually, photoluminescence excitation (PLE) spectra
of YJ1 in Cr(III)-free and -bound states are nearly identical,
except tiny increase in the MLCT absorption band (410 − 500 nm;
SI, Figure S7). In order to gain insight into a mechanism of
the phosphorescence modulation upon Cr binding, we performed quantum chemical calculations based on density functional theory and time-dependent density functional theory
(DFT/TD-DFT;
uB3LYP/LANL2DZ:6-31+G(d,p)//uB3LYP/
LANL2DZ:6-31+G(d,p)) for the triplet geometry of YJ1. The tertiary amino group of BTTA is quaternized by proton to mimic
the electronic structure of a Cr-bound form of YJ1. Details on
calculation results are listed in SI, Figure S8 and Table S1. The
TD-DFT results obtained under the conductor-like polarizable
continuum model (C-PCM, acetonitrile) indicate that the intraligand charge-transfer transition from BTTA to phen (ILphenCT)
mainly constructs the lowest triplet state (T1 = 2.30 eV)
in the metal-free YJ1, whereas the Ir(III)-to-phen ligand chargetransfer (MLphenCT) and the dfppy ligand-to-phen ligand
charge-transfer (LdfppyLphenCT) transitions become dominant in
the lowest triplet state (T1 = 1.60 eV) in the protonated form.
Actually, similar phosphorescence ratiometric change of a
5-fold turn-on and a 42 nm red-shift is experimentally observed
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Figure 2. Reversible phosphorescence ratiometric response to Cr(III) ions. (a) Change in phosphorescence spectrum of YJ1 (10 μM, CH3CN) with the
continuous addition of Cr(ClO4)3 (0 − 1.5 equiv). Inset graph is a titration isotherm plotting prompt increase in phosphorescence intensity as a function of total concentration of Cr(ClO4)3 (1 − 1.5 equiv). (b) Phosphorescence spectra of YJ1 (10 μM, CH3CN) in the absence and presence of Cr(III) ion
(5 equiv) and after subsequent addition of a strong metal chelator, TPEN (10 equiv). (c) Lippert-Mataga plot for the phosphorescence spectra of YJ1, the
Cr(III)-bound form of YJ1, and Ir5F in DMSO, CHCl3, EtOAc, CH2Cl2, and CH3CN. Solvent polarity parameter (f) is defined as f = (ε−1)/(2ε+1)−(n2−1)/
(2n2+1), where ε and n are dielectric constant and refractive index, respectively: DMSO (0.1352), CHCl3 (0.1481), EtOAc (0.1998), CH2Cl2 (0.2184),
and CH3CN (0.3055). Inset photo is phosphorescence emission of the Cr(III)-bound YJ1 in various solvents. (d) Photoluminescence decay profiles of
YJ1 (20 μM, CH3CN, deaerated) observed at 630 nm after 375 nm nanosecond laser excitation in the absence and presence of Cr(ClO4)3 (20 equiv)
and after subsequent addition of TPEN (50 equiv).
when HClO4 (10 equiv) is added to an YJ1 solution (10 μM,
CH3CN; SI, Figure S9). Subsequent addition of Bu4NOH (20
equiv) restores the original phosphorescence (SI, Figure S9),
further supporting that the Lewis basic tertiary amine of BTTA
plays a key role in the phosphorescence ratiometric response.
Specifically, the small red shift of 42 nm may be a consequence
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of +3 charge of the proximal [Cr(BTTA)] ionophore. In addition
to this, Cr binding to the tertiary amine destabilizes the nonphosphorescent ILphenCT transition state, yielding the MLCT
transition state being the lowest state at which an efficient
phosphorescent transition occurs. Strong solvatochromism
in the phosphorescence spectrum is therefore expected in the
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suggesting that the oxidation state of the Cr ion changed to +2
or +4 and that a Cr-mediated reaction at the [Cr(BTTA)] ionophore occurred. Actually, a positive mode ESI MS spectrum
of the 48 h-old mixture has three prominent peaks at m/z =
767.3, 781.3, and 797.3 (Figure 3d). These peaks correspond
to [Ir(dfppy)2(phen-R)]+ with the R being methyl (calcd m/z =
767.1), formyl (calcd m/z = 781.1), and carboxylic acid (calcd
m/z = 797.1) in place of BTTA (refer to inset structures in
Figure 3d). We also observe a tiny peak at m/z = 256.7 that can
be assigned to be a cleaved part of the [Cr(BTTA)] ionophore,
[K2CrII(BTTA)(ClO4)]2+ (calcd m/z = 256.4). Observation of this
Cr(II) species is in accordance with the EPR results (Figure 3b).
In contrast, such oxidative cleavage does not occur in the presence of other transition metal ions such as Cu(II) as evidenced
by the ESI MS spectra (SI, Figure S14). The oxidative cleavage
at the benzylic position is also supported by an appearance of
a formyl peak (δ = 10.63 ppm) in the 1H NMR spectrum of the
72 h-old mixture (1 mM, CD3CN; SI, Figure S15). The yield of
the oxidation reaction is quantitative as revealed by 1H NMR.
Collectively, the data strongly indicate occurrence of an oxidative
cleavage of [Cr(BTTA)]. The Cr center is critical to the oxidation
because protonated YJ1 does not undergo such an oxidation
reaction (SI, Figure S16). In addition, since phosphorescence
spectrum of Ir5F that lacks BTTA is almost identical to that of
the oxidized products, the second stage phosphorescence ratiometric response can be reasonably ascribed to the Cr-promoted
oxidative cleavage. It should be underscored that any additional
oxidants and base are not required for the cleavage. Therefore, the reaction mechanism for the oxidative cleavage may
involve activation of molecular oxygen by the Cr center. Actually, the oxidative cleavage did not occur when a CH3CN solution of Cr-bound YJ1 (10 mM) was left for 24 h under anaerobic
conditions (SI, Figure S17). Such molecular oxygen activation
and subsequent oxidation are frequently found in biological
systems, representative examples of which are methanol production by methane monooxygenase[38] and hydroxylation of
taurine by taurine α-glutarate dioxygenase (TauD).[39] Similar
reactions are also found in biomimetic systems,[40] such as
generation of high-valent Fe(IV)(O) species and their catalytic
oxidation of substrates.[41] Based on the spectroscopic evidences
described above, we propose a mechanism for our Cr-mediated
oxidative cleavage, which is depicted in Figure 3c.[42]
The double-stage phosphorescence ratiometric response
of YJ1 allows for detection of Cr(III) ion with excellent accuracy.[43] We have acquired phosphorescence spectra of YJ1
(10 μM, CH3CN) in the presence of various metal ions (1 equiv),
after subsequent addition of Cr(ClO4)3 (1 equiv) into the mixture,
and after additional 12 h at room temperature (Figures 4a−c).
Each of the phosphorescence spectra can be characterized
by a phosphorescence intensity ratio of 557 nm vs. 520 nm
(I557/I520) and integrated phosphorescence intensity. As shown
in Figure 4a, Na(I), Ca(II), Mn(II), Co(II), Ni(II), Zn(II), Ag(I),
Cd(II), Hg(II) ions exert almost no effect on the I557/I520 and
integrated phosphorescence intensity values, whereas Cu(II),
Pd(II), and Pb(II) ions influence the values. Subsequent addition of Cr(ClO4)3 shifts the points to the top right corner of the
map plotting I557/I520 and integrated phosphorescence intensity,
indicating that metal ions bound to the BTTA ligand were displaced by Cr(III) ion except Hg(II). The points that correspond
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Cr-bound form due to the enhanced MLCT character.[30–34]
As anticipated, a Lippert-Mataga plot reveals prominent positive solvatochromism (−7670 cm−1) in the phosphorescence of
the Cr-bound YJ1 (Figure 2c). In sharp contrast, the metal-free
form of YJ1 (−403 cm−1) and Ir5F (−633 cm−1) do not display
such strong solvatochromism.
The metal-free form of YJ1 (20 μM, Ar-saturated CH3CN)
exhibits a biphasic decay of the phosphorescence observed at
630 nm with time constants of 40 ns and 1.7 μs (Figure 2d).
The fast decay component (i.e., 40 ns component) has been
further identified by picosecond transient photoluminescence
measurements (SI, Figure S10). Presence of the fast component
may indicate occurrence of an emission-quenching process.
For its origin we presume photoinduced electron transfer (PeT)
from BTTA to Ir(IV) in the photoexcited state of YJ1. Actually,
a large positive driving force for the PeT of −ΔGPeT = 1.35 eV is
calculated through the Rehm-Weller equation by applying the
reduction potential of [Ir(dfppy)2phen]+ (−1.39 V vs. SCE; SI,
Figure S11), the oxidation potential of BTTA moiety (1.20 V vs.
SCE; SI, Figure S11) and photoexcitation energy (3.94 eV).[35,36]
Addition of Cr(III) ion (20 equiv) to the YJ1 solution clearly
eliminates the fast decay component, yielding an apparent
mono-exponential decay with a time constant of 1.5 μs
(Figure 2d). Subsequent addition of TPEN (50 equiv) regenerates the fast component (29 ns), the result of which provides
additional evidence for the reversibility for Cr binding. Taking
the steady-state and time-resolved photophysical results into
account, the reversible phosphorescence ratiometric response
of YJ1 to Cr(III) ion can be rationalized by a consequence of
enhanced MLCT contribution and suppression of PeT in the
stabilized triplet state of the Cr-bound form.
Air-equilibrated CH3CN solutions of the Cr-bound form of
YJ1 display slow but complete conversion of the yellow phosphorescence (λems = 557 nm) into the green phosphorescence
(λems = 515 nm; Figure 3a). Photoluminescence quantum yield
(PLQY) of an Ar-saturated CH3CN solution of the green phosphorescence is 0.018 ± 0.004,[37] which is comparable to the
PLQY (0.016) of the Cr-free YJ1. The green phosphorescence
observed at 630 nm follows a double-exponential decay with time
constants of 40 ns and 0.92 μs which are also similar to those of
the Cr-free form (SI, Figure S12). This second phosphorescence
ratiometric response is spontaneous and the rate is proportional
to temperature; increasing temperature from 0 °C to 50 °C
results in a 14-fold enhancement in the observed rate (1/tobs)
(SI, Figure S13). Applying the temperature-dependent rate constants into the Eyring-Polanyi equation yields enthalpy (ΔH )
and entropy (ΔS ) for the Cr-induced reaction to be 36.7 kJ mol−1
and −62.9 J K−1 mol−1, respectively. In order to figure out
a mechanism underlying this conversion, we measured X-band
CW EPR spectra of CH3CN solutions (1 mM) of fresh Cr(ClO4)3,
the metal-free form of YJ1, a fresh mixture of Cr(ClO4)3
(1 equiv) and YJ1, and a mixture of Cr(ClO4)3 (1 equiv) and YJ1
which was left for 48 h at room temperature for full conversion. As shown in Figure 3b, a CH3CN solution containing free
Cr(III) ion has two peaks at g values of 1.97 and 4.32, whereas
a CH3CN solution of YJ1 is EPR-silent. The fresh mixture has
EPR signals with g values of 1.98, 4.27 and 8.55, which indicates the Cr ion captured by YJ1 is high spin Cr(III). In contrast, no EPR signals are observed after the oxidative cleavage,
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Figure 3. Biomimetic phosphorescence ratiometric response to Cr(III) ions. (a) Phosphorescence spectra of YJ1 (10 μM, CH3CN) in the absence and
presence of Cr(ClO4)3 (1 equiv) and after full oxidative cleavage (24 h). (b) X-band CW EPR spectra of Cr(ClO4)3 (1 mM), YJ1 (1 mM), a freshly prepared mixture of YJ1 (1 mM) and Cr(ClO4)3 (1 mM), and a mixture of YJ1 (1 mM) and Cr(ClO4)3 (1 mM) incubated for 48 h allowing for full oxidative
cleavage. Refer to Experimental for measurement conditions. (c) Proposed mechanism of the oxidative cleavage of YJ1. Species in the square brackets
are proposed intermediates. The appearance of the methyl-terminated product is yet to be explained by this mechanism and remains elusive. There is
also a possibility for free-radical autooxidation. (d) Full ESI MS spectrum of the crude product of the Cr(III)-mediated oxidative cleavage of YJ1. Peaks
corresponding to oxidized products and the cleaved Cr(III) ionophore are magnified and compared with theoretical isotopic distributions (black bars).
Marked peaks at 573.4 (∗), 613.9 (∗∗), and 654.8 (∗∗∗) corresponds to [Ir(dfppy)2]+ (calcd m/z = 573.1), [Ir(dfppy)2·(CH3CN)]+ (calcd m/z = 614.1),
and [Ir(dfppy)2·(CH3CN)2]+ (calcd m/z = 655.1), respectively.
to Fe(III) and Hg(II) ions experience small but observable
changes.[44] The Cr(III)-induced oxidative cleavage finally generates the second phosphorescence ratiometric response except
Fe(III) ion. The double-stage phosphorescence ratiometric
response can be quantified by defining a fill factor (FF, FF = a
phosphorescence intensity ratio of I557/I520 × integrated phosphorescence intensity). As shown in Figure 4d, the fill factors
are not affected by Na(I), Ca(II), Mn(II), Co(II), Ni(II), Zn(II),
Ag(I), and Cd(II) ions. Fe(III) and Hg(II) ions seem to bind
stronger than Cr(III) ion. Noticeable results are Cu(II), Pd(II)
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and Pb(II) ions, which evoke an increase in fill factors similar
to Cr(III) ions. Subsequent oxidative cleavage specific to Cr(III)
clearly allows one can discriminate presence of Cr(III) ion
over these metal ions by observation of the FF changes being
increased and then decreased. In particular, signal specificity
toward Cr(III) ion over Cu(II) is worthwhile because Cu(II) ion
usually binds most strongly in usual cases.
To summarize, we have developed a novel strategy to detect
Cr(III) ion selectively. The new phosphorescent sensor containing the sulfur-rich BTTA receptor exhibits the double
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Figure 4. Double-stage phosphorescence ratiometric response of YJ1 for Cr(III) ions. Plot of phosphorescence intensity ratios (I557 nm/I520 nm) vs integrated phosphorescence intensities of YJ1 (10 μM, CH3CN) in the presence of various metal ions (1 equiv) (a), after subsequent addition of Cr(III) ion
(1 equiv) into the mixture (b), and after additional 12 h (c). (d) Corresponding fill factor (FF, FF = phosphorescence intensity ratio × integrated phosphorescence intensity) of YJ1. (e) Photo showing Cr(III) ion-selective double-stage phosphorescence ratiometric response of YJ1 (10 μM, CH3CN).
phosphorescence ratiometric response to Cr(III) ion. Steadystate and time-resolved photophysical experiments have established a mechanism for the first reversible phosphorescence
change which involves modulation of PeT and the ILCT transition state. In the Cr-bound state, the Cr(III) center evokes a
biomimetic oxidation reaction by activating molecular oxygen
to cleave the [Cr(BTTA)] ionophore. This biomimetic oxidative
cleavage produces the second phosphorescence ratiometric
response. Taking the double-stage phosphorescence ratiometric
response, the probe successfully serves to discriminate Cr(III)
ion among divalent transition metal ions such as Cu(II). The
novel sensing strategy developed in this work can be extended
to future photoluminescence sensors targeting oxophilic transition metal ions. Despite limited compatibility in aqueous environments, we envision that our strategy would provide the first
and valuable guidance to the development of a new sensing
methodology for transition metal ions.
Adv. Mater. 2012, 24, 2748–2754
Supporting Information
Supporting Information is available from the Wiley online library or from
the author.
Acknowledgements
This research was financially supported by National Research
Foundation (NRF) of Korea funded by the Ministry of Education,
Science and Technology (MEST) through the CRI, GRL (2010-00353),
and WCU program (R31-2008-000-10010-0) and by Ewha Womans
University (RP-Grant 2010). Y.Y. acknowledge Prof. Soo Young Park at
Seoul National University and Prof. Joan S. Valentine at Ewha Womans
University for use of a TCSPC system and helpful comments.
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: November 22, 2011
Revised: January 20, 2012
Published online: April 20, 2012
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[27] Among commercially available Cr(III) salts, only perchlorate salt is
soluble to give optically clear acetonitrile solutions.
[28] At high concentraion of 1 mM, addition of Cr(III) ion caused appearance of an absorption band at 610 nm (ε = 40 M−1 cm−1). See Fig.
S4 in the Supporting Information.
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[44] The interference exerted by Fe(III) and Hg ions can be overcome
with an excess amount of Cr(III) ion (> 20 equiv).
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2012, 24, 2748–2754