A sensitive phosphorescent thiol chemosensor based on an iridium(iii) complex with α,β-unsaturated ketone functionalized 2,2′-bipyridyl ligand

Article abstract of DOI:10.1039/c0dt00456a

An iridium(iii)-containing phosphorescent chemosensor Ir(ppy) ₂(L)(PF₆) (1, ppy = 2-phenylpyridine) containing a 2,2′-bipyridyl ligand (L) functionalized with an α,β- unsaturated ketone for selective detection of thiol was synthesize

Full text of DOI:10.1039/c0dt00456a

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PAPER
www.rsc.org/dalton | Dalton Transactions
A sensitive phosphorescent thiol chemosensor based on an iridium(III) complex
with a,b-unsaturated ketone functionalized 2,2¢-bipyridyl ligand†
Na Zhao,a Yu-Hui Wu,a Lin-Xi Shi,a Qi-Pu Lina and Zhong-Ning Chen*a,b
Received 11th May 2010, Accepted 17th June 2010
First published as an Advance Article on the web 9th August 2010
DOI: 10.1039/c0dt00456a
An iridium(III)-containing phosphorescent chemosensor Ir(ppy)2(L)(PF6) (1, ppy = 2-phenylpyridine)
containing a 2,2¢-bipyridyl ligand (L) functionalized with an a,b-unsaturated ketone for selective
detection of thiol was synthesized and characterized by spectroscopic and photophysical
measurements. The structure of complex 1 was determined by X-ray crystallography. In order to get an
insight into 1,4-addition reactions of thiol to complex 1, the adduct 2 from reaction of 1 with
benzenethiol was successfully prepared and characterized. Complex 1 shows a lowest energy absorption
at ca. 450 nm, primarily ascribable to an intraligand charge transfer (ILCT) transition from the HOMO
(p) resident on the fragment -C(O)C6H4N(C2H5)2 to the LUMO (p*) localized on the 2,2¢-bipyridyl
moiety in the functionalized 2,2¢-bipyridyl ligand as suggested from DFT computational studies.
Complex 1 is weakly emissive at ca. 587 nm at ambient temperature, arising likely from the 3ILCT
excited state. Upon addition of thiol to a semi-aqueous solution of complex 1, the lowest energy
absorption is obviously blue-shifted and the emission is remarkably enhanced due probably to a
conversion from the primary ILCT state to the predominant [p(ppy)→p*(L)] LLCT and the
[5d(Ir)→p*(L)] MLCT state caused by the formation of the 1-thiol adduct. The sensing properties of 1
to thiol were also investigated by ESI-MS spectrometry and 1H NMR spectroscopy.
they can be adapted to afford highly selective chemosensors.6
Utilizing iridium(III) complexes as selective chemosensors for
Introduction
anion,7 oxygen,8 biological labeling regents9 and heavy metal ions10
has been investigated. Phosphorescent chemosensors based on
iridium(III) complexes for recognition of thiols, however, are still
rarely explored.11
The development of chemosensors with favorable selectivity for
detection of thiol-containing amino acids has received consid-
erable attention because the biological thiols including cysteine
(Cys), homocysteine (Hcy), and glutathione (GSH) play very
crucial roles in physiological processes.1 It has been revealed
that many diseases are relevant to the unusual contents of Cys
or Hcy in the human body. For instance, an abnormal level of
cysteine may cause skin lesions and liver damage.2 Furthermore,
homocysteine is a risk factor for Alzheimer’s and cardiovascular
diseases.3 Accordingly, the exploitation of chemosensors with high
selectivity for thiol-containing amino acids is very important. In
fact, abundant organic fluorescent chemosensors for the detection
of thiols have been recently developed.4
Chemosensors based on phosphorescent transition-metal com-
plexes as signal groups afford advantages such as significant
Stokes shifts, long lifetimes, luminescence in the visible region,
and large emission shifts from changes in the local environments
compared with those of pure organic emitters.5 It is well known
that cyclometallated iridium(III) complexes are one of the best
families of phosphorescence dyes. The chelating ligands play a key
role on the luminescence of the iridium(III) complexes so that
We describe herein a selective phosphorescent thiol probe based
on a cyclometallated iridium(III) complex Ir(ppy)2(L)(PF6) (1)
(Scheme 1, ppy = 2-phenylpyridine) containing a functionalized
2,2¢-bipyridyl ligand using an a,b-unsaturated ketone moiety as a
conjugated spacer. Our synthetic strategy is inspired by the 1,4-
addition of thiols to a,b-unsaturated ketones in water without the
use of any catalyst.12 By proper design of cyclometallated irid-
ium(III) complex containing a 2,2¢-bipyridyl ligand functionalized
with an a,b-unsaturated ketone, it is possible to attain an “off–on”
phosphorescent chemosensor with high selectivity for detection
of thiol-containing amino acids owing to a conversion from the
3ILCT to the 3LLCT/3MLCT excited state.13 As anticipated,
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian,
350002, China; Fax: 86-591-8379-2346
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai, 200032,
China. E-mail: czn@fjirsm.ac.cn
† Electronic supplementary information (ESI) available: Figures giving
additional UV–vis absorption and emission spectral titration curves, tables
and figures regarding the TD-DFT calculations. CCDC reference number
776707. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt00456a
Scheme 1 Synthetic routes to 1 and 2.
8288 | Dalton Trans., 2010, 39, 8288–8295
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the weak emission in complex 1 is assignable primarily to an
3ILCT transition from the HOMO (p) resident on the electron-
rich fragment -C(O)C6H4N(C2H5)2 to the LUMO (p*) localized
on the 2,2¢-bipyridyl moiety in the functionalized 2,2¢-bipyridyl
ligand L.10e,13 Upon addition of thiol to a semi-aqueous solution
of complex 1, a thioether is created because of the 1,4-addition
6.77. Found: C, 53.04; H, 4.38; N, 6.55. ESI-MS (m/z): 872.2 ([M –
PF6]+). 1H NMR (400 MHz, d6-DMSO, 298 K): d 9.20 (s, 1H), 8.87
(s, 1H), 8.33 (d, J = 15.8 Hz, 1H), 8.26 (t, J = 3.6 Hz, 2H), 8.04–8.01
(m, 3H), 7.96-7.91 (m, 4H), 7.80 (d, J = 6.0 Hz, 1H), 7.73–7.64 (m,
4H), 7.56 (d, J = 5.6 Hz, 1H), 7.19–7.13 (m, 2H), 7.05–7.00 (m,
2H), 6.93–6.88 (m, 2H), 6.77 (d, J = 9.2 Hz, 2H), 6.20 (t, J = 5.4 Hz,
3
reaction of thiol with a,b-unsaturated ketone so that the ILCT
2H), 3.49–3.44 (q, 4H), 2.57 (s, 3H), 1.14 (t, J = 7.0 Hz, 6H). 13
C
character is significantly suppressed whereas [p(ppy)→p*(L)]
NMR (400 MHz, d6-DMSO, 298 K): d 184.90, 166.81, 166.77,
156.09, 154.90, 151.52, 151.49, 150.67, 150.58, 149.80, 149.11,
148.88, 145.56, 143.87, 143.79, 138.74, 136.43, 131.51, 131.12,
131.07, 130.23, 129.93, 129.42, 127.03, 125.81, 125.06, 123.95,
123.84, 123.01, 122.26, 120.07, 110.49, 43.97, 21.06, 12.38.
3
3LLCT and [5d(Ir)→p*(L)] MLCT transitions are most likely
converted to the lowest energy excited states, thus giving highly
enhanced luminescence.14
Experimental
Synthesis of the adduct of 1 with benzenethiol (2). A mixture of
benzenethiol (8 mg, 0.07 mmol) and CH3ONa (4 mg, 0.07 mmol)
in methanol (20 mL) was stirred in room temperature for 10 min.
Afterwards a dichloromethane solution (1 mL) of complex 1
(50 mg, 0.05 mmol) was added to the solution with stirring
for 30 min. The solution was then evaporated in vacuo and
the residue was dissolved in 40 mL of dichloromethane. To the
dichloromethane solution was added 20 mL of water, and the
organic layer was collected and dried overnight with MgSO4. The
product was purified by chromatography on a silica gel column
using dichloromethane–acetone (v/v = 50 : 1) as eluent. Yield:
90%. Anal. Calcd for C52H47F6IrN5OPS: C, 55.41; H, 4.21; N,
6.21. Found: C, 54.60; H, 4.72; N, 5.89. ESI-MS (m/z): 982.2 ([M
– PF6]+). 1H NMR (400 MHz, d6-DMSO, 298 K): d 8.71 (d, J =
9.6 Hz, 1H), 8.68 (s, 1H), 8.28–8.23 (m, 2H), 7.97–7.87 (m, 4H),
7.80 (d, J = 8.8 Hz, 2H), 7.66 (m, 1H), 7.62–7.55 (m, 2H), 7.52 (d,
J = 5.6 Hz, 2H), 7.32 (t, J = 7.2 Hz, 1H), 7.27–7.15 (m, 7H), 7.02–
6.98 (m, 2H), 6.91–6.86 (m, 2H), 6.70 (d, J = 4.8 Hz, 1H), 6.68 (d,
J = 4.8 Hz, 1H), 6.17 (d, J = 7.1 Hz, 1H), 6.14 (d, J = 6.7 Hz, 1H),
5.03–4.96 (m, 1H), 3.80 (d, J = 6.4 Hz, 2H), 3.46–3.38 (m, 4H),
2.55 (s, 3H), 1.13-1.08 (m, 6H).13C NMR (400 MHz, d6-DMSO,
298 K): d 193.00, 192.97, 167.06, 167.00, 166.87, 166.82, 155.27,
155.13, 154.87, 154.76, 151.62, 151.21, 150.70, 150.52, 149.07,
148.78, 148.70, 148.07, 147.97, 143.82, 143.74, 138.79, 133.48,
132.09, 131.94, 131.08, 130.67, 130.25, 129.31, 129.04, 129.01,
128.34, 128.26, 127.53, 127.25, 125.62,125.10, 124.04, 123.92,
123.79, 122.96, 122.26, 120.18, 120.03, 110.08, 47.29, 47.01, 43.91,
41.04, 40.96, 20.95, 12.35, 12.33.
General procedures and materials
The manipulations were carried out under an argon atmosphere
using Schlenk techniques and vacuum-line systems. The solvents
were dried, distilled and degassed prior to use except those
for spectroscopic measurements were of spectroscopic grade.
2-Phenylpyridine (ppy) and 4¢-diethylaminoacetophenone were
purchased from Alfa Aesar, Cysteine (Cys) and Homocysteine
(Hcy), and benzenethiol (PhSH) from TCI, and other amino
acids from Sinopharm Chemical Reagent Co., Ltd. (Shanghai).
4,4¢-Dimethyl-2,2¢-bipyridine and hydrate iridium(III) chloride
(IrCl3·3H2O) were commercially available and used as received.
4-Aldehyde-4¢-methyl-2,2¢-bipyridine and Ir2(m-Cl)2(ppy)4 were
synthesized according to literature procedures.15
Synthesis
Synthesis of ligand L. To an ethanol solution (5 mL) of 4¢-
diethylaminoacetophenone (191 mg, 1 mmol) and 4-aldehyde-4¢-
methyl-2,2¢-bipyridine (198 mg, 1 mmol) was added a 10% aqueous
solution of sodium hydroxide (2 mL) with stirring for 12 h at room
temperature. The solvents were then evaporated in vacuo and the
residue was dissolved in dichloromethane. 30 mL of water was then
added to the dichloromethane solution, and the organic layer was
collected and dried with MgSO4 for 1 d. The product was purified
by chromatography on a silica gel column using dichloromethane–
acetone (v/v = 10 : 1) as eluent. Yield: 80%. Anal. Calcd for
C24H25N3O·H2O: C, 74.01; H, 6.99; N, 10.79. Found: C, 74.52;
H, 7.02; N, 10.11. ESI-MS (m/z): 372.5 ([M + H]+). H NMR
1
(400 MHz, CDCl3, 298 K): d 8.71 (d, J = 4.8 Hz, 1H), 8.63 (s, 1H),
8.58 (d, J = 5.2 Hz, 1H), 8.27 (s, 1H), 8.02 (d, J = 8.8 Hz, 2H), 7.85
(d, J = 16 Hz, 1H), 7.74 (d, J = 16 Hz, 1H), 7.44 (d, J = 3.6 Hz,
1H), 7.17 (d, J = 5.2 Hz, 1H), 6.69 (d, J = 8 Hz, 2H), 3.49–3.43 (q,
4H), 2.46 (s, 3H), 1.23 (t, J = 7.0 Hz, 6H).
Physical measurements
1H NMR spectra were measured on a Bruker Avance III
(400 MHz) spectrometer with SiMe4 as the internal reference.
Electrospray ion mass spectra (ESI-MS) were performed on a
Finnigan LCQ mass spectrometer. Elemental analyses (C, H,
N) were recorded on a Perkin-Elmer model 240C automatic
instrument. UV–vis absorption spectra were measured on a
Perkin-Elmer Lambda 25 UV–vis spectrometer. Emission and
excitation spectra were recorded on a Perkin-Elmer LS55 lumi-
nescence spectrometer with a R928 red-sensitive photomultiplier.
Luminescence quantum yields in air-equilibrated solution were
measured with reference to rhodamine B (UF = 0.49 in ethanol).16
DMF–HEPES buffer solution (50 mM, pH 7.2, v/v = 4 : 1) was
selected for sensing studies due to the poor solubility of complex
1 in aqueous phase.
Synthesis of [Ir(ppy)2(L)](PF6) (1). A mixture of Ir2(m-
Cl)2(ppy)4 (267.7 mg, 0.25 mmol) and L (186.1 mg, 0.5 mmol)
in methanol–dichloromethane (30 mL, v/v = 1 : 2) was refluxed
with exclusion of light for 4 h. After cooling to the ambient
temperature, to the solution was added a methanol solution
(5 mL) of potassium hexafluorophosphate (93 mg) with stirring
for 30 min. The solution was evaporated to dryness and the residue
was dissolved in dichloromethane (5 mL). The solution was then
chromatographed on a silica gel column using dichloromethane–
acetone (v/v = 10 : 2) as eluent to give an orange product. Yield:
82%. Anal. Calcd for C46H41F6IrN5OP·H2O: C, 53.38; H, 4.19; N,
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Table 1 Crystallographic data for 1
the 6-31G(p,d) basis set was used for the remaining atoms. To
precisely describe the molecular properties, one additional f-type
polarization function was implemented for the iridium(III) atom
(a = 0.938).25
Empirical formula
C46H41IrN5OF6P
T/K
293(2)
P212121
13.256(3)
13.706(3)
31.379(7)
5701(2)
4
1.185
2.421
0.71073
0.0632
0.1831
1.103
Space group
˚
a/A
Results and discussion
˚
b/A
˚
c/A
3
Synthesis and characterization
˚
V/A
Z
rc/g cm-3
The ligand L was prepared by the reaction of 4-aldehyde-
4¢-methyl-2,2¢-bipyridine with 4¢-diethylaminoacetophenone
through aldol condensation. Cyclometallated iridium(III)
complex (1) was synthesized by the reaction of functionalized
2,2¢-bipyridyl ligand L with Ir2(ppy)4(m-Cl)2 in CH2Cl2–CH3OH
(v/v = 2 : 1) under reflux (Scheme 1), followed by metathesis with
KPF6 and chromatographic purification using silica gel columns.13
Attempts to isolate the cysteine adduct with 1 were unsuccessful
because of the difficulty in purification of the additive product
1-Cys by column chromatography. In order to get an insight into
the reaction of thiol with complex 1, the adduct 2 was prepared
and isolated successfully from the 1,4-additive reaction of 1 with
benzenethiol. Complexes 1 and 2 were characterized by positive
ion ESI-MS spectrometry, 1H NMR and 13C NMR spectroscopy,
elemental analyses, and X-ray crystallography for 1. Positive
ion ESI-MS showed that the molecular ion fragment [M-PF6]+
was detected as the base peak for 1 (m/z = 872.2) and 2 (m/z =
m/mm-1
˚
Radiation (l/A)
R1(Fo)a
wR2(Fo)b
GOF
a R1 = R|Fo - Fc|/RFo. b wR2 = R[w(Fo - Fc2)2]/R[w(Fo2)]1/2
.
2
Crystal structural determination
Crystals of 1 were obtained by diffusion of n-hexane into a
dichloromethane solution of 1. Data collection was performed
on a SATURN70 CCD diffractometer using the w scan technique
at room temperature using graphite-monochromated Mo-Ka (k =
˚
0.71073 A) radiation. Absorption corrections by SADABS were
applied to the intensity data. The structure was solved by direct
methods. The heavy atoms were located from E-maps, and the rest
of the non-hydrogen atoms were found in the subsequent Fourier
maps. All non-hydrogen atoms were refined anisotropically, and
the hydrogen atoms were generated geometrically and refined
with isotropic thermal parameters. The structures were refined
on F2 by full-matrix least-squares methods using the SHELXTL-
97 program package.17 The crystallographic data are summarized
in Table 1.
1
982.2). The H NMR spectrum of 1 indicated that the ethylenic
protons were observed at 8.32 (doublet, J = 15.8 Hz) and 7.66
ppm (multiplet), which were supported by the 1H–1H COSY
1
spectrum (Fig. S9, ESI†). In the H NMR spectrum of complex
2, however, two resonance signals at 3.80 (doublet, J = 6.4 Hz)
and 4.99 (multiplet) ppm can be ascribed to the methylene and
the thioether methine protons, respectively.
As shown in Fig. 1, the iridium(III) center adopts a distorted
octahedral geometry composed of C2N4 donors from two cy-
clometallated ppy and one 2,2¢-bpy ligand. The N donors from
two cyclometallated ppy are trans-arranged, and the C donors
are cis-arranged, as revealed by structural studies on iridium(III)
complexes containing two ppy and one 2,2¢-bipyridyl ligands.14,26
Theoretical calculations
All the calculations were carried out by using the Gaussian 03
program package.18 The computational method used was the
density functional theory DFT,19 where the gradient-corrected
correlation functional PBE1PBE20 was employed for the cal-
culations since the PBE1PBE gave the best results in terms
of structural parameters and simulation of UV–vis properties
by comparing different functions. The geometry structure of
[Ir(ppy)2(L)]+ (1) was optimized as an isolated molecule from
the solvent phase at the DFT level of theory. We performed
geometry optimization from the X-ray structure in the ground
state without symmetry constraints. The geometry structure of
1-Cys was selected from several possible geometry optimizations
with the minimum energy. Moreover, the vibration frequencies
were calculated for the optimized geometry of 1-Cys to verify the
minimum (NIMAG = 0) on the potential energy surface. Table S5
(ESI†) lists the DFT optimized coordinates for 1-Cys. Based on
these structures, sixty singlet states were obtained to determine
the vertical excitation energies for 1 and the adduct 1-Cys in DMF
solution by the time-dependent DFT (TD-DFT)21,22 calculations.
Solvent effects were taken into account by the conductor-like
polarizable continuum model (CPCM)23 with DMF as solvent.
In these calculations, the Hay–Wadt double-x with a Los Alamos
relativistic effect basis set (LANL2DZ)24 consisting of the effective
core potentials (ECP) was employed for the iridium(III) atom and
˚
The Ir–N distances [2.140(3) and 2.147(4) A] for 2,2¢-bipyridyl
˚
are obviously longer than those [2.037(4) and 2.027(5) A] for ppy
due most likely to remarkable trans influence of the C donors for
the former. The C34–C35 and C36–O1 distances are 1.256(13) and
˚
1.219(13) A, respectively, typical of C C and C O double bonds.
The ethylenic bond (C34 and C35) in the L ligand is trans-oriented
as depicted in Fig. 1.
Fig. 1 ORTEP drawing of the cation of 1 with atomic labeling scheme.
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at
30% probability level.
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Photophysical properties
Optical sensing properties
The UV–vis absorption spectrum of complex 1 in DMF–HEPES
buffer solution (50 mM, pH 7.2, v/v = 4 : 1) is shown in Fig. 2.
Complex 1 exhibits high-energy bands at 280–320 nm with
extinction coefficients (e) of ~105 mol-1 L-1 cm-1, arising most
probably from spin-allowed intraligand transition from ppy and
functionalized bpy ligand with reference to previous studies on
iridium(III) diimine complexes.9a,26c,27,28 A broad lowest energy
absorption band centered at 450 nm with extinction coefficients
(e) of 1.7 ¥ 104 mol-1 L-1 cm-1 could be tentatively assigned to
an intraligand charge transfer (ILCT) transition from the HOMO
(p) residing on the fragment -C(O)C6H4N(C2H5)2 to the LUMO
(p*) localized on the 2,2¢-bipyridyl moiety in the functionalized
2,2¢-bipyridyl ligand L,14 mixed probably with some character
from [5d(Ir)→p*(L)] MLCT and [p(ppy)→p*(L)] LLCT states.
Such an assignment is supported by DFT calculations (vide infra).
Compared with the corresponding ILCT absorption at 420 nm in
the free ligand L (Fig. S1, ESI†), a 20 nm red-shift in the iridium(III)
complex 1 could be ascribed to an increased electron-accepting
ability upon the 2,2¢-bipyridyl moiety (p*) in the L ligand bound
to iridium(III) ion.13a,13c It has been shown that the ILCT state
always causes lowest energy absorptions in iridium(III) complexes
when the chelating ligand contains an electron-donating amino-
substituent.10a,10e,13a,13c
The sensing selectivity of complex 1 in response to thiols was
investigated by UV-vis spectral titration. Fig. 2 shows the changes
of UV-vis absorption spectra of 1 in response to Cys. Upon
addition of Cys (0–80 equiv) to a DMF–HEPES buffer solution
(50 mM, pH 7.2, v/v = 4 : 1) of 1, the lowest energy absorption
at 450 nm decreases gradually with a concomitant growth in the
higher energy absorption band at 350 nm, in which the yellow
solution changes distinctly to colorless. The rapid colorimetric
response indicates that 1 can serve as a “naked-eye” indicator
for detection of Cys (Fig. 2, inset). Well-defined isosbestic points
occur at 325 nm and 372 nm, indicating that a new species is only
produced during the titration. A similar trend is also observed
upon addition of Hcy to a DMF–HEPES buffer solution (50 mM,
pH 7.2, v/v = 4 : 1) of 1 (Fig. S2, ESI†), but the band at 450
nm does not completely disappear when 80 equiv of Hcy is
added. A remarkable blue shift of the lowest energy absorption
for 1 results most likely from a 1,4-addition of Cys/Hcy to a,b-
unsaturated ketone in the L ligand. Upon addition of Cys/Hcy
to a DMF–HEPES buffer solution (50 mM, pH 7.2, v/v = 4 : 1)
of 1, a thioether is readily formed. The ILCT transition is thus
significantly suppressed whereas the [p(ppy)→p*(L)] LLCT and
[5d(Ir)→p*(L)] MLCT transitions are likely switched to the lowest
energy transitions. A weak band still remains at 450 nm when
titration of 1 with Hcy reaches an equilibrium due probably to the
lower reactivity of Hcy than that of Cys to the a,b-unsaturated
ketone. This suggests that complex 1 displays a better selectivity
to Cys. For the purpose of comparison, an absorption spectral
titration was performed by addition of Cys to the free L in DMF–
HEPES buffer solution (50 mM, pH 7.2, v/v = 4 : 1). As shown
in Fig. S3 (ESI†), the lowest energy absorption of the L ligand
is blue-shifted from 420 nm to 350 nm upon addition of Cys (0–
60 equiv) to the solution of the free ligand L, due probably to the
formation of the 1,4-addition product, thus inducing a significant
suppression of the ILCT state.
The sensing selectivity of complex 1 to thiol was also investigated
by emission spectral titration. Complex 1 is weakly emissive at ca.
587 nm in DMF–HEPES buffer solution (50 mM, pH 7.2, v/v =
4 : 1). Nevertheless, upon addition of thiol to a solution of complex
1, the emission intensity is gradually increased with increasing
concentration of thiol. Fig. 3 depicts the emission spectral changes
of complex 1 as well as a naked-eye detectable luminescence in
response to Cys. The luminescence intensity of complex 1 (20 mM)
in DMF–HEPES buffer solution (50 mM, pH 7.2, v/v = 4 : 1)
shows a 20-fold enhancement when 80 equiv of Cys is added. The
luminescence quantum yield in air-equilibrated solution becomes
0.055 and the emission lifetime monitored at 587 nm is 358 ns,
suggesting that complex 1 can serve as an “off–on” luminescence
dosimeter for Cys. The luminescence intensity at 587 nm displays
a good linear relationship with the Cys concentration in the range
of 0 to 10-4 M (Fig. 3 inset), implying that complex 1 is potentially
useful for quantitative measure of Cys concentrations. For Hcy,
the maximum enhancement of luminescence intensity is 14-fold.
The luminescence quantum yield in air-equilibrated solution is
0.044 and the emission lifetime increases to 127 ns upon addition
of 80 equiv of Hcy (Fig. S4, ESI†). The luminescence intensity
at 587 nm also shows a good linear relationship with the Hcy
concentration in the range of 0 to 8 ¥ 10-4 M (Fig. S5, ESI†). It is
Fig. 2 Changes in UV–vis absorption spectra of complex 1 (20 mM) in
DMF-HEPES buffer solution (50 mM, pH 7.2, 4 : 1, v/v) upon titration
with Cys (0–80 equiv). The equilibration time is ca. 40 min. Inset: Color
image of complex 1 and the adduct 1-Cys.
Complex 1 shows a weak broad emission at ca. 587 nm in
DMF–HEPES buffer solution (50 mM, pH 7.2, v/v = 4 : 1) at
ambient temperature. The luminescence quantum yield of complex
1 in air-equilibrated solution is ~ 0.013. The emission lifetime
monitored at 587 nm is 109 ns in air-equilibrated solution at
ambient temperature, suggesting a phosphorescent character of
complex 1. With reference to previous spectroscopic studies on
a series of cyclometallated iridium(III) diimine complexes con-
taining an amino-substituent ligand,13a,13c,14,29 the emission origin
3
is tentatively ascribed as a predominately ILCT triplet excited
state, mixed probably with some character from 3[p(ppy)→p*(L)]
3LLCT and 3[5d(Ir)→p*(L)] 3MLCT triplet excited state. The
weak emission of complex 1 suggests the effective non-radiative
deactivation in 3ILCT excited state process.
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3
a significant suppression of the ILCT state, thus activating the
highly luminescent 3MLCT/3LLCT triplet states.
The luminescence kinetics of complex 1 to thiol were carried
out. The time course of emission intensity for complex 1 at 587
nm in the absence or presence of Cys, Hcy, or GSH (80 equiv) in
DMF–HEPES buffer (50 mM, pH 7.2, 4 : 1, v/v) was determined
at an interval of 2 min. As shown in Fig. S6 (ESI†), the emission
intensity of complex 1 at 587 nm remains unchanged in the absence
of thiol whereas exhibits a little increase in the presence of 80 equiv
of GSH. Upon addition of 80 equiv Cys to a solution of complex
1, however, the emission intensity at 587 nm increases significantly
and reaches a maximum after 40 min. When 80 equiv of Hcy
is added to a solution of complex 1, it takes about 300 min to
reach the emission maximum at 587 nm, in which the emission
enhancement is less remarkable than that in the presence of Cys.
As a result, the reactivity of the thiols to complex 1 follows Cys
> Hcy > GSH, which is consistent to the order in the steric-
hindrance effect.12a, 30
Fig. 3 Changes in emission spectra of complex 1 (20 mM) in DM-
F-HEPES buffer solution (50 mM, pH 7.2, 4 : 1, v/v) upon titration with
Cys (0–80 equiv). The equilibration time is ca. 40 min. Top inset: Plot of
the emission intensity at 587 nm as a function of Cys concentration (0–10-4
M). Bottom inset: Luminescence images before (a) and after (b) addition
of 80 equiv Cys.
The sensing properties of 1 to Cys were also investigated by
1
ESI-MS spectrometry and H NMR spectroscopy. As shown in
Fig. S7 (ESI†), the molecular ion peak of complex 1 occurs at
m/z = 872.2 ([M-PF6]+). Upon addition of Cys to the solution of
complex 1, a new fragment peak at 993.3 is observed as the base
peak, corresponding to the molecular ion fragment of the adduct
1-Cys with a 1 : 1 binding ratio between Cys and 1.
noteworthy that the luminescence enhancement is well consistent
with the corresponding UV-vis spectral changes during titration of
complex 1 with Cys/Hcy. It is likely that the 3ILCT excited state
in 1 is significantly suppressed upon formation of the thioether
in the adduct 1-Cys or 1-Hcy. Thus, the lowest energy exited
state is probably converted from the 3ILCT state in 1 to the
In the 1H NMR spectrum of free ligand L (Fig. S8, ESI†), the
resonance signals at d = 8.18 and 7.68 ppm are due to the Ha
and Ha¢ of the ethylenic bond with a big coupling constant (J =
16 Hz). Upon addition of 5 equiv Cys to a d6-DMSO solution
of the ligand L, the two original signals of the ethylenic bond
disappear, whereas a new peak occurs at d = 4.60 ppm (Fig. S8,
ESI†), which is most likely attributable to the thioether methine
3[p(ppy)→p*(L)] LLCT and [5d(Ir)→p*(L)] MLCT states in
the adduct 1-Cys or 1-Hcy, which is highly emissive.13 As both the
free ligand L and the adduct of L with Cys are non-emissive beyond
400 nm, complexation of the phosphorescent Ir(ppy)2 component
to the L ligand is of significance in improving the emissive sensing
properties and detecting sensitivity in response to thiols.
3
3
3
1
proton Hd in the L-Cys adduct. In the H NMR spectrum of
Since the additive product 1-Cys was unattainable due to
the purifying difficulty, the analog adduct 2 was prepared from
the 1,4-addition reaction of 1 with benzenethiol. As shown in
Fig. 4, complex 2 shows an intense emission in contrast with the
weakly emissive complex 1. The luminescence quantum yield of
complex 2 in air-equilibrated solution is 0.046 and the emission
lifetime monitored at 587 nm is 131 ns. This suggests further
that the 1,4-addition of thiol to the ethylenic bond in 1 induces
complex 1 (Fig. 5a), the resonance signals at around d = 8.32
and 7.66 ppm correspond to the ethylenic proton Ha and Ha¢ with
Fig. 4 The UV-vis absorption (solid lines) and emission (dash lines)
spectra of 1 (red), 2 (blue), and 1-Cys adduct (green) from the reaction
of 1 with 80 equiv Cys in DMF–HEPES buffer solution (50 mM, pH 7.2,
v/v, 4 : 1) at ambient temperature.
Fig. 5 1H NMR spectra of 1 (a) and 2 (b) in d6-DMSO solution, and
1-Cys adduct (c) formed by addition of 5 equiv Cys to 1 in d6-DMSO–H2O
solutions (9 : 1), and 13C NMR spectra of 1 (d) and 2 (e) in d6-DMSO
solution.
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a big coupling constant (J > 9.0 Hz), which are supported by
the cross-peak of 8.32 and 7.66 ppm in 1H–1H COSY spectrum of
complex 1 (Fig. S9, ESI†). In contrast, the ethylenic proton signals
are unobserved in the 1H NMR spectrum of complex 2, but two
new resonance signals at d = 4.99 and 3.80 ppm distinctly occur
as indicated in Fig. 5b. Combining the cross-peak of 4.99 and
1
3.80 ppm in the H–1H COSY spectrum of complex 2 (Fig. S10,
ESI†), the two new observed resonance signals are ascribable
to the thioether methine proton Hb and the methylene proton
Hc, respectively. Upon addition of 5 equiv Cys to a d6-DMSO–
H2O solution (9 : 1) of complex 1 (Fig. 5c), the peaks at d = 8.32
and 7.66 ppm disappear, whereas two new peaks are observed at
1
d = 4.62 and 3.75 ppm. Furthermore, the corresponding H–1H
COSY spectrum shows the new cross-peaks of 4.62 and 3.75 ppm
(Fig. S11, ESI†), indicating that the new peaks at d = 4.62 and
3.75 ppm are due to thioether methine proton Hd and methylene
proton He, respectively, consistent with the formation of the 1-Cys
adduct.
The 13C NMR spectrum of complexes 1 and 2 were recorded
in d6-DMSO, but the 13C NMR spectrum of the mixture of 1
and Cys was unobtainable due to the poor solubility. In the 13C
NMR spectrum of complex 1 (Fig. 5d), the signals at d = 145.6
and 136.4 ppm are due to the C atoms of the ethylenic bond. In
contrast, the two C peaks of the ethylenic bond disappear entirely
in the 13C NMR spectrum of complex 2 (Fig. 5e), whereas two new
signals are distinctly observed at 47.6 and 41.4 ppm, attributable to
the methylene and thioether methine, respectively. Thus, both 1H
and 13C NMR spectrum studies demonstrate unambiguously that
1,4-addition of thiol to the a,b-unsaturated ketone of complex 1
indeed occurs during the titration.
Fig. 7 Emission response of complex1 (20 mM) in the presence of different
amino acids (80 equiv) in DMF–HEPES buffer solution (50 mM, pH 7.2,
4 : 1, v/v). The excitation wavelength was 358 nm.
1 exhibits insignificant UV–vis and emission responses to the
potential competitive amino acids. This indicates that complex 1
has a high selectivity to thiol-containing amino acids, particularly
to Cys. Moreover, the equilibration time of Cys response is ca.
40 min. The interference experiments were performed by emission
spectral titration, in which complex 1 was titrated with Cys
(80 equiv) in the presence of potentially competitive amino acids
(80 equiv) in DMF–HEPES buffer solution (50 mM, pH 7.2, 4 : 1,
v/v). As shown in Fig. S12 (ESI†), the emission intensity of 1-Cys
adduct was not disturbed obviously by other competitive amino
acids, indicating that complex 1 displays an exclusive response to
Cys.
DFT computational studies
Selectivity studies
To understand the nature of absorption transition characters,
time-dependent DFT (TD-DFT) calculation was performed to
estimate the corresponding transition energy of complex 1. The
calculated frontier orbitals for the low-lying transitions revealed
that both the Highest Occupied Molecular Oribital (HOMO) and
the Lowest Unoccupied Molecular Orbital (LUMO) of 1 reside
on the L ligand as depicted in Fig. 8. The HOMO (Table S1,
ESI†) is mainly resident on the fragment -C(O)C6H4N(C2H5)2
(97.3% compositions) of the L ligand. The HOMO-1, HOMO-
6 are mostly localized on the Ir(5d) and the ppy (p). Both LUMO
The selectivity of complex 1 toward thiol over other amino
acids was investigated by the UV–vis and emission spectra in
DMF–HEPES buffer solution (50 mM, pH 7.2, 4 : 1, v/v). These
elementary amino acids include L-glycine, L-alanine, L-serine,
L-threonine, L-valine, L-leucine, L-isoleucine, L-methionine, L-
phenylananine, L-tryptophane, L-tyrosine, L-aspartiac acid, L-
aspartate, L-glutamic acid, L-glutamine, L-lysine, L-arginine, L-
histidine, and L-proline. As depicted in Fig. 6 and Fig. 7, complex
Fig. 6 Effects on the UV–vis absorption spectra of complex 1 (20 mM)
upon addition of different amino acids (80 equiv) in DMF–HEPES buffer
solution (50 mM, pH 7.2, 4 : 1, v/v).
Fig. 8 Electron-density diagrams of the HOMO (left) and LUMO (right)
of 1.
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and LUMO+1 are predominately contributed by the 2,2¢-bpy (p*)
moiety in the L ligand. The LUMO+5 and LUMO+6 are resident
on both the L and ppy ligands.
3ILCT character (weakly emissive) in complex 1 to predominant
3LLCT/3MLCT (highly emissive) states in the 1-thiol adduct.
As shown in Table S2 (ESI†), although a calculated low-energy
absorption at 513 nm with a low oscillator strength of 0.0066
exhibits 1MLCT/1LLCT character, the much stronger low-energy
absorption at 474 nm with the highest oscillator strength of 0.4502
due to HOMO→LUMO is primarily featured with intraligand
charge transfer (ILCT) transition from the HOMO (p) resident
on the fragment -C(O)C6H4N(C2H5)2 to the LUMO (p*) localized
on the 2,2¢-bipyridyl moiety in the functionalized 2,2¢-bipyridyl
ligand L. The absorption at 379 nm from HOMO→LUMO+1
also displays 1ILCT character. The high-energy absorption at
287 nm from HOMO→LUMO+5, HOMO-6→LUMO+1, and
HOMO→LUMO+6 can be assigned mainly to a 1ILCT transition
together with some contribution from 1MLCT/1LLCT states. As
indicated in Fig. S16 (ESI†), the calculated absorption character
(bars) accords well with the measured UV–vis absorption spec-
trum of 1 in DMF solution.
In order to explore the influence on the UV–vis absorption
character by the 1,4-addition of Cys to a,b-unsaturated ketone
of the L ligand in complex 1, time-dependent DFT (TD-DFT)
calculation was also performed to estimate the corresponding
transition energy of the adduct 1-Cys. As listed in Table S4
(ESI†), the calculated lowest energy absorption at 462 nm with an
oscillator strength of 0.0006 due to HOMO-1→LUMO transition
is typical of 1MLCT and 1LLCT states. Although a weaker absorp-
tion at 402 nm with a lower oscillator strength of 0.0003 due to
HOMO→LUMO transition is mainly featured with 1ILCT state,
the stronger absorption at 361 nm with higher oscillator strength
of 0.1079 due to HOMO-2→LUMO and HOMO-1→LUMO+3
transitions is ascribable to substantial 1MLCT/1LLCT character.
The calculated (red bars) absorptions in DMF media by the
TD-DFT calculation coincide well with the measured (blue line)
UV–vis spectrum of the adduct 1-Cys in DMF–HEPES buffer
solution (50 mM, pH 7.2, 4 : 1, v/v) as depicted in Fig. S17
(ESI†). Consequently, the DFT computational results suggest
that a significant blue shift of the low-energy absorption and a
remarkable emission enhancement from 1 to 1-Cys is most likely
induced by a conversion of the primary ILCT transition in 1 to
the predominant MLCT/LLCT states in the adduct 1-Cys upon
1,4-addition of Cys to a,b-unsaturated ketone in complex 1.
Acknowledgements
We thank the NSFC (20625101, 20821061, 20931006, and
U0934003), the 973 project (2007CB815304) from MSTC, and
NSF of Fujian Province (2008I0027 and 2008F3117) for financial
support.
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Conclusions
A new iridium(III)-containing phosphorescent chemosensor for
thiol was prepared and characterized and the sensing properties
to thiol was investigated by spectroscopic and photophysical mea-
surements. In this iridium(III) complex with an a,b-unsaturated
ketone functionalized 2,2¢-bipyridyl ligand, the Ir(ppy)2(2,2¢-bpy)
chromophore serves as a signaling emitter and the a,b-unsaturated
ketone moiety as a receptor to thiol. This iridium(III) complex acts
as an “off–on” luminescent switch and a naked-eye perceivable
sensor for detection of thiol based on 1,4-addition of thiol to
a,b-unsaturated ketone. Upon formation of the 1-thiol adduct by
1,4-addition of thiol with 1 in a DMF–HEPES buffer solution
(50 mM, pH 7.2, 4 : 1, v/v), the emission intensity at ca. 587 nm
is significantly increased, which could be reasonably elucidated
by the lowest energy excited state being switched from primary
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