A ruthenium(ii) complex with environment-responsive dual emission and its application in the detection of cysteine/homocysteine

Article abstract of DOI:10.1039/c2dt31180a

A ruthenium polypyridine complex [Ru(phen)₂(IPBA)](PF ₆)₂ (complex 1) (IPBA = 4-(1H-imidazo[4,5-f][1,9] phenanthroline-2-yl)benzaldehyde), which displays environment-responsive dual emissive properties, was designed and synthesized. In aprotic solvent, such as DMSO, DMF or CH₃CN, the complex emits strong cyan light. When in protic solvent, it emits orange light. Similarly, the response of the complex to homocysteine and cysteine (Hcy/Cys) also shows obvious solvent dependence. TDDFT calculations reveal that the protonation of imidazole nitrogen in protic solution is responsible for the luminescent red shift. Hence, the complex could be used to detect Hcy/Cys in aprotic solvent, as well as in protic solvent.

Full text of DOI:10.1039/c2dt31180a

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PAPER
A ruthenium(II) complex with environment-responsive dual emission and its
application in the detection of cysteine/homocysteine†
Huili Chen,*a,b Xiaokai Li,a Yanbo Wu,a Wei Gaoa and Ruichun Baia
Received 1st June 2012, Accepted 10th July 2012
DOI: 10.1039/c2dt31180a
A ruthenium polypyridine complex [Ru(phen)2(IPBA)](PF6)2 (complex 1) (IPBA = 4-(1H-imidazo[4,5-f]-
[1,9]phenanthroline-2-yl)benzaldehyde), which displays environment-responsive dual emissive properties,
was designed and synthesized. In aprotic solvent, such as DMSO, DMF or CH3CN, the complex emits
strong cyan light. When in protic solvent, it emits orange light. Similarly, the response of the complex to
homocysteine and cysteine (Hcy/Cys) also shows obvious solvent dependence. TDDFT calculations
reveal that the protonation of imidazole nitrogen in protic solution is responsible for the luminescent red
shift. Hence, the complex could be used to detect Hcy/Cys in aprotic solvent, as well as in protic solvent.
having environment-responsive dual emissive properties, has
been designed and synthesized.
Introduction
The use of phosphorescent heavy-metal complexes as chemosen-
sors and bioimaging probes has recently attracted considerable
interest,1 because of advantageous photophysical properties such
as high luminescence efficiency, tunable excitation and emission
wavelength over the whole visible range, significant Stokes
shifts, and relatively long lifetimes. As one of the best families
of phosphorescent dyes, ruthenium complexes, particularly
ruthenium polypyridine complexes, have attracted widespread
interest due to their abundant optical-property-based MLCT.
They have been studied in many fields, such as chemical
sensors,2–10 chemistry analysis,11–13 catalyzation,14 electrolumi-
nescent devices,15–20 DNA structure probes,21–26 and so on.
Many ruthenium complexes with different properties have
already been synthesized and utilized. Most reported ruthenium
complexes emit red light attributed to 3MLCT at room
temperature.2–7,11,18–23 Their emissive states are insensitive to
their ligands and local environment.
Until now, the development of ruthenium complexes that
display environment-responsive dual emissive properties under
ambient conditions has not been reported. It has, however, been
reported that iridium(III) complexes with environment-responsive
dual emissive properties could be used as luminescent biosen-
sors.27 Establishing the relationship between the structure and
properties of ruthenium complexes is still one of the crucial pro-
blems for the chemical scientist. As reported here, a ruthenium
polypyridine complex [Ru(phen)2(IPBA)](PF6)2 (1) (IPBA =
4-(1H-imidazo[4,5-f][1,9]phenanthroline-2-yl)benzaldehyde),
The thiol-containing amino acids homocysteine (Hcy) and
cysteine (Cys) are essential biological molecules and play a
crucial multidimensional role in maintaining biological redox
homeostasis. An abnormal level of Hcy in plasma is an indicator
of various disorders, including cardiovascular and Alzheimer’s
disease, osteoporosis, etc.28 Cys deficiency has been linked to
slowed growth in children, liver damage, muscle and fat loss,
skin lesions, and weakness.29 Great effort has gone into the
detection of Hcy and Cys, including by high performance liquid
chromatography,30–33 capillary electrophoresis,34 electrochemical
detection,35–37 and optical detection.38 Due to the simple, easy
operation and high sensitivity, fluorescence technologies have
been applied for the specific detection of Hcy and Cys over the
past decade. Sensing approaches include indicator displacement
assays (IDA),39–41 cyclization with aldehyde,42–47 thiolysis of
sulfonate ester48 sulfonamide,49 cleavage of selenium–nitrogen
bonds by thiols,50 cleavage of disulfide by thiols51,52 and
others.53 However, most of these approaches are limited to fluore-
scent detection based on organic dyes as probes. Ruthenium
complexes as Hcy/Cys sensors are still limited. Recently, Yuan
reported a ruthenium complex as a turn-on luminescent probe
for Cys/Hcy.54 Herein, we report a novel dual-responsive
Cys/Hcy sensor based on the cyclization with aldehyde of a
ruthenium complex [Ru(phen)2(IPBA)](PF6)2 (1) (IPBA =
4-(1H-imidazo[4,5-f][1,9]phenanthroline-2-yl)benzaldehyde)
(Scheme 1). As expected, the complex might preferentially react
with Hcy/Cys over other amino acids, as confirmed by lumine-
scence spectra, absorption spectra, 1H NMR spectra and ESI.
aInstitute of Molecular Science, Key Laboratory of Chemical Biology
and Molecular Engineering of Ministry of Education, Shanxi University,
Taiyuan 030006, P.R. China. E-mail: huilichen@sxu.edu.cn
bState Key Laboratory of Coordination Chemistry (SKLCC), Nanjing
University, Nanjing 210093, P.R. China
Results and discussion
Photophysical properties of 1
The absorption spectrum (Fig. 1) of 1 shows intense high-energy
absorption bands at approximately 270–300 nm, assigned to
†Electronic supplementary information (ESI) available: Absorption and
emission spectra and ESI mass spectra. See DOI: 10.1039/c2dt31180a
13292 | Dalton Trans., 2012, 41, 13292–13297
This journal is © The Royal Society of Chemistry 2012

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Table 1 Photophysical properties of 1, 1-Cys and 1-Hcy in different
solvents at 298 K
Complex
Solvent
λem/nm
τ/ns
1
DMF
HEPES buffer
480
495
2.2 ( χ2 = 1.017)
0.88 (84.46%), 4.88 (15.54%)
( χ2 = 1.062)
588
428
588
428
588
352, 575 ( χ2 = 1.135)
1.61, 7.45 ( χ2 = 1.115)
438, 754 ( χ2 = 1.12)
1.70, 9.60 ( χ2 = 1.061)
406, 739 ( χ2 = 1.079)
1-Cys
1-Hcy
DMF
HEPES buffer
DMF
HEPES buffer
cyan range (Φ = 0.038 in DMF), while in protic solvents, such
as EtOH, the HE band becomes much weaker, and the LE band
is much higher (Φ = 0.023 in EtOH). In aqueous buffer, the
emission intensity of the HE band is comparable to that of the
LE band (Φ = 0.003 in H2O), with an orange emission light.
With increasing solvent polarity, the HE band shifts to red from
440 nm in EtOH to 495 nm in the HEPES buffer. Interestingly,
an analogue complex 2 does not display dual emission in
aqueous solution (Fig. S3, ESI†). The only broad band of this
complex (at around 550–650 nm) is assigned to a charge-transfer
Scheme 1 The proposed response mechanism of complex 1 with Hcy/
Cys and the photoluminescence in different solvents.
3
(CT) state of mixed MLCT and ligand-to-ligand charge-transfer
(3LLCT) character. The effect of the solvent hydrophilicity on
the luminescent spectrum indicates hydrogen bonding between
solute and solvent formed because hydrogen bonding presum-
ably enhances intersystem crossing.60 On the basis of the spectral
profiles, emission wavelengths, and lifetimes of the two bands of
1 (Table 1), we have tentatively assigned (1) the HE band to a
singlet intraligand IL (π–π*) excited state,59 and (2) the LE
3
feature to an excited state with high MLCT/3LLCT character.
The luminescence properties indicate that the aprotic solvent is
unfavorable for intersystem crossing (ISC) and prohibits the
phosphorescence of the complex.
Fig. 1 The absorption and emission spectra of 1 in different solvents.
For comparison, the intensities in DMF were divided by 800, while the
intensities in EtOH and HEPES buffer were divided by 50.
Solvent-dependent luminescent response of 1 to Cys/Hcy. The
luminescent response of 1 to Hcy/Cys shows solvent dependence
(Fig. 2). In DMF–HEPES buffer (10 : 1, v/v), probe 1 (4.5 μM)
shows a very strong fluorescence at 480 nm. However, upon
addition of Cys/Hcy, the fluorescence intensity decreases remark-
ably, and blue-shifts to 428 nm, corresponding to a change in
photo color from cyan to purple that can be observed by the
naked eye (Fig. 2b and 3). The changes of the emission intensi-
ties nearly become constant when the amount of Cys/Hcy added
reaches 400 equiv. (Φ = 0.014 for 1-Hcy, Φ = 0.016 for 1-Cys).
A good exponential decay relationship (Fig. 3, inset) is observed
in the conditions. Surprisingly, with the addition of Cys, the
color of the solution appears to be a deeper shade of purple than
with the addition of Hcy. Comparing the emission properties of
1-Cys and 1-Hcy, the emission intensity at 428 nm for 1-Cys is
stronger than that for 1-Hcy. While in HEPES buffer, 1 emits
orange light when excited by a 365 nm UV lamp. With the
addition of Cys/Hcy, the band at 500 nm disappears, while the
peak at 588 nm elongates somewhat. Simultaneously, the photo-
luminescence changes to red by “naked eye” (Fig. 2a). The
similar luminescence spectra of 1-Cys/Hcy with that of 2 both in
DMF and HEPES buffer indicate the aldehyde moiety of 1 is
missing after the interaction of Cys/Hcy (Fig. S1 and S3 in
ESI†).
intraligand π–π* transition, medium absorption bands at
300–370 nm, assigned to intraligand n–π* (CHO on IPBA), and
weak absorption bands at 400–470 nm, assigned to spin-allowed
metal-to-ligand charge-transfer (MLCT) transitions with refer-
ence to previous photophysical studies on related ruthenium(III)
diimine systems55–57 and the control complex [Ru(phen)2-
(PMIP)](PF6)2 (2) (PMIP = 2-(4-methylphenyl)imidazo[4,5-f]-
1,10-phenanthroline)58 (Fig. S1, ESI†). In protic solvent, the
bands assigned to intraligand n–π* at 300–370 nm show a blue-
shift and hypochromicity. It is possible that hydrogen bonds
form between the aldehyde group and protic solvent.59 In con-
trast, the bands derived from the MLCT transition are more
stable, and are not affected by solvents.
Upon irradiation, 1 exhibited intense luminescence in fluid
solutions (Fig. 1 and Fig. S2, ESI†). The photophysical data are
listed in Table 1. Interestingly, it shows dual emission peaks with
a high-energy (HE) structured band at about 440–500 nm and a
low-energy (LE) band at approximately 600 nm. Similarly, room
temperature luminescence of 1 also depends on the type of
solvent used (Fig. S2, ESI†). In aprotic solvents, such as DMF,
DMSO or CH3CN, the emission intensity of the HE band is
much higher than that of the LE band, with strong light in the
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Fig. 4 Changes in UV-Vis absorption spectra of 1 (4.5 μM) in DMF–
HEPES buffer (50 mM, pH 7.0, 10 : 1 v/v) with various amounts of
Hcy/Cys (0–2 mM).
Fig. 2 The luminescence response of 1 to Hcy/Cys in different sol-
vents. (a) HEPES buffer (50 mM, pH 7.0); (b) DMF–HEPES buffer
(50 mM, pH 7.0, 10 : 1 v/v).
(10 : 1, v/v) containing the probe 1 (4.5 μM). As the concen-
tration of Cys/Hcy increased, the bands at 325–400 nm
decreased gradually until the amount of Cys/Hcy added reached
400 equiv. Because the bands are related to the n–π* transition
of the aldehyde moiety, and the final absorption mode is similar
to that of 2, it can be inferred that the addition of Cys/Hcy leads
to a change in the aldehyde group.
1H NMR spectra and ESI-MS
1H NMR experiments were carried out (Fig. 5) to reveal the
sensing mechanism of 1 for Cys/Hcy. When excess Cys/Hcy is
added into a DMSO-d6 solution of 1, the aldehyde resonance
(9.98 ppm) of 1 disappears and two new peaks, centered at
5.84 ppm and 5.65 ppm for 1-Cys, 5.64 ppm and 5.42 ppm for
1-Hcy, corresponding to the methine protons of the thiazolidine
diastereomer, are observed, consistent with the previous
reports.42–47 This fact indicates that thiazolidine is formed by the
interaction of aldehyde with Cys/Hcy. This formation is further
characterized by mass spectrometry analysis, in which the peaks
at m/z 451.3 (calcd = 451.49) corresponding to [(1-Hcy)
− 2PF6]2+ and 444.4 (calcd = 444.48) corresponding to [(1-Cys)
− 2PF6]2+ were clearly observed, respectively (Fig. S4 in ESI†).
Fig. 3 Changes in the phosphorescence emission spectra of 1 (4.5 μM)
with various amounts of Hcy and Cys (0–2 mM) (λex = 360 nm).
Electronic absorption spectral response of 1 to Cys/Hcy. The
recognition of Cys/Hcy by 1 is studied here by UV-Vis absorp-
tion. Fig. 4 shows the change in the UV-Vis spectrum when the
Cys/Hcy solutions were added to the DMF–HEPES buffer
Selective optical response of 1 to various amino acids
The fluorescence responses of 1 (4.5 μM) to 250 equiv. of
various amino acids as well as thiol biomacromolecules such as
13294 | Dalton Trans., 2012, 41, 13292–13297
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Fig. 5 1H NMR spectra of 1 in DMSO-d6 in the absence and presence
of Hcy and Cys. The resonance centered at 9.98 ppm corresponds to the
aldehyde hydrogen of 1. The resonances centered at 5.84 ppm and
5.65 ppm for 1-Cys, and 5.64 ppm and 5.42 ppm for 1-Hcy correspond
to the methine protons of the thiazolidine diastereomer, respectively.
Fig. 7 Photoluminescence observed from the solutions of 1 (4.5 μM)
in the presence of different amino acids (2 mM). Excitation wavelength
was 365 nm from a portable lamp.
as that in completed experimental conditions, such as the effect
of HEPES buffer, hydrogen-bonding, etc. The structures are opti-
mized based on 1 and protonated 1 in a vacuum environment.
Both complexes were confirmed to be genuine minima on the
PES. The important orbitals are shown in Fig. 8. The calculation
results show that the emission peak is at 687 nm for the proto-
nated 1, corresponding to a HOMO − 1 to LUMO MLCT tran-
sition. While for 1, the emission is at 563 nm, corresponding to
a HOMO to LUMO LLCT and LC transition. The change trend
is consistent with that in the protic and aprotic solvents.
Conclusions
In summary, a novel luminescent ruthenium complex, which dis-
plays interesting environment-responsive dual emissive proper-
ties, has been discovered. Importantly, the complex could be
used to probe Cys/Hcy in both protic and aprotic environments,
in which it produces pronounced changes in the emission
profiles. We expect that the complex will act as a practical tool
for biological sample analysis.
Fig. 6 Luminescent responses of 1 (4.5 μM) in DMF–HEPES buffer
(50 mM, pH 7.0, 10 : 1 v/v) to various amino acids at 25 °C. Bars rep-
resent the ratio I480/I428 of luminescence intensity at 480 to 428 nm. Red
bars represent the addition of 2 mM various amino acids to a 4.5 μM sol-
ution of 1. Black bars represent the addition of Cys/Hcy (2 mM) to the
above solution, respectively. (λex = 360 nm).
Experimental section
2-thiobarbituric acid, thioglycol and GSH, were also investigated
(Fig. 6 and 7). In fact, these competitive species do not induce
any significant fluorescent changes of 1, and only Cys/Hcy
cause a dramatic change in the fluorescent intensity, and emit a
purple fluorescence. Importantly, as a structurally related thiol
biomolecule, reduced glutathione (GSH) hardly reacts with 1
under these conditions, which may be due to its steric hindrance.
Moreover, in the presence of these competitive species, Cys/Hcy
still result in similar fluorescent changes, suggesting that 1 is
highly selective for Cys/Hcy.
Materials and physical measurements
Ru(phen)2Cl2 and 4-(1H-imidazo[4,5-f][1,9]phenanthrolin-2-yl)-
benzaldehyde (IPBA) were synthesized using literature
methods.61,62 Unless otherwise stated, all chemical materials
were purchased from commercial sources and used without
further purification. Here, 1-Hcy and 1-Cys represent the adducts
of 1 with homocysteine and cysteine, respectively.
NMR spectra were recorded on a Bruker Avance spectrometer
(300 MHz). ESI-MS spectra were measured on a Finnigan LCQ
system. UV-Vis spectra were recorded on a Hewlett Packard
HP-8453 spectrophotometer. Luminescence spectra were
recorded on a Perkin-Elmer LS 50B luminescence spectrometer
with excitation slits of 5 nm and emission slits of 3 nm.
TDDFT calculations
To further explain the different emission properties of 1 in protic
and aprotic solutions, TDDFT calculations at the B3LYP level
with a 6-31G(d) basis set for nonmetals and a CC-PVDZ-PP
basis set for Ru were performed. Firstly it was supposed that 1
protonated on the imidazole N exists in protic solvent. For theor-
etical calculations it is difficult to model the same environment
Synthesis of [Ru(phen)2(IPBA)](PF6)2 (1)
Under a nitrogen atmosphere, 32.4 mg of IPBA (0.1 mmol) in
5 mL of EtOH was added to a solution of Ru(phen)2Cl2 (57 mg,
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Fig. 8 Important orbitals of protonated 1 and 1 based on TDDFT.
0.1 mmol) in 2.5 mL H2O. The reaction mixture was refluxed
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