Two novel six-coordinated cadmium(ii) and zinc(ii) complexes from carbazate β-diketonate: Crystal structures, enhanced two-photon absorption and biological imaging application

Article abstract of DOI:10.1039/c3dt51318a

To explore the photophysical properties of coordination compounds with enhanced two-photon absorption, two novel six-coordinated metal complexes (ML₂, M = Cd(ii), Zn(ii)) from carbazole β-diketone ligand (HL = 4,4,4-trifluoro-1-(9-butylcarbazole-3-yl)-1,3-butanedione) were prepared and fully characterized. Their crystal structures were determined by X-ray diffraction analysis. Both variable temperature ¹H NMR spectra and MALDI-TOF mass spectrometry proved that the coordination compounds exhibit good stability in solution. The results of time-dependent density functional theory (TD-DFT) calculations indicated that the complexation of the ligands with metal ion extends the electronic delocalization in the coordination compounds, leading to enhanced two-photon absorption. The photophysical properties for the coordination compounds were identified relying on both experimentally and theoretically studies. Finally, confocal microscopy and two-photon microscopy fluorescent imaging of HepG2 cells labeled with the Zn(ii) complexe revealed its potential applications as a biological fluorescent probe.

Full text of DOI:10.1039/c3dt51318a

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PAPER
Two novel six-coordinated cadmium(II) and zinc(II)
complexes from carbazate β-diketonate: crystal
structures, enhanced two-photon absorption and
biological imaging application†‡
Cite this: Dalton Trans., 2014, 43,
599
a
a
b
c
c
Cuiyun Nie,§ Qiong Zhang,§ Hongjuan Ding, Bei Huang, Xinyan Wang,
a
a
a
a
a,d
Xianghua Zhao, Shengli Li, Hongping Zhou, Jieying Wu* and Yupeng Tian*
To explore the photophysical properties of coordination compounds with enhanced two-photon absorp-
tion, two novel six-coordinated metal complexes (ML , M = Cd(II), Zn(II)) from carbazole β-diketone ligand
HL = 4,4,4-trifluoro-1-(9-butylcarbazole-3-yl)-1,3-butanedione) were prepared and fully characterized.
2
(
1
Their crystal structures were determined by X-ray diffraction analysis. Both variable temperature H NMR
spectra and MALDI-TOF mass spectrometry proved that the coordination compounds exhibit good stabi-
lity in solution. The results of time-dependent density functional theory (TD-DFT) calculations indicated
that the complexation of the ligands with metal ion extends the electronic delocalization in the coordi-
nation compounds, leading to enhanced two-photon absorption. The photophysical properties for the
coordination compounds were identified relying on both experimentally and theoretically studies. Finally,
confocal microscopy and two-photon microscopy fluorescent imaging of HepG2 cells labeled with the
Zn(II) complexe revealed its potential applications as a biological fluorescent probe.
Received 20th May 2013,
Accepted 24th September 2013
DOI: 10.1039/c3dt51318a
www.rsc.org/dalton
researchers focused to seek novel materials with large TPA
across-section (σ), leading to a great deal of work on building
Introduction
Two photons are simultaneously absorbed to an excited state π-conjugated dipolar, quadrupolar, and octupolar molecules
in a medium via a virtual state, which was first predicted by with π-centers and functional groups of electron-donating and/
1
5
Göppert-Mayer in 1931. Two-photon absorption (TPA) was or electron-withdrawing groups at the terminal sites. Their
possible only 30 years later with the advent of lasers. The first structure–property correlations have been systematically inves-
experimental evidence was performed by W. Kaiser and tigated, and molecular design strategies of organic compounds
2
6
C. G. Garret. The recent technologies that can exploit TPA with large σ were well-established.
materials have received significant applications in the areas of Apart from the studies of organic compounds for TPA,
chemistry, biology and photonics, such as optical limiting, there has been a growing interest in the study of coordination
7
two-photon laser scanning fluorescence imaging, micro-fabri- compounds as TPA materials. Previously, our group has de-
3
8
cation, 3D optical data storage and photodynamic therapy.
veloped various two-photon materials, and it is suggested that
The applications highly rely on the high TPA cross-section of large TPA cross sections can be brought about by their charac-
4
the specifically engineered molecules. Spurred by this, many teristic intra/inter-ligand and metal-to-ligand charge-transfer
transitions during photo-excitation. However, apart from the
large TPA values, it is also current challenge that TPA materials
should meet various requirements for the specific utility. With
a
Department of Chemistry, Key Laboratory of Functional Inorganic Materials
Chemistry of Anhui Province, Anhui University, Hefei 230039, P. R. China.
E-mail: jywu1957@163.com, yptian@ahu.edu.cn
the interests and efforts in the design and synthesis of series
8
of coordination compounds, a program was launched to
b
Department of Physics, Shandong Normal University, Jinan 250014, P. R. China
c
explore the functional organic ligands to construct novel
coordination compounds. Herein, the design, synthesis,
characterization, crystal structures, and photophysical pro-
perties of two novel coordination compounds based on carba-
zate β-diketonate ligand were described on the basis of both
experiments and theoretical calculation. The ligand (HL),
shown in Scheme 1, was designed based on the following
Department of Biology, Anhui University, Hefei 230039, P. R. China
d
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing,
2
†
‡
10093, P. R. China
Dedicated to Professor Xiao-Zeng You for His 80 Birthday.
th
Electronic supplementary information (ESI) available. CCDC 635449 for
Cd(L)
CIF or other electronic format see DOI: 10.1039/c3dt51318a
These authors contributed equally to this work.
2 2 2 2 2
·2H O and 635450 for Zn(II)L (H O) . For ESI and crystallographic data in
§
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Measurements
The fluorescence quantum yields (Φ) and fluorescence life-
time (τ). The fluorescence quantum yields (Φ) were deter-
mined by using coumarin 307 as the reference according to
1
0,11
Scheme 1 Synthesis routes for the preparation of HL.
the literature method.
follows:
Quantum yields were corrected as
ꢀ
ꢁ
2
considerations: (1) carbazole moiety contains conjugated
system, bearing good optical properties, carbazole-based
materials have potential applications as an electron donor,
hole-transport and photoconductive materials, molecular
r s s
A η D
Φs ¼ Φr
2
A η D
r
s
r
where the s and r indices designate the sample and reference
samples, respectively, A is the absorbance at λexc, η is the
average refractive index of the appropriate solution, and D is
9
probes and photosensitizer; (2) β-diketonate unit as a chelate
ligand has an interesting coordination chemistry, unusual
1
2
1
0
the integrated area under the corrected emission spectrum.
3
structural features; (3) –CF group as an electron-withdraw-
Fluorescence lifetime is traditionally considered to be a
kinetic parameter and is determined as being inversely pro-
portional to the sum rate constants of a radiative process kr
and the nonradiative processes knr collectively known as
ing group to make β-diketonate unit easier to coordinate with
metal ion, forming an extended conjugated system; (4) for
practical applications, the advantages of the ligand (HL)
should be lower molecular weight and easier to be synthesized
7
,8
quenching.
than those previously reported. The present results proved
that the two coordination compounds exhibit significantly
enhanced TPA cross-section values compared with those of the
free ligand. Furthermore, the connections between the struc-
ture and TPA properties were firstly studied depending on
both experimentally and theoretically for the novel Zn(II) and
Cd(II) complexes, according to our knowledge. Finally, a study
of the zinc coordination compound as a biological fluorescent
probe was carried out.
X
τ ¼ 1=ðkr þ
kiÞ
Φ ¼ kr=ðkr þ knrÞ
where ∑k is the sum of the rate constant of non-radiative tran-
sition process, k is the rate constant of a radiative process and
nr is the cumulative rate constant of a nonradiative processes.
TPA cross-section (σ). TPEF (two photon excited fluo-
i
r
k
rescence) spectra were measured using femtosecond laser
pulse and Ti:sapphire system (680–1080 nm, 80 MHz, 140 fs,
Chameleon II) as the light source. All measurements were
carried out in air at room temperature. TPA cross sections were
measured using two-photon-induced fluorescence measure-
ment technique. The TPA cross sections (σ) are determined by
comparing their TPEF to that of fluorescein in different sol-
Experimental section
Materials and apparatus
All chemicals were commercially available and all solvents
were purified by conventional methods before use.
1
3
vents, according to the following equation:
Elemental analyses were performed with a Perkin Elmer
1
2
40C elemental analyzer. The H NMR spectra were recorded
Φref cref nref F
σ ¼ σref
at 25 °C on a Bruker Avance 400 spectrometer, and the chemi-
cal shift are reported as parts per million from TMS (δ). Coup-
ling constants J are given in hertz. Mass spectra were acquired
on a Micromass GCT-MS (EI source). IR spectra were recorded
Φ
c
n Fref
Here, the subscripts ref stands for the reference molecule.
σ
ref is the TPA cross-section value, c is the concentration of
solution, n is the refractive index of the solution, F is the TPEF
integral intensities of the solution emitted at the exciting wave-
length, and Φ is the fluorescence quantum yield. The σref value
on
4
a NEXUS 870 (Nicolet) spectrophotometer in the
00–4000 cm− region using a powder sample on a KBr plate.
1
XPS spectra were recorded on PHI 5000 Versaprobe spectro-
photometer. UV-vis absorption spectra were recorded on a
UV-265 spectrophotometer. Fluorescence measurements were
1
4
of reference was taken from the literature.
Fluorescence lifetime
carried out on
a Hitachi F-7000 fluorescence spectro-
For time-resolved fluorescence measurements, the fluo-
rescence signals were collimated and focused onto the
entrance slit of a monochromator with the output plane
equipped with a photomultiplier tube (HORIBA HuoroMax-
photometer. All the fluorescence spectra were collected. The
single-photon excited fluorescence (SPEF) quantum yields
were measured by using a standard method under the same
experimental conditions for all compounds. Two-photon
absorption (TPA) cross sections (σ) of the sample were
obtained at femtosecond laser pulse and Ti sapphire system
4
P). The decays were analyzed by ‘least-squares’. The quality of
2
the exponential fits was evaluated by the goodness of fit (χ ).
(
680–1080 nm, 80 MHz, 140 fs) as the light source. All
The Stokes shift
measurements were carried out in air at room temperature.
TPA cross-sections were measured using fluorescein as The Stokes shift is defined as the loss of energy between
reference.
absorption and emission of light, which is a result of several
600 | Dalton Trans., 2014, 43, 599–608
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dynamic processes. These processes include losses arising wells treated with various concentration of the compounds
from the dissipation of vibrational energy, redistribution of and OD490(control) represents that of the wells treated with
electrons in the surrounding solvent molecules induced by the DMEM + 10% FCS. Three independent trials were conducted,
altered dipole moment of the excited compound, reorientation and the averages and standard deviations are reported. The
of the solvent molecules around the excited state dipole, and reported percent cell survival values are relative to untreated
specific interactions between the compound and the solvent or control cells.
solutes. The Lippert equation is the most widely used equation
to describe the effects of the physical properties of the solvent
on the emission spectra of a compound.
Density functional theory (DFT) calculation
1
5
The two-photon absorption cross-section that can be directly
compared with the experimental results is defined as:
3
2
Δν ¼ νabs ꢀ νem ¼ ð2=eha ÞΔ f ðμe ꢀ μgÞ þ const
2
5
2
In this equation, h is Planck’s constant, e is the speed of light,
and a is the radius of the cavity in which the compound
4π a α ω gðωÞ
0
σtp ¼
ꢁ
δtp
15e0
Γf
resides. The wave numbers of the absorption and emission are
νabs and νem (in cm−1).
0 0
Here a is the Bohr radius, e is the speed of light, α is the
fine structure constant, ω is the photon frequency of the inci-
dent light, and g(ω) denotes the spectral line profile, which is
Cell image
assumed to be a δ function here, Γ
of the final state, which is commonly assumed to be 0.1 eV,
f
is the lifetime broadening
5
HepG2 cells were seeded in 6 well plates at a density of 2 × 10
1
6
cells per well and grown for 96 hours. For live cell imaging cell
cultures were incubated with the complexes (10% PBS : 90%
cell media) at concentrations 20 μM and maintained at 37 °C
and δtp is orientation average value of the two-photon absorp-
tion probability in gas and solution, which is written as
1
7
follows,
2
in an atmosphere of 5% CO and 95% air for incubation
X ꢂ
*
*
*
times ranging for 2 hours. The cells were then washed with
PBS (3 × 3 mL per well) and 3 mL of PBS was added to each
well. The cells were imaged using confocal laser scanning
microscopy and water immersion lenses. Excitation energy of
δtp ¼
F ꢁ Sαα ꢁ Sββ þ G ꢁ Sαβ ꢁ Sαβ þ H ꢁ Sαβ ꢁ Sβαꢂ
αβ
where F, G, and H are coefficients dependent on the polari-
zation of the light. For the linearly polarized light, F, G and H
are 2 ,2, 2, for the circularly case, they are −2, 3, 3. Sαβ is the
two-photon transition matrix element. For the absorption of
7
4
20 nm was used and the fluorescence emission measured at
61–511 nm.
two photons with the same frequency ω /2 it can be written
as,
f
Microscopy
18
HepG2 cells were imaged on a Zeiss LSM 710 META upright
confocal laser scanning microscope using magnification
ꢃ
k0jμβjilkijμαjflꢄ
X
k0jμαjilkijμβjfl
ωi ꢀ ωf =2
Sαβ ¼
þ
ωi ꢀ ωf =2
i
4
0× and 100× water-dipping lenses for monolayer cultures.
Image data acquisition and processing was performed
using Zeiss LSM Image Browser, Zeiss LSM Image Expert and
Image J.
where ω and ω denote the excitation frequency of the inter-
i
f
mediate state |i〉 and final state |f〉 respectively, α, β ∊ (x, y, z),
and the summation goes over all the intermediate states
including the ground state |0〉 and the final state |f〉.
Cytotoxicity assays in cells
Optimizations were carried out with B3LYP[LAN2DZ]
To ascertain the cytotoxic effect of all the compounds treat- without any symmetry restraints, and the TD-DFT {B3LYP-
ment over a 24 h period, the 5-dimethylthiazol-2-yl-2,5- [LAN2DZ]} calculations were performed on the optimized
1
9a
diphenyltetrazolium bromide (MTT) assay was performed. structure.
All calculations, including optimizations and
1
9b
HepG2 cells were trypsinized and plated to ∼70% confluence TD-DFT, were performed with the G03 software.
Geometry
in 96-well plates 24 h before treatment. Prior to the com- optimization of the singlet ground state and the TDDFT calcu-
pounds’ treatment, the DMEM was removed and replaced with lation of the lowest 25 singlet–singlet excitation energies were
fresh DMEM, and aliquots of the compound stock solutions calculated with a basis set composed of 6-31G(d,p) for C, N, H,
(500 μM DMSO) were added to obtain final concentrations of F, O atoms and the Lanl2dz basis set for Zn and Cd atom was
2
2
0, 40, 60, 80 and 100 μM. The treated cells were incubated for downloaded from the EMSL basis set library. An analytical fre-
4 h at 37 °C and under 5% CO . Subsequently, the cells were quency analysis provides evidence that the calculated species
2
−
1
treated with 5 mg mL MTT (40 μL per well) and incubated represents a true minimum without imaginary frequencies on
for an additional 4 h (37 °C, 5% CO ). Then, DMEM was the respective potential energy surface. In the calculation of
removed, the formazan crystals were dissolved in DMSO the absorption spectrum, the 25 lowest spin-allowed singlet–
150 μL per well), and the absorbance at 490 nm was recorded. singlet transitions, up to energy of about 5 eV, were taken into
The cell viability (%) was calculated according to the following account (Table S3‡) The calculation two-photon absorption is
2
(
2
0
equation: cell viability % = OD490(sample)/OD490(control) × carried out by the response theory method at the DFT level
1
2
1
00, where OD490(sample) represents the optical density of the implemented in DALTON program.
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Synthesis
(400 MHz, DMSO-d
6
): 8.27 (s, 1H), 8.13 (d, 1H), 7.65 (d, J = 10.0
Hz, 1H), 7.55–7.44 (m, 3H), 7.31 (d, J = 4.0 Hz, 1H), 6.84 (s,
HL (Scheme 1) was synthesized by a modified method of 1H), 4.35 (m, 2H), 3.76–3.70 (m, 2H), 1.44–1.37 (m, 2H),
2
2
typical Claisen condensation procedure.
-Butyl-carbazole. To a solution of 9-H-carbazole (16.7 g, cm ): 3415 (m), 2956 (w), 2871 (w), 1269 (s), 1616 (vs), 1494
00 mmol) and KOH (8.4 g, 150 mmol) in acetone (100 mL) (m), 1469 (m), 789 (m), 580 (w).
was refluxed for 2 hours. After cooling to room temperature, Zn(II)L (H O) . An aqueous of Zn (NO
4 mL 1-bromination butane (13.7 g, 100 mmol) were added 1 mmol) 3 mL was added dropwise to a solution of HL (0.72 g,
1.26–1.22 (m, 2H), 0.96 (t, J = 6.8 Hz, 3H). IR (KBr pellet,
−
1
9
1
2
2
2
3 2 2
) ·6H O (0.37g,
1
and refluxed for 24 hours. 200 mL water was added with stir- 2 mmol) in ethanol and tetrahydrofuran (15 mL, ethanol–tetra-
ring after removing the solvent. The residual solid was recrys- hydrofuran = 5 : 1), which had been neutralized with 2 mL tri-
tallized from ethanol to give colorless crystals (18.4 g, yield ethylamine. Then, the mixture was stirred at 80 °C for
8
3%). Ms m/z (%): 223.14(100) Anal. Calc. for C16
H
17N: C 12 hours. The precipitated product was filtered and washed by
8
6.05, H 7.67, N 6.27%; found C 86.02, H 7.65, N 6.23%.
ethanol and tetrahydrofuran. The product was dried at 80 °C
1
3
-Acetyl-9-butyl-carbazole. To a solution of 9-butyl-carbazole in vacuum for 12 hours. Yield: 0.66 g (80%). H NMR
6
(4.4 g, 20 mmol) in dichloromethane (70 mL) was rapidly (400 MHz, DMSO-d ): 8.76 (s, 1H), 8.38 (s, 1H), 8.04 (d, J =
added AlCl (5.2 g, 40 mmol) with stirring. After cooling to 8.7 Hz, 1H), 7.60 (d, J = 8.19 Hz, 2H), 7.45 (t, J = 7.6 Hz, 1H),
3
0
2
°C, a solution of acetic anhydride (2.04 g, 20 mmol) in 7.16 (s, 1H), 6.46 (s, 1H), 4.38 (t, J = 6.5 Hz, 2H), 3.59–3.56 (m,
0 mL of dichloromethane was added dropwise under stirring. 2H), 1.27–1.21 (m, 2H), 0.81 (t, J = 7.3 Hz, 3H).
After stirring over night at room temperature, a large amount
of water and HCl were added into the mixture and extracted
X-ray crystallography
with dichloromethane twice, washed by 1 M NaHCO
3
and
Intensity data of 1 was collected with Mo-Kα radiation (λ =
water. The combined organic phase was dried over anhydrous
0.71073 Å) on a Bruker SMART Apex CCD diffractometer at
MgSO and then filtered. The organic solvent was completely
4
293 K. Absorption corrections were applied by using
2
3
24
removed by rotary evaporation. The solid was purified by
column chromatograph (silica gel, ethyl acetate–petroleum
ether = 1 : 4 as eluent) to give the purified title compound
SADABS. The structures were solved by WinGX package and
refined by full-matrix least-squares with anisotropic thermal
2
5
parameters for all the nonhydrogen atoms using SHELXL-97.
(
4.0 g, 76%). Ms m/z (%): 265.13(100). Anal. Calc. for
19NO: C 81.47, H 7.22, N 5.28; found C 81.41, H 7.25,
N 5.26.
Hydrogens were included and located from difference Fourier
map but not refined anisotropically. Crystal data collections
18
C H
and refinement parameters of Cd(II)L
2 2 2
(H O) and Zn(II)-
4
,4,4-Trifluoro-1-(9-butylcarbazole-3-yl)-1,3-butanedione (HL).
L (H O) are shown in Table S1.‡ Selected bond lengths and
2
2
2
To a solution of ethyl trifluoroacetate (1.71 g, 12 mmol) in
2 2 2 2 2 2
angles for Cd(II)L (H O) and Zn(II)L (H O) are listed in
anhydrous ethanol (20 mL) was added t-BuOK (3.33 g,
Table S2.‡
1
1
5 mmol). A solution of 3-acetyl-9-butyl-carbazole (2.64 g,
0 mmol) in 10 mL of anhydrous ethanol was added dropwise,
and then stirring for 5 hours at room temperature. After 3 M
HCl was added to the mixture with stirring, the mixture was
extracted with dichloromethane and the organic layer was col-
Results and discussion
Characterizations of the coordination compounds
lected. The organic layer was dried over anhydrous MgSO and The pH value in the reaction solution is an important factor
4
2 2 2
filtered. The organic solvent was completely removed by rotary for the complexation formation of M(II)L (H O) , in which the
evaporation. The residue was purified by recrystallized from pH value was adjusted to 7.0 with triethylamine. The IR
the petroleum ether and ethyl acetate to give the yellowish crys- spectra of M(II)L (H O) show features attributed to each part
2
2
2
tals of the title compound (3.05 g, 85%). MS m/z (%): 361.02 of them, in which the coordinated water molecules cause a
1
−1
(
100). H NMR (400 MHz, DMSO-d
6
) δ 14.23 (s, 1H), 9.13 (s, stronger band at 3415 cm than that of the ligand. Herein, we
1
8
H), 8.34 (d, J = 7.6 Hz, 1H), 8.23 (d, J = 8.8 Hz, 1H), 7.77 (d, J = take Cd(II)L (H O) as an example to assign the bands. The
2
2
2
.80 Hz, 1H), 7.71 (d, J = 8.0, 1H), 7.55 (t, J = 7.6 Hz, 1H), 7.33 bands of 2956 cm− and 789 cm are assigned to νC–H of car-
1
−1
−1
(t, J = 7.4, 1H) 7.21 (s, 1H), 4.46 (s, 1H), 1.76 (t, J = 7.0, 2H), bazole ring. In addition, the bands at 1494 and 1469 cm are
.30 (d, J = 7.2, 2H), 0.87 (t, J = 7.2, 3H). IR (KBr pellet, cm− ): attributed to the stretching vibration of the carbazole ring.
1
1
3
1
414 (m), 2988 (w), 2966 (w), 1272 (s), 1599 (vs), 1496 (m), Then the band located at 1616 cm− is attributed to the
1
474 (m), 792 (m), 578 (w).
stretching vibration of carbonyl, which has a blue shift
Cd(II)L (H O) . To an aqueous of Cd(NO ) ·4H O (0.3085 g, because of the coordination with metal ion. The shape and
2
2
2
3 2
2
1
mmol) 3 mL was added dropwise to a solution of HL location of the other bands are consistent with those of the
(
0.7220 g, 2 mmol) in ethanol (15 mL) neutralized with 2 mL ligand.
triethylamine. Then, the mixture was stirred at 80 °C for To further prove the stability of the coordination com-
2 hours. The precipitated product was filtered and washed by pounds, the variable temperature H NMR spectra of Zn(II)-
ethanol and tetrahydrofuran. The product was dried at 80 °C (H O) , as an example, shown in Fig. S1.‡ MALDI-TOF
in vacuum for 12 hours. Yield: 0.70 g (80%). H NMR mass spectrometry was used to further verify the stability of
1
1
L
2
2
2
1
602 | Dalton Trans., 2014, 43, 599–608
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Zn(II)L in DMSO solution. The results suggest that the so Cd(II)L
Paper
2
(H
2
O)
2
2
(H
2
O)
2
has a larger conjugate plane. From the bond
coordination compounds are very stable in DMSO solution, lengths of M–O we can see that Cd(II)L (H O) is in a more dis-
2
2
2
which belongs to the molecular weight of the structure Zn(II)- torted octahedral geometry.
(H O) (more detailed information is shown in Fig. S2‡), The carbonyl bond lengths of Cd(II)L
that shows that the two coordinated water molecules in the and C24–O5 are 1.279 and 1.257 Å, respectively. The carbonyl
complex do not decompose in solution. The results of spectral bond lengths of Zn(II)L (H O) for C19–O2 and C17–O1 are
characterization are consistent with those of single crystal 1.282 and 1.256 Å, respectively. Interestingly, the two co-
L
2
2
2
2 2 2
(H O) for C22–O6
2
2
2
X-ray diffraction analysis as below.
ordinated ligands are almost co-planar (4.70° for Zn(II)-
(H O) and 8.10° for Cd(II)L (H O) ) in the coordination
L
2
2
2
2
2
2
compounds. Therefore, it was observed that the bond lengths
of C–O of the diketone become longer than those of the free
ligand after coordinating to metal ions Cd(II), Zn(II), resulting
in an extending conjugated system. The structural features
reveal that the two compounds will possess a good optical
properties.
Crystal structures of Cd(II)L (H O) and Zn(II)L (H O)
2
2
2
2
2
2
Cd(II)L (H O) crystallizes in the monoclinic space group C2/c,
2
2
2
8
b
as shown in Fig. 1. Cd(II) ion has distorted octahedral geome-
try with four oxygen atoms from two HL in the equatorial posi-
tions and two oxygen atoms from two water molecules at the
2
6
axial positions. The bond lengths of Cd(1)–O(3), Cd(1)–O(5),
Cd(1)–O(6) are 2.361(3), 2.227(3) and 2.230(3) Å, respectively.
Each bond angle around the cadmium atom is in the range
UV-vis spectra and TD-DFT studies
8
3.37(11)–96.63(11)°, indicating
geometry.
Zn(II)L (H O) also crystallizes in the monoclinic space seen, HL exhibits an absorption band at 349 nm with the
a
distorted octahedral The UV absorption spectra of HL, Cd(II)L
2
(H
O) and Zn(II)-
2 2
(1.0 × 10− M in THF) are shown in Fig. 3. As can be
6
2 2 2
L (H O)
2
2
2
4
−1
group C2/c, as shown in Fig. 2. Similar to Cd(II)L
ion center in the coordination compound Zn(II)L
2
(H
2
O)
2
, Zn(II) corresponding molar extinction coefficient (ε = 5.11 × 10 M
−
1
also cm ), which was assigned as the intramolecular charge trans-
2
(H
2
O)
2
adopts a slightly distorted octahedral geometry with four fer (ICT) transition [πcarbazole → π*COCHC(OH)CF3] due to the H →
8
b
oxygen atoms from HL on the equatorial plane, two oxygen L transition. The both compounds, Cd(II)L
donors from two water molecules at the apical positions. The
(H O) , exhibit an absorption band around 278 nm with the
bond lengths Zn(1)–O(1), Zn(1–O(2), Zn(1)–O(3) are 2.053(5), corresponding molar extinction coefficient (ε = 6.21 × 10 M
(H O) and Zn(II)-
2
2 2
L
2
2
2
4
−1
−
1
4
−1
−1
2
.033(5) and 2.239(5) Å, respectively. Each bond angle around cm for Cd(II)L
the zinc atom is in the range 89.4(2)–90.6(2)°.
(H O) ), which originates from the n → π* transition of the
As shown in Table S2,‡ the bond length of Cd–O is longer carbazole unit. Besides, the both UV-visible spectra of Cd(II)-
than that of Zn–O. In addition, the radius of Cd atom is larger,
(H O) and Zn(II)L (H O) exhibit one characteristic absorp-
tion band at λmax 339 and 334 nm assigned to M–L(L′)CT
2
(H
2
O)
2
and 6.50 × 10
M
cm for Zn(II)-
L
2
2
2
L
2
2
2
2
2
2
2
7
(
(Cd, Zn center)-to-carbazole excitation).
By comparing the UV-vis absorption spectra of Zn(II)-
(H O) , Cd(II)L (H O) and HL in the same concentration, it
(H O) and
Cd(II)L (H O) are 2-fold less than that of HL in solution. It
L
2
2
2
2
2
2
was found that the extinction coefficients of Zn(II)L
2
2
2
2
2
2
can be explained that the spectra observed should arouse from
the whole complex molecules rather than the dissociated
molecules in the solution (Fig. S1 and S2‡). The stability of the
coordination compounds was verified by the results of the vari-
1
able temperature H NMR spectra and MALDI-TOF mass spectro-
2 2 2
Fig. 1 Coordination environments of Cd(II)L (H O) with the atom num-
bering scheme.
metry. It should be noted that the structural changes have
Fig. 2 Coordination environments of Zn(II)L
bering scheme.
2
(H
2
O)
2
with the atom num-
Fig. 3 UV-vis absorption spectra of HL, Cd(II)L
2
(H
2
O)
2
and Zn(II)L
2
(H
2
O)
2
(1.0 × 10− M in THF).
6
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taken place when the complexation formation of M(II)L
after deprotonation of the free ligands.
2
(H
2
O)
2
H−1 → L+2 and H → L+3 transition [n → π*(COCHC(OH)C H )]. It
is inferred that the oscillator strengths of the high-energy
6
5
TD-DFT computational studies were performed to elucidate absorption bands of both ligands are larger than those of the
the electronic structures of the ground state of Cd(II)L (H O) low-energy bands. Basically, the calculated singlet–singlet tran-
2
2
2
and Zn(II)L (H O) . The molecular orbital diagrams of HL, sitions in Cd(II)L (H O) and Zn(II)L (H O) are in reasonable
2
2
2
2
2
2
2
2
2
Cd(II)L
2
(H
2
O)
2 2 2 2
and Zn(II)L (H O) were shown in Fig. 4, respect- agreement with the extended electron delocalization drawn
ively. The energies and compositions of some frontier orbitals from the crystal structure determination. Finally, XPS
are listed in Table S3.‡ (Fig. S3‡) determination results also show the atomic concen-
As shown in Fig. 4 and Fig. S5,‡ the HOMOs (H) and H−1 tration sequence of the coordination site O atom is Cd(II)-
of HL are of π orbitals localized on the carbazole and COCHC- (H O) (16.89) > Zn(II)L (H O) (13.78) > HL (12.12), which
OH)CF groups. The unoccupied molecular orbitals LUMO (L) was in accordance with the electron delocalization tendency
L
2
2
2
2
2
2
(
3
and L+1 are of the carbazole and COCHC(OH)C
HOMO and H−1 of Zn(II)L (H O) are of the π orbitals on
COCHC(OH)CF and Zn center. LUMO and L+1 are of π* orbi-
6 5
H units. The calculated.
2
2
2
Single-photon excited fluorescence (SPEF)
3
tals localized on the whole molecule. As shown in Table S3,‡ The SPEF spectra were measured at the same concentration as
the lowest-energy excitation bands of the two complexes (λ = that of the linear absorption spectra. Surprisingly, the SPEF
3
53.91 nm and f = 0.0011 for Cd(II)L (H O) , λ = 351.00 nm spectra of HL, Cd(II)L (H O) and Zn(II)L (H O) display pro-
2 2 2
2 2 2 2 2 2
2 2 2
and f = 0.0021 for Zn(II)L (H O) ) are assigned as the MLCT nounced negative solvatochromism blue shift with increasing
mixed with π(C(OH)CHCOCF3) → π*(COCHC(OH)C H ) transition due polarity of the solvents (Table 1). This can be explained by the
6
5
to the H−1 → L transition, whereas those of the transitions at fact that the ground state may possess a higher polarity than
77.16 nm for Cd(II)L (H O) and 283.63 nm for Zn(II)L (H O)
the excited state, for negative solvatochromism is associated
in the high-energy absorption regions are mainly due to the with a increasing of the energy levels. A decreased dipole–
2
2
2
2
2
2
2
Fig. 4 Molecular orbital diagram for HL (B3LYP/6-31G(d)), Zn(II)L
2 2 2 2 2 2
(H O) (B3LYP/LANL2DZ) and Cd(II)L (H O) (B3LYP/LANL2DZ) (the two colors of
electron clouds is representative of the square of the wave function of atomic orbitals).
Table 1 Single-photon-related photophysical properties of HL, Cd(II)L
2
(H
2
O)
2
and Zn(II)L
2
(H
2
O)
2
in several different solvents
a
b
c
Δν (cm−1)
d
9
9
Compound
HL
Solvents
λ
max
λ
max
ε
Φ
T (ns)
K
r
(10 )
Knr(10 )
CH Cl
2 2
357
349
394
362
364
330
339
364
345
345
339
334
366
354
339
477
461
460
459
450
475
462
458
461
442
479
468
462
467
455
23 301
51 089
23 786
29 845
19 800
26 529
48 000
56 798
47 700
47 000
42 500
31 800
35 237
41 900
68 200
0.23
0.24
0.29
0.46
0.37
0.29
0.55
0.35
0.57
0.63
0.39
0.76
0.36
0.61
0.74
4161
3561
3642
3339
7898
5685
5677
5638
5932
7137
5719
5945
5955
6520
7092
0.24
0.22
0.20
0.23
0.80
3.65
1.52
0.26
0.75
0.15
3.67
1.97
2.80
2.10
0.73
1.21
2.50
1.75
2.85
0.79
0.06
0.16
1.11
0.61
2.47
0.11
0.39
0.13
0.31
1.01
2.96
2.05
3.25
2.15
0.46
0.21
0.50
2.73
0.72
2.47
0.17
0.12
0.22
0.20
0.36
THF
CH
CH
3
COOC
2
H
5
5
5
3
CH OH
2
DMF
CH Cl
THF
2 2
Cd(II)L (H O)
2
2
2
CH
CH
3
COOC
2
H
3
CH OH
2
DMF
CH Cl
THF
2 2
Zn(II)L (H O)
2
2
2
CH
CH
3
2
H
DMF
a
b
c
Peak position of the largest absorption band. Peak position of SPEF, exited at the absorption maximum. Quantum yields determined by
d
−1
using RhB as standard. Stokes’ shift in cm . The experimental errors are estimated to be ±10% from sample concentrations and instruments.
604 | Dalton Trans., 2014, 43, 599–608
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in different solvents. (b) The one-photon fluorescence spectra of HL (1.0 × 10−
6
M
Fig. 5 (a) The one-photon fluorescence spectra of Cd(II)L
2
(H
2
O)
2
in ethyl acetate). (c) The time resolved fluorescence curves of HL (λex = 340 nm, λem = 461 nm), Cd(II)L
2
2
(H O)
2
(λex = 340 nm, λem = 462 nm) and
2 2 2
Zn(II)L (H O)
(λex = 340 nm, λem = 468 nm)(1.0 × 10− M in THF).
5
dipole interaction between the solute and solvent molecules evaluate the dipole moment changes of the dyes with
8
c,e
29,30
leads to an increasing the energy level.
photoexcitation:
As shown in Fig. 5a, the emission maximal of Cd(II)-
L (H O) is about 475 nm in CH Cl . In high polar solvent
2
Δ f
2
2
2
2
2
2
Δν ¼ 4
ðμ ꢀ μ Þ þ b
e
g
ð1Þ
ð2Þ
3
π ε0hca
DMF, the band is located at 442 nm, which is blue-shifted by
3 nm due to the different molecular environment. Such be-
2
3
ε ꢀ 1
Δ f ¼ 2
n ꢀ 1
ꢀ
haviour is consistent with a symmetry breaking in the ground
state. When D–A structural molecule is in a high polar solvent,
the molecule in the ground state tends to be polarized to form
a charge-separated molecule by the action of an electric field
formed by the surrounding solvent molecules. Large Stokes
shift (Table 1) is observed for HL, Cd(II)L (H O) and Zn(II)-
2
ε þ 1 2n þ 1
in which Δν = νabs − νem stands for Stokes shift, νabs and νem
are absorption and emission (cm ), h is the Planck’s constant,
c is the velocity of light in vacuum, a is the Onsager radius and
e g
b is a constant. Δf is the orientation polarizability, µ and µ
are the dipole moments of the emissive and ground states,
respectively and ε is the permittivity of the vacuum. (µ − µ )
is proportional to the slope of the Lippert–Mataga plot.
Plots of the Stokes shifts as a function of the solvent
polarity factor Δf are shown in Fig. 6. Only the data involving
the aprotic solvents are shown because application of this ana-
lysis with solvents where specific solute–solvent interactions
are present is not appropriate. As shown in Fig. 6, the Lippert–
Mataga plot of HL gave much larger slop than Cd(II)L
and Zn(II)L (H O) which infers larger dipole moment
changes for two complexes with photoexcitation. The slope of
the best-fit line is related to the dipole moment change
−
1
2
2
2
L (H O) in the five solvents due to the strong solvent–solute
2
2
2
2
0
e
g
dipole–dipole interactions, a manifestation of the large dipole
moment and orientation polarizability. And the fluorescent
quantum yield decrease along with the increasing polarity of
7
c
the solvents is consistent with the blue-shift of their spectra.
From Fig. 5b shown that the one-photon fluorescence intensity
of the coordination compounds (Cd(II)L (H O) Zn(II)L (H O) )
2
2
2
2
2
2
is stronger than that of the ligand because of the formation of
the higher electron delocalization in the coordination
compounds.
The enhanced fluorescence also showed corresponding sig-
nificant changes in emission lifetime, as illustrated by the
time resolved fluorescence curves in Fig. 5(c) in THF (0.22,
2 2 2
(H O)
2
2
2
,
between the ground and excited states (µ
all three lines are different: 4776, 3581 and 2748 cm for HL,
Cd(II)L (H O) and Zn(II)L (H O) , respectively. So the values of
µ − µ were calculated as 8.49 D for HL, 11.75 D for Cd(II)-
e
− µ
g
). The slopes of
−
1
1
2 2 2 2 2 2
.52, 1.97 ns for HL, Cd(II)L (H O) and Zn(II)L (H O) , respecti-
2
2
2
2
2
2
vely). The fluorescence lifetime is the time required by a popu-
lation of excited fluorophores to decrease exponentially to N/e
via the loss of energy through fluorescence and other non-
e
g
2
8
radiative processes. Since this process is affiliated with an
energetically unstable state, fluorescence lifetime can be sensi-
tive to a great variety of internal factors defined by the fluoro-
phore structure and external factors including temperature
and polarity. The coordination compounds (Cd(II)L (H O)
2
2
2
2 2 2
Zn(II)L (H O) ) contain more electron-donating group (carb-
azole) and an electron-withdrawing group (–CF ). Consequently,
3
it can be seen that a larger conjugated plane and higher polarity
result in the longer lifetime than that of the free ligand.
As seen in Table 1, the fluorescence spectra showed moder-
ate Stokes shifts depending on the solvent polarity. Further,
the solvatochromism is observed, suggesting a large change
in dipole moment between ground and excited states.
The Lippert–Mataga equation is the most widely used to Fig. 6 Lippert–Mataga regressions of the three compounds.
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L
2
(H
2
O)
2
and 9.87 D for Zn(II)L
2
(H
2
O)
2
, respectively (eqn (1)). and the bands show a red shift, compared to the SPEF spectra
The values of Cd(II)L (H O) and Zn(II)L (H O) indicates that of them. Furthermore, from the figure, large difference
2
2
2
2
2
2
the molecule in the excited state has an extremely polar struc- observed between the coordination compounds and its free
31
ture, an enhanced TPA response found in the complexes ligand, which may arise from their different electronic struc-
comparing with HL may be caused by that the equatorial ture, as a result of the ligands bonded to the metal ion to form
coordination sphere is more coplanar in Cd(II)L
Zn(II)L (H O) than that in HL, leading to a higher degree of pound molecules after deprotonation, which were consistent
π-electron delocalization, which is proved by the crystal struc- with the results of structural determination above.
2 2 2
(H O) and an extended π-conjugated system in the coordination com-
2
2
2
tures and in good agreement with their nonlinear optical pro-
perties as shown in Fig. 8.
It has been found that the two-photon absorption cross sec-
tions of coordination compounds (Cd(II)L (H O) Zn(II)-
L (H O) ) are remarkably enhanced compared with that of HL
2
2
2
2
2
2
Two-photon excited fluorescence (TPEF)
in the determined wavelength range (Fig. 7b). The behaviour
observed here in the TPA spectra can be attributed to the
extending conjugation after complexation of the ligand with
Cd(II) or Zn(II) ions, in which the ICT states also play an impor-
tant role for TPA sections of the coordination compounds. The
increased TPA values were observed from Zn(II) to Cd(II)
because a larger radius of Cd(II) compared with that of Zn(II)
ion possibly gives an extended delocalization. As shown in
Table 2, the experimental results are nearly in accordance with
those from DFT calculations. The largest two-photon absorp-
tion cross-sections of the molecules are 277.25 for Cd(II)-
L (H O) , 179.40 for Zn(II)L (H O) and 11.94 for HL, respecti-
As shown in UV-vis spectra of the compounds (Fig. 3), there is
no linear absorption in the wavelength range 680–900 nm for
them, indicating that there are no energy levels corresponding
to an electron transition in this spectral range. Fig. 7a (inset
figure) shows a log–log plot of the excited fluorescence signal
versus excited light power. It provides direct evidence for the
squared dependence of excited fluorescence power and input
laser intensity upon excitation with a tunable laser in this
range, therefore, it should be safely assigned to two-photon
excited fluorescence (TPEF). As shown in Fig. 7a, the TPA
2
2
2
2
2
2
2 2 2 2 2 2
spectra of HL, Cd(II)L (H O) and Zn(II)L (H O) are deter-
vely, which is also consistent with experimental trend (σ(Cd(II)-
(H O) ) > σ(Zn(II)L (H O) ) > HL). This large TPA response
comes from the marked core to periphery charge redistribu-
tion in Cd(II)L (H O) and Zn(II)L (H O) upon photoexcitation.
Therefore, complexation with Zn(II) and Cd(II) enhances the
electron-acceptor character of the central β-diketonate group,
which converts the ligand to a more strongly polarized D–π-A
unit, making the complexes a potential candidate for TPA
responses. Moreover, the variation tendency of fluorescence
mined in the wavelength range by investigating their two-
photon excited fluorescence (TPEF) in ethyl acetate with a con-
centration of 1.0 × 10 mol L at 720 nm. It can be seen
clearly that all the two-photon excited fluorescence (TPEF)
spectra of the compounds exhibited a broad emission band,
L
2
2
2
2
2
2
−
3
−1
2
2
2
2
2
2
intensity (Cd(II)L
with the results of the fluorescence lifetime (Cd(II)L
Zn(II)L (H O) > HL) because the ligand coordinates with the
2
(H
2
O)
2
> Zn(II)L
2
(H
2
O)
2
> HL) is consistent
2
2
(H O)
2
>
2
2
2
2
+
ion M (M = Cd(II), Zn(II)). At present condition, HL was
shown very weak two-photon excited fluorescence (TPEP). The
tendency was also supported by the results of crystal structures
and XPS determinations.
Fig. 7 (a) The two-photon fluorescence spectra of HL, Cd(II)L
and Zn(II)L (H O)
2 2 2
2
(H
(1.0 × 10− M in ethyl acetate) at 720 nm. Inset: output
fluorescence (Iout) vs. the square of input laser power (Iin) for Cd(II)-
2 2
O)
3
Two-photon microscopy biological imaging application of
2
(H
ethyl acetate. (b) Two-photon (from a 140 fs, 80 MHz, Ti:sapphire laser)
absorption cross sections of HL, Cd(II)L (H O) , Zn(II)L (H O)
versus excitation wavelengths of identical energy of 0.500 W.
2 2
O)
. Excitation carried out at 720 nm, with c = 1.0 × 10− mol L in
3
−1
L
Zn(II)L
in THF As Zn has relatively low toxicity toward live cells, Zn(II)L (H O)
2
2 2 2
(H O)
2
2
2
2
2
2
2
2
was selected for its bearing high quantum yield, large TPA
Table 2 The two-photon absorption cross-section σtp (GM = 10− cm s per photon) of the six lowest exited states with the excitation energy E
50
4
(
eV) and the corresponding wavelength λtp (nm) in the gas phase
Cd(II)L (H O)
2 2 2
Zn(II)L (H O)
2 2 2
HL
E
λ
tp
σ
tp
E
λ
tp
σ
tp
E
λ
tp
σ
tp
3
3
3
3
3
3
.41
.46
.55
.55
.65
.66
725.25
714.77
696.65
696.65
677.56
675.71
0.00
277.25
2.90
0.00
0.00
3.31
3.41
3.47
3.49
3.57
3.57
747.16
725.25
712.71
708.63
692.75
692.75
0.00
12.52
0.01
179.40
0.00
7.92
3.34
3.47
3.54
3.97
4.01
4.05
740.45
712.71
698.62
622.95
616.73
610.64
3.09
11.94
1.45
0.90
4.58
0.47
31.12
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L
2
(H
2
O)
2
. Higher concentrations result in decreased cell survi-
val, with 65% and 60% viability observed following 24 h of
treatment with 20 and 40 μM Zn(II)L (H O) . As a result, cyto-
2
2
2
toxicity tests definitely indicate that the low-micromolar con-
centrations of Zn(II)L (H O) have relatively low toxic effects on
2
2
2
living cells over a period of 24 h, and Zn(II)L
great potentials for biological studies.
2 2 2
(H O) indeed has
Conclusion
Two novel coordination compounds (Cd(II)L (H O) Zn(II)-
2
2
2
L (H O) ) were synthesized and fully characterized. Both the
2
2
2
results of time-dependent density functional theory (TD-DFT)
and of the experimental studies indicated that the complexa-
Fig. 8 (a) (I) Bright-field image of HepG2 cells. (II) One-photon image
2 2 2
of HepG2 cells incubated with 20 μM Zn(II)L (H O) after 20 min of incu-
bation, washed by PBS buffer. λex = 330 nm (emission wavelength from tion of the β-diketone ligand lost a proton with metal ion
4
2
λ
40 to 499 nm). (III) Two-photon image of HepG2 cells incubated with
0 μM Zn(II)L (H O) after 30 min of incubation, washed by PBS buffer.
ex = 720 nm (emission wavelength from 495 to 523 nm). (IV) The
overlay of (a) to (c). Scale bars represent 20 μM. (b) Time series showing
luminescence intensity of Zn(II)L (H O) in a HepG2 cytosol region
extending the electronic delocalization in the coordination
compounds, leading to dramatically enhanced two-photon
absorption, especially, for the complex Cd(II)L (H O) . There-
2
2
2
2
2
2
fore, it can be drawn that the coordination compounds for
2
2
2
(
20 μM, 20 min) under laser exposure over 100 s. Inset images from time enhanced TPA properties may be a better strategy. In addition,
point 0, 30, 60 and 90 s, scale bars represent 20 μM. (c) Cytotoxicity
data results obtained from the MTT assay.
Zn(II)L (H O) shows potential as potential biological imaging
2
2
2
probe for two-photon microscopy.
cross-section and long fluorescent lifetime. Fluorescent
images of confocal microscopy and two-photon microscopy of
HepG2 cells labeled with Zn(II)L (H O) were captured. A
bright-field image (Fig. 8a(I)) of each cell was taken immedi-
ately prior to the one-photon and two-photon microscopy
Acknowledgements
2
2
2
This work was supported by grants from the National Natural
Science Foundation of China (21071001, 21271004, 51372003
and 21271003), the Natural Science Foundation of Anhui Pro-
vince (1208085MB22, 1308085MB24), Ministry of Education
Funded Projects Focus on returned overseas scholar, Program
for New Century Excellent Talents in University (China) and
Doctoral Program Foundation of Ministry of Education of
China (20113401110004).
(TPM) imaging. The green channel reveal HepG2 cells success-
fully uptake Zn(II)L (H O) , clearly, the luminescence located
2
2
2
in cell cytosol (Fig. 8(b)(II) and (III)). The luminescence which
comes from the cytosol supposes that the complex permeated
the phospholipids bilayer of cellular membrane and bounded
with cellular cytosol organelle. To further demonstrate the
potential applications of Zn(II)L (H O) for two photon
2
2
2
microscopy (TPM) imaging in living cells, HepG2 cells were
Notes and references
cultured and stained with Zn(II)L
2 2 2
(H O) . The results showed
that Zn(II)L (H O) is clearly capable of detecting the cyto-
1 M. Göppert-Mayer, Fiber Elementarakte mit xwei Quan-
temspWwgem Von Maria Göppert-Mayer, Ann. Phys., 1931,
9, 273.
2
2
2
plasm section in HepG2 cells (Fig. 8(c)(IV)). Furthermore,
Zn(II)L (H O) (20 μM) in HepG2 cells cytosol region reveals
higher stability upon laser irradiation during the period over
00 seconds (Fig. 8(b)), which reveals almost no photo-bleach-
ing. The results showed that Zn(II)L (H O) is cell-permeable
and suitable for the cytosol staining, and exhibits the high
fluorescent sensitivity relative to Zn(II)L (H O) in live cells.
2
2
2
2 W. Kaiser and C. G. Garret, Phys. Rev. Lett., 1961, 7, 229.
3 (a) B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer,
J. E. Ehrlich and L. L. Erskine, Nature, 1999, 398, 51;
(b) P. A. Bouit, G. Wetzel, G. Berginc, B. Loiseaux, L. Toupet
and P. Feneyrou, Chem. Mater., 2007, 19, 5325;
(c) S. Kawata, H. B. Sun, T. Tanaka and K. Takada, Nature,
2001, 412, 697; (d) R. J. Xia, J. P. Malval, M. Jin,
A. Spangenberg, D. C. Wan, H. T. Pu and T. Vergote, Chem.
Mater., 2012, 24, 237; (e) H. M. Kim, C. Jung, B. R. Kim,
S. Y. Jung, J. H. Hong and Y. G. Ko, Angew. Chem., Int. Ed.,
2007, 46, 3460; (f) G. S. He, L. S. Tan, Q. D. Zheng and
P. N. Prasad, Chem. Res., 2008, 108, 1245; (g) H. M. Kim
and B. R. Cho, Chem. Commun., 2009, 153; (h) L. Li,
J. Y. Ge, H. Wu, Q. H. Xu and S. Q. Yao, J. Am. Chem. Soc.,
2012, 134, 12157; (i) B. Kong, A. W. Zhu, C. Q. Ding,
1
2
2
2
2
2
2
Considering their application in intracellular imaging, the
MTT assay was performed to ascertain the cytotoxic effect of
Zn(II)L (H O) against HepG2 cells over a 24 h period. Cytotox-
2 2 2
icity is a potential side effect of dyes that must be controlled
when dealing with living cells or tissues. Fig. 8c shows the cell
2 2 2
viability for HepG2 cells treated with Zn(II)L (H O) at different
concentrations for 24 h. The results clearly indicate that
HepG2 cells incubated with concentration of 5 μm of Zn(II)-
2 2 2
L (H O) remain more than 80% viable after 24 h of feeding
time, demonstrating the superior biocompatibility of Zn(II)-
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Paper
Dalton Transactions
X. X. Zhao, B. Li and Y. Tian, Adv. Mater., 2012, 24, 5844;
j) H. Y. Ahn, K. E. Fairfull-Smith, B. J. Morrow, V. Lussini,
B. Kim, M. V. Bondar, S. E. Bottle and K. D. Belfield, J. Am.
Chem. Soc., 2012, 134, 4721.
(a) B. Wang, Y. C. Wang, J. L. Hua, Y. H. Jiang, J. H. Huang,
S. X. Qian and H. Tian, Chem.–Eur. J., 2011, 17, 2647;
7997; (c) X. C. Wang, X. H. Tian, Q. Zhang, P. P. Su,
J. Y. Wu, H. P. Zhou and B. K. Jin, Chem. Mater., 2012, 24,
954; (d) Z. J. Hu, X. H. Tian, X. H. Zhao, P. Wang, Q. Zhang
and P. P. Sun, Chem. Commun., 2011, 47, 12467; (e) Y. Ren,
X.-Q. Yu, D.-J. Zhang, D. Wang, M.-L. Zhang, G.-B. Xu,
X. Zhao, Y.-P. Tian, Z.-S. Shao and M.-H. Jiang, J. Mater.
Chem., 2002, 12, 3431.
9 (a) N. Blouin and M. Leclerc, Acc. Chem. Res., 2008, 41,
1110; (b) W. S. Gevaerts, A. Furlan, M. M. Wienk and
M. Turbiez, Adv. Mater., 2012, 24, 2130.
(
4
(
b) X. J. Li, X. H. Zhang, W. Li, Y. L. Wang, T. L. Liu,
B. L. Zhang and W. J. Yang, J. Mater. Chem., 2011, 21, 3916;
c) Y. Hai, J. J. Chen, P. Zhao, H. B. Lv, Y. Yu, P. Y. Xu and
J. L. Zhang, Chem. Commun., 2011, 47, 2435;
(
(d) S. Sumalekshmy and J. F. Christoph, Chem. Mater., 2011, 10 D. B. A. Raj, B. Francis, M. L. P. Reddy, R. R. Butorac,
2
3, 483; (e) M. G. Zhu, G. F. Zhang, C. Li and M. P. Aldred,
V. M. Lynch and A. H. Cowley, Inorg. Chem., 2010, 49,
J. Am. Chem. Soc., 2011, 133, 365; (f) N. Lin, W. X. Zhang,
9055.
B. M. Koshel, J. X. Cheng and Y. Chen, J. Phys. Chem., 2011, 11 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
15, 3198; (g) A. L. Padilha, G. Nootz, P. D. Olszak, 12 T. G. Gray, C. M. Rudzinski, E. E. Meyer, R. H. Holm and
1
S. Webster and D. J. Hagan, Nano Lett., 2011, 11, 1227;
h) S. Yao and K. D. Belfield, Eur. J. Org. Chem., 2012, 3199.
D. G. Nocera, J. Am. Chem. Soc., 2003, 125, 4755.
13 C. Xu and W. W. Webb, J. Opt. Soc. Am. B, 1996, 13, 481.
(
5
6
(a) Y. H. Jiang, Y. C. Wang, J. L. Hua, J. Tang, B. Li, 14 O. Varnavski, T. Goodson, L. Sukhomlinova and R. Twieg,
S. X. Qian and H. Tian, Chem. Commun., 2010, 46, 4689; J. Phys. Chem. B, 2004, 108, 10484.
b) Y. H. Jiang, Y. C. Wang, B. Wang, J. B. Yang, N. N. He, 15 J. R. Lakowicz, Principles of Fluorescence Spectroscopy,
(
S. X. Qian and J. L. Hua, Chem.–Eur. J., 2011, 17, 2479.
Plenum Press, New York, 1983, p. 19.
(a) G. S. He, L.-S. Tan, Q. D. Zhang and P. N. Prasad, Chem. 16 J. H. Strickler and W. W. Webb, Opt. Lett., 1991, 16, 1780.
Rev., 2008, 108, 1245–1330; (b) M. Pawlicki, H. A. Collins, 17 R. Cammi, M. Cossi and J. Tomasi, J. Chem. Phys., 1996,
R. G. Denning and H. L. Anderson, Angew. Chem., Int. Ed.,
104, 4611.
009, 48, 3244; (c) M. Albota, D. Beljonne, J.-L. Brédas, 18 Y. R. Shen, The Principles of Nonlinear Optics, Wiley,
2
J. E. Ehrlich, J.-Y. Fu, A. A. Heikal, S. E. Hess, T. Kogej,
New York, 1984.
M. D. Levin, S. R. Marder, D. McCord-Maughon, 19 (a) L. Yang, J. K. Feng and A. M. Ren, J. Org. Chem., 2005,
J. W. Perry, H. Röckel, M. Rumi, G. Subramaniam, 70, 5987; (b) GAUSSIAN 03, see http://www.guassian.com
W. W. Webb, X.-L. Wu and C. Xu, Science, 1998, 281, 1653; 20 J. Olsen and P. Jorgensen, J. Chem. Phys., 1985, 82, 3235.
(
d) S. R. Marder, Chem. Commun., 2006, 131.
21 DALTON, Release Dalton, 2011, see http://daltonprogram.
7
(a) S. S. Zhou, X. Xue, J. F. Wang, Y. Dong, B. Jiang, D. Wei
org/cm
and M. L. Wan, J. Mater. Chem., 2012, 22, 22774; 22 P. He, H. H. Wang, S. G. Liu, J. X. Shi, G. Wang and
b) B. J. Coe, J. Fielden, S. P. Foxon, B. S. Brunschwig and M. L. Gong, J. Phys. Chem. A, 2009, 113, 12885.
I. A. Selberghs, J. Am. Chem. Soc., 2010, 132, 3496; 23 G. M. Sheldrick, SADABS, University of Göttingen, Göttin-
c) C. Feuvrie, O. Maury, H. L. Bozec, I. Ledoux, gen, Germany, 1997.
(
(
J. P. Morrall, G. T. Dalton, M. Samoc and M. G. Humphrey, 24 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
J. Phys. Chem. A, 2007, 111, 8980; (d) M. P. Cifuentes, 25 G. M. Sheldrick, SHELXL-97, Program for the Refinements of
C. E. Powell, J. P. Morrall, A. M. McDonagh, N. T. Lucas
Crystal Structure, University of Göttingen, Göttingen,
and M. G. Humphrey, J. Am. Chem. Soc., 2006, 128, 10819;
Germany, 1997, p. 40.
(
e) C. K. Koo, K. L. Wong, W. Y. Man and Y. W. Lam, Inorg. 26 B. M. Kariuki, K. D. M. Harris, D. Philp and
Chem., 2009, 48, 872; (f) V. Chandrasekhar, R. Azhakar,
J. M. A. Robinson, J. Am. Chem. Soc., 1997, 119, 12679.
B. Murugesapandian and T. Senapati, Inorg. Chem., 2010, 27 Q. Zhang, W. L. Song, A. M. Showkot Hossain, Z. D. Liu,
4
9, 4008; (g) C. K. M. Chan, C. H. Tao, H. L. Tam and
G. J. Hu, Y. P. Tian, J. Y. Wu, B. K. Jin, H. P. Zhou,
N. Y. Zhu, Inorg. Chem., 2009, 48, 2855; (h) W. F. Sun,
J. X. Yang and S. Y. Zhang, Dalton Trans., 2011, 40, 3510.
B. G. Zhang, Y. J. Li, T. M. Pritchett, Z. G. Li and J. E. Haley, 28 Y. M. Berezin and S. L. Achilefu, Chem. Res., 2010, 110,
Chem. Mater., 2010, 22, 6384; (i) Z. Q. Ji, Y. J. Li, 2641.
M. P. Timothy, S. M. Nikolay and E. H. Joy, Chem.–Eur. J., 29 S. A. Patel, M. Cozzuol, J. M. Hales, C. I. Richards,
2
011, 17, 2479.
M. Sartin, J. C. Hsiang, T. Vosch, J. W. Perry and
8
(a) Y. H. Gao, J. Y. Wu, Y. M. Li, P. P. Sun, H. P. Zhou,
R. M. Dickson, J. Phys. Chem. C, 2009, 113, 20264.
J. X. Yang, S. Y. Zhang, B. K. Jin and Y. P. Tian, J. Am. Chem. 30 S. M. Ji, J. Yang, Q. Yang, S. S. Liu, M. D. Chen and
Soc., 2009, 131, 5208; (b) D. M. Li, X. H. Tian, G. J. Hu, J. Z. Zhao, J. Org. Chem., 2009, 74, 4855.
Q. Zhang, P. Wang and P. P. Sun, Inorg. Chem., 2011, 50, 31 M. Irie and K. Sayo, J. Phys. Chem., 1992, 96, 7671.
608 | Dalton Trans., 2014, 43, 599–608
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