Synthesis, characterization and electrochemiluminescent properties of cyclometalated platinum(ii) complexes with substituted 2-phenylpyridine ligands

Article abstract of DOI:10.1039/c2dt32466k

Two neutral cyclometalated platinum(ii) complexes, Pt(DPP)(acac) and Pt(BPP)(acac) (DPP = 2,4-diphenylpyridine, BPP = 2-(4-tert-butylphenyl)-4- phenylpyridine, acac = acetylacetone), have been synthesized and characterized by ¹H NMR spectroscop

Full text of DOI:10.1039/c2dt32466k

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Synthesis, characterization and
electrochemiluminescent properties of cyclometalated
platinum(II) complexes with substituted
2-phenylpyridine ligands†
Cite this: Dalton Trans., 2013, 42, 4059
Chunxiang Li,*a Shuqin Wang,a Yangmei Huang,a Bin Zheng,a Ziqi Tian,c
Yonghong Wena and Feng Li*b
Two neutral cyclometalated platinum(II) complexes, Pt(DPP)(acac) and Pt(BPP)(acac) (DPP = 2,4-diphenyl-
pyridine, BPP = 2-(4-tert-butylphenyl)-4-phenylpyridine, acac = acetylacetone), have been synthesized and
characterized by 1H NMR spectroscopy, mass spectrometry, elemental analyses and by X-ray crystallogra-
phy for Pt(DPP)(acac). Electrogenerated chemiluminescence (ECL) of the two complexes in the absence
or presence of coreactant tri-n-propylamine (TPrA) in different solvents (CH3CN, CH2Cl2, DMF, CH3CN/
H2O (V, 50 : 50)) has been studied. The ECL spectra are identical to their own PL spectra, indicating that
ECL processes lead to the same metal-to-ligand charge-transfer (3MLCT) excited state that is generated
by light excitation. The ECL potentials of Pt(DPP)(acac) and Pt(BPP)(acac)/TPrA in CH3CN and CH3CN/H2O
solution were at ∼0.75 V vs. SCE, and significantly negatively shifted by about 0.6 V compared to that of
the Ru(bpy)32+/TPrA system. The ECL quantum efficiencies of the complexes are comparable to that of
the Ru(bpy)32+/TPrA system. The significant increase of the ECL signal in the coreactant system is due to
the formation of the strongly reducing intermediate TPrA˙. It is noteworthy that the ECL efficiencies of
the synthesized compounds are much higher than that of the tridentate polypyridyl ligands.
Received 17th October 2012,
Accepted 18th December 2012
DOI: 10.1039/c2dt32466k
www.rsc.org/dalton
a number of organometallic complexes such as Al(III), Re(I), Ir
(III), Os(II), Eu(II) and Pt(II) complexes have been studied as new
Introduction
Electrogenerated chemiluminescence (ECL) has now become a ECL reagents due to their thermal and photochemical stability
very powerful analytical technique and been widely used in and high photoluminescence.8–13
immunoassays, DNA analyses and food and water testing,
Recently, luminescent square-planar platinum(II) complexes
because of its inherent features, such as low background, high have gained major attention because of their extensive appli-
sensitivity, good reproducibility and selectivity.1–4 These appli- cations in many fields, such as chemosensors, photocatalysis
cations are predominantly based on the fundamental ECL and organic light-emitting diodes (OLEDs) in view of their
research work of a variety of ECL-active reagents. The design rich photoluminescence properties.14–18 The strong spin–orbit
and synthesis of new excellent ECL luminophores have coupling of the heavy metal atom allows for efficient inter-
attracted considerable interest since Bard et al. reported the system crossing (ISC) between singlet and triplet states, which
first ECL from the Ru(bpy)3(tris-(2,2′-bipyridyl) ruthenium(II))/ can lead to a high quantum yield of emission from the triplet
tri-n-propylamine (TPrA) system.3,5–7 In the past several years, state. Meanwhile, the electrochemistry properties of plati-
num(II) complexes have been studied, which revealed well-defined
redox behaviors.19,20 Effective ECL from the homoleptic
aState Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and
Pt(Thpy)2 complex (Thpy = 2-(2-thienyl)-2-pyridine) has also
Molecular Engineering, Qingdao University of Science and Technology, Qingdao
been demonstrated by Balzani et al. in coreactant systems.21
266042, PR China. E-mail: lichunxiang76@yahoo.com.cn; Tel: +86-532-84022681
bCollege of Chemistry and Pharmacy, Qingdao Agricultural University, 700
Changcheng Road, Qingdao 266109, PR China. E-mail: lifeng@qust.edu.cn;
Fax: +86-532-86080213; Tel: +86-532-86080213
Recent studies on the ECL of platinum(II) complexes were
focused on complexes containing tridentate polypyridyl
ligands,12,22–26 while the ECL properties of heteroleptic (C^N)-
Pt(LX) complexes (C^N represents a cyclometalating ligand
and LX represents an ancillary ligand) have not been
studied so far, although many exhibit more excellent
cSchool of Chemistry and Chemical Engineering, Nanjing University, Nanjing,
210093, PR China
†CCDC reference number 901672. For crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt32466k
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water and were subsequently reacted with 3 equiv. of the acetyl-
acetone ancillary ligand and 10 equiv. of Na2CO3 in 2-ethoxy-
ethanol at 120 °C for 16 h. The solvent was removed under
reduced pressure, and the compound was purified by flash
chromatography using dichloromethane. Subsequently, the
product was recrystallized with dichloromethane/methanol.
1
Pt(DPP)(acac), yellowish green, yield: 38%. H NMR (CDCl3,
300 MHz) δ: 9.02 (d, J = 6.1 Hz,1H), 7.81 (s, 1H), 7.74 (d, J =
5.7 Hz, 2H), 7.67 (d, J = 7.2 Hz, 2H), 7.53–7.56 (m, 4H), 7.24
(m, 1H), 7.15 (t, 1H), 5.50 (s, 1H), 2.0 (s, 6H). MS (EI) m/z:
523.8 (8.11, M+), 424.2 (9.73), 230.7 (45.10), 153.8 (4.65), 84.8
(30.11), 42.9 (100). Anal. Calcd for C22H19NO2Pt: C, 50.38; H,
3.65; N, 2.67. Found: C, 50.69; H, 3.78; N, 2.55. IR (KBr, cm−1):
3436 (vw), 3040 (vw), 1618 (s), 1561 (s), 1514 (s), 1480 (m), 1389
(s), 1270 (m), 1020 (w).
Scheme 1 Synthesis of Pt(DPP)(acac) and Pt(BPP)(acac).
photoluminescence and more tailorable properties than other
platinum(II) complexes.
Pt(BPP)(acac), yellow, yield: 45%. 1H NMR (CDCl3,
300 MHz): δ 8.97 (d, J = 6.2 Hz, 1H), 7.69–7.76 (m 5H), 7.55 (d,
J = 7.0 Hz, 3H), 7.48 (d, J = 8.2 Hz, 1H), 7.19 (d, J = 8.1 Hz, 1H),
5.50 (s, 1H), 2.0 (s, 6H), 2.4 (s, 9H). MS (EI) m/z: 580.0 (7.69,
M+), 272.0 (5.94), 84.9 (16.7), 63.9 (22.5), 43.9 (100). Anal.
Calcd for C26H27NO2Pt: C, 53.78; H, 4.68; N, 2.41. Found:
C, 53.69; H, 4.78; N, 2.35. IR (KBr, cm−1): 3444 (w), 3047 (w),
2954 (m), 2859 (w), 1619 (m), 1579 (s), 1517 (m), 1403 (m),
1270 (m), 1022 (w).
In this paper, two neutral cyclometalated heteroleptic plati-
num(II) complexes, Pt(DPP)(acac) and Pt(BPP)(acac) (DPP = 2,4-
diphenylpyridine, BPP = 2-(4-tert-butylphenyl)-4-phenylpyri-
dine, acac = acetylacetone) (as shown in Scheme 1), were syn-
thesized and characterized. Their photophysical properties
and ECL characteristics in the absence or presence of coreac-
tant TPrA were evaluated. The ECL intensity of the platinum(II)
complexes/TPrA system was found to be a function of the TPrA
concentration and is affected by experimental factors, such as
the nature of the solvent and pH. The possible pathway for the
ECL of the platinum(II) complexes/TPrA system was also
proposed.
X-ray crystallography
A suitable crystal of the Pt(DPP)(acac) complex was obtained
by slow evaporation of the CH2Cl2–isobutanol solution at
room temperature. Single crystal X-ray diffraction data of the
complex were collected using a Bruker SMART 1000 CCD dif-
factometer with graphite monochromated Mo Kα radiation (λ =
0.71073 Å) using the ω scan mode at 298(2) K. Intensity data
were corrected for Lp factors and empirical absorption. The
structure was solved by direct methods and expanded by using
Fourier differential techniques with SHELXTL.30 All non-hydro-
gen atoms were located with successive difference Fourier
syntheses. The hydrogen atoms were geometrically fixed and
allowed to ride on the parent atoms to which they are attached.
The structure was refined by full-matrix least-squares method
on F2 with anisotropic thermal parameters for all non-
hydrogen atoms. Atomic scattering factors and anomalous
dispersion corrections were taken from international tables for
X-ray crystallography.31
Experimental
Materials
The cyclometalated ligands 2,4-diphenylpyridine (DPP) and
2-(4-tert-butylphenyl)-4-phenylpyridine (BPP) were prepared
according to the literature procedure.27,28 K2PtCl4, tetra-
n-butylammoniumhexafluorophosphate (Bu4NPF6), tripropyl-
amine (TPrA) and 2-ethoxyethanol were purchased from
Aldrich and used without further purification. Other chemicals
and solvents were all of reagent grade for synthesis and used
as received. Solvents were purified and distilled using standard
procedures. All the aqueous solutions were prepared with de-
ionized water (Milli-Q, Millipore). The pH of the phosphate
buffer solution (PBS, 0.1 M) was adjusted with concentrated
NaOH or hydrochloric acid.
Density functional calculations
Synthesis
All calculations for Pt(DPP)(acac) were performed with
The synthesis of the platinum(II) complexes was carried out in Gaussian09 program package.32a Density functional theory
an inert gas atmosphere despite the air stability of the com- B3LYP32b,c including Becke’s three-parameter-exchange func-
plexes, the main concern being the oxidative and thermal stabi- tional and the Lee–Young–Parr correlation functional was used
lity of intermediates at the high temperatures in the reactions. for optimizations of the geometries of the ground state, with
The Pt(II)-dichloro-bridged dimers were prepared using a the standard 6-31+G(d) basis sets for C, H, O and N, and
modified method of that reported by Lewis et al.29 This LANL2DZ32d–f for Pt. Following the optimizations, time depen-
involves heating the K2PtCl4 salt with 2–2.2 equiv. of cyclome- dent density functional theory (TDDFT) calculations at the
talating ligand (DPP or BPP) in a 3 : 1 mixture of 2-ethoxyetha- B3LYP/6-31+G(d)/LANL2DZ level were carried out to evaluate
nol and water to 80 °C for 16 h. The dimers were isolated in the UV-Vis absorption spectrum. At the same level, the
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geometry of the first excited triplet (T1) state are optimized to determine10,35), Is and Ir are the integrated ECL intensities of
2+
investigate phosphorescence emission.
the Pt complexes and the reference Ru(bpy)3 under the same
conditions, respectively.
Physical measurements
1H NMR spectra were measured using a Bruker ARX-300
(300 MHz) spectrometer, using TMS as the internal standard.
Mass spectra (MS) were measured using a VG-ZAB-HS spec-
trometer with electron impact ionization. Elemental analysis
was performed using a PerkineElmer 240C elemental analyzer.
Infrared spectra were recorded using a Bruker Vector 22 with
KBr pellet. The UV-Vis spectra were recorded using a VARIAN
Cary 5000 spectrometer. Phosphorescence spectra were
recorded using a PerkineElmer LS 50B luminescence spectro-
photometer. Quantum efficiency measurements were carried
out using an optically diluted CH3CN solution (OD < 0.05 at
the excitation wavelength) and calibrated with degassed (ppy)-
Pt(acac) in 2-methyltetrahydrofuran (Φem = 0.15) as a refer-
ence.33 Eqn (1) was used to calculate quantum yields,
Results and discussion
Synthesis and characterization
As depicted in Scheme 1, the required cyclometalated DPP
and BPP ligands were synthesized by a reaction of the
corresponding aracylpyridinium bromide, cinnamaldehyde
and ammonium acetate in glacial acetic acid with moderate
yields.27,28 The platinum(II) complexes were prepared by a con-
ventional two-step method according to the literature.36 Treat-
ment with K2PtCl4 in a mixture of 2-ethoxyethanol and H2O
gave dichloro-bridged orthometalated dimers. Subsequent
treatment of the corresponding dimers with the ancillary acetyl-
acetone ligand in the presence of a base yielded the desired
platinum(II) complexes upon purification by column chromato-
graphy and crystallization. The obtained complexes were satis-
η2s ArIs
1
factorily characterized by H NMR, elemental analysis, IR and
Φs ¼ Φr
ð1Þ
η2AsIr
r
mass spectroscopy, and Pt(DPP)(acac) was also characterized
by X-ray crystallography.
where Φs is the quantum yield of the sample, Φr is the
quantum yield of the reference, η is the refractive index of the
solvent, As and Ar are the absorbance of the sample and
the reference at the wavelength of excitation, and Is and Ir are
the integrated areas of emission bands.
A single crystal of Pt(DPP)(acac) was grown from a dichloro-
methane/isobutyl alcohol solution and characterized using
X-ray crystallography. A summary of the key crystallographic
data and structural refinements for the complex is presented
in Table 1. Selected bond distances and angles are illustrated
in Table 2. The molecular structure of the complex with the
atomic numbering scheme is shown in Fig. 1. The complex
crystallizes in the monoclinic space group P21/c. The molecules
pack as head-to-tail dimers, each molecule of the dimer
related to the other by a center of inversion. The dimers have a
Electrochemical measurements were performed using the
CH Instruments model 660D electrochemical workstation.
Anhydrous CH3CN was used as the solvent and 0.1 M Bu4NPF6
was used as the supporting electrolyte. A three-electrode
system was employed with platinum wire as the auxiliary elec-
trode, Ag/Ag+ as the reference electrode, and a Au disk (2 mm
diameter) as the working electrode. The working electrodes
were polished with 0.3 and 0.05 μm alumina slurry to obtain a
mirror surface and then solicited and thoroughly rinsed with
ultrapure deionized water. To eliminate the influence of
oxygen, all solutions were deaerated by the bubbling of high-
purity (99.995%) nitrogen. All potentials reported were against
the SCE reference with the argotic system using ferrocene as
an internal standard (E°Fc/Fc+ = 0.424 V vs. SCE).34 The ECL
signals were measured using the ECL analysis system for the
multi-detector and an Electrochemical Analyzer (Xi’an ruimai
Analytical Instruments Co., Ltd.) at room temperature. The Au
disk (2 mm diameter) was employed as working electrode. The
relative ECL efficiencies of Pt(DPP)(acac) and Pt(BPP)(acac)
were estimated by comparing the integrated light intensity of
the present system to that of the Ru(bpy)32+ system under iden-
tical experimental conditions, and the electrical charges
through the complexes and standard system were taken as the
same. ECL efficiency (ΦECL) was obtained according to eqn (2),
Table 1 Crystal data and structure refinement information of the complex
Pt(DPP)(acac)
Formula
Fw
Crystal system
Space group
T (K)
a (Å)
b (Å)
C22H19NO2Pt
524.47
Monoclinic
P21/c
298.15
12.6018(15)
11.4459(14)
13.9239(17)
90
111.159(2)
90
1873.0(4)
4
1.86
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
Dc (g cm−3
)
M (mm−1
)
7.507
Crystal size (mm)
θ range (°)
0.32 × 0.26 × 0.24
3.46–52
10 048
3678
3678/237
1.061
Reflections collected
Unique reflections
Data/parameters
GOF on F2
Is
Ir
ΦECL ¼ Φr
ð2Þ
Ra, wRb (all data)
Ra, wRb [I > 2σ(I)]
0.0654, 0.1087
0.0458, 0.1050
2+
where Φr is the ECL efficiency of Ru(bpy)3 (taken as 1 since
a R = ∑||Fo| − |Fc||/∑|Fo|. b wR = [∑w(Fo2 − Fc2)/∑w(Fo2)2]1/2
.
2+
the absolute value of the Ru(bpy)3 efficiencies are difficult to
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Table 2 Selected bond lengths (Å) and angles (°) for the complex
Pt(1)–O(1)
Pt(1)–O(2)
C(1)–C(2)
C(1)–Pt(1)–O(1)
C(1)–Pt(1)–N(1)
N(1)–Pt(1)–O(2)
2.001(6)
2.098(7)
1.342(13)
92.6(3)
82.7(4)
92.8(3)
Pt(1)–N(1)
Pt(1)–C(1)
C(1)–C(6)
O(1)–Pt(1)–O(2)
C(1)–Pt(1)–O(2)
N(1)–Pt(1)–O(1)
1.965(7)
1.963(10)
1.456(13)
92.0(2)
175.3(3)
175.2(3)
Fig. 3 The dimer of the complex showing ring–metal interaction.
Fig. 4 The UV-Vis absorption and photoluminescence spectra of Pt(DPP)(acac)
and Pt(BPP)(acac) (10−5 M) in CH3CN at room temperature. Excitation wave-
lengths: 289 nm for Pt(DPP)(acac) and 320 nm for Pt(BPP)(acac).
Fig. 1 ORTEP diagram of the complex Pt(DPP)(acac) showing the atom label-
ing scheme.
N(1)–Pt(1)–O(2) being 360.1°. In the complex, DPP is a biden-
tate ligand which forms a stable five-membered ring. Three
aromatic rings are essentially planar, and the dihedral angles
between pyridine and the two benzene rings are 2.5(4) and
31.0(5)°, respectively. In addition, intramolecular C(11)–
H(11)⋯O(2) [C⋯O = 2.99(1) Å] hydrogen bonding exists in
the complex.
Absorption and fluorescent emission
The absorption spectra along with phosphorescence emission
spectra of the complexes Pt(DPP)(acac) and Pt(BPP)(acac) were
recorded at room temperature (shown in Fig. 4), and the data
are summarized in Table 3. The two complexes have similar
electronic absorption spectra. Low energy transitions in the
absorption spectrum between 300 and 400 nm are assigned as
metal-to-ligand charge transfer (MLCT) transitions and, to a
certain extent, the intra-ligand state (IL), as already observed
for comparable compounds.16,36,40–42 The intense bands (ε =
3.4–6.2 × 104 M−1 cm−1) observed at higher energy 200–300 nm
can be described as spin-allowed, ligand-centered (LC) π→π*
in nature. The two complexes show a strong green emission in
CH3CN at room temperature, with nearly identical emission
spectra. This is attributed to the presence of an excited-state
resulting from the mixing of comparable percentages of 3LC
and 3MLCT states, which is in agreement with Thompson’s
work concerning analogous platinum complexes.36 Strong
spin–orbit coupling of central metal atoms facilitates the spin-
forbidden 3MLCT transitions of the metal complexes. The
average value of ca. 500 nm for the emission maxima is red
Fig. 2 ORTEP diagram showing π–π stacking of the complex.
plane-to-plane separation of 3.333 Å indicative of moderate π–π
stacking interactions (Fig. 2). The distance between the pyri-
dine ring centroid and Pt is 3.585 Å which is indicative of
ring–metal interactions (Fig. 3). There are no metal–metal
interactions, the closest Pt–Pt distance being 4.683 Å. The
central ion Pt(II) is four-coordinated by two oxygen atoms from
acac, one nitrogen atom and one carbon atom from the DPP
ligand. All coordinated bond lengths (Table 2) show normal
values and are comparable to those in the related cyclometa-
lated Pt(II) complexes.36–39 The C(1)–C(2) bond length (1.34 Å)
is shorter than the normal benzene ring C–C bond length,
while the C(1)–C(6) bond length (1.46 Å) is longer than normal
benzene ring C–C bond length. This is due to the coordination
of C(1) with the Pt(II) ion. The coordination geometry around
Pt is best described as square planar, with the sum of the
angles C(1)–Pt(1)–O(1), O(1)–Pt(1)–O(2), C(1)–Pt(1)–N(1) and
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DFT calculations
Table 3 Absorption and emission data of Pt(DPP)(acac) and Pt(BPP)(acac)
b
Density functional theory (DFT) and time dependent DFT
(TDDFT) calculations are performed to investigate the photo-
physical properties of the complex Pt(DPP)(acac). B3LYP and
cam-B3LYP functionals were both tested with the standard
6-31+G(d) basis sets for the light elements and LANL2DZ for
Pt. The results obtained at the B3LYP/6-31+G(d)/LANL2DZ level
reproduced the UV-VIS absorption spectrum better than using
cam-B3LYP, thus in the following discussion, only the B3LYP
results will be discussed. Some features of the important fron-
tier orbitals are displayed in Fig. 5. The transition from T1 to
S1 state is mainly contributed with the LUMO → HOMO
(72.7%) transition. The HOMO consists of a mixture of the
phenyl from DPP (54.7%), Pt (33.6%), and acac (11.7%) orbi-
tals, while the LUMO is predominantly DPP (93.5%) in charac-
ter. The results clearly indicate that the observed emission has
mostly LC character with a mixture of MLCT. The calculated
phosphorescence line from T1 → S0 is at 544 nm, which is in
good agreement with the experimental measured emission
energies.
Complex
λabs/nma(ε × 10−4/M−1 cm−1
)
λem/nm
495, 527
499, 533
Φem
Pt(DPP)(acac)
Pt(BPP)(acac)
248(3.4), 283(3.5), 334(0.96),
370(0.98)
251(5.3), 268(5.3)286(6.2),
334(1.8), 368(1.5)
0.17
0.21
a Absorption measurements of the complexes were carried out in
anhydrous CH3CN. b Values obtained in thoroughly degassed CH3CN
using degassed (ppy)Pt(acac) in 2-methyltetrahydrofuran (Φem = 0.15)
as a reference.
Electrochemistry and electrochemiluminescence
The electrochemical properties of the complexes were deter-
mined by cyclic voltammetry (CV) using Bu4NPF6 (0.1 M) as
the supporting electrolyte in anhydrous CH3CN solution. All
electrochemical data vs. SCE for the two complexes are col-
lected in Table 4. Both of the complexes described here show a
quasi-reversible reduction wave at −1.80 V vs. SCE, an irrevers-
ible reduction wave at about −2.30 V, and irreversible oxidation
Fig. 5 Contour plots of the three highest occupied molecular orbitals (HOMOs)
and the three lowest unoccupied molecular orbitals (LUMOs) of complex
Pt(DPP)(acac).
shifted by ca. 20 nm from that of the similar complex [Pt(ppy)-
(acac)]. This is due to the enlarged conjugation system of the wave near 0.75 V (shown in Fig. 6). It is generally considered
ligands and corresponds to the lower π* orbital (LUMO) energy that the oxidation of this species can be mainly attributed to
of the substituted phenylpyridine ligand compared to that of
the metal center, with a substantial contribution from the
the ppy ligand which results in a decrease in the energy gap ligands. Since square planar Pt(I) and Pt(II) metal centers are
between HOMO and LUMO.
susceptible to nucleophilic attack by solvents, the Pt(II) redox
processes are usually irreversible.44 The reduction process are
The introduction of the t-butyl group makes the Φem of Pt-
(BPP)(acac) higher than that of Pt(DPP)(acac), which is pro- mainly localized on the C^N ligands, with only a partial contri-
posed to be due to a decrease of self-quenching. It is note- bution from the metal center.45
worthy that both of the Φem for the synthesized complexes,
The ECL performances of the complexes using a 10 μM
0.19 for Pt(DPP)(acac) and 0.21 for Pt(BPP)(acac), are substan- sample were studied in CH3CN, DMF and CH2Cl2, respectively,
tially greater than that of Ru(bpy)32+ (Φem = 0.095 in CH3CN43). and the results are collected in Table 4. A stable ECL of the
Table 4 Electrochemical and ECL properties of the complexes
Electrochemistry (V vs. SCE)a
Electrochemiluminescence
c
e
Rel ΦECL
Coreactant System (TPrA) Rel ΦECL
Oxd
pa
Red b
1/2
Complex
E
E
CH3CN
DMF
CH2Cl2
CH3CN
CH3CN–PBS (50 : 50, V)
d
d
d
d
Pt(DPP)(acac)
Pt(BPP)(acac)
+ 0.79
+ 0.75
−1.77
−1.79
−2.26
−2.29
0.19
0.21
0.87
0.91
0.86
0.89
a All potentials were determined at room temperature in deaerated DMF solutions (0.1 M Bu4NPF6) vs. SCE at scan rate of 100 mV s−1
.
b E1/2 refers
c
to [(Epa + Epc)/2] where Epa and Epc are the anodic and cathodic peak potentials. ΦECL were calculated with respect to the ECL efficiency of Ru-
2+
2+
(bpy)3 and the electrical charges through the systems were taken as the same (the ΦECL of Ru(bpy)3 was taken as 1) in CH3CN. d Too weak to
be measured. e Relative ECL efficiencies with respect to Ru(bpy)32+/TPrA and the electrical charges through the systems were taken as the same
(the ΦECL of Ru(bpy)3 was taken as 1), ECL solutions contained 10 μM complex and 25 mM TPrA. Reported values were averaged from at least
five scans with a relative standard deviation of ∼7%.
2+
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Fig. 8 ECL intensity-potential curves of 10 μM Pt(DPP)(acac), Pt(BPP)(acac) and
2+
Ru(bpy)3 in CH3CN containing 0.1 M Bu4NPF6; (A) in the absence of TPrA; (B)
in the presence of 25 mM TPrA.
Fig. 6 Successive cyclic voltammograms of the complexes (1 mM) in anhydrous
CH3CN containing 0.1 M Bu4NPF6 as the supporting electrolyte at the Au disk
electrode. Scan rate, 100 mV s−1. (a) Pt(DPP)(acac); (b) Pt(BPP)(acac); (c) the
blank cyclic voltammogram in CH3CN containing 0.1 M Bu4NPF6 under the
same experimental conditions.
Fig. 9 ECL spectra of the complexes using a 10 μM sample in deaerated
CH3CN solution: Bu4NPF6 as supporting electrolyte, TPrA as oxidative reductive
co-reactant, scan range: 0–1.5 V.
ECL spectra of the complexes were quite similar to their own
emission spectrum obtained on photoexcitation (Fig. 9 and 4),
indicating that the same MLCT states are probably formed in
Fig. 7 ECL intensity-time curves of 10 μM Pt(DPP)(acac), scanning voltage
range from 0.0 to 0.80 V. (a) in CH3CN solution containing 25 mM TPrA and
0.1 M Bu4NPF6 (b) in CH3CN–PBS (PH = 7.5, 50 : 50 ,V) and (c) in CH3CN solution
containing 0.1 M Bu4NPF6.
3
both experiments.21,46 The ECL intensity peaks of the complex
Pt(DPP)(acac) and Pt(BPP)(acac) with TPrA as the coreactant
appear at potentials of 0.76 and 0.77 V vs. SCE, respectively, as
shown in Fig. 8(B). Compared with that of [Ru(bpy)3]2+/TPrA,
the potential for a redox couple of the two complexes is shifted
by ∼0.6 V towards a more negative potential. A lower redox
potential is helpful to decrease background interference. The
potentials of the maximum ECL intensity of the Pt(BPP)(acac)/
TPrA system are slightly shifted by about 0.01 V towards a
more negative potential compared with those of the Pt(DPP)-
(acac)/TPrA system, which is consistent with that obtained by
cyclic voltammogram scans.
It is found that the ECL intensities of Pt(DPP)(acac) and
Pt(BPP)(acac) in CH3CN depend on the concentration of the
coreactant. As shown in Fig. 10, the ECL intensities of the two
complexes noticeably increase with an increase in the concen-
tration of TPrA up to 25 mM and then decrease slowly upon a
further increase of the TPrA concentration. The ECL intensity
changes of the two complexes with different TPrA concen-
trations are in agreement with those found in the [Ru(bpy)3]2+/
TPrA coreactant system.47,48
complex Pt(DPP)(acac) and Pt(BPP)(acac) solution containing
0.1 M Bu4NPF6 was observed in CH3CN when the potential of
the Au working electrode was pulsed from 0 to 1.5 V (Fig. 7(c)
for Pt(DPP)(acac) as an example), while in DMF and CH2Cl2
media, the ECL emission of the two complexes is unobserved
or very weak, which indicates that DMF and CH2Cl2 can inten-
sively quench the ECL emission. Therefore, the following
investigation for ECL of the two complexes was carried out in
CH3CN solution. The ECL intensity peaks of the complex
Pt(DPP)(acac) and Pt(BPP)(acac) appear at potentials of 1.19
and 1.20 V vs. SCE, respectively, as shown in Fig. 8(A). Com-
pared with that of [Ru(bpy)3]2+, the potential for a redox couple
of the two complexes is shifted by ∼0.26 V towards a more
negative potential.
The coreactant system ECL also has been studied using a
10 μM sample of the complexes in CH3CN at the Au working
electrode with TPrA as the coreactant. A significant increase of
the ECL signal of the two complexes was observed in presence
of TPrA, as listed in Fig. 7(a) taking the complex Pt(DPP)(acac)
as an example. When the potential is scanned from 0 to 1.5 V
for 10 times, the ECL signals of the Pt(DPP)(acac) and Pt(BPP)-
(acac)/TPrA systems are almost unchanged, indicating these
new ECL systems could be used as novel ECL reagents. The
To further estimate whether these types of compounds can
be applied in immunoassays and DNA analysis, the ECL
performances in mixed CH3CN–H2O (50 : 50, v/v) solutions
were also studied. Strong and stable ECL appeared as observed
4064 | Dalton Trans., 2013, 42, 4059–4067
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generation of luminescence from Pt complexes solutions on
pulsing the potential from −2.0 to 1.25 V can be explained by
the following reactions:
Pt þ eꢀ ! Ptꢀ˙
Pt ꢀ eꢀ ! Ptþ˙
Ptꢀ˙ þ Ptþ˙ ! Pt ꢁ þPt
ð3Þ
ð4Þ
ð5Þ
ð6Þ
Ptꢁ ! Pt þ hν
where Pt represents the Pt(DPP)(acac) or Pt(BPP)(acac)
complex. When TPrA is employed as a coreactant, the emitter
complex (Pt) is oxidized to Pt+˙ at an electrode and then inter-
acts with TPrA˙, which is irreversibly generated by the oxi-
dation and decomposition of TPrA at the electrode, to receive
an electron and forms an excited-state species, as shown in the
following reaction sequences:
Fig. 10 ECL intensity changes of 10 μM Pt(DPP)(acac) or Pt(BPP)(acac) with
different concentrations of the TPrA coreactant at a Au working electrode.
Pt ꢀ eꢀ ! Ptþ˙
TPrA ꢀ eꢀ ! TPrAþ˙
TPrAþ˙ ! TPrA˙ þ Hþ
TPrA˙ þ Ptþ˙ ! Pt ꢁ þTPrA
Ptꢁ ! Pt þ hν
ð4Þ
ð7Þ
ð8Þ
ð9Þ
ð10Þ
Fig. 11 ECL intensity changes of 10 μM Pt(DPP)(acac) or Pt(BPP)(acac) with
different pH at a Au working electrode. TPrA (25 mM) in 0.10 M PBS solution
In the coreactant system, the intense ECL is ascribed to the
strong reducing power of TPrA˙ (E(TPrA˙) = −1.7 V), reducing
Pt+˙ directly into Pt*.51 The generated cation radicals of
TPrA˙+ (eqn (5)) play key roles in light emission since the
deprotonation process of TPrA˙+ to TPrA˙ (eqn (7)) can occur
extremely quickly and the oxidation of the emitter complexes
(eqn (2)) could occur relatively fast.
(50% MeCN). Scan rate, 100 mV s−1
.
in pure CH3CN (listed in Fig. 7(b)) with only a slight decrease.
The ECL emission is also pH dependent in 50 : 50 (v/v)
CH3CN–PBS solutions, and maximum intensities were
observed at pH ∼8 for Pt(DPP)(acac) and Pt(BPP)(acac) (shown
2+
in Fig. 11). Similar trends are observed for the Ru(bpy)3 co-
reactant system.4,49 This is important for potential applications
since the pH of the environmental and biological systems is
∼7.4 and would require less sample preparation prior to analy-
sis. It also suggests that both ruthenium- and platinum-based
complexes can be used in the same sample solution for multi-
analyst ECL determination.
The ECL quantum efficiencies of the Pt(DPP)(acac) and
Pt(BPP)(acac)/TPrA coreactant systems in different media were
measured using Ru(bpy)32+/TPA (ΦECL was taken as 1) as the
standard, and the data were given in Table 4. Under the
same experimental conditions, the ECL quantum efficiency
of Pt(BPP)(acac) (Rel ΦECL = 0.91) is slightly higher than that of
Pt(DPP)(acac) (Rel ΦECL = 0.87), which are comparable to that
of the Ru(bpy)32+/TPrA system.
Conclusions
Two neutral cyclometalated platinum(II) complexes bearing a
substituted 2-phenylpyridine and a β-diketone, have been syn-
thesized and characterized. The photophysical and electro-
chemical behaviors have been investigated. The ECL
characteristics in the absence or presence of coreactant TPrA
in different solvents have also been investigated. Stable and
intense ECL for the Pt(II) complexes has been observed with
TPrA as a coreactant at reasonable voltage and pH levels. The
ECL potentials of Pt(DPP)(acac) and Pt(BPP)(acac) in CH3CN
and CH3CN/H2O solution were at ∼0.75 V vs. SCE, and signifi-
cantly negatively shifted by about 0.6 V compared to that of
the Ru(bpy)32+/TPrA system. Moreover, the ECL quantum
efficiency of Pt(BPP)(acac) is slightly higher than that of
ECL mechanism
It is clear from Fig. 4 and 9 that the ECL spectra of the com- Pt(DPP)(acac) and approximates to that of the Ru(bpy)32+/TPrA
plexes/TPrA system are almost identical with their PL spectra, system. The fact that the electrochemical and ECL behavior of
2+
indicating that the excited state obtained on photoexcitation is the Pt(II) complexes are very similar to those of Ru(bpy)3
/
also generated in the ECL experiment. By analogy with the TPrA indicates that the synthesized Pt(II) complexes can be an
2+
2+
well-known Ru(bpy)3 system and other platinum(II) com- alternative for conventional Ru(bpy)3 complexes. They could
plexes,21,50 in the absence of the redox coreactant, the be used as novel ECL luminophores which are used in many
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Foundation of China (No. 21205065 and 20971076), Postdoc-
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the Natural Science Foundation of Shandong Province
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