Electrochemiluminescent dinuclear Ru(II) complexes assembled with 1,1′-(1,2-ethynediyl)- or dimethlyene-bridged bis(bipyridine) ligands: Synthesis and photophysical and electrochemical properties

Article abstract of DOI:10.1016/j.ica.2012.10.028

1,2-Di(2,2′-bipyridin-5-yl)ethane (BL1) and 1,2-di(2,2′- bipyridin-5-yl)ethyne (BL2) were synthesized as new bridging ligands and coordinated to (RuL₂(acetone)₂)(PF₆) ₂ for the preparation of various [Ru(L)₂(BL)Ru(L) ₂](PF₆)₄-type dinuclear ruthenium complexes (where BL = BL1, BL2 and L = bpy, o-phen, DTDP). The electrochemical redox potentials, spectroscopic properties, and relative electrochemiluminescence intensity of BL1 and BL2 were characterized and compared to those of well-known tris(1,10-phenanthroline)rutheniun(II) [Ru(o-phen)₃](PF ₆)₂] complex as a reference. Dinuclear Ru(II) complexes containing the conjugated bridging ligand (BL2) showed much more intense electrochemiluminescent responses than dinuclear Ru(II) complexes with the non-conjugated bridging ligand (BL1). Among the complexes with conjugated bridging ligands, [(DTDP)₂Ru(bpy-CC-bpy)Ru(DTDP)₂](PF ₆)₄ exhibited enhanced ECL intensities as high as 3.6 times greater than that of the reference, [Ru(o-phen)₃](PF ₆)₂.

Full text of DOI:10.1016/j.ica.2012.10.028

Inorganica Chimica Acta 395 (2013) 145–150
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Inorganica Chimica Acta
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Electrochemiluminescent dinuclear Ru(II) complexes assembled with
1,10-(1,2-ethynediyl)- or dimethlyene-bridged bis(bipyridine) ligands: Synthesis
and photophysical and electrochemical properties
a,
Minki Kim a,1, Chang Hoon Kang b,1, Subong Hong a, Won-Yong Lee b, , Byeong Hyo Kim
⇑
⇑
a Department of Chemistry, Kwangwoon University, Seoul 139-701, Republic of Korea
b Department of Chemistry and Center for Bioactive Molecular Hybrids, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
1,2-Di(2,20-bipyridin-5-yl)ethane (BL1) and 1,2-di(2,20-bipyridin-5-yl)ethyne (BL2) were synthesized as
new bridging ligands and coordinated to (RuL2(acetone)2)(PF6)2 for the preparation of various [Ru(L)2
(BL)Ru(L)2](PF6)4-type dinuclear ruthenium complexes (where BL = BL1, BL2 and L = bpy, o-phen, DTDP).
The electrochemical redox potentials, spectroscopic properties, and relative electrochemiluminescence
intensity of BL1 and BL2 were characterized and compared to those of well-known tris(1,10-phenanthro-
line)rutheniun(II) [Ru(o-phen)3](PF6)2] complex as a reference. Dinuclear Ru(II) complexes containing the
conjugated bridging ligand (BL2) showed much more intense electrochemiluminescent responses than
dinuclear Ru(II) complexes with the non-conjugated bridging ligand (BL1). Among the complexes with
conjugated bridging ligands, [(DTDP)2Ru(bpy-CC-bpy)Ru(DTDP)2](PF6)4 exhibited enhanced ECL intensi-
ties as high as 3.6 times greater than that of the reference, [Ru(o-phen)3](PF6)2.
Article history:
Received 9 July 2012
Received in revised form 24 October 2012
Accepted 31 October 2012
Available online 15 November 2012
Keywords:
Electrochemiluminescence
Dinuclear ruthenium complexes
Bridging ligands
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
plexes in a single molecular framework, such as tri- to hepta-
ruthenium metallodendrimers and di-ruthenium complexes,
Since the first report of tris(2,20-bipyridyl)ruthenium(II)
(Ru(bpy)32+) as an electrochemiluminescent material [1], the elec-
trochemiluminescence (ECL) emitted by transition metal com-
plexes has been recognized as a powerful tool. It has been
applied for the analysis of a wide range of compounds such as oxa-
lates, alkylamines, amino acids, NADH, organic acids, and pharma-
ceutical compounds. This concept is also used for immunoassay
and DNA probe assays by employing ECL-active species as labels
on biological molecules [2].
The inherent molecular structure of central metal, ligand, and
functional group attached on the ligand is the most important fac-
tor for determining ECL characteristics. In addition to using various
ligands and central metals, several research groups have investi-
gated the use of multi-metallic systems containing multiple redox
centers in a molecule [3]. Further improvements can possibly be
made if multi-metallic complexes can exist within a single molec-
ular framework, even though the ECL intensity may not be propor-
tional to the number of metals. We previously examined the
application of multi-nuclear metallic complexes for improving
ECL efficiency and found that some multi-nuclear metallic com-
exhibited improved ECL intensities compared to simple mono-
ruthenium complexes [4]. In our previous studies, we used poly-
pyridyl ligands connected with polyamidoamine [4a], amide [4b],
or ester groups [4c,d], in which spacers are bridged with extended
single bonds. Most spacers that we previously used were extended
with single bonds that can rotate freely, indicating that the metal-
lic site of multi-nuclear complexes may possibly affect the ECL
properties of complexes. We are interested in how ECL properties
will change if the free rotation of the spacer is restricted using a
proper spacer. For example, the 1,2-di(2,20-bipyridin-5-yl)ethyne
ligand will be ligated with metals in two positions opposite of
the spacer without dangling around the spacer. The 1,2-di(2,20-
bipyridin-5-yl)ethyne ligand can not only fix the position of the
spacer, but can also influence the degree of bipyridyl ligand conju-
gation. This affects the extent of metal-to-ligand charge transfer
(MLCT) or localized p–
p⁄ transition. It is of great interest to design
ECL materials that are connected with 1,2-di(2,20-bipyridin-5-
yl)ethyne ligands and ligated with other proper ligands (Fig. 1).
These ECL materials are expected to show improved ECL proper-
ties. Herein, we report the synthesis and physical properties of
newly designed di-ruthenium complexes connected with 1,2-
di(2,20-bipyridin-5-yl)ethyne ligand. We also synthesized di-ruthe-
nium complexes connected with 1,2-di(2,20-bipyridin-5-yl)ethane
ligand to compare physical properties. Novel dinuclear ruthenium
complexes, [Ru(L)2(BL)Ru(L)2](PF6)4 (BL = bridging ligand, L = bpy,
⇑
Corresponding authors. Tel.: +82 2 2123 2649 (W.-Y. Lee), tel.: +82 2 940 5247;
fax: +82 2 942 4635 (B.H. Kim).
E-mail addresses: wylee@yonsei.ac.kr (W.-Y. Lee), bhkim@kw.ac.kr (B.H. Kim).
These authors contributed equally to this work.
1
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.10.028

146
M. Kim et al. / Inorganica Chimica Acta 395 (2013) 145–150
2.2.2. Synthesis of 1,2-di(2,20-bipyridin-5-yl)ethane (6) [9]
5-Methyl-2,20-bipyridine (0.600 g, 3.5 mmol) was dissolved in
THF (20 mL degassed under N2) in a 250 mL round-bottomed flask.
1.8 M LDA (4 mL, 7 mmol) was used to added dropwise using a
cannula at ꢁ78 °C. After stirring at ꢁ78 °C for 2 h under N2, 5-(bro-
momethyl)-2,20-bipyridine (868 mg, 3.5 mmol) dissolved in a de-
gassed THF solution (20 mL) was added dropwise. After warming
to room temperature and stirring for 24 h, the mixture was
quenched with a saturated aqueous NaCl (20 mL) solution. The or-
ganic solvent was mostly evaporated and the remaining was ex-
tracted with CH2Cl2. The organic layer was dried over MgSO4 and
concentrated by rotary evaporator to give a crude product. The
coupling product was purified by flash column chromatography
(silica gel, MeOH/CH2Cl2 = 1/9). Yield: 67% (792 mg), TLC (SiO2):
Rf 0.57 (10% methanol/dichloromethane); IR (KBr, cmꢁ1) 3051
(Ar–H), 3006 (Ar–H), 2916 (C–H), 2856 (C–H), 1458 (aromatic
C@C), 1H NMR (400 MHz, CDCl3) d 3.04 (d, J = 4.0 Hz, 4H, CH2),
7.27–7.31 (m, 2H, PyH), 7.58–7.61 (m, 2H, PyH), 7.78–7.82 (m,
2H, PyH), 8.29–8.36 (m, 4H, PyH), 8.48–8.49 (m, 2H, PyH), 8.65–
8.67 (m, 2H, PyH); MS/EI m/z 338 [M+, 51%], 169 [M+ꢁCH2bpy,
100%], 141 [M+ꢁNCH2CH2bpy, 20%].
Fig. 1. UV–Vis absorption spectra of synthesized dinuclear ruthenium complexes.
o-phen, DTDP), containing new ethynyl or ethylene bridging li-
gands (BL) were synthesized in this study and their structures were
characterized. The electrochemical redox potentials, spectroscopic
properties, and relative electrochemiluminescence intensities of
the complexes were also investigated in detail.
2. Experimental
2.2.3. Synthesis of 5-[(trimethylsilyl)ethynyl]-2,20-bipyridine (7) [10]
(Trimethylsilyl)acetylene (0.400 g, 4.07 mmol), [PdCl2(PPh3)2]
(0.123 mg, 0.18 mmol), CuI (0.052 g, 0.27 mmol), and 6 mL of tri-
ethyl amine were added in sequence to 5-bromo-2,20-bipyridine
(0.400 g, 1.71 mmol) in 30 mL of THF (degassed under N2) in a
100 mL round-bottomed flask equipped with a septum. The solu-
tion was stirred at room temperature for 24 h. During that time,
the solution turned black with the formation of an abundant pre-
cipitate of the salt. After complete consumption of the starting
material (determined by TLC), the mixture was treated with acti-
vated carbon (ca. 200 mg) and filtered over Celite. The filtrate
was concentrated by rotary evaporator to give a crude product,
which was purified through flash silica gel column chromatogra-
phy using CH2Cl2 as eluent. Yield: 80% (345 mg), TLC (SiO2): Rf
0.45 (20% ethyl acetate/hexane); IR (KBr, cmꢁ1) 3049 (Ar–H),
3006 (Ar–H), 2960 (Ar-H), 2898 (C–H), 2160 (C„C), 1H NMR
(400 MHz, CDCl3) d 7.00–7.03 (m, 1H, PyH), 7.50–7.59 (m, 2H,
PyH), 8.00–8.12 (m, 2H, PyH), 8.38–8.39 (m, 1H, PyH), 8.44–8.45
(m, 1H, PyH); MS/EI m/z 252 [M+, 38%], 237 [M+ꢁCH3, 100%], 221
[M+ꢁ2(CH3)H, 7%], 207 [M+ꢁ3(CH3), 5%].
2.1. Materials and instrumentation
Most chemical reagents were purchased from Aldrich Chemical
Co. (St. Louis, Missouri, USA) and were used as received without
further purification. All the reactions were carried out under a
dry nitrogen atmosphere, unless stated otherwise. Solvents were
dried using the standard method. 5-Carboxylate-, 5-bromo-, and
5-methyl- substituted 2,20-bipyridine [5], 5-(hydroxymethyl)-
2,20-bipyridine (bpy-(CH2OH)) [6], 1,3-dihydro-1,1,3,3-tetra-
methyl-7,8-diazacyclopenta[1]phenanthren-2-one (DTDP) [4d],
and cis-Ru(L)2Cl2ꢀ2H2O [7] (L: bpy, o-phen, DTDP) were prepared
using known literature methods.
1H NMR spectra were recorded on a 400 MHz (Jeol, Tokyo,
Japan), and chemical shifts were reported in ppm relative to the
residual solvent as an internal standard. GC/MS was recorded on
an HP 5973 mass spectrometer connected with HP 6890 GC
(Hewlett–Packard Co., Palo Alto, California, USA) and MALDI-TOF
mass spectra were recorded on a JMS-DX303 (Jeol, Tokyo, Japan).
Infrared spectra (IR) were recorded on a MB104 FT-IR (ABB Bomen
Inc., Zurich, Switzerland) and UV–Vis spectra were recorded on a
S-3100 (Scinco, Seoul, Korea). Emission spectra were obtained with
the use of a luminescence spectrometer LS 50B (excitation source
at 400 nm; Perkin Elmer, Waltham, Massachusetts, USA). Flow
injection analysis (FIA) was performed with the previously described
ECL detection system [8].
2.2.4. Synthesis of 5-ethynyl-2,20-bipyridine (8) [10]
5-[(Trimethylsilyl)ethynyl]-2,20-bipyridine (0.360 g, 1.43 mmol)
was dissolved in 30 mL of THF (degassed under N2). A solution of
KOH (0.320 g, 5.72 mmol) in methanol (30 mL) was then added.
The reaction was stirred at room temperature for 6 h and the
solvent was removed under vacuum. The residue was purified
through flash silica gel column chromatography using CH2Cl2 as
eluent. Yield: 95% (245 mg), TLC (SiO2): Rf 0.45 (10% methanol/
dichloromethane); IR (KBr, cmꢁ1) 3298 (C„C–H), 3190 (C„C–H),
3049 (Ar–H), 3001 (Ar–H), 2964 (Ar–H), 2933 (Ar–H), 2096
(C„C), 1H NMR (400 MHz, CDCl3) d 3.29 (s, 1H, C„CH), 7.26–
7.33 (m, 1H, PyH), 7.79–7.84 (m, 1H, PyH), 7.88–7.90 (m, 1H,
PyH), 8.37–8.41 (m, 2H, PyH), 8.67–8.69 (m, 1H, PyH), 8.77 (m,
1H, PyH); MS/EI m/z 180 [M+, 100%] 153 [M+ꢁNCH, 48%], 126
[M+ꢁ2(NCH), 9%].
2.2. Synthesis
2.2.1. Synthesis of 5-bromomethyl-2,20-bipyridine (5) [5]
The 5-(hydroxymethyl)-2,20-bipyridine (0.837 g, 4.5 mmol) was
dissolved in a mixture of 48% HBr (1 mL) and concentrated sulfuric
acid (2 mL). The resulting solution was refluxed for 6 h and then al-
lowed to cool to room temperature, after which 10 mL of water
was added. The pH was adjusted to neutral with NaOH solution
and the resulting precipitate was filtered, washed with water (pH
7), and air-dried. The product was dissolved in chloroform and fil-
tered. The solution was dried over magnesium sulfate and evapo-
rated to dryness. Yield: 48% (536 mg), TLC (SiO2): Rf 0.21 (20%
ethyl acetate/hexane); IR (KBr, cmꢁ1) 3039 (Ar–H), 2999 (Ar–H),
2968 (C–H), 1460 (aromatic C@C), 649 (Ar–C), 1H NMR
(400 MHz, CDCl3) d 4.54 (s, 2H, CH2Br), 7.30–7.33 (m, 1H, PyH),
7.80–7.86 (m, 2H, PyH), 8.39–8.41 (m, 2H, PyH), 8.68 (m, 2H,
PyH); MS/EI m/z 248 [M+, 8%], 169 [M+ꢁBr, 100%], 141 [M+ꢁNCH2-
Br, 21%].
2.2.5. Synthesis of 1,2-di(2,20-bipyridin-5-yl)ethyne (9) [10]
For the cross-coupling reaction leading to 5-ethynyl-2,20-bipyr-
idine (0.150 g, 0.83 mmol), the 5-bromo-2,20-bipyridine (0.200 g,
0.83 mmol) was dissolved in benzene at 80 °C. When a clear solu-
tion was obtained, [Pd(PPh3)4] (0.092 g, 0.08 mmol) and 5 mL of
triethylamine were added. After 24 h of heating at 80 °C, the sol-
vent was removed under vacuum and the residue was purified

M. Kim et al. / Inorganica Chimica Acta 395 (2013) 145–150
147
by flash column chromatography (silica gel, MeOH/CH2Cl2 = 1/9).
Yield: 48% (133 mg), TLC (SiO2): Rf 0.28 (10% methanol/dichloro-
methane); IR (KBr, cmꢁ1) 3053 (Ar-H), 3008 (Ar–H), 2923 (Ar–H),
2850 (Ar–H), 1456 (aromatic C@C), 798 (Ar–C), 1H NMR
(400 MHz, CDCl3) d 7.32–7.35 (m, 2H, PyH), 7.82–7.86 (m, 2H,
PyH), 7.97–8.00 (m, 2H, PyH), 8.43–8.46 (m, 4H, PyH), 8.70–8.71
(m, 2H, PyH), 8.85–8.86 (m, 2H, PyH); HRMS: m/z calc. for
(acetone) 627 nm, IR (KBr, cmꢁ1) 3085 (Ar–H), 2925 (Ar–H), 2850
(Ar–H), 2225 (C„C), 1699 (aromatic C@C), 1465 (aromatic C@C),
840 (Ar–C), 1H NMR (400 MHz, acetone-d6) d 7.44–7.47 (m, 1H,
PyH), 7.70–7.74 (m, 2H, PyH), 7.93–7.99 (m, 2H, PyH), 8.01–8.03
(m, 1H, PyH), 8.14–8.18 (m, 3H, PyH), 8.20–8.24 (m, 2H, PyH),
8.36–8.42 (m, 4H, PyH), 8.50–8.51 (m, 1H, PyH), 8.65–8.66 (m,
1H, PyH), 8.72–8.74 (m, 2H, PyH), 8.82–8.86 (m, 4H, PyH); MAL-
DI-TOF: m/z 1718 [M+ꢁPF6+Na+3H].
C22H14N4: calc. for 336.1375, found 336.1369.
2.2.6. General procedure for syntheses of dinuclear ruthenium
complexes
2.2.6.6. [(DTDP)2Ru(bpy-CC-bpy)Ru(DTDP)2](PF6)4 (18). Yield: 44%
(100 mg), UV (acetone) kmax (e
) 436 nm (26,200 Mꢁ1 cmꢁ1); PL
(acetone) 590 nm, IR (KBr, cmꢁ1) 3110 (Ar–H), 2974 (Ar–H), 2935
(Ar–H), 2873 (Ar–H), 1967 (C„C), 1602 (aromatic C@C), 1467 (aro-
matic C@C), 840 (Ar–C), 1H NMR (400 MHz, acetonitrile-d3) d 1.61–
1.73 (m, 48H, CH3), 7.29–7.32 (m, 2H, PyH), 7.53–7.62 (m, 6H, PyH),
7.71–7.81 (m, 10H, PyH), 7.94–7.96 (m, 2H, PyH), 8.03–8.10 (m, 4H,
PyH), 8.20–8.21 (m, 2H, PyH), 8.48–8.52 (m, 4H, PyH), 8.74–8.77
(m, 4H, PyH), 8.84–8.88 (m, 4H, PyH); MALDI-TOF: m/z 2137
[M+ꢁPF6+4H].
AgPF6 (202 mg, 0.8 mmol) was added to a solution of cis-Ru(L)2-
Cl2ꢀ2H2O (L = bpy, o-phen, DTDP, 0.4 mmol) dissolved in acetone
(10 mL, degassed under N2), and the mixture was stirred for 2 h
at room temperature. After reaction completion, AgCl was removed
by filtration. After acetone evaporation, the residue was dissolved
in DMF (5 mL), a bridging ligand (BL = bpy-CH2CH2-bpy, bpy-CC-
bpy, 0.1 mmol) was added to the solution, and the mixture was
heated for 3 h at 90 °C. After cooling to room temperature, the res-
idue was treated with a saturated aqueous solution of NH4PF6,
which gave a red precipitate. The solid was filtered and recrystal-
lized from acetone/ethyl acetate. Red crystals were obtained in
32–52% yield.
2.3. Electrochemical and ECL measurements
Cyclic voltammetric experiments were performed with an EG &
G 273A potentiostat (Oak Ridge, TN, USA). A conventional three-
electrode system was employed with a platinum wire as counter
electrode, glassy carbon (0.07 cm2) electrode as a working elec-
trode, and an Ag/AgCl (3 M NaCl) reference electrode. The photon
counting system used was a Hamamatsu Photonics HC 135-02
photon counting module (Hamamatsu city, Japan) in conjunction
with a computer for recording the output. The electrochemical cell
was also used in the ECL experiments. The ECL cell was placed di-
rectly in front of the photomultiplier tube (PMT) window. Prior to
the electrochemical and ECL experiments, the working electrode
2.2.6.1. [(bpy)2Ru(bpy-CH2CH2-bpy)Ru(bpy)2](PF6)4 (13). Yield: 39%
(68 mg), UV (acetone) kmax (e
) 430 nm (21,200 Mꢁ1cmꢁ1); PL (ace-
tone) 581 nm, IR (KBr, cmꢁ1) 3118 (Ar–H), 3087 (Ar–H), 2923 (C–
H), 1708 (C@C), 1604 (aromatic C@C), 1465 (aromatic C@C), 1446
(aromatic C@C), 840 (Ar–C), 761 (Ar–C), 1H NMR (400 MHz, ace-
tone-d6) d 2.63 (s, 4H, CH2), 7.38–7.39 (m, 10H, PyH), 7.48 (m,
2H, PyH), 7.65–7.73 (m, 12H, PyH), 8.03–8.07 (m, 10H, PyH),
8.33–8.35 (m, 2H, PyH), 8.43–8.49 (m, 10H, PyH); MALDI-TOF:
m/z 1601 [M+ꢁPF6].
was polished with 0.05 lm alumina, sonicated, and rinsed with
2.2.6.2.
[(o-phen)2Ru(bpy-CH2CH2-bpy)Ru(o-phen)2](PF6)4
428 nm
methanol followed by water. Ru(II) complex solution and tripro-
pylamine (TPA) solutions were prepared in the same 50 mM pH
7.0 phosphate buffer containing acetonitrile (v/v, 80%). TPA solu-
tions (1 mM) were mixed with 0.5 mM synthesized Ru(II) complex
solutions (1:1 v/v) and also blank solutions were prepared by mix-
ing the given concentration of Ru(II) complex solution and the
same buffer (1:1 v/v) without TPA. During the course of the ECL
measurement, the potential of the working electrode was cycled
from 0.7 V to +1.4 V with a scanning rate of 100 mV/s. ECL mea-
surements were also performed for blank solutions in all studies.
Corrected ECL signals were obtained by subtracting the ECL signals
for blank solutions from the observed ECL signals for TPA.
(14). Yield: 32% (59 mg), UV (acetone) kmax
(e)
(25,400 Mꢁ1 cmꢁ1); PL (acetone) 629 nm, IR (KBr, cmꢁ1) 3085
(Ar–H), 2923 (Ar–H), 1703 (C@C), 1469 (aromatic C@C), 1429 (aro-
matic C@C), 838 (Ar–C), 721 (Ar–C), 1H NMR (400 MHz, acetone-
d6) d 2.54 (s, 2H, CH2), 7.38–7.42 (m, 1H, PyH), 7.68–7.77 (m, 3H,
PyH), 7.81–7.82 (m, 1H, PyH), 7.88–7.99 (m, 3H, PyH), 8.13–8.20
(m, 3H, PyH), 8.34–8.49 (m, 6H, PyH), 8.63–8.65 (m, 1H, PyH),
8.70–8.75 (m, 3H, PyH), 8.80–8.85 (m, 2H, PyH); MALDI-TOF: m/z
1697 [M+ꢁPF6ꢁ3H].
2.2.6.3. [(DTDP)2Ru(bpy-CH2CH2-bpy)Ru(DTDP)2](PF6)4 (15). Yield:
52% (119 mg), UV (acetone) kmax (e
) 426 nm (22,000 Mꢁ1 cmꢁ1);
PL (acetone) 587 nm, IR (KBr, cmꢁ1) 3110 (Ar–H), 2974 (Ar–H),
2935 (Ar–H), 2873 (C–H), 1749 (C@C), 1708 (C@O), 1602 (aromatic
C@C), 1467 (aromatic C@C), 842 (Ar-C), 727 (Ar-C), 1H NMR
(400 MHz, acetone-d6) d 1.62–1.77 (m, 48H, CH3), 2.79 (s, 4H,
CH2), 7.37–7.40 (m, 2H, PyH), 7.73–7.85 (m, 9H, PyH), 7.92–8.01
(m, 5H, PyH), 8.11–8.25 (m, 6H, PyH), 8.40–8.48 (m, 4H, PyH),
8.66–8.68 (m, 2H, PyH), 8.75–8.77 (m, 2H, PyH), 8.97–9.02 (m,
4H, PyH), 9.10–9.13 (m, 4H, PyH); MALDI-TOF: m/z 2137 [M+ꢁPF6].
3. Results and discussion
3.1. Synthesis
To examine the influence of the bridging ligands on ECL proper-
ties, new dinuclear ruthenium complexes that are covalently con-
nected with 1,10-(1,2-ethynediyl)- or dimethylene-bridged
bis(bipyridine) ligands were designed and synthesized. While a
dimethylene-bridged spacer is more freely rotated between the
bipyridine ligands, 1,2-ethynediyl-bridged spacers can restrict
the free rotation of the spacer and extend the degree of bipyridyl
ligand conjugation in the spacer. ECL properties may be affected
accordingly.
Multi-step synthesis of these two bridging ligands was carried
out as outlined in Scheme 1. The first bridging ligand, dimethyl-
ene-bridged bis(bipyridine) ligand 6, was synthesized using
modified literature methods. This ligand’s spacer has a flexible
geometry because of the compound’s dimethylene unit. First,
2.2.6.4. [(bpy)2Ru(bpy-CC-bpy)Ru(bpy)2](PF6)4 (16). Yield: 47%
(82 mg), UV (acetone) kmax (e
) 441 nm (24,800 Mꢁ1 cmꢁ1); PL (ace-
tone) 579 nm, IR (KBr, cmꢁ1) 3120 (Ar–H), 3083 (Ar–H), 2962 (Ar–
H), 2925 (Ar–H), 2867 (Ar–H), 1992 (C„C), 1604 (aromatic C@C),
1465 (aromatic C@C), 838 (Ar–C), 1H NMR (400 MHz, acetone-d6)
d 7.53–7.62 (m, 5H, PyH), 7.97–8.06 (m, 5H, PyH), 8.12–8.30 (m,
7H, PyH), 8.77–8.82 (m, 6H, PyH); MALDI-TOF: m/z 1597 [M+ꢁPF6].
2.2.6.5. [(o-phen)2Ru(bpy-CC-bpy)Ru(o-phen)2](PF6)4 (17). Yield:
41% (75 mg), UV (acetone) kmax (e
) 438 nm (24,200 Mꢁ1 cmꢁ1); PL

148
M. Kim et al. / Inorganica Chimica Acta 395 (2013) 145–150
CO2Me
(1)
(2)
CO2Me
N
Cl
N
N
1 (80%)
or
(a)
+
SnMe3
N
N
N
Br
Br
N
or
2 (59%)
Br
Br
(3)
N
N
N
3 (58%)
BL1
(b)
(c)
CO2Me
(4)
N
N
Br
N
N
N
N
OH
1
4 (51%)
5 (48%)
(d)
N
N
N
N
(5)
N
N
2
6 (67%) BL1
BL2
(f)
(e)
+
Br
H
TMS
H
TMS
N
N
N
N
N
N
3
7 (80%)
8 (95%)
(g)
N
N
(6)
N
N
9 (48%) BL2
Scheme 1. Synthesis of bridging ligands, BL1 and BL2. (a) (PPh3)2PdCl2, m-Xylene, reflux, 10 h; (b) NaBH4, EtOH, reflux, 4 h; (c) 48% HBr, H2SO4, rt, 6 h; (d) 1) 1.8 M LDA, THF,
ꢁ78 °C, 2 h, 2) 5, THF, ꢁ78 °C ? rt, 24 h; (e) (PPh3)2PdCl2, CuI, Et3N, THF, rt, 24 h; (f) KOH, MeOH, THF, rt, 7 h; (g) 3, Pd(PPh3)4, Et3N, benzene, reflux, 24 h.
5-bromomethyl-2,20-bipyridine (5) was prepared from 2,20-bipyri-
dine-5-carboxylate (1) (Eq. 4, Scheme 1). 2,20-Bipyridine-5-carbox-
ylate (1) was reacted with NaBH4/EtOH to obtain precursor 5-
(hydroxymethyl)-2,20-bipyridine (4). Then, 4 was transformed into
5 by treatment with 48% HBr and H2SO4 at room temperature. Lith-
iated 5-methyl-2,20-bipyridine (2) was added to the THF solution of
5 at ꢁ78 °C. The temperature was then raised to room temperature
and the solution was stirred for 24 h. As a result, bridging ligand
(BL1), 1,2-di(2,20-bipyridin-5-yl)ethane (6) was obtained in rea-
sonable yield (Eq. 5, Scheme 1).
For the coordination reactions of bridging ligands to the Ru me-
tal shown in Scheme 2, dinuclear ruthenium complexes [Ru(L)2
(BL)Ru(L)2](PF6)4 (L = bpy, o-phen, DTDP) 13–18 were obtained in
reasonably good yields. This was done by the complexation of
bridging ligands 6 and 9 to [cis-Ru(L)2(acetone)2]2+(L = bpy, o-phen,
DTDP), 10–12 in DMF at 90 °C. The final products, 13–18, were
fully characterized with 1H NMR, IR, UV–Vis, and MALDI-TOF mass
spectra.
3.2. Absorption and emission spectra of dinulear Ru(II) complexes
A three step process was performed to synthesize the second
bridging ligand (BL2), 1,2-di(2,20-bipyridin-5-yl)ethyne (9), which
has geometry fixed with a triple bond. At room temperature, 3
was coupled with (trimethylsilyl)acetylene in the presence of
PdCl2(PPh3)2, CuI, and diisopropylamine in THF to produce 5-[(tri-
methylsilyl)ethynyl]-2,20-bipyridine (7) in 80% yield. 1,2-Di(2,20-
bipyridin-5-yl)ethyne (9) was obtained by desilylation of 7–8 using
KOH and MeOH in THF. This was followed by a coupling reaction
with 3 in the presence of Pd(PPh3)4 and diisopropylamine in ben-
zene at reflux (Eq. 6, Scheme 2).
Absorption and photoluminescence emission spectral data of
the newly synthesized dinuclear Ru(II) complexes, 13–18, were
obtained at the concentration of 0.5 ꢂ 10ꢁ4 M in acetone. Metal-
to-ligand charge transfer (MLCT) transitions were observed be-
tween 426 and 441 nm depending upon the bridging ligand and
the terminal ligand in monomeric Ru(II) complexes (Fig. 1). The
maximum absorption wavelengths of dinuclear Ru(II) complexes
containing conjugated 1,2-di(2,20-bipyridin-5-yl)ethyne bridging
ligand (BL2) (16, 17, and 18) were red shifted about 10 nm

M. Kim et al. / Inorganica Chimica Acta 395 (2013) 145–150
149
10 L = bpy
(a)
(b)
+
RuCl3 xH2O
2L
Ru(L)2Cl2 2H2O
[Ru(L)2(acetone)2](PF6)2
11 L = o-phen
12 L = DTDP
4+
L
L
Ru
N
N
-
(c)
4PF6
BL
N
N
N
N
BL
6, BL1
9, BL2
N
N
O
Ru
L
L
L =
13, BL1, L = bpy (39%)
14, BL1, L = -phen (32%) 17, BL2, L =
15, BL1, L = DTDP (52%) 18, BL2, L = DTDP (44%)
16, BL2, L = bpy (47%)
N
N
bpy
N
o
N
-phen
N
DTDP
N
o
o
-phen (41%)
Scheme 2. Synthesis of various bridged dinuclear ruthenium complexes. (a) DMF, reflux, 10 h; (b) AgPF6, acetone, rt, 2 h; (c) 1) 10, 11, or 12, DMF, 90 °C, 3 h; 2) aq. NH4PF6.
700
13
14
15
16
17
18
800
600
400
200
0
600
500
400
300
200
100
0
-100
500
550
600
650
700
750
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. PL data of synthesized dinuclear ruthenium complexes.
compared to those of dinuclear Ru(II) complexes with non-conju-
gated dimethlyene-bridged bis(bipyridine) bridging ligand (BL1)
(13, 14, and 15). These results indicate that the degree of bridging
in the range of 60–100 mV. The ratios of anodic to cathodic cur-
rents, Ipa/Ipc, were close to 1.0, confirming the quasi-reversible re-
dox behavior of newly synthesized dinuclear Ru(II) complexes. In
addition, all the dinuclear Ru(II) complexes showed single quasi-
reversible waves. This result indicates that the Ru(II) centers in
dinuclear Ru(II) complexes are electrochemically equivalent.
Although all the dinuclear Ru(II) complexes exhibited similar
electrochemical properties, as shown in the CV data in Table 1,
the half-wave potentials were slightly different depending on the
bridging ligand and the ligand in each monomeric Ru(II) complex.
For dinuclear Ru(II) complexes containing o-phen and DTDP li-
gands in a monomeric Ru(II) complex, the BL2 bridging ligand
caused the oxidation potential of dinuclear Ru(II) complexes to
shift cathodically compared to those obtained with a BL1 bridging
ligand. This result indicates that the BL2 bridging ligand with a tri-
ple bond increases the electron density of the resulting Ru(II) com-
plex relative to that of a Ru(II) complex containing the BL1 bridging
ligand with a single bond. The donor abilities of dinuclear Ru(II)
complexes with a BL2 bridging ligand are stronger than those of
Ru(II) complexes with a BL1 bridging ligand, leading to a cathodic
potential shift.
ligand conjugation affects the extent of MLCT or localized
p–
p⁄
transition. The maximum absorption wavelengths of dinuclear
Ru(II) complexes with the same bridging ligand seems to be af-
fected mainly by the nature of the terminal ligands in monomeric
Ru(II) complexes. Thus, their absorption bands are slightly differ-
ent within a 5 nm range.
However, the maximum PL wavelengths of BL2 bridged dinu-
clear Ru(II) complexes containing a DTDP terminal ligand are
slightly red-shifted compared to the BL1 bridged dinuclear Ru(II)
complexes. Other BL2 bridged dinuclear Ru(II) complexes contain-
ing bpy and o-phen are slightly blue-shifted compared to the BL1
bridged dinuclear Ru(II) complexes (Fig. 2).
3.3. Electrochemical and ECL characteristics
Cyclic voltammograms of dinuclear Ru(II) complexes, 13–18,
were obtained at the concentration of 0.5 mM prepared in a
mixture solution (1:1, v/v) of 50 mM phosphate buffer (pH 7)
and acetonitrile. All the dinuclear Ru(II) complexes showed quasi-
reversible one-electron processes for Ru(II)/Ru(III) oxidation–
reduction with half-wave potentials (E1/2 = (Epa + Epc)/2) within
1.13 – 1.31 V versus Ag/AgCl (3 M NaCl), as summarized in Table 1.
The ECL characteristics of newly synthesized dinuclear Ru(II)
complexes were studied in the presence of tripropylamine as a
co-reactant for the Ru(II) ECL oxidation–reduction system. Tripro-
pylamine was used in this study because it is the best character-
ized among a variety of coreactants [2]. ECL emissions were
Peak separation,
DEp, between the anodic and cathodic peaks was

150
M. Kim et al. / Inorganica Chimica Acta 395 (2013) 145–150
Table 1
UV, PL, CV and ECL data of synthesized dinuclear ruthenium complexes.
Entry
Ru(II) cpd
BLn
UV (nm)a
PL (nm)a
Epa (V)b
Epc (V)b
ECLc
1
2
3
4
5
6
[(bpy)2Ru(BL1)Ru(bpy)2](PF6)4(13)
[(o-phen)2Ru(BL1)Ru(o-phen)2](PF6)4(14)
[(DTDP)2Ru(BL1)Ru(DTDP)2](PF6)4(15)
[(bpy)2Ru(BL2)Ru(bpy)2](PF6)4(16)
[(o-phen)2Ru(BL2)Ru(o-phen)2](PF6)4(17)
[(DTDP)2Ru(BL2)Ru(DTDP)2](PF6)4(18)
BL1:bpy-(CH2CH2)-bpy
430
428
426
441
438
436
581
629
587
579
627
590
1.16
1.36
1.36
1.22
1.20
1.20
1.09
1.26
1.26
1.16
1.14
1.14
0.2
0.1
0.1
1.2
1.2
3.6
BL2:bpy-(C„C)-bpy
a
b
c
In acetone (0.5 ꢂ 10ꢁ4 M).
Measured in acetonitrile/50 mM phosphate buffer (pH 7.0) at a glassy carbon electrode vs Ag/AgCl (3 M NaCl).
ECL relative to [Ru(o-phen)3](PF6)2.
obtained for each dinuclear Ru(II) complex upon sufficiently
sweeping the positive potential to simultaneously oxidize the com-
plex and tripropylamine in static solution systems. The dinuclear
Ru(II) complexes containing the conjugated bridging ligand (BL2)
showed a much more intense electrochemiluminescent response
than the dinuclear Ru(II) complexes with the non-conjugated
bridging ligand (BL1). In addition, the ECL intensities of dinuclear
Ru(II) complexes with the conjugated bridging ligand (BL2) were
stronger than those of the reference [Ru(o-phen)3](PF6)2 complex.
As summarized in Table 1, the ECL intensities of the dinuclear
ruthenium(II) complexes, 16 and 17, were around 20% higher than
that of the complex. Among the newly synthesized complexes, 18
shows the strongest electrochemiluminescent intensity. Its ECL
intensity was 3.6 times higher than that of the reference complex.
However, the ECL intensities of dinuclear Ru(II) complexes with
the non-conjugated ligand (BL1) were much weaker than that of
the reference complex. The ECL intensity of dinuclear ruthe-
nium(II) complex 13 was 20% that of the reference complex. Also,
ECL intensity for the dinuclear ruthenium(II) complexes 14 and
15 was only 10% that of the reference complex. These results indi-
cate that the nature of the bridging ligand in dinuclear Ru(II) com-
plexes strongly affects the ECL response of resulting dinuclear
Ru(II) complexes. The conjugated bridging ligand used in the
present study can hinder free rotation of the metallic sites of the
dinuclear complexes and increase the degree of conjugation of
the bipyridyl ligand. This affects the extent of metal-to-ligand
conjugated BL1 bridging ligand (13, 14, 15). The ECL intensities
of the dinuclear Ru(II) complexes were affected not only by the
kind of ligand with in monomeric Ru(II) complexes, but also by
the nature of the bridging ligand. Due to their very strong ECL
intensity, newly prepared dinuclear ruthenium(II) complexes (18)
could be utilized as sensitive ECL materials for the biochemical
analysis of a variety of bioactive amine compounds and substrates
that can produce NADH in dehydrogenase enzyme reactions.
Acknowledgements
Financial support from Kwangwoon University in 2012 is
acknowledged. The Basic Science Research Program through the
National Research Foundation of Korea (NRF), funded by the Min-
istry of Education, Science and Technology (2011-0008829), also
supported this research.
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