A new dinucleating bridging ligand 1,2-bis(2-(1H-imidazo[4,5-f]-[1,10] phenanthrolin-2-yl)phenoxy)ethane and its dinuclear ruthenium(II) complexes: Syntheses, characterization, luminescence, and electrochemical properties

Article abstract of DOI:10.1016/j.ica.2011.12.014

A new imidazo[4,5-f][1,10]phenanthroline-based dinucleating bridging ligand 1,2-bis(2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxy)ethane (L) and its dinuclear Ru(II) complexes [(bpy)₂Ru(L)Ru(bpy)₂](ClO ₄)₄

Full text of DOI:10.1016/j.ica.2011.12.014

Inorganica Chimica Acta 384 (2012) 247–254
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Inorganica Chimica Acta
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A new dinucleating bridging ligand 1,2-bis(2-(1H-imidazo[4,5-f]-
[1,10]phenanthrolin-2-yl)phenoxy)ethane and its dinuclear ruthenium(II)
complexes: Syntheses, characterization, luminescence, and electrochemical
properties
⇑
N. Arockia Samy, V. Alexander
Department of Chemistry, Loyola College, Chennai 600034, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2011
Received in revised form 2 December 2011
Accepted 8 December 2011
Available online 22 December 2011
A new imidazo[4,5-f][1,10]phenanthroline-based dinucleating bridging ligand 1,2-bis(2-(1H-imidazo
[4,5-f][1,10]phenanthrolin-2-yl)phenoxy)ethane (L) and its dinuclear Ru(II) complexes [(bpy)2Ru(L)Ru
(bpy)2](ClO4)4 (2) and [(phen)2Ru(L)Ru(phen)2](ClO4)4 (3) (bpy = 2,20-bipyridine and phen = 1,10-phe-
nanthroline) and their characterization, photophysical, and electrochemical properties have been
reported. The symmetric nature of the bridging ligand L enables the formation of dinuclear Ru(II) com-
plexes 2 and 3 with equivalent metal centers. The compounds 2 and 3 exhibit the spin-allowed 1MLCT
Keywords:
Ruthenium
(d –p⁄) transition at 459 and 456 nm and Ru(II) metal centered emission at 616 and 600 nm, respectively,
p
in fluid solution at room temperature. The emission profile and emission maxima are similar and inde-
pendent of the excitation wavelength for each complex. The free bridging ligand is a blue emitter, while
the complexes are red emitters in solution. The luminescence decay of the compounds 2 and 3 are mono-
exponential and the luminescence lifetime is 174 and 141 ns, respectively. The photoluminescence quan-
tum efficiency of the compound 2 is higher than that of [Ru(bpy)3]2+. The compounds 2 and 3 undergo
metal centered oxidation and the E1/2 value for the Ru(II)/Ru(III) redox couple is 1.32 V versus Ag/Ag+.
The study demonstrates the versatility of the imidazo[4,5-f][1,10]phenanthroline-based dinucleating
bridging ligand in forming dinuclear Ru(II) complexes.
Imidazole
Bridging ligand
Polypyridine complexes
Phenanthroline
Cyclic voltammetry
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
monly employed polypyridyl ligands [3b–d,5]. Because of the con-
venient features of pyridine as a coordinating molecule, its higher
The design and construction of metallosupramolecular architec-
tures have been the subject of considerable attention by many re-
search groups over the past decade. In particular, much attention is
devoted to 2,20-bipyridine (bpy) because of its high complexing
affinity with a variety of metal ions and interesting photophysical,
photochemical, and electron transfer properties of its metal com-
plexes [1]. Ru(II) complexes of polypyridine ligands [2], because
of their unique combination of chemical stability, redox properties,
luminescence intensity, and excited state lifetime, have been used
extensively to construct supramolecular systems [3] to study
photoinduced energy- and electron transfer processes and as
photosensitizers in a variety of intermolecular electron transfer
processes [4].
oligomers have been used in the construction of bridging ligands
and polynuclear supramolecular assemblies [3b,6]. The shape, size,
and functions of polynuclear complexes depend to a large extent
on the nature of the bridging ligands which connect the metal cen-
ters. The crucial role played by bridging ligands in determining the
ground state metal-metal interaction is well recognized [7]. Poly-
nuclear complexes employing imidazole [8] and pyrazine [9] con-
taining bridging ligands are attracting considerable attention. In
particular, the introduction of imidazolyl [8c,10] or triazolyl [11]
rings in the backbone of bridging ligands has led to a number of
polypyridine analogs in which the multidentate character of the
N-heterocycles allowed the modulation of the spectroscopic and
redox properties of the resulting polynuclear Ru(II) complexes
[10a,b,12]. Incorporation of the imidazolyl moiety into a chelating
ligand does not affect the coordinating ability of the latter and
complexes of such ligands have been found to feature new
optical properties [13]. So far only a few imidazo[4,5-f][1,10]
phenanthroline-based dinucleating bridging ligands and their
Ru(II) complexes have been reported. We have designed a new
Since the luminescence and redox behavior of Ru(II) complexes
are strongly ligand-dependent, considerable research interest has
been dedicated to fine-tune these properties by changing the com-
⇑
Corresponding author. Tel.: +91 44 28175351; fax: +91 44 28175566.
E-mail address: valexander@rediffmail.com (V. Alexander).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.014

248
N. Arockia Samy, V. Alexander / Inorganica Chimica Acta 384 (2012) 247–254
(ClO4)4
H
N
N
N
N
N
N
O
N
N
Ru
N
N
N
N
Ru
N
N
O
N
N
H
2
(ClO4)4
H
N
N
N
N
N
O
N
N
Ru
N
N
N
N
N
Ru
N
O
N
N
N
H
3
Chart 1. The compounds [(bpy)2Ru(L)Ru(bpy)2](ClO4)4 (2) and [(phen)2Ru(L)Ru(phen)2](ClO4)4 (3) containing dinuclear Ru(II) complexes.
imidazo[4,5-f][1,10]phenanthroline-based dinucleating bridging
ligand by incorporating imidazolyl moieties in the ligand backbone
(Chart 1) and we report herein the syntheses of 1,2-bis(2-(1H-imi-
dazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxy)ethane (L) and its
dinuclear Ru(II) complexes with 2,20-bipyridine or 1,10-phenan-
throline as ancillary ligands and their photophysical and electro-
chemical properties.
cinnamic acid, dissolved in chloroform and acetonitrile/methanol,
was used as the matrix.
The electronic absorption spectra were recorded on a Shima-
dzu UV-2450 UV-Visible spectrophotometer controlled by the UV
Probe version-2.33 software. The spectra were recorded in the
region 190–900 nm in acetonitrile at 25 °C using a matched pair
of Teflon stoppered quartz cell of path length 1 cm. Fluorescence
spectra were recorded on a Fluorolog-3 FL3-221 spectrofluorom-
eter. The excitation source was a 450 W CW Xenon lamp. The
band pass for the excitation and double-grating emission mono-
chromator was set at 2 nm. A matched pair of quartz cell of path
length 10 mm was used. The emission spectra of the complexes
were recorded in oxygen free acetonitrile at room temperature.
Emission quantum yields (Uem) were calculated by comparing
the wavelength-integrated fluorescence intensities of the sam-
2. Experimental
2.1. Materials
Acetic acid, ammonium acetate, aqueous ammonia (25%), nitric
acid, 1,10-phenanthroline monohydrate, potassium bromide, so-
dium hydrogencarbonate, sodium hydroxide, and sulfuric acid
(Merck); 2,20-bipyridyl, ruthenium(III) chloride trihydrate, and so-
dium perchlorate (Aldrich); 1,2-dibromoethane, lithium chloride,
and salicylaldehyde (Fluka); and silica gel (TLC, 100-120 mesh,
ACME) were used as received. Acetonitrile, dichloromethane, and
diethyl ether were purified by the standard procedures [14].
1,10-Phenanthroline-5,6-dione [15], [Ru(bpy)2Cl2]ꢀ2H2O [16], and
[Ru(phen)2Cl2]ꢀ2H2O [16] were synthesized by the literature
methods.
ples to that of the standard fluorescent complex [Ru(bpy)3]2+
,
which has a quantum yield of 0.062 [17] in acetonitrile using
Eq. (1) [18].
Usample ¼ fODstandard ꢂ Asample
ꢂ
g2sampleg=fODsample ꢂ Astandard
ꢂ
g2standardg ꢂ Ustandard
ð1Þ
where, OD is optical density of the compound at the excitation
wavelength (455 nm), A is the area under the emission spectral
2.2. Physical measurements
curve and g is the refractive index. The emission lifetimes were re-
corded on a Spectra-Physics LAB-150 laser flash photolysis spec-
trometer. The samples were irradiated using the third harmonic
pulses of the Nd3+:YAG laser (Quanta-Ray) and 8 ns pulse width
was used to excite the sample. The beam emerging through the
sample was focused onto a Czerny-Turner monochromator using a
pair of lenses. The detection was carried out using a Hamama-
tsu R-928 photomultiplier tube. The signals were captured with
an Agilent infiniium digital storage oscilloscope interfaced to a
computer.
CHN microanalyses were carried out using a Perkin–Elmer 2400
Series II CHNS/O Elemental Analyzer interfaced with a Perkin-El-
mer AD 6 Autobalance. Helium was used as the carrier gas. Infrared
spectra were recorded on a Perkin–Elmer Spectrum RX-I FT IR
spectrometer in the range 4000–400 cmꢁ1 using KBr pellets.
NMR spectra were recorded on a Bruker AVANCE III 500 MHz (AV
500) multinuclear NMR spectrometer working at 500 MHz at
25 °C. Standard 5 mm probe was used for the 1H and 13C NMR mea-
surements. The electrospray ionization mass spectra (ESI MS) were
performed using a Micromass Quattro-II Triple Quadrupole mass
Cyclic voltammetry was performed on a EG&G PAR 273A Poten-
tiostat/Galvanostat using RDE0018 Analytical Cell Kit consisting of
a thermostated cell bottom, EG&G G0229 glassy carbon disk
spectrometer. 2,5-Dihydroxybenzoic acid or
a-cyano-4-hydroxy-

N. Arockia Samy, V. Alexander / Inorganica Chimica Acta 384 (2012) 247–254
249
milli-electrode, platinum counter electrode, and EG&G K0265 Ag/
2.4. Synthesis of heteroleptic ruthenium(II) complexes
Ag+ reference electrode. The auxiliary electrode was connected to
the test solution through the counter electrode bridge tube. The
reference electrode was separated from the test solution through
the bridge tube containing AgCl-KCl filling solution. The cyclic vol-
tammograms were recorded using 10ꢁ3 M solution of the com-
Caution: Perchlorate salts are potentially explosive and, there-
fore, should be handled in small quantities with care!
2.4.1. [(bpy)2Ru(L)Ru(bpy)2](ClO4)4 (2)
plexes in
oxygen free acetonitrile containing 0.1 M
A solution of [Ru(bpy)2Cl2]ꢀ2H2O (1.09 g, 2.10 mmol) and 1,2-
bis(2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxy)ethane
(L) (0.68 g, 1.05 mmol) in ethylene glycol (20 mL) was refluxed for
6 h under argon atmosphere. The solution was cooled to room tem-
perature, water (40 mL) was added, and filtered. An aqueous solu-
tion of sodium perchlorate was added to the filtrate dropwise with
stirring whereupon a reddish orange solid separated out. The prod-
uct was filtered, dried in air, and recrystallized in hot acetonitrile-
water (1:1 v/v), (1.56 g, 79%). Anal. Calc. for C80H58Cl4N16O18Ru2: C,
51.24; H, 3.12; N, 11.95. Found: C, 51.20; H, 3.11; N, 11.92%. IR
tetraethylammonium perchlorate as the supporting electrolyte.
Oxygen free argon, saturated with the solvent vapor, was flushed
through each sample solution through the purge tube assembly
for 30 min before voltammetry was performed and all measure-
ments were carried out in an atmosphere of argon at 25 °C. All
operations were performed through a computer using EG&G Model
270 Software and all electrochemical parameters were obtained
using the EG&G PowerSuite software. The instrument was cali-
brated by recording the cyclic voltammograms of ferrocene (pur-
um, Fluka) in oxygen free acetonitrile under the same
experimental conditions.
(cmꢁ1, KBr): 3394
1243 as(C–O–C), 1087
m
(N–H), 1601
m
(C@N), 1446
m
(C–N) (pyridine),
(C–
m
m(ClO4), 766
ms(C–H) (pyridine), 625 m
C). dH(500 MHz; DMSO-d6; 298 K) 4.51 (s, 4H, –CH2–), 7.09–7.13
(m, 4H, Hh, Hf), 7.36 (t, 4H, J = 6.75 Hz, H02), 7.43 (t, 2H, J = 7.5 Hz,
Hg), 7.61 (t, 4H, J = 6.5 Hz, H2) 7.64 (d, 4H, J = 5.5 Hz, H01), 7.87 (d,
4H, J = 5.5 Hz, H1), 7.94 (t, 4H, J = 6.5 Hz, Hb), 8.10–8.13 (m, 6H,
He, H03), 8.20–8.24 (m, 8H, Ha, H3), 8.84 (d, 4H, J = 8 Hz, H04), 8.88
(d, 4H, J = 8 Hz, H4), 9.14 (d, 4H, J = 7 Hz, Hc). ESI MS: m/z 838
2.3. Synthesis of ligand
2.3.1. Stage-1: 1,4-Bis(20-formylphenyl)-1,4-dioxabutane (1)
The dial 1 was synthesized by modifying the procedure re-
ported by Armstrong et al. [19]. To a solution of salicylaldehyde
(20.80 mL, 200 mmol) in ethanol (40 mL) was added a solution of
sodium hydroxide (8 g, 200 mmol) in water (400 mL), warmed,
and 1,2-dibromoethane (8.60 mL, 100 mmol) was added followed
by ethanol (400 mL) to get a homogeneous solution. The solution
was refluxed under nitrogen atmosphere for 100 h, cooled to
0 °C, and the colorless needles that separated out was filtered,
washed with water, and recrystallized in ether–chloroform (1:1
v/v), (16.34 g, 61%). Anal. Calc. for C16H14O4: C, 71.10; H, 5.22.
[Mꢁ2ClO4]2+
,
788 [Mꢁ3ClO4]2+
,
738 [Mꢁ4ClO4]2+
,
493
[Mꢁ4ClO4]3+, 459 [(MꢁC59H40Cl4N12O16Ru)]+, 370 [Mꢁ4ClO4]4+
362 [(MꢁC41H31Cl4N8O17Ru)]2+, 237 [(MꢁC67H49Cl4N12O17Ru2)]+.
,
2.4.2. [(phen)2Ru(L)Ru(phen)2](ClO4)4 (3)
A solution of [Ru(phen)2Cl2]ꢀ2H2O (1.19 g, 2.10 mmol) and 1,2-
bis(2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxy)ethane
(L) (0.68 g, 1.05 mmol) in ethylene glycol (20 mL) was refluxed for
6 h under argon atmosphere. The solution was cooled to room tem-
perature, water (40 mL) was added, and filtered. An aqueous solu-
tion of sodium perchlorate was added to the filtrate dropwise with
stirring whereupon a reddish orange solid separated out. The prod-
uct was filtered, dried in air, and recrystallized in hot acetonitrile-
water (1:1 v/v), (1.55 g, 75%). Anal. Calc. for C88H58Cl4N16O18Ru2: C,
53.61; H, 2.97; N, 11.37. Found: C, 53.50; H, 2.89; N, 11.28%. IR
Found: C, 71.02; H, 5.15%. IR (cmꢁ1, KBr): 3076
as(C–O–C), 753 d(C–H) (aromatic), 2922 as(C–H), 2866
(aliphatic), 1652 s(C@O), 1597, 1484, and 1452 s(C@C) (aro-
matic), 944 (CH2), 860 (CH2), 834 (CH2) (methylene).
m
s(C–H), 1248
m
m
m
s(C–H)
m
m
x
s
q
dH(500 MHz; CDCl3; 298 K) 4.54 (s, 4H, –O-CH2–), 7.07 (d, 2H,
J = 8.5 Hz, Ph-H), 7.09 (t, 2H, J = 8.5 Hz, Ph-H), 7.59 (t, 2H,
J = 7.75 Hz, Ph-H), 7.86 (d, 2H, J = 7.5 Hz, Ph-H), 10.45 (s, 2H, –
CHO). dC(125 MHz; CDCl3; 298 K) 67.0, 112.8, 121.4, 125.2, 128.6,
135.9, 160.7, 189.3. FAB MS: m/z 271 [M+H]+, 149 [Mꢁ(C7H5O2)]+,
136 [Mꢁ(C8H6O2)]+, 121 [Mꢁ(C10H10O2)]+.
(cmꢁ1, KBr): 3431
1246 as(C–O–C) 1086
m
(N–H), 1627
m
(C@N), 1423
m
s(C–H) (pyridine),
m
m(ClO4), 722 (b-ring) (pyridine), 626 m
(C–
C). dH(500 MHz; DMSO-d6; 298 K) 4.55 (s, 4H, –CH2–), 7.13 (d,
4H, J = 5 Hz, Hh, Hf), 7.45 (t, 2H, J = 8 Hz, Hg), 7.73–7.76 (m, 8H,
H2, H7), 7.79–7.81 (m, 4H, Hb), 8.02 (d, 4H, J = 5 Hz, H1), 8.06 (d,
4H, J = 5 Hz, H8), 8.12 (d, 2H, J = 5 Hz, He), 8.21 (d, 4H, J = 8 Hz,
Ha), 8.36 (s, 8H, H4, H5), 8.74 (d, 8H, J = 7.5 Hz, H3, H6), 9.13 (d,
2.3.2. Stage-2: 1,2-Bis(2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-
yl)phenoxy)ethane (L)
4H, J = 5 Hz, Hc). ESI MS: m/z 1872 [MꢁClO4]+, 886 [Mꢁ2ClO4]2+
,
836 [Mꢁ3ClO4]2+
,
558 [Mꢁ3ClO4]3+
,
525 [Mꢁ4ClO4]3+
, 484
To a solution of 1,10-phenanthroline-5,6-dione (0.42 g, 2 mmol)
and ammonium acetate (3.08 g, 40 mmol) in hot glacial acetic acid
(10 mL) was added a solution of 1,4-bis(20-formylphenyl)-1,4-
dioxabutane (1) (0.27 g, 1 mmol) in glacial acetic acid (10 mL)
dropwise over a period of 30 min with stirring in argon atmo-
sphere, refluxed for 5 h, cooled to room temperature, and poured
in water (200 mL). Aqueous ammonia (25%) was added dropwise
with stirring whereupon a pale yellow precipitate separated out.
The product was filtered, washed with large portions of water,
and dried in vacuo (0.37 g, 57%). Anal. Calc. for C40H26N8O2: C,
73.83; H, 4.03; N, 17.22. Found: C, 73.79; H, 3.92; N, 17.14%. IR
[Mꢁ(C62H43Cl4N12O18Ru)]+, 394 [Mꢁ4ClO4+H]4+
.
3. Results and discussion
3.1. Synthesis of the bridging ligand
The dial derivative 1 is synthesized by modifying the procedure
reported by Armstrong et al. [19] by the O-alkylation of salicylalde-
hyde (2 equiv) with 1,2-dibromoethane (1 equiv) (Williamson’s
synthesis) in the presence of sodium hydroxide (2 equiv) in
water–ethanol as colorless needles. The bridging ligand 1,2-bis(2-
(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxy)ethane (L) is
synthesized on the basis of the imidazole ring preparation estab-
lished by Steck et al. [20] by a two-step reaction involving the for-
mation of a quinone–diimine intermediate by the reaction of 1,10-
phenanthroline-5,6-dione with ammonium acetate followed by an
aldol-type condensation of the intermediate with 1 in acetic acid.
(cmꢁ1, KBr): 3394
s(C@C), 1237 as(C–O–C), 1062
743 (b-ring) (pyridine), 663 (C–C). dH(500 MHz; DMSO-d6; 298 K)
m
(N–H), 2936
m
as(C–H), 1598
s(C–O–C), 804
m(C@N), 1454
s(C–H) (pyridine),
m
m
m
m
m
4.68 (s, 4H, –O–CH2–), 6.77 (t, 2H, J = 7.5 Hz, Hf), 7.14 (d, 2H,
J = 7.75 Hz, Hh), 7.22–7.27 (m, 2H, Hg), 7.44 (d, 4H, J = 8.5 Hz, Hb),
7.60 (t, 2H, J = 7.5 Hz, He), 8.19 (d, 4H, J = 7.5 Hz, Hc), 9.12 (s, 4H,
Ha), 14.03 (s, 2H, –NH). ESI MS: m/z 689 [M+K]+, 651 [M]+.

250
N. Arockia Samy, V. Alexander / Inorganica Chimica Acta 384 (2012) 247–254
H
N
O
N
O
N
O
O
OH
N
O
N
N
2
EtOH/H2O
O
Br
Br
2
+
O
O
N
N
O
N
reflux, 100 h
CH3COONH4
CH3COOH
reflux, 5 h
N
H
1
L
Scheme 1. Synthetic routes for the preparation of the bridging ligand (L).
The reaction involving the synthesis of the dinucleating ligand L is
depicted in Scheme 1. The ligand L is a polyaromatic bridging
ligand containing two equivalent chelating sites of phenanthroline
ring. In the ligand L, the two imidazo[4,5-f][1,10]phenanthroline
moieties are bridged by a flexible ethoxyphenyl spacer moiety. It
is characterized by IR, ESI MS, 1H NMR and elemental analysis
and is hardly soluble in common organic solvents, but partly solu-
ble in DMF and DMSO.
may be due to the effect of the ring current of the bpy and phen
ancillary ligands. The He, Hf, and Hg protons of the phenyl spacer
encounter downfield shift, while the Hh protons do not show sig-
nificant shift. The NH proton resonance of the ligand is not ob-
served in the complexes because of the fast exchange between
the nitrogens of the imidazole ring, characteristic of an active pro-
ton. In the compound 2, the two pyridine rings of each bpy are not
chemically and magnetically equivalent due to the shielding influ-
ence of the adjacent coordinated bpy and the imidazo[4,5-
f][1,10]phenanthroline moiety of the bridging ligand leading to
eight signals for the bpy protons: four signals (H01–H40 ) for the pyr-
idine ring adjacent to the imidazo[4,5-f][1,10]phenanthroline and
the other four signals (H1–H4) for the pyridine ring adjacent to
the coordinated bpy. The protons of the pyridine rings of the coor-
dinated bpy ligands are highly deshielded compared to that of the
pyridine rings of the ligand framework and hence the chemical
shifts for the former proton is higher than that of the latter. Such
a behavior has been reported for the Ru(II) polypyridine complexes
containing imidazo[4,5-f][1,10]phenanthroline [21]. The 1H NMR
spectra of the compounds 2 and 3 are presented in Fig. 1.
3.2. Synthesis of complexes
The heteroleptic dinuclear compounds [(bpy)2Ru(L)Ru(bpy)2](-
ClO4)4 (2) and [(phen)2Ru(L)Ru(phen)2](ClO4)4 (3) are prepared
by the reaction of the preformed dinucleating imidazo[4,5-
f][1,10]phenanthroline-based bridging ligand
L with [Ru(b-
py)2Cl2]ꢀ2H2O and [Ru(phen)2Cl2]ꢀ2H2O in ethylene glycol. The
compounds are isolated as perchlorate salts and stable in air and
in solution. In the ESI-MS of the compound 2, the peaks at m/z
838, 788, and 370 are due to the species [Mꢁ2ClO4]2+
,
[Mꢁ3ClO4]2+, and [Mꢁ4ClO4]4+ formed by the loss of two-, three-,
and four perchlorate ions, respectively, from the molecular ion.
In the ESI-MS of the compound 3, the peaks at m/z 1872, 886,
558, and 525 are due to the species [MꢁClO4]+, [Mꢁ2ClO4]2+
,
3.4. Electronic absorption spectra
[Mꢁ3ClO4]3+, and [Mꢁ4ClO4]3+ formed by the loss of one-, two-,
three-, and four perchlorate ions, respectively, from the molecular
ion.
The electronic absorption spectral data of the dinuclear Ru(II)
complexes are presented in Table 2. The compounds 2 and 3 exhi-
bit intense absorption bands at 200-300 nm assignable to the spin-
3.3. NMR spectra of the ligand and complexes
allowed ligand centered (1LC)
work and a broad band at 459 (
p
–
p⁄ transition of the ligand frame-
e
/dm3 molꢁ1 cmꢁ1 20,322) and
The 1H NMR spectral data of the ligand and complexes are pre-
sented in Table 1. In the 1H NMR spectrum of L, the resonances at
9.12, 8.19, and 7.44 ppm are assigned to the Ha, Hc, and Hb protons,
respectively, of the phenanthroline ring. The resonances at 7.60,
7.24, 7.14, and 6.77 ppm are assigned to the He, Hg, Hh, and Hf pro-
tons, respectively, of the phenyl spacer. The Hi (methylene) protons
of the linker moiety resonate at 4.68 ppm.
On coordination to the metal ion, the chemical shift of the li-
gand protons in the compounds 2 and 3 are shifted when com-
pared to that of the free ligand. The Ha protons of the
imidazo[4,5-f][1,10]phenanthroline moiety in the compounds 2
and 3 encounter upfield shift, while the Hb and Hc protons experi-
ence downfield shift. The dramatic upfield shift of the Ha proton
456 nm (25,443), respectively, assignable to the spin-allowed
1MLCT (d –p⁄) transition [22]. The MLCT band of the compound
p
2 with bpy as ancillary ligands and that of the compound 3
with phen as ancillary ligands are bathochromically shifted with
respect to that of [Ru(bpy)3]2+ (kmax = 451 nm) and [Ru(phen)3]2+
(kmax = 446 nm) which can be attributed to the extended structural
framework of the bridging ligand [23]. However, the MLCT band of
the complexes are not significantly shifted compared to that of the
mononuclear counterpart [Ru(bpy)2(PIP)]2+ (PIP = 2-phenyl-imi-
dazo[4,5-f][1,10]phenanthroline) [24]. The Ru(II) complexes are
bright red in solution and in the solid state due to the strong 1MLCT
absorption band at 456–459 nm. The electronic absorption spectra
of the compounds 2 and 3 are presented in Fig. 2.
Table 1
The 1H NMR spectral data of the ligand and complexes.a
Compound entry no.
Chemical shift (ppm)
Ha
Hb
Hc
Hd
He
Hf
Hg
Hh
Hi
L
2
3
9.12
8.22
8.21
7.44
7.94
7.80
8.19
9.14
9.13
14.03
-
-
7.60
8.12
8.12
6.77
7.11
7.13
7.24
7.43
7.45
7.14
7.11
7.13
4.68
4.51
4.55
a
Data obtained from 500 MHz 1H NMR spectra recorded in DMSO-d6 at 298 K.

N. Arockia Samy, V. Alexander / Inorganica Chimica Acta 384 (2012) 247–254
251
f
(ClO4)4
g
h
e
d
c
b
H
N
'
'
3
2
N
N
N
N
1'a
'
4
N
O
N
Ru
N
N
N
i
4
2
a
3
Ru
N
N
O
N
1
N
H
N
N
(ClO4)4
f
g
h
e
d
c
b
2
H
3
N
N
N
b
4
a
1
N
O
N
N
5
Ru
N
N
N
i
N
N
6
N
Ru
N
O
N
7
8
N
H
N
Fig. 1. 500 MHz 1H NMR spectra of [(bpy)2Ru(L)Ru(bpy)2](ClO4)4 (2) (a) and [(phen)2Ru(L)Ru(phen)2](ClO4)4 (3) (b) in DMSO-d6 at 298 K (resonances in the aromatic region
only are shown).
Table 2
Photophysical and electrochemical data of the complexes.a
sc
krd 104 knre 105 E1/2 (V)
b
f
Complex
Absorption kmax (nm)
Excitation
kex (nm)
Emission
kem (nm)
Uem
(e
ꢂ 104, Lmolꢁ1 cmꢁ1
)
(ns) (sꢁ1
)
(sꢁ1
)
Metal
Ligand
centered
oxidation
centered
reductions
[(bpy)2Ru(L)Ru(bpy)2](ClO4)4 (2)
[(phen)2Ru(L)Ru(phen)2](ClO4)4 (3)
243 (4.0)
254 (3.7)
285 (10.3)
459 (2.0)
200 (12.0)
222 (11.4)
264 (13.4)
456 (2.5)
244 (3.6)
253 (3.5)
283 (10.0)
458 (1.8)
272
299
460
616
600
615
0.0845 174 49
0.0612 141 43
53
1.32
1.32
1.28
ꢁ0.55, ꢁ1.37,
ꢁ1.58
279
457
67
ꢁ0.56, ꢁ1.41,
ꢁ1.72
g
[Ru(bpy)2(PIP)](ClO4)2
ꢁ0.85, ꢁ1.43,
ꢁ1.63
a
b
c
d
e
f
Absorption and emission spectra were recorded using 1.5 ꢂ 10ꢁ5 M solution of the complexes in acetonitrile at 298 K.
Relative quantum yield calculated by the method given under experimental section using [Ru(bpy)3]2+ as the standard.
The emission decays were calculated as single exponential (kexc = 355 nm).
kr = Uem
knr = 1/
/s.
s
ꢁ kr.
The data are computed from the cyclic voltammograms recorded on a glassy carbon millielectrode in acetonitrile (10ꢁ3 M) using tetraethylammonium perchlorate as the
supporting electrolyte (0.1 M) at 298 K at the scan rate of 50 mVsꢁ1. Potentials are reported in volts versus Ag/Ag+.
g
From Ref. [24].

252
N. Arockia Samy, V. Alexander / Inorganica Chimica Acta 384 (2012) 247–254
exhibit an emission band at 616 and 600 nm, respectively. The
emission profile and emission maxima are similar and independent
of the excitation wavelength. The free bridging ligand is a blue
emitter, while the complexes are red emitters in solution. The
emission maxima of the compound 2 (kem = 616 nm) containing
bpy ancillary ligands is not significantly shifted compared to that
of [Ru(bpy)2(PIP)]2+ (kem = 615 nm).
The luminescence bands of the compounds 2 and 3 are quite
similar in shape to the luminescence bands of their mononuclear
counterpart [Ru(bpy)2(PIP)]2+. Thus, the photophysical properties
of the heteroleptic compounds 2 and 3 with bpy and phen as ancil-
lary ligands are not significantly different from that of the mono-
nuclear complex [Ru(bpy)2(PIP)]2+. The emission of the Ru(II)
centers in the complexes is, therefore, assigned to the 3MLCT state
involving the bridging ligand as the acceptor site and the emitting
MLCT state of the acceptor ligand does not extend over to the flex-
ible spacer of the bridging ligand [25], but it may involve only the
phenyl group directly connected to the imidazo[4,5-f][1,10]phe-
nanthroline moiety. The photoluminescence quantum efficiency
of the compound 2 is higher than that of [Ru(bpy)3]2+ which can
be attributed to the higher energy of the emission maxima of the
former as expected from the energy gap law [4b]. The lumines-
cence decay of the compounds 2 and 3 are monoexponential and
the luminescence lifetime is 174 and 141 ns, respectively. The
emission and the excitation spectra of the compounds 2 and 3
are depicted in Fig. 3.
Fig. 2. Electronic absorption spectra of [(bpy)2Ru(L)Ru(bpy)2](ClO4)4 (2) (a) and
[(phen)2Ru(L)Ru(phen)2](ClO4)4 (3) (b) in acetonitrile at room temperature.
3.5. Luminescence spectra
The emission spectral data (wavelength of excitation and emis-
sion), luminescence quantum yield, emission lifetime, and radia-
tive and nonradiative decay rates are presented in Table 2. All
measurements are made with solutions deoxygenated by purging
argon. The excitation spectra of the compounds are monitored at
the emission maxima of each compound at room temperature.
The excitation spectrum of the free ligand L contains bands at
286, 302, and 343 nm. The free ligand emits at 426 nm upon exci-
tation onto the excitation maxima. This fluorescence band of the li-
gand is no longer observed in its metal complexes. The excitation
spectrum of [(bpy)2Ru(L)Ru(bpy)2](ClO4)4 (2) contains two intense
bands at 272 and 299 nm and a weak band at 460 nm, while the
excitation spectrum of [(phen)2Ru(L)Ru(phen)2](ClO4)4 (3) con-
tains an intense band at 279 nm and a weak band at 457 nm. The
compounds 2 and 3 upon excitation onto their excitation maxima
3.6. Electrochemistry of complexes
The redox behavior of the complexes has been investigated in
acetonitrile solution to complement the spectroscopic data. The
investigation of electrochemical behavior is very useful to deter-
mine the extent of electronic interaction between the metal centers.
The cyclic voltammetric data of the complexes are presented in Ta-
ble 2. In the present study only one oxidation process is observed for
the compounds 2 and 3 and the E1/2 values for the Ru(II)/Ru(III)redox
couple is 1.32 V versus Ag/Ag+ which is anodically shifted with re-
spect to that of [Ru(bpy)3]2+ and [Ru(phen)3]2+ (E1/2 = 1.27 V versus
Ag/Ag+) [23]. From the separation between the anodic and cathodic
peak potential of ca. 63 mV, it is apparent that the oxidation process
Fig. 3. Emission spectra of [(bpy)2Ru(L)Ru(bpy)2](ClO4)4 (2) (a) and [(phen)2Ru(L)Ru(phen)2](ClO4)4 (3) (b) in acetonitrile at room temperature, inset: excitation spectra of 2
(a) and 3 (b).

N. Arockia Samy, V. Alexander / Inorganica Chimica Acta 384 (2012) 247–254
253
involves two electrons for each one of the dinuclear complexes,
Appendix A. Supplementary material
which are attributed to two simultaneous independent one-electron
metal-centered processes. This indicates that the electrostatic inter-
action between the metal centers is negligible due to the large dis-
tance of separation of the Ru(II) centers. The occurrence of a single
oxidation wave for these complexes confirms that each metal-cen-
tered subunit is electronically ‘‘isolated’’ from the electrochemical
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.12.014.
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This research work was carried out with the financial support
from the Department of Science and Technology (DST), Govern-
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providing the facilities to study the emission lifetimes.

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