Demonstration of intramolecular energy transfer in asymmetric bimetallic ruthenium(II) complexes

Article abstract of DOI:10.1039/c6dt03250h

A new family of bimetallic Ru(ii) complexes derived from an asymmetric bridging ligand (tpy-Hbzim-dipy) consisting of both bipyridine and terpyridine chelating sites covalently connected via phenyl-imidazole spacer were designed in this work to demonstrate intramolecular energy transfer from one component to the other in asymmetric dyads. To fine tune the photo-redox properties, both bidentate and tridentate terminal ligands in the complexes were varied systematically. Both steady state and time-resolved luminescence spectral results indicated photo-induced intramolecular energy transfer from the excited MLCT state of the [(bpy/phen)₂Ru^II(dipy-Hbzim-tpy)] component to the MLCT state of the tpy-containing unit [(dipy-Hbzim-tpy)Ru^II(tpy-PhCH₃/H₂pbbzim)] in dyads with rate constant values on the order of 10⁶-10⁷ s^-1. Temperature-dependent luminescence studies indicated an enhancement in the luminescence intensity and excited state lifetimes upon decreasing the temperature.

Full text of DOI:10.1039/c6dt03250h

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DOI: 10.1039/C6DT03250H
Demonstration of intramolecular energy transfer in asymmetric
bimetallic ruthenium(II) complexes
Manoranjan Bar, Dinesh Maity, Shyamal Das, and Sujoy Baitalik*
_______________________________________________________________________
* To whom correspondence should be addressed. Department of Chemistry, Inorganic Chemistry
Section, Jadavpur University, Kolkata 700032, India. E-mail: sbaitalik@hotmail.com and
sujoyb@chemistry.jdvu.ac.in; Fax: + 91 3324146584; Tel: + 91 33 24146666.
†Electronic Supplementary Information (ESI) available: Fig. S1-S5. For ESI see
DOI: 10.1040/b000000x/
1

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DOI: 10.1039/C6DT03250H
Abstract
A new family of bimetallic Ru(II) complexes derived from an asymmetric bridging ligand (tpy-
Hbzim-dipy) consisting of both bipyridine and terpyridine chelating sites covalently connected
via phenyl-imidazole spacer have been designed in this work to demonstrate intramolecular
energy transfer from one component to the other in the asymmetric dyads. For fine tuning of
photo-redox properties, both bidentate and tridentate terminal ligands in the complexes have
been varied systematically. Both steady state and time-resolved luminescence spectral results
indicate photo-induced intramolecular energy transfer from the excited MLCT state of the
[(bpy/phen)2RuII(dipy-Hbzim-tpy)] component to the MLCT state of the tpy-containing unit
[(dipy-Hbzim-tpy)RuII(tpy-PhCH3/H2pbbzim)] in dyads with the values of the rate constant in
the order of 106-107 s-1. Temperature-dependent luminescence studies indicate enhancement of
luminescence intensity and excited state lifetimes upon decreasing the temperature.
Introduction
Photo-induced directional flow of energy or/and electron in a multi-component system
which in turn can generate long-lived charge-separated species is very important with regard to
the development of artificial leaf.1-5 To this end, a large number of donor-acceptor systems have
been designed synthetically. Among the synthetically designed systems, complexes of Ru(II)
derived from pyridine-based ligands are of particular interest because of their unique
combination of photophysical and redox properties as well as their remarkable stability.6-15 In
order to induce directional flow of electron or energy, an asymmetry should be incorporated in a
bi- or multimetallic system. This sort of asymmetry can be incorporated either by using bridging
ligands differing in donor-acceptor properties of the coordinating sites or by using heterometallic
sites. The structure and electronic nature of the bridge play important roles in this context as
cooperative interactions between metal centers in the multimetallic system give rise to properties
that are very much essential for the designing of photochemical molecular devices1,14-18 Among
the pyridine-based ligands, generally two types of chelates are employed for this sort of study,
viz. bidentate bpy-type (2,2'-bipyridine) or tridentate tpy-type (2,2′:6′,2′′-terpyridine)type
chelating sites, either directly connected or covalently linked by a suitable spacer. Complexes
derived from both category of ligands are very large in number and were found to be utilized in
2

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DOI: 10.1039/C6DT03250H
performing various light-induced functions.5-15 It has been observed that bpy-type chelates
produced complexes possessing rich photophysical behaviors than their tridentate tpy analogues,
particularly with regard to their room temperature luminescence characteristics.6 On the other
hand, in constructing beautiful achiral linear architectures, tpy-type bridges are more appealing
than bpy-type ligands as bpy-type ligands give rise of mixture of diastereomers.11-13 But the main
problem of the tpy-type Ru(II) complexes is their inferior room temperature luminescence
properties which are the major constraint to act as suitable photosensitizer.11-13 Although the
Ru(II) complexes derived from solely bpy-type or tpy-type ligands are very much abundant, but
those derived from a combination of bpy-tpy ligands are very few in the literature.19-24 It is
expected that the photophysical properties of Ru(II) complexes derived from bpy-type ligands
would be very interesting as they will be composed of the unique attributes of each category of
complex. To this end, we recently designed a bpy-tpy type bridging ligand, 4'-[4-(4,5-Di-pyridin-
2-yl-1H-imidazol-2-yl)-phenyl]-[2,2';6',2'']terpyridine (tpy-Hbzim-dipy) functionalized with
phenyl-imidazole spacer and its monometallic Ru(II) complex of composition [(bpy/phen)2Ru
(tpy-Hbzim-dipy)](ClO4)2 (Chart 1).25 In this work, we will report the synthesis, characterization
and detailed photophysical and redox properties of a new family of homobimetallic Ru(II)
complexes wherein one Ru(II) center is coordinated to bpy-type chelates, while the second
Ru(II) center is coordinated to tpy-type chelates of the heteroditopic bridge. For fine tuning of
the physicochemical properties, both bidentate and tridentate terminal ligands in the complexes
have been varied systematically resulting in four bimetallic complexes as shown in Chart 1.
Incorporation of the asymmetry in the bridging ligand induces difference in the excited-state
energy of two Ru(II) centers and thereby photo-induced intramolecular energy transfer from one
center to the other is thermodynamically feasible. We will be interested to address detailed
thermodynamic and kinetic aspects of this photo-induced intramolecular and will also see the
influence of the terminal ligands, if any, on the energetics of the energy transfer process. Prior to
this work, we reported another series of bimetallic Ru(II) complexes derived from heteroditopic
phen-Hbzim-tpy, wherein a phenanthroline unit was coupled with terpyridine moiety through
phenyl-imidazole spacer.23a The beauty of the present system is that it provides two bidendate
bpy-type chelating sites compared with one phen-type bidentate chelate and thus opening up a
wide domain for designing homo- and heteromultinuclear assemblies.
3

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DOI: 10.1039/C6DT03250H
NH
N
N
N
N
N
N
N
H
N
H
N
N
N
N
N
N
Ru
Ru
N
N
N
N
N
N
NH
[(dipy-Hbzim-tpy)Ru(tpy-PhCH3]4+ (1a)
[(dipy-Hbzim-tpy)Ru(H2pbbzim]4+ (1b)
N
N
H
N
N
N
N
tpy-Hbzim-dipy
N
N
N
N
H
N
N
H
N
N
N
N
N
N
N
N
N
N
N
N
Ru
Ru
N
N
N
N
N
[(phen)2Ru(dipy-Hbzim-tpy)]2+ (2)
[(bpy)2Ru(dipy-Hbzim-tpy)]2+ (1)
N
N
N
N
H
N
H
N
N
N
N
N
Ru
Ru
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Ru
Ru
N
N
[(bpy)2Ru(dipy-Hbzim-tpy)Ru(tpy-PhCH3]4+ (3) [(phen)2Ru(dipy-Hbzim-tpy)Ru(tpy-PhCH3]4+ (4)
NH
N
NH
N
N
H
N
N
N
N
H
N
N
N
N
N
Ru
N
N
Ru
N
N
N
N
N
N
N
N
NH
N
N
N
N
Ru
NH
Ru
N
N
[(bpy)2Ru(dipy-Hbzim-tpy)Ru(H2pbbzim)]4+ (5)
[(phen)2Ru(dipy-Hbzim-tpy)Ru(H2pbbzim]4+ (6)
Chart 1. Molecular structures of the complexes
4

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Results and discussion
Synthesis and characterization of the complexes
The monometallic complexes which are utilized for the synthesis of bimetallic dyads were
synthesized by following our previous reports.25 Each of the monometallic complex having a free
terpyridine site is allowed to react with appropriate precursor ([Ru(tpy-PhCH3)Cl3] or
[Ru(H2pbbzim)Cl3]) in ethylene glycol in the temperature range 180-200°C for the synthesis of
the unsymmetrical bimetallic complexes. The new complexes were purified by silica gel column
chromatography using appropriate eluent followed by recrystallization from proper solvent(s)
under slightly acidic condition to retain the imidazole NH proton(s) in the complexes intact.
Complexes were then fully characterized by elemental (C, H and N) analyses, ESI mass
1
spectrometry, and H NMR spectroscopic measurements and the relevant data have been
provided in the Experimental Section.
1H NMR spectra of the complexes recorded in DMSO-d6 are displayed in Fig. 1. Though
the spectra are extremely complicated, tentative assignment of different proton resonances
according to the numbering scheme were made possible with the aid of their {1H–1H} COSY
spectra along with by comparing the spectra of related complexes (Fig. S1-S2, ESI).23 Most of
the observed peaks in the complexes lie in aromatic region of 6.04-9.61 ppm. Three distinct
peaks were found to appear outside this range. Among three peaks, the singlet at 2.50 ppm in 3
and 2.49 ppm in 4 (not shown in Fig. 1) is certainly designating the -CH3 protons of the
coordinated tpy-PhCH3 moiety. The doublet at 6.06 ppm for 5 and at 6.04 ppm for 6 accounting
two protons each is clearly due to H22 of H2pbbzim moiety as these protons are under the
influence of anisotropic ring current effect of neighboring pyridine rings inducing remarkable
up-field shift. The most down-fielded resonance that appeared either as a singlet or as a broad
feature in range of 14.22-15.11 ppm corresponds to imidazole NH proton(s) of either dipy-
Hbzim-tpy bridging unit or H2pbbzim terminal ligand. For complexes 1-4, only one NH signal
appeared, while in case of 5 and 6, occurrence of two distinct NH signals are observed in the said
range. By comparing the chemical shifts of NH proton of 1-4, the peak at 14.34 ppm in 5 and
14.31 ppm in 6 can be attributed to NH of dipy-Hbzim-tpy, while the peak observed at 15.11
ppm in 5 and 15.04 ppm in 6 is due to NH of H2pbbzim. The singlet peak due to H3' protons
undergo remarkable down-field shift in the bimetallic complexes compared with monometallic
precursor complexes. In addition, two closely situated singlet at 9.46 and 9.26 ppm in 3 and 9.48
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DOI: 10.1039/C6DT03250H
and 9.17 ppm in 4 is indicative of differences of chemical environment of the H3' protons in
dipy-Hbzim-tpy and tpy-PhCH3.
4
3
9
5
N
17
N
10
11
N
N
N
18
3"
6
H
N
H
H
3'
8
7
3"
N
N
4'
4'
19
12
N
5
N
5'
N
N
5'
20
21
N
8
3'
N
N
N
7
N
N
4
6'
6'
22
6
3
N
H(4')+H(5')+H(6')+H(3)
+H(7)+H(12)+H(19)
H(3'')+H(18)+H(9)
H(8)+H(16)+H(17)
H(4)+H(10)
+H(11)
H(3')
H(5')
H(3'')
+
H (6)
H(4')+H(6')+
H(8)+H (15)
H(5')+H(21)
H(22)
NH
NH
6
H(3'')+H(9)+H(16)
H(17)+H(18)
H(3'')
H(5')+H(6')+H(3)+
H(7)+ H(12)+H(19)
H(3')
H(4')+H(6')+H(8)+
H(10)+ H(11)+H(15)
+
H(6)
H(22)
H(21)
NH
H(8)
NH
H(5')
5
H(6')+H(4)+H(5)
+H(11)+H(14)
H(3')+H(4')+
H(6)+H(8)
H(5')+H(4)+H(5)
+H(12)+H(13)
H(3')+H(3'')+
H(6)+ H(9)
H(3')
H(3)
H(5)
NH
4
H(6')+H(3)+
H(12)+H(15)
H(7)
+
H(3'')
+
H(8)
H(4')+H(4)+H(10)
+H(11)+H(14)
H(6')
H(3')
H(3')+H(3'')+
H(6)+ H(9)
H(3)
H(5)
H(5)
NH
3
15.5
15.0
14.5
14.010.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
Fig. 1 1H NMR spectra of 3-6 in DMSO-d6 at room temperature.
6

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ESI mass spectrometric studies were also employed to characterize the complexes. ESI
mass spectra along with their simulated isotopic patters are presented in Fig. 2 and Fig. S3-S4
(ESI). ESI mass spectra of the complexes consist of many informative peaks with their isotopic
distribution patterns very close to that of simulated one. The bimetallic complexes exhibit
beautiful ESI mass spectra and many of the fragmented ions show only sequential removal of
imidazole NH proton and/or counter anion maintaining the principal bimetallic core intact,
making the assignments of the different peaks easier. Mass Spectrum of 4 exhibits a cluster of
peaks with m/z = 471.81 which clearly corresponds to [(phen)2Ru(dipy-bzim-tpy)Ru(tpy-
PhCH3)]3+ species. Complex 5, on the other hand, shows two peaks at m/z = 451.76 and m/z =
677.15 corresponding to [(phen)2Ru(dipy-Hbzim-tpy)Ru(Hpbbzim)]3+ and [(phen)2Ru(dipy-
Hbzim-tpy)Ru(pbbzim)]2+ species, respectively.
451.76
451.76
471.81
100
100
50
0
471.81
451.09
452.10
(b) 450.76
472.14
(a)
471.14
677.15
676.65
472.81
473
470.47
676.17
677.65
678.17
449 450 451 452 453
m/z
50
675.65
469
470
471
m/z
472
674
676
678
680
m/z
677.15
0
450
500
550
600 400
600
800
1000
m/z
m/z
Fig. 2 ESI-MS (positive) for the complex cations of 4 ([(phen)2Ru(dipy-bzim-tpy)Ru(tpy-Ph
3+
CH3)]3+ with m/z = 471.81) (a) and 5 ([(bpy)2Ru(dipy-Hbzim-tpy)Ru(Hpbbzim)] with m/z =
451.76 and [(bpy)2Ru(dipy-Hbzim-tpy)Ru(pbbzim)]2+ with m/z = 677.15) (b) in acetonitrile
showing experimental and simulated isotopic distribution patterns.
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Absorption spectral studies
UV-vis absorption spectra of equimolar solutions of the four bimetallic and two monometallic
precursor complexes recorded in acetonitrile are displayed in Fig. 3 and relevant spectral data are
summarized in Table 1. Absorption spectra of the monometallic precursors (1-2) consist of
several highly intense bands in the range of 220-375 nm due to ILCT and π-π* transitions of
coordinated dipy-Hbzim-tpy, bipyridine, and phenanthroline moieties and moderately intense
overlapping bands in the range between 420 nm and 470 nm due to Ru(dπ)→bpy/phen(π*) and
Ru(dπ)→dipy-Hbzim-tpy(π*) MLCT transitions. Each of the bimetallic complexes (3-6) exhibits
an addition strong MLCT band in the lower energy region (486 nm to 492 nm) which arises due
1.2
1.5x105
1
2
3
6
4
(b)
(a)
RT
5
3
0.9
0.6
0.3
0.0
5
1.0x105
6
4
1
2
5.0x104
0.0
300
450
600
600
700
/nm
800
900
/nm
2

1

3
5
6
4
(c)
(d)
1
2
3
4
5
6
116
50.3
77K
1
1000
100
10
2
0.9
89
40
22
20
8.8 18
5
6
4
3
9.2 20.6
0.6
0.3
0.0
600
700
/nm
800
0
200
400
/ns
600
800
Fig. 3 UV-vis absorption spectra (a), steady state luminescence spectra (b) and time-resolved
photoluminescence decays (c) of 1-6 in acetonitrile at room temperature and their luminescence
spectra in methanol-ethanol (1:4) glass at 77K (d).
8

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Table 1 Spectroscopic and photophysical data of 1-6 in acetonitrile
Absorption
Luminescence
298Ka
77Kb
max
max
,
Φ×1 kr×105/ s-1
knr,×107/ s-1 ken×
Φ
Com
pds

max/n /ns
/
0-3
nm (,M-1cm-1)
m
107s
nm
-1c
468(7420),423(8170),326
(sh)(30500),290((79490),
239(41480)
461(7330),421(11110),372
(br)(16070),321(br)(30000),2
85(sh)(50000),264(83000),22
4(68330)
637
620
116
46.0 3.96
21.5 8.09
0.82
5.63
--
--
605
592
0.18
0.16
1
2
50.3
490(46940),441(sh)(28650),3
37(br)(58820), 290 (149320)
655
22(74.5%) ,
89(25.5%)
15.5 7.02 , 1.73
4.47, 1.10
0.26 633
0.52 635
0.39
3
492(34760),310(sh)(75230),
285(88560)
652
682
20(28.2%) ,
24.0 11.97, 5.98 4.88, 2.44
1.9 2.1, 1.02
0.28
0.41
4
5
40(71.8%)
8.8(37.5%),
486(31740),348(sh)(79730),
331(sh)(73990), 313(sh)
(85760),288(109580),243
(75790)
11.34, 5.54 4.69 663
18(62.5%)
487(34250),348(70700),
331(sh)(78000),315(89060),
285 (98110), 265 (94600)
492(40320),351(br)(48950),3
30(sh)(74610),311(88770),28
2(72840),230(br)(50710)
685
658
9.2(48.9%),
20.6(51.1%)
2.0
20.5 12.43, 5.55 10.74, 4.79 2.86 665
0.44
0.12
6
1.8
9.20
49.91
8.21
----
----
637
1a
490(16230),347(53780),333(5 678
1360),315(49900),282(sh)(36
570),242(br)(37310)
12.0
14.7 12.24
672
628
0.22
1b
7e
8f
490(28000)
640
<5.0
------
≤0.0 ----
3
681(sh)
----
475(17400)
---
---
---
---
----
---
b
aIn CH3CN, MeOH-EtOH(1:4 v/v) glass.c ken corresponds to rate constant for energy transfer in 3-6, e
[Ru(tpy-Ph-CH3)2]2+ , f [Ru(H2pbbzim)2]2+
to Ru(dπ)→tpy-Hbzim-dipy/tpy-PhCH3(π*) MLCT transitions. The MLCT band ~500 nm region
is found to be typical signature of bis-terpyridine complexes of Ru(II).10-15 Coordination of
[Ru(tpy-PhCH3/H2pbbzim)]2+ unit to the free terpyridine site in 1 and 2 also leads to red-shifts of
ILCT bands of coordinated tpy-Hbzim-dipy bridge.
Luminescence spectral studies
Photoluminescence spectra of equimolar solutions of the bimetallic complexes (3-6) along with
the monometallic model complexes (1-2) in acetonitrile solution at room temperature and in
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ethanol-methanol (4:1 v/v) glass at 77K are presented in Fig. 3b and 3d. Relevant photophysical
data were already been summarized in Table 1. On excitation to any of the MLCT band maxima,
each monometallic complex exhibits strong room temperature luminescence with band
maximum at 637 nm for 1 and 620 nm for 2. Each of the four asymmetric bimetallic complexes
(3-6), on excitation at its MLCT maximum, shows relatively weak and broad luminescence band
at room temperature in the range between 652 nm and 685 nm, depending upon the coligand
(Fig. 3b). Thus, coordination of [Ru(tpy-PhCH3/H2pbbzim)]2+ unit to the tpy-site of the bridge
leads to red-shift of the luminescence maximum along with its significant quenching compared
with the monometallic precursor complexes (1 and 2). By comparing with structurally related
complexes, it can be concluded that the observed luminescence in all the complexes arise from
3
their respective MLCT state.6,11,12 Comparatively weak and red-shifted emission in the
asymmetric bimetallic complexes is due to the presence of bis-terpyridine Ru(II) moiety.
Unfavorable bite angle of tpy moiety compared with bpy/phen unit induces weaker crystal field
strength in Ru(tpy)2-type complexes resulting in smaller energy difference between the emitting
3
3MLCT and non-emitting MC excited states which in turn is responsible for inferior
luminescence characteristics in the bimetallic complexes.6,11,12 Excited state luminescence decays
of the four dyads (3-6) as well as two monometallic model complexes (1 and 2) are displayed in
Fig. 3c. 1 and 2 exhibits mono-exponential decay with lifetimes 116.0 ns and 50.3 ns,
respectively, while each of the four dyads exhibits bi-exponential decay with lifetime shorter
than the corresponding monometallic precursor (Table 1). The bi-exponential decay probably
arise due to the excited-state energy transfer from the Ru→bpy/phen MLCT state into lower-
energy Ru→tpy (of tpy-Hbzim-dipy) MLCT state and due to larger delocalization of the
"promoted" electron on the acceptor ligand (tpy-Hbzim-dipy), Franck-Condon factors for
radiationless decay of the MLCT became worse resulting in increase of the second component
lifetime. In frozen glass at 77K, the luminescence maximum of both bi- and monometallic
complexes shifts to lower wavelength region with substantial enhancement of luminescence
intensity and quantum yield which are similar to typical 3MLCT emitters.6
Temperature-dependent luminescence spectroscopic studies
In order to understand the kinetics of the excited state deactivation, temperature dependent (260-
350 K) steady state luminescence and excited state lifetimes of the bimetallic (3-6) as well as the
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monometallic (1-2) complexes measured in acetonitrile. The experimental outcomes are
displayed in Fig. 4, Fig. 5 and Fig. S5 (ESI). Both lifetimes and quantum yield of the complexes
were found to increase gradually with the decrease of temperature, although the extent of change
is different.

Temp(K)
K1=(7.71 ± 0.027)×106
(a)
141
121
268
293
(c)
1.8x107
1.2x107
6.0x106
1000
K2=(1.10 ± 0.12)×1013
100
303
308
=4716 ± 41
88
74
61
48
34
318
328
333
353
100
10
3.0x10-3
3.6x10-3
0
250
500
/ns
750
1/T(K-1)
K1=(1.19 ± 0.02)×107
Temp(K)

(b)
(d)
1000
100
10
73
64
54
42
268
278
K2=(1.31 ± 0.14)×1014
288
298
=4930 ± 40
1.0x108
35
23
13
08
308
323
343
353
5.0x107
0.0
3.0x10-3
3.5x10-3
1/T(K-1)
0
200
400
/ns
Fig. 4 Excited state decay profiles along with their lifetime values for 1 (a) and 2 (b) in MeCN as a
function of temperature. The nonlinear fits of the temperature-dependent lifetime data using equation
1 are shown in (c) for 1 and (d) for 2. Insets of (c) and (d) show the values of different parameters.
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2.0x107
 
Temp(K)
266
K1=(7.85 ± 0.034)×106
(a)
1000
100
10
131
14
(c)
K2=(1.14 ± 0.17)×1013
20 115
283
23
95
=4745 ± 53
293
25 77
69
25
23 60
23 53
313
1.5x107
1.0x107
323
333
343
3.0x10-3
3.5x10-3
4.0x10-3
0
250
500
/ns
750
1000
(1/T)/K-1
9.0x107
6.0x107
3.0x107
K1=(1.25 ± 0.013)×107


Temp(K)
(b)
1000
100
10
(d)
268
278
288
298
308
318
333
353
38 87
K2=(2.63 ± 0.22)×1013
28
22 57
15 43
10
07
5.5
3.7
70
=4493 ± 31
30
23
17
12
2.8x10-3 3.2x10-3 3.6x10-3
0
150
300
450
/ns
1/T(K-1)
Fig. 5 Excited state decay profiles along with their lifetime values for 3 (a) and 6 (b) in MeCN as a
function of temperature. The nonlinear fits of the temperature-dependent lifetime data using equation
1 are shown in (c) for 3 and (d) for 6. Insets of (c) and (d) show the values of different parameters
The lifetime data of the complexes can be fitted with the following equation.6,26
(τ(T))-1 = (k1 + k2 exp[-ΔE2/RT])/(1 + exp[-ΔE2/RT])
(1)
where k1 is the temperature-independent rate constant which is the summation of radiative (kr),
3
and nonradiative (knr) rate constants corresponding to the decay from the MLCT state to the
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ground state at low temperature (77K). However, it is assumed that both kr and knr are
independent of temperature at T>77 K. k2 is the dependent rate constant which corresponds to the
3
population of the ligand-field (3MC) state from MLCT state and the activation energy required
for this process is designated as ΔE2. Non linear regression analysis of the experimental data
allowed us to calculate the values of k1, k2 and ΔE2 and the calculated values are displayed in Fig.
4, Fig. 5 and Fig. S5 (ESI). Calculated ΔE2 value lie in the range between 4279±45cm-1 (5) and
4901±26 cm-1 (4) for the bimetallic complexes, while for the monometallic precursor complexes
the value is 4716±41cm-1 for 1 and 4930±40cm-1 for 2. The calculated values of k1 lie in the
range of (1.48±0.025)×106 - (1.25±0.013)×107s-1, while the k2 values vary between
(1.14±0.17)×1013 and (2.63±0.22)×1013 s-1 for 3-6. The variation of luminescence lifetime of the
monometallic model complexes (1-2) follow similar trend as that of bimetallic complexes (3-6)
with the exception that they exhibit mono-exponential decay profiles throughout the studied
temperature range compared with the bi-exponential decays of the asymmetric bimetallic
complexes (Fig. 5 and S5, ESI). It is to be mentioned that values of ΔE2 increase remarkably in
all the four bimetallic complexes compared with parent [Ru(tpy)2]2+ (ΔE2=1500 cm-1) and
[Ru(tpy-PhCH3)2]2+ (ΔE2=1800 cm-1) and the increase is probably due to extensive
delocalization induced by the bridge as well as by the terminal Ru(bpy/phen)2 units.6,7,10-14
Moreover, the value of ΔE2 varies with the terminal ligand (both bidentate and tridentate) in the
bimetallic complexes. In addition, experimentally observed room temperature lifetimes and
quantum yields of the complexes are nicely correlated with the calculated values of ΔE2.
Redox behaviors
Cyclic (CV) and square wave voltammetry (SWV) were used to study the redox behaviors of the
complexes and the voltammograms that are recorded in accetonitrile solutions are displayed in
Fig. 6 and the relevant redox data are summarized in Table 2. Each of the monometallic complex
exhibits reversible oxidation at ~1.2 V due to RuII/RuIII and several reversible or quasi-reversible
reduction waves due to coordinated ligands in the negative potential window (0 to -2.4 V). The
bimetallic complexes, on the other hand, exhibit two successive reversible oxidative redox
couples overlapped with each other. By comparing the potential of RuII/RuIII oxidation in the
monometallic complexes, the couple with E1/2 value in the vicinity of 1.25 V corresponds to the
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oxidation of the Ru(II) center coordinated to the bidentate site of dipy-Hbzim-tpy, while the
couple having E1/2 value around 1.1 V corresponds to the oxidation of the Ru(II) center
coordinated to the terpyridine unit of dipy-Hbzim-tpy. Due to relatively long distances, the
extent of electronic interaction between the two metal centers is relatively weak. Two closely
associated oxidation waves in the bimetallic complexes probably arise due to small difference of
the electronic environment around the two metal centers. Coordination of the [Ru(tpy-
PhCH3/H2pbbzim)]2+ unit to the free terpyridine site of 1 and 2 tend to stabilize the π* anti-
bonding orbital of dipy-Hbzim-tpy which results in the positive shift of the reduction potential of
dipy-Hbzim-tpy in the bimetallic complexes.
(6)
(4)
(3)
(1)
(2)
1.6
1.4
1.2
1.0
0.8
0.6
1.6
1.4
1.2
1.0
0.8
0.6
Pot(E/V)
Pot(E/V)
Fig. 6 CVs (solid lines) and SWVs (dotted lines) of complexes 3-6 in acetonitrile using Pt as the
working and Ag/AgCl as the reference electrode at room temperature. The scan rate is 100mVs-1.
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Table 2 Electrochemical data[a] for 3–6 in acetonitrile
Compds
Oxidationb
E1/2(ox)/V
1.22
Reductionc
E1/2(red)/V
1
2
3
4
5
6
-1.04,-1.39 , -1.69
1.21
-1.44, -1.71, -1.93, -2.16
-1.14, -1.27, -1.45, -1.82, -2.29
-1.24, -1.41, -1.78, -2.09
-1.18, -1.55, -1.8, -1.99
-1.37, -1.58, -1.88, -2.13
1.18, 1.25
1.13, 1.25
1.08, 1.19
1.11, 1.23
[a] All potentials are referenced against Ag/AgCl electrode with E1/2= 0.36 V
for the Fc/Fc+ couple.[b] Reversible electron-transfer process with a Pt working electrode.
[c]
E
1/2
values obtained SWV with glassy carbon electrode.
Photoinduced intramolecular energy transfer in the asymmetric bimetallic
complexes
In the previous section we have observed that luminescence intensity, quantum yields and the
excited state lifetimes of the bimetallic complexes got substantially reduced relative to that of
their monometallic model complexes (1 and 2) indicating that coordination of the [Ru(tpy-
PhCH3/H2pbbzim)]2+ moiety to the terpyridine site of dipy-Hbzim-tpy induces significant
quenching on the [(bpy)2RuII(dipy-Hbzim-tpy)]-centered emission. The extent of luminescence
quenching can be estimated by recording the luminescence intensities of isomolar solutions of
both bimetallic as well as monometallic model complexes under identical experimental
condition. Observed results indicate that the luminescence intensity of the [(bpy/phen)2Ru(dipy-
Hbzim-tpy)]2+chomophore in 3 and 4 is quenched up to the extent of ~80%, while the extent of
quenching is much greater (~97%) for 5 and 6. The excited state energy of the [(bpy)2Ru(bpy-
Hbzim-tpy)]- and [(phen)2Ru(bpy-Hbzim-tpy]-based donor component in the bimetallic
complexes can be approximated as the E00 value (estimated from the luminescence maximum at
77 K) of the monometallic model complex 1 and 2, respectively, and the value is 2.05 eV for 3
and 5, while 2.09 eV for 4 and 6. The excited state energy of the acceptor component [(dipy-
Hbzim-tpy)Ru(tpy-PhCH3)]- and [(dipy-Hbzim-tpy)Ru(H2pbbzim)] in the dyads, on the other
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hand, can be estimated from the 77 K luminescence maximum of the monometallic model
complex 1a and 1b, respectively, and the estimated value is 1.94 eV for 3 and 4 while 1.84 for 5
and 6 (Table 1). Interestingly, the observed luminescence maxima of the asymmetric dyads
match closely with those of their corresponding model bis-tridentate Ru(II) complexes (1a and
1b). Thus, the free energy change of photo-induced energy transfer from [(bpy/phen)2Ru(bpy-
Hbzim-tpy]–based unit to the [(tpy-PhCH3/H2pbbzim)Ru(dipy-Hbzim-tpy)]-based component in
the bimetallic complexes were found to lie in the range between -0.09 eV and -0.23 eV and thus
the energy transfer process is thermodynamically favorable in all the four dyads. The observed
luminescence quenching may also be due to photo-induced electron transfer process. But
calculation of excited state redox potentials shows that free energy change (lying in the range
between +0.17 and +0.39 eV) for the electron transfer process are thermodynamically
unfavorable and thus ruling out the photo-induced electron transfer process in the dyads.
The rate constant, ken for energy transfer can be calculated by using equation (2)
ken = 1/ – 1/0
(2)
where 0 is the emission lifetime of the monometallic model complexes 1 and 2 (116 ns for 1 and
50.3 ns) and  is the luminescence lifetime of the bimetallic complexes (3-6). Luminescence
decay profiles of the complexes at room temperature show that all the four bimetallic complexes
(3-6) have shorter lifetimes (varying between 18.0 ns and 89.0 ns) compared with the
monometallic model complex (116 ns for 1 and 50.3 ns for 2). Thus, the estimated values of
intramolecular energy transfer (ken) were found to in the range of 2.6 106– 4.69 107 s-1 at room
temperature.
The occurrence of intramolecular energy transfer in few asymmetric binuclear Ru(II)
complexes derived from heteroditopic bpy-tpy or phen-tpy type bridging ligands have previously
been reported.20-24,27 Energy transfer processes usually occur in two ways, viz. the Dexter–type28
through bond electron exchange or the Förster-type29 through space Coulombic interaction.
However, in most cases it is not easy to ascertain which particular mechanism plays the
dominant role. In general, electron exchange becomes important where the spacer is either highly
conjugated or relatively short, while Coulombic mechanism becomes important when the spacer
is either saturated or the transition dipoles are well–separated.30
Constable and coworkers reported the synthesis, electronic communication and inter-
component energy transfer in bimetallic Ru(II) complexes based on a bridge which contain both
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2,2′-bipyridine and 2,2′:6′,2″-terpyridine type coordinating units. In the bimetallic complex, only
one red-shifted luminescence band was observed compared with the monometallic Ru(bpy)2-
based model complex.21a From electrochemical results, they showed that LUMO is localized
primarily on the {Ru(terpy)2}-fragment and they suggested that observed luminescence in the
bimetallic complexes is due to tpy-coordinated Ru(II) center. Although they indicated the
occurrence of energy transfer from the bpy-coordinated to the tpy-coordinated Ru(II) center, but
they did not report the thermodynamic and kinetic aspect of the photo-induced intramolecular
energy transfer process.
Vos and coworkers designed a series of mono- and bimetallic Ru(II) polypyridyl
complexes based on the heteroditopic bridging ligand (PT) incorporating bidentate 1,10-
phenanthroline (phen) and terdentate 2,2′:6′,2″-terpyridine (tpy) units.22a For the asymmetric
bimetallic complex, a single red-shifted luminescence band at 634 nm with very low quantum
yield (Φ=0.0014) and short lifetime <10 ns was observed compared with tris-bidentate
monometallic precursor complex having λemi = 624 nm, Φ = 0.062 and τ = 1580 ns. The
emission lifetime data suggest that the lowest-energy triplet excited state in the bimetallic Ru(II)
complex is based on the tpy moiety and that energy transfer from the phen to the tpy unit is
effective.
Previously, we have reported a series of bimetallic Ru(II) complexes of the type
[(bpy)2RuII(phen-Hbzim-tpy)RuII(tpy/tpy-PhCH3/H2pbbzim)]4+ where phen-Hbzim-tpy is a
heteroditopic phen-tpy bridge, 2-(4-(2,6-di(pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5-
f][1,10]phenanthroline.23a Steady state and time resolved luminescence data at room temperature
show that an efficient intramolecular electronic energy transfer from the MLCT excited state of
the [(bpy)2RuII(phen-Hbzim-tpy)] chromophore to the MLCT state of the tpy-containing
chromophore [(phen-Hbzim-tpy)RuII(tpy/tpy-PhCH3/H2pbbzim)] occurs in all the three
unsymmetrical bimetallic complexes and the values of the rate constant were found to vary in the
range between 6.4  106 s-1 and 5.7  107 s-1, depending on the nature of the tridentate terminal
ligand in the complexes.
Hammarström, Johansson and coworkers investigated detailed photophysical properties
of N6–N5C bichromophoric systems of the type [(bpy)2Ru(I)Ru(ttpy)][PF6]3 (bpy is 2,2′-
bipyridine and ttpy is 4′-p-tolyl-2,2′:6′,2″-terpyridine) and [(bpy)2Ru(II)Ru(ttpy)][PF6]3
(I and II are bpy-dipyridylbenzene ditopic ligands bridged by an ethynyl and phenyl unit,
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respectively) and compared with the model mononuclear complexes [(bpy)2Ru(I)][PF6]2,
[(bpy)2Ru(II)][PF6]2, [Ru(III)(ttpy)][PF6] (III is 3,5-di(2-pyridyl)-biphenyl) and [Ru(dpb)(ttpy)]
[PF6] (Hdpb is 1,3-di(2-pyridyl)-benzene).27 Selective excitation of the N5C unit in the
bichromophoric systems leads to luminescence typical for a bis-tridentate cyclometallated Ru(II)
complex and is similar to the [Ru(III)(ttpy)][PF6] model complex. In contrast, the luminescence
from the tris-bidentate N6 unit is efficiently quenched by energy transfer to the N5C unit. They
determined the energy transfer rate by femtosecond pump-probe measurements to 0.7 ps in the
ethynyl-linked [(bpy)2Ru(I)Ru(ttpy)][PF6]3 and to 1.5 ps in the phenyl-linked [(bpy)2Ru(II)Ru
(ttpy)][PF6]3 (in acetonitrile solution at 298 K), and concluded the energy transfer occur via a
Dexter mechanism.
From the above discussions it is probable that intramolecular energy transfer in our
present systems also occur via electron-exchange Dexter mechanism. It is evident that rate
constant of the energy transfer of the present system is much less compared to those reported by
the research group of Hammarström, Johansson and coworkers. Although we are not very sure
for slower rate but the relatively long distance between the donor (Ru-bpy based component) and
acceptor component (Ru-tpy based unit) is probably responsible for slower energy transfer rate
in our system.
Conclusions
A new family of luminescent and redox-active bimetallic Ru(II) complexes derived from
asymmetric bridge consisting of both bipyridine and terpyridine chelating sites covalently
coupled via phenyl-imidazole spacer have been designed and fully characterized in this work.
The main motivation of the work is to demonstrate photo-induced intramolecular energy transfer
from one component to the other in the asymmetric dyads. Both bidentate and tridentate terminal
ligands in the asymmetric complexes were varied for fine tuning of their photo-redox properties.
Steady state and time-resolved luminescence spectral measurements unequivocally suggests
efficient energy transfer from the excited MLCT state of the [(bpy/phen)2RuII(dipy-Hbzim-tpy
phen-Hbzim-tpy)] component to the tpy-containing unit, [(dipy-Hbzim-tpy)RuII(tpy-PhCH3/
H2pbbzim)] of all the four dyads at room temperature. The rate constants of the energy-transfer
process were determined by time-resolved emission spectroscopy and were found to dependent
upon the nature of the coligand in the complexes.
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Experimental Section
Materials
2,2'-Bipyridine (bpy), 1,10-phenanthroline (phen), 1,2-phenylenediamine, and 2,6-pyridine
dicarboxylic acid were purchased from Sigma-Aldrich. Other reagent grade chemicals were
procured from local vendors and were used as received. 4'-(p-methylphenyl)-2,2':6',2''-
terpyridine (tpy-PhCH3),31 4′-(p-formylphenyl)-2,2':6', 2''-terpyridine (tpy-PhCHO),32 2,6-bis
(benzimidazole-2-yl)pyridine (H2pbbzim)33, asymmetric bridging ligand 4'-[4-(4,5-Di-pyridin-2-
yl-1H-imidazol-2-yl)-phenyl]-[2,2';6',2'']terpyridine (tpy-Hbzim-dipy)25a, cis-[Ru(bpy)2Cl2]·
2H2O34 and cis-[Ru(phen)2Cl2]34 were prepared following reported literature procedures. The
tridentate precursors, [(tpy-PhCH3)RuCl3] and [(H2pbbzim) RuCl3] were synthesized by reacting
RuCl3·3H2O with tpy-PhCH3 and H2pbbzim in 1:1 molar ratio in refluxing ethanol. The
mononuclear complexes of the type [(bpy/phen)2Ru(tpy-Hbzim-dipy)](ClO4)2 (1/2) were
synthesized according to our recent reports.25b,c Synthesis and characterization of two other
model bis-tridentate complexes, viz. [dipy-Hbzim-tpy)Ru(tpy-PhCH3)](ClO4)22H2O (1a) and
[(dipy-Hbzim-tpy)Ru(H2pbbzim)](ClO4)2·H2O (1b) were provided in the experimental section of
ESI.
Synthesis of the metal complexes
Synthesis of the bimetallic Ru(II) complexes were carried out under argon atmosphere.
[(bpy)2Ru(dipy-Hbzim-tpy)Ru(tpy-PhCH3)](ClO4)42H2O (3). [(tpy-PhCH3)RuCl3]
(50 mg, 0.09 mmol) and [(bpy)2Ru(tpy-Hbzim-dipy)](ClO4)2 (1) (100 mg, 0.09 mmol) were
dispersed in 20 mL ethylene glycol and heated at 200C with continuous stirring for ~6h. After
cooling at room temperature the resulting deep red solution was filtered and the filtrate was
slowly added to an aqueous solution of NaClO4H2O (1.0 g in 10 mL of water) to induce
precipitation of the complex as the perchlorate salt. After ~10m of stirring, the precipitated
complex was separated out by filtration and washed couple of times with cold water and finally
dried under vacuum. The complex was purified by loading its acetonitrile solution into silica gel
column and gradually eluting with acetonitrile-methanol (1:1 v/v) solvent mixture. On rotary
evaporation of the combined filtrate to a volume of 5 mL, a bright red compound was deposited
on the walls of the round bottomed flask. The compound was collected by filtration and
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recrystallized from acetonitrile–methanol (1:1 v/v) mixture in presence of a few drops of
aqueous 10-4 M HClO4 (Yield: 105 mg, 63 %). Anal. Calcd. for C76H60N14Cl4O18Ru2: C, 50.69;
H, 3.35; N, 10.88. Found: C, 50.80; H, 3.45; N, 10.60. 1H NMR (500 MHz, DMSO-d6,  / ppm:
14.25 (s, 1H, NH imidazole), 9.46 (s, 2H, 2H3), 9.42 (d, 1H, J = 5.0 Hz, H3), 9.26 (s, 2H,
2H3), 9.14 (d, 2H, J = 7.5 Hz, 2H6), 9.07 (d, 2H, J = 7.0 Hz, 2H6), 8.92 (d, 1H, J = 7.5 Hz,
H16), 8.83 (d, 1H, J = 7.5 Hz, H9), 8.76 (d, 1H, J = 7.5 Hz, H3), 8.72-8.69 (m, 4H , 2H8+
2H3), 8.65 (d, 1H, J = 8.0 Hz, H8), 8.61 (d, 1H, J = 8.0 Hz, H8), 8.36-8.35 (m, 3H, 2H7+ H6),
8.23 (d, 2H, J = 6.0 Hz, 2H4), 8.14 (t, 3H, J = 7.5 Hz, H14+2H4), 8.07-8.0 (m, 4H,
2H4+H10+H11), 7.72 (t, 1H, J = 5.5 Hz, H5), 7.66 (t, 4H, J = 5.5 Hz, 2H4+H12+ H5), 7.60-
7.54 (m, 4H, 2H3+2H6), 7.48-7.47 (m, 5H, H5+2H7+2H5), 7.35 (d, 1H, J = 6.0 Hz, H6), 7.34
(d, 1H, J = 6.0 Hz, H15), 7.32-7.27 (m, 3H, 2H5+H13), 7.30-7.19 (m, 1H, H5), 7.02 (d, 2H, J =
7.5 Hz, 2H3), 2.50 (s, 3H, CH3). ESI-MS (positive, CH3CN) m/z = 455.78 (100 %)
[(bpy)2Ru(dipy-bzim-tpy)Ru(tpy-PhCH3)]3+.
[(phen)2Ru(dipy-Hbzim-tpy)Ru(tpy-PhCH3)](ClO4)42H2O (4). Complex 4 is obtained
in the same way as 3 except using [(phen)2Ru(tpy-Hbzim-dipy)](ClO4)2 (2) instead of
[(bpy)2Ru(tpy-Hbzim-dipy)](ClO4)2 (1). Yield: 120 mg, 64%. Anal. Calcd. for
1
C80H60N14Cl4O18Ru2: C, 47.29; H, 2.97; N, 9.65. Found: C, 47.15; H, 3.01; N, 9.58. H NMR
(500 MHz, DMSO-d6,  / ppm): 14.22 (s, 1H, NH imidazole), 9.48 (s, 2H, H3 ), 9.24 (d, 1H, J =
6.5 Hz, H3), 9.17 (s, 2H, H3 ), 9.11 (d, 1H, J = 8.5 Hz, H6 ), 9.04 (d, 2H, J = 8.0 Hz, H6+ H9),
8.94-8.91 (m, 2H, H6+H16), 8.82 (d, 1H, J = 8.5 Hz, H6), 8.68-8.55 (m, 6H, 4H8+ 2H3’), 8.48-
8.30 (m, 6H, H4+H10+H3+H4+2H5), 8.24 (d, 1H, J = 5.5 Hz, H4), 8.21-8.16 (m, 4H,
2H7+H15+H4), 8.10-8.03 (m, 3H, 2H4+H11), 7.94 (d, 2H, J = 8.0 Hz, 2H6), 7.86 (d, 1H, J =
5.0 Hz, H3), 7.79 (d, 2H, J = 8.5 Hz, 2H6), 7.66-7.57 (m, 5H, 2H3+2H7+H4), 7.74-7.70 (m,
2H, H5+H14), 7.52-7.47(m, 2H, H12+H5), 7.37 (t, 1H, J = 6.0 Hz, H5), 7.32-7.28 (m, 4H,
H4+2H5+H13), 7.13-7.11 (m, 1H, H5), 7.02-6.98 (m, 1H, H3) 2.49 (s, 3H, CH3). ESI-MS
(positive, CH3CN) m/z = 471.81 (100 %) [(phen)2Ru(dipy-bzim-tpy)Ru(tpy-PhCH3)]3+.
[(bpy)2Ru(dipy-Hbzim-tpy)Ru(H2pbbzim)](ClO4)42H2O (5). [(bpy)2Ru(tpy-Hbzim-
dipy)](ClO4)2 (1) (100 mg, 0.09 mmol) was suspended into 25 mL ethylene glycol and heated at
~100C with stirring until the complex get dissolved. To the solution, powdered
[(H2pbbzim)RuCl3] (50 mg, 0.09 mmol) was added and the temperature was raised gradually to
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~220C and kept at this temperature for ~8h with continuous stirring when a clear red-violet
solution was obtained. The perchlorate salt of the complex was obtained by pouring the solution
into an aqueous solution of NaClO4H2O (1.0 g in 10 mL of water). The resulting red-violet
precipitate was collected by filtration and washed with cold water and then vacuum dried. The
resulting complex was purified by silica gel column chromatography using a mixture of CH3CN
and 10% aqueous KNO3 (10:1 v/v) as the eluent. Rotary evaporation and subsequent anion
metathesis reaction with NaClO4H2O leads to desired compound. Further purification was done
by recrystallization of the compound from acetonitrile–methanol (1:1 v/v) mixture in presence of
a few drops of aqueous 10-4 M HClO4. Yield: 105 mg, 68%. Anal. Calcd. for
C73H56N16Cl4O18Ru2: C, 49.02; H, 3.15; N, 12.53. Found: C, 48.97; H, 3.24; N, 12.60. 1H NMR
(500 MHz, DMSO-d6,  / ppm): 15.11 (s, 2H, NH imidazole, H2pbbzim), 14.34 (s, 1H, NH
imidazole), 9.39 (s, 2H, 2H3 ), 8.98 (d, 3H, J = 8.5 Hz, H6+2H3), 8.93(d, 1H, J = 4.5 Hz, H6),
8.84 (d, 1H, J = 8.5 Hz, H9), 8.80-8.75 (m, 4H, 2H18+ 2H3), 8.71 (d, 1H, J = 5.5 Hz, H16),
8.65 (t, 1H, J = 5.5 Hz, H17), 8.47 (d, 1H, J = 8.0 Hz, H8), 8.28-8.23 (m, 1H, H6), 8.20-8.17
(m, 4H, H8+3H4), 8.0-7.96 (m, 6H, 2H4+H10+H11+H15+H4), 7.77 (t, 1H, J = 5.5 Hz, H5),
7.68-7.65 (m, 5H, H12+2H5+2H6), 7.54-7.52(m, 4H, 2H7+2H19), 7.45(t, 1H, J = 5.5 Hz, H5),
7.37 (d, 2H, J = 8.0 Hz, 2H3), 7.29-7.25 (m, 6H, H5+H13+H14+2H20+ H6), 7.19(t, 1H, J = 5.5
Hz, H5), 7.02(t, 2H, J = 7.95 Hz, 2H21), 6.06(d, 2H, J = 8.0 Hz, 2H22). ESI-MS (positive,
CH3CN) m/z = 451.76 (100 %) [(bpy)2Ru(dipy-Hbzim-tpy)Ru(Hpbbzim)]3+ and m/z = 677.15
(12 %) [(bpy)2Ru(dipy-Hbzim-tpy)Ru(pbbzim)]2+.
[(phen)2Ru(dipy-Hbzim-tpy)Ru(H2pbbzim)](ClO4)42H2O (6). Complex
6
was
synthesized by adopting the same procedure as 5 except using [(phen)2Ru(tpy-Hbzim-dipy)]
(ClO4)2 (2) instead of [(bpy)2Ru(tpy-Hbzim-dipy)](ClO4)2 (1) as the starting material. Yield: 105
mg, 69%. Anal. Calcd. for C77H56N16Cl4O18Ru2: C, 50.35; H, 2.85; N, 12.20 Found: C, 50.43; H,
1
2.95; N, 12.11. H NMR (500 MHz, DMSO-d6,  / ppm): 15.08 (s, 2H, NH imidazole,
H2pbbzim), 14.31 (s, 1H, NH imidazole), 9.61 (s, 2H, 2H3 ), 9.10-8.97 (m, 4H, 2H6+2H3),
8.80-8.75 (m, 5H, H9+2H18+2H3), 8.70 (d, 1H, J = 8.5 Hz, H16), 8.66-8.62 (m, 2H,
H17+H4), 8.46(d, 1H, J = 9.0 Hz, H8), 8.37 (d, 4H, J = 8.5 Hz, 2H4+2H6), 8.18-8.13 (m, 2H,
H8+H15), 7.95 (t, 4H, J = 7.75 Hz, 2H4+H10+H11), 7.66 (t, 5H, J = 7.5 Hz, 2H3+H12+2H5),
7.51 (t, 5H, J = 6.25 Hz, 2H7+2H19+H6), 7.43 (d, 1H, J = 4.5 Hz, H4), 7.29-7.24 (m, 7H,
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2H5+H13+H14+2H20+H6), 7.20 (t, 1H, J = 5.5 Hz, H5), 7.03-6.98(m, 3H, 2H21+H5), 6.04(d,
2H, J = 8.0 Hz, 2H22). ESI-MS (positive, CH3CN) m/z = 467.77 (100 %) [(bpy)2Ru(dipy-
Hbzim-tpy)Ru(Hpbbzim)]3+ and m/z = 701.17 (22 %) [(bpy)2Ru(dipy-Hbzim-tpy)Ru(pbbzim)]2+.
Physical measurements
The instruments used and descriptions of different physicochemical measurements employed in
this work have been provided in the experimental section of Electronic supplementary
information.
Acknowledgements
Financial supports received from the Department of Science and Technology, New Delhi, India
[Grant No. (EMR/2015/001163 (SERB)] and the Council of Scientific and Industrial Research,
New Delhi, India [Grant No. 01(2766)/13/EMR-II] is gratefully acknowledged. TCSPC facility
under the DST-PURSE program of Department of Chemistry (JU) is also gratefully
acknowledged. Manoranjan Bar acknowledges UGC for his research fellowship.
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References
1
V. Balzani, Electron Transfer in Chemistry, in: Biological and Artificial Supramolecular
Systems, vol. 3, VCH Wiley, Weinheim, 2001; b) V. Balzani, A. Credi and M. Venturi,
Molecular Devices and Machines; Wiley-VCH: Weinheim, 2003.
P. Jordan, P. Fromme, H. T. Witt, O. Klukas, W. Saenger and N. Krauss, Nature, 2001, 411,
909.
2
3 J. R. Winkler and H. B. Gray, Chem. Rev., 1992, 92, 369.
4 H. B. Gray and J. R. Winkler, Proc. Natl. Acad. Sci. USA, 2005, 102, 3534.
5 J. Stubbe, D. G. Nocera, C. S. Yee and M. C. Y. Chang, Chem. Rev., 2003, 103, 2167.
6
(a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and A. von Zelewsky, Coord.
Chem. Rev., 1988, 84, 85; (b) V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni,
Chem. Rev., 1996, 96, 759 ; (c) S. Welter, N. Salluce, P. Belser, M. Groeneveld and L. De
Cola, Coord. Chem. Rev., 2005, 249, 1360.
7 (a) T. J. Meyer, Acc. Chem. Res., 1989, 22, 163; (b) M. H. V. Huynh, D. M. Dattelbaum and
T. J. Meyer, Coord. Chem. Rev., 2005, 249, 457.
8 E. Baranoff, J.- P. Collin, L. Flamigni and J.- P. Sauvage, Chem. Soc. Rev., 2004, 33,147.
9 O. S. Wenger, Coord. Chem. Rev., 2009, 253, 1439.
10 (a) A. Arrigo, A. Santoro, F. Puntoriero, P. Lain and S. Campagna, Coord. Chem. Rev., 2015,
304-305, 109; (b) M. Natali, S. Campagna and F. Scandola, Chem. Soc. Rev., 2014, 43, 4005;
(c) P. P. Laine, S. Campagna and F. Loiseau, Coord. Chem. Rev., 2008, 252, 2552.
11 (a) E. A. Medlycott and G. S. Hanan, Coord. Chem. Rev., 2006, 250, 1763; (b) A. K. Pal and
G. S. Hanan, Chem. Soc. Rev., 2014, 43, 6184; (c) E. A. Medlycott and G. S. Hanan, Chem.
Soc. Rev., 2005, 34, 133.
12 (a) H. Hofmeier and U. S. Schubert, Chem. Soc. Rev., 2004, 33, 373; (b) E. C. Constable,
Chem. Soc. Rev., 2004, 33, 246.
13 A. Harriman and R. Ziessel, Chem. Commun., 1996, 1707.
14 (a) A. Hagfeldt and M. Grätzel, Acc. Chem. Res., 2000, 33, 269; (b) L. Sun, L.
Hammarström, B. Akermark and S. Styring, Chem. Soc. Rev., 2001, 30, 36; (c) W. R.
Browne, N. M. O'Boyle, J. J. McGarvey and J. G. Vos, Chem. Soc. Rev., 2005, 34, 641; (d)
A. Inagaki and M. Akita, Coord. Chem. Rev., 2010, 254, 1220; (e) Z. Q. Chen, Z. Q. Bian
and C. H. Huang, Adv. Mater., 2010, 22, 1534; (f) S. Baitalik, X. Wang and R. H. Schmehl,
23

Dalton Transactions
Page 24 of 26
View Article Online
DOI: 10.1039/C6DT03250H
J. Am. Chem. Soc., 2004, 126, 16304; (g) X.-Y. Wang, A. Del Guerzo and R. H. Schmehl, J.
Photochem. Photobiol. C 2004, 5, 55; (h) X. Wang, A. Guerzo, S. Baitalik, G. Simon, G. B.
Shaw, L. X. Chen and R. H. Schmehl, Photosynth. Res., 2006, 87, 83 and references therein.
15 (a) G. Chelucci and R. P. Thummel, Chem. Rev., 2002, 102, 3129; (b) N. Kaveevivitchai, R.
Chitta, R. Zong, M. E. Ojaimi and R. P. Thummel, J. Am. Chem. Soc., 2012, 134, 10721; (c)
R. Zong and R. P. Thummel, J. Am. Chem. Soc., 2004, 126, 10800.
16 (a) J. J. Warren, T. A. Tronic and J. M. Mayer, Chem. Rev., 2010, 110, 6961; (b) J. M.
Mayer, Acc. Chem. Res., 2011, 44, 36.
17 (a) T. Ohno, K. Nozaki, N. Ikeda and M. Haga, Advances in Chemistry. 2009, 228, 215; (b)
M. Haga, M. Ali and R. Arakawa, Angew. Chem. Int. Ed., 1996, 35, 76.
18 (a) K. M-C. Wong and V. W. W. Yam, Acc. Chem. Res., 2011, 44, 424; (b) V. W. W. Yam,
V. K. M. Au and S. Y. L. Leung, Chem. Rev., 2015, 115, 7589.
19 (a) K. J. Arm and J. A. G. Williams, Chem. Commun., 2005, 230; (b) K. J. Arm and J. A. G.
Williams, Dalton Trans., 2006, 2172.
20 (a) A. Harriman, M. Hissler, A. Khatyr and R. Ziessel, Eur. J. Inorg. Chem., 2003, 955; (b)
A. Harriman, A. Khatyr and R. Ziessel, Dalton Trans., 2003, 2061; (c) A. Harriman, A.
Mayeux, C. Stroh and R. Ziessel, Dalton Trans., 2005, 2925.
21 (a) E. C. Constable, E. Figgemeier, C. E. Housecroft, J. Olsson and Y. C. Zimmermann,
Dalton Trans., 2004, 1918; (b) E. Figgemeier, E. C. Constable, C. E. Housecroft and Y. C.
Zimmermann, Langmuir, 2004, 20, 9242.
22 (a) E. Coronado, P. Gavi a, S. Tatay, R. Groarke and J. G. Vos, Inorg. Chem., 2010, 49,
6897; (b) Y. Halpin, D. Dini, H. M. Y. Ahmed, L. Cassidy, W. R. Browne, J. G. Vos, Inorg.
Chem. 2010, 49, 2799.
23 (a) D. Maity, C. Bhaumik, S. Karmakar and S. Baitalik, Inorg. Chem., 2013, 52, 7933; (b) D.
Maity, C. Bhaumik, D. Mondal and S. Baitalik, Dalton Trans., 2014, 43, 1829; (c) D. Maity,
C. Bhaumik, S. Mardanya, S. Karmakar and S. Baitalik, Chem. Eur. J., 2014, 20, 13242.
24 (a) C. Herrero, A. Quaranta, S. Protti, W. Leibl, A. W. Rutherford, Dr R. Fallahpour, M-F.
Charlot and A. Aukauloo, Chem. Asian J., 2011, 6, 1335; (b) C. Herrero, A. Quaranta, R-A.
Fallahpour, W. Leibl and A. Aukauloo, J. Phys. Chem. C, 2013, 117, 9605.
25 (a) S. Karmakar, S. Mardanya, S. Das and Baitalik, S. J. Phys. Chem. C, 2015, 119, 6793; (b)
S. Das, S. Karmakar, S. Mardanya and S. Baitalik, Dalton Trans., 2014, 43, 3767; (c) S.
24

Page 25 of 26
Dalton Transactions
View Article Online
DOI: 10.1039/C6DT03250H
Karmakar, M. Nandi, S. Mukherjee and S. Baitalik, Inorg. Chim. Acta, DOI.org/10.
1016/j.ica.2016.02.041.
26 D. P. Rillema, C. B. Blanton, R. J. Shaver, D. C. Jackman, M. Boldaji, S. Bundy, L. A. Worl
and T. J. Meyer, Inorg. Chem., 1992, 31, 1600.
27 S. Ott, M. Borgström, L. Hammarström and O. Johansson, Dalton Trans., 2006, 1434.
28 D. L. Dexter, J. Chem. Phys., 1953, 21, 836.
29 T. H. Förster, Discuss. Faraday Soc., 1996, 27, 7.
30 L. De Cola and P. Belser, Coord. Chem. Rev., 1998, 177, 301.
31 (a) C. Bhaumik, S. Das, D. Saha, S. Dutta and S. Baitalik, Inorg. Chem., 2010, 49, 5049; (b)
W. Spahni and G. Calzaferri, Helv. Chim. Acta, 1984, 67, 450.
32 K. T. Pott, D.A. Usifer and H. D. Abruna, J. Am. Chem. Soc., 1987, 109, 3961.
33 A. W. Addison, T. N. Rao and C. G. Wahlgren, J. Heterocycl. Chem., 1983, 20, 1481.
34 B. P. Sullivan and T. J. Meyer, Inorg. Chem., 1978, 17, 3334.
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Graphical abstract
Demonstration of intramolecular energy transfer in asymmetric
bimetallic ruthenium(II) complexes
Manoranjan Bar, Dinesh Maity, Shyamal Das, and Sujoy Baitalik*
Asymmetric bimetallic Ru(II) complexes exhibit photo-induced intramolecular energy transfer
with the values of rate constant in the order 107 s-1.
hν
N
N
N
N
Ru
NH
N
N
N
N
N
N
N
N
Ru
N
H
N
600
700
800
NH
0
250
500
750
26