A high molar extinction coefficient mono-anthracenyl bipyridyl heteroleptic ruthenium(II) complex: Synthesis, photophysical and electrochemical properties

Article abstract of DOI:10.3390/molecules16064615

In our quest to develop good materials as photosensitizers for photovoltaic dye-sensitized solar cells (DSSCs), cis-dithiocyanato-4-(2,3-dimethylacrylic acid)-2,2'- bipyridyl-4-(9-anthracenyl-(2,3-dimethylacrylic)-2,2'-bipyridyl ruthenium(II) complex, a high molar extinction coefficient charge transfer sensitizer, was designed, synthesized and characterized by spectroscopy and electrochemical techniques. Earlier studies on heteroleptic ruthenium(II) complex analogues containing functionalized oligo-anthracenyl phenanthroline ligands have been reported and documented. Based on a general linear correlation between increase in the length of p-conjugation bond and the molar extinction coefficients, herein, we report the photophysical and electrochemical properties of a Ru(II) bipyridyl complex analogue with a single functionalized anthracenyl unit. Interestingly, the complex shows better broad and intense metal-to ligand charge transfer (MLCT) band absorption with higher molar extinction coefficient (δmax = 518 nm, σ = 44900 M^-1cm^-1), and appreciable photoluminescence spanning the visible region than those containing higher anthracenyl units. It was shown that molar absorption coefficient of the complexes may not be solely depended on the extended p-conjugation but are reduced by molecular aggregation in the molecules.

Full text of DOI:10.3390/molecules16064615

Molecules 2011, 16, 4615-4631; doi:10.3390/molecules16064615
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
A High Molar Extinction Coefficient Mono-Anthracenyl
Bipyridyl Heteroleptic Ruthenium(II) Complex: Synthesis,
Photophysical and Electrochemical Properties
Adewale O. Adeloye * and Peter A. Ajibade
Department of Chemistry, Faculty of Science and Agriculture, University of Fort Hare, P.M.B. X1314,
Alice 5700, South Africa; E-Mail: pajibade@ufh.ac.za (P.A.A.)
* Author to whom correspondence should be addressed; E-Mail: aadeloye@ufh.ac.za;
Tel.: +27-761-423-313; Fax: +27-406-022-366.
Received: 27 April 2011; in revised form: 31 May 2011 / Accepted: 1 June 2011 /
Published: 3 June 2011
Abstract: In our quest to develop good materials as photosensitizers for photovoltaic
dye-sensitized solar cells (DSSCs), cis-dithiocyanato-4-(2,3-dimethylacrylic acid)-2,2'-
bipyridyl-4-(9-anthracenyl-(2,3-dimethylacrylic)-2,2'-bipyridyl ruthenium(II) complex, a
high molar extinction coefficient charge transfer sensitizer, was designed, synthesized
and characterized by spectroscopy and electrochemical techniques. Earlier studies on
heteroleptic ruthenium(II) complex analogues containing functionalized oligo-anthracenyl
phenanthroline ligands have been reported and documented. Based on a general linear
correlation between increase in the length of π-conjugation bond and the molar extinction
coefficients, herein, we report the photophysical and electrochemical properties of a Ru(II)
bipyridyl complex analogue with a single functionalized anthracenyl unit. Interestingly, the
complex shows better broad and intense metal-to ligand charge transfer (MLCT) band
absorption with higher molar extinction coefficient (λmax = 518 nm, ε = 44900 M−1cm−1),
and appreciable photoluminescence spanning the visible region than those containing
higher anthracenyl units. It was shown that molar absorption coefficient of the complexes
may not be solely depended on the extended π-conjugation but are reduced by molecular
aggregation in the molecules.
Keywords: Ru(II) complex; bipyridine; extended π-bond conjugation; spectroscopy; molar
extinction coefficient; molecular aggregation

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1. Introduction
The quest for new materials that efficiently harvest solar light continues to be an important
goal. Recently, considerable efforts have been focused on new photosensitizers, including ruthenium
complexes [1–10] and organic dyes [11−14], in dye-sensitized nanocrystalline solar cells (DSSCs)
since cis-dithiocyanato bis(4,4'-dicarboxy-2,2'-bipyridine) ruthenium(II) [Ru(dcbpy)2(NCS)2] (referred
as N3) anchored on porous nanocrystalline TiO2 electrode has exhibited 10% light-to-electric power
conversion efficiency. The efficiency of sensitization is critically dependent on electron injection from
a photoexcited state of the dye into the conduction band of the semiconductor.
It has been shown that one of the best way to enhance both the absorption coefficient and red-shift
of the metal-to ligand charge transfer (MLCT) band in a ruthenium-based photosensitizer was to
extend the π-conjugation length of the colorant’s ancillary [15] or anchoring [16] ligands. Other classes
of ligands such as carboxylated terpyridine and phenanthroline showed enhanced UV-Vis absorption
over a broad range due to their large conjugated backbone structure. These ligands can be utilized as
efficient light harvesting sensitizers as well [17]. Various studies have also shown that it is not an
impossible task to increase the excited state lifetime of Ru(II) polypyridyl complexes based on
polypyridyl ligands. However, if all the requirements are to be fulfilled and the driving force for
3
further reactions be high enough, a new approach that does not stabilize the MLCT state but rather
destabilized the 3MC state has to be developed [18,19].
Existing design strategies for the preparation of these advanced multicomponent molecules follow
two principal methods. One approach exploits the use of metallosynthons bearing reactive functions
such as triflates, halides and carbonyls as the intermediates for producing the final species with the
expected features. In another approach, the ligand is first constructed and subsequently coordinated to
the appropriate metal. In both cases, the quest for new and alternative approaches for easy building and
organizing various photoactive partners around photoactive metals is one of the main aims of this
field [20]. Some authors reported the synthesis of heteroleptic ruthenium complexes by extending the
conjugation length of the ancillary ligand [21,22]. Such heteroleptic ruthenium complexes have a
strong MLCT band and dye solar cells devices based on them display very good photovoltaic
performance. In spite of this, the main drawback of these sensitizers is the lack of absorption in the red
region of the visible spectrum and also relatively low molar extinction coefficient [23]. Many
researchers have tried to overcome these shortcomings without significant success [24−26]. The
molecular engineering of ruthenium complexes for TiO2-based solar cells presents a challenging task
as several stringent requirements have to be fulfilled by the sensitizer and these are very difficult to be
met simultaneously, including absorption of all the visible light and function as an efficient charge
transfer sensitizer [27]. Our strategy in turning the excited state properties of the Ru(II) complexes is
based on the new design of bidentate polypyridyl ligands by extending the π-system of the ligands
using both unsaturated alkylene and polyaromatic molecules of which the anchoring ligand group carry
carboxylic acid functionality which is to serve as linkage to the semiconductor nanocrystalline titanium
dioxide in an application such as the dye-sensitized solar cells.
The emphasis of the present work is the synthesis of a new ruthenium(II) bipyridyl complex
incorporating a single anthracenyl unit as substituent on a ligand, thus showing the effect in the
reduction of molecular aggregation as found in the previous reports for functionalized oligo-anthracenyl

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Ru(II) phenanthroline complexes [28,29]. Herein, we present the synthetic methodology, photophysical,
spectroscopical characterization, and the electroredox properties of the complex.
2. Results and Discussion
2.1. Chemistry
Scheme 1 (i−iii) show the stepwise synthetic pathways for L1 and L2, and outline the chemistry of
the present study. 9,10-Dibromoanthracene, 2,3-dimethylacrylic acid and 2,2'-bipyridine are the
starting materials for 1 and 2. 9-Bromo-10-(2,3-dimethylacrylic acid)-anthracene (1) was obtained when
9,10-dibromoanthracene and 2,3-dimethylacrylic acid were refluxed in a benzene/dichloromethane
mixture under basic condition using triethylamine, potassium hydroxide and palladium carbide.
Product 2 was obtained by the initial introduction of required bromine functions on bipyridine under an
environmentally benign condition as reported by Vyas and co-workers [30]. L1 and L2 were
synthesized following slight modifications of established procedures reported in the literature [29]. L1
was obtained by one-step nucleophilic aromatic substitution reaction of the bromide group of product 2
with 2,3-dimethylacrylic acid. The synthesis of L2, essentially follows two major reaction steps: the
initial synthesis of a 2,3-dimethylacrylic acid functionalized anthracenyl group followed by a
palladium cross-catalyzed reaction of two halide compounds. This reaction was aided by the use of
an equivalent volume ratio of dichloromethane-benzene as solvent to overcome the poor solubility
of anthracenyl derivatives in common organic solvents such as methanol and chloroform. It was
found that this reaction could be reversed by first reaction of the halogenated polypyridine with
9,10-dibromoanthracene and a subsequent dehydrohalogenation reaction with 2,3-dimethylacrylic acid.
The synthesis of the metal precursor [RuCl2(dmso)4] [31], and the complex [RuL1L2(NCS)2] followed
the general synthetic route shown in Scheme 1 (iv and v). Sequential substitution of the DMSO
coordinating ligand from the metal precursor with L1 and L2 led to formation of intermediate complex
[RuL1L2Cl2] which was not isolated. Final reaction was carried out with chloride exchange with
thiocyanate group [32].
2.2. Infrared Spectra
The FT-IR spectra of the starting materials, the ligands and the complex showed certain
characteristic absorption bands that were compared and assigned on careful comparison. Due to
structural similarities between the ligands, and the complex, a strong band in the region 3427–3054 cm−1
was found. This gave indication of the presence of an O−H group possibly from the carboxylic acid
moiety. This band, however, shifted to higher frequency at 3551 cm−1 in the complex (Figure 1). The
vibration frequency bands between 3478−3066 cm−1 may be due to presence of α,β-unsaturated
carboxylic acid and/or aromatic C−H stretching characteristics of the molecules. The band at 2927 cm−1
shows the presence of C−H stretching of methyl groups. The complex shows a broad absorption
frequency at 2103 cm−1 for stretch vibrational modes due to the N-coordinated υ(CN). This band is
very close in intensity compared to the band at 839 cm−1, due to υ(CS). The bands at 1635 cm−1 and
1239 cm−1 were assigned to the υ(C=O) and υ(C–O) stretching of carboxylic acid groups, respectively.

Molecules 2011, 16
Scheme 1. Synthetic pathways for Ligands L1, L2 and [RuL1L2(NCS)2].
4618
Br
Br
CH3
COOH
+
H
(i)
H3C
Br
CH3
H3C
COOH
CH3
HOOC
H3C
(1)
Br
CH3HC=C(CH3)COOH
(1:1)
HBr/H2O2
MeOH, rt, 20 h
(ii)
MeOH, Pd/C, Et3N, KOH, 8 h
N
N
N
N
N
N
(2)
(L1)
COOH
CH3
Br
H3C
(iii)
CH2Cl2-C6H6 (50%)
Br
N
+
Pd/C, Et3N, KOH, 12 h
N
CH3
H3C
HOOC
N
N
(L2)
1. reflux, 100 oC
10 min
(iv)
(v)
[RuCl2((CH3)2SO)4]
(yellow powder)
[RuCl3(H2O)] +
4 (CH3)2SO
2. washed in acetone
HOOC
H3C
CH3
DMF, reflux, 120 oC
[RuCl2((CH3)2SO)4]
+
+
+
2NH4NCS
L1
L2
8 h, Chromatography
N
N
N
N
Ru
N
C
H3C
N
C
S
S
COOH
CH3
The three bands at 1584, 1533 and 1443 cm−1 are due to ring stretching modes of the ligands. The
band at 1309 cm−1 was assigned to the carboxylate symmetric υ(–COO–) of the carboxylic acid group.
A comparison of the infrared spectra of L2 and 9,10-dibromoanthracene showed that a strong
vibrational band in the former was conspicuously absent in the latter, confirming the loss of C–Br bond
and the formation of C–C bond linkages of the anthracenyl group. Furthermore, the C–C bond linkage
between anthracene and bipyridine was affirmed by the absorption frequency at 805 cm−1. Peaks in the
region 760 and 718 cm−1 demonstrate the existence of four adjacent hydrogen atoms common to L2
and complex [RuL1L2(NCS)2]. All vibrational peaks in the region are found to be relatively weak and
broad in the complex, which may be ascribed to the loss of crystallinity and the broad distribution of

Molecules 2011, 16
4619
the anthracene chain length [33]. The weak absorption frequencies at 473 and 426 cm−1, respectively,
show the coordination of nitrogen atoms of the ancillary ligands to ruthenium central metal atom [34].
Figure 1. FT-IR spectrum of [RuL1L2(NCS)2] complex in KBr.
926.78
965.95
473.60
682.45
619.92
1309.64
1155.66
1533.36
426.06
557.68
728.63
1239.24 1089.49
1014.51
3066.82
2922.38
%T
3239.92
1635.46
1584.98
1459.30
1617.64 1443.71
1419.43
3551.16
3478.79
839.68
760.31
3414.72
2103.81
2000
4000.0
3600
3200
2800
2400
1800
1600
1400
1200
1000
800
600
370.0
cm-1
2.3. 1H and 13C-NMR Spectroscopic Studies
1
The aromatic region of the H-NMR spectrum of L1 gave six peaks at 8.66 (d, 1H), 8.41 (d, 1H),
7.86 (dd, 1H), 7.36 (1H), 1.73 (s, 3H), 1.66 (d, 3H) ppm. The 1H peaks are very similar to those of the
bromo-bipyridine starting material. A principal difference is due to the inclusion of the methyl
resonance at the aliphatic region of the spectrum. The 13C-NMR spectrum gave the anticipated peaks at
169.76, 156.21, 149.98, 137.86, 136.88, 129.74, 124.81, 121.32, 14.81 and 12.71. The bipyridine
peaks due to chemical equivalency were observed in the range 156–128 ppm. The peak at 169.76 ppm
was assigned to the carbonyl carbon; the two peaks at 124.81 and 121.32 were assigned to the alkenyl
1
carbons, while the methyl groups were found at 14.81 and 12.71 ppm. The H-NMR of L2 show five
signals at the aromatic region at δ 9.20 (d), 8.57 (dd), 8.26 (d), 8.24 (d), 7.62 (dd) were assigned to the
bipyridine and anthracene protons. The two singlet peaks at the aliphatic region were assigned to the
trans- methyl protons of the carboxylic acid group at δ 2.17 and 1.67 ppm. The 13C spectrum of L2 was
similar to that of L1, except for those additional peaks at 131.03, 128.25, 127.44 and 126.49 ppm that
were assigned to the anthracenyl carbons signals.
The proton NMR spectrum of the [RuL1L2(NCS)2] complex in CD3OD (Figure 2) is consistent with
the structure shown in Scheme 1. Due to the presence of two different anchoring ligand substitutions,
such that all the protons are electronically found in different environments, gave complications in the
spectrum most especially at the aromatic region. The spectrum showed six proton signals between δ
8.71−7.50 ppm integrating for twelve protons. The bipyridyl protons were observed as doublets at δ

Molecules 2011, 16
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8.71, 8.61 7.50 and a singlet at 8.14 ppm, while the anthracenyl protons were assigned to signals at δ
7.83 and 7.73 ppm. At the aliphatic region, the two conspicuous singlet peaks at δ 2.16 and 1.29 ppm,
integrating for six protons due to chemical equivalence were unambiguously assigned to the methyl
groups. In comparison to the individual ligands, the proton peaks in the complex were shifted
downfield. The deshielding pattern is ascribed to the lone pair-lone pair electron donation of the
13
nitrogen atoms to the d-orbital of the ruthenium metal. The C-NMR spectrum data of the complex
could not be adequately assigned to individual carbon atoms due to poor signal resolutions.
Figure 2. 1H-NMR spectrum of [RuL1L2(NCS)2] in CD3OD.

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2.4. Electronic Absorption and Emission Spectra
2.4.1. Electronic absorption spectroscopy
The UV-Vis absorbance and emission spectra of complex [RuL1L2(NCS)2] in dimethylformamide
are shown in Figures 3 and 4. In the UV-region, [RuL1L2(NCS)2] displays four distinct vibronic peaks
for the intra ligand (π→π*) charge transfer transitions characteristics of anthracene derivatives at 366,
383, 406 and 430 nm. The complex shows broad and intense absorption bands between 419 and 600 nm
with wavelength maxima at 518 nm. This absorption band was due to the metal-to-ligand charge transfer
transition (MLCT) of the complex with the molar extinction coefficient (ε = 44900 M−1 cm−1). The effect
of extending the π-conjugation length through anthracene was observed when [RuL1L2(NCS)2]
complex spectrum was compared to a similar [Ru(L1)2(NCS)2] complex with no anthracene
substitution (Figure 3). The molar extinction coefficient of the lowest energy MLCT band in the
[Ru(L1)2(NCS)2] complex at 508 nm (ε ≈ 20350 M−1 cm−1) which is 45.3% lower than that of the
[RuL1L2(NCS)2] complex. However, it terms of increase in the number of anthracenyl units, molar
extinction was found to decrease proportionally as previously reported by us for two phenanthroline
ruthenium(II) complexes. Thus it could be taken that the distance transfer of electronic energy through
anthracene may have been hindered due to molecular aggregation [28,35]. Large pi-aromatic
molecules, such as prophyrins, phthalocyanines, and perylenes have been reported to be important
classes of potential sensitizers for highly efficient dye-sensitized solar cells, owing to their
photostability and high light-harvesting capabilities that can allow applications in thinner, low-cost
dye-sensitized solar cells [36].
Figure 3. Comparison of the UV-Vis absorption spectrum of [RuL1L2(NCS)2] (blue line)
and [Ru(L1)2(NCS)2] (red line) at a concentration of 1 × 10−3 M−1 in DMF solution
showing effect of extended π-conjugation using anthracene.

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2.4.2. Emission study
The emission spectrum of complex [RuL1L2(NCS)2] is displayed in Figure 4. Upon excitation into
1
1
the LC and MLCT bands (λexc = 500 nm), the complex displays appreciable luminescence at room
temperature. An emission wavelength maximum was found at 673 nm. A direct correlation has been
found between emission intensity and the molecular weight of compounds [35]. It is well known that
conjugated functional organic molecules are useful for the study of electron transport at the molecular
scale and that the use of fused-ring systems is a powerful and practical approach. It could be seen in
the complex that the choice of ligand has significant effects on the energy ordering of the low energy
excited state and, in particular to the orbital nature of the lowest excited state which also influence the
positions of the metal centre (MC), ligand centre (LC) as well as the MLCT of the complexes. The
energy of the MC excited state depends on the ligand field strength, which in turn depend on the
σ-donor and π-acceptor properties of the ligands, the steric crowding around the metal (that can
preclude a sufficient close approach between metal and ligand) [37−39], and the bite angle of the
polydentate ligands (which in some cases cannot be optimized because of molecular constraints [40]. It
has been shown that the energy of the MLCT excited state depends on the reduction potential of the
ligand involved in the MLCT transition, the oxidation potential of the metal in the complex (which is
affected by the electron donor and acceptor properties of all the ligands), and by the charge separation
caused by the transition. The energy of the LC excited states depends on intrinsic properties of the
ligands, such as the HOMO-LUMO energy gap and the singlet-triplet splitting. The intense emission is
a significant contribution to the excited state from an interaction between the metal d-orbital and the
ligand π-systems [41].
Figure 4. Emission spectrum of [RuL1L2(NCS)2] in DMF solution.

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2.5. Electrochemical Study
The cyclic voltammogram of [RuL1L2(NCS)2] complex in DMF, containing 0.1 M tetrabutyl-
ammonium tetrafluoroborate as supporting electrolyte and Ag|AgCl electrode was studied in the
potential range +1.5 to −1.5 V and at a scan rate 50 mV s−1 is shown in Figure 5. The voltammogram
displays an irreversible oxidation peak (process III) at 0.84 V, this was assigned to the metal centre
Ru(III)/Ru(II) couple. The oxidation potential of the complex [RuL1L2(NCS)2] is close to that
of Ru2+ as reported by Grätzel and co-workers for other thiocyanate containing ruthenium(II)
dyes [6]. The redox wave of the complex [RuL1L2(NCS)2] were observed at E1/2 = −0.95 and −0.53 V
for processes I and II, respectively. The electrochemical properties of these types anthracenyl
containing Ru(II) polypyridine complexes have been shown to possess good electroredox properties
and that the redox processes could be both single- and multi- electronic in nature due to the
incorporation of anthracene moiety [28]. By comparison, the number of electrons transfer by
[RuL1L2(NCS)2] complex in the three redox processes were determined accurately by
chronocoulometry method with the equation:
ꢅ
ꢅ
ꢅ
⁄
Q = 2ꢀꢁꢂꢃꢄꢆꢇꢆ πꢆ
Figure 5. Cyclic voltammogram for [RuL1L2(NCS)2] complex at 1 × 10−3 M in freshly
distilled DMF containing 0.1 M TBABF4 supporting electrolyte. Step potential = 5 mV,
amplitude = 50 mV vs. Ag|AgCl, frequency = 10 Hz. Scan rate = 100 m Vs−1 vs. Ag|AgCl.
From the slope of the data when the quantity of electricity was plotted against square root of time
(Figures 6 and 7), the number of electrons (n) for processes I, II and III were found to be in ratio
(1:1:1), suggesting that a single- electron process occurred in the redox wave characteristics of the
molecule unlike the ruthenium(II) phenanthrolyl complex containing oligo-anthracenyl moieties [28].

Molecules 2011, 16
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Figure 6. Plots of charge vs. time response for processes I, II and III; (line a–c, respectively).
Scan rate = 200 m Vs−1.

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Figure 7. Anson plot of charge vs. square root of time (s) for processes I, II and III. Scan
rate = 200 m Vs−1.
3. Experimental
3.1. Materials and General Physical Measurements
All chemical and reagents were analytically pure and used without further purification. 4-Bromo-
2,2'-bipyridine was synthesized as described in the literature [30]. 4-(2,3-Dimethylacrylic acid)-2,2'-
bipyridine (L1) and 4-(9-anthracenyl-10-(2,3-dimethylacrylic acid)-2,2'-bipyridine (L2) were synthesized
by the literature procedure [28] with slight modifications (Scheme 1). All thin layer chromatography

Molecules 2011, 16
4626
(TLC) analyses were done with aluminium sheet precoated with normal phase silica gel 60 F254
(Merck, 0.20 mm thickness) unless otherwise stated. The TLC plates were developed using any of the
following solvent systems: Solvent system A: Dichloromethane-Methanol (9:1); Solvent system B:
Dichloromethane-Methanol (7:3); Solvent system C: Dichloromethane-Benzene (1:1); Solvent system
D: Diethyl ether-Methanol (1:1). Gel filtration was performed using Sephadex LH-20 previously
swollen in specified solvent (s) prior to loading of extract onto the column (3.5 cm × 8.5 cm).
Melting points were determined using a Gallenkamp electrothermal melting point apparatus.
Microanalyses (C, H, N, and S) were carried out with a Fisons elemental analyzer and Infrared spectra
were obtained with KBr discs or nujol on a Perkin Elmer System 2000 FT-IR Spectrophotometer. UV-Vis
and fluorescence spectra were recorded in a 1 cm path length quartz cell on a Perkin Elmer Lambda 35
1
13
spectrophotometer and Perkin Elmer Lambda 45 spectrofluorimeter, respectively. H- and C-NMR
spectra were run on a Bruker EMX 400 MHz spectrometer for 1H and 100 MHz for 13C. The chemical
shift values were reported in parts per million (ppm) relative to (TMS) as internal standard. Chemical
shifts were reported for the ligands with respect to CDCl3 at δc 77.00 and δH CDCl3 at 7.25; DMSO-d6
at δc 40.98 and DMSO-d6 at δH 2.50. The complex was run in CD3OD at δH 3.31. Electrochemical
experiment was performed using a PGSTAT 302 Autolab potentiostat (EcoChemie, Utrecht, The
Netherlands) driven by the general purpose Electrochemical System data processing software (GPES,
software version 4.9). A conventional three-electrode system was used. The working electrode was
a bare glassy carbon electrode (GCE), Ag|AgCl wire and platinum wire were used as the pseudo
reference and auxiliary electrodes, respectively. The potential response of the Ag|AgCl
pseudo-reference electrode was less than the Ag|AgCl (3 M KCl) by 0.015 ± 0.003 V. Prior to use, the
electrode surface was polished with alumina on a Buehler felt pad and rinsed with excess millipore
water. All electrochemical experiments were performed in freshly distilled dry DMF containing
TBABF4 as supporting electrolyte.
3.2. Synthesis of 4-(2,3-DMAA)-2,2'-bipyridine (L1)
4-Bromo-2,2'-bpy (1.05 g, 3.38 mmol) and 2,3-dimethylacrylic acid (DMAA, 0.34 g, 3.38 mmol)
were dissolved in MeOH (40 mL) in a 250 mL flask. Et3N (1.0 mL) and palladium-carbide (0.050 g)
were added and the mixture was reflux for 8 h at a temperature between 110−120 °C. The reaction was
allowed to cool to room temperature and the solvent removed under reduced pressure. The residue was
dissolved in degassed water and then extracted with chloroform. The chloroform extract was
concentrated in vacuo to obtain a brilliant colourless liquid which solidified after about 48 h at room
temperature. The resultant residue was recrystallized from Et2O (30 mL). Colour: White crystalline
solid; Percentage yield: 0.90 g, 67%, Melting point: ND; IR (KBr, νcm−1): 3054, 2927, 2676, 1965,
1690, 1648, 1581, 1559, 1456, 1419, 1346, 1251, 1141, 1089, 1040, 992, 893, 757, 653, 631, 619, 555.
1H-NMR (DMSO): δ 8.66 (d, J = 4.0 Hz, H−6, 6’), 8.41 (d, J = 8.0 Hz, H−3, 3’), 7.86 (dd, J = 7.6, 8.0 Hz,
H−5, 5’), 7.36 (dd, J = 5.2, 7.2 Hz, H−4’), 1.73 (s, CH3), 1.66 (d, CH3). 13C-NMR (DMSO): δ 169.76,
156.21, 149.98, 137.86, 136.88, 129.74, 124.81, 121.32, 14.81, 12.71. Elemental Analysis: Calculate H
5.55, C 70.85, N 11.02; required H 5.55, C 70.60, N 11.43. Molecular Formula: C15H14N2O2.

Molecules 2011, 16
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3.3. Synthesis of 4-(9-Anthracenyl-10-(2,3-DMAA))-2,2'-bipyridine (L2)
4-Bromo-2,2'-bpy (1 g, 4.82 mmol) and 9-bromo-10-(2,3-dimethylacrylic acid)-anthracene (1.72 g,
4.82 mmol) were dissolved in benzene-dichloromethane (50 mL, v/v, 1:1), followed by the addition of
Et3N (1 mL), KOH (0.27 g, 4.82 mmol) and palladium-carbide (0.05 g). The reaction was carried out
under reflux for 12 h at temperature 110−120 °C. Colour: Yellow crystalline solid. Melting point:
167−169 °C, Percentage yield: 1.93 g, 71%, IR (KBr): 3427, 3056, 2926, 1952, 1802, 1690, 1622,
1582, 1558, 1524, 1456, 1437, 1420, 1349, 1304, 1256, 1162, 1149, 1089, 1040, 1028, 995, 926, 747,
676, 654, 619, 605, 578. 1H-NMR (DMSO): δ 9.20 (2d, J = 1.6, 4.4 Hz, H−6, 6'), 8.57 (dd, J = 3.2, 6.8 Hz,
H−a, b), 8.26 (d, J = 2.0 Hz, H−3, 5), 8.24 (d, J = 1.6 Hz, H−4'), 7.62 (dd, J = 3.2. 6.8 Hz, H-c, d), 2.17
(s, CH3), 1.67 (s, CH3). 13C-NMR (DMSO): δ 156.76, 150.31, 146.27, 144.21, 135.94, 131.03, 128.52,
128.25, 127.44, 126.49, 124.21, 123.51, 123.04, 30.90 and 21.92. Elemental Analysis: Calculate H
5.15, C 80.91, N 6.51; required H 5.51, C 80.67, N 6.23. Molecular Formula: C29H22N2O2.
3.4. Synthesis of cis-Dithiocyanato-4-(2,3-dimethylacrylic acid)-2,2'-bipyridyl-4-(9-anthracenyl-(2,3-
dimethylacrylic acid)-2,2'-bipyridyl-ruthenium(II) complex [RuL1L2(NCS)2]
In a 250 mL flask, [RuCl2(dmso)4] (0.24 g, 0.49 mmol) was dissolved in N,N-dimethyl-
formamide/MeOH (80 mL, 1:1, v/v) followed by the addition of ligand L1 (0.25 g, 0.98 mmol) and L2
(0.42 g, 0.98 mmol). The mixture was refluxed initially at 120 °C for 2 h in the dark and excess of
NH4NCS (1.50 g, 1.95 mol) was added. The reaction was left to reflux for 10 h. The solution was
allowed to cool to room temperature and then filtered to remove unreacted starting material. The
filtrate was concentrated to dryness and 40 mL of 0.05 M NaOH solution was added to give a dirty
brown precipitate which was filtered off. The pH of the resulting solution was adjusted to 3 with 0.5 M
HNO3. The solution was left to stand in the fridge (−2 °C) for 12 h before being filtered and
concentrated in vacuo. The crude residue product was adsorbed onto Sephadex LH-20 adsorbent in a
glass column and eluted using solvent system D (diethyl ether-methanol, 50%, v/v, 250 mL). Colour:
Dark brown solid, Melting point: >260 °C, Percentage yield: 0.60 g, 36%. IR (KBr) νmax/cm−1: 3551,
3478, 3414, 3239, 3066, 2922, 2103, 1635, 1617, 1584, 1533, 1459, 1443, 1419, 1309, 1239, 1155,
1089, 1039, 1014, 961, 926, 839, 805, 760, 718, 682, 619, 611, 557, 473, 426, 399. UV-Vis (λmax/nm,
ε = M−1 cm−1, DMF): 366 (64000), 383 (62800), 406 (48650), 430 (36977), 518 (44900). Emission
wavelength: (λexc. = 500 nm, λem = 680 nm). 1H-NMR (DMSO-d6): δ 8.71 (d, J = 7.6 Hz, 2H), 8.61 (d,
J = 6.8 Hz, 2H), 8.14 (s, br, 2H), 7.83 (br, 2H), 7.73 (br, 2H), 7.50 (d, J = 1.2 Hz, 2H), 2.16 (s, 3H),
1.29 (s, 3H). 13C-NMR (CDCl3): ND. Cyclic voltammetry Data: Ru2+/Ru3+ = 0.84 V; Ecathodic = −0.53 V,
E1/2 = −0.95 V. Elemental Analysis: Calculate H 4.02, C 61.25, N 9.32, S 7.11; required H 4.43,
C 61.51, N 9.66, S 7.57. Molecular Formula: RuC46H36N6O4S2.
4. Conclusions
This work has reported the synthesis and characterization of two new 4-monosubstituted
2,2'-bipyridine ligands and the corresponding heteroleptic ruthenium(II) complex. The photophysical,
spectroscopic and electroredox properties of the complex were studied. One of the ligands in
particular, with a substituted anthracene derivative, was introduced to function as a source of increased

Molecules 2011, 16
4628
π-conjugative bonds in the complex, with a view to enhancing the molar extinction coefficient of the
molecule. The complex showed enhanced and better photophysical, photoluminescence and
electrochemical properties that are common to ruthenium(II) polypyridine complexes. However, in
comparison to our previous observations towards the study of a systematic increase in the number of
anthracene units in the ruthenium(II) phenanthroline complexes, the UV-Vis absorption and
luminescence property of this complex revealed that extension of the π-bond conjugation on these
types of complexes may not be the only contributory factor responsible for the shift in absorption
wavelengths to the red region, with corresponding increase in the molar absorptivity coefficient, but
the types, position of substituents on ligands, use of heteronuclear ligands and/or the molecular weight
of compounds could as well be very vital or the determining factor in the overall characteristics
displayed by the complexes.
Particularly for this molecule, further work is ongoing in our laboratory to establish the solar-to-
electrical energy conversion efficiency (η) in the dye-sensitized solar cells (DSSCs). However, due to
π-electron cloud overlaps in anthracene derivatives, the molecule could find use in applications,
including organic electroluminescence material for biosensitizers, and display devices such as Organic
Light Emitting Diode (OLED), Organic Thin Film Transistor (OTFT), wearable display, photochromic
agents and electronic paper.
Supplementary Materials
Supplementary materials can be accessed at http://www.mdpi.com/1420-3049/16/6/4615/s1.
Acknowledgments
The authors are grateful to the South African National Energy Research Institute (SANERI) and
Govan Mbeki Research Centre and University of Fort Hare, Alice, South Africa for financial support.
AOA acknowledges Obafemi Awolowo University, Ile-Ife, Nigeria for study leave.
Conflict of Interest
The authors hereby declare no conflict of interest.
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Sample Availability: Samples of the compounds L1, L2 and [RuL1L2(NCS)2] are available from the
authors.
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