Synthesis, characterisation, electrochemical study and photovoltaic measurements of a new terpyridine and pyridine-quinoline based mixed chelate ruthenium dye

Article abstract of DOI:10.1016/j.poly.2015.10.040

A novel ruthenium-terpyridine based photosensitizer, Ru[(p-F-tpy)(pcqH)Cl] PF₆ (1); (p-F-tpy = 4′-(4-fluorophenyl)-2,2′:6′,2″-terpyridine, pcqH = 2-(2-pyridyl)-4-carboxyquinoline) was synthesized and spectroscopically characterized. The electro

Full text of DOI:10.1016/j.poly.2015.10.040

Polyhedron 102 (2015) 615–626
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterisation, electrochemical study and photovoltaic
measurements of a new terpyridine and pyridine-quinoline based mixed
chelate ruthenium dye
Binitendra Naath Mongal , Arunava Pal , Tarun Kanti Mandal , Jayati Datta , Subhendu Naskar a,
a
b
c
b,
⇑
⇑
a
Department of Chemistry, Birla Institute of Technology, Mesra, Ranchi 835215, Jharkhand, India
Electro-chemistry and Non-Conventional Energy Laboratory, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India
Department of Biotechnology, Haldia Institute of Technology, Hatiberia, Haldia 721 657, West Bengal, India
b
c
a r t i c l e i n f o
a b s t r a c t
0
Article history:
6
A novel ruthenium-terpyridine based photosensitizer, Ru[(p-F-tpy)(pcqH)Cl] PF (1); (p-F-tpy = 4 -(4-flu-
Received 29 June 2015
Accepted 16 October 2015
Available online 29 November 2015
0
0
00
orophenyl)-2,2 :6 ,2 -terpyridine, pcqH = 2-(2-pyridyl)-4-carboxyquinoline) was synthesized and spec-
troscopically characterized. The electronic spectrum of the complex shows the lowest energy MLCT
band at 536 nm in DMSO. The ground state pK
were spectrophotometrically determined. The pK
reduces to 2.89 when bound to the metal. The cyclic voltammetry of the complex shows a reversible
a
values of the acidic ligand (pcqH) and the Ru complex
a
of the COOH group in the free ligand is 3.56, and this
Keywords:
Ruthenium photosensitiser
MLCT
Pourbaix diagram
Ru-aquo species
DFT
II/III
Ru
oxidation at 0.825 V with respect to Ag/AgCl reference electrodes in DMF. The Cl group is very
O generates aqua species, which has been shown spectrophoto-
metrically. A Pourbaix diagram for the Ru-aqua species has been constructed to show the detailed redox
properties of the complex by means of cyclic voltammetry and differential pulse voltammetry at variable
labile, substitution of the Cl group by H
2
pH. Non-linear regression analysis was performed to generate the pK
a
values for the Ru-aqua species as
II
III
1
1.89 (Ru ) and 4.00 (Ru ). Photovoltaic measurements with the dye were performed after anchoring
ꢀ
ꢀ
3
2
onto a TiO surface with the I /I redox electrolyte. Photovoltaic properties, like open-circuit photo-voltage
ꢀ4
ꢀ2
(
Voc = ꢀ0.35 V), short-circuit photocurrent density (Jsc = 1.428 ꢁ 10 amp cm ), fill factor (ff = 39.4%) and
solar-to-electric conversion efficiencies ( = 0.13%), of the DSSCs constructed from the [Ru(p-F-tpy)(pcqH)Cl]
PF sensitized TiO electrodes were measured. A DFT and TDDFT study has been performed on the complex.
The TDDFT calculated absorption spectrum nicely matches the experimental spectrum.
Ó 2015 Elsevier Ltd. All rights reserved.
g
6
2
1
. Introduction
lifetime of 1100 ns) [19], structural robustness and close proximity
II/III
of the Ru
oxidation to that of the Photosystem-II [1,2]. However
+
2
Photosynthesis has inspired the scientific community to design
3
for the Ru(bpy) system with substituted bpy ligands, the Ru cen-
molecular systems to effectively trap sunlight as a source of alter-
native energy. Polypyridyl complexes of transition metals with a d
electronic configuration have the capability of absorbing light.
Therefore these complexes can act as effective photosensitisers,
just like chlorophyll-b or b-carotenoid in PSII [1–3]. With this goal,
extensive research has been carried out on Ru-bipyridyl, phenan-
tered inherent stereogenicity makes it hard to get pure compounds
[20]. This synthetic difficulty disappears for Ru-terpyridyl com-
pounds, but the terpyridyl compounds of Ru are photophysically
not so appealing due to the very low excited state lifetime (for
6
+
2
2
Ru(terpy) , s = 0.25 ns) at room temperature [21]. The rigid tri-
dentate moiety of terpyridine results in a distorted octahedral
geometry in their Ru(II) complexes. In coordinated terpyridine,
the N–Ru–N trans angles reduce to 158.6° from 173.0° in the anal-
ogous Ru(II)bpy complexes [22,23]. Hence the ligand field strength
weakens, reducing the energy of the d–d metal-centered triplet
throline and other related diimine complexes [4–18]. Ru
2
(
bipyridine)+
3
has evolved as the most effective photosensitiser
over the years due to its photophysical novelty (an excited state
3
3
state ( MC) [24]. Consequently the energy gap between the MLCT
⇑
Corresponding authors. Tel.: +91 33 2668 4561/4563x514; fax: +91 33 2668
916 (J. Datta). Tel.: +91 651 2276 531; fax: +91 651 2276 052 (S. Naskar).
3
3
and MC states decreases and thus the MC state becomes ther-
mally accessible from the MLCT state, causing easy non-radiative
2
3
E-mail addresses: jayati_datta@rediffmail.com (J. Datta), subnaskar@yahoo.co.in
(
S. Naskar).
decay back to the ground state. For a better excited state property
http://dx.doi.org/10.1016/j.poly.2015.10.040
277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
0

6
16
B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
3
of the sensitizer, thermal non-radiative deactivation from the MC
state should be minimised. This can be achieved either by destabi-
one –COOH linker site may seem counterproductive, but merely
increasing the number of linker groups does not always enhance
the efficiency of the solar cell. It has been reported that although
black dye has three potential COOH linkers, it attaches to the semi-
conductor surface via only one carboxylic acid group due to steric
congestion [39]. Our aim was to get rid of this stereochemical con-
gestion for the attachment of the dye to the semiconductor as well
as to study the electrochemical and photovoltaic behaviour of this
new heteroleptic Ru complex with a new anchoring moiety. There
is a possibility that these new dye molecules will be better
attached on the semiconductor surface.
We have also studied the spectroscopic and electrochemical
properties of the dye by varying the pH from 0.04–13.17. It has
been shown that both the MLCT and redox properties can be signif-
icantly tuned by changing the pH. Also an interesting observation
is that the Ru–Cl bond is labile in aqueous media [34,40] and gets
3
3
lizing the MC state or by stabilizing the MLCT state. Proper sub-
stitution (electron withdrawing or/and electron donating group) at
the para position of the central pyridine ring of the terpyridine can
achieve the desired effect [25]. The 3MLCT state can be stabilised
by introducing an electron withdrawing group at the para position
of the central pyridine of the terpyridine ligand [26,27], whereas an
3
electron donating group cannot destabilise the MC state to a great
extent [28]. Nevertheless, presence of a phenyl substituent at the
3
1
para position stabilises the MLCT state more than the MLCT state
29,30].
Approaches have been made to develop thiocyanate free Ru
dyes in order to improve the light absorption capacity as well as
to avoid the possibility of getting a mixture of products due to
the ambidentate nature of the thiocyanate ligand. One methodol-
ogy in this respect is to use a cyclometallating ligand [31–33],
which fulfills both purposes. Another approach that may be used
is to change the ancillary ligand [34]. Substitution of SCN by Cl
red shifts the MLCT band by 20–30 nm.
[
2
substituted by H O molecules in acidic media (within pH 4). This
can provide an idea about the optimal conditions required for
dye stability and effective dye regeneration for this class of photo-
sensitisers in Dye Sensitized Solar Cells (DSSCs).
Pioneered by Professor Michael Gratzel, these ruthenium poly-
pyridyl complexes are widely used as photosensitizer dyes in wide
band gap solar cells. The most efficient photosensitizer to date is
N719, with an efficiency of 11.18% [35]. Based on the above
approach, we report a new Ru-polypyridyl complex (Fig. 1) which
consists of a substituted terpyridine ligand, an anchoring group,
2
. Experimental
.1. Materials and methods
-F-benzaldehyde, 2,3-indolinedione, tetrabutyl ammonium
2
4
2
-(2-pyridyl)-4-carboxyquinoline, and an ancillary Cl ligand. We
bromide and silver nitrate were purchased from Spectrochem, 2-
acetylpyridine and ammonium hexafluoro phosphate were pur-
chased from Sigma Aldrich and ruthenium chloride was obtained
from Arora Mathey India Limited. The ethanol and DMF used were
HPLC grade, whereas methanol was dried according to literature
procedures [41]. The water used for the spectrophotometric and
electrochemical studies was purified by a Milli-Q system. Tetra-
butyl ammonium perchlorate was synthesised according to the lit-
erature [42].
Infrared spectra were recorded as KBr pellets on a Shimadzu IR-
Prestige21 spectrometer. UV–Vis spectra were recorded using a
Perkin Elmer Lambda 750 spectrophotometer. The pH values were
measured in a Thermos Scientific Orion 4 star pH Benchtop. Ther-
mogravimetric analysis of the dye was executed in a DTG-60 (Shi-
madzu Corporation, Kyoto, Japan) from 30 to 800 °C in a platinum
were successful in our attempt to modulate the energy gap
between the d -p HOMO and the ligand based LUMO by red shift-
6
ing the MLCT absorbance maxima up to 536 nm using Cl as an
⁄
ancillary ligand. The coordinated Cl reduces the d
p
?p
energy
gap and therefore causes a significant red shift in the MLCT absor-
bance [36]. For the majority of terpyridine based Ru complexes the
low energy MLCT absorbance remains below 500 nm [34,37,38],
whereas the complex reported here exhibits the MLCT absorbance
at 536 nm. Here, we have used a pyridine ligand and a quinoline
based anchoring group, 2-(2-pyridyl)-4-carboxyquinoline, for the
dye. The anchoring group in this dye is quite non-conventional
as there are no reports, to our knowledge, where such a pyri-
dine-quinoline moiety has been used as the linker group. The
extended aromatic heterocycle in this ligand may help in better
delocalisation of the electron density involved in the metal to
ligand charge transfer (MLCT), thus helping in better electron
2
pan at a heating rate of 10 °C per minute under flowing N (30 ml
per min.). Cyclic and differential pulse voltammograms were
recorded in a CHI6003E potentiostat, either in DMF or DMF-water
solutions, containing 0.1 M TBAP as a supporting electrolyte, with
glassy carbon as a working electrode, a Pt wire as a counter elec-
2
injection in the conduction band of TiO . The presence of only
trode and an Ag/AgCl non-aqueous reference electrode. The fer-
0
rocene/ferrocenium couple was observed at
(
E
(DEp) = 0.4 V
1
100 mV) under these experimental conditions. H NMR spectra
were recorded on a Bruker AVANCE DPX 400 MHz spectrometer
using Si(CH as an internal standard. ESI-MS spectra of the sam-
3 4
)
ples were recorded on a JEOL JMS 600 instrument. Fluorescence
spectra at room temperature were recorded using a Shimadzu
RF-5301 PC spectrofluorometer.
2.2. Synthesis
2
2
.2.1. Preparation of the ligands
.2.1.1. 4 -(p-Fluorophenyl)-2,2 :6 ,2 -terpyridine (p-F-tpy). The ter-
0
0
0
00
pyridyl ligand has been synthesized by following the reported
procedure [43]. 4-F-benzaldehyde (0.620 g, 5 mmol) and 2-acetyl-
pyridine (1.21 g, 10 mmol) were dissolved in ethanol. KOH (0.77 g,
1
0 mmol) was added and the mixture was vigorously stirred. After
the potassium hydroxide pellets completely dissolved, ammonia
(excess, ca. 20 ml) was added and the mixture was stirred at room
6
Fig. 1. Labeled structure of [Ru(p-F-tpy)(pcqH)Cl]PF .

B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
617
temperature for 16 h under a N
tion was filtered under vacuum and washed with ethanol to give
a very light blue colored fluffy solid, yield 0.656 g (40%). The crude
2
atmosphere. The resulting solu-
it electrically conducting) using a glass rod with quick downward
sweeping motions by the Doctor’s Blade technique, the cell area
being maintained at 0.5 cm . The films were made fairly smooth,
2
product was recrystallised from methanol. Mass: 328 (M+1).
avoiding any inconsistencies or streaks in the applied TiO
2
paste.
1
H NMR (CDCl
3
, d, ppm): 8.61 (2H, d, Ph3,5-H), 8.59 (2H, d,
The thickness of the film (ꢃ14–15
l
m) was gravimetrically mea-
00
Ph2,6-H), 8.55 (2H, d, 6,6”-H), 7.75 (2H, ddd, 5,5 -H), 7.8 (2H,
sured using a highly sensitive digital micro-balance, Metler-
Toleodo [Model No.AB265-S], Switzerland and further verified with
a Mitutoyo ABSOLUTE (No. 547-301) thickness meter. In order to
00
0
0
00
13
ddd, 4,4 -H), 7.15 (2H, s, 3 ,5 H), 7.05–7.1 (2H, m, 3,3 -H).
NMR (CDCl , d, ppm): 155.76 (C ), 154.80 (C ), 149.34 (C
), 137.47 (C ), 136.29 (C ), 134.35 (C ), 129.36 (C
), 124.38 (C ), 120.94 (C ), 117.82 (C ).
C
),
),
3
L
F
E
1
1
48.19 (C
28.78 (C
H
A
C
I
J
2
dry and strengthen the TiO coating, the coated plates were sin-
B
D
G
K
tered at 500 °C for ꢂ30 min, until a transformation from white to
a brownish colour and back to white again took place. Finally the
2.2.1.2. 2-(2-pyridyl)-4-carboxyquinoline (pcqH). This ligand was
2
TiO thin films were cooled to room temperature and made ready
prepared according to the literature method [44]. 18 g (0.12 mol)
of 2,3-indolinedione was crushed to a powder and mixed with
for dye sensitization.
1
5 g (0.12 mol) of 2-acetylpyridine for ꢂ30 min. 60 g (ꢂ60 ml) of
2.5. Fabrication of the Dye Sensitized Solar Cell
3
3% NaOH were added at 5 °C with stirring. The solution was stir-
red continuously for 30 min, whereupon the temperature raised to
ꢂ50 °C. Ice flakes were added to the mixture. Stirring the mixture
with a glass rod produced a purple red solid. The solid was filtered,
washed with water followed by cold acetone. The crude product
In order to form the photo-anode, the thin films of TiO were
2
sensitized with the synthesised dye (1) and a reference dye
(N719) by immersing the films overnight in a 1 mM ethanolic solu-
tion of the appropriate dye. The counter electrode of the proposed
DSSC cell was prepared by galvanostatic electro-deposition of Pt
nanoparticles onto the FTO glass substrate from a precursor
was recrystallised from water to give a light purple crystal. Yield:
+
1
2
2
8 g (67%) Mass: 295 (M+23+23) (base peak) (23 = mass of Na ).
ꢀ2
H
2
PtCl
300 s. The two electrodes were assembled together using Bynel
SX1170-60, 50 m thick, Solaronix) as the sealant, heating at
ꢂ80 °C. Subsequently, the working electrolyte, composed of
0.5 M NaI (Sigma–Aldrich), 0.05 M I (Sigma–Aldrich), 0.05 M
6
solution, applying a current density of 5 mA cm
for
.3. Preparation of the complexes
(
l
.3.1. Ru(p-F-tpy)Cl
p-F-tpy (1.2 g, 3.67 mmol) and RuCl
dissolved in 20 ml dry methanol and heated to reflux under N
3
3
(0.95 g, 3.67 mmol) were
2
2
TPMPI (Triphenylmethylphosphonium iodide effectively used in
low light ambience [46]) and 0.5 M 4-tertbutyl pyridine (TBP,
Sigma–Aldrich) in acetonitrile [47], was injected into the cell sys-
tem and the cell was sealed properly. It may be noted that in the
for 3 h. The resulting deep brown solution was allowed to cool at
room temperature, after which the solution was cooled in an ice
bath for 0.5 h. The brown solid that formed was collected by vac-
uum filtration and washed with cold methanol until the filtrate
ꢀ
ꢀ
I /I redox electrolyte, TBP acts as an important additive by
3
was colorless, and then it was washed with Et
The product (yield: 0.691 g, 35.24%) was used without further
purification.
2
O and air dried.
improving the open circuit potential (Voc) of the cell [48]. It has
been further reported that TBP increases the electron lifetime in
the TiO conduction band thereby arresting the recombination
2
effect. The fabricated DSSCs were subjected to performance screen-
ing with the help of electrochemical techniques.
2
.3.2. [Ru(p-F-tpy)(pcqH)Cl](PF
Ru(p-F-tpy)Cl (0.267 gm, 0.5 mmol) and pcqH (0.125 gm,
.5 mmol) were taken in a round bottomed flask in 25 ml dimethyl
. The reaction mix-
ture solution was then reduced in rotary evaporator to 5 ml and a
saturated aqueous solution of NH PF was added to the solution.
6
) (1)
3
0
2.6. Electrochemical impedance spectroscopy (EIS)
formamide (DMF) and refluxed for 6 h under N
2
In order to determine the charge-transfer resistance and double
layer capacitance across the dye-sensitized semiconductor elec-
trode – redox electrolyte interface, the fabricated DSSC cells were
subjected to EIS measurements at room temperature with the help
of AUTOLAB 302 N PG-stat, Eco-Chemie BV (The Netherlands) com-
bined with the NOVA-v1.10 software package, under background
illumination from a high-power white light emitting diode (LED).
The frequency dispersion impedance spectra (Nyquist plot) for
the DSSC cells were recorded by applying a sinusoidal perturbation
of 5 mV amplitude at the working electrode over a frequency range
of 100 kHz to 10 mHz at the respective open circuit potentials and
4
6
On addition of more water, a precipitate appeared, which was col-
lected by filtration in a G4 sintered glass filter. Yield: 0.346 g,
8
using silica as the stationary phase and DCM as the mobile phase.
The violet colored product (0.088 g, 22.5%) was eluted with 1:1
4.18%. The product was purified by column chromatography
DCM/CH
3
OH eluent. ESI-Mass, m/z: 712.5.
H NMR (d-6 DMSO, d, ppm): 10.3 (1H, d, H
.04 (1H, d, H ), 8.86 (4H, d, H , H ), 8.6 (2H, s, H
), 8.35 (1H, d, H ), 8.3 (1H, m, H ), 8.00 (2H, m, H
m, H ), 7.56 (2H, d, H ), 7.50 (1H, m, H ), 7.35 (2H, m, H
1H, d, H ), 7.1 (1H, m, H ). C NMR (d-6 DMSO, d, ppm):
64.698 (C ), 162.232 (C ), 158.699 (C ), 158.485 (C ), 153.062
), 152.707 (C ), 151.774 (C ), 151.889 (C ), 150.447 (C
), 137.031 (C and C ), 136.786 (C ), 130.565 (C
), 128.114 (C ), 127.632 (C ), 127.427 (C
), 126.454 (C ), 126.020 (C ), 124.478 (C ), 124.280 (C
23.814 (C ), 120.099 (C ), 116.352 (C ), 116.130 (C ).
1
a
), 9.15 (1H, d, H
), 8.42 (1H, dd,
), 7.60 (2H,
), 7.30
j
),
9
H
g
G
D
K
b
d
m
C
2
A
J
l
B
under illumination of 30 mW/cm measured with an optical pow-
13
(
1
k
c
ermeter (Newport, 1916-R, Canada).
o
L
F
E
(
C
e
f
H
A
a
),
),
2.7. I–V characteristics, transient photoresponse and incident photon-
to-current conversion efficiency (IPCE) records
1
1
37.403 (C
n
C
c
I
h
30.130 (C
l
m
k
K
), 126.549
),
(
C
g
b
J
D
d
Performance output of the DSSC cell was derived in terms
of photo-conversion efficiency with the help of current–voltage
1
i
B
G
j
ꢀ
2
(
I–V) measurements under a light intensity of 30 mW cm from
2
.4. TiO
2
paste and thin film preparation
a white LED light source. The photoresponse of the films, in terms
of rise and decay of the short-circuit current (Isc) at 20 s intervals in
the dark and illuminated conditions, was recorded with a similar
cell setup. IPCE spectra of the cells were recorded with a xenon
lamp light source using a grating monochromator (Sciencetech
9055, Canada).
TiO
grade TiO
45]. The nano-crystalline TiO
over FTO glass plates (Fluorine doped tin oxide on one side making
2
paste was prepared in the laboratory using nanocrystalline
powder, acetic acid, de-ionized water and surfactants
paste was applied as a thin film
2
[
2

6
18
B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
2.8. Computational details
to the central pyridine of p-F-tpy. In the present case we get the
isomer where the pyridin-N atom of pcqH is present at the trans
The quantum chemical calculations were performed using den-
position of central pyridine ring of p-F-tpy. This has been con-
firmed by both the DFT optimized structure and the H NMR spec-
trum of the complex. The H atom at the ortho position of the
1
sity functional theory (DFT) implemented in GAUSSIAN 09 [49]. A split
basis set was used for the optimization of the complex using the
B3LYP hybrid functional and 6–31 g (d) basis set for hydrogen, car-
bon, oxygen, nitrogen and fluorine atoms and the LANL2DZ basis
set for the ruthenium atom. The absorption spectra were simulated
using Time Dependent Density Functional Theory (TD-DFT). All the
computational studies were carried out in DMSO solvent using the
Polarizable Continuum Model (PCM) implemented in GAUSSIAN 09.
The orbital contribution was calculated by GaussSum [50].
pyridine-N atom (H
quently the signal for H
a
in Fig. 1) of pcqH remains close to Cl. Conse-
is shifted to higher field (d = 10.3 ppm)
a
than the other aromatic protons. Similar observations have been
reported by D.J. Wasylenko et al. [40].
3.2. Thermogravimetric analysis
Thermogravimetric analysis of the dye 1 was monitored in the
range 30–800 °C to measure the thermal stability of this new
dye. As seen in the thermogram (Fig. 2) the atmospheric moisture
absorbed in the dye is removed by 60 °C. There is no weight loss up
to 216 °C, when carbon dioxide elimination starts and this gets
completed at 300 °C. Thereafter the compound starts to decompose
on further heating. The thermogram thus clearly indicates that the
dye is fairly stable up to 216 °C.
3
. Results and discussion
3.1. Synthesis
The synthesis of the novel complex [Ru(p-F-tpy)(pcqH)Cl]PF
6
0
0
0
00
(1) (F-TPY = 4 -(4-fluoro phenyl)-2,2 :6 ,2 terpyridine, pcqH = 2-
(
2-pyridyl)-4-carboxyquinoline) has been outlined in Scheme 1.
The ligands p-F-terpyridine and 2-(2-pyridyl)-4-carboxyquinoline
have been synthesized according to the published procedures.
The ruthenium complex has been synthesized first by the reac-
3.3. Absorption spectra
tion of the terpyridine ligand with RuCl
3
in dry methanol for 3 h
In DMSO, complex 1 shows two intense ligand centered bands
at 277 and 321 nm in the UV region (Fig. 3a). As per the literature
[51], these peaks may also have a contribution from a high energy
MLCT band arising from a Ru to p-F-tpy/pcqH ligand transition.
There is also a moderately strong low energy MLCT band at
536 nm (Fig. 3c).
The absorption spectra of the complex 1 (Fig. 3b) have been
studied at various pH values (0.04–13.17). At pH 0.04 the complex
exhibits a MLCT band at 547 nm. With an increase in pH, the
energy of the MLCT band increases and appears at 522 nm at pH
3.54. The position of the band remains constant at 522 nm up to
a pH value of 11.27. On further increasing the pH value, the MLCT
band is red shifted to 527 nm at pH 13.17.
and then, after subsequent washing with cold methanol and filtra-
tion and drying of the solid formed, by further reaction of this solid
with the anchoring group (pcqH) in DMF for 6 h. The product was
4 6
precipitated by treatment with excess NH PF and addition of
water to a reduced volume of the DMF mixture. The solid so
obtained was filtered, dried and further purified by column chro-
matography in silica, where the desired complex was eluted by
4
0–50% methanol in dichloromethane as a deep reddish violet
coloured solution. The molecular structure has been characterised
by mass and NMR spectroscopy. There is the possibility of the for-
mation of two isomers for the molecule, depending upon the coor-
dination of the pcqH ligand; the pyridine or quinoline N atom trans
6
Scheme 1. Synthetic routes for (a). p-F-tpy (b) pcqH (c) [Ru(p-F-tpy)(pcq)Cl]PF .

B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
619
determined by plotting the change in absorbance with pH at a
ꢀ3
fixed wavelength. 1.4 ꢁ 10 M TBAP (tetrabutyl ammonium per-
chlorate) was added to maintain the ionic strength of the solution.
The spectrophotometric titration of the complex shows interesting
features.
ꢀ
5
A 200 ml 7 ꢁ 10 molar solution (1:1 DMF-H
2
O) of complex 1
values
was prepared for the spectrophotometric titration. The pK
a
were obtained from the inflection point of the plot for absorptions
at 285, 315 and 516 nm versus pH (Fig. 5). From all the three plots,
two clear inflection points were obtained at 2.89 and 12.20. The
former is assigned to be the deprotonation of the ꢀCOOH group
of pcqH, while the later may be due to the deprotonation of the
water molecule attached to Ru that replaced the Cl ligand during
the addition of aqueous NaOH. The pK
a
value of the coordinated
2
H O molecule has been previously reported by Mayer [54,55] and
Gratzel [34]. The pK
ity of the spectator ligand. The presence of a –COO group in the
a
values increase with an increase in the basic-
ꢀ
2
molecule increases the basicity of the H O ligand to 12.20.
Fig. 2. Thermogram of the dye [Ru(p-F-tpy)(pcqH)Cl]PF
6
.
3.5. Electrochemical studies
At pH 0.04, the carboxylic acid of pcqH remains protonated.
Deprotonation of the –COOH group takes place within the pH win-
dow of 1.56–3.54, evident in the large blue shift of the MLCT peak
position from 547 to 522 nm. This blue shift of 25 nm may be due
Cyclic voltammetry (Fig. 6) of the Ru complex (1) in DMF med-
ium shows a reversible oxidation wave at 0.85 V with i /i ꢂ 1 and
a
c
D
p
E = 70 mV. In the cathodic scan the complex shows a quasire-
versible reduction process at ꢀ0.96 V and two additional peaks at
ꢀ1.56 and ꢀ1.73 V. Cyclic voltammetry of the free ligands (Fig. 7)
shows similar reduction process: pcqH (ꢀ1.01 V), 4F-terpy
(ꢀ1.14, ꢀ1.79 V). Therefore, on the cathodic side the reduction of
the complex may be due to a ligand based electron transfer. The
reversibility of the oxidation process of complex 1 is verified by
the constant E1/2 value at different scan rates (20–500 mV/Sec)
to the combined effect of -COOH deprotonation [52] as well as
⁄
replacement of Cl by H
in pcq while the later stabilises the d
2
O. The former destabilizes the
p
orbital
ꢀ
p
(Ru) orbital. It has already
been reported that the Ru–Cl bond is not robust in aqueous med-
ium. This in-situ formed aqua complex is stable up to pH 11.27.
An interesting feature in the plot of absorbance at fixed wave-
length versus pH is the appearance of a 2nd inflection point after
pH 11.27. This later inflection point marks the deprotonation of
1
/2
(Fig. 8a) as well as from the plot of i/(
m)
versus m, (where i = cur-
rent and = scan rate) (Fig. 8b).
m
the coordinated H
medium, conversion of H
the t2g orbital of Ru due to the stronger
2
O that replaced the Cl ligand. In highly alkaline
The detailed redox properties of complex 1 were studied by
means of cyclic voltammetry and differential pulse voltammetry
at variable pH (0–13) (SM7). The complex exhibits pH dependent
redox behavior. Throughout the whole pH range on the anodic side
ꢀ
2
O to OH again increases the energy of
ꢀ
p
donor property of OH .
Similar results have been reported by Wrighton et al. [53] and M.
Gratzel et al. [34]. A change in the MLCT band position with the
variation of pH value is also reflected by the naked eye colour
change of the complex. The colours at pH 0 and pH 13 are more
intense than those at the other pH values (Fig. 4).
0
one reversible couple is observed in the range 0.44–0.87 V (EI ),
0
whereas in the pH range 6.08–6.53 a second reversible peak (E ) is
II
0
I
obtained at 0.91 V. The first oxidation wave (E ) shows pH depen-
dence in the range 6.08–11.36, but the second reversible electron
transfer process (E I0 I) is independent of pH. Here we assign the first
II/III
III/IV
3.4. Spectrophotometric determination of pK
a
values
reversible peak as Ru
process.
and the second peak as the Ru
oxidation
The pK
a
values of the Ru complex 1 were determined by
values can be
At pH 0, the –COOH group of the anchoring ligand remains
protonated. The spectrophotometric titration shows that the
spectrophotometric titration. The ground state pK
a
6 6 2
Fig. 3. Absorbance spectra of (a) [Ru(p-F-tpy)(pcqH)Cl]PF in DMF (b) [Ru(p-F-tpy)(pcqH)Cl]PF in (1:1) DMF:H O in the pH range 0.04–13.17.

6
20
B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
Fig. 4. DMF-H
2
6
O (1:1) solution of the dye ([Ru(p-F-tpy)(pcqH)Cl]PF ) at different pH.
ꢀ
Water is deprotonated to OH . Therefore the process may be
assigned as a proton coupled electron transfer. On further increasing
0
the pH, the E value remains constant. Hence the redox process is
independent of pH.
The complete redox behavior of the Ru complex 1 is shown by
the Pourbaix diagram (Fig. 9a). The molecule contains two acidic
2
protons in -COOH and H O. So two different proton coupled elec-
tron transfer process are expected. In the pH range 0–4, since the
deprotonation of –COOH is accompanied by the removal of Cl by
II–III
2
H O, the expected trend in E1/2 for THE Ru
oxidation is not
obtained experimentally. This could only be achieved by starting
with an aqua complex instead of the Chloro complex (1). The Pour-
baix diagram displays experimental E1/2 versus pH graph within
the pH range 4–13. In the pH range 4–6.25, two chemically rever-
II
III
sible species, Ru H
2 2
O and Ru H O, exist. This process is indepen-
II
Fig. 5. Absorptivity vs. pH at 516 nm, 315 nm and 285 nm of the complex ([Ru(p-
Ftpy)(pcqH)Cl]PF ).
dent of pH. From pH 6.25–11.36 the two species are Ru H O and
Ru OH. A 2nd redox process appeared in anodic side of the CV in
2
III
6
the range pH 6.08–6.53 at a relatively high potential (0.905 V). This
III/IV
may be due to the Ru
oxidation.
a I
deprotonation of the –COOH group has a pK value
of 2.89. The E0
The relation of E1/2 with pH can be expressed by the following
continuously increases from pH 0 to 4.31. With the increase in pH
from 0 to 4.31 two process simultaneously operate: deprotonation
of the –COOH group and the replacement of the Cl ligand by a water
0
equations [56] where E1/2 is the standard redox potential of the
II/III
Ru
couple at pH 4.31;
a
red is the concentration of the reduced
is
is that in
species and
aox is the concentration of the oxidised species, K
1
ꢀ
molecule. Formation of –COO increases the electron density on the
II
the H O deprotonation constant in the Ru state and K
2
2
Ru atom, reducing the E
reduces the electron density on the Ru atom and increases the E
ues. Since H O is directly connected to Ru, H O addition dominates
0
values, while replacement of Cl by H
2
O
III
the Ru state (Scheme 2).
0
val-
0
1
E
1=2 ¼ E þ ðRT=FÞ lnð
a
red
=
a
ox
Þ
2
2
=2
the deprotonation of the –COOH group. Consequently the redox
potential increases. After pH 6.08, the potential gradually decreases
up to pH 11.36. So in this range the redox process is pH dependent.
þ
where
a
red ¼ 1K =½Hꢄ
1
þ
a
ox ¼ 1 þ K =½Hꢄ
2
a
b
6
Fig. 6. Cyclic voltammetry and differential pulse voltammetry in DMF solvent of [Ru(p-Ftpy)(pcqH)Cl]PF at a scan rate of 100 mV/s of (a) anodic side (b) cathodic side.

B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
621
Fig. 7. Cyclic voltammetry (Cathodic Scan) of p-F-tpy (left) and pcqH (right) in DMF.
a
b
at different scan rates and (b) i/ 1
m
/2
vs. m (scan rates).
Fig. 8. (a) Cyclic voltammetry of [Ru(p-F-tpy)(pcqH)Cl]PF
6
Fig. 9. (a) Pourbaix diagram (E1/2 vs. pH) of [Ru(p-F-tpy)(pcqH)Cl]PF
6
6
and (b) plot of calculated and observed E1/2 vs. pH for [Ru(p-F-tpy)(pcqH)Cl]PF .
Non-linear regression analysis (Fig. 9b) by using the Microsoft
Excel Solver program was done according to the above equations
3.6. Photovoltaic study
II
III
to generate pK
pK ).The pK value conforms to the pK
acid-base spectrophotometric titration (pK
in the pK value by almost 8 units upon changing from the Ru to
a
values for Ru as 11.89 (pK
value obtained from the
= 12.20). The decrease
1
) and for Ru as 4
The anchoring group in this dye with a single carboxylic acid is
quite unconventional. It has been reported [58] that the black dye,
despite having three potential COOH linkers, attaches to the semi-
conductor surface via only one carboxylic acid group due to steric
congestion. Therefore, there exists a fair possibility of the dye
molecule (1) to get better attached on the semiconductor surface
(
2
1
a
1
II
a
III
Ru state for the aqua ligands in a polypyridylic type of complex is
in accord with the literature [57].

6
22
B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
Scheme 2. Outline of the electrode process for the RuIII–RuII couple, involving both
redox and acid–base equilibria.
[
59–61], creating more reaction sites and enhancing the absorption
capability of the matrix.
The functional properties like Voc, short-circuit current density
(
(
Isc), fill factor (FF) and solar-to-electrical conversion efficiencies
) were derived for the fabricated DSSCs, employing the respec-
g
tive electrochemical techniques. The power output characteristics
of 1-TiO and N719-TiO based DSSCs are presented in Fig. 10. In
the case of 1-TiO matrices the Voc values and percent efficiency
are rather compromised compared to those of N719-TiO based
Fig. 11. Nyquist plot of the dye 1 and N719 dye (Inset) sensitized DSSC system,
scanned from 105 to 0.1 Hz.
2
2
2
2
DSSC (inset). However, even with a moderate photon to energy
conversion efficiency of 0.13%, 1-TiO2 exhibits a fill factor of
3
9.4%, which is promising.
The impedance spectra (Fig. 11) of the films at the respective
open circuit potentials (OCP) were recorded for the cell configura-
3ꢀ
ꢀ
tion FTO/TiO
upon the simple equivalent circuit (EC) model constituting several
circuit components. The circuit included a solution resistance (R ),
2
/1/I – I /Pt–FTO and were further analyzed based
s
charge transfer impedance (Rct) and a constant phase element (Q)
as the non-ideal capacitor, which substitutes the double layer
capacitance of the electrode–electrolyte interface and Warburg
impedance [30]. It has been observed that the cell exhibited high
charge transfer resistance (5.71 kΩ) and moderate values of the
constant phase element (Q) (4.87 h), whereas N719 based DSSC
shows charge transfer resistance (73.1 Ω) with a constant phase
element of 217 h (Fig. 11 inset). This may be rationalized due to
the presence of a lower number of anchoring groups in the sensi-
tizer 1.
In Fig. 12 an instantaneous photo response is featured with the
rise and decay curves of photocurrent density (Isc) during succes-
sive exposure of the films to illuminated and dark conditions. The
overall photoresponse behaviour of such type of unconventional
dye in the device is much more than expected and can also be
modified by extending the anchoring site through a preferable
Fig. 12. Short-circuit current density (Isc) – rise and decay curves for the cell
system FTO–1- /I3–I –/ Pt–FTO. Inset shows polynomial fit chrono-amperometric
plot.
group. The inset figure depicts the chrono-amperometric study
through a polynomial fit over a specified time barrier. The slow
decay indicates that the stability of the 1-TiO based DSSC device
2
is also satisfactory.
The IPCE measurement provides information about the absorp-
tion capability of the photo-anode and the kinetic efficiency of the
electron transport during the energy conversion process. Fig. 13
2
shows the action spectrum of IPCEs for the 1-TiO solar cell, which
is quite impressive compared to the N719 based system in the
inset. In the case of the N719 sensitizer, the maximum IPCE exhib-
ited was of the order of 52%, whereas for the dye (1) the IPCE
attains a considerable value of 40%, which is indicative of the facile
electron transfer process from the excited 1 to the conduction band
of the semiconductors. It appears 1 synthesized DSSC is particu-
larly effective in the spectral range 420–550 nm. However, the
spectral responses of this new dye can be expanded with further
modifications, co-sensitization in the matrix and optimizing the
electrolyte composition.
3.7. DFT calculations
Fig. 10. Current–voltage and power output characteristics, measured in 0.3 AM
white LED illumination for a DSSC using 1 and N719 (Inset) as the molecular
sensitizer over titania matrix respectively.
A computational study using density functional theory with
the B3LYP hybrid functional was performed to get the optimized

B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
623
structures of the ligands and the Ru(II) complex in DMSO for a bet-
ter understanding of their electronic structures. The compositions
and energies of the frontier molecular orbitals of the complex are
given in Table 1 while the metrical parameters are displayed in
Table 2. The method used for the optimization is perfect, as is evi-
dent from close matching with the literature for similar molecules.
The molecule adopts a distorted octahedral geometry where the cis
angles vary from 77.35–106.29° and the trans angles are in the
range 157.45–176.28°. Due to its rigidity, the terpyridine ligand
is expected to coordinate to the metal centre in a meridional fash-
ion, which is also observed in our optimized structure. In a number
of related 4 substituted terpyridine ligands, Y. Tanaka et. al.
reported the bond length for Ru-Npy(central) is smaller (ꢂ1.97 Å)
than that of Ru-Npy(terminal) (ꢂ2.07 Å). In our case we also find
a similar trend, where the Ru-Npy(central) bond length is 2.00 Å
and the Ru-Npy(terminal) bond length is 2.11 Å [62]. A comparison
+
with the optimized geometry of the [Ru(bpy)(terpy)Cl] system
[
63] also shows a similar trend in the Ru-Npy distances.
The other basal coordination is made by the pyridine-N atom of
Fig. 13. Photocurrent action spectra of the 1 and N719 (Inset) sensitized DSSC
device.
the anchoring ligand, whereas the axial sides are coordinated by
Table 1
Compositions and energies of the frontier molecular orbitals of the complex.
Energy(eV)
ꢀ6.84
ꢀ6.04
ꢀ5.88
ꢀ5.74
ꢀ2.98
ꢀ2.51
ꢀ2.3
ꢀ1.82
Atom/ligand %
Ru
Cl
p-F-tpy
pcqH
HOMOꢀ3
HOMOꢀ2
HOMOꢀ1
HOMO
70
11
12
7
LUMO
5
0
1
LUMO+1
7
1
91
1
LUMO+2
2
0
98
1
LUMO+3
1
0
2
3
75
1
66
11
20
3
13
81
3
8
16
93
98
Table 2
Metrical parameters of the optimized structure of the complex.
Geometry (Å, °)
Bond
(Å)
Angle
(°)
Angle
(°)
Angle
(°)
Ru–Cl
2.49
2.12
2.00
2.11
2.11
2.16
Cl–Ru–N3
Cl–Ru–N4
Cl–Ru–N5
Cl–Ru–N6
Cl–Ru–N7
N7–Ru–N3
89.18
83.36
88.58
92.96
169.87
89.72
N7–Ru–N4
N7–Ru–N5
N7–Ru–N6
N6–Ru–N3
N6–Ru–N4
N6–Ru–N5
106.29
96.24
77.35
100.76
176.28
101.76
N5–Ru–N3
N5–Ru–N4
N4–Ru–N3
157.45
78.75
78.70
Ru–N3
Ru–N4
Ru–N5
Ru–N6
Ru–N7
Fig. 14. DFT optimised structure of Ru[(p-F-tpy)(pcqH)Cl] PF
6
(1).

6
24
B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
the quinoline-N atom of the anchoring ligand and the ancillary Cl
ligand. A comparison with the tris chelate Ru complex of the
geometry and the structure in DMSO. Composition of the frontier
molecular orbitals shows that HOMO, HOMOꢀ1 and HOMOꢀ2
have maximum contributions from Ru, whereas HOMOꢀ4 is
terpyridine based. On the other hand, LUMO and LUMO+3 are rich
with 2-(2-pyridyl)-4-carboxyquinoline ligand character and
LUMO + 1 and LUMO + 2 are mostly comprised of the terpyridine
ligand. The optimized structure of the molecule is given in Fig. 14.
2
-(2-pyridyl)-4-carboxyquinoline) ligand [64] also shows a similar
trend in the Ru-Npy and Ru-Nquinoline bond distances. Literature
values for the two bonds are 2.06 and 2.15 Å, whereas for the
present molecule those values are 2.11 and 2.16 Å, respectively.
The small deviation is explainable considering the crystal
Fig. 15. The calculated UV–Vis spectrum of Ru[(p-F-tpy)(pcqH)Cl] PF
6
(1) (left) in DMSO(Green vertical lines correspond to calculated oscillator strengths) and a plot of the
experimental and calculated UV–Vis spectrum in DMSO(right). (Colour online.)
Fig. 16. Electronic transitions assigned to the major peaks.

B.N. Mongal et al. / Polyhedron 102 (2015) 615–626
625
Table 3
Vertical excitations with band position, oscillator strength (f) and character assignment.
Expt
peak (nm)
Calculated
peak (nm)
Oscillator
strength (f)
Maximum orbital
contribution
From
pcqH
To
Assignment
p-F-tpy
Cl
Ru
pcqH
p-F-tpy
Cl
Ru
536
458
330
321
288
271
510.62
440.2
330.4
324.45
294.04
273.54
0.2101
0.023
0.1767
0.1455
0.1592
0.3515
H-2->LUMO (80%)
H-1->L+2 (81%)
HOMO->L+5 (42%)
H-3->L+1 (88%)
H-9->L+1 (61%)
H-4->L+3 (31%)
16
3
7
3
4
8
1
75
66
70
3
10
2
93
1
79
1
1
98
1
0
0
0
1
1
0
5
2
3
7
7
1
Ru?pcqH
Ru?p-F-tpy
Ru?pcqH
p-F-tpy?p-F-tpy
Cl?p-F-tpy
20
12
81
21
6
11
11
13
66
3
98
18
91
91
2
89
pcqH?pcqH
3
.8. TDDFT studies
The dye shows a very prominent red shifted MLCT (536 nm),
3
ꢀ1
ꢀ1
e
= 17 ꢁ 10 M cm . This dye is a promising candidate for PV
The consensus between the computed and experimental
energy conversion in terms of IV, %FF and %IPCE character. It is
envisaged that with suitable modification in the anchoring sites,
better substitutions on the phenyl group attached to the central
pyridine ring of the terpyridine ligand and coordination with dif-
ferent ancillary ligands, there is further scope of improvement of
the efficiency of this dye.
absorption spectra in DMSO is presented in Fig. 15. The experimen-
tal peaks at 277 and 321 nm have been matched nicely with the
theoretical observations at 273 (f = 0.35) and 324 nm (f = 0.145),
respectively. The former band involves a pcqH centered transition
while the later has been found to involve a p-F-tpy based electronic
transition. The band at 536 nm have been computed with a slight
blue shift at 510 nm (f = 0.21) which can be assigned a MLCT char-
acter from the Ru atom to the anchoring ligand pcqH. Moreover the
shoulders accompanying all the major peaks have been calculated
with great accuracy at 294 (kexpt = 288), 330 (kexpt = 330) and
Acknowledgements
SN acknowledges AICTE, New Delhi, India. This work was
funded from the AICTE-RPS grant: 8023/RID/RPS-10/Pvt (II Pol-
icy)/2011-12. SN also acknowledges the NMR spectral facility and
Computational Chemistry facility established in the Chemistry
Dept. BIT-Mesra through the DST FIST Program (SR/FST/CSI-
4
40 nm (kexpt = 458 nm). Here, an interesting observation is that
the electronic transitions for the shoulder are quite different in
nature from the corresponding major peaks, as observed from
the simulated UV–Vis spectrum. The absorption at 277 nm is pcqH
based, whereas the shoulder at 288 nm involves a transition from
the ancillary Cl ligand to the terpyridine ligand. The band at
2
42/2012). Different analytical services obtained from CIF, BIT-
Mesra is greatly acknowledged. JD acknowledges DST-SERI, New
Delhi-110016. T.K.M. thanks CSIR for research funding through
CSIR-EMR (Project Nos. 01(2470)/11/EMR-II.
3
3
21 nm is terpyridine based, whereas the very broad shoulder at
30 nm is actually a high energy MLCT involving Ru and the pyri-
dine moiety of pcqH. Unlike the major peak at 536 nm, a low inten-
sity shoulder at around 458 nm, which has been observed very
prominent in TDTDFT, is a MLCT from Ru to p-F-tpy, i.e. away from
the anchoring ligand.
Appendix A. Supplementary data
All the structural characterizations, namely mass, 1H and 13
C
Hence, it can be seen that there are only two transitions where
the electron density shifts towards the dye semiconductor inter-
face through the anchoring 2-(2-pyridyl)-4-carboxyquinoline
ligand. The remaining transitions are away from the linker group
and serve no purpose in enhancing the probability of electron
injection into the conduction band of the semiconductor. This
can be one reason behind the lower efficiency of the dye. The cor-
responding molecular orbitals (shown in SM 8) involved in the
transitions of the major peaks are shown in Fig. 16.
NMR data, molecular orbital diagrams of the optimized structure
and redox potential data. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1
016/j.poly.2015.10.040.
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[
[
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