Novel 4′-functionalized 4,4′′-dicarboxyterpyridine ligands for ruthenium complexes: Near-IR sensitization in dye sensitized solar cells

Article abstract of DOI:10.1039/c4dt01598c

Novel ruthenium complexes (MC113-MC117), obtained by modifying the terpyridine ligand of the black dye (N749), have been evaluated as sensitizers for dye sensitized solar cells (DSSCs). The modification is carried out by attaching selected chromophores, with varying electron donating strength, covalently to the central ring of the ligand. The complexes, compared to the parent dye, show red shifted absorption covering visible and near IR regions and higher molar extinction coefficients. We report in this work synthesis of a series of these ruthenium complexes with chromophores such as tert-butyl phenyl, triphenylamine, bithiophene, phenoxazine and phenothiazine. Detailed experimental characterization using optical, electrochemical and photovoltaic techniques has been carried out and complemented by density functional theory studies. The fill factors (ff) obtained for these dyes are larger than those of the parent black dye. In spite of these superior properties, the dyes show only moderate to good power conversion efficiencies. The possible reasons for this have been investigated and discussed.

Full text of DOI:10.1039/c4dt01598c

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Novel 4’-functionalized 4,4’’-dicarboxyterpyridine
ligands for ruthenium complexes: near-IR
sensitization in dye sensitized solar cells†
Cite this: Dalton Trans., 2014, 43,
14992
Ganesh Koyyada,a,b Vinayak Botla,a Suresh Thogiti,a Guohua Wu,c Jingzhe Li,c
Xiaqin Fang,c Fantai Kong,*c Songyuan Dai,c,d Niveditha Surukonti,a
Bhanuprakash Kotamarthia,b and Chandrasekharam Malapaka*a,b
Novel ruthenium complexes (MC113–MC117), obtained by modifying the terpyridine ligand of the black
dye (N749), have been evaluated as sensitizers for dye sensitized solar cells (DSSCs). The modification is
carried out by attaching selected chromophores, with varying electron donating strength, covalently to
the central ring of the ligand. The complexes, compared to the parent dye, show red shifted absorption
covering visible and near IR regions and higher molar extinction coefficients. We report in this work syn-
thesis of a series of these ruthenium complexes with chromophores such as tert-butyl phenyl, triphenyl-
amine, bithiophene, phenoxazine and phenothiazine. Detailed experimental characterization using
optical, electrochemical and photovoltaic techniques has been carried out and complemented by density
functional theory studies. The fill factors (ff) obtained for these dyes are larger than those of the parent
black dye. In spite of these superior properties, the dyes show only moderate to good power conversion
efficiencies. The possible reasons for this have been investigated and discussed.
Received 30th May 2014,
Accepted 24th June 2014
DOI: 10.1039/c4dt01598c
www.rsc.org/dalton
extends into the red and near-infrared region of the solar spec-
Introduction
trum, contributing to very high efficiencies of N3,11 N71912
Dye sensitized solar cells (DSSCs) have attracted increasing and N749.13 In this context, much attention has been devoted
attention as promising alternatives to conventional silicon to enhance the performance of DSSCs by engineering sensi-
based solar cells due to their low cost and ease of fabrica- tizers. For example, ancillary ligands of the bipyridyl ruthe-
tion.1,2 The optimum efficiency of a DSSC device depends nium complexes have been modified for tuning the
upon several parameters and particularly the structure of the photophysical properties of the sensitizers leaving the dicarb-
sensitizer plays a major role. The ground and excited state oxy bipyridine for performing its duty of grafting the sensitizer
energy levels of the dye should be well matched with the onto the TiO2 for effective electronic communication.31–42 In
highest occupied molecular orbital (HOMO) level of the redox contrast, the structural variation of ruthenium-terpyridyl com-
mediator and the conduction band of the semiconductor, plexes warrants the presence of an anchoring group as well as
respectively, in order to facilitate easy regeneration of the sen- the tunable chromophores on the same terpyridyl ligand and
sitizer and facile electron injection onto TiO2.3–11 Among a are comparatively less addressed because of the synthetic chal-
large variety of dyes investigated for DSSC application,11–30 Ru- lenges involved. Ruthenium complexes with an electron donat-
sensitizers11–17 have attracted more attention because of their ing group on the terminal ring of the dicarboxy terpyridines
low lying metal–ligand charge transfer (MLCT) transition that have been reported,43–45 while terpyridine ligands carrying the
two carboxy groups on the terminal rings and the electron
donating group on the middle ring have not been reported for
DSSC applications.
aNetwork of Institutes for Solar Energy, CSIR-Indian Institute of Chemical
Technology, I&PC Division, Uppal Road, Tarnaka, Hyderabad-500 607, India.
The application of the black dye (N749) was successfully
E-mail: chandra@iict.res.in; Fax: +914027193186
bAcademy of Scientific and Innovative Research, CSIR-IICT, India
cKey Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics,
Chinese Academy of Sciences, Hefei, 230031, P. R. China
dState Key Laboratory of Alternate Electrical Power System with Renewable Energy
Sources, North China Electric Power University, Beijing, 102206, P. R. China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt01598c
explored as a base-dye along with small organic molecules
as co-sensitizers achieving the highest certified record
efficiency.46 The broad absorption properties of the terpyridyl
ruthenium complexes offer a large space for structural modifi-
cation to further improve the DSSC performance. Such modifi-
cations can extend the absorption of a sensitizer further into
14992 | Dalton Trans., 2014, 43, 14992–15003
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the near infrared region. While not all such modifications lead R3-aldehyde (1) was synthesized using 5-hexylthiophene-2-
to highly efficient sensitizers, many of these give us an oppor- boronoic acid pinacol ester and 5-bromothiophene-2-carb-
tunity to understand in detail the structure and property aldehyde under Suzuki reaction conditions. The aldehydes 4
relationship. These studies also give us a deep insight into and 5 were synthesized by N-alkylation followed by the
more fundamental issues like charge transfer, polarization, Vilsmeier–Haack formylation on phenoxazine and phenothia-
binding, etc. Keeping this in view, we in this work have modi- zine respectively. The commercially available aldehyde part-
fied the terpyridine ring by substituting chromophores co- ners (R1-CHO and R2-CHO) and the aldehydes 1, 4–5 were
valently on the central ring while retaining the anchoring further reacted with 7 in the presence of sodium hydroxide
groups on the terminal rings to create novel ligands that can and ammonia in methanol affording 40–50% yields of the
form complexes with the Ru metal. These new ruthenium ter- corresponding 4′-functionalized 4,4″-dicarboxyterpyridine (8).
pyridyl sensitizers are shown as MC113–MC117 in Fig. S1 The terpyridine ligands (8a–e) on complexation with
(ESI†).
RuCl3·3H2O, followed by treatment with NH4NCS results in the
The functionalization of the central ring in these complexes formation of diester, which upon hydrolysis using triethyl-
has been systematically studied by attaching chromophores amine afforded the desired ruthenium complexes (MC113–
with different electron donating strength. In MC113 the MC117) in 39–47% yield. The new complexes were character-
chromophore is a tert-butyl phenyl group which is shown to ised by nuclear magnetic resonance spectroscopy (NMR) and
have a moderately electron donating moiety.47,48 In MC114, electrospray-ionization mass spectra (ESI-MS).
the triarylamine group, a widely used chromophore in electro
Geometries
and optical materials, is incorporated owing to its strong elec-
tron donating and transporting capability and the propeller Knowledge of the geometry is important to understand the
starburst molecular shape.48,49 Hexyl bithiophene, which has properties and also the effect of substitution. The optimized
been used as the chromophore in MC115, is expected to ground state geometries of the complexes MC113–MC117 are
increase the solubility and red shift the absorption peak, apart shown in Fig. S2 (ESI†). Bulky alkyl groups of the ancillary
from increasing the molar extinction coefficient of the sensi- ligands are replaced with methyl groups so as to minimize the
tizer.50,51 Diheteroanthracenes (phenoxazine and phenothia- computational complexity without compromising the quanti-
zines) are heterocyclic compounds with electron-rich oxygen/ tative picture. The optimized geometries reveal that the basic
sulfur and nitrogen heteroatoms, and these molecules are Ru[3 + 3] skeleton of MC113–MC117 is like that of an octa-
non-planar with a butterfly conformation in the ground state, hedron. The carboxylic acid groups and the pyridines of all the
which can impede the molecular aggregation and the for- complexes are in the same plane. The optimized geometry
mation of intermolecular excimers.52–55 Phenoxazine and parameters are tabulated in Table 1 and Table S1 (ESI†). It has
phenothiazines are also potential hole transport materials in been observed that there is not much variation of the geometry
organic devices, presenting unique electronic and optical pro- with regard to both bond lengths and bond angles on substi-
perties. Hence these moieties were selected as chromophores tution with different chromophores.
in MC116 and MC117.
Molecular orbitals
Detailed experimental characterization of the new ruthe-
nium complexes MC113–MC117 has been carried out using Analysis of the molecular orbitals, obtained using compu-
optical, electrochemical and photovoltaic techniques while the tational methods, would be helpful to understand some funda-
standard quantum chemical software has been employed for mental properties of the molecules, like the nature of charge
DFT studies. A structure–property relationship between the transfer of the electronic transitions. Additionally the HOMO
substituted terpyridyl group in the ruthenium complexes and and the LUMO energies can also be compared to the values
the efficiency of the DSSC device has been systematically obtained by differential pulse voltammetry (DPV) experiments
studied.
to obtain a complementary picture of the frontier orbitals. The
molecular orbitals obtained at the M06/LANL2DZ level of
theory along with the percentage contributions of the individ-
ual groups are shown in detail in Fig. S3 (ESI†) and the impor-
tant ones (mainly HOMO, LUMO and LUMO+1) in Fig. 1. The
molecular orbital pictures indicate that the HOMOs of
Results and discussion
Synthesis
We introduced different electron donor groups at the central the molecules MC113–MC115, MC117 are distributed on the
pyridine (4′-position) of terpyridine by the Krohnke Ru atom (∼40%), NCS (∼40%), and terpyridine ligands
method.56,58 The synthetic route for MC113–MC117 is shown (∼20%), while HOMO of MC116 is distributed evenly on all the
in Scheme 1. Isonicotinic acid was used as the starting three groups, viz., Ru (32%), NCS (30%), and terpyridine
material. The ethylisonicotinate derivative 6 was synthesized, (38%). HOMO−1 of MC113–MC117 has similar contribution
according to a previously published procedure, in 90% yield.57 from all three groups, viz., Ru (∼43%), NCS (∼45%), and
Compound 6 was subjected to acylation using paraldehyde terpyridine (∼12%). HOMO−2 of MC116 is mainly located on
and CF3COOH in acetonitrile medium under reflux for 4 h to terpyridine (76%). HOMO−3 of MC113 is mainly located on
give 2-acetyl isonicotinate (7) in 61% yield. The intermediate NCS (100%), and for the molecules MC114, MC115, MC117 it
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Scheme 1 (i) Pd(PPh3)4 NaCO3, DME, 80 °C 6 h; (ii) NaH, DMF, 1-bromodecane, RT, 12 h; (iii) DMF, POCl3, 1,2-dichloroethane reflux, 12 h; (iv)
CH3CH2OH, conc. H2SO4, 6 h, reflux; (v) paraldehyde, t-BuOOH, FeSO4·7H2O, CF3COOH, CH3CN, reflux, 4 h; (vi) NaOH, NH4OH (aq), CH3OH; (vii)
EtOH, conc. H2SO4, reflux, 6 h; (viii) RuCl3·3H2O, EtOH/CHCl3, reflux under dark, 4 h; (ix) (a) DMF reflux, 4 h, (b) NH4NCS, reflux, 2 h, (c) TEA, H2O,
reflux, 48 h.
is delocalized on terpyridine (>70%), NCS (<30%), and Ru
It is also clear from these pictures that the HOMO is slightly
atom (<5%). LUMO as well as LUMO+1 and LUMO+3 of all the delocalized on the chromophore attached to the central ring in
molecules have major contributions from terpyridine ligands all the molecules. Inspecting the antibonding orbitals we note
(>90%).
that in LUMO and LUMO+1 the contribution from the chrom-
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Table 1 Optimized geometry parameters of MC113–MC117 calculated using the PBE0 functional, in gas phase
MC113
MC114
MC115
MC116
MC117
Bond lengths (Å)
Ru1–N9
Ru1–N10
Ru1–N11
Ru1–N12
Ru1–N13
Ru1–N14
2.035
1.932
2.035
2.047
2.022
2.022
Ru1–N9
2.035
1.932
2.035
2.047
2.022
2.022
Ru1–N9
2.035
1.929
2.035
2.045
2.019
2.019
Ru1–N9
2.035
1.930
2.034
2.041
2.022
2.021
Ru1–N9
2.035
1.932
2.034
2.042
2.022
2.021
Ru1–N10
Ru1–N11
Ru1–N12
Ru1–N13
Ru1–N14
Ru1–N10
Ru1–N11
Ru1–N12
Ru1–N13
Ru1–N14
Ru1–N10
Ru1–N11
Ru1–N12
Ru1–N13
Ru1–N14
Ru1–N10
Ru1–N11
Ru1–N12
Ru1–N13
Ru1–N14
Fig. 1 HOMO, LUMO orbital pictures at M06/LANL2DZ level of MC113–MC117 and respective HOMO–LUMO gaps calculated at M06/LANL2DZ
level of theory in DMF solvent. In parenthesis B3LYP/LANL2DZ calculated values are given.
phore is drastically reduced, thus indicating that if the excited
state transition involves HOMO to LUMO or LUMO+1, then
this transition would involve charge transfer from the chromo-
phore to the central part of the terpyridine ligand.
Optical properties
The UV-Vis absorption spectra obtained for MC113–MC117 in
DMF solvent is shown in Fig. 2 and relevant data summarized
in Table 2. It is seen that all the molecules have an absorption
in the range 250–900 nm. Compared to the N749 dye the new
sensitizers show red shifted broad absorption covering visible
and near-IR regions with higher molar extinction coefficients.
The bands in the range 250–450 nm can be broadly attributed
to the ligand centred π–π* electronic transition and/or metal to
ligand charge transition (MLCT). The bands corresponding to
the longer wavelength (λmax) at 620–655 nm can be broadly
Fig. 2 Absorption spectra of MC113–MC117 and N749 in DMF solution.
attributed to the MLCT from the occupied 4d orbitals of ruthe-
nium to the lowest unoccupied π* orbitals of the terpyridine
ligand.13 The λmax of MC113–MC117 in the low-energy region
spans absorption range from 632 nm to 654 nm and is a result wavelength order follows MC113 (632 nm) < MC114 (638 nm) <
of chemical tuning of the complexes by structural design. The MC115 (646 nm) < MC116 (654 nm) ∼MC117 (654 nm), which
λmax bands of all the five sensitizers are red shifted around is in line with the increasing conjugation length of the 4′-func-
10–30 nm relative to that of N749 (620 nm). The absorption tionalized terpyridine ligands. The molar extinction coefficient
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Table 2 Optical, electrochemical and photovoltaic data of sensitizers MC113–MC117
λmax (ε × 104 M−1
Dye
cm−1)a [nm]
Eoxd b [V]
E0–0 c [eV]
E*oxd d [V]
Voc e [V]
Jsc f [mA cm−2
]
ffg
ηh (%)
MC113
MC114
MC115
MC116
MC117
N749
632 (0.7886)
638 (0.8495)
646 (1.6476)
654 (1.1380)
654 (1.0452)
620 (0.6884)
0.67
0.66
0.67
0.64
0.67
1.75
1.65
1.61
1.64
1.62
−1.08
−0.99
−0.94
−0.99
−0.95
0.64
0.61
0.60
0.67
0.68
0.66
8.28
4.66
6.16
5.94
3.31
15.41
0.677
0.731
0.722
0.689
0.706
0.637
3.59
2.08
2.65
2.72
1.59
6.51
a Absorption spectra were recorded in DMF solution. b Oxidation potentials were measured by DPV. c The bandgap, E0–0, was derived from the
intersection of the absorption and emission spectra. d E*oxd was calculated by subtracting E0–0 from Eoxd
.
e Open-circuit voltage. f Current density.
g Fill factor. h Photoelectric conversion efficiency.
(ε) for the MLCT band of these ruthenium dyes are in the from HOMO−3, which is mainly distributed on ancillary part
order MC115 (16 476 M−1 cm−1) > MC116 (11 637 M−1 cm−1) > of the terpyridine ligand to LUMO, LUMO+1, LUMO+3, which
MC117 (10 502 M−1 cm−1) > MC114 (8335 M−1 cm−1) > MC113 are located mainly on the terpyridine ligand. From the orbital
(7794 M−1 cm−1), which are comparable and higher than that pictures, it can be inferred that the main allowed charge trans-
of N749 (8930 M−1 cm−1). Thus, these results indicate that fer transitions in the molecules are of metal–ligand (MLCT)
extension of π-conjugation at the 4′- position of the terpyridyl and ligand–ligand (LLCT) as also inferred from the experi-
ligand can lower the MLCT energy and also increase the mental data. The band gap, E0–0, obtained from the inter-
absorption intensity and the effect is clearly seen in the case of section of the absorption and emission spectra is also shown
MC115 absorption spectra with a bithiophene chromophore in Table 2.
on the central ring (εmax = 16 476 M−1 cm−1). In the case of tri-
cyclic isosters (phenothiazine and phenoxazine), phenoxazine
Redox properties
based MC116 showed a higher extinction coefficient compared
to phenathiazine based MC117. This could be attributed to
strong electron donating nature. The bathochromic shift and
hyperchromic effects of the absorption in these new sensitizers
indicate that in addition to e-donating character, the extension
of conjugation at the 4′-position of the terpyridyl ligand
through the incorporation of an aromatic segment is beneficial
to the light-harvesting ability of the sensitizer.
The differential pulse voltammetry measurements were carried
out to obtain the oxidation potentials of the new sensitizers.
The main aim is to obtain a picture of the availability of ade-
quate driving energy for dye regeneration to occur and also the
excited state oxidation potentials. The oxidation potentials of
the RuIII/II couple are calculated to be 0.67, 0.66, 0.67, 0.64 and
0.67 V (vs. the normal hydrogen electrode (NHE)) for MC113,
MC114, MC115, MC116 and MC117, respectively (Fig. 3a and
Table 2). This potential is sufficiently more positive than that
TDDFT calculations were carried out to analyze the photo
physical behaviour of these complexes and also for under-
standing the transitions in terms of molecular orbitals. The
normalized plots of simulated and experimental UV-Vis
spectra of MC113–MC117 are given in Fig. S4 (ESI†). The sum-
marized data of important allowed transitions within the
range of 450–850 nm in comparison with experimental λmax
are given in Table S2 (ESI†). The results are in good agreement
with experimental UV-Vis spectra. The most intense singlet
transitions of MC113–MC117 in this range are shown. In
MC113 it is dominant from HOMO to LUMO+1 (89%) at
657 nm, with oscillator strength, f = 0.1274. In MC114, it is
from HOMO to LUMO+1 (95%) at 662 nm, with f = 0.1227. In
MC115, it is from HOMO to LUMO+1 (89%) at 666 nm with f =
0.1231. In MC116, it is from HOMO to LUMO+1 (80%) at
665 nm with f = 0.1231, and for MC117 it is from HOMO to
LUMO+1 (87%) at 660 nm with f = 0.1238. All these obser-
vations suggest that this transition is dominated by metal to
terpyridine ligand and additionally it is also strengthened by
the small charge transfer transition from the chromophore to
the terpyridine ligand.
of the I−/I3 redox couple (∼0.4 V vs. NHE) and provides
−
enough driving force for efficient dye regeneration.59–64
Excited state oxidation potentials must be sufficiently more
negative than the TiO2 conduction band for efficient electron
injection. The excited-state oxidation potentials (Eox*) of
MC113, MC114, MC115, MC116 and MC117 calculated from
EHOMO − E0–0, are −1.08, −0.99, −0.94, −0.99 and −0.95 V vs.
NHE, respectively. The Eox* values here are more negative than
the conduction band edge of TiO2 (−0.5 V vs. NHE) ensuring
an efficient electron injection process from the excited state of
the dyes into the TiO2 electrode (Fig. 3b).65–69
Thermogravimetric analysis
Thermogravimetric analysis was performed to evaluate the
thermal stability of ruthenium sensitizers. The measurements
were carried out in a TGA/SDTA 851 °C Thermal system
(mettler Toledo, Switzerland) at a heating rate of 10 °C min−1
in the temperature range 25–600 °C under a N2 atmosphere
(flow rate, 30 mL, min−1). Film samples ranging from 1 to
7 mg were placed in the sample pan and heated. During the
heating period the weight loss was recorded as a function of
temperature. The incorporation of different electron donating
groups (shown in Scheme 1, Fig. S1†) at the 4′-position of the
In general, the charge transfer in the molecules under
study is dominant from their HOMO and HOMO−1 compris-
ing largely of the Ru(II) atom and the NCS ligand, and partly
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10.25%, 49.32%, respectively, and under similar conditions
black dye recorded 64.57%.
Photovoltaic properties
The photovoltaic performance of the new sensitizers MC113–
MC117 in DSSCs was tested by constructing the 0.25 cm2
active area TiO2 electrodes using a thermally stable electrolyte
composed of 0.5 M 1,2-dimethyl-3-propylimidazole iodide
(DMPII), 0.05 M I2 and 0.1 M LiI in acetonitrile. Fig. 5 shows
the incident photon-to-current conversion efficiency (IPCE) as
a function of excitation wavelength for DSSCs based on
MC113–MC117, compared with N749. Notably, the IPCE of
MC113 is the highest as compared to the other dyes reaching
34% at 590 nm, while it decreases to 18% at 590 nm, 20% at
590 nm, 24% at 590 nm and 15% at 540 nm for MC114,
MC115, MC116 and MC117, respectively.
The lower IPCE values observed for MC113–MC117 when
compared with N749 might be attributed to the steric con-
straints associated with the 4′-substitution units on the terpyri-
dine resulting in the loss of co-planarity and conjugation while
anchored on TiO2. Further bulkiness of these 4′-substitutions
might have prevented the effective grafting onto the TiO2
surface slowing down the electron injection. Generally, it is
known from the equation IPCE = (1240 × Jsc)100/(λ × Pin) (in %)
that the IPCE value is closely related to Jsc at a specific wave-
length and a fixed input power (Pin). The IPCE values of the
devices constructed from the new complexes are in the range
of 310–750 nm and roughly follow the order of the Jsc values
observed (MC113 > MC115 > MC116 > MC114 > MC117).
The Jsc, Voc and ff of the DSSCs were measured under stan-
dard global AM 1.5 solar light (100 mW cm−2). The current–
voltage (J–V) curves for DSSCs based on MC113–MC117 are
shown (Fig. 6) and the detailed device performance data are
listed in Table 2. The Jsc values of the solar cells for MC113–
MC117 are 8.28, 4.66, 6.16, 5.94 and 3.31 mA cm−2, respecti-
Fig. 3 (a) Oxidative differential pulse voltammetry of MC113–MC117.
(b) The schematic energy levels of MC113–MC117 based on absorption
and electrochemical data.
terpyridine ligand makes the dye more hydrophobic and
increases the thermal stability. According to the thermogram
(Fig. 4) MC113 and MC116 are more stable than all other sen-
sitizers. Percentage of conversion of the sensitizers (MC113–
MC117) at 600 °C are recorded as 10.82%, 41.94%, 33.83%,
Fig. 5 Photocurrent action (IPCE) curves of the TiO2 electrodes sensi-
tized by MC113–MC117 and standard N749.
Fig. 4 Thermograms of the sensitizers MC113–MC117 relative to N749.
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Fig. 6 Photocurrent–voltage (J–V) curves for the DSSCs based on
MC113–MC117 and standard N749.
Fig. 7 EIS Nyquist plots for DSSCs based on MC113–MC117 sensitizers
and the N749 dye (measured under dark conditions with an external
potential of −0.60 V. The inset shows the equivalent circuit).
vely. The high Jsc observed for the MC113 is attributed to its
high IPCE value in the range of 400–650 nm. It is obvious that
the dark current of MC113–MC117 dyes is increased in the
order of MC117 < MC116 < N749 < MC113 < MC114 < MC115
under the same bias leading to a consequent increase of
electrolyte interface is decreased in the order of MC117
(1159 Ω) > MC116 (402 Ω) > N749 (261 Ω) > MC113 (208 Ω) >
MC114 (115 Ω) > MC115 (69 Ω), which indicates that the
recombination reaction rate of the injected electron with the
–
−
recombination rate between the injected electrons and I3 in
I3 in the electrolyte is increased in the order of MC117 <
electrolytes, which causes a consequent decrease in Voc values.
An interesting point is that all these dyes show larger fill
factors. The larger fill factors of the novel compounds MC113–
MC117 are assumed to be arising from stronger intermolecular
interactions suggesting better pathways for charge carriers to
the electrodes. In addition this may be also attributed to the
lower series resistances compared with N749 dye. The power
conversion efficiencies (η) of MC113, MC114, MC115, MC116
and MC117 sensitized solar cells are moderate at 3.59%,
2.08%, 2.65%, 2.72% and 1.59%, respectively (vs. 6.51% for
N749 under the same device-fabrication process and measur-
ing conditions). The dye with the moderate electron donating
group (MC113) exhibited a better photovoltaic performance
with power conversion efficiency (η) of 3.59% than the other
four dyes because of the increase in the Jsc value. The lower η
is largely due to the lower Jsc of the sensitizers as compared to
N749 that is associated with their lesser IPCE.
MC116 < N749 < MC113 < MC114 < MC115 in the dark. The
simulated electron lifetime (τe) could be estimated by τe = Rct
×
Cμ. The corresponding τe values for the DSSCs based on
MC113–MC117 and N749 dyes are 115 ms, 60 ms, 39 ms,
229 ms, 615 ms and 128 ms, respectively. The order of the
above lifetime values is consistent with that of the Rct and Voc.
Therefore, the enhanced electron lifetime may be the intrinsic
reason for the higher Voc values of the DSSCs based on MC117
and MC116 compared to that for N749.
Conclusions
Replacement of the carboxy group with a chromophore on the
central ring of the terpyridine ligand in the ruthenium
complex gives rise to enhanced absorption and higher molar
absorption coefficient compared to N749. The novel, struc-
turally modified ruthenium complexes also show better
thermal stability. On the other hand when sensitized on the
TiO2 surface, there is a drop in the efficiency of the test cells
Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) was performed which is resulting from lower IPCE values. This is probably
in the dark under a bias of −0.60 V to characterize the charge due to the slowdown of electron injection capacity of the sensi-
transfer resistances of the cells with a frequency range of tizer, as the bulky substitution on the central ring of the terpyr-
50 mHz to 1000 kHz. The Nyquist plots for MC113–MC117 idine ligand may be hindering proper grafting of carboxylic
and N749 dye sensitized solar cells are shown in Fig. 7. It is groups on the TiO2. Most interestingly the fill factors of all the
observed that there are two semicircles in the Nyquist plots. new sensitizers are higher compared to the parent N749 dye
The first smaller semicircle at higher frequency represents the and are assumed to be arising from stronger intermolecular
charge transfer resistances at the Pt/electrolyte interface and interactions suggesting better pathways for charge carriers to
the second larger one at lower frequency indicates charge the electrodes. The experimental results complimented with
recombination resistance at the TiO2/dye/electrolyte interface. DFT studies provided insights into the structure–property
It is obvious that the recombination resistance (Rct) obtained relationship. The exact reasons for the higher ff values and
from the Z-view software and the inset model at the TiO2/dye/ moderate efficiencies are under further investigation.
14998 | Dalton Trans., 2014, 43, 14992–15003
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removed under reduced pressure and the residue was purified
by silica gel column chromatography using n-hexane–ethyl-
Experimental section
The starting materials phenothiazine, phenoxazine, isonicoti- acetate (9/1; v/v) as the eluent to give a viscous liquid (2.89 g,
1
nic acid, bromodecane, 4-tertiary butyl benzaldehyde 82%). H NMR (300 MHz, CDCl3, δ in ppm): 6.76 (t, 3H), 6.60
(R1-CHO), 4-(diphenylamino) benzaldehyde (R2-aldehyde) and (m, 3H), 6.44 (m, 2H), 3.4 (t, 2H), 1.64 (m, 2H), 1.5 (m, 2H), 1.3
5-hexyl-2-thiopheneboronic acid pinacol ester were purchased (m, 12H), 0.88 (t, 3H). 13C NMR: 143.4, 139.2, 118.3, 117.5,
from Sigma Aldrich. The solvents were purified by a standard 114.1, 48.5, 29.2, 28.1, 27.3, 22.7, 14.1. ESI-MS (C22H29NO):
procedure and purged with nitrogen before use. All other calcd 323.22, found: 324 (M + H)+.
chemicals used in this work were of analytical grade and were
10-Decyl-10H-phenothiazine (3)
used without further purification, and all reactions were per-
formed under an argon atmosphere. Chromatographic separ- Compound 3 was synthesized by following the procedure
ations were carried out on silica gel (60–120 mesh). 1H NMR described for 2 from phenothiazine (2.00 g 10.050 mmol),
and 13C NMR spectra were recorded on an Avance 300 MHz NaH (0.289 g, 12.060 mmol) and 1-bromodecane (3 mL,
spectrometer using tetramethylsilane (TMS) as an internal 12.060 mmol) and the product was obtained as a viscous
standard. Mass spectra were recorded on a Shimadzu model liquid (2.75 g, 81%). 1H NMR (300 MHz, CDCl3, δ in ppm):
LCMS-2010EV system that was equipped with an electrospray 7.13 (m, 4H), 6.8 (m, 3H), 3.84 (t, 2H), 1.78 (m, 2H), 1.4
ionization (ESI) probe. Absorption spectra were recorded on a (m, 2H), 1.2 (s, 12H), 0.87 (t, 3H). 13C NMR: 145.2, 127.3,
Shimadzu ultraviolet-visible light (UV-vis) spectrometer. 127.0, 124.8, 122.2, 115.3, 47.3, 31.8, 29.49, 29.44, 29.2, 22.6,
Electrochemical data were recorded using an Autolab potentio- 14. ESI-MS (C22H29NS): calcd 339.20, found: 340 (M + H)+.
stat/Galvanostat PGSTAT30. The DPV curves were obtained
Synthesis of 10-decyl-10H-phenoxazine-3-carbaldehyde (4)
from a three electrode cell in 0.1 M Bu4NPF6, N,N-dimethyl-
formamide solution at a scan rate of 100 mV s−1, Pt wire as a To
a solution of 10-decyl-10H-phenoxazine (2) (0.800 g,
counter electrode and an Ag/AgCl reference electrode and cali- 2.476 mmol) and dry DMF (0.228 mL, 2.972 mmol) in 1,2-
brated with ferrocene. Emission spectra were recorded on a dichloroethane (DCE) (10 mL) kept at 0 °C (ice water bath),
J. Y. Horiba model Fluorolog3 fluorescence spectrometer.
POCl3 (0.277 mL, 2.972 mmol) was added slowly. After the
addition was complete the mixture was heated overnight at
reflux. The reaction mixture was quenched with water and
Synthesis of 5′-hexyl-[2,2′-bithiophene]-5-carbaldehyde (1)
A 50 mL two neck round bottom flask was charged with hexyl- extracted three times with chloroform. The combined organic
thiopheneboronic acid pinacol ester (0.511 mL, 1.7 mmol), fractions were washed with brine and dried over Na2SO4. The
Pd(PPh3)4 (0.196 g, 0.17 mmol), dimethoxy ethane (8 mL) and solvent was removed under reduced pressure and the residue
2 M aqueous sodium carbonate (2 mL), then the tube was was purified by silica gel column chromatography using pet-
purged with argon gas. Under an inert atmosphere, 5-bromo- roleum ether–ethylacetate (8/2; v/v) as the eluent to give yellow
1
2-thiophenecarboxaldehyde (0.261 mL, 2.04 mmol) was added powder (0.608 g, 70%). H NMR (300 MHz, CDCl3, δ in ppm):
as a neat liquid. The tube was sealed and refluxed for 12 h at 9.63 (s, 1H), 7.27–7.23 (m, 1H), 7.4 (m, 1H), 6.79–6.59 (m, 2H),
90 °C with vigorous stirring. The organic layers were extracted 6.48–6.44 (m, 2H), 3.5 (t, 2H), 1.6 (m, 2H), 1.3 (m, 2H), 1.2 (s,
three times with EtOAc. The combined organic fractions were 12H), 0.8 (t, 3H). 13C NMR: 189.6, 145.0, 144.5, 139.2, 131.3,
washed with brine and dried over Na2SO4. The solvent was 129.7, 128.7, 123.8, 122.4, 115.6, 114.1, 112.0, 110.3, 44.2, 31.8,
removed under reduced pressure and the residue purified by 29.5, 29.3, 26.8, 24.9, 22.6, 14.0. ESI-MS (C23H29NO2): calcd
silica gel column chromatography using petroleum ether– 351.48, found: 352 (M + H)+.
ethylacetate (8/2; v/v) as the eluent to give the product as a
yellow powder (0.457 g, 70%). H NMR (300 MHz, CDCl3, δ in
1
10-Decyl-10H-phenothiazine-3-carbaldehyde (5)
ppm): 9.83 (s, 1H), 7.64 (d, 1H), 7.17 (dd, 2H), 6.74 (d, 1H), Compound 5 was synthesised by following the procedure
2.81 (t, 2H), 1.68 (m, 2H), 1.38 (m, 2H), 1.32 (m, 4H), 0.89 (t, described for 4 starting from 3 (0.8 g, 2.359 mmol), DMF
3H). 13C NMR: 182.4, 148.6, 140.9, 137.4, 133.3, 126.0, 125.4, (0.218 mL, 2.831 mmol), and POCl3 (0.264 mL, 2.831 mmol)
123.3, 31.4, 30.2, 28.6, 22.5, 14.0. ESI-MS (C15H18OS2): calcd and the product was obtained as a yellow liquid (0.606 g,
278.08, found: 279 (M + H)+.
72%). 1H NMR (300 MHz, CDCl3, δ in ppm): 9.78 (s, 1H),
7.65–7.62 (dd, 1H), 7.58–7.57 (d, 1H), 7.19–7.09 (m, 2H),
6.98–6.93 (m, 1H), 6.90–6.86 (m, 2H), 3.8 (t, 2H), 1.85–1.75 (m,
Synthesis of 10-decyl-10H-phenoxazine (2)
A 100 mL round bottom flask was charged with phenoxazine 2H), 1.43 (m, 2H), 1.23 (s, 12H), 0.87 (t, 3H). 13C NMR: 187.2,
(2.00 g, 10.928), NaH (0.314 g, 13.114 mmol) and 30 mL of 146.1, 144.5, 129.5, 128.2, 126.7, 125.4, 121.3, 117.2, 115.6,
DMF. The mixture was stirred for 30 min and 1-bromodecane 114.2, 43.4, 30.7, 29.3, 29.1, 28.5, 22.3, 14.1. ESI-MS (m/z): 368
(2.70 mL, 13.114 mmol) was then added. The mixture was (M + H)+.
stirred overnight at room temperature. The reaction mixture
was quenched with ice water (400 mL) and extracted three
Synthesis of ethyl 2-acetyl isonicotinate (7)
times with ethylacetate. The combined organic fractions were To a stirred solution of paraldehyde (47 g, 331.1 mmol) and
washed with brine and dried over Na2SO4. The solvent was ethyl isonicotinate (10 g, 66.2 mmol) in acetonitrile (140 mL)
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at ambient temperature, FeSO4·7H2O (0.312 mg, 1.12 mmol), Diethyl 4′-(5′-hexyl-[2,2′-bithiophen]-5-yl)-[2,2′:6′,2″-
trifluoroacetic acid (7.49 g, 67.5 mmol) and 70% t-BuOOH terpyridine]-4,4″-dicarboxylate (9c)
(11.9 g, 132.4 mmol) were added and the mixture was refluxed
Yield (0.384 g, 70%) 1H NMR (300 MHz, CDCl3, δ in ppm): 9.17
for 4 h. The solvent was removed under reduced pressure and
(s, 2H), 8.8 (d, 2H), 8.6 (s, 1H), 7.9 (d, 2H), 7.6 (d, 2H), 7.2 (s,
the residue was taken up in a saturated sodium carbonate
1H), 7.14 (dd, 2H), 6.7 (s, H), 2.8 (q, 4H), 1.32 (m, 8H), 0.9 (t,
aqueous solution (50 mL). The aqueous layer was extracted
3H); 13C NMR: 165.5, 157.7, 156.3, 151.2, 146.4, 136.5, 136.2,
with toluene (3 × 75 mL). The combined organic fractions were
127.2, 126.1, 125.8, 124.6, 123.4, 123.0, 118.5, 60.7, 32.9, 31.8,
dried (Na2SO4), filtered and the solvent was removed under
28.7, 22.7, 14.1; ESI-MS (m/z): calcd for (C35H35N3O4S2), 625.21;
reduced pressure. The residue was then purified by bulb-to-
found, 626 (M + H)+, 648 (M + Na)+.
bulb distillation, giving 4.77 g (61%) of pure 7 as white crystals
(mp = 56 °C). 1H NMR (300 MHz, CDCl3, δ in ppm): 1.25 (t,
Diethyl 4′-(10-decyl-10H-phenoxazin-3-yl)-[2,2′:6′,2″-
3H), 2.76 (s, 3H), 4.44 (q, 2H), 8.40 (dd, 1H), 8.55 (s, 1H), 8.84
(d, 1H); 13C NMR (CDCl3): δ ppm: 25.9, 62.1, 120.9, 126.1,
138.9, 149.8, 154.5, 164.6, 199.4; ESI-MS (C23H29NOS): calcd
367.20, found: 368 (M + H)+.
terpyridine]-4,4″-dicarboxylate (9d)
Yield (0.337 g, 62%) 1H NMR (300 MHz, CDCl3, δ in ppm): 9.17
(s, 2H), 8.9 (d, 2H), 8.6 (s, 2H), 7.9 (d, 2H), 7.4 (d, 1H), 7.1 (s,
1H), 6.7 (t, 1H), 6.66 (t, 1H), 6.60 (d, 1H), 6.4 (m, 2H), 3.4 (t,
2H), 1.6 (m, 2H), 1.2 (m, 14H), 0.8 (q, 3H); 13C NMR: 165.2,
157.1, 155.0, 149.6, 148.8, 145.2, 144.6, 138.8, 134.4, 132.4,
129.7, 123.6, 122.5, 121.1, 120.7, 117.9, 115.3, 113.5, 111.4,
General procedure for the synthesis of 4′-substituted
4,4″-dicarboxyterpyridine (8a–8e)
A round bottomed flask was charged with ethyl 2-acetyliso- 111.2, 61.7, 44.0, 31.8, 29.59, 29.52, 29.3, 29.2, 26.8, 24.9, 22.6,
nicotinate (1.0 mmol) and aldehyde (0.5 mmol) in 120 mL 14.21, 14.09; ESI-MS (m/z) calcd for (C43H46N4O5), 698.35;
methanol and to it was then added sodium hydroxide found, 699 (M + H)+, 721 (M + Na)+.
(3.0 mmol) and 25% aqueous ammonia (30 mL, 2.2 mol). The
Diethyl 4′-(10-decyl-10H-phenothiazin-3-yl)-[2,2′:6′,2″-
terpyridine]-4,4″-dicarboxylate (9e)
mixture was heated to reflux for 20 h. The resulting suspension
was cooled to r.t. The precipitate obtained was collected by
filtration and subsequently dissolved in hot water. After the
solution cooled to r.t., hydrochloric acid (37%) was added
until pH < 3. The precipitate was collected by filtration and
was subjected to esterification without further purification.
Yield (0.325 g, 60%) 1H NMR (300 MHz, CDCl3, δ in ppm): 9.17
(s, 2H), 8.93 (d, 2H), 8.65 (s, 2H), 7.96 (d, 2H), 7.43 (d, 2H),
7.18 (s, 1H), 6.81 (m, 1H), 6.67–6.59 (m, 3H), 6.52–6.47 (m,
2H), 4.51 (q, 4H), 3.47 (t, 2H), 1.65 (m, 2H), 1.49 (t, 6H).
13C NMR: 165.2, 155.3, 149.7, 138.8, 127.4, 127.3, 126.3, 125.7,
125.4, 122.8, 122.8, 122.6, 120.7, 118.4, 115.4, 61.7, 47.5, 31.8,
29.5, 29.2, 26.8, 22.6, 14.2, 14.0ESI-MS (m/z) calcd for
(C43H46N4O5S), 714.32; found, 715 (M + H)+, 737 (M + Na)+.
General procedure for the synthesis of 4′-substituted
4,4″-dicarboxyterpyridine esters (9)
A suspension of 4′-functionalized, 4,4″-dicarboxy-2,2′:6′,2″-ter-
pyridine (3.12 mmol) in methanol (absolute, 150 mL) and sul-
phuric acid (1 mL) was refluxed for 3 days. After cooling to
room temperature, the precipitated white crystals were filtered
and washed with methanol and ether.
General procedure for the synthesis of [RuCl3(4′-substituted
terpyridine dicarboxylate)] (10a–10e)
Ruthenium trichloride (130 mg, 0.84 mmol) was dissolved in
EtOH (30 mL), and the solution was stirred for 2 min at 50 °C.
To this solution, a solution of ligand (4′-substituted terpyridine
dicorboxylate, 9a–9e) (0.51 mmol) in dichloromethane (20 mL)
was added, and the mixture was refluxed for 3 h. The solution
was concentrated to ca. 10 mL, and then cooled to room tem-
pareture. The precipitate was filtrered, washed with cold EtOH
to remove unreacted ruthenium trichloride, and the product
was air-dried to yield as a dark brown powder.
Dimethyl 4′-(4-(tert-butyl)phenyl)-[2,2′:6′,2″-terpyridine]-
4,4″-dicarboxylate (9a)
Yield (0.345 g, 65%), 1H NMR (300 MHz, CDCl3, δ in ppm):
9.15 (d, 2H), 8.42 (s, 2H), 8 (d, 2H), 7.76 (d, 2H), 7.4 (d, 2H),
3.3 (s, 6H), 1.3 (s, 9H). 13C NMR: 165.8, 157.3, 155.3, 152.4,
150.2, 149.8, 138.4, 135.1, 126.9, 125.9, 122.8, 120.7, 119.3,
52.7, 34.7, 31.2; ESI-MS (C27H23N3O4): calcd 481.20, found: 482
(M + H), 504 (M + Na)+.
General procedure for the synthesis of MC113–MC117
complexes
Dimethyl 4′-(4-(diphenylamino)phenyl)-[2,2′:6′,2″-terpyridine]-
4,4″-dicarboxylate (9b)
A mixture of [RuCl3(4′-substituted terpyridine dicorboxylate)]
(0.39 mmol), aqueous ammonium thiocyanate (113 mmol) in
Yield (0.356 g, 68%) 1H NMR (300 MHz, CDCl3, δ in ppm): 9.17 H2O (5 mL) in DMF (25 mL) was refluxed at 130 °C for 4 h
(s, 2H), 8.86 (d, 2H), 8.7 (s, 2H), 7.9 (d, 2H), 7.7 (d, 2H), 7.3 (m, under an argon atmosphere. Triethylamine (10 mL) and H2O
4H), 7.1 (m, 6H), 7.08 (t, 2H); 13C NMR: 165.8, 157.3, 155.2, (5 mL) were then added, and the solution was refluxed for a
149.8, 148.9, 147.2, 138.4, 131.2, 129.3, 129.0, 128.0, 124.8, further 24 h to hydrolyze the ester groups on the terpyridine
123.7, 123.4, 122.9, 122.8, 120.7, 118.8, 52.7; ESI-MS (m/z) ligand. The reaction mixture was allowed to cool, and the
(C36H28N4O4): calcd 592.21 593 (M + H)+, 615 (M + Na)+.
solvent volume was reduced on a rotary evaporator to about
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5 mL. Water was added to the flux, and the insoluble solid was 2 × 10−3 M chenodeoxycholic acid as the coadsorbent for at
filtered and dried under vacuum. The isolated solid was recrys- least 12 h. After soaking into the dye solution, the dye-
tallized by methanol and diethylether. It was further purified adsorbed TiO2 working electrodes were rinsed with anhydrous
on a Sephadex LH-20 column with 3 : 1 methanol and dichloro- acetonitrile and dried. The DSSCs used for photovoltaic
methane. The main band was collected and concentrated to measurements consists of a dye-adsorbed TiO2 working elec-
give the corresponding ruthenium complex.
trode, a 45 μm thermal adhesive film (Surlyn®, USA), an
MC113: 1H NMR (300 MHz, CDCl3, δ in ppm); (yield; 0.13 g, organic electrolyte and a counter electrode. The Pt catalyst was
47%) 9.1 (d, 1H), 9.0 (d, 1H), 8.7 (s, 1H), 8.6 (s, 1H), 8.3 (s, 2H), deposited on the FTO glass by spraying H2PtCl6 solution and
8.1 (s, 1H), 8.0 (d, 1H), 7.85 (s, 1H), 7.6 (t, 3H), 1.42 (s, 9H). pyrolysis at 410 °C for 20 min to prepare the counter electrode.
ESI-MS (m/z) calcd for 729.0; found, 729 (M+); FT-IR (KBr) The organic electrolyte solution was a mixture of 0.5 M 1,2-
(cm−1); 3418, 2958, 2108, 1715, 1602, 1543, 1483, 1398, 1323, dimethyl-3-propylimidazole iodide (DMPII), 0.05 M I2 and 0.1 M
1292, 1233, 1164, 1112, 1007, 914, 852, 834, 787, 725, 665, 548, LiI in acetonitrile. The active area of the TiO2 film electrodes
455. Anal calcd for C30H23N6O4RuS3, C, 48.32; H, 3.21; was 0.25 cm2. The DSSCs were tested under an ambient atmos-
N, 11.24; S, 12.85. Found: C, 48.03; H, 3.12; N, 11.24; S, 12.79.
phere with a 3A grade solar simulator (Newport, USA, 94043A)
MC114: 1H NMR (300 MHz, CDCl3, δ in ppm): (yield; 0.14 g, under AM 1.5 (100 mW cm−2) illumination to obtain the photo-
45%) 9.04 (s, 1H), 8.2 (m, 3H), 7.7 (s, 2H), 7.7 (s, 2H), 7.56 (s, current density–photovoltage (J–V) curves. The incident mono-
2H), 7.4 (m, 6H), 7.2 (m, 9H). ESI-MS (m/z) calcd for 840.01, chromatic photon to current conversion efficiency (IPCE)
found, 840 (M+); FT-IR (KBr) (cm−1): 3422, 2924, 2767, 2103, spectra were recorded as a function of wavelength from 300 to
1719, 1586, 1541, 1513, 1488, 1463, 1407, 1366, 1327, 900 nm, using a QE/IPCE measurement kit (Newport, USA).
1283,1199, 1173, 1108, 1007, 864, 828, 791, 757, 697, 617, 494. Electrochemical impedance spectroscopy (EIS) measurements
Anal calcd for C38H24N7O4RuS3, C, 54.17; H, 2.74; N, 11.51; were performed on a electrochemical workstation (Autolab 320,
S,11.17. Found C, 53.92; H, 2.94; N, 11.51; S, 10.97.
Metrohm, Switzerland) in the frequency region from 50 mHz to
MC115: 1H NMR (300 MHz, CDCl3, δ in ppm): (yield; 0.12 g, 1000 kHz at perturbation amplitude of 10 mV in the dark.
39%) 9.1–90 (m, 2H), 8.4 (m, 1H), 8.2–8.1 (m, 2H), 7.6–8 (m,
DFT calculations
4H), 7.04 (d, 2H), 6.7 (d, 1H), 2.8 (t, 2H), 1.7 (m, 2H), 1.3 (m,
6H), 0.9 (t, 3H); ESI-MS (m/z) calcd for 844.97 found, 845 (M+);
FT-IR (KBr) (cm−1): 3420, 2924, 2852, 2677, 2102, 1720, 1603,
1537, 1468, 1365, 1301, 1247, 1162, 1109, 1064, 1008, 860, 797,
764, 724, 695, 440. Anal calcd for C34H27N6O4RuS5, C, 48.23; H,
3.12, N, 9.63; S, 18.82. Found C, 48.09; H, 3.34; N, 9.56; S, 18.56.
MC116: 1H NMR (300 MHz, CDCl3, δ in ppm): (yield; 0.14 g,
41%)9.1–9.0 (m, 2H), 8.5–8.4 (m, 2H), 8.2–7.8 (m, 4H), 7.0–6.7
(m, 5H), 3.2 (t, 2H), 1.8 (m, 2H), 1.3 (m, 14H), 0.8 (t, 3H);
ESI-MS (m/z) calcd for, 918.11; found, 918 (M+); FT-IR (KBr)
(cm−1): 3422, 2923, 2851, 2679, 2105, 1721, 1589, 1524, 1494,
1466, 1371, 1273, 1164, 1111, 1009, 861, 794, 744, 561. Anal
calcd for C42H38N7O4RuS3. C, 54.85; H, 4.19; N, 10.54; S, 10.37.
Found C, 54.72; H, 3.98; N, 10.34; S, 10.26.
Density functional theory (DFT) and time-dependent DFT
(TDDFT) calculations were carried out to gain deeper under-
standing of the electronic structure and photophysical behaviour
of the molecules, using the Gaussian09 program package.70 The
closed shell configurations of the five ruthenium(II) sensitizers,
MC113–MC117, with a charge of −1 were fully optimized in the
gas phase with PBE071,72 hybrid functional and LANL2DZ73–76
basis set (double zeta quality), which uses Dunning D95V basis
functions on first row atoms, Los Alamos, ECP plus DZ on Na–
La, Hf–Bi, was adopted on all atoms. Vibrational frequency ana-
lysis has been carried out to ensure that there are no imaginary
frequencies. Thus, the optimized structures correspond to real
minima on the potential energy surface.
MC117: 1H NMR (300 MHz, CDCl3, δ in ppm): (yield; 0.15 g,
43%) 9.1–90 (m, 2H), 8.6–8.5 (m, 2H), 8.2–7.9 (m, 4H), 7.2–6.8
(m, 5H), 3.3 (t, 2H), 1.8 (m, 2H), 1.3 (m, 14H), 0.8 (t, 3H);
ESI-MS (m/z) calcd for 934.09; found, 934 (M+); FT-IR (KBr)
(cm−1): 3422, 2926, 2852, 2738, 2677, 2491, 2105, 1722, 1599,
1539, 1464, 1392, 1367, 1297, 1253, 1169, 1110, 1036, 1010,
864, 799, 749, 552, 458. Anal calcd for C42H38N7O4RuS4,
C, 53.88; H, 4.06; N, 10.37, S, 13.67. Found C, 53.71; H, 3.97;
N, 10.26; S, 13.36.
At the optimized geometry, TDDFT calculations were per-
formed at the M0677/LANL2DZ level of theory in DMF solvent
by means of the Polarizable Continuum Model (PCM),78,79 as
implemented in Gaussian09. 80 singlet–singlet excitations at
S0 optimized geometry were calculated. The software Gauss-
Sum2.2.580 was used to simulate the major portion of the
absorption spectrum and to interpret the nature of transitions.
The molecular orbital surfaces were generated by Gaussview,81
and the percentage contributions of the Ru(II) and the ligands
to the respective molecular orbitals were calculated using
GaussSum.
Fabrication and characterization of DSSCs
The dye-sensitized TiO2 electrode was prepared as follows.
After cleaning, a ∼10 μm double layer of TiO2 particles was de-
posited by screen-printing on the fluorine tin oxide (FTO)
coated glass (12–14 Ω per square, TEC 15, USA). Then the fab-
Acknowledgements
ricated TiO2 thin-film electrodes were sintered at 450 °C for GK thanks CSIR, New Delhi for senior research fellowship.
30 min. After that, the electrodes were immersed in aceto- This work was financially supported by the National Basic
nitrile solution (2 × 10−4 mol L−1 sensitizers) containing Research Program of China under grant no. 2011CBA00700.
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25 W. H. Howie, F. Claeyssens, H. Miura and L. M. Peter,
J. Am. Chem. Soc., 2008, 130, 1367–1375.
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