Synthesis, characterization and photoelectrochemical performance of a tris-heteroleptic ruthenium(II) complex having 4,7-dimethyl-1,10-phenanthroline

Article abstract of DOI:10.1016/j.ica.2014.02.002

The novel cis-[Ru(Me₂-phen)(dcbH₂)(NCS)₂] complex was prepared using a one-pot route and characterized based on ¹H NMR, FTIR, absorption and emission spectra. The properties of this complex were compared with those of cis-[Ru(R₂-phen)(dcbH ₂)(NCS)₂], where R = Ph or H, to establish a relationship between the donor/acceptor character of the R substituent and the observed behavior of these compounds. Computational methods were also employed to gain a comprehensive understanding of this relationship. By using electron-donating/ accepting substituents, it was possible to tune both the ground and excited states of the cis-[Ru(R₂-phen)(dcbH₂)(NCS)₂] complexes. This compound is capable of sensitizing the TiO₂ surface and efficiently converting the sunlight into electricity. In comparison to cis-[Ru(Ph₂-phen)(dcbH₂)(NCS)₂] it is observed that the presence of phenyl groups provide higher efficiency. The conversion efficiency was improved in comparison to cis-[Ru(phen)(dcbH₂)(NCS) ₂]. This increase in efficiency is attributed to an increase in electron density, thus favoring electron transfer to TiO₂.

Full text of DOI:10.1016/j.ica.2014.02.002

Inorganica Chimica Acta 414 (2014) 145–152
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization and photoelectrochemical performance
of a tris-heteroleptic ruthenium(II) complex having 4,7-dimethyl-1,
10-phenanthroline
⇑
F. Carvalho, E. Liandra-Salvador, F. Bettanin, J.S. Souza, P. Homem-de-Mello, A.S. Polo
Federal University of ABC, Av. dos Estados, 5001, Santo André 09210-580, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The novel cis-[Ru(Me2-phen)(dcbH2)(NCS)2] complex was prepared using a one-pot route and character-
ized based on 1H NMR, FTIR, absorption and emission spectra. The properties of this complex were com-
pared with those of cis-[Ru(R2-phen)(dcbH2)(NCS)2], where R = Ph or H, to establish a relationship
between the donor/acceptor character of the R substituent and the observed behavior of these
compounds. Computational methods were also employed to gain a comprehensive understanding of this
relationship. By using electron-donating/accepting substituents, it was possible to tune both the ground
and excited states of the cis-[Ru(R2-phen)(dcbH2)(NCS)2] complexes. This compound is capable of sensi-
tizing the TiO2 surface and efficiently converting the sunlight into electricity. In comparison to cis-
[Ru(Ph2-phen)(dcbH2)(NCS)2] it is observed that the presence of phenyl groups provide higher efficiency.
The conversion efficiency was improved in comparison to cis-[Ru(phen)(dcbH2)(NCS)2]. This increase in
efficiency is attributed to an increase in electron density, thus favoring electron transfer to TiO2.
Ó 2014 Elsevier B.V. All rights reserved.
Received 9 December 2013
Received in revised form 28 January 2014
Accepted 5 February 2014
Available online 14 February 2014
Keywords:
Molecular engineering
Dye-sensitized solar cells
tris-Heteroleptic ruthenium
DFT
1. Introduction
and its bis-deprotonated analogue, N719, which are considered
standards for these investigations. Density functional theory
tris-Heteroleptic ruthenium compounds, such as cis-
[Ru(L)(L0)(X)2], where L – L0, are very interesting because their
properties, in both the ground and excited states, can be modu-
lated by the choice of the ligands. The preparation of these com-
pounds is not simple, and several routes can be used to obtain
the desired compound. These routes can start from Ru(DMSO)4
(Cl)2 [1] or polymeric [Ru(CO)2(Cl)2]n [2]. In early 2000s, a one-
pot route using a dichloro(p-cymene)ruthenium(II) dimer was
used to prepare a series of tris-heteroleptic ruthenium dye sensitiz-
ers, cis-[Ru(dRbpy)(dcbH2)(NCS)2] (bpy = 2,20-bipyridine deriva-
tives, R = methyl, nonyl, hexyl or tridecyl substituents) [3], and
since this time, several similar compounds, cis-[Ru(L)(dcbH2)
(NCS)2], L = 2,20-bipyridine derivatives, have been prepared and
investigated as sensitizers for use in dye-sensitized solar cells,
DSSCs [4]. Among these complexes, the C101 complex [5], which
has a thiophene derivative as the bipyridine substituents, exhibited
better performance than cis-[Ru(dcbH2)2(NCS)2], also known as N3,
(DFT) has been successfully employed to describe the electronic
properties of these compounds and to establish a relationship with
their behavior as dye-sensitizers in DSSCs [6–14]. Few compounds
have been prepared using other ligands similar to 2,20-bipyridine,
such as 1,10-phenanthroline.
Some cis-[Ru(L)(dcbH2)(NCS)2] compounds where L = 1,10-phe-
nanthroline or its derivatives were prepared and evaluated as
dye-sensitizers for DSSCs. Ligands including phen, dppz [15], 5,6-
dimethyl-phen [16], Ph2-phen [17] have been evaluated previously
and, especially the complex having the Ph2-phen ligand, exhibit a
better performance than N719. However, there has been no inves-
tigation of the relationship between the electron donor or acceptor
character of the ligand substituent and the properties of the
ground and excited states of the complexes.
In this work, cis-[Ru(R2-phen)(dcbH2)(NCS)2] complexes where
R = Me, H or Ph, Fig. 1, were synthesized and characterized. The
relationship between the R group and the donor or acceptor char-
acteristics was evaluated based on their electronic properties
determined by quantum chemistry as well as their FTIR, 1H NMR,
absorption and emission spectra. Their performances as dye-sensi-
tizers for solar cells are also compared and discussed in terms of
the phenanthroline substituents.
⇑
Corresponding author. Tel.: +55 11 4996 0164; fax: +55 11 4996 0090.
E-mail address: andre.polo@ufabc.edu.br (A.S. Polo).
http://dx.doi.org/10.1016/j.ica.2014.02.002
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

146
F. Carvalho et al. / Inorganica Chimica Acta 414 (2014) 145–152
raphy column containing Sephadex LH-20 as the stationary phase.
Methanol was used as the eluent. The pure fraction was concen-
trated, precipitated by the addition of HNO3 and filtered, and the
solid was dried in a desiccator.
Using this procedure, it was possible to prepare
cis-[Ru(phen)(dcbH2)(NCS)2] (yield = 78%; Anal. Calc. for C26H21
N6O7RuS2: C, 44.83; H, 3.49; N, 12.31. Found: C, 44.95; H, 3.05;
N, 12.10%), cis-[Ru(Ph2-phen)(dcbH2)(NCS)2] (yield = 65%; Anal.
Calc. for C50H44N8O11RuS2: C, 57.49; H, 3.05; N, 10.59. Found: C,
57.18; H, 4.11; N, 9.63%) and cis-[Ru(Me2-phen)(dcbH2)(NCS)2]
(yield = 76%; Anal. Calc. for C28H26N6O7RuS2: C, 46.38; H, 3.54; N,
11.53. Found: C, 46.47; H, 3.62; N, 11.6%).
Fig. 1. Structure of the prepared compounds; R = Me, H or Ph.
2.3. Dye-sensitized solar cells
2. Experimental details
Dye-sensitized solar cells were prepared in a sandwich-type
arrangement, as shown in Fig. 2. Nanocrystalline TiO2 mesoporous
films were prepared as described elsewhere [19] and deposited
onto FTO conducting glasses to provide an active area of
0.16 cm2 and sintered for 30 min at 450 °C. The films were sensi-
tized by immersing the processed electrodes into an ethanolic
solution of each prepared compound for a minimum duration of
12 h. The counter-electrodes have a thin film of platinum as a cat-
alyst layer on their conductive sides. The solar cells were sealed
using a film of sealant, and the mediator was placed between pho-
toanode and counter-electrode through holes made on the coun-
ter-electrodes. The mediator was prepared by dissolving iodine
(76 mg), guanidinium thiocyanate (118 mg), 4-tert-butylpyridine
(0.76 mL) and 1-butyl-3-methylimidazolium iodide (1.60 g) in a
mixture of acetonitrile:valeronitrile (85:15) to produce a solution
with a final volume of 10 mL.
4,40-dimethyl-2,20-bipyridine (Aldrich, 99%), H2SO4 (Sigma–
Aldrich, 97%), [Ru(p-cymene)Cl2]2 (Aldrich), 1,10-phenanthroline,
phen (Synth), 4,7-dimethyl-1,10-phenanthroline, Me2-phen (Alfa
Aesar, 98%), 4,7-diphenyl-1,10-phenanthroline, Ph2-phen (Aldrich,
98%), a methanolic solution of tetrabutylammonium hydroxide
(Acros Organics), Sephadex LH20 (Aldrich), N,N0-dimethylformam-
ide-D7, (Aldrich), NaNCS (Merck), Na2Cr2O7 (Synth), HCl (Fluka),
HNO3 (Sigma–Aldrich), methanol (Synth), ethanol (Synth), isopro-
panol (Synth), N,N0-dimethylformamide, DMF (Synth), H2PtCl6 (Ac-
ros), fluorine-doped tin oxide (FTO – 8 ohm squareꢀ1, Aldrich), the
low temperature sealant – Surlyn (30 lm – Dyesol), acetonitrile
(Lichrosolv – Merck), valeronitrile (HPLC – Aldrich), iodine (Sig-
ma–Aldrich), guanidinium thiocyanate (Sigma–Aldrich) 4-tert-
butylpyridine (Aldrich) and 1-butyl-3-methylimidazolium iodide
(Aldrich) were used as received.
2.1. Synthesis of 2,20-bipyridine-4,40-dicarboxylic acid (dcbH2)
2.4. Methods
The compound 2,20-bipyridine-4,40-dicarboxylic acid (dcbH2)
was synthesized as described in the literature [18]. Briefly, 1.60 g
(8.6 mmol) of 4,40-dimethyl-2,20-bipyridine was added to a solu-
tion of 6.28 g (19.4 mmol) of Na2Cr2O7 dissolved in 21 mL of con-
centrated H2SO4, and the mixture was stirred for 30 min. The
resulting solution was added to 220 mL of cold water, and the solid
was removed and dissolved in 10% NaOH. The final product was
precipitated by the addition of HCl to the solution until it reached
pH 2, and the white solid was collected. Yield: 70–80%. Anal. Calc.
for C12H10N2O5: C, 57.60; H, 3.49; N, 11.20. Found: C, 57.84; H,
3.34; N, 11.27%.
NMR spectra were recorded at 25.0 °C on a DRX-500 Bruker
Avance spectrometer at 500.13 MHz using DMF-D7 as the solvent.
2.2. Synthesis of cis-[Ru(R2-phen)(dcbH2)(NCS)2]
The tris-heteroleptic complexes were synthesized using the
procedure reported in the literature with slight modifications
[15,17]. In general, the ruthenium p-cymene dimer, [Ru(p-cymene)
Cl2]2, was added to N,N0-dimethylformamide (DMF) and 2 equiva-
lents of (R2-phen) was added. The mixture was kept at 80 °C for 2 h
under an inert atmosphere. After this period, 2 equivalents of
dcbH2 was added to the mixture, and the temperature was in-
creased to 160 °C. The mixture was kept at this temperature for
4 h. Finally, NaNCS was added to the mixture in a 10-fold excess,
and the reaction was kept under these conditions for 4 h to allow
the reaction to proceed to completion. All the reactions were mon-
itored by UV–Vis spectrophotometry and thin layer chromatogra-
0
phy (silica gel, 60 ÅA, as the stationary phase and methanol
saturated with NaCl as the eluent). To obtain pure compounds,
the complexes were deprotonated using a methanolic solution of
tetrabutylammonium hydroxide (1 mol Lꢀ1), centrifuged to re-
move any residual particulates and applied to a liquid chromatog-
Fig. 2. Schematic representation of a dye-sensitized solar cell.

F. Carvalho et al. / Inorganica Chimica Acta 414 (2014) 145–152
147
The residual DMF peaks were used as an internal standard. FTIR
spectra were recorded at 25.0 °C in KBr pellets on a Bomem MB
100 spectrometer (4 cmꢀ1 resolution). Electronic absorption spec-
tra were recorded on an Agilent 8453 diode-array spectrophotom-
[10–14,24–26] and simulate the vibrational and electronic spectra
of ruthenium complexes [12–14,26,27]. The absence of imaginary
frequencies ensures that the structures obtained correspond to
minima in the potential energy surfaces. TD-DFT simulations were
performed for 80 states.
eter using quartz cuvettes with
a 1.000 cm path length.
Uncorrected emission spectra were recorded on a Cary Eclipse
spectrofluorimeter using quartz cuvettes with a 1.000 cm path
length after the samples had been purged with argon. Photocurrent
action spectra were obtained using a Newport system. This system
consists of a 300 W Xe lamp (model 6258) placed in a lamp hous-
ing that illuminates the entrance slit of a 0.25 m Czerny–Turner
monochromator (Cornerstone 260), which has a diffraction grating
of 1200 lines mmꢀ1 and band-pass filters to avoid second-order ef-
fects. The quasi-monochromatic light beam exits the monochroma-
tor, and a beam-splitter splits the incident light to the sample and a
silicon detector, model 818-UV, which was connected to a virtual
power-meter, model 841-p-USB. The photocurrent was measured
using a Keithley – 2410 source-meter. Current–potential curves
were obtained by measuring the current response for the applied
voltage provided by a Keithley – 2410 source-meter, from 0 V to
the open-circuit photovoltage, under simulated solar irradiation.
The incident ligth was provided by a Newport solar simulator,
model 96000, using a 150 W Xe lamp (6255) placed in a lamp
housing. The housing includes a water filter to remove the near
IR region and a filter to provide the simulated A.M. 1.5G solar spec-
trum. The intensity was measured using a thermopile detector,
model 818P-001-12, connected to a power-meter, model 842-PE.
The results presented for the photoelectrochemical experiments
are the average of at least 5 distinct experiments.
All calculations were performed with the LANL2DZ [28–30] ba-
sis set as implemented in the GAUSSIAN 09 [31] package. Solvation ef-
fects (acetonitrile) were introduced in all calculations with the
Integral Equation Formalism of the Polarizable Continuum Model
(IEFPCM) [32–37] using the GAUSSIAN 03 defaults.
3. Results
3.1. 1H NMR spectra
The proton signals of the tris-heteroleptic compounds in the 1H
NMR spectra were difficult to distinguish because both ligands ex-
hibit signals in the same region, from 10 to 7 ppm. To avoid misin-
terpretation, H–H COSY bidimensional correlation experiments
were performed for cis-[Ru(Me2-phen)(dcbH2)(NCS)2], Fig. 3. The
assignments for cis-[Ru(phen)(dcbH2)(NCS)2] and cis-[Ru(Ph2-
phen)(dcbH2)(NCS)2] were based on this interpretation as well as
their coupling constants and on their previous assignments de-
scribed in the literature [15,17], Table 1.
The protons of the aromatic rings trans to NCS were shifted up-
field in comparison to the protons of the aromatic rings that were
trans to each other, as shown by the data listed in Table 1. This ef-
fect was attributed to the anisotropic effect, resulting in the shield-
ing of these protons.
2.5. Computational details
3.2. FTIR
The computational calculations employed the B3LYP [20–23]
functional to obtain geometries and vibrational spectra and, in
the time-dependent implementation, to obtain absorption spectra.
Several studies have used the same functional to obtain geometries
The FTIR spectra of cis-[Ru(R2-phen)(dcbH2)(NCS)2], Fig. 4, exhi-
bit peaks in the aromatic region, from 1650 to 1410 cmꢀ1, which
were attributed to the
m(CC) and m(CN) of the aromatic ligands
[38]; however, it was not possible to distinguish their characteristic
Fig. 3. H–H COSY bidimensional NMR spectrum of cis-[Ru(Me2-phen)(dcbH2)(NCS)2] in DMF-d7. (500 MHz).

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F. Carvalho et al. / Inorganica Chimica Acta 414 (2014) 145–152
Table 1
Chemical structure and 1H NMR chemical shifts (d) and coupling constants (J) for the
protons of cis-[Ru(Me2-phen)(dcbH2)(NCS)2]. (DMF-d7; 500 MHz).
Proton
type
d
J (Hz)
(ppm)
Ha (d, 1H) 9.77
b (d, 1H) 8.44
5.8
5.8
CH
3
H
6
Hc (s, 1H) 9.24
Hd (s, 1H) 9.06
5
8
O
e
H
H
H
H
H
H
H
H
e (d, 1H) 7.56
6.0
6.0
5.3
5.3
5.5
5.5
9.3
9.3
H C
3
9
f
(d, 1H) 7.99
N
OH
OH
f
2 (d, 1H) 9.58
3 (d, 1H) 8.24
5 (d, 1H) 8.02
6 (d, 1H) 7.51
8 (d, 1H) 8.37
9 (d, 1H) 8.50
N
N
3
d
c
2
Ru
SCN
N
NCS
a
Fig. 5. Absorption spectra of cis-[Ru(R2-phen)(dcbH2)(NCS)2], R = Me (—), H (_ _ _
)
b
or Ph (ꢂ ꢂ ꢂ), in acetonitrile.
O
Fig. 6. Theoretical lines determined by TD-B3LYP/LanL2DZ/IEFPCM and experi-
mental absorption spectra for cis-[Ru(Me2-phen)(dcbH2)(NCS)2] in
acetonitrile.
(
)
Fig. 4. FTIR spectra of cis-[Ru(R2-phen)(dcbH2)(NCS)2], R = Ph (a), H (b) or Me (c), in
KBr. (⁄ indicates the peak ascribed to the NCS peak at 770 cmꢀ1.).
bands were primarily attributed to charge transfer transitions, and
the peaks in the UV region were attributed to the internal elec-
tronic transitions of the ligands. These absorption features are typ-
ical of ruthenium polypyridyl complexes [15,17].
The cis-[Ru(Ph2-phen)(dcbH2)(NCS)2] complex exhibited two
maxima, at 445 and 530 nm. For cis-[Ru(phen)(dcbH2)(NCS)2],
these peaks were centered at 420 and 530 nm, whereas for cis-
[Ru(Me2-phen)(dcbH2)(NCS)2], the peaks were observed at 430
and 535 nm.
The TD-DFT peaks can be grouped into sets that resemble the
experimental absorption bands, Fig. 6, with contributions to an
absorption band in the ultraviolet region and two absorption bands
in the visible range.
signals. The distinct
m(CO) of the carboxylic groups of the dcbH2 li-
gand were observed in all spectra between 1740 and 1715 cmꢀ1
.
FTIR is a valuable tool to evaluate the presence of N or S linkage
isomers of cis-[Ru(R2-phen)(dcbH2)(NCS)2] because the coordina-
tion of the NCSꢀ ion through the N or S atom results in stretches
with distinct maxima. Special attention was given to CN stretching,
m
(NCS), which can be observed in the 2100 and 770 cmꢀ1 regions.
Only one peak was observed for each complex in both regions due
to the existence of only cis-[Ru(R2-phen)(dcbH2)(NCS)2] complexes.
The existence of both isomers would be indicated by peaks at 2100
and 770 cmꢀ1 as well as 2050 and 700 cmꢀ1, ascribed to coordina-
tion through nitrogen and sulfur, respectively [39,40]. The FTIR spec-
tra of the prepared complexes exhibited only one peak, indicating
that the isothiocyanate isomer was the main product [41].
The isothiocyanate peaks were observed at 2101 and 770 cmꢀ1
for cis-[Ru(Ph2-phen)(dcbH2)(NCS)2]2ꢀ, at 2109 and 780 cmꢀ1 for
cis-[Ru(phen)(dcbH2)(NCS)2] and at 2117 and 771 cmꢀ1 for
cis-[Ru(Me2-phen)(dcbH2)(NCS)2]. The simulation of the IR spectra
gave frequencies for the symmetric stretching of the NCS ligand of
2130.1, 2130.5 and 2131.0 cmꢀ1 for R = Ph, H and Me, respectively,
in cis-[Ru(R2-phen)(dcbH2)(NCS)2].
3.4. Emission spectra
The emission spectra of the complexes in acetonitrile exhibited
broad and non-structured profiles, which are typical of 3MLCT
transitions, Fig. 7. These characteristics have also been observed
for similar compounds [15,17].
The emission maxima for cis-[Ru(Ph2-phen)(dcbH2)(NCS)2], cis-
[Ru(phen)(dcbH2)(NCS)2] and cis-[Ru(Me2-phen)(dcbH2)(NCS)2]
were at 755, 790 and 805, respectively.
3.3. Absorption spectra
3.5. Computational analyses
The cis-[Ru(R2-phen)(dcbH2)(NCS)2] compounds exhibited two
absorption bands with high molar absorptivity (ꢁ104 L molꢀ1
cmꢀ1) in the visible region (350–700 nm), Fig. 5. These absorption
DFT calculations were performed for geometry optimization,
followed by the simulation of the FTIR spectra, electrostatic surface
potentials and electronic absorption spectra.

F. Carvalho et al. / Inorganica Chimica Acta 414 (2014) 145–152
149
chemical parameters determined from these experiments are
listed in Table 2.
The photoelectrochemical experiments demonstrate that the
synthesized compounds have energy levels that are adequate for
promoting electron injection into the semiconductor conducting
band of TiO2, converting the sunlight into electrical output. It is
also observed that the presence of phenyl or methyl substituents
on the phenanthroline ring enhances the performance of the solar
cells. This performance increase is evident in both the overall effi-
ciency and IPCEMax when compared to the unsubstituted phenan-
throline ring.
4. Discussion
The influence of the R groups at the 4 and 7 positions of the phe-
nanthroline ligand on the properties of cis-[Ru(R2-phen)(dcbH2)(-
NCS)2] are notable and are related to their electron donor
(methyl) or acceptor (phenyl) character. The values of the R group’s
Hammett constants and their main spectroscopic signals are listed
in Table 3.
Fig. 7. Emission spectra of cis-[Ru(R2-phen)(dcbH2)(NCS)2], R = Me (—); H (_ _ _) or
Ph (ꢂ ꢂ ꢂ), in acetonitrile. (kex = 500 nm; = 30 nm minꢀ1).
m
The influence of the R groups on the 1H NMR spectrum can be
primarily observed on d2. The observed upfield shift in the order
of Ph, H and Me is consistent with the increase in the electron
donation character of the substituents. There was a correlation be-
tween the presence of the electron-withdrawing group (phenyl)
and a decrease in the electron density of the aromatic rings, thus
deshielding H2 and shifting the signal downfield relative to that
for phen. In contrast, the electron donation caused by methyl
groups increased the electron density of the aromatic ring, shield-
ing H2 and resulted in the signal being shifted upfield. The electron
density changes caused by the R groups did not have significant
influences on long-range protons signals, such as that for Ha, as
there was no correlation observed for their electron donor/accep-
tor character. The electron density was also evaluated using DFT
calculations. The calculated atomic charges derived from the
Octahedral-like structures were found as the minima for the po-
tential energy surfaces after the geometry optimization calculation
for each compound, Fig. 8.
The electrostatic potential surfaces for the three compounds
were calculated, and the results are presented in Fig. 9.
The charge distributions for all atoms are presented in the
Supporting Info.
3.6. Photoelectrochemical experiments
The performances of these compounds as dye-sensitizers for so-
lar cells were evaluated using current–voltage (J–V) curves, Fig. 10,
and using photocurrent action spectra, Fig. 11. The photoelectro-
Fig. 8. Optimized quantum mechanics structures for cis-[Ru(R2-phen)(dcbH2)(NCS)2]; R = Ph (a),
H (b) or Me (c). (cyan = carbon, blue = nitrogen, red = oxygen,
white = hydrogen, yellow = sulfur, brown = ruthenium). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.)
Fig. 9. Electrostatic surface potentials determined for cis-[Ru(R2-phen)(dcbH2)(NCS)2]; R = Ph (a), H (b) or Me (c).[42,43].

150
F. Carvalho et al. / Inorganica Chimica Acta 414 (2014) 145–152
Table 3
Hammett constants for the R substituents and the main spectroscopic signals of these
substituents determined for cis-[Ru(R2-phen)(dcbH2)(NCS)2].
Ph
H
Me
rp [44]
0.05
9.80
9.85
2101; 770
445; 535
755
0
9.71
9.74
2109; 780
420; 530
790
ꢀ0.14
9.77
9.58
2117; 770
430; 535
805
da (ppm)
d2 (ppm)
m(NCS) (cmꢀ1
kmax-abs (nm)
kmax-em (nm)
)
Table 4
ChelpG charges determined for the nitrogen atoms of the investigated compounds
(L = R2-phen: R = Ph, H or Me).
Ph
0.261
ꢀ0.116
ꢀ0.258
0.255
H
Me
0.187
ꢀ0.113
ꢀ0.370
0.112
Fig. 10. Current–potential curves determined for DSSCs in which the photoanode
was sensitized by cis-[Ru(R2-phen)(dcbH2)(NCS)2], R = H (__); Me ( ) or Ph ( ).
N1
N2
N3
N4
N5
N6
0.215
N3
dcbH
L
N2
Ru
2
ꢀ0.117
ꢀ0.273
0.207
N1
N4
ꢀ0.222
ꢀ0.261
ꢀ0.250
ꢀ0.308
ꢀ0.360
ꢀ0.377
SCN
6
NCS
5
on FTIR shift. In contrast, the presence of methyl groups in cis-
[Ru(Me2-phen)(dcbH2)(NCS)2] resulted in a shift of 8 cmꢀ1 to a
higher energy region in comparison with the signal for cis-
[Ru(phen)(dcbH2)(NCS)2] due to their electron donor character
which results in the opposite effect. The calculated data for the
NCS symmetric stretch are in agreement with the trend observed
in the experimental data and can be explained by the change in
the electron density due to distinct effect of the substituents.
A low influence of the R groups was observed for the lower en-
ergy absorption maxima of the compounds because the electronic
transitions responsible for these absorption bands are primarily
Fig. 11. Photocurrent action spectra of solar cells in which the photoanode was
sensitized by cis-[Ru(R2-phen)(dcbH2)(NCS)2], R = H (_d_); Me (
) or Ph (
).
CTNCS-dcbH [45], which are not directly connected to the R substitu-
2
ent. Thus, the energies of these electronic transitions are not sensi-
tive to characteristics of the R group. Higher molar absorptivity
values in the visible region were observed for cis-[Ru(Ph2-
phen)(dcbH2)(NCS)2] due to the presence of phenyl groups, which
increased the light harvesting by this compound [17].
electrostatic potential (ChelpG) for the three compounds, Table 4,
confirmed the 1H NMR observations: the electron density of the
aromatic rings of phen increased in the order Ph < H < Me. The
atomic charges on the nitrogen atoms of phen, N3 and N4, show
that these atoms had higher electron densities as the donor charac-
ter of R increased. The effect also extended to the N atoms of the
opposite ligand: N1 and N6. The atomic charges on N2 and N5 were
less affected.
Theoretical calculations show that both bands in the visible re-
gion have MLCT character, Fig. 12. In some transitions, the most
likely charge transfer is to dcbH2, and in others, it is to R2-phen.
Transfers to R2-phen have smaller oscillator strengths and are
more important for transitions below 480 nm, corresponding to
the absorption band in the higher energy region of the experimen-
tal spectra. The second band in the visible region is primarily due
to transfers to dcbH2. Because the molecular orbitals of the
R2-phen ligand are not directly involved, the calculations suggest
an association between the inductive effect of the R group and
the wavelength/intensity of the band. However, this difference
was not observed in the experimental spectra because these
absorption bands are broad, and the difference may have been
too small to be detected.
The distinct characteristics of the R groups also resulted in
changes in the
sity of the metal center. It is interesting to note that the changes in
(CN) are in accordance with the distinct electron donor/acceptor
m(CN) stretching due to changes in the electron den-
m
character of the R groups. The presence of phenyl groups in
cis-[Ru(Ph2-phen)(dcbH2)(NCS)2] resulted in a shift of 8 cmꢀ1 to a
lower energy in comparison with the signal for the cis-
[Ru(phen)(dcbH2)(NCS)2] complex. This electron withdrawing sub-
stituent on phenanthroline decreases the electronic density on
ruthenium center and as a consequence, the electron donation of
NCSꢀ ligand through the N-Ru
interaction to t2g⁄, and weak the CN bond as can be observed
r bond is enhanced as well as its
Using different substituents in polypyridinic ligands leads to
changes on the emission spectra of homoleptic ruthenium(II) poly-
p
Table 2
Photoelectrochemical parameters determined for solar cells sensitized by cis-[Ru(R2-phen)(dcbH2)(NCS)2]; R = Ph, H or Me.
R
VOC (Volts)
JSC (mA cmꢀ2
)
Pmax (mW cmꢀ2
ff
g
(%)
IPCEmax (%) (k (nm))
Ph
H
Me
0.738
0.733
0.683
13.28
8.67
12.72
6.20
3.88
4.73
0.63
0.61
0.55
6.18
3.78
4.60
67 (485)
56 (485)
60 (515)

F. Carvalho et al. / Inorganica Chimica Acta 414 (2014) 145–152
151
pyridyl complexes since these complexes have the 3MLCT as the
lowest lying excited state and the use of electron donor or acceptor
substituent changes both the HOMO and LUMO of these com-
pounds, leading to an increase or decrease on emission energy.
An interesting behavior was observed for the emission properties
of the cis-[Ru(R2-phen)(dcbH2)(NCS)2] complexes investigated.
The emission energy increased as the electron-withdrawing char-
acter increased. This apparent contradictory effect can be ex-
to the previous discussion. It is expected that the performance of
solar cells should be lower due to the presence of an electron-with-
drawing substituent on the phenanthroline ring. However, valu-
able information is obtained from the photocurrent action
spectra. In these spectra, an increase in performance is observed
for the cis-[Ru(Ph2-phen)(dcbH2)(NCS)2] compound in the 380–
600 nm region in comparison to other compounds.
The IPCE can be expressed as a function of the electron-injection
quantum efficiency, UEI; the efficiency of collecting electrons in the
plained by the presence of the 3MLCTdRu-
thexi (thermally
pꢃdcbH2
equilibrated excited) state for all the complexes presented. In this
case, the thexi state energy is almost the same for all compounds
investigated, while the electron-withdrawing effect of the phenyl
groups reduces the electron density at the metal center, and conse-
quently, reduces the energy of its HOMO, resulting in the observed
increase in energy. In contrast, the donor effect of the methyl
groups changes the energy of its HOMO to higher energy and re-
duces the energy observed for emission from this state.
The photoelectrochemical performance of the solar cells is im-
proved by the presence of the methyl electron donor group. These
results are consistent with those from the other experiments that
were performed. This substituent increases the electron density
on dcbH2, which led to a shift of the photocurrent action spectrum
to a lower energy region when compared to cis-[Ru(phen)(dcbH2)
(NCS)2]. Since the ratio between the integrated photocurrent action
spectra of cis-[Ru(Me2-phen)(dcbH2)(NCS)2] and cis-[Ru(phen)
(dcbH2)(NCS)2] is 1.14 and the ratio of their JSC is 1.47, are quite
similar under the experimental conditions, the improvement in
the overall performance of the DSSCs can be ascribed to the shift
on the photocurrent action spectrum of cis-[Ru(Me2-phen)
(dcbH2)(NCS)2].
external circuit, gEC; and the light harvesting efficiency, LHE, at a
determined wavelength. These terms have the mathematical rela-
tionship described by Eq. (1) [4].
IPCEðkÞUEI
ꢂ
gEC ꢂ LHE
ð1Þ
The UEI can be considered constant for all dyes since the same
anchoring group was used for the compounds investigated. The
unexpected performance of cis-[Ru(Ph2-phen)(dcbH2)(NCS)2] can
be attributed to its higher molar absorptivity values when com-
pared to the other compounds, Fig. 5, and to a lower recombination
of the injected electron. It was already observed the influence of
electron donor or acceptor groups on charge recombination
dynamics in DSSCs, which can enhance the performance of the de-
vice [46]. Thus, the presence of phenyl groups on phenanthroline
ligand can enhance both LHE and gEC terms and further investiga-
tion is being carried out.
5. Conclusion
The new tris-heteroleptic cis-[Ru(Me2-phen)(dcbH2)(NCS)2]
complex was synthesized, characterized and compared with other
phenanthroline complexes with substituents with distinct charac-
ters. The results indicate that the R substituents of the phenanthro-
line ligands affect some of the characteristics of the ground and
excited states of cis-[Ru(R2-phen)(dcbH2)(NCS)2]. The 1H NMR,
FTIR and emission experiments were found to depend on the
acceptor or donor character of the R groups. The calculated absorp-
tion spectrum resembled the experimental spectrum, allowing the
comprehensive interpretation of the absorption spectra of these
complexes. The calculated atomic charges demonstrated that the
inductive effects involve one nitrogen atom of the dcbH2 ligand
in the sequence Ph, H and Me, thus explaining the sequence
observed for the experimental 1H NMR and FTIR data and demon-
strating a viable way to tune the properties of cis-[Ru(R2-phen)
(dcbH2)(NCS)2]. The photoelectrochemical performance of the
compounds as dye sensitizers were evaluated, and it was observed
that the improvement in the performance of the solar cells was due
to electronic effects of methyl groups. The use of phenyl groups re-
vealed that the light-harvesting efficiency or the reduction on elec-
tron recombination of this dye has more influence on the
performance of DSSCs than its electron-withdrawing effects.
The use of phenyl substituents also improves the performance
of the prepared solar cells. This behavior is apparently in contrast
Acknowledgments
The authors would like to acknowledge FAPESP (2011/11717-8
and 2007/05370-0), UFABC and CAPES for financial support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2014.02.002.
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Fig. 12. Schematic representation of selected electronic transitions and the
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