Synthesis and Characterization of a Series of Bis-homoleptic Cycloruthenates with Terdentate Ligands as a Family of Panchromatic Dyes

Article abstract of DOI:10.1021/acs.inorgchem.7b01412

A series of six homoleptic bis-cyclometalated ruthenium complexes, Ru(N^N^C)₂, is reported where N^N^C is a 6-(2,4-difluoro-3-R³-phenyl)-4-R²-4′-R¹-2,2′-bipyridine with R³ = -H or -CF₃ and R² and R¹ = -COOEt or -CF₃. An effective synthesis of the ligands and the complexes is described. The UV-visible absorption studies demonstrate that these complexes are panchromatic dyes absorbing up to 900 nm. Importantly, the onset of absorption depends only on the substitution on the metalated phenyl, whereas the intensity of absorption throughout the spectra is a function of substituents on both the phenyl and the bipyridine moieties. The same trend is observed in electrochemistry as the redox gap depends only on the substitution on the metalated phenyl, whereas the oxidation and reduction potentials are a function of substituents on both the phenyl and the bipyridine moieties. Preliminary tests as sensitizer for dye-sensitized solar cells demonstrate that the number of anchoring groups on the dye has a major influence on the device efficiency.

Full text of DOI:10.1021/acs.inorgchem.7b01412

Article
pubs.acs.org/IC
Synthesis and Characterization of a Series of Bis-homoleptic
Cycloruthenates with Terdentate Ligands as a Family of
Panchromatic Dyes
†
†,‡
§
†
∥
Thomas W. Rees, JinFeng Liao, Alessandro Sinopoli, Louise Male, Giuseppe Calogero,
Basile F.E. Curchod,⊥ and Etienne Baranoff*,†
†
School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K.
‡
Sun Yat-sen University, Guangzhou 510275, P.R. China
§
Qatar Environment & Energy Institute (QEERI), Hamad bin Khalifa University (HBKU), Doha Qatar
∥
CNR-IPCF, Istituto per i Processi Chimico-Fisici, Messina 98158, Italy
⊥
Centre for Computational Chemistry, School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
*
S Supporting Information
ABSTRACT: A series of six homoleptic bis-cyclometalated
ruthenium complexes, Ru(N^N^C) , is reported where N^N^C is
2
3
2
1
3
a 6-(2,4-difluoro-3-R -phenyl)-4-R -4′-R -2,2′-bipyridine with R =
2
1
−
H or −CF and R and R = −COOEt or −CF . An effective
3
3
synthesis of the ligands and the complexes is described. The UV−
visible absorption studies demonstrate that these complexes are
panchromatic dyes absorbing up to 900 nm. Importantly, the onset
of absorption depends only on the substitution on the metalated
phenyl, whereas the intensity of absorption throughout the spectra is
a function of substituents on both the phenyl and the bipyridine
moieties. The same trend is observed in electrochemistry as the
redox gap depends only on the substitution on the metalated phenyl,
whereas the oxidation and reduction potentials are a function of
substituents on both the phenyl and the bipyridine moieties. Preliminary tests as sensitizer for dye-sensitized solar cells
demonstrate that the number of anchoring groups on the dye has a major influence on the device efficiency.
INTRODUCTION
interest for use in emissive devices such as organic light-
emitting diodes (OLEDs). Cyclometalated complexes of
■
6
Organometallic complexes based on 2,2′-bipyridine (bpy) and
,2′;6′,2′′-terpyridine (tpy) are some of the most well-known in
the literature. In particular, [Ru(bpy) ] and [Ru(tpy) ] have
been thoroughly investigated due to their strong absorption of
light in the UV and visible region, intense emission and
interesting redox properties.
ruthenium are also well-known in the literature. Notable
2
7
−11
2
+
2+
examples include potential anticancer agents
efficient dye molecules for photovoltaics.
and highly
3
2
12−17
Although there are numerous literature examples of cyclo-
1
,2
metalated complexes of transition metals using N^N^C or
Ruthenium(II) polypyridyl
18−33
N^C^N cyclometalated terdentate ligands,
there are no
complexes and their derivatives are particularly suited to a
role as dyes in photovoltaics due to their Ru(II)/(III) reversible
reported bis-homoleptic terdentate examples of ruthenium and
34−36
3
−5
only a few examples of rhodium and iridium.
This is
redox couple and broad absorption profiles.
Derivatives of
Ru(bpy)3]2 and [Ru(tpy)2] polypyridyl complexes of other
+
2+
therefore a relatively unknown area of chemical space to
explore.
Herein, we report a series of Ru(N^N^C)2 complexes
[
metals such as osmium have also been found to be of particular
interest in supramolecular chemistry.
(
Figure 1) where N^N^C represents a cyclometalated 6-
Alteration of a bpy or tpy ligand by replacing a pyridyl with a
phenyl moiety gives a ligand that is isoelectronic and capable of
cyclometalation. This modification results in complexes with
generally lower oxidation potentials, red-shifted absorption, and
more metal-to-ligand charge-transfer (MLCT) character in the
excited state of the complex. One of the most well-known
complexes of this type is iridium tris(2-phenylpyridine),
phenyl-2,2′-bipyridine terdentate ligand. Our design is based on
a 6-(2,4-difluorophenyl)-2,2′-bipyridine core ligand substituted
on positions 4 and 4′ of the bipyridine with −COOEt or −CF
3
and on position 3 of the phenyl with −H or −CF . In the
3
literature, there is a broad interest in the effect of fluorinated
Ir(ppy) . Due to its intense and highly efficient green
Received: June 2, 2017
3
phosphorescence, Ir(ppy) and its derivatives are of great
3
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01412
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Article
Figure 1. Chemical structure of bis-homoleptic bis-cycloruthenated complexes 1a−1f.
a
Scheme 1. Synthesis of Phenylbipyridine Ligands
a
t
Reagents and conditions: (i) mCPBA (1 equiv), 2a: CH Cl rt, 16 h. 2b: CHCl , reflux, 16 h; (ii) POCl , reflux, 2 h; (iii) Pd(OAc) , P Bu -HBF,
2
2
3
3
2
3
K CO , toluene, reflux, 24 h; (iv) 2,4-difluorophenylboronic acid (7a, 7c, and 7e) or 2,4-dilfluoro-3-(trifluoromethyl)phenylboronic acid (7b, 7d,
2
3
and 7f), Pd(PPh ) , Na CO , THF/H O, 70 °C, 16 h.
3
4
2
3
2
ligands on the electronic properties of metal complexes. 7−40
Our particular interest in these substituents is multifaceted.
First, the use of electron-withdrawing groups is expected to
increase the oxidation potential of the complexes and, hence,
offset the decrease of oxidation potential expected from two
cyclometalations. This would lead to more air-stable complexes,
easier to manipulate, hence facilitating the development of a
new synthetic method. Second, we have a long-standing
interest in the effect of substituents on the optoelectronic
properties of photoactive transition metal complexes. Our
recent work on this topic has focused on phosphorescent
cyclometalated iridium complexes, in which case electron-
withdrawing substituents have been widely used to induce a
blue shift of the emission. However, insights are limited because
information is obtained mostly from the emissive triplet state,
whereas the absorption spectra are made of largely overlapping
bands very much restricted to the 350−400 nm range.
3
Expecting Ru(C^N^N) complexes to absorb across the visible
2
spectrum with more defined absorption bands, we anticipate
that these complexes will act as a suitable platform to study the
effect of substituents. Third, the use of ester substituents would
allow for the preparation of the carboxylic acid derivative, a
widely used anchoring group for dyes for dye-sensitized solar
42
cells and to control lipophilicity of biologically relevant
43
photoactive drugs.
4
Since the first report of the black dye N749, ruthenium
complexes with terdentate ligands have attracted attention due
44
to their ability to harvest photons in the near-infrared. The
use of thiocyanate ligands affords an effective tuning of the
energy of the highest occupied molecular orbital (HOMO), yet
the ligand itself has two disadvantages. First, the ligand is
ambidentate; coordination of the sulfur instead of the nitrogen
causes linkage isomerism. Second, the ligand is monodentate
and can more easily disassociate than a polydentate ligand.
41
B
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Overall, thiocyanate ligands are expected to reduce the stability
would mean introducing the variation on the first rather than
last step, greatly reducing the efficiency of the synthesis by
effectively doubling the number of steps required to access the
two ligands.
45
and therefore the longevity of the sensitizer. Utilization of a
terpyridine ligand as in N749 not only allows an increased
range of absorption but also a greater stability compared to
ligands of a lower denticity. However, the use of two
terpyridine ligands has an overall negative effect. The excited
states become too low for injection of electrons to be efficient;
the distorted octahedral geometry causes a decrease in the gap
between the MLCT and metal-centered (MC) states, allowing
In order to solve this two-fold problem and achieve an
efficient synthesis of these ligands, as well as avoid toxic
stannanes, we turned to the cross-coupling method developed
54
by the Fagnou group. In this method, pyridyl oxides are
coupled at the 2-position with aryl halides, and in the product,
the original oxidized nitrogen remains oxidized. Applying this
method to 5 and 6 then allows access to 3c, which can then be
chlorinated in the same way as previously described to yield 4b.
With the three chlorides 4a−c in hand, Suzuki coupling with
the relevant phenylboronic acids gave the ligands 7a−f in yields
from 55 to 93%.
46
decay of photoexcited states, and the increased symmetry
4
7
causes a narrowing of the absorption spectrum. The van
Koten group demonstrated the first use of terdentate
cyclometalating ligands in a ruthenium dye. Although the
efficiencies of the devices were only in the region of 2%, it
nevertheless revealed an effective method to overcome the
11
deficiencies present in a bis-terpyridine complex. The use of
terdentate cyclometalated ligands not only gives the advantage
of stability over thiocyanate ligands but also allows independent
The next challenge was to synthesize the complexes. As this
is a new family of compounds, no specific literature exists;
however, synthetic routes to terdentate cyclometalates in the
literature include a broad range of solvents and conditions
(Table 1). Generally, polar solvents and bases to aid
48
tuning of the HOMO and π*-based LUMO. A further
improvement is that the HOMO of the dye is expanded over
the metal and anionic section of the ligand, while the lowest
unoccupied molecular orbital (LUMO) remains on the
polypyridyl ligands. This gives an array of metal-to-ligand and
ligand-to-ligand transitions resulting in a broad absorption of
Table 1. Example Literature Methods for Synthesizing
Terdentate Cyclometalates
47
temp
(°C)
duration
(h)
incident photons.
metal
solvent
base
ref
Ir
ethylene glycol
glycerol
175
200
118
200
45
90
2
36
55
56
57
47
58
59
RESULTS AND DISCUSSION
Synthesis. The synthesis of the complexes begins with
accessing the ligands 7a−f. All six are phenylbipyridine ligands
with variation introduced at the R , R , and R positions
Scheme 1). The strategy determined to be most efficient for
■
Ir
K CO3
2
Ru
Ru
Ru
Ru
Os
n-butanol
3
ethylene glycol N-methylmorpholine
1
1
2
3
MeOH/H O
N-ethylmorpholine
NaOH
14
72
72
2
(
CH CN
82
3
all six was to proceed via the bipyridyl chlorides 4a−c. The
CH CN
NaOH
82
3
synthesis of 4a and 4b was relatively simple and follows the
1
6
method described by Kim et al. First, the symmetrical
bipyridine (2a or 2b) was oxidized by reaction with meta-
chloroperoxybenzoic acid. For 2a, this reaction occurred at
room temperature in CH Cl . In the case of 2b, which has
cyclometalation are used, and the ligands are added stepwise.
We were initially able to access 1b using a method utilized by
58
the Pfeffer group. First, the reaction of 2 equiv of ligand with
ruthenium(II) para-cymene dichloride dimer, KOH, and KPF6
in MeCN gave the intermediate 8, which was then combined
with another equivalent of ligand and refluxed in EtOH with N-
ethylmorpholine to furnish 1b in 5% overall yield (Scheme 2).
In order to improve upon this method, we tried to devise a
one-pot synthesis. To prevent trans-esterification and saponi-
fication, we would avoid alcohols and strong bases. Initially, we
tried reflux of 4 equiv of ligand in MeCN with N-
ethylmorpholine as base for 144 h, which gave 1b in 23%
yield. At this point, we identified the major side product of this
reaction as the monocyclometalated intermediate 9, which was
formed in a 27% yield. In order to optimize this reaction, it was
repeated with triethylamine to determine if a stronger base
would aid cyclometalation. The yield of both 1b and 9 was,
however, greatly reduced to 2.7 and 1.4%, respectively, due to a
large proportion of de-esterification. A third attempt using
butyronitrile as a higher boiling point analogue of MeCN and
the less basic N-ethylmorpholine gave 1b in only 11% yield but
9 in 72% yield. From this information, we hypothesized that the
formation of 9 is favored at higher temperature as the activation
energy barrier of its formation is more easily overcome;
however, butyronitrile is not as efficient as MeCN at aiding the
removal of the last remaining chloride. In the work by Koga et
al. (the second entry in Table 1), a similar problem converting a
bis-cyclometalated iridium chloride to the desired tris-cyclo-
metalated product was overcome by heating in glycerol at 200
2
2
more electron-withdrawing trifluoromethyl groups, the reaction
would not proceed at room temperature. Instead, reflux in
CHCl was required to access 3b. Then, chlorination by reflux
3
in phosphorousoxychloride followed to furnish 4a and 4c.
Synthesis of 4b proved to be a more challenging procedure.
First, was the issue of forming the carbon−carbon bond
between the two pyridyl moieties; a standard Suzuki coupling
method is not possible in this case as boronic acids at the 2-
4
9
pyridyl position undergo rapid protodeborylation. A classic
coupling in this situation would be achieved via the Stille
method, stannylation of the 2-pyridyl halide, and coupling with
50
the complementary 2-pyridyl halide. Tin compounds are,
51
however, notoriously toxic and if possible should be avoided.
Another solution would be to use a modified Suzuki style cross-
coupling method with a stabilized boronic acid analogue, such
as the boronic esters of methyl iminodiacetic acid developed by
49,52,53
the Burke group.
Having accessed the asymmetric
bipyridine, we would, however, be presented with a second
problem. Oxidation of this asymmetric bipyridine would at best
yield a mixture of bipyridyl-N-oxides and at worst furnish
mostly the undesired product, oxidized on the nitrogen closest
to the ester. Utilizing either Stille coupling or a stabilized
boronic acid, we could instead approach the ligands 7c and 7d
by forming the phenylpyridine moiety first. Oxidation followed
by chlorination would then give a suitable halide for cross-
coupling. As the ligands differ on the phenyl ring only, this
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a
Scheme 2. Development of the Synthesis of 1b
a
Reagents and conditions: (i) KOH, KPF , MeCN, reflux, 72 h; (ii) N-ethylmorpholine, ethanol, reflux, 16 h; (iii) N-ethylmorpholine, MeCN,
6
reflux, 144 h; (iv) NEt , MeCN, reflux, 162 h; (v) N-ethylmorpholine, butyronitrile, reflux, 120 h; (vi) ethylene glycol, reflux, 2 h.
3
°
C with K CO . In an analogous approach, 9 was refluxed in
purified with chromatography in air and without protection
from light, which demonstrates their stability in normal
conditions.
2
3
ethylene glycol for 2 h, and complete conversion to 1b was
observed.
Armed with these results, we were able to devise an
optimized procedure. First, reflux of the ligand in butyronitrile
with ruthenium(II) para-cymene dichloride dimer and N-
ethylmorpholine for 16 h followed by reflux in ethylene glycol
for 2 h gave 1a−f in moderate to excellent yields (Scheme 3),
X-ray Crystal Structures. Single crystals of 1a, 1b, and 1d
were obtained by slow diffusion of hexane into a dichloro-
methane solution of the complexes. Structures are shown in
Figure 2, and selected bond lengths and angles are given in
Table 2. All of the compounds are octahedral, which are
distorted to a greater or lesser extent by the bite angle of the
ligands and the relative bond strengths of the metal with the
phenyl carbon and pyridyl nitrogens. The structure of 1a
contains two crystallographically independent molecules, in
which the ethyl group C(311)−C(312)/C(31′)−C(32′) is
disordered over two positions with a refined occupancy ratio of
3
0−80%. Although inert atmosphere is necessary during the
synthesis to avoid oxidation to Ru(III), the complexes can be
a
Scheme 3. Synthesis of Cycloruthenated Complexes 1a−f
0
.67(2):0.33(2). 1b also contains a disordered ethyl group
C(109)−C(110)/C(09′)−C(10′) over two positions with the
refined percentage occupancy ratio of 56(2):44(2). A molecule
of dichloromethane solvent is present at 20% occupancy and is
disordered over two positions. In the case of 1d, the ethyl
groups are not disordered; however, the fluorine atoms F(10),
F(11), and F(12)/F(10′), F(11′) and F(12′) are each
disordered over two positions with a refined occupancy ratio
of 0.681(17):0.319(17).
Photophysical and Electrochemical Properties. Com-
plexes 1a−f were studied by UV−vis absorption spectroscopy
(
2
Figure 3). All six complexes show very broad absorption from
50 to above 850 nm as well as very intense absorption with
−1
−1
extinction coefficients mostly in excess of 10 000 M cm . 1a
and 1b show particularly strong absorption with extinction
coefficients in excess of 20 000 M− cm . For all the
complexes, we see two main series of peaks: the peaks
attributed to π−π* transitions between 250 and 400 nm and
1
−1
a
Reagents and conditions: (i) [Ru(p-cymene)Cl ] (0.25 equiv), N-
2
ethylmorpholine, butyronitrile, reflux, 16 h; (ii) ethylene glycol, reflux,
2
h.
D
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Figure 2. X-ray crystal structures of 1a, 1b, and 1d with ellipsoids drawn at the 50% probability level. For 1a, only one of the crystallographically
independent molecules is shown. For 1b, the minor component of the disordered ethyl group and the solvent molecule have both been omitted for
clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Complexes 1a, 1b, and 1d
the peaks associated with MLCT transitions between 400 and
800 nm. As we move through the series from 1a to 1f and from
the least to most electron-withdrawing ligand, we can see a
clear trend that, relative to the π−π* transitions (which remain
much the same), the intensity of the MLCT bands decreases.
This is to be expected as the more electron-withdrawing the
ligand is the more electron density lies on the ligands, and the
absorption profile is more ligand centered in character.
Theoretical calculations (DFT and LR-TDDFT using the
PBE0 functional; see Supporting Information for full computa-
tional details) on the complete series provide similar trends
between the different compounds (Figure 4), notably the
a
1
a
1b
1d
bond distance
Ru−N(1)
Ru−N(2)
Ru−N(3)
Ru−N(4)
1.980(4)
2.124(5)
1.987(4)
2.122(4)
2.039(5)
2.026(6)
1.983(5)
2.139(6)
1.982(5)
2.109(5)
2.015(8)
2.020(7)
1.979(4)
2.116(4)
1.987(4)
2.125(4)
2.026(5)
2.031(5)
Ru−C(1)
Ru−C(2)
bond angle
C(1)−Ru−N(2)
C(2)−Ru−N(4)
C(1)−Ru−C(2)
N(2)−Ru−N(4)
N(1)−Ru−N(3)
157.3(2)
157.1(2)
91.7(2)
157.6(2)
157.6(2)
92.9(3)
157.88(18)
157.5(2)
88.40(19)
95.10(14)
178.13(15)
96.56(17)
177.87(18)
96.75(19)
178.66(18)
a
For one of the two crystallographically independent molecules.
Figure 4. Theoretical absorption spectra for compounds 1a−1f (LR-
TDDFT/PBE0 with implicit solvation). Insets show the assignment of
selected transitions in terms of natural transition orbitals (singular
value given in parentheses).
higher absorption of 1a and 1b. At this level of theory, the
Kohn−Sham HOMO orbital is mostly composed of a 4d(Ru)
orbital with small contributions from the phenyl moiety,
whereas the LUMO mostly resides on the pyridine part of the
ligands (see Supporting Information for graphical representa-
tion). The assignment of vertical transitions computed at the
LR-TDDFT/PBE0 level of theory is greatly facilitated by the
use of “natural transition orbitals” (NTOs), which offer a
Figure 3. UV−vis absorption spectra of 1a−f in CH Cl at room
2
2
temperature.
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Table 3. Photophysical and Electrochemical Data
λmaxabs (nm) (ε, M−1 cm−1
)
a
Eox (V vs NHE)
b
Ered (V vs NHE)
b
Eox‑red (V vs NHE)
b
complex
1
1
1
1
1
1
a
b
c
d
e
f
475 (28109), 617 (19252), 729sh (4030)
463 (25373), 600 (21639), 691sh (5010)
454 (18710), 618 (12711), 729sh (3665)
437 (15063), 600 (12219), 691sh (3592)
441 (15099), 617 (11418), 729sh (2571)
0.70
0.90
0.75
0.94
0.79
0.99
−1.22
−1.13
−1.17
−1.06
−1.15
−1.04
1.92
2.03
1.92
2.00
1.94
2.03
408 (10402), 468 (9455), 599 (9340), 691sh (2374)
a
b
Data obtained from a CH Cl solution at room temperature. Redox data were obtained in a 0.1 M tetrabutylammonium hexafluorophosphate
solution in CH Cl . Fc/Fc was used as an internal standard (Fc/Fc = 0.70 vs NHE).
2
2
+
+
2
2
compact description of the donating and receiving orbitals in an
electronic transition. LR-TDDFT/PBE0 indicates that the
bands between 400 and 800 nm indeed exhibit a strong
MLCT character, where the electron mostly departs from a
mostly pure 4d(Ru) orbital with a weak contribution of the
phenyl moiety (Figure 4). Looking at the theoretical spectra for
Furthermore, Eox‑red, 1.92/2.03 eV, is very close to the energy of
the absorption peak at 600/617 nm. Based on the theoretical
calculations, this confirms that the oxidation occurs on the
ruthenium ion and the reduction on the bipyridine part of the
ligand.
Dye-Sensitized Solar Cells. To perform a preliminary
study of solar cells sensitized with these new complexes, we
selected the ester complexes 1b and 1d and prepared the
corresponding acid complexes 10 and 11 by hydrolysis with
potassium hydroxide (Scheme 4). The choice was based on
1a and 1b, we observe a trend between the two main MLCT
bands in the range of 400−600 nm similar to what is observed
experimentallythe two bands for 1a are both red-shifted with
respect to the ones of 1b, and 1b’s low-energy band is stronger
than 1a, whereas the opposite is true for the higher-energy one.
The MLCT region can be further split into three main areas.
First, between 400 and 500 nm, we see a single peak for 1a,
which is blue-shifted and broadened as we go through the series
until in 1f it splits into two distinct peaks. This trend would
indicate that the bipyridyl portion of the ligand is strongly
involved in this transition. Investigation of the NTOs of 1a
confirms this assignment for the receiving orbital in the
brightest transition to S13 (Figure 4), with the donating orbital
also displaying a weak contribution from the central pyridine
moiety. The second section in the MLCT region lies between
a
Scheme 4. Synthesis of 10 and 11
500 and 650 nm. We see that as we go through the group from
the 2,4-difluorophenyl to the more electron-withdrawing 2,4-
difluoro-3-(trifluoromethyl)phenyl cyclometalating group, the
peak is blue-shifted. The pattern repeats through the series as
the ligands alternate between the two cyclometalating moieties,
the peak shifts from 617 to 600 nm and back again,
independently of the substitution on the bipyridine part of
the ligand (Table 3). The final part of the MLCT region is the
shoulder observed for all six complexes between 650 and 800
nm. Theoretical calculations indicate that this band is due to
weakly allowed MLCT transitions, even if we cannot rule out
the presence of spin-forbidden MLCT transitions (calculations
do not include spin−orbit coupling).
The complexes were also analyzed by cyclic voltammetry,
and the resulting data are presented in Table 3. We can see that
the main effect on the oxidation potential is the change in
cyclometalating moiety. As the phenyl group alternates from
less to more electron-withdrawing, the oxidation potential
changes from 0.63−0.72 to 0.83−0.92 V vs NHE. This trend
among the oxidation potential is almost perfectly reproduced
by the trend among the Kohn−Sham HOMO energies (see
Figure S3 in Supporting Information). We also see that as we
go through the group from least to most electron-withdrawing
a
Reagents and conditions: (i) KOH, acetone, 60 °C, 16 h; (ii) HCl, rt.
their oxidation potential, about 0.9 V vs NHE, which appeared
similar to efficient ruthenium dyes using terdentate ligands,
between 0.8 and 1 V vs NHE.
complex with four acid moieties, 10, and one complex with
only two acid moieties, 11.
14,16,44,60,61
This gave us one
Photovoltaic measurements were performed on an AM 1.5
−2
solar simulator (100 mW cm ). The incident light was
calibrated by using a Si photodiode reference. Photocurrent−
voltage (JV) curves were obtained by applying an external bias
to the cells and measuring the generated photocurrent with a
Keithley digital meter. The main photovoltaic parameters for
the constructed DSSC devices, using 10 and 11, are listed in
Table 4 with N719 as a benchmark comparison. The overall
conversion efficiencies η were derived from the equation η =
J V FF/I , where J is the short-circuit current density, V is
sc oc
s
sc
oc
a
Table 4. Photovoltaic Parameters of Dyes 10, 11, and N719
Jsc (mA cm−2
dye
Voc (V)
)
FF (%)
η (%)
ligand, as ester moieties are replaced by −CF , there is an
3
1
0
1
0.2
0.35
11.2
18.4
0.33
0.61
0.58
0.024
4.3
overall increase in oxidation potential.
1
0.59
0.63
The same trend is observed with reduction potentials as the
more electron-withdrawing the substituents on the ligand, the
N719
6.84
a
less negative is E . However, within the uncertainty of the
red
TiO -based DSSCs; the electrolyte contained 0.1 M LiI and 0.05 M
2
I2, in acetonitrile/valeronitrile (1:1, v/v) mixture together with 0.6 M
N-methyl-N-butyl imidazolium (BMII) and 0.5 M tert-butylpyridine.
measurements, the redox gap Eox‑red = E − E depends only
ox
red
on the presence/absence of −CF on the metalated phenyl.
3
F
DOI: 10.1021/acs.inorgchem.7b01412
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the open-circuit voltage, FF is the fill factor, and I is the
intensity of the incident light. Figure 5 shows the JV and
groups could facilitate the interaction between the oxidized
dyes and the iodide, enhancing the regeneration constant.
s
37
Further investigation will include the entire set of carboxylated
complexes and shed light on the charge-transfer kinetics inside
the cells.
CONCLUSIONS
■
A new family of bis-homoleptic cycloruthenated complexes was
synthesized. A synthetic strategy was devised to efficiently
access phenylbipyridines containing an asymmetrical bipyridine
moiety, avoiding the use of toxic stannanes. A rational
methodology was also developed to access bis-homoleptic
ruthenium complexes, which was optimized to give moderate to
excellent yields depending on the ligand. Selected complexes
were characterized by X-ray crystallography; the data from
which show the distorted octahedral structure present in this
type of complex. Analysis of the absorption profiles of 1a−f
showed them to be extremely broad and intense. Cyclic
voltammetry studies revealed the oxidation potentials to be
between 0.63 and 0.90 V vs NHE. These properties of intense
broad absorption and oxidation potentials around 0.75 V are
also of interest as these compounds may have potential as dyes
in photovoltaic devices and anticancer photoactive drugs.
EXPERIMENTAL SECTION
■
Materials, Synthesis, and Characterization. All reactions were
performed under argon. Compounds 2a, 2b, 5, and 6 were synthesized
using literature methods described in the Supporting Information. All
commercial chemicals were used without further purification with the
exclusion of phosphorousoxychloride, which was distilled before use.
1
Column chromatography was performed on 40−60 μm mesh silica. H
NMR, 19F NMR, and C NMR spectra were recorded in CDCl
13
3
unless specified otherwise on a Bruker AVIII400 (400 and 100 MHz)
or a BrukerAVIII300 (300 MHz). Chemical shifts are reported as δ
values (ppm) referenced to the following solvent signals: CHCl , δ
Figure 5. Top: JV curves of DSSC constructed using complexes 10
blue), 11 (red), and N719 (black). Bottom: IPCE traces for complex
1 and N719.
3
H
(
1
7.26, CDCl , δ 77.0, CH CN, δ 1.94, CD CN, δ 118.3. Multiplicity
3 C 3 H 3 C
of signals and coupling constants were obtained by processing on
MestReNova. Mass spectra were recorded on a Micromass ZABspec
spectrometer utilizing electrospray ionization with a MeOH or MeCN
mobile phase. Infrared spectra were recorded neat as thin films on a
PerkinElmer “Spectrum” spectrometer. The intensity of each band is
described as s (strong), m (medium), or w (weak) where appropriate.
UV−vis spectra were recorded on a Cary-5000 spectrometer in
CH Cl solution. Electrochemical data were obtained using a
incident photon-to-current efficiency (IPCE) traces of the
DSSCs based on the new dyes. Note that in this preliminary
work, JV curves cannot be quantitatively compared to the IPCE
traces because of the different measurement conditions.
The IPCE trace for 11 showed a maximum of 25% at 500
nm, whereas the current generated by 10 was too low for
recording an IPCE trace. In the same experimental condition
and setup, N719 resulted in an IPCE value of 57%. The best
result between the two tested dyes is obtained for complex 11
with an efficiency of 4.3%, compared to 0.024% for complex 10
and 6.84% for the benchmark dye N719 under the same
conditions.
2
2
Metrohm Autolab Pgstat101 potentiostat with a glassy carbon disk
as working electrode and Pt wire reference and counter electrodes.
Cyclic voltammetry was performed in 0.1 M solutions of n-BuNPF in
6
+
argon-degassed CH Cl . The ferrocene/ferrocenium couple (Fc/Fc )
2
2
was used as an internal standard. Crystallographic data were recorded
at the EPSRC UK National Crystallography Service at the University
of Southampton by Diamond Light Source on beamline I19.
General Procedure for the Synthesis of Phenylbipyridine
Ligands. The bipyridyl chloride (1.0 equiv), boronic acid (1.2 equiv),
and Na CO (1.2 equiv) were combined with THF and H O (5:1).
Interestingly, although the complex 1d (ester version of 10)
has less favorable absorption properties compared to those of
2
3
2
The solvent was degassed with argon for 20 min before tetrakis-
1b (ester version of 11), because of much lower absorption
(triphenylphosphine)palladium(0) (5 mol %) was added. The mixture
coefficient, about half of 1b, the device performances are
significantly different with dye 11 being the one giving the
highest efficiency. Being a preliminary test, the reasons for this
discrepancy could not be fully understood, and detailed studies
of the devices are outside of the scope of this paper. As a
hypothesis, we attribute the higher efficiency for complex 11 to
a better directionality of its anchoring groups, hence a higher
injection rate, together with a higher dye loading related to its
greater solubility with respect to the tetracarboxylated complex
was then degassed with argon for a further 10 min before stirring
under argon at 70 °C for 16 h. After being cooled to rt, the solvent was
removed under reduced pressure, CH Cl and H O were added, and
2 2 2
the organic phase was extracted. The aqueous phase was extracted with
a further 2 portions of CH Cl . The organic phases were combined
2
2
and dried (MgSO ), and the solvent was removed under reduced
4
pressure. The resulting solid was purified by column chromatography
on silica.
General Procedure for the Synthesis of Cyclometalated
Ruthenium Complexes. A two-neck round-bottom flask fitted with
stirrer, condenser, and stopcock was charged with the ligand (4.0
10. Also, the replacement of two carboxylic groups with −CF3
G
DOI: 10.1021/acs.inorgchem.7b01412
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
equiv) and [Ru(p-cymene)Cl ] (1.0 equiv). The apparatus was filled
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2
2
with argon before the addition of n-BuCN and N-ethylmorpholine.
The resulting orange solution was degassed with argon for 20 min
before heating and stirring at 120 °C, for 16 h under argon and
reduced light. After being cooled to rt, solvent was removed under
reduced pressure and the crude black solid was dried in vacuo.
Ethylene glycol was then added and the resulting mixture degassed for
AUTHOR INFORMATION
■
Corresponding Author
*
E-mail: eb.chimie@gmail.com.
2
0 min with argon. The reaction mixture was then heated and stirred
ORCID
at 200 °C for 2 h under argon and reduced light. After being cooled to
rt, the crude black mixture was combined with H O and CH Cl . The
organic phase was separated and the aqueous phase extracted with
Basile F.E. Curchod: 0000-0002-1705-473X
Etienne Baranoff: 0000-0003-3780-0733
2
2
2
further portions of CH Cl . The organic phases were combined and
Notes
2
2
dried (MgSO ), and solvent was removed under reduced pressure.
The authors declare no competing financial interest.
4
The crude product was purified by column chromatography on silica.
Dye-Sensitized Solar Cell Fabrication. The FTO glass, used for
the preparation of transparent electrodes, was first cleaned with a
detergent and then rinsed with water and ethanol. Clean conductive
ACKNOWLEDGMENTS
Thanks to the School of Chemistry, University of Birmingham,
for a studentship, and the EPSRC for underpinning funding.
■
glass plates were then immersed in 40 mM TiCl aqueous solution at
4
62
7
8
0 °C for 30 min, washed with water and ethanol, and dried in oven at
Thanks to the EPSRC U.K. National Crystallography Service
0 °C for 30 min. The TiO layers were deposited on the FTO glass
at the University of Southampton for the collection of the
crystallographic data for compound 1d and at the Diamond
Light Source for compound 1b. We acknowledge support from
the Centre for Chemical and Materials Analysis in the School of
Chemistry at the University of Birmingham.
2
plates by screen-printing (frame with polyester fibers having 43.80
mesh/cm ). This procedure, involving two steps (coating and drying
2
at 125 °C), was repeated three times. The TiO -coated plates were
2
gradually heated to 500 °C and sintered at this temperature for 30
min. After being cooled, the TiO film was further treated with 40 mM
2
TiCl solution following the previously described procedure. Then a
4
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