Functionalized ruthenium dialkynyl complexes with high second-order nonlinear optical properties and good potential as dye sensitizers for solar cells

Article abstract of DOI:10.1021/om5009378

Various dipolar π-delocalized Ru(II) dialkynyl complexes were prepared and characterized. Their second-order nonlinear optical (NLO) properties were investigated by the electric-field-induced second harmonic generation (EFISH) technique working in CH

Full text of DOI:10.1021/om5009378

Article
pubs.acs.org/Organometallics
Functionalized Ruthenium Dialkynyl Complexes with High Second-
Order Nonlinear Optical Properties and Good Potential as Dye
Sensitizers for Solar Cells
Filippo Nisic,† Alessia Colombo,*,† Claudia Dragonetti,†,‡ Eleonora Garoni,† Daniele Marinotto,‡
Stefania Righetto,† Filippo De Angelis,*,§ Maria Grazia Lobello,§ Paolo Salvatori,§ Paolo Biagini,*,∥
and Fabio Melchiorre∥
̀
†Dipartimento di Chimica dell’Universita degli Studi di Milano, UdR-INSTM, and ‡ISTM-CNR, via Golgi 19, I-20133, Milano, Italy
§Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari, Via Elce
di Sotto 8, I-06213, Perugia, Italy
∥Centro Ricerche per le Energie non Convenzionali, Istituto eni Donegani, via G. Fauser 4, 28100, Novara, Italy
S
* Supporting Information
ABSTRACT: Various dipolar π-delocalized Ru(II) dialkynyl
complexes were prepared and characterized. Their second-
order nonlinear optical (NLO) properties were investigated by
the electric-field-induced second harmonic generation
(EFISH) technique working in CH2Cl2 solution with an
incident wavelength of 1907 nm, whereas the dipole moments
were determined by density functional theory (DFT)
calculations. All the investigated complexes are characterized
by a negative value of μβ1.907 EFISH, in agreement with a
negative value of Δμeg (difference of the dipole moment in the
excited and ground state) upon excitation. Their second-order
NLO response can be easily modulated by the nature of the alkynyl substituents. Besides, the most promising “push−pull”
ruthenium diacetylide complex, adequately functionalized for anchoring to TiO2, was tested as photosensitizer in dye-sensitized
solar cells (DSSCs).
redox properties which afford the possibility of NLO
switching.15 Ruthenium σ-acetylides have been largely studied
through hyper-Rayleigh (HRS) scattering measurements, but,
to our knowledge, only in two cases through electric field
second harmonic generation (EFISH) measurements.16,17
Thus, a few years ago, an EFISH study showed that dipolar
alkynyl ruthenium complexes with a phenyleneethynylene or
phenylenevinylene bridge between a donor “ClRu” moiety and
a nitro acceptor group, such as trans-[Ru(4,4′-CCC6H4C
CC6H4NO2)Cl(dppe)2] (dppe = Ph2PCH2CH2PPh2) and
trans-[Ru{(E)-4,4′,4″-CCC6 H4 CCC6 H4 CH
CHC6H4NO2}Cl(dppm)2] (dppm = Ph2PCH2PPh2), are
characterized by good second-order NLO properties.16 Also,
very recently, some of us reported that a dinuclear Ru(II)
complex, where two “phenylalkynyl-Ru” moieties are separated
by a bridge consisting of 2,1,3-benzothiadiazole (BTD) flanked
on either side by 2,5-thienyl units (Th), is characterized by an
unexpected high EFISH NLO response where the benzothia-
diazole moiety acts as an acceptor.17 These observations
prompted us to prepare novel dipolar mononuclear ruthenium
INTRODUCTION
■
Compounds with second-order nonlinear optical (NLO)
properties are of great interest as molecular building block
materials for optical communications, optical data processing
and storage, or electrooptical devices.1 Among them, organo-
metallic complexes represent a fascinating and growing class of
second-order NLO chromophores that can offer additional
flexibility, when compared to organic chromophores, due to the
presence of metal−ligand charge-transfer transitions usually at
relatively low energy and of high intensity, tunable by virtue of
the nature, oxidation state, and coordination sphere of the
metal center.1−6
In particular, metal σ-acetylides constitute a widely
investigated class of second-order NLO chromophores, mainly
developed by Humphrey et al.,7−9 where the metal acts as the
donor group of a Donor-Acceptor system connected by a π-
linker. The almost linear M-CC-R structure allows a good
coupling between the d metal orbitals and the π* system of the
σ-acetylide bridge, affording a significant NLO response
controlled by low-energy MLCT excitations. Ruthenium σ-
alkynyl complexes are very attractive, due to their facile high-
yielding syntheses,10,11 enhanced NLO coefficients,12,13 easy
construction of multimetallic dendrimers,14 and reversible
Received: September 11, 2014
© XXXX American Chemical Society
A
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Chart 1. Structure of the Investigated Ru(II) Complexes
a
Scheme 1
a
(i) Trimethylsilylacetilene, Pd(PPh3)2Cl2, CuI, PPh3, TEA, 70 °C, 16 h, 80%; (ii) K2CO3, MeOH/THF, 24 h, 90%; (iii) trans-[ClRu(dppe)2C
CPh], TEA, NaPF6, CH2Cl2, rt, 20 h, 85%; (iv) CNCH2COOMe, piperidine, MeOH, 50°C, 3 h, 98%; (v) K2CO3, MeOH/THF,40 °C, 3 h, 90%;
(vi) trans-[ClRu(dppe)2CCPh], TEA, NaPF6, CH2Cl2, rt, 20 h, 70%.
dialkynyl complexes (Chart 1) and to study their second-order
NLO properties, using the EFISH technique, in order to get a
better understanding of the 2,1,3-benzothiadiazole as acceptor
moiety and to investigate new “Donor-phenylalkynyl-Ru-
alkynylphenyl-Acceptor” architectures. The dipole moments
and the optical properties were evaluated by density functional
theory (DFT) calculations.
But, thanks to the linear geometry of the alkynyl unit and its
π-unsaturated character, metal alkynyls are attractive building
blocks not only for the preparation of materials with optical
nonlinearity but also for molecular wires and polymeric
B
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a
Scheme 2
a
(i) tBuO−K+, THF, rt, 16 h, 80%; (ii) trimethylsilylacetilene, Pd(PPh3)2Cl2, CuI, PPh3, TEA, 70 °C, 16 h, 60%; (iii) K2CO3/MeOH, 24 h (95%).
a
Scheme 3
a
(i) trans-[Ru(dppe)2Cl2], NaPF6, CH2Cl2, 40 °C 24 h, TEA, rt, 24 h 85%; (ii) p-ethynyl-benzaldehyde, NaPF6, TEA, CH2Cl2, rt, 24 h 90%; (iii)
CNCH2COOCH3, piperidine, CH2Cl2, rt, 18 h, 90%.
organometallic materials, which can possess interesting proper-
ties such as luminescence, liquid crystallinity, and electrical
conductivity.18 Metal−alkynyl complexes have been readily
employed as simple electronic components with the ability to
transport electronic charges between two points, allowing the
study of intramolecular charge-transport mechanisms over long
distances.19−21 The ability of ruthenium to operate as a
connector allowing electron flow to occur between different
elements in trans-ditopic architectures was demonstrated,22−25
and its exceptional propensity to serve as efficient electron
relays in nanoscale oligomeric structures was reported.26−29
Metal alkynyls also showed their potential as donor
components of bulk heterojunction solar cells.30−34
We asked ourselves whether the trans-ruthenium diacetylide
motif could act as an efficient relay within a “push−pull”
architecture such as that of the new complex D and, therefore,
could find application in the field of DSSCs. For this reason, we
prepared the related complex E bearing a carboxylic acid
anchoring group instead of the methyl ester function and
studied in a preliminary way its potential as dye sensitizer for
solar cells. For comparison, F, the carboxylic acid derivative of
B, was also investigated.
A and B were synthesized according to the simple synthetic
strategy reported in Scheme 1: the starting materials trans-
Ru(dppe)2Cl2 and 4-(5-bromothiophen-2-yl)-7-(thiophen-2-
yl)benzo[c][1,2,5]thiadiazole derivatives 1 and 2 were synthe-
sized following reported methods.35 A and B were obtained
using an adaptation of the method of Humphrey et al.,36 by
reaction of the known [ClRu(dppe)2CCPh]37 with the
deprotected alkyne-derivative of 3 and with 5. F, similar to B
but with a carboxylic acid function instead of the methyl ester,
was prepared starting from the potassium salt derivative of 5;
final acidification of the potassium salt of B with trifluoroacetic
acid gave F.
In order to obtain C and D, we first prepare the acetylene
derivative 10 as reported in Scheme 2; it was then reacted with
trans-[Ru(dppe)2Cl2] as described in Scheme 3.
The 4-ethynyl-benzaldehyde was prepared according to the
literature,38 whereas (E)-4-(4-ethynylstyryl)-N,N-diphenyl-
aniline 10 was synthesized as reported in Scheme 2. The
trans isomer 8 was obtained by Horner−Hammonds reaction
between 4-diphenyl-amminobenzaldehyde 6 and the phospo-
nate 7.
The synthesis of the desired complexes C and D was
accomplished by using terminal alkyne activation by ruthenium
complexes,39 thereby providing selective formation of vinyl-
idene complexes that are the key intermediates for the
formation of monoalkynyl complex 11 (by deprotonation
with TEA) and unsymmetric bis-alkynyl complex 12 (by
RESULTS AND DISCUSSION
■
Synthesis. Various novel ruthenium acetylide complexes
(Chart 1) were prepared following the synthetic pathways
shown in Schemes 1−3.
C
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Figure 1. Optimized geometries of complexes A−D.
Table 1. Electronic Spectra, μβ1.907 EFISH, μ, and β1.907 EFISH of the Investigated Ru(II) Complexes
a
b
c
dg
β1.907 EFISH (×10−30) esu
,
complex
absorbance λmax/nm [ε/M−1 cm−1
]
μβ1.907 EFISH (×10−48) esu
μ (×10−18) esu
A
365 [36200], 489 [31400]
397 [28400], 497 [64400]
289 [5600], 427 [9800]
399 [21500], 437 [16300]
−1550
−1700
−345
6.9
18.1
5.9
−225
−94
−58
−99
e
B
C
f
D
−780
7.9
a
b
c
In CH2Cl2. Working in CH2Cl2 10−3 M with an incident radiation wavelength of 1.907 μm; the error is 10%. Calculated at the B3LYP level in
CH2Cl2; in vacuo, μ = 5.7, 13.54, 4.4, and 6.0 × 10−18 esu for complexes A, B, C, and D, respectively. At the CAM-B3LYP level, μ values are slightly
d
lower; see the Supporting Information. By using μ calculated in CH2Cl2; by using μ calculated in vacuo, β1.907 EFISH = −272, −126, −78, and −130
e
× 10−30 esu for complexes A, B, C, and D, respectively. At the CAM-B3LYP level; see the Supporting Information. For complex F: λ (nm) [ε (M−1
f
g
cm−1)] = 395 [22500], 493 [59600]. For complex E: λ (nm) [ε (M−1 cm−1)] = 267 [17900], 428 [10700]. β0 values are reported in Table S12 in
the Supporting Information.
introduction of a second alkynyl moiety at the ruthenium
center of the monoalkynyl complex).
between the phenyl and the thiophene planes. Similarly to B,
complexes C−D show a dihedral angle of about 60° between
the donor and the acceptor groups.
The absorption maxima of the four complexes are reported
in Table 1. TD-DFT calculations, by using two different
exchange-correlation functionals, have been performed to gain
information on the spectral features of the considered
[RuCl(dppe)2][PF6] was generated from trans-[Ru-
(dppe)2Cl2] by chloride abstraction using sodium hexafluo-
phosphate and subsequently treated with the stoichiometric
amount of (E)-4-(4-ethynylstyryl)-N,N-diphenylaniline 10 and
TEA in order to form the monoalkynyl complex trans-[Ru(C
C-4-(E)-ethynylstyryl-N,N-diphenylaniline)Cl(dppe)2] 11.
Using an analogous approach, the substitution of the
remaining chloride ligand of 11 with 4-ethynylbenzaldehyde
afforded C; subsequent condensation with methyl cyanoacetate
in the presence of piperidine gave the target complex D in good
yields. E was obtained in a similar manner by Knoevenagel
condensation between C and cyanoacetic acid.
compounds. The B3LYP functional underestimates the λmax
,
in a range between 0.30 and 1.25 eV, for all the considered
compounds. On the other hand, by considering a rigid shift of
the excitation energies, a well-balanced description of the whole
spectra, both in terms of energies and intensities, is obtained.
The CAM-B3LYP functional better performs for complexes A
and B, where a longer intramolecular charge transfer occur,
while, for C and D, the calculated excitation energies are
overestimated by about 0.4 eV. A detailed list of the calculated
excitation energies is reported in the Supporting Information.
In all cases, the absorption band in the visible region is mainly
characterized by HOMO (highest occupied molecular
orbital)−LUMO (lowest unoccupied molecular orbital) tran-
sitions, with some contribution of the lower occupied molecular
orbitals (HOMO-1/HOMO-2). The HOMOs of all the
The molecular structures of the new compounds were
1
characterized by using H NMR and mass spectroscopy.
Optimized Geometry, Optical Properties, and Sec-
ond-Order NLO Measurements. In Figure 1, we report the
optimized geometry in vacuum of A−D. Complex A is
characterized by a quite planar arrangement of the donor-
metal-acceptor backbone. The insertion of the hexyl chains on
the thiophene moieties causes an increase of the dihedral angle
D
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Figure 2. Schematic representation of the energy levels of complexes A−D. Isodensity surface plots (isodensity contour: 0.035) of HOMO and
LUMO molecular orbitals are also shown.
investigated complexes are Ru orbitals combined with the π
orbitals of thiophene and benzothiadiazole groups (complexes
A−B) or of the (E)-4-(4-ethynylstyryl)-N,N-diphenylaniline
(complexes C−D). The lowest unoccupied molecular orbitals
(LUMOs) are instead completely localized on the acceptor
moieties without any contribution from the metal (Figure 2). In
particular, for complexes A−B, LUMO is strongly stabilized,
due to the delocalization on the thiophene and benzothiadia-
zole groups. The presence of the methyl-cyanoacrylate moieties
on the acceptor part of the complexes (complexes B−D) causes
a further stabilization of the LUMO orbital.
In order to investigate their second-order NLO properties,
the various ruthenium(II) complexes were investigated by the
EFISH technique (see Table 1). It is known that this
technique40−42 can provide direct information on the intrinsic
molecular NLO properties through eq 1
value (−900 × 10−48 esu) was also reported for the related
dinuclear complex [Ph-CC-Ru(dppe)2-CC-Th-BTD-Th-
CC-Ru(dppe2)-CC-Ph] (BTD = 2,1,3-benzothiadiazole;
Th = 2,5-thienyl),17 but A has a much higher absolute value of
μβ1.907 EFISH (−1550 × 10−48 esu) due to a much higher dipole
moment in the ground state (μ of complex A is 6.9 × 10−18 esu,
whereas μ of [Ph-CC-Ru(dppe)2-CC-Th-BTD-Th-CC-
Ru(dppe2)-CC-Ph]17 is 1.6 × 10−18 esu) . The value
determined for B (−1700 × 10−48 esu) is even higher due to a
large increase of the dipole moment, attributed to the
functionalization of the thienyl end-group with a methylcya-
noacetate moiety, although the absolute value of β1.907 EFISH is
lower (Table 1). The simple complex C with a “Donor-
phenylalkynyl-Ru-alkynylphenyl-Acceptor” architecture (Donor
= Ph2NPhCHCH; Acceptor = CHO; see Chart 1) is
characterized by the lowest β1.907 EFISH, in agreement with the
particularly high HOMO−LUMO gap (Figure 2),5 which
increases by a factor of 2.3 upon substitution of the aldehyde
function by the more electron-withdrawing methylcyanoacetate
moiety (D) due to an increase of both μ and β1.907 EFISH. It is
worth pointing out that the μβ1.907 EFISH value of B is more than
twice that of D, but this is due to a much higher dipole
moment, the quadratic hyperpolarizability of both complexes
being similar. Remarkably, A is characterized by the highest
β1.907 EFISH value (−225 × 10−30 esu) reported for a
mononuclear alkynyl ruthenium complex,16 in agreement with
a quite planar arrangement of the donor-metal-acceptor
backbone (Figure 1) and a low HOMO−LUMO gap (Figure
2). Calculation of the dipole moments with the CAM-B3LYP
functional provides slightly lower values of μ, and consequently
higher β1.907 EFISH (see the Supporting Information). No
significant differences were observed along the series of
investigated complexes, making us confident about the
employed computational setup.
γEFISH = (μβEFISH/5kT) + γ(−2ω; ω, ω, 0)
(1)
where μβEFISH/5kT is the dipolar orientational contribution and
γ(−2ω; ω, ω, 0), a third-order term at frequency ω of the
incident light, is a purely electronic cubic contribution to γEFISH
which can usually be neglected when studying the second-order
NLO properties of dipolar compounds.5
In Table 1 are reported the μβ1.907 EFISH values of all the
investigated complexes, measured in CH2Cl2 solution with an
incident wavelength of 1.907 μm. To obtain β1.907 EFISH, the
projection along the dipole moment axis of the vectorial
component of the tensor of the quadratic hyperpolarizability, it
is necessary to know the dipole moment, μ. In the present
study, we used the theoretical dipole moments calculated as
described in the Experimental Section (see also the Supporting
Information).
All the novel investigated complexes are characterized by a
negative value of μβ1.907 EFISH, in agreement with a negative
value of Δμeg (difference of the dipole moment in the excited
and ground state) upon excitation.5 A negative μβ1.907 EFISH
Complexes E and F in DSSCs. Photoelectrochemical cells,
capable of directly converting sunlight into electrical power, are
E
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a b
,
one of the most promising devices in the search for sustainable
and renewable sources of clean energy. Dye-sensitized solar
cells (DSSCs)43−52 are considered a realistic solution for
harnessing the energy of the sun and converting it into
electrical energy, with power conversion efficiencies now
exceeding the value of 12.3%.53
Table 2. Main Photovoltaic Parameters of DSSCs Based
on Complexes E and F, in Comparison with Those Based on
the Ruthenium Benchmark N719
dye
area/cm2
Jsc/mA cm−2
Voc/mV
FF
η/%
E
0.0979
0.0898
0.1025
4.6
1.5
578
432
745
56.1
46.5
71.9
1.5
0.3
8.1
F
Solar light harvesting is the most important step of the
photovoltaic process; consequently, the photosensitizer plays a
key role in DSSCs. It might be characterized by an absorption
spectrum that spans the entire visible range and extends into
the near-IR region with a sufficiently high molar absorption
coefficient. Moreover, the HOMO/LUMO energies of the dye
must fit the conduction band of the photoanode semiconductor
and the redox couple Nerst potential, in order to provide
efficient charge transfer mechanisms. Ruthenium based
coordination compounds bearing polypyridine ligands such as
cis-di(thiocyanato)bis(2,2′-bipyridine-4,4′-dicarboxylate)Ru(II)
(N354,55 and N71956,57) are among the most studied and
performing photosensitizers. However, a serious drawback of
such Ru(II) complexes is the presence of the anionic
thiocyanate ligands which can be easily replaced, in working
conditions, by other ligands which generally are contained, in
high concentration, into the electrolyte solution, yielding a
decrease of the efficiency during the lifetime of the device.58
Therefore, a lot of efforts have been devoted to the preparation
of thiocyanate-free ruthenium complexes for DSSCs.59−69
Following our work on the use of diruthenium alkynyl
complexes, as donor components of bulk heterojunction solar
cells,34 we anticipated that the trans-ruthenium diacetylide
motif of B and D might act as an efficient relay within a “push−
pull” architecture and, therefore, could find application in the
field of DSSCs. For this reason, we prepared two new trans-
diacetylide ruthenium compounds, E and F, both bearing the
required cyano acrylic acid anchoring group, and we studied, in
a preliminary way, their potentiality as dye sensitizers for solar
cells.70 The UV−vis spectra of E and F in dichloromethane
solution show two main absorption bands at 267 nm (ε = 17
900 M−1 cm−1), 428 nm (ε = 10 700 M−1cm−1) and 395 nm (ε
= 22 500 M−1 cm−1), 493 (ε = 59 600 M−1 cm−1), respectively.
It should be pointed out that the introduction of a carboxylic
acid group produces a noticeable lowering of the ε values and a
slight blue shift of the absorption band (Table 1; compare E
and F with D and B) with respect to those measured for the
corresponding methyl ester derivatives, D and B.
Complex F shows a LUMO level of −3.43 eV, very similar to
the methyl ester analogue, suggesting an unfavorable energy
level alignment with the TiO2 conduction band edge for the
electron injection. Similar LUMO values were found, at the
same calculation level, for another class of sensitizers for which
an inefficient electron injection process was highlighted as the
main cause for the low photovoltaic performances.71 On the
other hand, the LUMO level of E (−2.78 eV) appears to be
adequate to allow the electron injection into TiO2 conduction
band.
The main photovoltaic parameters of E and F are listed in
Table 2, and they are compared, under the same experimental
conditions, with those of the benchmark ruthenium dye N719.
As predicted from our theoretical calculations, F fails to act as
a good dye sensitizer. The very low overall power conversion
efficiency observed (0.3%, Table 2) could be ascribed to the
incorrect matching of the energetic levels between the dye and
TiO2 which practically impedes the passage of the electrons in
the device. In fact, the low short-circuit photocurrent (1.5 mA/
N719
15.2
a
b
By using a black mask (0.1672 cm2). Electrolyte composition: 0.6 M
N-methyl-N-butylimidazolium iodide, 0.04 M iodine, 0.025 M LiI, 0.05
M guanidinium thiocyanate, 0.28 M tert-butylpyridine in 15/85 (v/v)
mixture of valeronitrile/acetonitrile.
cm2) and photovoltage levels are coherent with a poor charge
transfer process.
A much better efficiency was obtained with E (η = 1.5%),
although it does not reach a high value, as expected on the basis
of its absorption spectrum (Table 1), which makes it capable to
harvest only a little fraction of the sunlight. Moreover, the value
measured in the IPCE curve (Figure 3), which does not exceed
20% in the visible region, suggests that, in this case, charge
recombinations are very favorable processes and the attempt to
mitigate them by employing 3 equiv of deoxycolic acid as
coadsorbent does not lead to satisfactory results; in fact, the
corresponding device shows, under the same conditions, an
overall power conversion efficiency of 0.8%.
These results, together with those recently published by Tian
and co-workers72−74 on platinum diacetylide compounds and
by Olivier et al.75 on analogous ruthenium derivatives, show
that it would be correct to propose D-π-M-π-A structured metal
acetylide complexes as sensitizers for DSSCs with the aim to
obtain the right compromise between the best efficiency
together with a satisfactory stability. However, our results
would indicate that, in such organometallic structures, the
introduction of selected functional groups along the push−pull
chain is a very subtle choice and seems to be crucial to obtain
good efficiencies in DSSCs; in fact, by simple chemical changes
in the donor moiety of the organometallic dye (introduction of
a carbazole unit instead of a triphenylamine and removal of the
styryl moiety), Olivier et al.75 were able to overcome an overall
power conversion efficiency of 7%, outperforming fully organic
dyes of analogous structure. The main cause of the higher
photovoltaic performance of their ruthenium sensitizer,
compared to our complex E, can be highlighted in the better
light harvesting ability (λmax of 520 nm, with an ε value of 30
200 M−1 cm−1). This difference in the absorption spectrum is
readily converted in a remarkable increase in the photocurrent
value, reaching comparable levels with the standard N719
ruthenium dye.
CONCLUSION
■
In summary, ruthenium(II) dialkynyl complexes are fascinating
second-order NLO chromophores whose NLO response can be
easily modulated by the nature of the alkynyl substituents, as
confirmed by the electric-field-induced second harmonic
generation technique. Remarkably, A and B, bearing a 2,1,3-
benzothiadiazole flanked on either side by 2,5-thienyl units, are
characterized by the largest values of μβ1.907 EFISH ever reported
for alkynyl metal complexes and among the best reported for
organometallic complexes.2−6 They are excellent candidates for
the preparation of second harmonic generation active
polymeric films.76,77 In addition, π-delocalized “push−pull”
ruthenium diacetylide complexes, where a donor and an
F
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Figure 3. JV and IPCE curve for E measured in the conditions of Table 2.
4 was prepared starting from the bromo derivative 2 (121.5 mg).
The crude product was purified using hexane/acetate 8:2 as eluent,
affording the product as a red power in 78% yield.
acceptor units are separated by the metallic center, exhibit a
promising application for the next generation of dye-sensitized
solar cells.
1H NMR (400 MHz, CDCl3, δ (ppm)) 1H NMR (400 MHz,
CDCl3, δ (ppm)) 10.01 (s, 1H), 8.08 (s, 1H), 7.98 (d, J = 7.7 Hz, 1H),
7.95 (s, 1H), 7.87 (d, J = 7.7 Hz, 1H), 3.05 (t, J = 7.6 Hz, 2H), 2.78 (t,
J = 7.6 Hz, 2 H), 1.83−1.66 (m, 4H), 1.42−1.35 (m, 12H), 0.92−
0.91(m, 6H), 0.30 (s, 9H).
EXPERIMENTAL SECTION
■
General Comments. Solvents were dried by standard procedures:
THF freshly distilled from Na/benzophenone under a nitrogen
atmosphere; N,N-dimethylformamide (DMF) was dried over activated
molecular sieves; toluene was distilled over Na/benzophenone, and
triethylamine (Et3N) freshly distilled over KOH. All reagents were
purchased from Sigma-Aldrich and were used without further
purification. Reactions requiring anhydrous conditions were performed
MS (FAB+) calcd for C32H40N2OS3Si m/z 592.21, found 592. Anal.
Calcd: C, 64.82; H, 6.80; N, 4.72. Found: C, 64.89; H, 6.79; N, 4.73.
Synthesis of Derivative 5. To a solution of 4 (67.4 mg, 0.11 mmol)
and methyl cyanoacetate (12 μL 0.125 mmol) in MeOH (5 mL) was
added piperidine (13 μL, 0.55 mmol). After stirring at 50 °C for 3 h
and overnight at room temperature, the solvent was removed under
reduced pressure. The residue was purified by flash chromatography
on silica gel (hexane/ethyl acetate 95:5). 1H NMR (400 MHz, CDCl3,
δ (ppm)) 8.50 (s, 1H), 8.22 (s, 1H), 8.03 (d, J = 7.8 Hz, 1H), 7.95 (s,
1H), 7.86 (d, J = 7.8 Hz, 1H), 2.93−2.91 (m, 2H), 2.80−2.76 (m,
2H), 1.74−1.55 (m, 6H), 1.35−1.27 (m, 10H), 0.94−0.91 (m, 6H),
0.30 (s, 9H).
1
under nitrogen. H and 13C NMR spectra were recorded at 400 MHz
on a Bruker AVANCE-400 instrument. Chemical shifts (δ) for 1H and
13C spectra are expressed in ppm relative to internal Me4Si as standard.
Signals were abbreviated as s, singlet; bs, broad singlet; d, doublet; t,
triplet; q, quartet; m, multiplet. Mass spectra were obtained with an
FT-ICR Mass Spectrometer APEX II & Xmass software (Bruker
Daltonics) - 4.7 Magnet and Autospec Fission Spectrometer (FAB
ionization). MALDI-TOF mass spectra were obtained using a
MICROFLEX-LT (BRUKER) with trans-2-[3-(4-tert-butylphenyl)-2-
methyl-2-propenylidene] malononitrile (DCTB) as matrix. Samples
for MALDI: 1 μL of complex solution + 49 μL of matrix solution + 0.5
μL of LiCl solution + 200 μL of CH2Cl2. The solutions were prepared
as follows: 1 mg/mL in CH2Cl2 for the complex, 10 mg of DCTB/mL
for the matrix solution, and 10 mg of LiCl/mL in methanol for the salt
solution.
Electronic absorption spectra were obtained using a Jasco V-530
spectrophotometer.
Thin-layer chromatography (TLC) was carried out with precoated
Merck F254 silica gel plates. Flash chromatography (FC) was carried
out with Macherey-Nagel silica gel 60 (230−400 mesh).
Synthesis of Complexes A and B (see Scheme 1 in Results
and Discussion). Synthesis of Derivatives 3 and 4. Under a
nitrogen atmosphere, ethynyltrimethylsilane (33 μL, 0.23 mmol) was
added to a degassed solution of the opportune bromo-derivative 1 or 2
(0.21 mmol), [PdCl2(PPh3)2] (4.0 mg, 0.02 mmol), CuI (2.0 mg, 0.04
mmol), and PPh3 (2.0 mg, 0.02 mmol) in THF/triethylamine (12 mL,
1/2 v/v). The mixture was stirred for 20 h at 70 °C, and then the
solvent was removed under reduced pressure. The residue was
dissolved in dichloromethane and washed with water. The organic
layer was dried over Na2SO4, and the solvent was removed at reduce
pressure. The crude product was purified by flash chromatography on
silica gel.
50.0 mg (0.83 mmol) of alkyne was deprotected with K2CO3 (126
mg, 0.91 mmol) in THF/MeOH (1:1) 6 mL at r.t for 1 h. The solvent
was removed under reduced pressure, and the residue was purified by
flash chromathography on silica gel (hexane/ethyl acetate 95:5). The
1
product was obtained as a dark-red powder in 90% yield. H NMR
(400 MHz, CDCl3, δ (ppm)) 8.51 (s, 1H), 8.22 (s, 1H), 8.03 (d, J =
7.8 Hz, 1H), 7.96 (s, 1H), 7.84 (d, J = 7.8 Hz, 1H), 3.98 (s, 3H), 3.63
(s, 1H), 2.93−2.91 (m, 2H), 2.80−2.76 (m, 2H), 1.74−1.55 (m, 6H),
1.35−1.27 (m, 10H), 0.93−0.91 (m, 6H).
MS (FAB+) calcd for C33H35N3O2S3 m/z 601.19, found 601. Anal.
Calcd: C, 65.86; H, 5.86; N, 6.98. Found: C, 65.90; H, 5.87; N, 6.96.
Synthesis of Complexes A and B: General Procedure. A mixture of
alkyne (0.07 mmol), trans-[Ru(dppe)2Cl2] (79.7 mg, 0.077 mmol),
and NaPF6 (26.9 mg, 0.16 mmol) in CH2Cl2 (35 mL) was stirred in
the dark for 18 h; triethylamine (45 μL) was added and stirring was
maintained for 3h. The solvent was removed under reduced pressure,
and the residue was taken up in CH2Cl2 and purified by column
chromatography on neutral alumina.
Complex A was prepared starting from the free alkyne 4-(5-
ethynylthiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole
(22.7 mg) obtained from 3 by deprotection with K2CO3 in THF/
MeOH as described in the synthesis of 5.
1H NMR (400 MHz, CD2Cl2, δ (ppm)) 8.16 (m, 1H), 8.12 (d, J =
4.8 Hz, 1H), 8.08 (d, J = 5.4 Hz, 1H), 7.98 (d, J = 5.4 Hz, 1H), 7.88
(d, J = 4.8 Hz, 1H), 7.31−7.00 (m, 47H), 2.67 (m, 8H).
MS (MALDI-TOF, 2-[(2E)-3-(4-tert-butylphenyl)-2 methylprop-2-
enylidene]malonitrile as the matrix) calcd for C76H60N2P4RuS3 m/z
1322,19 found 1323. Anal. Calcd: C, 69.02; H, 4.57; N, 2.12. Found:
C, 69.06; H, 4.55; N, 2.12.
3 was prepared starting from 4-(5-bromothiophen-2-yl)-7-(thio-
phen-2-yl)benzo[c][1,2,5]thiadiazole (79.4 mg). The crude product
was purified using hexane/acetate 9:1 as eluent, affording the desired
product as a red powder in 80% yield.
1H NMR (400 MHz, CDCl3, δ (ppm)) 1H NMR (400 MHz,
CDCl3, δ (ppm)) 8.15 (d, J = 3.8 Hz, 1H), 8.00 (d, J = 3.8 Hz, 1H),
7.88 (d, J = 7.6 Hz, 1H), 7.86 (d, J = 7.6 Hz, 1H), 7.49 (d, J = 4.8 Hz,
1H), 7.32 (d, J = 3.8 Hz, 1H), 7.23 (t, J = 4.6 Hz, 1H), 0.30 (s, 9H).
MS (FAB+) calcd for C19H16N2S3Si m/z 396.02, found 396. Anal.
Calcd: C, 57.54; H, 4.07; N, 7.06. Found: C, 57.58; H, 4.06; N, 7.03.
Complex B was prepared starting from the free alkyne 5 (42.13
mg).
Elution with hexane/ethyl acetate 8:2 gave a purple band that was
collected and dried in vacuo, affording the desired product in 75%
yield.
G
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Article
1H NMR (400 MHz, CD2Cl2, δ (ppm)) 8.52 (s, 1H), 8.20 (s, 1H),
8.11 (d, J = 7.8 Hz, 1H), 8.00 (s, 1H), 7.95 (d, J = 7.8 Hz, 1H), 7.29−
7.01 (m, 45H), 3.98 (s, 3H), 2.71 (m, 8H), 1.55−1.39 (m, 16H),
1.58−1.55 (m, 10H).
MS (MALDI-TOF, 2-[(2E)-3-(4-tert-butylphenyl)-2 methylprop-2-
enylidene]malonitrile as the matrix) calcd for C93H87N3O2P4RuS3 m/z
1599.40, found 1600.7. Anal. Calcd: C, 69.82; H, 5.48; N, 2.63. Found:
C, 69.99; H, 5.39; N, 2.60.
Complex F was prepared starting from 45 mg of the potassium salt
derivative of 5.
The intermediate was obtained in 95% yield in more basic condition
than previous described for 5.
(E)-4-(4-ethynylstyryl)-N,N-diphenylaniline (8.0 mg, 0.0216 mmol)
was added, and the solution was heated at 40 °C for 2 h. After cooling
to room temperature, the reaction mixture was filtered, and the
inorganic salts were washed with CH2Cl2 (1 mL). Et2O was added to
the CH2Cl2 extract to precipitate the vinylidene salt as a violet powder,
which was collected by filtration and then under nitrogen was
redissolved in CH2Cl2 (1 mL). TEA (50 μL) was added and stirring
was maintained for 2 h, the solvent was removed under reduced
pressure, and the residue was taken up in CH2Cl2 and purified by
column chromatography (basic alumina). Elution with CH2Cl2 gave a
yellow band that was collected and dried in vacuo. The yellow product
was obtained in 85% yield
1H NMR (400 MHz, CD2Cl2): 7.77 (d, JH‑H = 8.6 Hz, 2H), 7.40−
7.19 (m, 20H), 7.13 (d, JH‑H = 8.6 Hz, 2H), 7.11−6.98 (m, 36H), 2.68
(m, 8H). Elemental Analysis: Calcd for C80H68ClNP4Ru: C, 73.70; H,
5.26; N, 1.07. Found: C, 74.05; H, 5.29; N, 1.09.
Starting from 4 (50 mg, 0.074 mmol), K2CO3 (20.4 mg, 0.148
mmol) in THF/MeOH (1:1, 6 mL) at 30 °C for 1 h. The solvent was
removed under reduced pressure, and the residue was purified by flash
chromathography on silica gel (hexane/ethyl acetate 8:2). The acid
was obtained dissolving the residue in CH2Cl2 and adding the
Synthesis of Complex C. 11 (25.0 mg, 0.019 mmol), 4-ethynyl-
benzaldehyde (2.6 mg, 0.02 mmol), and NaPF6 (1.0 mg, 0.076 mmol)
were dissolved in CH2Cl2 (2 mL), and the solution was stirred for 1 h.
TEA (50 μL) was added, and stirring was maintained for 24 h at room
temperature with exclusion of light. The reaction mixture was filtered,
the inorganic salts were washed with CH2Cl2 (1 mL), and the solvent
was removed under reduced pressure.
1
stoichiometric amount of trifluoroacetic acid. H NMR (400 MHz,
CD2Cl2, δ (ppm)) 8.51 (s, 1H), 8.20 (s, 1H), 8.08 (d, J = 7.8 Hz, 1H),
7.96(s, 1H), 7.95 (d, J = 7.8 Hz, 1H), 7.28−7.03 (m, 45H), 2.72 (m,
8H), 1.55−1.39 (m, 16H), 1.58−1.55 (m, 10H).
MS (MALDI-TOF, 2-[(2E)-3-(4-tert-butylphenyl)-2 methylprop-2-
enylidene]malonitrile as the matrix) calcd for C92H84KN3O2P4RuS3
m/z 1623.34, found 1624.0. Anal. Calcd: C, 68.04; H, 5.21; N, 2.59.
Found: C, 68.02; H, 5.21; N, 2.60.
The residue was taken up in CH2Cl2 and purified by
chromatography on neutral alumina, elution with hexane/CH2Cl2
9:1, and then 1:1 gave an orange band that was collected and
evaporated under reduced pressure. The orange product was obtained
in 90% yield.
Synthesis of Complexes C, D, and E (see Schemes 2 and 3 in
Results and Discussion). Synthesis of (E)-4-(4-Bromostyryl)-N,N-
diphenylaniline (8). To a solution of diethyl 4-bromobenzyl-
phosphonate (364.0 mg, 1.2 mmol, 1.1 equiv) and 4-(diphenyl-
amino)benzaldehyde (295.0 mg, 1.08 mmol, 1 equiv) in THF (9 mL),
under nitrogen, and cooled to 0 °C, was added, in small portions,
potassium tert-butoxide (316.0 mg, 2.8 mmol, 2.6 equiv). The cool
bath was then removed and the mixture stirred for 18 h at room
temperature. After hydrolysis with water, the mixture was stirred for a
further 30 min. The reaction mixture was diluted with AcOEt and
washed with water: the organic layer was dried over Na2SO4 and
concentrated. The crude product obtained was purified by flash
chromatography, using hexane/dichloromethane 1:1 as eluant, to give
the product as a pale yellow solid (378 mg; yield 85%). Character-
1H NMR (400 MHz, CD2Cl2): 9.95 (s, 1H), 7.74 (d, JH‑H = 8.6 Hz,
2H), 7.40−7.19 (m, 19H), 7.13 (d, JH‑H = 8.6 Hz, 2H), 7.11−6.98 (m,
37H), 6.93 (d, JH‑H = 8.9 Hz, 2H), 6.78 (d, JH‑H = 8.9 Hz, 2H), 2.68
(m, 8H). Elemental Analysis: Calcd for C89H73NOP4Ru: C, 76.49; H,
5.27; N, 1.00. Found: C, 76.89; H, 5.25; N,0.98.
Synthesis of Complexes D and E. To a solution of Complex C
(10.0 mg, 0.007 mmol) and methylcyanoacetate (2.52 μL, 0.02 mmol)
or cyanoacetic acid (13.0 mg, 0.02 mmol) in acetonitrile/CH2Cl2 1:1
(4 mL) was added piperidine (10 μL, 0.431 mmol, 7.0 equiv). After
stirring at room temperature overnight, the solvent was removed
under reduced pressure. The residue was taken up in CH2Cl2 and
diethyl ether was added, and the precipitate was filtered, and was
washed with diethyl ether (1 mL). The filtrate was evaporated under
reduced pressure, and the product was recrystallized.
1
ization was performed by H and 13C NMR, and it is consistent with
published data.
Synthesis of (E)-N,N-Diphenyl-4-(4-((trimethylsilyl)ethynyl)styryl)-
aniline (9). To a solution of (E)-4-(4-bromostyryl)-N,N-diphenyl-
aniline (8) (280.0 mg, 0.47 mmol, 1 equiv) and ethynyltrimethylsilane
(69.2 mg, 0.7 mmol, 1.5 equiv) in a degassed tetrahydrofuran (7.8
mL), under a flow of nitrogen, were added [PdCl2(PPh3)2] (13.2 mg,
4 mol %), CuI (5.4 mg, 6 mol %), and triethylamine (2 mL). The
reaction mixture was left under stirring at 70 °C overnight. The solvent
was removed under reduced pressure, and the residue was purified by
flash chromatography, using hexane/dichloromethane 9:1 as eluant, to
give 9 as a yellow solid (157 mg; yield 75%). Characterization was
Complex D: Orange/red product was obtained by slow diffusion of
1
pentane into CH2Cl2 solution (8.0 mg, 80%). H NMR (400 MHz,
CD2Cl2): 8.03 (s, 1H), 7.72 (d, JH‑H = 8.6 Hz, 2H), 7.55−7.44 (m,
19H), 7.30 (d, JH‑H = 8.6 Hz, 2H), 7.32−7.13 (m, 39H), 6.82−6.77
(m, 2H), 3.95 (s, 3H), 2.70 (m, 8H). Elemental Analysis: Calcd for
C93H76N2O2P4Ru: C, 75.55; H, 5.18; N, 1.89. Found: C, 75.59; H,
5.16; N, 1.89 MS(MALDI) calcd. m/z: 1478.34 found: 1478.0
(product), 897.2 (Ru(dppe)2).
1
performed by H and 13C NMR, and it is consistent with published
Complex E: The acid was obtained dissolving the residue in CH2Cl2
and adding the stoichiometric amount of trifluoroacetic acid. Orange
crystals of product were obtained by slow diffusion of pentane into
data.
Synthesis of (E)-4-(4-Ethynylstyryl)-N,N-diphenylaniline (10). To a
solution of (E)-N,N-diphenyl-4-(4-((trimethylsilyl)ethynyl)styryl)-
aniline (9) (71.3 mg, 0.16 mmol, 1 equiv), in methanol (5.3 mL),
under a flow of nitrogen, was added K2CO3 anhydrous (88.4 mg, 0.64
mmol, 4 equiv). The reaction mixture was left under stirring at room
temperature overnight. The solvent was removed under reduced
pressure, and the residue was diluted with CH2Cl2 and washed with
water: the organic layer was dried over Na2SO4 and concentrated. The
crude product obtained was purified by flash chromatography, using
hexane/dichloromethane 8:2 as eluant, to give 5 as a yellow solid (54.0
mg; yield 90%). MS(FAB+): m/z 371. Characterization was performed
1
CH2Cl2 solution (9.0 mg, 91%). H NMR (400 MHz, CD2Cl2): 8.05
(s, 1H), 7.78 (d, JH‑H = 8.6 Hz, 2H), 7.60−7.44 (m, 19H), 7.35 (d, JH‑H
= 8.6 Hz, 2H), 7.32−7.13 (m, 39H), 6.82−6.77 (m, 2H), 2.70 (m,
8H). Elemental Analysis: Calcd for C92H74N2O2P4Ru: C, 75.45; H,
5.09 N, 1.91. Found: C, 75.65; H, 5.07; N, 1.89 MS(MALDI) m/z:
1469.6 (COO−Li+), 1270.3 (Ru(CC-4-(E)-ethynylstyryl-N,N-
diphenylaniline) (dppe)2), 897.2 (Ru(dppe)2).
EFISH Measurements. All EFISH measurements40−42 were
̀
carried out at the Dipartimento di Chimica of the Universita degli
Studi di Milano, in CH2Cl2 solutions at a concentration of 1 × 10−3 M,
working with a nonresonant incident wavelength of 1.907 μm,
obtained by Raman-shifting the fundamental 1.064 μm wavelength
produced by a Q-switched, mode-locked Nd3+:YAG laser manufac-
tured by Atalaser. The apparatus for the EFISH measurements is a
prototype made by SOPRA (France). The μβEFISH values reported are
the mean values of 16 successive measurements performed on the
1
by H and 13C NMR, and it is consistent with published data.
Synthesis of trans-[Ru(CC-4-(E)-Ethynylstyryl-N,N-diphenyl-
aniline)Cl(dppe)2][PF6] (11). trans-[Ru(dppe)2Cl2] (20.0 mg, 0.019
mmol) and NaPF6 (3.6 mg, 0.0216 mmol) were dissolved in CH2Cl2
(1 mL), and the solution was stirred for 12 h at room temperature
with exclusion of light.
H
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Article
same sample. The sign of μβ is determined by comparison with the
reference solvent (CH2Cl2).
ACKNOWLEDGMENTS
■
We deeply thank Mattia Fontana for some experimental help.
This work was supported by MIUR and by eni (Contract
number 3500005452).
Computational Details. All the calculations have been performed
by the Gaussian 09 program package.78 Geometry optimizations of the
various ruthenium complexes were performed in vacuo, using the
B3LYP exchange-correlation functional79 and using a LANL2DZ basis
set80 for all atoms along with the corresponding pseudopotentials for
Ru. On the optimized geometries, dipole moments have been
calculated by single point, both in vacuo and in CH2Cl2, using the
B3LYP functional and the LANL2DZ basis set (see the Supporting
Information). Solvation effects were included by means of the
conductor-like polarizable continuum model (C-PCM),81,82 as
implemented in G09. Then, the first 50 singlet excitations have been
computed using TD-DFT at the same calculation level (B3LYP/
LANL2DZ/C-PCM) on the optimized geometries. For comparative
purposes, also the long-range corrected CAM-B3LYP functional was
tested both in the dipole moments and in the excitation energies
calculations.
Fabrication and Evaluation of Solar Cells. TiO2 electrodes
were prepared by spreading (doctor blading) a colloidal TiO2 paste
(20 nm sized; “Dyesol” DSL 18NR-T) onto a conducting glass slide
(FTO, Hartford glass company, TEC 8, having a thickness of 2.3 mm
and a sheet resistance in the range 6−9 Ω/cm2) that had been cleaned
with water and EtOH treated with a plasma cleaner at 100 W for 10
min, dipped in a freshly prepared aqueous TiCl4 solution (4.5 × 10−2
M), at 70 °C, for 30 min, and finally washed with ethanol. After a first
drying at 125 °C for 15 min, a reflecting scattering layer containing
>100 nm sized TiO2 (“Solaronix” Ti-Nanoxide R/SP) was bladed over
the first TiO2 coat and sintered until 500 °C for 30 min. Then, the
glass coated TiO2 was dipped again into a freshly prepared aqueous
TiCl4 solution (4.5 × 10−2 M), at 70 °C for 30 min, then washed with
ethanol, and heated once more at 500 °C for 15 min. At the end of
these operations, the final thickness of the TiO2 electrode was in the
range 8−12 μm, as determined by SEM analysis. After the second
sintering, the FTO glass coated TiO2 was cooled at about 80 °C and
immediately dipped into a CH2Cl2 solution [1.5 × 10−4 M] of the
selected dye at r.t. for 18 h.The dyed titania glasses were washed with
EtOH and dried at r.t. under a N2 flux. Finally, the excess of TiO2 was
removed with a sharp Teflon penknife, and the exact active area of the
dyed TiO2 was calculated by means of microphotography. A 50 μm
thick Surlyn spacer (TPS 065093-50 from Dyesol) was used to seal the
photoanode and a platinized FTO counter electrode. Then, the cell
was filled up with the desired electrolyte solution. The photovoltaic
performance of the cells was measured with a solar simulator (Abet
2000) equipped with a 300 W xenon light source; the light intensity
was adjusted with a standard calibrated Si solar cell (“VLSI Standard”
SRC-1000-RTD-KG5). The current−voltage characteristics were
acquired by applying an external voltage to the cell and measuring
the generated photocurrent with a “Keithley 2602A” (3A DC, 10A
Pulse) digital source meter.
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* Supporting Information
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AUTHOR INFORMATION
■
Corresponding Authors
*Tel: +39 0250314399. Fax: +39 0250314405. E-mail: alessia.
colombo@unimi.it (A.C.).
*Tel: +39 075 5855522. Fax: +39 075 585 5606. E-mail:
filippo@thch.unipg.it (F.D.A.).
*Tel: +39 0321447817. Fax: +39 03217241. E-mail: paolo.
biagini@eni.com (P.B.).
̀
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Notes
The authors declare no competing financial interest.
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