New dinuclear hydrido-carbonyl rhenium complexes designed as photosensitizers in dye-sensitized solar cells

Article abstract of DOI:10.1039/c5nj03000e

The possible use of some dinuclear rhenium complexes as sensitizers for dye sensitized solar cells (DSSCs) has been investigated. They have general formula [Re₂(μ-X)(μ-Y)(CO)₆(μ-pyridazine-4-COOH)], with X = Y = Cl (1), X = H, Y = benzoato (2), and X = H, Y = 4-diphenylaminobenzoato (3). An original synthetic strategy has been set for preparing the hydrido-carboxylato derivatives 2 and 3. They have been indicated by DFT and TD-DFT computations as the most promising dyes, endowed with good light harvesting capability. The complexes have absorption maxima in the range of 405-443 nm, on TiO₂ films, arising from metal-to-ligand-charge transfer transitions. Cyclic voltammetry experiments have been performed on the derivatives containing the methyl ester of the pyridazine-4-COOH acid, showing electrochemical band gaps in the range of 2.25-1.63 eV. The best DSSC results have been obtained using complex 3, with an overall solar-to-electric conversion efficiency of 1.0%. Noteworthy the presence of a hydrido ligand did not show any detrimental effect on the stability of the sensitizers under the operating conditions.

Full text of DOI:10.1039/c5nj03000e

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New dinuclear hydrido-carbonyl rhenium
complexes designed as photosensitizers in
dye-sensitized solar cells
Cite this:DOI: 10.1039/c5nj03000e
Lorenzo Veronese,a Elsa Quartapelle Procopio,a Francesca De Rossi,b
Thomas M. Brown,b Pierluigi Mercandelli,acd Patrizia Mussini,a Giuseppe D’Alfonsoac
and Monica Panigati*acd
The possible use of some dinuclear rhenium complexes as sensitizers for dye sensitized solar cells
(DSSCs) has been investigated. They have general formula [Re2(m-X)(m-Y)(CO)6(m-pyridazine-4-COOH)],
with X = Y = Cl (1), X = H, Y = benzoato (2), and X = H, Y = 4-diphenylaminobenzoato (3). An original
synthetic strategy has been set for preparing the hydrido-carboxylato derivatives 2 and 3. They have
been indicated by DFT and TD-DFT computations as the most promising dyes, endowed with good light
harvesting capability. The complexes have absorption maxima in the range of 405–443 nm, on TiO2
films, arising from metal-to-ligand-charge transfer transitions. Cyclic voltammetry experiments have
been performed on the derivatives containing the methyl ester of the pyridazine-4-COOH acid, showing
electrochemical band gaps in the range of 2.25–1.63 eV. The best DSSC results have been obtained
using complex 3, with an overall solar-to-electric conversion efficiency of 1.0%. Noteworthy the
presence of a hydrido ligand did not show any detrimental effect on the stability of the sensitizers under
the operating conditions.
Received (in Montpellier, France)
27th October 2015,
Accepted 22nd January 2016
DOI: 10.1039/c5nj03000e
www.rsc.org/njc
a [Re2(m-Cl)2(CO)6(m-pyridazine)] complex constitutes the chromo-
phoric subunit, covalently linked to a fullerene unit by a carbo-
Introduction
A new a family of dinuclear rhenium complexes with general cyclic molecular bridge. In this dyad the rhenium complex does
formula [Re2(m-X)(m-Y)(CO)6(m-diazine)], containing two ‘‘Re(CO)3’’ not exhibit any emission, since its strongly emissive triplet charge-
units connected by a bridging 1,2-diazine ligand and two anionic transfer state is efficiently quenched by the fullerene subunit, via
ancillary ligands, such as halides,1 hydrides,2 alcohoxides or oxidative electron transfer, with a time constant of about 110 ps.
sulphides,3 has been recently reported. Upon optical excitation,
These results prompted us to investigate the possible use of
many of these complexes exhibit broad and featureless photo- dinuclear rhenium complexes as dyes in dye sensitized solar
luminescence, centred between 520 and 660 nm, arising from cells (DSSCs).7–9 Actually sensitizers suitable for such applica-
the lowest-lying triplet-manifold excited state (T1 - S0), with tions must be able to transfer electrons from an excited state to
metal-to-ligand charge transfer (3MLCT) nature. Emission the conductive band of an oxide (most often TiO2) acting as the
quantum yields up to 0.5, both in solution and in the solid anode of the solar cell. The most commonly used dyes are
state, have been measured for some members of this family,4 ruthenium complexes containing poly-pyridine ligands function-
which have also found applications in OLED devices as a alized with carboxylic substituents for anchoring to TiO2.10
dopant in the emissive layer.5
These complexes exhibit broad absorptions, due to MLCT transi-
Moreover very recently the first molecular dyad containing a tions, long excited state lifetime, and high chemical stability.11
dinuclear rhenium(I) moiety has been reported.6 In this species The highest efficiency reported for small area DSSCs based on the
ruthenium dye and iodide/triiodide electrolyte is 12.1%,12,13 while
a
an efficiency as high as 13% was achieved using a molecularly
engineered porphyrin dye in combination with a cobalt-based
electrolyte.14
Alternative dyes based on different metal complexes, such as
iron,15 copper,16 and platinum,17 mainly containing bipyridine
ligands, have been investigated so far.18 In this framework, the use of
rhenium polypyridine complexes has not been largely investigated.
`
Dipartimento di Chimica, Universita degli Studi di Milano, Via Golgi, 19, 20133,
Milano, Italy. E-mail: monica.panigati@unimi.it
b
`
CHOSE, Dipartimento di Ingegneria Elettronica, Universita degli Studi di Roma
Tor Vergata, Via del Politecnico 1, 00133 Roma, Italy
c Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali
(INSTM), Via G. Giusti 9, 50121 Firenze, Italy
d Istituto per lo Studio delle Macromolecole, Consiglio Nazionale delle Ricerche
(ISMAC-CNR), Via E. Bassini, 15, 20133 Milano, Italy
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spectra were obtained using an Agilent 8453 spectrophotometer,
fused quartz cuvettes (10 mm optical path), and CHROMASOL V
grade solvents. IR spectra were acquired on a Bruker Vector 22
FT spectrophotometer. MS spectrum of 1 was carried out on a
Finnigant LCQt Advantage MAX ion trap instrument using
methanol as the solvent (ESI-MS: m/z 736). FAB spectrum of 3
was carried out on a mass spectrometer VG Autospec M246 using
nitrobenzyl alcohol as the matrix.
[Re2(l-H)(l-OOCPh)(CO)6(l-ppd)] 5. A sample of benzoic acid
(6.4 mg, 0.052 mmol) was added to a solution of [Re2(m-H)2(CO)6-
(m-ppd)] (4, 40 mg, 0.052 mmol) previously dissolved in 15 mL of
anhydrous toluene. The reaction mixture was stirred overnight,
and the completeness of the reaction was monitored by IR
spectroscopy. Then the solution was evaporated to dryness under
vacuum, and the solid was dissolved in CH2Cl2. The addition of
n-hexane caused the precipitation of the product, which was
purified through column chromatography (eluent ethylacetate/
hexane 9 : 1), then dried under vacuum, yielding 30 mg (0.034 mmol)
of white powder (isolated yield 65%). IR (CH2Cl2) n(CO): 2041 (m),
2022 (vs), 1935 (vs), 1914 (s) cmÀ1. 1H NMR (CD2Cl2, 300K, 400 MHz)
Chart 1 Molecular structure of the three dyes.
Indeed, beside some preliminary studies concerning the electron
transfer from Re polypyridyl complexes to nanocrystalline TiO2,19,20
only one study briefly exploring the photovoltaic performances
of tricarbonyl rhenium complexes can be found.21
The dirhenium complexes here presented (Chart 1) have
been designed by the use of combined density functional (DFT)
and time-dependent density functional (TD-DFT) studies. All of
1
them show MLCT transitions, in which the excited electron is
transferred to the diazine ligand, where the LUMO is mainly
localized.1–4 The diazine has been functionalized by a carboxylic
group, to allow the anchoring of the dye to the TiO2 surface and
the fast injection of the electron from the diazine to the TiO2
conduction band. The ancillary ligands have been selected in
order to maximize the light harvesting capability of the dye.
The computational studies showed that complexes containing
one hydrido and one benzoate bridging ligands were the most
promising dyes. The synthesis of hydrido-carboxylato derivatives
required the setting of a new general synthetic procedure, which
has been exploited for the preparation of complexes 2 and 3 of
Chart 1. Their electrochemical and spectroscopical properties
have been measured and compared with those of the previously
known complex 1, containing two bridging chloride anions.
Finally, solar cells with each of the three complexes have been
prepared and their performances are discussed herein, in relation
to the molecular structure of the dyes and to the nature of the
ancillary ligands.
dH (ppm) 8.15 (d, 4H, Hortho), 7.87 (t, 2H, Hpara), 7.77 (t, 4H, Hmeta),
8.03 (d, 2H, Hortho), 7.51 (t, 1H, Hpara), 7.41 (dt, 2H, Hmeta), À7.08
(s, 1H, hydride). Elemental anal. calcd for C27H16N2O9Re2:
C 36.65, H 1.82, N 3.17. Found: C 36.70, H 1.96, N 3.14.
[Re2(l-H)(l-OOCPh)(CO)6(l-pydz-4-COOH)] 2. A sample of
[Re2(m-H)(m-OOCPh)(CO)6(m-ppd)] (5, 30 mg, 0.034 mmol) dis-
solved in freshly distilled THF solution (5 mL) was treated with
4-pyridazine-carboxylic acid (6.3 mg, 0.050 mmol). The tempera-
ture was set at 333 K for 2 h. The color of the solution progressively
becomes deep red and the progress of the reaction was monitored
by IR spectroscopy. Then the solution was evaporated to dryness
under vacuum, the crude product dissolved in CH2Cl2 and
addition of n-hexane affords the precipitation of red powder.
The solution was removed and the remaining powder was
washed with n-hexane (5 times with 3 mL), then dried under
vacuum. The solid was dissolved again in CH2Cl2 and complex
2 was collected as red powder after precipitation with n-hexane
(13 mg, 0.165 mmol, isolated yield 56%). IR (CH2Cl2) n(CO):
2039 (m), 2018 (s), 1935 (s), 1918 (s) cmÀ1. 1H NMR (THF, 300K,
400 MHz) dH (ppm) 9.75 (dd, 1H, H2 4-COOH-pydz), 9.51 (d, 1H,
H3 4-COOH-pydz), 8.34 (dd, 1H, H5 4-COOH-pydz), 7.88 (dd, 2H,
Experimental section
Synthesis
H
ortho), 7.50 (t, 1H, Hpara), 7.38 (dt, 2H, Hmeta), À6.71 (s, 1H,
All reactions were performed under a nitrogen atmosphere hydride). Elemental anal. calcd for C18H10N2O10Re2: C 27.48, H
unless specified otherwise. All solvents were deoxygenated and 1.28, N 3.56. Found: C 27.42, H 1.30, N 3.55.
dried by standard methods before use, while toluene has been
4-(Diphenylamino)benzoic acid (TPA-COOH). In a dried flask,
distilled on Na(s)-benzophenone and dichloromethane on P2O5, a sample of 4-(diphenylamino)benzaldehyde (500 mg, 1.8 mmol)
both under a N2 atmosphere. The reagents were purchased from was dissolved in 60 mL of acetone. Separately, 40 mL of aqueous
Aldrich, Fluka and Lancaster and used as received. Commercial solution of KMnO4 (0.157 M, 3.5 equiv.) was prepared, filtered,
deuterated solvents were used as received. Column chromato- and then added dropwise to the solution containing the alde-
graphy was performed using Alfa Aesar silica gel 60 (0.032– hyde. The reaction mixture was left at room temperature for 72 h,
0.063 mm). [Re4(m3-H)4(CO)12](C6H6)2,22 [Re2(m-H)2(CO)6(m-ppd)] then filtered on celite to remove MnO2. The filtered solution was
(4, ppd = 2,5-diphenyl-1,3,4-oxadiazole)23 and [Re2(m-Cl)2(CO)6(m- collected in Àan open vessel and left at room temperature until
pydz-4-COOH)] (1, pydz-4-COOH = pyridazine-4-carboxylic acid)24 excess MnO4 was decomposed and the solution becomes yellow
were synthesized according to the literature procedures. 1H NMR and cloudy. Then it was filtered again on celite. The yellow solution
spectra were recorded on Bruker DRX-300 and Bruker DRX-400 was treated with 2.5 mL of HCl/H2O 1 : 1, a pale yellow precipitate
NMR spectrometers. Solution and thin-film UV/Vis absorption appears, then stirred for 30 minutes in an ice bath, and
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filtered again. The resulting solution was dried under vacuum with 0.1 M TBAPF6 (TBA = NBu4, Aldrich) as the supporting
affording 270 mg (0.93 mmol) of pure microcrystalline product electrolyte, at 298 K. The ohmic drop has been compensated by
(isolated yield 50%). 1H NMR (CD2Cl2, 300 K, 400 MHz) dH the positive feedback technique. The experiments were carried
(ppm) 7.91 (d, 2H), 7.35 (t, 4H), 7.17 (m, 6H), 7.01 (d, 2H).
out using an AUTOLAB PGSTAT potentiostat (EcoChemie, The
[Re2(l-H)(l-4-OOC-TPA)(CO)6(l-ppd)] 6. A sample of [Re2(m-H)2- Netherlands) run by a PC with GPES software. The working
(CO)6(m-ppd)] (4, 42 mg, 0.055 mmol) was dissolved in freshly electrode was a glassy carbon one (AMEL, diameter 1.5 mm)
distilled toluene (20 mL) and treated with a slight excess of cleaned by diamond powder (Aldrich, diameter 1 mm) on a wet
4-(diphenylamino)benzoic acid (19 mg, 0.066 mmol). The reaction cloth (STRUERS DP-NAP); the counter electrode was a platinum
mixture was refluxed overnight, then the solution was evaporated to wire; the reference electrode was an aqueous saturated calomel
dryness under vacuum. The white residue was dissolved in CH2Cl2 electrode, having in our working medium a difference of
and precipitated with n-hexane, affording a white powder which was À0.385 V vs. the Fc+|Fc couple (the intersolvent redox potential
further purified through column chromatography (eluent toluene/ reference currently recommended by IUPAC).
ethyl acetate 9 : 1), affording the desired product as white powder
Computational details
(32 mg, 0.034 mmol, isolated yield 55%). IR (CH2Cl2) n(CO):
2041 (m), 2022 (vs), 1936 (vs), 1915 (s) cmÀ1. H NMR (CD2Cl2, Ground state geometries were optimized by means of density
1
300 K, 400 MHz) dH (ppm) 8.16 (d, 4H), 7.77 (m, 4H), 7.89 (t, 2H), functional calculations. The parameter-free hybrid functional
7.84 (dd, 2H), 7.32 (t, 4H), 7.14 (m, 6H), 6.95 (dd, 2H), À7.09 (s, 1H, PBE025,26 was employed along with the standard valence double-z
hydride). Elemental anal. calcd for C39H25N3O9Re2: C 44.52, H 2.40, polarized basis set 6-31G(d,p) for C, H, Cl, N, O and S. For Re the
N 3.99. Found: C 44.93, H 2.45, N 3.96.
Stuttgart–Dresden effective core potentials were employed along
[Re2(l-H)(l-4-OOC-TPA)(CO)6(l-pydz-4-COOH)] 3. A sample with the corresponding valence triple-z basis set. Preliminary
of [Re2(m-H)(m-4-OOC-TPA)(CO)6(m-ppd)] (6, 12 mg, 0.011 mmol) calculations were done without imposing any symmetry. However,
dissolved in freshly distilled THF (5 mL) was treated at room almost all the optimized structures are symmetric (apart from 2, 3
temperature with 4-pyridazine-carboxylic acid (1.7 mg, 0.014 mmol). and TPA-COOH), as reported in Table 1. The nature of all the
The solution was refluxed for 3 h. The solution was evaporated stationary points was checked by computing vibrational frequen-
to dryness under vacuum and the crude product was dissolved cies and all the geometries were found to be true minima. In order
in CH2Cl2 and precipitated with n-hexane. The supernatant to simulate the absorption electronic spectrum down to 300 nm the
solution was removed and the remaining powder was washed lowest 20 singlet excitation energies were computed by means of
with n-hexane (five times with 3 mL), then dried under vacuum, time-dependent density functional calculations. For complexes 1–3
affording the desired product 3 (9 mg, 0.009 mmol, isolated calculations were done also in the presence of solvent (toluene,
yield 83%). IR (CH2Cl2) n(CO): 2038 (m), 2018 (s), 1936 (s), used in the photophysical characterizations) described by the
1
1917 (s) cmÀ1. H NMR (THF, 300 K, 400 MHz) dH (ppm) 9.70 conductor-like polarizable continuum model (CPCM). All the
(s, 1H), 9.45 (d, 1H), 8.33 (d, 1H), 7.74 (d, 2H), 7.31 (m, 4H), 7.12 calculations were done with Gaussian 09.27
(m, 6H), 6.91 (d, 2H), À6.70 (s, 1H, hydride). FAB-MS: m/z 955,
Fabrication and characterization of DSSCs
FTO glass substrates (Pilkington, 8 U sqÀ1) were ultrasonically
elemental anal. calcd for C30H19N3O10Re2: C 37.77, H 2.01,
N 4.41. Found: C 37.56, H 2.01, N 4.37.
Methyl ester of pydz-4-COOMe and of complexes 1-COOMe, cleaned sequentially in acetone and in isopropyl alcohol for
2-COOMe and 3-COOMe. A sample of pyridazine-4-carboxylic 10 min and then immersed in a 40 mM aqueous solution of
acid (pydz-4-COOH, 100 mg, 0.805 mmol) was dissolved in MeOH TiCl4 at 70 1C for 30 min. The as treated substrates were rinsed
(2 mL) and was treated with H2SO4 (40 mL, 95% w/w) at room with deionized water and ethanol and dried at room tempera-
temperature. The reaction mixture was heated at reflux tempera- ture. An opaque TiO2 single-layer was screen-printed on clean
ture and was stirred overnight. Then the solution was cooled at FTO glass substrates using a commercial TiO2 paste (18 NR-AO
room temperature and the reaction was quenched by the addi- paste, Dyesol). Over each 4 Â 2 cm2 glass sample, three 8 mm-thick
tion of saturated solution of Na2CO3 until pH 8. The product was TiO2 active areas of 0.5 Â 0.5 cm2 were deposited and electrically
extracted with Et2O and the organic fractions were collected, isolated one from another by laser scribing the FTO. The electrodes
washed with brine, dried with Na2SO4 and evaporated to dryness were then sintered in an oven with a final step at 525 1C for 30 min.
to leave a pale yellow solid (isolated yield 30%). 1H NMR (DMSO, The sintered photoanodes were left in a 40 mM aqueous solution
300 K, 400 MHz) dH (ppm) 9.58 (s, 1H, Hortho), 9.51 (d, 1H, Hortho), of TiCl4 at 70 1C for 30 min. After rinsing with deionized water
8.09 (dd, 1H, Hmeta), 3.94 (s, 3H, CH3). This pyridazine was used and ethanol, the samples were treated at 500 1C for 30 min. For
to synthesize the corresponding derivatives 1-COOMe, 2-COOMe each dye, two 0.3 mM ethanol solutions were prepared, respec-
and 3-COOMe, according to the previous procedures.
tively, with and without adding 3 mM of chenodeoxycholic acid
(CDCA from Sigma Aldrich). After 2 h of immersion in the
dye solution, photoelectrodes were assembled with platinized
Electrochemical measurements
The cyclovoltammetric study of the complexes has been per- counter electrodes, prepared by pyrogenic decomposition at
formed at scan rates typically ranging from 0.02 to 10 V sÀ1 in 420 1C for 15 min of a screen-printed platinum-based paste
HPLC-grade acetonitrile (MeCN) solutions at 2.5 Â 10À4
M
(Pt10 004F-05, Chimet), using a thermoplastic gasket (Surlyn 25,
concentration in each substrate, deaerated by N2 bubbling, Solaronix). For comparison, a reference sample was sensitized
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by a commercial N719 dye (Dynamo AB). Finally, either a commer-
cial electrolyte (Dyesol’s EL-HSE High Stability Electrolyte) or the
EL14 electrolyte, containing smaller amounts of the redox couple
IÀ/I3À, was injected by vacuum back-filling. All devices were
assembled in air and no secondary encapsulants were utilized.
Photovoltaic performance of the cells was measured using a
computer-controlled Keithley 2420 source meter, under a solar
simulator (SolarTest 1200 KHS Class B) at 1000 W mÀ2, AM 1.5.
The incident power was measured using a Skye SKS 1110 sensor;
before measurement, a black 0.7 Â 0.7 cm2 adhesive mask was
applied over each cell to avoid overestimations by light collection
from areas surrounding the active area of the cell. Incident
photon-to-current efficiency (IPCE) measurements were carried
Fig. 1 Isodensity surface plots of some relevant molecular orbitals of
[Re2(m-Cl)2(CO)6(m-pydz-4-COOH)] (1): HOMOÀ3 (left) and LUMO (right).
out using a set-up consisting of a 150 W xenon lamp (Newport correspond to the ‘‘t2g’’ set of the two rhenium atoms in a
Model 70612) coupled with a monochromator (Cornerstone 130) pseudo-octahedral environment (three of them are combi-
and a Keithley 2420 source meter.
nation of non-bonding d orbitals and the remaining three are
Re-(m-Cl) p* orbitals) while the two lowest unoccupied molecu-
lar orbitals are the lowest-lying p* orbitals of the diazine. These
orbitals lie at much lower energy than in the parent compound
containing unsubstituted pyridazine (À0.47 eV, Table 1), in
accordance with the electron-withdrawing nature of the carboxylic
Results and discussion
Computational study
DFT and TD-DFT computations have been previously used to group. In agreement with the nature of the frontier orbitals, the
investigate in detail the frontier orbitals and the electronic HOMO level is poorly affected (À0.07 eV).
transitions of many [Re2(m-X)(m-Y)(CO)6(m-diazine)] complexes.1–4
The orbitals involved in the strongest MLCT electronic
None of them contained a carboxylic group on the bridging diazine transition computed for 1 (see Table 1) are represented in
ligand, therefore the first aim of the present study was to under- Fig. 1. This transition is red-shifted with respect to the corres-
stand the effects caused by the presence of a COOH substituent ponding transition computed for the pyridazine derivative, due
on the diazine. The b position was chosen, since, as previously to the stabilization of the LUMO. In addition, Fig. 1 clearly
reported,1 the presence of substituents in the a position has shows that the LUMO in 1 is delocalized over the carboxylic
detrimental effects on the stability of the complex, due to the substituent, used to anchor the sensitizer to the nanocrystalline
steric hindrance with the equatorial carbonyl ligands.
TiO2, possibly allowing a fast charge-injection process into the
To enlighten the effects of the COOH substituent on the surface. We have then investigated the possibility to further
molecular energy levels, we have investigated the [Re2(m-Cl)2(CO)6- shift to longer wavelengths the absorption band, in order to
(m-pydz-4-COOH)] complex (1) and compared its data with those of increase the light harvesting ability of the dye.
the progenitor [Re2(m-Cl)2(CO)6(m-pydz)] complex.1
Taking into account the possibility to tune the energy-gap
In agreement with the results previously reported for some for these species not only modifying the substituents on the
other members of this family of complexes,1,4b DFT calcula- diazine (i.e. shifting the LUMOs) but also changing the nature
tions on 1 show that the six highest occupied molecular orbitals of the ancillary ligands (i.e. shifting the HOMOs), we have
Table 1 Computed orbital energies (EHOMO and ELUMO), energy gap (DE), most intense MLCT excitation energies (l) and oscillator strengths (f, in
parentheses) for the compounds 1–3 here presented and for a series of related rhenium complexes (each molecule is labeled with its symmetry group)
Compound
EHOMO [eV]
ELUMO [eV]
DE [eV]
l [nm, eV] ( f )
Transitiona
Cs-[Re2Cl2(CO)6(pydz-4-COOH)] (1)
C1-[Re2H(OOCPh)(CO)6(pydz-4-COOH)] (2)
C1-[Re2H(OOC-TPA)(CO)6(pydz-4-COOH)] (3)
À6.91
À6.53
À5.48
À3.99
À3.78
À3.71
2.92
2.75
1.77
428, 2.90 (0.178)
526, 2.36 (0.118)
530, 2.34 (0.079)
344, 3.60 (0.672)b
385, 3.22 (0.136)
387, 3.20 (0.130)
415, 2.98 (0.059)
446, 2.78 (0.091)
411, 3.01 (0.090)
406, 3.06 (0.113)
397, 3.12 (0.096)
386, 3.21 (0.128)
469, 2.64 (0.117)
327, 3.79 (0.473)b
HÀ3 - L
HÀ1 - L
HÀ3 - L
H - L+2
HÀ3 - L
HÀ9 - L
HÀ5 - L
HÀ2 - L
HÀ4 - L
HÀ3 - L
HÀ6 - L
HÀ1 - L+1
HÀ1 - L
H - L
C2v-[Re2Cl2(CO)6(pydz)]
À6.84
À6.31
À6.27
À6.50
À3.52
À3.36
À3.22
À3.19
3.32
2.95
3.05
3.32
C2v-[Re2(OPh)2(CO)6(pydz)]
C2-[Re2(SPh)2(CO)6(pydz)]
C2-[Re2(OOCPh)2(CO)6(pydz)]
Cs-[Re2HCl(CO)6(pydz)]
Cs-[Re2H(OPh)(CO)6(pydz)]
Cs-[Re2H(SPh)(CO)6(pydz)]
Cs-[Re2H(OOCPh)(CO)6(pydz)]
TPA-COOH
À6.69
À6.39
À6.27
À6.44
À5.48
À3.45
À3.38
À3.29
À3.31
À1.04
3.24
3.00
2.97
3.13
4.44
a
A description of the electronic transition in terms of one-electron excitation between the pair of ground-state orbitals mainly involved
b
(H = HOMO, L = LUMO). p–p* transition.
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systematically computed the energies of the frontier molecular for the free ligand, even if their energies are quite different, as a
orbitals and the electronic transition energies for two series consequence of their different charge distribution.
of complexes containing different ancillary ligands and an
unsubstituted pyridazine: [Re2(m-X)2(CO)6(m-pydz)] and [Re2(m-H)-
Synthesis of the designed molecules
(m-X)(CO)6(m-pydz)] (X = Cl, OPh, SPh, OOCPh). It is important to
A new synthetic strategy was designed for obtaining the hydrido-
compute both the energy gap and the strongest MLCT electronic
carboxylato derivatives 2 and 3 (whereas the dichloride complex 1
transition energy because in these species the HOMO - LUMO
was prepared according to a literature procedure, involving the
transition is either symmetry forbidden or shows a very low
high temperature reaction of ReCl(CO)5 with 0.5 equivalents of
oscillator strength. Therefore no clear correlation between the
pyridazine-4-carboxylic acid).1,24
energy gap (as measured for instance from electrochemical data)
The three step procedure shown in Scheme 1 was followed,
and the maximum of the absorption spectra can be found. More-
which is based on the peculiar reactivity of the tetrahedral
over, the ‘‘t2g’’ set of orbitals of the two rhenium atoms (involved in
hydrido-carbonyl cluster [Re4(m3-H)4(CO)12] with bridging donor
the MLCT transition) are not necessarily the six HOMOs for these
ligands.2 Indeed this tetrahedral cluster, which possesses four
species, since the presence of aromatic moieties in the ancillary
electrons less than that required by the effective atomic number
ligands (OPh, SPh and OOCPh) determines the intercalation of
some p orbitals between them. The data reported in Table 1 show
that a more red-shifted absorption can be attained by using the
hydrido derivative containing a m-benzoato bridging ligand.
On this ground, we focused our attention on complex 2,
in which the pyridazine-4-carboxylic acid ligand is combined
with one hydrido and one benzoato ligand, and on the analo-
rule (56 instead of 60 valence electrons, v.e.s),29 easily reacts with
any (even weakly) donor species L.30 By using pyridazine as a
donor ligand, the main reaction products are the tetrametallic
derivatives [Re4(m-H)4(CO)12(m-pydz)2], with a square geometry of
the Re(m-H)Re skeleton and two pyridazine molecules bridging
on opposite edges of the squares, in trans or cis positions with
respect to the plane of the cluster.2 The reaction produces also
gous derivative containing the 4-diphenylamino-benzoate
the dinuclear unsaturated complex [Re2(m-H)2(CO)6(m-pydz)],
anion (compound 3), due to the recognized ability28 of the
which possesses 32 v.e.s, instead of the 34 v.e.s required by
triarylamine moiety to promote charge separation after UV
the effective atomic number rule.2 Such dinuclear species is not
formed in high yields in this reaction. In contrast dinuclear
species of this kind are the only reaction products using
3,6-disubstitued pyridazine ligands, indicating the occurrence
excitation. Fig. 2 shows the electronic density difference maps
for the two most intense electronic transitions computed for 3.
In the MLCT transition it is clearly recognizable the electronic
transfer from the Re2(m-H)(m-X) moiety to the pyridazine ligand
of a [2+2] fragmentation pathway of the parent tetranuclear
(and its carboxylic substituent). Apparently the diphenylamino
cluster, without any spectroscopically recognizable intermediate.
substituent is not directly involved in this transition, however it
This is probably due to steric hindrance, which prevents the
drastically changes the energy of the HOMO (not a rhenium
formation of the square planar species by destabilizing some
key intermediate of the addition reaction. Analogously the
selective formation of a dinuclear product is observed by
reacting [Re4(m3-H)4(CO)12] with 2,5-diphenyl-1,3,4-oxadiazole
(ppd), which contains two aromatic substituents in the a position
of the oxadiazole ring.23 The labile ppd ligand can be easily
replaced by a diazine molecule, and this provides a general
route to [Re2(m-H)2(CO)6(m-diazine)] complexes.
centred orbital anymore). Its presence gives rise to an additional
p–p* transition, that can be described as a charge transfer from
the diphenylamino moiety to the benzoato bridging ligand,
and is very similar to the corresponding transition computed
Therefore, at first the complex [Re2(m-H)2(CO)6(m-ppd)] (4)
was prepared in high yields by reacting [Re4(m3-H)4(CO)12] with
2 equiv. of ppd, at room temperature in dichloromethane
solution.23 Then the mixed hydrido-carboxylato complex [Re2(m-H)-
(m-OOCPh)(CO)6(m-ppd)] (5, Scheme 1) was obtained by reaction 1,
which exploits the reactivity of the hydride ligands of 4 towards
the acidic proton of the carboxylic acid.
[Re2(m-H)2(CO)6(ppd)] + PhCOOH
- [Re2(m-H) (OOCPh)(CO)6(ppd)] + H2
(1)
Finally, substitution of the oxadiazole by the pyridazine-4-
carboxylic acid afforded the desired product 2. It is important
to note that the introduction of the diazine ligand must be
the last step of the synthesis, since both [Re4(m3-H)4(CO)12]
and [Re2(m-H)2(CO)6(m-ppd)] (4) could react with the carboxylic
substituent of the diazine ring, affording undesired byproducts.
Fig. 2 Electronic density difference maps for the two most intense
electronic transitions computed for the rhenium complex [Re2H(OOC-
TPA)(CO)6(pydz-4-COOH)] (3): the MLCT at 530 nm (left) and the p–p* at
344 nm (right). Light blue indicates a decrease in electron density, blue
indicates an increase.
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Scheme 1 Synthetic pathways for the preparation of rhenium complexes 2 and 3. Reaction conditions: (i) CH2Cl2, room temperature, 24 h; (ii) toluene
reflux, 2 h; (iii) THF reflux, 12 h.
The same synthetic procedure has been exploited to carry out Table 2 First reduction and oxidation peak potentials (Ep,c and Ep,a) and
electrochemical (DEe) and spectroscopic (DEs)a energy gaps of the com-
the synthesis of complex 3, containing the 4-diphenylamino-
plexes. Potentials are referred to the Fc+|Fc coupleb in the operating
medium (MeCN, 0.1 M [TBA][PF6]). Scan rate 0.2 V sÀ1
benzoate anion as an ancillary ligand.
Ep,c
Ep,a
[V]
DEe DEs ELUMO EHOMO
[eV] [eV] [eV] [eV]
Electrochemical characterization
Compound [V]
The cyclic voltammetry analysis has been performed on the
derivatives 1-COOMe, 2-COOMe and 3-COOMe, containing the
methyl ester of the pydz-4-COOH ligand, in order to avoid
the reduction of the acidic proton, affording H2 evolution.
The cyclic voltammetry behaviour in acetonitrile, at 298 K, is
reported in Fig. 3 and the peak potentials, together with the
related HOMO and LUMO energy values, are collected in Table 2.
In the cathodic region all the complexes show one monoelectronic
reduction peak at about À0.9 V (vs. Fc+|Fc, see Table 2), which is
centred on the diazine ligand and whose chemical reversibility is
confirmed by the presence of its anodic counterpart. Moreover,
the position of this peak is unaffected by the scan rate,
in agreement with the electrochemical reversibility of this
1-pydz
À1.34 1.31
2.66 3.31 À3.46 À6.11
2.25 2.98 À3.88 À6.14
2.00 2.56 À3.90 À5.91
1-COOMe
2-COOMe
3-COOMe
À0.91 1.34
À0.89 1.11, 1.32
À0.97 0.66,c 1.07, 1.32 1.63 2.56 À3.82 À5.46
a
The spectroscopic (DEs) energy gaps is the energy associated with the
electronic transition determined from the maximum of the 1MLCT
absorption band. Fc+|Fc potential is 0.385 V vs. SCE in acetonitrile
b
c
solution. First oxidation peak potential not located on the metal core.
reductive process. The peak potential is slightly modulated
by the different nature of the ancillary ligand (see Table 2).
Comparing the CV curve of the complexes 1–3 with those of
analogous complexes of this family containing an unsubsti-
tuted pydz ligand, a shift in the positive direction of the
reduction potential can be observed (see for instance 1-pydz
in Table 2),31 consistently with the presence of the electron-
withdrawing substituent on the aromatic ring. These features
are in agreement with the localization of the reduction process
on the diazine ring, as indicated also by the description of the
LUMO provided by the DFT computations.
In all cases, the first reduction peak is followed by another
reversible reduction peak, which is probably centred on the
diazine ligand, as suggested by the invariance of the potential
on varying the ancillary ligands. This agrees with DFT compu-
tations, that indicated that the two lowest LUMOs are both p*
orbitals of the diazine.
In the anodic region significant differences have been
observed for the three complexes. 1-COOMe exhibits an oxidation
potential at 1.34 V (vs. Fc+|Fc), which corresponds to a metal-
centred bi-electronic oxidation and appears to be irreversible.1
Fig. 3 Normalized CV features of the methyl-ester derivatives of the
three dyes on GC electrodes, in MeCN +0.1 M (NBu4)PF6 solution, at
0.2 V sÀ1 with ohmic drop compensation.
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However, it becomes partially reversible at a higher scan rate, as Photophysical characterization
demonstrated by the presence of the return peak.1 This clearly
The UV-Vis absorption spectra of the three dyes in diluted
toluene and EtOH solution are reported in Table 3, while
Fig. 4 shows the spectra in toluene solution. In toluene solution
all the complexes exhibit a broad and featureless absorption
band that covers a large part of the visible spectrum between
350 nm and 550–600 nm. The predicted red-shift in the
absorption maximum (see the Computational study section)
for 2 and 3, with respect to complex 1, is observed. This feature
indicates that this band is ascribable to the metal-to-ligand
dp(Re) - p*(diazine) charge transfer transition (1MLCT). This
attribution is also supported by the weak intensity of these
bands and by the observed solvatochromic behaviour. Indeed a
significant blue-shift of the absorption maximum has been
observed in a more polar solvent as EtOH (see Table 3). This
broad band arises from the convolution of multiple transitions,
as testified by the more or less pronounced shoulders observed
at longer wavelengths. As evidenced by TD-DFT computations,
the electronic transitions responsible for the MLCT absorption
band involve as starting orbitals the metal-centred HOMOÀn
orbitals and, as final orbitals, the two low lying p* orbitals of
the diazine, LUMO and LUMO+1.
points to an electrochemically quasi-reversible electron transfer
step and a subsequent chemical step. The resulting electro-
chemical HOMO–LUMO gap was 2.25 eV and the HOMO and
the LUMO energy levels of 1 are calculated to be À6.14 eV and
À3.88 eV respectively.
Interesting differences appear in the anodic behaviour of the
complexes 2-COOMe and 3-COOMe. As already observed in
the derivatives containing alcoholate or phenolate anions as
ancillary ligands,3 also in these complexes a close sequence of
two monoelectronic oxidation peaks is observed, instead of the
bi-electronic one. These two oxidation processes are clearly
localized on the metal core (as confirmed also by the computa-
tional analysis)3 and strongly indicate that the oxidation pro-
cess is markedly different in the case of the oxygen-bridged
derivatives with respect to the dichloride ones, also when only
one oxygenated ancillary ligand is present. This might be
related to the closer Re(m-X)2Re scaffold and to the harder
nature of the bridging ligands in the case of the derivatives
containing oxygen ligands. This feature could afford a less-
efficient stabilization of the cationic products, with respect to
the softer anions, such as halides.
Complex 3 displays another high energy absorption band, at
350 nm, whose position is independent of the polarity of the
solvent. In agreement with the electrochemical and computa-
tional data, this band is attributed to the p - p* transition
involving the triarylamine moiety.
The first oxidation peak for 2-COOMe is observed at +1.11 V
(vs. Fc+|Fc), and it appears chemically irreversible. It is followed
by a second peak, at 1.32 V, which is reversible in the whole
scan rate range explored. This lower oxidation potential affords
a higher HOMO level, in agreement with the DFT calculations,
and the resulting electrochemical energy gap is reduced to
2.00 eV.
DSSC performances
Photovoltaic measurements have been carried out to evaluate
the potential of the three new rhenium complexes as dyes in
DSSC devices. The main photovoltaic parameter performances
On the other hand, for complex 3-COOMe, beside the two
oxidation peaks centred on the metal core (at 1.07 V and at
1.32 V), another chemically reversible oxidation peak at +0.66 V
is observed. This peak is clearly attributed to the formation of a
radical cation on the triarylamine (TPA) moiety, as indicated by
the comparison with the electrochemical behaviour of the free
4-diphenylamino-benzoic acid (Ep,a = 0.70 V). This confirms the
lack of communication between the pyridazine ring and the
triarylamine moiety, which is verified also in the UV-Vis absorp-
tion data (see the following section).32 In spite of the lack of
electronic communication, the presence of the TPA moiety
affects (even if slightly) the position of the reduction of the
pyridazine ring, which results at more negative potential values
than in 2. The slight destabilization of the LUMO level is also
confirmed by the computational data (see Table 1).
Even so, the introduction of the TPA moiety in 3 affords a
strong reduction of the electrochemical energy gap, which does
not parallel the spectroscopic (absorption) gap, as in the case of 1
and 2. This is consistent with the fact that the electronic absorp-
tion arises from a 1MLCT transition that in 3 does not involve the
HOMO level, which is centred on the TPA moiety. In the case of
dyes 1 and 2, in contrast, the electrochemical and spectroscopical
energy gaps does parallel each other, although the spectroscopical
gaps are always higher than the electrochemical ones, since the
electronic transition responsible for the 1MLCT absorption band is
not a pure HOMO–LUMO transition.
of the solar cells under AM 1.5 G at 1 Sun (1000 W mÀ2
)
illumination are presented in Table 4 and the current–voltage
( J–V) curves are reported in Fig. 5. Two different electrolytes
were used, both based on the iodide/triiodide redox couple. The
first one (HSE, Dyesol) is commercially available, while the
latter one (labeled El14) was developed in-house, reducing the
concentration of iodine (and thus of iodide), which strongly
absorbs below 450 nm, competing with the rhenium complexes
absorption. Actually, an increase in the photogenerated currents
can be observed (see Table 4 and Fig. 5) for all the rhenium-
based dyes when El14 substitutes HSE and it is confirmed by the
incident photon-to-current conversion efficiency (IPCE) plots for
the DSSCs sensitized by 1, 2 and 3, in the presence of the two
different electrolytes shown in Fig. 6.
Table 3 UV-Vis MLCT absorption data of the dyes in solution and on TiO2
film
Compound
lmaxa/nm
ea (104 MÀ1 cmÀ1
)
lmaxb/nm
l
maxc/nm
1
2
3
416
487
487
0.65
0.67
0.43
375
430
425
405
440
443
a
Toluene solution (1 Â 10À4 M). b EtOH solution (5 Â 10À4 M). c TiO2 film.
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Fig. 6 IPCE spectra for DSSC devices sensitized by different dyes: 1 with a
HSE electrolyte (thick red dash-dotted line) and with an El14 electrolyte
(thin red dash-dotted line); 2 (dotted blue lines); 3 (solid green lines); N719
as reference dye (dashed black lines). Two different iodide/triiodide
electrolytes were used, a commercially available one (HSE, Dyesol) and
another developed in-house (labeled El14).
Fig. 4 UV-Vis absorption spectra of the three dyes in toluene solution.
A comparison with excitation energies and oscillation strengths computed
in toluene is reported (vertical lines).
Table 4 Photovoltaic parameters using homemade E114 electrolyte (first
line for each entry) and commercial HSE electrolyte (Dyesol)
from those obtained under similar conditions by cells assembled
using the conventional ruthenium N719 dye. Indeed these dyes
absorb in a narrow spectral range compared to N719 and in
addition, below 450 nm, they compete with iodide/triiodide-based
electrolytes in the absorption of light. However, the maximum of
the absorption spectrum is not the only factor which determines
the overall characteristics of the cell. Indeed, despite the red-shift
observed in the absorption spectrum, mainly due to the raise of
the HOMO level, complex 2 displays the worst performances. This
result is probably due to the particularly low LUMO level (see
Table 2) which, as in the case of complex 1, hampers the electron
injection into the TiO2 conduction band (À3.9 eV).
Compound
JSC (mA cmÀ2
)
VOC (mV)
Fill factor
Z (%)
N719
12
0.78
0.32
0.63a
0.61
0.70a
0.6
3.0
13.6a
2.2
0.73a
0.54
6.3a
0.8
1
2
3
1.8a
1.7
0.53a
0.48
0.6a
0.45
0.38a
0.97
1.0a
1.2a
3.3
0.46a
0.51
0.67a
0.57
0.70a
2.9a
0.51a
a
Photovoltaic performances based on the commercial HSE electrolyte
(Dyesol).
The influence of the ancillary ligand is confirmed by the
results obtained with complex 3. The replacement of the simple
benzoate anion with the carboxylate derivative of the triaryl-
amine passing from 2 to 3 has a double role. The LUMO level
of the complex was slightly raised, then allowing the injection
on the TiO2. Most importantly, the hole transport unit, now
localized on the TPA moiety, is placed far away from TiO2 and
can better interact with the electrolyte. Moreover the higher
steric hindrance of the TPA unit, with respect to the simple
benzoate anion, reduces the recombination of the electrons,
with the redox species, on the surface of TiO2. This insulating
effect is in agreement with the slightly higher VOC value
obtained for complex 3.
Conclusions
Fig. 5 Current–voltage curves for the DSSCs sensitized by the rhenium-
based dyes under 1 Sun illumination, AM 1.5. Two different iodide/triiodide
electrolytes were used, a commercially available one (HSE, Dyesol) and
another developed in-house (labeled El14).
Dirhenium complexes designed as potential sensitizers in
DSSCs have been prepared, characterized and tested in operat-
ing solar cells. Light-to-current conversion has been really
observed, although with a low efficiency. The best results have
The best performing cell, sensitized by complex 3, gives an been obtained for the hydrido-carboxylato complex 3, in which
overall power conversion efficiency of 1% (with short-circuit the introduction of the triarylamine moiety further suppresses
photocurrent density JSC = 2.9 mA cmÀ2, open-circuit photo- the recombination of the injected electron with the oxidized
voltage VOC = 0.51 mV, fill factor ff = 0.70). These values are far state of the complex, thus improving the charge separation on
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4 (a) E. Quartapelle Procopio, M. Mauro, M. Panigati, D. Donghi,
P. Mercandelli, A. Sironi, G. D’Alfonso and L. De Cola, J. Am.
Chem. Soc., 2010, 132, 14397–14399; (b) M. Panigati, M. Mauro,
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TiO2. Moreover the bulky triarylamine moiety also reduces the
back reaction of the injected electron with the electrolyte.
Compared to the standard ruthenium-based N719 dye, the
maximum power conversion efficiency of 1% obtained for 3 is
obviously too low. However, the data here collected have
provided a sufficiently clear picture of the critical factors and
of the lines to follow for improving the performances. The
future molecular design will take advantage from the possibility
to independently tune the ground-state energy levels of the
dirhenium diazine complexes, since their frontier orbitals are
clearly localized on different regions of the molecule.
6 F. Nastasi, F. Puntoriero, M. Natali, M. Mba, M. Maggini,
P. Mussini, M. Panigati and S. Campagna, Photochem.
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So a different choice of the diazine ligand will permit to raise
the LUMO level, to overcome the most serious problem con-
cerning the electron injection into TiO2, which is certainly
represented by the excessive stabilization of the p* orbitals of
the diazine in the complexes here investigated.
On the other hand, wider light harvesting and efficient hole-
transfer towards the periphery of the molecule could be obtained
through the introduction of a more conjugated moiety on the
triarylamine ancillary ligand. Moreover metal oxide photoelec-
trodes other than TiO2, such as SnO2, and different electrolytes,
such as those based on cobalt complexes, could be used. Work
towards these directions is in progress in our laboratory.
However, it must be pointed out that, due to the novelty of the
employed dyes, the present work did not aim to optimize the cell
performances, but rather to understand the relationships between
the molecular structure of the complexes and their spectral and
photovoltaic properties. This result has been attained.
¨
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Clearly the bridging coordination of the hydride contributes
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32 The same behaviour has been observed in the complex
containing, as an ancillary ligand, the carboxylate deriva-
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New J. Chem.
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