Synthesis, photovoltaic performances and TD-DFT modeling of push-pull diacetylide platinum complexes in TiO2 based dye-sensitized solar cells

Article abstract of DOI:10.1039/c4dt00301b

In this joint experimental-theoretical work, we present the synthesis and optical and electrochemical characterization of five new bis-acetylide platinum complex dyes end capped with diphenylpyranylidene moieties, as well as their performances in dye-sensitized solar cells (DSCs). Theoretical calculations relying on Time-Dependent Density Functional Theory (TD-DFT) and a range-separated hybrid show a very good match with experimental data and allow us to quantify the charge-transfer character of each compound. The photoconversion efficiency obtained reaches 4.7% for 8e (see TOC Graphic) with the tri-thiophene segment, which is among the highest efficiencies reported for platinum complexes in DSCs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00301b

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Synthesis, photovoltaic performances and TD-DFT
modeling of push–pull diacetylide platinum
complexes in TiO2 based dye-sensitized
solar cells†
Cite this: DOI: 10.1039/c4dt00301b
Sébastien Gauthier,*a Bertrand Caro,a Françoise Robin-Le Guen,a
Nattamai Bhuvanesh,b John A. Gladysz,b Laurianne Wojcik,c Nicolas Le Poul,c
Aurélien Planchat,d Yann Pellegrin,d Errol Blart,d Denis Jacquemin*d,e and
Fabrice Odobel*d
In this joint experimental–theoretical work, we present the synthesis and optical and electrochemical
characterization of five new bis-acetylide platinum complex dyes end capped with diphenylpyranylidene
moieties, as well as their performances in dye-sensitized solar cells (DSCs). Theoretical calculations
relying on Time-Dependent Density Functional Theory (TD-DFT) and a range-separated hybrid show a
very good match with experimental data and allow us to quantify the charge-transfer character of each
compound. The photoconversion efficiency obtained reaches 4.7% for 8e (see TOC Graphic) with the tri-
thiophene segment, which is among the highest efficiencies reported for platinum complexes in DSCs.
Received 28th January 2014,
Accepted 17th April 2014
DOI: 10.1039/c4dt00301b
www.rsc.org/dalton
platinum acetylide complexes have found widespread appli-
cations as structural building blocks in the areas of supramole-
Introduction
Dye sensitized solar cells (DSCs) are attractive photovoltaic cular self-assembly5 and functional materials science.3h,i,6
devices for low cost electricity production.1 Transition metal Moreover, due to the linear geometry of the alkynyl unit and
coordination complexes, especially polypyridine ruthenium its conjugated character, these platinum acetylide complexes
complexes, have proven to be particularly efficient sensitizers have displayed interesting properties such as non-linear
for this technology.2 Compared to ruthenium complexes, plati- optical (NLO) responses, luminescence, and electrical conduc-
num complexes remain rather unexplored as sensitizers for tivity.5a,e,7 Recently, platinum acetylide complexes have been
DSCs,3 despite their valuable photoredox properties.4 In fact, used as dyes in photoinduced energy and electron transfer
arrays and for hydrogen production.4a,8 In two previous
reports, Tian and co-workers developed a series of platinum
aInstitut des Sciences Chimiques de Rennes, UMR CNRS 6226, IUT de Lannion,
acetylide complexes showing promising photoconversion
rue Edouard Branly, BP 30219, F22302 Lannion Cedex, France.
efficiencies.3h,i
E-mail: sebastien.gauthier@univ-rennes1.fr; Fax: +33 296469354;
We have combined the proaromatic pyranylidene ligand9
Tel: +33 296469344
bDepartment of Chemistry, Texas A&M University, PO Box 30012, College Station,
with bis-acetylide platinum units and systematically varied the
nature and the length of the spacer connected to the cyano
acrylic anchoring group (Chart 1). The pyranylidene which
Texas 77842-3012, USA
cLaboratoire de Chimie, Electrochimie Moléculaires et Chimie Analytique UMR CNRS
6521, Université de Bretagne Occidentale, 6 Avenue Le Gorgeu, 29238 Brest Cedex
acquires a pyrylium aromatic character on charge transfer has
been successfully used in the push–pull NLO-chromophore10
and in organic dyes for solar cells.11 The D-π-M-π-A combi-
nation resulted in dyes exhibiting an effective broadening of
the light absorption spectrum and, consequently, in a higher
03, France
dUniversité LUNAM, Université de Nantes, CNRS, Chimie et Interdisciplinarité:
Synthèse, Analyse, Modélisation (CEISAM), UMR 6230, 2 rue de la Houssinière,
44322 Nantes Cedex 3, France. E-mail: Fabrice.Odobel@univ-nantes.fr;
Fax: +33 251125402; Tel: +33 251125429
eInstitut Universitaire de France, 103, bd Saint-Michel, F-75005 Paris Cedex 05,
photocurrent production and
a higher photoconversion
France. E-mail: Denis.Jacquemin@univ-nantes.fr
†Electronic supplementary information (ESI) available: Synthetic and character-
efficiency (PCE).
ization data of new compounds, general crystallographic data of complex 5, vol-
tammetry cyclic and density difference plots for 8b and 8e. Crystallographic data
have been deposited in the Cambridge database. CCDC 982741. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt00301b
In the field of DSCs, like in many others, joint theoretical–
experimental analyses are particularly useful considering the
complexity of the processes involved in these devices. Indeed,
the electron injection into the semiconductor comes after the
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ating the Pt unit from the anchoring cyanoacetic group. These
complexes were prepared, fully characterized and evaluated in
TiO2-based DSCs.
Experimental
Chemicals and materials
All reactions were conducted under a dry argon atmosphere
using Schlenk techniques, but workups were carried out in air.
THF was dried using a solvent dispensing system (Seca Solvent
System). Hexanes, CH2Cl2, CHCl3, ethyl acetate (3 × ACS
grade), K2CO3 (99% Alfa Aesar), silica gel (Acros or Fluoro-
flash), alumina (Al2O3, neutral, Brockmann I, for chromato-
graphy, 50–200 μm, Acros), copper(I) iodide (99.999%, Aldrich),
diethylamine (99.5%, Aldrich), piperidine (99% Alfa Aesar)
and cyanoacetic acid (99%, Alfa Aesar) were used as received.
The complexes cis-[Pt(P(Tol)3)2Cl2] and tributyl(2,6-diphenyl-
4H-pyran-4-yl)phosphonium tetrafluoroborate (1) were pre-
pared as previously described.14,15 The aldehydes 6a–e were
prepared from commercially available aryl bromide precursors:
4-bromobenzaldehyde, 5-bromo-2-furaldehyde, 5-bromothio-
phene-2-carboxaldehyde, 5-bromo-2,2′-bithiophene-5′-carbox-
aldehyde and 5″-bromo-2,2:5′,2″-terthiophene-5-carboxaldehyde
(5 × Aldrich).16 A CH2Cl2 solution of 5 was layered with pen-
tanes and kept at room temperature under an inert atmos-
phere. After 3 days, colorless column crystals of suitable
size for single crystal X-ray diffraction measurements were
obtained.
Chart 1 Chemical structures of platinum complexes 8a–e investigated
in this work.
electronic excitation of the dye: the light-to-electricity conver-
sion is therefore triggered by the formation of an electronically
excited state. In particular, charge-transfer (CT) excited states
where the absorption of a photon induces a strong intramole-
cular separation between the electron and the hole are particu-
larly beneficial to DSC’s efficiency. Therefore, the optimisation
of the CT properties of the dyes is of particular interest.
However, excited states are extremely short-lived, making their
complete experimental characterization particularly difficult
and often costly. For this reason, the use of theoretical tools
to investigate these electronically excited states is becoming
increasingly popular. In particular, Time-Dependent Density
Functional Theory (TD-DFT), which provides an accurate result
while maintaining reasonable computational time, has
emerged as the most applied theory in the field. Indeed,
numerous recent publications on DSCs have reported results
combining theoretical and experimental data.12 As an example
of the growing interest in TD-DFT simulations, theoretical
approaches have been developed to quantify the CT nature of
the excited states of dipolar dyes.13 One of these theoretical
approaches is also used in this work to provide a CT distance
(expressed in Å), subsequently paving the way for an in-silico
optimisation of the CT properties.
Physical characterization
Elemental analysis was performed at the Centre Régional de
Mesures Physiques de l’Ouest (CRMPO, University of Rennes 1)
using a Microanalyseur Flash EA1112 CHNS/O Thermo Elec-
tron. MALDI spectra were recorded on an Applied Biosystems
Voyager-DE STR spectrometer with DCTB as the matrix at the
Laboratory for Biological Mass Spectrometry, Texas A&M Uni-
versity, USA. NMR spectra were recorded on a Varian NMRS
500 MHz spectrometer or a Bruker Advance NMRS 300 MHz
spectrometer and referenced as follows: 1H NMR, residual
CHCl3 (δ, 7.24 ppm); 13C{1H} NMR, internal CDCl3 (δ,
77.0 ppm); 31P{1H} external H3PO4 (δ, 0.00 ppm). IR spectra
were recorded on a Shimadzu IRAffinity-1 spectrometer with a
Pike MIRacle ATR system (diamond crystal). UV-visible spectra
were recorded on a Shimadzu UV-1800 spectrometer. Emission
spectra were recorded using a PTI QuantaMasterTM 40 fluo-
rescence spectrofluorometer. Thin-layer chromatography (TLC)
was carried out on EMD Silica Gel 60 F254 or EMD aluminum
oxide 60 F254 (neutral) plates that were visualized with
254 nm or 365 nm UV light.
X-ray crystallographic data collection and refinement
Here we present a synthesis route of five new di-acetylide
platinum-based complexes 8a–e comprising a pyranylidene
ligand and functional groups (i.e. phenyl, furan, bi- and tri-
thiophene) of different nature and lengths as linkers separ-
Single crystal X-ray diffraction data were collected using a
Bruker GADDS (Cu-Kα) diffractometer.17 Cell-now18 was used
to determine unit cell from 180 data frames. After a careful
examination, an extended data collection procedure (26 sets)
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was initiated using omega and phi scans. Integrated intensity anharmonicity effects. In the second step, the first forty singlet
information for each reflection was obtained by reduction of lowest-lying excited-states were determined within the vertical
the data frames with APEX2.19 SADABS was employed to PCM-TD-DFT approximation using the same basis set, but
correct the data for absorption effects.20 The structure was augmented with diffuse orbitals.28 The latter calculations were
solved by direct methods (SHELXS) and refined (SHELXL-97, performed with the standard non-equilibrium LR-PCM
weighted least squares refinement on F2) using APEX219 and (Linear-Response PCM) approach.26 For the first strongly
OLEX2, respectively.20,21 Non-hydrogen atoms were refined dipole allowed states, we determined the difference in elec-
with anisotropic thermal parameters. The parameters were tronic density between ground and excited-states with TD-DFT,
refined by weighted least squares refinement on F2 to conver- which allows for chemically sound representations of the
gence.20 The absence of additional symmetry was verified impact of the electronic transitions. A threshold of 0.0004 u.a.
using PLATON,22 which also indicated a void of about 33 Å19 was used to plot these differences. A recently developed
with a residual electron density (1.03 e− Å−3) suggesting the approach for quantifying the charge transfer13 has been
presence of partially occupied solvent, which was assumed to applied in order to further characterize the dyes (see the
be water (10% occupancy). The crystallographic data and the Introduction).
result of refinements of 5 (see below for structure) are sum-
marized in ESI.†
Results and discussion
Cyclic voltammetry
Synthesis of the sensitizers
The electrochemical studies were performed in a glovebox
The synthesis of the complexes requires the starting material,
4-[(4-ethynyl-phenyl)-phenyl-methylene]-2,6-diphenyl-4H-pyran
4, whose preparation was accomplished in two steps as shown
in Scheme 1. The diphenylpyranylidene unit 3 was produced
by a Wittig reaction of tributyl(2,6-diphenyl-4H-pyran-4-yl)-
phosphonium tetrafluoroborate 1 15 with 4-(2-trimethylsilyl-
ethynyl)benzophenone 2 29 in the presence of n-BuLi in good
yield.30 The trimethylsilyl compound 3 was converted to the
corresponding terminal alkyne 4 quantitatively by desilylation
under basic conditions.31
(Jacomex) (O2 < 1 ppm, H2O < 1 ppm) with a home-made
3-electrode cell (WE: Pt, RE: Ag wire, CE: Pt). Ferrocene stan-
dard was added at the end of each experiment. The redox
potential of the Fc+/Fc couple in CH2Cl2/NBu4PF6 was
measured experimentally with reference to the standard
calomel electrode (SCE): E0(Fc+/Fc) = 0.47 V vs. SCE, and re-
calibrated vs. NHE assuming that E0(SCE) = 0.24 V vs. NHE.
The potential of the cell was controlled by an AUTOLAB
PGSTAT 100 (Metrohm) potentiostat monitored by the NOVA©
software (Metrohm). Dichloromethane was freshly distilled
from CaH2 and kept under Ar in the glovebox. The supporting
salt NBu4PF6 was synthesized from NBu4OH (Fluka) and HPF6
(Aldrich). It was then purified, dried under vacuum for
48 hours at 100 °C, and then kept under N2 in the glovebox.
The synthetic approach to platinum complexes 8a–e, shown
in Scheme 2, was based upon the well documented substi-
tution of platinum chloride complexes by terminal acetyle-
nes.3i,32 The precursor complex 5 was prepared by reaction of 4
with cis-[Pt(P(Tol)3)2Cl2] in the presence of a catalytic amount
of copper(I) iodide and diethylamine at 60 °C. In this reaction,
an excess of cis-[Pt(P(Tol)3)2Cl2]14 was necessary to minimize
the formation of the undesirable oligomeric byproducts. Sub-
sequent coupling with the corresponding aldehydes 6a–e16
under a conventional cross-coupling condition resulted in
compounds 7a–e. The final products 8a–e were then obtained
in good yields (80–95%) by Knoevenagel reactions of the
corresponding precursors with cyanoacetic acid in the pres-
ence of piperidine.
Theoretical
All quantum mechanical simulations have been performed
with the latest version of the Gaussian09 program.23 The
optical spectra of the structures represented in Chart 1 have
been analyzed. However, since the tolyl groups bonded to the
phosphorus atoms do not play an important role in the optical
features of individual dyes, methyl groups were used in the
calculation in order to reduce the computational burden. We
followed a methodology similar to the one successfully used
previously for similar DSC dyes.13 Therefore, we selected the
PBE0 global hybrid exchange-correlation functional24 for deter-
mining structural parameters and the CAM-B3LYP25 range-sep-
arated hybrid functional to compute the optical properties
(TD-DFT part). This choice was motivated by the charge-trans-
fer nature of several excited-states. First, geometry optimi-
zations and frequency calculations in dichloromethane were
performed using the Polarizable Continuum Model (PCM)
solvent model,26 and the LanL2DZ basis with additional polari-
zation functions.27 This allowed the nature of the structures
(true minima) to be ascertained and the infrared spectra to be
obtained. The theoretical (harmonic) vibrational frequencies
The new compounds were characterized by IR, NMR (1H,
31P, and 13C), high-resolution mass spectrometry and satis-
reported below were scaled by a factor of 0.96 to account for Scheme 1 Preparation of compound 4.
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phosphines are coordinated to the platinum center in a trans
arrangement. The bond lengths and angles about platinum
are unexceptional.33 The C2uC1–Pt–Cl chain is almost linear,
with angles at C2uC1–Pt and C1–Pt–Cl of 174.2(3)° and
178.33(9)°, respectively. The Pt–C1 distance of 1.952(3) Å and
the C1uC2 bond length of 1.209(4) Å lie in the range observed
for other platinum coordinated acetylide ligands.34
The IR spectra of the alkynyl complexes 8a–e contain a
νCuC band at ca. 2112 cm−1 assigned to the metal-bonded
alkynyl group. The νCuN (around 2160 cm−1), νCvO (around
1694 cm−1) and νCvC (around 1575 cm−1) bands are character-
1
istic of the cyanoacrylic acid function. The H NMR spectrum
of the ligand 4 shows characteristic signals such as the vinylic
H resonance of the methylenepyran core which appears as two
4
doublets (δ = 6.72 and 6.69 ppm; JH,H = 2.0 Hz), a multiplet
signal for the disubstituted phenyl ring (δ = 7.67–7.37 ppm)
and a singlet for the alkyne proton (δ = 3.14 ppm). In the
complex 5, due to the inductive effect of the platinum coordi-
nation, the hydrogens of the disubstituted phenyl (δ = 6.77
and 6.04 ppm) and the vinylic protons (δ = 6.61 and 6.60 ppm)
are upfield shifted. A similar shift effect has been observed for
the series 7a–e and 8a–e. This series of complexes display
essentially identical 1H NMR signals for the tri(p-tolyl)phos-
phines, the pyranylidene ligand, the π-conjugated spacer and
the aromatic protons. The 1H and 13C NMR spectra of the com-
plexes 7a–e show expected signals such as a singlet at ca. δ =
9.80 ppm assigned to the aldehyde proton and a peak of the
Scheme 2 Synthesis route to the dyes 8a–e.
1
aldehyde carbon at ca. δ = 180 ppm. The H NMR spectra of
the cyanoacetic acid complexes 8a–e exhibit a characteristic
singlet at ca. δ = 8.10 ppm assigned to the proton attached to
the β-carbon atom of the cyanoacetic acid group. Finally, the
31P NMR spectra of all complexes 8a–e exhibit a sharp singlet,
diagnostic of trans geometries of the phosphine ligands
factory microanalyses (see the ESI†). These data fully sup-
ported the proposed structures. Single crystals of complex 5
were grown by slow diffusion of pentane into a concentrated
solution of 5 in dichloromethane under an inert atmosphere.
The X-ray structure was determined, and the resulting thermal
ellipsoid plot is given in Fig. 1. Complex 5 was crystallized in
1
around the platinum (for example δ = 17.4 ppm, JPt–P = 2600
Hz, for 8a). The approximately 3 ppm shift of this singlet upon
going from 5 to 8a can be attributed to the electron π-conju-
gated bridges.
ˉ
the P1 space group. The platinum atom is four-coordinated in
a slightly distorted square planar geometry. As expected, the
molecular structure shows that the platinum center is attached
to a chloride, a pyranylidene ligand and that two tri(p-tolyl)-
Electronic absorption and emission spectra
The UV-Vis absorption and emission spectra of dyes 8a–e
recorded in a dilute dichloromethane solution are shown in
Fig. 2 whereas the spectroscopic data are collected in Table 1.
The dye 8a exhibits only one major band at 415 nm, but in
contrast 8b–d dyes display two major prominent bands on
their UV-visible spectra, λambasx(2) in the 300–400 nm range and
λambasx(1) in the 400–600 nm range. As shown below, the higher-
energy absorption bands [λambasx(2)] correlate with intra-ligand
charge transfer (ILCT) transitions (π–π*) centered on the pyran
part, whereas the lower-energy absorption band [λambasx(1)]
corresponds to ILCT transitions (π–π*) more localized on the
cyanoacetic part (see TD-DFT calculations below for more
details). The latter band corresponds to the light harvesting of
the complexes in the visible region. The red-shift of the
maximum absorption of 8d and 8e, as compared with that of
8c, can be explained by the enhanced electron delocalization
over the oligothiophene spacer of increasing length. Moreover,
Fig. 1 Perspective view of 5 with thermal ellipsoids drawn at the 50%
probability level; solvent molecules and hydrogen atoms are omitted for
clarity.
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Fig. 2 UV-Vis absorption spectra of dyes 8a–e in dichloromethane
solution (3 × 10−5 M) at room temperature.
Fig. 3 Emission spectra recorded in CH2Cl2 solution (5 × 10−6 M) at
room temperature for dyes 8a–e.
Table 1 Absorption and fluorescence properties of the dyes 8a–e
recorded in dichloromethane at room temperature
Table 2 Voltammetry data for compounds 4, 5, 8a–e and 9 (1 mM) in
CH2Cl2/NBu4PF6 0.1 M (E/V vs. Fc+/Fca, v = 0.1 V s−1); platinum working
electrode
λambasx(2)/nm
λambasx(1)/nm
Dye
(ε × 104 M−1 cm−1
)
(ε × 104 M−1 cm−1
)
λemmaix/nm
Dye
Epa(1)
E0(1)
Epa(2)
E0(2)
Epc(3)
Epc(4)
8a
8b
8c
8d
8e
—
415 (5.0)
463 (4.7)
469 (4.4)
504 (3.7)
514 (4.3)
539
535
545
645
736
342 (1.7)
342 (1.5)
363 (2.2)
390 (3.9)
8a
8b
8c
8d
8e
4
0.18
0.18
0.17
0.19
0.18
0.30
0.30
0.18
0.14
0.14
0.13
0.14
0.14
0.26
0.41
0.43
0.42
0.42
0.45
0.62
0.38
0.39
0.37
0.38
0.41
−1.37
−1.39
−1.25
−1.35
−1.43
−0.85
−0.86
−0.86
−0.84
−0.85
−0.83
−0.78
−0.85
the phenyl spacer in complex 8a spans a shorter π-conjugation
length than the furan or the thiophene linker as evidenced by
the blue shift of the ILCT band. In addition, these dye mole-
cules exhibit molar absorption coefficients about three times
greater than that of the well-known N719 ruthenium sensitizer
(N719: cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′ dicarboxylato)
ruthenium(II), di-tetrabutylammonium) which displays a molar
absorption coefficient of 1.47 × 104 M−1 cm−1 at 535 nm,35 while
8d and 8e absorption coefficients are 3.7 × 104 M−1 cm−1 at
504 nm and 4.37 × 104 M−1 cm−1 at 514 nm, respectively.
All the complexes 8a–e are emissive at room temperature
(Fig. 3), a feature that allows the determination of the zero–
zero excited state energy (E00) with the wavelength at the inter-
section of the electronic absorption and emission spectra (see
below). We note that two dyes, 8a and 8c, seem to present a
second emission band at longer wavelengths, and this is
clearly unexpected. Nevertheless, as we use only the emission
spectra to determine the zero–zero energies, this outcome is
not relevant for our purposes and was not further investigated.
9
5
0.14
V
0.41
0.37
a E0(Fc+/Fc)
=
0.47
vs. SCE in CH2Cl2/NBu4PF6 (measured
experimentally) and E0(SCE) = 0.24 V vs. NHE.
plexes displayed an irreversible reduction peak at ca. Epc(3) =
−1.4 V (Fig. 4 and Table 2). When scanning up to 1.0 V, an
additional peak at 0.80 V (see Fig. S1B, in the ESI,† curve b,
peak *), and two reduction peaks on the second cycle at
Epc(6) = −0.65 and Epc(4) = −0.80 V (Fig. 4 and Table 2) were
detected. The former reduction peak was not observed for all
complexes whereas the latter systematically appeared. In order
to identify the redox processes, the analogous compound 4-[(4-
ethynyl-benzylidene)-2,6-diphenyl-4H-pyran 9 was investigated
by CV.37 It underwent irreversible oxidation at Epa(1) = 0.30 V
and displayed irreversible reduction at Epc(4) = −0.78 V on the
backscan (Fig. S1A,† curve b, and Table 2). This reduction
peak can be assigned to the reduction of the bis-pyrylium
species, resulting from a C–C coupling reaction after mono-
electronic oxidation of the pyran, as previously reported.38
Compound 4, which contains an additional phenyl ring on the
ethynylene bridge, behaved differently than 9. For this com-
pound, the first oxidation process was no longer irreversible at
the timescale of the experiment (ipa/ipc = 0.6 at 0.1 V s−1) and a
second oxidation process was detected at Epa(2). For the Pt-
complex 5, cyclic voltammetry showed two reversible systems
at E0(1) = 0.14 V and E0(2) = 0.41 V (Table 2) and one irrevers-
Electrochemical measurements
The redox behavior of each of the platinum complexes 8a–e
(Chart 1) was examined by cyclic voltammetry (CV) in CH2Cl2/
NBu4PF6 0.1 M on a platinum working electrode (see Table 2
for the electrochemical data). In oxidation, two reversible
systems at ca. E0(1) = 0.14 V and E0(2) = 0.38 V vs. Fc+/Fc were
systematically detected (see Table 2).36 In addition, all com-
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rationalize the exact influence of the linker on the reduction
electron transfer process.
In conclusion, the voltammetry studies provided not only
information about the redox properties of the complexes 8a–e
to be obtained, but also the determination of injection (ΔGinj
)
and regeneration (ΔGreg) Gibbs free energies (Table 3). Com-
plexes 8a–e can be mono-electronically oxidized at Epa(1) in
this electrolyte. The resulting pyrylium species are relatively
stable as shown by exhaustive electrolysis. This property is
valuable for DSC applications since electrochemical stability
of the oxidized species is a key feature for achieving a long
lifespan of the cell. For all complexes, very negative values of
ΔGinj were calculated, implying that the injection yields for
8a–e were likely to be very similar and close to unity (Table 3).
Likewise, the regeneration step is endowed with an identical
Gibbs free energy, regardless of the complex. All these thermo-
dynamic considerations indicate that complexes 8a–e could
potentially be efficient sensitizers in TiO2 based DSCs.
Fig. 4 Cyclic voltammograms of 8c (1 mM) (v = 0.1 V s−1, 2 cycles, E/V
vs. Fc+/Fc) at a Pt working electrode in CH2Cl2/NBu4PF6 (0.1 M).
TD-DFT calculations
First principle calculations were performed to further charac-
terize the nature of the excited-states in 8a–e. A preliminary
step to optical simulations is to ascertain the quality of the
structures. The connecting unit and the donor group of 8a–e
and 5 being the same, we have first used 5 for this assessment.
For compound 5, the computed Pt–C1 (C1uC2) distance is
1.926 (1.236) Å, which deviates from experiments by 0.026
(0.023) Å. This lies in the typical error range of DFT calculations.
Secondly, we compared the theoretical IR frequencies to the
experimental IR data. In compound 8a, DFT gives an intense
band at 2090 cm−1 for the stretching of the carbon–carbon
triple bonds which corresponds to the band at 2112 cm−1
observed experimentally. Likewise, the theoretical νCuN band
calculated by DFT at 2231 cm−1 reasonably matches the experi-
mental value found at 2160 cm−1 (see above). Lastly, the νCvO
and νCvC stretching, measured at ca. 1694 cm−1 and
ible cathodic peak at Epc(4) = −0.85 V. Unlike complexes 8a–e,
neither an oxidation peak at 0.8 V nor a reduction peak at
Epc(3) was detected for this complex (Fig. S1B,† curve a).
Exhaustive electrolysis was performed on compounds 4 and 5
at a constant potential value between the two oxidation
processes (1) and (2). For these experiments, coulometric
measurements indicated that the first oxidation wave corres-
ponded to a one-electron process. Interestingly, Rotating-Disk
Electrode Voltammetry (RDEV) showed the presence of pyry-
lium cations in solution for the complex 5 after full electrolysis
(Fig. S1C†), while those cations were not detected in com-
pounds 4 (inset of Fig. S1C†) and 9. From these results, we can
conclude the following:
(i) Both reversible processes at E0(1) and E0(2) can be
ascribed to the successive monoelectronic oxidations of the
pyrylidene moiety. They are only slightly affected by the vari-
ation of the linker (phenyl, furanyl, thiophenyl, etc.) within the
electron-withdrawing part of the platinum complexes. The
potential difference value ΔE0(1–2) = E0(2) − E0(1) is low
(240–250 mV), indicating that the oxidation of the pyrylium
radical cation requires a relatively small quantity of energy.
(ii) Epa(1) and Epa(2) values are significantly lower for com-
plexes 8a–e and 5 than for compound 4, suggesting a stabiliz-
ation of the radical pyrylium cation and dicationic species in
the presence of platinum.
(iii) The phenyl moiety present in the ethylene bridge of
the pyran for compounds 8a–e, 4 and 5 has a strong influence
on the C–C coupling reaction. This might be due to both
steric (hindrance of the phenyl and metallic moiety) and elec-
tronic effects, since the stabilization of the radical cation
through delocalization of the charge may deactivate the C–C
coupling.
1575 cm−1, were computed at 1739 cm−1 and 1560 cm−1
,
respectively. The theoretical simulation of the electronic
absorption spectra is displayed in Fig. 5. The computed and
the experimental absorption spectra (Fig. 2) are in very good
agreement with each other, and this holds for both the posi-
tions and the relative intensities of all absorption peaks,
Table 3 ΔGinj and ΔGreg values for the complexes 8a–e
Eox (V vs.
Dye
Fc+/Fc)
E00 (eV)
ΔGinj (eV)
ΔGrega (eV)
ΔGregb (eV)
8a
8b
8c
8d
8e
0.14
0.13
0.14
0.14
0.14
2.66
2.51
2.47
2.14
2.02
−1.37
−1.23
−1.18
−0.99
−0.87
−0.49
−0.48
−0.49
−0.49
−0.49
−0.04
−0.03
−0.04
−0.04
−0.04
a ECB,TiO = −1.15 V vs. Fc+/Fc; E0(I3−/I−) = −0.35 V vs. Fc+/Fc. b E0(I2
/
•−
(iv) The Epc(3) irreversible peak can be ascribed to the
reduction of the Pt-cyano-acrylic moiety of the complexes 8a–e.
However, the broad shape of the peaks does not enable us to
2
I−) = 0.1 V vs. Fc+/Fc. E00 calculated from the wavelength (λinter) at the
intersection of normalized emission and absorption spectra, with the
equation E00 (eV) = 1240/λinter
.
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Fig. 5 Computed absorption spectra obtained from convolution with a
0.40 eV Gaussian of the TD-DFT (vertical) stick spectrum.
except for the intensity of the second absorption band of 8e,
which is slightly underestimated by the calculations. The com-
puted maximum absorption wavelengths are gathered in
Table 4. The order of these wavelengths 8a < 8b < 8c < 8d < 8e
is in full agreement with the experimental data (Table 1),
whereas the mean absolute deviation of the λambasx(1) is as
small as 12 nm, a trifling deviation for TD-DFT calculations on
large dyes, thus confirming the accuracy of the selected
protocol.39
The density difference plots representing the electronic
changes between the ground and excited states are shown in
Fig. 6. As expected, the cyanoacetic moiety acts as an electron
acceptor (mostly in red), which is favorable for charge injection
as it pulls the electronic density close to the TiO2 surface.
Interestingly, the excited state is highly delocalized. The pyran
donor only plays a major role in the first transition of 8a and
in the second transition of 8e. For the first transition in the
other dyes, the changes are mainly localized to the thiophene
rings and anchoring groups. It is also noteworthy that all CT
distances are ca. 5 Å (Table 4). While this indicates that the CT
strength is not relevant to discriminate among the dyes in the
present case, a 5 Å CT should be considered as particularly
large within the selected model.13a–c In other words, the
nature of the excited states of these dyes is favorable for DSCs.
For 8e, the second strongly dipole-allowed excited-state that
Fig. 6 Density difference plots for 8a, 8c and 8e. For 8e two plots
corresponding to the first (top) and second (bottom) bands are dis-
played. The blue (red) regions indicate a decrease (increase) of the
density upon photoabsorption.
corresponds to an experimental peak at ca. 390 nm is localized
to the pyran side and the ethynyl segment. However, the
computed CT distance in that case is trifling (1.09 Å), and it is
unlikely that this band significantly contributes to energy con-
version process. It can then be inferred that the CT character
is therefore not the determining factor controlling the photo-
voltaic efficiency of these series of dyes. However, note that 8d
and 8e present two specific features: absorption at a wave-
length closer to the optimum for light harvesting and larger
dipole moments (both at the ground and excited states), both
characteristics being helpful to bend the conduction band of
the semi-conductor and hence to inject charges.40
Photovoltaic properties in TiO2 DSCs
The platinum complexes 8a–e along with the reference bench-
mark sensitizer N719 were tested in TiO2 based solar cells with
an iodide/triiodide based electrolyte (for composition see the
Experimental part) and their performances were evaluated
under AM 1.5 calibrated artificial sunlight. The photovoltaic
characteristics (Voc, Jsc, ff and PCE) of the solar cells are dis-
played in Table 5 and the photoaction spectra are shown in
Fig. 7.
The photoconversion efficiencies (PCEs) steadily increase in
the following order 8e > 8d > 8c > 8b > 8a ranging from 1.70%
(for 8a) to 4.25% (for 8e). Since the photopotentials and fill
factors stay similar throughout the entire series, this constant
rise in the PCE is essentially due to an improved short circuit
current, increasing from 4.89 mA cm−2 (8a) to 12.54 mA cm−2
Table 4 Computed wavelengths (oscillator strength between brackets),
CT distance (Å), CT charge (e) and dipole moments (D) of the ground
and excited states of the five dyes
λambasx(1)/nm
( f in a.u.)
Dye
d
CT/Å
q
CT/e
μ
GS/D
μ
ES/D
8a
8b
8c
8d
8e
407 (2.6)
442 (2.1)
452 (2.1)
494 (2.0)
516 (2.3)
5.2
4.7
4.4
4.7
5.1
0.6
0.6
0.6
0.6
0.6
7.4
8.7
9.6
12.2
10.2
22.6
22.5
22.5
25.8
25.2
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the IPCE value around 400 nm is not as high as that around
510 nm probably because the charge shift of the excited-state
is not optimal for electron injection (taking the place of the
pyran moiety).
Table 5 Photovoltaic performances for complexes 8a–e in DSCs
recorded under stimulated AM 1.5 irradiation (100 mW cm−2
)
Dye
Voc (mV)
Jsc (mA cm−2
)
FF (%)
PCE (%)
8aa
8ab
8ba
8bb
8ca
505
553
504
578
496
563
513
562
496
554
721
4.89
4.46
6.45
6.38
6.89
6.84
10.0
10.15
12.54
12.01
15.23
68.7
74.2
72.8
72.4
72.2
72.3
68.5
70.9
68.3
70.4
69.4
1.70
1.83
2.36
2.67
2.47
2.78
3.52
4.04
4.25
4.69
7.60
The values of the photopotentials were rather low
(ca. 500 mV); in order to increase these values, tert-butylpyri-
dine 0.1 M was added into the electrolyte.41 An increase of
50–60 mV was recorded for all devices, with a very moderate
loss in Jsc, implying an overall significant rise of the PCE
(Table 5). The simultaneous effects of conduction band
bending and surface defects passivation by tert-butylpyridine
are likely responsible for this observation. Addition of greater
amounts of tert-butylpyridine (0.5 M) led to an important
decrease of the short circuit current (ca. 30%), likely because
of a more sluggish electron injection process.
To sum up, the worst PCE was obtained for 8a while the
best was obtained for 8e, featuring respectively the narrowest
and broadest coverage of the visible spectrum among the
whole series. Noticeably, Voc values remained rather low for all
complexes; the peak value reaches 578 mV for 8b; by compari-
son benchmark N719 exhibited more than 700 mV. This could
explain a lower band bending induced by the permanent
dipole moment of the chemisorbed dye, or – more likely – due
to a faster charge recombination. Moreover, since the oxi-
dation of iodide into triiodide is a multielectronic process, the
use of the redox potential of the E(I2−/I−) couple over E(I3−/I−)
is considered a better approximation to estimate the regener-
ation of the photo-oxidized dye.42 In this case (Table 3), low
regeneration driving forces were calculated and kinetics for
this step could be slower than geminate charge recombination,
thus entailing low values for the Voc.
8cb
8da
8db
8ea
8eb
N719c
a Without tert-butylpyridine in the electrolyte. b In the presence of
0.1 M tert-butylpyridine in the electrolyte. c In the presence of 0.5 M
tert-butylpyridine in the electrolyte. For details see experimental
details in ESI.
Fig. 7 Photoaction spectra (IPCE as a function of the incident wave-
length) of the solar cells sensitized with dyes 8a–e and N719.
Conclusions and outlook
(8e) (Table 4). As mentioned above, the reason for such differ-
ences cannot be assigned to thermodynamics (see Electro-
chemical measurements). However, UV-Vis absorption spectra
show a steady red-shift of the main absorption peaks upon
increasing the conjugation of the spacer, from 8a to 8e. This
feature is observed both in solution and on TiO2 transparent
electrodes with onset absorptions ranging from 470 nm (8a) to
610 nm (8e). A red-shifted absorption of the photoelectrode is
always considered a significant advantage since it allows for a
better spectral matching with incident solar light and a con-
comitant increase of the photocurrent density. This logically
translates into a constantly broadening LHE (light harvesting
efficiencies) from 8a to 8e, and it is obviously illustrated by the
IPCE, increasing from 53% at 440 nm to 71% at 500 nm, and
spreading until 550 nm to 700 nm, for 8a and 8e respectively
(Fig. 7). Interestingly, the incident photon to current efficiency
(IPCE) of 8d and 8e is as high as that of the N719 reference,
meaning that the injection efficiency of the latter is quite opti-
mized. On the other hand, as noted in the computational
study, the second band of 8e around 400 nm is not as effective
as the first one around 510 nm to produce electricity. Indeed,
Five new platinum complexes (8a–e) were prepared and charac-
terized in TiO2 based DSCs. These five structures present a
common pyran unit but differ by the linker separating the
platinum from the cyanoacetic group. Voltammetry studies
showed that, unlike their photoconversion efficiency, the oxi-
dation potential of the compounds was not significantly
changed by the nature or the size of the linker. In addition,
exhaustive electrolysis indicated that the radical pyrilium
cations for 8a–e were relatively stable due to steric and elec-
tronic effects. Compound 8e, which contains a trithiophene
segment, gives a PCE of 4.7% in a dye-sensitized solar cell.
The theoretical calculations provided us with simulations
of the experimental data and with insights into the nature of
the excited states of the dyes, and showed that all five com-
pounds present large but similar CT characteristics. In other
words, the differences in the experimentally obtained PCE
were not related to different CT efficiencies, but rather to
differences in dipole moments and LHE.
The photovoltaic performances of the dye 8e are relatively
high for such platinum complexes and they are essentially
limited by the LHE. Experiments are currently underway to
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modify the structure of 8e in order to enhance the LHE by red-
shifting the maximum absorbance wavelength and conse-
quently improving the photovoltaic performances.
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Acknowledgements
We thank the US National Science Foundation (CHE-1153085),
the European Research Council (ERC) and the Région des Pays
de la Loire (Marches – 278845) for support. This research used
resources of (1) the GENCI-CINES/IDRIS (c2014085117); (2)
CCIPL (Centre de Calcul Intensif des Pays de Loire); and (3) the
Troy cluster.
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J. Phys. Chem. Lett., 2012, 3, 3581–3584; (b) G. Boschloo and
A. Hagfeldt, Acc. Chem. Res., 2009, 42, 1819–1826.
Dalton Trans.
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