Efficient light harvesters based on the 10-(1,3-dithiol-2-ylidene) anthracene core

Article abstract of DOI:10.1021/ol401841u

Three new push-pull chromophores based on the 10-(1,3-dithiol-2-ylidene) anthracene core were synthesized and fully characterized. The new chromophores display broad absorption spectra, nearly covering the whole visible region, with high extinction coeffi

Full text of DOI:10.1021/ol401841u

ORGANIC
LETTERS
2013
Vol. 15, No. 16
4166–4169
Efficient Light Harvesters Based on the
10-(1,3-Dithiol-2-ylidene)anthracene Core
†
Pierre-Antoine Bouit, Lourdes Infantes, Joaquın Calbo, Rafael Viruela,
§
´
Enrique Ortı,* Juan Luis Delgado,* and Nazario Martın*
,
,†
,†,‡
´
´
IMDEA-Nanociencia, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain,
´
Instituto de Quımica-Fısica “Rocasolano”, CSIC, 28006 Madrid, Spain, Instituto de
Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Spain, and Departamento
´
´
de Quımica Organica, Facultad de Ciencias Quımicas, Universidad Complutense de
Madrid, 28040 Madrid, Spain
ꢀ
´
juanluis.delgado@imdea.org; enrique.orti@uv.es; nazmar@quim.ucm.es
Received July 1, 2013
ABSTRACT
Three new pushꢀpull chromophores based on the 10-(1,3-dithiol-2-ylidene)anthracene core were synthesized and fully characterized. The new
chromophores display broad absorption spectra, nearly covering the whole visible region, with high extinction coefficients. Electrochemistry and
theoretical calculations allowed the understanding of these singular electronic properties. The molecular structures were unambiguously
confirmed by X-ray diffraction.
Solar energy currently represents one of the most realis-
tic alternatives to the use of fossil fuels. Photovoltaic
devices based on organic compounds (OPV) have experi-
enced tremendous development in the past years,1 and
recently efficiencies around 8ꢀ10% have been reported.2
One of the crucial points to understanding this improve-
ment is the careful election of suitable organic materials
able to fulfill some energetic and electronic requirements.
Among these requisites, the ability of the material to
harvest sunlight in a broad range of the visible and NIR
spectrum is essential in order to obtain a suitable photo-
response. In this regard, organic chromophores such as
pushꢀpull systems, displaying outstanding absorption
properties, are promising materials to obtain highly effi-
cient OPV3 and hybrid cells.4
† Imdea Nanociencia.
In this communication, we report the synthesis and the
optical and electrochemical properties of new pushꢀpull
§ CSIC.
Universidad de Valencia.
‡ Universidad Complutense de Madrid.
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´
´
r
10.1021/ol401841u
Published on Web 07/30/2013
2013 American Chemical Society

molecules based on the 10-(1,3-dithiol-2-ylidene)-
anthracene core (see Scheme S1 in the Supporting Infor-
mation (SI)), absorbing in a broad region of the visible
spectrum. The chemical structures of the new pushꢀpull
materials have been unambiguously confirmed by single-
crystal X-ray diffraction, and their electrochemical and
optical properties have been characterized with the help
of density functional theory (DFT) calculations. Recently,
we have described the preparation of 10-(1,3-dithiol-2-
ylidene)anthracene derivatives for their application in
dye-sensitized solar cells.5 Molecular engineering of
this appealing core allows the design of materials with
improved light-harvesting properties. The new dyes
synthesized here bear the same electron-donor unit, the
10-(1,3-dithiol-2-ylidene)anthracene group, but a different
conjugated π-bridge, 3,4-ethylenedioxythiophene (EDOT)
for 6 or phenyl for 4 and 5, as well as two kinds of electro-
accepting units, an ester group for 4 and a dicyanovinylene
group for 5 and 6 (Figure 1).
organic solvents. The structures of all the new compounds
and the new sensitizers were confirmed by 1H, 13C NMR,
and high resolution mass spectrometry.
Slow evaporation of chloroform solutions of 2, 5, and 6
allowed the obtention of single crystals suitable for its
study by X-ray diffraction (Figures 2 aswell asS10and S11
in the SI).7 The compounds adopt the typical butterfly- or
saddle-like shape observed for exTTF (2-[9-(1,3-dithiol-2-
ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole) derivatives.8
The existence of several CꢀH π and π π stacking
3 3 3
3 3 3
3 3 3
interactions plus other weaker CꢀH NtC and
CꢀH S hydrogen bonds led to the formation of supra-
3 3 3
molecular networks of compounds 2, 5, and 6.
The synthesis of the key synthon 1, featuring two
terminal alkyne groups (Scheme S1), was carried out in
four steps in good yield.6 The conjugated bridges were then
introduced by the Sonogashira cross-coupling reaction
using PdCl2(PPh3)2 and CuI as catalysts. The introduction
of the accepting units was achieved by 2-fold Knoevenagel
condensation of aldehydes 2 and 3 with malononitrile,
to afford the new dyes 5 and 6 in good to moderate yields
(75% and 50%, respectively; see the SI). Dye 4, with the
two decyl chains, displays excellent solubility in the main
Figure 2. (a) X-ray crystal structure of 6. (b) Unit cell.
The molecular geometries of the three pushꢀpull systems
were optimized using DFT calculations at the B3LYP/
6-31G** level (see the SI for computational details). The
decyl chains in 4 were replaced by a simple methyl group to
simplify the calculation. As obtained from X-ray analysis,
theoretical calculations predict concave saddle-like struc-
tures, in which the central ring of the anthracene unit folds
up in a boat conformation and the dithiole rings are tilted
down. The planes defined by the external benzene rings of
the anthracene unit are predicted to form an angle of 40.5°
for 5 and 38.6° for 6 slightly overestimating the angles
obtained by X-ray diffraction (36.2° and 34.5°, respectively)
due to the packing forces present in the crystal that tend to
reduce the folding. The dihedral angles of 35.5° computed
for the dithiole tilting in 5 and 6 nicely fit the X-ray values of
34.2° (5) and 34.1° (6). The same considerations apply for
the tilting of the acceptor moiety (see Table S1 in the SI).
The acceptor arms remain mostly planar in all dyes thus
favoring the π-electron communication along the acceptor
moiety. This planarity is even more pronounced in the
crystal due to the efficient πꢀπ coplanar interactions in
the packing (see Figure 2b and Table S1).
Figure 1. Pushꢀpull systems 4, 5, and 6.
Figure 3 shows the atomic orbital (AO) composition
of the highest-occupied (HOMOꢀ2 to HOMO) and
(4) (a) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J. H.; Fantacci,
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Chen, Q.; Teuscher, J.; Di Censo, D.; Humphry-Baker, R.; Moser, J. E.;
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Chem. Soc. 2010, 132, 5164–5169.
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S.; De Angelis, F.; Di Censo, D.; Nazeeruddin, M. K.; Gratzel, M.
(7) CCDC 900353 (2), CCDC 890337 (5), and CCDC 890785 (6)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
(8) Batsanov, A. S.; Bryce, M. R.; Coffin, M. A.; Green, A.; Hester,
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Org. Lett., Vol. 15, No. 16, 2013
4167

(>2.07 V), 5 (1.60 V), and 6 (1.51 V), which is in perfect
agreement with theoretical predictions and with the trend
observed from optical data (see below).
Table 1. Electrochemical and Optical Data for 4ꢀ6
cv
E°ox
E°red
Eg
(V)b
λmax
(nm)c
ε
compd
(V)a
(V)a
(L molꢀ1 cmꢀ1
)
3
3
4
5
6
0.27
0.29
0.25
<ꢀ1.80
ꢀ1.31
ꢀ1.26
>2.07
1.60
491
536
590
19 000
14 000
19 000
1.51
a Measured by CV (CH3CN, 0.1 M Bu4ClO4, v = 100 mV sꢀ1, V vs
3
Ag/AgNO3). b Electrochemical gap determined as Eox ꢀ Ered. c Measured
in DCM, 10ꢀ5 mol Lꢀ1
.
3
Figure 3. Electron-density contours (0.03 e bohrꢀ3) and orbital
energies calculated for the frontier molecular orbitals of 6 at the
B3LYP/6-31G** level. H and L denote HOMO and LUMO,
respectively.
The absorption properties of compounds 4ꢀ6 were
measured in dichloromethane solution (Figure 4a). 4 dis-
plays two main absorption bands in the UV and visible
range (λ1 = 364 nm; λ2 = 491 nm). Both transitions
are red-shifted, first when a better acceptor is inserted in
the molecule (5, λ1 = 366 nm; λ2 = 536 nm), then with the
lowest-unoccupied (LUMO to LUMOþ2) molecular
orbitals of 6, as a representative example, The HOMO
(ꢀ5.10 eV) and HOMOꢀ1 (ꢀ5.65 eV) are localized on the
electron-donor 10-(1,3-dithiol-2-ylidene)anthracene unit,
hereafter named hemi-exTTF, whereas the HOMOꢀ2
(ꢀ6.01 eV) spreads over the electron-acceptor moiety with
no hemi-exTTF participation. The LUMO (ꢀ3.06 eV) and
LUMOþ1 (ꢀ2.81 eV) are located on the electron-acceptor
arms, and the LUMOþ2 (ꢀ1.85 eV) comprises both the
hemi-exTTF moiety and the acceptor fragment. Identical
topologies are predicted for the frontier molecular orbitals
of 4 and 5 (see Figure S13). A pronounced stabilization of
the LUMO is predicted in passing from 4 (ꢀ2.46 eV) to 5
(ꢀ3.01 eV) and 6 (ꢀ3.06 eV) due to the higher electro-
negative character of the dicyanovinylene group compared
to the ester unit. The presence of the electron-donor EDOT
fragment in 6 slightly increases the energy of the HOMO
(ꢀ5.10 eV) compared to 5 (ꢀ5.17 eV). Theoretical calcula-
tions therefore predict small HOMOꢀLUMO energy gaps
of ꢀ2.64, ꢀ2.16, and ꢀ2.04 eV for 4, 5, and 6, respectively,
and suggest the presence of low-energy charge-transfer
(CT) absorption bands in the electronic spectrum.
The redox properties of 4ꢀ6 were examined by cyclic
voltammetry (CV) (Table 1). The anodic region of dye 4 is
characterized by one reversible wave (E°ox = 0.27 V)
attributed to the hemi-exTTF unit, whereas no reduction
is observed in the experimental conditions. The presence of
the stronger electron-accepting dicyanovinylene groups in
5 shifts the reduction potential to ꢀ1.31 V, whereas the
oxidation potential is only weakly affected (E°ox = 0.29 V).
This trend nicely correlates with the slight stabilization
of the HOMO (0.07 eV) and the significant stabilization
of the LUMO (0.55 eV) predicted by theoretical calulations
in passing from 4 to 5 (Figure S13). The presence of the
auxiliary electron-donor EDOT groups in6 slightly reduces
Figure 4. (a) Absorption spectra of 4 (red), 5 (blue), and 6 (green)
recorded in dichloromethane at room temperature. (b) Theo-
retical simulation of the UVꢀvis absorption spectra for
pushꢀpull compounds 4ꢀ6. Vertical lines indicate the vertical
excitation energies and oscillator strengths calculated for the
electronic transitions to the different singlet excited states Sn.
States S1 to S5 are explicitly denoted for compound 6.
the oxidation (E°ox = 0.25 V) and reduction (E°red
=
ꢀ1.26 V) potentials. CV measurements therefore obtain
a decrease of the electrochemical gap along the series 4
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Org. Lett., Vol. 15, No. 16, 2013

presence of the EDOT units (6, λ1 = 443nm; λ2 = 590 nm).
As expected, a decrease of the optical band gap is observed
when the pushꢀpull effect in the compound is reinforced.
To investigate the nature of the electronic transitions
that give rise to the absorption bands observed in the
electronic spectra, the lowest-energy singlet excited states
(Sn) were calculated for 4ꢀ6 using the time-dependent
DFT (TD-DFT) approach. TD-DFT calculations predict
that the lowest-energy absorption band observed for 5 and
6 above 500 nm is due to electronic transitions to the first
two excited singlets S1 and S2 (see Table S2 and Figure 4b).
These states originate from the HOMOfLUMO and
HOMOfLUMOþ1 monoexcitations, respectively, and
imply an electron density transfer from the hemi-exTTF
moiety, where the HOMO resides, to the acceptor moiety,
where the LUMO and LUMOþ1 are mainly located
(Figure 3). In compound 4, S2 stands higher in energy
(2.89 eV) due to the higher energy of the LUMOþ1
(Figure S13) and does not contribute to the lowest-energy
band at 491 nm (Figure 4b). Calculations therefore con-
firm the CT nature of the lowest-energy absorption band
of 4ꢀ6. The moderately high intensities observed for this
band are in agreement with the oscillator strengths (f)
around 0.4ꢀ0.5 calculated for S1 and S2 (Table S2) and
are due to the significant overlap between the HOMO and
the LUMO/LUMOþ1.
In summary, a new family of anthracene-based chromo-
phores has been synthesized and fully characterized. The
new dyes capture light in a broad range of the solar
spectrum, compound 6 being the best light-harvesting
chromophore, absorbing in the UV and visible spectrum
(from 300 to 750 nm). The outstanding absorption proper-
ties stem from the efficient electronic communication
between the donor and acceptor moieties, giving rise to
very intense charge-transfer bands. The remarkable elec-
tronic and optical properties exhibited by the new com-
pounds make them promising materials for their further
incorporation in molecular solar cells.9,10
Acknowledgment. This work has been supported by the
MINECO of Spain (CTQ 2011-27934, CTQ2011-24652,
CTQ2012-31914, and Consolider-Ingenio CSD 2007-
00010 on Molecular Nanoscience), the Comunidad
de Madrid (MADRISOLAR-2, S2009/PPQ-1533), the
Generalitat Valenciana (Prometeo/2012/053), and the
EU project (FUNMOLS FP7-212942-1). J.L.D. thanks
ꢀ
the MINECO for a Ramon y Cajal Fellowship. P.-A.B.
thanks IMDEA-Nanociencia for a postdoctoral research
grant. J.C. acknowledges MECD (Spanish Ministry of
Education, Culture, and Sport) for a FPU grant. L.I.
thanks Comunidad de Madrid (Spain) for support
(BIPEDD-2: S2010-BMD-2457).
The intense absorption band observed at higher energies
mainly results from the HOMOꢀ2fLUMO excitation
(state S6 for 5 and S5 for 6, Table S2 and Figure 4b), which
mainly implies the electron-acceptor part of the molecule
(Figure 3). This electronic transition is predicted to have
a high intensity (f ≈ 1.00) and shifts to lower energies in
passing from 5 to 6 due to the EDOT groups that desta-
bilize the HOMOꢀ2 from ꢀ6.46 (5) to ꢀ6.01 eV (6). For
compound 4, the transition is calculated to show a lower
intensity (f = 0.33) in good agreement with the experi-
mental spectra (Figure 4a). The HOMOfLUMOþ2
excitation involving the hemi-exTTF moiety and the
CT HOMOꢀ1fLUMO and HOMOꢀ1fLUMOþ1 ex-
citations also contribute to this intense absorption band
(Table S2).
Supporting Information Available. Synthetic proce-
dure, chemical characterization, X-ray structures, and
computational details (geometrical data and TD-DFT
results). This material is available free of charge via the
Internet at http://pubs.acs.org.
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Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.; Roncali, J.
Chem. Commun. 2009, 1673–1675.
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P.-A.; Rauh, D.; Neugebauer, S.; Delgado, J. L.; Di Piazza, E.; Rigaut,
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The authors declare no competing financial interest.
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