Near-IR photoresponse of ruthenium dipyrrinate terpyridine sensitizers in the dye-sensitized solar cells

Article abstract of DOI:10.1021/ic5006538

Coordination of bidentate 5-pentafluorophenyldipyrrinate (pfpdp) or 5-(2-thienyl)dipyrrinate (2-tdp) to a Ru^II center bearing 2,2:6,2″-terpyridine-4,4,4″-tricarboxylate (tctpy) and a NCS ^- ligand results in strongly light-absorbing complexes [Ru(tctpy)(L)(NCS)] (L = pfpdp or 2-tdp). Anchored to a mesoporous TiO ₂ electrode, these complexes afford a photoaction spectral response at wavelengths of up to 950 nm, one of the most red-shifted values reported to date for molecular dyes in the dye-sensitized solar cell (DSSC).

Full text of DOI:10.1021/ic5006538

Communication
pubs.acs.org/IC
Near-IR Photoresponse of Ruthenium Dipyrrinate Terpyridine
Sensitizers in the Dye-Sensitized Solar Cells
Guocan Li,† Aswani Yella,‡ Douglas G. Brown,§ Serge I. Gorelsky,# Mohammad K. Nazeeruddin,‡
Michael Gratzel,*,‡ Curtis P. Berlinguette,*,§ and Michael Shatruk*,†
̈
†Department of Chemistry & Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States
‡Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole
Polytechnique Federale de Lausanne, Lausanne, Switzerland
§Departments of Chemistry and Chemical & Biological Engineering, The University of British Columbia, Vancouver, British
Columbia V6T 1Z1, Canada
#Centre for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
S
* Supporting Information
corresponding Ru(II) complexes are characterized by very high
ABSTRACT: Coordination of bidentate 5-pentafluoro-
phenyldipyrrinate (pfpdp) or 5-(2-thienyl)dipyrrinate (2-
tdp) to a RuII center bearing 2,2′:6′,2″-terpyridine-4,4′,4″-
tricarboxylate (tctpy) and a NCS− ligand results in
strongly light-absorbing complexes [Ru(tctpy)(L)(NCS)]
(L = pfpdp or 2-tdp). Anchored to a mesoporous TiO2
electrode, these complexes afford a photoaction spectral
response at wavelengths of up to 950 nm, one of the most
red-shifted values reported to date for molecular dyes in
the dye-sensitized solar cell (DSSC).
extinction coefficients. We previously demonstrated that
appropriate consideration of the electronic properties of
ruthenium dipyrrinato complexes containing polypyridyl anchor-
ing ligands can render a dye motif capable of sensitizing TiO2, but
absorption for these dyes was limited to λ < 750 nm. Herein we
report two new dyes, [Ru(tctpy)(pfpdp)(NCS)] (1) and
[Ru(tctpy)(2-tdp)(NCS)] (2) (Figure 1), that differ in their
ye-sensitized solar cells are a promising alternative to
Dconventional (micro)crystalline silicon (c-Si) solar cells
because of their relative ease of fabrication, environmental
considerations, and high tolerance toimpurities.1 Nevertheless, c-
Si solar cells still dominate the global photovoltaic market
because, among other factors, the small band gap of silicon (1.1
eV) enables effective light-harvesting out to 1100 nm, while
conventional DSSCs typically suffer from the poor collection of
photons at λ > 800 nm. This scenario provides the impetus to
develop sensitizers capable of more effectively capturing infrared
(IR) light to increase the device efficiency.
While an efficient single-junction DSSC should convert all
photons with λ < 920 nm (the threshold derived from the
Shockley−Queisser equation2), very few dyes have been reported
to provide long-wavelength light absorption.3 The most prolific
example is the so-called “black dye”, (Bu4N)[Ru(tctpy)(NCS)3]
(N749),4 which long stood as the most efficient DSSC sensitizer
owing to the relatively broader spectral absorption of the
complex. Nonetheless, the low-energy tail of the absorption band
of N749 suffers from a relatively low extinction coefficient, and
thus moving the dominant metal-to-ligand charge-transfer
(MLCT) band further into the IR region stands as a reasonable
strategy for a higher current collection at λ < 920 nm. This goal
has been realized through partial or complete substitution of the
NCS− ligands of N749 with chelating anionic cyclometalating
and pyrazolate ligands among others.5,6
Figure 1. Molecular representations of 1 and 2.
meso substituents on the dipyrrinates while maintaining charge
distributions in the ground and excited states that are conducive
to charge collection in the DSSC. Complexes 1 and 2 harvest light
out to 950 nm, which is one of the lowest-energy photon-
collection thresholds achieved with ruthenium sensitizers to date.
The synthesis of 1 involves reaction of the deprotonated
dipyrrin with [(p-cymene)RuCl2]2 to furnish [(p-cymene)Ru-
(pfpdp)Cl] (Scheme S1 in the Supporting Information). A
subsequent halide abstraction with AgNO3 in MeCN affords
[Ru(pfpdp)(MeCN)4]NO3, which was successively treated with
trimethyl-2,2′:6′,2″-terpyridine-4,4′,4″-tricarboxylate
(Me3tctpy) and KNCS to produce [(Me3tctpy)Ru(pfpdp)-
(NCS)] (1a). A facile hydrolysis step enables quantitative
conversion of 1a to 1. The NCS− ligand can bind to ruthenium
through either the nitrogen or sulfur atom, but the N-bound
species is typically the thermodynamically favorable isomer for
Ru(II) complexes. The NMR spectrum of 1a indicated exclusive
formation of the N-bound isomer (Figure S1 in the Supporting
Information, SI), but ∼16% of the S-bound isomer was present in
Dipyrrinate ligands have hitherto received far less attention in
Received: March 21, 2014
this context, which is somewhat surprising considering that their
© XXXX American Chemical Society
A
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Communication
the case of 1, which was confirmed by the IR spectrum of 1
(Figure S2 in the SI). Complex 2 was synthesized following a
similar protocol, also yielding the N- and S-bound linkage
isomers in a 10:1 ratio.
The absorption spectra of 1 and 2 in MeCN are congruent
(Figure 2). The negligible influence of the meso substituent on
Figure 3. Selected frontier molecular orbitals of 1. Isosurface contour
values are 0.05 au. Hydrogen atoms are omitted for clarity. Color
scheme: Ru, green; S, yellow; O, pink; N, blue; C, gray; F, maroon.
significantly by the meso substituent, and thus their character is
similar in both 1 and 2; namely, LUMO, LUMO+1, and LUMO
+3 are localized onthe tctpy π* orbitals and LUMO+2 is localized
on the dipyrrinato π* orbital. In both cases, the tctpy-based
LUMO is extended onto the carboxylic groups and well separated
in energy from the higher-lying molecular orbitals, which
provides the well-defined lowest excited state with strong
electronic coupling to the TiO2 acceptor states.
The simulated optical absorption spectrum of 1 reproduces the
main features of the experimental spectrum (Figure 2 and Table
S2 in the SI). The absorption band at 470 nm corresponds to the
pfpdp π → π* and interligand π → π* (pfpdp → tctpy)
transitions with a nominal MLCT contribution. The lower-
energy band at ∼600 nm is mainly of MLCT (Ru → tctpy)
character. The shoulder at 700−800 nm is due to the HOMO →
LUMO MLCT excitation process. The absorption bands of 2 are
similar in nature; the complete excitation assignments are
provided in Figure S6 and Table S3 in the SI.
Figure 2. Experimental absorption spectra of 1 (red) and 2 (green) and
the calculated spectrum of 1 (dotted gray) in MeCN at room
temperature. The oscillator strengths of calculated transitions are
shown as gray vertical lines with the major transitions highlighted in blue.
the optical properties is rationalized by the limited conjugation
between the substituent and the dipyrrinato moiety because of
the lack of coplanarity. The strong absorption band at 470 nm (ε
∼ 40000 M−1 cm−1) considerably improves the absorptivity of 1
and 2 in the visible region compared to that of N749.4 A
moderate band at ∼600 nm is attributed to the MLCT transition
from the ruthenium ion to the tctpy ligand. The MLCT band has
a distinctive shoulder at ∼750 nm, a feature not observed for
N749. The low-energy extension of this shoulder provides
substantial absorption of 1 and 2 in the IR region, with an onset at
∼950 nm (Figure S3 in the SI). Emission was not detected for 1 at
room temperature, consistent with our observations for other
Ru(II) dipyrrinates,8 but a prominent emission signal centered at
870 nm was observed at 77 K in an EtOH/MeOH glass [4:1 (v/
v); Figure S4 in the SI].
To assign the absorption bands to specific electronic
transitions, density functional theory (DFT)/time-dependent
DFT calculations were performed on 1 and 2 using the implicit
solvent model for MeCN (Table S1 in the SI). The highest
occupied molecular orbital (HOMO) for both complexes
contains contributions from the Ru 4d orbital (46%) and the
NCS− π orbital (31−33%), which is consistent with other NCS-
containing Ru sensitizers.9 This metal−ligand orbital mixing
ostensibly benefits dye regeneration because the oxidized dye will
have a substantial amount of hole density on the NCS− ligand to
better interact with the redox mediator in the electrolyte.10 The
HOMO−1 of 1 is also a mixed Ru-NCS orbital, while the
HOMO−2 is mainly located on the dipyrrinate (Figure 3). The
order of these two orbitals is reversed in the case of 2 because the
electron-donating nature of the meso substituent destabilizes the
dipyrrinato-based π orbital. The energies of the dipyrrinato-based
orbitals in 1 and 2 are, nonetheless, similar, highlighting the
limited conjugation between the meso substituent and the
dipyrrinato fragment (Figure S5 in the SI). The lowest
unoccupied molecular orbitals (LUMOs) are not affected
Cyclic voltammograms were recorded on 1 and 2 in a MeCN
solution using (Bu4N)PF6 as the supporting electrolyte at a scan
rate of 0.1 V s−1. The complexes exhibit a reversible oxidation
wave centered at 0.93 and 0.88 V vs NHE, respectively (Figure S7
in the SI). Analysis of the DFT-calculated spin-density
distributions for 1+ and 2+ (Figure S8 in the SI) indicates that
the electron is removed from the HOMO of the neutral
complexes upon oxidation. The DFT-optimized structures of 1+
and 2+ feature the Ru atom with a spin density of 0.62. The NCS−
ligand contributes 0.24 and 0.21 to the spin density in 1+ and 2+,
respectively. The oxidation of 2 is cathodically shifted by 0.05 V
relative to 1 (corroborated by a 0.06 V shift modeled by DFT)
because of the more electron-donating 2-thienyl substituent in 2.
+
The excited-state reduction potential, ES /S*, was estimated
+
+
+
from ES /S* = ES /S − E0−0, where ES /S is the Ru(III) reduction
potential and E0−0 was determined by the intersection of the
absorption and normalized emission spectra (Figure S4 in the
+
SI). This procedure led to ES /S* = −0.56 and −0.61 V for 1 and 2,
respectively, and confirmed that the lowest excited state is
positioned above the TiO2 conduction band edge (−0.5 V vs
+
NHE). The ES /S values for 1 and 2 are also lower than the
reduction potential for the I2•−/I− couple (+0.8 V),11 and thus
both the ground and excited states of 1 and 2 are positioned
B
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Communication
appropriately for use in a conventional DSSC-containing TiO2
electrode and iodide-based electrolytes.
ASSOCIATED CONTENT
* Supporting Information
Synthesis, additional spectra and voltammograms, and results of
DFT calculations on 1 and 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
The dyes were evaluated in the DSSC under AM 1.5 conditions
using cells constructed with an electrode containing a 12-μm
active layer of TiO2 and a 3-μm scattering overlayer of TiO2. The
electrolyte contained 0.7 M LiI and 0.06 M I2 in MeCN/
valeronitrile (85:15, v/v) with 0.1 M guanidinium thiocyanate
and 0.3 M 1,3-dimethylimidazolium iodide as additives. The
incident photon-to-current efficiency (IPCE) curve (Figure 4)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: michael.graetzel@epfl.ch.
*E-mail: cberling@chem.ubc.ca.
*E-mail: shatruk@chem.fsu.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was supported, in part, by the Council on Research
and Creativity at Florida State University.
■
REFERENCES
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In summary, two novel heteroleptic ruthenium(II) complexes
bearing a dipyrrinato ligand that differs in the meso substituent
have afforded substantially improved optical absorptivities in the
visible and IR regions. The IPCE curves for 1 and 2 show
sensitization out to 950 nm, a rare observation for ruthenium
dyes, which points to the possibility of developing efficient near-
IR sensitizers relying on ruthenium dipyrrinates.
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