β-(Ethynylbenzoic acid)-substituted push-pull porphyrins: DSSC dyes prepared by a direct palladium-catalyzed alkynylation reaction

Article abstract of DOI:10.1039/c3cc44847a

The palladium-catalyzed oxidative alkynylation of β-borylated porphyrins allows for concise preparation of push-pull structured ethynylbenzoic acid porphyrin derivatives. The resulting β-singly- and doubly-substituted porphyrin dyes are regarded as isomeric derivatives of the corresponding meso-substituted reference systems, and were found to give rise to nearly equal power conversion efficiencies when analyzed in DSSCs.

Full text of DOI:10.1039/c3cc44847a

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b-(Ethynylbenzoic acid)-substituted push–pull porphyrins:
DSSC dyes prepared by a direct palladium-catalyzed
alkynylation reaction†
Cite this: Chem. Commun., 2013,
49, 9164
Received 28th June 2013,
Masatoshi Ishida,ab Deasub Hwang,ac Young Bean Koo,a Jooyoung Sung,a
Dong Young Kim,c Jonathan L. Sessler*ad and Dongho Kim*a
Accepted 12th August 2013
DOI: 10.1039/c3cc44847a
www.rsc.org/chemcomm
The palladium-catalyzed oxidative alkynylation of b-borylated
porphyrins allows for concise preparation of push–pull structured
ethynylbenzoic acid porphyrin derivatives. The resulting b-singly-
and doubly-substituted porphyrin dyes are regarded as isomeric
derivatives of the corresponding meso-substituted reference systems,
and were found to give rise to nearly equal power conversion
efficiencies when analyzed in DSSCs.
Porphyrins are an important family of organic chromophores
that has attracted attention as photosensitizers in TiO2-based
dye-sensitized solar cells (DSSCs).1 Relatively high photo-
voltaic efficiencies have been demonstrated in DSSCs based on
‘‘push(donor)–pull(acceptor)’’ structured porphyrin dyes.2 A key
feature of successful push–pull porphyrin sensitizers is the
Chart 1 Molecular structures of YDn series and our porphyrins, ZnEPn used in
this study; n = 1 and 2.
unidirectional ‘‘D–p–A’’ arrangement of the electron donating of acceptor subunits. In previous work, we reported the pre-
substituent, typically a diarylamine, and the electron-acceptor, paration of doubly b-vinylogous-linked porphyrins where the
an ethynylbenzoic acid anchoring group, across the 5- and alkenyl spacer was thought to play a role in improving the
15-(meso) positions of the porphyrin core. A breakthrough in overall efficiency by mediating effective charge separation.4
the world of DSSCs was attained recently, wherein a power con- In the present study, we report a new set of porphyrin-based
version efficiency (Z) of 11.9% was obtained using porphyrin- photosensitizers prepared through a direct palladium-catalyzed
sensitized cells of this general structure (viz., YD2-o-C8; Chart 1) alkynylation reaction of b-borylated porphyrins with various
and a cobalt-based redox electrolyte system.3 The outstanding terminal alkyne derivatives. It was expected that the presence
results obtained using YD2-o-C8 provide an incentive to develop of the rigid CRC triple-bond and the resulting enhancement of
synthetic methods that would allow a range of ‘‘D–p–A2’’ porphyrin electronic communication through the b-positions would facilitate
sensitizers to be prepared such that the effect of structural effective electron injection into the conduction band of the TiO2.
modification on functional efficiency can be probed in detail. As detailed below, the DSSC efficiencies of b-(4-ethynylbenzoic
Of particular interest within the cadre of this generalized approach acid)-substituted derivatives (singly- and doubly-substituted;
would be elucidating the effect of b-pyrrolic vs. meso-substitution ZnEP1 and ZnEP2, respectively) proved to be nearly equal to the
corresponding meso-linked derivative (i.e., YD15) as measured
a Department of Chemistry, Yonsei University, Seoul, 120-749, Korea.
using a sensitized TiO2 photoelectrode (Chart 1).
The iridium(I) catalyst-assisted borylated porphyrins (1a: mono-
boryl and 1b: di-boryl porphyrins) are potentially useful precursors
E-mail: dongho@yonsei.ac.kr
b Education Center for Global Leaders in Molecular Systems for Devices,
Kyushu University, Fukuoka 819-0395, Japan
c Optoelectronic Materials Lab, IMCM, Korea Institute of Science and Technology,
for direct alkynyl coupling reactions.6 We thus sought to effect the
cross-coupling between the porphyrin borylester derivative (1a)
and the readily available terminal acetylene derivative 2b to access
the corresponding zinc porphyrin ZnEPn (n = 1 or 2). To date, there
have been few studies involving the oxidative alkynylation reac-
tions of analogous borylated precursors, although it was found
that Pd/Ag7 or Cu catalysts8 were not effective when tested with
Seoul 130-650, Korea
d Department of Chemistry & Biochemistry, University of Texas at Austin,
Texas 78712-1224, USA. E-mail: sessler@cm.utexas.edu
† Electronic supplementary information (ESI) available: Synthetic experimental
procedure, characterization data, a proposed reaction mechanism, spectroscopic
data, calculations, and results obtained from test DSSC experiments. See DOI:
10.1039/c3cc44847a
c
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The time-resolved fluorescence decay profiles of ZnEP1 and
ZnEP2 recorded in CH2Cl2 could fit well as a single exponential
decay with a time constant of 1.46 and 1.35 ns, respectively
(Fig. S3, ESI†). It is worth noting that the observed excited state
lifetimes are longer than the electron injection rate (meaning
deactivation is unlikely to compete with the electron injection
into TiO2).12
The electrochemical properties of ZnEP1 and ZnEP2 were
investigated by cyclic voltammetry in THF containing 0.1 M
tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the
supporting electrolyte (Table S1, ESI†). The oxidation potentials
for ZnEP1 and ZnEP2 were determined to be approximately
1.0 V (vs. NHE), which is comparable to those of the other
diarylamine-substituted porphyrins.5,13 The injection potential
Scheme 1 Palladium-catalyzed alkynylation of 1 with various terminal alkyne
derivatives 2.
large p-conjugated porphyrins (cf. ESI†). After extensive screening
of reaction conditions, we found that mild palladium-catalyzed
aerobic oxidative coupling of b-borylated porphyrins, 1 with various
terminal alkynes, 2 allows access to the desired the cross-linked
products (3) in satisfactory yields (Scheme 1 and Fig. S1, ESI†).
Electron deficient alkynes (e.g., 2e and 2f) proved to be less reactive
compared to electron rich ones (e.g., 2c and 2d); however, a variety
of terminal alkynes were tolerated. The procedure was sufficiently
robust that the doubly-(methyl ethynylbenzoate)-substituted
derivative (3g) could be isolated in 15% yield starting from
the diborylated derivative (1b).9 A proposed mechanism for the
coupling reaction is provided in the ESI† (cf. Scheme S1).10
The isolated methyl ester derivatives (3f and 3g) could be easily
hydrolyzed under basic conditions. The final products, ZnEP1 and
ZnEP2, were characterized by 1H- and 13C-NMR spectroscopy and
high resolution FAB mass spectrometry (ESI†).
(Eox*) was evaluated by subtracting the excitation energy (E0–0
)
from the oxidation potential (Eox) of the porphyrins, giving
values of À1.14 V and À1.08 V (vs. NHE) for ZnEP1 and ZnEP2,
respectively. Based on the optical and electrochemical para-
meters, the driving force for electron injection (DGinj) from the
porphyrin excited state to the conduction band of the TiO2
(À0.5 V vs. NHE) and the regeneration (DGreg) of the porphyrin
radical cation by the Co(II/III) redox couple (+0.56 V vs. NHE) for
DSSCs built from ZnEP1 and ZnEP2 are expected to be thermo-
dynamically favoured (DG o 0).
Density functional theory (DFT) calculations were carried
out at the B3LYP/6-31G(d) level to gain insight into the charge
transfer character of the dyes (Fig. S4 and Table S3, ESI†).
The overall electron distribution pattern of the two ZnEPns of
this study is essentially similar to that of YD1.5 Despite the
lower molecular symmetry of ZnEP1 compared to that of YD1,
the characteristic orbital densities and phases are retained.
Specifically, ZnEP1 and ZnEP2 were predicted to be dipolar in
nature, which is expected to facilitate efficient and rapid electron
injection into the CB of TiO2, as seen in other push–pull
porphyrin derivatives.2 The time-dependent DFT analyses of
ZnEP1 and ZnEP2 were also consistent with the experimental
absorption spectra (Fig. S5, ESI†).
The light-harvesting ability of ZnEP1 and ZnEP2 was confirmed
by steady-state absorption spectroscopy in CH2Cl2 (Fig. 1a). These
two ZnEPns exhibited Soret bands and Q-bands in 400–500 and
550–700 nm spectral ranges, respectively, which were broadened
and red-shifted as the number of b-linkages increased (lmax
=
422 nm; e = 1.59 Â 105 MÀ1 cmÀ1 for ZnEP1 and lmax = 439 nm;
e = 1.31 Â 105 MÀ1 cmÀ1 for ZnEP2).11 The absorption spectra of
ZnEPn are also characterized by a shoulder at 450–500 nm,
which is not present in the spectrum of the meso-linked
analogue YD1.5 The spectral features are thought to reflect
the electronic interaction between the porphyrin macrocyclic
core and the b-ethynylaryl substituents resulting in an effective
extension in the overall p-conjugation.
To clarify the effect of the number (1 vs. 2) and orientation
(b- vs. meso-) of the linkage, DSSCs were fabricated using ZnEPns
as photosensitizers and mesoporous anatase TiO2 spheres
(diameter B 600 nm) prepared by an electrostatic spray method.14
The photovoltaic properties of these two porphyrins were then
In accord with what was seen in the absorption spectral
studies, the fluorescence maxima of ZnEP1 and ZnEP2 also shift
to lower energy as the degree of substitution increases (Fig. 1b).11
The zero–zero excitation energies (E0–0) are estimated to be 1.91
and 1.85 eV for ZnEP1 and ZnEP2, respectively (Table S1, ESI†).
investigated under AM 1.5 irradiation conditions (100 mW cmÀ2
)
using a Co(bpy)3-based electrolyte.15 The photocurrent density–
voltage ( J–V) curves of the sandwich type cells sensitized with
ZnEP1 and ZnEP2 in the presence of 2 equiv. of chenodeoxy-
cholic acid (CDCA) as a co-adsorbent allowed the short circuit
photocurrent density ( JSC, mA cmÀ2), open circuit voltage
(VOC, V), and fill factor (FF) to be calculated. This yielded an
overall conversion efficiency (Z) of 5.9% for ZnEP1 and 4.0% for
ZnEP2 according to the equation, Z = JSC Â VOC Â FF (Fig. 2a
and Table 1).
The efficiency of the doubly b-ethynylphenyl-substituted ZnEP2
used in the present study proved to be inferior to that of singly-
substituted ZnEP1 as manifest in the lower JSC value. The photo-
current action spectra of the ZnEP1-sensitizer cell exhibited a
maximum incident photon-to-current efficiency (IPCE) of 54.5%
Fig. 1 (a) Steady-state absorption and (b) fluorescence spectra of ZnEP1 (black)
and ZnEP2 (red) recorded in CH2Cl2. Excitation was made in the Soret band.
c
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YD1, confirming the viability of the present substitution strategy.
It was also found that the power conversion efficiency of the
monosubstituted ZnEP1 is superior to that of its doubly function-
alized ZnEP2, leading us to suggest that further design optimiza-
tion could produce yet-improved systems.
This work was supported by the Global Frontier R&D Program
on Center for Multiscale Energy System funded by the National
Research Foundation under the Ministry of Science, ICT & Future,
Korea (2012-8-2081; D.K.) and the U.S. NSF (CHE-1057904; J.L.S.).
Fig. 2 (a) Current–voltage characteristics and (b) action spectra at incident
photon-to-current (IPCE) conversion efficiencies of DSSCs sensitized by ZnEP1
(black) and ZnEP2 (red). Conditions: [Co(bpy)3]2+ based electrolyte in CH3CN and
AM 1.5 under simulated solar light irradiation (100 mW cmÀ2).
Notes and references
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JSC/mA cmÀ2
VOC/V
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Z/%
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ZnEP2
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11.1
8.2
12.7
0.77
0.77
0.71
0.69
0.64
0.68
5.9
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9 The reduced overall yield is ascribed in part to the partial hydrolysis
of the methyl ester product (3g) formed under the reaction condi-
tions. Unfortunately, in our hands the polar hydrolyzed product
could not be isolated via silica gel column chromatography.
at 450 nm that is also larger than in cells built up from ZnEP2
(27.3% at 470 nm) (Fig. 2b). This difference matched the
absorption spectral features of the corresponding porphyrins
adsorbed on the electrodes (Fig. S6, ESI†).
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encies of (i) light harvesting (LHE), (ii) charge injection (jinj),
and (iii) charge collection (Zcol).16 Among these factors, the LHE
defined as LHE = 1 À 10ÀeG (where e is the molar extinction
coefficient of the dye and G is the molar concentration of the
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11 Optical data for the unfunctionalized porphyrin (4) are also given in
dominant in accounting for the photocurrent generated by
the cells prepared using ZnEPn of this study. Considering the
Fig. S2 (ESI†) for the purpose of comparison.
similar absorption spectral features of the dyes, the larger 12 Electron injection from the excited state of a sensitized dye to the
G value of ZnEP1 (1.2 Â 10À7 mol cmÀ2) as determined on
TiO2 surface has been reported to take place on a time scale of
1012–1013 À1. See, Y. Tachibana, S. A. Haque, I. P. Mercer, J. R.
s
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consider it possible that the tilted dye geometry of ZnEP2 serves
to accelerate the back electron transfer process (Fig. S7, ESI†).
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the acceptor ethynylphenyl rings is larger in ZnEP2 than in ZnEP1
based on the optimized structures (Fig. S8, ESI;† 17.41 vs. 0.71,
respectively). This may disrupt the effective electronic communi-
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´
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In summary, we have demonstrated the use of a direct
palladium-catalyzed oxidative alkynylation reaction of b-borylated
porphyrins to obtain b-ethynylphenyl-substituted porphyrin deri-
vatives. This has allowed us to obtain new alternative porphyrin-
based sensitizers, ZnEPn for DSSCs. The photophysical and
electrochemical properties of two ZnEPns were investigated and
proved to be similar to those of a meso-substituted control system,
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c
9166 Chem. Commun., 2013, 49, 9164--9166
This journal is The Royal Society of Chemistry 2013