A new method for the synthesis of β-cyano substituted porphyrins and their use as sensitizers in photoelectrochemical devices

Article abstract of DOI:10.1039/c5ta07226c

β-Cyanoporphyrins have high positive potentials for oxidation and absorb light at longer wavelengths than most porphyrins, making them potential candidates for sensitizers in photoelectrosynthetic cells for water oxidation. In order to begin to evaluate this potential, two Zn(ii) tetra-β-cyanoporphyrins have been synthesized and evaluated as sensitizers in dye sensitized solar cells using I^-/I₃^- as the redox mediator. To prepare such specialized β-cyanoporphyrins, a new synthetic method has been developed. This approach involves reaction of Zn(CN)₂ with β-brominated zinc porphyrins in the presence of tris-(dibenzylideneacetone)dipalladium. The tetra-cyanation reaction is complete under milder conditions as compared to those usually employed in previous methods and gives improved yields of up to ～50%. The procedure allows for the cyanation of porphyrins with relatively sensitive functional groups. Examples of its application to a range of substituted tetra-arylporphyrins are reported, and the absorption and electrochemical properties of the compounds prepared are given. The results from using two of the molecules as sensitizers in dye sensitized solar cells are presented. It was found that the porphyrins produced no photocurrents in nanoparticulate TiO₂-based cells, but both molecules produced photocurrents in SnO₂-based cells, and are potential candidates for sensitizers in photoelectrosynthetic cells for water oxidation.

Full text of DOI:10.1039/c5ta07226c

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Graphical Abstract
The high positive potentials for oxidation and long‐wavelength absorption properties of tetra‐‐
cyanoporphyrins make them good candidates for sensitizers in photoelectrosynthetic cells for
water splitting. A new synthetic procedure for preparation of these porphyrins that gives high
yields under relatively mild conditions has been developed and used to evaluate two tetra‐‐
cyanoporphyrins as sensitizers for photoanodes of photoelectrochemical cells.

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A new method for the synthesis of β‐cyano substituted porphyrins
and their use as sensitizers in photoelectrochemical devices
Received 00th January 20xx,
Accepted 00th January 20xx
A. Antoniuk‐Pablant,a Y. Terazono,a B. J. Brennan,b B. D. Sherman,a J. D. Megiatto Jr.,a G. W.
Brudvig,b A. L. Moore,a† T. A. Moore,a† D. Gusta†
DOI: 10.1039/x0xx00000x
‐Cyanoporphyrins have high positive potentials for oxidation and absorb light at longer wavelengths than most
porphyrins, making them potential candidates for sensitizers in photoelectrosynthetic cells for water oxidation. In order to
begin to evaluate this potential, two Zn(II) tetra‐‐cyanoporphyrins have been synthesized and evaluated as sensitizers in
dye sensitized solar cells using I‐/I3‐ as the redox mediator. To prepare such specialized ‐cyanoporphyrins, a new synthetic
method has been developed. This approach involves reaction of Zn(CN)2 with ‐brominated zinc porphyrins in the
presence of tris‐(dibenzylideneacetone)dipalladium. The tetra‐cyanation reaction is complete under milder conditions as
compared to those usually employed in previous methods and gives improved yields of up to ~50%. The procedure allows
for the cyanation of porphyrins with relatively sensitive functional groups. Examples of its application to a range of
substituted tetra‐arylporphyrins are reported, and the absorption and electrochemical properties of the compounds
prepared are given. The results from using two of the molecules as sensitizers in dye sensitized solar cells are presented. It
was found that the porphyrins produced no photocurrents in nanoparticulate TiO2‐based cells, but both molecules
produced photocurrents in SnO2‐based cells, and are potential candidates for sensitizers in photoelectrosynthetic cells for
water oxidation.
www.rsc.org/
One approach to achieving photochemical water oxidation
is through the design of dye‐sensitized photoelectrosynthetic
cells. Such cells couple the fundamental chemistry of the dye‐
Introduction
In this energy dependent society, it is important to find
sustainable and clean alternative energy sources to reduce our
dependence on fossil fuels. Sunlight is a viable source of
energy as it is globally available and delivers enough energy to
the Earth’s surface to not only exceed current global energy
requirements, but to meet the energy demands for the
foreseeable future. Methods to convert sunlight to a usable
energy source have been widely sought. A system that can
convert sunlight directly into chemical fuels would be most
advantageous because such fuels would provide a practical
energy source that could be stored and transported.1
Hydrogen gas is one such energy source, but in order to use
hydrogen for our energy needs, a sustainable method to
generate it must be found. One promising route is through
photochemical water oxidation. While promising, this method
has proven difficult to develop, in part because it requires
combining single‐photon excitation with multiple electron and
proton transfer events to drive the half reactions.2–7 One of the
more difficult steps of this process is the oxidation of water.
sensitized nanoparticulate wide band gap solar cell to catalysts
for water oxidation and hydrogen evolution.5,8–10 The
pioneering report11 by O’Regan and Grätzel in 1991 has
generated a huge number of investigations of approaches to
dye sensitized solar cells (DSSC) for electricity production with
a view toward improving performance.12–15 The knowledge
gained from these studies can be applied to the design of
efficient photoelectrosynthetic cells for water splitting,
although water splitting requires an aqueous environment,
specific wavelength sensitivity, and the generation of suitable
redox potentials. The excited state redox potential of the
sensitizer must have the thermodynamic driving force
necessary to inject an electron into the conduction band of a
semiconductor such as TiO2 or SnO2, and the radical cation
produced by such injection must be able to accept an electron
from the water‐oxidation catalyst.
Of the many types of sensitizers for DSSC that have been
investigated,16–31 porphyrins have recently been found to be
particularly promising. Porphyrins have long been used as light
absorbers and electron donors in artificial photosynthetic
molecules, as many of the properties of synthetic porphyrins
are generally similar to those of their natural chlorophyll
relatives,32–42 and porphyrins have given some of the highest
efficiencies yet observed for DSSC.16,43 Porphyrins absorb in
the UV and visible regions of the solar spectrum with several
a. School of Molecular Sciences, Arizona State University, Tempe, AZ 85287
b. Energy Sciences Institute and Department of Chemistry, Yale University, P. O. Box
208107, New Haven, CT 06520
†E‐mail: gust@asu.edu, tmoore@asu.edu, amoore@asu.edu.
Electronic Supplementary Information (ESI) available: Synthetic details for
preparation of new compounds. See DOI: 10.1039/x0xx00000x
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distinctive bands typically from 400 to 650 nm which arise insertion by first incorporating nickel into the porphyrin,54 but
from π‐π* transitions.42,44 Porphyrins are electroactive and can removal of the Ni(II) also requires strong acid. We found this
undergo multiple redox processes, and the potentials at which cyanation method to give low yields and produce a number of
these processes occur can be tailored for different side products which complicated purification. We therefore
applications, as they depend on several factors including the sought another synthetic route to β‐cyanoporphyrins.
substituents, the planarity of the macrocycle, and the type and
oxidation state of a central metal ion.45 However, most nitriles via palladium‐catalyzed cross‐coupling reactions,55–69
porphyrins have several shortcomings as ideal dyes for water and in 2006 Liu and Chen70 reported synthesis of a
It is known that aromatic halides may be converted to

‐
oxidation. For example, the most common porphyrins do not trifluoromethylated porphyrin from a tetra‐β‐bromoporphyrin
absorb light in the far red and infrared spectral regions, and using such a reaction.70 Based on these reports, we have
sunlight at these wavelengths is wasted, thus reducing investigated the preparation of β‐cyanoporphyrins via
efficiency. In addition, the radical cations of most common palladium‐catalyzed cross coupling. The palladium‐catalyzed
porphyrins are not oxidizing enough to drive catalysts for cyanation we have developed gives yields of ca. 50% for simple
water oxidation.
porphyrins. The conditions of the reaction are relatively mild,
Porphyrins with cyano groups in β‐positions absorb light at employing temperatures up to 110oC. This method (Fig. 1)
longer wavelengths than most porphyrins lacking such eliminates the use of CuCN, which avoids the insertion of
substituents, and thus in principle can make more efficient use copper into the porphyrin macrocycle, and therefore
of the solar spectrum in energy conversion applications. eliminates the copper demetallation step, which requires
Furthermore, addition of electron withdrawing cyano groups harsh conditions. The new approach opens up the synthesis of
at the β‐positions of a porphyrin stabilizes both the lowest β‐cyanoporphyrins that have relatively sensitive functional
unoccupied molecular orbital (LUMO) and the highest groups.
occupied molecular orbital (HOMO). As
a
result,
Using this new method, several β‐cyanoporphyrins were
cyanoporphyrins are more easily reduced than their synthesized and two were investigated for use in DSSC
unsubstituted analogs, making them well suited as electron applications. The investigations were geared to test the β‐
acceptors in photoinduced electron transfer reactions. They cyanoporphyrins for their eventual use in light‐driven water‐
can replace more commonly used electron acceptors such as oxidation systems. First, we investigated the spectroscopic and
fullerenes or quinones.46 Their radical cations are very strong electrochemical properties of model porphyrins bearing either
oxidants, which could in principle make them useful for driving three or four
‐cyano groups, and their zinc analogs, in order
water oxidation in solar fuel production systems. to determine which were most suitable for DSSC applications.
In spite of these advantages, β‐cyanoporphyrins have been Based upon these results, we selected two porphyrins as
little explored for such uses, due mainly to the difficulty of sensitizers for DSSC. These two compounds differ in the
preparing them, and the synthetic methods available have position of the carboxylic acid group that serves to link the
limited the selection of substituents to serve as anchoring porphyrin to the semiconductor oxide. It has been
groups to semiconductors. The first tetrasubstituted β‐ hypothesized that having the anchoring group at a meta
cyanoporphyrin was reported by Callot in 1973.47 Bromination position to the macrocycle on a meso‐aryl ring can allow for
of 5,10,15,20‐tetraphenylporphyrin with N‐bromosuccinimide more efficient net electron injection into the semiconductor
(NBS) yielded a porphyrin with bromine atoms at four β‐ than can be obtained with a para linkage.18,71,72 Finally, we
positions, and treatment of the nickel derivative of this prepared and tested several electrochemical cell types in order
compound with CuCN in pyridine yielded the tetracyano to determine whether or not
analog. Using the same synthetic method, Callot and function in DSSC.
coworkers synthesized various cyanoporphyrins.48–51 In 1991,
‐cyanoporphyrins could
Crossley and coworkers reported that the tetrabromo‐, and
Results and discussion
General cyanation method
consequently the tetracyano‐tetraarylporphyrins formed in
this way have the 2,3,12,13 substituent pattern, wherein
substitution occurs only on transannular double bonds,
opposite one another on the porphyrin macrocycle.52 A few
additional reports of the synthesis of tetracyanoporphyrins
have appeared since that time.53,54
In these previous reports, the same basic method was
employed to prepare the porphyrins. The method requires
harsh conditions which are unsuitable for some kinds of
substituted porphyrins. The cyanation step necessitates a
copper catalyst, and if free base porphyrins or porphyrins
containing zinc or other easily replaced metals are used in the
cyanation step, copper is introduced into the macrocycle
during the reaction. Removal of the copper requires the use of
concentrated sulfuric acid. Bhyrappa et al. avoided the copper
The general method of synthesis of the β‐cyanoporphyrins
from the corresponding β‐bromoporphyrins is illustrated in Fig.
1.
tris(dibenzylideneacetone)dipalladium(0) and a ligand such as
triphenylarsine, tri‐(o‐tolyl)phosphine, or 1,1’‐
The
catalyst
is
generated
in
situ
from
bis(diphenylphosphino)ferrocene (dppf). Such catalysts are
commonly used for cyanation of aromatic bromides.56,67 The
dppf resulted in the highest yields in our hands, and this ligand
was used for most of the preparations. This ligand has been
found to enhance the yields in other cyanation reactions.56,65,69
A variety of additional considerations had to be taken into
account in order to obtain reasonable yields. One contribution
2 | J. Name., 2012, 00, 1‐3
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Synthesis of β‐cyanoporphyrins
The overall scheme for preparation of β‐cyanoporphyrins is
shown in Fig. 2, and the compounds prepared are depicted in
Fig. 3. The requisite dipyrromethanes
A were prepared from
the corresponding aromatic aldehyde and pyrrole by addition
of trifluoroacetic acid (TFA), and then triethylamine.
Condensation of the dipyrromethane with the appropriate
aldehydes under the conditions described previously40‐42
yielded the porphyrins
B. Treatment of the porphyrins with
recrystallized N‐bromosuccinimide (NBS) in chloroform or
dichloromethane solution beginning with one addition of 4.5
mol equivalents of NBS per mol of porphyrin in solution,
followed by several portions of ~2.5 equivalents of NBS (with
the number of portions varying among the different
porphyrins) until a complete reaction was observed, yielded
the 2,3,12,13‐tetrabromoporphyrins. The introduction of zinc
Fig. 1. General synthetic scheme for cyanation of β‐bromoporphyrins.
to low yields with this catalyst is deactivation by oxygen, and
so the reaction was carried out under an argon atmosphere.
We also employed zinc dust and zinc acetate, which have been
shown to help keep the catalyst active during the course of the
reaction.62,69,73
yielded the zinc 2,3,12,13‐tetrabromoporphyrins
were converted to the corresponding zinc β‐cyanoporphyrins
as discussed above. In the case of the compounds where the
R1 groups are aryl groups bearing methoxycarbonyl
substituents, hydrolysis of with base as discussed above
yielded the corresponding acid derivatives. Finally, removal of
the zinc with TFA gave the free base porphyrins
C, which
Catalyst deactivation is also caused by reaction with
cyanide ion to form a palladium cyanide complex.58,59,73,74 We
employed Zn(CN)2 in our work, as the metal‐cyanide bond is
less ionic than that of the more commonly employed NaCN or
KCN, and this reduces the formation of the palladium cyanide
complex.60,75 Potassium ferrocyanide was also used as a source
of cyanide, and worked successfully in several reactions.
The choice of solvent is very important. The solvent must
dissolve the organic precursor and catalyst, and in addition the
cyanide source, which is a salt. If the solvent is too non‐polar,
sufficient cyanide will not dissolve. On the other hand, too
much cyanide ion in solution can exacerbate the problem of
catalyst cyanation.57,58,60,61 Although dimethylformamide
(DMF) is often used as a solvent for reactions of this type, we
observed significant debromination of the porphyrin precursor
with this solvent, resulting in the formation of porphyrin side
products. It has been reported that DMF can be a hydride
source in palladium catalyzed reactions.59 In addition, it is
known that the thermal and base catalyzed decomposition of
dimethylformamide gives dimethylamine and carbon
monoxide,59,74,76 and that in the presence of secondary acyclic
amines, the reduction of aryl bromides occurs under Pd(0)
catalysis.59 It is possible that these reactions also contribute to
the debromination. After investigation of several solvent
possibilities, N,N‐dimethylacetamide (DMA) purified by
distillation from barium oxide was found to give the best
yields. The reactions were carried out at 110 °C. Yields are the
best when the β‐bromoporphyrin component is very soluble in
DMA, and solubility is in general facilitated by substituents on
the phenyl rings.
D
D
E
.
The demetallation of the zinc β‐cyanoporphyrins was
carried out using a 10% (v/v) solution of trifluoroacetic acid
(TFA) in dichloromethane and the free base porphyrins were
obtained in nearly quantitative yields. Some of the β‐
cyanoporphyrins were prepared bearing methoxycarbonyl
groups on meso‐aryl rings. It was found that to obtain the
corresponding carboxylic acid, the ester must be hydrolyzed
under base‐promoted conditions while the Zn(II) is still in the
center of the macrocycle in order to prevent
hydrolysis/degradation of the cyanide groups.
Fig. 2. Synthetic scheme for zinc and free base β‐cyanoporphyrins. See the
text for reagents and conditions.
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Electrochemical and spectroscopic properties
We determined the absorption and emission spectra and
selected electrochemical properties of the compounds
synthesized, as these properties are relevant for possible
applications of these materials to solar energy conversion and
other optoelectronic applications.
The electrochemical properties of the cyanoporphyrins were
studied by cyclic voltammetry, and the redox potentials are
reported vs. SCE in Table 1. The potentials for acid derivatives
and were not measured, but are assumed to be essentially
identical to those of the corresponding esters and , based on
4
7
3
6
studies of other porphyrins. Cyclic voltammetry was performed
under an argon atmosphere in purified dichloromethane
containing 0.1 M tetrabutylammonium hexafluorophosphate as
supporting electrolyte. A Pt disc working electrode, Pt mesh
counter electrode, and an Ag+/Ag quasi‐reference electrode
were employed. Potentials were determined using a ferrocene
internal standard and the results were converted to SCE by
referencing peaks to the first oxidation wave of ferrocene, which
is assigned to be 0.45 V vs SCE in this solvent.77 A typical
voltammogram, that for free base compound 20, is shown in Fig.
4. The free base cyanoporphyrins show two reversible reduction
waves. The first oxidation wave is reversible. When higher
potentials are applied, a quasi‐reversible second oxidation wave
is also observed, and the peaks for the first oxidation are no
longer of equal intensity (Fig. 4). The potentials show the
anticipated45,50,52,78 anodic shift due to the electron withdrawing
cyano groups. For example, the potential for the first oxidation
of 5, 10, 15, 20‐tetrakis‐(4‐methylphenyl)porphyrin is 0.94 V vs
SCE and the potential for the first reduction is ‐1.27 V.79 The
corresponding numbers for 22, with three cyano groups, are
1.19 V and ‐0.53 V vs SCE. Thus addition of the three cyano
groups has a very large effect on the stabilization of a negative
Fig. 3. Porphyrins prepared using the cyanation procedure.
Fig. 4. Cyclic voltammogram of free base porphyrin 20 in dichloromethane obtained
under the conditions described in the text.
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Table 1
Spectroscopic and cyclic voltammetric data for the synthesized β‐cyanoporphyrins
Cmpd Ox1
Soret Bya Soret Bxa Q bandsa Emissionb
(E1/2, V vs (E1/2, V vs (E1/2, V vs (E1/2, V vs
Ox2
Red1
Red2

max (nm) max (nm) max (nm) max (nm)



SCE) SCE) SCE) SCE)
1
2
443
441
459
454
660, 685
556, 600,
670, 738
657, 677
659, 681
555, 602,
672, 738
661, 685
740
781
1.01 1.42 ‐0.59 ‐0.98
1.22
‐‐
‐0.34
‐0.61
3
4
5
443
443
445
454
458
459
739
‐‐
781
1.19
‐‐
1.32
‐‐
‐‐
‐0.38
‐‐
‐0.31
‐0.75
‐‐
‐0.61
6
443
459
737, ~827
0.94
‐‐
‐0.64
‐1.02
(sh)
‐‐
781
7
8
443
441
460
453
662, 689
556, 601,
668, 735
655, 677
‐‐
1.27
‐‐
‐‐
‐‐
‐0.42
‐‐
‐0.69 (Ep)
9
443
440
458
453
711, ~776
(sh)
768
1.04
1.41
‐‐
‐‐
‐0.62
‐0.38
‐1.02
‐0.67
10
553,599,
667, 730
655, 679
657, 681
554, 599,
667, 729
655, 680
661, 685
555, 600,
669, 730
659, 680
556, 601,
679, 737
11
12
13
443
443
441
458
458
452
713
‐‐
768
1.18
1.10
1.38
‐‐
‐‐
‐‐
‐0.45
‐0.54
‐0.32
‐0.88
‐0.92
‐0.62
14
15
16
444
445
443
456
460
453
709
712
767
1.22
‐‐
~1.29
‐‐
‐‐
1.47
‐0.48
‐‐
‐0.37
‐0.93
‐‐
‐0.65 (Ep)
17
18
441
441
458
453
737
783
~0.87
1.28
1.47
1.51
‐0.47
‐0.30
‐0.88
‐0.58
19
20
444
439
460
451
662, 686
703, 770
1.22
1.48
1.65
1.80
‐0.49
‐0.32
‐0.95
‐0.75
552, 599,
673, 733
742, ~823
(sh)
21
444
445
416
415
632
695
1.02
1.19
1.33
1.11
1.41(Ep)
1.40(Ep)
1.54
‐0.69
‐0.53
‐0.97
‐1.15
‐1.05
‐0.89
‐1.35
‐1.57
22
544, 586,
650, 708
755
PF10
PF10Zn
510, 543,
587, 641
646, 709
590, 642
545, 578
1.35
aSpectra were obtained in dichloromethane. For the porphyrins bearing a carboxylic acid, a solution containing 2% methanol was used to prevent
aggregation.
bEmission spectra were obtained in dichloromethane, with excitation into the Soret band. For the porphyrins bearing a carboxylic acid, a solution of 2%
methanol in dichloromethane was used.
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charge on the porphyrin during reduction, and a smaller but
still substantial effect on oxidation. Addition of the fourth
cyano group in 18 increases the effect with a ~230 mV shift
positive for the first reduction and about ~100 mV shift
positive for the first oxidation, relative to 22. Addition of zinc
to the porphyrin makes the macrocycle somewhat easier to
oxidize and more difficult to reduce, as can be seen in Table 1
by comparing free base porphyrins with their zinc analogs.
Addition of an electron withdrawing carboxyl group to one of
the meso‐aryl rings has little effect on the potentials for
oxidation and makes the potentials for reduction more
negative by less than 100 mV. Table 1 also shows data for 5‐(4‐
methoxycarbonylphenyl)‐15‐(4‐carboxyphenyl)‐10,20‐
bis(pentafluorophenyl)porphyrin, PF10, and its zinc analog
PF10Zn which have been explored for use in light‐driven water
oxidation due to their strong oxidizing ability.80,81 The
tetracyanoporphyrins are roughly comparable to the PF10
compounds in terms of oxidation, whereas reduction of the
cyanoporphyrins is considerably more facile. This last fact
suggests that cyanoporphyrins may be useful as electron
acceptors in molecular artificial photosynthetic reaction
centers and related compounds in place of the more usually
employed quinones or fullerenes. Such applications have been
recently demonstrated.46,82,83
Fig. 5. Spectrum of 13 in dichloromethane. The inset shows absorbance
(solid line), and fluorescence with excitation at 451 nm (dotted line).
character in the excited state. It should be noted that the
cyanoporphyrins absorb at significantly longer wavelengths
than the fluorinated porphyrins and thus have lower‐energy
excited states.
Photoelectrochemical cells
The UV‐visible absorption and emission maxima for the
cyanoporphyrins in dichloromethane are given in Table 1. The
absorption spectrum of 13 is shown in Fig. 5. In general, the
UV‐visible maxima of the cyanoporphyrins are shifted to
longer wavelengths as compared to porphyrins without β‐
Both DSSC and photoelectrosynthetic cells for water splitting
share the concept of a dye‐sensitized nanoparticulate wide
band gap semiconductor as a photoanode. Thus, we have
evaluated the cyanoporphyrins as sensitizers for DSSC as a step
toward their use in water splitting cells. The redox potential
for water oxidation at pH 0 is 0.99 V vs SCE, and at pH 8 it is
substituents. Due to the electron withdrawing ‐substituents,
the longest wavelength Q‐band of 18 has a maximum at 737
nm, whereas typical free base meso‐tetra‐arylporphyrins have
this maximum around 650 nm. In 17 the Soret bands appear in
the same spectral region as those of 18, but the longest‐
wavelength Q‐band absorption is at 680 nm. As can be seen in
Fig. 5, the Soret band is split into two peaks. In terms of the
Gouterman four orbital theory,44 the Soret band is due to
electronic transitions from the b2 orbital (HOMO) to the c1 and
c2 LUMO orbitals. These transitions consist of two transition
dipole moments in directions along a set of axes, x and y, on
the porphyrin which pass through the two sets of opposing
pyrrole rings. At room temperature these transitions are
observed as a single peak in most porphyrins.44 Due to the
electronic effects of the 4 cyano substituents on the antipodal
β‐pyrrole positions on only one axis, the energies of the c1 and
c2 orbitals are no longer degenerate in these compounds, and
this in turn leads to the observed split in the Soret band. The
cyanoporphyrins with only three cyano groups do not show
the splitting of the Soret band, and have less pronounced
shifts of the Q‐bands to longer wavelengths (see literature84
and Experimental Section). The fluorescence spectrum of 13 is
also shown in Fig. 5. The spectrum features a relatively large
Stokes shift and is not the mirror image of the longest‐
wavelength Q‐band. This is in contrast to most porphyrins,
which show small Stokes shifts, and indicates that the relaxed
excited singlet state has significant electronic differences from
the ground state. It may indicate some charge‐transfer
0.52 V. Thus, from Table
thermodynamic point of view, any of the tetra‐
1 it is clear that from a
‐
cyanoporphyrins investigated can oxidize water, and they have
significant excess oxidizing power to supply reasonable
overpotentials to drive the reaction. The sensitizer must also
be able to inject an electron into the CB of the semiconductor.
This ability is a function of both the potential for the first
oxidation and the excited state energy. Table 2 shows the
excited state potentials for oxidation of the various porphyrins.
It also shows the thermodynamic driving force values for
electron injection into the conduction bands of
nanoparticulate TiO2 and SnO2 based on the CB potentials of
the semiconductors. These data show that with the possible
exception of 21, none of the porphyrins are expected to be
able to sensitize TiO2. With the exception of 18, all of the
porphyrins have some driving force for electron injection into
the CB of SnO2. On the other hand, the tri‐‐cyanoporphyrins,
especially 21, do not absorb at especially long wavelengths
compared to other tetra‐arylporphyrins, and thus do not have
the advantage of absorbing more of the solar spectrum. We
decided to evaluate the zinc tetra‐‐cyanoporphyrins as
potential sensitizers for DSSC. We also decided to examine
both the commonly used TiO2 and the less common SnO2 as
semiconductors. Because the CB potentials of nanoparticulate
semiconductors are difficult to determine and are a function of
the solvent in the cell, and because trap states below the CB
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may be present, we decided to study sensitization of both prepared using one of two slightly different protocols. Details
semiconductors.
of cell construction are given in the Experimental Section.
The absorption maxima of the sensitizers on both TiO2 and
SnO2 were slightly shifted to longer wavelengths relative to the
spectra in solution. Fig. 6a shows the light harvesting efficiency
Table 2. Longest wavelength absorption maximum in dichloromethane (1P
(nm)), energy of the first excited singlet state (1P (eV)), excited state
potentials for oxidation vs SCE (*Ox), and free energy changes (G (eV)) for
injection into the conduction bands of TiO2 and SnO2 for selected porphyrins.
for electrodes sensitized with
7 and 4. The absorption spectra
from which these plots were derived were corrected for
scatter and absorbance by the unsensitized electrode. The
electrodes for the two sensitizers were prepared under
c
d
Cmpd
1P
1P
*Oxb
G TiO2
G SnO2
(nm)
(eV)a
identical conditions, but electrodes bearing
4 consistently
showed slightly stronger absorption than those sensitized with
17
18
680
737
1.75
1.59
‐0.55
+0.32
+0.56
‐0.36
‐0.12
7
4
. This is likely because the para carboxylate linkage group on
allows the molecules to attach to the semiconductor oxide
‐0.311
with the macrocycle plane more perpendicular to the local
semiconductor surface than does the meta‐carboxylate linkage
21
632
1.87
‐0.85
+0.02
‐0.66
in
7
, and thus
Fig. 7 shows the current‐voltage plots for DSSC with a TiO2
photoanode sensitized with and two cells with SnO2
electrodes sensitized with either or and prepared using the
4 achieves a greater packing density.
22
3
708
677
1.70
1.75
‐0.51
‐0.56
+0.36
+0.31
‐0.32
‐0.37
7
7
4
electrodes characterized in Fig. 6. The TiO2 cell was prepared
according to protocol 1 (see Experimental Section) and the
SnO2 cells were prepared according to protocol 2. Illumination
was carried out with simulated AM 1.5 G sunlight. The TiO2‐
based cell produced no significant photocurrent. This is
6
685
641
578
1.75
1.93
2.12
‐0.57
‐0.60
‐1.01
+0.30
+0.27
‐0.14
‐0.38
‐0.41
‐0.82
PF10
PF10Zn
consistent with the positive ΔG value for injection of an
electron into the conduction band of the semiconductor
shown in Table 2.
aCalculated from the wavenumber average of the longest wavelength
absorption maximum and shortest wavelength emission maximum.
bThe excited state potentials for oxidation were estimated by
subtracting the energy of the first excited singlet state from Ox1.
cCalculated using the literature value of ‐0.63 V vs. SHE for the CB of
nanoparticulate TiO2 at pH 8.85
dCalculated using the literature value of 0.05 V vs. NHE for the CB of
nanoparticulate SnO2 at pH 8.80
The sensitizer dye in a DSSC must bear at least one
substituent that serves to bind the sensitizer to the
semiconductor nanoparticle. Although various choices are
possible,18 we selected carboxylic acid moieties, as these have
been extensively employed in the DSSC literature.18,71,86,87 The
effect of the position of the anchoring group on the
macrocycle has been studied and it has been hypothesized
that having the anchoring group at the meta position on a
meso‐phenyl group allows the porphyrin to lie flatter on the
semiconductor surface than does a substituent at the para
position, and that this increases the efficiency of electron
injection.71,72 Therefore, we have studied both
sensitizers.
4 and 7 as
For construction of the cells, the photoanode featured
nanoparticulate TiO2 or SnO2 deposited on glass substrates
bearing a coating of conductive fluorine‐doped tin oxide (FTO).
After sensitizing the photoanode from a solution of dye, the
electrode was used in preparation of a DSSC device that
employed a platinized FTO cathode and the I‐/I3‐ redox couple
in an acetonitrile‐based electrolyte. The DCCS cells were
Fig. 6. (a) Light‐harvesting efficiency (LHE) of SnO2 electrodes bearing 7 (dashed) and 4
(solid). The absorption spectra were corrected by subtraction of the spectra of the
corresponding unsensitized electrodes. (b) Incident photon to current efficiency (IPCE)
as a function of wavelength for DSSC constructed using the electrodes whose LHE are
shown in (a). The curves are from electrodes bearing 7 (dashed) and 4 (solid).
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Journal Name
thermodynamically more facile than injection into TiO2. This in
turn permits the use of more oxidizing long wavelength light
absorbing sensitizers than can be used with TiO2.
Experimental
Synthesis
Synthetic details are reported in the Electronic Supplementary
Information.
DSSC preparation and testing
Two related protocols were used to prepare and evaluate the
cells.
Protocol 1. A titanium dioxide based electrode was used
for constructing DSSC. Transparent conductive fluorine‐doped
tin oxide (FTO) glass electrodes (Hartford Glass) were coated
with a TiO2 paste97 and sintered at 450 °C for 45 min. After
sintering, the FTO‐semiconductor electrode was allowed to
cool in air, and was then immersed in a porphyrin solution in
ethyl acetate while still warm. Electrodes were soaked for at
least 18 h.
Fig. 7. Current density‐voltage plots for DSSC with SnO2 photoanodes sensitized by 7
(red) and 4 (black). The solid lines show performance under illumination (simulated AM
1.5 G solar irradiation) and the dotted lines show behavior in the dark. The blue curve
shows the behavior of a DSSC using 7 deposited on a TiO2 electrode. The inset is an
expansion of the vertical axis for 7 on TiO2.
A DSSC was constructed using the TiO2 electrode in
combination with a platinum‐coated FTO counter electrode.
Heat shrink film (Solaronix) was used to seal the electrodes
together. Electrolyte was introduced to the internal volume of
the cell via holes drilled in the FTO‐Pt counter electrode. The
electrolyte consisted of an acetonitrile:valeronitrile (1:1 by
volume) mixture containing 0.2 M lithium iodide, 0.05 M
iodine, 0.2 M tetrabutylammonium iodide, and 0.5 M 4‐t‐butyl
pyridine. A mask with a 0.2 cm2 opening placed over the solar
cell defined the illuminated area used for photo‐based
measurements. A 350 W Xenon arc lamp (Osram) light source
with a 400 nm cutoff filter (Thor Labs) and an AM 1.5 solar
spectrum filter (Oriel) was used to provide 100 mW cm‐2
illumination. The 400 nm cutoff filter was used to avoid direct
bandgap excitation of the semiconducting layer. A Keithley
Sourcemeter was used for photocurrent and photovoltage
experiments. A monochromator (Jobin‐Yvon) was placed
between the Xe lamp and solar cell to provide monochromatic
light for incident photon to current (IPCE) measurements. The
light intensity at each wavelength used in the IPCE
measurements was determined using a silicon photodiode
(Newport).
The SnO2 electrodes, on the other hand, both produced
photocurrents. The para‐linked compound
more photocurrent than the meta‐linked
4
produced slightly
7
. For , the short‐
4
circuit photocurrent density Jsc was 2.09 mA cm‐2 whereas for
7
Jsc was 1.97 mA cm‐2. The open‐circuit voltages Voc for the
cells were nearly identical at 0.118 V for and 0.116 V for
and the fill factor (FF) was 0.31 for both cells. These values give
4
7,
a very slightly higher efficiency η for
4 (0.08%) than for 7
(0.07%).
More detailed information concerning performance comes
from the incident photon to current efficiency (IPCE) plots
shown in Fig. 6b. In the Soret band region around 430 nm
where both electrodes absorb essentially all of the available
photons, meta sensitizer
para isomer . This increase is consistent with the findings of
other workers for other porphyrins sensitizing TiO2, as
mentioned above.18,71,72 On the other hand, the para isomer
has efficiencies comparable to or greater than those of in the
region from ca. 550 – 800 nm due to the increased LHE in this
spectral region shown in Fig. 6a. Thus, in all spectral regions
is more efficient than in net electron injection into the cell
circuit, but achieves higher packing density on the electrode.
The efficiencies of and reported above are significantly
7 performs slightly better than the
4
4
7
7
Protocol 2. Conductive FTO‐coated glass (TEC 7, Hartford
Glass) was cleaned by sonication in acetone and then water,
and rinsed with ethanol. Using tape (Scotch magic tape) as a
mask and spacer to outline a defined area on the FTO, a film of
SnO2 paste was spread using a glass rod. The film was dried at
4
4
4
7
less than those observed for porphyrins in TiO2‐based dye
sensitized solar cells. This is expected, as DSSC based on SnO2
are in general less efficient than those based on TiO2, and
o
80 C for 9 min, and the process was repeated to produce a
especially those that use an I‐/I3 electron mediator.94,95,96
‐
double layer. This layer was sintered (temperature ramped
from 180 oC to 370 oC, held for 10 min, ramped from 180 oC to
470 oC, held for 30 min, and slowly cooled). An additional SnO2
layer was added as described above, and the sintering step
repeated. The thickness of the nanoparticulate porous SnO2
films was ~ 12 μm as determined using profilometry (Tencor
Alpha‐step 200). The colloidal SnO2 paste used in this
However, the performance of
4 and 7 is similar to that
obtained in similarly‐constructed, SnO2‐based DSSC using Zn(II)
5‐(4‐carbomethoxyphenyl)‐15‐(4‐carboxyphenyl)‐10,20‐
bis(pentafluorophenyl)porphyrin as sensitizer.80 Optimized
SnO2‐based anodes are theoretically advantageous for use in
photoelectrochemical cells for water oxidation because
electron injection into the SnO2 conduction band is
experiment was prepared using
a modification of the
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procedure of Ito and co‐workers.97 Acetic acid (200 μL) and methods, this approach still requires the use of more than a
water (1 mL) were added to 1.2 g of Sn(IV) oxide nanoparticles catalytic amount of palladium in order to obtain high yields.
(Nanoarc ‐ Alfa Aesar, 13‐19 nm). Once the particles were
The ‐tetracyanoporphyrins have absorption maxima at
hydrated, 10 mL of ethanol was added and the mixture was considerably longer wavelengths than do their unsubstituted
sonicated using a sonic horn (Branson 450 with ¼” tip; 30 s × 3, analogs. For example, as shown in Table 1, the longest‐
50% duty cycle, power level 4). α‐Terpineol (4 mL, Alfa Aesar) wavelength Q‐bands in the free base compounds are found
was added, and the mixture was sonicated using the sonic around 740 nm. Absorption in this region is important for solar
horn (30 s × 3, 50% duty cycle, power level 4). In 20 mL ethanol energy‐harvesting applications, as there is ample sunlight in
was dissolved 300 mg of ethylcellulose (Acros Organics, 22 cps) this spectral region that is not captured by many other
and 300 mg of ethylcellulose (Acros Organics, 10 cps), and the common dyes. In addition, these porphyrins, especially the
resulting solution was added to the colloid. The colloid was freebase versions, are very good electron acceptors; indeed
sonicated using the sonic horn (30 s × 3, 50% duty cycle, power they are better acceptors than the quinones that are found in
level 4). Under reduced pressure, the volatile ethanol was natural photosynthesis and have often been used in artificial
removed from the colloid by evaporation to yield a viscous photosynthetic molecules.
paste. The SnO2 electrodes were sensitized in 0.150 mM
solutions of porphyrin in ethyl acetate overnight, rinsed, and DSSC show that the zincated tetra‐
The results from using the cyanoporphyrins as sensitizers in
‐cyanoporphyrins can

dried under a nitrogen atmosphere in the absence of light. A inject electrons into the conduction band of nanoparticulate
defined electrode area was formed by removal of the SnO2, but are not thermodynamically competent to inject into
undesired SnO2 film using a plastic razor blade. Platinized FTO TiO2. This is due to a combination of the relatively long
was prepared by adding a drop of 20 mM H2PtCl6 in ethanol to wavelength absorption of the porphyrin Q‐bands, which
FTO coated glass (TEC 7, Hartford Glass), drying the glass in air, decreases the energy of the first excited singlet state relative
o
and then heating it at 450 C for 30 min. A hole was drilled in to many other porphyrins, and the high potentials for
the FTO to allow for incorporation of electrolyte.
oxidation of these molecules, which is due to the electron
DSSC devices were prepared by sandwiching a 25 μm withdrawing properties of the cyano groups. The
thermoplastic spacer (Surlyn, Solaronix) between the dye‐ performances of the DSSC prepared here are inferior to those
sensitized electrode and the counter electrode and sealing at of optimized TiO2 based cells, but are similar to those obtained
o
115 C for 45 s. Electrolyte (600 mM LiI, 50 mM I2, 10 mM t‐ in similarly constructed DSSC with Zn(II) 5‐(4‐
butylpyridine in acetonitrile) was introduced into the device by carbomethoxyphenyl)‐15‐(4‐carboxyphenyl)‐10,20‐
vacuum‐assisted addition, and the Pt/FTO hole was sealed bis(pentafluorophenyl)porphyrin.80 No attempt was made to
using Surlyn.
optimize the cells reported here, but in general, cells made
Incident photon to current efficiency (IPCE) was measured with SnO2 electrodes perform more poorly than those that use
by using monochromatic light from a xenon lamp and single‐ TiO2.94,95 Losses in efficiency relative to TiO2 cells could be
grating monochromator. Power density was determined using related to inefficient injection of electrons into the
a calibrated silicon diode (Newport 818‐UV). Current‐voltage semiconductor, enhanced recombination between the
(I‐V) curves were obtained under simulated sunlight at 100 semiconductor conduction band and the oxidized porphyrin, or
mW cm‐2 at the electrode by filtering light from a xenon arc enhanced recombination reactions involving the I‐/I3
‐
lamp with an AM 1.5 G filter (Newport Corp). The power mediator.96 The use of cobalt complexes as mediators would
density was determined using a calibrated silicon diode (ASTM be expected to increase Voc in such cells; however, our efforts
E948‐09 / ASTM E1021‐06). The IPCE and I‐V data were are toward incorporation of these molecules into
obtained using a Keithley 2400 Sourcemeter and LabView photoelectrosynthetic devices.98
software.
These same properties – long wavelength absorption and
strong oxidizing power – make the cyanoporphyrins good
candidates for use in photoelectrosynthetic cells for solar fuel
production. Their redox potentials ensure that their radical
cations possess significant overpotential for water oxidation
via suitable catalysts. The long wavelength absorption ensures
that they can absorb significant amounts of light at longer
wavelengths than can most porphyrin sensitizers. These two
properties are especially promising for the application of these
porphyrins in tandem photoelectrosynthetic cells for solar fuel
production.
Conclusions
We have reported an improved synthesis of
cyanoporphyrins and carried out photoelectrochemical
experiments to determine whether porphyrins bearing
cyano groups could act as sensitizers in DSSC, with a view
toward their eventual use in photoelectrosynthetic cells for
water splitting or other solar fuel production. The newly
developed method for the synthesis of β‐cyano substituted
porphyrins is better suited for the production of specialized β‐
cyano substituted porphyrins containing sensitive functional
groups than previously known methods. This new method uses
‐
‐
Acknowledgements
a palladium catalyzed reaction to give yields of up to ca. 50%, This work was supported by the Office of Basic Energy
while employing milder conditions than the previous methods Sciences, Division of Chemical Sciences, Geosciences, and
used for cyanation of ‐tetrabromoporphyrins. Like previous Energy Biosciences, Department of Energy under contract DE‐
This journal is © The Royal Society of Chemistry 20xx
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14
15
16
D. Cahen, G. Hodes and M. Gra, J. Phys. Chem. B, 2000, 9,
2053–2059.
FG02‐03ER15393 and the Center for Bio‐Inspired Solar Fuel
Production, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DE‐SC0001016. Work at
Yale University was funded by the U.S. Department of Energy,
Chemical Sciences, Geosciences, and Biosciences Division,
Office of Basic Energy Sciences, Office of Science (DE‐FG02‐
07ER15909) and by a generous donation from the TomKat
Charitable Trust
S. Ardo and G. J. Meyer, Chem. Soc. Rev., 2009, 38, 115–
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M. K. Nazeeruddin and M. Grätzel, Nat. Chem., 2014, 6,
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17
18
N. Robertson, Angew. Chem. Int. Ed. Engl., 2006, 45, 2338–
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