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

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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,^aY. Terazono,^aB. J. Brennan,^bB. D. Sherman,^aJ. D. Megiatto Jr.,^aG. W.

Brudvig,^bA. L. Moore,^a†T. A. Moore,^a†D. Gust^a†

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)₂with ‐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.^2–7One 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.^5,8–10The

pioneering report¹¹by 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.^12–15The 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 TiO₂or SnO_2,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,^16–31porphyrins 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,^32–42and porphyrins have given some of the highest

efficiencies yet observed for DSSC.^16,43Porphyrins 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,⁵⁴but

from π‐π* transitions.^42,44Porphyrins 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.⁴⁵However, most nitriles via palladium‐catalyzed cross‐coupling reactions,^55–69

porphyrins have several shortcomings as ideal dyes for water and in 2006 Liu and Chen⁷⁰reported 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.⁷⁰Based 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 110^oC. 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.⁴⁶Their 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 1973.⁴⁷Bromination 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.^18,71,72Finally, 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.^48–51In 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.⁵²A 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.^56,67The

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 previously^40‐42

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

R₁groups 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.^58,59,73,74We

employed Zn(CN)₂in 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.^60,75Potassium 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.^57,58,60,61Although 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.⁵⁹In addition, it is

known that the thermal and base catalyzed decomposition of

dimethylformamide gives dimethylamine and carbon

monoxide,^59,74,76and that in the presence of secondary acyclic

amines, the reduction of aryl bromides occurs under Pd(0)

catalysis.⁵⁹It 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

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.⁷⁷A 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

anticipated^45,50,52,78anodic 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.⁷⁹The

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 By^aSoret Bx^aQ bands^aEmission^b

(E_1/2, V vs (E_1/2, V vs (E_1/2, V vs (E_1/2, 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

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 (E_p)

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

441

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 (E_p)

17

18

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(E_p)

1.40(E_p)

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

PF_10Zn

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

PF_10Znwhich have been explored for use in light‐driven water

oxidation due to their strong oxidizing ability.^80,81The

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,⁴⁴the Soret band is due to

electronic transitions from the b₂orbital (HOMO) to the c₁and

c₂LUMO 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.⁴⁴Due to the

electronic effects of the 4 cyano substituents on the antipodal

β‐pyrrole positions on only one axis, the energies of the c₁and

c₂orbitals 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 TiO₂and SnO₂based 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 TiO₂and the less common SnO₂as

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 TiO₂and

SnO₂were 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 TiO₂and SnO₂for 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

*Ox^b

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

4

electrodes characterized in Fig. 6. The TiO₂cell was prepared

according to protocol 1 (see Experimental Section) and the

SnO₂cells 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

PF_10Zn

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 TiO₂at pH 8.⁸⁵

^dCalculated using the literature value of 0.05 V vs. NHE for the CB of

nanoparticulate SnO₂at 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,¹⁸we selected carboxylic acid moieties, as these have

been extensively employed in the DSSC literature.^18,71,86,87The

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.^71,72Therefore, we have studied both

sensitizers.

4 and 7 as

For construction of the cells, the photoanode featured

nanoparticulate TiO₂or SnO₂deposited 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 SnO₂electrodes 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 TiO₂paste⁹⁷and 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 SnO₂photoanodes 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 TiO₂electrode. The inset is an

expansion of the vertical axis for 7 on TiO₂.

A DSSC was constructed using the TiO₂electrode 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 cm²opening 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 SnO₂electrodes, 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 J_scwas 2.09 mA cm^‐2whereas for

7

J_scwas 1.97 mA cm^‐2. The open‐circuit voltages V_ocfor 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.^18,71,72On 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

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

SnO₂paste was spread using a glass rod. The film was dried at

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^‐/I₃electron 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 SnO₂paste 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.⁸⁰Optimized

SnO₂‐based anodes are theoretically advantageous for use in

photoelectrochemical cells for water oxidation because

electron injection into the SnO₂conduction band is

experiment was prepared using

a modification of the

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procedure of Ito and co‐workers.⁹⁷Acetic 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 SnO₂electrodes 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 SnO₂film 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 H₂PtCl₆in 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 TiO₂based 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.⁸⁰No 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 SnO₂electrodes perform more poorly than those that use

by using monochromatic light from a xenon lamp and single‐ TiO₂.^94,95Losses in efficiency relative to TiO₂cells 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^‐2at 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.⁹⁶The use of cobalt complexes as mediators would

density was determined using a calibrated silicon diode (ASTM be expected to increase V_ocin 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‐

J. Name., 2013, 00, 1‐3 | 9

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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

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M. K. Nazeeruddin and M. Grätzel, Nat. Chem., 2014, 6,

242–7.

17

18

N. Robertson, Angew. Chem. Int. Ed. Engl., 2006, 45, 2338–

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

DOI: 10.1039/c5ta07226c

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