Structure-function relationships in unsymmetrical zinc phthalocyanines for dye-sensitized solar cells

Article abstract of DOI:10.1002/chem.200801778

A series of unsymmetrical zinc phthalocyanines bearing an anchoring carboxylic function linked to the phthalocyanine ring through different spacers were designed for dye-sensitised solar cells (DSSC). The modification of the spacer group allows not only a

Full text of DOI:10.1002/chem.200801778

DOI: 10.1002/chem.200801778
Structure–Function Relationships in Unsymmetrical Zinc Phthalocyanines
for Dye-Sensitized Solar Cells
Juan-Josꢀ Cid,[a] Miguel Garcꢁa-Iglesias,[a] Jun-Ho Yum,[b] Amparo Forneli,[c]
Josep Albero,[c] Eugenia Martꢁnez-Ferrero,[c] Purificaciꢂn Vꢃzquez,[a] Michael Grꢄtzel,[b]
Mohammad K. Nazeeruddin,*[b] Emilio Palomares,*[c] and Tomꢃs Torres*[a]
Abstract: A series of unsymmetrical
zinc phthalocyanines bearing an an-
choring carboxylic function linked to
the phthalocyanine ring through differ-
ent spacers were designed for dye-sen-
sitised solar cells (DSSC). The modifi-
cation of the spacer group allows not
only a variable distance between the
dye and the nanocrystalline TiO2, but
also a distinct orientation of the phtha-
locyanine on the semiconductor sur-
face. The photovoltaic data show that
the nature of the spacer group plays a
significant role in the electron injection
from the photo-excited dye into the
nanocrystalline TiO2 semiconductor,
the recombination rates and the effi-
ciency of the cells. The incident mono-
chromatic photon-to-current conver-
sion efficiency (IPCE) for phthalocya-
nines bearing an insulating spacer is as
low as 9%, whereas for those with a
conducting spacer an outstanding IPCE
80% was obtained.
Keywords: electron transfer · meso-
porous materials · photochemistry ·
phthalocyanines · sensitizers · zinc
Introduction
Moreover, through the rational design of their structure, we
have previously shown the possibility of controlling their
electrochemical and photophysical properties.[14,15] Also, we
have demonstrated the possibility of controlling the electron
recombination dynamics between nanocrystalline TiO2 parti-
cles and axially anchored titanium phthalocyanines.[15] How-
ever, despite the number of groups working on the develop-
ment of phthalocyanines for molecular photovoltaic devices,
only recently have there been reports of efficiencies higher
than 3% under standard conditions (100 mWcmÀ2, air mass
(AM) 1.5).[9,10] Previously, Hagfeldt et al. showed that the
formation of molecular aggregates of zinc phthalocyanines
was a major issue for the achievement of reasonable effi-
ciencies.[7] Such aggregates are formed on the surface of the
nanoparticles, avoiding efficient electron injection into the
semiconducting band when the sensitised film is irradiated.
In fact, the use of co-adsorbents, such as chenodeoxycholic
acid, reduces the formation of such aggregates, but does not
drastically increase the device efficiency. Hence, other key
factors might also have an important influence over the
charge-transfer reaction that takes place at the different in-
terfaces of the dye-sensitised solar cells. We recently showed
that the design of the peripheral substituents such as tert-
butyl groups on the phthalocyanine ring was paramount to
reducing the formation of aggregates onto the TiO2 surface
and thereby achieving incident photon-to-current efficien-
cies (IPCE) over 80%.[9] In this paper we describe the syn-
Phthalocyanines[1] are an active focus of intense research for
the development of efficient light-to-energy conversion devi-
ces.[2–13] Such molecules have unique properties in terms of
light absorption in the far visible region of the solar spec-
trum (l=690 nm) with molecular extinction coefficients (e)
higher than 100000 dm3 mÀ1 cmÀ1 and good chemical stability.
[a] Dr. J.-J. Cid, M. Garcꢀa-Iglesias, Prof. P. Vꢁzquez, Prof. T. Torres
Departamento de Quꢀmica Orgꢁnica
Universidad Autꢂnoma de Madrid, Cantoblanco
28049, Madrid (Spain)
Fax : (+34)914-973-966
E-mail: tomas.torres@uam.es
[b] Dr. J.-H. Yum, Prof. M. Grꢃtzel, Prof. M. K. Nazeeruddin
Laboratory for Photonics and Interfaces
Institute of Chemical Sciences and Engineering
School of Basic Sciences
Swiss Federal Institute of Technology
1015 Lausanne (Switzerland)
Fax : (+41)216-934-111
E-mail: mdkhaja.nazeeruddin@epfl.ch
[c] A. Forneli, J. Albero, Dr. E. Martꢀnez-Ferrero, Prof. E. Palomares
Institute of Chemical Research of Catalonia
ICIQ, Avda. Paꢄsos Catalans 16, 43007 Tarragona (Spain)
Fax : (+34)977-920-223
E-mail: epalomares@iciq.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801778.
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FULL PAPER
thesis and characterisation of a series of unsymmetrical zinc
phthalocyanines TT1–TT5, and the effect the spacer be-
tween the anchoring group and the TiO2 surface has on pho-
tovoltaic properties.
regioisomers from the reaction mixture. Further chemical
modifications consisting of two oxidation steps were accom-
plished using the hydroxyl compound 2 to obtain carboxyph-
thalocyanine TT4. Two consecutive oxidations were necessa-
ry, since the direct oxidation from the alcohol to the carbox-
ylic acids did not work under the common synthetic proce-
dures. A very soft oxidation of the hydroxyl group in 2 with
the periodinane derivative IBX (1-hydroxy-1,2-benziodox-
ole-3ACTHNURTGNEU(GN 1H)-one-1-oxide) in dimethyl sulfoxide (DMSO) af-
forded the formyl derivative 3 in 81% yield, after chromato-
graphic isolation. Compound 3 was then treated with
NaClO2 in water, in the presence of sulfamic acid as a chlor-
ine atom scavenger, leading to carboxyphthalocyanine TT4
in 54% yield, after purification by chromatography on the
reverse phase.
On the other hand, compound TT5 was synthesised by
means of palladium-catalysed cross-coupling reactions in a
three-step synthetic sequence depicted in Scheme 2: Suzuki
cross coupling reaction of iodophthalocyanine 4[18,19] with 4-
bromophenylboronic acid and PdACHTNUTRGNEUNG(PPh3)4 (Ph=phenyl), af-
forded the bromo derivative 5 in moderate yield (34%).
Compound 5 was then treated under Heck reaction condi-
tions with [PdACTHUNTRGNE(UNG PPh3)4] in ethyl acrylate heated to reflux,
giving rise to the ester derivative 6 in a 69% yield. Com-
pound 6 was transformed in the last step to carboxyphthalo-
cyanine TT5 by hydrolysis with KOH, in high yield (90%)
after chromatographic purification on the reverse phase.
Results and Discussion
Phthalocyanine synthesis: Phthalocyanines TT1,[9] TT2,[16]
and TT3[16] were prepared following a slightly modified of
procedure previously described by us. The phthalocyanines
TT4 and TT5 were synthesised following the routes depicted
in Schemes 1 and 2, respectively. Thus, unsymmetrical
phthalocyanine 2 (Scheme 1) was prepared by a cyclotetra-
merisation reaction of 4-tert-butylphthalonitrile and substi-
tuted phthalonitrile 1[17] in a 4:1 ratio in the presence of zinc
UV/Vis measurements: All the sensitisers are soluble in var-
ious polar solvents, such as dichloromethane, chloroform,
tetrahydrofuran, methanol and ethanol. As an example, the
normalised UV/Vis absorption spectrum for TT3 sensitiser
is shown in Figure 1. The different molecules have maxi-
mum absorbances in the near infrared region. Compounds
TT2 and TT3 show a maximum absorbance in solution at
670 and 675 nm, respectively, while TT4 and TT5 show max-
imum absorbance peaks at 672 and 684 nm, respectively.
Moreover, the solid-state spec-
acetate (Zn
ethanol (DMAE) to give 33% yield. The desired phthalo-
cyanine was separated chromatographically as a mixture of
ACHTUNGTRENNUNG(OAc)2) and heated at reflux in dimethylamino-
ACHTUNGTRENNUNG
trum shows a broadening of
the phthalocyanine Q-band,
but no shift of their absorbance
maximum. This enlargement is
mainly due to the adsorption
of the dye molecules onto the
transparent mesoporous TiO2
or Al2O3 films and suggests the
lack of significant molecular
aggregation, as will be illustrat-
ed by the IPCE measurements
later in the paper.[9]
Electrochemical, steady-state
and lifetime emission measure-
ments: When all phthalocya-
nines were excited within the
Q-band (Qb =670–684 nm) in
solution under ambient condi-
Scheme 1. Synthesis of phthalocyanine TT4. a) 4-(Hydroxymethyl)phenol, Cs2CO3, DMF; 65%. b) 4-tert-Bu-
tylphthalonitrile, Zn
54%.
ACHTUNGTRENNUNG(OAc)2, DMAE; 33%. c) IBX, DMSO; 81%. d) i) NaClO2, acetone, ii) H3NO3S, H2O;
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5131

T. Torres, E. Palomares, M. K. Nazeeruddin et al.
duction, as it has been ob-
served before.[23] Table 1 con-
tains all the relevant lumines-
cence emission and electro-
chemical data for the phthalo-
cyanine dyes in solution.
As previously reported by
several
groups,
including
ours,[24–27] the use of time-cor-
related single-photon-counting
(TCSPC) measurements is a
convenient technique to esti-
mate the yield of electron in-
jection and also to approxi-
mately calculate the electron-
injection kinetics when a sensi-
tised wide-band-gap metal
oxide is used as a reference for
comparison purposes. The use
Scheme 2. Synthesis of phthalocyanine TT5. a) 4-Bromophenylboronic acid, Pd
Ethyl acrylate, [Pd(PPh3)4], K2CO3; 69%. c) KOH, dioxane; 90%.
ACHTNUGTRNEN(UGN PPh3)4, K2CO3, DMF; 34%. b)
ACHTUNGTRENNUNG
of
a
large-band-gap metal
oxide such as ZrO2 or Al2O3 prevents electron injection
from an excited dye (lowest unoccupied molecular orbital:
LUMO) into the metal oxide and, therefore, the energy of
the excited state is radiatively dissipated before the dye re-
laxes to the ground state. Hence, if we use sensitised films
with the same absorbance value at lex (and hence similar
dye coverage), and during the measurement of the emission
lifetime we keep the acquisition time constant for the con-
trol and the sample, in our case the dye/Al2O3 and the dye/
TiO2, we can evaluate the yield of electron injection, as illus-
trated for TT4 in Figure 2. The yields of electron injection
for all of the dyes tested are higher than 90% with electron-
injection kinetics in the range of 173–277 ps, calculated ac-
cording to Koops et al.[24] We note that the estimated life-
time corresponds to the fit of the experimental data, since
the TCSPC instrument response is 325 ps measured at
FWHM. In fact, this time range is in good agreement with
the electron-injection studies carried out previously on Zn-
based phthalocyanines, which showed injection dynamics of
ꢀ250 ps. It is worth noting that for TT1, the phthalocyanine
dye gives higher light-to-energy conversions that are faster
with respect to other dyes (170 ps). Therefore, we conclude
Figure 1. UV/Vis spectra of TT3 phthalocyanine in ethanol (4ꢆ10À6 m)
and adsorbed onto 4 mm thick films of Al2O3 and TiO2.
tions, they exhibited a luminescence maximum between 680
and 695 nm. It is worth mentioning that the emission is
strongly quenched when the dyes are adsorbed onto the
TiO2 film, which, in principle,
is indicative of efficient elec-
Table 1. Luminescence emission and electrochemical properties of the tested phthalocyanines.
tron injection. On the other
hand, the cyclic voltammogram
of the phthalocyanine dyes, for
lem [nm][b]
t (solution)
t (Al2O3 films)
E = vs. SCE
1
Sample
lmax [nm]/
2 [d]
e[a] [molÀ1 cmÀ2
]
[ns][c]
[ns][c]
[V]
TT1[e]
680
695
4 (40.5%)
1.1 (81.4%)
2.8 (18.6%)
2.8 (100%)
3.3 (49.4%)
1.1 (50.6%)
0.9 (100%)
0.79
example, TT3, exhibited
a
2.8 (59.5%)
4.04 (100%)
3.5 (100%)
quasi-reversible oxidation at
TT2
TT3
670/142500
675/125000
680
685
0.67
0.74
À1.01
0.8
1
E = =0.74 V versus SCE as-
2
signed to the phthalocyanine-
TT4
TT5
672/125893
684/173780
680
695
3.2 (89.1%)
3.8 (10.9%)
3.42 (100%)
ring oxidation processes. When
scanning towards negative po-
tentials, one reversible wave
À1.05
1.5 (100%)
–
[a] lmax =absorption maxium; e: absorption extinction coefficient. [b] lem =Emission maxmimum; the samples
were dissolved in THF. The excitation wavelength was lex =650 nm. [c] t=Emission lifetime: Samples were
excited at lex =635 nm and the emission was monitored at lem =700 nm. [d] The samples were measured in
THF with TBAP (0.1m) as electrolyte and SCE as reference electrode. [e] From reference [9].
1
was observed at E = =À1.03 V,
2
which we assigned to the
phthalocyanine-ring-based re-
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Zinc Phthalocyanines
FULL PAPER
Figure 3. Electron recombination dynamics for a 4 mm thick TT2- and
TT3-sensitised mesoporous TiO2 film. The sensitised film optical absorb-
Figure 2. Time-correlated single photon counting measurements for TT4-
sensitised Al2O3 and TiO2 films (lex =635 nm, lem =700 nm). IR stands
for the instrument response (325 ps). The acquisition time was 1500 s.
The sensitised film optical absorbance at l=635 nm was 0.7 a.u. for both
films.
ance at the laser excitation wavelength (lex =520 nm for TT3, lex
=
550 nm for TT2) was 0.7 a.u. for each film. The probe wavelength for the
measurement was fixed at lprobe =600 nm. The fitting corresponds to a
stretched exponential (see text for details).
that, in the case of phthalocyanine dyes, faster electron in-
jection is a requisite and, thus, good orbital coupling and in-
jection directionality between the dye and the titanium orbi-
tals is a must.
Table 2. Electron recombination kinetic parameters and efficiency meas-
urements of the tested phthalocyanines (see text for details).
a[a] JSC
[mAcmÀ2
VOC
FF[b]
h
1
Sample t =
2
[b]
[ms]
]
[mV][b]
[%][b]
TT1
TT2
TT3
TT4
TT5
3.2
11.7
0.11
5.2
0.62 7.60Æ0.20
0.9 0.90Æ0.20
0.18 4.80Æ0.20
0.97 1.44Æ0.20
0.52 6.80Æ0.20
617Æ20
550Æ10
610Æ10
611Æ10
613Æ10
0.75Æ0.02 3.52
0.72Æ0.02 0.4
0.74Æ0.02 2.20
0.75Æ0.01 0.67
0.74Æ0.01 3.10
Electron recombination kinetics: Once the electron-injec-
tion dynamics were determined, we turned to the recombi-
nation dynamics for the Zn–phthalocyanine, nanocrystal-
line–TiO2-sensitised films. We employed laser transient ab-
sorption spectroscopy (TAS) to measure the decay and the
excited spectrum of the light-induced cation for the sensi-
tised samples. To ensure similar dye coverage, all of the
measured samples showed similar absorbance values at lex.
Figure 3 shows the typical data for the charge-recombination
process measured by TAS, and can be fitted to a stretched
3.9
[a] Calculated from the stretched exponential [DO.D. =exp(À
ACHTUNGTRENNUNG
(t/t)a].
[b] Short circuit photocurrent density (JSC), open circuit voltage (VOC),
fill factor (FF), and overall conversion efficiency (h) related in the equa-
tion: h=JSC VOC FF/Pin (Pin incident radiation power, mWcmÀ2).
that the recombination process begins before we can mea-
sure. However, the fast electron injection dynamics for TT3
avoids the potential kinetic competition between electron
injection and electron recombination between the photo-in-
jected electrons and the oxidised TT3.
The spectrum of the excited state for TT2/TiO2 and TT3/
TiO2 was also recorded under ambient conditions. Both
samples show similar features with a broad maximum at l=
525 nm, which we assign to the phthalocyanine cation after
injection of one electron into the semiconducting band as
other groups have previously reported.[7,32]
exponential [DO.D. =exp(À
(t/t))a]. The stretched exponen-
ACHTUNGTRENNUNG
tial kinetics has been discussed for a wide range of sensitis-
ers employed in dye-sensitised solar cells (DSSC) and has
been assigned to the presence of an inhomogeneous distri-
bution of electron traps in the film.[28–30] The recombination
dynamics (lifetime, measured at half maximum of the signal,
and a value) for the samples are shown in Table 2. In the
cases of TT1, TT2, TT4 and TT5, the electron recombina-
tion dynamics is ten times slower than that of the widely
used ruthenium dye N719 (cis-bis(isothiocyanato)bis(2,2’-bi-
pyridyl-4,4’-dicarboxylate)ruthenium(II)
bis-tetrabutylam-
On the other hand, we have carried out electron recombi-
nation kinetic studies in complete functional devices.
Figure 4 shows the measured recombination lifetimes at dif-
ferent electron density for each device. It is worth noting
that all devices were made by using the same TiO2 thickness
(3 mm) as well as the same electrolyte (0.6M 1-butyl-3-meth-
1
monium) that we found to give t = =0.28 ms (a=0.34). The
2
fact that the recombination dynamics for the TT samples,
except TT3, show a high value of a implies that the injected
electrons had sufficient time to thermally equilibrate among
all the available trap sites[30,31] before they recombined with
the oxidised dye, and, therefore, a quasi-monoexponential
decay is observed. In the case of TT3, we did not observe a
plateau at faster recombination timescales (ms), indicating
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
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T. Torres, E. Palomares, M. K. Nazeeruddin et al.
Figure 4. Electron recombination dynamics for a 4 mm thick TT-sensitized
1 cm2 TiO2 solar cell.
the differences between the devices are significant. For ex-
ample, under the same experimental conditions, TT2 and
TT4 show less charge (electrons) density than TT1, TT3 and
TT5 devices. This fact makes the comparison between the
different devices very difficult. However, we can already
foresee that in the case of TT2 and TT4, the latter shows
faster recombination dynamics and, therefore, we should
expect lower device performance not only in photocurrent
but also in photovoltage. Moreover, according to the mea-
sured electron density, we can expect low photocurrent for
the TT2 devices when compared to TT1, TT3 and TT5.
Figure 5. Photocurrent action spectrum (top) and current-voltage charac-
teristics (bottom) of TT1 (e), TT2 (a), TT3 (c), TT4 (b), and TT5 (d) ob-
tained with a nanocrystalline TiO2 film supported on a conducting glass
sheet and derivatised with a monolayer of phthalocynine sensitisers in
the presence of chenodeoxycholic acid. A sandwich-type cell configura-
tion was used to measure these spectra.
Dye-sensitised solar cells: The modification of the selected
linker groups within the series TT1–TT5 allows not only a
variable distance between the dye and the photosensitised
nanocrystalline TiO2, but also a distinct orientation of the
dye molecular plane with regard to the semiconductor sur-
face, which modifies the orbital coupling between the dyes
and the semiconductor.[33,34] To reduce aggregation of dye
molecules, dye solution involving 60 (TT1) or 120 mm (TT2–
TT5) 3a,7a-dihydroxy-5b-cholanic acid (CDCA) is used for
sensitization process. Figure 5 shows the IPCE obtained
with a sandwich cell by using N-methyl-N-butylimidazolium
iodide (0.6m), iodine (0.04m), LiI (0.025m), guanidinium
with a rigid and conducting bridge in TT3, the incident
monoACTHNUGRTENUNGchromatic photon-to-current conversion efficiency in-
creased phenomenally from 9 to 56%. In fact, TT3, which
has a rigid connection between the phthalocyanine moiety
and the phenyl carboxylic unit, exhibits remarkable IPCE
spectra due to the high directionality in the excited state of
the sensitiser, which must be located perpendicular to the
TiO2 surface. On the other hand, TT5 also has a rigid bridge
with an extended conjugation and showed higher photovol-
taic performance when compared to TT3. This result is
caused by enhanced light-harvesting yield (molar extinction
coefficient) of TT5 due to the mentioned extended conjuga-
tion and slower recombination processes. These observations
are in good agreement with the recombination kinetics car-
ried out in the devices as explained above
To prove the “directional effect,” TT4-sensitised solar
cells were prepared. This low result is in total agreement
with previous findings and is caused by poor electronic cou-
pling of the phthalocyanine donor with TiO2 d orbitals, be-
cause of the flexible and non-directional oxygen linker. Fi-
nally, when there is no bridge between the anchoring car-
boxylic acid group and the zinc phthalocyanine, like in TT1,
the increase in the IPCE value is outstanding (85%) when
compared to the TT4 (IPCE 16%) and TT5 (IPCE 65%)
sensitisers. The TT1 sensitiser under standard global AM 1.5
solar conditions yielded a total efficiency of 3.55%. The im-
thio
ACHTUNGTRENNUNGcyanate (0.05m) and tert-butylpyridine (0.28m) in 15:85
ACHTUNGTRENNUNG
trolyte. The IPCE data of all sensitisers plotted as a function
of excitation wavelength exhibit as low as 9% efficiency for
TT2 to extraordinarily high 85% efficiency for TT1.
Under standard global air mass (AM) 1.5 solar conditions,
the TT2-sensitised cell gave a poor performance. We believe
that this discouraging result is most probably due to the low
directionality in the excited state of the sensitiser as well as
the intrinsic properties of the linker, which has a non-conju-
gated and flexible moiety as an anchoring extension from
the aromatic core of the phthalocyanine to the nanocrystal-
line surface of the semiconductor nanoparticle. In contrast,
when the flexible and insulating bridge in TT2 was replaced
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Chem. Eur. J. 2009, 15, 5130 – 5137

Zinc Phthalocyanines
FULL PAPER
9(10),16(17),23(24)-Tri-tert-butyl-2-[4-(hydroxymethyl)phenoxy]phthalo-
proved efficiency of the TT1 compared to the TT5, TT3,
TT4 and TT2 sensitisers (in this order) demonstrates the in-
fluence the nature of the bridge between the anchoring car-
boxylic acid group and zinc phthalocyanine.
To the best of our knowledge, these data provide a major
breakthrough in the design and development of near-infra-
red-based sensitisers. Therefore, we believe that the findings
of this study should spark a broad spectrum of interest in
molecular engineering of low-band-gap sensitisers for solar
cells.
cyaninatozinc(II) (mixture of regioisomers) (2): Phthalonitrile
(1.23 mmol), 4-tert-butylphthalonitrile (907 mg, 4.92 mmol) and Zn-
(OAc)2 (300 mg,1.64 mmol) were stirred in DMAE (7.0 mL) and heated
1
AHCTUNGTRENNUNG
at reflux under an argon atmosphere for 18 h. After cooling to room tem-
perature, the intense blue solution was poured into water (150 mL) and
the precipitate collected by filtration through celite. The resulting blue
solid was washed with water, methanol/water mixtures (1:2) and (1:1)
(100 mL each) and finally with MeOH (50 mL). Afterwards, the dried
solid was redissolved in THF and purified by column chromatography on
silica gel using hexane/dioxane (2:1) and (1:1) as eluent. The isolated
blue solid was triturated in hot hexane, filtered and dried to afford 2 as a
dark blue solid (352 mg, 33%). M.p. >2508C; 1H NMR (500 MHz,
[D6]DMSO, 258C, TMS): d=1.9–1.7 (s, 27H; C
CH2OH), 5.32 (m, 1H; CH2OH), 7.8–7.5 (m, 4H; ArH), 8.4–8.1 (m, 4H;
PcH), 9.3–8.6 ppm (m, 8H; PcH); UV/Vis (THF): lmax(loge)=350 (4.9),
ACHTUGNTRENUN(NG CH3)3), 4.66 (m, 2H;
Conclusion
AHCTUNGTRENNUNG
608 (4.5), 673 nm (5.3); MS (MALDI-TOF, dithranol): m/z (%): 873–866
(100) [M]+; HR MALDI-TOF MS, dithranol: m/z calcd for
C51H46N8O2Zn: 866.30352; found: 866.30297 [M]+; elemental analysis
calcd (%) for C51H46N8O2Zn (866.30352): C 70.54, H 5.34, N 12.90;
found: C 70.60, H 5.35, N 12.89.
In summary, we have developed a series of phthalocyanine
sensitisers, which demonstrate the impact of anchoring
group on photovoltaic performance, yielding power conver-
sion efficiencies from 0.4 to 3.55% under light-simulated 1.5
AM irradiation conditions. Also, our finding demonstrates
that creating directionality in the excited state of the sensi-
tiser by adjusting the electron densities of donor moieties
and the anchoring group is the key for the unprecedented
efficiency of TT1 and probably for the design of future red-
light-absorbing molecular materials. Moreover, we found
that faster electron injection kinetics are needed with these
dyes in order to achieve higher efficiencies. These faster
electron-injection kinetics are directly related to the direc-
tionality (orbital coupling) of these dyes. Further work is
being carried out to increase the overlap between the dyesꢇ
LUMO and the 3d titanium orbitals through modification of
the peripheral anchoring ligands.
9(10),16(17),23(24)-Tri-tert-butyl-2-[(4-formyl)phenoxy]phthalocyanin-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
DMSO (47.0 mL) at room temperature. The solution was stirred at room
temperature for 5 h and then, poured over brine (200 mL) and extracted
with Et2O (3ꢆ40 mL). The organic layer was separated, washed with a
saturated aqueous solution of NaHCO3 (2ꢆ60 mL) and brine (2ꢆ
60 mL), and dried over Na2SO4. After filtration of the drying agent, the
solvent was evaporated and the blue solid was purified by column chro-
matography on silica gel using hexane/dioxane (3:1) as eluent, affording
aldehyde 3 as a dark blue solid (137 mg, 81%). M.p. >2508C; 1H NMR
(500 MHz, [D6]DMSO, 258C, TMS): d=1.8–1.7 (m, 27H; CACHTNUGRTNEUNG(CH3)3), 7.8–
7.6 (m, 4H; ArH), 9.3–8.1 (m, 12H; PcH), 10.1–9.9 ppm (m, 1H; CHO);
FTIR (KBr): n˜ =2961, 2907, 2866, 1705 (C=O), 1597, 1485, 1404, 1337,
1242, 1163, 1095, 1055, 947, 831, 758, 696, 530 cmÀ1; UV/Vis (THF):
lmax(loge)=349 (4.9), 607 (4.6), 672 nm (5.4); MS (MALDI-TOF, dithra-
nol): m/z (%): 871–864 (100) [M]+; HR MALDI-TOF MS, dithranol:
m/z calcd for C51H44N8O2Zn: 864.28787; found: 864.28732 [M]+; elemen-
tal analysis calcd (%) for C51H44N8O2Zn (864.28787): C 70.71, H 5.12, N
12.93; found: C 70.73, H 5.10, N 12.98.
Experimental Section
9(10),16(17),23(24)-Tri-tert-butyl-2-[(4-carboxy)phenoxy]phthalocyanin-
AHCTUNGTRENGaNNU tozinc(II) (mixture of regioisomers) (TT4): NaClO2 (47 mg, 0.50 mmol)
Phthalocyanines TT1,[9] TT2,[16] and TT3[16] were prepared following
slightly modified procedures previously described by us.
was added in a few portions to a vigorously stirred solution of aldehyde 3
(0.17 mmol) cooled to 08C in acetone (96 mL). Then, a solution of sulfa-
mic acid (51 mg, 0.50 mmol) in Milli-Q grade deionised water (12 mL)
was added all in one portion. The reaction was allowed to proceed at
room temperature for 4 h. After the starting compound had disappeared,
the solution was poured into aqueous HCl (0.1m, 400 mL) and a blue-
green solid precipitated. The solid was filtered over celite and washed
with water and mixtures of water/MeOH (3:1 and 2:1, 100 mL each).
Then, it was dried under vacuum and extracted with THF. The solvent
was evaporated, and the crude product was triturated in hexane, filtered,
and washed with a solution of water/MeOH (1:1, 50 mL) and finally with
cold MeOH (25 mL) to afford a dark blue solid. Further purification was
accomplished by column chromatography (reverse phase) employing
water/THF (1:1) as eluent affording TT4 (81 mg, 54%). M.p. >2508C;
1H NMR (500 MHz, [D6]DMSO, 258C, TMS): d=1.9–1.5 (s, 27H; C-
4-[4-(Hydroxymethyl)phenoxy]phthalonitrile (1): 4-Nitrophthalonitrile
(1.00 g, 5.77 mmol) and Cs2CO3 (3.76 g, 11.54 mmol) in anhydrous DMF
(50.0 mL) were stirred under argon for 15 min. Then, 4-(hydroxymethyl)-
phenol (1.43 g, 11.54 mmol) was added slowly in a few portions, and the
mixture was heated at 408C for 72 h. After cooling, the yellow-greenish
suspension was added to brine (300 mL) and extracted with diethyl ether
(5ꢆ50 mL). The organic extracts were washed with brine (50 mL) and
dried over Na2SO4. The drying agent was removed by filtration and the
solvent evaporated in vacuum. The resulting yellow-green oily crude was
purified by column chromatography over silica gel by using ethyl acetate
(EtOAc) as an eluent. Compound 1 was obtained as a yellow oil (0.94 g,
65%) that solidifies upon standing. M.p. 75–778C; 1H NMR (300 MHz,
CDCl3, 258C, TMS): d=2.25 (brs, 1H; CH2OH), 4.74 (s, 2H; CH2OH),
7.07 (d, J=8.6 Hz, 2H; H-3’, H-5’), 7.24 (dd, J=8.3 Hz, J=2.6 Hz, 1H;
H-5), 7.26 (d, 4J=2.6 Hz, 1H; H-3), 7.47 (d, 3J=8.6 Hz, 2H; H-2’, H-6’),
7.72 ppm (d, 3J=8.3 Hz, 1H; H-6); 13C NMR (75 MHz, CDCl3): d=68.1
(CH2OH), 108.9 (C-1), 115.1 (C-2), 115.5 (2ꢆCN), 117.7 (C-2’, C-6’),
120.8 (C-3), 121.6 (C-5), 129.3 (C-3’, C-5’),135.6 (C-6), 139.3 (C-4’), 152.9
(C-1’),162.0 ppm (C-4); FTIR (KBr): n˜ =3472 (O-H, associated dimer),
3
3
4
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(4.9), 607 (4.6), 672 nm (5.3); MS (MALDI-TOF, dithranol): m/z (%):
887–880 (100) [M]+; HR MALDI-TOF MS, dithranol: m/z calcd for
C51H44N8O3Zn: 880.28278; found: 880.28224 [M]+; elemental analysis
calcd (%) for C51H44N8O3Zn (880.28278): C 69.42, H 5.03, N 12.70;
found: C 69.45, H 5.00, N 12.78.
ꢁ
3072, 3045 (O-H, non-associated dimer), 2945, 2878, 2235 (C N), 1603,
1495, 1427, 1263, 1247, 1207, 1099, 1045, 1018, 951, 897, 843 cmÀ1; MS
(EI): m/z (%): 251 (97) [M]+, 249 (38) [MÀH]+, 234 (19) [MÀOH]+, 221
(96) [M+HÀCH2OH]+, 192 (40) [M2+HÀHCN]+, 107 (100) [C7H7O]+.
Chem. Eur. J. 2009, 15, 5130 – 5137
ꢅ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5135

T. Torres, E. Palomares, M. K. Nazeeruddin et al.
9(10),16(17),23(24)-Tri-tert-butyl-2-[(4-bromo)phenyl]phthalocyaninato-
zinc(II) (mixture of regioisomers) (5): A mixture of iodophthalocyanine
(20% in H2O, colloidal dispersion). The colloidal dispersion (15 mL) was
mixed with hydroxypropyl cellulose (0.35 g; 2 wt% of Al2O3). The mix-
ture was stirred for 7 d at 658C. For the TiO2 colloids, we followed previ-
ous literature reports.[20] To prepare the transparent mesoporous metal
oxide films, transparent glass cover slides were cleaned and a drop of the
corresponding metal oxide paste was spread using a glass rod. After
drying the films in air, the films were calcined at 4508C for 30 min. The
measured thickness was 4 mm.
4
(40.0 mg, 0.046 mmol), 4-bromophenylboronic acid (10.2 mg,
0.051 mmol), [Pd(PPh3)4] (26.6 mg, 0.023 mmol) and K2CO3 (17.7 mg,
ACHTUNGTRENNUNG
0.128) in anhydrous DMF (4.0 mL) was heated at 458C under argon for
18 h. The solution was then poured into brine (150 mL) and extracted
with Et2O (2ꢆ50 mL). The organic extracts were washed with brine (2ꢆ
30 mL), dried over MgSO4 and filtered. After evaporation of the solvent,
compound 5 was separated from the remaining iodophthalocyanine by
using a chromatographic column (SiO2, hexane/dioxane (3:1); the eluent
polarity was gradually increased to 2:1). The obtained solid was triturat-
ed in methanol and hexane, filtered and dried, yielding 5 as a dark blue
solid (14.0 mg, 34%). M.p. >2508C; 1H NMR (500 MHz, [D6]DMSO,
Electron recombination measurements: Laser transient absorption spec-
troscopy was used to determine the recombination lifetimes of TT1–TT5
phthalocyanine-sensitised TiO2 films. All the films had the same absorb-
ance value. The experiments were carried out as reported before.[15]
Briefly, a PTI nitrogen dye laser model was used as excitation source
with the appropriated dye solution. The laser power was kept constant
during the experiment to 0.05 mJcmÀ2 with a repetition rate of 1 Hz
(pulse duration less than 1 ns). The resulting photoinduced change in op-
tical density was monitored by using a 150 W tungsten lamp with 20 nm
bandwidth PTI monochromators before and after the sample, a photo-
diode-based detection system from Costronic Electronics and a TDS-220
Tecktronic DSO oscilloscope.
258C, TMS): d=1.9–1.7 (s, 27H, C
ACHTUNGTRENNUNG
8.9 ppm (m, 3H; PcH); UV/Vis (THF): lmaxAHCTUNTGRENNUNG
676 nm (5.0); MS (MALDI-TOF, dithranol): m/z (%): 908–899 (100)
[M]+; elemental analysis calcd (%) for C50H44BrN8Zn: C 66.56, H 4.92, N
12.42; found: C 66.66, H 4.92, N 12.39.
(E)-9(10),16(17),23(24)-Tri-tert-butyl-2-{4-[2-(ethyl)acryloyl]phenyl}ph-
thalocyaninatozinc(II) (mixture of regioisomers) (6): Bromophenylphtha-
locyanine 5 (20.0 mg, 0.022 mmol), [PdACTHNUTRGNEUNG(PPh3)4] (12.7 mg, 0.011 mmol)
Electron injection measurements: Time-correlated single-photon-count-
ing measurements of complete DSSC devices were carried out by using
an Edinburgh Instruments system model LifeSpec-PS. As an excitation
and K2CO3 (8.3 mg, 0.060 mmol) were heated to reflux in ethyl acrylate
(3.0 mL) for 18 h in an argon atmosphere. After cooling to room temper-
ature, excess ethyl acrylate was removed under vacuum and the crude
was purified by column chromatography on silica gel using hexane/diox-
ane (2:1) as eluent to yield 6 (14.0 mg, 69%) as a dark blue solid. M.p.
>2508C; 1H NMR (500 MHz, [D6]DMSO, 258C, TMS): d=1.7–1.6 (s,
27H; (CH3)3), 6.51 (d, 3J=16.8 Hz, 1H; ArCH=CHCO2Et), 7.62 (d, 3J=
16.8 Hz, 1H; ArCH=CHCO2Et), 7.79 (br s, 2H; ArH), 8.4–8.0 (m, 6H;
PcH, ArH), 9.5–9.0 ppm (m, 8H; PcH); FTIR (KBr): n˜ =2959, 2876,
1715, 1634, 1487, 1460, 1393, 1366, 1313, 1261, 1167, 1126, 1099, 980, 806,
source we used
a picosecond laser diode lex =635 nm (laser power
2 nJcmÀ2 by pulse) with an instrument response of 325 ps FWHM (full
width at half maximum). The devices were made using a 4 mm thick
transparent film sensitised with the corresponding phthalocyanine dyes.
The electrolyte was composed of a solution of 1-propyl-2,3-dimethylimi-
dazolium iodide (DMPII; 0.60m), iodide (0.04m), lithium iodide
(0.025m), in a mixture of acetonitrile and valeronitrile (85:15 volume
ratio). As control samples, we utilised Al2O3-sensitised films with identi-
cal absorbance at the excitation wavelength. The high conduction band
of Al2O3 prevents the electron injection from the dye-excited state. The
calculation of the injection yield was done by comparison of the area
under the signal area of the control and the sample.
459 cmÀ1; UV/Vis (THF): lmax
ACTHUNRTGNE(GNU loge)=352 (4.9), 610 (4.5), 672, (5.2),
685 nm (5.2); MS (MALDI-TOF, dithranol): m/z (%): 925–918 (100)
[M]+; elemental analysis cald. (%) for C55H50N8O2Zn: C 71.77, H 5.48, N
12.17; found: C 71.73, H 5.55, N 12.21.
(E)-9(10),16(17),23(24)-Tri-tert-butyl-2-[(4-acryloyl)phenyl]phthalocyani-
natozinc(II) (mixture of regioisomers) (TT5): An aqueous solution
(3.0 mL) of KOH (8.4 mg, 0.150 mmol) was added dropwise to a stirred
solution of ester 6 (14.0 mg, 0.015 mmol) in dioxane (25 mL). The mix-
ture was then heated at reflux in an argon atmosphere for 1 h. After
cooling, a solution of HCl (0.1m) was added dropwise until pH <5. The
dark green mixture was poured into brine (125 mL), extracted with Et2O
(3ꢆ20 mL) and dried over Na2SO4. After filtration of the drying agent,
the solvents were removed under and the crude product was purified on
a chromatographic column (reverse phase, THF/water 5:2). In this way
carboxyphthalocyanine TT5 was afforded as a dark blue-green solid
(12.0 mg, 90%). M.p. >2508C; 1H NMR (500 MHz, [D6]DMSO, 258C,
Electrochemical measurements: Measurements were done by cyclic vol-
tammetry in a conventional thee-electrode cell connected to a CH Instru-
ments 660c potentiostat–galvanostat. We used tetrabutyl ammonium per-
chlorate (TBAP) as the electrolyte, a platinum working electrode, a calo-
melan reference electrode (SCE) and a platinum wire as an auxiliary
electrode. The 0.5ꢆ10À4 m solutions of the samples in THF were purged
with Ar for 5 min prior to the measurements.
Dye-sensitised solar cells: The screen-printed double-layer film of TiO2
consisted of a 9–10 mm transparent layer and a 4 mm scattering layer and
were prepared and treated with titanium tetrachloride solution (0.05m)
using a previously reported procedure.[21,22] The film was heated to 5008C
in air and calcined for 20 min before use. Dye solutions were prepared in
the concentration range of 0.5–1ꢆ10À4 m solution in ethanol containing
TMS): d=1.7–1.6 (s, 27H; CACHTNUGRTNEUNG
(CH3)3), 6.49 (d, 3J=16.8 Hz, 1H; ArCH=
CHCO2H), 7.55 (d, 3J=16.8 Hz, 1H; ArCH=CHCO2H), 7.79 (m, 2H;
ArH), 8.3–8.0 (m, 6H; PcH, ArH), 9.5–9.0 (m, 8H; PcH), 11.97 ppm
(brs, 1H; COOH); FTIR (KBr) n˜ =3412, 2957, 2930, 2862, 1697, 1634 ,
1447, 1433, 1393, 1367, 1329, 1288, 1261, 1198, 1157, 1090, 1049, 930, 825,
3a,7a-dihydroxy-5b-cholanic acid (Cheno; 60 (TT1) or 120 mm ACHTUNGTRENNUNG(TT2–
TT5)). The electrodes were soaked in the dye solution for 4 h at 228C
and the dye-coated electrodes were rinsed quickly with ethanol and used
as such for photovoltaic measurements. The electrolyte was composed of
N-methyl-N-butyl imidiazolium iodide (0.6m), iodine (0.04m), LiI
(0.025m), guanidinium thiocyanate (0.05m) and tert-butylpyridine (0.28m)
in 15:85 (v/v) mixture of valeronitrile and acetonitrile. The dye-adsorbed
TiO2 electrode and the thermally platinised counter electrode were as-
sembled into a sealed sandwich type cell with a gap of a hot-melt iono-
756, 698 cmÀ1; UV/Vis (THF): lmax
ACTHUNTGRENNG(U loge)=352 (4.9), 611 (4.5), 673 (5.2),
684 nm (5.2); MS (MALDI-TOF, dithranol): m/z (%): 897–890 (100)
[M]+; HR MALDI-TOF MS, dithranol: m/z calcd for C53H46N8O2Zn:
890.30352; found: 890.30297 [M]+; elemental analysis calcd (%) for
C53H46N8O2Zn (890.30352): C 71.33, H 5.20, N 12.56; found: C 71.39, H
5.23, N 12.61.
AHCTUNGERTGmNNUN er film (Surlyn 1702, 25 mm thickness, Du-Pont). To reduce scattered
UV/Vis and emission fluorescence measurements: The UV/Vis spectra of
all the phthalocyanine dyes in THF (HPLC degree) were recorded on a
Shimadzu UV/Vis system model 1700. The fluorescence emission proper-
ties of the dyes, either in solution or adsorbed onto the metal oxide films,
were measured under ambient conditions using an Aminco Bowman
Series 2 luminescence spectrometer equipped with a temperature control-
ler and a holder for films and solid samples.
light from the edge of the glass electrodes of the dyed TiO2 layer, a light-
shading mask was used on the DSSCs, so the active area was fixed to
0.2 cm2. For photovoltaic measurements of the DSSCs, the irradiation
source was a 450 W xenon light source (Osram XBO 450, USA) with a
Tempax 113 solar filter (Schott). The output power of an AM 1.5 solar
simulator was calibrated by using a reference Si photodiode equipped
with a coloured matched IR cut-off filter (KG-3, Schott) in order to
reduce the mismatch in the region of 350–750 nm between the simulated
light and AM 1.5 to less than 2%. The measurement delay time of photo
I-V characteristics of DSSCs was fixed to 40 ms. The measurement of in-
Nanocrystalline TiO2 and Al2O3 films for optical measurements: The
metal oxide nanoparticles were sensitised as reported previously.[20] In
brief, Al2O3 nanoparticles were purchased from Alfa-Aesar chemical
5136
www.chemeurj.org
ꢅ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 5130 – 5137

Zinc Phthalocyanines
FULL PAPER
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This work has been supported by the Spanish MEC (CTQ-2008-00418-
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Received: August 28, 2008
Revised: December 31, 2008
Published online: March 31, 2009
Chem. Eur. J. 2009, 15, 5130 – 5137
ꢅ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5137