Synthesis and characterization of silicon phthalocyanines bearing axial phenoxyl groups for attachment to semiconducting metal oxides

Article abstract of DOI:10.1142/S1088424611003847

A series of axial phenoxy substituted octabutoxy silicon phthalocyanines bearing ethyl carboxylic ester and diethyl phosphonate groups have been prepared from the corresponding phenols in pyridine. Axial bis-hydroxy silicon phthalocyanine was prepared using an adaptation of a reported protocol [1, 2] from the octabutoxy free-base phthalocyanine. The phenols bear either carboxylic ester or phosphonate groups, which upon deprotection can serve as anchoring groups for attaching the phthalocyanines to semiconducting metal oxides used in dye sensitized solar cells (DSSCs). All the phthalocyanines of the series absorb in the near infra-red region: 758-776 nm. The first oxidation potential for each phenoxy derivative occurs near 0.55 V vs. SCE as measured by cyclic voltammetry, with all falling within a 10 mV range. This indicates that these dyes will have sufficient energy in the photo-excited state to drive the reduction of protons to hydrogen. Taking into account the absorption and electrochemical potentials, these dyes are promising candidates for use in dual-threshold photo-electrochemical cells.

Full text of DOI:10.1142/S1088424611003847

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2011; 15: 943–950
DOI: 10.1142/S1088424611003847
Published at http://www.worldscinet.com/jpp/
Synthesis and characterization of silicon phthalocyanines
bearing axial phenoxyl groups for attachment to
semiconducting metal oxides
Jesse J. Bergkampa, Benjamin D. Shermana, Ernesto Mariño-Ochoab,
Rodrigo E. Palaciosc, Gonzalo Cosad, Thomas A. Moore*a, Devens Gust*a and
Ana L. Moore*a
a Department of Chemistry and Biochemistry, Center for Bio-Inspired Solar Fuel Production,
Arizona State University, Tempe, Arizona 85287-1604, USA
b Department of Chemistry, Tecnológico de Monterrey, Campus Monterrey, Monterrey, NL, 64849, México
c Departamento de Química, Facultad de Ciencias Exactas Físico-Químicas y Naturales,
Universidad Nacional de Río Cuarto, Río Cuarto, Córdoba 5800, Argentina
d Department of Chemistry, McGill University, Otto Maass Chemistry Building # 314, 801 Sherbrooke Street West,
Montreal, QC, H3A 2K6, Canada
Dedicated to Professor Karl M. Kadish on the occasion of his 65th birthday
Received 1 May 2011
Accepted 14 June 2011
ABStrAct: A series of axial phenoxy substituted octabutoxy silicon phthalocyanines bearing ethyl
carboxylic ester and diethyl phosphonate groups have been prepared from the corresponding phenols
in pyridine. Axial bis-hydroxy silicon phthalocyanine was prepared using an adaptation of a reported
protocol [1, 2] from the octabutoxy free-base phthalocyanine. The phenols bear either carboxylic
ester or phosphonate groups, which upon deprotection can serve as anchoring groups for attaching
the phthalocyanines to semiconducting metal oxides used in dye sensitized solar cells (DSSCs). All
the phthalocyanines of the series absorb in the near infra-red region: 758–776 nm. The first oxidation
potential for each phenoxy derivative occurs near 0.55 V vs. SCE as measured by cyclic voltammetry,
with all falling within a 10 mV range. This indicates that these dyes will have sufficient energy in the
photo-excited state to drive the reduction of protons to hydrogen. Taking into account the absorption
and electrochemical potentials, these dyes are promising candidates for use in dual-threshold photo-
electrochemical cells.
KEYWordS: silicon phthalocyanines, synthesis, electrochemistry, axial phenoxy linkage.
energy demands while also mitigating our impact on the
IntroductIon
environment [3]. Using the energy of sunlight to drive
Developing artificial photosynthetic technologies for
the conversion of water to oxygen and hydrogen offers an
converting solar energy to transportable, energy-dense
attractive means of converting and storing solar energy.
fuels would be a major step towards meeting future human
A device capable of splitting water with light requires
several key components, including catalysts for the oxi-
SPP full member in good standing
dation of water and production of hydrogen and photo-
chemical systems capable of generating and stabilizing
charge separated states. The latter are of interest in this
work and require light absorbing materials judiciously
chosen for their absorbance and redox properties.
*Correspondence to: Ana L. Moore, email: amoore@asu.edu,
tel: +1 480-965-2953, fax: +1 480-965-2747; Devens Gust,
email: gust@asu.edu and Thomas A. Moore, email: tom.
moore@asu.edu
Copyright © 2011 World Scientific Publishing Company

944
J.J. BergkamP et al.
To date, artificial photosynthetic systems have suf-
fered from high cost, a need for rare materials, and/or
low overall efficiencies for generating hydrogen [4, 5].
An appealing route for incorporating potentially lower
cost materials that enable fine-tuning of light absorption
and the generation of long-lived, stable charged sepa-
rated states lies in the use of organic dyes. Organic chro-
mophores such as porphyrins, chlorins, perylenes, and
phthalocyanines that bear carboxylic acid or phosphonate
groups share many properties with natural dyes involved
in photosynthesis and have demonstrated their viability
in capturing and converting solar energy to electricity in
dye sensitized solar cells as shown in recent reviews
(Refs. 6 and 9) [6–9].
Producing an efficient device for the generation of
hydrogen from water with the only energy input coming
from light will likely require a design utilizing two light
absorbing photosystems, each of which harvests distinct
regions of the solar spectrum [10, 11]. When considering
organic dyes for use in such a system, careful balance of
the absorption and redox properties of the dyes plays an
integral role in the design; in a dual-threshold cell, the
dyes must interact with separate parts of the spectrum
while still accessing similar solar flux, and the excited
state energies and redox properties of each must provide
sufficient driving force for its electrochemical reaction
of interest.
adsorption to metal oxide semiconductors. We have intro-
duced phenyl or biphenyl axial substituents with ethyl
carboxylic ester or diethyl phosphonate functional groups
via a phenoxy linkage. Upon deprotection, the resulting
carboxylate or phosphonate moieties would serve as
anchoring groups to metal oxide semiconductors.
rESultS And dIScuSSIon
Joyner et al. [17] first reported a protocol for displac-
ing axial hydroxy ligands on silicon phthalocyanines
using molten phenol and a few drops of pyridine. Herein
we report an adaption of this method which yields mono-
phenoxy silicon phthalocyanines. Silicon insertion into
the free-base octabutoxy phthalocyanine involved first
using trichlorosilane in a mixture of dichloromethane
and tributylamine to yield the dichloride silicon phthalo-
cyanine in situ (Scheme 1). Displacement of the chloride
ligands in a mixture of water and triethylamine produced
the dihydroxy silicon phthalocyanine 1 with a 53% yield
(Scheme 1). Monosilation of 1 took place in refluxing
toluene with one equivalent of chlorotriethylsilane to
yield 2 with a 76% yield (Scheme 1). The reaction was
monitored via thin layer chromatography and quenched
upon formation of traces of the disilylated silicon phtha-
locyanine. Diethyl 4′-hydroxybiphenyl-4-ylphosphonate
3 was prepared by an adaptation of the Hirao palla-
dium-catalyzed cross-coupling reaction [18] in dry
dimethylformamide using tris(dibenzylideneacetone)-
dipalladium(0) as the catalyst, diisopropylethylamine as
the base, and 1,1′-bis(diphenylphosphino)ferrocene as
the ligand (Scheme 2). The linkage of the corresponding
phenols in compounds 4, 5, and 6 (Schemes 2 and 3) was
done in pyridine at 55 °C for 48–60 h. In all cases, two
column-chromatography steps were necessary for purifi-
cation of 4, 5, and 6 due to the tendency for streaking on
silica gel.
Phthalocyanines are good candidates for use in dual-
threshold photoelectrochemical cells because of their
high extinction coefficients, good chemical stability, and,
most importantly, excited state energies that provide suf-
ficient driving force for the cathodic reduction of protons
to hydrogen. Octabutoxy silicon phthalocyanines are
of particular interest because of their absorbance in the
visible and near-IR and the opportunity to functionalize
them by covalent attachment to the axial position of the
central silicon atom.
Herein we explore axially modifying silicon phthalo-
cyanines to introduce desirable functional groups for inte-
grating these dyes into photoelectrochemical cells. Axial
substitution is an attractive feature for multiple synthetic
reasons: (i) having substituents above and below the
macrocycle prevents aggregation and increases the solu-
bility of these compounds [12], (ii) the ligands can be
highly functionalized, and (iii) silicon phthalocyanines
are robust under harsh chemical treatments. Axially coor-
dinated ruthenium phthalocyanines have been shown to
photo-inject electrons into TiO2 [13], demonstrating that
the electronic coupling of axially-linked phthalocyanines
is suitable for electron injection into semiconducting
metal oxides. It is also worth noting that the use of non-
peripheral octabutoxy groups helps to reduce aggregation
[14], bathochromically-shifts the absorbance consider-
ably relative to peripherally substituted analogs [15], and
shifts the redox values to more negative potentials [16].
Here we report synthetic strategies for functionalizing
far red absorbing octabutoxy silicon phthalocyanines for
We started with a non-peripheral octaalkoxy substi-
tuted phthalocyanine for several reasons. Most impor-
tantly, alkoxy substituents aid in increasing the solubility
of the phthalocyanine in organic solvents while inhibit-
ing the tendency for aggregation [12, 14, 19]. Secondly,
we have chosen octabutoxy groups located at the non-
peripheral positions (1,4,8,11,15,18,22,25) because sub-
stitution at these sites has been shown to induce a greater
shift on the absorption spectrum to longer wavelengths
than octabutoxy groups on the peripheral positions [15].
We have inserted silicon into the macrocycle of the
phthalocyanine as this allows for axial covalent modifica-
tion. Typically, axial modification of silicon phthalocya-
ninesproceedsfromthedichloridesiliconphthalocyanine.
However, we found the dichloride species to be unstable,
possibly due to greater lability of the chloride ligands
resulting from electron donating effects and increased
solubility from the presence of the octabutoxy groups
at the non-peripheral positions of the phthalocyanine
macrocycle. Others have achieved the phenoxy linkage
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 944–950

SyntheSIS anD CharaCterIzatIOn Of SIlICOn PhthalOCyanIneS BearIng axIal PhenOxyl grOuPS
945
OC4H9
N
OC4H9
OH
C4H9O
C4H9O
N
N
OC4H9
OC4H9
OC4H9
OC4H9
N
C4H9O
C4H9O
C4H9O
C4H9O
NH
N
N
(i), (ii)
53%
N
Si
N
N
N
HN
N
N
N
N
OH
C4H9O
OC4H9
OC4H9
C4H9O
1
OC4H9
OH
OC4H9
OH
C4H9O
C4H9O
N
N
OC4H9
OC4H9
OC4H9
OC4H9
N
N
C4H9O
C4H9O
C4H9O
C4H9O
N
N
(iii)
Si
N
Si
N
N
N
76%
N
N
N
N
N
N
OH
OC4H9
OC4H9
C4H9O
C4H9O
O
Si
2
Scheme 1. Reaction conditions: (i) HSiCl3, TBA, CH2Cl2, rt, 18 h; (ii) TEA, H2O, rt, 3 h; (iii) HSiEt3, toluene, pyridine, reflux,
45 min
O
O
O
P
OC4H9
N
C4H9O
O
N
O
P
OC4H9
OC4H9
C4H9O
C4H9O
(i)
N
(ii)
HO
Br
HO
O
Si
N
N
53%
31%
N
N
O
3
N
OH
OC4H9
C4H9O
4
Scheme 2. Reaction conditions: (i) dimethylformamide, diethylphosphite, diisopropylethylamine (Hünig’s base), 1,1′-
bis(diphenylphosphino)ferrocene, tris(dibenzylideneacetone)dipalladium(0), 110 °C, 24 h; (ii) 1, pyridine, 55 °C, 48 h
OC4H9
C4H9O
OH
N
OC4H9
OC4H9
N
C4H9O
C4H9O
N
Si
N
N
N
N
N
OH
OC4H9
C4H9O
O
(ii)
54%
(i)
38%
O
O
O
OC4H9
N
OC4H9
N
C4H9O
C4H9O
O
O
N
N
OC4H9
OC4H9
OC4H9
OC4H9
C4H9O
C4H9O
C4H9O
C4H9O
N
N
Si
N
Si
N
N
N
N
N
N
N
N
N
OH
OH
OC4H9
OC4H9
C4H9O
C4H9O
5
6
Scheme 3. Reaction conditions: (i) ethyl 4′-hydroxybiphenyl-4-carboxylate, pyridine, 55 °C, 48 h; (ii) ethyl 4-hydroxybenzoate,
pyridine, 55 °C, 60 h
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 945–950

946
J.J. BergkamP et al.
1
The H NMR spectra of 4, 5, and 6
show an upfield shift of the aromatic
protons that are axial to the plane of the
phthalocyanine. Aromatic protons ortho
to the phenoxy linkage show signals at
~2.9 ppm, consistent with results from
other studies [23]. The magnitude of the
up field shift decreases as the location of
the proton moves further away from the
center of the phthalocyanine macrocycle
which is in accordance with the literature
[24–27]. The OH signal in the 1H NMR of
2, 4, 5, and 6 could not be detected; Cook
et al. have observed similar effects [28].
Each target compound was character-
ized by matrix assisted laser desorption/
ionization-time of flight (MALDI-TOF)
mass spectroscopy. Trans-2-[3-(4-tert-
butylphenyl)-2-methyl-2-propenylidene]-
malononitrile was found to be the best
matrix and free-base 1,4,8,11,15,18,22,25-
octabutoxyphthalocyanine was used as an
Fig. 1. Normalized absorption spectra for phthalocyanine derivatives 1, 2, 4, 5,
and 6. Inset shows an expanded view of the longest wavelength Q-band of each
derivative. Spectra were taken in distilled dichloromethane
internal reference.
[12, 16, 20, 21], but started from the non-substituted
An important aspect of evaluating dyes
dichloride silicon phthalocyanine and allowed it to react
with a strong base such as sodium hydride. Our method
starts from the dihydroxy species, as demonstrated by
Joyner and others [17, 22], but requires only mild reaction
conditions (such as lower temperatures), limited amounts
of starting phenol, and allows use of phenols with chemi-
cally sensitive functional groups. Furthermore, it allows
for the synthetic control necessary to produce the mono
axial phenoxy substituted silicon phthalocyanine.
Initially we set out to synthesize mono-phenoxy
phthalocyanine with the opposite face protected with a
triethylsiloxyl group. This strategy resulted in two issues
of concern: (i) the longest wavelength Q-band was blue
shifted in 2 with respect to 1 by 5 nm (Fig. 1), and,
(ii) after the phenoxy group is coupled, the stability of
the triethylsiloxyl group decreases as evidenced by dif-
ficulties in isolation. Therefore we decided to leave the
second axial hydroxyl group unprotected.
for photoelectrochemical applications lies in the deter-
mination of their redox properties in order to ensure that
they can carry out the photo-initiated electrochemical
processes of interest. We are targeting silicon phthalo-
cyanines in this work to drive the cathodic reduction of
protons to hydrogen; their excited state redox potentials
must be sufficiently negative to carry out this process,
which at pH 7 occurs at -0.65 V vs. SCE. Determin-
ing the potentials for the oxidation and reduction of a
dye, coupled with its absorption characteristics, allows
approximation of its excited state redox potentials. Cyclic
voltammetry experiments on the silicon phthalocyanine
Ultraviolet-visible spectral analysis reveals a shift to
longer wavelengths of the last Q-band upon phenoxy
displacement of the hydroxide. Compound 6 shows a
bathochromic shift of 12 nm with respect to 1 (Fig. 1)
with the last Q-band occurring at 776 nm. Both 4 and 5
have the biphenyl linker and, although they bear different
functional groups at the para position, their absorption
properties are similar in that the last Q-band is slightly
shifted to shorter wavelengths in comparison with 6
but longer wavelengths with respect to 1 (Fig. 1). The
absorbance of the last Q-bands of 4 and 5 indicates that
the ethyl carboxylic ester or diethyl phosphonate groups
do not have as strong an influence on the absorbance
presumably due to decoupling caused by the biphenyl
linker.
Fig. 2. Cyclic voltammograms for compounds 4, 5, and 6. All
scans shown were taken at 100 mV/s in dichloromethane with
0.1 M tetrabutylammonium hexafluorophosphate. The identity
of the phenoxy ligand had only subtle influence on the redox
potentials as evidenced by the similarity of each scan
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 946–950

SyntheSIS anD CharaCterIzatIOn Of SIlICOn PhthalOCyanIneS BearIng axIal PhenOxyl grOuPS
947
table 1. Redox values for the compounds 1, 4, 5, and 6. Potentials given
are in reference to a standard calomel electrode (SCE). Peak separation
(∆Ep) of the anodic and cathodic scans is given for each process
materials
Dichloromethane used for synthesis and elec-
trochemistry was refluxed over calcium hydride
followed by distillation and storage over acti-
vated 4 Å molecular sieves. Pyridine and dim-
ethylformamide were dried over activated 4 Å
molecular sieves. Tributylamine was passed
though activated alumina and stored overnight
over activated 4 Å molecular sieves. Toluene for
synthesis was distilled from phosphorus pen-
toxide and stored over activated 4 Å molecular
sieves. Toluene, ethyl acetate and hexane used for col-
umn chromatography were distilled. Triethylamine was
purchased from Alfa Aesar and used without further
purification. Trichlorosilane, 1,4,8,11,15,18,22,25-oc-
tabutoxy-29H,31H-phthalocyanine, chlorotriethylsilane,
diethylphosphite, diisopropylethylamine (Hünig’s base),
1,1′-bis(diphenylphosphino)ferrocene, 4′-bromo-(1,1′-
biphenyl)-4-ol, ethyl 4′-hydroxybiphenyl-4-carboxylate,
ethyl 4-hydroxybenzoate, and tris(dibenzylideneacetone)-
dipalladium(0) were purchased from Sigma-Aldrich and
used without further purification. Tetrabutylammonium
hexafluorophosphate was also purchased from Sigma-
Aldrich. Thin layer chromatography plates (250 micron),
both fluorescent and non-fluorescent, were purchased
from Analtech, Inc. Silica gel (SiliaFlash F60 40–63 µm)
used for column chromatography was purchased from
SILICYCLE.
Compound
Ox(1), (∆Ep)
Ox(2), (∆Ep)
Red(1), (∆Ep)
1
4
5
6
0.55 V, (64 mV) 0.95 V, (67 mV) -0.84 V, (80 mV)
0.55 V, (63 mV) 0.96 V, (64 mV) -0.90 V, (98 mV)
0.54 V, (64 mV) 0.96 V, (64 mV) -0.83 V, (98 mV)
0.54 V, (68 mV) 0.93 V, (67 mV) -0.82 V, (80 mV)
dyes synthesized for this work showed them to be good
candidates for driving proton reduction. Figure 2 features
cyclic voltammagrams for compounds 4, 5, and 6; the
redox potentials for these compounds and dye 1 are listed
in Table 1. Overall, the identity of the axial ligand to the
silicon had minimal influence on the redox potentials of
the phthalocyanines. For example, the potentials for the
first oxidation differed by only 10 mV across the series.
These molecules showed reversible first and second oxi-
dations with anodic and cathodic peak separations (∆Ep)
near 60 mV, while the first reductions gave greater peak
separations, implying more quasi-reversible character.
With an eye toward the eventual testing of these dyes
in a dual-threshold photoelectrochemical water splitting
cell, we are most interested in the potential of the first
oxidation. With midpoint potentials near 0.55 V vs. SCE
and longest-wavelength photon absorption of ~1.6 eV,
these dyes will have adequate excited state energies to
sensitize TiO2 or other semiconducting metal oxides
with a more negative conduction band [29]. Future
work will aim at investigating the efficiency of elec-
tron injection into metal oxide semiconductors by these
axial functionalized phthalocyanines, and their ability
to drive the formation of hydrogen in a photoelectro-
chemical cell.
General
1H NMR spectra were recorded on a 400 MHz Varian
Liquid-State spectrometer. NMR samples were dissolved
in deuteriochloroform with 0.03% tetramethylsilane as
an internal reference. Mass spectra were obtained on an
Applied Biosystems Voyager-DE STR matrix-assisted
laser desorption/ionization time-of-flight spectrometer
(MALDI-TOF). The matrix used for all mass spectra
samples was trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-
propenylidene]malononitrile. Ultraviolet-visible ground
state absorption spectra were measured using a Shimadzu
UV2100U spectrophotometer. All ultraviolet-visible sam-
ples were dissolved in distilled dichloromethane.
ExpErImEntAl
Electrochemistry
All electrochemical experiments were carried out with
a CH Instruments 760D potentiostat. All samples were
analyzed in a closed glass cell under an argon atmo-
sphere with a three electrode setup. A working platinum
disc electrode was used in concert with a platinum mesh
counter electrode and a silver quasi-reference electrode.
The potential of the silver quasi-reference electrode was
calibrated vs. the ferrocenium/ferrocene (Fc+/Fc) couple
at the conclusion of each series of experiments with the
Fc+/Fc couple taken as 0.45 V vs. SCE. Distilled dichlo-
romethane was used as the working solvent along with
0.1 M tetrabutylammonium hexafluorophosphate as sup-
porting electrolyte, which was doubly recrystallized from
ethanol and dried under reduced pressure with heat prior
to use.
Synthesis
dihydroxy-1,4,8,11,15,18,22,25-octabutoxyphth-
alocyaninatosilicon(IV) (1). A portion of 1,4,8,11,
15,18,22,25-octabutoxy-29H,31H-phthalocyanine (1.1 g,
1.0 mmol) was dissolved in dry dichloromethane (240 mL)
and dry tributylamine (24 mL). The solution was degassed
with argon for 10 min, followed by addition of trichlo-
rosilane (2.4 mL, 23.8 mmol) and allowed to stir at room
temperature for 18 h under argon. Triethylamine (80 mL)
was added carefully, followed by H2O (40 mL) and the
mixture was allowed to stir at room temperature for an
additional 3 h. To neutralize the amines, aqueous HCl
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 947–950

948
J.J. BergkamP et al.
(12 M, 58 mL) was added until the solution was acidic,
and the solution was allowed to stir for an additional 1 h.
The mixture was filtered and the solid was washed with
dichloromethane (500 mL). The mother liquor was sepa-
rated and the organic layer was washed twice with HCl
(1M,100mL)followedby3washeswithH2O.Theorganic
layer was dried over sodium sulfate and the solvent was
removed under reduced pressure. Column chromatogra-
phy was done using silica gel with ethyl acetate/toluene
(50:50) as eluent, which afforded a green solid. Yield
615mg(53%).1HNMR(400MHz;CDCl3;0.03%Me4Si):
δH, ppm -2.54 (2H, bs, –OH), 1.08 (24H, t, –CH2–CH2–
CH2–CH3, J = 7.2 Hz), 1.66 (16H, sex., –CH2–CH2–CH2–
CH3, J = 7.6 Hz), 2.20 (16H, p, –CH2–CH2–CH2–CH3,
J = 7.6 Hz), 4.87 (16H, t, –CH2–CH2–CH2–CH3, J =
7.2 Hz), 7.64 (8H, s, PcH). UV-vis (CH2Cl2): λmax, nm
334, 470, 687, 764. MS (MALDI-TOF): m/z calcd. for
C64H82N8O10Si 1150.59, obsd. 1150.59.
triethylsilylhydroxy-1,4,8,11,15,18,22,25-octabu-
toxyphthalocyaninatosilicon(IV) (2). A portion of (1)
(70 mg, 0.06 mmol) was dissolved in dry toluene (30
mL) and dry pyridine (4 mL) and degassed for 15 min.
Chlorotriethylsilane (10.2 µL, 0.06 mmol) was added and
the solution was allowed to reflux for 45 min. Once for-
mation of bis-triethylsilyl phthalocyanine was observed
(monitored by TLC), the solvent was removed under
reduced pressure and column chromatography was done
using silica gel with toluene/ethyl acetate (65:35) as elu-
ent, which afforded a green solid. Yield 59 mg (76%).
1H NMR (400 MHz; CDCl3; 0.03% Me4Si): δH, ppm -2.14
(6H, q, –CH2–CH3, J = 8 Hz), -0.98 (9H, t, –CH2–CH3, J =
8 Hz), 1.04 (24H, t, –CH2–CH2–CH2–CH3, J = 7.6 Hz),
1.63 (16H, sex., –CH2–CH2–CH2–CH3, J = 7.6 Hz), 2.15
(16H, p, –CH2–CH2–CH2–CH3, J = 7.2 Hz), 4.88 (16H,
m, –CH2–CH2–CH2–CH3), 7.65 (8H, s, PcH). UV-vis
(CH2Cl2):λmax,nm335,465,679,759.MS(MALDI-TOF):
m/z calcd. for C70H96N8O10Si2 1264.68, obsd. 1264.68.
diethyl (4′-hydroxy-[1,1′-biphenyl]-4-yl)phospho-
nate (3). A portion of 4′-bromo-(1,1′-biphenyl)-4-ol
(996 mg, 4.0 mmol) was dissolved in dimethylforma-
mide (15 mL) and the solution was degassed with argon
for 15 min. To that solution was added diethylphosphite
(662 mg, 4.8 mmol), diisopropylethylamine (Hünig’s
base) (0.9 mL), 1,1′-bis(diphenylphosphino)ferrocene
(24.3 mg, 0.044 mmol) and tris(dibenzylideneacetone)-
dipalladium(0) (26.9 mg, 0.040 mmol, 1 mol.%). The
solution was allowed to stir at 110 °C for 24 h under
argon. The solvent was removed under reduced pressure
and the red-yellow oil dissolved in ethyl acetate, H2O was
added (50 mL) and the aqueous layer was washed four
times with ethyl acetate followed by one wash with brine,
dried over magnesium sulfate and concentrated. Column
chromatography was done using silica gel with ethyl ace-
tate/hexane (70:30) as eluent, which afforded a yellow
solid. Yield 378 mg (31%). 1H NMR (400 MHz; CDCl3;
0.03% Me4Si): δH, ppm 1.35 (6H, t, –CH2 –CH3, J = 7.2
Hz), 4.15 (4H, m, –CH2–CH3), 6.87 (2H, d, Ar-phenoxy,
J = 8.8 Hz), 7.47 (2H, d, Ar-phenoxy, J = 8.8 Hz), 7.63
(2H, m, Ar-phosphonate), 7.84 (2H, m, Ar-phosphonate).
MS (MALDI-TOF): m/z calcd. for C16H19O4P 306.10,
obsd. 307.11.
4′-(diethoxyphosphoryl)biphenyl-4-olatehydroxy-
1,4,8,11,15,18,22,25-octabutoxyphthalocyaninato-
silicon(IV) (4). A portion of (1) (31 mg, 0.027 mmol)
was dissolved in dry pyridine (10 mL), to this solution
was added diethyl 4′-hydroxybiphenyl-4-ylphosphonate
(3) (82 mg, 0.27 mmol) and the solution was degassed
with argon for 10 min. The solution was allowed to stir
at 55 °C for 48 h under argon. The solvent was removed
under reduced pressure and two column chromatography
steps were needed to purify the desired product using sil-
ica gel. The first chromatography step was with toluene/
ethyl acetate (60:40) and the second with toluene/ethyl
acetate (75:25) to afford a green solid.Yield 20 mg (53%).
1H NMR (400 MHz; CDCl3; 0.03% Me4Si): δH, ppm 1.04,
(24H, t, –CH2–CH2–CH2–CH3, J = 7.6 Hz), 1.22 (6H, t,
–CH2–CH3, J=6.8Hz), 1.62(16H, sex., –CH2–CH2–CH2–
CH3, J = 7.6 Hz), 2.14 (16H, p, –CH2–CH2–CH2–CH3,
J = 8 Hz), 2.94 (2H, d, Ar-phenoxy, J = 8.8 Hz), 3.98 (4H,
m, –CH2–CH3), 4.82 (16H, m, –CH2–CH2–CH2–CH3),
5.96 (2H, d, Ar-phenoxy, J = 8.8 Hz), 6.87 (2H, m, Ar-
phosphonate), 7.49 (2H, m, Ar-phosphonate) 7.65 (8H, s,
PcH). UV-vis (CH2Cl2): λmax, nm 339, 472, 691, 774. MS
(MALDI-TOF): m/z calcd. for C80H99N8O13PSi 1438.68,
obsd. 1438.69 and 1133.59 (M-phenoxy).
4′-(Ethoxycarbonyl)biphenyl-4-olatehydroxy-
1,4,8,11,15,18,22,25-octabutoxyphthalocyaninato-
silicon(IV) (5). A portion of (1) (48 mg, 0.042 mmol)
was dissolved in dry pyridine (10 mL) and the solution
degassed for 10 min. To this solution was added ethyl
4′-hydroxybiphenyl-4-carboxylate (51 mg, 0.21 mmol)
and allowed to stir at 55 °C for 48 h under argon. The
solvent was removed under reduced pressure and col-
umn chromatography was done using silica gel with
toluene/ethyl acetate (65:35) and then a second column
chromatography step using toluene/ethyl acetate (75:25)
as eluent, afforded a green solid. Yield 22 mg (38%).
1H NMR (400 MHz; CDCl3; 0.03% Me4Si): δH, ppm 1.04,
(24H, t, –CH2–CH2–CH2–CH3, J = 7.6 Hz), 1.31 (3H, t,
–CH2–CH3, J=7.2Hz), 1.62(16H, sex., –CH2–CH2–CH2–
CH3, J = 7.6 Hz), 2.14 (16H, p, –CH2–CH2–CH2–CH3,
J = 8 Hz), 2.93 (2H, d, Ar-phenoxy, J = 8.8 Hz), 4.27 (2H,
q, –CH2–CH3, J = 7.2 Hz), 4.81 (16H, m, –CH2–CH2–
CH2–CH3), 5.98 (2H, d, Ar-phenoxy, J = 8.4 Hz), 6.83
(2H, d, Ar-benzoate, J = 8.8 Hz), 7.64 (8H, s, PcH), 7.72
(2H, d, Ar-benzoate, J = 8.4 Hz). UV-vis (CH2Cl2): λmax
,
nm 338, 471, 690, 774. MS (MALDI-TOF): m/z calcd.
for C79H94N8O12Si 1374.68, obsd. 1374.68.
4-(Ethoxycarbonyl)phenolatehydroxy-1,4,8,11,
15,18,22,25-octabutoxyphthalocyaninatosilicon(IV) (6).
A portion of (1) (48 mg, 0.042 mmol) was dissolved
in dry pyridine (12 mL) and the solution degassed for
10 min. To this solution was added ethyl 4-hydroxy-
benzoate (35 mg, 0.21 mmol) and was allowed to stir at
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 948–950

SyntheSIS anD CharaCterIzatIOn Of SIlICOn PhthalOCyanIneS BearIng axIal PhenOxyl grOuPS
949
55 °C for 60 h under argon. The solvent was removed
under reduced pressure and column chromatography was
done using silica gel with toluene/ethyl acetate (70:30)
as solvent. A second column chromatography step using
toluene/ethyl acetate (80:20) as eluent, afforded a green
Argentina (PRH 23 PICT 140/08), the Consejo Nacional
de Ciencia y Tecnología, México (101939), the National
Science and Engineering Research Council (Canada) and
a Tomlinson Award, McGill University. The research of
J.J.B, B.D.S, T.A.M, D.G. and A.M. was supported as part
of the Center for Bio-Inspired Solar Fuel Production, an
Energy Frontier Research Center funded by the US Depart-
ment of Energy, Office of Science, Office of Basic Energy
Sciences under Award Number DE-SC0001016. R.E.P. is
a permanent research staff of CONICET (Consejo Nacio-
nal de Investigaciones Científicas y Técnicas, Argentina).
We would also like to acknowledge John Lopez and Zach
Laughrey from the ASU Proteomics and Protein Chem-
istry Facility for obtaining MALDI-TOF mass spectra.
B.D.S thanks Science Foundation Arizona for their sup-
port of this work through a graduate research fellowship.
1
solid. Yield 29 mg (54%). H NMR (400 MHz; CDCl3;
0.03% Me4Si): δH, ppm 1.07, (24H, t, –CH2–CH2–CH2–
CH3, J = 7.2 Hz), 1.34 (3H, t, –CH2–CH3, J = 7.2 Hz),
1.64 (16H, sex., –CH2–CH2–CH2–CH3, J = 7.2 Hz), 2.16
(16H, p, –CH2–CH2–CH2–CH3, J = 8 Hz), 2.87 (2H, d,
Ar-phenoxy, J = 8.8 Hz), 3.91 (2H, q, –CH2–CH3, J =
6.8 Hz), 4.80 (16H, m, –CH2–CH2–CH2–CH3), 6.41 (2H,
d, Ar-phenoxy, J = 8.8 Hz), 7.65 (8H, s, PcH). UV-vis
(CH2Cl2): λmax, nm 337, 471, 692, 776. MS (MALDI-
TOF): m/z calcd. for C73H90N8O12Si 1298.64, obsd.
1298.65 and 1133.58 (M-phenoxy).
rEFErEncES
concluSIon
1. Ranta J, Viljanen R, Lemmetyinen H and Efimov A.
Dyes Pigm. 2009; 83: 317–323.
2. Aoudia M, Cheng GZ, Kennedy VO, Kenney ME
and Rodgers MAJ. J. Am. Chem. Soc. 1997; 119:
6029–6039.
3. Hambourger M, Moore GF, Kramer DM, Gust D,
Moore AL and Moore TA. Chem. Soc. Rev. 2009;
38: 25–35.
4. Khaselev O and Turner JA. Science 1998; 280:
425–427.
5. Youngblood WJ, Lee SHA, Kobayashi Y, Hernan-
dez-Pagan EA, Hoertz PG, Moore TA, Moore AL,
Gust D and Mallouk TE. J. Am. Chem. Soc. 2009;
131: 926–927.
6. Walter MG, Rudine AB and Wamser CC. J. Porphy-
rins Phthalocyanines 2010; 14: 759–792.
7. Muthukumaran K, Loewe RS, Ambroise A, Tamaru
SI, Li QL, Mathur G, Bocian DF, Misra V and Lind-
sey JS. J. Org. Chem. 2004; 69: 1444–1452.
8. Loewe RS, Ambroise A, Muthukumaran K, Padm-
aja K, Lysenko AB, Mathur G, Li QL, Bocian DF,
Misra V and Lindsey JS. J. Org. Chem. 2004; 69:
1453–1460.
The synthesis of silicon octabutoxy phthalocyanines
bearing axial ethyl carboxylic esters and diethylphospho-
nate groups via a phenoxy linkage can be accomplished
in a few synthetic steps, from a variety of phenols, and
under mild conditions. Additionally, this approach allows
for isolation of the mono axial functionalized phenoxy
phthalocyanines. Strongly red absorbing phthalocya-
nines are good candidates for dual-threshold solar cells
and the use of non-peripheral octabutoxy groups as well
as the substitution of phenols on the axial position of sili-
con phthalocyanines, as demonstrated with 4, 5, and 6,
helps to further shift their absorption to the near infrared.
Furthermore, the phenoxy substituted phthalocyanines
prepared in this study demonstrated potentials for the
first oxidation near 0.55 V vs. SCE, making them good
candidates for photosensitizing low-potential conduc-
tion band metal oxide semiconductors. The ability to use
low-potential conduction band materials will provide
more driving force for the generation of hydrogen from
water. By incorporating phenoxy ligands with functional
groups that can anchor to metal oxide semiconductors,
we are progressing toward the use of these dyes in dual-
threshold photoelectrochemical cells designed to carry
out overall water splitting with sunlight. Moreover the
ability to bind a variety of phenoxy derivatives in the
axial position can be exploited to control the macrocy-
cle to metal-oxide electronic coupling and consequently
the efficiency of the sensitization process. The synthetic
strategy presented offers a route to obtaining phthalocya-
nines with axial groups for anchoring to semiconduct-
ing metal oxides from the readily obtainable bis-hydroxy
octabutoxy silicon phthalocyanines.
9. Martinez-Diaz MV, de la Torrea G and Torres T.
Chem. Commun. 2010; 46: 7090–7108.
10. Hanna MC and Nozik AJ. J. Appl. Phys. 2006;
100.
11. Blankenship RE, Tiede DM, Barber J, Brudvig
GW, Fleming G, Ghirardi M, Gunner MR, Junge
W, Kramer DM, Melis A, Moore TA, Moser CC,
Nocera DG, Nozik AJ, Ort DR, Parson WW, Prince
RC and Sayre RT. Science 2011; 332: 805–809.
12. Cosut B,Yesilot S, Durmus M, Kilic A and Ahsen V.
Polyhedron. 2010; 29: 675–682.
13. O’Regan BC, Lopez-Duarte I, Martinez-Diaz MV,
Forneli A, Albero J, Morandeira A, Palomares E,
Torres T and Durrant JR. J. Am. Chem. Soc. 2008;
130: 2906–2907.
Acknowledgements
This work was supported in part by a grant from the
National Science Foundation, USA (DMR-0908656), the
Agencia Nacional de Promoción Científica yTecnológica,
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 949–950

950
J.J. BergkamP et al.
14. Cook MJ, Dunn AJ, Daniel MF, Hart RCO, Rich-
ardson RM and Roser SJ. Thin Solid Films 1988;
159: 395–404.
15. Ke MR, Huang JD and Weng SM. J. Photochem.
Photobiol. A 2009; 201: 23–31.
16. Ermilov EA, Tannert S, Werncke T, Choi MTM,
Ng DKP and Roder B. Chem. Phys. 2006; 328:
428–437.
17. Joyner RD, Cekada J, Linck RG and Kenney ME.
J. Inorg. Nucl. Chem. 1960; 15: 387–388.
18. Belabassi Y, Alzghari S and Montchamp JL. J.
Organomet. Chem. 2008; 693: 3171–3178.
19. Li ZY and Lieberman M. Inorg. Chem. 2001; 40:
932–939.
22. Maree MD and Nyokong T. J. Porphyrins Phthalo-
cyanines 2001; 5: 555–563.
23. Leng XB and Ng DKP. Eur. J. Inorg. Chem. 2007;
4615–4620.
24. Koyama T, Suzuki T, Hanabusa K, Shirai H and
Kobayashi N. Inorg. Chim. Acta 1994; 218: 41–45.
25. Janson TR, Kane AR, Sullivan JF, Knox K and Ken-
ney ME. J. Am. Chem. Soc. 1969; 91: 5210.
26. Davison JB and Wynne KJ. Macromolecules 1978;
11: 186–191.
27. Palacios RE, Kodis G, Herrero C, Ochoa EM, Ger-
valdo M, Gould SL, Kennis JTM, Gust D, Moore
TA and Moore AL. J. Phys. Chem. B. 2006; 110:
25411–25420.
20. Huang JD, Jiang XJ, Shen XM and Tang QQ. J.
Porphyrins Phthalocyanines 2009; 13: 1227–1232.
21. Maree MD, Nyokong T, Suhling K and Phillps D.
J. Porphyrins Phthalocyanines 2002; 6: 373–376.
28. Zhao ZX, Cammidge AN, Hughes DL and Cook
MJ. Org. Lett. 2010; 12: 5138–5141.
29. Durr M, Rosselli S,Yasuda A and Nelles G. J. Phys.
Chem. B 2006; 110: 21899–21902.
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 950–950