New peripherally-substituted naphthalocyanines: Synthesis, characterisation and evaluation in dye-sensitised photoelectrochemical solar cells

Article abstract of DOI:10.1039/b201273a

A series of novel tetra-substituted naphthalocyanines featuring hydroxy and carboxylate substituents in peripheral positions have been synthesised in reasonable yields and characterised by spectroscopic techniques. The photochemical properties of the tetra-acid derivative 7 were examined following immobilisation on a nanocrystalline TiO₂ surface and photoinduced dye cations were observed following optical excitation. 7 has a redox potential of 0.25 V (vs. Ag/AgCl), which is lower than is required for an efficient regeneration reaction and, therefore, results in limited photocurrent flow when 7 is used in photoelectrochemical solar cells.

Full text of DOI:10.1039/b201273a

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New peripherally-substituted naphthalocyanines: synthesis,
characterisation and evaluation in dye-sensitised
photoelectrochemical solar cells
Xiyou Li, Nicholas J. Long,* John N. Clifford, Colin J. Campbell and James R. Durrant
Department of Chemistry, Imperial College of Science, Technology and Medicine,
South Kensington, London, UK SW7 2AY. E-mail: n.long@ic.ac.uk
Received (in London, UK) 4th February 2002, Accepted 14th May 2002
First published as an Advance Article on the web 26th June 2002
A series of novel tetra-substituted naphthalocyanines featuring hydroxy and carboxylate substituents in
peripheral positions have been synthesised in reasonable yields and characterised by spectroscopic
techniques. The photochemical properties of the tetra-acid derivative 7 were examined following
immobilisation on a nanocrystalline TiO2 surface and photoinduced dye cations were observed following
optical excitation. 7 has a redox potential of 0.25 V (vs. Ag/AgCl), which is lower than is required for
an efficient regeneration reaction and, therefore, results in limited photocurrent flow when 7 is used in
photoelectrochemical solar cells.
Introduction
Results
Although the first preparation of metal naphthalocyanines
(MNc) was reported in 1936,1 there is still only limited che-
mical and physical data on these compounds, especially
when compared to phthalocyanines (Pc). Nc are of interest
because of the extension of the p-electron system, compared
to Pc, which can effectively modify the optical spectra and
other electronic properties such as redox potentials, electrical
conductivity, photoconductivity and catalytic activity.2 In
unsubstituted Nc preparation, 1,2- and 2,3-dicyanonaphtha-
lenes can be used as starting materials, though the former
species will give rise to up to four isomers.3 To avoid any diffi-
culties associated with isomerism, the 2,3- starting material can
be utilised.4 (1,2-Naphthalocyaninato)iron(II) has been pre-
pared and one of the isomers structurally characterised,5
further derivatisation was carried out via axial substitution.6
Synthesis
Compound 1 was prepared from the bromination of 4-fluorox-
ylene (Scheme 1). A literature method9 indicated that the
reaction was efficiently carried out in the presence of N-bromo-
succinimide. We found that the reaction was very slow and
there was an incomplete bromination of starting material.
However, irradiation with UV light along with reflux of the
solution for 17 h enabled isolation of the product, after column
chromatography (silica gel, hexane eluant), in good yield
(60%). Compound 2 was synthesised following literature meth-
ods,10,17 but using anhydrous sodium iodide and heating the
reaction mixture at 70 ꢀC for 18 h. This gave the off-white pro-
duct in 95% yield. 2 was then treated with potassium carbonate
and benzyl alcohol under N2 and, after 72 h, the crude residue
purified by column chromatography (silica gel, dichloro-
methane eluant). The product 3 was removed as the first frac-
tion in 35% yield and characterised by 1H NMR and mass
spectrometry.
The cyclisation of 3 with zinc acetate in n-pentanol and
DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) resulted in the for-
mation of 4. The crude residue was washed thoroughly with
methanol and subjected to column chromatography (silica
gel, chloroform eluent), whereby a green fraction was collected
and evaporated to dryness to give the product in 26% yield.
Mass spectrometry (+ve FAB) showed the expected molecular
ion and UV-vis spectroscopy indicated formation of the
naphthalocyanine (Q-band at 765 nm, Soret band 330 nm).
Deprotection of the benzoxyl groups was effected by adding
4 to trifluoroacetic acid and tetramethylbenzene, followed by
reflux of the solution. The crude residue was repeatedly
washed with dichloromethane, and 5 was finally isolated via
reprecipitation from THF and dichloromethane. Again, mass
spectrometry indicated the presence of the molecular ion. To
aid solubility and purification, the hydroxy substituents were
replaced by ethyl acetate units via the reaction of 5 with ethyl
Other peripherally-substituted Nc (R ¼ OC5H11) have been
7–11
widely applied, most notably as IR-absorbing dyes.
A
good deal of effort has also been directed towards the
study of nonlinear optical (NLO) properties of Nc, e.g. InNc
has a Q-band shifted to the near IR region at 800 nm, with
a transmission window at 530–650 nm.12 Axially-13 and
peripherally-substituted14 Nc exhibit large third-order non-
linearities.
We are interested in dye-sensitised nanocrystalline solar cells
as low cost alternatives to conventional solid-state photo-
voltaic devices.15 Since phthalocyanines and naphthalo-
cyanines give rise to intense absorption bands in the near IR
region and, along with their excellent stability, this renders
them attractive for photovoltaic applications.16 We can
now report (i) the synthesis of some novel peripherally-
and tetra-substituted Nc featuring hydroxy and carboxylate
substituents, and (ii) the first evaluation, as far as we are
aware, of a naphthalocyanine dye in a photoelectrochem-
ical solar cell via immobilisation on nanocrystalline TiO2
and analysis of the electrochemical and photochemical pro-
perties.
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DOI: 10.1039/b201273a
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Scheme 1 Reagents and conditions: (i) N-bromosuccinimde, AIBN, CCl4 , reflux, UV irradiation, 17 h; (ii) fumarodinitrile, NaI, THF, 70 ꢀC, 18 h;
(iii) benzyl alcohol, K2CO3 , DMF, 60 ꢀC, 72 h; (iv) Zn(OAc)2 , DBU, n-pentanol, 80 ꢀC, N2 , 16 h; (v) trifluoroacetic acid, tetramethylbenzene,
reflux, 16 h; (vi) ethylchloroacetate, K2CO3 , DMF, 60 ꢀC, N2 , 24 h; (vii) KOH (aq), THF, 3 h, then HCl (aq).
chloroacetate and potassium carbonate in DMF. The crude
residue was washed thoroughly with methanol and purified
by column chromatography on silica gel with chloroform–
methanol (95:5) as eluent. A green fraction containing the
product in 42% yield was isolated and fully characterised.
Finally, a THF solution of 6 was added to a solution of
KOH in water. After stirring for 3 h, followed by acidification,
a green solid, 7, formed and was collected by filtration. This
was dried and isolated in 61% yield, and characterised by mass
spectrometry (high resolution), and IR and UV-vis spectrosco-
pies.
Steady-state spectroscopy
The carboxylated Zn Nc 7 showed a strong tendency to aggre-
gate in solution. Fig. 1 shows typical steady-state electronic
absorption, excitation and emission spectra of 7 in DMF.
The steady-state absorption spectra exhibits a broad band in
trum was recorded after excitation at 770 nm. The emission spectrum
Fig. 1 Steady-state absorption (—), fluorescence excitation (- - -) and
fluorescence emission (ꢁ ꢁ ꢁ) spectra of 7 in DMF. The excitation spec-
the near infrared, with maxima at 700 and 780 nm, character-
istic of extensive dye aggregation. In contrast, the fluorescence
was recorded after excitation at 730 nm. Excitation and emission were
normalised for comparative purposes.
New J. Chem., 2002, 26, 1076–1080
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Fig. 2 Steady-state absorption spectra of 7 immobilised on 4 mm
TiO2 film following sensitisation in a DMF solution only (—) and in
a DMF solution with 15% 4-tert-butylpyridine, 5% DMSO and 75
mM 3a,7a-dihydroxy-5b-cholic acid (- - -).
Fig. 3 Cyclic voltammogram of 7 in 0.1 M TBAP in DMF at a scan
.
rate of 5 mV sꢂ1
excitation and emission of this solution show spectra charac-
teristic of monomeric Nc dyes. We conclude that, in this solu-
tion, the dye consists primarily as molecular aggregates, with a
smaller proportion of monomeric dyes. The fluorescence of the
molecular aggregates appears to be strongly quenched, as is
typical of such aggregates. The fluorescence of the monomeric
dyes is, however, not quenched and these dyes therefore dom-
inate the observed excitation and emission spectra. Immobili-
sation of
7 on nanocrystalline TiO2 films was readily
achieved by immersion of the unsensitised film in the dye solu-
tion for 3–4 h. Fig. 2 shows the electronic absorption spectra of
7 immobilised on nanocrystalline TiO2 film after exposure to
two different sensitising solutions. It is apparent that the
absorption spectrum of the dye immobilised on the nanocrys-
talline TiO2 film shows significantly less aggregation than that
observed in solution. Aggregation of the immobilised dye was
further reduced by the inclusion of 4-tert-butylpyridine,
DMSO and 3a,7a-dihydroxy-5b-cholicacid in sensitising solu-
tions, as has been reported previously for the immobilisation
of phthalocyanine sensitiser dyes.18 Under these sensitising
conditions, the absorption spectrum appears to be that of
monomeric dye, and is indistinguishable from the solution
fluorescence excitation spectrum shown in Fig. 1.
Solution electrochemistry
Electrochemical investigations of 7 were carried out in 0.1 M
tetrabutylammonium perchlorate (TBAP) in DMF at a scan
rate of 5 mV sꢂ1 (see Experimental). Fig. 3 shows a voltammo-
gram for the oxidation of 7 under these conditions, showing
reversible electrochemistry with oxidation and reduction peaks
at 0.3 and 0.2 V vs. Ag/AgCl, respectively. We conclude that
the dye/dye+ mid-point potential of 7 is 0.25 V vs. Ag/AgCl.
Comparison with the excitation and emission spectra shown in
Fig. 1 indicates an excited-state oxidation potential for dye*/
dye+ of ꢂ1.4 V vs. Ag/AgCl.
Fig. 4 Transient absorption data monitoring charge recombination
dynamics for TiO2 films sensitised with 7 (a) and [Ru(dcbpy)2(NCS)2]
(b) in propylene carbonate solutions with 0.1 M LiClO4 (black) and 0.1
M LiI (grey). Probe wavelengths were 800 and 850 nm for [Ru(dcb-
py)2(NCS)2] and 7, respectively.
Transient absorption spectroscopy
Transient absorption spectroscopy was employed to assess the
ability of 7 to sensitise the nanocrystalline TiO2 to visible irra-
diation. Following pulsed laser excitation of the immobilised
dye at 450 nm, a long-lived transient absorption signal was
observed throughout the visible/near infrared. Typical data,
at a probe wavelength of 850 nm are shown in Fig. 4(a). The
spectrum of this signal is characteristic of the formation of
Nc cation species, exhibiting a narrow ground-state bleach
signal at 780 nm superimposed on a broad induced absorption
assigned to photoinduced dye cation absorption (data not
shown). No signal was observed for 7 in solution. This signal
is therefore assigned to dye cations generated by photoinduced
electron injection into the TiO2 .
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Photoelectrochemical solar cells fabricated employing dye 7
yielded only negligible photocurrent, as expected from our
transient absorption data. Our electrochemical and transient
absorption studies have identified that the primary reason
for this is the insufficiently positive oxidation potential of this
dye. Attempts to synthesise alternative phthalocyanines with
more positive oxidation potentials are currently in progress.
In dye-sensitised photoelectrochemical cells, the dye ground
state is typically regenerated from the photoinduced dye cation
by the inclusion of iodide ions in the electrolyte (kRR):
dyeþ þ 2Iꢂ ! dye þ I2
ð1Þ
ꢂ
For an efficient photoelectrochemical cell, this regeneration
must compete effectively with the wasteful charge recombina-
tion reaction between the oxidised dye and electrons injected
into the TiO2 (eꢂTiO ), kCR
:
2
Experimental section
dyeþ þ eꢂ
! dye
ð2Þ
TiO2
All solvents were distilled over standard drying agents under
nitrogen directly before use and, where stated, reactions were
carried out under an atmosphere of nitrogen. Silica gel (grade
II) was used for chromatographic separations.
The efficiency of the regeneration reaction can be monitored by
transient optical spectroscopy, as we have shown previously19
and as is illustrated in Fig. 4(b) for TiO2 films sensitised by
the well-established sensitiser dye cis-[RuII(dcbpy)2(NCS)2]-
(dcbpy ¼ 2,20-bipyridyl-4,40-dicarboxylate). In the absence of
iodide ions in the electrolyte, a long-lived signal is observed
and assigned to dye cation absorption. This signal decays with
a half time, t50%, of 0.4 ms due to the charge recombination
reaction kCR . Following the addition of 0.1 M Iꢂ to the elec-
trolyte, the decay becomes biphasic. The fast phase (t50% ꢃ 4
ms) has been assigned to the regenꢂeration reaction kRR . The
slow phase has been assigned to I2 species generated by this
regeneration reaction. It can be concluded that an efficient
regeneration reaction kRR in the presence of iodide ions can
be monitored both by the detection of a large reduction in
the lifetime of the dye cation photoinduced absorption and/
All NMR spectra were recorded using a Delta upgrade on a
Jeol EX270 MHz spectrometer operating at 250.1 MHz (1H)
and chemical shifts are reported in d using CDCl3 (1H, d
7.25 ppm) as the reference. Infrared spectra were recorded
using NaCl solution cells (CH2Cl2) with a Mattson Polaris
Fourier Transform IR spectrometer and UV-vis absorption
spectra were recorded on a Perkin-Elmer Lamba 20 UV-vis
spectrometer. Excitation and emission spectra were recorded
in DMF using a Spex Fluoromax spectrofluorometer. Mass
spectra were recorded using positive FAB methods on an
Autospec Q mass spectrometer. Microanalyses were carried
out by the SACS, University of North London. Electrochemi-
cal data were obtained using an Autolab PGSTAT12 potentio-
stat and a standard three-electrode cell (platinum working and
counter electrodes and an Ag/AgCl reference) at a scan rate of
5 mV sꢂ1. The voltammograms were recorded at room tem-
perature in a solution of 0.1 M tetrabutylammonium perchlo-
rate as the supporting electrolyte in DMF and all solutions
were N2 purged.
ꢂ
or by the observation of long-lived I2 product species.
Comparison of Fig. 4(a) and (b) indicates that, in contrast to
the data observed for the efficient sensitiser dye [Ru(dcb-
py)2(NCS)2], the data observed for the TiO2 films sensitised
with dye 7 are independent of the presence of iodide ions in
the electrolyte, with a t50% of 8 ms. We conclude that, for
dye 7, the regeneration reaction kRR is unable to compete effec-
Anatase phase nanocrystalline TiO2 films were prepared as
described elsewhere on transparent conducting glass slides.21
tively with the recombination reaction kCR
.
Dye immobilisation was achieved by immersion of 1 cm2
4
mm TiO2 films in dye solutions of at least 10ꢂ6 M concentra-
tion with 15% 4-tert-butylpyridine, 5% DMSO and 75 mM
3a,7a-dihydroxy-5b-cholic acid.
Discussion
We have demonstrated the synthesis of a novel carboxylated
naphthalocyanine dye and its immobilisation on a nanocrystal-
line TiO2 electrode. The strong red/near infrared absorption of
this dye makes it, in principle, attractive as a sensitiser dye for
dye-sensitised photoelectrochemical solar cells, complementing
the strong blue/green absorption of existing efficient sensitis-
ing dyes.20
For transient absorption experiments, films were placed in a
quartz cuvette at a fixed position and measurements carried
out in 0.1 M LiClO4 and 0.1 M LiI propylene carbonate solu-
tions. Details of the apparatus employed have been given
previously.22 All experiments were conducted at room tem-
perature.
Following optical excitation of the immobilised dye, photo-
induced dye cations are observed, attributed to electron injec-
tion into the TiO2 electrode. Such electron injection is
consistent with the ꢂ1.4 V excited-state oxidation mid-point
potential determined for this dye from solution phase electro-
chemistry and spectroscopy, this potential being typical of that
for other dyes employed successfully in dye-sensitised photo-
electrochemical solar cells. We note, however, that the magni-
tude of the transient signals attributed to dye 7 cation
formation was significantly smaller ( > 20 times) for this dye
than that observed for our control dye [Ru(dcbpy)2(NCS)2].
It appears unlikely that this difference can be explained solely
by a difference in cation extinction coefficients of these two
dyes, nor by differences in laser excitation intensity, and most
probably results from a significantly lower injection yield.
The primary limitation of the use of 7 in dye-sensitised
photoelectrochemical cells employing an iodide/iodine redox
couple is the inability of iodide to reduce photogenerated
dye cations. This inability is consistent with dye ground-state
oxidation potential, determined to be 0.25 V vs. Ag/AgCl in
solution. Comparison with alternative, efficient sensitiser dyes,
such as [Ru(dcbpy)2(NCS)2], indicates that a ground-state oxi-
dation potential of > 0.85 V is required for an efficient regen-
Synthesis
Compounds 1 and 2 were synthesised following methods
adapted from literature procedures.9,10,17
2,3-Dicyano-6-benzyloxynaphthalene (3). 2 (0.50 g, 2.55
mmol) was dissolved in dry DMF (30 cm3) and the solution
was treated with K2CO3 (2.00 g) and benzyl alcohol (1.00 g,
9.26 mmol) under N2 . The mixture was heated to 60 ꢀC for
72 h. The solvents were then evaporated to dryness under
reduced pressure and the residue subjected to column chroma-
tography on silica gel using CH2Cl2 as eluent. The first fraction
was collected and, after removal of the solvents, 3 was isolated
in 35% yield (0.25 g). MS (EI) m/z: 284 (M+), 191
(M+ ꢂ C7H7); HRMS: found m/z 284.0948, calculated for
C19H12N2O m/z 284.0950; dH (CDCl3): 8.24 (1 H, s, C10H5),
8.17 (1 H, s, C10H5), 7.85 (1 H, d, C10H5), 7.30–7.50 (7 H,
m, 2 of C10H5 , 5 of C6H5), 7.28 (1 H, d, C10H5), 5.22 (2 H,
s, CH2).
Tetra(benzyloxy)-2,3-naphthalocyaninatozinc(II) (4). 3 (0.10
g, 0.35 mmol) and zinc acetate (0.04 g, 0.19 mmol) were added
to freshly distilled n-pentanol (10 cm3) and the mixture heated
eration reaction kRR
.
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to 80 ꢀC under N2 , several drops of DBU were then added.
The mixture was heated to reflux for 16 h and the solvent eva-
porated to dryness under reduced pressure. The residue was
washed thoroughly with methanol (4 ꢄ 25 cm3) and then pur-
ified by column chromatography on silica gel with CHCl3 as
eluent. A green fraction was collected, the solvent removed
in vacuo and 4 was isolated in 26% yield (0.11 g). MS (FAB
+ve) m/z: 1202 (M+); UV-vis (CHCl3) l/nm: 765, 720, 675,
330.
films, Richard Monkhouse for assistance with the transient
absorption spectrometer and Emilio Palomares for help with
excitation and emission studies.
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(0.034 g). MS (FAB +ve) m/z: 776 (M+); UV-vis (THF) l/
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Acknowledgements
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We are very grateful to the EPSRC for funding. We thank
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