Spectroscopic and electrochemical studies on platinum and palladium phthalocyanines

Article abstract of DOI:10.1039/b401880j

The synthesis, spectroscopy and electrochemical characterisation of palladium and platinum phthalocyanines is reported. Electrochemical techniques were used in conjunction with electronic absorption spectroscopy to analyse the reduced and oxidised phthalo

Full text of DOI:10.1039/b401880j

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P A P E R
Spectroscopic and electrochemical studies on platinum and
palladium phthalocyanines
Richard J. C. Brown,y Anthony R. Kucernak,* Nicholas J. Long* and Carlos Mongay-Batalla
Department of Chemistry, Imperial College London, South Kensington, London, UK SW7 2AZ.
E-mail: n.long@imperial.ac.uk; Fax: þ44 (207) 5945804; Tel: þ44 (207) 5945781
Received (in Durham, UK) 9th February 2004, Accepted 6th April 2004
First published as an Advance Article on the web 4th May 2004
The synthesis, spectroscopy and electrochemical characterisation of palladium and platinum phthalocyanines is
reported. Electrochemical techniques were used in conjunction with electronic absorption spectroscopy to
analyse the reduced and oxidised phthalocyanine species in solid films and in solution, and significant and
extensive excitonic coupling resulting in broadening of the Q-band was observed in the film phase. The Q-band
was also shown to be affected by the electrolyte, changes in potential concomitant with the redox properties
of the films and structural changes of the phthalocyanines within the films.
chloride, at high temperatures (though a method starting from
Introduction
lithium phthalocyanine also gave similar yields), based on the
original methods of Linstead et al.11 However, ultra-pure sam-
ples were required so the crude products obtained were sub-
jected to sublimation at (i) 140 ꢀC and at atmospheric
pressures to remove unreacted phthalonitriꢁle3 and (ii) in a tub-
ular vacuum furnace at 550–650 ꢀC and 10 mm Hg to allow
collection of highly purified phthalocyanines. The synthetic
routes gave reasonable yields, though exhaustive sublimation
lowered these values to ca. 20%. The sublimed products were
microcrystalline, enabling easier application onto the electrode
surface than the usual powdered samples.
Precious metal phthalocyanines1 have been known for many
years, but such materials are of increasing interest in cataly-
sis2,3 and electrochemistry.4–8 Electronic absorption spectro-
scopy is an excellent tool for phthalocyanine investigation
and characterisation. Interpretation of the spectra is possible
by analogy with the 18p electron configuration of the por-
phyrin system and the mapping of the electronic molecular
orbitals of standard metallophthalocyanines by, for instance,
extended Huckel calculations.9 The technique is amenable to
in situ electrochemical studies as the profound affect on electro-
nic structure after electrochemical oxidation and reduction
should be easily apparent. Through analysis of excitonic cou-
pling theory,10 which is still in its infancy for the spectra of thin
films, information about the structure and the long-range
order of the films may be obtained.
Although significant work has been performed on the elec-
trochemistry of soluble phthalocyanine materials, there are a
large number of materials that have not been studied due
to their low solubility in most common solvents. Due to the
difficulty in preparing suitable thin films of phthalocyanines
on conductive substrates, there are relatively few reports of
the solid state electrochemistry of phthalocyanine films. Such
electrochemistry may be expected to be significantly different
from that of the soluble materials, as there is significant inter-
molecular interaction between the individual phthalocyanine
molecules within the film.
Electronic absorption spectroscopy
Appleby et al.12 were the first to attempt to correlate the spec-
troscopic properties of absorbed iron phthalocyanine with
their catalytic activities. Yeager et al.13 reported the reflectance
spectra of monolayers of tetra-sulfonated transition metal
phthalocyanines adsorbed on electrode surfaces. Gavrilov
et al.14 have studied the absorption spectra and the electro-
chemistry of the tetra-t-butyl derivative of cobalt, zinc and
lutetium phthalocyanines by ‘wiping’ the compounds on opti-
cally transparent electrodes and observed completely reversible
electrochemical and spectro-electrochemical transitions for all
three of the compounds under test.
Minami15 has recorded photocurrent and UV-vis. spectra
for CuPc, ZnPc, NiPc, VOPc, PbPc and H2Pc. A significant
change was observed for the absorption spectra of the metal-
lophthalocyanines when they were in contact with aqueous
solution. Free base analogues of metallophthalocyanines
have often been studied in the literature,16,17 both electroche-
mically and spectroscopically, to provide a better indication
of the mechanisms of Pc redox chemistry. Several studies18–20
have investigated the electrochromism of transition metal
phthalocyanines, especially CuPc.
In this work, we describe the synthesis, purification, spectro-
scopic and electrochemical characterisation of palladium and
platinum phthalocyanines. As far as we are aware, this is the
first time that electrochemical techniques have been used in
conjunction with electronic absorption spectroscopy to study
reduced or oxidised platinum and palladium phthalocyanine
species in solid films.
Few studies have measured the absorption spectra of PtPc.
Hamnett et al.7 measured the absorption spectra before elec-
trochemical polarisation of a vacuum sublimed 200 nm PtPc
film on a gold/carbon substrate. The result was a sharp
absorption peak at about 620 nm with only a small shoulder
at longer wavelengths suggesting a predominantly a-phase in
the deposited film. Kogan and Yakushi21,22 have fabricated
a composite electrode from PtPc and inert, conductive organic
Results and discussion
Synthesis and purification
The synthesis of the metal-containing phthalocyanines utilised
the reaction of phthalonitrile with the appropriate metal
y Current address: Analytical Science Group, National Physical
Laboratory, Teddington, UK TW11 0LW
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binders and carbon. The spectra show significant excitonic
coupling within the electrodes and a dramatic change upon
oxidation of the composite. By contrast, little study has been
carried out on PdPc. Its crystallographic forms were described
by Kempa et al.23 and the redox behaviour suggested by Nyo-
kong.24 However in this paper, its electrochemical behaviour
and the effect on UV-vis. spectroscopy is discussed fully.
Film and solution spectra
A listing of the wavelengths at which Q-band absorption
occurs for the phthalocyanines in a-chloronaphthalene solu-
tion and as solid thin films prior to polarisation is shown in
Table 1. It is clear that the nature of the phthalocyanine and
its physical state has a significant bearing on the wavelength
of the Q-band and even how many peaks are seen. In solution,
just one Q-band transition is observed for PtPc and PdPc. The
free-base phthalocyanine (H2Pc) shows two absorptions in
solution owing to the lower symmetry of the compound and
the splitting in the p–p* transition that this induces. The small
peaks observed for PtPc, PdPc and H2Pc at lower wavelengths
than the main Q-band are additional components of the
Q-band resulting from vibronic fine structure. It is known that
vibronic structure in phthalocyanines is sensitive to the devia-
tion of the phthalocyanine ring from planarity.25 Co-ordina-
tion of a metal to the phthalocyanine ring (as opposed to
free base Pc) shifts the Q-band to shorter wavelengths (known
as a ‘hypsochromic shift’). This has been interpreted as a
strong metal d-orbital interaction with the phthalocyanine26
(strong metal to ligand p back-bonding). This interaction then
increases the separation between the phthalocyanine HOMO
and LUMO orbitals. The effects of applying the phthalocya-
nines as solid thin films is quite pronounced resulting in multi-
ple Q-band peaks and peak broadening owing to extensive
excitonic coupling. This phenomenon is often referred to as
‘Davydov splitting’. This effect is clearly seen when the solu-
tion and solid phase spectra are displayed together. This has
been completed for the phthalocyanines under study and the
results are shown below (Fig. 1). The absorptions have been
normalised for easy comparison between the spectra. PdPc
solution and film spectra are virtually identical to those for
PtPc.
Fig. 1 Absorption spectra for a-chloronaphthalene solutions of
PtPc (a) and H2Pc (c) and as thin solid films of PtPc (b) and H2Pc
(d) deposited by mechanical abrasion on F-doped SnO coated glass.
splitting of the free base phthalocyanine Q-band owing to the
lowered symmetry compared to an MPc is not great enough to
be directly distinguished but is probably responsible for signi-
ficant broadening of the absorption peaks. Indeed the H2Pc
film shows significantly more extensive broadening with three
peaks observed whilst, the MPc films show two distinct peaks.
This is thought to be a function of their polycrystalline/disor-
dered structure. Chau et al.28 note that any significant disorder
within such layers will lead to additional exciton splitting pat-
terns. These spectra also suggest that there may be significant
longer range interaction between non-adjacent Pc molecules.
The shape of the absorption spectra for these films is consistent
with literature examples.29,18,19 Peak fitting of the H2Pc film
absorption spectrum by Bayliss30 suggests that at least 5 dis-
crete absorptions are observed in the Q-band. Despite this,
the structural interpretation of polycrystalline Pc films has
been previously attempted in the literature.31
The electrochemistry of similarly deposited PtPc films has
been extensively studied in both non aqueous32–35 and aqueous
electrolytes.36 It is noteworthy that after electrochemical
cycling the absorbance of the films is generally lower. This
occurs over the first few electrochemical scans (always more
extensively for PdPc) and then the absorbance remains con-
stant. Since electrically disconnected phthalocyanine should
still absorb even if it does not participate in the electrochemis-
try, this is an indication of two possibilities. It is possible that
material is physically lost from the substrate, which is common
during colour switching,37 or that changes occur in the film
structure during electrochemistry, which result in different
spectral characteristics and a different extinction coefficient
per unit length. It is suggested that both of these factors con-
tribute to the observed changes. Conversely, Kogan and
Yakushi22 observed an increase in the absorption for their
composite PtPc electrodes upon repetitive scanning. Toshima18
observes a decrease in the absorption of the phthalocyanine
layer depending on the extent of its reversibility.
A cofacial arrangement and interaction of molecules leads
to a blue-shift in absorption, whereas an edge-to-edge config-
uration leads to a red-shift. The interaction of the transition
moments (dimer interactions) of molecules at an angle to each
other leads to the observation of both red-shifted and blue-
shifted bands. The Q-band is no longer degenerate and two
distinct transitions can be observed. This theory can be
extended to describe the absorption of infinite stacks, but the
interactions are often more complicated. Bayliss et al.27 have
observed different contributions of these two Q-band transi-
tions for a- and b-H2Pc phases grown under UHV conditions
which are well explained by Davydov splitting. The additional
Spectral changes with potential
Table 1 Listing of the Q-band absorption maxima for several
phthalocyanines in a-chloronaphthalene solution and as thin sold
films. (sh) refers to the presence of a shoulder rather than a distinct
peak and the lMAX has thus been estimated (no meaningful extinction
coefficients could be obtained due to insolubility problems)
The fact that the electrolyte has some effect on the wavelength
of maximum absorption after electrochemical cycling shows
that intercalation of solution into the film has an effect on
the structure, packing and intermolecular interactions of the
phthalocyanine molecules. It is also possible that the phthalo-
cyanine molecule may have its symmetry altered by this pro-
cess. Indeed, there is a universal negative shift in lMAX from
the pre-electrochemical cycling Q-band absorption of 605 nm
for PtPc and 612 nm for PdPc. This suggests that less intermo-
lecular interaction and less dense phthalocyanine packing is
seen after electrolyte intercalation. The concentration of elec-
trolyte seems to have little effect on lMAX , although these mea-
surements were all taken after several electrochemical cycles
Solution phase
Q-band absorption,
Solid phase
Q-band absorption,
Phthalocyanine
lMAX/nm
lMAX/nm
PtPc
PdPc
H2Pc40
LiPc34
650
660
605, 658
612, 667
664, 699
—
632, 694, 782 (sh)
690, 632 (sh)
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
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and any over-swelling phenomena occurring on the first sweep
may have dissipated.
Fig. 2 shows the absorption spectra recorded on the oxida-
tive sweep for PtPc film on F-doped SnO2-coated glass in 0.2
M sulfuric acid. As expected, the shape of the spectra shows
little change until the first bulk oxidation at 1.1 V. However
the intensity of the lMAX peak decreases slightly and the spec-
trum becomes slightly broader from about 0.6 V to 1.0 V coin-
ciding with the intra-domainal intercalation of anions and
solvent and the oxidation of loosely attached crystals. After
the first oxidation peak, the absorption shows a dramatic
change in shape with the peaks moving to lower wavelength.
In addition, the absorption intensity decreases and the peak
width increases markedly. This change continues until the first
oxidation is complete at 1.2 V. Ignoring for the moment the
structural changes that will definitely be occurring within the
film, one would expect a shift of the Q-band absorption to
lower wavelengths since the positive charge and electron defi-
ciency of the phthalocyanine ring increases the HOMO–
LUMO separation. In this way more energy is now required
to facilitate the p–p* transition. These changes in absorption
continue as the second oxidation is traversed from 1.2 V to
1.4 V. Additionally, the shoulder observed at longer wave-
lengths after the first oxidation disappears after the second
oxidation suggesting further structural changes. One may
conclude that since the absorption spectra at 1.2 V is an inter-
mediate step of the shape observed at 1.4 V, the two oxidation
peaks in the cyclic voltammetry represent different extents of
the same process. A clear isobestic point can be observed up
until 1.1 V. After this potential no isobestic point is obvious.
This has been previously19 taken as evidence that the film
may be oxidised reversibly until 1.1 V. During the oxidative
sweep the film is seen to change from a dark purple to a sky
blue colour.
Fig. 3 Absorption spectra for a PtPc film on F-doped SnO coated
glass in 0.2 M sulfuric acid, polarised at 1.4 V then at 100 mV incre-
ments down to 0.0 V. Inset is the cyclic voltammogram for this mate-
rial in the same electrolyte showing the points at which measurements
were made.
The lMAX (which gives an indication of where the bulk of
the Q-band absorption lies) shows little change with potential
until the first bulk oxidation peak after which it drops signifi-
cantly to about 570 nm indicating a greater proportion of cofa-
cial intermolecular interaction than seen in the fully reduced
film. This then stays almost unchanged until the first reduction
peak when lMAX increases to approximately 615 nm strongly
suggesting that the film is now exhibiting edge-to-edge packing
to a greater extent than observed in the original film. Upon the
second reduction peak, the value of lMAX returns to its value
in the original reduced film indicating that the structural
changes occurring during scanning are reversible despite the
fact that the film may physically deteriorate and lose material
since the absorption intensity is not completely reversible.
The main features of the absorption response over the
reductive part of the voltammetry are shown in Fig. 3. As
the potential decreases the peak at about 560 nm decreases
and a new peak grows at about 610 nm. Soon after the reduc-
tion peak at 1.15 V this new peak increases in height over that
of the peak at 560 nm. This new peak shows a slight increase in
intensity and little change in lMAX until the reduction peak at
0.6 V, after which a large increase in intensity is observed and
the peak shifts down in wavelength to 594 nm (the original
The effect of electrolyte
Toshima et al.19 postulate that the electrochromic behaviour of
the phthalocyanine films they studied, prepared by vacuum
sublimation, could be placed in four categories. The materials
we have studied seem to broadly fall within this framework.
The first category included those films showing no reversible
electrochemistry and no changes in absorption spectra with
potential. The second type also show no reversible electro-
chemistry and a decrease in the intensity of absorption without
a change in shape after the first electrochemical scan. The third
type observes irreversible electrochromism whilst the fourth
category describes those films showing at least partly reversible
electrochromism. The films examined in this work would
appear to fall only into the second or fourth categories. This
restriction may be because the studies have been confined to
non-first row transition metal phthalocyanines. These cate-
gories are not definitive though. The behaviour of the PtPc
lMAX) and there is growth of a shoulder at about 660 nm. This
result is particularly interesting as the absorption spectra (and
thus structure) of the film is significantly different at the same
extents of oxidation depending on whether the potential is
being stepped positively or in the negative direction. This phe-
nomenon is shown in Fig. 4 and suggests the structure of the
film is exhibiting some hysteresis behaviour.
Fig. 2 Absorption spectra for a PtPc film on F-doped SnO coated
glass in 0.2 M sulfuric acid, polarised at 0 V, then 0.5 V, then at 100
mV increments up to 1.4 V. Inset is the cyclic voltammogram for this
material in the same electrolyte showing the points at which measure-
ments were made.
Fig. 4 Current response (solid line) and lMAX/nm against potential
for a PtPc film on F-doped SnO coated glass in 0.2 M sulfuric acid.
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electrolyte. The lMAX of absorption is seen to change even in
the absence of phthalocyanine electrochemistry i.e. in the inert
regions of the cyclic voltammogram especially between 1.1 V
and 0.5 V on the reverse scan. This may be taken as unequi-
vocal evidence that purely structural changes are occurring
within these films with potential and the spectral changes are
not solely governed by the onset of redox processes within
the film.
films in HCl falls between the second and third categories.
Electrochemical studies have shown that in HCl electrolyte, a
large irreversible oxidation peak is observed and then very lit-
tle further electrochemistry on subsequent scans. However, the
absorption spectra of the PtPc film in HCl shows slight
changes even on the third scan. These results are shown below
(Fig. 5).
Upon polarisation to more positive potentials the peak
absorption intensity of the Q-band at 600 nm decreases
slightly. This intensity recovers when the potential is stepped
back to ꢁ0.3 V. Fig. 5 indicates that the PtPc film in HCl
undergoes only very minor and superficial structural changes
at oxidative potentials. It is suggested that it is just the loosely
attached PtPc that is taking part here since the size of the anion
is not as important to the loosely packed material. However,
the system essentially shows almost no electrochromic or
electrochemical behaviour.
Clearly this behaviour is in stark contrast to the reversibility
shown by the PtPc films in sulfuric acid. Similar reversible
behaviour is observed in perchloric acid.36 However the elec-
trochemistry in perchloric acid is not as well-defined but the
trends in the absorption of the film with potential remain simi-
lar. It is observed however that variations in intensity occur
more continuously than the sharp changes seen in sulfuric acid.
This is simply a consequence of more well-defined phase tran-
sitions occurring in sulfuric acid. It is noteworthy that upon
reduction of the film the absorbance returns to almost exactly
the same value as before the oxidation, indicating that no
material has been lost and film degradation is less severe than
that experienced in sulfuric acid.
Differences in absorption spectra with different electrolytes
are thought to be caused by the different sizes of the Stokes
radii associated with different anions,38 although it is difficult
to explicitly relate these sizes to the structural changes within
the film. The effect of an electrolyte with a larger cation on
the electrochemistry of the phthalocyanine films has been
described. The effect of the cation on the UV-vis. properties
of the film is less pronounced. These results suggest that it is
the anion that is responsible in the major part for controlling
structural changes accompanying redox processes.
Conclusion
The platinum and palladium Pcs synthesised show a high
degree of purity and the solids are microcrystalline, thus pro-
viding useful extra roughness for mechanical application onto
the electrode. The UV-vis. study of the phthalocyanine system
has centred on the nature of the Q-band transition. It has been
clearly shown that the solution phase spectra are very different
from the spectra obtained from thin solid films. The absorp-
tion spectra of the films show significant and extensive exci-
tonic coupling resulting in extensive broadening of the Q-band.
The Q-band absorptions have been shown to shift slightly to
shorter wavelengths after electrochemical cycling. This effect
has been attributed to structural changes induced in the film
by the intercalation of electrolyte between and within the Pc
microcrystals. The nature of the electrolyte’s anion has been
shown to have significant effect on changes in the Q-band
absorption. This effect seems to be much greater than that of
the cation.
Investigation of the change of the absorption spectra with
potential has shown additional changes in the absorption spec-
tra occurring concomitantly with redox processes within the
film. These changes have been attributed to further structural
changes in the film accompanying the redox processes. Shifts
of the Q-band absorption to shorter wavelengths upon oxida-
tion have been explained by an increased gap between the
HOMO and LUMO orbitals. More energy is then required
for the p–p* transition. Interesting hysteresis has been
observed in the lMAX against potential between the spectra
obtained whilst stepping to positive and to negative potentials.
This has been attributed to structural hysteresis within the
film during the redox processes. This is thought to be caused
by changes in the herringbone crystal structure of the Pc as
electrolyte intercalates within the film.
It seems that the PdPc films experience a greater drop in
absorption intensity with cycling than their PtPc counterparts.
This clearly suggests that the PdPc films are more susceptible
to film degradation upon intercalation suggesting structural
packing differences between the PtPc and PdPc crystals. This
is certainly a feature where the differences between the PtPc
and PdPc films are most apparent. Apart from this observa-
tion, the UV-vis. absorption behaviour of the two films is
almost identical. However, the variation of lMAX of the PdPc
absorption against potential shows some additional differences
seen with changes in electrolyte concentration, again ascribed
to the greater swelling caused by the less concentrated
Experimental
General
The solvents used were commercial grade and used as supplied
(unless otherwise stated, when they were dried and distilled
over standard drying agents). The precious metal salts were
provided by Johnson Matthey plc and organic reagents were
purchased from Aldrich or Lancaster Chemical Companies
and used without further purification. Aerobic reaction condi-
tions were normally used, unless otherwise stated where a
nitrogen atmosphere was employed. 1H NMR spectra were
taken using a Jeol EX 270 MHz Delta upgrade spectrometer
and referenced to each solvent i.e. DMSO-d6 (2.70 ppm) and
D2O (4.82 ppm). Infrared spectra were recorded using either
a KBr disc or Nujol mull on a Mattson Polaris Fourier Trans-
form IR spectrometer. All microanalyses were carried out
using Scientific Analysis and Consultancy Services, University
of North London.
Synthesis (based on literature methods11 but including
extra purification)
Fig. 5 Absorption spectra for a PtPc film on F-doped SnO coated
glass in 0.2 M hydrochloric acid, polarised at ꢁ0.3 V then at 100
mV increments up to 1.1 V and then back down to ꢁ0.3 V.
Platinum phthalocyanine. Platinum chloride (1 g, 3.8 mmol)
and excess phthalonitrile (20 g, 156 mmol) were ground and
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
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precious metal salts and the University of London Central
Research Fund for the purchase of a UV-Vis. spectrometer.
mixed thoroughly and placed in a 100 mL round-bottomed
flask fitted with a reflux condenser. The temperature was
quickly raised to 200 ꢀC and held there for 1 h. The blue solid
formed was cooled and washed with methanol (100 mL) to
remove soluble impurities. The crude mixture was placed in
a laboratory sublimation vessel, packed with dry ice and acet-
one, and unreacted phthalonitrile was removed by sublimation
at 140 ꢀC and atmospheric pressures. Following this, the crude
product (ca. 50% yield) was subjected to sublimation in a high
vacuum furnace. Optimal conditions were 550 ꢀC, 1 ꢂ 10ꢁ3
mm Hg and 8 h and the sublimation process was most efficient
when using small batches (0.25 g) of material. The purified
phthalocyanine, which was violet with a metallic lustre, was
isolated in ca. 20% yield.
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Electrochemical studies
The working electrode was generally a 30 ꢂ 8 mm piece of
fluorine-doped tin oxide coated glass. Electrical contact was
made by means of a copper wire joined to the glass with silver
conductive paint and shielded from the electrolyte by an epoxy
resin or silicone sealant covering. The electrode was housed in
a 20 ꢂ 20 ꢂ 40 mm quartz cuvette with a platinum flag counter
electrode resting on the cuvette’s bottom and a calomel refer-
ence electrode in solution contact with the electrolyte from
above. The assembly was placed in a Hewlett Packard 8453
UV-vis. spectrometer with the cuvette face and the working
electrode’s face orthogonal to the path of the incident light.
Phthalocyanine layers were applied by means of mechanical
abrasion of the microcrystals onto the electrode surface. All
electrolytes were saturated with argon ( > 99.9%) by bubbling
the gas through the solution for 30 min prior to each experi-
ment and keeping a positive pressure above the solution during
experiments. Potentiostatic control was achieved with an
Oxford Electrodes manual potentiostat and sweep generator.
The experiments were conducted in a sequential potential step-
ping fashion by polarising the electrode at a given potential
(monitored by a digital voltmeter attached to the voltage out-
put channel of the potentiostat) and recording the Electronic
absorbance spectra approximately 5 s after each step. The
applied potential was then increased/decreased to the next
measurement point. Generally spectra were recorded at inter-
vals of 100 mV.39 Potentials are reported referenced to the
saturated calomel reference electrode (SCE).
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Acknowledgements
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This project has been funded by the United Kingdom EPSRC
and MOD/DERA under Grant GR/L 57920. We acknowl-
edge and thank Johnson Matthey plc for the loan of the
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
680
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 7 6 – 6 8 0