Visible photostability of some ruthenium and platinum phthalocyanines in water and in the presence of organic substrates

Article abstract of DOI:10.1142/S1088424610002306

Water-soluble, metal-tetrasubstituted phthalocyanines (-SO₃H, MPcS and -COOH, MPcC) of platinum and ruthenium were synthesized and their photostability to visible light irradiation was determined. For the ruthenium phthalocyanines, the characte

Full text of DOI:10.1142/S1088424610002306

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 499–508
DOI: 10.1142/S1088424610002306
Published at http://www.worldscinet.com/jpp/
Visible photostability of some ruthenium and platinum
phthalocyanines in water and in the presence of
organic substrates
Manuela Carchesioa, Lucia Tonuccia, Nicola d’Alessandro*a, Antonino Morvillob,
Piero Del Boccioc and Mario Bressana
a Department of Science, University G.d’Annunzio Chieti Pescara, Viale Pindaro 42, 66013 Chieti Scalo, Italy
b Department of Chemical Science, University of Padua, Via Marzolo 1, 35100 Padua, Italy
c Department of Biomedical Science, University G.d’Annunzio ChietiPescara, Italy and Center for Excellence
on Ageing (CeSI), 66013 Chieti, Italy
Received 2 November 2009
Accepted 9 February 2010
ABSTRACT:Water-soluble, metal-tetrasubstituted phthalocyanines (-SO3H, MPcS and -COOH, MPcC)
of platinum and ruthenium were synthesized and their photostability to visible light irradiation was
determined. For the ruthenium phthalocyanines, the characteristic visible Q band of the phthalocyanines
was almost totally suppressed after five days of irradiation. The platinum derivatives were instead more
resistant to photodegradation, and the Q band did not decrease by more than 25%. The addition of
carbonyl compounds to the phthalocyanine solution in water (at concentrations at least 1400-fold those
of the phthalocyanines) dramatically accelerated the photobleaching of these phthalocyanine complexes.
PtPcS turned from blue to green and to colorless with one day of visible-light irradiation in the presence
of acetone. This effect decreased with the increase in molecular weight of the ketones (from acetone to
2-pentanone). The addition of alcohols (i.e. 1-butanol) or other organics (i.e. phenylacetic acid) did not
affect the photostability of these metal-tetrasubstituted phthalocyanines. Dioxygen also had an important
role, as when the solutions of phthalocyanines were carefully deaerated before irradiation, the visible
spectra were preserved. The platinum phthalocyanines, as with the palladium analogs, sensitize the
photoproduction of 1O2, as shown by the formation of endoperoxide and its rearranged products in the
presence of furfuryl alcohol (a singlet oxygen trapping agent).
KEYWORDS: singlet oxygen, photochemistry, platinum, ruthenium.
of which is characterized by the production of oxygen in
INTRODUCTION
its singlet-excited state [2].
The photochemical properties of the metal phthalocy-
Generally, photochemical parameters include pho-
anines (MPcs) have been intensively studied over the last
todegradation and photostability, and singlet oxygen
decade or so because of their increasing importance as
quantum yields. Photostability refers to the resistance
photosensitizers and photocatalysts [1]. Ultraviolet (UV)
of MPcs to degradation upon irradiation by UV or vis
and visible (vis) radiation can result in the stimulation of
light, as can easily be followed through the fading of the
excited states, which are involved in oxidation processes
color of MPc solutions. Experimentally, photodegrada-
via well-known type I and type II mechanisms, the latter
tion is characterized by an intensity decrease in the spec-
tral absorption bands (both Q and B), which sometimes
includes a shift in the maxima to give isosbestic points,
and/or with the formation of new absorption bands [3].
SPP full member in good standing
The literature on MPc photochemical and photophysi-
cal data is extensive, but it has mainly been focused on
*Correspondence to: Nicola d’Alessandro, email: dalessan@
unich.it, tel: +39 0871-3555365, fax: +39 0871-3555364
Copyright © 2010 World Scientific Publishing Company

500
M. CarCheSIO et al.
MPcs dissolved in organic solvents, rather than in water.
Moreover, there is little data available on carboxylated
phthalocyanines, probably due to their low solubilities in
aqueous media, which can be improved only at higher
pH values. Furthermore, most of the MPcs that have been
investigated as photosensitizers in photodynamic thera-
pies (PDT) have non-transition metals by nature (closed
shell, diamagnetic ions, such as Zn2+, Al3+ and Ga3+,
which are expected to give complexes with both high
triplet yields and long lifetimes) [4]. On the other hand,
transition metals have been successfully used for other
medical applications. This can be seen, for example, with
cis-platinum applications as chemotherapeutics [5] and
all of the ruthenium derivatives tested recently as anti-
cancer agents [6].
conditions, and in neutral and alkaline water solutions
(when possible, depending upon solubilities).
EXPERIMENTAL
Materials
Chemicals (acetone, 2-pentanone, 1-butanol, phenyla-
cetic acid, ethanol) were purchased from Sigma-Aldrich.
Furfuryl alcohol was purchased from Aldrich, although
accurate distillation at reduced pressure was necessary
before use (boiling point around 100 °C at 20 mmHg).
Synthesis of metal phthalocyanines
Photobleaching in PDT can be interpreted in two dif-
ferent ways. The new generations of photosensitizers
have very large and red-shifted extinction coefficients
(the Q band for MPcs), which facilitate the deep pen-
etration of light. However, despite the high efficiency
for destroying superficial cancer cells, the sensitizer effi-
ciently filters the radiation, thus making the penetration
of light into deeper cell layers very difficult. Therefore, a
moderate and perfectly balanced degree of photobleach-
ing can enhance the deep penetration of light, thus mak-
ing a cancer treatment more efficient. For ruthenium
phthalocyanine and naphthalocyanine, a potential control
method has been published recently which consists of
irradiating with a pulsed dye laser [7].
The mechanism of PDT is also not completely clear,
and the detection of singlet oxygen under in vivo experi-
mental conditions is still difficult to demonstrate. Model
studies performed in water are not exhaustive, since the
aqueous environment of biological media where MPcs
are active in PDT is far from free of organics. Therefore,
the photostability can be modified dramatically in the
presence of organic derivatives, even in small amounts.
The degree of aggregation of MPcs has a key role: limited
aggregation, achieved by adding specific organic agents,
can noticeably reduce photostability. However, other
kinds of interactions can also be active. For instance,
Caronna [8] recently reported an interesting dependence
of phthalocyanine photostability upon the media used,
exclusively not involving the concept of aggregation.
Thus acetone, methanol, ethanol and dimethylformamide
added to aqueous reaction mixtures as such or as mix-
tures in different ratios can profoundly modify photo-
bleaching. Many studies have tried to explain the whole
mechanism involved here [9], but at present several reac-
tion pathways can be hypothesized, where the nature of
the central metal is certainly important, along with that
of the media.
The tetrasulfonated phthalocyanines of platinum
(PtPcS) and ruthenium (RuPcS) were prepared by
template synthesis, starting from K2PtCl4 (or RuCl3),
4-sulfopthalic acid and urea, following previously
reported general procedures for the synthesis of metal-
sulfophthalocyanines [10]. For their characterization, see
reference 11. Similarly, the tetracarboxylated phthalocy-
anine of ruthenium (RuPcC) and platinum (PtPcC) were
prepared by analogous synthetic procedure, replacing
4-sulfopthalic acid with trimellitic anhydride [12].
PtPcC. In a 100 mL flask, a mixture of trimellitic
anhydride (5.0 g, 26 mmol), K2PtCl4 (2.7 g, 6.5 mmol),
NH4Cl (0.7 g, 13 mmol), ammonium molybdate (0.6 g,
0.5 mmol) and urea (2.34 g, 39 mmol) was brought to
reflux in nitrobenzene (5 mL) for 4 h. The recovered
mixture was then washed with methanol to eliminate
the nitrobenzene. The tetraformamide phthalocyanine
derivative was obtained as a black solid. After boiling
for about 5 min in a saturated sodium chloride solution,
the solid was filtered. The solid was dissolved in a 2.0 M
NaOH solution and the reaction was stirred at 100 °C for
18 h. After cooling to room temperature, the resulting
solution was diluted with appropriate amounts of water
and acidified to pH < 3 with concentrated hydrochloric
acid. The resulting solid was filtered and then dissolved
in a 0.5 M NaOH solution. It was then precipitated again
with hydrochloric acid, filtered and washed several times
with water, and finally with methanol. The molecular
ion (MALDI-TOF) was observed at m/z = 883.198 [M]+
(calcd. for C36H16N8O8Pt 883.07) with a perfectly super-
imposable isotopic pattern to a simulated mass spectrum
(see Fig. 1). Q band, in alkaline water solution, appeared
at 598 nm (λmax; ε = 9500 M-1.cm-1) and 655 nm (72% of
the λmax) while in DMSO it appeared at 600 nm (λmax; ε =
9200 M-1.cm-1) and 658 nm (86% of λmax). Anal. calcd. for
C36H16N8O8Pt: C, 48.93; H, 1.83; N, 12.68, Pt, 22.08%.
Found: C, 48.63; H, 1.95; N, 13.40; Pt 21.65%. Pt analy-
sis was performed by ICP MS.
Our study has moved on from these observations: we
have synthesized a pool of transition-metal-tetrasubsti-
tuted phthalocyanines (-COOH and -SO3H), of platinum
and ruthenium, which we have tested for photostabil-
ity to UV and visible light under aerated or deaerated
RuPcC. The same procedure as above was carried out
except for the metal salt which was RuCl3·nH2O instead
of K2PtCl4 (only one molecule of H2O was considered
in the molecular weight). In this last case, before being
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VISIble PhOtOStabIlIty Of SOMe ruthenIuM anD PlatInuM PhthalOCyanIneS
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Fig. 1. MALDI mass spectrum of PtPcC. Signals centered at m/z 901 and 919 are attributable to the mono and dihydrated forms of
PtPcC. Evidences for the present attribution were obtained by several mass experiments at different laser power energies. The inset
shows the isotopic pattern of the molecular ion
used in the synthesis of the phthalocyanine, the metal
salt was refluxed in a 0.3 M HCl solution in the presence
of 2–3 mL Hg, until a blue color was observed (1–8 h).
After cooling to room temperature, the solid metal
was filtered, evaporated under vacuum and used in the
abovementioned synthetic procedure. The molecular
ion (MALDI-TOF) was observed at m/z 790.07 [M]+
(calcd. for C36H16N8O8Ru 790.02) with a perfectly
superimposable isotopic pattern to a simulated mass
spectrum (see Fig. 2). Q band, in alkaline water solution,
5 mm BBO probe (30–300 MHz). Water suppression was
determined by a presaturation sequence using a compos-
ite pulse (zgcppr; Bruker sequence). A co-axial capillary
tube containing a 30 mM solution of 3-(trimethylsilyl)
propionic-2,2,3,3-tetradeuterium acid, as the sodium salt
in water (D2O), was used as the reference and for the
lock procedure. The UV-vis spectra were recorded with
a 6505 UV-vis Jenway spectrophotometer. All MALDI
analyses were performed with a Reflex IV MALDI-TOF
mass spectrometer (Bruker Daltonics, Germany). 100 µL
of MPcC, dissolved in DMSO, were mixed with 100 µL
of saturated solution of α-ciano-hydroxy-cinnamic acid
matrix in water/acetonitrile (1:1). This solution was spot-
ted directly on the MTP Target Ground Steel 384 (Bruker
Daltonics). Spots were left in air for 6 h. After the sol-
vent evaporation, the target was kept on the MALDI
source overnight before analysis (under high vacuum).
MALDI-TOF spectra were acquired in reflectron mode
at a voltage of 20.00, 16.80 and 8.90 kV for the first
and second ion extraction stages and lens, respectively,
and 23.00 kV for reflectron in the 500–3000 m/z range.
appeared at 588 nm (42% of the λmax) and 645 nm (λmax
;
ε = 11700 M-1.cm-1) while in DMSO it appeared at 583 nm
(36% of λmax) and 642 nm (λmax; ε = 10200 M-1.cm-1).
Anal. calcd. for C36H16N8O8Ru: C, 54.76; H, 2.04; N,
14.19, Ru, 12.80%. Found: C, 54.47; H, 2.20; N, 14.82;
Ru 12.25%. Ru analysis was performed by ICP MS.
Apparatus
The NMR measurements were performed with a
Bruker Avance 300 MHz spectrometer equipped with a
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502
M. CarCheSIO et al.
Fig. 2. MALDI mass spectrum of RuPcC. The inset shows the isotopic pattern of the molecular ion
The sample was irradiated with a nitrogen laser (λ =
337 nm). A 5 × 10-5 M mix solution of the Angiotensin I
(1,296.68 Da), ACTH 18-39 (2,465.19 Da), Bradikynin
(1,060.57 Da) and Renin Substrate (1,758.93 Da) were
used for the external calibration standards. Ultrapure
water was produced by a Millipore Milli-Q plus 185
apparatus. GLP 22 Crison pH meter working at room
temperature was used to measure pH of solutions; calibra-
tion was performed by Crison standard buffer calibration
solutions of pH 7.0 and 9.0. A photochemical multi-ray
apparatus (Helios Italquartz, Milano, Italy) with 10 UV
(254 nm) or visible (450–600 nm) lamps of 15 W each
was used to irradiate the water solutions.
test-tubes. One mL aliquot was sampled, and the reac-
tion mixtures were analysed by UV-vis spectroscopy and
1H NMR and 13C NMR. Experiments in the presence of
furfuryl alcohol were performed by lowering the concen-
trations of the MPcs from 0.5 mM to 0.1 mM.
RESULTS
Characterization of metal phthalocyanines was per-
formed by visible spectroscopy, MALDI-TOF spec-
troscopy and elemental analysis. Mass spectra of PtPcS
and RuPcS were reported elsewhere [11], while Figs 1
and 2 show the MALDI mass spectra of PtPcC and
RuPcC.
In the present study, we investigated the photostability
of PtPcS, RuPcS, PtPcC and RuPcC in water solutions
in the UV (254 nm) and visible (400–700 nm) regions.
All of the irradiation was carried out in aerated or deaer-
ated water solutions with the pH adjusted (neutral and
alkaline) to enhance the solubility of the carboxylated
derivatives.
Typical procedure for the photochemical experi-
ments
10 mL of water (or D2O) containing the MPcs (0.5 mM)
in the absence or presence of the organic substrates
(0.05, 0.70, 1.40 M) were irradiated in the UV or vis-
ible region for up to seven days. The solutions were at
natural pH or were taken to pH 12 by addition of NaOH
pellets. The reactions were carried out inside a photore-
actor chamber at a temperature of about 35 °C in quartz
UV irradiation always resulted in high reaction
rates, with the complete disappearance of the Q band in
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VISIble PhOtOStabIlIty Of SOMe ruthenIuM anD PlatInuM PhthalOCyanIneS
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L
O
N
N
N
N
N
L
hν (254 nm)
L
L
N
N
O
H
+
NH3 + Mn+
N
M
N
6 h
+ small amounts of CO2, HCOOH etc.
L
-L = -COOH or -SO3H
Scheme 1. Reaction scheme of UV degradation of platinum and ruthenium tetrasubstituted phthalocyanines
less than 24 h for all of the substrates examined. This
bleaching resulted in the almost complete degradation
to phthalimide, ammonia and the hydrated metal ion
(Scheme 1) [11].
The behavior of the complexes examined in the
presence of visible light in neutral and alkaline pHs is
reported in Fig. 3. The platinum derivatives were always
more stable than the ruthenium ones, and they showed
good preservation of the macrocycle for up to five days of
continuous irradiation. Oxygen aeration had a key role,
as shown in Fig. 4 (to simplify the graph, only the ruthe-
nium data are reported): indeed, both metals had good
Fig. 4. Absorbance of RuPc in water solution under visible light
in aerated (continuous lines) or deaerated (dashed lines) con-
photostability under deaerated conditions, whereas the
twisted behavior (dashed lines, particularly circles and
triangles) that was sometimes seen was probably due to
the difficulty of performing the whole degassing proce-
dure efficiently.
ditions over seven days. Squares: RuPcS in neutral solution;
circles: RuPcC in alkaline solution; triangles: RuPcS in alkaline
solution
Acetone, 1-butanol and phenylacetic acid were added
to the water solutions, at concentrations 100-fold, 1400-
fold and 2700-fold those of the complexes. In the pres-
ence of 1-butanol or phenylacetic acid, PtPcS showed
good photostability over five days of irradiation, with
only a 5% to 10% decrease in the Q band. On the con-
trary, in the presence of acetone and in a neutral medium,
there was strong photobleaching, with the disappearing
Fig. 5. Absorbance of PtPcS under visible light over five days:
without acetone (dotted line); in the presence of acetone (trian-
gles: 100-fold), (stars: 1400-fold), (circles: 2700 fold); visible
spectra were recorded diluting the irradiated solution (0.5 mM
of PtPcS) 10-fold with water
of the Q band (Fig. 5). The concentration of acetone is
extremely important, since only in the presence of sig-
nificant amounts (0.7 M, i.e. 1400-fold the PtPcS con-
centration, or more) did the bleaching reach 90% after
one day of irradiation. During the reaction, the solutions
turned green (from blue), as is also seen by the red shift
of the shoulder of about 28 nm, from 632 nm to 660 nm
(Fig. 6). It should be noted that PtPcS was stable with
Fig. 3. Absorbance of MPcC and MPcS in water solution under
visible light over five days. Squares: MPcC in alkaline solution;
circles: MPcS in neutral solution; triangles: MPcS in alkaline
solution. Dotted line: M = Ru; continuous line: M = Pt
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504
M. CarCheSIO et al.
amounts were extremely small (less than 5% with respect
to the conversion of furfuryl alcohol). Furthermore, it was
not possible to assign many other NMR signals, mak-
ing the whole interpretation of the reactivity difficult.
When performed in D2O, the product distribution did not
change for either PtPcS or RuPcS. However, conversions,
and consequently yields, of the rearranged endoperoxide
products noticeably increased, at least for PtPcS.
DISCUSSION
Across the two metals investigated, the platinum deriv-
atives showed better stability to visible light in aerated
water solution, even if it was in the presence of acetone,
where fast degradation of PtPcS occurred, with the color
gradually turning from blue to green, and finally becom-
ing completely colorless. Bleaching was also seen in the
presence of the other two water-soluble alkylketones, i.e.
2-butanone and 2-pentanone. Furthermore, deoxygen-
ated solutions that contain ketones conserved integrally
the Q band without any changes. Therefore, as previously
observed [8], the presence of aliphatic carbonyl deriva-
tives appears to accelerate the attack to the macrocycle.
Moreover, bleaching appears to decrease with increasing
molecular weight of the ketone, thus making acetone the
most effective bleaching inducer of PtPcS. With alcohols,
such as 1-butanol, but confirmed also by supplementary
experiments with ethanol, bleaching did not occur.
Fig. 6. Visible spectra of 0.5 mM PtPcS in D2O in the presence
of acetone (1400-fold). Data at 0 h, 24 h and 48 h of visible
irradiation
regard to irradiation in alkaline media, and also in the
presence of large amounts of acetone; PtPcC behaved
similarly.
Addition of the organic substrates to the RuPcS and
RuPcC solutions did not result in any major changes,
either under neutral or alkaline conditions. Again, when
oxygen was removed, the bleaching was noticeably
reduced.
Since singlet oxygen is usually considered to be
involved in PDT [13], we carried out all of the experi-
ments (without and with added organics) also in deuter-
ated water, which is typically expected to enhance
the lifetime of singlet oxygen. RuPcS and RuPcC
were degraded with no appreciable changes in
the reaction rates, and neutral and alkaline PtPcS
solutions were totally insensitive to the irradia-
tion, while after five days of irradiation, the alka-
line PtPcC solution showed a partial quenching of
the Q band (25–30% vs. 12–15% seen previously,
in H2O solution).
Our interpretation is oriented around the involvement
of an active species, which is radical in character or in
Table 1. Reaction conditions: 30 mM furfuryl alcohol in D2O,
1
0.1 mM MPcS, λ = 450–600 nm, 24 h. Data from H NMR; results
given in mol.%
Reagent/products
No photosens.a, RuPcSa, PtPcSb,
%
%
%
100
95
75
Tests for singlet oxygen generation were per-
formed using the furfuryl alcohol probe (usually
used in water under neutral conditions) [14]. In
the presence of 0.1 mM PtPcS and after 24 h of
visible light irradiation, we observed very clean
type II photooxidation of furfuryl alcohol (30
mM), with formation of the diagnostic products
coming from the rearrangement of furfuryl alco-
hol endoperoxide (Table 1, Figs 7 and 8) [15]; the
color of the sensitizer turned green, in agreement
with the decreased intensity and the red shift of
the Q band. In the presence of sodium azide, a
strong singlet oxygen quencher [16], endoper-
oxide formation was totally suppressed and the
reaction mixtures fully preserved their initial blue
color. RuPcS behaved differently: we noticed
weak NMR signals, attributable to the rear-
ranged endoperoxide products, even if the relative
OH
O
A
traces
0
0
0
0
0
HO
O
O
B
OH
O
6
O
OH
C
O
traces
17
HO
O
a Longer irradiation times (four days) lead to a complex mixture of reac-
b
tion products. Four days irradiation leads to the same reaction prod-
ucts but with different yields (A:B:C = 10:40:13).
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VISIble PhOtOStabIlIty Of SOMe ruthenIuM anD PlatInuM PhthalOCyanIneS
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Fig. 7. 1H NMR spectra of reaction mixture containing 30 mM furfuryl alcohol and 0.1 mM PtPcS, irradiated for one, two and four
days. Starting from the bottom: the first spectrum (one day) was recorded in water suppression mode (zgcppr NMR pulse sequence),
while the second and third spectra were recorded without suppression of the water signal (zg NMR pulse sequence) (FA = furfuryl
alcohol; B and C are the products reported in Table 1)
its excited state, and which is responsible for the attack
on the macrocycle. The formation of this active species
requires the presence of oxygen. Also, the water affinity
of ketone is important: acetone is the most active, prob-
ably because its active species can reach the macrocy-
cle more easily than 2-pentanone, the less polar of the
carbonyl derivatives investigated. Unfortunately, other
ketones are not soluble in water at the required concen-
trations (>0.7 mM).
The reactivity of furfuryl alcohol in the presence of
PtPcS strongly suggests the involvement of singlet oxygen,
which is further confirmed by the experiments in D2O.
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506
M. CarCheSIO et al.
Fig. 8. 13C NMR and DEPT 135 spectra of a one-day irradiation reaction mixture containing 30 mM furfuryl alcohol and 0.1 mM
PtPcS. The solution was too dilute to detect the NMR signals of the quaternary carbons
However, the macrocycle is apparently stable in the
presence of singlet oxygen, as shown by irradiating the
PtPcS alone (or in the presence of 1-butanol and phe-
nylacetic acid). Only in alkaline deuterated water did
PtPcC show partial degradation of the complex (see
Results section) and this was the only condition under
which we noted an increasing degradation rate going
from H2O to D2O.
The sharp color changes can be explained as per-
oxide attack on the phthalocyanine macrocycle [11].
Indeed, we must remember that hydrogen peroxide
can easily be formed, at least at the beginning of the
reaction, when the endoperoxide rearranges to C of
Table 1, where C is also the main reaction product [15].
Alternative hypotheses to explain the fading of the
color, which involves singlet oxygen, are, in our opin-
ion, unlikely, since in the presence of a singlet oxygen
trapping species, like furfuryl alcohol, the integrity of
the macrocycle, and consequently its color, should be
preserved.
Another issue to be considered is the staking. It is
clear that the presence of an organic substrate in the
aqueous media should change the degree of aggrega-
tion, even if a similar effect, although more or less
emphasized, should be observed also when butanol
and phenylacetic acid are added. Furthermore, the vis-
ible spectra of PtPcS solution containing acetone did
not show any change in shift and shape of the Q band.
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VISIble PhOtOStabIlIty Of SOMe ruthenIuM anD PlatInuM PhthalOCyanIneS
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These findings support the idea that the degradation of
Acknowledgements
PtPcS in the presence of acetone is not closely related
to the aggregation degree. The change of color observed
during the degradation steps can be easily attributed to
the change in the molar ratio between the dimer (blue)
and monomer (green) forms. We believe that the radical
attack occurs randomly at the periphery of the phthalo-
cyanine ring, resulting in a sort of stacking destruction
that leads to the abovementioned change of the dimer/
monomer ratio.
Contrary to PtPcS, singlet oxygen was not produced
with RuPcS (at least as the main reaction). A radical
mechanism involving dioxygen can be proposed, since
the reaction rate was completely insensitive to the pres-
ence of D2O, and in the presence of oxygen, the degra-
dation of the macrocycle occurred readily, both under
neutral and alkaline conditions; under anaerobic condi-
tions, the complex was absolutely photostable.
The different behavior between the two metals should
be finally discussed, taking into consideration both aggre-
gation and electronic effects. Ruthenium can coordinate
to apical positions, thus leading in principle to a less pro-
nounced aggregation in comparison to the corresponding
square-planar platinum derivatives: indeed, the Q band
of RuPcS is both red-shifted of about 30 nm and more
tiny in size. However, we maintain that aggregation alone
might be not sufficient to explain the behavior reported in
Fig. 3, since monoelectronic processes, which are more
likely to occur in ruthenium derivatives, equally support
the proposed type I mechanism.
The authors are grateful to the “G.d’Annunzio”
University of Chieti and Pescara for financial support.
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CONCLUSION
Visible light photostability studies of two transition
metal (Pt and Ru) tetrasulpho and tetracarboxyl phtha-
locyanines were carried out in water solution. Carboxyl
phthalocyanines were soluble only in alkaline media,
while the sulphonic phthalocyanines could also be
tested under neutral conditions. The platinum complexes
always showed good photostability, with the exception
of PtPcS in the presence of added carbonyl derivatives.
Tests with furfuryl alcohol demonstrated the involvement
of singlet oxygen, although at the same time they showed
that PtPcS is surprisingly stable in the presence of singlet
oxygen. Although further complementary photophysical
studies are necessary to demonstrate further and define
the involvement of singlet oxygen, we have obtained evi-
dence that under neutral conditions, PtPcS can generate
singlet oxygen without self-degradation. To our knowl-
edge, among the water-soluble transition metal phthalo-
cyanines, only the tetrasulfophthalocyanine of palladium
has previously been proposed as a singlet oxygen gen-
erator [17]. Together with the possibility to control the
degree of bleaching by addition of known amounts of
carbonyl derivatives, these findings make PtPcS particu-
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J. Porphyrins Phthalocyanines 2010; 14: 508–508

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