Water-soluble transition-metal-phthalocyanines as singlet oxygen photosensitizers in ene reactions

Article abstract of DOI:10.1002/ejic.201000784

The capability of platinum, palladium and ruthenium sulfophthalocyanines (PtPcS, PdPcS and RuPcS) to act as singlet oxygen [¹O ₂(¹δ_g)] photosensitizers in ene reactions in aqueous medium has been investigated by combining time-resolved and steady-state techniques. Laser flash photolysis experiments with nanosecond time resolution revealed the population of the lowest excited triplet state in the case of PtPcS and PdPcS upon light excitation. In both cases, this transient is effectively quenched by molecular oxygen leading to the formation of ¹O₂(¹δ_g) with a quantum yield Φ_δ = 0.24, as unequivocally demonstrated by time-resolved near-infrared luminescence. In contrast, RuPcS did not photosensitize ¹O₂(¹δ_g), in accordance with the lack of population of the precursor excited triplet state. These metal-sulfophthalocyanines (MPcSs) were further tested in the ene reaction. In line with the photophysical results, PtPcS and PdPcS photosensitized the formation of hydroperoxide by ¹O₂(¹δ _g) addition to the target α,β-unsaturated carboxylic derivatives whereas RuPcS was totally inactive in this respect. Supporting the MPcSs on Amberlite apparently made the ene reaction more rapid.

Full text of DOI:10.1002/ejic.201000784

FULL PAPER
DOI: 10.1002/ejic.201000784
Water-Soluble Transition-Metal-Phthalocyanines as Singlet Oxygen
Photosensitizers in Ene Reactions
Primiano D’Ambrosio,[a] Lucia Tonucci,[a] Nicola d’Alessandro,*[a] Antonino Morvillo,[b]
Salvatore Sortino,*[c] and Mario Bressan[a]
Keywords: Singlet oxygen / Photosensitizers / Ene reaction / Phthalocyanines / Platinum / Palladium / Ruthenium
The capability of platinum, palladium and ruthenium sulfo-
phthalocyanines (PtPcS, PdPcS and RuPcS) to act as singlet
oxygen [1O2(1Δg)] photosensitizers in ene reactions in aque-
infrared luminescence. In contrast, RuPcS did not photosen-
sitize 1O2(1Δg), in accordance with the lack of population of
the precursor excited triplet state. These metal–sulfophthalo-
ous medium has been investigated by combining time-re- cyanines (MPcSs) were further tested in the ene reaction. In
solved and steady-state techniques. Laser flash photolysis
experiments with nanosecond time resolution revealed the
population of the lowest excited triplet state in the case of
PtPcS and PdPcS upon light excitation. In both cases, this
transient is effectively quenched by molecular oxygen lead-
line with the photophysical results, PtPcS and PdPcS photo-
sensitized the formation of hydroperoxide by 1O2(1Δg) ad-
dition to the target α,β-unsaturated carboxylic derivatives
whereas RuPcS was totally inactive in this respect. Support-
ing the MPcSs on Amberlite® apparently made the ene reac-
tion more rapid.
ing to the formation of 1O2(1Δg) with a quantum yield ΦΔ
=
0.24, as unequivocally demonstrated by time-resolved near-
with partially filled d orbitals, if the triplet state is popu-
lated, it usually has too short a lifetime to interact with
molecular oxygen.[8]
Introduction
Phthalocyanines are an important class of derivatives
that are used in several fields.[1] Historically, they have been
used as pigments and dyes due to their very high extinction
coefficients in the visible region. However, over the last dec-
ade, new applications have become very popular, for exam-
ple, in optoelectronics,[2] new materials[3] and quantum
dots.[4] Several metal–phthalocyanines (MPcs) have also
been tested as photosensitizers for use in the destruction of
tumour cells (photodynamic therapy, PDT).[5]
Indeed, several closed valence shell (i.e., diamagnetic)
MPc derivatives are known to be efficient sensitizers for the
generation of singlet oxygen on irradiation with visible
light.[6] When water-soluble (i.e., sulfonated, Figure 1)
MPcs containing Zn or Al are irradiated they are excited
to the singlet state (1MPc*), which can undergo intersystem
crossing (ISC) to produce the excited triplet state (3MPc*).
3
This MPc* can interact with ground-state molecular oxy-
3
1
gen, O2, to generate singlet oxygen, O2(1Δg), which can
eventually oxidize organic substrates by a type II mecha-
nism[7] (Scheme 1). In contrast, in the presence of a metal
Figure 1. Metal tetrasulfophthalocyanines.
[a] Department of Science, University G. D’Annunzio of Chieti-
Pescara,
Viale Pindaro, 42, Pescara, Italy
Fax: +39-0871-3555365
E-mail: dalessan@unich.it
[b] Department of Chemical Science, University of Padua,
Padua, Italy
[c] Department of Chemical Science, University of Catania,
Catania, Italy
Scheme 1. Reaction pathway leading to a type II mechanism for
the oxidation of organic substrates.
E-mail: ssortino@unict.it
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Although MPcs with non-transition metals have been with a maximum at 520 nm. The decay of these bands fits
widely investigated for their ability to generate singlet oxy- well with monoexponential kinetics with lifetimes of around
gen,[6a,9] very few studies have dealt with transition metals. 190 μs (Figure 2, insets). Furthermore, the time evolution
Recently it was reported that palladium–tetrasulfophthalo- of the absorption does not reveal the formation of any new
cyanine (PdPcS) is a highly efficient and stable photosensi- transient. Experiments carried out in air-equilibrated and
tizer of singlet oxygen that can degrade chlorophenols and oxygen-saturated solutions showed that for both MPcSs the
oxidize cyclohexene and sulfides mainly in aqueous me- transients observed are effectively quenched by oxygen with
dia.[10] However, the quantum yields for 1O2 were measured a diffusion-controlled bimolecular rate constant (kq
≈
exclusively using dimethylformamide as the solvent.[11]
2ϫ109 m–1 s–1). On the basis of the literature data, these
1O2(1Δg) is a reactive oxygen species[12] that can react spectral and kinetic features can be assigned to the lowest
with electron-rich derivatives in three different ways: the ene excited triplet states of PtPcS and PdPcS.[26] On the other
reaction, [2+2] or [4+2] cycloaddition and the oxygenation hand, RuPcS did not show any significant transient absorp-
of compounds with nucleophilic heteroatoms.[13]
tion in the whole of the UV/Vis spectra region. Time-re-
Since it was first defined in 1943 by Schenck,[14] the ene solved near-infrared luminescence with sub-microsecond
reaction has represented a useful method for the diastereo- time resolution is the most suitable technique for unequivo-
selective oxyfunctionalization of simple organic deriva- cally demonstrating the generation of 1O2(1Δg). Indeed, this
tives.[15] Two alternative mechanisms have been proposed in species shows a typical luminescence signal at 1.27 μm with
the past: concerted or stepwise.[16] Other intermediates that a lifetime on the microsecond timescale[27] (see the Exp.
are worth a mention include biradicals,[17] zwitterions,[18] Sect. for details). The quantum yields of the singlet oxygen
exciplexes[19] and perepoxides.[20] A pivotal study that cou- (ΦΔ) produced by energy transfer from the excited triplet
pled theoretical and experimental data led to the formula-
tion of a two-step, no-intermediate mechanism with a rate-
limiting transition state that resembled that of the forma-
tion of a perepoxide in the initially proposed stepwise pro-
cess.[21] However, a large part of the aforementioned litera-
ture deals with reactions performed in organic media. Sten-
saas and co-workers carried out studies in water using α,β-
unsaturated derivatives as mechanistic probes and they pro-
posed a perepoxide intermediate that is stabilized by a hy-
drogen bond with the water solvent.[22] Recently, an inter-
esting overview of the factors controlling the stereoselecti-
vity of the 1O2/alkene ene reaction was published by Alberti
and Orfanopoulos.[23]
Our recent interest in the stability and photostability of
transition-metal-phthalocyanines,[24] which have also been
used as oxidation catalysts,[25] led us to carry out a more
complete photochemical investigation. In this contribution
we report a photophysical and -chemical study of three
metal–tetrasulfophthalocyanines (M = Pt, Pd, Ru) with the
aim of rationalizing their photobehaviour in both the
homogeneous and heterogeneous phases using Amberlite®
as the supporting agent.
Results and Discussion
3
As indicated above, the population of long-lived MPc*
is a fundamental pre-requisite to the generation of 1O2(1Δg).
Laser flash photolysis with nanosecond time resolution is a
powerful tool for obtaining spectroscopic and kinetic fea-
tures of excited triplets of phthalocyanines. Indeed, these
transient species show very intense spectral absorption in
the visible region and have lifetimes on the microsecond/
millisecond timescale. Figure 2 shows the transient absorp-
tion spectra obtained upon laser excitation at 532 nm of
PtPcS and PdPcS in Ar-saturated solution recorded at dif-
ferent delay times with respect to the initial laser pulse. In
Figure 2. Transient absorption spectra observed upon laser exci-
tation (532 nm) of (A) PdPcS and (B) PtPcS in Ar-saturated aque-
ous solution at 0.1 (squares), 100 (circles), 300 (triangles, up) and
800 μs (triangles, down) after the laser pulse. E532 ≈ 10 mJ/pulse.
Each point is the mean of 10 traces. Insets: decay traces monitored
both cases the spectra recorded at 0.1 μs show a main band at 520 nm and the related monoexponential fitting.
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Transition-Metal–Phthalocyanines in Ene Reactions
state of PtPcS and PdPcS to molecular oxygen were deter- tosensitizers in the aqueous medium. Indeed, for both
mined in partially deuterated water (30% D2O) solution PtPcS and PdPcS the visible spectra in water solution
using tetrakis(4-sulfonatophenyl)porphyrin (TPPS) as the showed clear deviations from the Lambert–Beer law at
reference compound. The luminescence intensity at zero about 2 μm (Figure 4). It is well known that an aggregation
time of the time-dependent signal (see the Exp. Sect.) was phenomenon commonly leads to the self-quenching effects
1
plotted as a function of the laser intensity at different molar that preclude the photogeneration of O2(1Δg).[29]
ratios.[28] Because all of the solutions were optically
matched at the excitation wavelength, the values of ΦΔ for
the phthalocyanines can be obtained directly from the dif-
ferent slopes, π, of the straight lines produced from Equa-
(TPPS)
tion (1) in which a value of ΦΔ
= 0.6 was assumed for
TPPS, in accord with the literature.[28] The results are
shown in Figure 3, together with a typical luminescence de-
1
cay trace for O2(1Δg).
(Pcs)
ΦΔ
= ΦΔ(TPPS)π(Pcs)/π(TPPS)
(1)
Figure 4. Absorbances recorded at several concentrations of metal–
phthalocyanines: PdPcS (squares) and PtPcS (triangles).
This hypothesis is well supported by the spectroscopic
data summarized in Table 1. Indeed, in the aqueous me-
dium, the positions of the Q-band absorption maxima of
both PtPcS and PdPcS are significantly blue-shifted in com-
parison with those found in DMF solution in which the
MPcSs are present in their monomeric forms.[30]
Table 1. Maximum absorption signals of the Q bands of PtPcS and
PdPcS in water, DMF, and when supported on Amberlite® (solid-
state spectra).
Q-band maximum absorption [nm]
Water
DMF
Amberlite-supported
PtPcS
PdPcS
603
590
651
662
656
663
We then studied the photostability of the MPcSs by irra-
diating PtPcS, PdPcS and RuPcS in air-equilibrated water
solutions in the presence of 0.1 m NaOH. Both the Pt and
Pd derivatives were very stable even after 5 d of continuous
irradiation. In contrast, RuPcS showed around 20% degra-
dation after irradiation for 1 d and around 50% degrada-
tion after 5 d, as confirmed by the dramatic changes in the
steady-state absorption spectra. To support the idea of the
involvement of aggregation in this photostability behaviour,
we also irradiated (visible light) the MPcSs in the presence
and absence of Triton X-100, a well-known agent that is
used to monomerize MPcSs in aqueous media. Indeed, the
1
Figure 3. (A) Representative decay kinetics of O2(1Δg) monitored
at 1270 nm and (B) dependence of the luminescence intensity of
1O2(1Δg) at t = 0 for optically matched solutions of TPPS (squares),
PtPcS (circles), PdPcS (triangles) and RuPcS (diamonds). Each
point is the mean of 10 traces.
According to the laser flash photolysis experiments, both solutions containing the surfactant showed greater bleach-
PtPcS and PdPcS photosensitized the formation of 1O2(1Δg) ing than in the absence of Triton X-100. After 5 d of irradi-
1
with ΦΔ = 0.24 in both cases. In contrast, no O2(1Δg) gen- ation, the PdPcS/surfactant system showed a decrease in
eration was observed for RuPcS, in agreement with the ab- the Q-band attenuated by 60%, 35% Q-band attenuation
sence of signal from the excited triplet state. The value of was seen for the PtPcS/surfactant system, and RuPcS
ΦΔ found for PtPcS and PdPcS is lower than those reported showed the same degradation trend without the surfactant.
for similar metal–phthalocyanines in DMF solution,[11] These findings strongly suggest that in the aggregated forms
which can be attributed to a partial aggregation of the pho- seen above, the MPcSs are stable in the presence of 1O2(1Δg)
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N. d’Alessandro, S. Sortino et al.
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and that the bleaching observed in the case of RuPcS
should be due to a photodegradation mechanism that is not
type II. There are potentially several reasons for the rela-
tively good photostability of PtPcS and PdPcS, and we be-
lieve that the aggregation, which tends to hinder the oxi-
1
dation by O2(1Δg), is one of these. As expected for the sol-
1
vent used (D2O), the lifetime of O2(1Δg) (ca. 20 μs) is in
good agreement with the hypothesis of aggregation contrib-
uting to the photostability, which reduces the role of singlet
oxygen in oxidation processes with the photosensitizers.
Ene reactions were performed by using tiglic acid and
cyclohex-1-ene-1-carboxylic acid as model compounds. The
reactions were carried out in both homogeneous and
heterogeneous phases. The formation of hydroperoxide and
its corresponding alcohol derivative was monitored by
NMR upon irradiation with visible light of an alkaline D2O
solution of these α,β-unsaturated acids (10 mm) in the pres-
ence of the MPcS (0.1, 0.025 mm). For the heterogeneous
reactions, the MPcS complexes were supported on Am-
berlite® anion-exchange resin (1%, w/w). In the homogen-
eous phase and for both tiglic acid (Figure 5) and cyclohex-
1-ene-1-carboxylic acid (Figure 6), the mass balances (con-
version of reactants vs. yield of products) were strictly re-
spected.
Figure 6. Photobehaviour in the homogeneous phase of (A) PtPcS
and (B) PdPcS in the ene reaction with cyclohex-1-ene-1-carboxylic
acid, showing cyclohex-1-ene-1-carboxylic acid concentrations
(circles), yields of alcohol + hydroperoxide (major isomer; squares)
and the mass balance (triangles).
Of the two potential isomers, only one was detected
(Scheme 2, a and aЈ), in good agreement with the results
published by Stensaas et al., who reported a maximum yield
of only 5% of the b derivative (Scheme 2).[22b] After 5 d,
PtPcS triggered quantitative conversion of tiglic acid,
whereas with PdPcS a small amount (Ͻ5%) of the unre-
acted reagent was still detectable; RuPcS was totally inac-
tive.
Cyclohex-1-ene-1-carboxylic acid showed significantly
lower conversion rates (PtPcS, 37%; PdPcS, 32%; after
120 h). Again, RuPcS was not active.
Experiments were also performed at a lower temperature,
0 °C rather than 35 °C (see the Exp. Sect.). For the reactions
performed at 0 °C, hydroperoxide a was the only product
after 24 h. At the higher temperature (35 °C), it is likely that
the disproportion of hydroperoxide a was very fast as the
alcohol derivative aЈ was the main reaction product, at least
after prolonged irradiation.
To investigate the regiochemistry of the reactions, we
synthesized 2-methyl-1-(1-methylethyl)cyclohexa-2,5-diene-
1-carboxylic acid (c), which has previously been used in ene
reactions performed with chemically or photochemically
generated 1O2(1Δg).[31] Upon irradiation under the same
conditions as described above and in the presence of both
PtPcS and PdPcS, only one stable product was detected (see
Scheme 3), which appears to be the same product as pre-
Figure 5. Photobehaviour in the homogeneous phase with
(A) PtPcS and (B) PdPcS in the ene reaction with tiglic acid, show-
ing tiglic acid concentrations (circles), yields of alcohol + hydroper-
oxide (major products, a + aЈ; squares) and the mass balance (tri-
angles).
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Transition-Metal–Phthalocyanines in Ene Reactions
tion process more efficient as a result of the increase in the
monomeric form, which is the most active in photogenerat-
1
ing O2(1Δg). Indeed, the solid-state visible spectra of these
Amberlite-supported phthalocyanines showed the Q bands
to be redshifted to 656 nm with PtPcS and to 663 nm with
PdPcS. Of note, these values are perfectly superimposable
on the Q bands recorded in DMF in which MPcSs are pref-
erentially present in their monomeric forms[30] (see Table 1).
Conclusions
The photobleaching of phthalocyanines is crucial for ap-
1
plication in photodynamic therapy and O2(1Δg) has often
been indicated as being at least partially responsible for the
self-bleaching of zinc and aluminium tetrasulfonated
phthalocyanines. Indeed, it is commonly accepted in the lit-
erature that the photobleaching mechanism of phthalocy-
anines is complex and that there is often an association with
several other kinds of degradative pathways.[32] In our case,
good photostability of PtPcS and PdPcS was observed, to-
gether with moderate but significant 1O2(1Δg) quantum
yields. The aggregation of MPcSs in water solution is shown
to be involved, although this would appear not to be the
only pathway. Note also, for example, that the data in the
literature is mainly on AlPc and ZnPc, and metallation by
other transition metals alters the π system and therefore
their redox properties. Indeed, aggregation of the photosen-
sitizer reduces the concentration of the monomeric active
1
Scheme 2. Oxidation of tiglic acid photosensitized by O2(1Δg).
viously reported[31a] with the carboxy and hydroxy groups
mutually cis-oriented in the cyclohexenyl ring. In the pres-
ence of RuPcS, no reaction was observed even after irradia-
tion for five days.
1
form in the photogeneration of O2(1Δg), although, on the
other hand, this can limit the autoxidation processes medi-
ated by this transient species. Thus, the systems reported
herein offer a useful compromise between photostability
1
and O2(1Δg) quantum yield. The synthetic point of view
must also be taken into consideration because the two pho-
tosensitizers PtPcS and PdPcS can be used efficiently in
both homogeneous and heterogeneous phases to function-
alize α,β-unsaturated carboxylic acids by the well-known
ene reaction.
Scheme 3. Oxidation of 2-methyl-1-(1-methylethyl)cyclohexa-2,5-
1
diene-1-carboxylic acid photosensitized by O2(1Δg).
Experimental Section
Amberlite-supported MPcSs were also used to photo-
oxidize tiglic acid and cyclohexenecarboxylic acid in alka-
line D2O solution. The product distributions in the presence
of PtPcS and PdPcS were exactly the same as those seen in
the homogeneous phase, although they were formed with
around five-fold higher rates of conversion. In fact, for ex-
Materials: Tiglic acid, cyclohex-1-ene-1-carboxylic acid, (anthra-
cene-9,10-diyl)bis(methylmalonate), Triton X-100 and Amberlite
IRA-900 were purchased from Sigma–Aldrich. Deuterated water
(99.8%) was purchased from Armar Chemicals. Ultrapure water
was produced by a Millipore Milli-Q plus 185 apparatus. 1-Methyl-
ample, 5 d were necessary for the quantitative transforma- ethyl-2-methylcyclohexa-2,5-dienecarboxylic acid was synthesized
following the literature procedure.[33] PtPcS, PdPcS and RuPcS
tion of tiglic acid under homogeneous conditions in com-
were prepared by template synthesis following previously reported
parison with only one day necessary under heterogeneous
general procedures for the synthesis of MPcS.[34] For their charac-
conditions. RuPcS showed negligible reactivity with the
alcohol yields never exceeding 10% and the hydroperoxide
derivative never detected.
terization, see the publications of dЈAlessandro[24] and Nicastro[25]
and their co-workers. The Amberlite-supported MPcSs were pre-
pared as indicated in the literature:[35] Amberlite IRA-900 (1 g) was
The enhanced reactivity observed for PtPcS and PdPcS
added to a solution of the MPcS (10 mg) in H2O/CH3CN (40 mL,
can be attributed to a lower level of stacking of the photo-
sensitizers induced by the Amberlite. Heterogenization
might indeed significantly reduce the aggregation level of
1:1) and the mixture was stirred at room temperature for 48 h. At
the end of reaction, the solution was colourless and the phthalocy-
anine B and Q bands were no longer observed in the UV/Vis spec-
the phthalocyanine derivatives, making the photosensitiza- trum.
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N. d’Alessandro, S. Sortino et al.
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Photochemical Experiments in the Homogeneous Phase: Water (or
D2O; 10 mL) containing the MPcS (0.5, 0.1, 0.025 mm) was basi-
fied to pH 13 by the addition of NaOH pellets. A GLP 22 Crison
pH-meter was used to measure the pH of the solutions at room
temperature, with the calibration performed by using Crison stan-
dard buffer calibration solutions of pH 7.0 and 9.0. The carboxylic
acids (10 mm) were added and the solutions were irradiated with
visible light for up to 5 days. The reactions were carried out inside
a photoreactor chamber at a temperature of about 35 °C in open
quartz test-tubes. Experiments at 0 °C were carried out by using a
cryostat apparatus. Aliquots (1 mL) were sampled and the reaction
mixtures were analysed by UV/Vis and 1H and 13C NMR spec-
troscopy.
ter (Ͼ1.1 μm) and an interference filter (1.27 μm). Pure signals of
1O2(1Δg) were obtained as differences between signals in air- and
Ar-saturated solutions. The temporal profile of the luminescence
was fitted to a single-exponential decay function with the exclusion
of the initial portion of the plot, which was affected by scattered
excitation light, fluorescence and the formation profile of singlet
oxygen itself. The initial luminescences were extrapolated from the
curve-fitting.
Analysis: NMR measurements were performed with a Bruker Av-
ance 300 MHz spectrometer equipped with a 5 mm BBO probe
(30–300 MHz). Water suppression was determined using a presatu-
ration sequence with a composite pulse (zgcppr; Bruker sequence).
A co-axial capillary tube containing a 30 mm solution of [2,2,3,3-
D4]3-(trimethylsilyl)propionic acid was used as reference and for
the lock procedure, as the sodium salt in water (D2O). The UV/Vis
spectra were recorded with a 6505 UV/Vis Jenway spectrophotome-
ter.
Photochemical Experiments in the Heterogeneous Phase: Water (or
D2O; 5 mL) containing the Amberlite-supported MPcS (31.25 mg,
1% w/w) was basified to pH 13 by the addition of NaOH pellets.
The carboxylic acids (10 mm) were added and the solutions were
irradiated with visible light for up to 5 days under continuous stir-
ring. The reactions were carried out inside a photoreactor chamber
at a temperature of about 35 °C in quartz test-tubes. Aliquots
(1 mL) were sampled and filtered, and the reaction mixtures were
analysed by UV/Vis and 1H and 13C NMR spectroscopy. Experi-
ments in the presence of ADMA (0.01 mm) were conducted in
50 mm phosphate buffer at pH 7.2, sampling the reaction mixture
at 1, 3, 5 and 7 min. Analyses were performed by spectrophotome-
try, following the decrease in the absorbance at 379 nm.
Acknowledgments
The authors are grateful for financial support from the Ministero
dell’Università e della Ricerca (MIUR) (PRIN 2008) and from the
Consorzio di Ricerca per l’Innovazione Tecnologica, la Qualità e
la Sicurezza degli Alimenti S.C.R.L. (grant number 28497/2006).
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Steady-State Irradiation: A photochemical multi-ray apparatus
(Helios Italquartz, Milano, Italy) equipped with 10 visible-light
lamps (15 W each, λ = 450–600 nm), was used to irradiate the water
solutions. These lamps (length 47 cm, diameter 2 cm) were fixed in
a special lamp-holder with IP 44 protection and were equipped
with a special aluminium reflector to increase their radiation
power.
Laser Flash Photolysis: All of the samples were excited with the
second harmonic of a Nd:YAG Continuum Surelite II-10 laser
(532 nm, 6 ns, ca. 5 mJ) using quartz cells with a path length of
1.0 cm. The excited solutions were analysed with a Luzchem Re-
search mLFP1-11 apparatus with an orthogonal pump/probe con-
figuration. The probe source was a ceramic xenon lamp coupled to
quartz fibre optic cables. The laser pulse and the mLFP1-11 system
were synchronized by a Tektronix TDS 3032 digitizer operating in
pre-trigger mode. The signals from a compact Hamamatsu photo-
multiplier were initially captured by the digitizer and then transfer-
red to a personal computer controlled by Luzchem Research soft-
ware operating in the National Instruments LabView 5.1 environ-
ment. For experiments in the absence of oxygen, the solutions were
deoxygenated by bubbling with a vigorous and constant flux of
pure argon (previously saturated with solvent). In all of these ex-
periments, the solutions were renewed after each laser shot (in a
flow cell of 1 cm optical path), to prevent probable auto-oxidation
processes. The sample temperature was 295Ϯ2 K. The energy of
the laser pulse was measured at each shot with a SPHD25 Scientech
pyroelectric meter.
1
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Received: July 19, 2010
Published Online: December 16, 2010
Eur. J. Inorg. Chem. 2011, 503–509
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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