Photocatalysis with Polyoxometalates Associated to Porphyrins under

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Photocatalysis with Polyoxometalates Associated to Porphyrins under Visible Light: An Application of Charge Transfer in Electrostatic Complexes Claire Costa-Coquelard, Se´bastien Sorgues,* and Laurent Ruhlmann Laboratoire de Chimie Physique, UMR 8000 CNRS, UniVersite´ Paris-Sud 11, 91405 Orsay cedex, France ReceiVed: February 9, 2010; ReVised Manuscript ReceiVed: April 20, 2010

Absorption spectrum of derived Dawson sandwich polyoxometalates (POM) [M4(H2O)2(P2W15O56)2]n- with n ) 16 for M ) ZnII, NiII, and n ) 12 for M ) FeIII have been extended in the visible range forming electrostatic complexes with a chromophore, the zinc tetracationic porphyrin [ZnTMePyP]4+. Formation of such complexes was followed by steady-state absorption and luminescence spectroscopies. The electrostatic complexation gives in all cases a strong, neutral, and nonluminescent complex. A charge transfer between the two units was shown by transient absorption spectroscopy. Upon a visible excitation of the porphyrin subunit, an electron transfer from the porphyrin to the POM occurs and imparts it a catalytic activity. This has been demonstrated studying a model reaction such as the reduction of silver cations leading to nanoparticles. In all cases, the reduction of the silver cations takes place. We showed that the catalytic activity depends of the nature of the metal of the tetraoxometallic central cluster of the Dawson sandwich POM. Introduction Over the past decade, the properties of polyoxometalates (POM) have been a subject of great interest due to their application range.1-4 The role of POM in catalysis, both electrocatalysis and photocatalysis, has received particular attention in the literature. Examples in photocatalysis include metals,5-7 nitrite,4,8,9 and O24,10 that can be reduced without degradation or poisoning of the catalyst. This remarkable property is due to the fact that POM are able to exchange many electrons without changing their structure.11,12 Nevertheless, according to the absorption spectrum, these compounds must be irradiated with UV light to induce photoactivity. This requirement makes their widespread use for environmental applications difficult using ambient sunlight. Some groups are attempting to circumvent this problem by developing new POM with visible absorption spectra.13 An alternative strategy consists of the elaboration of compounds associating POM and a chromophore. In this case, a photosensitizer with a visible absorption spectrum should be able to transfer electrons to the POM, the catalyst. Some publications relate the association of chromophores with POM: for example, investigations of association of POM with ruthenium or iron polypyridic complexes have shown the formation of electrostatic complexes.14-17 Moreover, complexes with porphyrins, an usual chromophore, have been realized. Most publications report associations between metalloporphyrins and POM by coordination18,19 or by covalence.19,20 However, only a few studies describe the formation of electrostatic porphyrin-POM complexes.21-24 In this contribution, we show the formation of electrostatic complexes in aqueous solution between the zinc tetracationic porphyrin [ZnTMePyP]4+ (Figure 1A) and derived Dawson sandwich polyoxometalates [M4(H2O)2(P2W15O56)2]n- (with n ) 16 for M ) ZnII, NiII, and n ) 12 for M ) FeIII) abbreviated M4-POM (Figure 1B). The stoichiometry and the global formation constant of the complexes formed have been obtained * To whom correspondence should be addressed, sebastien.sorgues@ u-psud.fr.

Figure 1. Representations of the studied tetracationic porphyrin [ZnTMePyP]4+ (A) and Dawson-derived sandwich complexes (B) [M4(H2O)2(P2W15O56)2]n- (n ) 16 for M ) ZnII, NiII and n ) 12 for M ) FeIII). The pink color corresponds to the tetraoxometallic central cluster.

using absorption and fluorescence spectroscopies. Transient absorption spectra obtained by nanosecond laser photolysis revealed a charge transfer between porphyrin and POM. As we will show in the last part, this charge transfer into the complexes shows potential for use as photocatalyst compounds in the visible range. This has been demonstrated by the model reaction of the reduction of the metallic cations Ag+ in nanoparticles. Experimental Section Chemical Products. Most common laboratory chemicals were of reagent grade purchased from commercial sources and used without further purification. [ZnTMePyP4+, 4Cl-], propan2-ol, and Ag2SO4 were purchased from Fluka and the compounds [M4(H2O)2(P2W15O56)2]n- with (n ) 16 for M ) ZnII, NiII and n ) 12 for M ) FeIII) were prepared by published methods.25,26 Water was obtained by passing through a MilliRO4 unit and subsequently through a Millipore Q water purification set. Steady-State Spectroscopy. Steady-state optical absorption spectra were recorded using a Perking Elmer Lambda 19 spectrophotometer. Steady-state fluorescence measurements were performed using a Spex Fluorolog 111 spectrofluorimeter equipped with a Hamamatsu R3896 photomultiplier cooled to -20 °C.

10.1021/jp101261n  2010 American Chemical Society Published on Web 05/18/2010

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Figure 2. (A) Steady-state UV-visible absorption spectra recorded during the titration of a 7 × 10-6 M aqueous solution of [ZnTMePyP]4+ by a 8.25 × 10-5 M solution of Zn4-POM. Insert: absorption spectrum of Zn4-POM (c ) 8 × 10-6 M). (B) Job’s plot of the absorption change ∆ε at λ ) 436 nm (b) as a function of the molar fraction of the added Zn4-POM.

All spectroscopic measurements reported were performed at room temperature using the following method: the titration was realized in a quartz cell (1 cm optical path length) by adding small quantities (10-50 µL) of a concentrated POM solution at a 3 mL porphyrin solution (≈10-5 M). Dilution effects were thus negligible. Job’s diagram allowed the graphical determination of both the stoichiometry and formation constants of the complexes. Transient Absorption Spectroscopy. Transient absorption spectra were recorded using the third harmonic at 355 nm of a Nd:YAG laser from BMI. A repetition rate of 2 Hz was used for all measurements reported. The transient absorption signals were recorded via a monochromator coupled to a photomultiplier (Hamamatsu R955S). All experiments were performed in deaerated solutions with a concentration of complexes of 8 × 10-5 M. Steady-State Photolysis Used for the Photocatalysis. Irradiation was performed using a 300 W Xe arc lamp equipped with a water cell to filter the near-IR radiation. The irradiance of the lamp in the 320-790 nm spectral region was approximately 50 mW/cm2/nm. A 385 nm cutoff filter was used to eliminate the UV light. All experiments were performed with propan-2-ol as electron sacrificial donor (0.13 M, equivalent at 1% in volume). Silver cations from Ag2SO4 at 8 × 10-5 M were added to a solution containing complexes at 2 × 10-6 M. The samples consisted of 4 mL of aqueous solutions with propan-2-ol, [ZnTMePyP4+]n/4s[M4(P2W15O56)2]n- and if present, Ag2SO4. They were contained in a spectrophotometer cell with 1 cm path length. Transmission Electronic Microscopy (TEM). The TEM observations were performed with a JEOL 100 CXII TEM instrument operated at an accelerating voltage of 100 kV. Samples for TEM analysis were prepared by solution drops deposited and dried on carbon-coated copper TEM grids. Results and Discussion 1. Evidence of the Formation of Electrostatic Complexes: UV-vis Absorption and Luminescence Spectroscopy. Titration of an aqueous solution of [ZnTMePyP]4+ by an aqueous solution of Zn4-POM is illustrated Figure 2A. Analysis of the absorption spectrum of the [ZnTMePyP]4+ reveals the characteristic bands of porphyrins: one B band at λ ) 436 nm and two Q bands at λ ) 564 and λ ) 574 nm which correspond to the S0 f S2 and S0 f S1 transitions, respectively. The Zn4POM absorbs in the UV domain below 370 nm (insert of Figure 2A). During the addition of the Zn4-POM, the absorbance below 370 nm is observed to increase strongly, whereas the absorbance

of B and Q bands decreases continuously. Moreover, a slight red shift of the porphyrin bands is observed. Two isosbestic points at λ ) 370 nm and λ ) 540 nm are indicative of the formation of a new species in solution. In order to determine the stoichiometry and association constant of the new compound, a variant of the Job’s method is used. The method requires the determination of the net deviation of additive absorbance ∆ε at a given wavelength as a function of the molar fraction of POM according to the following equation

∆ε )

A - (1 - XPOM)εPorphyrin - XPOMεPOM lCt

where A is the measured absorbance, l the optical path length, Ct the total concentration of compounds in the solution, XPOM the molar fraction of POM, and ε the different molar extinction coefficients. Job’s diagram at λ ) 436 nm (maximum of the porphyrin B band) is presented in Figure 2B. It shows a unique slope break at XPOM ) 0.2 indicating the formation of only one complex with a stoichiometry of 4:1, i.e., four porphyrins for one POM. This complex is electrostatically neutral according to the reaction

4[ZnTMePyP]4+ + [Zn4(P2W15O56)2]16- a [ZnTMePyP4+]4[Zn4(P2W15O56)2]16Moreover, the β association constant is estimated to be 1030 M-4 indicating a total reaction. It seems that the electrostatic interaction between the two subunits is very strong, more than the electrostatic interaction in heterotrimer of porphyrins.27 The molar extinction coefficient of the ground state of such complex has been estimated at 5000 L · mol-1 · cm-1 (summarized in Table 1). The formation of such an electrostatically neutral complex is confirmed by steady-state fluorescence measurements. After an excitation in the Q-band (570 nm), the [ZnTMePyP]4+ porphyrin fluoresces in 590-800 nm spectral region. The increasing of the Zn4-POM concentration in a solution of free [ZnTMePyP]4+ leads to a regular decrease of the fluorescence until a total quenching, without a shift of the spectrum (Figure 3). This behavior has already been reported by Seery et al.15 for an electrostatic complex between [Ru(bpy)3]2+ and [S2W18O62]4-. The evolution of the luminescence at λ ) 700 nm illustrated in Figure 3B clearly shows two parts that can be

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TABLE 1: Ground State Absorption Data for [ZnTMePyP]4+ and [ZnTMePyP4+]n/4s[M4-POM]nComplexes in Aqueous Solution stoichiometry (xporphyrins:yPOM)

compounds [ZnTMePyP4+] [ZnTMePyP4+]4s [Zn4(P2W15O56)2]16[ZnTMePyP4+]4s [Ni4(P2W15O56)2]16[ZnTMePyP4+]3s [Fe4(P2W15O56)2]12-

λmax (nm), ε (×103 M-1 · cm-1) B

Q1

Q2

1:0 4:1

436, 186 442, 336

564, 15 573, 40

606, 15 616, 20

4:1

441, 308

573, 40

615, 20

3:1

439, 246

573, 27

612, 10

fitted by a straight line intersecting at XPOM ) 0.2. This result is in agreement with the formation of one complex formed by four porphyrins and one POM. Absorption and luminescence behavior during the titration of a solution of [ZnTMePyP]4+ by a solution of [Ni4(H2O)2(P2W15O56)2]16- or [Fe4(H2O)2(P2W15O56)2]12- are similar (see Figures S1 and S2 in Supporting Information) and the ground state absorption data are summarized in Table 1. In these two cases a unique neutral nonluminescent complex is observed at XPOM ) 0.20 and XPOM ) 0.25 for Ni4-POM and Fe4-POM, respectively, according to the equilibriums

4[ZnTMePyP]4+ + [Ni4(H2O)2(P2W15O56)2]16- a [ZnTMePyP4+]4[Ni4(H2O)2(P2W15O56)2]163[ZnTMePyP]4+ + [Fe4(H2O)2(P2W15O56)2]12- a [ZnTMePyP4+]3[Fe4(H2O)2(P2W15O56)2]12In order to test the strength of the electrostatic interactions in the complex formed, titrations with ionic salts monitored by luminescence spectroscopy were completed. Following a preparation of 4 mL of a solution of [ZnTMePyP4+]4s[Fe4(P2W15O56)2]16-, the addition of a solution of (Li+, ClO4-) up to 0.17 M had no impact on the nonluminescence (see Figure S3 in Supporting Information). Therefore, we conclude that there is no formation of ion pair between Li+ and Fe4-POM or between ClO4- and [ZnTMePyP]4+. Consequently, the electrostatic interactions between the precursors are very strong as already indicated by the estimated association constant. The same experiment with a (Na+, ClO4-) solution confirms that the size of the cation has no influence on the electrostatic interactions. The same conclusions could result in the cases of Zn4-POM and Ni4-POM. 2. First Evidence of a Charge Transfer between the Subunits: Time Resolved Spectroscopy. The excited states of the complexes were studied by nanosecond laser photolysis. These spectra have been recorded in a deaerated solution at different temporal delays following laser excitation at λ ) 355 nm, corresponding to an excitation in the N band of the porphyrin. 2.1. Isolated Porphyrin [ZnTMePyP]4+. Transient absorption spectra of the [ZnTMePyP]4+ are presented in Figure 4. It shows the bleaching of the three bands at λ ) 440, 570, and 620 nm (corresponding to the B and Q bands) and two large transient bands, one centered at λ ) 480 nm while the second is in the near-infrared. No differences in the spectra are observed during the temporal evolution whatever the wavelength because all the spectra are homothetic with three isosbestic points at λ ) 460, 550, and 585 nm, as observed by Kalyanasundaram.28

The decay profile of the [ZnTMePyP]4+ at 520 nm is presented in the bottom of Figure 4; only one decay of 3.7 ( 0.1 ms is observed and attributed to the triplet state of the porphyrin.28 This observation is confirmed by the quenching of this state by O2 where a time decay of 3.8 ( 0.2 µs is recorded in aerated solution. 2.2. [ZnTMePyP4+]4s[Zn4(P2W15O56)2]16- and [ZnTMePyP4+]4s[Ni4(P2W15O56)2]16- Complexes. The evolution of the transient absorption spectra of the [ZnTMePyP4+]4s [Zn4(P2W15O56)2]16- complex is presented in Figure 5A. In the insert, the normalized spectrum 2 µs after the excitation is compared with the one of the porphyrins at the same time delay. First, ∆OD, 2 µs after the excitation, is approximately 30 times less that for the free porphyrin indicating a strong quenching of the triplet state by the POM. Second, according to the steady-state absorption spectrum, the bleaching of the B band is red-shifted by approximately 5 nm when compared to the free porphyrin. In addition, two transient bands are observed, one centered at λ ) 530 nm and a second one centered at λ ) 730 nm. The first band corresponds mainly to the absorption of the triplet state of the porphyrin in the complex. This assertion is supported by the fact that addition of dioxygen decreases, 2 µs after the excitation, the differential absorption by 70%. Also, the temporal profile (Figure 5B) of the absorbance at this wavelength is different than that of the free porphyrin. This is a multiexponential decay where 30 µs after the excitation the signal decreases by a factor 10. It shows that there are different relaxation channels for the triplet state. The second band centered at λ ) 730 nm has a different temporal behavior (See Figure S4 in Supporting Information), probably due to the presence of the reduced POM.11,12,29 Three arguments lead to this conclusion: first, as it is shown in Figure 5, the time evolution recorded at that wavelength is slightly different from that recorded at λ ) 530 nm. Second, quenching by dioxygen decreases the differential absorbance at λ ) 530 nm by 70%, 2 µs after the excitation, while less than 20% of the differential absorbance at 730 nm. Finally, it is known that reduced POM absorbs in this spectral region.11,12,29 It appears that the signal at this wavelength probably shows a charge transfer between the excited porphyrin and the POM. Results obtained for [ZnTMePyP4+]4s[Ni4(P2W15O56)2]16- are comparable to the [ZnTMePyP4+]4s[Zn4(P2W15O56)2]16- and lead to the same conclusions (see Figure S5 in Supporting Information). 2.3. Comparison between [ZnTMePyP4+]n/4s[M4(P2W15O56)2]n- Complexes with n ) 16 for M ) ZnII, NiII, and n ) 12 for M ) FeIII. Transient absorption spectra of [ZnTMePyP4+]3s[Fe4(P2W15O56)2]12- were recorded under the same conditions as those for the last complexes. The transient spectrum recorded 2 µs after the excitation is presented in Figure 6. This spectrum presents mainly one bleaching band corresponding to the Soret band of the porphyrin in the complex. Also, two transients centered around λ ) 530 nm and λ ) 730 nm are observed. Time profiles are quite similar whatever the wavelengths (as the one presented in the insert of the Figure 6). Comparing these transient absorption spectra with the one of the free porphyrin, the two transient bands can be indentified to the triplet state of the porphyrin. The weakness of the signal and the multiexponential character of the time decays show a strong quenching of the triplet state by the POM. Figure 6 compares also the transient absorption spectra of [ZnTMePyP4+]3s [Fe4(P2W15O56)2]12- and [ZnTMePyP4+]4s [Zn4(P2W15O56)2]16-, 2 µs after the laser excitation. Even if the global shape of both spectra is quite similar, the differential

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Figure 3. (A) Luminescence spectra recorded during the titration of a 7 × 10-6 M aqueous solution of [ZnTMePyP]4+ by a 8.25 × 10-5 M solution of Zn4-POM (λexcitation ) 570 nm). (B) Luminescence change at λ ) 700 nm (9) as a function of the molar fraction of the added [Zn4(H2O)2(P2W15O56)2]16-.

Figure 4. Top: Transient absorption spectra of a deaerated aqueous solution of [ZnTMePyP]4+ recorded upon λ ) 355 nm laser excitation at (a) 150 µs, (b) 2.9 ms, (c) 4.25 ms, (d) 15.1 ms, and (e) 35.1 ms. Insert: absorption spectrum of the [ZnTMePyP]4+ in aqueous solution. Bottom: decay curve recorded at λ ) 520 nm.

Figure 5. (A) Transient absorption spectra of a deaerated aqueous solution of [ZnTMePyP4+]4s[Zn4(H2O)2(P2W15O56)2]16- recorded upon λ ) 355 nm laser excitation at (a) 2.25, (b) 11, (c) 27.5, (d) 65, (e) 300, and (f) 825 µs. Insert: comparison of the spectra of the porphyrin (red line) and the complex (black line), 2 µs after the excitation. (B) Decays recorded at λ ) 530 nm on two different time scales.

absorbance of the first compound is 8 times less than the second. Moreover, the time decay of the triplet state of the porphyrin is faster when it is complexed with Fe4-POM (estimated at 80 10 µs) than with Zn4-POM (estimated at few hundred microseconds). It shows a more efficient quenched of the triplet state by the POM with Fe4-POM. Finally, in the near-infrared part of the transient spectra, a weak signal of charge transfer has been recorded in the [ZnTMePyP4+]4s [Zn4(P2W15O56)2]16- complex and not in the [ZnTMePyP4+]3s [Fe4(P2W15O56)2]12- complex. It does not signify that the charge transfer occurs in the first complex and not in the second. On the contrary, the charge transfer between the porphyrin and the POM is probably much

Figure 6. Comparison of transient absorption spectra of [ZnTMePyP4+]3s[Fe4(H2O)2(P2W15O56)2]12- (black line) and [ZnTMePyP4+]4s [Zn4(H2O)2(P2W15O56)2]16- (red line) in aqueous solution 2 µs after the excitation. Insert: decay of [ZnTMePyP4+]3s [Fe4(P2W15O56)2]12- recorded at λ ) 530 nm.

more efficient in the [ZnTMePyP4+]3s[Fe4(P2W15O56)2]12complex. It explains why the transient signal, a few microseconds after the excitation, is weaker and why at the scale of these experiments no signal characteristic of the charge transfer is detected in that complex. It is important to note that this system is perfectly reversible: after the excitation of the porphyrin, whatever the relaxation processes invoked (noticeably via a charge transfer), the system goes back to the initial state. This has been confirmed by steadystate photolysis experiments, where the system continuously illuminated by visible light shows no degradation. 3. Photocatalytic Applications Using Visible Light. If light is able to induce a charge transfer into the complexes, the formed reduced POM should be able to give its supplementary electron for the reduction reaction. In order to examine the photocatalytic activity of the complexes, steady-state photolysis experiments were performed in aerated solution with visible light (up to 400 nm, depending of the absorption spectrum of the used POM). Under these conditions, the porphyrin is the only absorbing species. Reduction of silver ions (Ag+) was investigated as the process is monoelectronic. Even if it is known that dioxygen is an efficient electron captor and acts as a competition reaction, the processes involved remain quite efficient.7 3.1. Photoreduction of SilWer Ions by the Porphyrin-POM Complexes. First, the porphyrins and POM were studied separately. Photolysis of the porphyrin in the presence of silver cations leads rapidly to its degradation, observing a continuous decrease in the absorption spectrum during irradiation. This is characteristic of the decomposition of the compound in chlorine and phlorin and further products of decomposition.30-32 Con-

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TABLE 2: Quantum Yields of the Catalytic Reactiona compounds 4+

Φ (%) 16-

[ZnTMePyP ]4s[Zn4(P2W15O56)2] [ZnTMePyP4+]4s[Ni4(P2W15O56)2]16[ZnTMePyP4+]3s[Fe4(P2W15O56)2]12-

0.005 0.005 0.045

a Φ is the quantum yield of the total catalytic reaction (number of reduced Ag+ divided by the number of absorbed photons).

cerning the POM, no evolution of the spectrum during the illumination was noted, according to their absorption spectrum. To be able to demonstrate the catalytic efficiency of the synthesized complexes, a control experiment was carried out: a solution containing only Ag2SO4 and propan-2-ol was illuminated under visible light and in the presence of dioxygen. No change of absorption spectrum was noticed, according a published work.33 Then, the photocatalytic activity of the [ZnTMePyP4+]n/4s [M4-POM]n- with (n ) 16 for M ) ZnII, NiII and n ) 12 for M ) FeIII) complexes have been studied separately and compared. The quantum yields for the catalytic reaction are reported in Table 2. For [ZnTMePyP4+]4s[Zn4(P2W15O56)2]16-, absorbance recorded as a function of the irradiation time at two wavelengths has been recorded (Figure 7, parts A and B). At λ ) 442 nm, the maximum of the Soret band of the porphyrin in the [ZnTMePyP4+]4s[Zn4(P2W15O56)2]16- complex, the absorption decreases continuously until a near complete vanishing of the signal. At the same time, the absorbance at λ ) 700 nm increases continuously until reaching a plateau after 50 min of irradiation. An identical observation has been found for the [ZnTMePyP4+]4s[Ni4(P2W15O56)2]16- (see Figure S6 in Supporting Information). A different behavior is observed using the [ZnTMePyP4+]3s [Fe4(P2W15O56)2]12- complex (Figure 7, parts C and D). At λ ) 442 nm and λ ) 700 nm the absorbance increases slightly gradually over a few hours. In both cases, the increasing of the absorbance at λ ) 700 nm is due to the formation of large silver nanoparticles nonhomogeneous in size34 as revealed by transmission electronic microscopy. This difference of behavior of the two complexes can be explained by the redox potentials of the compounds involved. The reasoning is done on the redox potential of the separate compounds because it has been shown that they are slightly shifted in the electrostatic complexes.16,18,19,35 They are summarized in Scheme 1. Two mechanisms could be involved to explain the formation of the nanoparticles, one involving the reduction of the porphyrin and the other involving the oxidation of the porphyrin. 3.2. Reduction Mechanism. After the excitation of the porphyrin in the complex, a charge transfer between the porphyrin (in the singlet or triplet state) and the alcohol occurs according the reaction

P* + (CH3)2CHOH f P•- + (CH3)2C•OH + H+

(1) A charge transfer between the reduced porphyrin and the POM is thermodynamically possible leading to the formation of reduced POM. Nevertheless, this reaction does not occur, or very slowly, since in absence of Ag+ cations an accumulation of reduced POM has never been observed, even in deaerated solution (result not shown).

Moreover, thermodynamically, the potentials E0(P/P•-) ) -0.6 V28 or E0((CH3)2C•OH/(CH3)2CHOH) ≈ 0.8 V36 do not allow the reaction of electron transfer between one of the reduced compounds and one Ag+ cation. Nevertheless, the reduction occurs via the reaction with the alcohol radical (reactions 2 and 3): the reduction process can be initiated via a reaction of complexation between Ag+ and the alcohol radical, as it has been demonstrated in radiolysis37,38

Ag+ + (CH3)2C•OH f [Ag((CH3)2C•OH)]+

(2)

(Ag(CH3)2C•OH)+ + Ag+ f Ag2+ + Ag(CH3)2CO + H+ (3) From that, silver nanoparticles are formed by association and coalescence reactions between Ag+ ions and primary clusters according the following reactions34

Ag+ + Ag0 f Ag2+

(4)

2Ag2+ f Ag42+

(5)

Agmx+ + Agny+ f Agm+n(x+y)+

(6)

It is interesting to note that in that mechanism the POM has no role in the formation of the nanoparticles. Consequently, this reactions occurs whatever the nature of the metal of the tetraoxometallic central cluster in the used POM. 3.3. Oxidation Mechanism. After the excitation of the porphyrin in the complex, a charge transfer occurs with the POM according to the following reaction

P*(S) + M4-POM f P•+ + M4-POM-

(7)

Then, the photocatalytic cycle is activated (Scheme 2) leading to the formation of silver nanoparticles. Thermodynamically, a direct charge transfer from the reduced POM to a silver cation is unfavorable. Nevertheless, as has been shown,39 two parallel mechanisms lead to the formation of silver nanoparticles. First, Ag+ ions could be electrostatically ligated with the complexes (via the POM negatively charged) without dissociation of the complexes (effect of ionic strength). This modifies the Agnm+/Agn-x(m-y)+ redox potentials leading to the formation of the silver clusters.40 Second, during the regeneration of the porphyrin in the catalytic cycle, an alcohol radical is produced. Then nanoparticles can be generated by reactions 2-7. 3.4. Discussion of the Influence of the Nature of the Metal of the Tetraoxometallic Central Cluster in the POM. Theoretically, both mechanisms occur at the same time and are competitive. It appears that according to that experimental work, the process involving the oxidation of the porphyrin is the dominant process since accumulation of reduced POM has not been observed when all the Ag(I) is reduced. Nevertheless, The degradation of the porphyrin takes place for [ZnTMePyP4+]4s[M4(P2W15O56)2]16- (M ) Zn2+ or Ni2+) but not for [ZnTMePyP4+]3s[Fe4(P2W15O56)2]12-. Two reasons could be invoked. First, the degradation appears during the reduction process, probably by the same process as in the isolated porphyrin.30-32 The other reason could be due to the reaction

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Figure 7. (A and C) Change in the UV-visible absorption spectrum and (B and D) evolution of the absorbances at (2) λ ) 442 nm and (b) λ ) 700 nm of 2 × 10-6 M aerated solutions of [ZnTMePyP4+]3s[Fe4(H2O)2(P2W15O56)2]12- and [ZnTMePyP4+]4s[Zn4(H2O)2(P2W15O56)2]16-, respectively, in presence of 8 × 10-5 M Ag2SO4 and 0.13 M propan-2-ol under visible irradiation.

SCHEME 1: Redox Potentials vs NHE of Involved Compoundsa

a Data concerning the [ZnTMePyP]4+ (abbreviated P) comes from ref 28, concerning the POM see ref 25, and concerning alcohol see ref 36.

SCHEME 2: Oxidation Mechanism: Representation of the Catalytic Cycle Leading to the Silver Atom

of O2 (E(O2/O2•-) ) -0.16 V vs NHE)25 with the reduced POM (E(POM/POM4-) ) -0.20 V vs NHE)41 leading to the radical O2•- and further reactive products HO2•, OH•, and H2O242 which can react with porphyrins and degrade them. Such degradation could be avoided by changing the redox properties of the M4POM. Indeed, in the case of Fe4-POM the potential of the couple FeIII4-POM/FeIII3FeII-POM- (+0.35 V vs NHE) does not allow such reaction with O2 (Scheme 1). The competition between the reduction and the oxidation mechanisms lies also in the equilibrium between the thermo-

dynamics and the kinetics factors of the reactions. Thermodynamically, according to the redox potentials, the reaction of electron transfer between the excited porphyrin and the POM is more exergonic with the Fe4-POM than for the two other compounds. Consequently, the quantum yield of electron transfer reaction is important for this compound. Then by a concentration effect which is a kinetic effect, it can be understood that for [ZnTMePyP4+]3s[Fe4(P2W15O56)2]12- the catalytic cycle involving the charge transfer between the porphyrin and the POM leads principally to the formation of the nanoparticles via the reduced POM. On the contrary for the two other complexes, this charge transfer becomes less favorable and when it takes place, the degradation of the porphyrin occurs. 3.5. Comparison between Transient Absorption Spectroscopy Measurements and Photocatalytic ObserWations. It is important to note that the previous considerations are in perfect agreement with those deduced from the transient absorption spectroscopy measurements where no Ag+ cations and isopropanol were added to the solutions. In the case of [ZnTMePyP4+]3s[Fe4(P2W15O56)2]12- complex, the electron transfer reaction is very efficient leading to the formation of silver nanoparticles by this channel. Considering the porphyrin, the quantum yield of the triplet state becomes very small. In addition, the “return electron transfer” reaction from the POM to the porphyrin is very rapid and invisible at the scale of the experiments. On the contrary, in the case of [ZnTMePyP4+]4s[Zn4(P2W15O56)2]16- and [ZnTMePyP4+]4s[Ni4(P2W15O56)2]16-, the electron transfer is less efficient. Consequently, Ag+ cations might also be reduced by the channel of the reduced porphyrin. The second consequence is that considering the excited states, the quantum yield of triplet state of the porphyrin is more important than that for the complex with Fe4-POM. The transient absorption bands associated to that states are also more intense. It is also understandable that the “return electron transfer” reaction from the POM to the porphyrin is less efficient leading to the observation of a band characteristic of the reduced POM. 3.6. Final Remark. In those systems, the choice of the nature of the metal of the tetraoxometallic central cluster of the Dawson sandwich POM (see Figure 1B) is essential for photocatalysis application. In all cases, the reduction of metallic cations as

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Ag+ is possible. But for further applications, the degradation of the catalyst is not desired. Then a metallic cation with a high reduction potential leading to an efficient electron transfer from the excited porphyrin to the POM should be chosen since in fact, it is demonstrated here that the catalysis involved the reduction of the metallic central cluster and finally not the complete structure of the POM. Conclusion In this work, the photocatalytic ability of derived Dawson sandwich polyoxometalates using visible light has been demonstrated for the first time using a photosensitizer, a porphyrin. Formation of such electrostatic complexes between [ZnTMePyP]4+ and derived Dawson sandwich polyoxometalates [M4(P2W15O56)2]n- with (n ) 16 for M ) ZnII or NiII and n ) 12 for M ) FeIII) has been demonstrated. In each case, association of these units leads to the formation of one single neutral complex. The calculation of the formation constant by the Job’s method and studies of the influence of ionic salts prove the stability and the strong interaction between the porphyrins and the polyoxometalate. Steady-state absorption, fluorescence emission, and transient absorption spectroscopies allow a clearer determination of the role of the excited states. The observations prove the existence of a charge transfer from the porphyrin to the POM accessible from the S2 or the S1 state of the porphyrin. This charge transfer is reversible. The formation of reduced POM after an excitation in the visible range allows the use of this system as a photocatalyst. The catalytic measurements were undertaken studying a model reaction such as the reduction of silver cation. In any case, the reduction of the cations in nanoparticles takes place. Nevertheless, different behaviors as a function of the nature of the metal of the tetraoxometallic central cluster of the POM have been observed, involving different types of catalytic processes: it depends on the first reduction potential of the derived Dawson sandwich polyoxometalates. Finally, it is important to note that the [ZnTMePyP4+]3s [Fe4(P2W15O56)2]12- complex is photoactive in aqueous solution, under air, after illumination with visible light and presents no degradation after its utilization. This result is very promising and could lead to its utilization in green chemistry as biphotocalyst replacing the sacrificial donor by polluting halogened aromatic compounds. Acknowledgment. The authors thank Professor Jacqueline Belloni from our laboratory for her help to understand nanoparticle formation. The authors thank also Patricia Beaunier, Laboratoire de Re´activite´ de Surface, UMR 7609 CNRSUniversite´ Paris VI (France) for the TEM observations. This research was supported by the CNRS and by the University Paris-Sud 11. This work was also supported by the ANR, Project Number JC05_52437, NCPPOM. Supporting Information Available: Figures showing UV-vis absorption spectra, luminescence spectra, and transient absorption spectra, luminescence change with salt concentration, and decay comparisons. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Katsoulis, D. E. Chem. ReV. 1998, 98, 359. (2) Rhule, J. T.; Hill, C. L.; Judd, D. A.; Schinazi, R. F. Chem. ReV. 1998, 98, 327.

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