PorphyrinâNanoclay Photosensitizers for Visible Light Induced

Article pubs.acs.org/JPCC

Porphyrin−Nanoclay Photosensitizers for Visible Light Induced Oxidation of Phenol in Aqueous Media Dominik Drozd,† Krzysztof Szczubiałka,*,† Michał Skiba,‡ Mariusz Kepczynski,† and Maria Nowakowska*,† †

Faculty of Chemistry, Jagiellonian University, 30-060 Kraków, Ingardena 3, Poland Institute of Geological Studies, Jagiellonian University, 30-063 Kraków, Oleandry 20, Poland

‡

S Supporting Information *

ABSTRACT: A new type of hybrid photosensitizer (Po−C30B) was obtained by efficient adsorption of a 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (Po) by Cloisite 30B (C30B)a monotallow bis(hydroxyethyl)ammonium-modified montmorillonite clay from acidic solution in methanol. Structural and spectroscopic properties of Po-nanoclay photosensitizer were determined using X-ray diffraction, laser scanning fluorescence confocal microscopy, and electronic absorption/emission spectroscopies. Po is present not only at the surface of the nanoclay but also in the interior of the Po-C30B hybrid material. The obtained material was found to be an efficient photosensitizer for the oxidation of phenol in aqueous solution under irradiation with the visible light (λ > 470 nm). The mechanism and the quantum yield of that process were shown to be strongly pHdependent. They were controlled by the acid−base equilibria of porphyrin associated with imine N-protonation as well as by the ionization of the phenol molecule. The quantitative information regarding these dependencies was obtained. The values of K3 and K4 acid− base equilibrium constants were determined (pK3 = 5.88 and pK4 = 2.46) from the absorption spectra recorded during acid−base titration and using an evolutionary factor analysis with the mathematical model including dicationic (H2Po2+), monocationic (HPo+), and neutral (Po) porphyrin forms. They were used to evaluate the importance of these forms in singlet oxygen generation by Po−C30B under defined pH conditions. Moreover, the hybrid photosensitizer can be used repeatedly, which makes it possible to use it in industrial applications.

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INTRODUCTION The world is facing a huge challenge of purification of toxic substances from industrial wastewater.1−4 Phenol is one of the main pollutants found in water. It is commonly present in industrial wastewater produced by many industries, for example, by coking plants, oil extraction plants and refineries, paper industry, pharmaceutical plants, etc.5−8 Since it is harmful to living organisms even at low concentrations, it is considered as the priority pollutant.9 The effective removal of that pollutant from wastewater is a problem of great importance and interest. An interesting method of removing pollutants is to use clay minerals. Natural clays such as montmorillonite are extensively used as adsorbents due to their high cation exchange capacity (CEC), swelling properties, and high surface area.10 Montmorillonite is a 2:1 type clay mineral which possesses two silica−oxygen tetrahedral sheets with a central alumina octahedral sheet (TOT layer). The isomorphic substitution within the layers (i.e., Al3+ replaced by Mg2+ or Fe2+ in the octahedral sheet, Si4+ replaced by Al3+ in the tetrahedral sheets) causes the surface of the clay layer to be charged negatively, which is counterbalanced by the presence of the exchangeable cations (e.g., Na+ or Ca2+) in the interlayer space. Because of the hydration of the inorganic cations, the clay surface is hydrophilic. This makes clay minerals to be ineffective adsorbents for organic compounds.11,12 By replacing © 2014 American Chemical Society

inorganic cations in the interlayer space with organic ones such as quaternary ammonium cations, hydrophilic clays are converted to hydrophobic organoclays which are effective adsorbents for organic compounds, including a variety of water pollutants.8,13−15 That method, however, does not detoxify the pollutants and creates the new problem of the safe disposal of the contaminated clay. Recently, the hybrid photosensitizers based on organoclays and various photosensitizers, both low molecular weight ones, like phthalocyanines,16,17 and polymeric ones,18 have been obtained. It has been demonstrated that these photosensitizers can induce photodegradation of pollutants when irradiated with visible light. The process occurs with the formation of reactive oxygen species such as singlet oxygen, and the hybrid photosensitizer can be easily separated from water after the purification process. In the current paper we present the novel hybrid system in which porphyrin, a dye frequently occurring in nature, was applied as the photoactive component and Cloisite 30B (C30B) was used as a support. C30B was chosen for that purpose because it is one of the most commonly used organoclay minerals. It is obtained through the modification of Received: January 2, 2014 Revised: April 5, 2014 Published: April 9, 2014 9196

dx.doi.org/10.1021/jp500024h | J. Phys. Chem. C 2014, 118, 9196−9202

The Journal of Physical Chemistry C

Article

POCh Gliwice), methanol (analytical grade, Lach-Ner), acetonitrile (POCh, Gliwice, Poland, pure), and hydrochloric acid (35−38 wt % POCh, Gliwice, Poland, pure) were used as received. Phosphoric acid (85 wt % in H2O) was received from Aldrich Chemical Co. (Milwaukee, WI).

the montmorillonite nanoclay with an ammonium surfactant bearing two functional groups: alkyl chains and hydroxyl groups. 5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrin (Po) was selected as it can absorb light in the visible spectral region and its spectral and photophysical/photochemical properties can be modified by changing pH of the medium. We have not found reports on that kind of the system in the literature. However, an interesting photoactive composite system was prepared by intercalating of Pd(II)−5,10,15,20-tetrakis(4carboxyphenyl)porphyrin between the lamellae of Zn2Al layered double hydroxide and dispersing into a polyurethane matrix. The photophysical studies have shown that the inorganic matrix enhances the chemical stability of the porphyrin by minimizing photobleaching and quenching aggregation effects.19 Photofunctional material was also obtained by the incorporation of tetrasulfonated porphyrins into rare-earth layered hydroxides. The material functioned as an effective sensitizer of singlet oxygen, O2(1Δg), which had a relatively long lifetime of 23 ± 1 μs in that system.20 The advances in synthesis and physicochemical, spectral, photophysical, catalytic, and sensing properties of layered hydroxide− porphyrin hybrids were presented in recently published review paper.21 The effect of adsorption of dicationic porphyrins, cisand trans-bis(N-methylpyridinium-4-yl)diphenyl porphyrins, on clay mineral templates on spectral properties of dyes was also investigated. It has been shown that although adsorption of the porphyrins resulted in significant changes in their spectral characteristics, the dyes preserved their photoactivity. The changes in the porphyrin absorption spectra were explained considering the structural changes (flattening) of the porphyrin molecules and molecular aggregation.22 Interestingly, the clay/ porphyrin complexes have been shown recently to be promising candidates for the construction of an efficient artificial light-harvesting system with the efficiency of the energy transfer between cationic porphyrins on an anionic clay surface reaching 100%.23 These reports indicate clearly that there is a lot of potential in porphyrin-containing hybrid systems. In the current paper we have demonstrated that the C30B− porphyrin hybrid synthesized can be used as the photosensitizer for oxidation of phenol in aqueous solution under irradiation with the visible light. Considering that C30B−Po dispersed in water is an effective generator of singlet oxygen, one can expect that it can be used as the photosensitizer in various photochemical reactions occurring in aqueous medium, including these of considerable importance for water purification and remediation.

Scheme 1. Structural Formula of 5,10,15,20-Tetrakis(4carboxyphenyl)porphyrin (Po) and Structure of Cloisite 30B

Synthesis of Po−Nanoclay Photosensitizers. The typical synthesis of Po−C30B hybrid photosensitizer was as follows. 5 mg of Po was dissolved in 15 mL of methanol containing 3.5 mM HCl. Then, 250 mg of C30B was dispersed in that solution. The suspension was stirred in the dark for 15 h and centrifuged, and the solid was washed with methanol until no Po was detected in solution by measuring its UV−vis absorption spectrum. The amount of Po present in the photosensitizer was determined from the absorption spectra of the solutions used for synthesis, and these left after removal by centrifugation of the clays saturated with porphyrin. The amount of Po introduced to 1 g of clay was found to be 0.0195 mg. Apparatus. UV−vis absorption spectra were recorded using a Hewlett-Packard HP 8452A diode-array spectrophotometer in 1 cm optical path quartz cuvettes. The steady-state fluorescence spectra were recorded using a PerkinElmer LS55 spectrofluorometer in 1 cm optical path quartz cuvettes. XRD patterns were obtained with a Philips X’Pert diffractometer using Cu Kα radiation (40 kV and 30 mA). The diffractometer was equipped with a PW3020 vertical goniometer, a 1° divergence slit, 0.2 mm receiving slit, incident and diffracted beam Sollers, and 1° antiscatter slit. The data were obtained by scanning from 2° to 52° 2 θ at a counting speed of 0.02° step/2 s. The XRD mounts were prepared by dispersing 20−100 mg solid samples in deionized water using an ultrasonic tip, depositing the suspensions on a zero background silicon wafer followed by air-drying. HPLC analyses were performed using a Waters system equipped with a Waters 2996 PDA detector and a Symmetry C18 5 μm (4.6 mm × 150 mm) column. The eluent was 1:1 (v/v) mixture of water and methanol containing 0.1 vol % of phosphoric acid. The flow rate was 0.5 mL min−1. Microstructures were analyzed with a A1-Si Nikon Inc. (Japan) confocal laser scanning system (LSCM) built onto a Nikon inverted microscope Ti-E using a Plan Apo 100×/1.4 Oil DIC objective. Images were acquired at a resolution of 2048 × 2048. The Al−Si system was equipped with a four-channel detection as well as LSBF imaging by diascopic detection of forward scattered excitation laser light during confocal laser scanning. The excitation for confocal microscopy was provided by a set of four diode lasers with excitation wavelengths at 405, 488, 561, and 638 nm.

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EXPERIMENTAL SECTION Materials. Cloisite 30B (C30B), a gift from Southern Clay Products, Inc., and 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (>97%, Porphyrin Products Inc., Po) were used as received. Potassium tetrathiocyanatodiaminechromate(III) (K[Cr(NH3)2(SCN)4]) used in the actinometric measurement s was obt ained from ammo nium t etrath iocyanatodiaminechromate(III) (Reinecke’s salt, NH 4 [Cr(NH3)2(SCN)4]·H2O, analytical grade, Sigma-Aldrich) and purified according to a literature procedure.24 Iron(III) nitrate nonahydrate (analytical grade, Sigma-Aldrich) and perchloric acid (70%, Riedel-de Haën AG) were also used for actinometry. Phenol (POCh, Gliwice, Poland, pure), imidazole (analytical grade, Sigma-Aldrich), N,N-dimethyl-4-nitrosoaniline (RNO, 97%, Sigma-Aldrich), potassium nitrate (analytical grade, 9197



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Fluorescence spectra were collected using a 32-channel spectral detector. Irradiation of the Sample. A 75 W xenon lamp with a cutoff filter (λ > 470 nm) was used to irradiate a system in which Po−C30B was used as a photosensitizer. For aerobic experiments solvents were bubbled with oxygen for 30 min. The photon flow was determined using the Reineckate actinometer24 under an argon atmosphere, the concentration of K[Cr(NH3)2(SCN)4] being 1.24 × 10−2 M. The reaction rate, Vr, was calculated using the equation: Vr = I> 470 Φ> 470

∫λ F>470(λ)(1 − 10−A (λ)) dλ R

Figure 1. XRD patterns for C30B (dashed line), C30B treated with a mixture of methanol and hydrochloric acid (solid line), and Po−C30B (small dashed line).

(1)

where I>470 is the maximum intensity of light emitted by the lamp at λ > 470 nm, Φ>470 is the quantum yield of the actinometric reaction, AR(λ) is the absorbance of K[Cr(NH3)2(SCN)4], and F>470(λ) is the spectral distribution of the lamp at λ > 470 nm.25 The quantum yield of singlet oxygen formation (ϕΔ) was determined by measuring the bleaching of N,N-dimethyl-4nitrosoaniline (RNO) at 440 nm resulting from the transannular peroxide formation in the reaction of singlet oxygen with imidazole, as presented previously.26,27 The suspension contained 40 mg of Po−C30B photosensitizer, 0.01 mol/dm3 imidazole, and RNO at the initial concentration of 5 × 10−5 mol/dm3. Under these conditions (imidazole was in large excess and the changes of RNO concentration did not exceed 10% of its initial concentration) RNO bleaching is a zero-order kinetics process with a slope proportional to ϕΔ. The value of ϕΔ was calculated as follows: GPor [RNO] = [RNO]0 − Iabs φΔt

rearranged the paraffinic structure of the surfactant molecules by increasing the tilting angle from 37° to 49°.28 Considering these facts, it is difficult to reach the conclusion regarding the location of Po in the nanoclay. To elucidate that problem, the visualization of the microstructures with the LSCM microscope was performed. The typical LSCM micrograph is shown in Figure 2. The clay forms flat flakes of

(2)

where GPor Iabs = I> 470

∫λ F>470(λ)(1 − 10−A

GPor (λ)

) dλ

(3)

and AGPor(λ) is the absorbance of Po−C30B photosensitizer at the given wavelength (λ).

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RESULTS AND DISCUSSION Structure and Spectroscopic Properties of Po−Nanoclay Photosensitizer. The synthetic procedure described above resulted in the formation of green products, suggesting that porphyrin was introduced into the nanoclay and hybrid systems were produced. Porphyrin can be adsorbed on the surface of nanoclay or intercalated between the interlayers. To differentiate between these two options and to obtain the structural information on Po−nanoclay photosensitizers, the XRD analyses were performed. Figure 1 presents the resulting diffraction patterns for Po−C30B and for C30B organoclay for a comparison. The distance between the layers in C30B was found to be 18.7 Å, while that for Po−C30B was equal to 15.8 Å. These results suggested that during the synthesis of Po− C30B hybrid most probably the rearrangement and/or partial leaching of the surfactant from the aluminosilicate occurs. That was confirmed experimentally. XRD analysis for C30B treated with the solvent used to incorporate Po (methanol−HCl mixture) indicated that the interlayer distance decreased from 18.7 to15.4 Å (red line in Figure 1). A similar effect was previously observed during the cation exchange intercalation of fluorescent dyes into C30B and explained as resulting from

Figure 2. (A) Confocal micrograph showing fluorescence of porphyrin (red) superimposed on the transmitted light image of the Po−C30B sample. (B) Cross sections of the Po−C30B sample. The bars shown in the pictures represent 50 μm. (B) Three-dimensional image of the Po−C30B sample constructed from a series of fluorescence micrographs.

a large size distribution. The microstructures showed strong red emission typical for the porphyrin. The emission was localized at the particles, and there was no fluorescence from the aqueous phase. The three-dimensional reconstruction of fluorescence micrographs (Figure 2C) and the cross sections of the microstructure (Figure 2B) indicate that porphyrin is inhomogeneously distributed forming clusters. However, these clusters extend over the entire volume of the clay flake. Thus, it is reasonable to assume that the dye is not only adsorbed on the surface of the microstructure but also can penetrate into its interior. The fluorescence spectrum measured for this material is shown in Figure 3. The shape of the spectrum is similar to that in acidic methanol, reflecting the acidity of the clay surface.27 Interestingly, we have not observed 9198



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red-shifted compared to the UV−vis absorption spectrum of the dye in methanol (red line) or water (not shown). The lowering of pH resulted in the shift of the Soret band to the higher wavelengths (452 nm). Concomitantly, four Q bands transformed into two bands centered at 625 and 662 nm. In acidic methanol solution, Po has the Soret band at 437 nm and two Q bands of lower intensities, at 599 and 650 nm. The changes in the spectra reflect the acid−base equilibrium inside the porphyrin ring, the protonation state of the peripheral carboxylic groups of Po showing little effect on its electronic spectrum.30−33 Two imine nitrogen atoms of a free-base porphyrin are able to attach protons forming mono- and dications. Using the generally accepted convention that K3 and K4 acid−base equilibrium constants are associated with imine-N protonation, while the constants K1 and K2 correspond to the deprotonation of amine groups,30,32 the equilibria can be represented by the following expression, where the charges of the peripheral groups are not represented:

Figure 3. Fluorescence spectra of Po−C30B in aqueous dispersion measured using confocal microscope (solid line, λexc = 488 nm) and spectrofluorometer (dashed line, λexc = 599 nm). Fluorescence spectrum of Po in acidic methanol solution (small dashed line, λexc = 599 nm).

the emission characteristic of H or J aggregates which could be expected in such system. Effect of pH on Spectral Properties of Po−C30B. We have noticed that the Po−C30B material changes its color depending on the acidity of the medium, from green in acidic to brown in the alkaline system. To get more detailed information on the spectral properties of Po embedded into the C30B clay, the absorption spectra of the hybrid material were measured and compared with those for Po in methanol and acidic methanol (Figure 4). The porphyrin studied has two

pK4

pK3

H 2Po2 + HoooI HPo+ HooI Po

(4)

where H2Po2+ and HPo+ denote the dicationic porphyrin species (two protons attached to the imino nitrogen atoms in the acidic medium) and the monocationic one, respectively. The pK3 of Po has been reported to have the value of 5.5, while the second constant, pK 4 , could not be determined experimentally due to the aggregation.25 The aggregation processes of ionic water-soluble tetraarylporphyrins (TArPs) have been investigated by Pasternack et al.30 They showed that Po undergoes dimerization in water. The equilibrium constant for dimerization in 0.1 M solution of KNO3 at pH = 7.5 was determined to be 4.55 × 104 M−1. To determine the type of Po species photochemically active in Po−C30B under various pH conditions, we have attempted to investigate the acid−base equilibrium of Po using a method that we have described previously.33 Figure 5A shows the absorption spectra of Po recorded during acid−base titration. On the basis of these data, one can conclude that with the lowering of pH the intensity of the band centered at 414 nm, which is characteristic of nonprotonated form of Po, decreases with the concomitant appearance of the band at 434 nm. However, the increase of the band at 434 nm is not as pronounced as the decrease of the band at 414 nm. We assigned this to formation of aggregates due to neutralization of the carboxylic groups. To extract information on the pK values from the absorption spectra, the evolutionary factor analysis34 with the mathematical model including H2Po2+, HPo+, and Po forms was applied, and the measured absorption spectra were fitted to this model. The best fit was obtained for pK3 = 5.88 and pK4 = 2.46. The extracted absorption spectra of dicationic, monocationic, and neutral forms are shown in Figure 5B. However, the intensity of the bands characteristic of dicationic and monocationic forms is much lower than expected. Furthermore, the shape of the spectra of these forms is different from that presented in the literature.33 Therefore, one may conclude that the spectra for these forms and the value of pK4 are strongly affected by the presence of aggregates. Their formation is facilitated by the protonation of carboxyl groups. Figure 5C presents the dependence of the fractions of dicationic (H2Po2+), monocationic (HPo+), and neutral form (Po) of the porphyrin chromophore on pH of the solution.

Figure 4. Electronic absorption spectra of aqueous dispersions of Po− C30B at pH = 5 (dotted line), pH = 10 (short dotted line), and pH = 12 (dashed-dotted line) and Po in acidic methanol (solid line) and in methanol (dashed line).

kinds of groups that can be protonated: four peripheral carboxylic groups and two imine nitrogen atoms in the porphine ring. The pKa value for the peripheral carboxylic groups can be approximated accepting the value for benzoic acid, i.e., 4.2.29 A similar approach was taken earlier by Pasternack et al.30 Using that value and the dissociation constants of pyrrole nitrogens determined in the current studies (vide inf ra), the acid−base equilibria for 5,10,15,20-tetrakis(4carboxyphenyl)porphyrin and the dependence of the fraction of different porphyrin ions on pH were determined and presented in the Supporting Information (S1 and S2). For practical reasons we are particularly interested in spectral and photophysical properties of our hybrid material in the neutral and alkaline systems. Under alkaline conditions, the spectrum of Po−C30B (green dashed-dotted line) is characterized by the presence of the Soret band (415 nm) and four Q bands (518, 554, 617, and 664 nm), and it is slightly 9199



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Figure 6. Dependence of phenol concentration on exposure time for Po−C30B + phenol systems under various pH (cPo−C30B = 1.0 g/dm3).

inset). The quantum yield of phenol photooxidation, ϕPhOH, can be calculated using eq 4:

φPhOH =

V r0 GPor Iabs

(5)

V0r

where is the initial rate of the reaction (consumption of phenol) determined at low conversion to avoid any possible interference from the products formed. The initial rate of the reaction (V0r ) was determined from dependence of phenol concentration on irradiation time at the early stage of the reaction. The results are listed in Table 1. The quantum yield of photooxidation of phenol increases with an increase in pH of the irradiated solution (see Table 1 and Figure S4 in Supporting Information). It has been shown before that this can be explained considering the effect of pH on the ionization of Po and the differences in the reactivity of the species present in the systems.25,35 The degree of phenol (PhOH) dissociation (α) at any given pH can be calculated as follows: α=

[PhO−] = 1/(1 + 10(pKα − pH)) [PhO−] + [PhOH]

(6)

36

where pH is a measured value and pKα = 9.99. The calculated values of α and the concentrations of PhOH and PhO− in solutions at various pH used in our experiments are collected in Table 1. Po−C30B was demonstrated to act as a photocatalyst in several consecutive cycles of phenol photooxidation. For that purpose Po−C30B was suspended in phenol solution (pH = 7.20; c = 1 × 10−3 M) and irradiated for a defined period of time. Then, Po−C30B was removed by centrifugation, fresh phenol sample was added, and the system was irradiated again. This procedure was repeated four times, and the changes of phenol concentration on irradiation time in each cycle were monitored by measurements of UV−vis spectra and HPLC analysis (see Figure S5 in Supporting Information). Mechanism of Po−C30B Photosensitized Oxidation of Phenol. Porphyrin derivatives are known to be efficient generators of singlet oxygen. The most important area of practical applications of that phenomenon is the photodynamic therapy (PDT).37−39 The mechanism of singlet oxygen formation by porphyrins is well-known. It involves electronic excitations of their molecules to the first singlet excited state, followed by the intersystem crossing with the formation of excited triplet state which is quenched by molecular oxygen. In the last step of that sequence the porphyrin molecule returns to the ground state and singlet oxygen is formed. Po used in current studies was found to be quite efficient generator of

Figure 5. (a) Changes in the absorption spectra of Po (cPo = 2.14 × 10−6 M) as a function of pH. (b) Calculated absorption spectra of the dicationic (H2Po2+), monocationic (HPo+), and free-base form (Po) of the porphyrin chromophore. (c) Dependence on pH of the relative fractions of porphyrin chromophore of different protonation degree.

Po−C30B Photosensitized Oxidation of Phenol. Aqueous dispersions of Po−C30B containing the constant concentration of phenol (c = 1 × 10−3 M) and differing in the value of pH (5.40, 7.20, 9.00, 10.06, and 12.30) were prepared. Then, oxygen-saturated dispersions were irradiated with light at λ > 470 nm that is absorbed only by the porphyrin chromophore present in the system. The reaction was followed by the measurement of the UV spectra and HPLC analysis. It was observed that the irradiation resulted in the decrease of the phenol concentration in all the systems studied (Figure 6). The rate of the process increased with an increase in pH (see Table 1). Irradiation of the system at pH = 7.20 resulted in the appearance of a new absorption band with maximum at λ = 246 nm (Figure 7). A similar effect was observed for acidic systems (pH = 5.40) (see Figure S3 in Supporting Information). Using HPLC, p-benzoquinone was identified as the main primary product formed under these conditions, which undergoes further oxidation at prolonged irradiation times (see Figure 7 9200



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Table 1. Experimental Data for Po−C30B Photosensitized Oxidation of Phenol under Various pH 5.40 7.20 9.00 10.04 12.30

[PhOH] × 104 (mol dm−3)

α

pH

−5

2.57 × 10 0.0016 0.0928 0.529 0.995

[PhO−] × 104 (mol dm−3) 2.57 × 10 0.0162 0.928 5.29 9.95

10.00 9.98 9.07 4.71 0.0487

V0r × 107 (mol dm−3 s−1)

ϕPhOH

0.0257 0.0351 0.637 1.99 3.35

0.0021 0.0028 0.0516 0.161 0.271

−4

important role in singlet oxygen generation (see Figure 5C and Table 2). Thus, although the observed effect of pH on the quantum efficiency of Po−C30B photosensitized oxidation of phenol reflects both the pH-induced change in the quantum yield of singlet oxygen formation and the difference in reactivity of PhO− and PhOH toward singlet oxygen, it is dominated by the latter.

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CONCLUSIONS Novel hybrid photocatalysts active in visible spectral region have been prepared. They have been obtained by introducing porphyrin into the Cloisite 30B nanoclay. Po is present in the whole volume of the sample as demonstrated using LSCM. The spectral properties of Po−C30B are strongly pH-dependent which was explained taking into account the acid−base equilibrium between H2Po2+, HPo+, and Po forms. Po−C30B has been shown to be effective photocatalyst in photooxidation of phenol in the aqueous medium. The process occurs with the participation of singlet oxygen. The quantum yield of singlet oxygen formation is strongly pH-dependent. In the pH range considered in the current studies HPo+ and Po participate in the singlet oxygen generation. 1,4-Benzoquinone was identified as a primary photochemical product. Quantum yield of phenol oxidation is pH-dependent and increases in alkaline solutions. That fact can be explained taking into account the higher reactivity of PhO− than PhOH toward singlet oxygen. The hybrid photosensitizers can be easily separated and reused in subsequent cycles of the reaction. Thus, that efficient and environmentally friendly photosensitizer can be of interest for possible industrial applications.

Figure 7. UV−vis spectra of oxygenated aqueous Po−C30B + phenol system irradiated with λ > 470 nm (pH = 7.20; c0 = 1.0 × 10−3 mol/ dm3, cPo−C30B = 1.0 g/dm3). Inset: dependence of p-benzoquinone concentration on irradiation time of Po−C30B + phenol system (pH = 7.20).

singlet oxygen in organic solvents, with the quantum yield equal to 0.47 in DMF solution.40 As was shown before35,41,42 the dye-sensitized photooxidation of phenol occurs with the participation of singlet oxygen. Thus, one can assume that singlet oxygen is also involved in the reaction in the system studied. That assumption was supported by the findings that in Po−C30B-sensitized oxidation of phenol the p-benzoquinone is formed as the primary photochemical product. Using the spectrophotometric method, we have determined the values of quantum yield of singlet oxygen formation by Po−C30B at various pH. The procedure was based on the secondary bleaching of RNO (N,N-dimethyl-4-nitrozoaniline) caused by the reaction of singlet oxygen with imidazole. The kinetics of the degradation of RNO were tracked by measuring the decrease in RNO absorption at 440 nm. The results are presented in Table 2.

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ASSOCIATED CONTENT

S Supporting Information *

Full description of the material. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 2. Quantum Yields of Singlet Oxygen Formation by Po−C30B in an Aqueous Medium at Various pH

AUTHOR INFORMATION

Corresponding Author

fraction of pH

H2Po2+

HPo+

Po

ϕΔ ± 5%

5.40 7.20 9.00 10.06 12.30

0.0009 0 0 0 0

0.7506 0.0457 0.0008 0.0001 0

0.2485 0.9543 0.9992 0.9999 1

0.288 0.211 0.034 0.027 0.069

*E-mail: [email protected] (M.N.).; szczubia@ chemia.uj.edu.pl (K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of National Science Centre in Poland (NCN) in the form of Grant UMO-2012/05/N/ST5/00857 and the financial support of Polish Ministry of Science and Higher Education in form of Grant N209144436. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract POIG.02.01.00-12-023/08).

The quantum yield of singlet oxygen formation in aqueous dispersion of Po−C30B is strongly pH dependent, reflecting the participation of different porphyrin forms (H2Po2+, HPo+, and Po) in that process. Under our experimental conditions (pH ≥ 5.40) participation of H2Po2+ can be neglected while both HPo+ and Po are present and active. Although the importance of Po is growing with the increase in pH of the medium, the equilibrium between HPo+ and Po plays an 9201



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