Environ Sci Pollut Res DOI 10.1007/s11356-013-2311-8 14TH EUCHEMS INTERNATIONAL CONFERENCE ON CHEMISTRY AND THE ENVIRONMENT (ICCE 2013, BARCELONA, JUNE 25 - 28, 2013)
Singlet oxygen generation by photoactive polymeric microparticles with enhanced aqueous compatibility Víctor Fabregat & M. Isabel Burguete & Francisco Galindo & Santiago V. Luis
Received: 13 September 2013 / Accepted: 28 October 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract Two new photoactive materials compatible with environmentally friendly solvents (water and methanol) have been synthesized and characterized. They are comprised of a porous matrix of polystyrene and divinylbenzene with bound Rose Bengal and additional pendant groups added to increase the hydrophilicity (ethylenediamine and γ-gluconolactone). The new polymers are efficient photocatalysts capable of generating singlet oxygen after irradiation with visible light. Photochemical oxygenations of 9,10-anthracenedipropionic acid and 2-furoic acid have been carried out. The measured conversions indicate that the new supported photosensitizers are more effective than the parent hydrophobic polymer. Keywords Singlet oxygen . Photocatalysis . Photosensitizer
Introduction Photochemical methodologies for environmental remediation are gaining increasing acceptance (Vasquez et al. 2013; Emeline et al. 2012; Benabbou et al. 2011; Chong et al. 2010; Arques et al. 2009; Malato et al. 2007). Treatment of wastewater with UV light is currently done in order to eliminate pollutants and pathogens (Guo et al. 2013; Matilainen and Sillanpaa 2010). However, the economic cost associated to the use of UV light has prompted researchers to look for Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-013-2311-8) contains supplementary material, which is available to authorized users. V. Fabregat : M. I. Burguete : F. Galindo (*) : S. V. Luis (*) Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, Av. Sos Baynat, s/n, 12071 Castellón, Spain e-mail: [email protected] e-mail: [email protected]
alternatives using visible light, taking into consideration that it can be obtained at no cost from sunlight. In this regard, visible light photocatalysts have been reported to degrade a number of pollutants (Marín et al. 2012; Amat et al. 2007; Miranda et al. 2000, 2001). The mechanisms of photocatalytic reactions are markedly different, ranging from electron transfer to energy transfer and involving a variety of reactive intermediate species, encompassing from hydroxyl radical (·OH) to superoxide radical anion (O2·−) or singlet oxygen (1O2). In the case of 1O2, it can be generated by energy transfer from the triplet state of the appropriate photosensitizer (Galian and Pérez-Prieto 2010; Ogilby 2010). Apart from the aforementioned environmental application, photochemically generated singlet oxygen is also used in synthetic applications (Clennan and Pace 2005) since it is an excellent electrophile which adds to unsaturated compounds such as, for instance, furane derivatives (Montagnon et al. 2008; Corey and Roberts 1997) or terpenes (Lamberts and Neckers 1985), yielding a series of important synthetic intermediates or final products of industrial importance. The potential use of solar visible light to generate 1O2 is also an ecological advantage for these practical applications, as compared to the generation of 1O2 by conventional thermal methods (Wahlen et al. 2004). Both remediation and synthetic application of singlet oxygen using visible light photosensitizers have as their main drawback the need to remove the dissolved photosensitizer from the reaction medium once the chemical transformation has been carried out. The most typical approach to overcome this inconvenience is the attachment of the photosensitizer to a solid support yielding an immobilized photocatalyst which is easily removable from the medium by simple filtration (Zhang et al. 2013; Lacombe and Pigot 2010; Ribeiro et al. 2008; Griesbeck et al. 2004; Wang et al. 2004; Benaglia et al. 2002). However, some loss of photoactivity can be concomitant to the attachment of the photoactive unit, which can be due, for
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instance, to quenching of the excited states by the polymeric matrix or shortening of the lifetime of the reactive species close to the solid support (Bae 2012; Ribeiro et al. 2008). In addition to those reasons, there is another cause of inefficient photochemical performance of some supported photocatalysts, especially in water as a solvent, which is related to the poor wettability of the immobilized photosensitizer. Supported photocatalysts based on hydrophobic matrices are hardly dispersed in aqueous environments and carry out the desired reactions with lower conversions than in organic solvents like chloroform or dichloromethane. Hence, the development of hydrophilic photocatalysts is an important current target in order to be able to surpass this problem and to carry out efficient phototransformations in aqueous media (Urakami et al. 2013). Herein we report on the synthesis, characterization and photochemical performance as singlet oxygen generators of two new photoactive materials. They are synthesized by modification of a previously reported photoactive porous monolithic polymer containing Rose Bengal (RB) as photosensitizer (Burguete et al. 2009, 2010b). We have converted a rather hydrophobic polymer into materials easily dispersible in water and alcoholic solutions and capable of performing efficiently two benchmark photochemical reactions such as the oxidation of 2-furoic acid (FA) and 9,10-anthracenedipropionic acid (ADPA).
Results and discussion Synthesis and characterization of hydrophilic polymers In previous papers, we have reported on the synthesis of a new type of polymeric photosensitizer (P1) by grafting RB to the surface of porous monolithic polymers (Burguete et al. 2009). In order to make these materials more compatible with water or alcohols, additional groups have been introduced. The group of Fréchet described the efficient hydrophilization of a porous cross-linked polystyrene matrix used for chromatographic separations. For this purpose, a two-step modification process involving reaction with ethylenediamine and γgluconolactone was developed (Wang et al. 1995). Following this strategy, we synthesized the hydrophobic photoactive polymer P1 (Scheme 1) and also two hydrophilic derivatives (P2 and P3 ). P1 was modified with ethylenediamine to yield P2, which in turn was reacted with γ-gluconolactone to afford P3 (see details in the “Experimental” section). The new materials were characterized by means of Fourier transform infrared (FT-IR), Raman, diffuse reflectance and fluorescence spectroscopies. To carry out the photochemical reactions in suspension, the polymers were crushed mechanically in order to get a powder. Scanning electron microscopy
(SEM) analysis showed that the morphology of the powdered P2 and P3 can be described as a series of aggregates with an average size of 24 μm. In Fig. 1, an example of such morphology for the case of P3 can be seen. The presence of the strongly absorbing RB on the surface of P1–P3 allows a comparison between the spectroscopic features displayed by each material and to deduce the environment in which this chromophore is situated. The absorption of P2 (561 nm) and P3 (562 nm) is blue-shifted relative to that of P1 (571 nm), and all three are broadened in comparison to the absorption of free RB in solution (Fig. 2a and Table 1). Moreover, there is a shoulder at shorter wavelengths (ca. 530 nm) that can be associated to the formation of aggregates, as described in the literature for other RB-containing polymers (Paczkowski and Neckers 1985). On the other hand, the 10-nm shift in the maxima recorded for P2 and P3 relative to P1 affords the first indication that the grafting with ethylenediamine and γ-gluconolactone altered the surface of each matrix. Paczkowski and Neckers (1985) has described that Merrifield resins loaded with RB display absorption maxima between 571 and 578 nm, depending on the load of the photosensitizer and the format of the sample (methylene chloride solution of the film), whereas RB dianion in methanolic solution absorbs at 558 nm (Lamberts et al. 1984) and the RB benzyl ester absorbs at 564 nm also in methanol (Lamberts et al. 1984). Hence, our measurement suggests that in P2 and P3, the photosensitizer is surrounded by a more polar environment than that found in P1 , as a consequence of the grafting by ethylenediamine and γ-gluconolactone. Fluorescence measurements corroborate also this idea: as it can be seen in Fig. 2b, the emission of P1 takes place with a maximum at 602 nm, and the spectra of P2 and P3 have maxima at 593 and 591 nm, respectively. This result is in accordance with the data reported for RB and derivatives: polymers containing this photosensitizer are described to emit at 593–595 nm (Paczkowski and Neckers 1985), but free RB dianion emits at 573 nm (MeOH) (Neckers 1989) and RB benzyl ester at 584 nm (MeOH) (Neckers 1989). The amount of RB loaded in each polymer was estimated by basic hydrolysis and spectrophotometric measurement of the released dye, using the appropriate calibration curve. In all the three cases, a loading of 2 μmol of RB per gram of polymer was calculated, which means that the reaction of P1 to yield P2 and P3 does not involve displacement of the photosensitizer out of the matrix. Finally, the ability of the polymers to be dispersed in water was qualitatively tested, prior to conducting the photochemical assays. Samples of 40 mg of P1–P3 were placed in a tube with 40 mL of water and were stirred magnetically for 10 min, and the dispersions were allowed to stand for 60 s. The results revealed that P1 is much more hydrophobic than P2 and P3 since it tends to separate from the aqueous phase very easily. On the other hand, P2 and P3 can be dispersed to a higher
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Scheme 1 Schematic representation of the synthesis of P2 and P3
extent, with no noticeable differences between them. Moreover, as one of the main advantages of supported photosensitizers is the possibility of their recovery after the reaction, the samples were centrifuged for 10 min at 10,000 rpm and only the new supported photosensitizers could be separated from the reaction medium by means of this method.
Photochemical reactions In order to test the ability of P2 and P3 to generate singlet oxygen and, more importantly, to promote sensitized photooxidations in water or other polar media, two prototypical oxygenations of aromatic compounds with 1O2 were used as benchmark reactions. The bleaching of the absorption of ADPA has been used to test the ability of a number of photosensitizing systems to generate 1O2 since ADPA is water soluble and its reaction with 1O2 is very fast (Scheme 2) (Qin et al. 2011; Tsay et al. 2007; Wieder et al. 2006; Moreno et al. 2003). Upon irradiation, the decreasing absorption of ADPA in the presence of the supported photosensitizers (including P1 for comparison) was monitored. The absorption changes of ADPA solutions can be safely transformed into ADPA conversions (to its corresponding endoperoxide) taking into account that the reaction product
Fig. 1 Scanning electron microscopy image of polymer P3
does not absorb in the spectral range where the measurements are made (350–400 nm). Additionally, the first-order rate constants (k ) for each reaction can be calculated from Eq. 1, as reported in the literature for other assays using ADPA (Qin et al. 2011; Moreno et al. 2003). In Eq. 1, C and C 0 are the concentrations of ADPA at a certain time (t) and at t = 0, respectively, which can be deduced from the absorption spectra recorded during the irradiation. In this way, both rate constants and conversions can be used as indicators of the efficiency of the polymeric photosensitizers. In Fig. 3, a representative example of the reactivity of ADPA with 1O2 can be seen. C ln ¼ −kt ð1Þ C0 Upon irradiation in pure water and water buffered with PBS (pH 7.4) with visible light, hydrophilic polymers P2 and P3 showed higher conversions (93–100 %) than hydrophobic P1 (84–85 %) after 1 h. In methanol, the differences are not so marked (84 % for P1 and 92–94 % for P2–P3), and in chloroform, the difference is lower (62 % for P1 and 66– 69 % for P2–P3). In terms of reaction rate, the same conclusion can be deduced, with higher reaction rates in water for P2–P3 (buffered or not) than for P1 (see Table 2 for details). These rates lie within the range of described values for other
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Scheme 2 Reaction of 9,10-antracenedipropionic acid with singlet oxygen
Fig. 2 a Normalized absorption spectra of solid P1–P3 and RB dianion in methanol (10 μM). Note that the diffuse reflectance spectra of polymers were transformed to absorbance units. b Fluorescence spectra of solid P1–P3 and RB dianion in methanol (10 μM). Excitation wavelength, 550 nm
photosensitizers promoting the photooxygenation of ADPA (Qin et al. 2011; Moreno et al. 2003). Reaction rates and conversions point to the same conclusion in our experimental data. It must be noted, however, that this is not always the case as it could be possible to record high reaction rates (at the beginning of the reaction) but poor conversions due to photodegradation of the photosensitizer after long exposure to light, which is not the situation for our system. Table 1 Spectral properties of photosensitizers P1–P3 Photosensitizer
Absorption λmax (nm)a
Emission λmax (nm)b
P1
571
602
P2 P3
561 562
593 591
a
From diffuse reflectance measurements of solid samples
b
From fluorescence measurements of solid samples (λexc = 550 nm)
Two important aspects can be highlighted, then, from the results obtained with this reaction: (1) introduction of ethylenediamine groups or γ-gluconolactone pendant residues gives rise to an enhanced aqueous compatibility for this particular photooxygenation, and (2) the ethylenediamine moieties seem to be enough to induce such hydrophilicity since no important differences can be noticed between P2 and P3. The long-term photostability and recyclability of these materials were evaluated in the case of P3 , being the one displaying the most complex chemical structure, with functional groups that could affect the fluorophore. A sample of P3 could be used up to ten times for the quantitative conversion of ADPA with no signs of RB degradation. After each irradiation cycle, the photocatalyst and the reaction medium were easily separated by simple centrifugation (see Fig. 3b). Reaction with ADPA is a good comparative test to evaluate the relative efficiency of several photosensitizers, as it has been shown above, but in order to prove the utility of the new photoactive polymers, a photooxygenation reaction with practical applications should be tested. In this regard, we have chosen the reaction of 2-furoic acid with singlet oxygen to yield the γ-hydroxybutenolide 5-hydroxy-5H -furan-2-one (Scheme 3) (White et al. 1982). The reactivity of furan derivatives with 1O2 is known since long ago and has been used in a great number of syntheses involving a butenolide skeleton. However, the vast majority of them require a dissolved molecular photosensitizer that must be removed from the medium by column chromatography (Montagnon et al. 2008; Corey and Roberts 1997). Samples of P1–P3 (40 mg in 10 mL) and 2-furoic acid (3× 10−2 M) were prepared in several media: pure water, buffered water with phosphate-buffered saline (PBS; pH 7.4), MeOH/ water (1:1), MeOH/water (9:1), MeOH and chloroform. The samples were irradiated with visible light and the evolution of the reaction was monitored by UV–vis, as previously described. A representative example can be seen in Fig. 4. In water (buffered or not), the reaction did not occur even after 7 h of irradiation, and in MeOH/water (1:1), the conversions were only 11–12 %. However, in MeOH/water (9:1), the yield reached 85–90 % in the presence of P2–P3 and 67 % for
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Fig. 3 Illustrative examples of ADPA reactivity with singlet oxygen promoted by P1–P3. a ADPA (1.2×10−4 M) + P2 (40 mg in 10 mL) in Milli-Q water as a solvent. b Conversions of ADPA using P1–P3 as photosensitizers in methanol as a solvent
P1. Remarkably, in pure MeOH, P2 and P3 yielded quantitative conversions (100 %), whereas P1 afforded 72 %. In chloroform, the difference between the new photosentitizers (90–92 %) and P1 (51 %) was even higher (Table 3). Regarding the absence of reactivity in media with high content of water, it could be considered that furan derivatives
react notably slower than other substrates. For instance, 9,10dialkylanthracenes are described to react to 1O2 with overall bimolecular reaction rates ranging from 107 to 108 M−1 s−1 (depending on the solvent) whereas the reactivity of furan derivatives can be up to several orders of magnitude slower, with rates ranging from 104 to 107 M−1 s−1 (Wilkinson and Brummer 1981). Additionally, the physical deactivation of 1 O2 in water is at least 1 order of magnitude faster than that in MeOH (5.0 × 10 5 s −1 vs. 3.1 × 104 s −1, respectively) (Wilkinson and Brummer 1981) which disfavours even more the reactivity of furan derivatives in aqueous media. Nevertheless, it must also be taken into account that the initial concentration of 2-furoic acid employed is low (30 mM). As a recent example of the importance of these factors, it can be mentioned that Zhang et al. (2013) and Urakami et al. (2013) have reported ca. 90 % conversion of 100 mM 2-furoic acid in water after 22 h of irradiation. Examination of yields in MeOH and chloroform affords the following conclusions. Firstly, the quantitative conversions attained with modified polymers P2 and P3 in MeOH demonstrate the effectiveness of the grafting using ethylenediamine and γ-gluconolactone for enhancing the wettability of the polymeric matrices in this polar medium, in comparison with the parent apolar polymer P1. More strikingly, the enhanced polarity of the surface of polymers P2 and P3 does not hamper their effective use even in a less polar solvent like chloroform. Yields in this medium reached up to 92 % and could be rationalized considering that the introduced groups, specially the amines from ethylenediamine, could act as basic centres capable of inducing the approach of the acid substrate by ion pairing (carboxylate–ammonium), hence increasing the local concentration of this reactant around the polymeric matrix where 1O2 is generated. Similar local concentration effect has been described for other supported photosensitizers and substrates (Burguete et al. 2010a; Suzuki et al. 2000; Neckers and Paczkowski 1986). Moreover, the longer lifetime of 1O2 in chloroform would favour reactivity in this medium (reported deactivation rate for 1O2 in CHCl3 is 1.7×104 s−1) (Wilkinson and Brummer 1981).
Table 2 Photosensitized oxygenation of ADPA in several media Photosensitizera Water
PBS (pH 7.4)
MeOH
Chloroform
Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c P1 P2 P3 a
85 100 100
7.3 10.8 10.1
85 100 93
5.1 10.1 6.9
Polymer (40 mg) dispersed in 10 mL of solvent containing 1.2×10−4 M of ADPA
b
From UV–vis measurement at 398 nm
c
From Eq. 1
84 94 92
5.2 7.2 6.7
62 66 69
2.8 3.0 3.3
Environ Sci Pollut Res Table 3 Photosensitized oxygenation of 2-furoic acid in several media Photosensitizera Scheme 3 Reaction of 2-furoic acid with singlet oxygen
In summary, a poorly dispersible polymer (in polar solvents) like P1, comprised of a matrix of highly cross-linked polystyrene and divinylbenzene with attached Rose Bengal, has been converted into two polymeric derivatives with enhanced compatibility with environmentally friendly solvents like water and methanol. The new polymers are obtained by grafting ethylenediamine and γ-gluconolactone and are active for the photocatalytic oxygenation of ADPA and 2-furioc acid. The use of both new materials allows attaining higher photochemical conversions with both substrates, not only in water and methanol but also in non-polar solvents like chloroform. Further work will be oriented towards the use of the
P1 P2 P3
Conversion (%)b MeOH/waterc
MeOH
CHCl3
67 85 90
72 100 100
51 92 90
Polymer (40 mg) dispersed in 10 mL of solvent containing 3×10−2 M of 2-furoic acid
a
b
From UV–vis measurements at 247 nm
c
MeOH/water = 9:1 (vol/vol)
systems herein described for specific phototransformations of environmental value (degradation of contaminants using solar light).
Experimental section Materials and methods
Fig. 4 Illustrative examples of furoic acid reactivity with singlet oxygen promoted by P1–P3. a Furoic acid (3×10−2 M) + P3 (40 mg in 10 mL) in methanol as a solvent. b Conversions of furoic acid using P1–P3 as photosensitizers in methanol as a solvent
All commercially available reagents and solvents were used as received: p -chloromethylstyrene (Aldrich, 90 %), divinylbenzene (DVB; Fluka, ~80 % mixture of isomers; the residual is composed mainly of 1,3- and 1,4-ethylstyrene isomers), 2,2′-azobis(isobutyronitrile) (AIBN; Fluka, ≥98.0 %), Rose Bengal sodium salt (Fluka), tetrabutylammonium hydroxide solution ~25 % in MeOH (~0.8 M) (TBAOH solution; Fluka), ethylenediamine (Sigma-Aldrich, ≥99 %), γ-gluconolactone (Sigma-Aldrich, ≥99 %), 9,10-anthracenedipropionic acid (ADPA; Aldrich, ≥98.0 %), 2-furoic acid (Merck, ≥99 %), 1-dodecanol (Aldrich, 98 %), tetrahydrofuran (Scharlab, synthesis grade), ethyl acetate (Scharlab, synthesis grade), ethanol (Scharlab, 96 %), methanol (Scharlab, synthesis grade), methanol (Scharlab, spectroscopy grade), 1,4-dioxane (Scharlab, spectroscopy grade), toluene (Scharlab, synthesis grade) and N, N ′-dimethylformamide (treated previously with anhydrous MgSO4). To characterize the polymeric materials, the following techniques were used: FT-IR spectra were acquired using a FT-IR-6200 type A JASCO spectrometer, with 4-cm−1 resolution and 50 scan accumulation. Fourier transform Raman (FT-Raman) spectra were recorded using a JASCO laser Raman spectrophotometer with 4-cm−1 resolution and 100 scan accumulation (λex = 632 nm). Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo TG-STDA instrument (30–1,000 °C at a heating rate of 5 °C min−1). UV– vis absorption spectra were recorded in a Hewlett-Packard 8453 apparatus. Steady-state fluorescence spectra were
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recorded in a Spex Fluorog 3–11 equipped with a 450-W xenon lamp. Scanning electron micrographs were taken on a LEO 440I microscope equipped with a digital camera. The samples were placed on top of a tin plate and sputtered with Au/Pd in a Polaron SC7610 Sputter Coater from Fisons Instruments. The particle size distribution of the synthesized polymer was determined by means a MASTERSIZER 2000 (MALVERN) laser diffraction instrument. To perform the measurements, the sample was suspended in MeOH. The data were analyzed with the software supplied with the instrument. Experimental procedure Synthesis of P1 Polymer P1 was prepared in a similar procedure to that described in the literature (Burguete et al. 2009) by thermal free radical-initiated polymerization of the monomers in the presence of a porogenic mixture and using a glass tube as the mould (1-cm diameter). AIBN (80 mg, 1 wt% with respect to monomers) was dissolved in p-chloromethylstyrene (90 %, 1.84 g) and divinylbenzene (80 % grade, 6.16 g). Then, the porogenic mixture, consisting of 1-dodecanol (10 g) and toluene (2 g), was added. The homogenized polymerization mixture was transferred to several glass tubes and purged with nitrogen in order to remove the dissolved oxygen. Then, the tubes were sealed with rubber septums and placed in a vertical position in a silicon bath heated at 80 °C. The polymerization was allowed to proceed at this temperature for 24 h. The glass tubes were carefully crushed and the polymer was then disaggregated mechanically and washed with tetrahydrofuran for 24 h in a Soxhlet apparatus in order to remove the porogenic mixture and any other soluble compounds remaining within the polymer; finally, the polymer was dried in a vacuum oven. FT-IR (cm−1): 3,018, 2,917, 1,629, 1,425, 1,265, 906, 795, 707. FT-Raman (cm−1): 3,058, 2,906, 1,631, 1,408, 1,267, 1, 181, 1,001. Decomposition temperature (°C): 500–510. The resulting polymeric material (5.77 g, 7.5 mmol of CH2Cl) and Rose Bengal sodium salt (8.81 g, 8.66 mmol) were mixed in a 500-mL round-bottom flask and stirred in 400 mL of dimethylformamide (DMF; treated previously with anhydrous MgSO4) at 80 °C for 8 h in a nitrogen atmosphere. Then, the reaction mixture was cooled to ambient temperature and filtered through a sintered glass funnel. The obtained resin, P1, was washed with 250-mL portions of the following solvents: DMF, ethyl acetate, ethanol, ethanol/water (1:1), water, methanol/water (1:1) and methanol. Next, the polymer was extracted with methanol in a Soxhlet apparatus until no visible colour appeared in the solvent. Finally, the light pink polymeric particles were dried in a vacuum oven. FT-IR (cm−1): 3,020, 2,920, 1,625, 1,453, 1,266, 902, 794, 709. FT-Raman (cm−1): 3,054, 2,907, 1,627, 1,411, 1,266, 1, 180, 1,003. Decomposition temperature (°C): 500–510. UV–
vis absorption spectroscopy (λmax): 571 nm. Fluorescence emission spectroscopy (λmax): 602 nm (λex = 572 nm). Rose Bengal loading: 2 μmol RB g−1 resin. Synthesis of P2 For the synthesis of polymer P2, 4.66 g of P1 was introduced with 15 mL of ethylenediamine (excess) in a 250-mL twoneck round-bottom flask. The mixture was dispersed in 100 mL of predried tetrahydrofuran (THF) and the flask was sealed and purged with nitrogen for 30 min. Then, the reaction mixture was refluxed for 8 h at 70 °C. After this time, the reaction mixture was cooled to ambient temperature and filtered through a sintered glass funnel. The obtained resin, P2, was washed with 200-mL portions of the following solvents: THF, dioxane, dioxane/water (1:1), water, dioxane/water (1:1), dioxane and ethanol. Finally, the polymer was dried in a vacuum oven at 55 °C. FT-IR (cm−1): 3,358, 3,046, 2,921, 1,619, 1,447, 990, 906, 798, 713. FT-Raman (cm−1): 3,055, 2,903, 1,632, 1,407, 1, 315, 1,179, 1,105, 1,003, 803. Decomposition temperature (°C): 500–510. UV–vis absorption spectroscopy (λmax): 561 nm. Fluorescence emission spectroscopy (λmax): 593 nm (λex = 564 nm). Rose Bengal loading: 2 μmol RB g−1 resin. Synthesis of P3 In a 250-mL two-neck round-bottom flask, 2.87 g of polymer P2 and 1.335 g of γ-gluconolactone (excess amount) were introduced. The mixture was dispersed in 100 mL of absolute ethanol and purged with nitrogen for 30 min. The reaction was carried out under reflux for 18 h at 80 °C. Then, the reaction mixture was cooled to ambient temperature and filtered through a sintered glass funnel. The obtained resin, P3, was washed with 250-mL portions of the following solvents: ethanol, methanol/water (1:1), water, methanol/water (1:1) and methanol. Finally, the obtained polymer was dried in a vacuum oven at 55 °C. FT-IR (cm−1): 3,367, 3,056, 2,928, 1,628, 1,451, 1,081, 902, 804, 704. FT-Raman (cm−1): 3,055, 2,903, 1,634, 1,408, 1,307, 1,188, 1,086, 1,001, 930, 803. Decomposition temperature (°C): 500–510. UV–vis absorption spectroscopy (λmax): 562 nm. Fluorescence emission spectroscopy (λmax): 591 nm (λex = 564 nm). Rose Bengal loading: 2 μmol RB g−1 resin. Average particle diameter: 26.3 μm. Analytical procedure to estimate the loading of Rose Bengal in P1–P3 The polymeric photosensitizers (20 mg) and tetrabutylammonium hydroxide solution ~0.8 M in MeOH (3 mL) were mixed in a 25-mL round-bottom flask containing 10 mL of 1,4-dioxane. The flask was sealed and the mixture
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was stirred for 24 h at room temperature. The reaction mixture was then filtered through a sintered glass funnel, and the resin was washed with MeOH until no visible colour appeared in the solvent. The filtrate was transferred into a 100-mL volumetric flask and diluted to 100 mL with MeOH. The final solvent ratio of the solution was 87:10:3 (MeOH/1,4-dioxane/ TBAOH solution). From the UV–vis absorption spectrum of the solution, the amount of free Rose Bengal was determined, using ε = 78,028±1,291 L mol−1 cm−1 at 556 nm from a previous calibration (Burguete et al. 2009) of Rose Bengal in MeOH/1,4-dioxane/TBAOH (87:10:3).
irradiation of 2-furoic acid with free Rose Bengal in solution (5 μM) as photosensitizer. The decreasing absorbance of 2furoic acid was monitored by means of UV–vis spectroscopy at 246 nm for 420 min. For every measurement, aliquots of 80 μL were removed from the reaction mixture and diluted in 25 mL of the appropriate solvent. Acknowledgments Financial support from the Spanish MINECO (CTQ2009-14366-C02-01) and Fundació Caixa Castelló-UJI (project P1 1B-2009-59, P1 1B2009-58, P1 1B2012-41) are acknowledged. V. F. thanks the financial support from UJI (predoctoral fellowship). We thank J. Javier Gómez (SCIC) for the technical assistance in SEM measurements.
Photochemical experiments: photooxidation of ADPA The rate of photooxidation of ADPA (1.2×10−4 M) was determined in several reaction media: water, PBS, MeOH and chloroform, for the polymeric photosensitizers P1, P2 and P3. In each of the experiments, 40 mg of the photosensitizer was added to 10 mL of solution of ADPA (1.2× 10−4 M) in a test tube. The heterogeneous mixture was kept under stirring and in equilibrium with air. The test tubes were irradiated at room temperature with a halogen lamp of 50 W, placed at a distance of 2 cm. The same experimental conditions were performed for the polymers in the dark in order to evaluate a possible adsorption of the photocatalyst into the polymeric substrate. Moreover, other control experiments were carried out with the absence of polymer and irradiation of ADPA with free Rose Bengal in solution (5 μM) as photosensitizer. The decreasing absorbance of ADPA was monitored by means of UV–visible spectroscopy at 398 nm for 60 min. Before every measurement, 3 mL of the reaction mixture was filtered (nylon syringe filter, 0.2 mm) to another cuvette in order to remove the polymeric particles in suspension. Photochemical experiments: synthesis of 5-hydroxy-5H -furan-2-one The oxidation of 2-furoic acid (3×10−2 M) to 5-hydroxy-5Hfuran-2-one was studied for photosensitizers P1–P3 in several reaction media: methanol/water (9:1), methanol and chloroform. The photosensitizer (40 mg) was added to 10 mL of solution of 2-furoic acid (3×10−2 M) in a test tube. The heterogeneous mixture was kept under stirring and in equilibrium with air. Such solutions were irradiated with a 125-W medium-pressure Hg vapour lamp for 6 h surrounded by an aqueous solution of 0.1 M FeCl3 used as a filter for wavelengths under 450 nm, and the tubes were placed at a distance of 2 cm from the solution. The same experimental conditions were used for the polymers in the dark in order to evaluate a possible adsorption of the photocatalyst into the polymeric substrate. Moreover, other control experiments were carried out: (a) irradiation in the absence of polymer and (b)
References Amat AM, Arques A, Galindo F, Miranda MA, Santos-Juanes L, Vercher RF, Vicente R (2007) Acridine yellow as solar photocatalyst for enhancing biodegradability and eliminating ferulic acid as model pollutant. Appl Catal B Environ 73:220–226 Arques A, Amat AM, Santos-Juanes L, Vercher RF, Marin ML, Miranda MA (2009) Abatement of methidation and carbaryl from aqueous solutions using organic photocatalysts. Catal Today 144:106–111 Bae B-c, Na K, (2012) Development of polymeric cargo for delivery of photosensitizer in photodynamic therapy. Int J Photoenergy 2012, Article ID 431975 Benabbou AK, Guillard C, Pigeot-Remy S, Cantau C, Pigot T, Lejeune P, Derriche Z, Lacombe S (2011) Water disinfection using photosensitizers supported on silica. J Photochem Photobiol A Chem 219: 101–108 Benaglia M, Danelli T, Fabris F, Sperandio D, Pozzi G (2002) Poly(ethylene glycol)-supported tetrahydroxyphenyl porphyrin: a convenient, recyclable catalyst for photooxidation reactions. Org Lett 4:4229–4232 Burguete MI, Galindo F, Gavara R, Luis SV, Moreno M, Thomas P, Russell DA (2009) Singlet oxygen generation using a porous monolithic polymer supported photosensitizer: potential application to the photodynamic destruction of melanoma cells. Photochem Photobiol Sci 8:37–44 Burguete MI, Gavara R, Galindo F, Luis SV (2010a) New polymersupported photocatalyst with improved compatibility with polar solvents. Synthetic application using solar light as energy source. Catal Commun 11:1081–1084 Burguete MI, Gavara R, Galindo F, Luis SV (2010b) Synthetic application of photoactive porous monolithic polymers. Tetrahedron Lett 51:3360–3363 Chong MN, Jin B, Chow CWK, Saint C (2010) Recent developments in photocatalytic water treatment technology: a review. Water Res 44: 2997–3027 Clennan EL, Pace A (2005) Advances in singlet oxygen chemistry. Tetrahedron 61:6665–6691 Corey EJ, Roberts BE (1997) Total synthesis of dysidiolide. J Am Chem Soc 119:12425–12431 Emeline AV, Kuznetsov VN, Ryabuchk VK, Serpone N (2012) On the way to the creation of next generation photoactive materials. Env Sci Pollut Res 19:3666–3675 Galian RE, Pérez-Prieto J (2010) Catalytic processes activated by light. Energy Env Sci 3:1488–1498 Griesbeck AG, El-Idreesy TT, Bartoschek A (2004) Photooxygenation in polystyrene beads with covalently and non-covalently bound tetraarylporphyrin sensitizers. Adv Synth Catal 346:245–251
Environ Sci Pollut Res Guo HG, Gao NY, Chu WH, Li L, Zhang YJ, Gu JS, Gu YL (2013) Photochemical degradation of ciprofloxacin in UV and UV/H2O2 process: kinetics, parameters and products. Env Sci Pollut Res 20: 3202–3213 Lacombe S, Pigot T (2010) Photochemistry. RSC Publications 38:313– 336. doi:10.1039/9781849730860 Lamberts JJM, Neckers DC (1985) Rose Bengal derivatives as singlet oxygen sensitizers. Tetrahedron 41:2183–2186 Lamberts JJM, Schumacher DR, Neckers DC (1984) Novel Rose Bengal derivatives—synthesis and quantum yield studies. J Am Chem Soc 106:5879–5883 Malato S, Blanco J, Alarcon DC, Maldonado MI, FernándezIbáñez P, Gernjak W (2007) Photocatalytic decontamination and disinfection of water with solar collectors. Catal Today 122:137–149 Marín ML, Santos-Juanes L, Arques A, Amat AM, Miranda MA (2012) Organic photocatalysts for the oxidation of pollutants and model compounds. Chem Rev 112:1710–1750 Matilainen A, Sillanpaa M (2010) Removal of natural organic matter from drinking water by advanced oxidation processes. Chemosphere 80:351–365 Miranda MA, Galindo F, Amat AM, Arques A (2000) Pyrylium salt-photosensitized degradation of phenolic contaminants derived from cinnamic acid with solar light—correlation of the observed reactivities with fluorescence quenching. Appl Catal B Environ 28:127–133 Miranda MA, Galindo F, Amat AM, Arques A (2001) Pyrylium saltphotosensitised degradation of phenolic contaminants present in olive oil wastewaters with solar light: part II. Benzoic acid derivatives. Appl Catal B Environ 30:437–444 Montagnon T, Tofi M, Vassilikogiannakis G (2008) Using singlet oxygen to synthesize polyoxygenated natural products from Furans. Acc Chem Res 41:1001–1011 Moreno MJ, Monson E, Reddy RG, Rehmtulla A, Ross BD, Philbert M, Schneider RJ, Kopelman R (2003) Production of singlet oxygen by Ru(dpp(SO3)(2))(3) incorporated in polyacrylamide PEBBLES. Sens Actuators B 90:82–89 Neckers DC (1989) Rose Bengal. J Photochem Photobiol A Chem 47:1–29 Neckers DC, Paczkowski J (1986) Microheterogeneous photooxidation. J Am Chem Soc 108:291–292 Ogilby PR (2010) Singlet oxygen: there is indeed something new under the sun. Chem Soc Rev 39:3181–3209
Paczkowski J, Neckers DC (1985) Polymer-based sensitizers for the formation of singlet oxygen—new studies of polymeric derivatives of Rose Bengal. Macromolecules 18:1245–1253 Qin M, Hah HJ, Kim G, Nie G, Lee YEK, Kopelman R (2011) Methylene blue loaded polyacrylamide nanoparticles for enhanced tumor-targeted photodynamic therapy. Photochem Photobiol Sci 10:832–841 Ribeiro SM, Serra AC, Rocha Gonsalves AMD (2008) Immobilised porphyrins in monoterpene photooxidations. J Catal 256:331–337 Suzuki M, Ohta Y, Nagae H, Ichinohe T, Kimura M, Hanabusa K, Shirai H, Wohrle D (2000) Synthesis, characterization and application of a novel polymer solid photosensitizer. Chem Commun 213–214 Tsay JM, Trzoss M, Shi L, Kong X, Selke M, Jung ME, Weiss S (2007) Singlet oxygen production by peptide-coated quantum dot–photosensitizer conjugates. J Am Chem Soc 129:6865–6871 Urakami H, Zhang K, Vilela F (2013) Modification of conjugated microporous poly-benzothiadiazole for photosensitized singlet oxygen generation in water. Chem Commun 49:2353–2355 Vasquez MI, Hapeshi E, Fatta-Kassinos D, Kummerer K (2013) Biodegradation potential of ofloxacin and its resulting transformation products during photolytic and photocatalytic treatment. Env Sci Pollut Res 20:3195–3201 Wahlen J, De Vos DE, Jacobs PA, Alsters PL (2004) Solid materials as sources for synthetically useful singlet oxygen. Adv Synth Catal 346:152–164 Wang QC, Svec F, Fréchet JMJ (1995) Hydrophilization of porous polystyrene-based continuous rod column. Anal Chem 67:670–674 Wang SZ, Gao RM, Zhou FM, Selke M (2004) Nanomaterials and singlet oxygen photosensitizers: potential applications in photodynamic therapy. J Mater Chem 14:487–493 White JD, Carter JP, Kezar HS (1982) Stereoselective synthesis of the macrocycle segment of verrucarin J. J Org Chem 47:929–932 Wieder ME, Hone DC, Cook MJ, Handsley MM, Gavrilovic J, Russell DA (2006) Intracellular photodynamic therapy with photosensitizer–nanoparticle conjugates: cancer therapy using a “Trojan horse”. Photochem Photobiol Sci 5:727–734 Wilkinson F, Brummer JG (1981) Rate constants for the decay and reactions of the lowest electronically excited singlet-state of molecular oxygen in solution. J Phys Chem Ref Data 10:809–1000 Zhang K, Kopetzki D, Seeberger RH, Antonietti M, Vilela F (2013) Surface area control and photocatalytic activity of conjugated microporous poly(benzothiadiazole) networks. Angew Chem Int Ed 52:1432–1436
Singlet oxygen generation by photoactive polymeric microparticles with enhanced aqueous compatibility Víctor Fabregat & M. Isabel Burguete & Francisco Galindo & Santiago V. Luis
Received: 13 September 2013 / Accepted: 28 October 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract Two new photoactive materials compatible with environmentally friendly solvents (water and methanol) have been synthesized and characterized. They are comprised of a porous matrix of polystyrene and divinylbenzene with bound Rose Bengal and additional pendant groups added to increase the hydrophilicity (ethylenediamine and γ-gluconolactone). The new polymers are efficient photocatalysts capable of generating singlet oxygen after irradiation with visible light. Photochemical oxygenations of 9,10-anthracenedipropionic acid and 2-furoic acid have been carried out. The measured conversions indicate that the new supported photosensitizers are more effective than the parent hydrophobic polymer. Keywords Singlet oxygen . Photocatalysis . Photosensitizer
Introduction Photochemical methodologies for environmental remediation are gaining increasing acceptance (Vasquez et al. 2013; Emeline et al. 2012; Benabbou et al. 2011; Chong et al. 2010; Arques et al. 2009; Malato et al. 2007). Treatment of wastewater with UV light is currently done in order to eliminate pollutants and pathogens (Guo et al. 2013; Matilainen and Sillanpaa 2010). However, the economic cost associated to the use of UV light has prompted researchers to look for Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-013-2311-8) contains supplementary material, which is available to authorized users. V. Fabregat : M. I. Burguete : F. Galindo (*) : S. V. Luis (*) Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, Av. Sos Baynat, s/n, 12071 Castellón, Spain e-mail: [email protected] e-mail: [email protected]
alternatives using visible light, taking into consideration that it can be obtained at no cost from sunlight. In this regard, visible light photocatalysts have been reported to degrade a number of pollutants (Marín et al. 2012; Amat et al. 2007; Miranda et al. 2000, 2001). The mechanisms of photocatalytic reactions are markedly different, ranging from electron transfer to energy transfer and involving a variety of reactive intermediate species, encompassing from hydroxyl radical (·OH) to superoxide radical anion (O2·−) or singlet oxygen (1O2). In the case of 1O2, it can be generated by energy transfer from the triplet state of the appropriate photosensitizer (Galian and Pérez-Prieto 2010; Ogilby 2010). Apart from the aforementioned environmental application, photochemically generated singlet oxygen is also used in synthetic applications (Clennan and Pace 2005) since it is an excellent electrophile which adds to unsaturated compounds such as, for instance, furane derivatives (Montagnon et al. 2008; Corey and Roberts 1997) or terpenes (Lamberts and Neckers 1985), yielding a series of important synthetic intermediates or final products of industrial importance. The potential use of solar visible light to generate 1O2 is also an ecological advantage for these practical applications, as compared to the generation of 1O2 by conventional thermal methods (Wahlen et al. 2004). Both remediation and synthetic application of singlet oxygen using visible light photosensitizers have as their main drawback the need to remove the dissolved photosensitizer from the reaction medium once the chemical transformation has been carried out. The most typical approach to overcome this inconvenience is the attachment of the photosensitizer to a solid support yielding an immobilized photocatalyst which is easily removable from the medium by simple filtration (Zhang et al. 2013; Lacombe and Pigot 2010; Ribeiro et al. 2008; Griesbeck et al. 2004; Wang et al. 2004; Benaglia et al. 2002). However, some loss of photoactivity can be concomitant to the attachment of the photoactive unit, which can be due, for
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instance, to quenching of the excited states by the polymeric matrix or shortening of the lifetime of the reactive species close to the solid support (Bae 2012; Ribeiro et al. 2008). In addition to those reasons, there is another cause of inefficient photochemical performance of some supported photocatalysts, especially in water as a solvent, which is related to the poor wettability of the immobilized photosensitizer. Supported photocatalysts based on hydrophobic matrices are hardly dispersed in aqueous environments and carry out the desired reactions with lower conversions than in organic solvents like chloroform or dichloromethane. Hence, the development of hydrophilic photocatalysts is an important current target in order to be able to surpass this problem and to carry out efficient phototransformations in aqueous media (Urakami et al. 2013). Herein we report on the synthesis, characterization and photochemical performance as singlet oxygen generators of two new photoactive materials. They are synthesized by modification of a previously reported photoactive porous monolithic polymer containing Rose Bengal (RB) as photosensitizer (Burguete et al. 2009, 2010b). We have converted a rather hydrophobic polymer into materials easily dispersible in water and alcoholic solutions and capable of performing efficiently two benchmark photochemical reactions such as the oxidation of 2-furoic acid (FA) and 9,10-anthracenedipropionic acid (ADPA).
Results and discussion Synthesis and characterization of hydrophilic polymers In previous papers, we have reported on the synthesis of a new type of polymeric photosensitizer (P1) by grafting RB to the surface of porous monolithic polymers (Burguete et al. 2009). In order to make these materials more compatible with water or alcohols, additional groups have been introduced. The group of Fréchet described the efficient hydrophilization of a porous cross-linked polystyrene matrix used for chromatographic separations. For this purpose, a two-step modification process involving reaction with ethylenediamine and γgluconolactone was developed (Wang et al. 1995). Following this strategy, we synthesized the hydrophobic photoactive polymer P1 (Scheme 1) and also two hydrophilic derivatives (P2 and P3 ). P1 was modified with ethylenediamine to yield P2, which in turn was reacted with γ-gluconolactone to afford P3 (see details in the “Experimental” section). The new materials were characterized by means of Fourier transform infrared (FT-IR), Raman, diffuse reflectance and fluorescence spectroscopies. To carry out the photochemical reactions in suspension, the polymers were crushed mechanically in order to get a powder. Scanning electron microscopy
(SEM) analysis showed that the morphology of the powdered P2 and P3 can be described as a series of aggregates with an average size of 24 μm. In Fig. 1, an example of such morphology for the case of P3 can be seen. The presence of the strongly absorbing RB on the surface of P1–P3 allows a comparison between the spectroscopic features displayed by each material and to deduce the environment in which this chromophore is situated. The absorption of P2 (561 nm) and P3 (562 nm) is blue-shifted relative to that of P1 (571 nm), and all three are broadened in comparison to the absorption of free RB in solution (Fig. 2a and Table 1). Moreover, there is a shoulder at shorter wavelengths (ca. 530 nm) that can be associated to the formation of aggregates, as described in the literature for other RB-containing polymers (Paczkowski and Neckers 1985). On the other hand, the 10-nm shift in the maxima recorded for P2 and P3 relative to P1 affords the first indication that the grafting with ethylenediamine and γ-gluconolactone altered the surface of each matrix. Paczkowski and Neckers (1985) has described that Merrifield resins loaded with RB display absorption maxima between 571 and 578 nm, depending on the load of the photosensitizer and the format of the sample (methylene chloride solution of the film), whereas RB dianion in methanolic solution absorbs at 558 nm (Lamberts et al. 1984) and the RB benzyl ester absorbs at 564 nm also in methanol (Lamberts et al. 1984). Hence, our measurement suggests that in P2 and P3, the photosensitizer is surrounded by a more polar environment than that found in P1 , as a consequence of the grafting by ethylenediamine and γ-gluconolactone. Fluorescence measurements corroborate also this idea: as it can be seen in Fig. 2b, the emission of P1 takes place with a maximum at 602 nm, and the spectra of P2 and P3 have maxima at 593 and 591 nm, respectively. This result is in accordance with the data reported for RB and derivatives: polymers containing this photosensitizer are described to emit at 593–595 nm (Paczkowski and Neckers 1985), but free RB dianion emits at 573 nm (MeOH) (Neckers 1989) and RB benzyl ester at 584 nm (MeOH) (Neckers 1989). The amount of RB loaded in each polymer was estimated by basic hydrolysis and spectrophotometric measurement of the released dye, using the appropriate calibration curve. In all the three cases, a loading of 2 μmol of RB per gram of polymer was calculated, which means that the reaction of P1 to yield P2 and P3 does not involve displacement of the photosensitizer out of the matrix. Finally, the ability of the polymers to be dispersed in water was qualitatively tested, prior to conducting the photochemical assays. Samples of 40 mg of P1–P3 were placed in a tube with 40 mL of water and were stirred magnetically for 10 min, and the dispersions were allowed to stand for 60 s. The results revealed that P1 is much more hydrophobic than P2 and P3 since it tends to separate from the aqueous phase very easily. On the other hand, P2 and P3 can be dispersed to a higher
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Scheme 1 Schematic representation of the synthesis of P2 and P3
extent, with no noticeable differences between them. Moreover, as one of the main advantages of supported photosensitizers is the possibility of their recovery after the reaction, the samples were centrifuged for 10 min at 10,000 rpm and only the new supported photosensitizers could be separated from the reaction medium by means of this method.
Photochemical reactions In order to test the ability of P2 and P3 to generate singlet oxygen and, more importantly, to promote sensitized photooxidations in water or other polar media, two prototypical oxygenations of aromatic compounds with 1O2 were used as benchmark reactions. The bleaching of the absorption of ADPA has been used to test the ability of a number of photosensitizing systems to generate 1O2 since ADPA is water soluble and its reaction with 1O2 is very fast (Scheme 2) (Qin et al. 2011; Tsay et al. 2007; Wieder et al. 2006; Moreno et al. 2003). Upon irradiation, the decreasing absorption of ADPA in the presence of the supported photosensitizers (including P1 for comparison) was monitored. The absorption changes of ADPA solutions can be safely transformed into ADPA conversions (to its corresponding endoperoxide) taking into account that the reaction product
Fig. 1 Scanning electron microscopy image of polymer P3
does not absorb in the spectral range where the measurements are made (350–400 nm). Additionally, the first-order rate constants (k ) for each reaction can be calculated from Eq. 1, as reported in the literature for other assays using ADPA (Qin et al. 2011; Moreno et al. 2003). In Eq. 1, C and C 0 are the concentrations of ADPA at a certain time (t) and at t = 0, respectively, which can be deduced from the absorption spectra recorded during the irradiation. In this way, both rate constants and conversions can be used as indicators of the efficiency of the polymeric photosensitizers. In Fig. 3, a representative example of the reactivity of ADPA with 1O2 can be seen. C ln ¼ −kt ð1Þ C0 Upon irradiation in pure water and water buffered with PBS (pH 7.4) with visible light, hydrophilic polymers P2 and P3 showed higher conversions (93–100 %) than hydrophobic P1 (84–85 %) after 1 h. In methanol, the differences are not so marked (84 % for P1 and 92–94 % for P2–P3), and in chloroform, the difference is lower (62 % for P1 and 66– 69 % for P2–P3). In terms of reaction rate, the same conclusion can be deduced, with higher reaction rates in water for P2–P3 (buffered or not) than for P1 (see Table 2 for details). These rates lie within the range of described values for other
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Scheme 2 Reaction of 9,10-antracenedipropionic acid with singlet oxygen
Fig. 2 a Normalized absorption spectra of solid P1–P3 and RB dianion in methanol (10 μM). Note that the diffuse reflectance spectra of polymers were transformed to absorbance units. b Fluorescence spectra of solid P1–P3 and RB dianion in methanol (10 μM). Excitation wavelength, 550 nm
photosensitizers promoting the photooxygenation of ADPA (Qin et al. 2011; Moreno et al. 2003). Reaction rates and conversions point to the same conclusion in our experimental data. It must be noted, however, that this is not always the case as it could be possible to record high reaction rates (at the beginning of the reaction) but poor conversions due to photodegradation of the photosensitizer after long exposure to light, which is not the situation for our system. Table 1 Spectral properties of photosensitizers P1–P3 Photosensitizer
Absorption λmax (nm)a
Emission λmax (nm)b
P1
571
602
P2 P3
561 562
593 591
a
From diffuse reflectance measurements of solid samples
b
From fluorescence measurements of solid samples (λexc = 550 nm)
Two important aspects can be highlighted, then, from the results obtained with this reaction: (1) introduction of ethylenediamine groups or γ-gluconolactone pendant residues gives rise to an enhanced aqueous compatibility for this particular photooxygenation, and (2) the ethylenediamine moieties seem to be enough to induce such hydrophilicity since no important differences can be noticed between P2 and P3. The long-term photostability and recyclability of these materials were evaluated in the case of P3 , being the one displaying the most complex chemical structure, with functional groups that could affect the fluorophore. A sample of P3 could be used up to ten times for the quantitative conversion of ADPA with no signs of RB degradation. After each irradiation cycle, the photocatalyst and the reaction medium were easily separated by simple centrifugation (see Fig. 3b). Reaction with ADPA is a good comparative test to evaluate the relative efficiency of several photosensitizers, as it has been shown above, but in order to prove the utility of the new photoactive polymers, a photooxygenation reaction with practical applications should be tested. In this regard, we have chosen the reaction of 2-furoic acid with singlet oxygen to yield the γ-hydroxybutenolide 5-hydroxy-5H -furan-2-one (Scheme 3) (White et al. 1982). The reactivity of furan derivatives with 1O2 is known since long ago and has been used in a great number of syntheses involving a butenolide skeleton. However, the vast majority of them require a dissolved molecular photosensitizer that must be removed from the medium by column chromatography (Montagnon et al. 2008; Corey and Roberts 1997). Samples of P1–P3 (40 mg in 10 mL) and 2-furoic acid (3× 10−2 M) were prepared in several media: pure water, buffered water with phosphate-buffered saline (PBS; pH 7.4), MeOH/ water (1:1), MeOH/water (9:1), MeOH and chloroform. The samples were irradiated with visible light and the evolution of the reaction was monitored by UV–vis, as previously described. A representative example can be seen in Fig. 4. In water (buffered or not), the reaction did not occur even after 7 h of irradiation, and in MeOH/water (1:1), the conversions were only 11–12 %. However, in MeOH/water (9:1), the yield reached 85–90 % in the presence of P2–P3 and 67 % for
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Fig. 3 Illustrative examples of ADPA reactivity with singlet oxygen promoted by P1–P3. a ADPA (1.2×10−4 M) + P2 (40 mg in 10 mL) in Milli-Q water as a solvent. b Conversions of ADPA using P1–P3 as photosensitizers in methanol as a solvent
P1. Remarkably, in pure MeOH, P2 and P3 yielded quantitative conversions (100 %), whereas P1 afforded 72 %. In chloroform, the difference between the new photosentitizers (90–92 %) and P1 (51 %) was even higher (Table 3). Regarding the absence of reactivity in media with high content of water, it could be considered that furan derivatives
react notably slower than other substrates. For instance, 9,10dialkylanthracenes are described to react to 1O2 with overall bimolecular reaction rates ranging from 107 to 108 M−1 s−1 (depending on the solvent) whereas the reactivity of furan derivatives can be up to several orders of magnitude slower, with rates ranging from 104 to 107 M−1 s−1 (Wilkinson and Brummer 1981). Additionally, the physical deactivation of 1 O2 in water is at least 1 order of magnitude faster than that in MeOH (5.0 × 10 5 s −1 vs. 3.1 × 104 s −1, respectively) (Wilkinson and Brummer 1981) which disfavours even more the reactivity of furan derivatives in aqueous media. Nevertheless, it must also be taken into account that the initial concentration of 2-furoic acid employed is low (30 mM). As a recent example of the importance of these factors, it can be mentioned that Zhang et al. (2013) and Urakami et al. (2013) have reported ca. 90 % conversion of 100 mM 2-furoic acid in water after 22 h of irradiation. Examination of yields in MeOH and chloroform affords the following conclusions. Firstly, the quantitative conversions attained with modified polymers P2 and P3 in MeOH demonstrate the effectiveness of the grafting using ethylenediamine and γ-gluconolactone for enhancing the wettability of the polymeric matrices in this polar medium, in comparison with the parent apolar polymer P1. More strikingly, the enhanced polarity of the surface of polymers P2 and P3 does not hamper their effective use even in a less polar solvent like chloroform. Yields in this medium reached up to 92 % and could be rationalized considering that the introduced groups, specially the amines from ethylenediamine, could act as basic centres capable of inducing the approach of the acid substrate by ion pairing (carboxylate–ammonium), hence increasing the local concentration of this reactant around the polymeric matrix where 1O2 is generated. Similar local concentration effect has been described for other supported photosensitizers and substrates (Burguete et al. 2010a; Suzuki et al. 2000; Neckers and Paczkowski 1986). Moreover, the longer lifetime of 1O2 in chloroform would favour reactivity in this medium (reported deactivation rate for 1O2 in CHCl3 is 1.7×104 s−1) (Wilkinson and Brummer 1981).
Table 2 Photosensitized oxygenation of ADPA in several media Photosensitizera Water
PBS (pH 7.4)
MeOH
Chloroform
Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c Conversion (%)b k (10−4 s−1)c P1 P2 P3 a
85 100 100
7.3 10.8 10.1
85 100 93
5.1 10.1 6.9
Polymer (40 mg) dispersed in 10 mL of solvent containing 1.2×10−4 M of ADPA
b
From UV–vis measurement at 398 nm
c
From Eq. 1
84 94 92
5.2 7.2 6.7
62 66 69
2.8 3.0 3.3
Environ Sci Pollut Res Table 3 Photosensitized oxygenation of 2-furoic acid in several media Photosensitizera Scheme 3 Reaction of 2-furoic acid with singlet oxygen
In summary, a poorly dispersible polymer (in polar solvents) like P1, comprised of a matrix of highly cross-linked polystyrene and divinylbenzene with attached Rose Bengal, has been converted into two polymeric derivatives with enhanced compatibility with environmentally friendly solvents like water and methanol. The new polymers are obtained by grafting ethylenediamine and γ-gluconolactone and are active for the photocatalytic oxygenation of ADPA and 2-furioc acid. The use of both new materials allows attaining higher photochemical conversions with both substrates, not only in water and methanol but also in non-polar solvents like chloroform. Further work will be oriented towards the use of the
P1 P2 P3
Conversion (%)b MeOH/waterc
MeOH
CHCl3
67 85 90
72 100 100
51 92 90
Polymer (40 mg) dispersed in 10 mL of solvent containing 3×10−2 M of 2-furoic acid
a
b
From UV–vis measurements at 247 nm
c
MeOH/water = 9:1 (vol/vol)
systems herein described for specific phototransformations of environmental value (degradation of contaminants using solar light).
Experimental section Materials and methods
Fig. 4 Illustrative examples of furoic acid reactivity with singlet oxygen promoted by P1–P3. a Furoic acid (3×10−2 M) + P3 (40 mg in 10 mL) in methanol as a solvent. b Conversions of furoic acid using P1–P3 as photosensitizers in methanol as a solvent
All commercially available reagents and solvents were used as received: p -chloromethylstyrene (Aldrich, 90 %), divinylbenzene (DVB; Fluka, ~80 % mixture of isomers; the residual is composed mainly of 1,3- and 1,4-ethylstyrene isomers), 2,2′-azobis(isobutyronitrile) (AIBN; Fluka, ≥98.0 %), Rose Bengal sodium salt (Fluka), tetrabutylammonium hydroxide solution ~25 % in MeOH (~0.8 M) (TBAOH solution; Fluka), ethylenediamine (Sigma-Aldrich, ≥99 %), γ-gluconolactone (Sigma-Aldrich, ≥99 %), 9,10-anthracenedipropionic acid (ADPA; Aldrich, ≥98.0 %), 2-furoic acid (Merck, ≥99 %), 1-dodecanol (Aldrich, 98 %), tetrahydrofuran (Scharlab, synthesis grade), ethyl acetate (Scharlab, synthesis grade), ethanol (Scharlab, 96 %), methanol (Scharlab, synthesis grade), methanol (Scharlab, spectroscopy grade), 1,4-dioxane (Scharlab, spectroscopy grade), toluene (Scharlab, synthesis grade) and N, N ′-dimethylformamide (treated previously with anhydrous MgSO4). To characterize the polymeric materials, the following techniques were used: FT-IR spectra were acquired using a FT-IR-6200 type A JASCO spectrometer, with 4-cm−1 resolution and 50 scan accumulation. Fourier transform Raman (FT-Raman) spectra were recorded using a JASCO laser Raman spectrophotometer with 4-cm−1 resolution and 100 scan accumulation (λex = 632 nm). Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo TG-STDA instrument (30–1,000 °C at a heating rate of 5 °C min−1). UV– vis absorption spectra were recorded in a Hewlett-Packard 8453 apparatus. Steady-state fluorescence spectra were
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recorded in a Spex Fluorog 3–11 equipped with a 450-W xenon lamp. Scanning electron micrographs were taken on a LEO 440I microscope equipped with a digital camera. The samples were placed on top of a tin plate and sputtered with Au/Pd in a Polaron SC7610 Sputter Coater from Fisons Instruments. The particle size distribution of the synthesized polymer was determined by means a MASTERSIZER 2000 (MALVERN) laser diffraction instrument. To perform the measurements, the sample was suspended in MeOH. The data were analyzed with the software supplied with the instrument. Experimental procedure Synthesis of P1 Polymer P1 was prepared in a similar procedure to that described in the literature (Burguete et al. 2009) by thermal free radical-initiated polymerization of the monomers in the presence of a porogenic mixture and using a glass tube as the mould (1-cm diameter). AIBN (80 mg, 1 wt% with respect to monomers) was dissolved in p-chloromethylstyrene (90 %, 1.84 g) and divinylbenzene (80 % grade, 6.16 g). Then, the porogenic mixture, consisting of 1-dodecanol (10 g) and toluene (2 g), was added. The homogenized polymerization mixture was transferred to several glass tubes and purged with nitrogen in order to remove the dissolved oxygen. Then, the tubes were sealed with rubber septums and placed in a vertical position in a silicon bath heated at 80 °C. The polymerization was allowed to proceed at this temperature for 24 h. The glass tubes were carefully crushed and the polymer was then disaggregated mechanically and washed with tetrahydrofuran for 24 h in a Soxhlet apparatus in order to remove the porogenic mixture and any other soluble compounds remaining within the polymer; finally, the polymer was dried in a vacuum oven. FT-IR (cm−1): 3,018, 2,917, 1,629, 1,425, 1,265, 906, 795, 707. FT-Raman (cm−1): 3,058, 2,906, 1,631, 1,408, 1,267, 1, 181, 1,001. Decomposition temperature (°C): 500–510. The resulting polymeric material (5.77 g, 7.5 mmol of CH2Cl) and Rose Bengal sodium salt (8.81 g, 8.66 mmol) were mixed in a 500-mL round-bottom flask and stirred in 400 mL of dimethylformamide (DMF; treated previously with anhydrous MgSO4) at 80 °C for 8 h in a nitrogen atmosphere. Then, the reaction mixture was cooled to ambient temperature and filtered through a sintered glass funnel. The obtained resin, P1, was washed with 250-mL portions of the following solvents: DMF, ethyl acetate, ethanol, ethanol/water (1:1), water, methanol/water (1:1) and methanol. Next, the polymer was extracted with methanol in a Soxhlet apparatus until no visible colour appeared in the solvent. Finally, the light pink polymeric particles were dried in a vacuum oven. FT-IR (cm−1): 3,020, 2,920, 1,625, 1,453, 1,266, 902, 794, 709. FT-Raman (cm−1): 3,054, 2,907, 1,627, 1,411, 1,266, 1, 180, 1,003. Decomposition temperature (°C): 500–510. UV–
vis absorption spectroscopy (λmax): 571 nm. Fluorescence emission spectroscopy (λmax): 602 nm (λex = 572 nm). Rose Bengal loading: 2 μmol RB g−1 resin. Synthesis of P2 For the synthesis of polymer P2, 4.66 g of P1 was introduced with 15 mL of ethylenediamine (excess) in a 250-mL twoneck round-bottom flask. The mixture was dispersed in 100 mL of predried tetrahydrofuran (THF) and the flask was sealed and purged with nitrogen for 30 min. Then, the reaction mixture was refluxed for 8 h at 70 °C. After this time, the reaction mixture was cooled to ambient temperature and filtered through a sintered glass funnel. The obtained resin, P2, was washed with 200-mL portions of the following solvents: THF, dioxane, dioxane/water (1:1), water, dioxane/water (1:1), dioxane and ethanol. Finally, the polymer was dried in a vacuum oven at 55 °C. FT-IR (cm−1): 3,358, 3,046, 2,921, 1,619, 1,447, 990, 906, 798, 713. FT-Raman (cm−1): 3,055, 2,903, 1,632, 1,407, 1, 315, 1,179, 1,105, 1,003, 803. Decomposition temperature (°C): 500–510. UV–vis absorption spectroscopy (λmax): 561 nm. Fluorescence emission spectroscopy (λmax): 593 nm (λex = 564 nm). Rose Bengal loading: 2 μmol RB g−1 resin. Synthesis of P3 In a 250-mL two-neck round-bottom flask, 2.87 g of polymer P2 and 1.335 g of γ-gluconolactone (excess amount) were introduced. The mixture was dispersed in 100 mL of absolute ethanol and purged with nitrogen for 30 min. The reaction was carried out under reflux for 18 h at 80 °C. Then, the reaction mixture was cooled to ambient temperature and filtered through a sintered glass funnel. The obtained resin, P3, was washed with 250-mL portions of the following solvents: ethanol, methanol/water (1:1), water, methanol/water (1:1) and methanol. Finally, the obtained polymer was dried in a vacuum oven at 55 °C. FT-IR (cm−1): 3,367, 3,056, 2,928, 1,628, 1,451, 1,081, 902, 804, 704. FT-Raman (cm−1): 3,055, 2,903, 1,634, 1,408, 1,307, 1,188, 1,086, 1,001, 930, 803. Decomposition temperature (°C): 500–510. UV–vis absorption spectroscopy (λmax): 562 nm. Fluorescence emission spectroscopy (λmax): 591 nm (λex = 564 nm). Rose Bengal loading: 2 μmol RB g−1 resin. Average particle diameter: 26.3 μm. Analytical procedure to estimate the loading of Rose Bengal in P1–P3 The polymeric photosensitizers (20 mg) and tetrabutylammonium hydroxide solution ~0.8 M in MeOH (3 mL) were mixed in a 25-mL round-bottom flask containing 10 mL of 1,4-dioxane. The flask was sealed and the mixture
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was stirred for 24 h at room temperature. The reaction mixture was then filtered through a sintered glass funnel, and the resin was washed with MeOH until no visible colour appeared in the solvent. The filtrate was transferred into a 100-mL volumetric flask and diluted to 100 mL with MeOH. The final solvent ratio of the solution was 87:10:3 (MeOH/1,4-dioxane/ TBAOH solution). From the UV–vis absorption spectrum of the solution, the amount of free Rose Bengal was determined, using ε = 78,028±1,291 L mol−1 cm−1 at 556 nm from a previous calibration (Burguete et al. 2009) of Rose Bengal in MeOH/1,4-dioxane/TBAOH (87:10:3).
irradiation of 2-furoic acid with free Rose Bengal in solution (5 μM) as photosensitizer. The decreasing absorbance of 2furoic acid was monitored by means of UV–vis spectroscopy at 246 nm for 420 min. For every measurement, aliquots of 80 μL were removed from the reaction mixture and diluted in 25 mL of the appropriate solvent. Acknowledgments Financial support from the Spanish MINECO (CTQ2009-14366-C02-01) and Fundació Caixa Castelló-UJI (project P1 1B-2009-59, P1 1B2009-58, P1 1B2012-41) are acknowledged. V. F. thanks the financial support from UJI (predoctoral fellowship). We thank J. Javier Gómez (SCIC) for the technical assistance in SEM measurements.
Photochemical experiments: photooxidation of ADPA The rate of photooxidation of ADPA (1.2×10−4 M) was determined in several reaction media: water, PBS, MeOH and chloroform, for the polymeric photosensitizers P1, P2 and P3. In each of the experiments, 40 mg of the photosensitizer was added to 10 mL of solution of ADPA (1.2× 10−4 M) in a test tube. The heterogeneous mixture was kept under stirring and in equilibrium with air. The test tubes were irradiated at room temperature with a halogen lamp of 50 W, placed at a distance of 2 cm. The same experimental conditions were performed for the polymers in the dark in order to evaluate a possible adsorption of the photocatalyst into the polymeric substrate. Moreover, other control experiments were carried out with the absence of polymer and irradiation of ADPA with free Rose Bengal in solution (5 μM) as photosensitizer. The decreasing absorbance of ADPA was monitored by means of UV–visible spectroscopy at 398 nm for 60 min. Before every measurement, 3 mL of the reaction mixture was filtered (nylon syringe filter, 0.2 mm) to another cuvette in order to remove the polymeric particles in suspension. Photochemical experiments: synthesis of 5-hydroxy-5H -furan-2-one The oxidation of 2-furoic acid (3×10−2 M) to 5-hydroxy-5Hfuran-2-one was studied for photosensitizers P1–P3 in several reaction media: methanol/water (9:1), methanol and chloroform. The photosensitizer (40 mg) was added to 10 mL of solution of 2-furoic acid (3×10−2 M) in a test tube. The heterogeneous mixture was kept under stirring and in equilibrium with air. Such solutions were irradiated with a 125-W medium-pressure Hg vapour lamp for 6 h surrounded by an aqueous solution of 0.1 M FeCl3 used as a filter for wavelengths under 450 nm, and the tubes were placed at a distance of 2 cm from the solution. The same experimental conditions were used for the polymers in the dark in order to evaluate a possible adsorption of the photocatalyst into the polymeric substrate. Moreover, other control experiments were carried out: (a) irradiation in the absence of polymer and (b)
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