Photoactive, Porous Honeycomb Films Prepared from Rose Bengal

Jul 15, 2013 - Honeycomb-structured porous polymer films based on photosensitizer-grafted polystyrene are prepared through the breath figure process. ...
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Photoactive, Porous Honeycomb Films Prepared from Rose BengalGrafted Polystyrene Laurence Pessoni,†,‡ Sylvie Lacombe,‡ Laurent Billon,† Ross Brown,*,‡ and Maud Save*,† †

CNRS, University of Pau and Pays de l'Adour, UMR 5254, IPREM, Equipe de Physique et Chimie des Polymères, 2 avenue du Président Angot, Pau, F-64053, France ‡ CNRS, University of Pau and Pays de l'Adour, UMR 5254, IPREM, Equipe de Chimie Physique, 2 avenue du Président Angot, Pau, F-64053, France S Supporting Information *

ABSTRACT: Honeycomb-structured porous polymer films based on photosensitizer-grafted polystyrene are prepared through the breath figure process. Rose Bengal (RB) photosensitizer is first attached to a well-defined poly(styrenestat-4-vinylbenzyl chloride) statistical copolymer, synthesized by nitroxidemediated radical polymerization. The RB grafted poly(styrene-stat-4-vinylbenzyl chloride) (ca. 20 000 g mol−1 molar mass, 1.2 dispersity) leads to porous polymer films, with a hexagonal pore pattern, while a simple mixture of poly(styrene-stat4-vinylbenzyl chloride) and the insoluble RB photosensitizer produced unstructured, nonporous films. The RB-grafted honeycomb films, compared with the corresponding nonporous flat films, are more efficient for oxidation of organic molecules via singlet oxygen production at a liquid/solid interface. The oxidations of 1,5-dihydroxynaphthalene to juglone and α-terpinene to ascaridole are followed in ethanol in the presence of both types of films. Oxidation of the organic molecules is a factor 5 greater with honeycomb compared to the nonporous films. This gain is ascribed to two factors: the specific location of the polar photosensitizer at the film interface and the greater exchange surface, as revealed by fluorescence and scanning electron microscopies.

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INTRODUCTION Organic photosensitizers producing singlet oxygen (1O2), particularly those active under visible light, are attracting wide interest in the fields of photodegradation of pollutants, antimicrobial applications, or specific oxidation.1−4 While immobilization of photosensitizers on solid substrates improves their handling and recycling,5 it may compromise efficient contact with the liquid or gaseous phase containing the compound to be oxidized. Porosity of the substrate is thus important. Various, mainly inorganic, porous materials such as for instance silica gels,6−8 silica nanoparticles,9,10 sepiolites,11 zeolites,12 carbon nanotubes,13 or titanium dioxide14,15 have been used to carry photosensitizers.16 Polymer materials are also good candidates for immobilizing photosensitizing dyes.17 Polymer substrates embedding photosensitizers include hydrophilic,18−20 hydrophobic,21,22 amphiphilic,23−25 or microporous polymers and polymer beads.18,19 Polymer films are readily processable onto a wide variety of substrates. The breath figure method, introduced in 1994,26 continues to attract attention.27−30 When a polymer is cast from solution under a humid air flow, fast evaporation of the solvent lowers the temperature of the solution, leading to condensation of water droplets at the surface. Bénard− Marangoni convection, driven by the temperature gradient within the solution, is thought to contribute to self-assembly of the droplets into a hexagonal array.31−33 Fast precipitation of © 2013 American Chemical Society

the thin surrounding polymer layer stabilizes them against coalescence.34 Complete evaporation of both solvent and water leaves behind a hexagonally organized array of pores with diameters in the micrometer range (Scheme 1). Several studies showed preferential positioning of the polymer polar functions at the surface of the pores, related to the presence of the water droplet “templates” during the breath figure process.35−39 Moreover, the larger specific surface area of structured porous honeycomb vs flat nonporous polymer films should provide greater activity. Here, we exploit these possibilities to obtain porous materials with improved photoactivity compared to the corresponding flat films. We immobilize Rose Bengal on a polystyrene honeycomb porous film and characterize its structure. The aim is to compare the photoactivity of the honeycomb-structured and the corresponding nonporous continuous films. Reasons for choosing Rose Bengal (RB) as the photosensitizer include its importance, based on its photochemical reactivity and photophysical parameters, reviewed by Neckers et al.40 Furthermore, RB is a good candidate here because its carboxylate function allows nucleophilic substitution41−43 and the polar sodium salt function favors exposure of RB at the Received: June 2, 2013 Revised: July 4, 2013 Published: July 15, 2013 10264

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Scheme 1. Schematic Representation of Honeycomb Film Formation by the Breath Figure Process: (〰) Hydrophobic Polymer, (Red ●) Hydrophilic Polymer Chain End, and (Green ●) Hydrophilic Photosensitizer

Scheme 2. (A) Synthesis of P(S-stat-VBC)-g-RB by NMP of S and VBC Followed by (B) Nucleophilic Substitution of Rose Bengal (RB) onto the P(S-stat-VBC) Copolymer

honeycomb film via self-assembly of the polystyrene ionomers into inverse micelles.32,47,48 Such inverse micelles are formed by aggregation of the insoluble polar chain end and for limited degree of polymerization. This justifies having recourse to controlled radical polymerization in order to control simultaneously the polymer chain end functionality and the polymer chain length. Indeed, the limitation of molar masses in conventional radical polymerization requires the use of transfer agents which produce new chains with noncontrolled chain-end functionality. The BlocBuilder alkoxyamine used in the present work directly produces polymers with a carboxylic acid chain end with a simultaneous control of molar masses (Scheme 2). The P(S-stat-VBC) copolymer is subsequently grafted with Rose Bengal through nucleophilic substitution of VBC units41 to produce the RB grafted copolymer (P(S-stat-VBC)-g-RB, Scheme 2). We determine the location of RB in the film and the pore ordering by fluorescence and scanning electron microscopies, respectively. Photosensitized production of singlet oxygen at a liquid/film interface is compared to that of nonporous film by following the oxidation in ethanol of 1,5-

surface of the pores. Although RB has been previously immobilized on different polymer materials,16 only one example of a hydrophobic polymer film has been reported, where colloidal structuring of Rose Bengal grafted polystyrene spheres enhanced fluorescence.44 To our knowledge, only one earlier report described a photosensitizer-grafted polymer honeycomb film for electrochemical and photochemical devices. It was an amphiphilic copolymer obtained by radical copolymerization of ruthenium (4-vinyl-4′-methyl-2,2′bipyridine)bis(2,2′-bipyridine)bis(hexafluorophosphate) with N-dodecylacrylamide and N-isopropylacrylamide.45 In the present study, we synthesize a well-defined, photosensitizer-grafted copolymer by a two-step procedure, described in Scheme 2. The poly(styrene-stat-4-vinylbenzyl chloride) statistical copolymer (Scheme 2), P(S-stat-VBC), is first synthesized by nitroxide-mediated radical polymerization (NMP), 46 which enables simultaneous control of the number-average molar mass, dispersity, and chain end of the polymer. Note that the ionic chain end of a linear polystyrene homopolymer previously favored growth of well-ordered 10265

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dihydroxynaphthalene (DHN) to juglone and of α-terpinene to ascaridole.

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determined by gas chromatography−mass spectroscopy (GC−MS) with a GC Clarus 680-MS Clarus 600S from PerkinElmer. Gas chromatography analyses were run at 50 °C/10 °C min−1/200 °C (20 min), at a detector temperature of 250 °C, an injector temperature of 150 °C, and an injector pressure of 1.6 atm, using an Elite-5MS column (60 m × 0.25 mm, film column of 0.25 μm). Mass spectra were obtained by electron impact at 70 eV. The transfer line temperature was 200 °C, ion source temperature was 150 °C, and the scan range (m/z) was 40−300. The chromatograms are displayed in Figure SI-10. Instrumentation. Proton nuclear magnetic resonance (1H NMR) spectra were recorded at 25 °C in deuterated chloroform on a Brüker 400 MHz spectrometer. The P(S-stat-VBC) copolymer was analyzed by size exclusion chromatography (SEC) with THF eluent, on four columns (Shodex KF801, 802.5, 804, and 806) each 300 mm × 8 mm, running at 30 °C with THF eluent at a flow rate of 1.0 mL min−1, controlled by a Malvern pump (Viscotek, VE1122). The SEC system was connected to a Malvern VE3580 refractometer. SEC with DMF eluent was used for analysis of the P(S-stat-VBC)-g-RB copolymer, on a PL-GPC 50 plus Integrated GPC (Polymer Laboratories-Varian). It was fitted with three columns (2 PLgel 5 μm MIXED-D (7.5 mm × 30 cm), 1 PLgel guard column (7.5 mm × 5 cm), Polymer Laboratories). The DMF HPLC grade solvent with 1 g L−1 of LiBr was used as eluent at 80 °C and a flow rate of 0.8 mL min−1. Toluene was used as flow marker, and the loop volume was 20 μL. The SEC system was connected to a refractometer and a UV detector (Polymer Laboratories-Varian). Number-average molar mass (Mn) and dispersity (Mw/Mn, where Mw is the mass-average molar mass) of the polymers were calculated from a calibration derived from polystyrene standards. The polymer samples were prepared at 5 g L−1 and filtered through 0.45 μm PTFE filters. The absorbances of P(S-stat-VBC)-g-RB copolymer, DHN, juglone, and α-terpinene were measured on a double-beam spectrophotometer (Cary 5000, Varian). Spectra were recorded at room temperature in the range 250−700 nm, using a 1 cm2 quartz optical cell (Hellma). Corrected steady-state emission and excitation spectra were measured using a photon counting Edinburgh FLS920 fluorescence spectrometer. They were recorded at 1 nm resolution in a 1 cm2 quartz cell (Hellma). Emission spectra were recorded at excitation wavelength 530 nm and excitation spectra at emission wavelength 580 nm. We recorded scanning electron micrographs of gold coated honeycomb films (Hirox SH-3000 microscope, acceleration voltage of 10 kV). Wide-field fluorescence images were taken with a CCD camera (DS-SMC, Nikon) on an inverted microscope (Nikon TiEclipse, ×60 oil immersion objective). A coverslip protected the front face of the films. Excitation and emission filters were respectively 470 ± 20 and 520 ± 20 nm. ImageJ was used for data analysis.57

MATERIALS AND METHODS

Materials. Styrene and 4-vinylbenzyl chloride (Sigma-Aldrich, S, ≥99% and VBC, 90%) were freshly passed before use over inhibitor remover column (Sigma-Aldrich). The BlocBuilder alkoxyamine (2methylaminoxypropionic-SG1, Scheme 2) used as the initiator was supplied by Arkema. Rose Bengal and α-terpinene (Sigma-Aldrich, 95% and 85%) were used as received. 1,5-Dihydroxynaphthalene (DHN, 97%, Sigma-Aldrich) was used after sublimation. Ethanol (ACS reagent ≥99%, absolute) was purchased from Sigma-Aldrich. Methanol, chloroform, N,N-dimethylformamide (DMF, Sigma-Aldrich/anhydrous 99.8%), and tetrahydrofuran (THF, VWR prolabo, GPR rectapur) were from commercial sources and used without purification. Synthesis of the Rose Bengal-Grafted Copolymer (P(S-statVBC)-g-RB). Step 1: Synthesis of the P(S-stat-VBC) Statistical Copolymer by NMP. The P(S-stat-VBC) copolymer was first synthesized by nitroxide-mediated polymerization as follows: S (18.6 g, 0.179 mol), VBC (1.371 g, 0.009 mol), and BlocBuilder alkoxyamine (0.232 g, 6.1 × 10−4 mol) as initiator were mixed in a round flask. The targeted degree of polymerization (DP) at full conversion was 310. The mixture was degassed by nitrogen bubbling for 30 min at 0 °C and immersed in a thermostated oil bath at 115 °C. The reaction was stopped after 3 h 30 min by cooling the polymer solution to room temperature. Monomer conversion was calculated by proton NMR of the crude solution in CDCl3 (see Figure SI-1 in Supporting Information). The mixture was dissolved in THF, and the P(S-stat-VBC) was precipitated in methanol before SEC analysis. The precipitated copolymer was analyzed by 1H NMR in CDCl3 (Figure SI-2). Step 2: Nucleophilic Substitution of Rose Bengal onto the P(Sstat-VBC) Copolymer. The experimental procedure was adapted from ref 41. The P(S-stat-VBC) copolymer (19 900 g mol−1, 1.0 g, 5.02 × 10−5 mol) and Rose Bengal (5.6 mg, 5.5 × 10−5 mol) were mixed in 50 mL of DMF. The mixture was degassed by nitrogen bubbling for 30 min and placed into an oil bath thermostated at 40 °C for 24 h. The P(S-stat-VBC)-g-RB chains were precipitated in methanol five times to purify polymer chains from free RB. Preparation of Honeycomb and Nonporous Flat Films from P(S-stat-VBC)-g-RB Copolymer. We prepared P(S-stat-VBC)-g-RB honeycomb films (HC) by casting 30 μL of P(S-stat-VBC)-g-RB in chloroform (solution A, 5 g L−1) on strips of glass microscope slides (Menzel-gläser, Superfrost) and evaporating under humid air flow at ambient temperature (2 L. min−1, 70−80% relative humidity). The slides were cut to fit into fluorescence cuvettes for monitoring the photoreactions (see below). Nonporous (NP) films were prepared similarly but in ambient conditions in the absence of humid air flow. The area of the films thus obtain was in the range 1.5−2 cm2. Photooxidation of 1,5-Dihydroxynaphthalene (DHN) or αTerpinene in the Presence of P(S-stat-VBC)-g-RB Films. Photooxidation of DHN49−52 and α-terpinene53−56 was achieved in ethanol solution in capped 1 cm2 quartz fluorescence cuvettes, thermostated at 24 °C. The dry polymer film on a glass substrate was set facing inward against a side face of the cuvettes and immersed in 3 mL of solution in ethanol ([DHN]0 = 1.34 × 10−4 mol L−1 and [αterpinene]0 = 1.98 × 10−4 mol L−1). The solution was stirred with a magnetic stirrer while the film was irradiated with a solar simulator including either a 495 nm long pass filter for DHN or a 395 nm long pass filter for α-terpinene. The irradiation spectra are displayed in the Supporting Information (Figures SI-3 and SI-4). The transmitted irradiance of the lamp in the visible was respectively 24 and 31 mW cm−2. The absorption of the solution was monitored hourly by UV− vis spectroscopy through the cuvette faces adjacent to the sample. We report averages and standard deviations of the activities of six films of each type. The molar absorption coefficients of DHN, juglone, and αterpinene in ethanol are in order 5200 L mol−1 cm−1 (λ = 331 nm), 3700 L mol−1 cm−1 (λ = 418 nm), and 7200 L mol−1 cm−1 (λ = 265 nm). The final product distribution of the α-terpinene reaction was

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RESULTS AND DISCUSSION Copolymer Synthesis. The poly(styrene-stat-vinylbenzyl chloride) (P(S-stat-VBC)) copolymer was synthesized by controlled radical nitroxide polymerization carried out at 115 °C. The polymerization was initiated by the BlocBuilder alkoxyamine and controlled by the SG1 nitroxide radical as depicted in Scheme 2. The initial fractions of 4-vinylbenzyl chloride was 4.8 mol %. We found final individual monomer conversions of 49% for S and 61% for VBC calculated by 1H NMR (Figure SI-1). The VBC fraction of the final purified copolymer was 6.3 mol % (Figure SI-2). The SEC chromatogram of the P(S-stat-VBC) copolymer is displayed in Figure SI5. The narrow molar mass distribution of the recovered P(Sstat-VBC) copolymer (Mw/Mn = 1.12) and the fair agreement between the SEC experimental and theoretical molar masses (Mn,SEC = 19 900 g mol−1 and Mn,theo = 16 800 g mol−1) both indicate good control of the polymerization. The theoretical molar mass is determined by eq 1 10266

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M ntheo = MBlocBuilder +

mS,0 + mVBC,0 nBlocBuilder,0

Cwt

RB copolymer (see Figure 2) confirmed successful grafting and the distribution of RB on the whole population of chain

(1)

where MBlocBuilder is the BlocBuilder molecular weight, mS,0 and mVBC,0 are the initial weights of styrene and VBC, and nBlocBuilder,0 is the initial number of moles of BlocBuilder. The weight conversion, Cwt, is calculated from the individual molar conversions (xS and xVBC) and the initial weight fractions of each monomer (wS,0 and wVBC,0): Cwt = xSwS,0 + xVBCwVBC,0. The well-defined P(S-stat-VBC) was subsequently grafted with Rose Bengal by nucleophilic substitution (Scheme 2). The RB grafted P(S-stat-VBC), hereafter P(S-stat-VBC)-g-RB, was thoroughly purified by several precipitations in order to extract free RB. Figure 1 compares the excitation and emission spectra Figure 2. SEC chromatograms of P(S-stat-VBC)-g-RB-grafted copolymer in DMF/LiBr eluent. UV trace in gray (λ = 570 nm) and refractive index trace in black.

lengths. Also, the RB-grafted polymer exhibited a low dispersity of 1.2 with Mn of 20 850 g mol−1, higher than that of the P(Sstat-VBC) precursor (Mn = 19 900 g mol−1). This difference agrees with the fraction of chains grafted with RB (molar mass 1017 g mol−1). However, a residual peak due to free RB is still present in the UV chromatogram (Figure 2). Therefore, in the P(S-stat-VBC)-g-RB used for film formation, there are 0.15 grafted and 0.02 free RB molecules per chain. Honeycomb Film Preparation and Characterization. Figure 3 shows cross-section views of a typical honeycomb film Figure 1. Excitation and emission spectra of Rose Bengal (RB) and polymer in solution in DMF in the same molar proportions as in the grafted polymer used to prepare HC films, [RB] = 7.5 × 10−5 M throughout. (A) Free RB alone; (B) physical mixture of free RB and P(S-stat-VBC) polymer ([polymer] = 4.2 × 10−4 M); (C) Rose Bengal-grafted polymer, P(S-stat-VBC)-g-RB.

of the P(S-stat-VBC)-g-RB copolymer dissolved in DMF with those of free Rose Bengal in the presence of the P(S-stat-VBC) ungrafted polymer. P(S-stat-VBC)-g-RB exhibits the excitation and emission bands characteristic of RB, but with a red-shift of ca. 10−15 nm, relative to the free dye, proving the grafting. The number of RB molecules grafted per polymer chain (NRB per P(S‑stat‑VBC)) was determined by absorption spectroscopy, using eq 2 nRB NRBper(S ‐ stat ‐ VBC) = nP(s ‐ stat ‐ VBC) ‐ g ‐ RB (2)

Figure 3. SEM images of a P(S-stat-VBC)-g-RB honeycomb film (a) and enlargement (b); oblique edge view of a torn film; (c, d) top and side view. A, B, C, and D: depths corresponding to parts of Figure 4.

where A and ε are the absorbance and the extinction coefficient at band peak (determined by calibration, Figure SI-6, and independent of grafting, cf. Figure 1), V is the volume of solvent, L the path length, and mP(S‑stat‑VBC)‑g‑RB is the mass of the polymer. The value of NRB per P(S‑stat‑VBC) was 0.18; i.e., about one in every six copolymer chains contains a grafted RB molecule. Although the initial ratio of reagents amounted to 1 RB per chain, this value is not as disappointing as it appears, considering that postmodification of polymers is rarely quantitative due to steric hindrance, particularly present with large molecules as RB. Furthermore, carrying hydrophilic functionalization of polystyrene too far is known to hinder formation of HC films by the breath figure method.35,58 In size exclusion chromatography, the correspondence between UV absorbance and refractive index traces of the P(S-stat-VBC)-g-

obtained under humid air flow. The film comprises three layers of roughly spherical pores, 2−2.5 μm in diameter, presenting a well-organized hexagonal patterned surface. SEM shows round windows between some pores, diameter ca. 0.4−0.8 μm. The front view shows hexagonally ordered pore openings to the film surface, of diameter ∼1.7 μm. Whereas the P(S-stat-VBC)-g-RB grafted copolymer formed ordered honeycomb films (Figures 3 and 4), insoluble free RB in simple mixture with P(S-stat-VBC) in chloroform (nRB,free = ngrafted+free RB in P(S‑stat‑VBC)‑g‑RB) disrupted the film ordering (Figure SI-7 left) with irregularly distributed RB (Figure SI-7 right). We conclude that despite the low concentration of RB, grafting the polar moiety to the polymer is mandatory to prepare organized honeycomb porous film with homogeneous distribution of RB photosensitizer.

=

AVM n,SEC,P(S ‐ stat ‐ VBC) ‐ g ‐ RB mP(S ‐ stat ‐ VBC) ‐ g ‐ RBεL

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Scheme 3. Photooxidation of (A) 1,5-Dihydroxynaphthalene to Juglone and (B) α-Terpinene (1) to Ascaridole (2), pCimene (3), and Isoascaridole (4)

Figure 4. Wide-field fluorescence microscopy images at increasing depths in a P(S-stat-VBC)-g-RB honeycomb film: (A) external surface of top layer of pores; (B) in between first and second layers; (C, D) the second and the third pore layers. See Figure 3d for schematic location of images (in a different film).

irradiation, the intensity of these three bands decreased, while a new absorption band at 418 nm appeared, due to the formation of juglone,59 indicating the oxidation of DHN in the presence of HC films. The formation of juglone from singlet oxygen addition to DHN was thus due to visible light irradiation of the HC films. We compared the production of singlet oxygen by the P(Sstat-VBC)-g-RB honeycomb and nonporous flat films by following the intensities of the bands of DHN at 331 nm and of juglone at 418 nm (Figure 6). While loss of DHN with the nonporous flat film was imperceptible, with no significant increase of the juglone band at 418 nm, 10% loss in 8 h was observed with the HC films. At first sight, this yield appears low, but it should be remembered that each HC film contains only ∼10−9 mol of RB. Furthermore, while RB was chosen here because it is a wellknown standard photosensitizer of 1O2, it has the disadvantage of significant susceptibility to photobleaching.40,60 Indeed, photobleaching of the films was clearly visible to the naked eye, further explaining the low final yield of juglone: 15% DHN oxidation at 16 h irradiation. Photobleaching was confirmed quantitatively by following under the same conditions, including irradiation and the presence of DHN, the diffuse reflectance of an HC film and the absorbance of a solution of free RB in ethanol. The concentration of free RB in ethanol and of grafted RB in the HC film dropped respectively by 85 and 75% in 8 h irradiation at the same intensity (see Figure SI-9). Photobleaching of RB is not related specifically to the present use of a nitroxide-terminated polymer, since photobleaching in solution is well-known, and was also observed here in commercial Merrifield resins. However, the higher RB content used in commercial Merrifield resins (typically ∼10−4 mol g−1 vs 10−5 mol g−1 here) probably hide loss by photobleaching. Here, one 2 cm2 HC film contains 0.15 mg of polymer and only 10−9 mol of RB. Photooxidation of α-Terpinene. Turning to photooxidation of α-terpinene in ethanol, Figure 5 (right scale) shows the drop of its absorption band at 265 nm. Control experiments, performed in the dark in the presence of HC film, showed no loss of α-terpinene. Self-photosensitization in the absence of HC films led to 5% loss under the same conditions. Significantly higher α-terpinene photooxidation was however achieved in the presence of P(S-stat-VBC)-g-RB honeycomb film as shown in Figure 7. The production of ascaridole, isoascaridole, and p-cymene as photooxidation products was

In addition to the lower quality of the porous film prepared from a mixture of P(S-stat-VBC) and free RB molecules, the covalent link obtained by the grafting step is crucial to avoid leaching of RB during the photo-oxidation experiments at the liquid/solid interface. Indeed, absorption bands of RB were observed in ethanol after immersion of the HC film prepared from the f ree RB−copolymer mixture but were undetectable with the RB-grafted copolymer HC film. Figure 4 shows four images from a series recorded by widefield fluorescence microscopy at 0.5 μm depth spacing into a P(S-stat-VBC)-g-RB honeycomb film. Like the film in Figure 3, this film exhibits strong hexagonal order in each of three layers of pores. Srinivasaro et al.32 also reported a 3D structured HC film, 10 μm thick, prepared from monochelic polystyrene via the breath figure process. RB is homogeneously distributed at the ∼10−100 m scale across the film, as deduced from the uniform level of signal in the digital images. Closer examination of Figure 4B shows a spot pattern consistent with the presence of RB at the surface of the pore floors, rather than in the bulk of the polymer walls: in the latter case, stepping through the image stack, one would expect at some point uniform fluorescence, which was not observed. This distribution of the grafted polymer might be expected from the affinity of hydrophilic RB with water droplets during the breath figure process. Photoactivity of Rose Bengal-Grafted Copolymer Honeycomb Films. The catalytic activity of the P(S-statVBC)-g-RB honeycomb film was determined for the photooxidations of 1,5-dihydroxynaphthalene (DHN) to juglone (see Scheme 3A) and of α-terpinene to ascaridole (see Scheme 3B), at the liquid−solid interface of films immersed in solution in ethanol (see Materials and Methods). Photooxidation of DHN. In order to prove the photoactivity of the films, control experiments were performed with DHN: (i) in the dark in the presence of HC films or (ii) with visible light, without HC films. No obvious change of the DHN absorption spectra was observed, indicating that no reaction occurred. On the contrary, DHN oxidation in the presence of P(S-stat-VBC)-g-RB honeycomb film was deduced from the absorption spectra (Figure 5 left). The absorption spectrum of DHN in ethanol has three bands in the UV region, at 300, 317, and 331 nm, attributed to the naphthalene rings. Under visible 10268

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Figure 5. Absorption spectra showing the oxidation of DHN to juglone (left) and of α-terpinene (to several products) (right), photosensitized by RB grafted HC films. Spectra recorded at times: 0, 2, 5, and 8 h (DHN), and 0, 2, 4, 6, and 8 h (α-terpinene).

consider the increase of the single Juglone absorption peak while the decrease of the α-terpinene absorbance is followed for the second reaction as no individual peak of the oxidation products has been identified (Figure 5). Figure 8 shows the

Figure 6. Photooxidation of DHN (black) and appearance of juglone (gray) under visible light irradiation: (NP) nonporous P(S-stat-VBC)g-RB flat film; (HC) P(S-stat-VBC)-g-RB honeycomb film.

Figure 8. Comparison of reaction yields for the photosensitized NP oxidation of DHN (triangles), G1 = ([J]HC t /[J]t , J = juglone, and of HC NP NP α-terpinene (diamonds), G2 = ([T]0 − [T]HC t )/([T]0 − [T]t ) with T = α-terpinene, using Rose Bengal-grafted honeycomb film (HC) and nonporous film (NP).

ratio of the reaction yields G(t) = YHC(t)/YNP(t) as a function of time. The maximum gain in both reactions is comparable, ∼5. The gain for the slow DHN reaction rises to the maximum ratio value at long times. The gain is immediately apparent for the much faster reaction of α-terpinene. For the second reaction, the apparent drop of gain at long times merely reflects the fact that the reaction using HC films is already over, when the reaction with nonporous films is still proceeding slowly. Two reasons for the high efficiency of HC films to produce singlet oxygen are their large interfacial surface areas (per unit substrate surface covered by the film) and the location of RB at the pore−liquid interface.

Figure 7. Photosensitized oxidation of α-terpinene under illumination with no film (BLANK), nonporous (NP), or honeycomb-structured porous film (HC).

shown by GC-MS (Figure SI-10). Formation of ascaridole and isoascaridole as main products indicates that α-terpinene was oxidized by singlet oxygen.54 Figure 7 confirms the higher activity of the HC compared to the nonporous film: after 8 h of irradiation, α-terpinene was fully oxidized in the presence of P(S-stat-VBC)-g-RB honeycomb films vs only 40% loss in presence of nonporous flat films. Comparison of Honeycomb and Nonporous Films. Ideally, the efficiencies of the HC and nonporous films would be determined from the rates of creation of products at short times, before significant depletion of the reactants and bleaching of the films. However, it is clear that under the present conditions the photosensitized oxidation of α-terpinene proceeds much faster than that of DHN, almost reaching completion in 8 h. Slow conversion of DHN on the other hand makes estimation of the reaction rates unreliable at short times. We therefore compare the films by plotting reaction yields, YHC(t) and YNP(t), as functions of time, t, directly from the product absorbance (Juglone) in the first reaction (DHN oxidation) and from the loss of α-terpinene absorbance in the second reaction. For DHN degradation, it is more accurate to

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CONCLUSIONS The present work shows the potential of honeycomb films as substrates for heterogeneous photosensitized reactions. To our knowledge, it is the first example of a photosensitizer grafted honeycomb-patterned polystyrene film inducing photooxidation of organic molecules. We identify two reasons for the significantly improved yield of singlet oxygen in the honeycomb compared to nonporous films: higher specific surface area due to the pore structure and improved exposure of the active dye, resulting from hydrophilic interactions with water droplets during the breath figure casting of the HC films. For the sake of this exploratory study, we used here a standard sensitizer of singlet oxygen, Rose Bengal, with well-documented properties. While cheap, this dye readily photobleaches, not a serious shortcoming in solution photochemistry or in standard substrates such as Merrifield resins. The present honeycomb 10269

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films come at the cost of more effort to produce a polymer that is both sensitizer-grafted and castable by the breath figure method. Future developments may therefore lie in the direction either of producing specific, hard to synthesize compounds by photosensitized oxidation, or in the use of more photostable photosensitizers.

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(11) Arques, A.; Amat, A. M.; Santos-Juanes, L.; Vercher, R. F.; Marin, M. L.; Miranda, M. A. Sepiolites as Supporting Material for Organic Sensitisers Employed in Heterogeneous Solar Photocatalysis. J. Mol. Catal. A: Chem. 2007, 271, 221−226. (12) Amat, A. M.; Arques, A.; Bossmann, S. H.; Braun, A. M.; Miranda, M. A.; Vercher, R. F. Synthesis, Loading Control and Preliminary Tests of 2,4,6-Triphenylpyrylium Supported onto YZeolite as Solar Photocatalyst. Catal. Today 2005, 101, 383−388. (13) Anbarasan, R.; Peng, C. A. Synthesis and Characterization of Rose bengal/Folic acid-Functionalized Multiwall Carbon Nanotubes. J. Mater. Sci. 2011, 46, 992−998. (14) Mele, G.; Garcia-Lopez, E.; Palmisano, L.; Dyrda, G.; Slota, R. Photocatalytic Degradation of 4-Nitrophenol in Aqueous Suspension by Using Polycrystalline TiO2 Impregnated with Lanthanide DoubleDecker Phthalocyanine Complexes. J. Phys. Chem. C 2007, 111, 6581− 6588. (15) Murphy, S.; Saurel, C.; Morrissey, A.; Tobin, J.; Oelgemoller, M.; Nolan, K. Photocatalytic Activity of a Porphyrin/TiO2 Composite in the Degradation of Pharmaceuticals. Appl. Catal. B: Environ. 2012, 119, 156−165. (16) Wahlen, J.; De Vos, D. E.; Jacobs, P. A.; Alsters, P. L. Solid Materials as Sources for Synthetically Useful Singlet Oxygen. Adv. Synth. Catal. 2004, 346, 152−164. (17) Rychtarikova, R.; Kuncova, G. Immobilized Singlet Oxygen Photosensitizers and Their Antimicrobial Effect. Chemicke Listy 2009, 103, 800−813. (18) Hong, K. H.; Sun, G. Photoactive Antimicrobial PVA Hydrogel Prepared by Freeze-Thawing Process for Wound Dressing. J. Appl. Polym. Sci. 2010, 116, 2418−2424. (19) Hurtgen, M.; Debuigne, A.; Mouithys-Mickalad, A.; Jerome, R.; Jerome, C.; Detrembleur, C. Synthesis of Poly(vinyl alcohol)/C-60 and Poly(N-vinylpyrrolidone)/C-60 Nanohybrids as Potential Photodynamic Cancer Therapy Agents. Chem.Asian J. 2010, 5, 859−868. (20) Wirotius, A. L.; Ibarboure, E.; Scarpantonio, L.; Schappacher, M.; McClenaghan, N. D.; Deffieux, A. Hydrosoluble Dendritic Poly(Ethylene Oxide)s with Zinc Tetraphenylporphyrin Branching Points as Photosensitizers. Polym. Chem. 2013, 4, 1903−1912. (21) Van Laar, F.; Holsteyns, F.; Vankelecom, I. F. J.; Smeets, S.; Dehaen, W.; Jacobs, P. A. Singlet Oxygen Generation Using PDMS Occluded Dyes. J. Photochem. Photobiol. A: Chem. 2001, 144, 141−151. (22) Urakami, H.; Zhang, K.; Vilela, F. Modification of Conjugated Microporous Poly-Benzothiadiazole for Photosensitized Singlet Oxygen Generation in Water. Chem. Commun. 2013, 49, 2353−2355. (23) Nowakowska, M.; Kepczynski, M.; Dabrowska, M. Polymeric Photosensitizers, 5 - Synthesis and Photochemical Properties of Poly(N-Isopropylacrylamide)-Co-(Vinylbenzyl Chloride) Containing Covalently Bound Rose Bengal Chromophores. Macromol. Chem. Phys. 2001, 202, 1679−1688. (24) Koizumi, H.; Shiraishi, Y.; Hirai, T. Temperature-Controlled Photosensitization Properties of Benzophenone-Conjugated Thermoresponsive Copolymers. J. Phys. Chem. B 2008, 112, 13238−13244. (25) Chen, C. Y.; Tian, Y. Q.; Cheng, Y. J.; Young, A. C.; Ka, J. W.; Jen, A. K. Y. Two-Photon Absorbing Block Copolymer as a Nanocarrier for Porphyrin Energy Transfer and Singlet Oxygen Generation in Micellar Aqueous Solution. J. Am. Chem. Soc. 2007, 129, 7220−+. (26) Widawski, G.; Rawiso, M.; Francois, B. Self-Organized Honeycomb Morphology of Star-Polymer Polystyrene Films. Nature 1994, 369, 387−389. (27) Stenzel, M. H.; Barner-Kowollik, C.; Davis, T. P. Formation of Honeycomb-Structured, Porous Films via Breath Figures with Different Polymer Architectures. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2363−2375. (28) Bunz, U. H. F. Breath Figures as a Dynamic Templating Method for Polymers and Nanomaterials. Adv. Mater. 2006, 18, 973−989. (29) Escale, P.; Rubatat, L.; Billon, L.; Save, M. Recent Advances in Honeycomb-Structured Porous Polymer Films Prepared via Breath Figures. Eur. Polym. J. 2012, 48, 1001−1025.

ASSOCIATED CONTENT

S Supporting Information *

NMR spectra, SEC trace of P(S-stat-VBC) copolymer, optical microscopy images of films: porous film prepared form mixture of P(S-stat-VBC) copolymer and free RB, P(S-stat-VBC)-g-RB HC and continuous films, GC of α-terpinene oxidation products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.S.); ross.brown@univ-pau. fr (R.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank V. Pellerin and S. Blanc for their help with scanning electron microscopy and fluorescence experiments. M. Schappacher and N. Guidolin are thanked for SEC DMF analyses with UV detector. Arkema kindly donated the BlocBuilder alkoxyamine. L.P acknowledges a PhD grant from Pau district council (CDAPP).

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REFERENCES

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