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Singlet Oxygen-Induced Photodegradation of the Polymers and Dyes in Optical Sensing Materials and the Effect of Stabilizers on These Processes Barbara Enko,† Sergey M. Borisov,‡ Johannes Regensburger,§ Wolfgang Baü mler,§ Georg Gescheidt,† and Ingo Klimant*,‡ †

Institute of Physical Chemistry, Graz University of Technology, Stremayrgasse 9, Graz 8010, Austria Institute of Analytical and Food Chemistry, Graz University of Technology, Stremayrgasse 9, Graz 8010, Austria § Institute of Dermatology, University Hospital Regensburg, Franz-Josef-Strauß-Allee 11, Regensburg 93042, Germany ‡

ABSTRACT: A comprehensive study of photodegradation processes in optical sensing materials caused by photosensitized singlet oxygen in different polymers is presented. The stabilities of the polymers are accessed in the oxygen consumption measurements performed with help of optical oxygen sensors. Polystyrene and poly(phenylsilesquioxane) are found to be the most stable among the polymers investigated, whereas poly(2,6-dimethyl-p-phenylene oxide) and particularly poly(methyl methacrylate) and their derivatives show the fastest oxygen consumption. The effect of the stabilizers (singlet oxygen quenchers) on the oxygen consumption rates, the photostability of the sensitizer, and the total photon emission (TPE) by singlet oxygen is studied. 1,4-Diazabicyclo[2.2.2]octane (DABCO) was found to significantly reduce both the TPE and the oxygen consumption rates, indicating its role as a physical quencher of singlet oxygen. The addition of DABCO also significantly improved the photostability of the sensitizer. The Nalkylated derivative of DABCO and DABCO covalently grafted to the polystyrene backbone are prepared in an attempt to overcome the volatility and water solubility of the quencher. These derivatives as well as other tertiary amines investigated were found to be inefficient as stabilizing agents, and some of them even negatively affected the oxygen consumption rates. Previous work13−17 considering photodegradation processes of optical chemical sensors was mostly focused on photobleaching of dyes, and the role of polymers was not investigated in detail. However, the stability of different polymers toward singlet oxygen attack is of the utmost importance for several reasons. First, oxidation of the polymer by photosensitized singlet oxygen can significantly alter physical properties and influence mechanical stability, hydrophobicity, and solubility.10 Second, because new functional groups are introduced during the oxidation, they may participate in quenching of the luminescence of the dye via electron transfer or, e.g., promote more efficient radiativeless deactivation via vibrations. This will result in a drift of the calibration. Third, because oxygen is consumed during the oxidation of the polymers, this process is particularly important for the performance of optical oxygen sensors that proved to be indispensible analytical tools in science and technology. Evidently, an ideal optical oxygen sensor does not consume the analyte, but in reality, oxygen consumption caused by oxidation of the polymer can significantly influence the measurement and result in underestimated values.

1. INTRODUCTION Polymers are widely used as matrix materials in optical chemical sensors and perform the function of a solvent and a support for an indicator dye and a permeation-selective membrane, which helps to tune the sensitivity to the analyte of interest and to minimize potential interference from other species.1−4 Polymers undergo photodegradation processes that have been investigated in detail since the 1960s.5−9 It is now generally accepted that most photodegradation processes in polymers proceed through photoinitiated radical chain reactions that generate reactive radical species such as hydroperoxides.7 Although singlet oxygen (1O2) may not directly initiate photodegradation chain reactions in polymers, it certainly enhances various processes contributing to photodegradation.6,10,11 When applied as matrix materials in optical chemical sensors, polymers may not only react with embedded dyes acting, e.g., as oxidants12 but also be exposed to much higher 1 O2 levels because most dyes act as sensitizers of singlet oxygen, albeit with rather different efficiencies. In most optical sensors, the absorption of visible light (also used for readout) involves dyes; on the other hand, the absorption of light by polymers is virtually negligible, so that the oxidation of the latter by photosensitized 1O2 can become a predominant mechanism of photodegradation. © 2013 American Chemical Society

Received: May 10, 2013 Revised: July 16, 2013 Published: August 21, 2013 8873

dx.doi.org/10.1021/jp4046462 | J. Phys. Chem. A 2013, 117, 8873−8882

The Journal of Physical Chemistry A

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Because of the high level of importance of 1O2 in photodegradation processes, two main stabilization strategies have been proposed.18 In the first strategy, generation of 1O2 is prevented by minimizing the population of the triplet state of a sensitizer achieved either via a synthetic modification of the dye or by the addition of quenchers. Evidently, the former strategy cannot be realized for optical oxygen sensors (which rely on dynamic quenching of the phosphorescence by molecular oxygen) but is also difficult in the cases of other optical sensors where synthetic modification can significantly affect the sensing properties of the indicator. The second strategy relies on addition of stabilizers that either act as 1O2 scavengers by removing it in a chemical reaction or deactivate singlet oxygen via physical quenching (e.g., via formation and decomposition of temporary charge-transfer complexes).19 In the case of optical sensors, deactivation via physical quenching is preferred because the process is fully reversible and neither the additive nor oxygen is consumed. Literature suggests a variety of 1O2 quenchers,20−34 most of which are thought to be physical quenchers. In this work, we present new insights into the role of 1O2 in photodegradation processes of optical chemical sensors. We investigate oxygen consumption in various polymers relevant for optical sensing. Different classes of 1O2 quenchers are studied via the measurements of 1O2 luminescence, oxygen consumption, and dye stability and the best candidates identified. Improvement of the long-term stability of the state-of-the-art stabilizers is attempted, and the results are critically discussed.

added dropwise to a solution of 3 g (26.7 mmol) of 1,4diazabicyclo[2.2.2]octane in 50 mL of THF. The solution was refluxed for 24 h and cooled to room temperature, and 300 mL of water was added. The desired product was extracted with dichloromethane and an excess of sodium tetrafluoroborate. The dichloromethane fraction was washed twice with water before the solvent was removed by distillation: yield 2.16 g (88%); 1H NMR (300 MHz, CDCl3) δ 3.35 (t, 6H), 3.22 (t, 8H), 1.72 (m, 2H), 1.40−1.20 (m, 18H), 0.88 (t, 3H). 2.1.2. Preparation of Poly(styrene-co-4-chloromethylstyrene) (P1). The inhibitors in styrene and 4-chloromethylstyrene were removed by filtration through a 3 cm aluminum oxide column. The mixture of 10 g (96 mmol) of styrene and 0.77 g (5.55 mmol) of 2-chloromethylstyrene was transferred into a Schlenk flask and was deoxygenated with argon for 30 min; 0.165 g (1 mmol) of AIBN were added, and the mixture was stirred for 6 h at 85 °C. The resulting polymer was dissolved in dichloromethane, and the solution was added dropwise into a 5-fold volume of ethanol. The precipitate was collected and further purified by repeating the precipitation procedure four times. After filtration, the product was washed twice with methanol and dried: yield 9.6 g (89%); 1H NMR (300 MHz, CDCl3) δ 7.25−6.8 (m, 3H), 6.75−6.25 (m, 2H), 4.50 (s, 0.1H, CH2-Cl), 2.10−1.75 (m, 1H), 1.43 (s, 2H). 2.1.3. Preparation of the DABCO−Polystyrene Conjugate (P2). The solution of 0.5 g of P1 in 14 mL of anhydrous THF was added dropwise to the solution of 0.26 g of 1,4diazabicyclo[2.2.2]octane (DABCO) in 14 mL of anhydrous THF. The solution was stirred for 6 h at 50 °C under a nitrogen atmosphere, cooled to room temperature, and diluted with THF until the precipitate was dissolved. The polymer solution was added dropwise to the solution of 2.6 g of sodium tetrafluoroborate in the mixture of 75 mL of methanol and 75 mL of water. The resulting precipitate was redissolved in THF, and the precipitation step was repeated four times to remove the unbound DABCO. Precipitation was promoted by addition of small amounts of a saturated solution of sodium chloride. Finally, the precipitate was washed with water and dried: yield 0.28 g (52%); 1H NMR (300 MHz, CDCl3) δ 7.25−6.8 (m, 3H), 6.75−6.25 (m, 2H), 4.40 (s, 0.1H, CH2-N), 3.33 (m, 0.3H, DABCO), 3.12 (m, 0.3H, DABCO), 2.10−1.75 (m, 1H), 1.46 (s, 2H). 2.1.4. Preparation of the Oxygen-Sensing Planar Foils. The polymers were dissolved in chloroform to obtain 13 wt % solutions for all the polymers except for D4 (10 wt %) and PSS (40 wt %). The sensor dye (1.5 wt % for S1 and S2 and 0.25 wt % for S3 with respect to the polymer) was added to the “cocktail” described above. Optionally, a 1O2 quencher was added to some of the cocktails. The cocktails were knife-coated on the polyethylene terephthalate support to give, after solvent evaporation, ∼10 μm thick polymer sensor films. A 10 wt % solution of PVOH in water was knife-coated on the sensor layer to obtain an ∼7.5 μm thick oxygen barrier layer after evaporation of the solvent. 2.2. Measurements. 2.2.1. Total Photon Emission (TPE) by Singlet Oxygen in Polymer Films. The detailed experimental setup is described elsewhere.36 Briefly, a sensor film was placed diagonally in a glass cuvette. The cuvette was positioned 50 cm from the irradiation laser (Nd:YAG, 532 nm, 50 mW, 2 kHz, pulse duration of 70 ns; 90° angle between the laser beam and the surface of the polymer film) and the detector (Hamamatsu R5509-42, equipped with a 1270 nm filter; 45° angle between the surface of the polymer film and the

2. EXPERIMENTAL SECTION 2.1. Materials. Pd(II) and Pt(II) meso-tetra(pentafluorophenyl)porphyrin (S1 and S2, respectively) were purchased from Triportech (Lübeck, Germany) and Frontier Scientific (Logan, UT), respectively. Pt(II) octaethylporphyrin (S3) was from Frontier Scientific. Ethylcellulose (EC) (ethoxyl assay, 45.0−46.5%), poly(styrene-co-acrylonitrile) (PSAN; MW = 185000), polyvinyl alcohol (PVOH; MW = 31000−50000), and the solvents were purchased from Sigma-Aldrich. Polyurethane Hydrogel D4 (available under the name HydroMed) was from AdvanSource biomaterials (Wilmington, DE). Poly(2,6-dimethyl-p-phenylene oxide) (PPO; MW = 50000) was acquired from Scientific Polymer Products, Inc. (Ontario, NY). Polystyrene (PS; MW = 250000) and poly(methyl metacrylate) (PMMA; MW = 35000) were from Acros Organics. Poly(phenylsilesquioxane) (PSS) was purchased from ABCR (Karlsruhe, Germany) and Eudragit RL100 (RL100) from Evonic Industries (Essen, Germany). All solvents, dodecyl bromide, sodium tetrafluoroborate, styrene, 4-chloromethylstyrene, azobisisobutyronitrile (AIBN), and the singlet oxygen quenchers 1,4-diazabicyclo[2.2.2]octane (DABCO, Q1), hexamethylenetetraamine (HMTA, Q3), 5,6-benzo4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacos-5-ene (Kryptofix 222B, Q5), N,N,N-trioctadecylamine (Q6), 1,2,2,6,6-pentamethyl-4-piperidinol (Q7), 9,10-anthraquinone (Q8), and duroquinone (Q9) were purchased from SigmaAldrich. HMTA stearate (Q4) was prepared according to the literature procedure.35 Poly(ethylene glycol terephthalate) support (Mylar) was purchased from Goodfellow (Coraopolis, PA). Gases were purchased from Air Liquide Austria. 2.1.1. Synthesis of N-Dodecyl-1,4-diazabicyclo[2.2.2]octane Tetrafluoroborate (Q2). A solution of 1.6 mL (6.2 mmol) of dodecyl bromide in 50 mL of anhydrous THF was 8874



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Figure 1. Structures of photosensitizers (S), investigated polymers, and quenchers (Q).

3. RESULTS AND DISCUSSION 3.1. Choice of Materials. 3.1.1. Polymers. Polymers of several different classes were chosen (Figure 1) and represent common matrices in a variety of optical chemical sensors.2−4 In particular, polystryrene (PS) and poly(methyl metacrylate) (PMMA) belong to the family of the most common polymers in optical oxygen sensors,37−41 and poly(styrene-co-acrylonitrile) (PSAN) was used for the preparation of less sensitive sensors.42−44 Ethylcellulose (EC) is commonly used as a matrix in plastic carbon dioxide sensors45−47 but also in oxygen sensors.48 Both poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and poly(phenylsilesquioxane) (PSS) possess high gas permeability49,50 and are of interest for designing trace oxygen sensors. PSS has a structure and properties very similar to those of some Ormosils (organically modified silicas).51 Finally, positively charged poly(methyl metacrylate) derivative Eudragit RL100 was found to be very promising for the preparation of cell-penetrating nanoparticles.52 3.1.2. Sensitizers. The platinum(II) and palladium(II) complexes with meso-tetra(pentafluorophenyl)porphyrin [PtTFPP and PdTFPP, respectively (Figure 1)] were used as singlet oxygen sensitizers because these phosphorescent dyes possess viable UV−vis absorption53 and their triplet states are efficiently quenched by molecular oxygen, which is the reason for the popularity of these dyes as phosphorescent oxygen indicators. Importantly, electron-withdrawing fluorine atoms render these sensitizers highly stable toward oxidation by singlet oxygen, which is the reason for the excellent photostability under air-saturated conditions.39 Thus, oxygen consumption due to photooxidation of the sensitizers is virtually negligible. On the other hand, to investigate the effect of singlet oxygen quenchers on the stability of a sensitizer, we used much more photolabile platinum(II) 2,3,7,8,12,13,17,18octaethyl-21H,23H-porphyrin [PtOEP (Figure 1)]. The much

detector). The laser beam was widened to a diameter of 2 cm by a lens. Photons emitted by singlet oxygen were detected by time-correlated single-photon counting (time resolution of 16 ns). 2.2.2. Investigation of Oxygen Consumption in Polymer Films. The measurements were performed at 25 °C on a dualphase lock-in amplifier (DSP 830, Stanford Research Inc.). The sensing foils containing an oxygen-sensitive layer and a barrier PVOH layer were excited with the light from a 525 nm LED (Roitner Lasertechnik, Vienna, Austria) filtered through a HQ515/30 long-pass filter (Analysentechnik, Tü bingen, Germany). The emission was detected by a photomultiplier module (H5701-02, Hamamatsu) after passing through an RG 630 long-pass filter (Schott, Mainz, Germany). A bifurcated fiber bundle was used to guide the excitation light to the sensing film and to guide the emission light back to the photodetector. The luminescence decay times (τ) were calculated according to the equation τ = tan Φ/2πf, where Φ is the luminescence phase shift and f is the modulation frequency (5 kHz). Prior to the oxygen consumption experiments, the oxygen-sensing films were calibrated with nitrogen/compressed air mixtures obtained using an MKS (Andover, MD) gas mixing device. 2.2.3. Investigation of the Photostability of the Dyes in Polymeric Films. The sensor films were fixed on the inner wall of a glass cuvette by a transparent silicone gel. The foil was irradiated with the light of a high-power 505 nm LED (operated at 3 W, photon flux of ∼400 μmol of photons s−1 m−2, estimated by a Li-250A light meter from Li-COR). The concentration of the dye in the film was accessed via absorption measurements of a Cary 50 UV−vis spectrophotometer (Varian). 8875



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Figure 2. TPE of singlet oxygen in investigated polymers containing sensitizer S1 and singlet oxygen quenchers Q1 and Q7−Q9 in a 1:200 S1:Q molar ratio for Q1 and Q7 and a 1:7 S1:Q molar ratio for Q8 and Q9. Note that Q8 and Q9 could not be used at higher concentrations because of their limited solubility.

stronger tendency of the dye to be photooxidized by 1O2 ensures relatively fast and therefore easily measurable photobleaching. 3.1.3. Singlet Oxygen Quenchers. The literature suggests a large variety of compounds as efficient singlet oxygen quenchers, most of which have been used for decades.20 In the work presented here, the main selection criteria were the reported effectiveness20 and processability of the 1O2 quenchers in optical chemical sensors rather than other criteria such as low cost or availability. Quenchers of several important classes were chosen for 1O2 total photon emission (TPE) experiments (Figure 1): 1,4-diazabicyclo(2.2.2)octane (DABCO, Q1), 1,2,2,6,6-pentamethyl-4-piperidinol (HAL, Q7), duroquinone (Q8), and 9,10-anthraquinone (Q9). Because the latter two quenchers proved to be inefficient, further experiments were conducted with Q1 and Q7. Because DABCO proved to be the most promising in both TPE experiments and oxygen consumption tests, other 1O2 quenchers based on tertiary amines were also investigated: hexamethylenetetraamine (HMTA, Q3), 5,6-benzo-4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacos-5-ene (Kryptofix 222B, Q5), and N,N,N-trioctadecylamine (Q6). The lipophilic derivatives of DABCO and HMTA, N-dodecyl-DABCO tetrafluoroborate (Q2) and HMTA stearate (Q4), respectively, were also investigated. 3.2. Total Photon Emission by 1O2 in Polymers. O2 1 ( Δg) emits at ∼1270 nm, and both the luminescence intensity and the decay time (τ) have been accessible in principle via the single-photon counting techniques since the early 1980s.54 These methods have constantly been improved for measurements in solution.55,56 Although some data for measuring the decay times of O2 (1Δg) in polymers are available,57−59 the decay time measurement remains rather challenging60 because of the low quantum yields, background (e.g., caused by photosensitizers emitting in the IR), microheterogeneity of the environment in polymers, etc. Moreover, the luminescence decay time of most sensitizers (which have lifetimes of microseconds) will affect the measured decay of singlet oxygen, and thus, sophisticated algorithms are necessary. Therefore, the

sum of the photons emitted by singlet oxygen (TPE) was used to estimate the efficiency of singlet oxygen quenchers. Figure 2 shows the TPE values for four investigated polymers with three distinct groups of quenchers, including DABCO (Q1), HAL (Q7), and quinones Q8 and Q9. Evidently, the quinones do not affect the TPE and are, therefore, not efficient as singlet oxygen quenchers. Notably, their solubility in the polymers was also a limiting factor. On the other hand, the addition of Q1 and Q7 significantly affects the TPE values that almost reach the noise level, albeit at rather high quencher concentrations. In fact, the TPE decreases by at least 10-fold compared to that of the polymers without quenchers. This indicates that virtually no emission from O2 (1Δg) is detected and both Q1 and Q7 are rather efficient quenchers of singlet oxygen. The effect of the amount of quencher (DABCO) on TPE was also investigated (Figure 3). Evidently, TPE is highest in the absence of DABCO and gradually decreases when the concentration of the quencher is increased. The concentrations of DABCO at a sensitizer:quencher molar ratio of ≥1:10 (i.e.,

Figure 3. TPE of singlet oxygen in the polymers containing DABCO (Q1) in different molar ratios with respect to S1. 8876



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concentration. This is achieved via measurements of the luminescence decay times of the indicator. In the ideal case, the singlet oxygen produced will nonradiatively or radiatively deactivate to the ground triplet state. The theoretical lifetime of 1O2 in the polymers varies from 10 μs to 40 ms.59 At low light intensities (such as in this experiment), the concentration of the excited sensitizer molecules is much lower than the concentration of 3O2 in the polymer, which is close to air saturation (210 hPa pO2). Thus, in the ideal case with no oxygen consumption, the pO2 remains constant at this level. However, in the real system, part of the 1O2 can be consumed in the reaction with the polymer or the sensitizer. Because the diffusion of oxygen from outside of the material is very slow, such reactions will result in a decrease in pO2 that can be detected with help of the oxygen indicator. Figure 4 demonstrates that oxygen consumption is indeed easily measurable in the case of polystyrene and poly(2,6dimethyl-1,4-phenylene oxide). The rates of oxygen consumption are rather different for these polymers. In fact, in the case of PS, ∼90% of the oxygen is consumed after 40 min but the process is much faster in PPO, which takes 1 min to achieve the same depletion. As one can see, the polymer becomes completely anoxic after only 3 min. Evidently, electron-rich PPO is more prone to oxidation by singlet oxygen than PS. This results correlate well with the observation of Kelleher and Gesner, who compared the stability of polystyrene, poly(2,6-dimethyl-1,4-phenylene oxide), and their blends toward thermal oxidation in air.72 Oxygen uptake was significantly faster (∼50-fold) for PPO than for PS, and the blends occupied the intermediate position. Thermal oxidation and photooxidation of PPO are believed to affect the methyl groups and result in cross-linking of the polymer.72 In the TPE experiments, DABCO Q1 and HAL Q7 were identified as efficient quenchers of singlet oxygen. As shown in Figure 4, the effect of these substances on oxygen consumption is opposite. DABCO reduces the oxygen consumption rates, and the reduction correlates well with the concentration of the quencher. This indicates that DABCO predominantly acts as a physical quencher and helps to accelerate the deactivation of 1 O2. On the other hand, HAL is likely to chemically react with singlet oxygen because the rate of oxygen consumption is greatly enhanced. In fact, the polystyrene films become completely anaerobic in the presence of HAL after only 20 s. HAL appears to be poorly suitable for application in optical sensors because it is consumed during the reaction with singlet oxygen and the reaction products may alter the polarity of the environment or the gas permeability of the polymer or act as quenchers for the luminescent compounds. In the case of optical oxygen sensors, the addition of HAL results in underestimated pO2 values caused by extremely fast oxygen consumption. Figure 5 shows the oxygen consumption profiles obtained for various polymers in the absence and presence of DABCO (1:10 S2:DABCO molar ratio, which corresponds to ∼1.5 wt % quencher in the polymer). In the absence of the quencher, oxygen consumption is very fast in poly(2,6-dimethyl-1,4phenylene oxide), poly(methyl metacrylate), and its derivative RL-100, ethyl cellulose, polyurethane hydrogel D4, and the copolymer of polystryrene and acrylonitrile (PSAN). Note that particularly in the case of ethyl cellulose and Rl-100 some oxygen consumption is observed already at the start of the experiment (compared to the barrier-free layers that show 100% air saturation). Although the form of the curve most

1.5 wt % DABCO with respect to the polymer) can be considered as rather efficient for all the polymers. The TPE values are rather close to the background level for ratios of ≥1:20. It should be mentioned here that the quenchers of singlet oxygen may also act as efficient quenchers for the sensitizer and the effect on the TPE can be similar in both cases. However, it is evidently not the case for DABCO, which was not found to significantly affect the phosphorescence decay times of the sensitizers. In fact, the decay times were 890 and 680 μs under nitrogen at room temperature in the absence of DABCO and at a molar ratio of 1:10 (polystyrene:DABCO). 3.3. Oxygen Consumption in Polymers. A broad variety of methods are employed to analyze and describe photodegradation processes in polymers,7,61,62 one of which is the measurement of oxygen consumption (also termed oxygen uptake) by polymers during degradation.63 Oxygen uptake can be measured in various ways64 such as measuring volume change,65 mass spectroscopy,66,67 thermal treatment, measuring oxidation product concentrations,68 and ultraviolet microscopy.69 Optical chemical sensing of oxygen represents a straightforward way to measure oxygen consumption in polymers because phosphorescent oxygen indicators are known to be efficient sensitizers of singlet oxygen. To the best of our knowledge, this method has not yet been applied in the investigation of oxygen consumption. The cross section of the sensing material for investigation of oxygen consumption is shown in Figure 4. A transparent

Figure 4. Cross section of the material for the investigation of oxygen consumption (top) and oxygen consumption profiles (bottom) in polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide), containing S2 without any quencher and with quencher Q7 or DABCO Q1 at different molar ratios.

poly(ethylene terephthalate) support is covered with an ∼10 μm thick layer containing a photosensitizer dye (S2) dissolved in an investigated polymer. Optionally, the polymer also contains a 1O2 quencher (Q). The polymer layer is covered with a layer of dry poly(vinyl alcohol) (PVOH) that is virtually oxygen-impermeable because of the extremely low gas permeability of the polymer.70,71 The PET support also acts as a gas barrier because of its large thickness (120 μm) and the rather low diffusion coefficients of oxygen in this polymer. Importantly, the dye functions both as a 1O2 photosensitizer and as an indicator allowing determination of oxygen 8877



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tration of oxygen and the efficiency of luminescence quenching) vary significantly. 3.4. Stability of the Singlet Oxygen Sensitizer in Polymers. As mentioned above, the photosensitized singlet oxygen is often responsible for fast bleaching of dyes in polymers. From a practical point of view, dye photostability represents an extremely important parameter, and its enhancement will prolong the lifetime of optical materials such as sensors. Evidently, investigation of the photostability of dyes represents yet another way to establish the optimal stabilization strategy. Sensitizers S1 and S2 are known to be highly photostable also in the presence of oxygen and are therefore poorly suited for this purpose. Therefore, we used the platinum(II) complex with octaethylporphyrin (S3), which is an excellent 1O2 sensitizer but is much less stable toward oxidation by singlet oxygen than the fluorinated porphyrins. Absorption spectroscopy is a straightforward method for accessing the dye concentration because the amount of dye is directly proportional to absorbance. As an example, Figure 6a

Figure 5. Oxygen consumption profiles for investigated polymers in the absence and presence of DABCO (1:10 S2:DABCO molar ratio). The dashed parts of the curves represent extrapolation in those cases where some oxygen consumption is already observed at the start of the experiment and the experimental data are missing.

likely indicates biphasic degradation (initial fast oxidation of labile groups or impurities and significantly slower consumption afterward), some consumption may also be caused by singlet oxygen production under ambient light conditions during the preparation of the experiment. It is important to note that PMMA and EC are common matrices in optical oxygen sensors,40,48,73,74 but our data indicate very fast oxidation by singlet oxygen that can result in errors in pO2 measurements and eventually change the properties of the polymer. For example, PMMA is known to be prone to photooxidation, which results in accumulation of the carbonyl groups in the polymer backbone,75 and the products may be similar during oxidation by singlet oxygen. On the other hand, oxygen consumption is found to be much slower in polystyrene, which appears to be relatively robust to oxidation by singlet oxygen. Poly(phenylsilesquioxane) shows significant oxygen consumption only in the first minutes of the experiment (which can be attributed to the oxidation of some unknown impurities) but is rather robust after this. As shown in Figure 5, the addition of DABCO significantly reduces the rates of oxygen consumption for all the polymers except PMMA and Rl-100, which still exhibit very high rates. It should be mentioned here that the data presented above allow only a rather rough comparison between various polymers because their oxygen permeabilities (and consequently the concen-

Figure 6. Photodegradation profiles for S3 in polystyrene without and with DABCO (1:10 and 1:5 S3:DABCO molar ratios) (a) and the amount of bleached sensitizer in investigated polymers after irradiation for 30 min in the absence (gray) and presence (black) of DABCO (b).

shows photodegradation profiles for S3 in polystyrene in the absence and presence of DABCO. As shown, DABCO significantly reduces the photobleaching rate and improves the photostability of the dye. Figure 6b compares the absorption changes in the absorption maximum of S3 obtained after continuous irradiation of the investigated materials for 30 min. DABCO significantly improves the photostability of the sensitizer in PS, EC, PSAN, and PSS and slightly improves the photostability in D4. However, the photobleaching rates in PMMA and its derivative Rl-100 remain virtually unaltered. Interestingly, the photostability of the dye in some polymers in the absence and presence of DABCO correlates rather well with the rates of oxygen consumption. Indeed, the oxygen consumption rates are the fastest in PMMA and Rl-100 and are barely affected by DABCO. On the other hand, the photostability of the dye in ethylcellulose appears to be better 8878



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with singlet oxygen. Unfortunately, DABCO appears to completely loose its quenching properties after the alkylation because the addition of Q2 does not affect the oxygen consumption rates. Unexpectedly, oxygen consumption rates in the polystyrene with covalently bound DABCO are much faster than in polystyrene and in the copolymer of styrene and 4chloromethylstyrene, which are similar. Thus, covalently bound DABCO appears not only to lose its quenching activity but also to react chemically with singlet oxygen. Figure 8 compares the amounts of the bleached sensitizer (S3) in ethylcellulose and polystyrene. As can be seen, only DABCO improves the photostability of the dye, and the alkylated derivative Q2 is not efficient. In good agreement with the oxygen consumption experiments, Q3 slightly improves the photostability of the sensitizer in polystyrene but has virtually no effect in ethylcellulose. Amines Q5 and Q6 significantly decrease the photostability of the sensitizer, which is in good agreement with the trend obtained in the oxygen consumption experiments. Finally, the photostabilities of the dye in polystyrene and in polystyrene with covalently coupled DABCO are very similar.

than in polystyrene, but the opposite situation is observed for the oxygen consumption rates. Evidently, the polarity of the environment may affect the photostability of the dye but also the activity of DABCO that can result in more or less efficient deactivation of singlet oxygen. 3.5. Investigation of Tertiary Amines as Singlet Oxygen Quenchers. The experiments described above have demonstrated the potential of DABCO as an efficient quencher of singlet oxygen. Both oxidation of most polymers and the degradation of the sensitizers are retarded in the presence of DABCO that, therefore, helps to prolong the lifetime of the materials. DABCO is a bridged tertiary diamine featuring a unique structure, and the 1O2 quenching mechanism is believed to be of a charge-transfer nature. First proposed by Ouannes and Wilson,29 this mechanism is generally accepted in numerous publications about DABCO as a 1O2 quencher.24,26,31,76 Because the proposed charge-transfer complex between 1O2 and the amino group involves a geometrically exposed electron pair located on the nitrogen atom, other amines and derivatives of DABCO are also of much interest. Therefore, the effect of several other representatives on the oxygen consumption rates and the sensitizer photostability was investigated. HMTA Q3, Kryptofix Q5, and N,N,N-trioctylamine Q6 (Figure 1) were chosen. Because the boiling point of DABCO is rather low (174 °C), slow evaporation of the quencher from the polymeric materials is likely, which can negatively effect prolonged measurements at room temperature. Leaching of DABCO in the aqueous environment is also possible because of its high solubility in water. Keeping practical applications in mind, we also investigated nonvolatile derivatives of DABCO and HMTA, namely, N-dodecylDABCO Q2 and HMTA stearate Q4 (Figure 1). In another approach, DABCO was covalently coupled to the copolymer of styrene and 4-chloromethylstyrene by alkylation of one of the DABCO nitrogen atoms. This completely eliminated the tendency of the quencher to undergo evaporation and leaching (Figure 1). Figure 7 shows the oxygen consumption curves for various amines in ethylcellulose and polystyrene. Interestingly, addition of Q3 to polystyrene reduces the rate of oxygen consumption, but it has virtually no effect in ethylcellulose. Among other amines, Q5 and Q6 greatly enhance oxygen consumption, which indicates that these compounds may chemically react

4. CONCLUSIONS The work demonstrated a crucial role of photosensitized singlet oxygen in photodegradation of sensing materials (polymers and dyes). In particular, stabilities of several polymers relevant for application in optical chemosensors were accessed via oxygen consumption measurements performed with help of optical oxygen sensors. Polystyrene and poly(phenylsilesquioxane) were found to have the lowest rates of consumption (and consequently the highest stability) in the presence of singlet oxygen; poly(methyl methacrylate)s have the lowest stability. Poly(2,6-dimethyl-1,4-phenylene oxide), ethylcellulose, and polyurethane hydrogel D4 occupy the intermediate position. The efficiency of several stabilizers that can act as quenchers of singlet oxygen is accessed via total photon emission measurements. 1,4-Diazabicyclo[2.2.2]octane (DABCO) and 1,2,2,6,6-pentamethyl-4-piperidinol were found to be the most efficient quenchers and reduced the TPE values almost to the noise level. The quinones (anthraquinone and duroquinone) were not efficient as quenchers. The rate of oxygen consumption dramatically increased in presence of 1,2,2,6,6pentamethyl-4-piperidinol, indicating that it chemically reacts with singlet oxygen. On the other hand, DABCO significantly decreased the rate of oxygen consumption in most polymers (PMMA and its derivative Rl-100 being exceptions), indicating physical quenching. The addition of DABCO also significantly improved the photostability of the sensitizer. In an attempt to overcome the volatility and water solubility of DABCO, we prepared and investigated the performance of the N-alkylated derivative of DABCO and the polystyrene-containing covalently grafted quencher. Unfortunately, these derivatives did not show any positive effect on the oxygen consumption rates or the photostability of the photosensitizer. Other nonvolatile tertiary amines, Kryptofix and N,N,N-trioctadecylamine, were found to negatively affect the oxygen consumption rates. This shows that the oxidation potential, the electronic structure, and the basicity of the amines are decisive for their 1O2 quenching ability. Further work addressing these factors in a systematic way will be of value. We can conclude that DABCO is highly efficient for stabilization of sensing materials with respect to the stability of the polymers and the dyes toward oxidation by photo-

Figure 7. Oxygen consumption in ethylcellulose and polystyrene in the absence and presence of different quenchers (1:10 S2:Q molar ratio). Note that the solubility of Q4 and Q6 in polystyrene is poor and homogeneous coatings could not be obtained. 8879



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Figure 8. Photostability of S3 in ethylcellulose and polystyrene in the absence and presence of different quenchers (1:10 S3:Q molar ratio). (12) Egerton, G. S. Action of light on dyes in polymer materials. Br. Polym. J. 1971, 3, 63−67. (13) Carraway, E. R.; Demas, J. N.; DeGraff, B. A.; Bacon, J. R. Photophysics and photochemistry of oxygen sensors based on luminescent transition-metal complexes. Anal. Chem. 1991, 63, 337− 342. (14) Fuller, Z. J.; Bare, W. D.; Kneas, K. a.; Xu, W. Y.; Demas, J. N.; DeGraff, B. A. Photostability of luminescent ruthenium(II) complexes in polymers and in solution. Anal. Chem. 2003, 75, 2670−2677. (15) Hartmann, P. Photobleaching of a ruthenium complex in polymers used for oxygen optodes and its inhibition by singlet oxygen quenchers. Sens. Actuators, B 1998, 51, 196−202. (16) Hartmann, P. Photochemically induced energy-transfer effects on the decay times of ruthenium complexes in polymers. Anal. Chem. 2000, 72, 2828−2834. (17) Oige, K.; Avarmaa, T.; Suisalu, A.; Jaaniso, R. Effect of long-term aging on oxygen sensitivity of luminescent Pd-tetraphenylporphyrin/ PMMA films. Sens. Actuators, B 2005, 106, 424−430. (18) Wiles, D. M. Stabilization of Polymers Against Singlet Oxygen. In Singlet Oxygen. Reactions with Organic Compounds & Polymers; Ranby, B., Rabek, J. F., Eds.; Wiley: New York, 1978; pp 320−327. (19) Young, R. H.; Brewer, D. R. The Mechanism of Quenching Singlet Oxygen. In Singlet Oxygen. Reactions with Organic Compounds & Polymers; Ranby, B., Rabek, J. F., Eds.; Wiley: New York, 1978; pp 36−60. (20) Bellus, D. Qunechers of Singlet Oxygen: A Critical Review. In Singlet Oxygen. Reactions with Organic Compounds & Polymers; Ranby, B., Rabek, J. F., Eds.; Wiley: New York, 1978; pp 61−110. (21) Ohara, K.; Kikuchi, K.; Origuchi, T.; Nagaoka, S. Singlet oxygen quenching by trolox C in aqueous micelle solutions. J. Photochem. Photobiol., B 2009, 97, 132−137. (22) He, S.; Jiang, L.; Wu, B.; Pan, Y.; Sun, C. Pallidol, a resveratrol dimer from red wine, is a selective singlet oxygen quencher. Biochem. Biophys. Res. Commun. 2009, 379, 283−287. (23) Ballardini, R.; Beggiato, G.; Bortolus, P.; Faucitano, A.; Buttafava, A.; Gratani, F. Quenching of Singlet Oxygen by Hindered Amine Light Stabilisers. A Flash Photolytic Study. Polym. Degrad. Stab. 1984, 7, 41−53. (24) Monroe, B. M. Quenching of singlet oxygen by aliphatic amines. J. Phys. Chem. 1977, 81, 1861−1864. (25) Lancaster, J. R.; Martí, A. A.; López-Gejo, J.; Jockusch, S.; O’Connor, N.; Turro, N. J. Nonradiative deactivation of singlet oxygen (1O2) by cubane and its derivatives. Org. Lett. 2008, 10, 5509−5512. (26) Das, K. C.; Misra, H. P. Hydroxyl radical scavenging and singlet oxygen quenching properties of polyamines. Mol. Cell. Biochem. 2004, 262, 127−133. (27) Nagaoka, S.; Fujii, A.; Hino, M.; Takemoto, M.; Yasuda, M.; Mishima, M.; Ohara, K.; Masumoto, A.; Uno, H.; Nagashima, U. UV protection and singlet oxygen quenching activity of aloesaponarin I. J. Phys. Chem. B 2007, 111, 13116−13123. (28) Jung, M. Y.; Min, D. B. ESR study of the singlet oxygen quenching and protective activity of Trolox on the photodecomposition of riboflavin and lumiflavin in aqueous buffer solutions. J. Food Sci. 2009, 74, C449−C455.

sensitized singlet oxygen. It benefits from low cost and good processability in polymeric films and can be a good choice in many cases. However, high volatility and water solubility may be serious drawbacks for prolonged measurements at elevated temperatures or in an aqueous environment. Therefore, preparation of a stabilizer that overcomes these shortcomings remains very important.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +43 316 87332500. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Ph.D. programme “Doktorandinnenkolleg FreChe Materie” (http:// frechematerie.tugraz.at/sites/frechematerie/index.php). Gerda Winterleiter (Institute of Analytical Chemistry and Food Chemistry, Graz University of Technology) is gratefully acknowledged for technical support in the synthesis of modified polymers.

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REFERENCES

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