Langmuir 2006, 22, 375-384
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Radical-Induced Generation of Small Silver Particles in SPEEK/PVA Polymer Films and Solutions: UV-Vis, EPR, and FT-IR Studies A. S. Korchev,† T. Konovalova,‡ V. Cammarata,*,† L. Kispert,‡ L. Slaten,§ and G. Mills*,† Department of Chemistry and Biochemistry, Department of Consumer Affairs, Auburn UniVersity, Auburn, Alabama 36849, and Department of Chemistry, UniVersity of Alabama, Tuscaloosa, Alabama, 35487 ReceiVed May 9, 2005. In Final Form: October 18, 2005 The present study is centered on the processes involved in the photochemical generation of nanometer-sized Ag particles via illumination at 350 nm of aqueous solutions and cross linked films containing sulfonated poly(ether ether ketone) and poly(vinyl alcohol). Optical and electron paramagnetic resonance experiments, including electron nuclear double resonance data, proved conclusively that the photogenerated chromophore exhibiting a band with λmax ) 565 nm is an R-hydroxy aromatic (ketyl) radical of the polymeric ketone. This reducing species was produced by illumination of either solutions or films, but the radical lifetime extended from minutes in the fluid phase to hours in the solid. Direct evidence is presented that this long-lived chromophore reduces Ag(I), Cu(II), and Au(III) ions in solution. A rate constant of k ) 1.4 × 103 M-1 s-1 was obtained for the reduction of Ag+ by the ketyl radical from the postirradiation formation of Ag crystallites. FTIR results confirmed that the photoprocess yielding polymeric ketyl radicals involves a reaction between the macromolecules. The photochemical oxidation of the polymeric alcohol, as well as the formation of light-absorbing macromolecular products and polyols, indicates that the sulfonated polyketone experienced transformations similar to those encountered during illumination of the benzophenone/2-propanol system.
Introduction Methods for the preparation of materials with structural features in the nanometer range have evolved significantly in recent years, starting from simple chemical transformations of precursors in solution to more complex strategies.1 Light has been frequently used as an energy source for initiation of such transformations, including generation of nanometer-sized metal crystallites with different morphologies,2,3 alteration of their sizes and shapes,4,5 and imaging.6 Utilization of light with λ > 300 nm is particularly useful for preparing metallic particles within dielectric matrixes that are transparent at these wavelengths. For instance, 350 nm irradiation has been utilized to form small Ag crystallites inside polymer films and fibers composed of poly(vinyl alcohol), PVA, and poly(acrylic acid), PAA,7 as well as to metallize polymeric gels or porous Vycor glasses with Au.8 An obvious limitation * To whom correspondence should be addressed. E-mail: cammavi@ auburn.edu (V.C.);
[email protected] (G.M.). † Department of Chemistry and Biochemistry, Auburn University. § Department of Consumer Affairs, Auburn University. ‡ University of Alabama. (1) For example, see Handbook of Nanostructured Materials and Nanotechnology Nalwa, H. S., Ed.; Academic Press: New York, 2000; Vol. 1-5. (2) (a) Mafune, F.; Kohno, J.-Y.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2002, 106, 7575. (b) Kameo, A.; Suzuki, A.; Torigoe, K.; Esumi, K. J. Colloid Interface Sci. 2001, 241, 289. (c) Han, M. Y.; Quek, C. H. Langmuir 2000, 16, 362. (3) (a) Akamatsu, K.; Ikeda, S.; Nawafune, H. Langmuir 2003, 19, 10366. (b) Sau, T. K.; Pal, A.; Jana, N. R.; Wang, Z. L.; Pal, T. J. Nanoparticle Res. 2001, 3, 257. (c) Rogash, A. L.; Shevchenko, G. P.; Afanas’eva, Z. M.; Sviridov, V. V. J. Phys. Chem. 1997, 101, 8129. (d) Yonezawa, Y.; Konishi, Y.; Hada, H.; Yamamoto, K.; Ishida, H. Thin Solid Films 1992, 218, 109. (4) (a) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (b) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 6152. (5) (a) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226. (b) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701. (6) (a) Bokhonov, B. B.; Burleva, L. P.; Whitcomb, D. R.; Usanov, Yu. E. J. Imaging Sci. Tech. 2001, 45, 259. (b) Sahyun, M. R. V. J. Imaging Sci. Tech. 1998, 42, 23. (c) Belloni, J. In Homogeneous Photocatalysis; Chanon, M., Ed.; John Wiley: New York, 1997; p 169. (7) (a) Gaddy, G. A.; McLain, J. L.; Korchev, A. S.; Slaten, B. L.; Mills, G.; J. Phys. Chem. B 2004, 108, 14858. (b) Gaddy, G. A.; Korchev, A. S.; McLain, J. L.; Slaten, B. L.; Steigerwalt, E. S.; Mills, G. J. Phys. Chem. B 2004, 108, 14850. (c) Gaddy, G. A.; McLain, J. L.; Steigerwalt, E. S.; Broughton, R.; Slaten, B. L.; Mills, G. J. Cluster Sci. 2001, 12, 457.
of this procedure is a requirement of metal precursors which can be easily photoreduced in the presence of electron donors. Such a requirement has recently been circumvented by means of a new synthetic procedure that employs 350 nm photons in conjunction with sulfonated poly(ether ether ketone), SPEEK, as a sensitizer and PVA as a reductant.9 This method has enabled generation of metal crystallites within SPEEK/PVA films, as well as in air-free aqueous solutions containing both polymers.10 The available data demonstrate that illumination induces a reaction between the polymers to yield long-lived macromolecular benzophenone ketyl (BPK) radicals, exhibiting an optical signal centered at 565 nm. Reduction of silver ions by polymeric BPK radicals produces small Ag crystallites within dry Ag+-doped films made from SPEEK/PVA blends.9 Optical measurements revealed a similar mechanism for the photogeneration of metal particles in solutions systems, where the crystallites are stabilized by the macromolecules from aggregation processes.10 Further investigations on the light-induced reaction between SPEEK and PVA, as well as on the subsequent attack on Ag+ ions by polymeric BPK radicals in aqueous solutions and solid films, were carried out employing electron paramagnetic resonance (EPR)/electron nuclear double resonance (ENDOR), UV-vis, and FTIR techniques. Results from these studies are presented here and are consistent with the notion that Ag crystallites form via reactions involving polymeric ketyl radicals and silver ions. Experimental Section SPEEK was synthesized by sulfonation of poly(ether ether ketone), PEEK, (average molar mass ) 4.5 × 104 g mol-1, Victrex USA, Inc.) according to standard procedures11,12 and was used as the sodium (8) (a) Malone, K.; Weaver, S.; Taylor, D.; Cheng, H.; Sarathy, K. P.; Mills, G. J. Phys. Chem. B 2002, 106, 7422. (b) Weaver, S.; Taylor, D.; Gale, W.; Mills, G. Langmuir 1996, 12, 4618. (c) Mu, R.; Ueda, A.; Wu, M. H.; Henderson, D. O.; Malone, K.; Mills G.; Meldrum, A. Proc. Electrochem. Soc. 1999, 98-19, 439. (9) Korchev, A. S.; Bozak, M. J.; Slaten, B. L.; Mills, G. J. Am. Chem. Soc. 2004, 126, 10. (10) Korchev, A. S.; Shulyak, T. S.; Slaten, B. L.; Gale, W. F.; Mills, G., J. Phys. Chem. B 2005, 109, 7733. (11) Bailly, C.; Williams, D. J.; Karasz, F. E.; MacKnight, W. Polymer 1987, 28, 1009.
10.1021/la0512345 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/25/2005
376 Langmuir, Vol. 22, No. 1, 2006 salt. PVA (99+% hydrolyzed, average molar mass ) 8.9-9.8 × 104 g mol-1), glutaraldehyde (GA, 25 wt% solution), and AgClO4‚H2O (Aldrich), HAuCl4‚3H2O (Alfa), CuSO4‚5H2O, HCl, and NaOH (Fisher) were used as received. All solutions were made from distilled water purified with a resin deionizer (U. S. Filter Service); the glassware used for generation of metal particles was cleaned with aqua regia. The SPEEK/PVA solutions employed for photochemical experiments in the liquid phase were prepared as described before.10 Films of SPEEK/PVA blends were made by first dissolving separately each of the polymers in water at 70 °C. Combination of the hot solutions produced a homogeneous mixture containing 5.2 wt% PVA and 2.2 wt% SPEEK that was stirred at 70 °C for 30 min and cooled to room temperature. After 3 mL of a 3 wt% GA solution and 3 mL of 0.5 M HCl were added, the reaction mixture was vigorously stirred for 5 min and then employed to cast films on glass plates using a Gardner knife. The films were cross linked in an oven at 55 °C for 3.5 h and cut into 5 cm × 2.5 cm strips. Unreacted GA cross linker and HCl were removed by soaking the strips in methanol for 16 h and washing extensively with CH3OH/H2O (90:10), as well as with deionized water. Neglecting the small amount of GA incorporated, the resulting insoluble films (∼50 µm thick) consisted of 30:70 SPEEK/PVA wt%. The films were stored at -77 °C prior to doping with silver ions, which was carried out by immersing them in a 0.01 M aqueous AgClO4 solution for at least 15 h. After doping, the films were washed with deionized water, mounted between two glass slides, and dried under vacuum for 3-4 h. Irradiation of films and solutions was carried out in a circular Rayonet PR-100 photochemical reactor using PR-3500A lamps. Light intensity (I0) values were obtained using the ferrioxalate actinometer.13 Illuminations of solutions free of air were performed under constant stirring in sealed Milton Roy no. 33-17-80 optical test tubes (path length ) 1 cm) modified as described before,10 whereas the films were photolyzed under air while mounted on quartz plates held together by metal frames.9 Optical spectra were acquired by means of a Shimadzu PC2501 spectrophotometer. Kinetic data derived from absorption measurements were typified by a maximum deviation of 30%. Transmission electron microscopy (TEM) of films was performed with a Zeiss EM10 microscope operating at 60 kV. For this purpose, irradiated cross-linked films containing silver were microtomed and collected on Cu-mesh grids. EPR measurements were carried out with an X-band (9.5 GHz) Varian E-12 EPR spectrometer equipped with a rectangular cavity. The magnetic field was measured with a NMR gaussmeter ER 035M, and microwave frequencies were determined with an HP 5245M frequency counter. Irradiation of polymer films was performed directly in EPR quartz tubes, previously flushed with argon for 20 min. Measurements of degassed polymer solutions were carried out at room temperature in thin quartz capillaries that were sealed under nitrogen. ENDOR spectra of irradiated SPEEK/PVA films were recorded at 120 K with a Bruker ESP 300-10/7 EPR instrument in conjunction with a DICE ENDOR ESP 350-U accessory. Infrared spectra of the films were acquired at a resolution of 4 cm-1 and averaging 500 scans using a Mattson RS-1 FTIR spectrometer equipped with a high D* MCT-A detector and a nitrogen-purged sample chamber.
Results and Discussion UV-Vis Measurements. Dilute aqueous solutions of SPEEK exhibit an absorption maximum at 300 nm that can be attributed to the benzophenone (BP) groups present in the polymer. This assignment is consistent with the typical optical signals of phenyl ketones centered between 280 and 300 nm resulting from singlet n f π* transitions of the carbonyl group.14 At the high SPEEK concentration (0.018 M) employed in the present photochemical (12) Bishop, M.; Karasz, F. E.; Russo, P.; Langley, K. Macromolecules 1985, 18, 86. (13) Murov, S. L. Handbook of Photochemistry; Marcel Dekker: New York, 1973; p 119.
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studies, the long absorption tail of the polyketone extending beyond 350 nm allowed light of this wavelength to induce n f π* transitions. Reaction 1 represents the proposed excitation
process of SPEEK carbonyl groups with 350 nm photons, where the structure on the left side corresponds to an SPEEK macromolecule and the structure on the right is a representation of the triplet excited state. Intersystem crossing from the initial singlet to triplet (n, π*) states is known to occur efficiently for BP and derivatives of this ketone,14 yielding a triplet state that is highly reactive toward hydrogen atom abstraction from different types of compounds, for example, alcohols. Photolysis studies of benzophenone incorporated in films of PVA and poly(methyl methacrylate), PMMA, have shown that these polymers can also serve as donors of H atoms.15 The abstraction process yields a transient BPK radical, (Ph)2C•OH (Ph ) phenyl), that displays an optical signal centered at 545 nm. By analogy to phenyl ketones, the triplet state of SPEEK benzophenone groups was anticipated to react with PVA through an H-atom abstraction process shown in reaction 2,
[-O-C6H4-CO*-C6H4-]z + [-CH2-CH(OH)-]y f [-O-C6H4-C•(OH)-C6H4-]z + [-CH2-C•(OH)-]y (2) where the first structure on the left side is a partial representation of the SPEEK excited state and the second structure represents the PVA macromolecule. The products of the reaction are polymeric BPK radicals of SPEEK (from now on referred to as SPEEK•) and the typical R-hydroxy radical of PVA formed during H-atom abstraction from the polyol.16 Irradiation of air-free solutions containing SPEEK/PVA mixtures results in absorption centered at 565 nm (Figure 1, curve b). These changes are accompanied by development of a pink color that decays slowly in the dark but is regenerated upon renewed exposure to 350 nm light. The 565 nm signal has been detected in previous studies involving illumination of SPEEK/PVA films or aqueous solutions of the polymers.9,10 This absorption was attributed to SPEEK• since photolysis of benzophenones containing alkyl sulfonate groups in the para position yields BPK radicals exhibiting a similar signal with λmax ) 560 nm.17 A different outcome was observed upon illuminating dry SPEEK/PVA films doped with either Ag+ or Cu2+ ions in the presence of air, and also of degassed aqueous solutions containing the polymers and AgClO4, HAuCl4, or CuSO4.10 The UV-vis spectra resulting from those experiments exhibited the characteristic plasmon signals of nanometer-sized Ag and Au crystallites, or the typical absorption from Cu+ ions. In all cases, the generated signals increased in intensity as illumination proceeded and the absorption band of the polymer radicals appeared only when the reduction of the metal ions was near completion. These results indicate that the photogenerated SPEEK• was consumed in the reduction process and that they started to accumulate only after most of the metal ions were reduced. Reaction 3 illustrates the (14) Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry; CRC Press: Boca Raton, 1991; Chapters 5 and 7. (15) (a) Horie, K., Ando, H., Mita, I. Macromolecules 1987, 20, 54. (b) Melhuish, W. H. Trans. Faraday Soc. 1966, 62, 3384. (16) (a) von Sonntag, C.; Bothe, E.; Ulanski, P.; Adhikary, A. Radiat. Phys. Chem. 1999, 55, 599. (b) Ulanski, P.; Bothe, K.; Rosiak, J. M.; von Sonntag, C. Macromol. Chem. Phys. 1994, 195, 1443. (17) Allen, N. S.; Chen, W.; Catalina, P.; Green, P. N.; Green, A. J. Photochem. Photobiol. A: Chem. 1988, 44, 349.
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Figure 1. Optical spectra from air-free aqueous solutions containing 0.36 M SPEEK and 0.018 M PVA: (a) prior to illumination; (b) after photolysis for 1 min with 350 nm light, I0 ) 6.8 × 1016 hv/s; (c) after addition of 0.01 M AgClO4; (d) after addition of 0.01 M HAuCl4 to a solution treated as in (b); (e) after addition of 0.01 M CuSO4 to a solution treated as in (b). In all cases, the final metal ion concentration was 0.7 mM.
reduction of Ag+ by polymeric ketyl radicals:
{R′RC•OH}z + Ag+ f {R′RCdO}z + Ag + H+
(3)
where R and R′ are the groups (shown in reaction 1) attached to the carbonyl function within SPEEK. Analogous reactions are expected for the reductions of Au(III) and Cu(II) ions. Support for reaction 3 was obtained from an experiment involving addition of a degassed solution of AgClO4 to a silver-free, pink-colored solution containing SPEEK•. The 565 nm signal decayed during the mixing process with simultaneous formation of an absorption band centered at 400 nm (Figure 1, curve c) corresponding to colloidal Ag. Similar results were obtained when irradiated films of SPEEK/PVA were immersed in AgClO4 solutions. Furthermore, addition of oxygenfree solutions of either Au(III) or Cu(II) ions to pre-irradiated, degassed solutions containing SPEEK• yielded the spectra shown in curves d and e, respectively. As shown before, the absorption at 520 nm in curve d corresponds to the surface plasmon of Au nanoparticles, whereas the 470 nm signal displayed in curve e has been assigned to the CuOH species.10 These results clearly demonstrate that reactions 1-3 take place when mixtures of the two polymers are exposed to 350 nm radiation either in solution or in the solid phase. Another interesting property of SPEEK radicals is their relatively long lifetime (∼40 min at room temperature) in degassed solution. This long lifetime allowed detection of the radical by simple optical means, therefore permitting elucidation of the role of this species in the reduction of the metal ions. The persistence of the macromolecular radicals is a consequence of their slow coupling reaction represented by reaction 4,
{R′RC•OH}z + {R′RC•OH}z f dimerization/disproportionation (4) which occurs with a second-order rate constant of k4 ) 7 × 102 M-1 s-1 in stirred solutions.10 As a comparison, the similar process for benzophenone radicals proceeds in water with a rate constant
Figure 2. Post-irradiation growth of the Ag plasmon signal in airfree solutions containing 60 µM AgClO4, 0.36 M SPEEK, and 0.018 M PVA; bottom trace, after 15 s of illumination; top trace, after 20 s of photolysis.
of k ) 1 × 109 M-1 s-1.18 The reason for the much slower decay in the polymer system is that this second-order reaction involves two negatively charged macromolecular radicals. This rationalization is supported by the long lifetime of anionic radicals produced via H-abstraction of PAA in solutions with pH > pKa of the polymeric acid.16 In contrast, anionic radicals of PAA rapidly decay in the presence of O2, which agrees with the lack of detection of SPEEK• (or of metal ion reduction) in air-saturated solutions. A further consequence of the long lifetime of SPEEK• is a post-irradiation reduction of Ag+ detected in solutions of the polymer blend. Evidence for such a process is presented in Figure 2, which shows an additional growth of the Ag plasmon (measured at 400 nm) after photolysis is terminated. The maximum quantum yield for formation of Ag crystallites was found to be 0.07,10 which excludes chain reactions as the origin for the post-irradiation process. Also, the kinetics of the photoreaction remained unchanged when Ag particles were added to the solution prior to irradiation, implying that catalysis of reaction 3 by the metal was not significant. No increase of the plasmon intensity was observed after adding Ag particles to an air-free solution of SPEEK/PVA containing AgClO4. This experiment demonstrated that the processes shown in Figure 2 were not a result of a particlecatalyzed thermal reduction of Ag+ ions by PVA. Displayed in this figure are kinetic traces corresponding to the reactions that followed illuminations for 15 and 20 s; the data were corrected for the small contribution of SPEEK measured at 400 nm prior to photolysis. Good first-order fits to the kinetic data were obtained (r2 ) 0.993), which are represented by the solid curves. Apparent first-order rate constants of 7.5 × 10-2 and 6.7 × 10-2 s-1 resulted for samples pre-irradiated for 15 and 20 s. The kinetics of metal particle formation was shown to be limited by the rate of Ag+ reduction in solutions containing gelatin concentrations high enough to bind most of the silver ions.19 Under such conditions, cluster aggregation occurred within gelatin domains faster than in solutions free of the polyelectrolyte. A similar situation is expected in the SPEEK/PVA solutions given that the concentration of sulfonated polyketone is much higher than [Ag+]. Thus, reaction 3 is anticipated to be the step (18) Ledger, M. B.; Porter, G.J. Chem. Soc., Faraday Trans. 1 1972, 68, 539. (19) Kapoor, S.; Lawless, D.; Kennepohl, P.; Meisel, D.; Serpone, N. Langmuir 1994, 10, 3018.
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that determines the rate of the post-irradiation metal formation. Such an assumption implies that particle generation follows pseudo-first-order kinetics because [Ag+] . [SPEEK•], which can be verified employing the data in Figure 2. Concentrations of unreduced Ag+ ions (52 and 47 µM after 15 and 20 s of pre-irradiation, respectively) were estimated from the analysis of the plasmon signals measured prior to interruption of photolysis using the extinction coefficient of the metal particles at 400 nm ( ) 2.4 × 104 M-1 cm-1) obtained recently in SPEEK/PVA solutions.10 On the other hand, the concentration change of Ag+ during the post-irradiation reaction (evaluated from the increase in absorbance during this process and the metal value) is equal to the steady-state [SPEEK•] and amounts to 0.5 µM independent of the pre-irradiation time. These results are in agreement with the first-order growth of the plasmon intensity depicted in Figure 2 according to reaction 3. Evaluation of the rate constant for the reduction of Ag+ by SPEEK• yielded k3 ) 1.4 × 103 M-1 s-1 in both experiments. Although k3 is only 2 times higher than k4, under the [Ag+] . [SPEEK•] condition, the reaction of the BPK radicals most likely proceeds through reaction 3. This allows efficient generation of particles during photolysis and also enables crystallite formation after illumination is terminated. Results from the earlier studies involving SPEEK/PVA solutions showed that the photogeneration of Ag crystallites could be understood in terms of the mechanism represented by reactions 1-5, where the last reaction involves reduction of benzophenone groups of SPEEK by PVA radicals:
{CH2C•OH}y + {R′RCdO}z f {R′RC•OH}z + {CH2CO}y (5) with further formation of SPEEK radicals.10 The second structure on the right side shows the resulting CdO functions introduced into the PVA macromolecules as a result of their oxidation. Reaction 5 is analogous to the reduction of benzophenone molecules by R-hydroxy radicals of 2-propanol, (CH3)2C•(OH), which is an important pathway for the generation of BPK radicals in the BP/2-propanol solution system.20 Rate constants for the reduction of BP by (CH3)2C•(OH) in different solvents range from 4 × 104 to 3 × 106 M-1 s-1,21 but reaction 5 involves polymeric species and is expected to be slower. High concentration of polymeric BP groups in the solutions (1.8 × 10-2 M) probably facilitates reaction 5. On the other hand, reactions 3-5 are anticipated to be slower in SPEEK/PVA films due to the lower mobility of the polymeric reactive intermediates in the solid matrixes. Earlier studies have shown divergences in the photochemical behavior of liquid and solid systems. For example, Ag crystallites are not generated in polymer solutions containing air, whereas no inhibiting effect of O2 on the particle formation process was detected for films.9,10 The different behavior of the solid matrix can be understood considering the low permeability/ mobility of O2 in PVA-based films below 60% relative humidity.22 As a result of the low mobility of reactive intermediates, the concentration of SPEEK• in films increases to millimolar levels after a few minutes of irradiation,9 that is about 100 times higher than the [SPEEK•] detected in solution.10 However, the sequence of reactions 1, 2, 4, and 5 was adopted from the simple mechanism involving BPK radicals that accounts for the photogeneration of benzopinacol and acetone with a maximum quantum yield (ø) (20) Beckett, A.; Porter, G. Trans. Faraday Soc. 1963, 59, 2038. (21) (a) Demeter, A.; Be´rces, T. J. Phys. Chem. 1991, 95, 1228. (b) Naguib, Y. M. A.; Steel, C.; Cohen, S. G. J. Phys. Chem. 1988, 92, 6574. (c) Hoshino, M.; Arai, S.; Imamura, M.; Ikehara, K.; Hama, Y. J. Phys. Chem. 1980, 84, 2576. (22) Lien, L.; Fellows, C. M.; Copeland, L.; Hawkett, B. S.; Gilbert, R, G. Aust. J. Chem. 2002, 55, 507.
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Figure 3. Optical spectra recorded from a film composed of 30:70 wt% SPEEK/PVA; solid line, prior to illumination; dashed line, after 48 h of photolysis under air with I0 ) 2.5 × 1017 hv/s.
of 1 in the benzophenone/2-propanol model system.20 Conditions that increase [(Ph)2C•OH], such as high light intensities, are known to favor the occurrence of several side-reactions responsible for altering the products of the BP/2-propanol system.23,24 Thus, we cannot exclude the formation of secondary products from sidereactions during photolysis of SPEEK/PVA films. Confirmation of this expectation was obtained from an extended exposure to light of a film free of Ag+ in the presence of air. Shown in Figure 3 is a broad band centered near 400 nm that resulted from such an experiment. Development of this band was accompanied by partial bleaching of the SPEEK absorption below 350 nm, implying that the polyketone served as a precursor of the produced chromophore. Benzopinacol products formed by dimerization of Ph2C•(OH) exhibit electronic transitions only in the deep-UV region.25 Similar optical properties are expected in the case of polymeric benzopinacol products formed by dimerization of SPEEK• (reaction 4), meaning that the macromolecular dimers are unable to account for the absorption with a maximum near 400 nm. Photoreduction of BP with 2-propanol yields intermediates (called LAT, or light-absorbing transients) with a broad signal centered at about 330 nm.26 Reaction 6 is a representation of the process believed to form these intermediates through insertion of (CH3)2C•(OH) into the ortho and para positions of the aromatic rings of Ph2C•(OH),27
Ph2C•(OH) + (CH3)2C•(OH) f LAT
(6)
The intermediates are stable for several days in the absence of O2, and typically account for 1-2% of the solution products.27c In analogy to the LAT,26 the absorption maximum of the chromophore shown in Figure 3 is red-shifted in comparison (23) (a) Schuster, D. I.; Karp, P. B. J. Photochem. 1980, 12, 333. (b) Rubin, M. B. Tetrahedron Lett. 1982, 23, 4615. (c) Demeter, A.; La´szlo´, B.; Be´rces, T. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1478. (24) Rubin, M. B. In CRC Handbook of Organic Photochemistry and Photobiology; Horspool, W. H., Song, P.-S., Eds.; CRC Press: Boca Raton, 1995; p 430. (25) Robinson, J. W. Practical Handbook of Spectroscopy; CRC Press: Boca Raton, 1991; p 643. (26) (a) Pitts, J. N.; Letsinger, R. L.; Taylor, R. P.; Patterson, J. M.; Recktenwald, G.; Martin, R. B. J. Am. Chem. Soc. 1959, 81, 1068. (b) Chilton, J.; Giering, L.; Steel, C. J. Am. Chem. Soc. 1976, 98, 1865. (27) (a) Demeter, A.; Bercer, T. J. Photochem. Photobiol. A, 1989, 46, 27. (b) Costa C. V.; Grela, M. A.; Churio, M. S. J. Photochem. Photobiol. A 1989, 46, 27. (c) Scaiano, J. C.; Abuin, E. B.; Stewart, L. C. J. Am. Chem. Soc. 1982, 104, 5673.
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Figure 4. Evolution of the room-temperature EPR spectra of an SPEEK/PVA film free of air: (a) after 5 min of a 350 nm irradiation at room temperature; (b) after 1 h in the dark; (c) after 5 h in the dark.
with that of SPEEK due to a more extended conjugation existing in the product. These results and the partial intensity decrease of the aromatic ring vibrations in the IR (see below) after extended illumination are evidence that polymeric LAT-type structures were produced in the films. Generation of the polymeric intermediate involves reactions between SPEEK• and PVA radicals similar to step 6, but such a process can only be a minor contributor to the overall phototransformation because the intensity decrease of the ring vibrations was small. Since the aromatic rings of the polymeric benzophenone groups are para substituted, insertion of a PVA radical into the ortho position is the most probable route for LAT formation. Photolysis of SPEEK/ PVA systems containing Ag+ resulted in the formation of metal particles exhibiting the surface plasmon band located in the same spectral region where the LAT product absorbs.9,10 However, the rate of polymeric LAT photogeneration was 1-2 orders of magnitude smaller compared to that of particle formation in films and even slower in degassed SPEEK/PVA solutions. In addition, Ag+ reacts with SPEEK• via reaction 3, scavenging the precursor of the polymeric LAT and making the formation of this product less likely. Hence, the contribution of LAT products to the photogenerated absorption in the range of the Ag plasmon absorption is anticipated to be negligible in the presence of Ag+, which enabled the monitoring of the kinetics of metal particle formation by optical means in SPEEK/PVA systems. EPR/ENDOR Measurements. EPR spectroscopy was used to confirm the free-radical nature of the reducing species exhibiting an absorption centered at 565 nm in Figure 1. Figure 4 shows room-temperature EPR spectra of an Ag+-free SPEEK/ PVA film obtained after 5 min of irradiation at 350 nm in the absence of air. Spectrum (a) exhibits a signal with a peak to peak line width (∆Hpp) of 11.2 G and a g value of 2.0033. The 1HENDOR spectrum of this sample (Figure 5) consists of a strong central peak at 14.5 MHz due to the matrix protons and three pairs of weak lines (AA′, BB′, CC′) with hyperfine coupling constants of 1.2, 2.5, and 4.4 G, respectively. The obtained hyperfine constants are in good agreement with those reported for phenyl ketyl radicals of BP.28,29 In the case of BPK radicals of benzophenone, the signals might originate from interactions of the unpaired electron with the aromatic ring protons located at the meta (peaks AA′) and ortho positions (peaks CC′). The BB′ signal arises from the corresponding interaction with the hydroxyl proton.28,29 The presently found hyperfine constants are in good agreement with those reported for phenyl ketyl radicals (28) Yoshida, H.; Warashida, T. Bull. Chem. Soc. Jpn. 1971, 44, 2950. (29) Rånby, B.; Rabek, J. F. ESR Spectroscopy in Polymer Research; SpringerVerlag: Berlin, 1977; p 106.
Figure 5. 1H ENDOR spectrum acquired at 120 K from a SPEEK/ PVA film illuminated at 77 K in the absence of air. The arrow marks the frequency of the free proton. Peaks AA′ and CC′ correspond to hyperfine couplings involving meta and ortho ring protons, respectively; the BB′ signals result from coupling with the hydroxyl proton.
of BP and provide evidence that aromatic ketyl radicals form via photolysis of SPEEK/PVA mixtures. As shown in the spectra labeled b and c (Figure 4), the intensity of the EPR signal decreased slowly over several hours in the dark. An analogous decay of the absorption centered at 565 nm was observed in parallel optical measurements, further confirming that the reducing chromophore is a radical species. Repeated irradiation of the film resulted in reformation of both EPR and optical absorptions. Analysis of the optical data indicated that the absorption decayed in two steps: the initial process was governed by mixed kinetics followed after ∼30 min by reaction obeying an apparent second-order rate law with kdecay ) 4 × 10-2 M-1 s-1. According to these results, SPEEK• radicals undergo dimerization (reaction 4) in the film with a rate constant more than 3 orders of magnitude smaller than that in solution.10 This is probably due to the low mobility of the radicals in the solid state. The initial process characterized by mixed kinetics is not unexpected since high radical concentrations make possible competing side-reactions similar to those found in the model system.23,24 Probable secondary reactions comprise various couplings of SPEEK• with PVA radicals to generate asymmetric diols or disproportionation products. Photolysis of degassed SPEEK/PVA solutions at room temperature produced the broad EPR signal depicted in Figure 6a, with a g value similar to that recorded in films. Spectra b-d demonstrate the progressive decay of the EPR intensity monitored in the dark at room temperature. The paramagnetic species persisted for tens of minutes, agreeing well with the optical decay data of SPEEK• measured in solution.10 According to reaction 2, formation of SPEEK• is anticipated to yield an R-hydroxy radical of PVA as a byproduct. However, attempts to detect paramagnetic species different from SPEEK• were unsuccessful. As shown in Figures 4 and 6, the decay of the EPR signal at room temperature was not accompanied by any spectral changes. No new features or resolution improvements were also detected in experiments performed with films at 77 K. A similar lack of radicals from cellulose triacetate was reported upon irradiation of benzophenone and 4-substituted benzophenones dispersed in films of the cellulosic material, which
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Figure 7. EPR spectra of an air-free SPEEK/PVA film doped with Ag+ ions; (a) after exposure to light at 77 K; (b) after warming the sample to room temperature; (c) after 15 min at room temperature. Figure 6. Room-temperature EPR spectra of a degassed aqueous solution containing 0.36 M SPEEK and 0.018 M PVA: (a) after exposure to light; (b), (c), and (d) after keeping the sample in the dark at room temperature for 4, 9, and 20 min, respectively.
yielded only a broad signal similar to that depicted in Figure 4.30 The absence of signals due to PVA radicals in the solution spectrum (Figure 6) is not surprising since most of them react according to reaction 5, which is anticipated to compete favorably with the radical dimerization analogous to reaction 4.16 PVA radicals are known to be stable in the solid polyol at least up to 290 K.31 Consequently, the lack of signals from these radicals in experiments with films at low temperatures can be understood assuming that the very large concentration of polymeric BP groups present in the solid matrix (5 mol%) enabled reaction 5 to occur under such conditions. A different rationalization has been proposed in the case of BPK and PVA radicals produced during photolysis of benzophenone molecules present inside films of the polyol.15a Illumination forms a BPK radical and a PVA radical next to each other (a geminate pair) via processes similar to reactions 1 and 2. The quantum yield of BP consumption (0.042) was explained considering that these radicals were unable to separate fast in the solid matrix, resulting in a back-reaction of the geminate pair with reformation of the starting materials. Such rationalization is not convincing since the quantum yield for the triplet BP excited state (the precursor of the radical pair) was not determined but was assumed to be ø ) 1 as in the solution BP/2-propanol system.20 In fact, the quantum yields for BPK radical formation and for phosphorescence were the same (0.05) in benzophenonecontaining PMMA films,15b suggesting that radiationless decay limited the number of BP triplet states able to transform into radicals. Interestingly, the triplet excited state of polymeric BP groups (3SPEEK*) were generated with ø ≈ 0.05 in SPEEK/ PVA solutions, and Ag+ ions were able to scavenge up to 70% of the SPEEK radicals predicted to form using the quantum yield of 3SPEEK* together with reactions 2 and 5.10 These results imply that the efficiency of SPEEK• formation is controlled mainly by the quantum yield of 3SPEEK* and that combination processes of the polymeric radicals occur mostly in solution. Data obtained from the BP/2-propanol system fully support such conclusion since radical dimerization processes were found to be more efficient in solution than those involving geminate pairs of Ph2C•(OH) and (CH3)2C•(OH) trapped in the solvent cage.23 A process that enables efficient escape of BPK radicals from the solvent cage at high [BP] is the self-exchange reaction via hydrogen atom transfer (HAT) involving Ph2C•(OH) and Ph2CdO,23a •
•
Ph2C (OH) + Ph2CdO f Ph2CO + Ph2C (OH)
(7)
with an estimated value of k7 ≈ 1 × 104 M-1 s-1,21b which is in good agreement with the HAT rate constants reported for
several acetophenones.32 Occurrence of a reaction analogous to 7 involving SPEEK• and polymeric BP groups allowed the polymeric ketyl radical to quickly consume the geminate pair in SPEEK/PVA solutions. These results in the large fraction of SPEEK radicals that were available to reduce Ag+ ions. In the absence of metal ions, the quantum yields of SPEEK• formation are similar for solutions (0.02) and films (0.017) of the SPEEK/ PVA blend, suggesting that an HAT reaction between the polyketone radical and the polymeric BP groups is also possible in the solid matrixes. No EPR-active species were detected during the photolysis of air-saturated solutions containing the two polymers. An exposure of the SPEEK• solution to air resulted in rapid disappearance of both the radical absorption at 565 nm and the EPR signal depicted in Figure 4. Molecular oxygen is a well-known scavenger of free radicals, as well as a quencher of excited states.14,16,18 Scavenging of SPEEK• by O2 in solution confirms the radical nature of the species absorbing at 565 nm. In contrast, photolysis of films in air yielded both the optical absorption and the EPR signal of SPEEK•, which decayed slowly for about 30 min. The simplest representation of the decay process is
{R′RC•OH}z + O2 f {R′RC(OH)OO•}z
(8)
with formation of an organic peroxyl radical. Selection of such species as a product of reaction 8 was considered reasonable because similar radical intermediates were detected during the addition of O2 to PVA radicals in solution, which then decay via elimination of HOO•/O2-.16 In contrast, the product of reaction 8 in the solid matrix may transform slowly into hydroperoxide derivatives since such compounds were found as stable products during the photolysis of PEEK films in air.33 According to optical measurements at 565 nm, the radical decays within 2 min after illumination is stopped, which was reminiscent but shorter than the initial complex process in the absence of air. At longer times, SPEEK• was consumed in a reaction that followed an apparent first-order rate law with a rate constant of k8 ) 1 × 10-3 s-1. Reaction 8 occurs in the films significantly faster than the decay of SPEEK• in the absence of O2 but much slower than the reaction of PVA radicals with oxygen in solution.16 Obviously, such effect can be related to the low mobility of SPEEK• and O2 in the solid matrixes.22 Presented in curve a of Figure 7 is the spectrum of an Ag+doped film photolyzed at 77 K in the absence of air, which consists of a single EPR line with a width of about 11 G similar to that depicted in spectrum a of Figure 4. Warming the film to (30) Iwakura, Y.; Takeda, K.; Nakazawa, T. Polym. Lett. 1968, 6, 115. (31) (a) Zainuddin; Hill, D. J. T.; Le, T. T. Radiat. Phys. Chem. 2001, 62, 283. (b) Suryanarayana, D.; Kevan, L. J. Phys. Chem. 1982, 86, 3834. (32) Wagner, P. J.; Zhang, Y.; Puchalski, A. E. J. Phys. Chem. 1993, 97, 13368. (33) Giancaterina, S.; Rossi, A.; Rivaton, A.; Gardette, J. L. Polym. Degrad. Stab. 2000, 68, 133.
Radical-Induced Generation of Small SilVer Particles
Figure 8. Cross-sectional TEM image of a SPEEK/PVA film doped with Ag+ ions that was illuminated for 70 min; the marker corresponds to 200 nm.
room temperature decreases significantly the signal intensity (Figure 7, spectrum b); the paramagnetic species was no longer detected after keeping the sample at room temperature for several minutes (see Figure 7, spectrum c). Preliminary experiments showed that SPEEK• started decaying in the presence of Ag+ at about 200 K without formation of EPR-active species. Furthermore, no paramagnetic species were detected upon irradiation of SPEEK/PVA films containing Ag+ ions at room temperature. Obviously, the different behavior of SPEEK• illustrated in Figure 7 compared to that given in Figure 4 is a consequence of reaction 3. The reduction of Ag+ by SPEEK• must be fast in the solid matrix because no metal formation was detected after stopping photolysis of the film. Such a conclusion is not unrealistic because in the films the metal ions are present in high concentration as counterions to the sulfonic groups of SPEEK and, therefore, close to the photogenerated radical sites of the macromolecules. An interesting result of these experiments was the absence of EPR signals due to Ag atoms or of paramagnetic clusters derived from them, which have been detected in alcoholic matrixes.34 Irrespective of irradiation temperature, both films doped with Ag+ exhibited the typical plasmon signal of Ag particles at room temperature.9 A simple way to explain these observations is to assume that the Ag atoms generated via reaction 3 were able to diffuse quickly through the solid producing larger diamagnetic species. Figure 8 shows a TEM image obtained from a sectioned film, demonstrating that spherical metal particles with an average diameter of 10 nm formed as a result of the illumination. The combined EPR and TEM observations support not only the idea that formation of metal crystallites in PVA-based films containing macromolecules able to bind Ag+ (such as PAA or SPEEK) occurs via aggregation of Ag atoms7 but also the assumption that reaction 3 is the rate-limiting step of particle formation in solution. FT-IR Measurements. Illumination of PEEK films with λ > 300 nm in air induces oxidation and degradation of the polymer.33 A different outcome was envisioned for SPEEK/PVA films because the sequence of reactions 1, 2, 5, and 8 predicted the photooxidation of the polymeric alcohol as the major transformation in the presence of O2. However, formation of the polymeric (34) (a) Matsuura, K.; Liu, M.; Hoshino, M.; Hase, H.; Arai, S.; Miyatake, Y. Chem. Phys. Lett. 1999, 301, 401. (b) Stevens, A. D.; Symons, M. C. R. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1439.
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LAT and the complex initial decay of the SPEEK• signal even in the presence of air were evidence that the radicals also engaged in secondary reactions analogous to those responsible for altering the products of the model BP/2-propanol system.23,24 Detection of products from such side-reactions was not considered a simple task in the highly concentrated SPEEK/PVA solutions. Instead, FTIR measurements were used to identify alterations of functional groups occurring when films with and without Ag+ ions were exposed to 350 nm irradiation in air. The physical stability of the films and their ability to produce radicals over extended exposures were examined as well. This information is considered important in evaluating potential applications of the films that take advantage of their capability of photoreducing chemical species. The strongest IR bands recorded from nonirradiated films will be assigned first followed by a presentation of the changes induced by light. Full spectra acquired before and after photolysis are included in the Supporting Information. Although the FTIR spectrum exhibited considerable overlap of bands from the polymers and cross-linker, signals due to the major PVA component were predominant. Figure 9a presents the twocomponent broad signal in the 3150-3550 cm-1 range that originates from stretching of O-H groups involved in several kinds of hydrogen bonding. Intermolecular polymeric and dimeric hydrogen-bonded O-H vibrations are detected at ∼3260 and ∼3460 cm-1, respectively.35 Other characteristic bands of PVA observed in the film spectrum are νas(CH2) (2940 cm-1), νs(CH2) (2910 cm-1), ν(CH) (2850 cm-1, weak shoulder),36 and the CH2 “scissoring” vibration (1470 cm-1). The last band overlapped partially with one of the breathing modes of the SPEEK phenyl rings. SPEEK bands that were clearly resolved in the IR spectrum include the ν(CdO) lowered by conjugation with the aromatic rings appearing at 1649 cm-1 and carbon-carbon stretching vibrations, ν(CdC) and skeletal in-plane vibrations of the rings observed at 1599, 1467, and 1416 cm-1 and a low-intensity band at 1500 cm-1. The symmetric S-O stretching of sulfonic groups, located in the 1080-1025 cm-1 range depending the surrounding medium in SPEEK containing blends,37 was partially masked by signals due to the C-H in-plane deformation (1030 cm-1) and the strong out-of-phase C-C-O stretching (1080 cm-1) of PVA. Introduction of Ag+ ions into the film as counterions to the sulfonic groups of SPEEK induced no spectral changes. Significant light-induced spectral changes of metal-free films occurred in the 3600-3000 and 1720-1600 cm-1 ranges and below 1500 cm-1, which points to permanent phototransformations of the polymer matrix. Compared in Figure 9a is the high-frequency region of spectra recorded prior to and after photolysis for 30 h. Substantial transformations of the hydroxyl stretching absorption occurred upon irradiation since intensity of the broad O-H stretching bands (at 3260 and ∼3460 cm-1) from PVA decreased progressively. Further confirmation that the concentration of hydroxyl groups from PVA declined was provided by the concomitant intensity drop of the vibrations corresponding to in-plane O-H bending (1310 cm-1) and outof-phase C-C-O stretching (1080 cm-1). This process was accompanied by the emergence of a weak signal, centered at 3425 cm-1, which can be assigned to the ν(O-H) of product(s) from the phototransformation. Figure 9b shows the evolution of the spectrum in the 1480-1900 cm-1 range with illumination (35) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: New York, 1991. (36) (a) Krimm, S.; Liang, C. Y.; Sutherland, G. B. B. M. J. Polym. Sci. 1956, 22, 227. (b) Ambrose, E. J.; Ellott, A.; Temple, R. B. Proc. R. Soc. 1949, A199, 183. (37) Lu, X.; Weiss, R. A. J. Polym. Sci. B 1996, 34, 1795.
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Figure 9. FTIR spectra of a SPEEK/PVA film free of metal ions: (a) stretching region of hydroxyl groups; discontinuous line, before photolysis; continuous line, after illumination for 30 h. (b) Changes of the carbonyl stretching bands upon irradiation of the film for 0, 10, 20, 40, 60, 100, 160, 260, 380, 560, 920, 1800, 2340, 3480, 4740, and 6120 min. The arrows indicate the direction of change; the marker corresponds to an absorption difference of 0.25.
time, illustrating the simultaneous intensity decrease of the characteristic SPEEK bands and the formation of new species. For instance, the decay of the carbonyl signal at 1649 cm-1 was correlated to the growth of new bands centered at 1708 and 1504 cm-1, as well as with the slow bleaching of PVA signals. A discernible loss of the signal corresponding to the antisymmetric C-C(O)-C stretching (1282 cm-1) took place in the same time scale, confirming further the irreversible transformation of the benzophenone groups of SPEEK. Combination of reactions 1, 2, 5, and 8 provides a possible overall change experienced by films free of Ag+ upon exposure to light in the presence of air,
{R′RCdO}z + {CH2CHOH}y + hν + O2 f {R′RC(OH)OOH}z + {CH2CO}y (9) where the first formula on the right-hand-side represents the hydroperoxide product derived from the hydroperoxyl radical generated in reaction 8. PVA radicals are known to reduce O2 in water;16 but such a process was considered less efficient than reaction 5 given the much larger concentration of polymeric BP groups present in the films. Although reaction 9 predicts the generation of CdO and OH groups as a result of the transformation of both SPEEK and PVA during photolysis, the initial decay of SPEEK• with mixed kinetics even in the presence of air is an indication that more than one process took place. The spectroscopic and kinetic results can be easily understood assuming that reaction 9 occurs in parallel with the overall transformation resulting from the sequence of reactions 1, 2, and 5 coupled with the dimerization pathway 4:
2{R′RCdO}z + {CH2CHOH}y + hν f {R′RC(OH)}z{C(OH)R′R}z + {CH2CdO}y (10) where the first structure on the right-hand-side represents the diol formed when SPEEK radicals dimerize. Thus, the band at 1708 cm-1 (Figure 9b) typical of carbonyl stretches can be assigned to the oxidation product of PVA. Both the hydroperoxide formed through process 9 and the cross-linked polymeric benzopinacol shown in reaction 10 can account for the appearance of the additional O-H signal in Figure 9a and also for the signal
at 1504 cm-1 in Figure 9b since absorptions in the 1540-1470 cm-1 range are typically associated with aromatic ring breathing modes.38 Para-substituted benzopinacols exhibit CdC aromatic stretching bands of medium/strong intensity in the 1520-1500 cm-1, but such signals are not specific.39 Hence, assignments of the signals at 3425 and 1504 cm-1 is presently not possible because additional products having OH groups and aromatic rings may form via alternative pathways. Examples of such products are asymmetric diols generated via various couplings of SPEEK• with PVA radicals and disproportionation products including those from reaction 4. Photolysis of Ag+-containing films induced decreases in the IR bands of both polymers but to a lesser extent than the changes encountered in the absence of metal ions. The effect induced by silver ions was followed as a function of time using two films (Ag+-doped and untreated) irradiated simultaneously. Extensive illumination of the film containing Ag+ resulted in spectral baseline drift because the photogenerated metal crystallites absorbed and scattered IR radiation. This problem was minimized using data gathered during the initial stages of the irradiation; the metal-induced interference was accounted for via spectra normalization and by restricting the analysis to the signals with the strongest intensity. Compared in Figure 10a are the decreases in intensities of carbonyl group stretching band during the first 6 h of photolysis in the presence and absence of Ag+ ions. The corresponding evolution of the phenyl ring mode band is presented in Figure 10b. The presence of CdO and CdC groups conjugated to aromatic rings results in a significant intensity enhancement of the ring modes at 1625-1575 cm-1.38 Both benzophenone and SPEEK exhibit a strong band at 1599 cm-1, whereas only weak peaks are observed for benzhydrol and benzopinacol derivatives between 1625 and 1575 cm-1.40 Thus, molecular transformations that alter the CdO signal also affect the ring skeletal mode, meaning that both of the signals employed in Figure 10a and b allow monitoring of the intensity decrease of the SPEEK carbonyl (38) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts; John Wiley: New York, 2001. (39) Zhang, Y.; Liu, T. Synth. Comm. 1988, 18, 2173. (40) The Aldrich Library of FT-IR Spectra; Pouchert, C. J., Ed.; Aldrich Chemical Co.: Milwaukee, 1985; pp 1125-1126.
Radical-Induced Generation of Small SilVer Particles
Figure 10. Evolution of the intensities of characteristic bands upon illumination of SPEEK/PVA films: squares, sample free of Ag+; circles, film doped with metal ions. (a) Carbonyl group stretching, (b) skeletal ring mode, and (c) stretching vibration of photogenerated CdO groups.
group. Consequently, the kinetic behavior of the photoreaction derived from the 1599 cm-1 absorption correlated to the one obtained from the data at 1649 cm-1 in each film. The change in intensity of both bands with irradiation time was 2-3 times more pronounced for the film devoid of metal ions, which also exhibited initial reaction rates 2 times higher than those of the matrix doped with Ag+. Furthermore, the decay of both the 1649 and 1599 cm-1 signals in the metal-free film followed approximately an apparent first-order rate law, whereas a biexponential function fitted best the kinetic results obtained in the presence of Ag+ ions. As shown in Figures 10a and b, the presence of Ag+ in the SPEEK/PVA film inhibited strongly the irreversible loss of carbonyl groups from the polyketone. This effect is a
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logical consequence of reaction 3 that reforms the aromatic ketyl groups while generating metal. The loss of polymeric CdO groups is also suppressed through quenching of the 3SPEEK* generated via reaction 1 by Ag+. Such a quenching process was demonstrated to be very fast when Ag+ is photoreduced either in SPEEK/ PVA solutions10 or in SDS micelles containing benzophenone.41 Quenching of the excited polymeric carbonyl groups is facilitated in films by the electrostatic interactions between sulfonic groups and Ag+ that keep this ion close to the ketyl functions. Depicted in Figure 10c is the development during photolysis of both films of the carbonyl band at 1708 cm-1 attributed to the oxidation product of PVA. The formation processes followed linear relationships with time for the first 6 h of the photoreaction. During that period, the drop in concentration of the photosensitizing polymeric BP groups was modest (e20%), the [PVA] remained constant since this polymer is the major component of the matrix, and the reaction took place at constant I0. These conditions lead to the apparent zero-order rate laws for the formation of the carbonyl band. The data from Figure 10c indicate that the reaction rate in the presence of Ag+ diminished by a factor of 2.5 as compared with the rates in the film without metal ions. Reduction of the metal ions by the PVA radicals produced via reaction 2 seems feasible because an analogous reaction has been reported for (CH3)2C•(OH).42 However, the fact that the polyol radicals are oxidized in any case, either via reaction 5 or by reaction with Ag+, makes the latter redox process of no kinetic consequence to the carbonyl formation. According to the zeroorder behavior of the PVA oxidation, the dissimilar rates must derive from different rates of the radical formation step 2. Given that both samples were exposed to the same I0, the logical conclusion is that quenching of 3SPEEK* by Ag+ was the determining factor in lowering the carbonyl formation rate of the doped film. A slower formation of PVA carbonyl groups was observed at longer times for the film free of metal ions, but the formation of SPEEK• remained unaffected. These findings suggest that some of the byproducts generated earlier were able to act as H-atom donors. No IR measurements were possible for the film doped with Ag+ ions after 6 h of exposure due to the strong scattering of light from the photogenerated metal crystallites. Further darkening of the sample occurred, indicating that formation of Ag particles continued with increasing photolysis. In fact, the photoactivity of both films persisted throughout the irradiation period that lasted 100 h, and no detrimental alterations of their flexibility or strength were noticed. These findings validated the strategy of selecting a combination of macromolecular components for a photosensitizing system that can induce reduction reactions.
Conclusions The optical and EPR data collected in the present study have shown unequivocally that illumination of systems consisting of SPEEK and PVA results in polymeric BPK radicals able to initiate the formation of metal nanoparticles. These findings have demonstrated that polymeric materials with appropriate functions can serve as analogues that reproduce the photoprocesses of the model system composed of BP and 2-propanol. Utilization of SPEEK and PVA offers several advantages as compared with the model system, including the much longer lifetime of the resulting polymeric BPK radicals. This, in turn, allows them to (41) Kometani, N.; Doi, H.; Asami, K.; Yonezawa, Y. Phys. Chem. Chem. Phys. 2002, 4, 5142. (42) (a) Yonezawa, Y.; Sato, T.; Kuroda, S.; Kuge, K.-I. J. Chem. Soc., Faraday Trans. 1991, 87, 1905. (b) Tausch-Treml, R.; Henglein, A.; Lillie, J. Ber. BunsenGes. Phys. Chem. 1978, 82, 1335.
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participate effectively in slow redox processes, such as the reduction of Ag+. Another advantage of the SPEEK/PVA blend systems is their ability to produce the same radicals either in solution or in the solid state with comparable efficiencies, which permits the photogeneration of Ag crystallites in both phases. The presence of sulfonic groups in the SPEEK macromolecules enables incorporation of Ag+ ions close to the radical sites, assisting the metal ion reduction in the solid matrixes. FTIR results showed that SPEEK/PVA films free of Ag+ experience irreversible reduction/dimerization of the polymeric BP groups, oxidation of PVA, and formation of byproducts analogous to those found for the BP/2-propanol system. However, these transformations are diminished by the metal ions through their reaction with SPEEK radicals (which reforms the ketyl groups), as well as by quenching the triplet excited states of the polymeric
KorcheV et al.
BP functions. Because phototransformation of the functional groups is minimal in such polymeric films, they can operate for extended periods without significant loss of their photochemical and physical properties. Acknowledgment. The authors are grateful to R. Leibfried (Victrex USA, Inc.) for a generous gift of PEEK, as well as to M. E. Miller and E. Duin for helping us with TEM and EPR measurements. We also thank A. Stove and N. Dalal for their help with low-temperature experiments and ARO for partial funding of this project. Supporting Information Available: FTIR data. This material is available free of charge via the Internet at http://pubs.acs.org. LA0512345
