Langmuir 2002, 18, 4195-4197
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Influence of Microencapsulation on the Stability and Reactivity of 2,4,6-Triphenylpyrylium Tetrakis(pentafluorophenyl)gallate as a Cationic Photoinitiator Elena Y. Komarova, Kangtai Ren, and Douglas C. Neckers*,† Center for Photochemical Sciences,‡ Bowling Green State University, Bowling Green, Ohio 43403 Received February 21, 2002. In Final Form: March 26, 2002 2,4,6-Triphenylpyrylium tetrakis(pentafluorophenyl)gallate (TPPGa) was synthesized and studied as a visible light cationic photoinitiator. TPPGa was encapsulated in polystyrene to increase thermal stability, and the microparticles obtained were characterized with scanning electron microscopy and laser scanning confocal microscopy. The properties of the microencapsulated initiator were investigated.
I. Introduction Diazonium, sulfonium, and iodonium salts with nonnucleophilic counterions such as SbF6- and (C6F5)4B- are widely used as cationic photoinitiators.1 Protic acids or positively charged species are released upon irradiation of such compounds, and these initiate the polymerization. Most onium salts require excitation with UV sources or the addition of sensitizers if longer wavelength light is required. Triphenylpyrylium and thiopyrylium salts have attracted attention due to the fact that they are strongly oxidizing. The longest wavelength absorption band of the 2,4,6-triphenylpyrylium cation is 420 nm in nonpolar solvents, so triphenylpyrylium salts can be used with longer wavelength sources than other cationic initiators such as diaryliodonium and triarylsulfonium salts. Ledwith2 reported triphenylpyrylium hexafluoroantimonate to be an effective photoinitiator for cyclohexene oxide polymerization. However, triphenylpyrylium tetrafluoroborate and hexafluorophosphate salts of similar cations were found to yield no polymer under the same experimental conditions. Recently, our research group3 has demonstrated a pronounced influence of the anion on the activity of cationic photoinitiators. Tetrakis(pentafluorophenyl)gallate (referred to as “gallate” in this paper) derivatives of both iodonium and triphenylpyrylium salts were found to be most efficient. The gallate anion possesses low nucleophilicity and does not interfere with photogenerated positively charged species that initiate polymerization. Moreover, the solubility of the pyrylium and iodonium tetrakis(perfluorophenyl)gallate salts in epoxy resin compositions was considerably increased over their hexafluoroantimonate counterparts. Triphenylpyrylium salts have been known for a hundred years and represent one of the fundamental oxygen* Corresponding author. † This paper is dedicated to Prof. Dr. J. W. Neckers, on the occasion of his 100th birthday. ‡ Contribution No. 460 from the Center for Photochemical Sciences. (1) Crivello, J. V. In Radiation Curing in Polymer Science and Technology; Fouassier, J. P., Rabek, J. F., Eds.; Chapman & Hall: London, 1993; Vol. 2, pp 435-471. (2) Ledwith, A. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1982, 23 (1), 323-324. (3) Ren, K.; Mejiritski, A.; Malpert, J. H.; Grinevich, O.; Gu, H.; Neckers, D. C. Tetrahedron Lett. 2000, 41, 8669-8672.
containing aromatic heterocycles. Positions 2 and 6 (Rpositions) have electrophilic reactivity. Nucleophiles are mostly added to these positions. At first, pyrylium salts had only academic attention as useful intermediates for conversion of aliphatic materials into aromatic compounds.4 Nowadays, the salts have numerous practical applications. For instance, the triphenylpyrylium cation is a widely used photosensitizer. Several successful efforts have been made to attach it to a polymeric backbone5 or to incorporate it inside a zeolite.6,7 The polymer-supported or zeolite-based triphenylpyrylium cation was used in the photoinduced electron transfer catalyzed dimerization and cycloaddition reactions of 1,3-hexadiene though a decrease in reactivity of encapsulated pyrylium cations in comparison with the reactivity of the corresponding salt in solution was reported. Polystyrene was chosen by several researchers as an encapsulating agent. For example, Kobayashi et al.8,9 used polystyrene to encapsulate scandium triflate and osmium tetroxide catalysts. Microencapsulation rendered these catalysts recoverable and reusable. Microencapsulated osmium tetroxide has a lower toxicity and is more stable under ambient conditions. In addition, copper phthalocyanine was microencapsulated with polystyrene. The encapsulated pigment flowed more readily and was more easily dispersed.10 The following work concentrates on 2,4,6-triphenylpyrylium tetrakis(pentafluorophenyl)gallate (TPPGa) (Figure 1). The major disadvantage of TPPGa as a visible light photoinitiator is its poor thermal stability. In the present work, TPPGa was encapsulated in a polystyrene sheath in order to increase its thermal stability and, therefore, its shelf life without significantly decreasing its reactivity. (4) Balaban, A. T.; Dinculescu, A.; Dorofeyenko, G. N.; Fischer, G. W.; Koblik, A. V.; Mezheritskii, V. V.; Schroth, W. Pyrylium Salts: Syntheses, Reactions, and Physical Properties. Advances in Heterocyclic Chemistry; Katritzki, A. R., Ed.; Academic Press: New York, 1982. (5) Mattay, J.; Vondenhov, M.; Denig, R. Chem. Ber. 1989, 122, 951958. (6) Fornes, V.; Garcia, H.; Miranda, M. A.; Mojarrad, F.; Sabater, M.-J.; Suliman, N. N. E. Tetrahedron 1996, 52 (22), 7755-7760. (7) Cano, M. L.; Cozens, F. L.; Garcia, H.; Marti, V.; Scaiano, J. C. J. Phys. Chem. 1996, 100, 18152-18157. (8) Kobayashi, S.; Nagayama, S. A. J. Am. Chem. Soc. 1998, 120, 2985-2986. (9) Kobayashi, S.; Masahiro, E.; Nagayama, S. J. Am. Chem. Soc. 1999, 121, 11229-11230. (10) Tianyong, Z.; Xuening, F.; Jian, S.; Chunlong, Z. Dyes Pigm. 2000, 44, 1-7.
10.1021/la025648z CCC: $22.00 © 2002 American Chemical Society Published on Web 04/25/2002
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Langmuir, Vol. 18, No. 11, 2002
Figure 1. 2,4,6-Triphenylpyrylium tetrakis(pentafluorophenyl)gallate (TPPGa).
II. Experimental Section Materials. All reagents were purchased from Aldrich and used without further purification unless noted otherwise. Polysiloxane-based resin with epoxy pendant groups (SE resin) was a gift from GE Silicones in Waterford, NY (resin #479-2008). UVR6110 was a gift from Union Carbide. All manipulations in the synthesis of TPPGa were performed under an argon atmosphere using standard techniques. Lithium tetrakis(pentafluorophenyl)gallate was prepared according to the procedure reported elsewhere.3 1H and 13C NMR spectra were taken with a Varian Gemini 200 NMR spectrometer. 19F NMR was taken with a Unity Plus 400 NMR spectrometer. Elemental analysis was performed by Atlantic Microlab Inc. in Norcross, GA. Silica gel chromatography was performed using silica gel (40 microns) purchased from Scientific Adsorbents Inc. Preparation of TPPGa. Lithium tetrakis(pentafluorophenyl)gallate (5.44 g, 6.09 mmol) was dissolved in 50 mL of methylene chloride. To this solution, 2,4,6-triphenylpyrylium tetrafluoroborate (2.01 g, 5.08 mmol) in 30 mL of methylene chloride was added at room temperature. The solution was stirred for 2 h in the dark. The precipitate was filtered, and the solvent was removed to give a yellow, viscous oil. The crude product was chromatographed through a short plug of silica gel by elution with mixture of dichloromethane and hexane (1:1) to give a yellow powder (4.57 g, 86% yield). The melting point was not determined since the compound decomposes at 72-75 °C. 1H NMR (200 MHz, DMSO-d6, δ): 9.21 (s, 2H), 8.59-8.65 (m, 6H), 7.72-7.96 (m, 9H). 13C NMR (DMSO-d6, δ): 170.0 (C), 165.1 (C), 135.0 (CH), 132 (C), 129.9 (CH), 129.0 (C), 128.8 (CH), 115.1 (CH). 19F NMR (CDCl3, δ): -163.4 (s, 2F), -158.0 (s, 2F), -122.7 (s, 2F). Elemental analysis calcd: 53.9% C, 1.6% H. Found: 53.8% C, 1.6% H. Cyclic Voltammetry. Cyclic voltammetry was performed with a BAS 100A potentiostat equipped with a platinum disk electrode and a platinum wire as an auxiliary electrode. A saturated calomel electrode (SCE) was used as a reference electrode for measurements in dichloromethane, and a Ag/AgNO3 electrode was used in acetonitrile.13,14 Tetrabutylammonium perchlorate (0.1 M) was a supporting electrolyte in both solvents. Polymerization of Cyclohexene Oxide. Solutions of TPPGa and triphenylpyrylium tetrafluoroborate (TPPBF) in cyclohexene oxide (1%) were irradiated in a 350 nm Rayonet reactor. The conversion of epoxy groups was monitored by NMR. Microencapsulation. An in-liquid drying technique was slightly modified.12 Microencapsulated TPPGa was prepared as follows: Polystyrene (0.2 g, 0.002 mol) and TPPGa (0.05 g, 4.8 × 10-5 mol) dissolved in 2 mL of dichloromethane were stirred into a 0.2% (w) solution of sodium dodecyl sulfate in water. The emulsion obtained was stirred overnight. The microcapsules obtained as a yellow powder were filtered, washed with methanol, and dried. The concentration of TPPGa inside the microcapsules was determined by UV/vis spectroscopy using a Hewlett-Packard 8452A diode array UV-visible spectrophotometer. Scanning (11) Miranda, M.; Garcı´a, H. Chem. Rev. 1994, 94, 1063-1089. (12) Thies, C. In Microcapsules and Nanoparticles in Medicine and Pharmacy; Donbrow, M., Ed.; CRC Press: Boca Raton, FL, 1992; Chapter 3, pp 47-71. (13) Pragst, F.; Ziebig, R. Electrochim. Acta 1978, 23, 735-740. (14) Akaba, R.; Sakuragi, H.; Tokumaru, K. J. Chem. Soc., Perkin Trans. 1991, 2, 291-297.
Letters electron microscopy (SEM) was performed with a Hitachi-S2700 microscope equipped with a LaB6 crystal as an electron emitter. Samples for SEM were coated with a Au-Pd alloy in a Polaron sputtercoater. Laser scanning confocal microscopy was conducted on a Zeiss 310 LSCM equipped with three lasers: a 543 nm He-Ne, a 488 and 514 nm multiline argon, and a 350 nm ultraviolet argon laser (Wayne State University, Center for Molecular and Cellular Toxicology with Human Applications). Thermal or Photopolymerizations. TPPGa (encapsulated or pure, 1%) was placed in the resin and kept in the dark at 60 °C or irradiated at 350 nm in the Rayonet reactor. The amount of cross-linked polymer was determined gravimetrically following 2 days of extraction with methylene chloride in a Soxhlet extractor. Extraction with Acetonitrile. A measured volume of acetonitrile was added to a suspension of TPPGa microcapsules in methanol. The quantity of TPPGa released was determined by UV/vis spectroscopy.
III. Results and Discussion Triphenylpyrylium Gallate. TPPGa was synthesized from commercially available TPPBF by metathesis with lithium tetrakis(pentafluorophenyl)gallate. The TPPGa obtained has photophysical properties similar to those of TPPBF. The compounds have identical absorption and fluorescence spectra as well as reduction potentials. Both salts are poor oxidizing agents in the ground state. In dichloromethane, the reduction potential is -0.23 V (SCE), while in acetonitrile it is -0.34 V (SCE). However, the singlet and the triplet excited states of the salts possess high reduction potentials (2.5 V, S1; 2.0 V, T1).11 As sensitizers, the compounds react by photoinduced electron transfer. TPPGa is more soluble in nonpolar solvents such as dichloromethane and toluene than is the widely used pyrylium tetrafluoroborate or perchlorate. The solubility of TPPGa in common epoxy resins (cyclohexene oxide, UVR6110, SE resin) is much higher as well. TPPGa has much greater reactivity in the bulk polymerization of cyclohexene oxide in that there is 95% conversion of epoxy groups after 1 min of irradiation in a Rayonet reactor (350 nm). In comparison, Ledwith2 reported 57% of polymer for the triphenylpyrylium hexafluoroantimonatecyclohexene oxide system after 5 min in Rayonet (350 nm) at 40 °C. However, the thermal stability of TPPGa is poor. In only 1 h in the dark at room temperature, 33% of the monomer is converted to polymer. There is 95% conversion in the dark after 2 h. Microencapsulation of Triphenylpyrylium Gallate. TPPGa was encapsulated in polystyrene using an in-liquid drying technique.12 Polystyrene was chosen as the encapsulating medium due to an anticipated relative inertness of the catalyst with the capsule wall. The reactivity of polystyrene was such that we predicted it would be unreactive even with an active encapsulated compound such as the triphenylpyrylium salt. The microcapsules obtained have a wide size distribution (from 5 to 100 microns) as shown by SEM (Figure 2). They are spherical in shape. Because the microencapsulated compound is highly fluorescent, laser scanning confocal microscopy (LSCM) could be used to determine the location of TPPGa inside the polystyrene bead. LSCM is a nondestructive technique making it possible to look at the details of a fluorescent interior without breaking the nonfluorescing wall of the microcapsule. TPPGa is evenly distributed within the polystyrene sphere (Figure 3). The high intensity of fluorescence in the middle of the microcapsule (cross-section view) shows that the excitation light is not scattered by the polystyrene wall, and it is successfully absorbed by the target TPPGa inside the microparticle. Polystyrene beads without interior contents
Letters
Langmuir, Vol. 18, No. 11, 2002 4197 Table 1. Comparison of Thermal Stability of Encapsulated and Pure TPPGa in Different Monomersa time (h) resin/initiator
0
0.5
1.0
1.5
2.0
UVR6110/capsules SE/capsulesb UVR6110/pure SE/pure
liq liq liq liq
vis liq liq liq sol
gel-sol liq gel sol
sol liq gel-sol sol
sol liq sol sol
a The degree of polymerization was roughly estimated by the change of the resin’s viscosity where “liq” means liquid, “sol” means solid, and “gel” means gel-like or soft solid. b An SE/capsules mixture was kept at these conditions for a week and did not solidify.
Figure 2. SEM image of TPPGa microparticles.
Figure 3. LSCM cross section of TPPGa microparticles.
were used as the control and emitted no light under similar conditions. The amount of microencapsulated material was determined by UV/vis spectroscopy following destruction of the capsule walls. The absorption of TPPGa dissolved in dichloromethane at 420 nm was monitored. Polystyrene absorbs below 300 nm, so it does not interfere with the measurements. The microcapsules contain about 15% (w) of TPPGa. Properties of Microencapsulated TPPGa. The thermal and photochemical stability of microencapsulated TPPGa was tested in two different epoxy resins. One polysiloxane-based resin having epoxy pendant groups (SE resin) does not dissolve the polystyrene wall of the microcapsule. The other, 3,4-epoxycyclohexylmethyl 3,4epoxycyclohexylcarboxylate (UVR6110), dissolves the polymeric material of the microparticle readily. TPPGa is readily soluble in either of the resins. Microencapsulated TPPGa, however, demonstrates excellent thermal stability in SE resin (Table 1). Moreover, TPPGa microcapsules initiated no crosslinking of the SE resin even after being kept for 1 week at 60 °C in the dark. In contrast, the dissolved TPPGa caused complete solidification of the same resin in 30 min. However, microencapulated TPPGa was completely devoid of photoreactivity and failed to initiate photopolymeri-
zation of the SE resin upon irradiation in a 350 nm Rayonet reactor. Though the polystyrene wall thoroughly protects TPPGa from contact with electron donors and/or nucleophiles that may induce the decomposition of the salt upon irradiation or in the dark, there is no decomposition of TPPGa inside the microcapsules either upon irradiation or when heated. Therefore, microencapsulated TPPGa can be stored under ambient conditions and in the mixture with SE resin for a long time. One can envision that the polystyrene sheath protects its contents even from reactive species found outside the microcapsular wall. Release of TPPGa from Microcapsules. An obvious way to use this advantage of encapsulation is to store the TPPGa in its microencapsulated form and release the reactive catalyst by dissolving the microparticulate wall with an appropriate solvent. TPPGa can be released from polystyrene microcapsules by dissolving the wall of the microcapsule in a solvent such as dichloromethane. Since CH2Cl2 is also miscible with SE resin, addition of methylene chloride converts a suspension of microcapsules in the resin to a homogeneous solution of TPPGa and polystyrene in the resin which polymerizes in the dark. The amount of the cross-linked polymer formed is 90%, and this is independent of the quantity of CH2Cl2. Other commonly used epoxy monomers such as cyclohexene oxide and UVR6110 will also dissolve the polystyrene wall to release the salt. In the latter case, as well as photopolymerization, thermal polymerization can be induced (Table 1). TPPGa microcapsules formed following dissolution of a microcapsule have a reactivity close to that of pure TPPGa. Another way to release TPPGa is to extract it from within the capsule with acetonitrile since acetonitrile can extract TPPGa from polystyrene microcapsules without dissolving the capsules. Addition of 50% (w) of acetonitrile to a methanol suspension of microcapsules releases ∼80% of the encapsulated salt. IV. Conclusions Triphenylpyrylium tetrakis(pentafluorophenyl)gallate was synthesized. TPPGA possesses significant reactivity as an initiator for the photopolymerization of epoxy resins, but its poor thermal stability is a major disadvantage. Microencapsulating TPPGa in polystyrene using an inliquid drying technique increases its stability. The microcapsules obtained were characterized with SEM and LSCM. Encapsulated TPPGa showed excellent thermal and photostability in an SE resin wherein the polymeric wall protects TPPGa from decomposition. Several methods were developed that release TPPGa from microcapsules. Acknowledgment. We thank the National Science Foundation Division of Materials Research (DMR 9803006) for financial support of this work. LA025648Z
