Triphenylsulfonium salts

The proposed mechanism is based on the basis of the fact that ylides (Scheme 23 and Scheme 24) undergo bond fission between the phosphorus atom and the phenyl group in TPPY as reported by Nagao et al. [51] and between the sulfur atom and the phenyl group in POSY as observed in triphenylsulfonium salts [52-55] when they are irradiated by a high-pressure mercury lamp. The phenyl radicals thus produced participate in the initiation of polymerization. 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

Bronsted Acid Generation from Triphenylsulfonium Salts in Acid-Catalyzed Photoresist Films... 

The development of new classes of cationic photoinitiators has played a critical role in the production of highly sensitive, acid-catalyzed deep-uv photoresists. Sulfonium salts have been widely used in this respect (4). These materials are relatively easy to prepare and structural modifications can be used to produce desired wavelength sensitivity. Triphenylsulfonium salts are particularly well suited for deep-uv application and in addition can be photosensitized for longer wavelength. These salts are quite stable thermally and certain ones such as the hexafluoroantimonate salt are soluble in casting solvents and thus easily incorporated within resist materials. 

Scheme 1. Mechanisms for acid generation from triphenylsulfonium salts.

Merocyanine Dye Method for Acid Analysis. Resist photochemistry can often be monitored by the changes in ultraviolet absorption spectra associated with a bleaching of the sensitizer absorbance. In the case of resist systems with triphenylsulfonium salts, no change in the film absorption is observed on irradiation. In order to determine the amount of acid produced, a direct method for acid analysis was required. A highly sensitive method was desirable since the amount of acid produced is approximately 10 6 mmol for a 1 micrometer thick film on a 2 inch wafer. Furthermore a nonaqueous technique is preferred in order to avoid hydrolysis of the hexafiuoroantimonate salt. Hydrolysis gives hydrogen fluoride (14) which makes accurate acid determination more difficult. 

Thus the quantum yield for acid production from triphenylsulfonium salts is 0.8 in solution and about 0.3 in the polymer 2 matrix. The difference between acid generating efficiencies in solution and film may be due in part to the large component of resin absorption. Resin excited state energy may not be efficiently transferred to the sulfonium salt. Furthermore a reduction in quantum yield is generally expected for a radical process carried out in a polymer matrix due to cage effects which prevent the escape of initially formed radicals and result in recombination (IS). However there are cases where little or no difference in quantum efficiency is noted for radical reactions in various media. Photodissociation of diacylperoxides is nearly as efficient in polystyrene below the glass transition point as in fluid solution (12). This case is similar to that of the present study since the dissociation involves a small molecule dispersed in a glassy polymer. 

The irradiation of films prepared from 1% triphenylsulfonium salts in poly(4-t-butoxycarbonyloxystyrene) with lithographically useful doses of 254 nm light generates acid which is less than 0.1% of the t-BOC groups. The efficiency of the photochemistry is several times less than the efficiency of acid generation from triphenylsulfonium salts in solution. The catalytic chain is about 1000 for the t-BOC deprotection step at 100°C. This implies that catalyst diffusion during postbake is on the order of 50A... 

Acid-catalyzed photoresist films acid diffusion, 35 acid generation, 303233/341 advantages, 28 catalytic chain length, 3435r development of classes of cationic photoinitiators, 28 experimental procedure, 35-36 generation mechanism from irradiation of triphenylsulfonium salts, 28-29 merocyanine dye method for acid analysis, 30,31/33/... 

Addition of an organic radical apparently takes place in the photochemical phenylation of naphthalene by triphenylsulfonium salts [187], a reaction which takes place also intramolecularly, with one of the aromatic substituents acting as the donor and the other one as acceptor (Scheme 33) [188]. Similarly the alkylation of aromatics by chloroacetonirile occurs [189], where the radical arises from chloride splitting. The radical can also result from the protonation of... 

Intermolecular PET from photoexcited JV-ethyl-2-ethylphenothiazine [87] and perylene [88], for example, to triphenylsulfonium salts produces C—S cleavage to provide diphenylsulfi.de and phenyl radical as well as the cation-radical of the sensitizer. 

In recent years, the photochemistry and polymerization behaviour of iodonium 12,37,38.39) triphenylsulfonium salts was investigated. Crivello and... 

Fig. 4. Mechani sm of Bronsted acid formation by photolysis of a triphenylsulfonium salt (X BF4, SbFg etc.)...

The photolysis of dialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenyl-sulfonium salts is different from that of triphenylsulfonium salts. The latter compounds undergo irreversible photoinduced carbon-sulfur bond cleavage the former compounds, however, react by reversible photodissociation and form resonance-stabilized ylids as shown in Fig. 5. Because of the slow thermally induced reverse reaction, only small equilibrium concentrations of the ylid and acid arc present during irradiation and the concentration will rapidly decrease when photolysis has been terminated. Therefore, in contrast to triarylsulfonium salt initiation, no dark reaction will continue after the irradiation step. 

Thermodynamic data of the epoxy polymerization with triphenylsulfonium salt photoinitiators can be obtained by differential scanning calorimetric measurements (DSC). Figure 6 shows the DSC diagram of the polymerization of Bisphenol-A di-glycidylether with triphenylsulfonium hexafluorophosphate, irradiation being carried... 

For applications, such hybrid systems are limited to either diaryliodonium salts or aryldiazonium photoinitiators and suitable radical initiators. Triphenylsulfonium salts, however, are not active as cocatalysts in the presence of free-radical initiators. 

Photopolymerizations. 5 ml Aliquots of a 0.02 mol/Iiter solution of the appropriate triphenylsulfonium salt photoinitiator in cyclohexene oxide were placed in sealed pyrex tubes. The tubes were then placed in a "merry-go-round" holder and irradiated using a Hanovia 450W medium pressure mercury arc lamp. The entire apparatus was immersed in a waterbath at 25°C. At appropriate times, the tubes were removed and poured into methanol containing a small amount of NH4OH. The precipitated polymers were isolated by filtration followed by washing with methanol and drying in vacuo at 60°C overnight. 

In 1952, Wittig and Fritz reported that treatment of a triphenylsulfonium salt with phenyllithium afforded directly diphenylsulfide (3) and biphenyl (4). They also showed that treatment of the triphenylsulfonium salt with trityl sodium led to tetraphenylmethane, although in poor yield. Later, in 1962, Sheppard described the reaction of phenyllithium with phenylsulfur trifluoride or with sulfur tetrafluoride at - 80 C, which afforded biphenyl and diphenylsulfide. At the same time, Franzen et al. suggested that a tetracoordinate sulfurane was intermediately formed and collapsed to lead to the ligand coupling products. i... 

It is only recently that the tetraphenylsulfurane (11) was detected by low temperature NMR experiments in the reaction of triphenylsulfonium salt with phenyllithium.28,29 2,2 -Biphenyl-ylenediphenylsulfurane (12), prepared by a similar method, was also recently detected by low temperature NMR experiments. predicted by Trost, tjjg bis(2,2 -biphenylylene)sulfurane (13) was eventually isolated and its X-ray structure determined. 

A number of ligand coupling reactions involving the treatment of an heteroaryllithium reagent with triphenylsulfonium salts leading to phenyl-heteroaryl compounds [(14) - (17) for example] have also... 

Dektar, J. L., Hacker, N. P., Triphenylsulfonium Salt Photochemistry New Evidence for Triplet Excited state Reactions, J. Org. Chem. 1988, 53, 1833 1835. 

Electron transfer photosensitization of iodonium salts and sulfonium salts is similar primary differences are a result of the stronger oxidizing power of iodonium salts compared to sulfonium salts ( (red) = —0.7 V and —1.2V versus SCE for diphenyliodonium and triphenylsulfonium salts respectively, see below). Triplet sensitization is again similar, including the triplet energies, and has been discussed above. 

Both diphenyliodonium and triphenylsulfonium salts quench anthracene singlet at near the diffusion controlled rate. Quantum yields at infinite onium salt concentration are identical for both as well, 0.10 0.02 and 0.09 0.02, respectively [91,93]. 

These NMR experiments at low temperature and the product analysis indicate that both triphenylsulfonium salt and diphenyl sulfoxide react with PhLi to give an identical tetraphenyl sulfurane 101 as a discrete intermediate at low temperature, which, on warming to room temperature, decomposes to diphenyl sulfide 102 and biphenyl 103. Interestingly, the four phenyl groups in the sulfurane 101 become spectroscopically identical, suggesting that the pseudorotation takes place rapidly even at low temperature such as -105 °C. 

The deprotection chemistry has been incorporated into the acid generator structure itself [177]. Phenolic hydroxyl groups pendant from triphenylsulfonium salts were protected with tBOC (Fig. 43). This dissolution inhibiting PAG mixed with PHOST becomes base soluble through photochemically-induced acid-catalyzed deprotection and thus the exposed area dissolves rapidly in aqueous base, which was named SUCCESS and promoted by BASF. A similar approach has been later reported on o-nitrobenzyl sulfonate acid generators, in which a tert-butyl ester was attached to the benzene ring for acid-generation and acid-catalyzed deprotection on one molecule (Fig. 43) [178]. 

Crivello and J.FI.W. Lam,. Polym. Set Polym. Lett. Ed. 17, 759 (1979) J.V. Crivello and J.L. Lee, Photosensitized cationic polymerizations using dialkylphenacylsulfonium and dialkyl(4 hydro xyphenyl)sulfonium salt photoinitiators, Macromolecules, 14, 1141 (1981) S.P. Pappas, Photo generation of acid Part 6 A review of basic principles for resist imaging applications, J. Imaging Technol. 11, 146 (1985) J.L. Dektar and N.P. Hacker, Triphenylsulfonium salt photochemistry. New evidence for triplet excited state reactions, J. Org. Chem., 53, (1988) J.L. Dektar and N.P. Hacker, Photochemistry of triarylsulfonium salts, J. Am. Chem. Soc. 112, 6004 (1990) G. Pohlers, J.C. Sciano, R.F. Sinta, R. Brainard, and D. Pai, Mechanistic studies of photoacid gen eration from substituted 4,6 bis(trichloromethyl) 1,3,5 triazines, Chem. Mater. 9, 1353 (1997). 

Initial attempts to demonstrate UV-curability of epoxysilicones were unsuccessful because crystalline, high melting unsubstituted diphenyl iodonium salts and triphenylsulfonium salts were completely immiscible in silicone matrices without the use of solvents. 

Wittig reported the isolation of biphenyl and diphenyl-sulfide upon treatment of triphenylsulfonium salt with phenyUithium. ) Although the origin of these products may be via the decomposition of a sulfurane, a... 

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