Nigel P. Hacker

Cyclopentadienylidene. A matrix isolation study exploiting photolysis with unpolarized and plane-polarized light.

Diazocyclopentadiene (1) was photolyzed in N(2), CO, and other low-temperature matrices. The resulting carbene, cyclopentadienylidene (2), was characterized by its UV and IR spectra, and its thermal dimerization and reaction with CO were observed. Photolysis of 1 with plane-polarized light gave matrices exhibiting linear dichroism. Comparison of dichroic IR and UV spectra revealed that the photolysis proceeds via an excited A(l) state of the diazo compound. Plane-polarized irradiation of the corresponding ketene (4) in CO matrices resulted in photoreorientation of the molecules of 4 without significant loss.

Novel photoinduced electron transfer reactions between naphthalene and triphenylsulphonium salts

Abstract Irradiation of solutions of naphthalene and triphenylsulphonium trifluoromethanesulphonate initiates an electron transfer reaction which gives 1- and 2-phenylnaphthalenes, 2- and 4-phenylthiobiphenyls, and diphenyl sulphide. The phenylnaphthalenes are formed by an in-cage reaction between phenyl radical and naphthalene radical cation, whereas diphenyl sulphide is an escape product. A second cage process, oxidation of diphenylsulphide by naphthalene radical cation, generates diphenylsulphinyl radical cation; reaction of this intermediate with phenyl radical gives the phenylthiobiphenyl isomers. However, irradiation of anthracene and triphenylsulphonium triflate yields only phenylated anthracenes and diphenylsulphide, because anthracene radical cation cannot oxidize diphenylsulphide.

Cationic Photoinitiators: Solid State Photochemistry of Triphenylsulfonium Salts

Abstract Triphenylsulfonium bromide, triflate, tetrafluoroborate, hexafluoroarsenate, hexafluorophosphate and hexafluoroantimonate were irradiate in solution and in the solid state. The photoproducts, 2-, 3- and 4-phenylthiobiophenyl are formed by an in-cage fragmentation-recombination mechanism, whereas diphenylsulfide is formed by a cage-escape reaction. In dilute solution, the ratio of cage to escape products is approximately one for each salt. Also in concentrated solutions, an identical cage : escape ratio is observed for all salts except for the bromide, which favors the escape reaction. Photolysis of the salts in the solid state shows remarkable counter ion dependence and cage : escape ratios as high as 5 : 1 are observed. The mechanism for both in-cage and escape reactions is described.

Photochemical reaction of onium salts with N,N-dimethylaniline: Evidence for photoinduced electron transfer

Abstract Direct photolysis of diphenyliodonium salts gives significantly more iodobiphenyls than in the in the presence of dimethylaniline. An electron transfer reaction from the singlet excited state of dimethylaniline is proposed

Base-catalyzed photosensitive polyimide

A scheme for imaging of polyimide films is described which is based on the amine-catalyzed imidization of poly(amic alkyl ester) precursor polymers. Films containing amine photogenerators along with poly(amic alkyl esters) are patterned by exposure followed by heating to partially imidize the exposed portion of the film. Negative images are developed by taking advantage of the greater solubility of the precursor polymer which is dissolved in an appropriate solvent mixture. A final cure is carried out to complete the imidization of the patterned film.

Mechanistic studies on the poly(4-tert-butoxycarbonyloxystyrene)/triphenylsulfonium salt photoinitiation process

Studies on the poly(4-tert-butoxycarbonyloxystyrene)/triphenylsulfonium salt (TBOC resist) photoinitiation process show strong evidence for a dual photoinitiation process. Photolysis studies on the relative quantum yields and also the ratios for in-cage versus cage-escape sulfide show that the TBOC polymer behaves differently from other polymers and likely sensitizes the decomposition of some of the triphenylsulfonium salt. Also the TBOC resist shows photoactivity at 300 nm, where the polymer absorbs but the triphenylsulfonium salt has only very weak absorbance, which suggests that the sensitization process is initiated by the polymer. However the fact that substantial amounts of in-cage products are also formed for photolysis of the TBOC resist, implicates a direct photodecomposition of the triphenylsulfonium salt. Fluorescence quenching studies on TBOC resist films show that the sensitization proceeds by electron transfer from the singlet excited state of the TBOC polymer and that a mainly static quenching mechanism is involved. The photoinitiation of the TBOC cleavage reaction proceeds by a dual initiation pathway which involves both the excited state of the polymer and the excited state of the triphenylsulfonium salt.

Photochemistry and fluorescence spectroscopy of polymeric materials containing triphenylsulfonium salts

Triphenylsulfonium salts (TPS) have been formulated with polymers to make photosensitive systems for optical and optoelectronic applications. Photolysis of these salts generates strong acid which has been used in cross-linking reactions, deprotection reactions, and depolymerization reactions for photosensitive polymers, photodeformable polymers, and photo-doped conducting polymers. In addition, materials best described as polymeric sulfonium salts have been found to become conducting after photolysis. We have studied the photochemistry of TPS in polymer films and in solution. TPS photodecomposes by a mechanism that gives both in-cage recombination reactions and cage-escape products, and by reaction with solvent or polymer matrix. These products give cage/escape (C/E) ratios which are sensitive to the viscosity, rigidity, and polarity of the environment, and also the excited state from which the photochemistry occurs. Details of the reactivity and C/E ratios from photolysis of TPS salts in solution, have made it possible to determine their reactivity in polymers. In some cases the polymer behaves as a viscous solvent leading to high C/E ratios and in other systems the polymer excited state can sensitize the decomposition of TPS salts to give lower C/E ratios. Fluorescence studies on these polymers and quenching studies with TPS salts have helped to determine which excited states of the polymer and TPS salts are involved, and whether there is static or dynamic quenching in these systems. The photochemistry of model compounds for the polymeric sulfonium salts is also described.