Singlet-oxygen quenching by a series of substituted diaryl tellurides
was investigated in
10% aqueous methanol by monitoring the competitive inhibition of
oxidation of 1,3-diphenylisobenzofuran. The extent of quenching of the sensitizer
triplet state was determined
by repeating the analysis in both air- and O2-saturated
solutions and by laser flash-photolysis
experiments. A Hammett analysis of the rate of quenching as a
function of substituent
constant (σ) gave a ρ value of −0.5, which is significantly lower
than the ones reported for
other chalcogenides. Although this value does not dispute the
existence of a charge-transfer
intermediate, it suggests that a strong contribution from the
heavy-atom effect of Te is
present and that Te is relatively insensitive to inductive electronic
substituent effects. The
efficiency of chemical quenching in the system was estimated to be
2−9% of the overall
quenching of 1O2.
Fluorescence probe techniques (FPT) have been used to obtain curing profiles for the polymerization of a model monomer, triethylene glycol diacrylate (TEGDA). A number of new chromophore ammonium salt triphenylbutyl and tetraphenylborates differing only in the structure of the ammonium salt moiety have been used as photoinitiators. The borates were chosen for study so that a comparison of their initiation efficiency would reveal information about the relative activity of possible initiating species produced upon borate photodecomposition. The results support our previous postulate that an R-aminoalkyl radical formed immediately after unimolecular decomposition from the tetraphenylborates is the most effective initiating species and that R-aminoalkyl radicals significantly increase the photoinitiator efficiency of triphenylbutylborates as well. The importance of other radicals to the initiation step is also discussed.
IntroductionThe photochemistry of tetrasubstituted borate salts has attracted substantial recent research interest. [2][3][4][5][6][7][8] Triphenylbutylborates have been studied most extensively, and the butyl radical formed when they are oxidized has been found to be the dominant initiating species for radical polymerization. 3,7-10 Interestingly, tetraphenylborates paired with a similar cationic partner have been observed to have similar initiating efficiencies. 11-13 Tetraphenylborates, when oxidized, are often prone to back electron transfer. Since phenyl radicals are produced from tetraphenylborates upon oxidation inefficiently, if at all, we have surmised that other initiating species must account for the efficiency of tetraarylborate salts. In our most recent work R-aminoalkyl, phenyl, and p-benzoylbenzyl radicals were trapped by methyl methacrylate when benzophenonemethylenetributylammonium triphenylbutylborate was photolyzed in its presence in solution. 14 Fluorescence probe techniques (FPT) have been recently introduced to measure and compare photoinitiator efficiencies. 12,13,15,16 FPT allow one to measure the polymerization kinetics from relative conversion data collected during the polymerization of systems that are similar. These methods are particularly useful for the evaluation of initiators that are similar in structure and for optimizing their performance. The possibility of collecting data over short time intervals (<1 s) makes the analysis simple and accurate. Several borates differing in the ammonium salt but possessing benzophenone and acetonaphthone chromophores have been chosen for the current work.
Aqueous colloidal silica suspensions modified by incorporation of tris(2,2′-bipyridyl)ruthenium(II) and zinc(II) tetrakis(N-methyl-4-pyridyl)porphyrin into the particle matrix were prepared. The photophysical properties of the complexes in this environment were investigated using laser flash photolysis and steadystate luminescence techniques. The excited states exhibit nonexponential kinetics and their lifetimes are increased compared to homogeneous aqueous solutions. The metal-to-ligand charge transfer state of the Ru(II) complex is not affected by the presence of dissolved oxygen, whereas a fraction of the triplet states of the porphyrin is quenched by O 2. Didodecyldimethylammonium ion was used to build surfactant bilayers supported on the silica surface. The possibility of using this system for transmembrane electron transfer is discussed.
A series of iodonium tetrakis(pentafluorophenyl)gallate photoinitiators were employed to catalyze the photopolymerization of three typical epoxy resins. Rates of photoacid release were compared using Quinaldine Red as acid indicator in either acetonitrile or ethanol. A new fluorescence probe technique was successfully employed to follow the cross-linking kinetics of the epoxysilicone oligomer in real time.The results show that these new salts are the most soluble, most reactive photoinitiators with the desirable stability in monomer formulations in this family.
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