Solvent dependence of electron‐transfer sensitized photolysis of onium salts

Abstract

The photoreactions of 9,10‐diphenylanthracene with two iodonium salts and a sulfonium salt in different solvents were investigated by fluorescence quenching, flash photolysis with UV/vis detection, measurements of the quantum yields of sensitizer decomposition, and photo‐CIDNP spectroscopy. Energy‐transfer sensitization is precluded by thermodynamics in these systems. In a solvent of lower polarity such as dimethoxyethane, the quenching rate constant is still about a third of the value in acetonitrile; the quantum yields of formation of radical cations and the quantum yields of sensitizer decomposition are decreased by about the same factor. In contrast, dramatic effects occur in the CIDNP spectra of the iodonium salts, which derive from two consecutive radicals pairs, the first containing the radical cation DPA± of the sensitizer and the radical On• of the onium salt, the second comprising DPA± and a phenyl radical produced by fragmentation of On•: In less polar solvents, the polarizations from the first pair are strongly reduced, and the polarizations from the second pair are larger by an order of magnitude. From a kinetic analysis, this effect was attributed to a higher rate of transformation of the first radical pair into the second in the less polar solvent. For the sulfonium salt, this transformation is faster than in the case of iodonium salts, so even in acetonitrile all polarizations stem from the second radical pair. S‐T±‐type CIDNP, which had been proposed in the literature for analogous chemical systems, could be excluded unambiguously, and all CIDNP effects could be explained by radical pair theory (S‐T0‐type CIDNP).

Photosensitized Reduction of Sulfonium Salts: Evidence for Nondissociative Electron Transfer

“…From our data, it is not possible to determine whether this forward rate is for fragmentation of the sulfuranyl radical within the primary cage to give a secondary intermediate, 4 , or for diffusive separation of the components of the primary cage. However, the photo-CIDNP results on the sensitized photoreductions of diphenyliodonium and triphenylsulfonium salts in CD 3 CN do permit a distinction . Both Ph 2 I + (return electron transfer) and C 6 H 5 D (forward reaction) are polarized to give emission.…”

“…Most directly, sulfuranyl radicals with electronegative groups such as alkoxy have been observed by ESR and absorption spectroscopy . In the absence of stabilizing substituents, alkyl and aryl sulfuranyl radicals are apparently short-lived and have not been observed . Even so, indirect tests have disclosed the intermediacy of sulfuranyl radicals in a variety of reactions.…”

“…For example, the quantum yield for the destruction of sulfonium salt by the photolysis of the triphenylsulfonium iodide charge-transfer complex is only 0.35 , …”

“…As noted above, the reductive cleavage of sulfonium salts can also be accomplished by photoinduced electron transfer ,, The forward electron transfer can be arranged so that all of the donor excited states are captured by acceptor sulfonium salt to give the ion-radical pair. Because return electron transfer can be a very fast reaction, it can be a very demanding probe of the structure and reactions of the primary ion-radical pair.…”

Chemical Transformations within the Paramagnetic World Investigated by Photo‐CIDNP

“…This discrepancy is especially pronounced with CIDNP where the polarizations are created during the lifetime of intermediate spin-correlated species, on a nanosecond timescale or faster, but persist in the diamagnetic products for a time on the order of T 1 , which means seconds for protons. Hence, CIDNP captures shorter lived intermediates and responds to faster processes than does EPR 8,9 although detection proper is by NMR spectroscopy, which is normally considered a very slow method.…”