Chao-Jen Chung

Time-resolved laser flash spectroscopic study of benzyl radical pairs in micelle cages

Photolysis of benzyl ketones provides a convenient means of producing triplet, spin-correlated geminate radical pairs.’ In nonviscous, homogeneous solution these radical pairs rapidly (5 s) become random free radicals.* Spin correlation is destroyed when the radical fragments have separated sufficiently to allow exchange forces to become negligible relative to weak, random, magnetic forces experienced by the individual radical centers, Le., the spin-correlated triplet radical pair develops singlet character. Micelles have been shown to provide a unique type of “solvent cage” for a radical pair.j The hydrophobic portion of the micelle provides a restricted volume of space of dimensions which allow the exchange interaction between the radical center to rapidly decrease to very small values, but a t the same time the micelle preserves the compositional correlation of geminate radical pairs for relatively long periods of time (5 s ) . ~ We report here a time-resolved laser flash spectroscopic investigation of triplet benzyl radical pairs produced in micelles. A study of transient absorption of benzyl radicals as a function of (1) isotopic composition and structure of the parent ketone, (2) magnetic field, and (3) the presence or absence of an aqueous phase radical trap provides support for the occurrence of two types of geminate benzyl radical pairs: a fast decaying pair and a slow decaying pair. The structures of the ketones6 employed in this study are shown in Chart I. Typically, a given ketone in aqueous solutions of hexadecyltrimethylammonium chloride (HDTCl) was excited by a 15-11s pulse of 249-nm light produced by an excimer laser.7 Under these conditions a transient, whose spectral characteristics (A, 320 nm) are in good agreement with those of benzyl radical in homogeneous solution, is produced.8a The decay of benzyl radical absorption in methanol and in isooctane cleanly follows second-order kinetics in the time range of 50-50 000 ns. Thus, as expected,8b the disappearance of benzyl radicals in homogeneous solution, in the time range of interest, occurs predominantly via combination of random free radicals. In the same time domain, the decay of benzyl radicals (monitored by absorbance or optical density, Figure 1) in micellar solution is complex, but can be analyzed in terms of a “fast” first-order decay (rate constant, k f ) and a “slow” first-order decay (rate constant, k,), Le., in terms of eq 1, where I(?) is the measured optical density a t time t, Zf

Photoreactions of biacetyl and tetramethylethylene. Solvent and temperature effects

Abstract The photoreaction of biacetyl and tetramethylethylene yields four products (1–4). A substantial variation in product yield and quenching kinetics with temperature and isotopic substitution is found for this photoreaction.

Magnetic and micellar effects in photoreactions. : 13C NMR determination of selective 13C enrichment in a dibenzyl ketone photoproduct.

Abstract The photolysis of dibenzyl ketone (DBK) in micellar solution results in formation of 1-phenyl-4-methyl acetophenone (PMAP) as a minor product; quantitative integration of the 13 C NMR spectrum of PMAP produced by photolysis of DBK in micellar solution demonstrates that significant selective 13 C enrichment has occurred at three carbon atoms.