William R. Bergmark

Photoisomerization of bis(9-anthryl)methane and other linked anthracenes. The role of excimers and biradicals in photodimerization

A series of linked anthracenes capable of storing photon energy through endoergic valence photo-isomerization have been studied. Photophysical and photochemical characteristics of the systems have been completely characterized by measurement of fluorescence quantum yields and lifetimes, and efficiencies for forward and reverse isomerization. The release of energy stored in photoisomers has been measured using kinetic and calorimetric techniques. From emission and lifetime data the respective roles of excimers and biradicals in anthracene photodimerization have been defined. (Author)

PHOTOPHYSICAL and PHOTOCHEMICAL PROPERTIES OF COUMARIN DYES IN AMPHIPHILIC MEDIA

Abstract— Photophysical properties of coumarin dyes solubilized in aqueous detergent solutions have been investigated including measurement of absorption and fluorescence emission maxima, and fluorescence quantum yields. Use of coumarin 4 as a fluorescence probe of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) solutions led to the conclusion that the sites for dye incorporation in micelles are significantly hydrogen‐bonded (hydrated). The inhibition of photochemical decomposition for detergent‐solubilizcd dyes has also been observed. Electron transfer from micelle‐bound dye to a water soluble acceptor, methyl viologen, has been investigated by flash photolysis.

Comment on “mechanism of photocycloadditon in in α,ω-bis-(9-anthryl)-n-alkanes”

Abstract The conclusions of the preceding paper concerning the mechanism of internal dimerization of linked anthracenes are reviewed. The new data are found consistent with a previously proposed biradical mechanism.

Photoinduced electron transfer reactions of chloranil with small-ring hydrocarbons. Contrasting reactivities of radical cation intermediates

Abstract Products that result from the irradiation of chloranil with phenylcyclopropane and with 1,2-diphenylcyclobutene in various solvents have been identified. The cyclopropane gave products that derive from nucleophilic attack on the small-ring radical cation that is formed on electron transfer quenching of the quinone excited triplet state. Contrastingly, the cyclobutene ring remains intact following an analogous radical cation formation, but undergoes an addition in which (formally) the quinone is added across an allylic CH bond. The results are consistent with the formation of radical ion pairs (triplet excited complexes, TECs) consisting of quinone radical anions and cations associated with the small-ring derivatives. Intra-ion-pair proton transfer and radical (ion) coupling reactions account for observation of the various products. Different pathways for photolysis of quinone and small-ring hydrocarbons in more polar media involving formation of free-radical ions were also established. The quenching of quinone excited triplet states and the appearance and decay of TEC-type and other intermediates were followed by laser flash photolysis.

The photochemistry of o-[(trimethylsilyl)methyl]acetophenone

Abstract The title compound was prepared and irradiated under a variety of conditions to produce no change. Irradiation in DMSO-d 6 and methanol-d 4 results in D incorporation into the o -(trimethylsilyl)methyl group. Irradiation in 2-propanol results in photoreduction (φ=0.012) which could be quenched with dienes (k q τ=4.5). These results are taken to suggest a rapid, efficient, and reversible H- transfer with no detectable silyl transfer.

The lack of photoreactivity for an ortho-tert.-butylvalerophenone

Abstract It is proposed that a rapid and reversible 1,6-hydrogen abstraction accounts for the lack of type II photoreaction in the title compound.