Cycloaddition Reactions of Carbonyl Compounds Possessing High Energy Content

Detailed structure and kinetic analyses of the photocycloaddition of alkanones to (a) α,β-unsaturated nitriles and (b) enol ethers have been performed. The excited singlet states of alkanones were found to add stereospecifically and regiospecifically to 3-unsaturated nitriles. Alkanone triplets do not add to unsaturated nitriles but instead transfer triplet excitation (at the diffusion controlled rate) and thereby sensitize dimerization and cis-trans isomerization of the nitrile. Even when all the alkanone singlets are quenched by a ground state nitrile, oxetane formation is relatively inefficient. The possibility is put forward that singlet exciplex of alkanone and nitrile is formed directly, but then dissociates (undergoes internal conversion) about ten times faster than it collapses to oxetane. In contrast, both S1 and T1 of alkanones add to enol ethers and effect both oxetane formation and cis-trans isomerization. Evidence is produced in support of singlet and triplet biradicals as intermediates to oxetanes. The possibility of a precursor to the biradicals is considered. Alkanone fluorescence quenching, by ethylenes known to form oxetanes, is used as a probe of the stereoelectronjc requirements of the quenching process and to study excited state conformations

Energy Transfer Processes

A discussion of the major mechanisms for electronic energy transfer for organic molecules is presented. Application of the techniques and ideas of electronic energy transfer to study the properties of polymers is given

Damage Control of DNA in Nucleosome Core Particles When a Histone's Loving, Protective Embrace Is Just Not Good Enough

AbstractPackaging DNA into nucleosome core particles generally offers protection from damage by molecules diffusing in solution. However, on page 403 of this issue, Barton and coworkers report that although noncovalently bound, activated Rh (Rhodium) does not readily bind within nucleosomal DNA, activated Rh that is covalently tethered to the 5′ terminus of a histone-associated oligonucleotide oxidizes guanine bases from a distance of up to 24 base pairs, demonstrating that histones do not protect DNA from long-range damage from the transport of charge through stacked bases. This implies that oxidative damage generated on DNA in vivo may spread from an initially damaged site to distal sites. Once created, such sites may persist and be resistant to repair because of the protective packaging by histones; they thus may result in permanent mutations

Thermal and Photochemical Generation of Electronically Excited Organic Molecules: Tetramethyl-1,2-dioxetane and Naphthvalene

In this paper some examples of reactions which yield electronically excited products are presented. In particular, the 1,2-dioxetanes are discussed. These molecules cleave cleanly into two carbonyl fragments when heated or irradiated. It will be shown that these simple, high energy, four atom arrays can efficiently generate electronically excited carbonyl fragments when they decompose. Surprisingly, tetramethyl-1,2-dioxetane (V) yields acetone triplet selectively upon thermolysis or photolysis. A study of the kinetics of thermal decomposition of V as a function of solvent, and the mechanistic implications of these observations will be discussed. Based on summation of available evidence, we propose that the thermolyses of 1,2-dioxetanes require specific vibrational motions which enhance spin—orbit coupling as the molecule fragments, and allow efficient decomposition into triplet states. In the photochemistry of V an exceptional 'anti-Stokes' sensitization is demonstrated, which suggests the possibility of efficient execution of 'blue light' photochemistry with 'red light', and a 'quantum chain' reaction. It is demonstrated that, at 77°K, naphthvalene undergoes efficient photochemical conversion to naphthalene triplets. Finally, the relationship of radiationless electronic relaxation and primary photochemical processes is discussed in the framework of our results

The Mechanism of Addition of Diazo-alkanes to Cyclopropanones

The addition of diazo-alkanes to cyclopropanones yields cyclobutanones whose structures may be predicted on the basis of conformational regulation of the migrating carbon-carbon bond

A TR ESR Study of the Quenching of Photoexcited Dioxouranium (VI) Salts by Stable Nitroxyl Free Radicals

TR ESR spectroscopy was applied to the study of the quenching of excited dioxouranium (VI) (uranyl) nitrate and sulfate by stable nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) family. Photoexcitation of uranyl in solutions of alcohols of moderate viscosity (η = 3-10 cP) in the presence of TEMPO leads to CIDEP signals of TEMPO due to a radical triplet pair mechanism (RTPM). Polarized nitroxyls were also observed in solutions of polyelectrolyte sodium poly(styrenesulfonate), NaPSS, in the presence of the nitroxyl with a positively charged trimethylammonium group. Photolysis of uranyl salts in solutions of alcohols leads to the generation of free radicals of alcohols. No CIDEP of these radicals was observed, distinguishing U2 2+* from its organic analog, the triplet benzophenone. The probable reason for the lack of polarization in uranyl photoreduction reactions is the difficult access of free radicals to the U atom of the solvated radical UO2+ (V); this atom bears the unpaired electron. The role of polyelectrolytes in the enhancement of the quenching of excited states is discussed. Results are in agreement with the statement that photoexcited uranyl has a triplet multiplicity