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The triplet-triplet extinction coefficients of seventeen compounds have been determined in cyclohexane and benzene at room temperature by a method involving energy transfer. Large changes in extinction coefficient at the wavelength maximum &(Am&) were observed upon changing solvent from cyclohexane to benzene. Values of &(Amax) were determined relative to a value of &(Amax) for the ketyl radical ( $zCOH) of 3220 M-' cm-' in water, assuming no change of oscillator strength with solvent. A number of triplet transfer rates, and other reactivities of benzophenone triplet and ketyl radicals, were also estimated.

The technique of time resolved laser flash spectroscopy has been used to study the photophysical and photochemical properties of the lowest triplet of benzophenone in water and acetonitrile. New estimates have been obtained for the triplet extinction coefficient, unimolecuIar and bimolecular rate constant of decay in both solvents, as well as a determination of the quantum yield of ketyl formation in pure water and the rate constants of hydrogen abstraction from various lactams in both solvents. Evidence for hydrogen abstraction from water by a triplet of benzophenone is found. Correlation between rate constants of hydrogen abstraction from lactams by the lowest benzophenone triplet and their ionization potential is discussed. Reversibility of these hydrogen abstractions is considered.

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The [6,6]-ring bridged methanofullerenes 61,61-bis[4-tert-butylbenzoate]-1,2-dihydro-1,2-methanofullerene [60] (1), 61,61-bis[4-(tert-butyldiphenylsilyloxymethyl)phenyl]-1,2-dihydro-1,2-methanofullerene[60] (2), and 61-[(ethoxycarbonyl)methylcarboxylate]-1,2-dihydro-1,2-methanofullerene[60] (3) were synthesized by the Diederich method (3) or the Wudl method via tosylhydrazone salts (1 and 2). Groundstate absorption spectra of the methanofullerenes in toluene and cyclohexane solutions are presented and possible assignments discussed. The sharp, 430 nm transition is associated with the existence of less than 60 p electrons, whereas a broad band peaking at 495 nm as well as weak features in the 700 nm region could be related to forbidden transitions of C 60 . The allowed transitions of C 60 in the UV region are modified relatively little in the methanofullerenes. Laser flash photolysis and pulse radiolysis techniques were used to obtain triplet ± triplet absorption spectra between 400 and 1100 nm and determine photophysical properties. The three methanofullerenes have very similar T ± T spectra, with a strong peak at 720 nm whose molar absorption coefficient is of the order of 14 000 m ) production by energy transfer from the triplet state of the three methanofullerenes, implying that the quantum yield of triplet production of each is % 1. The results indicate that it is possible for the spectroscopic and photophysical properties of methanofullerenes to vary little with the nature of the methano adduct and to undergo only slight modifications with respect to the corresponding properties of C 60 . Up to now, this has been found valid only when the functionalization does not involve an electron donor. The biological photosensitization efficiencies of these methanofullerenes are therefore expected to be similar to those of C 60 but with their hydrophobicity and intracellular site delivery modulated by the nature of the methano-adduct.

Thymine and uracil triplet-triplet absorption spectra and triplet excited state lifetimes have been observed in acetonitrile and water by nanosecond laser flash spectroscopy. A study of triplet energy transfer from these pyrimidines to retinol has also allowed an estimation of the triplet extinction coefficient ETT of thymine and uracil. These E~~ were then used to determine the triplet quantum yields @+ of both pyrimidines in acetonitrile and water.

Abstract— The triplet absorption spectra, lifetimes, extinction coefficients, eTT, and intersystem crossing quantum yields to the lowest triplet T1, øT1, of thymidine, thymidine monophosphate, uridine and uridine monophosphate, have been determined in various solvents at 300 K. The effect of H‐bonding on øT1, of these nucleosides and nucleotides and also of uracil has been determined and discussed. This effect allows, an ordering of l,3n, π* and 1,3π, π* states in protic and aprotic solvents.