No abstract

Triphenylpyrylium tetrafluoroborate (TPP ' BF,) has been found to sensitize oxygenation of adamantylideneadamantane to its dioxetane in good yield. Product studies, measurements of quantum yields for dioxetane formation, laser flash photolysis, and quenching experiments of TPP' fluorescence by various electron donors have been carried out to elucidate the mechanism for dioxetane formation. Cyclic voltammetry was also used to examine the reactivity of pyryl radicals (TPP') with molecular oxygen. The results are consistent with a mechanism involving electron transfer from the alkene to the excited states of TPP' to give radical pairs consisting of the alkene radical cations and TPP'. The alkene radical cation reacts with molecular oxygen to afford the dioxetane. A singlet oxygen mechanism is not likely since the quenching rate of the excited triplet state of TPP' by the alkene is 400 times faster than by dissolved oxygen under the p hotooxyg en at ion c o nd it i o ns.

Picosecond time-resolved Raman spectroscopy has been used to study the ultrafast relaxation dynamics of aromatic cation radicals following two-photon ionization. In acetonitrile, integrated Raman intensities due to the cation radicals rise in tens of picoseconds, and reach their maxima at a delay time of 40-60 ps from the pump pulse. Such a slow-rise component is observed in all the cation radicals treated (biphenyl, trans-stilbene and naphthalene), suggesting that the picosecond relaxation process increasing the cation Raman intensities occurs after the photoionization of aromatic molecules. In weak polar solvents such as ethyl acetate, on the other hand, only an instrumental-limited rise (<5 ps) is observed. The rise time of the cation Raman intensity does not correlate with the dielectric relaxation time but depends on the polarity of the solvent. This result indicates that the picosecond relaxation process is not controlled by the dielectric solvent relaxation alone. The positional changes and the band narrowings of the cation Raman bands occur on a 10-20 ps time scale. These are associated with intermolecular vibrational relaxation of the cation radical toward a thermal equilibrium with solvents. The time scale of the intermolecular vibrational relaxation is the same as that of the rise component of the cation Raman intensity. From these observations, it is suggested that the thermal excitation of the solvent shell disturbs the solvation structure of the cation radical, which causes the observed picosecond change in the cation Raman intensity.

Picosecond to submillisecond photochromic reactions of 2,4-diphenyl-2H-benzopyran and 2,2,4-triphenyl-2H-benzopyran have been investigated by time-resolved absorption spectroscopy. The C−O bond cleavage of the benzopyrans (closed forms) occurs via the first excited singlet state within 2 ps to produce vibrationally excited open forms in the ground electronic state. In the subnanosecond to submillisecond time domain, several decay components with almost the same spectral profiles are observed. These components are assigned to respective stereoisomers with respect to two double bonds and one single bond of the open enone forms. From the pump-laser power dependencies of the yields of the open forms, it is suggested that the photocleavage gives at first only the open forms revertible to the closed form by a single-bond rotation, and that the photoexcitation of the first generated open forms gives rise to other open forms which need a double-bond rotation for reversion to the closed form. The photochromic reactions of a series of 2H-benzopyrans bearing substituents on the pyran ring have also been studied using nanosecond time-resolved absorption spectroscopy. The size of a substituent in the 4-position fairly affects the rate constants of the thermal reversion of the open form to the closed form.