Reactive oxygen species are potentially damaging molecules. An important function of antioxidants is to intercept harmful triplet states, in order to prevent the formation of singlet oxygen, or to quench singlet oxygen directly. However, antioxidants are also reactive towards other active oxygen species such as the hydroxyl radical, the superoxide anion and the non-excited oxygen ground state in the presence of radical initiators. It is well known that¯avonoids and carotenoids show strong antioxidant properties. Polyenes and carotenoids are the best known among the compounds that quench singlet oxygen by ef®cient energy transfer. A large number of modi®ed, synthetic analogues and derivatives have been synthesised to prepare even better quenchers than the natural carotenoids. Phenols are also excellent chain-breaking antioxidants. Recently, many indigoid dyes (including bacterial indigoids) were studied, with the remarkable result that most, but not all, members of this class of chromophores quench singlet oxygen at the diffusion limit and some of them are excellent radical traps. It has been shown in this study that a quantitative assessment of antioxidant properties of avonoids, carotenoids, phenols and natural indigoids can be achieved using the following three assays: (1) oxygen pressure dependence; (2) peroxide formation; (3) singlet oxygen quenching. Reactivities towards both excited states and ground state radicals can be properly described by these assays. The remarkable role of b-carotene as an`unusual antioxidant' (Burton GW and Ingold KU, Science 224: 569±573 (1984)) in reactions using various oxygen pressures becomes clearer. The socalled`pro-oxidant effects' concern primarily the antioxidant itself and its degradation, since no or very little damage to the substrate occurs in this type of experiment. Three main categories of antioxidants may be classi®ed: (1) excellent antioxidants that perfectly quench excited states as well as ground state radicals (eg actinioerythrol, astaxanthin); (2) good antioxidants that strongly inhibit peroxide formation but are less ef®cient in quenching excited states (eg¯avonols, tocopherols) or lead to considerable degradation of the antioxidant itself (eg b-carotene, lycopene); (3) moderate antioxidants that fail to excel in both reactivities (eg z-carotene,¯avone).

The interplay of photochromism and fluorescence was studied by attaching anthracene as chromophore to dithienylperfluorocyclopentene (1,2-bis[5-anthryl-2-methylthien-3-yl]perfluorocyclopentene, Ac-BMTFP). The blue fluorescence of the open isomer of Ac-BMTFP is suppressed by the ring-closure reaction. The spectroscopic properties and the reaction dynamics of this compound were characterized by measurements of the fluorescence yield and decay dynamics, and the quantum yields of the photochromic ring-closure and ring-opening reactions, as well as the spectra and time evolution of reaction intermediates. The data are analyzed in terms of a model potential and single-electron density matrices, which are calculated using the collective electronic oscillator (CEO) approach and the INDO/S semiempirical Hamiltonian. For the ringopening reaction, single-exponential decays with a time constant of 8 ( 0.5 ps were determined for the photoinduced bleaching and absorption transients. In contrast, because of the presence of reacting and nonreacting conformers, the dynamics measured for ring closure are more complex. Both conformers of the open isomer undergo a fast electronic-conformational relaxation on a time scale of ≈0.9 ps after excitation of the S 1 or S 3 state. Nonreacting conformers fluoresce with a distribution of lifetimes ranging from less than 100 ps to more than 400 ps. Reacting conformers reach a precursor state with a lifetime of 10 ps from which the ring-closure reaction takes place. The rates of the ring-opening and ring-closure reactions are determined as 9.5 × 10 9 and 6 × 10 10 s -1 , respectively. Rather than being a drawback, the presence of different conformers in the sample is argued to be a requirement for applications relying on efficient switching of the fluorescence.

Abstract