No abstract

No abstract

No abstract

The photophysics and photochemical behavior of the phenoxazin‐3‐one dyes, resazurin and resorufin, have been studied in aqueous solutions. The irradiation of resazurin in the presence of amines leads to deoxygenation of the N‐oxide group, giving resorufin. This photoreaction is highly dependent on the amine structure and is efficient only in the presence of tertiary aliphatic amines. The absorption and fluorescence properties of these dyes are dependent on pH. At pH above 7.5 both dyes are in their anionic form. For resorufin this form is highly fluorescent (ΦF= 0.75). At lower pH the fluorescence is strongly reduced. The N‐oxide dye presents a very weak fluorescence quantum yield (0.11), which also is reduced at low pH. Flash photolysis experiments allowed characterization of the triplet state and the transients formed after irradiation of these dyes in the absence and presence of amines. The triplet quantum yields are 0.08 for resazurin and 0.04 for resorufin. The photodeoxygenation of N‐oxide in the presence of amines occurs from the triplet state.

The quenching of benzophenone by olefins having allylic hydrogens leads to hydrogen abstraction in addition to quenching by the ir system. The fraction of quenching events that leads to hydrogen abstraction in benzene at 27 °C ranges from ~0 for conjugated dienes to 94% for 1,4-cyclohexadiene. For cyclohexene the rate constant for abstraction is 1.7 X 107 M"1 s"1 and accounts for 23% of the quenching events. Deuteration leads to a decrease in the fraction of abstraction, which for cyclohexene-tf10 is 11%. Hydrogen abstraction has a higher activation energy than quenching by the double bond; as a result its importance increases with increasing temperatures. The absolute rate constants for the reactions of iert-butoxy radicals with the olefins were also measured.

The polymerization of 2-hydroxyethyl methacrylate photoinitiated by riboflavin in the presence of triethanolamine was investigated. The polymerization was also studied using as photoinitiator lumichrome, the major product obtained in the anaerobic photoreduction of riboflavin. Photopolymerization rates were measured as a function of amine concentration in the UV (366 nm) and visible (>450 nm) regions. The quenching of the excited states of the dyes by triethanolamine was investigated by fluorescence lifetime and laser flash photolysis experiments. Quenching rate constants were determined in the absence and the presence of monomer. These rate constants and singlet and triplet lifetimes were used to fit the polymerization rate vs amine concentration curves. From the fitting it was concluded that the interaction of both singlet and triplet excited states with the amine led to the 2-hydroxyethyl methacrylate polymerization.

No abstract

The quenching of excited singlet and triplet states of riboflavin, lumiflavin and lumichrome was investigated in methanol. The quenchers were aromatic electron donors and aliphatic amines. Bimolecular quenching rate constants were determined from static and dynamic fluorescence measurements. Triplet quenching was studied by laser flash photolysis. Transient absorption spectra showed the presence of semireduced flavins and lumichrome, and the radical cation of quenchers. The results confirm that the quenching rate constants for aliphatic donors are lower than those of aromatic donors of similar oxidation potential. Plots of the quenching rate constants vs. the free energy for the electron transfer reaction, DG , were fitted by the Rehm-Weller model of electron transfer quenching. The aliphatic quenchers needed a higher intrinsic barrier for the fitting that was ascribed to the internal reorganisation of the amines. For the aromatic donors the singlet quenching rate constants reach the diffusional limit at highly negative DG . However, for the triplet quenching the limiting value of the rate constants is lower than the plateau of the singlet quenching. This is explained in terms of the non-adiabaticity of the triplet quenching process.