Absorption, monitoring triplet state

A rather important aspeet that should be eonsidered is that interfaeial quenching of dyes does not neeessarily imply an eleetron-transfer step. Indeed, many photoehemieal reactions involving anthracene oeeur via energy transfer rather than ET [128]. A way to discern between both kinds of meehanisms is via monitoring the accumulation of photoproducts at the interfaee. Eor instance, heterogeneous quenehing of water-soluble porphyrins by TCNQ at the water-toluene interfaee showed a elear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the exeited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenehing rate eonstant of the order of 92M ems. ... 

Irradiation of a diazonium compound in EPA glass at 77°K and monitoring the reaction by UV revealed the presence of new absorption bands, which disappeared upon warming to room temperature. Little or no evolution of nitrogen occurred in these experiments. Hence the low-temperature intermediate has been proposed to be a rather stable triplet state of the diazonium compound. 

Reticulum ATPase [105,106], Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe s phosphorescence [107-112] and/or the recovery of the ground state absorption [113— 118] or fluorescence [119-122], The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. 

The triplet state decay kinetics of benzophenone has been monitored in fluorinated surfactants (sodium perfluorooctanoate, SPFO) where the surfactant does not quench the triplet40>. In this case, the excited benzophenone cannot react with the surfactant and the excited molecule escapes into the aqueous phase. When increasing amounts of SDS is added to solution, the observed triplet lifetime (by nanosecond transient absorption techniques) decreases indicating that hydrogen abstraction is occurring from the SDS (Fig. 17). 

The principle of laser flash photolysis and its oxygen-sensing application is shown in Fig. 10-7 [69]. In laser flash photolysis techniques, a strong xenon or laser lamp is used to generate Ti states via So-to-Si absorption, and this is followed by intersystem crossing from Si to T], The concentrations of species in the triplet state are determined via the transient T]-to-T2 absorption, which is monitored by means of a second light source. This method is particularly useful when the lifetimes of the triplet state are short (in the range of nanoseconds). 

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