Transient absorption spectroscopy kinetics analysis

Transient absorption spectroscopy has been a conventional technique for studying the configurational (conformational) changes that are reflected in the Tn <— T, optical transition. Picosecond to microsecond time-resolved, pump and probe measurement has been widely used to examine the triplet-state isomerizations as far as each ris-T, and the all-trans-Tj species are selectively observed. Even when their wavelengths are very similar to one another, the SVD and global-fitting analysis can successfully identify a set of triplet species appearing in different time-scales, when a correct kinetic model is built. 

Exciplex and Excimer Formation. The formation of excited complexes (excimers and exciplexes) in quenching reactions of excited organic compounds is a well-known phenomenon (cf. 70,230, 231,249). Evidence for the intermediacy of exciplexes is most readily obtained when exciplex emission is observed. However, other techniques. Including flash absorption spectroscopy (250-252) and kinetic analysis of both steady-state and dynamic quenching rate data (253) have also been used to obtain evidence for the existence of these transients. 

Electronic absorption spectroscopy is also a useful technique for the characterization of Cr intermediates. When coupled with global kinetic analysis, ligand stoichiometries, and spectra of transient species in the redox chemistry of Cr can be determined. The first applications of this global analysis to biologically relevant systems involved studies of the Cr(Vl) reactions with the main biological reductants Cys, GSH, and ascorbate (70, 97). The distinct electronic absorption features of the Cr(VI), Cr(V), Cr(lV), and Cr(lll) oxidation states... 

The conditions which determine whether flash photolysis can be used to smdy a given chemical system are (i) a precursor of the species of kinetic interest has to absorb light (normally from a pulsed laser) (ii) this species is produced on a timescale that is short relative to its lifetime in the system. Current technical developments make it easy to study timescales of nanoseconds for production and analysis of species, and the use of instrumentation with time resolution of picoseconds is already fairly common. In certain specific cases, as we will see in the last part of this chapter, it is possible to study processes on timescales greater than a few femtoseconds. Once the species of interest has been produced, it is necessary to use an appropriate rapid detection method. The most common technique involves transient optical absorption spectroscopy. In addition, luminescence has been frequently used to detect transients, and other methods such as time-resolved resonance Raman spectroscopy and electrical conductivity have provided valuable information in certain cases. 

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