Monitoring of Fast Reactions in SCFs using Time-resolved Vibrational Spectroscopy

4 Monitoring of Fast Reactions in SCFs using Time-resolved Vibrational Spectroscopy 

UV-Vis flash photolysis is the most widely used technique with which to follow fast photochemical reactions. However, although flash photolysis provides excellent kinetic information about excited states and reaction intermediates, UV-Vis spectra in solution are often broad and featureless and provide little structural information. Moreover, in experiments where several photoproducts are produced, overlapping of the broad absorption bands can make it very difficult to elucidate the mechanism. Transient vibrational spectroscopy provides more detailed structural information to probe photochemical reactions especially since Raman and IR bands of large molecules in solution are generally much narrower than UV-Vis bands. 

The most widely used vibrational spectroscopic technique is time-resolved resonance Raman spectroscopy (TR ) [65]. This has been used successfully to obtain structural information about organic excited states in SCCO2. McGar-vey and co-workers probed the excited triplet state of anthracene in SCCO2 [66]. However, TR experiments involve data collection over many laser pulses, with all of the problems associated with secondary photolysis. These problems have prevented TR being used effectively to follow chemical reactions apart from highly photoreversible processes. To our knowledge, TR has not yet been used to follow chemical reactions in SCFs. Recently, however. 

Tahara reported [67] the design of a flow system for obtaining ultrafast measurements, including picosecond-TR, in SCFs. Such flow systems should allow wider use of TR to monitor chemical reactions under supercritical conditions. 

Time-resolved infrared spectroscopy (TRIR) has been outstandingly successful in identifying reactive intermediates and excited states of both metal carbonyl [68,69] and organic complexes in solution [70-72]. Some time ago, the potential of TRIR for the elucidation of photochemical reactions in SCFs was demonstrated [73]. TRIR is particularly suited to probe metal carbonyl reactions in SCFs because v(CO) IR bands are relatively narrow so that several different species can be easily detected. Until now, TRIR measurements have largely been performed using tunable IR lasers as the IR source and this has restricted the application of TRIR to the specialist laboratory [68]. However, recent developments in step-scan FTIR spectroscopy promise to open up TRIR to the wider scientific community [74]. 

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