Flash photolysis 4-state model

The transition state model (13) for the Sn2 reaction of a vinyl bromide was detected using laser flash photolysis.18... 

Triplet, radical anion, and radical cation spectra of furocoumarins have been determined by flash photolysis and pulse radiolysis. Triplet spectra are modified by hydrogen bonding and in water the triplets may be regarded as exciplexes. " The triplet states, semi-oxidized and semi-reduced radicals of haematoporphyrin, and three O-acyl derivatives have also been characterized. Triplet-state participation of several 4-hydroxy- and 4-methoxybenzothiazoles has been studied by flash photolysis. " These compounds are models for pheomelonin. 

Indeed, photophysical studies by laser flash photolysis combined with the determination of MMA polymerization rate (Rp) in toluene solution, have allowed evaluation of the triplet state lifetime of the above systems, as well as their relative quantum yields of initiation ( j) and a-cleavage ( ). As reported in Table 22, 3, values for poly(MBA) and poly(MBVE) are appreciably lower than those for MBI and MBEE, respectively. On this basis, the polymeric photoinitiators would be expected to display lower activity than the models in the polymerization of acrylic monomers. On the contrary, poly(MBA) and poly(MBVE), together with the related copolymers, show higher values of Rp and hence i, in the UV initiated polymerization of MMA in toluene solution. These findings, therefore, confirm the previously obtained results in film matrix, where a HDDA/BA equimolar mixture was used as curing formulation (Table 21). 

However, flash photolysis and substituent-effect studies indicate that these complexes are orders of magnitude more photoactive when the lowest-energy ES of the complex is LF in character Complexes with low-energy MLCT states show little photolability. The photochemistry for these complexes can be explained by an ES model where initial MLCT excitation is followed by rapid internal conversion-intersystem crossing to the lowest energy ES, and the nature of the ES determines the overall photoactivity . ... 

Triplet Triplet Absorption Spectra. Triplet states of stilbenes and analogues with a fixed olefinic bond, which are unable to photoisomerize, were observed at low temperatures by flash photolysis (Tables 16a and b). T-T absorption spectra of a series of stilbenes were first reported by the group of Schulte-Frohlinde [16]. These findings were confirmed by Herkstroeter and McClure [96] and extended by others [412 415]. The properties of the "stiff stilbenes turned out to be quite consistent with those of the trans-stilbenes. cis-stilbene evaded spectral characterization even at — 196°C, probably because of rapid twisting, but an absorption maximum at 25.5 x 103cm 1 may be estimated on the basis of results with 5H-dibenzo[a,d]cycloheptane as model compound [96]. In EPA at — 196°C,... 

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