Bimolecular photoprocesses

The apparent bimolecular photoprocess of fourteen centers (44) would be even more remarkable, if it were concerted. Barltrop and Hesp (1965), who discovered it, write a stepwise mechanism for cyclization ... 

The Cr(VI) example quoted above illustrates, however, that besides the monomolecular gdso bimolecular photoprocesses are effective ... 

The lowest energy optical transition for the binudear systems is da - Q. The excitation results in formation of a formal metal-metal single bond in the excited state. The transition is metal localized and can be viewed as movement of an electron from an orbital localized on the exterior of the M2 unit (the da orbital) to an orbital localized in the interior of the dimer cage (the pa orbital). The excitation results in hole formation localized on a metal center at an open coordination site (Figure 2). The lifetime of the 3(da pa) excited state, normally in between 100 ns and 10 us, makes M2 systems attractive for bimolecular photoprocesses. 

In solution, the rate of the many bimolecular photoprocesses due to a polymer chain may be limited by the rate of mutual diffusion of the interacting species, but the most important factor in the case of a polymeric system is the chain conformation and flexibility. Guillet and coworkers [27,28] studied photophysical properties for naphthyl-substituted polymethacrylate (PNMA) solutions very extensively. The spectral properties of absorption and emission of PNMA studied using different solvents under different conditions are shown in Figure 4.5. Somersall and Guillet [27] observed delayed fluorescence, involving... 

Bimolecular quenching of the excited states of metal complexes generally involves electron transfer or energy transfer processes ( 1). Recently, however, Pt2(pop)4 " has been found to undergo a photochemical reaction involving atom abstraction as a primary photoprocess (.26). The reaction involves the catalytic conversion of isopropanol to acetone ... 

No attention is given to the mechanistic importance of a reaction rather, an attempt has been made to concentrate on reactions that have an actual (potential) synthetic role. This is not always an obvious selection, because photochemistry has not been sufficiently used in such syntheses, and mechanistic studies are not necessarily a reliable guide towards this aim. As an example, hydrogen abstraction by ketones (Scheme 1.3), which probably is the most thoroughly studied photochemical reaction, is not mechanistically discussed. Neither is presented the resultant photoreduction of ketones (Scheme 1.3, path a), because this will hardly ever become a sensible synthetic alternative for the reduction. However, other reactions arising from the same primary photoprocess, namely bimolecular reduction (path b) and... 

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