Exciplexes in Photopolymerizations and Cycloadditions

Photoreactions of donor/acceptor systems are conventionally initiated by using one of the following three routes  

As many photochemists have pointed out, photocopolymerizations and photo-cycloadditions usually involve charge-transfer complexes as intermediates, either in the ground state or in the excited state. The charge-transfer complexes that are formed only in the electronically excited state of either donor or acceptor molecules are termed exciplexes, which are stable in the electronically excited state, but 

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