Photoreduction in the triplet state

A polymer containing pe,ndant benzophenone groups (XV) is found to photocrosslink. The mechanism is inter.molecular coupling of radicals formed via photoreduction in the triplet state. In solution, crosslinking is concentration dependent as intramolecular cyclization competes (21). 

Many aromatic ketones are even more efficient in initiating photopolymerizations as a result of photoreductions in the triplet state. The mechanism of photoreduction is now conunonly accepted to be one of electron transfer. Upon irradiation, one of the n, nonbonding, electrons of the oxygen atom is promoted into either a a or n antibonding orbital. These are n- <7 and n- r transitions. They are the most readily observed transitions of carbonyl compounds, requiring less energy than n- n transitions, and can be illustrated as follows" ... 

Many aromatic ketones are very efficient initiators of photo polymerizations as a result of photoreductions in the triplet state Upon irradiation of the ketone, one of the n nonbonding electrons of the oxygen atom undergoes a n n transition. This was discussed in Chapter 1 and, as it is explained in the beginning of this Chapter, makes the oxygen in the exeited state eleetron deficient and gives it the ability to react similarly to an electrophilic alkoxy radical. This can be illustrated as follows ... 

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See also in sourсe #XX -- [ , ]

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