photoisomerization
noun
pho·to·isom·er·iza·tion
variants:
also British photoisomerisation \ ˌfōt-ō-ī-ˌsäm-ə-rə-ˈzā-shən 
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Medical Definition of photoisomerization
: the light-initiated process of change from one isomeric form of a compound, radical, or ion to another