The　photocycloadditions　of　1,4-dihydropyridines　(DHPs)　were　achieved　by　using　phenacylimidazoliums　(Plms)　as　photosensitizers.　Irradiation　of　DHPs　3a-g　in　the　presence　of　Plms　1a-e　and　2　performed　an　efficient　formation　of　3,9-diazatetraasteranes　in　shorter　times　under　a　lower　power　lamp.　The　mechanism　of　photocycloaddition　catalyzed　by　Plm　was　studied　by　laser　flash　photolysis　and　theoretical　OFT　computation.　These　time-resolved　results　showed　that　the　triplet　excited　states　of　Plms　were　generated　with　high　efficiency　and　detected　by　their　characteristic　ultraviolet　absorptions,　which　were　quenched　by　DHP　at　almost　diffusion　controlled　rate.　Theoretical　studies　suggest　that　Plm　is　involved　in　the　photocycloaddition　process　through　the　(3)(DHP　center　dot　center　dot　center　dot　Plm)*　triplet　complexes　and　assists　the　stabilization　of　intermediates.　All　subsequent　steps　are　predicted　to　be　favorable　and　exothermic,　leading　to　the　cage　dimers.　(C)　2012　Elsevier　B.V.　All　rights　reserved.