The　photochemical　properties　of　1-aryl-1,4-dihydropyrazines,　including　their　UV-vis　absorption,　photostability　and　photoreaction,　are　investigated.　The　photostability　of　the　1-aryl-1,4-dihydropyrazines　is　studied　in　conventional　solvents,　and　the　results　demonstrated　that　the　1-aryl-1,4-dihydropyrazines　are　unstable　under　irradiation　with　UV　light.　The　1-aryl-1,4-dihydropyrazines　undergo　photochemical　ring　contractions　to　1-aryl-1H-imidazoles,　as　determined　by　H-1　NMR,　C-13　NMR,　HRMS,　and　single　crystal　X-ray　diffraction　analyses.　The　photochemical　ring　contraction　is　proposed　to　occur　through　a　6　pi-electron　cyclisation　and　a　[1　+　2]　cycloreversion.　(C)　2013　Elsevier　B.V.　All　rights　reserved.