The photochemical reactivity of three 1, 1, 3, 3-tetrasubstituted 2-indanones was investigated in solution and in crystals. While solution irradiation of tetramethyl-2-indanone 2 leads to 1-isopropenyl-2-isopropyl benzene 5 in high yield, crystals of 2 were completely inert. In contrast, samples of 1, 3-bis (ethylenedioxy)-2-indanone 3 reacted efficiently in both media. While solution irradiation yielded benzocyclobutane 7 and o-xylylene 8, only the ...