led then, by oxidative cyclization processes, to the hexahelicenophanes 13, provided that the methylenedioxy chain of 12 is long enough (n = 8, 10). As competitive photoreaction, a twofold [2π+2π] cyclodimerization occurred, which furnished the belt cyclophanes 14. The latter process is the only photoreaction for 12 with n=6. The hexahelicenophanes 13 have lower racemization barriers and longer spinlattice
A drastic increase in the rate of racemization is found for the bridged hexahelicenophanes 1 (n=8, 10) relative to the unsubstituted parent [6]helicene. This unexpected result occurs from steric interactions between the polymethylenedioxy chains and the terminal rings, which causes the energy of the ground state to be raised and that of the transition state to be lowered.