Photochemistry of Bichromophoric Molecules. Intramolecular Excited-State Interactions in the cis- and trans-Decalin-Connected Aryl Olefins: 7-Methylene-4bβ-methyl-4bβ,5,6,7,8,8aβ,9,10-octahydrophenanthrene and Its 8aα Isomer

Article abstract of DOI:10.1021/jo01293a007

The photochemistry and photophysics of the title compounds (1,2) are reported.The trans-decalin 1 is relatively photoinert but the cis isomer (2) undergoes a singlet-derived, intramolecular, 1,3 photocycloaddition to give 13 (φ = 0.046).It is proposed that 13 is formed via an intramolecular exciplex (Scheme II) as in the acyclic analogue, 6-phenyl-2-hexene (3) , with a rate constant for complexation, k_ex, equal to 5.4*10⁶ s^-1.This is 185-fold slower than k_ex for 3 and corresponds to a ΔG₂₉₈ (=8.2 kcal/mol) which can be equated with a requisite half-chair/chair to half-chair/boat isomerization (eq 3).Though neither 1 nor 2 is phosphorescent at 77 K, 2 sensitizes the isomerization of 1-methylcyclohexene to methylenecyclohexane at room temperature in solution.Thus, intermolecular triplet-energy transfer is competitive with intramolecular energy transfer.Compound 13 efficiently (φ = 0.51) photocycloeliminates to give a carbene-derived product, 15.