Effect of alkyl substituents on photorelease from butyrophenone derivatives.

Photolysis of 1a yields 4a in argon-saturated methanol, whereas 1b is photostable. Laser flash photolysis of 1a in acetonitrile shows formation of biradical 2a (λmax = 340 nm, τ = ∼60 ns), which undergoes intersystem crossing to form Z-3a (λmax = 380 nm, τ = 270 ns) and E-3a (λmax = 380 nm, τ = 300 ms). Z-3a regenerates the starting material, whereas E-3b undergoes intramolecular lactonization to release the alcohol moiety and form 4a. Similar laser flash photolysis of 1b shows formation of biradical 2b (λmax = 340 nm, τ = 1.9 μs in acetonitrile), which is longer-lived than 2a is. However, 2b only undergoes intersystem crossing to form Z-3b (λmax = 380 nm, τ = 4.3 μs). Calculations demonstrate that intramolecular pseudo hydrogen bonding between the OH moiety and the radical centered on the isopropyl carbon in 2b and the bulkiness of the isopropyl group prevent the necessary rotation to form E-3b. In comparison, 2a does not form an intramolecular pseudo hydrogen bond between the methylene radical center an...