Reversible photodimerization of coumarin derivatives dispersed in poly(vinyl acetate)

Four photoreactive coumarin derivatives were successfully synthesized from 7‐hydroxy‐coumarin and 7‐hydroxy‐4‐methylcoumarin, i.e., 7‐propionyloxy‐4‐methylcoumarin (M₁), 7‐palmitoyloxy‐4‐methylcoumarin (M₂), 7‐propionyloxycoumarin (M₃), and 7‐palmitoy‐loxycoumarin (M₄). Reversible photodimerization (350 or 300 nm) and photocleavage (254 nm) of these coumarin derivatives dispersed in poly(vinyl acetate) (PVAc) were investigated by tracing their UV absorbance variations at 310 nm. The M₂ and M₄ with long palmitoyl chain show much better photoreaction reversibility than M₁ and M₃ with short propionyl chain. Moreover, photodimerization rate (under 350 nm) of M₂ is greater than 200 times of that of M₁. This has been explained by the formation of suitable conformation for revers‐ible photodimerization due to the hydrophobic interactions. Photodimerization of M₂ is ca. 3 times quicker than that of M₄, indicating 4‐methyl substitution enhances pho‐todimerization. The influence of photodimerization wavelength (350 and 300 nm) and photosensitizer (benzophenone) have also been investigated in detail. © 1995 John Wiley & Sons. Inc.