Laser flash photolysis study of N-alkylated phthalimides

Abstract

The photophysical and photochemical properties of N-phthaloylvaline methyl ester ( 2) in comparison with N-methylphthalimide ( 1a) and N-propylphthalimide ( 1b) were studied by nanosecond laser flash photolysis ( λ exc = 248 or 308 nm). The quantum yield of fluorescence of the three compounds is low (Φ f = 10 −2), whereas that of phosphorescence at −196°C is large (0.3–0.7). The triplet properties were examined in several solvents at room temperature and in ethanol at low temperatures. Formation of singlet molecular oxygen O 2( 1 Δ g ) was observed in several air- or oxygen-saturated solvents at room temperature; the quantum yield is substantial (up to 0.7) for 1a and only slightly smaller (Φ Δ = 0.3–0.5) for 2. Triplet acetone, acetophenone and xanthone in acetonitrile are quenched by 1 and 2 via energy transfer, the rate constant is almost diffusion-controlled and smaller for benzophenone. Product formation in preparative photolysis was not observed for 1a or 1b, whereas 2 (in the absence of a sensitizer) mainly gives a product of photoisomerization and two cyclization products in minor amounts. Formation of these products via the observed π,π triplet state is unlikely and an upper excited n,π triplet pathway is, therefore, proposed.