Laser flash photolysis study of N-alkylated phthalimides

The photophysical and photochemical properties of N-phthaloylvaline methyl ester (2) in comparison with N-methylphthalimide (la) and N-propylphthalimi de (Ib) were studied by nanosecond laser flash photolysis (lambda(exc) = 24 8 or 308 nm). The quantum yield of fluorescence of the three compounds is l ow (Phi(f) = 10(-2)), whereas that of phosphorescence at -196 degrees C is large (0.3-0.7). The tripler properties were examined in several solvents a t room temperature and in ethanol at low temperatures. Formation of singlet molecular oxygen O-2((1)Delta(g)) was observed in several air- or oxygen-s aturated solvents at room temperature; the quantum yield is substantial (up to 0.7) for la and only slightly smaller (Phi(Delta) = 0.3-0.5) for 2. Tri plet acetone, acetophenone and xanthone in acetonitrile are quenched by 1 a nd 2 via energy transfer, the rate constant is almost diffusion-controlled and smaller for benzophenone. Product formation in preparative photolysis w as not observed for la or Ib, whereas 2 (in the absence of a sensitizer) ma inly gives a product of photoisomerization and two cyclization products in minor amounts. Formation of these products via the observed pi,pi triplet s tate is unlikely and an upper excited n,pi tripler pathway is, therefore, p roposed. (C) 1999 Elsevier Science S.A. All rights reserved.