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-propylphthalimide (Ib) were studied by nanosecond laser flash photolysis (lambda(exc) = 248 or 308 nm). The quantum yield of fluorescence of the three compounds is low (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 at 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-saturated 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. 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 la or Ib, 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 pi,pi triplet state is unlikely and an upper excited n,pi tripler pathway is, therefore, proposed. (C) 1999 Elsevier Science S.A. All rights reserved.