Nitrogen-substituted phthalimides: fluorescence, phosphorescence, and the mechanism of photocyclization
From u.v. absorption and fluorescence measurements it is suggested that N-(dialkylaminomethyl)phthalimides fluoresce(weakly) from the second excited singlet state (π,π*), and that the lowest energy singlet state (n, π*) is non-fluorescent. Phosphorescence occurs from the lowest triplet state at 77 K, and in ethanol solution this state is unambiguously assigned a (π,π*) configuration. The second triplet state (n, π*) is quite close in energy to the first. The luminescence results point to the operation of an initial electron transfer step in the photochemical cyclization reaction of these phthalimides.