Effects of Solvents, N-Substituents and Acids on the Photocyclization and the Fluorescence Behavior of Diphenylmaleimides

Abstract

The fluorescence behavior of diphenylmaleimides shows the following characteristic features; 1) the Stokes' shift correlates linearly with the solvent polarity parameter, E_T value, 2) the emission is quenched intramolecularly when the imido-nitrogen is attached to the atoms having n- or π-electrons, and 3) the quenching is observed in strongly acidic solutions. The photocyclization of the imides to yield phenanthrenes and 9, 10-dihydrophenanthrene-9, 10-dicarboximides is influenced by the dual effect of acids; when pH ca. 3, the yield of the dihydrophenanthrenes increases with the decrease in pH without the change in the reaction rate, whereas in more strongly acidic solutions, H₀, ca. +2, the photoreactivity is reduced in proportion to the fluorescence quenching. Discussion is given on the cyclization mechanism based on these observations.