Conformations, Spectroscopy, and Photochemistry of Methyl Phenanthrene-9-carboxylate, Phenanthrene-9-carboxamides, and Their Lewis Acid Complexes

The spectroscopic properties and photochemical behavior of methyl phenanthrene-9-carboxylate and of a primary, secondary, and tertiary phenanthrene-9-carboxamides have been investigated in the absence and presence of strong Lewis acids. The ground-state conformations of the free and complexed molecules have been investigated by means of NMR and Gaussian 88 calculations. The dihedral angle between the phenanthrene and the carbonyl group is found to be dependent upon the bulk of the 9-substituent and upon Lewis acid complexation. Complexation also changes the secondary amide conformation from syn to anti. Both the phenanthrenes and their complexes are strongly fluorescent. Equilibrium constants for complex formation have been determined by means of fluorescence titrations. Rate constants for both radiative and nonradiative singlet-state decay increase upon complexation. The nonradiative rate constants for several boron halide complexes display a heavy-atom effect, which is larger for tertiary vs primary amides. Complexation results in an increase in both ester and amide singlet-state reactivity with simple alkenes.