ON THE SYNTHESES AND PHOTOCHEMICAL PROPERTIES OF HETEROCYCLIC KETONES AS PHOTOSENSITIZERS

The syntheses, and the electrochemical and photochemical properties of the fluorenone analogues 9H-pyrrolo[1,2-a]indole-9-one (1), and 9H-pyrido[3,4,b] pyrrolizin-9-one (2), which absorb in the visible region (epsilon400 nm = 4711 mol-1 cm-1 for 1 and epsilon400 nm = 8721 mol cm-1 for 2), are described. 9-Fluorenone as well as compounds 1 and 2 were able to reduce methyl viologen in the presence of 2-propanol under irradiation with UV light (lambda > 280 nm). Compounds 1 and 2 showed an interesting absorption and fluorescence behavior depending on concentration, solvent and pH value. Aqueous solutions of compound 1 gave two fluorescence maxima at 395 and 478 nm, when excited by lambda(EX) = 302 and 368 nm, respectively. In contrast, aqueous solutions of compound 2 showed only one emission maximum at 417 nm (excited by lambda(EX) = 296 or 378 nm). Fluorescence quantum yields (solvent: propanol, excitation wavelength lambda(EX) = 436 nm) for compounds 1 and 2 were 0.2%. Compounds 1 and 2 showed irreversible reduction potentials at -850 and -560 mV (vs NHE), respectively. The results were compared with benzophenone and 9-fluorenone.