PHOTOCYCLOADDITION OF PHENANTHRENE WITH ACETYLACETONATOBORON DIFLUORIDE - EXCIPLEX REACTIONS

In dioxane or ether singlet excited phenanthrene (PN) reacts with acetylacetonatoboron difluoride (AABF(2)) irreversibly to form a fluorescent exciplex with a well-defined isosbestic point in the low range of [AABF(2)] less than or equal to 0.03 M. This exciplex is shown to be the precursor to the observed product of cis-9-acetyl-9,10-dihydro-10-(2-oxopropyl)phenanthrene by quantitative quenching analysis of the product and fluorescence. In contrast, the excitation of AABF(2) in the presence of PN gives neither exciplex emission nor any products. The exciplex was studied on the basis-of its static and time-resolved fluorescence properties to give the dipole moment of 11.2 D and its reaction pattern. The exciplex undergoes self-quenching by AABF(2), and the quantum yield carries dependency on the square of its concentrations that would impose a nonlinear relation in the double reciprocal plot of 1/Phi vs 1/[AABF(2)]. The plot started to curve upward at [AABF(2)] as low as 0.02M owing to simultaneous complication from the competing and unproductive absorption by AABF2, particularly at a high concentration range. Experimentally, the isosbestic point is unfocused at [AABF(2)] greater than or equal to 0.025 M to indicate a small effect of self-quenching. In acetonitrile singlet excited PN preferentially undergoes electron transfer to give the phenanthrene cation radical without causing the cycloaddition.