Sterically tuned photoreactivity of an aromatic α-diketone family

The photophysics and photochemistry of five aromatic α-diketones exhibiting different geometries are reported. The ground state geometry is correlated to the absorption spectra and electron reduction properties. The emissive singlet states of transoidal α-diketones have a n,π* character and are weakly polar. The triplet states of all α-diketones exhibit a strong n,π* character which is consistent with the vibronic structure of the phosphorescence spectrum observed both in glassy matrix at 77 K and in oxygen-free solution at room temperature. The triplet state energies are influenced by the trans/cis conformation and by the degree of conjugation of the carbonyl groups with their adjacent phenyl moities. These two factors have important consequences on the α-diketone photoreactivities. The reaction constants ρ of the Hammett's plot for the triplet state quenching by several phenol derivatives indicate that the electrophilic character of the transient state is tunable by rotation of the intercarbonyl dihedral angle. After hydrogen abstraction, the intramolecular hydrogen exchange between the two vicinal carbonyl groups plays a major role in the stabilization of the produced ketyl radical. Consequently, the rate constants of hydrogen abstraction in the presence of 2-propanol vary by three orders of magnitude.