Laser flash photolysis study of the photochemistry of 4,5-diaza-9-fluorenone

• The triplet excited state of 4,5-diaza-9-fluorenone has ππ* character and its energy is located between 49.3 and 52.4 kcal/mol, with an intersystem crossing quantum yield of 0.41.
• Low quenching rate constants were obtained for quenchers where the primary process corresponds to hydrogen transfer, such as 1,4-cyclohexadiene, cyclohexane, 2-propanol and toluene .
• The 4,5-diaza-9-fluorenone triplet was efficiently quenched by electron donors such as triethylamine and DABCO.
• A Hammett plot for the quenching by phenols containing polar substituents led to a reaction constant ρ of −1.54 ± 0.10.

The triplet excited state of 4,5-diaza-9-fluorenone (1) shows absorption maxima at 410 and 470 nm and a lifetime of 3 μs, in acetonitrile. Its intersystem crossing quantum yield was determined using 9-fluorenone as a secondary standard and a value of 0.41 ± 0.01 was obtained. The reactivity of the triplet excited state of 1 towards several quenchers, in acetonitrile, was investigated employing the laser flash photolysis technique quenching rate constants ranging from 7.9 × 104 M−1 s−1 (2-propanol) to 1.0 × 1010 M−1 s−1 (triethylamine) were obtained. From the quenching rate constants obtained one can conclude that 4,5-diaza-9-fluorenone has a ππ* triplet excited state. A Hammett plot for the quenching rate constants of triplet 1 by phenols containing polar substituents against σ+ gave a reaction constant ρ of –1.54 ± 0.10, which demonstrates the electrophilic character of the 4,5-diaza-9-fluorenone triplet excited state.