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

Abstract 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 × 10 4  M −1  s −1 (2-propanol) to 1.0 × 10 10  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.