The reactivity of the triplet excited state of 1,4-diaza-9-fluorenones towards hydrogen and electron donors

The effect of the introduction of nitrogen atoms upon the triplet excited state reactivity of 1,4-diaza-9-fluorenone (1) and 1,4-diaza-9-benz[b]fluorenone (2), in acetonitrile, was investigated employing the nanosecond laser flash photolysis technique. The intersystem crossing quantum yield (φces) for 1 and 2 was determined using 9-fluorenone as a secondary standard (φces= 0.48, in acetonitrile) and for both diazafluorenones a value of φces= 0.28 was found. Quenching rate constants ranged from 8.17 x 104 L mol-1 s-1 (2-propanol) to 1.02 x 1010 L mol-1 s-1 (DABCO) for 1,4-diaza-9-fluorenone and from 6.95 x 105 L mol-1 s-1 (2-propanol) to 5.94 x 109 L mol-1 s-1 (DABCO) for 1,4-diaza-9-benz[b]fluorenone, depending if the quenching process involves energy, hydrogen or electron transfer. A comparison between quenching rate constants for both diazaflurenones and the parent compound, i.e. 9-fluorenone, a ketone with lowest triple state of ππ* configuration, lead to the conclusion that the reactive triplet excited state for 1,4-diaza-9-fluorenone and 1,4-diaza-9-benz[b]fluorenone has ππ* configuration. (author)