The intra- and intermolecular reactivity of triplet sumanenetrione 31, which contains a curved ?-system, were investigated to characterize the triplet aromatic ketone. The lowest ππ∗ triplet energy and lifetime were determined from phosphorescence measurements at 77 K to be 196 kJ mol-1 and 9.5 ms in methylcyclohexane, respectively. The transient π∗ triplet state 31 was detected by laser flash photolysis at 293 K and found to be significantly short-lived, e.g., 53.9 -s in degassed benzene, compared to a planar π∗ triplet ketone such as fluorenone triplet (500 -s). The fast intersystem crossing process (ISC) of 31 was attributed to the curved structure of the sumanenetrione, which enhances spin-orbit coupling (SOC). The π∗ character was proven by its negligible quenching by isopropanol, i.e., kq < 4.50 103 M-1 s-1 , although the quenching rate constants for molecular oxygen and triethylamine in benzene were high: 1.0 109 M-1 s-1 and 4.0 108 M-1 s-1 . The π∗ character of the triplet state was also proven by the relatively small electron spin resonance zero-field splitting parameter, «D»/hc, in toluene at 78 K, which was found to be 0.0621 cm-1 . The «E»/hc parameter was determined to be 0.0071 cm-1.
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