Intra- and Intermolecular Reactivity of Triplet Sumanenetrione

摘要

The intra- and intermolecular reactivity of triplet sumanenetrione (3)1, which contains a curved pi-system, were investigated to characterize the triplet aromatic ketone. The lowest pi pi* triplet energy and lifetime were determined from phosphorescence measurements at 77K to be 196 kJ mol(-1) and 9.5 ms in methylcyclohexane, respectively. The transient pi pi* triplet state (3)1 was detected by laser flash photolysis at 293 K and found to be significantly short-lived, e.g., 53.9 mu s in degassed benzene, compared to a planar pi pi* triplet ketone such as fluorenone triplet (500 mu 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 pi pi* character was proven by its negligible quenching by isopropanol, i.e., k(q) < 4.50 x 10(3) M-1 s(-1), although the quenching rate constants for molecular oxygen and triethylamine in benzene were high: 1.0 x 10(9) M-1 s-1 and 4.0 x 10(8) M-1 s(-1). The pi pi* character of the triplet state was also proven by the relatively small electron spin resonance zero-field splitting parameter, vertical bar DI vertical bar hc, in toluene at 78K, which was found to be 0.0621 cm(-1). The vertical bar E vertical bar/hc parameter was determined to be 0.0071 cm(-1).