PICOSECOND SPECTROSCOPIC MEASUREMENT OF VERY FAST INTERSYSTEM CROSSING FOR 9,10-DIOXA-ANTI-BIMANES

Picosecond spectroscopy, following the buildup of T1 → Tn absorption (maximum at 420 nm), shows that the T1 state of 1,5-diazabicyclo[3,3,0]octa-3,7-diene-2,6-diones(9,10-dioxa-anti-bimanes) is formed within about 10 ps. The nature of the T1 state was confirmed by decay rates of T1 → Tn absorption in acetonitrile (n = 0.375 cP, knr = 4.5 × 105 s-1), 1,2-ethanediol (n = 26 cP, knr = 1.5 × 104 s-1 and glycerol (n = 1400 cP, knr = 1.3 × 103 s-1). The very fast intersystem crossing is ascribed to the proximity of a 3nπ* state to the ππ* (S1 state produced by light absorption (El-Sayed rule). © 1979.