TRIPLET-STATES OF MOLECULES UNDERGOING INTERNAL DOUBLE-PROTON TRANSFER IN THE S1 STATE - 2,2'-BIPYRIDYL-DIOL AND ITS 5,5'-DIMETHYLATED DERIVATIVE

2,2'-bipyridyl-3,3'-diol and its dimethyl derivative undergo an ultrafast double-proton transfer upon excitation to S1 state, and no direct population of T1 state is observed. Thus, the primary (dienol) triplet (T1) was populated, both by sensitization from the toluene triplet and by enhancement of the forbidden T1 <-- S0 absorption. The low-energy phosphorescence of the phototautomeric (diketo) triplet was detected by irradiation in bromobenzene and was quenched by O2(3-SIGMA-g-) to yield O2(1-DELTA-g). The triplet state is located at 21300 +/- 300 and 12700 +/- 100 cm-1 for the primary and phototautomeric form, respectively, in good agreement with the values predicted by the INDO/S method.