TAUTOMERIZATION IN FLUORESCENT STATES OF BIPYRIDYL-DIOLS - A DIRECT CONFIRMATION OF THE INTRAMOLECULAR DOUBLE PROTON-TRANSFER BY ELECTROOPTICAL EMISSION MEASUREMENTS

Integral electro-optical emission measurements (IEOEM) and INDO/S calculations were carried out for 2,2'-bipyridyl-3,3'-diol (BP(OH)2), and its 5-methyl- (MeBP(OH)2) and 5,5'-dimethyl- (Me2BP(OH)2) derivatives. All three molecules were reported earlier as effective lasing dyes [5,6] undergoing phototautomerization in the S1 state. From the IEOEM experiment an upper limit of about 1.0 D (debye) is derived for the fluorescent state dipole moment, mu(e) in each case, while the INDO/S calculations predict mu(e) of the excited singlet (pi,pi*) state of the diketo and monoketo forms in the range: 0.0-0.5 D and 4.0-4.9 D, respectively. Comparison of both results leads to an unequivocal discrimination between the two possible photoreactions, single or double proton transfer (SPT or DPT), and shows that the fluorescent species are diketo tautomers produced by DPT.