ELECTROOPTICAL ABSORPTION-MEASUREMENTS OF PHOTOTAUTOMERIZING SYSTEMS - S(0) AND S(1) STATIC POLARIZABILITIES OF BIPYRIDINEDIOLS

Ground- and excited-state polarizabilities of 2,2-bipyridine-3,3-diol (BP(OH)2), 5-methyl-2,2'-bipyridine-3,3'-diol (MeBP(OH)2), and 5,5'-dimethyl-2,2'-bipyridine-3,3'-diol (Me2BP(OH)2) were determined by means of dielectric, refractometric, and electrooptical absorption measurements. The ground-state polarizability tensor is anisotropic, the main component lying in the long molecular axis. Nuclear contributions to the static polarizability were found to be small (5-10%). The experimental ground-state polarizabilities are compared with results of semiempirical and ab initio quantum chemical calculations. Upon electronic excitation to the first excited singlet state, the polarizability increases by a factor of about 1.5, the change being exclusively parallel to the direction of the transition dipole (long axis). The Franck-Condon excited-state dipole moments of the centrosymmetrical compounds BP(OH)2 and Me2BP(OH)2 were confirmed to be zero within experimental error of 2 x 10(-30) cm (0.6 D). The experimental results support the notion of a symmetric double proton transfer taking place in the S1 state of bipyridinediols.