New phototautomerizing systems: non-symmetric derivatives of [2,2′-bipyridyl]-3,3′-diol

Six new phototautomerizing compounds, structural analogues of [2,2′-bipyridyl]-3,3′-diol (BP(OH)2) and [2,2′-bipyridyl]-3-ol, were synthesized. The electron-donating or electron-withdrawing substituent introduced next to the reactive proton transfer site (N atom) can modify the status of the internal hydrogen bonds, controlling the excited state intramolecular proton transfer mechanism in a predictable way. Ab initio RHF/6-31 G** and semiempirical (PM3) calculations for the S0 ground state were performed to determine the structural features that `prepareʹ the molecular system for its fate in the S1 excited state. The calculated molecular geometries and Mulliken net charges were useful for interpreting photophysical data. The dramatic changes of phototautomeric fluorescence quantum yields are dependent on (1) molecular symmetry lowering of the substituted BP(OH)2 system and (2) electron density on the N atom(s).