A comparative study on the photochemistry of two bipyridyl derivatives: [2,2′-Bipyridyl]-3,3′-diamine and [2,2′-bipyridyl]-3,3′- diol

The two isoelectronic bipyridyl derivatives, [2,2′-bipyridyl]-3, 3′-diamine and [2,2′-bipyridyl]-3,3′-dial, are experimentally known to undergo very different excited-state double-proton-transfer processes, which result in fluorescence quantum yields that differ by four orders of magnitude. Herein, density functional theory (DFT), time-dependent DFT (TDDFT), and complete active space self-consistent field (CASSCF) calculations are used to study the double-proton-transfer processes in the ground and first singlet π→π* excited state. The quantum-chemistry calculations indicate 1) the existence of only one energy minimum in the ground electronic state corresponding to reactants (thus avoiding the possibility of a fast fluorescent relaxation process from the photoproducts region), 2) an endoergic process of the complete double proton transfer, and 3) the presence of a conical intersection in the excited intermediate region of [2,2′-bipyridyl]-3, 3′-diamine. These facts explain the very low fluorescence quantum yield in [2,2′-bipyridyl]-3,3′-diamine compared to [2,2′-bipyridyl]-3, 3′-diol. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.