摘要
The synthesis, characterization and fundamental of the dual excited-state proton-transfer properties of 3-hydroxy-2-(pyridin-2-yI)-4H-chromen-4-one (la) are reported. In the electronic ground state, there exist two competitive hydrogen bonding (HB) isomers for la. Conformer la(O) reveals a five-membered ring HB structure between O-H and carbonyl oxygen, while conformer la(N) possesses a six-membered ring HB formation between O-H and pyridyl nitrogen. In a single crystal, the X-ray crystallography unveils an exclusive formation of conformer la(N). In solution such as CH 2 C1 2 , la(O) and la(N) are in equilibrium, and their respective absorption chromophores are significantly different due to different degrees of hydrogen-bond induced π electron delocalization. Upon excitation, both conformers la(O) and la(N) undergo excited-state intramolecular proton transfer (ESIPT) reaction. Following ESIPT, la(O) gives rise to a tautomer emission maximized at 534 nm in CH 2 C1 2 . Conversely, due to dominant radiationless quenching processes the tautomer emission for la(N) cannot be obtained with a steady-state manner but can be resolved from time-resolved fluorescence. Time resolved fluorescence estimates an equilibrium constant of 27 ± 5 in favor of la(N) in CH 2 Cl 2 . Ultrafast ESIPT also takes place for the unique la(N) form in the crystal. Due to the prohibition of quenching processes in the solid state, bright tautomer emission maximized at 540 nm is resolved for la(N) (Φ f ∼ 0.3). The interplay between two HB conformers with on(la(O))/off(la(N)) character in tautomer emission may find future applications such as the recognition of organic Lewis acid/base in organic solvents. © 2009 American Chemical Society.