Intramolecular Excited-State Proton Transfer in Biologically Active Flavones: A Combined Experimental and Theoretical Study

摘要

The absorption and fluorescence characteristics of two biologically active flavone derivatives 6-hydroxy-7,3',4',5'-tetramethoxyflavone(6HTMF)and 7-hydroxy-6,3',4',5'-tetramethoxyflavone(7HTMF)were studied as a function of acidity(pH/H0)of the solution.Dissociation constants were determined for ground and first excited singlet states and the results were compared with those obtained from Forster-Weller calculations.The acidity constants for these two compounds(6HTMF and 7HTMF)obtained from fluorimetric titrations are in poor agreement with those obtained from Forster-Weller data but are in good agreement with ground state pKa values indicated the deactivation of excited state(Si)before the prototropic equilibrium is complete.The broad band emission of both me flavones in acidic aqueous solution suggests their use as tuneable dye lasers.6-Hydroxy-7,3',4',5'-tetramethoxyflavone undergoes adiabatic photo-dissociation in the singlet excited state indicating the formation of an exciplex or a photo-tautomer.The stability of photo-tautomer was confirmed by calculated value of enthalpy of formation using AMI method.