7-Hydroxyflavone Revisited. 2. Substitution Effect on Spectral and Acid-Base Properties in the Ground and Excited States.

Abstract

Three derivatives of 7-hydroxy-2-phenyl-4H-chromen-4-one (7-hydroxyflavone), containing chloro, methoxy, and dimethylamino substituents at position 4', were synthesized and investigated from the view of their ground-state and electronically excited-state behavior. Spectral and fluorescent properties in a wide range of pH/H0, thermodynamics of the ground and S1 states, and kinetics of the excited-state deactivation of the compounds were investigated by means of steady-state electronic absorption, steady-state, and time-resolved fluorescent spectroscopies as well as by computational methods. The results are rationalized from the point of view of the substituent effect. In spite of a similar structure and the same acid-base centers, the compounds strongly differ in fluorescence characteristics as well as in the dependence of fluorescent properties on pH/H0 of the media. Various protolytic/tautomeric forms of the compounds investigated absorb light in the 300-500 nm range and fluoresce in the whole visible range of spectra. The electron-releasing substituents at position 4' of 7-hydroxyflavone immensely affect spectral properties as well as the excited-state thermodynamics and kinetics, whereas the electron-withdrawing ones cause minimal effect.