Photophysical Properties of Pyrazino-Coumarin Derivatives

Abstract

Pyrazinocoumarin (PzC), 7-methoxypyrazinocoumarin (7-MOPzC) and 5,7-dimethoxypyrazinocoumarin (5,7-DMPzC) show the similar UV-visible absorption spectra as the corresponding coumarins except far more distinct shoulders in the near UV absorption band (300 similar to 350 nm), probably due to n --> pi* transition. Fluorescence excitation spectra of PzC derivatives in nonpolar solvents have the lowest energy maxima in the region of the shoulders of UV-visible absorption, indicating the lowest fluorescent state to have the same energy as these shoulders, Fluorescence quantum yields (Phi(F)) Of pyrazinocoumarins are on the order of 10(-2)-10(-4), much lower than the corresponding coumarins. The activation energies (E-a(nr)) for nonradiative processes (internal conversion, Intersystem crossing, etc.) determined by the temperature effect on the fluorescence are 10.2, 6.35 and 6.57 kJ/mol for PzC, 7-MOPzC and 5,7-DMPzC, respectively, Phosphorescence emission spectra of PzC and 7-MOPzC are structureless with Phi(P) of 0.12 and 0.17, respectively.