Photochemical of 5(E)-Styryl-1,3-dimethyluracil

Abstract

Fluorescence quantum yield and lifetime of 5(E)-styryl-1,3-dimethyluracil are relatively low and short, probably due to the contribution of (n,$π^*$) state to the fast radiationless decay. However, the effect of solvent polarity, salt, and pH on the fluorescence indicates that the $^1(n,π^*)$ level is not close enough to the lowest $^1(π,π^*)$ level. Fluorescence and fluorescence excitation polarization confirm that the lowest excited singlet state has the (π,$π^*$) configuration. Strong temperature dependence of fluorescence intensity may be due to the presence of the activated process, twisting in the first excited singlet state, competing with fluorescence and the rotation barrier is estimated to be 3.0 kcal/mole. The heavy atom effect on the fluorescence, very weak phosphorescence, and laser flash photolysis indicate that the intersystem crossing is inefficient. The lowest triplet state is considered to be $^3(π,π^*)$ state from the phosphorescence lifetime and its polarization. Photoisomerization quantum yield in various solvents of different viscosity and polarity, the effect of salt, pH, and heavy atoms on the photoisomerization, the triplet sensitization of the photoisomerization, and the azulene quenching of the direct and sensitized isomerization indicate the singlet mechanizm for the direct photoisomerization of 5-SDU. The photolysis of a dilute acetonitrile solution of 5(Z)-SDU in the presence of oxygen gives the dehydrocyclized product in 26.7% yield. Addition of benzophenone to the 5(E)-SDU solution improved the yield of the photocyclization owing to the efficient $E \rightleftarrows Z$ photoisomerization due to benzophenone sensitization. Studies of te salt, sensitization, and quenching effects on the photocyclization of the compound indicate that the photocyclization occurs from the lowest excited singlet $^1(π,π^*)$ state of 5(Z)-SDU. Photocyclodehydrogenation product of 5-SDU, 1,3-dimethylnaphtho[1,2-e]uracil, has the lower $S_1-S_0$ energy ga...