Synthesis and photochemistry of 5(E)-styryl-1,3-dimenthyluracil

Abstract

A new compound, 5(E)-styryl-1,3-dimethyluracil was synthesized using tetrachloropalladate as catalyst and characterized by spectroscopic methods and elemental analysis. Photophysical properties (fluorescence quantum yield, fluorescence life-time, phosphorescence quantum yield) of this compound was measured in ethanol solution. These values are 0.026, 1.15 nsec, but phosphorescence was not observed. From this, it is concluded that singlet excited state of trans styryl-DMU has one or more very efficient relaxation processes other than fluorescence, very likely an efficient intersystem crossing because of ($n,\pi^{\ast}$) and ($\pi, \pi^{\ast}$) mixing. The quantum yield of direct photochemical cis $\rightleftharpoons$ trans isomerization at 366 nm was 0.0874. The photolysis of trans styryl-DMU was carried out in acetonitrile solution in the prescence of oxygen as an oxidant. The major product of photolysis was photocyclized compound, a diazaphenanthrene. The structure of this compound has been elucidiated by nmr, IR, UV-Vis, and mass spectrometry as well as elemental analysis. The reaction is believed to involve an intermediate of dihydrophenanthrene via cis-isomer. The fluorescence of styryl-DMU was quenched by TME giving a linear Stern-Volmer plot, and the value of $K_q$ was $3.26\times10^9$ l/mole-sec. The photochemistry of this compound is expected to be much more complex than that of stilbene because of the numerous functional groups in the compound.