Photochemical and photophysical properties of cinnamate derivatives

Abstract

The syntheses and photochemical properties of ten methoxy substituted cinnamates were carried out and the photophysical properties of five selected methoxy substituted 2-ethylhexyl-cinnamates were studied. Both experimental and theoretical methods indicated that the fluorescence quantum yields varied strongly with the substituted position on the phenyl ring. A methoxy substitution at meta position gave strong fluorescence whereas tha para substituted compounds were strongly quenched. This observation could be correlated to the corresponding changes in the UV absorption spectra; the two lowest [pipi *] states were spited for the meta-substituted cinnamates but almost degenerate for the para compounds. Semin-empirical quantum calculation confirmed the observed state ordering and supported the difference in the experimentally determined activation energies for non-radiative decay. This “meta-effect” could be applied for the trimethoxy substituted compounds: strong fluorescence with relatively high barrier for non-radiative decay in 2,4,5- and weak fluorescence with relatively low barrier for non-radiative decay in 2,4,6- trimethoxycinnamate.