Substituent effects in the excited and ground states of methyl cinnamate derivatives

Abstract

Methyl cinnamate derivatives were selected as model compounds for the study of substituent effect in the ground and excited states. [2+2] Photocycloaddition reaction of methyl cinnamates with tetramethylethylene (TME) occurred from the first excited singlet state and substituent effects in the first excited singlet state were examined by the photocycloaddition and fluorescence quenching by olefins. Also, substituent effects in the ground state were determined by measuring the variation of chemical shift ($\delta H_{\alpha}$) of $\alpha$-proton. It was observed from the chemical shifts of $\alpha$-proton of methyl cinnamate derivatives that the degree of variation of $\delta H_{\alpha}$ value versus substituent in $\mbox{\underline{trans}}$-methyl cinnamate derivatives is similar to that of $\mbox{\underline{cis}}$-derivatives and that resonance contribution is larger in the $\mbox{\underline{trans}}$-derivatives than that in the $\mbox{\underline{cis}}$-derivatives, but inductive contribution is reversed. In the first excited singlet state($S_1$) the variation of electron density at double bond of $\mbox{\underline{trans}}$-methyl cinnamate derivatives can be correlated with ground state substituent constant $\sigma$ value approximately as measured by fluorescence quenching, but only $\mbox{\underline{trans}}$-methyl cinnamate and $\mbox{\underline{trans}}$-pnitro methyl cinnamate undergo a [2+2] photocycloaddition reaction with TME.