Lewis acid catalysis of Coumarin and 5,7-Dimethoxycoumarin photodimerization

Abstract

The spectroscopic properties and photodimerization reactions of coumarin and 5,7-dimethoxycoumarin in the presence and absence of Lewis acids have been investigated. Coumarin photodimerization quantum yields increased in the presence of $BF_3 \cdot OEt_2$ but those of 5,7-dimethoxycoumarin decreased with small amount of $BF_3\cdot OEt_2$. The enhanced quantum yields for coumarin photodimerization in the presence of $BF_3\cdot OEt_2$ is due to changes of spectroscopic and photophysical properties. UV absorbance of coumarin increased and fluorescence emission intensity is greatly increased with increasing $BF_3\cdot OEt_2$ concentration. Enhanced fluorescence emission intensity is due to a weakening of the vibronic interaction between the $^1(n,\pi^*)$ and $^1(\pi,\pi^*)$ state, as the $^1(n, \pi^*)$ state is raised in energy by $BF_3\cdot OEt_2$ complexation. Consequently, the excited $^1(\pi,\pi^* )$ of coumarin-$BF_3\cdot OEt_2$ complex will be kinetically and electronically more reactive than coumarin alone. In distinction from coumarin, 5,7-dimethoxycoumarin photodimerization quantum yields decreased in spite of increasing fluorescence emission intensity and UV absorbance at 366 nm in the presence of $BF_3\cdot OEt_3 \cdot BF_3 \cdot OEt_2$ inhibits the 5,7-dimethoxycoumarin photodimerization reaction because 5,7-dimethoxycoumarin-$BF_3\cdot OEt_2$ complex absorbs light more strongly but 3,4-double bond character is greatly decreased by $BF_3\cdot OEt_2$ complexation and charge transfer from 5,7-methoxy groups.