Coumaric amide rotaxanes: effects of hydrogen bonding and mechanical interlocking on the photochemistry and photophysics.

Secondary amide derivatives of coumaric and ferulic acid are shown to undergo photoisomerization, forming a photostationary mixture of E- and Z-isomers. When the same chromophores are incorporated in rotaxanes, the extent of conversion to the Z-isomers is much smaller. Low temperature fluorescence experiments show that the energy barrier for non-radiative decay of the excited state is higher in the rotaxanes than in the corresponding threads, but the barriers are low in all cases.