Photochemical E(trans)–Z(cis)–E Isomerization of an Amphiphilic Cholest-5-en-3β-yl(E)-9-anthraceneprop-2-enoate on Solid Substrate

Surface morphology and photochemical isomerization properties of monolayers of anthrancene acrylic acid derivative with cholesterol (a new class of bistable compound), cholest-5-en-3β-yl(E)-9-anthraceneprop-2-enoate (CAE), transferred onto quartz substrates were studied. The spectroscopic and photochromic behavior of CAE on solid substrates and in solution are compared keeping in mind the possible application of CAE in constructing molecular electronic devices. Monolayers of the trans(E)-isomer of CAE transferred from the air–water interface onto quartz plates show regular distribution of “holes” in the film, whereas similar monolayers of the cis(Z)-isomer of CAE (∼96%) show very smooth surfaces, free from any definite structures. The surface pressure–area (π–A) isotherms of both monolayers at the air–water interface are found to be irreversible, indicating formation of 2D/3D aggregates for both isomers. The surface potential–area (ΔV–A) isotherms of the two isomers predict the orientation of their molecular dipoles to be different. The fluorescence peak intensity of the E-isomer of CAE in transferred monolayers shows a sharp decrease upon irradiation with 405 nm light, indicating the successful E-to-Z isomerization in the monolayer. Fluorescence excitation and emission polarization studies on the solid substrate also confirm the change of molecular orientation resulting from the E-to-Z isomerization. The isomerization rate is found to be faster in solid substrates than that in the solution phase. Six alternate monolayers of E-CAE and triplet sensitizer (liphophilic porphyrin) film shows 5% efficiency of Z-to-E isomerization upon exciting on 550 nm, where porphyrin has substantial absorbance where as film of 24 monolayers of mixture solution of the E-isomer of CAE (1 mM) and liphophilic porphyrin (1 mM) in chloroform increases 5-fold efficiency of Z-to-E conversion. These results suggest that the E-CAE has the potential to be used in making optical data storage devices employing the trans–cis–trans isomerization process.