Morphology and molecular orientation and structure of Langmuir–Blodgett films of chlorophyll a studied by atomic force microscopy and UV–VIS and IR spectroscopies

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Abstract
Atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV–VIS absorption and IR spectroscopies have been used to investigate the morphological and molecular structure of Langmuir–Blodgett (LB) films of the title compound (Chl-a). The AFM and SEM measurements reveal that the surface of the LB films are very smooth and consist of rigid and condensed monolayers. The tilt angle of the chlorin plane in a six-monolayer LB film of Chl-a has been calculated to be 35° between the normal of the chlorin plane and the substrate normal and 44° between the dipping direction and projection of the normal of the chlorin plane, from measurements of its polarized UV–VIS spectra. Band frequencies in the 1750–1400 cm −1 region of the IR spectra of one-, six- and ten-monolayer (ML) LB films of Chl-a suggest that Chl-a exists in a five-coordinated monomer in the films, irrespective of the number of monolayers. Comparison of IR transmission and reflection–absorption spectra of 6 ML LB films of Chl-a indicates that the chlorin plane and the ester and keto CO groups are tilted considerably with respect to the surface normal. LB films of pheophytine a (Phe-a) have also been studied for comparison. Comparison of the UV–VIS and IR spectra of the LB films of Chl-a and Phe-a show that pheophytinization occurs little in the LB films of Chl-a. High-performance liquid chromatography (HPLC) examination has supported this conclusion. All the experiments carried out in the present study suggest that Chl-a molecules in the LB films are oriented in an orderly manner with face-to-face packing of the chlorin rings.

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