Structural and morphological studies on Langmuir-Blodgett films of pheophytine a by ultraviolet–visible and infrared spectroscopies and atomic force microscopy

Abstract

The structure concerning the molecular aggregation in Langmuir-Blodgett (LB) films of pheophytine a (Phe- a) has been studied by ultraviolet–visible (UV–Vis) and infrared (IR) spectroscopies and their morphology has been delineated by atomic force microscopy (AFM). UV–Vis spectra were measured for a chloroform (CHCl 3) solution of Phe- a, cast film from the CHCl 3 solution and six-layer LB films of Phe- a prepared in air and argon (Ar) atmospheres. A comparison of these UV–Vis spectra indicates the existence of two kinds of aggregates of Phe- a in the cast and LB films. One has a Qy band at 683 nm and another has a corresponding band at 700 nm. Soret bands of the cast and LB films also show a marked broadening. It is suggested from the UV–Vis measurements that the LB film prepared in the Ar atmosphere includes the aggregate with the 700 nm band at the highest ratio among the three kinds of films. On the other hand, the IR spectra of the solution, and cast and LB films resemble each other, suggesting that Phe- a exists in a monomer in them. Therefore, it seems that the aggregates are composed of stacked Phe- a monomers with a π– π interaction. AFM images for the LB films of Phe- a prepared in the air and Ar atmosphere reveal that they have very smooth and flat surface.