Molecular packing in LB films of a new porphyrin investigated by atomic force microscopy

A phenylalanylamino-linked tetraphenylporphyrin was synthesized and the molecular-resolution atomic force microscopy (AFM) image of its Langmuir-Blodgett (LB) films was obtained, The molecular area of 77 +/- 6 Angstrom(2) observed from AFM is in excellent agreement with the limiting molecular area from the pi-A isotherm. The orientation angle estimated from the monolayer thickness is 31 degrees with respect to the surface normal. The absorption spectra of LB films show considerable line broadening and red-shift compared with the solution spectra, These results indicate the porphyrin molecules in its LB films adopt "edge-ony" packing which results in pi-orbital overlap and forms an extended and conjugated system.