PRESSURE-INDUCED CHANGES IN THE ABSORPTION-SPECTRUM OF MONOLAYERS AT THE AIR-WATER-INTERFACE - COMPARISON OF CALCULATIONS WITH EXPERIMENTS

A shift to longer wavelengths and a change in the intensity of the Soret band spectrum of monolayers of 5-(N-tetradecyl-4-pyridino)-10,15,20-tri-p-tolylporphyrin (p-C14PyTTP) at the air/water interface were observed when the film was compressed. Considering the pi-A curve and the polarized absorption measurement, this change is interpreted as a rearrangement of the molecular packing within the layer. We investigated the mechanism of this spectral change by semiempirical MO INDO/S using a model porphyrin dimer system in which substituents on the pyridinium and benzene rings were replaced with hydrogen atoms. We calculated the spectra for various geometrical conformations produced by rotation and translation of one ring relative to the other. On the basis of the calculated spectral changes of the dimers and the observed polarization absorption measurement, a molecular level interpretation of the spectral change is presented.