INFLUENCE OF MOLECULAR-ORIENTATION AND PROXIMITY ON SPECTROSCOPIC LINE-SHAPE IN ORGANIC MONOLAYERS

Within a monomolecular layer, molecules may be present as isolated species that behave independently or in an aggregated form that acts collectively. Theoretical derivations of the energy shift expected for the aggregate transition relative to the isolated monomer are based on transition moment coupling and predict a significantly different orientational dependence for point-and extended-dipole approximations. In contrast to the commonly invoked transformation from H- to J-aggregate behavior as the molecular orientation increases from surface normal, purely H-aggregate behavior is predicted at small intermolerular separation distances using the extended-dipole approximation, regardless of axial orientation. This extended-dipole model is more accurate at the short separation distances commonly encountered in organic monolayers and shows excellent agreement with the experimentally measured energy shift of +9000 cm-1 for an intermolecular distance between hemicyanine dye molecules of 3.96 +/- 0.04 angstrom. As a further development to this model, orientational contributions to the energy shift and the resultant spectral band profile are derived. A Gaussian axial angular distribution for the range 0.5-degrees less-than-or-equal-to sigma(theta) less-than-or-equal-to 5-degrees is found to have a considerable effect on the predicted band profile, resulting in a variation in the transition wavelength from 3 to 10 nm and an increase in the spectroscopic line width contribution by more than 1 order of magnitude. Thus, small variations in the angular distribution have a significant effect on the spectral properties of aggregates and must be considered in correlating molecular structure with electronic transitions.