SPECTRAL LINESHAPE IN EXTENDED 2-DIMENSIONAL ELECTRONIC SYSTEMS

The close proximity and relative orientation of molecules within the aggregated portion of a monomolecular layer may lead to collective behavior, where molecules no longer act independently but create an extended electronic network. In most previous discussions, the axial orientation has been presumed to be discrete, although this condition is unlikely to be realized in any practically fabricated organic monolayer. The lineshape expressions developed here, based on the dipole-dipole coupling of adjacent transition moments, are general in nature and indicate that the presence of even a narrow orientational distribution can significantly affect the spectral lineshape of the aggregated species. Utilizing molecular parameters from hemicyanine dye molecules, electronic spectral lineshapes are determined as a function of intermolecular spacing, a1, ensemble average orientation, [theta], and orientational distribution standard deviation, sigma(theta). The transition frequency of the aggregate is shown to be dependent on the axial distribution within the associated structure, causing significant variations for [theta] > 45-degrees from surface normal. In fact, a 1.43-eV decrease in the transition energy is calculated for sigma(theta) from 0.5-degrees to 5.0-degrees at [theta] = 75-degrees and with an intermolecular spacing equal to the length of the transition dipole. The aggregate linewidth is also dependent on the orientational distribution, with sigma(theta) causing an increase in linewidth under all conditions examined. For the same conditions described above, the variation in the linewidth increase partial derivative DELTAnu/partial derivative sigma(theta) > 1800 cm-1/degrees. Although the orientational distribution does not have this profound effect under all structural conditions, this contribution to the overall aggregate lineshape is clearly more pronounced than has been previously appreciated. (C) 1994 Academic Press, Inc.