SPECTRAL LINESHAPE IN EXTENDED 2-DIMENSIONAL ELECTRONIC SYSTEMS

The close proximity and relative orientation of molecules within the a ggregated portion of a monomolecular layer may lead to collective beha vior, where molecules no longer act independently but create an extend ed electronic network. In most previous discussions, the axial orienta tion has been presumed to be discrete, although this condition is unli kely to be realized in any practically fabricated organic monolayer. T he lineshape expressions developed here, based on the dipole-dipole co upling of adjacent transition moments, are general in nature and indic ate 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, electr onic spectral lineshapes are determined as a function of intermolecula r spacing, a1, ensemble average orientation, [theta], and orientationa l distribution standard deviation, sigma(theta). The transition freque ncy of the aggregate is shown to be dependent on the axial distributio n 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-degre es to 5.0-degrees at [theta] = 75-degrees and with an intermolecular s pacing equal to the length of the transition dipole. The aggregate lin ewidth 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 linewi dth increase partial derivative DELTAnu/partial derivative sigma(theta ) > 1800 cm-1/degrees. Although the orientational distribution does no t have this profound effect under all structural conditions, this cont ribution to the overall aggregate lineshape is clearly more pronounced than has been previously appreciated. (C) 1994 Academic Press, Inc.