Optical Response of Linear Polyenes: Extension of the Simple Model for Linear Polyene Electronic Structure

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Abstract
A simple model based on Hückel theory that quantitatively accounts for linear polyene 21Ag and 11Bu excitation energies has been used to predict nonlinear optical response of this class of molecules. The essential elements of the model are the use of alternating resonance integrals to obtain the proper chain-length dependence of the 11Bu highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) excitation energy and mixing of the HOMO to LUMO + 1, HOMO-1 to LUMO and HOMO to LUMO double-excitation configurations to describe the 21Ag state. Even though one of the appealing features of this model is the ease with which it can be extended to include vibrational levels, this contribution focuses on calculations carried out in a purely electronic basis. Despite the extreme simplicity of the model, it reproduces many of the results of much more elaborate computations including the fact that a higher lying 1Ag state has substantially larger 2-photon absorptivity than does the 21Ag state and the fact that two photon resonance with the 21Ag state can provide significant enhancement of the nonlinear optical response of these molecules at wavelengths well below the one-photon absorption edge.