OPTICAL-RESPONSE OF LINEAR POLYENES - EXTENSION OF THE SIMPLE-MODEL FOR LINEAR POLYENE ELECTRONIC-STRUCTURE

A simple model based on Huckel theory that quantitatively accounts for linear polyene 2(1)A(g) and 1(1)B(u) excitation energies has been use d to predict nonlinear optical response of this class of molecules. Th e essential elements of the model are the use of alternating resonance integrals to obtain the proper chain-length dependence of the 1(1)B(u ) highest occupied molecular orbital (HOMO) to lowest unoccupied molec ular orbital (LUMO) excitation energy and mixing of the HOMO to LUMO 1, HOMO-1 to LUMO and HOMO to LUMO double-excitation configurations t o describe the 2(1)A(g) state. Even though one of the appealing featur es 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 th e model, it reproduces many of the results of much more elaborate comp utations including the fact that a higher lying (1)A(g) state has subs tantially larger 2-photon absorptivity than does the 2(1)A(g) state an d the fact that two photon resonance with the 2(1)A(g) state can provi de significant enhancement of the nonlinear optical response of these molecules at wavelengths well below the one-photon absorption edge.