Two-form two-state analysis of polarizabilities of push pull molecules

We describe push–pull molecules, using a two-form two-state model, and define a parameter, MIX, that characterizes the mixing between the two limiting-resonance forms. MIX is related to the difference between the weight of each form in either the ground or the first-excited state. We have established analytical relationships between static polarizabilities and MIX. The optimum MIX value that yields maximized quadratic polarizability βmax(0) occurs when the mixing is intermediate between equivalent contributions of the two limiting forms and the strong predominance of either form. By examining experimental results obtained with push–pull polyenes, we show that not only MIX but also βmax(0) is affected by the nature of the donor–acceptor end groups as well as by the size of the polyenic linker. The results also help us to understand the length dependences of the quadratic polarizability measured in solution for series of push–pull polyenes. These dependences result from a combination of MIX and βmax(0) variations with increasing chain length.