Nonlinear optical properties of organic molecules. XII. Calculations of the hyperpolarizabilities of donor–acceptor polyynes

The hyperpolarizabilities of polyynes containing a donor and acceptor group situated at either end of the molecular have been calculated at the sum‐over‐states CNDOVSB level using geometries derived from crystallographic data and ab initio studies. The calculated values increase with increasing chain length, though the maximum value obtained are considerably less than those for the related polyynes. Although the hyperpolarizability of the polyynes is dominated by one excited π‐electron state, there are a significant number of additional low‐lying excited states in the polyynes that contribute to the overall value by opposing the usual direction of charge transfer along the chain from donor to acceptor. © 1993 John Wiley & Sons, Inc.