Chromophores bond length alternation

The principal experimental techniques to characterize the LCP radical cations have been photoelectron spectroscopy (PES) and electron absorption (EA) in matrices [142-150]. The production of radicals in matrices is a slow process and hence EA probes the electronic structure of the chromophore at the equilibrium geometry of the radical ion. In PES the chromophore has no time to relax. However, even ionization results in a remarkable attenuation of the bond-length alternation in comparison with... 

The degree of bond length alternation determined from spectroscopy for the bR chromophore with the use of Fig. 28.33 allows prediction of the values of the molecular nonlinear optical responses /8 and y. Indeed, in the cyanine-like range u 0) /8 values are small and cross zero with negative slopes and y can have negative values. The positive contributions to y have been measured according to ECC applied to NLO responses. 

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