A MO Theoretical Study of Organic Dyes I. Effect of Chemical Softness on the Electronic Spectra

In order to obtain reasonable results for the π-electronic spectra of organic dyes with simple semi-empirical MO methods, a new approximation for the two-center electron repulsion integral (new gamma) is proposed which takes the chemical softness of the π-conjugated system into account. New gamma was successfully applied to PPP calculations of the electronic spectra of linear polyenes, simple cyanines, and simple merocyanines. The calculated results agree satisfactorily with the experimental data. On the contrary, a calculation based on the conventional PPP parameters gives shorter wavelengths for the absorption maxima of the electronic spectra of charge-resonance systems, such as cyanines and merocyanines. Furthermore, they give very strange electronic spectral shifts for merocyanines (see text), due to an inbalance between the one- and two-electron parts of the electronic transition energy.