A MO THEORETICAL-STUDY OF ORGANIC-DYES .1. EFFECT OF CHEMICAL SOFTNESS ON THE ELECTRONIC-SPECTRA

In order to obtain reasonable results for the pi-electronic spectra of organic dyes with simple semi-empirical MO methods, a new approximati on for the two-center electron repulsion integral (new gamma) is propo sed which takes the chemical softness of the pi-conjugated system into account. New gamma was successfully applied to PPP calculations of th e electronic spectra of linear polyenes, simple cyanines, and simple m erocyanines. The calculated results agree satisfactorily with the expe rimental data. On the contrary, a calculation based on the conventiona l PPP parameters gives shorter wavelengths for the absorption maxima o f the electronic spectra of charge-resonance systems, such as cyanines and merocyanines. Furthermore, they give very strange electronic spec tral shifts for merocyanines (see text), due to an inbalance between t he one- and two-electron parts of the electronic transition energy.