Study on the prediction of visible absorption maximum of phthalocyanine compounds by semiempirical quantum methods

To improve the prediction accuracy of visible absorption maximum, a new idea is introduced in this article, that is, OWF(pi-pi) (pi-pi overlap weighting factor, one of the adjustable parameters in ZINDO/S method) is considered as a measurement of the extent of pi-electron delocalization, rather than a certain fixed value applicable to all molecules. The results of this study show that the new idea is feasible in improving the prediction accuracy and there is some relationship between OWF(pi-pi) and molecular structural parameters for phthalocyanine compounds. The relationship could be extrapolated to compounds of the same series. From the point of view of electron transition, the electron transition from regions around one covalent bonded nitrogen to those around the other covalent bonded nitrogen and the two coordinate bonded nitrogens, i.e., the transition from HOMO to LUMO and LUMO+1, results in the visible absorption maximum of phthalocyanine compounds.