The Origin of the Halogen Effect on the Phthalocyanine Green Pigments

The structure and the electronic and optical properties of halogenated copper-phthalocyanine (n alpha,m beta(Hal)-CuPc) molecules are investigated, according to the variation in the substituted halogen-atom species (Hal = Cl or Br) at the alpha and beta positions of isoindole ring with different numbers (n and m = 0, 4, 8, or 16). Our results show that the halogen effect mainly results from a structural deformation rather than caused by electron-effects. A nonplanar deformation of the phthalocyanine chromophore of the n alpha,m beta(Hal)-CuPc molecule causes a significant change only in the HOMO and HOMO-1 levels, rather than in the LUMO levels, which leads to the appearance of a green color arising from the large red-shifts of the Soret and Q bands. The present result may serve as an important reference point for designing novel halogen-free green pigments, in accordance with the environmental regulations for the restriction of hazardous substances (RoHS) in electronic and electrical devices.