The Origin of the Halogen Effect on the Phthalocyanine Green Pigments

The structure and the electronic and optical properties of halogenated copper‐phthalocyanine (nα,mβ(Hal)‐CuPc) molecules are investigated, according to the variation in the substituted halogen‐atom species (Hal=Cl or Br) at the α and β 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 electronic effects. A nonplanar deformation of the phthalocyanine chromophore of the nα,mβ(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.