Heavy atom effect on Zn(II) phthalocyanines derivatives: a theoretical exploration of the photophysical properties

Absorption electronic spectra, singlet–triplet energy gaps and spin–orbit matrix elements have been computed at DFT and TDDFT levels of theory for a series of substituted Zn(II)-phthalocyanines (ZnPcs), recently proposed as potential photosensitizers in photodynamic therapy (PDT). Their photophysical properties have been rationalized in the light of the substitution pattern which includes the position, the donor or withdrawing nature, and the relative donating force of peripheral and non-peripheral ligands. Moreover, the effects of heavy substituents on these properties have been investigated by introducing a different number of iodine atoms on the phthalocyanine macrocycle. The results show that the substitution pattern significantly affects the absorption spectra, but just slightly modifies the DES–T values. The presence of heavy atoms produces a significant effect on the photophysical properties of the investigated compounds enhancing the spin–orbit coupling (SOC) values.