Comparative study of ROS generation by ZnPc, AlPcCl, SiPcCl₂ and CuPc

In this study, the photophysical properties and singlet oxygen generation efficiencies of ZnPc, CuPc, SiPcCl₂, and AlPcCl derivatives were systematically investigated in DMSO. UV-Vis absorption spectra showed characteristic Q-bands at 670-680 nm. Progressive band broadening and increased spectral complexity revealed an aggregation tendency: ZnPc ≈ AlPcCl < SiPcCl₂ < CuPc. Fluorescence measurements revealed emission maxima between 680 and 710 nm, with fluorescence lifetimes ranging from 3.50 ns (ZnPc) to 6.71 ns (AlPcCl) and fluorescence quantum yields ranging from 10.78% (ZnPc) to 28.50% (AlPcCl), reflecting differences in the excited-state deactivation pathways among the investigated complexes. Singlet oxygen generation (Φ_Δ), evaluated by the DPBF assay under red LED irradiation (λ ≈ 625 nm), followed the order ZnPc (0.56) > SiPcCl₂ (0.37) > AlPcCl (0.21) > CuPc (0.13). The fluorescence and singlet oxygen quantum yields did not exhibit a strictly inverse relationship, indicating that singlet oxygen generation depends on the combined contributions of radiative and non-radiative excited-state deactivation pathways, molecular aggregation, and the central-atom coordination environment. Overall, the results demonstrate that both molecular aggregation and the identity of the central metal ion critically govern the excited-state dynamics and photodynamic efficiency of phthalocyanines, providing valuable structure-property insights for the rational design of next-generation photosensitizers for cancer photodynamic therapy.