Theoretical study of Zinc Tetracarboxy-Phthalocyanine: geometry optimization and UV-VIS properties

Abstract

Metal phthalocyanines are good macrocyclic photosensitizers with useful photophysical and photochemical properties for photodynamic therapy (PDT) applications. Motivated by these remarkable properties, this study investigates both experimentally and theoretically the properties of tetracarboxy ZnPc. The focus is on understanding how these properties make ZnPc-tetracarboxy suitable for PDT applications, and the research compares these findings to existing literature [1].

The aim of our work is the simulation of molecular structure and optical absorption properties using Gaussian 09 [2] software. The geometry of zinc tetracarboxy phthalocyanine ZnPc(COOH)₄ was optimized using the B3LYP functional and the def2-SVP Gaussian basis set within the framework of density functional theory (DFT). DFT calculations were used to optimize the structure by finding the minimum energy configuration. Based on the optimized structure, time-dependent DFT (TDDFT) calculations were performed to simulate the UV-VIS absorption spectrum. The relevant electronic transitions were identified in the range of 300–800 nm, and the corresponding oscillator strengths allowed the assignment of the main absorption bands. The most intense transitions were found at approximately 729.7 nm (HOMO → LUMO, 87%) and 656.0 nm (HOMO → LUMO+1, 85%). The HOMO–LUMO energy gap was estimated to be around 1.61 eV, indicating good electronic stability. The B3LYP-based calculated absorption spectrum is in very good agreement with the experimental spectra of ZnPc(COOH)₄. Thus, this study predicts the electronic transitions responsible for UV-VIS absorption, which can be useful for understanding the photophysical properties of the molecule.