Overview of cationic phthalocyanines for effective photoinactivation of pathogenic microorganisms

Phthalocyanine (Pc) dyes are photoactive compounds that can absorb and emit light in a large range of the UV–vis spectrum, with recognized potential for medical applications. Considering the low solubility of Pc macrocycles in water, it is important to use cationic symptoms on their skeleton to improve their amphiphilicity for biomedical applications. The use of suitable pyridinium groups on Pc is a good strategy to solve this drawback and make them more eff ;ective to photoinactivate microorganisms via a photodynamic inactivation (PDI) approach. This review focuses the synthesis of quaternized Pc dyes, their photophysical and photochemical properties, and their antimicrobial photoinactivation efficiency. This innovative study compares, for the first time, different cationic moieties on Pc taking into account the efficiency of singlet oxygen (¹O₂), quantum yield (Φ_Δ) generation, fluorescence quantum yield (Φ_F), (photo)stability, light irradiation type (visible/white and/or red light), maximized overlapped absorption effect of Pc (S- and/or Q-band) vs light system irradiation type, and water solubility (n-octanol/water partition coefficient, P_o/w), when these parameters are determined and provided in the multidisciplinary reports. This approach is also relevant to conjugate free-base (H₂Pc) and metalated phthalocyanines (MPc, M = Zn²⁺, Mg²⁺, In³⁺, Ga³⁺, Ge³⁺, Si⁴⁺, etc.) with aromatic or aliphatic substituents linked by N, O or S atoms on the peripheral or axial positions of the Pc structures, such as e.g. (methoxy, oxy, or thio)pyridinium, ammonium, or benzimidazolium units, etc. Here, the influence of the structural peripheral (α- and/or β-position of Pc) or axial substituents type, number and positive charge position that can affect the PDI process will be analysed. These aspects are important to design versatile molecules that can interact with pathogenic microorganisms of variable size, subcellular architecture, biochemical composition, and susceptibility to externally added chemical agents. This review highlights the important developments of several modifications of cationic Pc dyes for the PDI of microorganisms, such as pathogenic bacteria, fungi, and virus.