ZnPc (Zinc Phtahlocyanine) derivatives as medicinal photosensitizers for photodynamic therapy

Abstract

Tetra-carboxy zinc phthalocyanine (ZnPc(COOH)2) is a functionalized metallophthalocyanine that exhibits strong optical absorption and characteristic fluorescence, making it an important compound for applications in photodynamic therapy, photocatalysis, and optoelectronic devices. Its photophysical behavior is dominated by the highly conjugated phthalocyanine macrocycle and the presence of a central zinc ion, which enhances its stability and electronic transitions. In the UV–visible absorption spectrum, tetra-carboxy zinc phthalocyanine typically displays two major absorption regions: the B band (Soret band) and the Q band. The B band appears in the ultraviolet region around 300–400 nm and is associated with higher energy π–π* transitions. The most intense feature is the Q band, observed in the visible to near-infrared region, usually between 650–700 nm that arised from electronic transitions within the macrocyclic ring and is highly sensitive to molecular aggregation and solvent environment. The introduction of carboxylic acid groups increases solubility in polar solvents and can reduce aggregation, leading to sharper and more intense Q-band absorption. Fluorescence emission of tetra-carboxy zinc phthalocyanine occurs when the molecule relaxes radiatively from its first excited singlet state back to the ground state. Upon excitation at the Q-band wavelength, ZnPc(COOH)₄ emits fluorescence typically in the range of 680–750 nm, with a relatively small Stokes shift. The fluorescence intensity depends strongly on concentration, solvent polarity, and aggregation state. In aggregated form, fluorescence is often quenched due to excitonic interactions and non-radiative energy transfer pathways.