Hypocrellin B, a perylenequinonoid pigment, and its complexes with lanthanide ions: Optical characterization and enhancements in its photodynamic properties

Hypocrellins, perylenequinonoid pigments isolated from Hypocrella bambusae, are known photodynamic agents. They exhibit several advantages over the other photosensitizers, such as ready preparation, easy purification, low aggregation tendency, high singlet oxygen generation quantum yield and rapid metabolism in vivo. However, hypocrellins do not exhibit absorption at wavelengths longer than 600 nm, which limits their application for PDT. To solve this problem our efforts were focused towards modifying the properties of hypocrellin B. In this study, hypocrellin B complexes formed with the lanthanide ions lanthanum (La3+), terbium (Tb3+) and europium (Eu3+), using ethanol as solvent, had their optical properties characterized through spectroscopic methods. We observed that all the lanthanide ions change the HB molecular structure, since displacements in the bands were noted in both absorption and emission spectra. Different molar ratios between the HB and the lanthanide ions were tested, and it was verified that the complexes with 1 mol of HB for 2 mol of lanthanide ions present the larger red-shift comparatively to pure HB. We also determined the dimerization constants for these complexes. Under excitation at 532 nm the singlet oxygen generation quantum yields were estimated by the emission at 1270 nm, where singlet oxygen has a well known emission peak. It can be affirmed that such complexations were responsible for an improvement in the optical properties of the HB molecule, which suggests that these complexes may find a good application at in vivo studies.