Synthesis of a novel hypocrellin B derivative and its photogeneration of semiquinone anion radical

In order to improve the photosensitizing activity of HB further, the complex of 5,8-di-Br-HB with Al3+ was first designed and synthesized in high yield. 5,8-di-Br-HB forms a 2:1 type (metal–ligand ratio) complex with Al3+ measured by molar ratio and continuous variation methods. The new photosensitizer was characterized by UV–Vis, IR, 1H NMR and elemental analysis measurements. Based on the above experimental results, we first proposed a polymer-like structural model of the complex. The water-solubility and red absorption of the [Al2(5,8-di-Br-HB)Cl4]n complex are both enhanced over hypocrellin B. In addition, the EPR and spectrophotometric measurements demonstrate that semiquinone anion radical of [Al2(5,8-di-Br-HB)Cl4]n can be produced by [Al2(5,8-di-Br-HB)Cl4]n photosensitization. The generation efficiency of {[Al2(5,8-di-Br-HB)Cl4]n}⋅− is almost equal to that of HB⋅−. These results obtained indicated that [Al2(5,8-di-Br-HB)Cl4]nwas at least a favorable Type I phototherapeutic agent.