Photochemical Reactivities and Photodynamic Effects of an Organorhodium Phthalocyanine

Abstract

The light-triggered uncaging of bioactive molecules is attractive for a new type of photodynamic therapy (PDT) as it enables the tumor-selective cell attack in various mechanism of action. In particular, photochemical release of formaldehyde is promising as they exhibit cytotoxicity by the alkylation and crosslinking of biological tissues.[1] Meanwhile, methyl-cobalt porphyrinic compounds are known to give formaldehyde by the homolytic cleavage of a metal-carbon bond under visible light irradiation in the presence of molecular oxygen.[2] However, there has been no report on the photochemical formation of formaldehyde under irradiation of red light which shows high permeability toward biological tissues. In this study, the organometallic phthalocyanine complexes possessing a group 9 metal center were synthesized, and their photophysical properties, photochemical reactivities and photodynamic effects were investigated under red light irradiation.Through the screening of reaction conditions, the methyl-rhodium phthalocyanine complex (1) was found to give formaldehyde by irradiation of pulsed laser of red light in the presence of oxygen. The laser-power dependency on the conversion rate indicated that the metal-carbon bond of 1 is activated through the stepwise, two-photon excitation. In order to realize the photochemical formation of formaldehyde in biological environment, 1 was encapsulated into a liposome of 1,2-Dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC). By irradiation of red light, the liposomal 1 released formaldehyde in water under air. Finally, the phototoxicity and cytotoxicity of liposomal 1 toward HeLa cells were investigated. The HeLa cells treated by liposomal 1 were successfully killed by light irradiation, indicating that 1 exhibits the photodynamic effect.