A Luminescent Poly(amidoamine)−Iridium Complex as a New Singlet-Oxygen Sensitizer for Photodynamic Therapy

A polymer complex (1P) was synthesized, by binding bis(cyclometallated) Ir(ppy)2+ fragments (ppy = 2-phenylpyridyl) to the phenanthroline (phen) pendants of a poly(amidoamine) copolymer (PhenISA, in which the phen pendants involved ca. 6% of the repeating units). The corresponding molecular complex [Ir(ppy)2(bap)]+ (1M, bap = 4-(butyl-4-amino)-1,10-phenanthroline) was also prepared for comparison. In water solution 1P gives nanoaggregates with hydrodynamic diameter of 30 nm, in which the lipophilic metal centers are supposed to be segregated within polymer tasks, to reduce interaction with water. Such confinement, joined to the dilution of the triplet emitters along the polymer chains, led to higher photoluminescence quantum yield for 1P with respect to 1M (0.061 vs. 0.034 in aerated water solution), with longer lifetime of the 3MLCT excited states and blue shifted emission (595 nm vs. 604 nm). The NMR data supported the segregation of the metal centers. The photoreaction of O2 with 1,5-dihydroxynaphthalene showed that 1P is able to sensitize 1O2 generation, but with halved quantum yields with respect to 1M. Cellular uptake experiments showed that both 1M and 1P are efficient cell staining agents endowed with two-photon excitation (TPE) imaging capability. TPE microscopy at 840 nm indicated that both the complexes penetrate the cellular membrane of HeLa cells, locating in the perinuclear region. Cell PDT tests showed that both 1M and 1P are able to induce cell apoptosis upon Xe lamp irradiation. The fraction of apoptotic cells was higher for 1M than for 1P (74% for 1M, 38% for 1P, 6 h after irradiation for 5 min), but cells incubated with 1P showed much reduced necrosis and also a lower toxicity in the absence of irradiation. More generally, the results evidenced that the cell damage induced by 1M was avoided by binding the Ir sensitizers to the poly(amidoamine).