Dendrimers with tetrabenzoporphyrin cores: near infrared phosphors for in vivo oxygen imaging

Pd complexes of tetrabenzoporphyrins (PdTBPs) are strong near infra-red phosphors, suitable for tissue oxygen imaging by phosphorescence quenching. For in vivo use, PdTBP phosphors have to be water-soluble and protected from interactions with biomacromolecules in the blood. An approach to the construction of biologically compatible PdTBP-phosphors based on dendritic encapsulation is proposed. A series of polyglutamic PdTBP-dendrimers was synthesized and their optical and quenching properties were studied. While retaining the excellent optical characteristics (λmax Q=630 nm, φphos=12%) of basic PdTBPs and in addition being highly water soluble, PdTBP-dendrimers still exhibit excessively high oxygen quenching constants (2000–3000 mmHg−1 s−1) and form complexes with lipophilic proteins, such as serum albumin. Modification of the dendrimer periphery with polyethyleneglycol chains somewhat reduces the quenching constants but does not prevent binding of the phosphors to albumin.