The thiolato complex [Pt(II)(bipyridine)(N,S-aminoethanethiolate)]+ Cl- (1) undergoes sequential reactions with singlet oxygen to initially form the corresponding sulfenato complex [Pt(II)(bipyridine)(N,S(=O)-aminoethansulfenate)]+ (2) followed by a much slower reaction to the corresponding sulfinato complex. In contrast with many Pt dithiolato complexes, 1 does not produce any singlet oxygen, but its rate constant for singlet oxygen removal (kT) is quite large (3.2 × 107 M-1sec-1) and chemical reaction accounts for ca. 25 % of the value of kT). The behavior of 1 is strikingly different from the complex Pt(II)(bipyridine)(1,2-benzenditholate) (4). The latter complex reacts with 1O2 (either from an external sensitizer or via a self-sensitized pathway) to form a sulfinato complex. These two very different reactivity pathways imply different mechanistic pathways: The reaction of 1 with 1O2 must involve O-O bond cleavage and intermolecular oxygen atom transfer, while the reactive intermediate in complex 4 collapses intramolecularly to the sulfinato moiety.

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Rf-PEG (fluoroalkyl double-ended poly(ethylene glycol)) hydrogel is potentially useful as a drug delivery depot due to its advanced properties of sol–gel two-phase coexistence and low surface erosion. In this study, 1H molecular diffusion nuclear magnetic resonance (NMR) and 19F spin diffusion NMR were used to probe the drug loading and diffusion properties of the Rf-PEG hydrogel for small anticancer drugs, 5-fluorouracil (FU) and its hydrophobic analog, 1,3-dimethyl-5-fluorouracil (DMFU). It was found that FU has a larger apparent diffusion coefficient than that of DMFU, and the diffusion of the latter was more hindered. The result of 19F spin diffusion NMR for the corresponding freeze-dried samples indicates that a larger portion of DMFU resided in the Rf core/IPDU intermediate-layer region (where IPDU refers to isophorone diurethane, as a linker to interconnect the Rf group and the PEG chain) than that of FU while the opposite is true in the PEG–water phase. To understand the experimental data, a diffusion model was proposed to include: (1) hindered diffusion of the drug molecules in the Rf core/IPDU-intermediate-layer region; (2) relatively free diffusion of the drug molecules in the PEG-water phase (or region); and (3) diffusive exchange of the probe molecules between the above two regions. This study also shows that molecular diffusion NMR combined with spin diffusion NMR is useful in studying the drug loading and diffusion properties in hydrogels for the purpose of drug delivery applications.

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