Brightly Phosphorescent, Environmentally Responsive Hydrogels Containing a Water-Soluble Three-Coordinate Gold(I) Complex

Stimuli-responsive phosphorescent hydrogel microspheres have been synthesized by incorporating a water-soluble phosphorescent Au(I) complex, Na(8)[Au(TPPTS)(3)], TPPTS = tris(3,3',3''-trisulfonatophenyl)phosphine, into the polymer network of poly(N-isopropylacrylamide) (PNIPAM). Remarkable sensitization of the Au-centered emission takes place in the resulting phosphorescent hydrogels (by up to 2 orders of magnitude!) compared to that of the gold complex alone in pure water. Results of pH- and temperature-dependent luminescence titrations show that the sensitization is further magnified at physiological conditions, which is desirable for biomedical applications that will include bioimaging and drug delivery. The physical properties of PNIPAM microgels are not negatively impacted by the presence of the gold luminophore, as the colloidal crystallinity and phase transition properties remain intact. Phosphorescent microspheres have been further cross-linked by covalently bonding to neighboring particles, leading to brightly phosphorescent/high-water-content crystalline hydrogel networks with more stable crystallinity vs microgel soft crystals. These gel networks exhibit the same green phosphorescence seen in the hydrogel microspheres and pure Na(8)[Au(TPPTS)(3)] aqueous solutions with a broad unstructured profile and peak maximum at ∼525 nm. Dehydration leads to further emission sensitization and gradual blue shifts that can be fine-tuned to ultimately reach a turquoise emission at ∼490 nm in the freeze-dried form of the gel, corresponding to the emission of single crystals of Na(8)[Au(TPPTS)(3)], in agreement with the photoinduced Jahn-Teller distorted excited state model we reported earlier. Remarkable sensitivity to temperature and pH takes place in the emission enhancement with particularly favorable results at physiological conditions. The work herein represents a unique example of a stimulus-responsive phosphorescent hydrogel from a transition metal-based as opposed to lanthanide-based phosphor in an aqueous medium.