Metal Nanoparticle Sensitized Molecular Triplets for Efficient Photon Upconversion

ABSTRACT Metal nanoparticles have recently been explored as sensitizers for molecular triplet generation via charge transfer and recombination. A key feature of metals is their gapless band structure with a wide range of electronic states available for charge transfer, potentially relaxing energy alignment constraints in semiconductor sensitizers. Here we show efficient triplet generation in metal (Au or Pt) nanoparticle‐Rhodamine B (RhB) hybrids after exciting RhB, followed by hole transfer from singlet 1 RhB * to the metal and charge recombination to form triplet 3 RhB * , revealed by ultrafast time‐resolved spectroscopy. Hole transfer from 1 RhB * to the Fermi level of the metal leads to products with insufficient energy to sensitize 3 RhB * thermodynamically. This contrasts with the high triplet generation yield and suggests the possibility of hole transfer to electronic states below the Fermi level, inducing a transition from inverted to normal Marcus regime that is thermodynamically and kinetically favorable for triplet generation. We show efficient phosphorescence emission, singlet oxygen generation, antibacterial applications, and triplet‐triplet annihilation photon upconversion sensitized by nontoxic metal nanoparticles, which is crucial for practical application. The gapless band structure, low toxicity, and high stability of metal nanoparticles make them promising for mediating molecular triplet generation for a variety of applications.