Ultrafast Intersystem Crossing in Icosahedral-Core Metal Nanoclusters

Atomically precise metal nanoclusters (NCs) occupy a unique chemical regime between molecular chromophores and nanoscale materials, yet their excited-state relaxation pathways remain fundamentally unresolved. In particular, the timing and mechanistic role of triplet-state formation at the earliest stages of photoexcitation remain unclear, limiting the development of predictive structure-photophysics relationships. Here, we establish a revised mechanistic picture of excited-state energy dissipation in icosahedral-core metal NCs by directly resolving primary relaxation pathways previously inaccessible to experiment. By combining broadband transient absorption spectroscopy with femtosecond-nanosecond measurements, we show─using prototypical Au₂₅, Ag₂₅, and Au₁₃ NCs as model systems─that ultrafast intersystem crossing on a sub-100 fs time scale rapidly populates triplet manifolds, as evidenced by distinct transient spectral signatures. Subsequent relaxation pathways diverge depending on cluster structure, with the flexibility of metallic staple motifs enabling additional vibrational relaxation channels prior to phosphorescence. These findings unify the excited-state energy dissipation framework of triplet-emissive NCs and provide mechanistic guidelines for the rational design of NC-based materials for energy conversion and photochemical applications.