Optical Properties and Structural Relationships of the Silver Nanoclusters Ag₃₂(SG)₁₉ and Ag₁₅(SG)₁₁

The recent discovery of stable Ag nanoclusters presents new opportunities to understand the detailed electronic and optical properties of the metal core and the ligands using ultrafast spectroscopy. This paper focuses on Ag₃₂ and Ag₁₅ (with thiolate ligands), which are stable in solution. The steady state absorption spectra of Ag nanoclusters show interesting quantum size effects, expected for this size regime. Using a simple structural model for Ag₃₂, TDDFT calculations show absorption at 480 nm and 680 nm that are in reasonable correspondence with experiments. Ag₃₂(SG)₁₉ and Ag₁₅(SG)₁₁ have quantum yields up to 2 orders of magnitude higher than Au nanoclusters of similar sizes, with an emission maximum at 650 nm, identified as the metal-ligand state. The emission from both Ag nanoclusters has a common lifetime of about 130 ps and a common energy transfer rate of K_EET ≥ 9.7 × 10⁹ s^-1. A "dark state" competing with the emission process was also observed and was found to be directly related to the difference in quantum yield (QY) for the two Ag clusters. Two-photon excited emission was observed for Ag₁₅(SG)₁₁, with a cross-section of 34 GM under 800 nm excitation. Femtosecond transient absorption measurements for Ag₃₂ recorded a possible metal core state at 530 nm, a metal-ligand state at 651 nm, and ground state bleaches at 485 and 600 nm. The ground state bleach signals in the transient spectrum for Ag₃₂ are 100 nm blue-shifted in comparison to Au₂₅. The transient spectrum for Ag₁₅ shows a weak ground state bleach at ∼480 nm and a broad excited state centered at 610 nm. TDDFT calculations indicate that the electronic and optical properties of Ag nanoclusters can be divided into core states and metal-ligand states, and photoexcitation generally involves a ligand to metal core transition. Subsequent relaxation leaves the electron in a core state, but the hole can be either ligand or core-localized. This leads to emission/relaxation that is consistent with the observed photophysics.