Size-Dependent Light Harvesting from Nonthermalized Excited States of Gold Clusters

Light harvesting is central to many photonic materials. The light harvesting efficiency in these materials is, however, generally reduced because upper excited-state energy is lost by energy-dissipating internal conversion (IC) and vibrational relaxation processes. Herein, size-dependent energy harvesting from nonthermalized, upper excited states of glutathione-protected gold clusters of various sizes, including Au₁₈, Au₂₂, Au₂₅, Au₆₇, Au₁₀₂, and Au_≈940, is reported. Femtosecond transient absorption measurements reveal that the IC processes of Au₁₈, Au₂₂, and Au₂₅ are relatively slow wherein the upper excited-state energy of gold clusters efficiently is harvested by an energy acceptor, aminofluorescein (AF), covalently attached to the gold cluster on the time scales of 150 fs (Au₁₈), 220 fs (Au₂₂), and 380 fs (Au₂₅). Steady-state photoluminescence measurements of AF-conjugated Au₁₈, Au₂₂, and Au₂₅ clusters show notable AF emission upon high-energy excitation of gold clusters. Herein, a new avenue for energy harvesting from nonthermalized upper excited states of gold clusters, which would otherwise be lost as heat, is opened up.