Excitation-Wavelength-Dependent Dual Emission in Au42 Quantum Rods

Abstract

Dual emissive materials are of particular importance in chemical and biological assays. Herein, we focus on a rod-shaped Au42(SCH2CH2Ph)32 nanocluster that exhibits dual emission at 875 and 1040 nm owing to fluorescence (FL) and phosphorescence (PH), respectively. Transient absorption analysis shows a slow intersystem crossing (ISC, ∼500 ps) from lowest singlet S1 to triplet T1 upon direct S1 excitation, leading to dual emission from both S1 and T1. However, upon excitation of the higher S n state, we observe a metastable singlet state (S′). We rationalize two competing decay pathways from S′: (i) internal conversion to S1 and (ii) ISC to high triplet T n≥2. These processes alter the populations of the S1 and T1 states, which explains the excitation-wavelength-dependent dual emission and FL/PH ratios. The revealed excited-state mechanism and the dual emissive Au42 material will serve as a paradigm for future research on atomically precise nanoclusters.