Excitation-Wavelength-Dependent Dual Emission in Au42 Quantum Rods

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 Sn 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 Tn≥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.