A single-gold-atom addition regulates sharp redshift in the fluorescence of atomically precise nanoclusters

The manipulation of emission peaks at the atomic level and the investigation of the fluorescent origin mechanism are important issues. In this study, a phosphine-mediated modification method was employed on Au₃₆(TBBT)₂₄ nanocluster to produce a new gold nanocluster Au₃₇(TBBT)₂₁(TPP)₂. The structural comparison revealed that Au₃₇(TBBT)₂₁(TPP)₂ has a structural framework similar to that of Au₃₆(TBBT)₂₄ except for the reconstruction of its surface motifs, the addition of one gold atom into the kernel, and local structural distortion. Interestingly, compared with Au₃₆(TBBT)₂₄, the emission peak of Au₃₇(TBBT)₂₁(TPP)₂ is red-shifted into the NIR-II windows (972 nm vs. 1152 nm in CDCl₃) with a quantum yield of 1.5%. Furthermore, the origin of the NIR-II fluorescence in Au₃₇(TBBT)₂₁(TPP)₂ and the red-shift mechanism of the emission peak were explored by combining the crystal structure and DFT calculations. The results reveal that the insertion of the 37^th gold atom into the core can increase the contribution of the gold atoms to the HOMO orbitals and change the origin of their fluorescence from local excitation (LE) to inter fragment charge transfer (IFCT).