Elucidating the Near-Infrared Photoluminescence Mechanism of Homometal and Doped M 25 (SR) 18 Nanoclusters.

More than a decade of research on the photoluminescence (PL) of classic Au 25 (SR) 18 and its doped nanoclusters (NCs) still leaves many fundamental questions unanswered due to the complex electron dynamics. Here, we revisit the homogold Au 25 (ligands omitted hereafter) and doped NCs, as well as the Ag 25 and doped ones, for a comparative study to disentangle the influencing factors and elucidate the PL mechanism. We find that the strong electron-vibration coupling in Au 25 leads to weak PL in the near-infrared region (∼1000 nm, quantum yield QY = 1% in solution at room temperature). Heteroatom doping of Au 25 with a single Cd or Hg atom strengthens the coupling of the exciton with staple vibrations but reduces the coupling with the core breathing and quadrupolar modes. The QYs of the three MAu 24 NCs (M = Hg, Au, and Cd) follow a linear relation with their PL lifetimes, suggesting a mechanism of suppressed nonradiative decay in PL enhancement. In contrast, the weaker electron-vibration coupling in Ag 25 leads to higher PL (QY = 3.5%), and single Au atom doping further leads to a 5× enhancement of the radiative rate and a suppression of nonradiative decay rate (i.e., twice the PL lifetime of Ag 25 ) in AuAg 24 (hence, QY 35%), but doping more Au atoms results in gold distribution to staple motifs and thus triggering of strong electron-vibration coupling as in the MAu 24 NCs, hence, counteracting the radiative enhancement effect and giving rise to only 5% QY for Au x Ag 25- x ( x = 3-10). The obtained insights will provide guidance for the design of metal NCs with high PL for lighting, sensing, and optoelectronic applications.