Structurally Flexible Au–Cu Alloy Nanoclusters Enabling Efficient Triplet Sensitization and Photon Upconversion

Ligand-protected noble-metal nanoclusters exhibit an innately triplet nature and have been recently recognized as emerging platforms for triplet sensitizers of photon upconversion (UC) via triplet-triplet annihilation. Herein, we report that a structurally flexible Au-Cu alloy nanocluster, [Au₄Cu₄(S-Adm)₅(DPPM)₂]⁺ (<b>Au₄Cu₄</b>; S-Adm = 1-adamantanethiolate, DPPM = bis(diphenylphosphino)methane), exhibited favorable sensitizer properties and superior UC performance. Contrary to the structurally rigid Au₂Cu₆(S-Adm)₆(TPP)₂ (<b>Au₂Cu₆</b>, TPP = triphenylphosphine), <b>Au₄Cu₄</b> exhibited significantly better sensitizer characteristics, such as a near-unity quantum yield for intersystem crossing (ISC), long triplet lifetime (ca. 8 μs), and efficient triplet energy transfer (TET). The efficient ISC of <b>Au₄Cu₄</b> was attributed to the practically negligible activation barriers during the ISC process, which was caused by the spin-orbit interaction between the two isoenergetic isomers predicted by theoretical calculations. A series of aromatic molecules with different triplet energies were used as acceptors to reveal the driving force dependence of the TET rate constant (<i>k</i>_TET). This dependency was analyzed to evaluate the triplet energy and sensitization ability of the alloy nanoclusters. The results showed that the maximum value of <i>k</i>_TET for <b>Au₄Cu₄</b> was seven times larger than that for <b>Au₂Cu₆</b>, which presumably reflects the structural/electronic fluctuations of <b>Au₄Cu₄</b> during the triplet state residence. The combination of the <b>Au₄Cu₄</b> sensitizer and the 9,10-diphenylanthracene (DPA) annihilator/emitter achieved UC with internal quantum yields of 14% (out of 50% maximum) and extremely low threshold intensities (2-26 mWcm^-2). This performance far exceeds that of <b>Au₂Cu₆</b> and is also outstanding among the organic-inorganic hybrid nanomaterials reported so far.