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, [Au 4 Cu 4 (S-Adm) 5 (DPPM) 2 ] + ( Au 4 Cu 4 ; S-Adm = 1-adamantanethiolate, DPPM = bis(diphenylphosphino)methane), exhibited favorable sensitizer properties and superior UC performance. Contrary to the structurally rigid Au 2 Cu 6 (S-Adm) 6 (TPP) 2 ( Au 2 Cu 6 , TPP = triphenylphosphine), Au 4 Cu 4 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 Au 4 Cu 4 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 ( k 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 k TET for Au 4 Cu 4 was seven times larger than that for Au 2 Cu 6 , which presumably reflects the structural/electronic fluctuations of Au 4 Cu 4 during the triplet state residence. The combination of the Au 4 Cu 4 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 Au 2 Cu 6 and is also outstanding among the organic-inorganic hybrid nanomaterials reported so far.