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

Area of Science:

• Materials Science
• Photochemistry
• Nanotechnology

Background:

• Ligand-protected noble-metal nanoclusters are emerging as triplet sensitizers for photon upconversion (UC) via triplet-triplet annihilation.
• Understanding the structure-property relationships of these nanoclusters is crucial for optimizing UC efficiency.

Purpose of the Study:

• To investigate the photon upconversion properties of a structurally flexible Au-Cu alloy nanocluster, [Au4Cu4(S-Adm)5(DPPM)2]+.
• To compare its performance as a triplet sensitizer against a structurally rigid analogue.
• To elucidate the factors contributing to its enhanced UC efficiency.

Main Methods:

• Synthesis and characterization of the flexible Au-Cu alloy nanocluster.
• Photophysical measurements including intersystem crossing (ISC) quantum yield and triplet lifetime determination.
• Triplet energy transfer (TET) studies using various aromatic acceptors.
• Theoretical calculations to understand ISC mechanisms.
• UC experiments using the nanocluster as a sensitizer with 9,10-diphenylanthracene (DPA) as an annihilator/emitter.

Main Results:

• The flexible [Au4Cu4(S-Adm)5(DPPM)2]+ nanocluster exhibited a near-unity ISC quantum yield and a long triplet lifetime (ca. 8 μs).
• It demonstrated significantly more efficient triplet energy transfer compared to a rigid Au2Cu6(S-Adm)6(TPP)2 nanocluster, with a maximum k_TET seven times larger.
• UC experiments achieved internal quantum yields of 14% with very low threshold intensities (2-26 mWcm-2).

Conclusions:

• Structural flexibility and electronic properties of the Au-Cu alloy nanocluster significantly enhance its triplet sensitizer capabilities.
• The nanocluster's efficient ISC and TET are attributed to negligible activation barriers and potential structural/electronic fluctuations.
• This flexible nanocluster represents a highly promising platform for efficient organic-inorganic hybrid photon upconversion systems.