Conformationally Constrained Bidentate Ligands Drive Record-High NIR Quantum Yield in Cu Nanoclusters

Area of Science:

• Materials Science
• Nanotechnology
• Photochemistry

Background:

• Atom-precise copper nanoclusters (Cu NCs) offer potential in biomedical and optoelectronic fields due to their unique properties.
• Existing Cu NCs suffer from low room-temperature (RT) solution photoluminescence quantum yields (PLQYs) and oxidation sensitivity, limiting practical applications.
• Developing Cu NCs with high NIR luminescence in solution is crucial for advancing these applications.

Purpose of the Study:

• To synthesize and characterize novel 15-nuclear copper nanoclusters (Cu NCs) with distinct ligand shells.
• To investigate the impact of ligand structure on the photoluminescence properties, particularly NIR emission.
• To establish a strategy for enhancing the performance of solution-phase NIR-emitting Cu NCs.

Main Methods:

• Synthesis of two structurally defined 15-nuclear copper-thiolate clusters: [Cu15(TPP)6(PET)13]2+ (Cu15-TPP) and [Cu15(DPPB)3(PET)12H]2+ (Cu15-DPPB).
• Single-crystal X-ray diffraction (SC-XRD) for structural determination.
• Photoluminescence quantum yield (PLQY) measurements in solution and solid states.
• Excited-state dynamics studies to elucidate luminescence mechanisms.

Main Results:

• Cu15-DPPB, featuring a rigidified ligand shell due to a cis-cis conformation of the diphosphine chelator, exhibited a significantly enhanced NIR PLQY (37.2% in solution, 46% in solid state) compared to Cu15-TPP (0.2% in solution).
• The achieved 37.2% solution PLQY for Cu15-DPPB represents the highest reported for solution-phase NIR-emitting Cu-thiolate NCs.
• Structural analysis revealed comparable Cu9 cores but distinct surface ligand arrangements influencing photophysical properties.
• Excited-state dynamics indicated that surface rigidification in Cu15-DPPB accelerates intersystem crossing (ISC) and boosts radiative decay while suppressing nonradiative decay.

Conclusions:

• Ligand conformational engineering is a viable strategy to overcome intrinsic limitations of Cu-based emitters, such as weak spin-orbit coupling and slow ISC.
• Rigidifying the ligand shell of Cu NCs can dramatically enhance NIR photoluminescence efficiency in solution.
• This work paves the way for developing high-performance, solution-phase, room-temperature NIR luminescent Cu NCs for various applications.