Brightly emissive octahedral Cu₄X₄ clusters showing polymorphic-dependent and mechanochromic phosphorescence

Area of Science:

• Inorganic Chemistry
• Materials Science
• Photophysics

Background:

• Octahedral copper(I) halide clusters (Cu4X4) are of interest due to their unique structural and photophysical properties.
• Understanding structure-property relationships in these clusters is crucial for developing advanced luminescent materials.

Purpose of the Study:

• To synthesize and characterize novel octahedral [Cu4X4L2] clusters using specific phosphine ligands (L1 and L2).
• To investigate the photoluminescent properties, including phosphorescence, quantum yields, and lifetimes.
• To explore polymorphic-dependent and mechanochromic luminescence phenomena in these copper clusters.

Main Methods:

• Synthesis of [Cu4X4L2] clusters (X = Cl, Br, I) employing bis(t-butyl)(2-pyridyl)phosphine (L1) and bis(2-pyridyl)-t-butylphosphine (L2).
• Room temperature phosphorescence spectroscopy to determine emission maxima (λmax), quantum yields, and lifetimes.
• Investigation of solid-state luminescence properties, including polymorphism and response to grinding and solvent addition (mechanochromism).

Main Results:

• Synthesized series of [Cu4X4L2] clusters exhibiting bright 3(M+X)LCT phosphorescence (λmax = 514-570 nm) with high quantum yields (up to 84%) and lifetimes (3-22 μs).
• Discovered polymorphic-dependent luminescence in [Cu4Br4(L1)2], showing green and yellow-green emission with distinct efficiencies and lifetimes.
• Observed mechanochromic luminescence in [Cu4I4(L2)2], with grinding causing emission band merging and solvent addition restoring original bands.

Conclusions:

• The synthesized octahedral copper clusters display efficient phosphorescence and novel polymorphic and mechanochromic luminescence behaviors.
• These findings highlight the potential of Cu4X4 clusters as tunable luminescent materials, sensitive to structural and environmental changes.
• The study provides a foundation for designing new luminescent materials based on octahedral copper halide cores.