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Two heteroctanuclear Au 4 Ag 4 cluster complexes of 4,5-diethynylacridin-9-one (H 2 L) were prepared through the self-assembly reactions of [Au(tht) 2 ](CF 3 SO 3 ), Ag(tht)(CF 3 SO 3 ), H 2 L and PPh 3 or PPh 2 Py (2-(diphenylphosphino)pyridine). The Au 4 Ag 4 cluster consists of a [Au 4 L 4 ] 4- and four [Ag(PPh 3 )] + or [Ag(PPh 2 Py)] + units with Au 4 L 4 framework exhibiting a twisted paper clip structure. In CH 2 Cl 2 solutions at ambient temperature, both compounds show ligand fluorescence at ca. 463 nm as well as phosphorescence at 650 nm for 1 and 630 nm for 2 resulting from admixture of 3 IL (intraligand) of L ligand, 3 LMCT (from L ligand to Au 4 Ag 4 ) and 3 MC (metal-cluster) triplet states. Crystals or crystalline powders manifest bright yellow-green phosphorescence with vibronic-structured emission bands at 530 (568sh) nm for complex 1 and 536 (576sh) nm for complex 2 . Upon mechanical grinding, yellow-green emission in the crystalline state is dramatically converted to red luminescence centered at ca. 610 nm with a drastic redshift of the emission after crystal packing is destroyed.

Reaction of [Au(tht)2](ClO4) (tht = tetrahydrothiophene), [Cu(CH3CN)4](ClO4), 3,6-di-tert-butyl-1,8-diethynyl-9H-carbazole (H3decz), and bis(2-diphenylphosphinophenyl)ether (POP) in the presence of triethylamine (NEt3) gave the cluster complex Au4Cu2(decz)2(POP)2 as yellow crystals. As revealed by X-ray crystallography, the Au4Cu2 cluster exhibits scissor-like structure sustained by two decz and two POP ligands and stabilized by Au-Cu and Au-Au interactions. The Au4Cu2 cluster shows bright yellow to orange photoluminescence upon irradiation at >300 nm, arising from 3[π (decz)→5d (Au)] 3LMCT (ligand-to-metal charge transfer) and 3[π→π* (decz)] 3IL (intraligand) triplet states as revealed by theoretical and computational studies. When it is mechanically ground, reversible phosphorescence conversion from yellow to red is observed owing to more compact molecular packing and thus stronger intermetallic interaction. Variable-temperature luminescence studies reveal that it displays distinct red-shifts of the emission whether the temperature is elevated or lowered from ambient temperature, suggestive of exceptional thermochromic phosphorescence characteristics. [ABSTRACT FROM AUTHOR]

[Display omitted] • The origin of luminescence thermochromism is briefly discussed. • The relationship between clusters and their electronic structures is described in detail. • The mechanism of luminescence thermochromism in silver clusters is highlighted. • The outcomes and important future aspects of silver(I) cluster are proposed. Stimuli-responsive materials have garnered significant attention in technology, sensors, and biological applications because of their quick response to various external factors. These include light, temperature, change in solvent, pressure fluctuations, mechanical friction, and alteration in pH values. Luminescence thermochromism, in particular, refers to the ability of materials to vary the color of their fluorescence emission as a function of temperature. Interestingly, silver clusters ranging from discrete to higher-dimensional structures show luminescence thermochromism due to strong argentophilic interactions and protecting ligands, affecting their emission position, intensity, and lifetime. These ultra-small materials possess superior photostability, biocompatibility, and low toxicity, holding great promise for potential applications. This review article provides valuable insights into the structural features of silver clusters for designing and developing luminescent thermochromic silver clusters. Additionally, this highlights the mechanisms behind luminescence thermochromism, including structural rigidity, competition between bathochromic and hypsochromic shifts, and thermally activated delayed fluorescence. The aim of this review is to inspire new ideas and concepts for developing new luminescent thermochromic silver cluster materials. [ABSTRACT FROM AUTHOR]