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1. Aggregation‐Enhanced Photoluminescence and Photoacoustics of Atomically Precise Gold Nanoclusters in Lipid Nanodiscs (NANO 2 )

   https://doi.org/10.1002/adfm.202009750  

   Tahmasbi Rad, Armin ; Bao, Yue ; Jang, Hyun‐Sook ; Xia, Yan ; Sharma, Hari ; Dormidontova, Elena E. ; Zhao, Jing ; Arora, Jaspreet ; John, Vijay T. ; Tang, Ben Zhong ; et al ( December 2020 , Advanced Functional Materials)

   The authors designed a structurally stable nano‐in‐nano (NANO²) system highly capable of bioimaging via an aggregation‐enhanced NIR excited emission and photoacoustic response achieved based on atomically precise gold nanoclusters protected by linear thiolated ligands [Au₂₅(SC_nH_2n+1)₁₈,n =4–16] encapsulated in discoidal phospholipid bicelles through a one‐pot synthesis. The detailed morphological characterization of NANO²is conducted using cryogenic transmission electron microscopy, small/wide angle X‐ray scattering with the support of molecular dynamics simulations, providing information on the location of Au nanoclusters in NANO². The photoluminescence observed for NANO²is 20–60 times more intense than that of the free Au nanoclusters, with both excitation and emission wavelengths in the near‐infrared range, and the photoacoustic signal is more than tripled. The authors attribute this newly discovered aggregation‐enhanced photoluminescence and photoacoustic signals to the restriction of intramolecular motion of the clusters’ ligands. With the advantages of biocompatibility and high cellular uptake, NANO²is potentially applicable for both in vitro and in vivo imaging, as the authors demonstrate with NIR excited emission from in vitro A549 human lung and the KB human cervical cancer cells.

    

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2. Bright NIR‐II Photoluminescence in Rod‐Shaped Icosahedral Gold Nanoclusters

   https://doi.org/10.1002/smll.202007992  

   Li, Qi ; Zeman, IV, Charles J. ; Ma, Zhuoran ; Schatz, George C. ; Gu, X. Wendy ( February 2021 , Small)

   Fluorophores with high quantum yields, extended maximum emission wavelengths, and long photoluminescence (PL) lifetimes are still lacking for sensing and imaging applications in the second near‐infrared window (NIR‐II). In this work, a series of rod‐shaped icosahedral nanoclusters with bright NIR‐II PL, quantum yields up to≈8%, and a peak emission wavelength of 1520 nm are reported. It is found that the bright NIR‐II emission arises from a previously ignored state with near‐zero oscillator strength in the ground‐state geometry and the central Au atom in the nanoclusters suppresses the non‐radiative transitions and enhances the overall PL efficiency. In addition, a framework is developed to analyze and relate the underlying transitions for the absorptions and the NIR‐II emissions in the Au nanoclusters based on the experimentally defined absorption coefficient. Overall, this work not only shows good performance of the rod‐shaped icosahedral series of Au nanoclusters as NIR‐II fluorophores, but also unravels the fundamental electronic transitions and atomic‐level structure‐property relations for the enhancement of the NIR‐II PL in gold nanoclusters. The framework developed here also provides a simple method to analyze the underlying electronic transitions in metal nanoclusters.

    

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3. Enhanced deep red to near-infrared (DR-NIR) phosphorescence in cyclometalated iridium( iii ) complexes

   https://doi.org/10.1039/d2qi02058k  

   Yoon, Sungwon ; Teets, Thomas S. ( December 2022 , Inorganic Chemistry Frontiers)

   The design of deep-red to near-infrared (DR-NIR) phosphorescent compounds with high photoluminescence quantum yields ( Φ PL ) is a significant fundamental challenge that impacts applications including optoelectronic devices, imaging, and sensing. Here we show that bis-cyclometalated iridium complexes with electron-rich ancillary ligands can have exceptional quantum yields for DR-NIR phosphorescence (peak λ > 700 nm). Six bis-cyclometalated iridium( iii ) complexes with DR-NIR phosphorescence are described in this work, pairing highly conjugated cyclometalating ligands with electron-rich and sterically encumbered β-ketoiminate (acNac), β-diketiminate (NacNac), and N , N ′-diisopropylbenzamidinate (dipba) ancillary ligands. The photoluminescence spectra and quantum yields are solvent-dependent, consistent with significant charge-transfer character in the emissive excited state. The ancillary ligands perturb the excited-state kinetics relative to closely related compounds, which can lead to enhanced Φ PL values in the DR-NIR region, particularly in toluene solution and in doped polymer films. 

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4. Photoluminescence of the Au38(SR)26 nanocluster comprises three radiative processes

   https://doi.org/10.1038/s42004-023-00819-3  

   Luo, Lianshun ; Liu, Zhongyu ; Du, Xiangsha ; Jin, Rongchao ( February 2023 , Communications Chemistry)

   Photoluminescence of ultrasmall, atomically precise gold nanoclusters constitutes an area of significant interest in recent years for both fundamental research and biological applications. However, the exploration of near-infrared photoluminescence of gold nanoclusters is still in its infancy due to the limitations of synthetic methods and characterization techniques. Herein, the photoluminescence properties of an Au₃₈(PET)₂₆(PET = 2-phenylethanethiolate) nanocluster are investigated in detail. The Au₃₈(PET)₂₆exhibits an emission peak at 865 nm, which is revealed to be a mix of fluorescence, thermally activated delayed fluorescence, and phosphorescence via the combined analyses of time-resolved and temperature-dependent photoluminescence measurements. The quantum yield of Au₃₈(PET)₂₆is determined to be 1.8% at room temperature under ambient conditions, which increases to above 90% by suppressing the non-radiative relaxation pathway at a cryogenic temperature (80 K). Overall, the results of this work discover the coexistence of three radiative processes in thiolate-protected Au nanoclusters and will pave the way for understanding the intriguing photoluminescence properties of gold nanoclusters in future studies.

    

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5. Isomerization-induced enhancement of luminescence in Au 28 (SR) 20 nanoclusters

   https://doi.org/10.1039/d0sc01270j  

   Chen, Yuxiang ; Zhou, Meng ; Li, Qi ; Gronlund, Harrison ; Jin, Rongchao ( August 2020 , Chemical Science)

   null (Ed.)

   Understanding the origin and structural basis of the photoluminescence (PL) phenomenon in thiolate-protected metal nanoclusters is of paramount importance for both fundamental science and practical applications. It remains a major challenge to correlate the PL properties with the atomic-level structure due to the complex interplay of the metal core ( i.e. the inner kernel) and the exterior shell ( i.e. surface Au( i )-thiolate staple motifs). Decoupling these two intertwined structural factors is critical in order to understand the PL origin. Herein, we utilize two Au 28 (SR) 20 nanoclusters with different –R groups, which possess the same core but different shell structures and thus provide an ideal system for the PL study. We discover that the Au 28 (CHT) 20 (CHT: cyclohexanethiolate) nanocluster exhibits a more than 15-fold higher PL quantum yield than the Au 28 (TBBT) 20 nanocluster (TBBT: p-tert -butylbenzenethiolate). Such an enhancement is found to originate from the different structural arrangement of the staple motifs in the shell, which modifies the electron relaxation dynamics in the inner core to different extents for the two nanoclusters. The emergence of a long PL lifetime component in the more emissive Au 28 (CHT) 20 nanocluster reveals that its PL is enhanced by suppressing the nonradiative pathway. The presence of long, interlocked staple motifs is further identified as a key structural parameter that favors the luminescence. Overall, this work offers structural insights into the PL origin in Au 28 (SR) 20 nanoclusters and provides some guidelines for designing luminescent metal nanoclusters for sensing and optoelectronic applications. 

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