Tailoring sensitization properties and improving near-infrared photon upconversion performance through alloying in superatomic molecular Au25 nanoclusters.

Noble-metal nanoclusters (NCs) protected by organic ligands have recently come to the forefront as potent triplet sensitizers for photon upconversion (UC) via triplet-triplet annihilation (TTA), owing to their capacity for atomic-level photophysical property customization. Among these, the rod-shaped bi-icosahedral [Au25(PPh3)10(S-C2H4Ph)5Cl2]2+ (Au-rod) NC is a particularly iconic superatomic molecular NC, recently identified as a near-infrared (NIR)-absorbing sensitizer for TTA-UC. In this study, we synthesized Cu-doped NCs, [Au25-xCux(PPh3)10(S-C2H4Ph)5Cl2]2+ (AuCu-rod), and paired them with 9,10-bis(phenylethynyl)anthracene (BPEA) annihilator/emitter to explore the impact of Cu-doping on the triplet sensitization and NIR-UC performance. The triplet state of AuCu-rod, with lifetime of 3 µs, exhibited a modest blue shift compared to the Au-rod, resulting in the increment in the driving force for triplet energy transfer (TET) to the BPEA acceptor. The TET rate constant was determined to be 5.0 × 107 M-1 s-1, which is an order of magnitude higher than the rate constant for the Au-rod/BPEA pair. This improvement has led to a remarkable increase in the TET efficiency. Notably, the AuCu-rod/BPEA pair facilitated the efficient UC of 805 nm NIR light into 510 nm visible light, realizing a large anti-Stokes shift close to 0.9 eV. The UC internal quantum yield of this combination was determined to be 2.33 ± 0.05%, marking a fivefold enhancement over the Au-rod sensitizer (0.49%). Thus, alloying NC sensitizers offers a promising route to enhance UC performance by tuning the triplet state energy and optimizing the compatibility between the sensitizer and annihilator. Additionally, in this series of experiments, the formation of small amounts of BPEA microaggregates was observed. These aggregates did not undergo singlet fission and could retain multiple long-lived triplet excitons. This characteristic facilitated TTA among triplet excitons, resulting in efficient NIR-to-visible UC emission.

Triplet-Mediator Ligand-Protected Metal Nanocluster Sensitizers for Photon Upconversion.

Triplet-triplet annihilation photon upconversion (TTA-UC) is attracting a great deal of attention as a viable approach to exploit unutilized wavelengths of light in solar-driven devices. Recently, ligand-protected metal nanoclusters have emerged as a compelling platform for serving as triplet sensitizers for TTA-UC. In this study, we developed an atomically precise, triplet-mediator ligand (TL)-protected metal nanocluster, Au2Cu6(S-Adm)6[P(DPA)3]2 (Au2Cu6DPA; S-Adm = 1-adamanthanethiolate, DPA = 9,10-diphenylanthracene). In Au2Cu6DPA, the excitation of the Au2Cu6 core rapidly generates a metal-to-ligand charge transfer state, followed by the formation of the long-lived triplet state (approximately 150 µs) at a DPA site in the TL. By combining Au2Cu6DPA with a DPA annihilator, we achieved a red-to-blue upconversion quantum yield (ΦUCg) of 20.7 ± 0.4% (50% max.) with a low threshold excitation intensity of 36 mW cm-2 at 640 nm. This quantum yield almost reaches the maximum limit achievable using a DPA annihilator and establishes a record-setting value, outperforming previously reported nanocrystal and nanocluster sensitizers. Furthermore, strong upconversion emission based on a pseudo-first-order TTA process was observed under 1 sun illumination, indicating that the Au2Cu6DPA sensitizer holds promise for applications in solar-energy-based systems.

Dissecting the Triplet-State Properties and Intersystem Crossing Mechanism of the Ligand-Protected Au13 Superatom.

Icosahedral Au13 nanoclusters are among the most typical superatoms and are of great interest as promising building blocks for nanocluster-assembled materials. Herein, the key parameters involved in the intersystem crossing (ISC) process of [Au13(dppe)5Cl2]3+ (Au13; dppe = 1,2-bis(diphenylphosphino)ethane) were characterized. Quenching experiments using aromatic compounds revealed that the T1 energy of Au13 is 1.63 eV. An integrative interpretation of our experimental results and the relevant literature uncovered important facts concerning the Au13 superatom: the ISC quantum yield is unity due to the ultrafast ISC (∼1012 s-1), the lowest absorption band includes contributions of direct singlet-triplet transitions, and there exists a large S1-T1 gap of 0.73 eV. To explain the efficient ISC, the El-Sayed rule was applied to the superatomic orbitals corresponding to the excited-state hole/electron distributions obtained from theoretical calculations. The strong spin-orbit coupling between the S1 and T2-T4 states offers a reasonable explanation for the ultrafast ISC.

Nested Keplerian architecture of [Cu58H20(SPr)36(PPh3)8]2+ nanoclusters.

This article explores the challenges in synthesizing highly symmetric Cu(I)-thiolate nanoclusters and reports a nested Keplerian architecture of [Cu58H20(SPr)36(PPh3)8]2+ (Pr = CH2CH2CH3). The structure is made up of five concentric polyhedra of Cu(I) atoms, which create enough space to accommodate five ligand shells all within a range of 2 nm. This fascinating structural architecture is also linked to the unique photoluminescence properties of the nanoclusters.

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

Ligand-protected noble-metal nanoclusters exhibit an innately triplet nature and have been recently recognized as emerging platforms for triplet sensitizers of photon upconversion (UC) via triplet-triplet annihilation. Herein, we report that a structurally flexible Au-Cu alloy nanocluster, [Au4Cu4(S-Adm)5(DPPM)2]+ (Au4Cu4; S-Adm = 1-adamantanethiolate, DPPM = bis(diphenylphosphino)methane), exhibited favorable sensitizer properties and superior UC performance. Contrary to the structurally rigid Au2Cu6(S-Adm)6(TPP)2 (Au2Cu6, TPP = triphenylphosphine), Au4Cu4 exhibited significantly better sensitizer characteristics, such as a near-unity quantum yield for intersystem crossing (ISC), long triplet lifetime (ca. 8 µs), and efficient triplet energy transfer (TET). The efficient ISC of Au4Cu4 was attributed to the practically negligible activation barriers during the ISC process, which was caused by the spin-orbit interaction between the two isoenergetic isomers predicted by theoretical calculations. A series of aromatic molecules with different triplet energies were used as acceptors to reveal the driving force dependence of the TET rate constant (kTET). This dependency was analyzed to evaluate the triplet energy and sensitization ability of the alloy nanoclusters. The results showed that the maximum value of kTET for Au4Cu4 was seven times larger than that for Au2Cu6, which presumably reflects the structural/electronic fluctuations of Au4Cu4 during the triplet state residence. The combination of the Au4Cu4 sensitizer and the 9,10-diphenylanthracene (DPA) annihilator/emitter achieved UC with internal quantum yields of 14% (out of 50% maximum) and extremely low threshold intensities (2-26 mWcm-2). This performance far exceeds that of Au2Cu6 and is also outstanding among the organic-inorganic hybrid nanomaterials reported so far.

Charge-Transfer-Mediated Mechanism Dominates Oxygen Quenching of Ligand-Protected Noble-Metal Cluster Photoluminescence.

Photoluminescence (PL) quenching of ligand-protected noble-metal clusters (NMCs) by molecular oxygen is often used to define whether the PL of NMC is fluorescent or phosphorescent, and only energy transfer has been always considered as the quenching mechanism. Herein, we performed the Rehm-Weller analysis of the O2-induced PL quenching of 13 different NMCs and found that the charge-transfer (CT)-mediated mechanism dominates the quenching process. The quenching rate constant showed a clear dependence on the CT driving force, varied markedly from 106 to 109 M-1s-1. Transient absorption spectroscopy and photon upconversion measurements confirmed the triplet sensitization of aromatic molecules by NMCs regardless of the quenching degree by O2, establishing that the PL of NMCs under investigation originates from the excited triplet state (i.e., phosphorescence). The results herein provide an essential indicator for correctly determining whether the PL of an NMC is fluorescent or phosphorescent.

Morphological variation and phylogeny of Karenia selliformis (Gymnodiniales, Dinophyceae) in an intensive cold-water algal bloom in eastern Hokkaido, Japan.

Harmful algal blooms responsible for mass mortalities of marine organisms have been rare in Hokkaido, northern Japan, although fish-killing blooms have been frequently reported from western Japanese coasts. In September-November 2021, a huge and prolonged cold-water bloom occurred along the Pacific coast of eastern Hokkaido, and was associated with intensive mortalities of sea urchin, fish, octopus, shellfish, etc. In this study, morphology and phylogeny of the dominant and co-occurring unarmored dinoflagellates of the Kareniaceae in the bloom were examined by using light microscopy, scanning electron microscopy and molecular phylogeny inferred from ITS and LSU rDNA (D1-D3) sequences. Morphological observation and molecular phylogeny showed that the dominant species was Karenia selliformis, with co-occurrences of other kareniacean dinoflagellates, Kr. longicanalis, Kr. mikimotoi, Karlodinium sp., Takayama cf. acrotrocha, Takayama tuberculata and Takayama sp. The typical cell forms of Kr. selliformis in the bloom were discoid, dorsoventrally flattened, and 35.3-43.6 (39.4  ±  2.1) µm in length, which was larger than the cell sizes in previous reports. Transparent cells of Kr. selliformis, lacking chloroplasts or having a few shrunken chloroplasts and oil droplets, were also found. Cells of Kr. selliformis showed morphological variation, but the species could be distinguished from other co-occurring Karenia species by the nucleus positioned in the hypocone and chloroplasts numerous (46-105) in number and small (2.9-4.6 µm) in diameter. Cell density of Kr. selliformis exceeding 100 cells mL-1 was recorded in the temperature range of 9.8-17.6 °C. The rDNA sequences determined from Kr. selliformis in the blooms of Hokkaido, Japan in 2021 were identical to those from the bloom in Kamchatka, Russia in 2020.

Evidence for triplet-state-dominated luminescence in biicosahedral superatomic molecular Au25 clusters.

In photoluminescence (PL) quenching and triplet fusion upconversion experiments with fluorescent organic-molecule quenchers, it was revealed that a rod-shaped, phosphine- and thiolate-protected biicosahedral Au25 cluster (a representative di-superatomic molecule) exhibits only phosphorescence, not fluorescence, at room temperature with an intersystem crossing quantum yield of almost 100%. By virtue of these photophysical properties, this cluster can be used as a triplet sensitizer that undergoes direct singlet-triplet transitions in the near-infrared (NIR) region (730-900 nm), inducing photon upconversion from NIR to visible light.

Spatial and temporal changes in zooplankton abundance, biovolume, and size spectra in the neighboring waters of Japan: analyses using an optical plankton counter.

BACKGROUND: An optical plankton counter (OPC) was used to examine spatial and temporal changes in the zooplankton size spectra in the neighboring waters of Japan from May to August 2011. RESULTS: Based on the zooplankton biovolume of equivalent spherical diameter (ESD) in 45 bins for every 0.1 mm between0.5 and 5.0 mm, a Bray-Curtis cluster analysis classified the zooplankton communities into six groups. Thegeographical distribution of each group varied from each of the others. Groups with a dominance of 4 to 5 mm ESD were observed in northern marginal seas (northern Japan Sea and Okhotsk Sea), while the least biovolume with a dominance of a small-size class (0.5 to 1 mm) was observed for the Kuroshio extension. Temporal changes were observed along the 155° E line, i.e., a high biovolume group dominated by 2 to 3 mm ESD during May shifted to other size spectra groups during July to August. These temporal changes were caused by the seasonal vertical descent of dominant large Neocalanus copepods during July to August. As a specific characteristic of the normalized biomass size spectra (NBSS), the slope of NBSS was moderate (-0.90) for the Neocalanus dominant spring group but was at -1.11 to -1.24 for the other groups. Theoretically, the slope of the NBSS of the stable marine ecosystem is known to settle at approximately -1. CONCLUSIONS: Based on the analysis by OPC, zooplankton size spectra in the neighboring waters of Japan were separated into six groups. Most groups had -1.11 to -1.24 NBSS slopes, which were slightly higher than the theoretical value (-1). However, one group had a moderate slope of NBSS (-0.90) caused by the dominance of large Neocalanuscopepods.