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.

Ambipolar Nickel Dithiolene Complex Semiconductors: From One- to Two-Dimensional Electronic Structures Based upon Alkoxy Chain Lengths.

Air-stable single-component ambipolar organic semiconductors that conduct both holes and electrons are highly desired but have been rarely realized. Neutral nickel bis(dithiolene) complexes are promising candidates that fulfill the stringent electronic requirements of shallow HOMO levels and deep LUMO levels, which can reduce the carrier injection barrier to overcome the work function of gold electrodes and ensure air stability. However, most nickel bis(dithiolene) analogs that have been characterized as ambipolar semiconductors have twisted molecular structures that hinder the effective intermolecular interactions required for carrier conduction. To address this issue, we synthesized planar alkoxy-substituted nickel bis(dithiolene) analogs that facilitate dense packing with effective intermolecular interactions. Remarkably, changing the methoxy substituents to ethoxy or propoxy groups led to a dramatic change in the packing mode, from one-dimensional to herringbone-like, while maintaining effective intermolecular interactions. These materials overcome the usual trade-off between crystallinity and solubility; they are highly crystalline, even in their film forms, and are highly soluble in organic solvents. They are therefore readily solution-processable to form semiconducting layers with well-defined and well-ordered structures in field-effect transistors. Devices based on these compounds exhibited efficient ambipolar characteristics, even after several months of exposure to air, achieving high carrier mobilities of up to 10-2 cm2 V-1 s-1 and large on/off ratios of up to 105, which are the top-class performances achieved for a single-component ambipolar semiconductor material driven in air.

Proton-electron-coupled functionalities of conductivity, magnetism, and optical properties in molecular crystals.

Proton-electron-coupled reactions, specifically proton-coupled electron transfer (PCET), in biological and chemical processes have been extensively investigated for use in a wide variety of applications, including energy conversion and storage. However, the exploration of the functionalities of the conductivity, magnetism, and dielectrics by proton-electron coupling in molecular materials is challenging. Dynamic and static proton-electron-coupled functionalities are to be expected. This feature article highlights the recent progress in the development of functionalities of dynamic proton-electron coupling in molecular materials. Herein, single-unit conductivity by self-doping, quantum spin liquid state coupled with quantum fluctuation of protons, switching of conductivity and magnetism triggered by the disorder-order transition of deuterons, and their external responses under pressure and in the presence of an electric field are introduced. In addition, as for the functionalities of proton-d/π-electron coupling in metal dithiolene complexes, magnetic switching with multiple PCET and vapochromism induced by electron transfer through hydrogen-bond (H-bond) formation is introduced experimentally and theoretically. We also outlined the basic and applied issues and potential challenges for development of proton-electron-coupled molecular materials, functionalities, and devices.

Effect of Alkyl Chain Length on Charge Transport Property of Anthracene-Based Organic Semiconductors.

Anthracene, a simple planar building block for organic semiconductors, shows strong intermolecular interactions and exhibits strong blue fluorescence. Thus, its derivatives have a great potential to integrate considerable charge carrier mobility and strong emission within a molecule. Here, we systematically studied the influence of alkyl chain length on the crystal structures, thermal properties, photophysical characteristics, electrochemical behaviors, and mobilities for a series of 2,6-di(4-alkyl-phenyl)anthracenes (Cn-Ph-Ants, where n represents the alkyl chain length). Among them, Cn-Ph-Ants (n = 0, 1, 2, and 3) display similar layered herringbone (LHB) packing motifs, which facilitate two-dimensional charge transport and thereby enables high-performance organic field-effect transistors (OFETs). All Cn-Ph-Ants exhibit similar work functions and show strong blue fluorescence with photoluminescence quantum yields (PLQY) of approximately 40% in toluene. In addition, the absolute powder PLQYs of C0-, C2-, C3-, C4-, and C6-Ph-Ants are 24.6, 8.2, 5.7, 10.9, and 8.6%, respectively. Note that the alkyl chain length shows a significant effect on the charge mobilities of Cn-Ph-Ants. Our newly synthesized C1-, C3-, and C4-Ph-Ants show hole mobilities of up to 2.40, 1.34, and 1.00 cm2 V-1 s-1, respectively, with mobilities of 3.40, 1.57, and 0.82 cm2 V-1 s-1 for C0-, C2-, and C6-Ph-Ants, indicating an increasing tendency of mobility with shorter alkyl chain length. This feature is related to the microstructures of the thin films, which reveal the enhanced film order, crystallinity, and grain size with a decrease in the alkyl chain length. Moreover, we theoretically analyze the intermolecular transfer integrals of HOMOs, which increase at T-shaped contacts as the alkyl chain length decreases, which improves the intermolecular charge transport properties, leading to the increases in mobility. Interestingly, the anisotropy of the transfer integral tends to decrease upon substitution with longer alkyl chains, suggesting that alkyl chain adjustments may facilitate isotropic charge transport property in 2,6-alkylated anthracenes.