Unconventional Luminescent Centers in Metastable Phases Created by Topochemical Reduction Reactions

High-nuclear polyoxovanadates assembled from pentagonal building blocks

The design and synthesis of high-nuclear polyoxometalates (POMs) constructed from pentagonal molecular building blocks (MBBs) are attractive and challenging. Herein, three new high-nuclear polyoxovanadates, including {V₂₀W₂P₂₀} (1), {V₁₈W₄P₁₄} (2), and {V₂₆W₆P₁₆} (3), have been successfully synthesized under solvothermal conditions. All these structures are assembled from pentagonal MBB {WV₅(PhPO₃)₅} with different configurations. Compound 1 exhibits efficient, stable, and versatile catalytic activity for sulfide oxidation.

Charge Transfer-Triggered Bi3+ Near-Infrared Emission in Y2Ti2O7 for Dual-Mode Temperature Sensing

Trivalent bismuth is a popular main group ion showing versatile luminescent behaviors in a broad spectral range from ultraviolet to visible, but barely in the near-infrared (NIR) region. In this study, we have observed unexpected NIR emission at ∼744 nm in a Bi³⁺-doped pyrochlore, Y₂Ti₂O₇ (YTOB). Our first-principles electronic structure calculation and analysis of the Bi local structure via extended X-ray absorption fine structure indicate that only Bi³⁺ species appears in YTOB and it has a similar local environment to that of Y³⁺. The NIR emission is assigned to a Ti⁴⁺ → Bi³⁺ metal-to-metal charge transfer process. Moreover, we have demonstrated dual-mode luminescence thermometry based on the luminescence intensity ratio (LIR) and lifetime (τ) in 0.5% Bi³⁺ and 0.5% Pr³⁺ co-doped Y₂Ti₂O₇ (YTOB0.5P0.5). It exhibits high thermometric sensitivity simultaneously in the cryogenic temperature range from 78 to 298 K based on τ of the NIR emission of Bi³⁺ at 748 nm and in the temperature range of 278-378 K based on the LIR of Bi³⁺ to Pr³⁺ emissions (I₇₄₈/I₆₁₅). As a novel LIR-τ dual-mode thermometric material over a wide temperature range, the maximum relative sensitivities of the YTOB0.5P0.5 reach 3.53% K^-1 at 298 K from the τ mode and 3.52% K^-1 at 318 K based on the LIR mode. The dual-mode luminescence thermometry with high responsivity from our Bi³⁺-based pyrochlore Y₂Ti₂O₇ phosphor opens a new avenue for more luminescent materials toward multi-mode thermometry applied in complex temperature-sensing conditions.

Ionic-caged heterometallic bismuth-platinum complex exhibiting electrocatalytic CO2 reduction

An air-stable heterometallic Bi-Pt complex with the formula [BiPt(SAc)5]n (1; SAc = thioacetate) was synthesized. The crystal structure, natural bond orbital (NBO) and local orbital locator (LOL) analyses, localized orbital bonding analysis (LOBA), and X-ray absorption fine structure (XAFS) measurements were used to confirm the existence of Bi-Pt bonding and an ionic cage of O atoms surrounding the Bi ion. From the cyclic voltammetry (CV) and controlled potential electrolysis (CPE) experiments, 1 in tetrahydrofuran reduced CO2 to CO, with a faradaic efficiency (FE) of 92% and a turnover frequency (TOF) of 8 s-1 after 30 min of CPE at -0.79 V vs. NHE. The proposed mechanism includes an energetically favored pathway via the ionic cage, which is supported by the results of DFT calculations and reflectance infrared spectroelectrochemistry data.

Defective [Bi2 O2 ]2+ Layers Exhibiting Ultrabroad Near-Infrared Luminescence

Aurivillius phases have been routinely known as excellent ferroelectrics and have rarely been deemed as materials that luminesce in the near-infrared (NIR) region. Herein, it is shown that the Aurivillius phases can demonstrate broadband NIR luminescence that covers telecommunication and biological optical windows. Experimental characterization of the model system Bi_2.14 Sr_0.75 Ta₂ O_9-x , combined with theoretical calculations, help to establish that the NIR luminescence originates from defective [Bi₂ O₂ ]²⁺ layers. Importantly, the generality of this finding is validated based on observations of a rich bank of NIR luminescence characteristics in other Aurivillius phases. This work highlights that incorporating defects into infinitely repeating [Bi₂ O₂ ]²⁺ layers can be used as a powerful tool to space-selectively impart unusual luminescence emitters to Aurivillius-phase ferroelectrics, which not only offers an optical probe for the examination of defect states in ferroelectrics, but also provides possibilities for coupling of the ferroelectric property with NIR luminescence.

Two-Dimensional-Layered Perovskite ALaTa2O7:Bi3+ (A = K and Na) Phosphors with Versatile Structures and Tunable Photoluminescence

Topological chemical reaction methods are indispensable for fabricating new materials or optimizing their functional properties, which is particularly important for two-dimensional (2D)-layered compounds with versatile structures. Herein, we demonstrate a low-temperature (∼350 °C) ion exchange approach to prefabricate metastable phosphors ALa_{1- x}Ta₂O₇: xBi³⁺ (A = K and Na) with RbLa_{1- x}Ta₂O₇: xBi³⁺ serving as precursors. The as-prepared ALa_0.98Ta₂O₇:0.02 Bi³⁺ (A = Rb, K, and Na) share the same Dion-Jacobson type 2D-layered perovskite phase, and photoluminescence analyses show that ALa_0.98Ta₂O₇:0.02 Bi³⁺ (A = Rb, K, and Na) phosphors exhibit broad emission bands peaking at 540, 550, and 510 nm, respectively, which are attributed to the nonradiative transition of Bi³⁺ from excited state ³P₁ or ³P₀ to ground state ¹S₀. The various Bi³⁺ local environments at the crystallographic sites enable the different distributions of emission and excitation spectra, and the photoluminescence tuning of ALa_0.98Ta₂O₇:0.02 Bi³⁺ (A = Rb, K, and Na) phosphors are realized through alkali metal ion exchange. Notably, the combination of superior trivalent bismuth emission and low-temperature ion exchange synthesis leads to a novel yellow-emitting K(La_0.98Bi_0.02)Ta₂O₇ phosphor which is successfully applied in a white LED device based on a commercially available 365 nm LED chip. Our realizable cases of this low-temperature ion exchange strategy could promote exploration into metastable phosphors with intriguing properties.

Transformation of Perovskite BaBiO3 into Layered BaBiO2.5 Crystals Featuring Unusual Chemical Bonding and Luminescence

Engineering oxygen coordination environments of cations in oxides has received intense interest thanks to the opportunities for the discovery of novel oxides with unusual properties. Herein, the synthesis of stoichiometric layered BaBiO_2.5 by a nontopotactic phase transformation of perovskite BaBiO₃ is presented. By analyzing the synchrotron X-ray diffraction data by the maximum-entropy method/Rietveld technique, it was found that Bi is involved in an unusual chemical bonding situation with four oxygen atoms featuring one ionic bond and three covalent bonds, which results in an asymmetric coordination geometry. Photophysical characterization revealed that this peculiar structure shows near-infrared luminescence differing from that of conventional Bi-containing compounds. Experimental and theoretical results led to the proposal of an excitonic nature of the luminescence. This work highlights that synthesizing materials with uncommon Bi-O bonding and Bi coordination geometry provides a pathway to the discovery of systems with new functionalities. This could inspire interest in the exploration of a range of materials containing heavier p-block elements with prospects for finding systems with unusual properties.

Ultrabroad Photoluminescence and Electroluminescence at New Wavelengths from Doped Organometal Halide Perovskites

Doping of semiconductors by introducing foreign atoms enables their widespread applications in microelectronics and optoelectronics. We show that this strategy can be applied to direct bandgap lead-halide perovskites, leading to the realization of ultrawide photoluminescence (PL) at new wavelengths enabled by doping bismuth (Bi) into lead-halide perovskites. Structural and photophysical characterization reveals that the PL stems from one class of Bi doping-induced optically active center, which is attributed to distorted [PbI6] units coupled with spatially localized bipolarons. Additionally, we find that compositional engineering of these semiconductors can be employed as an additional way to rationally tune the PL properties of doped perovskites. Finally, we accomplished the electroluminescence at cryogenic temperatures by using this system as an emissive layer, marking the first electrically driven devices using Bi-doped photonic materials. Our results suggest that low-cost, earth-abundant, solution-processable Bi-doped perovskite semiconductors could be promising candidate materials for developing optical sources operating at new wavelengths.

Superbroad near-infrared photoluminescence covering the second biological window achieved by bismuth-doped oxygen-deficient gadolinium oxide

We demonstrated that bismuth-doped oxygen-deficient gadolinium oxides, produced through a low-temperature topochemical reduction strategy using CaH₂ as a solid-state reducing agent, show superbroad NIR PL covering the second biological window.