Discrete inorganic molecular upconversion: a dinuclear ytterbium–polyoxometalate complex exhibits cooperative upconversion luminescence

Abstract

We present a novel molecular upconversion material based on a discrete lanthanide-polyoxometalate (Ln-POM) complex. A lacunary-type silicotungstate, [γ-SiW₁₀O₃₆]⁸⁻ (SiW), forms a dinuclear Yb-POM complex, [Yb₂(μ₂-OH)₂(γ-SiW₁₀O₃₆)₂]¹²⁻ (Yb-SiW), which exhibits upconversion luminescence via a cooperative luminescence mechanism in the visible region upon near-infrared excitation of its two Yb ions in CD₃CN.

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A myriad of inorganic materials are utilized as luminescent materials owing to their excellent thermal and chemical stability, ease of synthesis, and resistance to non-radiative deactivation. Among the existing luminescent inorganic materials, light upconversion (UC) in lanthanide (Ln) co-doped inorganic matrices, such as oxides and fluorides, has drawn significant attention. This is due to the non-linear optical nature of UC, wherein the emission of higher-energy photons occurs upon excitation with lower-energy incident light.^1–5 UC luminescence is a unique feature for bioimaging applications, particularly owing to its ability to be excited by near-infrared (NIR) light, which allows deeper tissue penetration with minimal photodamage. However, for bio-applications, luminescent inorganic materials often require nanoscaling to improve their biocompatibility.^2,6,7

In contrast, discrete molecular metal complexes offer an alternative strategy for achieving UC luminescence at the molecular level, distinct from nanoparticle-based systems. However, discrete molecular metal complexes also face considerable challenges, including the formation of poly- or hetero-nuclear Ln complexes having a close Ln–Ln distance and significant vibrational deactivation caused by X–H (X = C, O, N) oscillators in the ligand or solvent. To overcome these limitations, recent advances dealing with discrete metal complexes exhibiting molecular UC, such as those by Piguet (d–f heteronuclear complex), Charbonnière (supramolecular assembly), Murugesu (molecular aggregate cluster), and our laboratory (heteronuclear cluster complex), highlight the growing potential of molecular-level UC systems in this emerging field.^8–21 These discrete molecular systems emphasize the importance of controlling the coordination number and Ln–Ln distances within the molecular framework to achieve efficient UC luminescence.

Polyoxometalates (POMs) are discrete anionic inorganic clusters formed by oxo-bridged metal centers, often incorporating heteroatoms.^22–24 The unique electronic structure of POMs, coupled with their redox activity and magnetic/photophysical properties, has led to their use in various applications, such as catalysis, magnetism, and luminescence.^25,26 Among the existing POMs, lacunary-type POMs exhibit the potential to serve as anionic inorganic ligands that can coordinate with Ln ions to form Ln-POM complexes.^27,28 The Ln-POM complexes can exhibit synergistic properties arising from the interplay between the POM ligands and Ln centers. In fact, several heteronuclear Ln-POM systems have been shown to exhibit Ln-centered or UC luminescence, although most reported UC-active Ln-POM materials are either in the solid state or incorporated into nanoparticle-based MOF-type frameworks.^26,27,29

This study aims to advance the development of Ln-POM systems at the molecular level and establish a new class of purely inorganic UC-active molecules. Some Ln-POM complexes are soluble in solution and consist only of inorganic molecules/ligands without organic ligands, forming discrete Ln-POM molecules. Suzuki et al. reported dinuclear Dy–POM complexes, denoted as Dy–SiW ([Dy₂(μ₂-OH)₂(γ-SiW₁₀O₃₆)₂]¹²⁻, Fig. 1).³⁰ Two hydroxide ions bridge the two Dy³⁺ centers to form a {Dy(μ₂-OH)₂Dy} core, with a Dy–Dy distance of 3.6457 Å.³⁰ In this study, we present a new discrete dinuclear ytterbium-POM complex ([Yb₂(μ₂-OH)₂(γ-SiW₁₀O₃₆)₂]¹²⁻, Yb-SiW), in which hydroxide ions similarly bridge the two Yb³⁺ centers. The Yb–Yb distance is expected to be short, based on the analogous Dy-SiW complex, which is favorable for UC luminescence. In fact, the designed Yb-SiW complex exhibits two distinct luminescence behaviors in acetonitrile: (i) NIR luminescence via energy transfer and (ii) visible UC luminescence driven by cooperative luminescence (CL).

Fig. 1 Structure of the Dy–SiW complex (purple: Dy, blue: W, yellow: Si, red: O; hydrogen atoms have been omitted for clarity).³⁰

The Yb-SiW complex was synthesized according to a previously reported procedure and characterized by Fourier-transform infrared (FT-IR) spectroscopy, elemental analysis, and electrospray ionization mass spectrometry (ESI-MS).^30,31 The FT-IR spectra (Fig. S1†) and elemental analysis supported the formation of the Yb-SiW complex. Furthermore, ESI-MS analysis confirmed the presence of the Yb-SiW complex in acetonitrile solution, showing an isotopic distribution centered at m/z 2645.21595, which could be assigned to [2Yb³⁺ + 2(γ-SiW₁₀O₃₆)⁸⁻ + 11TBA⁺ + 4H⁺ + 2OH⁻]³⁺ (m/z 2645.2183, TBA⁺: tetrabutylammonium). The experimental results collectively confirm the successful synthesis and presence of the Yb-SiW complex in acetonitrile solution. As a non-emissive reference, the Lu–SiW complex was also prepared and analyzed similarly, confirming its formation (Fig. S1 and S3†).

The Yb-SiW complex in acetonitrile solution exhibited a sharp emission band at 980 nm, corresponding to the ²F_5/2 → ²F_7/2 transition of the Yb³⁺ ion (Fig. 2). The excitation spectrum of the Yb-SiW complex showed a broad band below 350 nm, which is consistent with its absorption spectrum (Fig. 2). These bands were attributed to the O → W charge-transfer transitions within the silicotungstate ligand, (γ-SiW₁₀O₃₆)⁸⁻. These observations indicate that the observed NIR luminescence originates from the energy transfer between the POM ligand and the Yb³⁺ center.

Fig. 2 Absorption (blue), excitation (red), and emission (red) spectra of the Yb-SiW complex in CH₃CN. Absorption spectrum: [Yb-SiW] = 11 μM, excitation and emission spectra: [Yb-SiW] = 14 μM, λ_ex = 260 nm, λ_em = 980 nm.

The luminescence decay curve of Yb-centered luminescence fitted well using a single-exponential function (Fig. S4†), suggesting that the NIR emission from the Yb-SiW complex in acetonitrile originates from a single component. The observed luminescence lifetime (τ_obs) was determined as 1.27 μs, which falls within the range of lifetimes associated with long-lived Ln-centered luminescence. Moreover, the radiative luminescence lifetime (τ_rad) of the Yb center was estimated to be 0.593 ms based on the absorption spectrum of the ²F_5/2 ← ²F_7/2 transition (Fig. S5†). Next, the quantum yield of the Yb center (ϕ_Yb) in the Yb-SiW complex was calculated using the following equation (eqn (1)):

ϕ_Yb = τ_obs/τ_rad, (1)

yielding a value of 0.21%. The ϕ_Yb value of the Yb-SiW complex was approximately one order of magnitude lower than those of other cooperative UC systems, including dinuclear complexes (ϕ_Yb = 1.7, 2.6%),¹⁵ supramolecular assemblies (ϕ_Yb = 1.5, 2.3%),^14,32 and nonanuclear cluster complexes (ϕ_Yb = 1.7, 2.1, 2.6%).^12,13,20 It should be noted that the τ_rad values of the Yb-SiW complex and other existing cooperative UC systems are similar (typically 0.5–1.13 ms); therefore, a relatively lower ϕ_Yb value of the Yb-SiW complex can be attributed to its shorter τ_obs. The shorter τ_obs of the Yb-SiW complex indicates the presence of nonradiative deactivation pathways, such as high-energy O–H vibrations within the {Yb(μ₂-OH)₂Yb} core or intrinsic vibrational modes of the SiW ligand in the Yb-SiW complex.

Although the Yb³⁺ centers contain O–H groups, the presence of two Yb³⁺ centers in close proximity in the Yb-SiW complex makes it a promising candidate for CL-based visible emission. Upon NIR laser excitation, the Yb-SiW complex in CD₃CN exhibited visible emission at 488 nm (Fig. 3a). To rule out the contribution of any alternative sources behind the emission at 488 nm, control experiments were conducted using only the CD₃CN solvent in the absence of the Yb-SiW complex. The CD₃CN solvent showed a weak background signal with a slope of approximately one in a log–log plot of the signal at 488 nm versus excitation power, consistent with the scattering of the excitation light (Fig. S6†). After subtracting the background contribution, the corrected emission intensity of the Yb-SiW complex versus excitation power yielded a slope of approximately two in the log–log plot, indicating that the 488 nm emission proceeds via a two-photon excitation process (Fig. 3b). Since the {Ln–μ₂OH–Ln} unit in a structurally related Ln-SiW complex has been reported to exhibit an intermetallic distance of 3.6457 Å (for Ln = Dy), the Yb-SiW complex is expected to possess a comparable Yb–Yb distance. According to Auzel's criterion, observation of CL requires Ln–Ln distances below 5 Å,³³ which supports the feasibility of a CL mechanism in the Yb-SiW complex (Fig. 3c). Furthermore, the absence of residual emissions in the corrected emission spectra of the free SiW ligand and the Lu–SiW complex confirms that the 488 nm band emitted by the Yb-SiW complex in CD₃CN is attributable to UC involving two Yb³⁺ ions (Fig. S7†). We are currently attempting to obtain direct structural data on the Yb–Yb distance in the Yb-SiW complex, which will be reported in future work.

Fig. 3 (a) CL spectra of the Yb-SiW complex in CD₃CN, (b) log–log plot of the emission intensity at 488 nm versus the excitation power of the NIR laser, and (c) mechanism of CL. [Yb-SiW] = 3.0 mM, λ_ex = 972 nm.

In summary, we have synthesized and characterized a discrete dinuclear Yb-SiW complex that exhibits UC luminescence in CD₃CN. The UC emission is attributed to a CL mechanism, as supported by the excitation power dependence and the presence of two Yb³⁺ centers. The Yb-SiW complex represents a fully inorganic, molecularly discrete system composed entirely of inorganic components without any organic ligands. These findings pave the way for the rational design of UC-active materials based on discrete Ln-POM architectures. Given that Ln-SiW systems can form heteronuclear complexes (LnLn′–SiW),³⁴ we are currently extending this approach for developing heteronuclear Ln-POM systems with tunable UC luminescence.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

We thank Mr Shunsuke Kayamori and Mr Hiroyuki Momma for the ESI-MS measurements. We thank Dr Tatsuo Nakagawa and Mr Hiroaki Hanada for the measurements of the luminescence decay curve. This work was supported by JSPS KAKENHI (grant numbers: 20K15308 and 23K04801).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterization, and photophysical properties. See DOI: https://doi.org/10.1039/d5dt01369k

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