Intensive near-infrared emitting Au₇Cu₁₀ nanoclusters for both energy and electron harvesting

Abstract

Triplet excitons have gained increasing recognition as inherent characteristics of various nanomaterials. However, the practical application of triplet excitons is limited due to their confinement within the material where they originate, posing significant challenges to harnessing their potential. In this work, we report direct extraction of both triplet energy and electrons from newly synthesized atomically precise Au₇Cu₁₀ nanoclusters (NCs). These NCs exhibit intensive near-infrared (NIR) emission with a quantum yield of 31% at room temperature. They also display near-unity quantum yield of intersystem crossing (ISC) with strong spin–orbit coupling (SOC) up to 864 cm⁻¹. Consequently, Au₇Cu₁₀ NCs can act as triplet sensitizers, facilitating efficient triplet–triplet annihilation and achieving upconverted emission with an efficiency of 18.4% in perylene. Furthermore, rapid electron injection from Au₇Cu₁₀ NCs in the triplet state to methyl viologen was clearly observed. This study represents the first direct extraction of both triplet energy and electrons from the same metal NCs, indicating their potential as molecular triplet energy and electron surrogates in optoelectronics, photocatalysis, and solar energy conversion.

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Introduction

At the forefront of materials science, there is significant interest in the intricate interplay between nanomaterials and organic molecules, particularly in the context of triplet energy transfer (TET) and photoinduced electron transfer (PET).^1–11 Nanomaterial–organic hybrids play a central role in facilitating the precise transfer of triplet excitons and electrons,^12–16 which is fundamental to fields ranging from optoelectronics to photocatalysis.^17–20 The exploration of PET from nanomaterials to organic molecules has emerged as a promising avenue, offering an alternative pathway alongside TET for manipulating triplet exciton utilization.^21–26 Despite significant efforts to understand triplet exciton utilization in quantum nanomaterials, the mechanisms for directly extracting triplet energy and electrons from nanomaterials remain elusive.

The advent of atomically precise metal nanoclusters (NCs) has opened new opportunities for exploring and refining triplet exciton utilization. Metal NCs represent an innovative category of luminescent nanomaterials, which exhibit molecular-like behaviors, characterized by tunable band gaps, unique optical and electronic properties.^27–40 These distinctive attributes can be finely tuned by adjusting parameters such as size, structure, and metal composition, which have found tremendous applications.^41–51 While there have been some instances of PET processes from metal NCs to organic materials,^52–59 these occurrences remain limited. Furthermore, the utilization of metal NCs as a platform for triplet sensitizers was reported recently, particularly in triplet–triplet annihilation-induced upconversion (TTA-UC) emission.^60–64 Therefore, advancing the practical applications of metal NCs requires a deep dive into the mechanisms of electron and energy transfer within these materials, crucial for their development and utilization in triplet sensitizers, optical sensing, and catalysis applications.

To achieve triplet energy and electron extraction from metal NCs, we designed and synthesized a new Au nanocluster alloyed with Cu heteroatoms. This design strategy was based on two key factors: (1) the introduction of Cu atoms could enhance the PLQY;³⁰ (2) heteroatom doping is an effective strategy to engineer and enrich the electronic structures, potentially yielding unique physicochemical properties and functionalities.^46,60,65 Here, we report the direct extraction of both triplet energy and electrons from atomically precise Au₇Cu₁₀ NCs that exhibit intense NIR emission. The crystal structure of Au₇Cu₁₀, elucidated through single-crystal X-ray diffraction (SC-XRD), reveals an icosahedral core comprising seven Au atoms and six Cu atoms, with an additional four Cu atoms forming the cap. The PL quantum yield (PLQY) of Au₇Cu₁₀ NCs reaches 31% in deaerated solution and 24% in oxygen saturated solutions. Theoretical calculations confirm the presence of strong spin–orbit coupling (SOC) and a minimal energy gap (25 meV) between S₁ and T₂/T₃ states, accelerating the intersystem crossing (<0.1 ps) process. Significant TET from Au₇Cu₁₀ to perylene (Pery for short) was observed, confirming the phosphorescent nature of the PL from Au₇Cu₁₀, and the triplet state lifetime is up to 3.9 μs. Additionally, the Au₇Cu₁₀ NCs act as efficient triplet energy donors, facilitating upconverted emission of Pery through efficient triplet–triplet annihilation (TTA) with an impressive efficiency of up to 18.4%. Furthermore, femtosecond transient absorption spectroscopy (fs-TA) captures the rapid PET process within 12 picoseconds between Au₇Cu₁₀ NCs and methyl viologen. The successful extraction of triplet energy and electrons from Au₇Cu₁₀ can be attributed to its unique structure featuring a significant number of exposed metal atoms, which eliminates the shielding effect and promotes efficient energy and electron exchange between the metal NCs and energy/electron acceptors.

Results and discussion

Crystal structure and steady state optical properties

Details of the synthesis of Au₇Cu₁₀ NCs are provided in the ESI.† The structure in Fig. 1A reveals a core composition of the Au₇Cu₁₀ kernel, which comprises an Au₇Cu₆ icosahedron that is capped by one up-Cu atom and three down-Cu atoms. Within the Au₇Cu₆ icosahedron, the average Au_core–M_shell and M_shell–M_shell (M = Au or Cu) distances measure 2.663 Å and 2.857 Å, respectively, which are notably shorter than those observed in an Au₁₃ icosahedron (2.757 and 2.908 Å),⁶⁶ indicating that Cu-doping induces a contraction in the entire icosahedron. Regarding surface-protecting ligands, the Au₇Cu₁₀ kernel is co-capped by three 1-adamantanethiol (1-Adm), six disphenyl-2-pyridylphosphine (Ph₂PyP), and three Cl atoms through Cu–N/S and Au–Cl/P coordination (Fig. 1A). The incorporation of Cu–N enhances the structural rigidity, thereby facilitating the improvement of its luminescence efficiency.³⁰ Furthermore, the presence of three [SbF₆]⁻ ions around the entire Au₇Cu₁₀ nanocluster suggests a 3+ charge for this nanocluster. Consequently, the precise chemical formula of this nanocluster is expressed as [Au₇Cu₁₀(1-Adm)₃(PPh₂Py)₆Cl₃](SbF₆)₃, abbreviated as Au₇Cu₁₀. Besides, the chemical composition of Au₇Cu₁₀ was also testified by electrospray ionization mass spectrometry (ESI-MS). As shown in Fig. 1B, a main peak at 1400.257 Da was observed, which can be assigned to [Au₇Cu₁₀(1-Adm)₃(PPh₂Py)₆Cl₃]³⁺ (cal. m/z = 1400.250 Da). The experimental isotopic pattern (red line) is in good agreement with the simulated one (black line).

Fig. 1 (A) Crystal structures of Au₇Cu₁₀. Color codes: yellow, Au; green, Cu; magenta, P; light green, Cl; red, S; gray, C; purple, N. (B) ESI-MS of Au₇Cu₁₀. (C) The UV-vis absorption spectrum (solid line) and emission spectra (dashed lines) of Au₇Cu₁₀ in diluted THF solution. The inset shows the cluster solution picture without and with UV excitation. (D) Emission decay curves (2.5 ns bin time and probe 740 nm) of Au₇Cu₁₀ measured under N₂-degassed, air-saturated and O₂-saturated conditions (λ_ex = 375 nm), respectively. The inset shows the PL decay profiles measured with a bin time of 25 ps (monitored at λ_em ∼ 650 nm and λ_em ∼ 740 nm).

The steady-state electronic characteristics of Au₇Cu₁₀ NCs were explored using the absorption and PL spectra (Fig. 1C) in THF. The absorption spectrum of Au₇Cu₁₀ NCs shows distinctive peaks centered at 365 nm and 635 nm. Notably, Au₇Cu₁₀ NCs exhibit a pronounced emission band at 740 nm in solution, with a high PLQY of 26% in air. It is observed that only one single peak is observed in the PL spectrum (Fig. 1C) of Au₇Cu₁₀ and the time-resolved PL measurements exhibit a long lifetime of 3.5 μs in air (Fig. 1D). To further reveal the emission characteristics, the time-resolved PL decay curves at short-wavelength and long-wavelength of the emission band were examined, wherein we confirmed that there is only one emission component across the entire emission band because all the PL decay curves remain the same on different time scales. Additionally, PL excitation spectra at different emission wavelengths (Fig. S1†) closely resemble the absorption spectrum, providing further evidence that Au₇Cu₁₀ possesses only one luminescent excited state.

To discern the intricate details governing the emission behavior of Au₇Cu₁₀, a comprehensive investigation involving PLQY and lifetime measurements was performed under varied atmospheric conditions – oxygen (O₂), air, and nitrogen (N₂) environments. The PLQY of Au₇Cu₁₀ exhibits a reduction of 16% in air (QY = 26%) compared to that in N₂ (QY = 31%, see Fig. 1C). In addition, the PLQY exhibits a relatively minor decrease of only 8% when exposed to O₂ as opposed to air (Fig. 1C). Concomitant PL lifetime measurements were conducted, yielding values of 3.9 μs, 3.5 μs, and 3.2 μs in N₂, air, and O₂ atmospheres (Fig. 1C), respectively. These findings underscore that the emission of Au₇Cu₁₀ remains largely unaffected by the presence of O₂. It is crucial to note that the absence of PL quenching by O₂, indicative of no discernible formation of singlet oxygen , does not conclusively establish fluorescence as the primary emission mechanism of Au₇Cu₁₀ NCs. The low solubility and slow diffusion rate of O₂ in organic solvents, the degree of ligand shielding of the metal core, and inefficient charge transfer or energy exchange between the metal NCs and O₂ could potentially impede the triplet energy transfer to O₂.⁶³ The existence of a microsecond lifetime for Au₇Cu₁₀ in the O₂ atmosphere further supports the notion that the PL is not completely quenched. Additionally, the PL decay curves at 650 nm and 740 nm in the nanosecond regime were collected, as illustrated in the inset of Fig. 1D, which further demonstrates a single emissive excited state.

Triplet energy transfer

Direct triplet–triplet energy transfer is an effective route to utilize the triplet exciton for solar-associated applications. Fig. 2A illustrates the schematic diagram of the photon up-conversion mechanism based on TTA. Pery was chosen as an appropriate triplet annihilator and blue-light emitter, leveraging its high fluorescence quantum yield and the strategic positioning of its lowest-lying triplet energy level at 1.53 eV.^67,68 Notably, the energy gap of T₁ energy levels between Au₇Cu₁₀ NCs (Table 1) and Pery is less than 0.15 eV (1.67–1.53 = 0.14 eV), which can reduce the energy loss during TET from the NCs to the Pery molecules.

Fig. 2 (A) Schematic energy-level diagram of TTA-UC. Steady-state (B) absorption and (C) emission spectra (excitation at 600 nm) of Au₇Cu₁₀ NCs in the presence of Pery. The inset of (C) shows the Stern–Volmer plot with the dynamic quenching rate constant K_SV. (D) Ns-TA color maps of Au₇Cu₁₀/Pery dissolved in deaerated THF with 630 nm excitation; the Pery concentration was set at 3 mM. (E) The kinetic traces of Au₇Cu₁₀ in THF. (F) The kinetic traces of Au₇Cu₁₀/Pery in THF with 630 nm excitation.

Table 1 Photophysical parameters of Au₇Cu₁₀ in THF

Sensitizer	τ^a/μs	Eedge^b/eV (nm)	ET^c/eV (nm)	ΦISC^d	kISC^d/10^12 s^−1	ΦP^e
a Obtained from the PL lifetime via the PL decay profiles as shown in Fig. 1D.b Absorption edge energy.c T1 energy level (ET) is evaluated using the photoluminescence peak.d ISC quantum yield and the ISC rate constant for Au7Cu10 are obtained from the fs-TA experiment.e Absolute PLQY in solution with saturated N2.
Au7Cu10	3.9	1.77 (700)	1.67 (740)	1	>10	0.31

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Upon the introduction of Pery, the absorbance and spectral shape of the absorption band at 600–700 nm for Au₇Cu₁₀ undergo minimal changes (Fig. 2B). This observation rules out the formation of complexes between Au₇Cu₁₀ and Pery, suggesting that Au₇Cu₁₀ NCs maintain their size and composition in the presence of Pery. A gradual decrease in the PL intensity of Au₇Cu₁₀ NCs was observed with the increment of the Pery concentration, as depicted in Fig. 2C. A maximum quenching of approximately 94% was observed with the addition of 6.6 mM Pery. It should be noted that the addition of Pery didn't change the entire PL profile even with a Pery concentration of 6.6 mM. The Föster energy transfer and inner filter effect mechanisms that may cause the PL suppression can be safely ruled out because Pery does not display any appreciable absorption at the photoexcitation and photoemission wavelengths of Au₇Cu₁₀ (Fig. S2†). Therefore, we can conclude that the TET reaction occurs between Au₇Cu₁₀ and Pery as a result of Dexter energy transfer. Based on the TET results, the PL in Au₇Cu₁₀ can be safely assigned to phosphorescence.

To quantify the effective TET between Au₇Cu₁₀ NCs and Pery, we analyzed PL quenching data using the Stern–Volmer equation, F₀/F = 1 + K_sv[Pery],^60,61 where F₀ and F represent the fluorescence intensities before and after the addition of Pery, respectively, K_sv is the quenching constant. The resultant Stern–Volmer plot, characterized by a linear dependence (inset of Fig. 2C), signifies a typical hallmark of dynamic quenching. This implies that diffusion predominantly governs the PL quenching processes, with a determined Stern–Volmer constant of 1903 M⁻¹. Besides, the observed decrease in the PL lifetime (from 3.9 to 0.3 μs) of Au₇Cu₁₀ upon interaction with Pery, as depicted in Fig. S3,† stems from the quenching of the luminescent state via the TET process. Based on the relation of k_TET = K_sv/τ₀,⁶⁰ the corresponding k_TET in the presence of Au₇Cu₁₀ NCs and Pery was calculated to be 4.8 × 10⁸ M⁻¹ s⁻¹, showing an efficient TET process.

To unravel the origin of Au₇Cu₁₀ PL and confirm the efficient energy transfer with Pery, a time-resolved spectroscopic investigation of PL quenching was conducted. Fig. 2D and S4A† present the ns-TA spectra of the Au₇Cu₁₀ NCs in the presence and absence of Pery, respectively. The ns-TA spectra (Fig. S4A and B†) show the excited-state absorption (ESA) with a maximum at 500 nm and another band with several features extending over the range of 670–950 nm. The ground-state bleaching (GSB) at 630 nm is not observable due to overlapping with the excitation beam. The fitting of TA kinetic traces at different wavelengths for pure Au₇Cu₁₀ NCs reveals time constants of 2.9 μs (Fig. 2E). It is observed that the lifetime obtained from ns-TA is slightly shorter than that from PL decay profiles (Fig. 1D). This slight difference is attributed to the fact that the two lifetimes were measured using different instruments, each with distinct instrument response functions and detection limits, leading to the small variation in the observed lifetimes. By comparing the ns-TA kinetic traces of Au₇Cu₁₀ NCs before and after the addition of Pery (Fig. 2F), we found that the decay of ESA bands between 700 and 900 nm is much accelerated with Pery addition. Besides, two new ESA bands appear at 450–500 nm (Fig. 2D), which can be assigned to the triplet–triplet absorption of Pery. The rise time (≈0.36 μs, Fig. 2F) of the triplet signal of Pery matches well with the decay time of the ESA bands of the Au₇Cu₁₀ NCs (0.36 μs), indicating that the triplet state of Pery (³Pery*) was solely generated via TET from the Au₇Cu₁₀ NCs. The triplet signal of ³Pery* in Fig. 2D, in accordance with previous reports,^69–71 further confirms the accuracy of the assignment. Global analysis (Fig. S4D†) suggests a two-species model as the best fit for the data, and the first component can be assigned to the TET process.

Using the obtained triplet lifetime from ns-TA, the TET quantum efficiency (Φ_TET) was calculated according to Φ_TET = 1 − τ/τ₀, where τ₀ and τ represent the triplet lifetime of Au₇Cu₁₀ in the absence and presence of Pery, respectively. A satisfactory Φ_TET value approaching 90% was achieved. The kinetic trace (Fig. S5†) further confirms that the TET rate increases significantly with an increase in the Pery concentration, which supports the conclusion of diffusion-controlled energy transfer. The k_TET value (4.8 × 10⁸ M⁻¹ s⁻¹) is two orders of magnitude smaller than the estimated diffusion-limited rate constant (k_d) of ∼1.51 × 10¹⁰ M⁻¹ s⁻¹ (see the ESI†). This is because the rate constant for Dexter-type energy transfer decays exponentially with the distance between the donor and acceptor. As illustrated in Fig. S6,† the spin density distribution of the lowest triplet state of Au₇Cu₁₀ is mainly localized on the metal core rather than the surface ligand moieties, which sterically inhibits the electron exchange between Au₇Cu₁₀ and Pery and thus reduces the TET rate.

Ultrafast intersystem crossing

To thoroughly understand the excited state relaxation mechanism of Au₇Cu₁₀ NCs, we focused on the ISC process from singlet to triplet states by employing time-resolved fs-TA experiments and theoretical calculations. Fs-TA data maps of Au₇Cu₁₀ in THF obtained upon 360 nm excitation are presented in Fig. S7B.† The initial TA spectrum displays two ESA peaks at ∼500 and ∼590 nm, along with a broad ESA band at ∼700–770 nm. Notably, the ESA bands at 590 and 685 nm rapidly disappear within the first picosecond (∼360 fs), accompanied by the rapid decay at 685 nm and the generation at 750 nm (Fig. 3A), while the remaining characteristic peaks persist for up to ∼10 μs (Fig. S7A and E†). These long-lived ESA bands are assigned to the T₁ state.

Fig. 3 (A) Fs-TA kinetic traces and fittings at selected probe wavelengths pumped at 360 nm. (B) Comparison of the TA kinetic trace of 750 nm at different excitation wavelengths of 360 and 600 nm. (C) Molecular orbitals corresponding to the hole and electron distributions of the S₁ and T_1–3 states. SOC values between S₁ and T_n are shown. (D) Simplified energy diagram showing the ultrafast ISC processes of Au₇Cu₁₀. The low-lying excited states energy level values are derived from TD-DFT. ISC: intersystem crossing; IC: internal conversion.

Further assignment of the sub-picosecond (∼360 fs) component observed in Au₇Cu₁₀ with 360 nm excitation was conducted by ultrafast spectroscopy with a long-wavelength (λ = 630 nm) excitation (Fig. S7D†), revealing only a long-lived component with the absence of the sub-picosecond component. A comparison of fs-TA with excitation at 360 nm and 630 nm (Fig. 3B) suggests that the pure ISC process of Au₇Cu₁₀ is too fast to be resolved using the current TA spectrometer (IRF = 100 fs). The 360 fs process is then attributed to the internal conversion coupled ISC process. Therefore, it can be deduced that the k_ISC is much faster compared to k_fluor so that fluorescence is completely suppressed, which indicates an almost unity ISC yield. A near unity ISC yield makes Au₇Cu₁₀ an ideal triplet sensitizer for TTA-UC.

To delve into a comprehensive understanding of the near unity ISC yield of Au₇Cu₁₀, an analysis of decisive factors determining the ISC was carried out through density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations (Fig. S8†). According to El-Sayed's rules,⁷² an efficient ISC is characterized by the simultaneous presence of strong spin–orbit coupling and a small singlet–triplet energy gap of different character states. Electronic structure calculations predict sizable spin–orbit coupling (SOC) values between S₁ and T₂/T₃ states (≥860 cm⁻¹, very large) for Au₇Cu₁₀ (Fig. 3C). Beside the intensive SOC, these two triplet states present small vertical energy gaps with respect to S₁ (only 25–28 meV), hence providing the necessary ingredients for efficient ISC. Moreover, the electron/hole pair densities for the S₁ and T_1–3 states of Au₇Cu₁₀ (Fig. 3C and S9†) show that the electrons of T_1–3 and S₁ are almost the same, mainly delocalized on the surface of the metal core. We noted that the ligands show minimal impact on the molecular orbital isosurface of Au₇Cu₁₀ as shown in Fig. 3C. The electron, hole, and charge density difference (CDD) analyses in Fig. S9† further support this observation. Since the investigation of ISC pathways is the primary focus of this work, we have not extensively discussed ligand-associated charge transfer.

As illustrated in Fig. S9,† the CDD indicate a charge transfer character, clearly demarcating the boundary between “electrons” and “holes” in these states. Specifically, the hole distribution in the T₁ state closely resembles that in the S₁ state, whereas the hole distributions in T₂ and T₃ states are perpendicular to that in S₁, respectively. This alteration in orbital angular momentum associated with a 90° rotation can trigger effective SOCs and rapid ISC between S₁ and T₂/T₃, as it can compensate for the change in the spin angular momentum associated with ISC. Consequently, these calculations indicate that the ultrafast ISC process from S₁ to the T₂/T₃ represents the most probable ISC pathways (Fig. 3D), owing to the relatively small ΔE_ST energy gaps and substantial SOC values. Moreover, these triplet states exhibit distinct spatial localization relative to the relaxed S₁ state (Fig. 3C and S9†). The presence of metal atoms, along with a non-zero overlap in singlet–triplet wave functions, engenders a robust SOC, facilitating efficient singlet-to-triplet ISC in Au₇Cu₁₀. Meeting all El-Sayed's criteria, Au₇Cu₁₀ undergoes an ultrafast spin-flip process driven by inherently strong spin–orbit coupling, complemented by a small ΔE_ST.

TTA-UC performance

The broad absorption in the visible region, coupled with efficient ISC and a long-lived triplet excited state, positions Au₇Cu₁₀ as a promising candidate for triplet photosensitizers in TTA-UC. To validate this potential, TTA-UC experiments were conducted on Au₇Cu₁₀ dissolved in deaerated THF, employing Pery as the annihilator and blue emitter (Fig. 4). Upon excitation at 663 nm, triplet Au₇Cu₁₀ NCs efficiently sensitize ³Pery^* with a noteworthy anti-Stokes shift exceeding 0.88 eV (Fig. 4A). Relative to other NIR to visible UC systems, the anti-Stokes shift of 0.88 eV observed in this work represents an effective UC performance.^73,74 The intensity of upconverted fluorescence progressively increases with increasing Pery concentration, saturating when the concentration exceeds 2.5 mM. The highest upconversion quantum yield is estimated to be 18.4% at a Pery concentration of 2.5 mM (the Φ_UC calculation can be found in the ESI and Fig. S10†). It's worth mentioning that a decline in quantum yield is observed with further increments in the Pery concentration, attributed to the formation of Pery excimers, as reported previously.^75,76 Besides, compared to the previously reported Au₄Cu₄-DPA (9,10-diphenylanthracene) system,⁶⁰ which achieved an upconversion quantum yield of 28%, higher than the efficiency in our work, several factors could account for this difference: (1) the triplet energy gap between Au₄Cu₄ and DPA is near zero, whereas for Au₇Cu₁₀ and Pery it is approximately 0.14 eV. (2) The DPA concentration in the previous study was 40 mM, significantly higher than the 2.5 mM concentration of Pery used in our study. (3) The excited-state lifetime of Au₄Cu₄ was 7.9 μs, longer than the 3.9 μs lifetime of Au₇Cu₁₀, and a longer excited-state lifetime allows more excited nanoclusters to participate in TET with organic acceptors (note that both lifetimes were obtained from photoluminescence decay curves in degassed THF).

Fig. 4 (A) PL spectra of Au₇Cu₁₀ and Pery at various concentrations with excitation at 663 nm, where the inset shows the photograph of the TTA-UC emission process. (B) Relationship of the TTA-UC quantum yield with the Pery concentration. Note that the theoretical maximum of Φ_UC is defined as 100%.⁷³ (C) Dependence of the TTA-UC fluorescence intensity on the excitation power density. (D) Double logarithmic plot of the integrated TTA-UC fluorescence intensity with the excitation power density, where the inset shows the lifetime measurement of delayed fluorescence of Pery.

Furthermore, the upconversion emission spectral intensity demonstrates a direct correlation with the excitation power density. Fig. 4C shows the integrated upconversion emission intensity of Pery within the 450 to 600 nm range as a function of the laser power density. The integrated upconversion emission intensity of Pery from 450 to 600 nm exhibits a quadratic-to-linear dependence on the laser power density (Fig. 4D), indicating typical behavior observed in TTA-UC systems. Specifically, the slope of the linear fit changes prominently from 2.1 (77 to 2000 mW cm⁻²) to 1.08 (2290 to 5200 mW cm⁻²), showcasing an approximate threshold excitation intensity (I_th) value of approximately 2065 mW cm⁻². In TTA-UC systems, the quadratic-to-linear dependence is a distinctive signature of two-photon-induced upconversion luminescence, signifying the TTA-UC process. Additionally, we observe a millisecond time-scale photoluminescence lifetime of Pery in the mixed solution with Au₇Cu₁₀ nanoclusters (inset of Fig. 4D), significantly longer than the fluorescence lifetime of pure Pery (Fig. S11†). Such long delayed fluorescence behavior signifies that the observed emission stems from the long-lived TTA process of the emitter molecules, suggesting that the NIR emission from Au₇Cu₁₀ can be successfully upconverted to the blue emission of Pery through efficient TET induced upconversion.

Photoinduced electron transfer

Au–Cu alloyed nanoclusters have been well known for their wealth of rich redox chemistry in addition to photophysical properties.^60,77,78 Since doping heteroatoms into gold nanoclusters can modulate the energy level scheme and excited state properties, it is reasonable to expect that Au₇Cu₁₀ would engage in the PET process. In the context of metal nanoclusters, most research has focused on the triplet energy aspects, neglecting the importance of electrons. Electrochemical measurements (Fig. S12†) indicate that Au₇Cu₁₀ can effectively participate in both donating and accepting electrons, demonstrating its capability of acting as an intermediate in the electron transfer process. In addition, the rapid ISC process and long-lived triplet state lifetime of Au₇Cu₁₀ provide ample time for electron transfer processes to occur in the triplet state. Such a PET is essential for converting light energy into chemical energy, including applications in dye-sensitized solar cells, organic photovoltaics, artificial photosynthesis, etc.^79–81

Methyl viologen (MV²⁺), with strong electron-accepting properties and suitable oxidation potential in methanol (MeOH), was employed to probe the excited-state intermolecular electron-transfer between Au₇Cu₁₀ (Fig. 5A). Furthermore, MV²⁺ can engage in noncovalent interactions with counter ions on the surface of Au₇Cu₁₀ nanoclusters via coulombic attraction, enhancing the feasibility of subsequent PET processes. Fig. 5B illustrates the steady state absorption and emission spectra of Au₇Cu₁₀ NCs with varying concentrations of MV²⁺. The absorption band of Au₇Cu₁₀ located at 630 nm remains unchanged upon MV²⁺ addition, providing clear evidence of no ground-state interaction between the two components. Concurrently, the emission intensity of Au₇Cu₁₀ NCs decreases with an increasing MV²⁺ concentration (Fig. 5C), with over 90% fluorescence intensity quenched at 15 mM MV²⁺. This efficient quenching indicates that the excited-state deactivation occurs through the PET pathway. The absorption spectra of MV²⁺ show no spectral overlap with Au₇Cu₁₀ nanocluster emission, ruling out Förster resonance energy transfer from Au₇Cu₁₀ to MV²⁺.

Fig. 5 (A) Schematic illustration of PET between excited Au₇Cu₁₀ and MV²⁺. Steady-state (B) absorption and (C) emission spectra (excitation at 600 nm) of Au₇Cu₁₀ NCs in the presence of MV²⁺. The inset of (B) shows the absorption spectra of Au₇Cu₁₀ and MV²⁺ in MeOH. The inset of (C) shows a Stern–Volmer plot. (D) fs-TA of Au₇Cu₁₀ NCs in deaerated MeOH after 400 nm laser excitation in the presence of MV²⁺. (E) Top: TA kinetic traces after laser excitation at 400 nm of Au₇Cu₁₀ NCs in the presence of MV²⁺ collected from fs-TA monitored at 600 nm (absorption of MV⁺ radical) and 700 nm (ESA of Au₇Cu₁₀). Below: kinetic traces of Au₇Cu₁₀ NCs in the absence of MV²⁺. (F) Global analysis results derived from fs-TA of Au₇Cu₁₀ NCs in the presence of MV²⁺.

We then employed the Stern–Volmer relationship to analyze the PL quenching process (inset of Fig. 5C), which demonstrates a diffusion-controlled mechanism with a quenching constant (K_SV) of 689 M⁻¹. Examining the thermodynamic feasibility of the PET, a negative free energy of −0.98 eV (ΔG_ET) is estimated by applying the Rehm–Weller equation (Table S1†), confirming the thermodynamically favorable electron transfer from Au₇Cu₁₀ to MV²⁺. To further clarify the PET mechanism, fs-TA spectroscopy was carried out to investigate the excited-state dynamic processes between the Au₇Cu₁₀ and viologen units. It is observed that the initial spectral evolution of Au₇Cu₁₀ in the presence of MV²⁺ (Fig. 5D) corresponds to the formation of the triplet signal of Au₇Cu₁₀ , closely resembling the fs-TA spectra of pure Au₇Cu₁₀ in the initial 1 ps (Fig. S13†), which can be attributed to the rapid internal conversion-coupled ISC process as we discussed above.

Due to the broad ESA of Au₇Cu₁₀ in the visible region, it remains challenging to identify the absorption peak of the oxidized . Compared to the spectra of pure Au₇Cu₁₀, new ESA emerges at approximately 600 nm within about 30 picoseconds (Fig. 5D), which is attributed to the reduction of MV⁺˙ by Au₇Cu₁₀ NCs. The kinetic trace monitored at 700 nm reveals an additional decay (∼12 ps, Fig. 5E) which is not observed in pure Au₇Cu₁₀. Simultaneously, a distinct raised signal from the reduced methyl viologen radical (MV⁺˙) is evident with absorption maxima around 600 nm (Fig. 5D and E).⁵⁵ However, the triplet signal of pure Au₇Cu₁₀ persists even at longer time delays (Fig. S14C†), indicating an equilibrium process between the triplet state of Au₇Cu₁₀ NCs and the PET process. The charge-separated state can undergo back either electron transfer to reform the triplet state or charge recombination to the ground state. Consequently, the fs-TA data of Au₇Cu₁₀ in the presence of MV²⁺ can be well reproduced through global fitting using two decay components.

The corresponding evolution-associated difference absorption spectra (EADS) and time constants are depicted in Fig. 5F. It is observed that the first EADS corresponds to the triplet Au₇Cu₁₀. 12 ps later, the second EADS, indicative of the equilibrium state between the triplet state of Au₇Cu₁₀ NCs and the PET process, becomes apparent. Besides, noteworthy differences are observed in the ns-TA spectra of the Au₇Cu₁₀ NCs in the absence and presence of MV²⁺ (Fig. S14†): (1) the lifetime of triplet Au₇Cu₁₀ in the presence of MV²⁺ (370 ns) is much shorter than that in the absence of MV²⁺ (2.8 μs), indicating effective quenching of by MV²⁺; (2) a distinct band at around 600 nm emerges in the presence of MV⁺˙, and the kinetic trace fitting at 600 nm reveals a lifetime of 1.2 μs, which can be assigned to the relaxation to the ground state of MV⁺˙. One might wonder why the quenching efficiency of Au₇Cu₁₀ by O₂ is low, while its quenching efficiencies by Pery and MV²⁺ are relatively high. As the degree of ligand shielding of the metal core shows similar effects on both O₂ and Pery/MV²⁺ quenching, we tentatively assign the differences to factors including the CT driving force, O₂ solubility, slow diffusion of O₂, and the triplet energy level matching between the donor nanocluster and acceptor O₂, all contributing to the low-efficiency quenching of Au₇Cu₁₀ by O₂.

It is essential to clarify that the PET process between Au₇Cu₁₀ and MV²⁺ (12 ps⁻¹) is much faster than the TET with the Pery molecule (360 ns⁻¹). This is because TET involves a two-step electron exchange process,⁸² while MV²⁺ can be adsorbed on the counterions around the nanocluster, accelerating the one-step PET. In the crystal structure of Au₇Cu₁₀, multiple Cu atoms are exposed to the ambient environment, and the spin density distribution of the triplet state shows that the electrons primarily localize on the copper atoms (Fig. S6†), allowing for efficient TET with suitable molecules and achieving high-efficiency upconversion luminescence. Additionally, our results indicate that both PET and TET can quench the PL of Au₇Cu₁₀. The key distinction between these two mechanisms lies in the nature of the quenching product. PET involves electron transfer, leading to the formation of an anion radical of the organic molecule (MV⁺˙), whereas TET involves energy transfer, resulting in the formation of the triplet state of the organic molecule (³Pery*). Compared to previously reported Au_xCu_y nanoclusters,⁶⁰ the newly synthesized Au₇Cu₁₀ stands out as a dual-function nanocluster capable of serving as both a triplet energy sensitizer and an electron donor (Scheme 1). These advances, supported by extensive experimental and theoretical evidence, position Au₇Cu₁₀ as a versatile platform for applications in optoelectronics, photocatalysis, and solar energy conversion. Additionally, directly extracting electrons and the generation of long-lived charge-separated excited states make the systems a potential light-harvesting sensitizer and photoinduced catalyst, opening avenues for numerous applications.^83–85

Scheme 1 Schematic diagram of the (A) TET and (B) PET processes between Au₇Cu₁₀ NCs and organic acceptors, represented by a single Au₇Cu₁₀ NC for simplicity. S: singlet state; T: triplet state; GS: ground state. In the TET process, the NIR emission of Au₇Cu₁₀ is quenched as triplet energy is transferred to Pery, resulting in TTA and the subsequent blue fluorescence of Pery. In the PET process, the NIR emission of Au₇Cu₁₀ is quenched due to electron transfer from Au₇Cu₁₀ to MV²⁺. The numbers in (B) indicate the sequence of electron motion: (1 → 2) excitation of an electron from the ground state to the singlet state in Au₇Cu₁₀ upon photoexcitation, (2 → 3) spin flip via ISC from the singlet state to the triplet state, and (3 → 4) PET from Au₇Cu₁₀ to MV²⁺.

Conclusion

In summary, we present the direct extraction of triplet energy from Au₇Cu₁₀ NCs through the TET process, inducing triplet–triplet annihilation and upconverted blue fluorescence from Pery. Meanwhile, the PET process enables efficient electron transfer from Au₇Cu₁₀ to MV²⁺, resulting in the formation of an MV⁺˙ radical and quenching of the nanocluster's NIR emission. The crystal structure analysis revealed an icosahedral core of Au₇Cu₆ (M₁₃) surrounded by four Cu atoms in Au₇Cu₁₀. These NCs exhibited a high NIR PLQY exceeding 31% under N₂ conditions at room temperature. Transient spectroscopy of Au₇Cu₁₀ unveils the remarkably ultrafast (<1 ps) internal conversion coupled ISC process and a long-lived triplet state (3.5 μs in the air). DFT calculations prove that the strong SOC (864.8 cm⁻¹) significantly promotes ultrafast ISC (<100 fs). Despite the modest quenching influence of O₂ on photoluminescence, the robust TET corroborates the near unity ISC yield, affirming the origin of emission in Au₇Cu₁₀ from the triplet state. When combined with the blue light emitter Pery, Au₇Cu₁₀ exhibited remarkable upconversion photoluminescence, converting red excitation to blue emission with an efficiency of 18.4%. Additionally, we demonstrated that Au₇Cu₁₀ can serve as a light absorber, enabling rapid and efficient electron injection into organic molecules. These findings highlight the tailored design of nanoclusters to facilitate efficient triplet energy and electron transfer, promising advancements in light harvesting and photoenergy conversion applications.

Data availability

Data available on request from the authors.

Author contributions

W. Z. and J. K. performed the spectral measurements and the theoretical calculations. T. X. and Y. S. synthesized and characterized the metal nanoclusters. Y. L. and X. Z. provided help in the measurements of TTA-UC experiments. J. Z and Q. Z. provided help in measuring photoluminescence quantum yield of metal nanoclusters. Y. S., Y. L. and M. Z. designed the project. W. Z., and J. K. wrote the draft. W. Z., J. K., Y. S., Y. L. and M. Z. revised the manuscript. All authors approved the final version of the manuscript.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

We acknowledge the financial support from the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0450202), the Fundamental Research Funds for the Central Universities (WK9990000145), the National Natural Science Foundation of China (22273095 and 22171007), the Innovation Program for Quantum Science and Technology (2021ZD0303303), and the Chinese Academy of Sciences (YSBR-007).

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Footnotes

† Electronic supplementary information (ESI) available: Materials and methods, supplementary text, Fig. S1–S15, and Table S1. CCDC 2350273. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc00671f

‡ These authors contributed equally to this work.

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