Sensitization experiments of ultrasmall gold nanoclusters: determination of triplet quantum yields and molar absorption coefficients

Abstract

We demonstrated for the first time the determination of triplet quantum yields and molar absorption coefficients of ultrasmall gold nanoclusters specifically [Au₂₅(PET)₁₈]⁻ with phenylethanethiolate (PET) ligands using two sensitization experiments.

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Recently, metal nanoclusters have attracted considerable interest in various basic and applied research fields owing to their size-dependent structures and properties derived from their discrete electronic structures.^1–12 In particular, ligand-protected ultrasmall metal nanoclusters exhibit characteristic molecular behaviours associated with the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).¹³ For example, regarding [Au₂₅(PET)₁₈]⁻ (denoted as Au₂₅(PET)₁₈) with phenylethanethiolate (PET) ligands, single-crystal X-ray diffraction demonstrated that Au₂₅(PET)₁₈ is composed of an icosahedral Au₁₃ kernel and six Au₂(PET)₃ staple motifs on the surface.^14,15 The molecular behaviour of these metal nanoclusters is attributed to singlet or triplet excited states,¹⁶ but the detailed excited-state dynamics associated with the molar absorption coefficients (ε_T) and quantum yields (Φ_T) in the triplet states are yet to be determined. The singlet–triplet (S₁–T₁) energy gaps (E_S–T) of ultrasmall gold nanoclusters such as Au₂₅ clusters (e.g., E(S₁) = ∼1.4 eV and E(T₁) = ∼1.2 eV) have been reported to be much smaller than those of organic materials according to theoretical calculations and spectroscopic measurements.^17,18 Thus, the values of E(S₁) and E(T₁) can be controlled simply and mainly by the number of metal atoms and types of ligands, making it much easier to establish a strategy for material design as compared to organic materials. Therefore, gold nanoclusters are useful for photo-functionalities based on absorption, emission,^19–24 and electron/energy transfer in the visible (red) and NIR regions, and are expected to be explored in various fields such as energy conversion, catalysis, electronics, and biomedical applications.^25–28

The photophysical behaviours associated with triplet excited states (T₁) of ultrasmall gold nanoclusters (e.g., Au_n: n = 25, 38) include phosphorescence emission through intersystem crossing (ISC),^19,20 reverse intersystem crossing (RISC)²⁹ and singlet oxygen (¹O₂) generation in oxygen-containing solutions.³⁰ Additionally, the bimolecular rate constant (∼10⁶ M⁻¹ s⁻¹) is extremely small, although the intermolecular triplet-triplet energy transfer (T-TEnT) from Au_n to organic chromophores was observed.³¹ Time-resolved spectroscopy, such as transient absorption spectra (TAS) and emission lifetimes, suggested that TAS of Au₂₅ from the longest lifetime component and photoluminescence decay at ∼1100 nm involve the excited state derived from the same T₁.^16,32–34 These lifetimes of Au₂₅ (ca. 50–200 ns) are much shorter than those of the typical T₁ lifetimes of organic chromophores (∼microsecond region). However, no attention has been paid to the determination of the ε_T and Φ_T of such gold nanoclusters. Although sensitization experiments using T-TEnT with an energy donor (a sensitizer) are useful for assigning the triplet TAS and corresponding ε_T values, the T₁ lifetimes of Au_n are extremely short. Accordingly, conventional methods are difficult to sufficiently evaluate.

Here we demonstrate two different sensitization experiments to evaluate the triplet character of [Au₂₅(PET)₁₈]⁻ (denoted as ) for the determination of the ε_T and Φ_T. First, the diffusion-controlled intermolecular T-TEnT from a sensitizer (C₆₀) to Au₂₅(PET)₁₈ (∼10¹⁰ M⁻¹ s⁻¹) was observed in toluene. Then, the diffusion-controlled quenching trend of in O₂-saturated toluene (∼10¹⁰ M⁻¹ s⁻¹) demonstrated the intermolecular T-TEnT from Au₂₅(PET)₁₈ (energy donor) to ³O₂ (energy acceptor), yielding ¹O₂ generation with the triplet yield (Φ_T: 24%). Based on these results, the ε_T of were successfully assigned. Additionally, the triplet behaviour of Au₃₈(PET)₂₄ was compared with Au₂₅(PET)₁₈.

The syntheses and characterization of Au₂₅(PET)₁₈ and Au₃₈(PET)₂₄ were performed according to the reported method (Schemes S1 and S2 and Fig. S1 and S2 and Table S1 in ESI†).^35,36 To determine the singlet and triplet energies (E(S₁) and E(T₁)), we also measured luminescence spectra of Au₂₅(PET)₁₈ at low temperatures following the reported methods²⁰ (Fig. S2 and S3 in ESI†). Consequently, E(S₁) and E(T₁) of Au₂₅(PET)₁₈ were determined to be 1.5 eV and 1.2 eV, respectively. These values are comparable to the above-mentioned theoretical and experimental values of the related Au₂₅.^18,20 Additionally, the E(S₁) and E(T₁) of Au₃₈(PET)₂₄ should be slightly smaller than those of Au₂₅(PET)₁₈ according to previous report.¹⁸

According to previous reports,^16,37 the ISC pathways of Au₂₅(PET)₁₈ and Au₃₈(PET)₂₄ are expected to be extremely fast. To examine the singlet and triplet characters, femtosecond transient absorption spectra (fs-TAS) were systematically measured in toluene (Fig. 1 and Fig. S4–S10 in ESI†). Although the origin of the absorption properties is generally dependent on the excitation wavelengths, we mainly chose 650 nm because there was no significant difference in the triplet behaviours upon either 350 or 650 nm excitation (Fig. S4–S6, ESI†). In the case of Au₂₅(PET)₁₈, the ultrafast relaxation occurred within ca. 4.0 ps after laser-pulse excitation (Fig. 1A and B), and long-lived excited species, extending from the visible to near-infrared (NIR) region (ca. 400–900 nm), remained within the measurement time-range of fs-TAS up to ∼6.0 ns. Assuming that this process is associated with ISC, the corresponding species-associated spectra (SAS) of Au₂₅(PET)₁₈ were analysed by target analysis (see: Fig. S11 and S12 in ESI†),³⁸ yielding the SAS of S₁ and T₁ (Fig. 1C). The time-dependent concentration profiles (inset figure in Fig. 1C) demonstrate the ultrafast deactivation process. Note that the behaviour of the initial species is beyond the time-resolution of fs-TAS. The first species (trace a) and second species (trace b) should be attributed to S₁ and T₁, respectively. The longer and second species continue to be undecayed in the above-mentioned time-range (∼6 ns) (Fig. S4 in ESI†). Regarding Au₃₈(PET)₂₄, a similar relaxation process occurs within ca. 1 ps, whereas second and longer species decayed with the lifetime of ∼6 ns. This is in sharp contrast with Au₂₅(PET)₁₈. Noted that the singlet character derived from RISC process from T₁ to S₁ could not be obtained in Au₂₅(PET)₁₈ and Au₃₈(PET)₂₄ (see: the temperature-dependent fs-TAS in Fig. S13 in ESI†).

Fig. 1 (A) fs-TAS of Au₂₅(PET)₁₈ in toluene (λ_ex: 650 nm) and (B) the time profile at 500 nm (298 K). (C) Species-associated spectra (SAS) of (a) first (black) and (b) second (red) species. Inset: The time-dependent concentration profiles of first and second species.

To further observe the longer timescale in Au₂₅(PET)₁₈, we also measured picosecond transient absorption spectra (ps-TAS) of Au₂₅(PET)₁₈ (Fig. S14 and S15 in ESI†). The resulting excited species of Au₂₅(PET)₁₈ remained and the lifetime was calculated to be 160 ns at 298 K. No excitation wavelength-dependent spectral changes of S₁ were observed for both Au₂₅(PET)₁₈ and Au₃₈(PET)₂₄ (Fig. S14–S17 in ESI†). From these results, triplet lifetimes of 160 ns for Au₂₅(PET)₁₈ and 5.0 ns for Au₃₈(PET)₂₄ were predicted, which are much shorter than those of typical organic molecules.

Thus, the above-mentioned fs-TAS allowed us to predict the triplet TAS of Au_n (n = 25, 38), although it is necessary to confirm whether these predictions are essentially correct or not. Therefore, we first observed triplet TAS of Au₂₅(PET)₁₈ using T-TEnT with a sensitizer (C₆₀). Fig. 2A presents ps-TAS in a mixed toluene solution of C₆₀ and Au₂₅(PET)₁₈. The excitation wavelength chosen was 355 nm to mainly excite C₆₀. The molar ratio between C₆₀ and Au₂₅(PET)₁₈ was also optimized after preliminary experiments in which the ps-TAS were measured by varying the ratios. After laser-pulse excitation, we initially observed the triplet spectrum of C₆₀ at ca. 750 nm. Note that 355 nm excitation includes a small amount of absorption in Au₂₅(PET)₁₈, resulting in the appearance of due to the direct excitation of Au₂₅(PET)₁₈ in addition to produced by T-TEnT from C₆₀ (Fig. 2A and Fig. S2 in ESI†). Another serious problem arises from the fact that the lifetime of is much shorter than that of . Therefore, it is difficult to distinguish the triplet absorption spectrum of Au₂₅(PET)₁₈ by the direct excitation from that by intermolecular T-TEnT from C₆₀. Although the ε_T cannot be determined by this method, the strong quenching process of was clearly observed at 750 nm (Fig. 2B). The time profile at 750 nm showed the significant quenching of with increasing the concentrations of Au₂₅(PET)₁₈. Fig. 2C also shows a linear plot of the observed decay rate constants (k_obs) at 750 nm based on the concentrations of Au₂₅(PET)₁₈. From the slope of the linear plot, the second-order rate constant of intermolecular T-TEnT from C₆₀ to Au₂₅(PET)₁₈ was calculated to be 1.0 × 10¹⁰ M⁻¹ s⁻¹. This is quite similar to the diffusion-limited value in toluene (k_diff: 1.1 × 10¹⁰ M⁻¹ s⁻¹).³⁹ The similar photophysical trend of Au₃₈(PET)₂₄ was successfully observed (Fig. S18 in ESI†). These results clearly demonstrate the triplet characters of Au₂₅(PET)₁₈ and Au₃₈(PET)₂₄.

Fig. 2 (A) ps-TAS of C₆₀ (32 μM) in the presence of Au₂₅(PET)₁₈ (20 μM) in toluene (298 K). λ_ex: 355 nm. (B) The corresponding time-profiles at 750 nm in the presence of different concentrations of Au₂₅(PET)₁₈. Inset: Pseudo-first order plot of k_obs monitored at 750 nm versus concentrations of Au₂₅(PET)₁₈.

Then, another way to reveal the triplet character of Au_n involves a sensitization experiment in oxygen-saturated toluene because of the occurrence of intermolecular T-TEnT from Au_n to the molecular oxygen (³O₂). In addition to the ³O₂- and Ar-saturated toluene solutions, air-saturated (21% ³O₂)⁴⁰ toluene solution was prepared according to the established method.⁴¹Fig. 3A shows ps-TAS of Au₂₅(PET)₁₈ in ³O₂-saturated toluene. Comparing these triplet absorption spectra with ps-TAS of Au₂₅(PET)₁₈ in Ar-saturated toluene (Fig. S14 and S15 in ESI†), no change in shape was observed. Fig. 3B shows strong decays in ³O₂-saturated and air-saturated toluenes compared to that in Ar-saturated toluene. These resulted in the occurrence of intermolecular T-TEnT because the triplet lifetimes in ³O₂-saturated (τ_T: 47 ns) and air-saturated (τ_T: 120 ns) toluenes were much shorter than that in Ar-saturated toluene (τ_T: 160 ns). Since there is a linear relationship between k_obs and ³O₂ concentrations, the second-order rate constant can be calculated to be 1.0 × 10¹⁰ M⁻¹ s⁻¹. This is very close to the above-mentioned k_diff in toluene, which is in sharp contrast with the lack of quenching of with ³O₂ (Fig. S19 in ESI†). At the saturated oxygen concentration in toluene (∼10 mM), the decay rate constant (k_d) of with ³O₂ was determined to be 1.0 × 10⁸ s⁻¹. This is nearly two orders of magnitude greater than the rate constant of the triplet deactivation process of Au₂₅(PET)₁₈ (1/(160 ns) = 6.3 × 10⁶ s⁻¹), whereas the k_d of with ³O₂ should be smaller than that of Au₂₅(PET)₁₈ (1/(5.0 ns) = 2.0 × 10⁸ s⁻¹). As a result, we can conclude that quantitative T-TEnT occurs between and ³O₂.

Fig. 3 (A) ps-TAS of Au₂₅(PET)₁₈ in O₂-saturated toluene (λ_ex: 355 nm) (298 K). (B) The time profiles at 600 nm in (a) Ar-saturated, (b) air-saturated (21% ³O₂), (c) ³O₂-saturated toluene. Inset: Pseudo-first order plot of k_obs monitored at 600 nm vs.³O₂ concentrations.

The above-mentioned quantitative intermolecular T-TEnT from Au₂₅(PET)₁₈ to ³O₂ resulted in ¹O₂ generation for evaluations of the Φ_T. To examine the Φ_T of Au₂₅(PET)₁₈via ISC, we employed ¹O₂ luminescence measurements using intermolecular T-TEnT from Au₂₅(PET)₁₈ to ³O₂ in ³O₂-saturated toluene at excitation wavelengths of 350 nm (Fig. 4A) and 650 nm (Fig. S20 in ESI†). By using intermolecular T-TEnT, we detected ¹O₂ luminescence at ca. 1270 nm, assuming negligible quenching processes from S₁. The Φ_T of Au₂₅(PET)₁₈ was calculated based on the reference compound: C₆₀ (Φ_T: 100%).⁴² Consequently, the Φ_T of Au₂₅(PET)₁₈ (λ_ex: 350 nm) was determined to be 24 ± 1.8%. Additionally, the Φ_T of Au₂₅(PET)₁₈ (λ_ex: 650 nm) is 24 ± 2.2%, and these values does not depend on the excitation wavelengths (Tables S2–S4 in ESI†). In contrast, in the case of Au₃₈(PET)₂₄, no T-TEnT occurred, as discussed above. Thus, no ¹O₂ peak at 1270 nm was observed in toluene (Fig. S21 in ESI†). The relatively small Φ_T of Au₂₅(PET)₁₈ is likely attributable to the nonradiative process from S₁ based on vibrational motion (vide infra). Based on the above Φ_T and ps-TAS of Au₂₅(PET)₁₈, the ε_T were obtained (Fig. 4B, calculation process in ESI†) together with the kinetic parameters (Table 1).

Fig. 4 (A) A luminescence differential spectrum of Au₂₅(PET)₁₈ in ³O₂-saturated toluene (298 K). The inset shows the parent luminescence spectra of Au₂₅(PET)₁₈ in (a) O₂- and (b) Ar-saturated toluenes. λ_ex: 350 nm. (B) The estimated ε_T values of Au₂₅(PET)₁₈ in toluene.

Table 1 Summarized photophysical parameters in toluene at room temperature (298 K)

	τ S ps	τ T ns	Φ T %	k EnT × 10^−9^d M^−1 s^−1	k EnT × 10^−9^e M^−1 s^−1
a Estimated by luminescence spectra. b Estimated by target analysis of fs-TAS. c Estimated by ^1O2 generation. d T-TEnT from C60 to Aun (n = 25, 38). e T-TEnT from Aun to ^3O2.
Au25(PET)18	4.0	160	24	10	10
Au38(PET)24	0.87	5.0	—	9.9	—

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To further examine the detailed dynamics of , we also measured the temperature-dependent ps-TAS (Fig. S22 in ESI†). Although the T₁ lifetimes increased with decreasing temperatures, the RISC process was negligible considering the single-exponential decay curves of together with the above-mentioned S₁–T₁ gap of Au₂₅(PET)₁₈ (∼0.3 eV). This energy gap is much larger than those of the reported Au_n with observations of RISC (∼0.03 eV).²⁹ Hence, considering the above results of the temperature-dependent fs-TAS (Fig. S13 in ESI†) and ps-TAS, the origin of the changes in the excited lifetimes of the singlet and triplet states in temperature-dependent experiments is mainly due to the vibrational motions originating from Au₁₃ kernel and/or six Au₂(PET)₃ staple motifs. This should be associated with the small Φ_T.⁴³ The proposed excited-state dynamics of Au₂₅(PET)₁₈ is also shown in Fig. S23 in in ESI.†

In conclusion, we demonstrated the triplet behavior of ultrasmall gold nanoclusters together with the determination of the Φ_T and ε_T using sensitization experiments. The simple and easy control of excited states in metal nanoclusters will stimulate basic and applied researches on excited triplet states in the future.

This work was partially supported by JSPS KAKENHI Grant-in-Aid for Transformative Research Areas, “Materials Science of Meso-Hierarchy”(JP23H04876 to T. H.).

Data availability

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

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, synthesis, spectroscopic measurements. See DOI: https://doi.org/10.1039/d4cc06083k

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