IrAu₁₂ superatom modified by chiral diphosphines: doping-induced enhancement of chiroptical activity

Abstract

Gold superatoms modified by chiral ligands are a new class of chiroptical nanomaterials, but improvement of their chiroptical properties, such as circular dichroism (CD) and circularly polarized luminescence (CPL), remains a challenge. In this work, we studied the effects of single-atom doping on the chiroptical properties of a representative Au₁₃ superatom using [Au₁₃((R,R)-DIPAMP)₅Cl₂]³⁺ and [Au₁₃((S,S)-DIPAMP)₅Cl₂]³⁺ (Au₁₃-R/S; DIPAMP = 1,2-bis[(2-methoxyphenyl)phenylphosphino]ethane). We synthesized an enantiomeric pair of superatoms: [IrAu₁₂((R,R)-DIPAMP)₅Cl₂]⁺ and [IrAu₁₂((S,S)-DIPAMP)₅Cl₂]⁺ (IrAu₁₂-R/S). Single-crystal X-ray diffraction analysis revealed that the icosahedral Ir@Au₁₂ core of IrAu₁₂-R/S was more twisted along the Cl–Au–Ir–Au–Cl axis compared with the Au₁₃ core of Au₁₃-R/S, leading to a larger absorption anisotropy factor. IrAu₁₂-R/S exhibited a much higher photoluminescence quantum yield (∼70%) compared with Au₁₃-R/S (15%) due to a larger energy gap between the highest occupied and the lowest unoccupied molecular orbitals. Although Ir doping did not appreciably enhance the photoluminescence anisotropy factors, the brightness of the CPL of IrAu₁₂-R/S was five times higher than that of Au₁₃-R/S. This work provides a rational guide for improving the chiroptical activity of Au superatoms via the doping-mediated manipulation of the geometric and electronic structures.

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Introduction

Ligand-protected gold (Au) and silver (Ag) superatoms with chiral structures have attracted growing interest as chiroptical nanomaterials because their atomically-precise structures make them ideal models for elucidating chiral structure–property correlations.^1–4 Since the first preparation of the optically active Au superatom protected by L-glutathione,⁵ various enantiomeric superatoms have been synthesized using chiral ligands such as thiolates,^6–9 diphosphines,^10–14 alkynyls,¹⁵ and N-heterocyclic carbenes.¹⁶ Chiral superatoms also are obtained via the asymmetric arrangement of achiral ligands.^17–21 Chiral counterions have often been used for the optical resolution and asymmetric synthesis of such chiral superatoms with achiral ligands.^22–24 These superatoms and assemblies of them exhibit unique chiroptical properties such as circular dichroism (CD) and circularly polarized luminescence (CPL). The CD activity is determined by the absorption anisotropy factor (g_abs), whereas the CPL is evaluated by the brightness (B_CPL) as given via the following equation:²⁵where ε, Φ_PL, and g_PL are the molar absorption coefficient at the excitation wavelength, the photoluminescence (PL) quantum yield, and the PL anisotropy factor, respectively. For these superatoms, the anisotropy factors (g_abs and g_PL) are typically of the order of 10⁻³, and the Φ_PL is less than 10%. Previous studies on the crystallization, self-assemby,^11,15,19,21 and doping of superatoms²⁶ have suggested the following strategies for improving these key parameters: (1) the g_abs value can be enhanced via torsion of the superatom¹⁴ and by placing π-electron ligands in the vicinity of the superatom;^10,27 (2) the Φ_PL value can be enhanced through rigidification of the superatom and widening of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).²⁶

To test the effectiveness of the above strategies, we herein studied the effects of single-atom doping on the chiroptical properties of a representative Au₁₃ superatom protected by chiral diphosphine ligands (Chart 1; dppe = 1,2-bis(diphenylphosphino)ethane, DIPAMP = 1,2-bis[(2-methoxyphenyl)phenylphosphino]ethane), i.e., [Au₁₃((R,R)-DIPAMP)₅Cl₂]³⁺ (Au₁₃-R) and [Au₁₃((S,S)-DIPAMP)₅Cl₂]³⁺ (Au₁₃-S).^13,14 Single-crystal X-ray diffraction (SCXRD) analysis showed that the icosahedral Au@Au₁₂ core consisted of a pair of Au₅ rings bridged by five DIPAMP ligands with a linear Cl–Au–Au–Au–Cl axis at their center (Fig. 1a). The degree of structural distortion of the Au@Au₁₂ core was evaluated via the dihedral angle of the two Au₅ rings (θ) (Fig. 1b). The average deviation of θ from that of a perfect icosahedron (Δθ = |36° − θ|) was 1.19 ± 0.61° and 1.24 ± 0.59° for Au₁₃-R/S, respectively.¹⁴ The dopant chosen in this study was iridium (Ir), based on the following findings obtained using achiral diphosphine ligands (Chart 1), for [Au₁₃(dppe)₅Cl₂]³⁺ (Au₁₃-dppe) and [IrAu₁₂(dppe)₅Cl₂]⁺ (IrAu₁₂-dppe). First, the Δθ value increased from 1.03 ± 0.95° for Au₁₃-dppe to 1.69 ± 0.50° for IrAu₁₂-dppe, probably due to the smaller atomic radius of Ir (2.0 Å) compared with that of Au (2.1 Å).^28,29 Second, the Φ_PL value increased markedly from 11% for Au₁₃-dppe to ∼70% for IrAu₁₂-dppe²⁶ due to the expansion of the HOMO–LUMO (HL) gap from 1.9 eV for Au₁₃-dppe to 2.3 eV for IrAu₁₂-dppe. These results suggest that the chiroptical properties of the Au₁₃ superatom can be improved through manipulation of the geometric and electronic structures via single-atom doping with Ir. To test this hypothesis, we fleshly synthesized an enantiomeric pair of Ir@Au₁₂ superatoms: [IrAu₁₂((R,R)-DIPAMP)₅Cl₂]⁺ (IrAu₁₂-R) and [IrAu₁₂((S,S)-DIPAMP)₅Cl₂]⁺ (IrAu₁₂-S). The B_CPL values of the IrAu₁₂-R/S enantiomers were five times larger than those of the non-doped Au₁₃-R/S enantiomers, mainly due to the enhancement of Φ_PL (eqn (1)).

Chart 1 Diphosphine ligands used in this study.

Fig. 1 (a) Side and (b) top view of the chiral structure of MAu₁₂-dppe (M = Ir, Au). Color code: yellow, Au the on Cl–Au–M–Au–Cl axis; red and blue, Au in the Au₅ rings; purple, M; orange, P; light green, Cl; gray, C.

Experimental section

Synthesis and characterization of IrAu₁₂-R/S·(PF₆)

The PF₆-salt of IrAu₁₂-R/S was synthesized according to the synthesis method for IrAu₁₂-dppe.²⁶ The Au₁₃-R/S enantiomers were also synthesized as a reference according to the method in ref. 14. Single crystals of IrAu₁₂-R/S·(PF₆) suitable for SCXRD measurements were obtained via recrystallization from CH₂Cl₂/cyclopentyl methyl ether. Details of the synthesis and the characterization of IrAu₁₂-R/S are provided in the ESI.†

Optical and chiroptical measurements

The UV–Vis absorption spectra and CD spectra were recorded using a Jasco V-670 spectrometer and a JASCO J-725 spectropolarimeter, respectively. The g_abs values were calculated using the equation g_abs = θ [mdeg]/(32 980·Abs) using the CD (θ: ellipticity) and absorbance (Abs) data. PL spectra were recorded using a Jasco FP-8500 spectrofluorometer. The absolute value of Φ_PL was measured at ambient temperature using a Hamamatsu Photonics CC9920-02G assembly with an integration sphere. The PL lifetime measurements were carries out using a C4780 streak camera. The excitation source was generated using a Nd:YVO₄ laser (Verdi, Coherent) pumped Ti:sapphire laser system (Mira-900, Coherent) equipped with a cavity dumper (Pulse Switch, Coherent), which delivers 100 fs pulse trains at 800 nm. After doubling the frequency with a LiB₃O₅ crystal, the incident pulses were focused onto the samples (excitation wavelength = 400 nm). The PL decay curves were extracted from the 2D streak images in the range of 95 nm centered at the PL peak of each temperature. CPL spectra were recorded using a homemade CPL spectroscopy system equipped with a UV laser diode (375 nm) as the excitation source.³⁰ The g_PL values were calculated using the equation g_PL = 2(I_L − I_R)/(I_L + I_R), where I_L and I_R are the PL intensity of the left and right circularly polarized light, respectively. An Ar-saturated solution was used for the PL and CPL measurements. The temperature for the PL and PL lifetime measurements was controlled in the range of 80–300 K using an Oxford Instruments OptistatDN liquid N₂ cryostat. For variable-temperature (VT) UV-Vis, CD, and CPL studies, the temperature of the samples was controlled in the range of 200–300 K using a Unisoku CoolSpeK cryogenic cell holder.

Results and discussion

Geometric structure

Isotopically resolved electrospray ionization (ESI) mass spectra of the IrAu₁₂-R/S samples confirmed the composition and charge state of [IrAu₁₂(DIPAMP)₅Cl₂]⁺ (Fig. S1, ESI†). The mass spectra also exhibited small peaks at m/z ≈ 4963, assigned to [IrAu₁₂(DIPAMP)₅ClBr]⁺ (IrAu₁₂-Br) that originated from impurities in the chemicals used during the synthesis. Ratios of the peak area of IrAu₁₂-Br to IrAu₁₂-R/S were 0.05 and 0.04, respectively, indicating that the purity of IrAu₁₂-R/S was ∼95 mol%. The purities of the IrAu₁₂-R/S samples were further confirmed using ¹H NMR and elemental analysis. The ¹H NMR charts of the IrAu₁₂-R/S samples showed signals only for the DIPAMP ligands and the cyclopentyl methyl ether (CPME) used for crystallization (Fig. S2, ESI†). The ratio of the integrated signals indicated that the samples contained two equivalents of CPME to IrAu₁₂-R/S. The elemental analysis demonstrated that IrAu₁₂-R/S contained 1.31 wt% of Cl, which was reproduced by considering that the samples consisted of 95 mol% of IrAu₁₂-R/S·(PF₆)·(CPME)₂ and 5 mol% IrAu₁₂-Br·(PF₆)·(CPME)₂. Nevertheless, based on a previous report that the optical absorption profile and Φ_PL value of Au₁₃-dppe were hardly changed by replacing the Cl ligands with Br,³¹ we concluded that the effect of the IrAu₁₂-Br impurity on the optical and chiroptical measurements was negligible.

The geometric structures of IrAu₁₂-R/S were solved via SCXRD analysis. The unit cell contained a single IrAu₁₂-R/S molecule and a PF₆ anion, supporting the monovalent charge state for IrAu₁₂-R/S as determined via ESI mass spectrometry. As shown in Fig. 2a, the IrAu₁₂-R/S enantiomers have an icosahedral IrAu₁₂ core ligated by five DIPAMP ligands and two Cl ligands. The position of the Ir atom could not been determined via SCXRD analysis due to the inability of distinguishing the electron density of Ir from Au. ³¹P{¹H} NMR analysis was performed to reveal the location of Ir in the IrAu₁₂ core (Fig. S3, ESI†). The single peak in the ³¹P{¹H} NMR charts of IrAu₁₂-R/S indicated that all ten phosphorus nuclei of the DIPAMP ligands were equivalent on the NMR time scale. Thus, the Ir atom was located at the center of the icosahedral core. The metal–metal bond lengths of IrAu₁₂-R/S and Au₁₃-R/S are summarized in Fig. S4 and Table S1 (ESI†). The average lengths of the Ir–Au bonds in IrAu₁₂-R/S were 2.726 ± 0.012 Å and 2.729 ± 0.017 Å, respectively, which were shorter than those of the corresponding radial Au–Au bonds in Au₁₃-R/S (2.761 ± 0.033 Å and 2.761 ± 0.034 Å, respectively).¹⁴ The average lengths of the peripheral Au–Au bonds of IrAu₁₂-R/S (2.866 ± 0.026 Å and 2.870 ± 0.034 Å, respectively) were also shorter than those of Au₁₃-R/S (2.904 ± 0.030 Å and 2.903 ± 0.032 Å, respectively).¹⁴ The contraction of the Ir@Au₁₂ core with respect to Au@Au₁₂ is due to the smaller van der Waals radius of Ir (2.0 Å) compared with that of Au (2.1 Å).³² The torsion of the Ir@Au₁₂ core was quantified using the average value of Δθ (Fig. 2b). The values were 1.47 ± 0.30° and 1.89 ± 0.67° for IrAu₁₂-R/S, respectively, which were larger than those of Au₁₃-R/S (1.19 ± 0.61° and 1.24 ± 0.59°, respectively).¹⁴ These results indicated that the Au₅ rings of the Ir@Au₁₂ core were more twisted around the Cl–Au–Ir–Au–Cl axis compared with those of the Au₁₃ core. In conclusion, the introduction of a smaller Ir atom to the Au₁₃ core induced further distortion.

Fig. 2 (a) SCXRD structures of IrAu₁₂-R (left) and IrAu₁₂-S (right). Phenyl rings, ethylene bridges, and methoxy groups are depicted as sticks and hydrogen atoms are omitted for simplicity. (b) Top view of the core structures of IrAu₁₂-R (left) and IrAu₁₂-S (right). Phenyl rings and methoxy groups are omitted for simplicity. Color code: yellow, Au; dark blue, Ir; orange, P; light green, Cl; red, O; gray, C.

UV–Vis absorption and CD properties

The UV–Vis absorption spectra of IrAu₁₂-R/S (Fig. 3a) exhibited a distinct peak at 418 nm with an ε value of 2.08 × 10⁴ M⁻¹ cm⁻¹ and 2.01 × 10⁴ M⁻¹ cm⁻¹, respectively. These ε values were larger than those of Au₁₃-R/S at 495 nm (1.61 × 10⁴ M⁻¹ cm⁻¹ and 1.76 × 10⁴ M⁻¹ cm⁻¹, respectively) (Fig. 3b).¹⁴ The onsets of the spectra revealed that the HL gaps of IrAu₁₂-R/S were 2.3 eV, which are larger than those of Au₁₃-R/S (1.9 eV) (insets of Fig. 3).¹⁴ These results indicated the expansion of the HL gap upon Ir doping, as observed in IrAu₁₂-dppe and Au₁₃-dppe (Table 1).²⁶ Fig. S5 (ESI†) shows the VT UV–Vis spectra of IrAu₁₂-R/S in 2-methyltetrahydrofuran (MeTHF) recorded at 300 and 200 K. The spectrum at 200 K exhibited a sharper profile than that of the spectrum at 300 K because the hot band transition was suppressed at the lower temperature.^33–35 As a result, the two peaks at 420 and 470 nm became more pronounced and a new shoulder structure appeared at ∼520 nm (the insets of Fig. S5, ESI†).

Fig. 3 UV–Vis absorption spectra of (a) IrAu₁₂-R (red) and IrAu₁₂-S (blue) in MeTHF and (b) Au₁₃-R (red) and Au₁₃-S (blue) in acetonitrile at ambient temperature. Arrows indicate the HL gaps estimated from the spectral onsets. Insets show the expanded spectra in the energy scale.

Table 1 Optical and chiroptical properties of IrAu₁₂-S and Au₁₃-S

Superatom	HL gap (eV)	ε (× 10^4 M^−1 cm^−1)	|gabs| (× 10^−3)	λ PL (nm)	Φ PL (%)	τ PL (μs)	λ CPL (nm)	|gPL| (× 10^−3)	Ref.
The numbers in parentheses represent the excitation wavelength.
IrAu12-S	2.3	2.01 (418 nm)	∼4 (475 nm, 300 K)	∼609 (400 nm)	65.9	5.87	∼600	∼2 (600 nm, 300 K)
			∼5 (475 nm, 200 K)					∼4 (600 nm, 200 K)
Au13-S	1.9	1.76 (495 nm)	∼2 (400 nm, 300 K)	∼792	15	3.19	∼760	∼2 (760 nm, 300 K)	13
			∼3 (400 nm, 200 K)					∼(4–5) (760 nm, 200 K)

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The VT CD spectra of IrAu₁₂-R/S are shown in Fig. 4a. The spectral profiles of the enantiomeric pair show a mirror image relationship, indicating the enantiopurity of the samples. The peak positions in the CD spectra at ∼310, 340, 370, 400, 470, and 530 nm corresponded to the absorption peaks and shoulder at ∼305, 350, 420, 470 and 520 nm, respectively. The CD response for IrAu₁₂-R/S in the visible region is associated with the optical transition within the chiral Ir@Au₁₂ core since the frontier orbitals are assigned to superatomic orbitals according to density functional theory calculations.²⁶ The |g_abs| spectra of IrAu₁₂-R/S and Au₁₃-R/S are presented in Fig. 4b and c, respectively. The |g_abs| values of IrAu₁₂-R/S are ∼3 × 10⁻³ at 365 nm and ∼4 × 10⁻³ at 475 nm, respectively, at 300 K (Table 1). By contrast, the maximum |g_abs| value for Au₁₃-R/S was ∼2 × 10⁻³ at 400 nm at 300 K (Table 1), which is consistent with the reported value.¹³ According to the theoretical study in ref. 13, the higher |g_abs| value for IrAu₁₂-R/S compared with that for Au₁₃-R/S is ascribed to the larger torsion of the Ir@Au₁₂ core. The CD intensity of IrAu₁₂-R/S was enhanced by lowering the temperature from 300 to 200 K (Fig. 4a). The |g_abs| value increased at 200 K to ∼4 × 10⁻³ at 365 nm and ∼5 × 10⁻³ at 474 nm, respectively (Fig. 4b, Table 1). The enhancement at the lower temperature is probably associated with the suppression of the thermal fluctuation of the Ir@Au₁₂ core structure. A similar temperature dependence was observed for the |g_abs| values of Au₁₃-R/S (Fig. 4c). The |g_abs| value at 400 nm was enhanced to ∼3 × 10⁻³ by lowering the temperature to 200 K (Table 1).

Fig. 4 (a) CD spectra and (b) g_abs spectra of IrAu₁₂-R (red) and IrAu₁₂-S (blue) in MeTHF. (c) g_abs spectra of Au₁₃-R (red) and Au₁₃-S (blue) in a MeOH–EtOH mixture (1 : 1 v/v). Solid and dotted lines denote the spectra recorded at 200 and 300 K, respectively.

PL and CPL properties

The PL spectra of IrAu₁₂-S were recorded in de-aerated MeTHF (Fig. 5a). The PL peak of IrAu₁₂-S appeared at 609 nm at ambient temperature, which was shorter than that of Au₁₃-R/S (792 nm).¹⁴ This blueshift reflects the expansion of the HL gap by the doping with Ir. The Φ_PL values of IrAu₁₂-R/S in MeTHF at 300 K were 69.9% and 65.9%, respectively(Fig. S6, ESI† and Table 1), and were much higher than the reported value of Au₁₃-R/S (15%).¹⁴ The time-resolved PL decay of IrAu₁₂-S and Au₁₃-S was also measured (Fig. 5b and Fig. S7, ESI†). The PL lifetime (τ_PL), obtained through fitting with a single exponential function, of IrAu₁₂-S (5.87 μs) was longer than that of Au₁₃-S (3.19 μs) (Table 1). The increase in Φ_PL and τ_PL associated with the expansion of the HL gap was consistent with the case of IrAu₁₂-dppe and Au₁₃-dppe. According to the energy gap law, an increase in the PL energy suppresses the nonradiative relaxation process.³⁶

Fig. 5 (a) PL spectrum of IrAu₁₂-S in MeTHF recorded at the excitation wavelength of 400 nm. (b) Time-resolved PL decay of IrAu₁₂-S probed at 400 nm in MeTHF. The black line represents the fitting result.

The VT CPL spectra of IrAu₁₂-R/S and Au₁₃-R/S were recorded in de-aerated MeTHF and a MeOH–EtOH mixture (1 : 1 v/v), respectively (Fig. 6). The spectral profiles of the enantiomeric pairs of both samples are mirror images. The CPL bands peaked at ∼600 and ∼760 nm for IrAu₁₂-R/S and Au₁₃-R/S, respectively, which are comparable to the positions of the corresponding PL bands at 300 K. The |g_PL| values for IrAu₁₂-R/S at 300 K were ∼3 × 10⁻³ and ∼2 × 10⁻³, respectively (Table 1), whereas those for Au₁₃-R/S were ∼2 × 10⁻³, which are comparable to the reported values (2.5 × 10⁻³ and −2.3 × 10⁻³ for Au₁₃-R/S, respectively).¹⁴ Note that the different |g_PL| values between IrAu₁₂-R/S are due to the limited sensitivity of the CPL measurement setup. The similarity of the |g_PL| values between IrAu₁₂-R/S and Au₁₃-R/S is probably associated with the similar structures in the photoexcited state responsible for PL. The |g_PL| values of IrAu₁₂-R/S and Au₁₃-R/S increased to ∼4 × 10⁻³ and ∼(4–5) × 10⁻³, respectively, upon lowering the temperature to 200 K (Fig. 6 and Table 1). The |g_PL| values of IrAu₁₂-R/S were of the same order (∼10⁻³) as those reported for other superatoms, whereas the Φ_PL values of IrAu₁₂-R/S were much higher than those of other chiral superatoms in either the solid or self-assembled state.^11,15,19,21 Therefore, IrAu₁₂-R/S are superior in terms of the CPL brightness (B_CPL) defined using eqn (1).²⁵ The B_CPL value of IrAu₁₂-S was 26 M⁻¹ cm⁻¹ (λ = 418 nm) at 300 K. This value is approximately five times higher than that of Au₁₃-S (B_CPL = 5.3 M⁻¹ cm⁻¹, λ = 495 nm) at 300 K, indicating that doping with Ir brightened the CPL, mainly by increasing the molar absorption coefficient at 495 nm and the HL gap of Au₁₃-R/S.

Fig. 6 CPL spectra of (a) IrAu₁₂-R (red) and IrAu₁₂-S (blue) in MeTHF and (b) Au₁₃-R (red) and Au₁₃-S (blue) in a MeOH–EtOH mixture (1 : 1 v/v) recorded at the excitation wavelength of 375 nm. Solid and dotted lines denote the spectra recorded at 200 and 300 K, respectively.

Conclusions

We successfully synthesized the enantiomeric pair of Ir@Au₁₂ superatoms protected by DIPAMP chiral diphosphine ligands and studied the single-atom doping effect on the chiroptical properties of the Au@Au₁₂ superatoms. The icosahedral Ir@Au₁₂ superatom was distorted more than the corresponding Au@Au₁₂, resulting in larger |g_abs| values for the former. Doping with the Ir atom expanded the HOMO–LUMO gap of the Au₁₃ superatom, leading to the marked increase in Φ_PL from 15% to ∼70%. By contrast, the |g_PL| values of the IrAu₁₂ and Au₁₃ superatoms were comparable. As a whole, the CPL brightness of the Au₁₃ superatom was enhanced by nearly five times upon introducing Ir at the center. This work provides a new strategy to enhance the CPL brightness of Au superatoms via the doping-mediated control of geometric and electronic structures.

Author contributions

Haru Hirai: data curation, investigation, writing – original draft. Takuya Nakashima: formal analysis, investigation. Shinjiro Takano: conceptualization, investigation. Yukatsu Shichibu: data curation, methodology. Katsuaki Konishi: data curation, methodology. Tsuyoshi Kawai: resource. Tatsuya Tsukuda: funding acquisition, project administration, supervision, writing – review and editing. Each author contributed substantially to the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was financially supported by JST, CREST (Grant No. JPMJCR20B2) and JSPS KAKENHI (Grant No. JP20H00370 and JP20J21726). This work was partly supported by the Photoexcitonix Project at Hokkaido University.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2225808 and 2225809. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2tc05321g

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