Potential of N-heterocyclic carbene derivatives from Au₁₃(dppe)₅Cl₂ gold superatoms. Evaluation of electronic, optical and chiroptical properties from relativistic DFT

Abstract

Atomically precise gold superatoms offer useful templates to evaluate tunable properties via ligand engineering. Herein, the role of different linked N-heterocyclic carbene (NHC) protecting ligands ranging from strong to weak σ-donors was evaluated according to the Tolman electronic parameter (TEP), in species related to the classical [Au₁₃Cl₂(dppe)₅]³⁺ nanocluster (1). Our results show a strong dependency on the nature of NHC, providing a useful design principle for the efficient tuning of the structural, optical, chiroptical and emission properties of the Au₁₃Cl₂ core. A sizable decrease is observed in the HOMO–LUMO gap for weaker σ-donor ligand cases, with a change in the LUMO nature from core-based orbitals in 1, to a π*-ligand nature. Furthermore, a shorter bridge results in interesting structural changes between the eclipsed ↔ staggered Au₁₃Cl₂ core unraveling the potential to convert light energy into mechanical work. Thus, the noticeable modulation of [Au₁₃Cl₂(NHC)₅]³⁺ properties by different ligands underlies design rules for tunable clusters towards nanostructured materials, by taking advantage of the recent introduction of NHC-protected gold clusters.

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Introduction

The last decade has seen a major advancement in ligand-protected gold nanoclusters owing to their novel optical, electronic and structural features resulting from their unique size-dependent molecule-like properties.^1–8 Their physical and chemical properties make them suitable nanomaterials for a wide range of potential applications in catalysis,^9,10 biomedicine,^11–18 and nanoelectronics,^19–21 among others.^4,22 These aggregates exhibit rich structural diversity featuring a specific electronic shell structure,^23–28 providing the ability to achieve tailorable properties for further applications.^{10,11,18,29–35,36–40} In pursuit of such development, numerous experimental and theoretical research studies are currently underway towards the rationalization of their structure and stability.^{41,42,43–50,51–54} Such structures stand on an inner metallic core passivated by ligands,^33,55–58 where the electronic and structural characteristics of the core explain the stability of the overall cluster.^59–62

The 8-ve closed-shell Au₁₃ icosahedron is one of the most recurring motifs, as observed in the earlier [Au₁₃Cl₂(dppe)₅]³⁺ cluster characterized by Mingos and coworkers,⁶³ and in the last decade for [Au₂₅(SR)₁₈]⁻.^40,64–66 [Au₁₃Cl₂(dppe)₅]³⁺ (1) exhibits interesting optical, luminescence and chiroptical properties as accounted for in separate reports by Konishi and Li research groups,^67,68 in comparison with Au₉, Au₁₁Cl₂ and Au₅₅Cl₆ phosphine-protected clusters,^69–71 owing to the use of the 1,2-bis(diphenylphosphino)ethane (dppe) ligand as the protecting ligand. 1 possesses singlet-oxygen (¹O₂) photogeneration capabilities from its lower triplet state (T₁), with higher quantum yields than organic dyes and related clusters useful for polymer science to biomedical applications.^72,73

The recent introduction of N-heterocyclic carbenes (NHC) as protecting ligands in [C(AuNHC)₆]²⁺ and [Au₁₃(NHC)₉Cl₃]²⁺ clusters by Shionoya and Crudden groups,^74,75 and explored for Au₁₃Cl₂ and Au₂₅Cl₂ cores,⁷⁶ precludes further development of more versatile ligands owing to the NHC characteristics ranging from strong to weak donor ligands according to the Tolman electronic parameter (TEP).^77–81 Hence, the incorporation of electron-donor or -withdrawing NHC ligands may strongly influence the molecular properties by tailoring the electronic structure via ligand engineering efforts.^82–91

Such features trigger further evaluation and modification of the ligand decorated clusters, by introducing NHC ligand counterparts of the parent dppe ligand in 1, offering a modification of its structural, chiroptical, luminescence and excited state properties. Herein, we explore a proposed series of 8-ve Au₁₃Cl₂ cluster-core derivatives involving linked NHC moieties related to dppe, ranging from strong to weak σ-donor ligands, resulting in a chiral arrangement of the ligand-protecting shell related to dppe, which enables the evaluation of their electronic, optical and chiroptical properties. In addition, excited state properties were obtained in order to account for their luminescence behavior in relation to the characterized T₁ → S₀ emission for 1,⁶⁸ to explore the tenability owing to the nature of bridged NHC derivatives.

Computational details

All calculations were carried out at the relativistic density functional theory level of theory⁹² by using the ADF code,⁹³ incorporating scalar corrections via the ZORA Hamiltonian.⁹⁴ We employed the triple-ξ Slater basis set, plus two polarization functions (STO-TZ2P) for valence electrons, within the generalized gradient approximation (GGA) according to the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional^95,96 because of its reliable performance at reasonable computational cost for similar clusters, allowing a direct comparison with other computational studies of medium-sized gold nanoclusters.^97–101

The use of the PBE-GGA functional provides accurate results for both structural and optical properties of gold nanoclusters allowing the use of realistic ligands or minimal ligand simplifications,^60–65 as denoted by Muniz-Miranda and coworkers,¹⁰¹ when comparing performance between GGA and hybrid functionals. For comparison the calculated HOMO–LUMO gap for [Au₁₃Cl₂(dppe)₅]³⁺ amounts to 1.876 eV at the GGA-PBE level, which agrees with the experimentally determined gap of 1.9 eV.⁶⁷ The gap obtained at the hybrid-PBE0 level amounts to 3.235 eV (ESI, Table S1‡).

The frozen core approximation was applied to the [1s²–4f¹⁴] shells for Au, [1s²] for C, and [2s²] for P and Cl, leaving the remaining electrons to be treated variationally. Geometry optimizations of both ground and related excited states were performed without any symmetry restraint, via the analytical energy gradient method implemented by Versluis and Ziegler.¹⁰² An energy convergence criterion of 10⁻⁴ hartree, gradient convergence criteria of 10⁻³ hartree per Å and radial convergence criteria of 10⁻² Å were employed for the evaluation of the relaxed structures. Electronic excitation energies were calculated via TD-DFT considering the van Leeuwen–Baerends (LB94) xc-functional as in previous similar studies,^101,103,104 which offers more accurate values because of its correct asymptotic behavior.¹⁰⁵ The current approach has been tested on the description of dual photoluminescence properties of a ligand-supported hexanuclear Au(I) framework, with good agreement with experimental emission and absorption energies.¹⁰⁶ Solvent and counterion effects were considered by using the conductor-like screening model^107,108 (COSMO) with acetonitrile as the solvent.

Results and discussion

Selected bond lengths for [Au₁₃Cl₂(dppe)₅]³⁺ (1) and a related 1,2-bis(N-isopropylimidazolidene) ethane derivative, namely, [Au₁₃Cl₂((IiPrNHC)₂Et)₅]³⁺ (2), are given in Table 1. The resulting relaxed structures (Fig. 1) exhibit typical Au–Au distances related to ligand-protected gold clusters with an average distance between Au_(staple)–Au_(center) of 2.622 Å for 1, which compares well with the experimental data available, 2.552 Å,⁶⁷ with an increase to 2.697 Å when 1,2-bis(N-isopropylimidazolidene)ethane is introduced as a ligand, as calculated for 2. The calculated Au_(Cl)–Au_(Cl) distance increases from 5.096 to 5.299 Å, denoting an expansion of the icosahedral cage, and similarly, the Au–Cl distances vary from 2.201 to 2.249 Å, respectively.

Fig. 1 Side (left) and top (right) views of [Au₁₃Cl₂(dppe)₅]³⁺ (1) and [Au₁₃Cl₂((IiPrNHC)₂Et)₅]³⁺ (2). Color code: Au, yellow; Cl, green; P, orange; N, blue; C, black; H, grey.

Table 1 Structural parameters obtained for [Au₁₃Cl₂(L)₅]³⁺ involving different ligands (L) along the 1–7 series

	1	1 exp.^a	2	3	4	5	6	7
a Experimental values from ref. 67.
Au–Au	2.622	2.552	2.697	2.742	2.704	2.700	2.696	2.729
Au–Au(L)	2.637	2.559	2.706	2.771	2.715	2.710	2.705	2.740
Au–Au(Cl)	2.548	2.486	2.650	2.597	2.649	2.650	2.654	2.675
Au(Cl)–Au(Cl)	5.096	4.974	5.299	5.195	5.298	5.300	5.309	5.351
Au–L	2.185	2.071	1.961	2.075	1.966	1.958	1.950	1.964
Au–Cl	2.201	2.169	2.249	2.267	2.246	2.237	2.217	2.246

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The possibility to achieve carbene ligands with characteristics ranging from strong to weak σ-donor capabilities with different π-acceptor strengths is an advantageous approach to modulate the ligand-protecting layer and tailoring molecular properties. The different σ-donor and π-acceptor character of NHC ligands can be evaluated experimentally by the Tolman electronic parameter (TEP),⁷⁷ which denotes acyclic diaminocarbene derivatives (ADC, cluster 3, Fig. 2) in the region of strong σ-donor ligands (TEP = 2037 cm⁻¹) with similar steric hindrance as in cluster 2 (TEP = 2050 cm⁻¹). The benzoimidazole derivative (1,2-bis(N-isopropylbenzoimidazolidene) ethane) leads to cluster 4, located in the weak donor region (TEP = 2054 cm⁻¹) with increased π-acceptor capabilities which can account for such NHC-ligand characteristics,¹⁰⁹ with a related imidazopyrazine-derivative (ImPy) resulting in cluster 5. Lastly, the oxalamide-based NHC ligand (NHC-O, for short) is included to account for a weaker donor character in cluster 6, with a TEP value of 2069 cm⁻¹, resulting in a higher π-acceptor character,^77,109 above 4 and 5. Such ligands lead to the series of ligand-protected clusters, with ligand-donor capabilities in the following order 3 > 4 > 5 > 6, with isopropyl groups as terminal side arms (attached to N). 2 is an intermediate situation between 3 and 4.

Fig. 2 Structures of calculated [Au₁₃Cl₂((NHC)₂Et)₅]³⁺ and [Au₁₃Cl₂((NHC)₂Me)₅]³⁺ derivatives (2–7), and their protecting bridged ligands. Color code: Au, yellow; Cl, green; P, orange; O, red; N, blue; C, black; H, grey.

Structurally, from 3 to 6, the average Au_(staple)–Au_(center) distance decreases from 2.742 to 2.696 Å, where the Au_(center)–Au_(NHC) distances decrease and Au_(center)–Au_(Cl) increases, leading to an increase of the Au_(Cl)–Au_(Cl) separation. The Au–NHC distances decrease from 2.075 to 1.950 Å from strong to weak donor ligand capabilities. In addition, the effect of shortening the bridge from two to one carbon atom is evaluated by introducing 1,2-bis(N-isopropylimidazolidene) methane, leading to cluster 7 which exhibits a ∼D_5h Au₁₃Cl₂ core (eclipsed) in contrast to the related cluster 2 with slightly larger averaged Au_(staple)–Au_(center) and Au_(Cl)–Au_(Cl) separations, retaining similar Au-NHC distances.

Such species can be formally viewed as 8-ve clusters, denoting a 1S²1P⁶ electronic structure according to the superatom model, where the 1P⁶ shell remains as the highest-occupied orbital (HOMO) split into two subshells (1P_z²1P_x,y⁴) owing to their symmetry (Fig. 3).^33,110,111 For the parent [Au₁₃Cl₂(dppe)₅]³⁺ (1) cluster, the first low-lying unoccupied levels (LUMO) account for the 1D shell followed by ligand centered orbitals of a π* character. Such features remain similar for cluster 2 (ESI, Fig. S1‡), denoting a related situation between phosphine and NHC-based ligands with a HOMO–LUMO gap of 1.876 and 1.901 eV, respectively, which is also observed for 3 with a gap of 1.909 eV. The calculated HOMO–LUMO gap for the parent cluster 1 is in agreement to the reports by Li and coworkers, denoting an experimental HOMO–LUMO gap of 1.9 eV obtained from ultraviolet photoemission spectroscopy and the measured optical gap.⁶⁷ Next, 7 exhibits a similar electronic structure to 2, denoting a smaller gap of 1.540 eV. Interestingly, for ligands with weak σ-donor and strong π-acceptor capabilities from 4 to 6, the π*-ligand centered levels are located between the frontier 1P and 1D shells resulting in noteworthy HOMO–LUMO gap decrease from 1.731, 1.201 and 0.380 eV, respectively. Thus, the small frontier orbital gap induced along the series can be an attractive target for further physical studies of unusual optoelectronic properties and multielectron-transfer phenomena.^112,113

Fig. 3 Schematic representation of the frontier electronic structure for 1–7. Black levels denote 1P and 1D shells, grey levels account for ligand centered orbitals. Grey box denotes the 5d-Au block. Levels of 6, were shifted +1 eV for clarity.

Optical properties were evaluated to unravel characteristic patterns for different NHC-protected species, reflecting the versatility^114,115 introduced by different ligands in the protecting layer.^{51,60,116–118} For [Au₁₃Cl₂(dppe)₅]³⁺ (1), the characterized UV/vis spectra exhibit a peak at 488 nm with a moderate shoulder between 620 to 520 nm, and a larger peak at 383 nm.⁶⁷ The calculated spectrum (Fig. 4) exhibits three peaks at 590, 473 and 375 nm, in which the first two give rise to the peak with a moderate shoulder with a maximum at 488 nm, and the latter accounts for the larger peak at 383 nm, denoting a good agreement with the experiment supporting the current approach.⁶⁷ The first peak is given by 1P_x,y → π*-ligand manifold transitions, followed by 1P_z → 1D transitions for the second peak, and 5d-Au → 1D transitions for the third peak. For the related [Au₁₃Cl₂((IiPrNHC)₂Et)₅]³⁺ (2) counterpart, the first peak is calculated at 625 nm denoting a red-shift in comparison with 1, owing to the stabilization of the π*-ligand manifold for NHC derivatives, while the second and third peaks are blue-shifted, at 469 and 336 nm, respectively. Thus, 2 retains similar optical absorption properties to its parent cluster 1.

Fig. 4 Calculated UV/vis within the 850–320 nm range for the studied species. Gaussian broadening of 0.6 eV was employed for 1 and 2, to match the experimentally observed UV/vis spectrum for 1 (background grey line). A broadening of 0.2 eV was employed in other cases.

For stronger σ-donor species (cluster 3), the calculated optical spectrum exhibits a similar shape to 1 and 2, showing the first peak at 576 nm with subsequent peaks at 472 and 373 nm, where for the weaker σ-donor species (4 to 6), a noteworthy red-shift is found owing to the marked decrease in the 1P–π*-ligand gap. For 4, four peaks are present at 690, 614, 517 and 380 nm, the first two peaks are related to 1P_x,y → π*-ligand transitions, while the third peak is related to 1P_z → 1D transitions. In 5, the 1P_x,y → π*-ligand transitions are now observed as three peaks at 1001, 937 and 876 nm (ESI, Fig. S2‡), followed by the 1P_z → π*-ligand transition at 706 nm, and 1P_z → 1D transitions at 505 and 442 nm, denoting the red-shift of the lower energy peaks. For 6, the observed patterns in the UV/vis spectrum are highly red-shifted according to the lower HOMO–LUMO gap found between 1P_x,y and π*-ligand shells (see above), with relevant peaks at 1830 and 1298 nm, besides the related 1P_x,y → 1D and 1P_z → 1D transitions at 640, 492 and 423 nm. Far infrared range patterns are given in the ESI.‡

Such features exhibit the strong tuning capabilities of introducing NHC-ligand derivatives along the σ-donor range guided in principle by TEP values, which introduces a noteworthy red-shift for the low-energy peaks of Au₁₃Cl₂ core superatoms, owing to their 1P → π*-ligand character. This shows enhanced tunable capabilities in comparison with the related optical properties of thiolate-protected Au₂₅(SR)₁₈ superatoms when different substituents are introduced via SPhX ligands according to the Hammet parameter of –X, denoting that the frontier-orbital gap varies to a small extent, leading to an almost unaffected optical absorption spectrum.¹¹⁸

Furthermore, the related photoluminescence properties of cluster 1,^67,68 which provide singlet-oxygen sensitization applications due to the related excited triplet-state (T₁)⁶⁷ are explored for the NHC series. The emission process is related to the T₁ → S₀ decay⁶⁷ which is characterized to originate upon excitation from two bands at 490 and 360 nm (as given by its excitation spectrum in acetonitrile)⁶⁸ of 1P → 1D and 5d-Au → 1D character (Fig. 5). The calculated T₁ state exhibits interesting geometrical changes denoted by the increase of the Au(Cl)-Au(Cl) distance from 5.069 to 5.184 Å (Fig. 5 and Table S2‡), leading to a T₁–S₀ emission calculated at 809 nm (1.533 eV) in line with what was measured experimentally in acetonitrile, 766 nm (1.619 eV), by Konishi and Shichibu.⁶⁸ Such an emission is solvent dependent owing to the different molecular packing obtained between different solvents, as depicted by the emission at 982 nm (1.263 eV) in dichloromethane.⁶⁷ This observation shows that molecular aggregation is lower in MeCN than CH₂Cl₂, which can be further explored as a useful approach to induced emission band variation.

Fig. 5 Variation of Au(Cl)–Au(Cl) at the emissive excited T₁ state. Color code: Au, yellow; Cl, green; P, orange; N, blue; C, black; H, grey.

For the related NHC derivative, [Au₁₃Cl₂(IiPr)₅]³⁺ (2), the T₁-geometry features greater Au(Cl)–Au(Cl) elongation from 5.299 to 5.711 Å (Δ_{Au(Cl)–Au(Cl)} = 0.412 Å), which is larger than that calculated for 1 (Δ_{Au(Cl)–Au(Cl)} = 0.088 Å). The obtained T₁ → S₀ emission is located at 1015 nm (1.221 eV), which despite the similar ground-state electronic structure and the larger geometrical distortion at T₁, leads to a decrease in the 1D–1P gap, resulting in an emission red-shift, as expected from our calculations. For 3, the similar frontier orbital characteristics to 1 result in a similar T₁ → S₀ emission at 875 nm (1.417 eV) with slight structural modifications at the T₁ excited state. Moreover, for the 4–6 series, the calculated emission is related to the decrease of the HOMO–LUMO gap at the ground state (S₀), with values of 0.682, 0.686 and 0.342 eV, respectively, which can be interesting far-infrared emitter species to probe experimentally, with similar excitation wavelengths to 1 owing to the similar 1P → 1D transition values.

For 7, the effect of bridge shortening from –CH₂CH₂– to –CH₂– results in a lower HOMO–LUMO gap (1.540 eV) in comparison with 2 (1.901 eV), which leads to a red-shift of the first peaks related to 1P → 1D and 5d-Au → 1D transitions (725 and 645 nm). The T₁ → S₀ emission is calculated at 0.821 eV (1510 nm) and exhibits a ∼D_5h-/∼D_5d-Au₁₃Cl₂ core transformation, which can be an interesting feature to explore further light-driven molecular motors (ESI, Fig. S3‡).¹¹⁹

Lastly, the circular dichroism (CD) spectrum is evaluated owing to the chiral arrangement of the protecting ligands supporting the achiral Au₁₃Cl₂ core, which results from the use of dppe (1), and related bridged NHC derivatives (2–7). All the studied species exhibit optical activity denoting the effective chiral pattern constructed by the protecting units (Fig. 6). The calculated CD for the right-handed enantiomer of 1 shows similar patterns to that observed experimentally for enantiomer 2 by Li and coworkers,⁶⁷ with a positive peak at 560 nm (exp.: 563 nm), followed by a negative peak at 462 nm (exp.: 472 nm), and a last signal as a positive peak at 345 nm (exp.: 386 nm). For 2, two negative peaks are expected at 578 and 462 nm, before a positive peak at 331 nm, in line with the trend observed for the UV/vis (see above). For the series from 3 to 7, the variation of the CD patterns is related to the red-shift variation observed for the 1P → π*-ligand transitions, where it is particularly modified for 5, 6 and 7, denoting the higher tuning capabilities introduced by metal-to-ligand centered transitions.

Fig. 6 Calculated circular dichroism spectra or right-handed isomers for 1–7. Gaussian broadening of 0.6 eV was employed for 1 and 2, to match the experimentally observed CD for 1 (background grey line), a broadening of 0.2 eV was employed in other cases. Values between ±500 × 10⁻⁴⁰ esu² cm². Lower energy peaks for 5 and 6 are shown in ESI Fig. S4.‡

Conclusions

In summary, the role of different NHC-linked protecting ligands ranging from strong to weak σ-donors according to TPE values was evaluated on the structural, optical, chiroptical and emission properties of the Au₁₃Cl₂ core, derived from the [Au₁₃Cl₂(dppe)₅]³⁺ nanocluster. Our findings provide a useful design principle for efficient tuning by selected NHC ligands resulting in a noteworthy variation of electronic properties, and thus, modification of related cluster characteristics. A comparison between [Au₁₃Cl₂(dppe)₅]³⁺ and [Au₁₃Cl₂((IiPrNHC)₂Et)₅]³⁺ reveals similar characteristics of related 8-ve superatoms, denoting a red-shift for the lower energy peaks in the optical absorption spectrum ascribed to 1P_x,y → π*-ligand manifold transitions, owing to the stabilization of the π*-ligand levels for the later. Also, a blue-shift is observed for 1P_z → 1D and 5d-Au → 1D transitions. The calculated emission peaks denote a red-shift from 809 to 1015 nm, respectively, owing to a larger distortion at the emissive excited state for the later. From 3 to 6, it is shown that weaker ligand-donor capabilities significantly decrease the frontier orbital gap with a change in the 1P–1D to a 1P–π*-ligand character. Lastly, the effect of shortening the bridge between linked NHC in [Au₁₃Cl₂((IiPrNHC)₂Me)₅]³⁺ results in a ∼D_5h eclipsed Au₁₃Cl₂ core, in contrast to [Au₁₃Cl₂((IiPrNHC)₂Et)₅]³⁺, denoting interesting structural changes in the eclipsed ↔ staggered Au₁₃Cl₂ core unraveling the potential to convert light energy into mechanical work.

Such a ligand-tailored behavior of [Au₁₃Cl₂(NHC)₅]³⁺ derivatives appears as an addition to the useful strategies to reduce HOMO–LUMO separation, which underlies design rules towards novel clusters for building blocks of nanostructured materials, for optical, chiroptical and structural modification tuning towards further applications of superatomic clusters.

Author contributions

The manuscript was written through the contributions of all authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by FONDECYT 1180683.

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Footnotes

† This work is dedicated to Professor Jean-François Halet on the occasion of his 60th Birthday.

‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/C9QI00513G

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