Chiral and Achiral Crystal Structures of Au25 (PET)18 0 Reveal Effects of Ligand Rotational Isomerization on Optoelectronic Properties

The crystal structures of 4 ligand-rotational isomers of Au25 (PET)18 are presented. Two new ligand-rotational isomers are revealed, and two higher-quality structures (allowing complete solution of the ligand shell) of previously solved Au25 (PET)18 clusters are also presented. One of the structures lacks an inversion center, making it the first chiral Au25 (SR)18 structure solved. These structures combined with previously published Au25 (SR)18 structures enable an analysis of the empirical ligand conformation landscape for Au25 (SR)18 clusters. This analysis shows that the dihedral angles within the PET ligand are restricted to certain observable values, and also that the dihedral angle values are interdependent, in a manner reminiscent of biomolecule dihedral angles such as those in proteins and DNA. The influence of ligand conformational isomerism on optical and electronic properties was calculated, revealing that the ligand conformations affect the nanocluster absorption spectrum, which potentially provides a way to distinguish between isomers at low temperature.

Metal nanoclusters: from fundamental aspects to electronic properties and optical applications

Monolayer-protected noble metal clusters, also called nanoclusters, can be produced with the atomic precision and in large-scale quantity and are playing an increasingly important role in the field of nanoscience. To outline the origin and the perspectives of this new field, we overview the main results obtained on free metal clusters produced in gas phase including mainly electronic properties, the giant atom concept, the optical properties, briefly the role of the metal atom (alkali, divalent, noble metal) and finally the atomic structure of clusters. We also discuss the limitations of the free clusters. Then, we describe the field of monolayer-protected metal clusters, the main results, the new offered perspectives, the added complexity, and the role of the ligand beyond the superatom concept.

Origins of the pH-Responsive Photoluminescence of Peptide-Functionalized Au Nanoclusters

Ultrasmall peptide-protected gold nanoclusters are a promising class of bioresponsive material exhibiting pH-sensitive photoluminescence. We present a theoretical insight into the effect peptide-ligand environment has on pH-responsive fluorescence, with the aim of enhancing the rational design of gold nanoclusters for bioapplications. Employing a hybrid quantum/classical computational methodology, we systematically calculate deprotonation free energies of N-terminal cysteine amine groups in proximity to the inherently fluorescent core of Au₂₅(Peptide)₁₈ nanoclusters. We find that subtle changes in hexapeptide sequence alter the electrostatic environment and significantly shift the conventional N-terminal amine pK_a expected for amino acids free-in-solution. Our findings provide an insight into how the deprotonation equilibrium of N-terminal amine and side chain carboxyl groups cooperatively respond to solution pH changes, explaining the experimentally observed, yet elusive, pH-responsive fluorescence of peptide-functionalized Au₂₅ clusters.

Structural Prediction of Anion Thiolate Protected Gold Clusters of [Au28+7n(SR)17+3n]− (n = 0-4)

Structural prediction of thiolate-protected gold nanocluster (AuNCs) with diverse charge states can enrich the understanding of this species. Till now, most expementally synthesized or theoretically predicted AuNCs structures own neutral total charge. In this work, a series of gold nanoclusters with negative total charge including [Au28(SR)17]−, [Au35(SR)20]−, [Au42(SR)23]−, [Au49(SR)26]−, and [Au56(SR)29]− are designed. Following crystallized [Au23(SR)16]- prototype structure, the inner core of the newly predicted clusters are obtained through packing crossed Au7. Next, proper protecting thiolate ligands are arranged to fullfil the duet rule to obtain Au3(2e) and Au4(2e). Extensive analysis indicates these cluster own high stabilities. Molecular orbital analysis shows that the orbitals for the populations of the valence electron locate at each Au3(2e) and Au4(2e), which demonstrates the reliability the GUM model. This work should be helpful for enriching the structural diversity of AuNCs.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

Effects of ligands on (de-)enhancement of plasmonic excitations of silver, gold and bimetallic nanoclusters: TD-DFT+TB calculations

Metal nanoclusters can be synthesized in various sizes and shapes and are typically protected with ligands to stabilize them. These ligands can also be used to tune the plasmonic properties of the clusters as the absorption spectrum of a protected cluster can be significantly altered compared to the bare cluster. In this paper, we computationally investigate the influence of thiolate ligands on the plasmonic intensity for silver, gold and alloy clusters. Using time-dependent density functional theory with tight-binding approximations, TD-DFT+TB, we show that this level of theory can reproduce the broad experimental spectra of Au₁₄₄(SR)₆₀ and Ag₅₃Au₉₁(SR)₆₀ (R = CH₃) compounds with satisfactory agreement. As TD-DFT+TB does not depend on atom-type parameters we were able to apply this approach on large ligand-protected clusters with various compositions. With these calculations we predict that the effect of ligands on the absorption can be a quenching as well as an enhancement. We furthermore show that it is possible to unambiguously identify the plasmonic peaks by the scaled Coulomb kernel technique and explain the influence of ligands on the intensity (de-)enhancement by analyzing the plasmonic excitations in terms of the dominant orbital contributions.

Toward quantitative electronic structure in small gold nanoclusters

Ligand-protected gold nanoclusters (AuNCs) feature a dense but finite electronic structure that can be rationalized using qualitative descriptions such as the well-known superatomic model and predicted using quantum chemical calculations. However, the lack of well-resolved experimental probes of a AuNC electronic structure has made the task of evaluating the accuracy of electronic structure descriptions challenging. We compare electronic absorption spectra computed using time-dependent density functional theory to recently collected high resolution experimental spectra of Au₉(PPh₃)₈ ³⁺ and Au₈(PPh₃)₇ ²⁺ AuNCs with strikingly similar features. After applying a simple scaling correction, the computed spectrum of Au₈(PPh₃)₇ ²⁺ yields a suitable match, allowing us to assign low-energy metal-metal transitions in the experimental spectrum. No similar match is obtained after following the same procedure for two previously reported isomers for Au₉(PPh₃)₈ ³⁺, suggesting either a deficiency in the calculations or the presence of an additional isomer. Instead, we propose assignments for Au₉(PPh₃)₈ ³⁺ based off of similarities Au₈(PPh₃)₇ ²⁺. We further model these clusters using a simple particle-in-a-box analysis for an asymmetrical ellipsoidal superatomic core, which allows us to reproduce the same transitions and extract an effective core size and shape that agrees well with that expected from crystal structures. This suggests that the superatomic model, which is typically employed to explain the qualitative features of nanocluster electronic structures, remains valid even for small AuNCs with highly aspherical cores.

Nonradiative relaxation dynamics in the [Au25-nAgn(SH)18]-1 (n = 1, 12, 25) thiolate-protected nanoclusters

Evaluation of the electron-nuclear dynamics and relaxation mechanisms of gold and silver nanoclusters and their alloys is important for future photocatalytic, light harvesting, and photoluminescence applications of these systems. In this work, the effect of silver doping on the nonradiative excited state relaxation dynamics of the atomically precise thiolate-protected gold nanocluster [Au_25-nAg_n(SH)₁₈]^-1 (n = 1, 12, 25) is studied theoretically. Time-dependent density functional theory is used to study excited states lying in the energy range 0.0-2.5 eV. The fewest switches surface hopping method with decoherence correction was used to investigate the dynamics of these states. The HOMO-LUMO gap increases significantly upon doping of 12 silver atoms but decreases for the pure silver nanocluster. Doped clusters show a different response for ground state population increase lifetimes and excited state population decay times in comparison to the undoped system. The ground state recovery times of the S₁-S₆ states in the first excited peak were found to be longer for [Au₁₃Ag₁₂(SH)₁₈]^-1 than the corresponding recovery times of other studied nanoclusters, suggesting that this partially doped nanocluster is best for preserving electrons in an excited state. The decay time constants were in the range of 2.0-20 ps for the six lowest energy excited states. Among the higher excited states, S₇ has the slowest decay time constant although it occurs more quickly than S₁ decay. Overall, these clusters follow common decay time constant trends and relaxation mechanisms due to the similarities in their electronic structures.

Towards elucidating structure of ligand-protected nanoclusters

Developing a centralized database for ligand-protected nanoclusters can fuel machine learning and data-science-based approaches towards theoretical structure prediction.