DFT and TD-DFT Assessment of the Structural and Optoelectronic Properties of an Organic–Ag14 Nanocluster

A Theoretical Benchmark of the Geometric and Optical Properties for 3d Transition Metal Nanoclusters via Density Functional Theory

Understanding structure-property relationships in atomically precise metal nanoclusters is vital in finding selective and tunable catalysts. In this study, density functional theory (DFT) was used to benchmark seven exchange correlation functionals at different basis sets for 17 atomically precise nanoclusters against experimentally determined geometries, band gaps, and optical gaps. The set contains both monometallic and bimetallic clusters that possess at least two types of 3d transition metals (specifically, Cu, Ni, Fe, or Co). The benchmark highlights that PBE0 is a good functional to use regardless of the basis set, and Minnesota functionals do well with respect to specific metals. Further, while long-range corrected functionals overestimate band and optical gaps, they model absorption features better than the other considered functionals. The study additionally looks at the photoinduced hydrogen evolution reaction (HER) and the CO2 reduction mechanism on nanoclusters reported from the literature.

Iron nuclearity in mineral fibres: Unravelling the catalytic activity for predictive modelling of toxicity

Chronic inflammation induced in vivo by mineral fibres, such as asbestos, is sustained by the cyclic formation of cytotoxic/genotoxic oxidant species that are catalysed by iron. High catalytic activity is observed when iron atoms are isolated in the crystal lattice (nuclearity=1), whereas the catalytic activity is expected to be reduced or null when iron forms clusters of higher nuclearity. This study presents a novel approach for systematically measuring iron nuclearity across a large range of iron-containing standards and mineral fibres of social and economic importance, and for quantitatively assessing the relation between nuclearity and toxicity. The multivariate curve resolution (MCR) empirical approach and density functional theory (DFT) calculations were applied to the analysis of UV-Vis spectra to obtain information on the nature of iron and nuclearity. This approach led to the determination of the nuclearity of selected mineral fibres which was subsequently used to calculate a toxicity-related index. High nuclearity-related toxicity was estimated for chrysotile samples, fibrous glaucophane, asbestos tremolite, and fibrous wollastonite. Intermediate values of toxicity, corresponding to a mean nuclearity of 2, were assigned to actinolite asbestos, amosite, and crocidolite. Finally, a low nuclearity-related toxicity parameter, corresponding to an iron-cluster with a lower catalytic power to produce oxidants, was assigned to asbestos anthophyllite.

SERS-Based Sensor Using Subnanometric Copper-Silver Mixed Clusters Ag(8-n)Cun (n = 0-8) for Pyridine: A DFT Study

Surface-enhanced Raman spectroscopy (SERS) is a powerful Raman technique that provides high selectivity and sensitivity in analyzing the intermolecular interaction of a target compound adsorbed on the surface of a noble nanomaterial, i.e., silver, gold, or copper. Although copper presents a better SERS enhancement than gold and silver, its oxidation in the air is much easier than that of gold and silver. A mixed material between these metals may potentially improve the SERS signal enhancement in this context. In this work, we evaluated the SERS spectra of pyridine (Py) adsorbed on the copper-silver mixed clusters Ag(8-n)Cun (n = 0-8) using density functional theory (DFT) at the PBE functional. The cc-pVDZ-PP basis set was chosen for Ag and Cu, while the cc-pVDZ basis set was used for C, N, and H atoms. Geometrical and electronic structures of the mixed clusters and the Py adsorption configuration on these clusters were computed. The calculated SERS spectra then revealed the influence of the Ag/Cu mixing ratio on the SERS enhancement. As a result, the substituted copper atoms on the silver cluster turned out to be favorable adsorption sites for Py. Interestingly, when the number of Cu atoms increased from n = 0 (pure Ag8 cluster) to n = 5 (Ag3Cu5 cluster), the ring stretching peak (1590 cm-1) of Py significantly increased from 20 to 120 au and then saturated around this value despite increasing the Cu atom number to 8 (pure Cu8 cluster). This observation was extended for other ligands such as pyrazine and 3H-pyrrole. TD-DFT was then employed to clarify the chemical enhancement mechanism. The results obtained hopefully provide helpful information for the design of analytical sensors with lower costs.

Computational Approaches to the Electronic Properties of Noble Metal Nanoclusters Protected by Organic Ligands

Organometallic nanoparticles composed by metal cores with sizes under two nanometers covered with organic capping ligands exhibit intermediate properties between those of atoms and molecules on one side, and those of larger metal nanoparticles on the other. In fact, these particles do not show a peculiar metallic behavior, characterized by plasmon resonances, but instead they have nonvanishing band-gaps, more along molecular optical properties. As a consequence, they are suitable to be described and investigated by computational approaches such as those used in quantum chemistry, for instance those based on the time-dependent density functional theory (TD-DFT). Here, I present a short review of the research performed from 2014 onward at the University of Modena and Reggio Emilia (Italy) on the TD-DFT interpretation of the electronic spectra of different organic-protected gold and/or silver nanoclusters.

Electronic Structure of Superoxidized Radical Cationic Dodecaborate-Based Clusters

The expanding field of boron clusters has attracted continuous theoretical efforts to understand their diverse structures and unique bonding. We recently discovered a new reversible redox event of B₁₂(O-3-methylbutyl)₁₂ in which the superoxidized radical cationic form [B₁₂(O-3-methylbutyl)₁₂]^•+ was identified and isolated for the first time. Herein, comprehensive (TD-)DFT studies in tandem with electrochemical experiments were employed to demonstrate the generality of the reported behavior across perfunctionalized B₁₂(OR)₁₂ clusters (R = aryl or alkyl). While the spin density of radical cationic clusters is delocalized in the core region, the oxidation brings about notable gains of positive partial charges on the supporting groups whose electronics can readily tune the redox potential of the 0/•+ couple. The underlying changes of frontier orbitals were elucidated, and the resulting [B₁₂(OR)₁₂]^•+ species manifest a general diagnostic absorption as a consequence of mixed local/charge-transfer excitations.

Aggregation Effects on Pigment Coatings: Pigment Red 179 as a Case Study

Here, we have studied, with a combined experimental and computational approach, the effect of the crystal environment and aggregation on the electronic properties of Pigment Red 179, which affect both its color and optical energy gap. Spectra acquired in the near-infrared and visible range of energies suggest that this molecule is indeed a "cool" dye, which can be employed as a red pigment that provides effective color coverage to different substrates without contributing to their heating during light irradiation. Spectra acquired on different polymer mixtures at different pigment concentrations (i.e., 2.5-10 wt %) suggest that absorption features depend on chromophoric arrangements promoted by the strong intermolecular π-π interactions. Calculations, performed at the time-dependent density functional theory level, allowed to both attribute the nature of the electronic transitions causing the observed spectra involved and understand the effect of the environment. Indeed, the visible spectra of the pigment is dominated by two localized transitions, with negligible charge transfer for both a dye monomer and dimer either in vacuum or acetonitrile solution. Instead, models including the crystal environment of the pigment show the presence of a high-wavelength S1 ← S0 charge transfer transition between two adjacent molecules, in quantitative agreement with the experimental absorption energy of the crystal pigment.

Face-Centered-Cubic Ag Nanoclusters: Origins and Consequences of the High Structural Regularity Elucidated by Density Functional Theory Calculations

Face-centered-cubic (FCC) silver nanoclusters (NCs) adopting either cubic or half-cubic growth modes have been recently reported, but the origin of these atomic assembly patterns and how they are achieved, which would inform our understanding of larger FCC silver nanomaterials, are both unknown. In this study, the cubic and half-cubic growth modes have been unified based on common structural characteristics, and differentiated depending on the starting blocks (cubic vs. half cubic). In both categories, the silver atoms adopt octahedral Ag₆ , linear AgS₂ (in projection drawing), or tetrahedral AgS₃ P binding modes, and the sulfur atoms adopt T-shaped SAg₃ and orthogonal SAg₄ modes. An additional T-shaped AgS₃ mode is oriented on the surface edge in cubic NCs to complete the cubic framework. Density functional theory calculations indicated that the high structural regularity originates from the strong diffusing capacity of the Ag(5d) and S(3p) orbitals, and the angular momentum distribution of the formed superatomic orbitals. The equatorial orientation of μ⁴ -S or μ⁴ -Ag determines whether growth stops or continues. In particular, a density-of-states analysis indicated that the octahedral silver atoms are chemically more reactive than the silver atoms in the AgS₃ P motif, regardless of whether the parent NC functions as an electron donor or acceptor.

Optical Properties of Isolated and Covalent Organic Framework-Embedded Ruthenium Complexes

Heterogenization of RuL₃ complexes on a support with proper anchor points provides a route toward design of green catalysts. In this paper, Ru(II) polypyridyl complexes are investigated with the aim to unravel the influence on the photocatalytic properties of varying nitrogen content in the ligands and of embedding the complex in a triazine-based covalent organic framework. To provide fundamental insight into the electronic mechanisms underlying this behavior, a computational study is performed. Both the ground and excited state properties of isolated and anchored ruthenium complexes are theoretically investigated by means of density functional theory and time-dependent density functional theory. Varying the ligands among 2,2′-bipyridine, 2,2′-bipyrimidine, and 2,2′-bipyrazine allows us to tune to a certain extent the optical gaps and the metal to ligand charge transfer excitations. Heterogenization of the complex within a CTF support has a significant effect on the nature and energy of the electronic transitions. The allowed transitions are significantly red-shifted toward the near IR region and involve transitions from states localized on the CTF toward ligands attached to the ruthenium. The study shows how variations in ligands and anchoring on proper supports allows us to increase the range of wavelengths that may be exploited for photocatalysis.

The Ligand-Exchange Reactions of Rod-Like Au25-n Mn (M=Au, Ag, Cu, Pd, Pt) Nanoclusters with Cysteine - A Density Functional Theory Study

The atomic precision of ultrasmall noble-metal nanoclusters (NMNs) is fundamental for elucidating structure-property relationships and probing their practical applications. So far, the atomic structure of NMNs protected by organic ligands has been widely elucidated, whereas the precise atomic structure of NMNs protected by water-soluble ligands (such as peptides and nucleic acid), has been rarely reported. With the concept of "precision to precision", density functional theory (DFT) calculations were performed to probe the thermodynamic plausibility and inherent determinants for synthesizing atomically precise, water-soluble NMNs via the framework-maintained two-phase ligand-exchange method. A series of rod-like Au_25-n M_n (M=Au, Ag, Cu, Pd, Pt) NMNs with the same framework but varied ligands and metal compositions was chosen as the modeling reactants, and cysteine was used as the modeling water-soluble ligand. It was found that the acidity of the reaction remarkably affects the thermodynamic facility of the ligand exchange reactions. Ligand effects (structural distortion and acidity) dominate the overall thermodynamic facility of the ligand-exchange reaction, while the number and type of doped metal atom(s) has little influence.

Bismuth as a versatile cation for luminescence in coordination polymers from BiX3/4,4'-bipy: understanding of photophysics by quantum chemical calculations and structural parallels to lanthanides

Coordination polymers (CPs) with bismuth(iii) as a connectivity centre have been prepared from BiX3 (X = Cl-I) and 4,4'-bipyridine (bipy) in order to implement Bi-based luminescence. The products were obtained via different synthetic routes such as solution chemistry, melt syntheses or mechanochemical reactions. Five neutral and anionic 1D-CPs are presented that show a chemical parallel to trivalent lanthanides forming isostructural or closely related 1D-CPs, of which five additional compounds are described. Bi3+ proves to be a versatile cation for luminescence resulting from energy transfer processes between a metal and a ligand in the presented CPs. Quantum chemical calculations were carried out to investigate Bi3+-participation in the luminescence processes. The calculated results allow an assignment of the bright transitions composed of mainly metal-to-ligand-charge transfer (MLCT) character. These results show that Bi3+ can form strongly luminescent coordination compounds with N-donor ligands.

TD-DFT Benchmark on Inorganic Pt(II) and Ir(III) Complexes

We report in the present paper a comprehensive investigation of representative Pt(II) and Ir(III) complexes with special reference to their one-photon absorption spectra employing methods rooted in density functional theory and its time dependent extension. We have compared nine different functionals ranging from generalized gradient approximation (GGA) to global or range-separated hybrids, and two different basis sets, including pseudopotentials for 4 iridium and 7 platinum complexes. It turns out that hybrid functionals with the same exchange part give comparable results irrespective of the specific correlation functional (i.e., B3LYP is very close to B3PW91 and PBE0 is very close to MPW1PW91). More recent functionals, such as CAM-B3LYP and M06-2X, overestimate excitation energies, whereas local functionals (BP86 -GGA-, M06-L -Meta GGA-) strongly underestimate transition energies with respect to experimental results. As expected, basis set effects are weak, and the use of a triple-ζ polarized (def2-TZVP) basis set does not significantly improve the computed excitation energies with respect to a classical double-ζ basis set (LANL2DZ) augmented by polarization functions, but it significantly raises the computational effort.

Raman and DFT study of methimazole chemisorbed on gold colloidal nanoparticles

The SERS/DFT study of methimazole chemisorbed on Au nanoparticles paves the way for the use of these nanohybrids in biomedicine.

Modeling the photosensitizing properties of thiolate-protected gold nanoclusters

An accurate computational strategy for studying the structural, redox and optical properties of thiolated gold nanoclusters (GNCs) using (time-dependent) density functional theory is proposed.

Heavily doped Au25–xAgx(SC6H11)18− nanoclusters: silver goes from the core to the surface

The Au₂₅(SR)₁₈ nanocluster (where R = c-C₆H₁₁) can be heavily doped with silver through Ag(i)–thiolate complex induced size/structure transformation of Au₂₃(SR)₁₆⁻ into Au_25–xAg_x(SR)₁₈⁻.