Why Do Silver Trimers Intercalated in DNA Exhibit Unique Nonlinear Properties That Are Promising for Applications?

Our investigation of one-photon absorption (OPA) and nonlinear optical (NLO) properties such as two-photon absorption (TPA) of silver trimer intercalated in DNA based on TDDFT approach allowed us to propose a mechanism responsible for large TPA cross sections of such NLO-phores. We present a concept that illustrates the key role of quantum cluster as well as of nucleotide bases from the immediate neighborhood. For this purpose, different surroundings consisting of guanine–cytosine and adenine–thymine such as (GCGC) and (ATAT) have been investigated that are exhibiting substantially different values of TPA cross sections. This has been confirmed by extending the immediate surroundings as well as using the two-layer quantum mechanics/molecular mechanics (QM/MM) approach. We focus on the cationic closed-shell system and illustrate that the neutral open-shell system shifts OPA spectra into the NIR regime, which is suitable for applications. Thus, in this contribution, we propose novel NLO-phores inducing large TPA cross sections, opening the route for multiphoton imaging

Tuning Structural and Optical Properties of Thiolate-Protected Silver Clusters by Formation of a Silver Core with Confined Electrons

We present a systematic theoretical investigation of the structural and optical properties of thiolate-protected silver clusters with the goal to design species exhibiting strong absorption and fluorescence in the UV–vis spectral range. We show that the optical properties can be tuned by creating systems with different counts of confined electrons within the cluster core. We consider liganded silver complexes with <i>n</i> silver atoms (Ag_<i>n</i>) and <i>x</i> ligands (L_<i>x</i>) in anionic complexes [Ag_<i>n</i>L_<i>x</i>]⁻ with L = SCH₃. Variation of the composition ratio gives rise to systems with (i) zero confined electrons for <i>x</i> = <i>n</i> + 1, (ii) two confined electrons for <i>x</i> = <i>n</i> – 1, and (iii) four confined electrons for <i>x</i> = <i>n</i> – 3. We show that the number of confined electrons within the cluster core and the geometric structure of the latter are responsible for the spectral patterns, giving rise to intense absorption transitions and fluorescence in the visible or even infrared range. Our results open a perspective for the rational design of stable ligand-protected silver cluster chromophores that might find numerous applications in the field of biosensing

Au₁₀(SG)₁₀: A Chiral Gold Catenane Nanocluster with Zero Confined Electrons. Optical Properties and First-Principles Theoretical Analysis

We report facile synthesis of the Au₁₀(SG)₁₀ nanoclusters, where SG stands for glutathione, found to be promising as a new class of radiosensitizers for cancer radiotherapy. The homoleptic catenane structure with two Au₅SG₅ interconnected rings, among different isomer structures, gives the best agreement between theoretical and experimental optical spectra and XRD patterns. This catenane structure exhibits a centrosymmetry-broken structure, resulting in enhanced second harmonic response and new characteristic circular dichroism signals in the spectral region of 250–400 nm. This is the first determination of the nonlinear optical properties of a ligated cluster with an equal Au-to-ligand ratio, thus without a metallic core and therefore zero confined electrons. Insight into the nonlinear and chiroptical efficiencies arising from interplay between structural and electronic properties is provided by the TD-DFT approach

Gas-Phase Structural and Optical Properties of Homo- and Heterobimetallic Rhombic Dodecahedral Nanoclusters [Ag_{14–<i>n</i>}Cu_<i>n</i>(CC<i>t</i>Bu)₁₂X]⁺ (X = Cl and Br): Ion Mobility, VUV and UV Spectroscopy, and DFT Calculations

The rhombic dodecahedral nanocluster [Ag₁₄(CC<i>t</i>Bu)₁₂Cl]⁺, which has been structurally characterized using X-ray crystallography, was transferred to the gas phase using electrospray ionization, where it was characterized by ion mobility (IM), vacuum ultraviolet (VUV), and ultraviolet (UV) spectroscopies in conjunction with DFT calculations. IM reveals a single peak, and modeling of the collision cross-section with the X-ray structure suggests that the cluster maintains its condensed phase structure upon transfer to the gas phase. The VUV spectra exhibit rich fragmentation, including: photoionization to give [Ag₁₄(CC<i>t</i>Bu)₁₂Cl]^2+• with an onset of 8.84 ± 0.08 eV; cluster fission fragmentation via losses of (AgCC<i>t</i>Bu)_<i>n</i> and (AgCC<i>t</i>Bu)_<i>n</i>−1(AgCl); and via reductive elimination of (<i>t</i>BuCC)₂. Apart from channels associated with photoionization, similar fragment ions are observed in the UVPD spectra, although their relative intensities differ. The TDDFT absorption spectra are symmetry-allowed transitions including A_u → A_g, E_u → A_g, and E_u → E_g irreducible representations. Comparing the collision cross-sections with the X-ray structures for the related clusters [Ag₈Cu₆(CC<i>t</i>Bu)₁₂Cl]⁺, [Ag₁₄(CC<i>t</i>Bu)₁₂Br]⁺, and [Ag₈Cu₆(CC<i>t</i>Bu)₁₂Br]⁺ suggests that they maintain their condensed-phase structures in the gas phase. The VUV spectra of [Ag₈Cu₆(CC<i>t</i>Bu)₁₂Cl]⁺ and [Ag₁₄(CC<i>t</i>Bu)₁₂Br]⁺ exhibit similar fragmentation channels and ionization onsets (8.86 ± 0.03 and 8.86 ± 0.05, respectively) compared with [Ag₁₄(CC<i>t</i>Bu)₁₂Cl]⁺