Spectroscopic Studies of “Exciplex Tuning” for Dicyanoaurate(I) Ions Doped in Potassium Chloride Crystals

Excimer and Exciplex Formation in Gold(I) Complexes Preconditioned by Aurophilic Interactions

Excimers and exciplexes are defined as assemblies of atoms or molecules A/A' where interatomic/intermolecular bonding appears only in excited states such as [A2 ]* (for excimers) and [AA']* (for exciplexes). Their formation has become widely known because of their role in gas-phase laser technologies, but their significance in general chemistry terms has been given little attention. Recent investigations in gold chemistry have opened up a new field of excimer and exciplex chemistry that relies largely on the preorganization of gold(I) compounds (electronic configuration AuI (5d10 )) through aurophilic contacts. In the corresponding excimers, a new type of Au⋅⋅⋅Au bonding arises, with bond energies and lengths approaching those of ground-state Au-Au bonds between metal atoms in the Au0 (5d10 6s1 ) and AuII (5d9 ) configurations. Excimer formation gives rise to a broad range of photophysical effects, for which some of the relaxation dynamics have recently been clarified. Excimers have also been shown to play an important role in photoredox binuclear gold catalysis.

Understanding the vapochromic response of mixed copper(i) iodide/silver(i) Iodide nanoparticles toward dimethyl sulfide

We report on the vapochromic behavior of a series of homo- and heterometallic copper(i) iodide/silver(i) iodide nanoparticles when exposed to dimethyl sulfide (DMS) vapor. These systems show remarkable colorimetric sensing behavior via emission color upon DMS exposure, shifting from pink to green emission. Kinetics measurements of CuI/AgI nanoparticle reactions with DMS show a significant rate increase with increasing Ag(i) content. However, luminescence spectroscopy and X-ray diffraction of the post-exposure samples with varying Ag(i) content reveal that the luminophore is identical in all cases and contains no Ag(i) ions. To rationalize the experimental observations and determine the vapochromic response mechanism, molecular dynamic calculations were performed on model (111) cation-terminated surfaces of copper iodide crystals doped with variable amounts of silver. Computational studies indicate that heterometallic Cu/Ag systems have a stronger binding affinity towards DMS vapor molecules than homometallic CuI and that embedding of the DMS molecules into the surface is the primary intermediate by which the vapochromic response occurs.

d8⋯d10 RhI⋯AuI interactions in Rh 2,6-xylylisocyanide complexes with [Au(CN)2]−: bond analysis and crystal effects

The well-known [RhL₄]_n(anion)_n structures, with Rh^I⋯Rh^I d⁸⋯d⁸ interactions, are replaced by others with Rh^I⋯Au^I d⁸⋯d¹⁰ interactions such as [{RhL₄}{Au(CN)₂}] (L = 2,6-xylylisocyanide) or [{RhL₄}{Au(CN)₂}{RhL₄}{Au₂(CN)₃}·4(CHCl₃)]_∞ when the anion is [Au(CN)₂]⁻.

Metal-Metal Bond Formations in [Au(CN)2-] n ( n = 3-5) Oligomers in Water Identified by Coherent Nuclear Wavepacket Motions

Large oligomers of [Au(CN)₂^-] _n including the pentamer were favorably formed in an aqueous solution containing tetra-ethylammonium chloride (1.0 mol/dm³), and intense transient absorption in the visible region was recorded by selective photoexcitation of the oligomers. Distinct oscillations at ∼40-100 cm^-1 were clearly observed in the temporal profile of the excited-state absorption signal, and the frequency-wavelength two-dimensional analysis of the oscillation clearly distinguishes the coherent nuclear motion of different oligomers. The observed nuclear motions were assigned to Au-Au stretch vibrations in the trimer, tetramer, and pentamer induced by the bond formation in the excited states. The transient absorption exhibits significant changes with a time constant of 3-20 ps, reflecting intersystem crossing and structural change.

Considerable Enhancement of Emission Yields of [Au(CN)2(-)] Oligomers in Aqueous Solutions by Coexisting Cations

The photophysical properties of [Au(CN)2(-)] oligomers in aqueous solutions were investigated as functions of coexisting cations as well as the viscosity and temperature of solutions. A solution of [Au(CN)2(-)] in the concentration range of 0.03-0.2 mol/dm(3) exhibited emission peaks at 460-480 nm because of the presence of oligomers larger than trimers. Although the emission yields (ϕem) of K[Au(CN)2] solutions were <1%, it considerably increased to 43% when 1.0 mol/dm(3) tetraethylammonium chloride (Et4NCl) was added. The lifetimes of the main emission bands were also significantly varied with additional salts, e.g., KCl, 15 ns; Et4NCl, 520 ns. The time-resolved emission measurements of [Au(CN)2(-)] in a water/glycerol mixture indicated that the lifetimes were almost directly proportional to the inverse of the viscosity of the solution. On the other hand, the intrinsic lifetimes of dimers and trimers with weak emission in shorter wavelength regions were very short and independent of the viscosity of the solutions and coexisting cations (dimer, ∼25 ps; trimer, ∼2 ns). These results indicated that the deactivation of the excited-state [Au(CN)2(-)]n oligomers (n ≥ 4) was dominated by the dissociation of the oligomers to a shorter species (dimer or trimer). The hydrophobic interactions between tetraalkylammonium cations and CN ligands remarkably stabilized the larger oligomers and suppressed the dissociation of the excited-state oligomers, which enhanced the emission yield of the oligomers. This work provides a new method of "exciplex tuning" by changing the environment of excited-state [Au(CN)2(-)]n oligomers.

Coherent vibration and ultrafast dynamics upon bond formation in excited dimers of an Au(i) complex

Au–Au bond strengthening in photoexcited dimers of an Au(i) complex is captured in solution as oscillations of femtosecond absorption signals.

A Review of Luminescent Anionic Nano System: d10 Metallocyanide Excimers and Exciplexes in Alkali Halide Hosts

Dicyanoaurate, dicyanoargentate, and dicyanocuprate ions in solution and doped in different alkali halide hosts exhibit interesting photophysical and photochemical behavior, such as multiple emission bands, exciplex tuning, optical memory, and thermochromism. This is attributed to the formation of different sizes of nanoclusters in solution and in doped hosts. A series of spectroscopic methods (luminescence, UV-reflectance, IR, and Raman) as well as theoretical calculations have confirmed the existence of excimers and exciplexes. This leads to the tunability of these nano systems over a wide wavelength interval. The population of these nanoclusters varies with temperature and external laser irradiation, which explains the thermochromism and optical memory. DFT calculations indicate an MLCT transition for each nanocluster and the emission energy decreases with increasing cluster size. This is in agreement with the relatively long life-time for the emission peaks and the multiple emission peaks dependence upon cluster concentration.

Modeling the photophysics of Zn and Cd monomers, metallophilic dimers, and covalent excimers

Density functional (Xα, BLYP, BPW91, B3LYP, and B3PW91), MP2, and coupled cluster with singles, doubles, and quasiperturbative triple excitations (CCSD(T)) methods in combination with LANL2DZ, cc-pVxZ, cc-pVxZ-PP, or cc-pVxZ-DK (where x = D (double), T (triple) Q (quadruple), and 5 (quintuple)) basis sets have been employed in computations of excited states of zinc and cadmium dimers and monomers. The spectroscopic constants r(e), D(e), T(e), ω(e), ω(e)x(e), and B(e) are compared among group 12 dimers in their ground and low-lying phosphorescent excited states. The weak metallophilic bonding in the ground state requires MP2 or CCSD(T) description, whereas the excimeric bonding is well described by CCSD(T), BLYP, B3LYP, BPW91, and B3PW91. Consistent with our previous work on Hg(n) species, absorption and phosphorescence transition energies that agree well with experimental results can be obtained with CCSD(T) and B3PW91 methods in combination with the correlation consistent basis sets of triple-ζ or higher level. This is the case for monomers and *Cd(2) and *Zn(2) lowest triplet excimers (in either the (3)Σ(u)(+) or (3)Π(g) state). The CCSD(T)/aug-cc-pV5Z-PP combination results in values that are only 150 and 240 cm(-1) (<1% errors) removed from the experimental values for the (1)S → (3)D atomic transitions for Zn and Cd, respectively. Such spectroscopic constants provide the most accurate theoretical values known to date for bonding and photophysical parameters characteristic of Zn(n) and Cd(n) species.

Structure and Spectroscopy of Tb[Au(CN)2]3·3H2O

The compound Tb[Au(CN)2]3.3H(2)O crystallizes in a layered structure in the hexagonal space group P6(3)/mcm with the 9-coordinate environment of Tb3+ comprising six (CN)- and three OH2 in a tricapped trigonal prism. The shortest Au...Au distance is 3.31 angstroms. The vibrational spectra show that the series Ln[Au(CN)2]3.3H2O (Ln = Y, Pr, Sm, Eu, Tb) are isostructural. The electronic spectra of Eu[Au(CN)2]3.3H2O clearly show that Eu3+ occupies one site of spectroscopic site symmetry D3h, in agreement with the crystallographic data. The electronic emission and absorption spectra of Tb[Au(CN)2]3.3H2O have been recorded at temperatures down to 1.5 K, and the f-f pure electronic transitions are interpreted in detail as arising from the lowest electronic states (in D3h symmetry) (7F6)E' in absorption and (5D4)E'' in emission. At low energy, further bands are assigned to the vibronic structure of the CN stretching and water stretching modes, with the latter more predominant. Although the CN stretching vibrations show exclusive infrared or Raman activity in Tb[Au(CN)2]3.3H2O, both of these infrared and Raman active modes are observed in the two-center vibronic transitions. The reasons for this are discussed.

Electronic Structure of Mercury Oligomers and Exciplexes: Models for Long-Range/Multicenter Bonding in Phosphorescent Transition-Metal Compounds

Spectroscopic and bonding properties of Hg(n) oligomers and *Hg(n) exciplexes are determined by rigorous theoretical treatments. Reliable values that agree well with experimental data have been computed for the luminescence energies and other molecular spectroscopic parameters by making a careful selection of theoretical methods and basis sets. The calculations clarified the assignments for several phosphorescence bands in the mercury vapor based on calculated energies and other parameters that quantify the large excited-state distortion in the emitting states. Both the weak ground-state mercurophilic bonding and the stronger covalent bonding in the triplet and quintet excited states studied are found to be cooperative, which is important for fundamental and applied research for luminescent and magnetic materials that have spectral behavior similar to that of Hg(n) systems.

Metallophilic Interactions in Closed-Shell d10 Metal−Metal Dicyanide Bonded Luminescent Systems Eu[AgxAu1-x(CN)2]3 and Their Tunability for Excited State Energy Transfer

We report on the heterobimetallic system, Eu[Ag(x)Au(1-x)(CN)(2)](3) (x = 0-1) in which sensitization of europium luminescence occurs by energy transfer from [Ag(x)Au(1-x)(CN)(2)](-) donor excited states. The donor states have energies which are tunable and dependent on the Ag/Au stoichiometric ratio. These layered systems exhibit interesting properties, one of which is their emission energy tunability when excited at different excitation wavelengths. In this paper, we report on their use as donor systems with Eu(III) ions as acceptor ions in energy transfer studies. Luminescence results show that the mixed metal dicyanides with the higher silver loading have a better energy transfer efficiency than the pure Ag(CN)(2)(-) and Au(CN)(2)(-) donors. The better energy transfer efficiency is due to the greater overlap between the donor emission and acceptor excitation. Additionally, more acceptor states are available in the high silver loading mixed metal Eu(III) complexes. The results from a crystal structure determination and Raman experiments are also presented in this paper and provide information about metallophilic interactions in the closed-shell d(10) metal-metal [Ag(x)Au(1-x)(CN(2)](-) dicyanide clusters.

Orange luminescence and structural properties of three isostructural halocyclohexylisonitrilegold(I) complexes

The preparation of three isonitrile complexes (CyNC)Au(I)Cl, (CyNC)Au(I)Br, and (CyNC)Au(I)I, along with their structural and spectral characterization, are reported. X-ray crystal structures reveal that these crystallize in the same space group and have closely related structures. The structures involve pleated chains of linear, two-coordinate monomers that are arranged in a head-tail fashion. However, these chains vary significantly in the degree of aurophilic interactions among the individual molecules. Thus, (CyNC)Au(I)Cl forms infinite chains with alternating Au...Au distances of 3.3894(7) and 3.5816(7) A. Within the chains of (CyNC)Au(I)Br, however, the alternation of Au.Au distances is more pronounced so that there are dimers, with an Au.Au distance of 3.4864(9) A, and neighboring gold centers at 3.7036(9) A. In (CyNC)Au(I)I, the gold-gold contacts do not lie within the range of significant aurophilic bonding. The closest Au...Au distance is 3.7182(11) A while every other Au...Au distance is 3.9304(12) A. The steric factor of the X ligand and dipole-dipole interactions between the antiparallel complexes is much more significant than aurophilic interactions in governing the self-association of the complexes in this series. The colorless crystals of each solid display an orange luminescence band with a strikingly large Stokes' shift ( approximately 21000 cm(-)(1), 2.6 eV). However, considerable care had to be taken to ensure that the crystals used for the study of the luminescence were free of a surface impurity that produced a turquoise-green luminescence in (CyNC)Au(I)Cl. The diffuse reflectance spectra for the solids show a similar three-band pattern in the 200-330 nm range.