Synthesis and Photophysical Properties of Laterally Asymmetric Digold(I) Alkynyls and Triazolyl: Ancillary Ligand and Organic Functionality Dictate Excited-State Dynamics

Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

Intra- vs Intermolecular Aurophilic Contacts in Dinuclear Gold(I) Compounds: Impact on the Population of the Triplet Excited State

Two series of dinuclear gold(I) complexes that contain two Au-chromophore units (chromophore = dibenzofurane or dimethylfluorene) connected through a diphosphane bridge that differs in the flexibility and length (diphosphane = dppb for 1,4-bis(diphenylphosphino)butane, DPEphos for bis[(2-diphenylphosphino)phenyl]ether, xanthphos for 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, and BiPheP for 2,2'-bis(diphenylphosphino)-1,1'-biphenyl) have been synthesized and structurally characterized. Their photophysical properties have been carefully investigated, paying attention to the role of the presence, or absence, of aurophilic contacts and their nature (intra- or intermolecular character). This analysis was permitted due to the X-ray crystallographic determination of all of the structures of the compounds discussed herein. The quantum yields of the triplet population, ϕ_T, have been calculated by nanosecond-laser flash photolysis measurements, and we could determine the main role of the character of the aurophilic contacts in the resulting ϕ_T, being especially favored in the presence of intermolecular contacts. Time-dependent density functional theory (TD-DFT) calculations support the absorption and emission assignments and the shorter distance between S₁ and the closest triplet excited state energy in the case of the compounds with a higher triplet-state population.

Synthetically Tunable White-, Green-, and Yellow-Green-Light Emission in Dual-Luminescent Gold(I) Complexes Bearing a Diphenylamino-2,7-fluorenyl Moiety

The syntheses and photophysical characterization of five new gold(I) complexes bearing diphenylamine-substituted fluorenyl moieties are reported; four are characterized by X-ray diffraction crystallography. Ancillary ligation on gold(I) is provided by organophosphine and N-heterocyclic carbene ligands. Two complexes, Au-DPA0 and Au-DPA1, are σ-aryls, two, Au-ADPA0 and Au-ADPA1, are σ-alkynyls, and one, Au-TDPA1, is a σ-triazolyl bound through carbon. All complexes show vibronically structured absorption and luminescence bands that are assignable to π-π* transitions localized on the diphenylamine-substituted fluorenyl π system. The excited-state dynamics of all five chromophores are governed by selection of the ancillary ligand and σ attachment of the diphenylamine-substituted fluorenyl moiety. All of these chromophores are dual luminescent in a toluene solution at 298 K. The luminescence from the aryl derivatives, Au-ADPA0 and Au-DPA1, appears green. The alkynyl derivative containing a phosphine ancillary ligand, Au-ADPA0, is a white-light emitter, while the alkynyl derivative containing an N-heterocyclic carbene ancillary ligand, Au-ADPA1, is a yellow-light emitter. The luminescence from the triazolyl-linked chromophore, Au-TDPA1, appears as yellow-green. Spin-restricted density functional theory calculations support the assignments of ligand-centric optical transitions but with contributions of ligand-to-metal charge transfer involving the vacant Au 6p orbital.

Two-photon absorption characterization and comparison of Au(I) fluorenyl benzothiazole complexes in the visible wavelength regime

We use the two-photon excited fluorescence method to determine the two-photon absorption (2PA) cross sections of three series of (fluorenyl benzothiazole) gold(I) complexes in the visible wavelength range from 570 to 700 nm. We compare the effect of ancillary ligand substitutions on the 2PA magnitudes and find that the ancillary ligand does not drastically affect either the magnitude or the shape of 2PA. Even so, moderate 2PA cross sections were measured that ranged from 10 to 1000 s of GM (Göppert-Mayer, =10^-50cm⁴s/photon), making these types of complexes nonlinear optical materials for two-photon absorbing applications.

Synthesis and photophysics of gold(i) alkynyls bearing a benzothiazole-2,7-fluorenyl moiety: a comparative study analyzing influence of ancillary ligand, bridging moiety, and number of metal centers on photophysical properties

Three new gold(i) alkynyl complexes (Au-ABTF(0-2)) containing a benzothiazole fluorenyl moiety, with either an organic phosphine or N-heterocyclic carbene as ancillary ligand, have been synthesized and photophysically characterized. All three complexes display highly structured ground-state absorption and luminescence spectra. Dual-luminescence is observed in all three complexes at room temperature in toluene after three freeze-pump-thaw cycles. The phosphine complexes (Au-ABTF(0-1)) exhibit similar photophysics with fluorescent quantum yields ∼0.40, triplet-state quantum yields ∼0.50, and fluorescent lifetimes ∼300 ps. The carbene complex Au-ABTF2 displays different behavior; having a fluorescent quantum yield of 0.23, a triplet-state quantum yield of 0.61, and a fluorescent lifetime near 200 ps, demonstrating that the ancillary ligand alters excited-state dynamics. The compounds exhibit strong (on the order of 105 M-1 cm-1) and positive excited-state absorption in both their singlet and triplet excited states spanning the visible region. Delayed fluorescence resulting from triplet-triplet annihilation is also observed in freeze-pump-thaw deaerated samples of all the complexes in toluene. DFT calculations (both static and time-resolved) agree with the photophysical data where phosphine complexes have slightly larger S1-T2 energy gaps (0.28 eV and 0.26 eV) relative to the carbene complex (0.21 eV). Comparison of the photophysical properties of Au-ABTF(0-2) to previously published dinuclear gold(i) complexes and mononuclear gold(i) aryl complexes bearing the same benzothiazole-2,7-fluorenyl moiety are made. Structure-property relationships regarding ancillary ligand, bridging moiety, and number of metal centers are drawn.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.