Aurophilicity in Action: Fine-Tuning the Gold(I)-Gold(I) Distance in the Excited State To Modulate the Emission in a Series of Dinuclear Homoleptic Gold(I)-NHC Complexes.

The solution-state emission profiles of a series of dinuclear Au(I) complexes 4-6 of the general formula Au2(NHC-(CH2)n-NHC)2Br2, where NHC = N-benzylbenzimidazol-2-ylidene and n = 1-3, were found to be markedly different from each other and dependent on the presence of excess bromide. The addition of excess bromide to the solutions of 4 and 6 leads to red shifts of ca. 60 nm, and in the case of 5, which is nonemissive when neat, green luminescence emerges. A detailed computational study undertaken to rationalize the observed behavior revealed the determining role aurophilicity plays in the photophysics of these compounds, and the formation of exciplexes between the complex cations and solvent molecules or counterions was demonstrated to significantly decrease the Au-Au distance in the triplet excited state. A direct dependence of the emission wavelength on the strength of the intracationic aurophilic contact allows for a controlled manipulation of the emission energy by varying the linker length of a diNHC ligand and by judicial choice of counterions or solvent. Such unique stimuli-responsive solution-state behavior is of interest to prospective applications in medical diagnostics, bioimaging, and sensing. In the solid, the investigated complexes are intensely phosphorescent and, notably, 5 and 6 exhibit reversible luminescent mechanochromism arising from amorphization accompanied by the loss of co-crystallized methanol molecules. The mechano-responsive properties are also likely to be related to changes in bromide coordination and the ensuing alterations of intramolecular aurophilic interactions. Somewhat surprisingly, the photophysics of NHC ligand precursors 2 and 3 is related to the formation of ground-state associates with bromide counterions through hydrogen bonding, whereas 1 does not appear to bind its counterions.

HSA-based phosphorescent probe for two-photon in vitro visualization.

Two-photon microscopy reveals several advantages over conventional one since it provides higher spatial resolution as well as deeper penetration into the sample under study. The development of suitable two-photon probes is one of the most challenging tasks in this area. Here we present phosphorescent non-covalent adduct of human serum albumin and Au-Ag alkynyl-diphosphine complex, [Au14Ag4(C2Ph)12(PPh2C6H4PPh2)6][PF6]4, which exhibits high cross section of two-photon-induced luminescence (δTPE) within large near-infrared excitation wavelength region (700-800 nm) with maximum δTPE about 38 GM at 740 nm. This feature makes it a promising probe for multiphoton bioimaging as demonstrated by successful visualization of glioma C6 cells and various tissues by two-photon confocal microscopy both in planar and z-stacking modes. Additionally, the broad excitation region enables optimization of the signal-to-background auto-fluorescence ratio via variation of excitation wavelength.

Triphosphine-supported bimetallic Au(I)-M(I) (M = Ag, Cu) alkynyl clusters.

The reactions of gold acetylides (AuC2R)n with triphosphine ligands PPh2-(CH2)n-PPh-(CH2)2-PPh2 (n = 1, dpmp; 2, dpep) in the presence of M(+) ions (M = Cu, Ag) lead to an assembly of the heterometallic clusters, the composition of which is determined by the steric bulkiness of the alkynyl groups and the flexibility of the phosphine motifs. For R = Ph, an unprecedented hexanuclear complex [Au5Cu(C2R)4(dpmp)2](2+) (1) was isolated, while for the aliphatic alkynes (R = 1-cyclohexanolyl, 2-borneolyl, 2,6-dimethyl-4-heptanolyl) a family of compounds based on a tetrametallic framework was prepared, [Au3Cu(C2R)3(dpmp)](+) (2, R = 1-cyclohexanolyl), [Au3M(C2R)3(dpep)]2(+2) (3, M = Cu, R = 1-cyclohexanolyl; 4, M = Cu, R = 2-borneolyl; 5, M = Ag, R = 2-borneolyl), and [Au3Ag(C2R)3(dpep)](+) (6, R = 2,6-dimethyl-4-heptanolyl). Clusters 1, 2, and 4-6 were studied by X-ray crystallography. The NMR spectroscopic investigations showed that 1 and 3-5 are stereochemically non-rigid in solution and reversibly undergo possible dissociation (3) or isomerization (4, 5) processes. All the title complexes exhibit room temperature luminescence in the solid state in the spectral region from 414 to 566 nm, showing a dependence of emission energy on the structure and composition of the metal core. Computational studies with density functional methods were carried out to rationalize the dynamic and photophysical behavior of these compounds.