Luminescent Gold(III) Thiolates: Supramolecular Interactions Trigger and Control Switchable Photoemissions from Bimolecular Excited States

Abstract A new family of cyclometallated gold(III) thiolato complexes based on pyrazine‐centred pincer ligands has been prepared, (C^Npz^C)AuSR, where C^Npz^C=2,6‐bis(4‐ButC6H4)pyrazine dianion and R=Ph (1), C6H4 tBu‐4 (2), 2‐pyridyl (3), 1‐naphthyl (1‐Np, 4), 2‐Np (5), quinolinyl (Quin, 6), 4‐methylcoumarinyl (Coum, 7) and 1‐adamantyl (8). The complexes were isolated as yellow to red solids in high yields using mild synthetic conditions. The single‐crystal X‐ray structures revealed that the colour of the deep‐red solids is associated with the formation of a particular type of short (3.2–3.3 Å) intermolecular pyrazine⋅⋅⋅pyrazine π‐interactions. In some cases, yellow and red crystal polymorphs were formed; only the latter were emissive at room temperature. Combined NMR and UV/Vis techniques showed that the supramolecular π‐stacking interactions persist in solution and give rise to intense deep‐red photoluminescence. Monomeric molecules show vibronically structured green emissions at low temperature, assigned to ligand‐based 3IL(C^N^C) triplet emissions. By contrast, the unstructured red emissions correlate mainly with a 3LLCT(SR→{(C^Npz^C)2}) charge transfer transition from the thiolate ligand to the π⋅⋅⋅π dimerized pyrazine. Unusually, the π‐interactions can be influenced by sample treatment in solution, such that the emissions can switch reversibly from red to green. To our knowledge this is the first report of aggregation‐enhanced emission in gold(III) chemistry.


Synthesis and Characterization
When required, manipulations were performed using standard Schlenk techniques under dry nitrogen or in a MBraun glove box. Nitrogen was dried by passing through columns of supported P 2 O 5 , with moisture indicator, and activated 4 Å molecular sieves. Anhydrous solvents were freshly distilled from appropriate drying agents. (C^N pz^C )AuCl and (C^N py^C )AuCl were synthesised using literature methods. S1 1 H and 13 C{ 1 H} spectra were measured on a Bruker Avance DPX-300 spectrometer using CD 2 Cl 2 as solvent and signals were referenced to the residual protons of the deuterated solvent. IR spectra were recorded using a Perkin Elmer Spectrum 65 FT-IR spectrometer with a diamond ATR attachment. Elemental analyses were carried out at London Metropolitan University with a Perkin-Elmer 2400 CHNS/O microanalyzer.

Bu-4 (2)
Method 2: Under a nitrogen atmosphere, a flask was charged with (C^N pz^C )AuCl (56.0 mg, 0.097 mmol), K 2 CO 3 (26.9 mg, 0.191 mmol) and degassed acetone (5 mL). 4-tert-butylbenzenethiol (15 µl, 0.097 mmol) was added and the mixture was stirred at room temperature for 3 hours. The acetone was removed in vacuo and the resulting solid was extracted into dichloromethane (5 mL). The suspension was filtered through celite and washed with dichloromethane (5 mL). The solvent was removed. The solid residue was washed with cold hexane to give 2 as a yellow solid (56.7 mg, 0.081 mmol, 83%).

Synthesis of (C^N pz^C )AuSC 5 H 4 N-2 (3)
Method 1: (C^N pz^C )AuCl (50.0 mg, 0.087 mmol) and K 2 CO 3 (24.0 mg, 0.174 mmol) were dissolved in acetone (5 mL). Then, 1-mercaptopyridine (9.8 mg, 0.087 mmol) was added and the mixture was stirred at room temperature for 3 h. The acetone was removed in vacuo and the resulting solid was extracted into dichloromethane (5 mL). The suspension was filtered through celite and washed with dichloromethane (5 mL). The solvent was removed to give a yellow solid. This was washed with cold hexane to give 3 as a yellow powder (41.0 mg, 0.063 mmol, 72%). Single crystals suitable for X-ray diffraction were obtained from slow evaporation of a dichloromethane and isopropanol (4:1) solution.

Synthesis of (C^N pz^C )AuSAd (8)
Under an atmosphere of nitrogen, a flask was charged with (C^N pz^C )AuCl (55.0 mg, 0.097 mmol), K 2 CO 3 (26.9 mg, 0.191 mmol) and 1-adamantanethiol (16 mg, 0.097 mmol). Degassed acetone (5 mL) was added and the mixture was stirred at room temperature for 24 hours (the reaction was incomplete after 6 h, so an excess 1-adamantanethiol was added and the reaction was left overnight).

Aggregation of (C^N Pz^C )AuX complexes in solution
where I is the intensity of the observed spin echo, I 0 the intensity of the spin echo in the absence of gradient, D t the self-diffusion coefficient, Δ the delay between the midpoints of the gradients, δ the length of the gradient pulse, and γ the magnetogyric ratio. The shape of the gradients was rectangular, their length δ was 4-5 ms, and their strength G was varied during the experiments. All spectra were acquired for 64K points and a spectral width of 6200 Hz and processed with a line broadening of 1.0.
The semi-logarithmic plots of ln(I/I 0 ) versus G 2 were fitted by using a standard linear regression algorithm. Different values of G and number of transients were used for different samples.
The self-diffusion coefficient D t , which is directly proportional to the slope of the regression line where k is the Boltzmann constant, T is the temperature,  is the solution viscosity, c is the "size factor", which depends on the solute-solvent radius ratio and f is the "shape factor", which takes into account the deviation from sphericity of the diffusing molecule. a, b and d are the semi-axes of the ellipsoid that better approximates the shape of the diffusing molecule. S8 Structural parameter values (P= fc abd 3 = kT / phD t ) were derived from the measured D t values at different concentration in CD 2 Cl 2 and the data are collected in Table S3.1 and Figure S22.
Assuming a monomer/dimer equilibrium, the observed P values can be treated as the average between the parameters of both monomer (P M ) and dimer (P D ), according to the following relationship: where x M and x D are the molar fractions of monomer and dimer at a given concentration. P M and P D parameters are depending on the dimensions of monomer (a M , b M , d M ), dimer (a D , b D , d D ) and solvent (r solv , approximated at the van der Waals radius of CD 2 Cl 2 ). P M parameters can be derived from X-ray structures of monomers, by approximating their shape to ellipsoids. P D parameters have been calculated by using packing data derived by X ray diffraction (see below).  Figure S23. Evolution of the aromatic region of the 1 H NMR spectrum of (C^N Pz^C )AuSNp-2 as a function of the concentration in CD 2 Cl 2 .
The amount of dimers was quantified by using standard equations for monomer-dimer equilibrium: S9 where f m and f d are the molar fractions of monomers and dimers,  M and  D are the limit chemical shift values of monomer and dimer, respectively, K is the association constant. After fitting chemical shift versus concentration trends, the molar fractions of dimers were obtained as: Table S3.2.

Photophysical Properties
UV−visible absorption spectra were recorded using a Perkin-Elmer Lambda 35 UV/vis spectrometer.
Excitation and emission spectra were measured using a (

Cryogenic time resolved photoluminescence (ICCD and TCSPC)
ICCD : An electrically gated intensified CCD ( ICCD ) camera (Andor iStar DH740 CCI-010) with a calibrated grating spectrometer (Andor SR303i) were used for the time resolved PL spectra. 400nm laser light pulsed at 1 KHz was used to pump the sample. This was produced from the second harmonic generation of a 800nm femtosecond laser using a BBO crystal. The fundamental output had a 80 fs pulse width and 1.5 eV pule energy from a Ti:Sapphire laser system (Spectra Physics Solstice).
Time resolution of the emission was achieved by changing the delay of the gate relative to the 1 KHz pulsed pump laser. The minimum gating width was ~2.5 ns. A 495 nm long pass filter was used to remove any scattered light from the pump laser. This setup was used for the green emitting film.
Time correlated single photon counting : A pulsed 407 nm laser was used to photo-excite the samples, with pulse width <200 ps and a rep rate of 2.5MHz. The detector was a Si based single photon avalanche photodiode. The instrument response function was ~200 ps also. A 495 nm long pass filter was used as in the ICCD. This setup was used for the red emitting film.

Theoretical Calculations
All structures were optimised using density functional theory, within the approximation of the PBE0 exchange and correlation functional S11 as implemented within the ORCA quantum chemistry program. S12 A def2-SVP basis set S13 was used throughout. To account for the weak π−π interactions, the calculations were supplemented with Grimme's D3 dispersion correction with the Becke−Johnson damping scheme. S14 The absorption spectrum of complex 5 was calculated using pertubrative spin orbit coupling TDDFT (pSOC-TDDFT) S15 calculated using the ZORA Hamiltonian S16 as implemented within ADF. S17 The PBE0 exchange and correlation functional was used, and a DZP basis set for all atoms, except gold, for which a TZP basis was used. All emission energies were calculated using both uDFT and pSOC-TDDFT within ADF.