Thermodynamic and kinetic origins of Au25(0) nanocluster electrochemiluminescence.

Au clusters with protecting organothiolate ligands and core diameters less than 2 nm are molecule-like structures, suitable for catalysis, optoelectronics and biology applications. The spectroscopy and electrochemistry of Au25(0) (Au25[(SCH2CH2Ph)18], SCH2CH2Ph = 2-phenylethanethiol) allowed us to construct a Latimer-type diagram for the first time, which revealed a rich photoelectrochemistry of the cluster and the unique relationship to its various oxidation states and corresponding excited states. The occurrence of cluster electrochemiluminescence (ECL) was examined in the presence of tri-n-propylamine (TPrA) as a co-reactant and was discovered to be in the near-infrared (NIR) region with peak wavelengths of 860, 865, and 960 nm, emitted by Au25(+), Au25(0), and Au25(-*), respectively. The light emissions, with an efficiency up to 103% relative to that of the efficient Ru(bpy)3(2+)/TPrA system, depended on the kinetics of the reactions between the electrogenerated TPrA radical and Au25(z) (z = 2+, 1+, 1-, and 2-) in the vicinity of the electrode or the bulk Au25(0). These thermodynamic and kinetic origins were further explored by means of spooling ECL and photoluminescence spectroscopy during a sweep of the potential or at a constant potential applied to the working electrode. NIR-ECL emissions of the cluster can be tuned in wavelength and intensity by adjusting the applied potential and TPrA concentration based on the above discoveries.