Thermodynamic and Kinetic Origins of Au250 Nanocluster Electrochemiluminescence

Au clusters with protecting organothiolate ligands and core diameters less than 2nm are molecule-like structures, suitable for catalysis, optoelectronics and biology applications. The spectroscopy and electrochemistry of Au-25(0) (Au-25[(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 960nm, emitted by Au-25(+*), Au-25(0*), and Au-25(-*), 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 Au-25(z) (z=2+, 1+, 1-, and 2-) in the vicinity of the electrode or the bulk Au-25(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.