Highly Efficient Electrogenerated Chemiluminescence of Au38 Nanoclusters

An investigation of mechanisms for the near-infrared (NIR) electrogenerated chemiluminescence/electrochemiluminescence (ECL) of Au-38(SC2H4Ph)(24) (Au-38, SC2H4Ph = 2-phenylethanethiol) nanoclusters both in annihilation and coreactant paths is reported. Essentially, no ECL emission was produced in the annihilation route over the potential range of the accessible redox states of Au-38, because of the short lifetime and/or low reactivity of the electrogenerated Au-38 intermediates necessary for ECL. Highly effluent light emission with a nominal peak wavelength of 930 nm in the NIR region was observed in the anodic region upon addition of tri-n-propylamine (TPrA) as the coreactant. The ECL mechanisms were elucidated by means of ECL potential curves and spooling ECL spectroscopy. It was discovered that the Au-38(+center dot) (and also Au-38(3+center dot)) were electrogenerated as the major excited species in the light emission processes. Benzoyl peroxide was also used as a coreactant in the cathodic potential range from which benzoate radicals, with a high oxidizing power, were formed. These radicals accepted electrons from the electrogenerated Au-38(2-) HOMO, resulting in the Au-38(-center dot) excited state that emitted light at 930 nm. The photoluminescence of the various Au-38 charge states, namely, Au-38(2-), Au-38(-), Au-38(0), Au-38(+), Au-38(2+), and Au-38(4+), electrogenerated in situ, indicated no significant difference in the emission peak wavelength. This information allowed a careful mapping of the relevant ECL mechanisms. It was found that the ECL efficiency could reach an efficiency of 3.5 times as high as that of the Ru(bpy)(3)(2+)/TPrA system.