Photopolymerization of gold nanoparticles: Size-related charge separation and emission

Nanoparticles in the diameter range of 1-10 nm have very unique properties compared to those of bulk metals and molecular compounds. Particle size and shape, interparticle distance, and the nature of organic shell have significant effects on their properties. Though much work on metal nanoparticles has been reported in recent years, little has dealt with direct detection of particle size effects on reaction processes. In fact, this is because the synthesis of nanoparticles in the desired two- or three-dimensional structures remains a challenge. We report the photopolymerization of bithiophene (BT)-functionalized gold nanoparticles (AuNPs) of different diameters (2, 4, and 6 nm). Photoinduced charge separation involving the BT ligand and the Au was directly detected using ferntosecond transient absorption measurements. The charge separation rate constants, 4.35 +/- 0.17 x 10(12), 1.27 +/- 0.07 x 10(12), and 6.99 +/- 0.16 x 10(11) s(-1) for 2, 4, and 6 nm AuNPs, respectively, decrease with increasing NP size. Also, emission was observed with an increase in particle size. Furthermore, the ensuing photopolymerization of the BT-functionalized AuNPs provides a useful method for making two- or three-dimensional designs of conductive polymeric nanomaterials. We envision potential applications in various areas, such as chemical and biosensors, transistors, photovoltaic cells, electrochromic devices, optical conducting materials, surface protection, and so on.