Quantitative analysis of thiolated ligand exchange on gold nanoparticles monitored by 1H NMR spectroscopy.

We use nuclear magnetic resonance spectroscopy methods to quantify the extent of ligand exchange between different types of thiolated molecules on the surface of gold nanoparticles. Specifically, we determine ligand density values for single-moiety ligand shells and then use these data to describe ligand exchange behavior with a second, thiolated molecule. Using these techniques, we identify trends in gold nanoparticle functionalization efficiency with respect to ligand type, concentration, and reaction time as well as distinguish between functionalization pathways where the new ligand may either replace the existing ligand shell (exchange) or add to it ("backfilling"). Specifically, we find that gold nanoparticles functionalized with thiolated macromolecules, such as poly(ethylene glycol) (1 kDa), exhibit ligand exchange efficiencies ranging from 70% to 95% depending on the structure of the incoming ligand. Conversely, gold nanoparticles functionalized with small-molecule thiolated ligands exhibit exchange efficiencies as low as 2% when exposed to thiolated molecules under identical exchange conditions. Taken together, the reported results provide advances in the fundamental understanding of mixed ligand shell formation and will be important for the preparation of gold nanoparticles in a variety of biomedical, optoelectronic, and catalytic applications.

Evolution of the electron mobility in polymer solar cells with different fullerene acceptors.

We investigate the evolution of the electron mobility of two different acceptors, [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and indene-C60 bisadduct (ICBA), in a poly(3-hexylthiophene) blend solar cell during a prolonged thermal aging process. High electron mobility does not correlate with the best device performance in our study of the P3HT:PC71BM and P3HT:ICBA systems. Very little changes are observed in the polymer crystallinity as a function of time. The evolution of the acceptor appears to be the dominant factor that leads to long-term changes in the device performance. The electron mobility evolves differently in PC71BM and ICBA systems, which highlights the importance of the fullerene molecular structure.

An Apparent Size-Exclusion Quantification Limit Reveals a Molecular Weight Limit in the Synthesis of Externally Initiated Polythiophenes.

We report the linear and nonlinear regions of the relationship between number average molecular weight determined by gel permeation chromatography (GPC) and 1H NMR end-group analysis for a series of o-tolyl-initiated poly(3-hexylthiophene)s (P3HTs). For conjugated polymers with chains that are 39-138 repeat units in length (6.5-23 kDa), GPC systematically overestimates the number average molecular weight (Mn) by a factor of 1.3 ± 0.1 (standard error), and GPC and 1H NMR end-group analysis correlate in a linear manner. For chains 138-1130 (23-188 kDa) repeat units in length, we observe a nonlinear relationship between GPC and end-group analysis. Static light-scattering experiments confirm that at high molecular weight (>70 kDa) decreasing the catalyst loading does not appreciably increase the polymer chain length. Thus, we conclude that there is a molecular weight limit in the synthesis of externally initiated polythiophenes and a propensity for the growth of nonexternally initiated chains which increases as a function of Mn. This is significant as external initiation has been reported to result in nearly 100% externally initiated chains as well as reduce the possibility of chain-chain coupling in a typical synthesis. Our data show that 100% external initiation only holds true for polymers that are less than 40 kDa and encourages caution when determining Mn by NMR using this synthetic methodology at high molecular weights.