N-Heterocyclic Carbenes (NHCs) as Conductive and Stable Anchors for Au-Based Hybrid Materials and Devices

Gold nanoparticles (Au NPs) have unique surface plasmonic resonances (SPR) and promise a broad variety of optical applications in sensors and imaging,¹ while combining them with organic components can give electronically functional hybrid materials.²Protecting surface ligands, carrying one or more functional anchor groups with a high affinity for gold, are needed to stabilize the Au NPs and prevent their irreversible aggregation. Thiols are ubiquitously used, yet the thermal instability and comparably low conductance of the S-Au contact limit the applications of the resulting Au NPs.³ <i>N</i>-heterocyclic carbenes (NHCs) have been used in surface chemistry as an high-performing alternative to sulfur-based ligands⁴: The NHC-Au linkage shows excellent stability under conditions that destroy the S-Au bond (e.g. high temperature, variable pH, and electrochemical redox), and is furthermore highly conductive.<br/><br/>We use NHC anchors as robust and conductive anchors to bind charge-conducting conjugated polymers (CPs) to Au NPs to obtain electronically functional hybrid materials.⁵Br-NHC-Au-X (X=Br, Cl) complexes were synthesized, converted to Grignard monomers and used for a chain-growth Kumada polymerization to obtain regioregular poly-3-hexylthiophenes (P3HT) with a low polydisperity index. Direct reduction gave spherical Au NPs with P3HT as surface ligands. The presence of the NHC on the surface was confirmed by XPS. The P3HT-NHC@Au NPs have a remarkable thermal stability up to 24 h in 100 °C toluene solutions, while Au NPs stabilized by sulfur-based PEG fully degrade under the same conditions. Also, continuous CV scans show only a slight decay of peak current after 50 cycles, indicating the good electrochemical stability of P3HT-NHC@Au NPs. To investigate the conductive linker, electrochromism (EC) experiments were carried out. P3HT is electrochromically active, and one can assume that binding the polymer to Au NPs via a conductive NHC anchor group will increase the electrochromic response speed, allowing to estimate the organic/inorganic electron transfer of P3HT-NHC@Au NPs. We found that blending 10% of P3HT-NHC@Au NP into P3HT films improves the responsive speed signiffcantly, from 1.21/1.05 s for neat P3HT to 0.79/0.52 s for the blend.<br/><br/><br/>Reference<br/>1 N. J. Halas, S. Lal, W.-S. Chang, S. Link, P. Nordlander, <i>Chem. Rev.</i><b> 2011</b>, 111, 3913<br/>2 A. M. Steiner, F. Lissel, A. Fery, J. Lauth, M. Scheele, <i>Angew. Chem. Int. Ed.</i> <b>2020</b>, 60, 1152<br/>3 M. S. Inkpen, Z.-F. Liu, H. Li, L. M. Campos, J. B. Neaton, L. Venkataraman, <i>Nat. Chem.</i> <b>2019</b>, 11, 351<br/>4 P. Bellotti, M. Koy, M.N. Hopkinson, F. Glorius Nat Rev Chem 2021, 5, 711; C. M. Crudden, J. H. Horton, I. I. Ebralidze, O. V. Zenkina, A. B. McLean, B. Drevniok, Z. She, H.-B. Kraatz, N. J. Mosey, T. Seki, E. C. Keske, J. D. Leake, A. Rousina-Webb, G. Wu, <i>Nat. Chem. </i><b>2014</b>, 6, 409<br/>5 N. Sun, S.-T. Zhang, F. Simon, A. M. Steiner, J. Schubert, Y. Du, Z. Qiao, A. Fery, F. Lissel, <i>Angew. Chem. Int. Ed. </i><b>2021</b>, 60, 3912