Mini-zippers: Determination of oligomer effects for the assembly of photoactive supramolecular rod/stack architectures on gold nanoparticles and gold electrodes

Abstract

The objective of this study was to evaluate the importance of oligomer effects for the assembly of functional rod/stack architectures on solid substrates. The original approach to the ‘zipper assembly’ of functional rod/stack architectures focused on p-quaterphenyl initiators and p-octiphenyl propagators with naphthalenediimides (NDIs) attached along their scaffold. Here, we describe the synthesis and evaluation of ‘mini-zippers’ composed of the homologous biphenyl initiators and p-quaterphenyl propagators. Whereas the synthesis of shorter oligomers is less demanding, their less efficient assembly on gold nanoparticles and electrodes produces overall less convincing results. On gold nanoparticles, the increase in absorption during the assembly of mini-zippers stops around notional π-stacks of six NDIs; original zippers provide access to at least three times higher NDI absorption and notional π-stacks of 30 NDIs before reaching saturation. On gold electrodes, the increase in photocurrent per NDI with mini-zippers is 50% of that with original zippers. Moreover, photocurrents of mini-zippers saturate at notional π-stacks of 40 NDIs, whereas original zippers can produce larger photocurrents and notional π-stacks of 80 NDIs without reaching saturation. These results suggest that for functional supramolecular rod/stack architectures, oligomer effects are essential and cannot be ignored.

Keywords:

• self-assembly
• layer-by-layer assembly
• photocurrent
• artificial photosynthesis
• length dependence
• oligomer effects

Acknowledgements

We thank D.-H. Tran for contributions to synthesis, and the University of Geneva and the Swiss NSF for financial support.