Self-organization of gold nanoparticles capped with liquid crystalline azobenzene ligands: the effect of terminal groups on particle arrangements

The ligands immobilized on the surface of gold nanoparticles (AuNPs) can be classified into two main types: end-on and side-on ligands. In order to investigate the effect of the terminal group of the end-on ligand on the nanoparticle arrangement, three types of AuNPs were employed by using azobenzene ligands with three different terminal groups (ethyl, acetyl, and nitrile). The obtained AuNPs with ethyl group (Au-L1e/d) are in a weakly ordered state, those with acetyl group (Au-L2e) show a hexagonal columnar liquid crystalline phase upon heating, and those with nitrile groups (Au-L3e) are highly aggregated and do not melt even when heated up to 220°C. According to DFT calculations, the dipole moments of each ligand are in the order of ethyl < acetyl < nitrile. There is a trade-off between the magnitude of the dipole moment and the aggregation/assembly behavior of the AuNPs. Au-L1e/d and Au-L2e exhibit trans-cis isomerization of the azobenzene ligands upon photoirradiation both in dispersed solution and in the film state. Furthermore, they also exhibit a gradual color change from orange to purple in solution, when the solutions are allowed to stand at rt for 24 h after photoirradiation. By means of tunneling electron microscopy (TEM), it is found that the nanoparticles irreversibly coalesce into a non-uniform morphology after being left to stand for 24 h. Prior to photoirradiation, the ligands are found to be densely packed on the gold surface; this dense packing may be responsible for the desorption after the isomerization to cis forms and the particle growth.