Optimized Photoisomerization on Gold Nanoparticles Capped by Unsymmetrical Azobenzene Disulfides

Gold nanoparticles capped by an unsymmetrical azobenzene disulfide, 4-hexyl-4‘-(12-(dodecyldithio)dodecyloxy)azobenzene (C6AzSSC12), were synthesized in order to investigate the efficiency of azobenzene photoisomerization on colloidal gold surfaces. The nanoparticles were synthesized by a two-step method to avoid the direct contact of azobenzene units with a reducing agent. The average size of the particles was determined to be ∼5.2 ± 1.3 nm from transmission electron microscope (TEM) images. The CH₂ antisymmetric (∼2919 cm^-1) and symmetric (∼2850 cm^-1) stretching bands in the FTIR spectra of the nanocomposite confirmed the all-trans conformation of alkyl chains in the C6AzSSC12 on the colloidal gold. The photoisomerization reaction of the C6AzSSC12-capped gold nanoparticles was studied by UV−vis absorption spectroscopy in toluene. The reaction kinetics was identical to that of the free C6AzSSC12 molecules dissolved in toluene, with no deviations from a first-order plot for both trans-to-cis and cis-to-trans photoisomerization, suggesting no steric hindrance throughout the whole reaction process. The free volume guaranteed by the 50% dilution of the dye functions due to the unsymmetrical disulfide structures, as well as their noncompact molecular tails owing to the assembly on the curved colloidal gold, must be responsible for such a highly efficient photoreaction. Sedimentation of the nanoparticles arose in toluene subsequent to the photoisomerization of the capping azobenzene molecules from trans to cis isomers. This phenomenon can be interpreted as resulting from differences in the degree of solvation between the azobenzene isomers.