Controlling Spin Filtering through Molecular Length in Helical Aromatic Oligomers

Chiral 8-amino-2-carboxylic acid quinoline-based aromatic oligoamide foldamers with lengths up to circa 5 nm were synthesized to investigate their spin filtering properties. A thiolated quinoline monomer was designed and incorporated at one extremity of the chiral helices to enable their immobilization on gold substrates. Scratching atomic force microscopy (AFM) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements confirmed the formation of the organic monolayers, with thicknesses ranging from approximately 1.1 to 5.4 nm depending on oligomer length. Spin filtering behavior was evaluated using magnetic conductive AFM, revealing that the spin-selective signal increases with helix length, achieving values up to ∼85%. Notably, increasing the oligomer length did not result in a decrease in conductivity along the helix axis. Magnetoresistance measurements performed using a dedicated device showed a temperature-dependent response, with the magnetoresistance signal increasing as the temperature rises.