Lateral π-extended helical nanographenes with large spin polarization

Abstract

The possibility that current passing through an organic molecule becomes spin-polarized is highly intriguing. Amongst these molecules, helicene units have recently been shown to exhibit such a chiral-induced spin selectivity (CISS) effect. Thus, helical nanographenes (NGs), whose core building block is a helicene unit, are natural candidates for generating CISS. However, reports on the CISS effect in helical nanographenes (NGs) remain limited, primarily due to the lack of a suitable molecular platform for detecting spin-selective transport. In this work, we have developed a synthetic strategy using pre-fused key bonds in oligophenylene precursors and successfully synthesized lateral extended NGs that incorporate either single or double undecabenzo[7]helicene units with high yields. The resultant lateral extended helical NGs display excellent chiroptical properties including strong circular dichroism and large dissymmetry factors. Furthermore, magneto-conductive atomic force microscopy (mc-AFM) and magnetoresistance (MR) measurements show clear evidence for spin polarization of the current with a large spin polarization of up to 80% and a robust MR of 1.5% at room temperature. Together with theoretical modeling, our results identify lateral extended helical NGs as promising quantum materials for future organic spintronic devices.