Challenges in the direct detection of chirality-induced spin selectivity: investigation of foldamer-based donor/acceptor dyads

Over the past two decades, the chirality-induced spin selectivity (CISS) effect was reported in several experiments disclosing a unique connection between chirality and electron spin. Recent theoretical works have highlighted time-resolved Electron Paramagnetic Resonance (trEPR) as a powerful tool to directly detect the spin polarisation resulting from CISS. Because of the absence of interfaces with conducting electrodes, such spectroscopic evidence could provide a clear understanding of how CISS works at the intramolecular level. Experimental results have demonstrated the potential of this approach for detecting a spin-polarised photoinduced electron transfer (ET) in hybrid systems comprising a CdSe quantum dot as an electron donor (D) connected by a chiral linker (χ) to a fullerene derivative as an electron acceptor (A). However, the study of the ET process in fully organic D-χ-A dyads holds tremendous potential for the unambiguous detection of CISS. Here, we report a first attempt performed using novel D-χ-A dyads, comprising pyrene (D) and fullerene (A) connected by chiral saturated peptide bridges (χ) of different length and electric dipole moment. The dyads are investigated by an array of techniques, including cyclic voltammetry, optical spectroscopies, and trEPR. Despite the promising energy alignment of the electronic levels and the evidence of luminescence quenching, trEPR does not detect a significant ET highlighting the challenges of spectroscopic detection of CISS. However, the analysis allows the formulation of guidelines for the design of chiral organic model systems suitable to directly probe CISS-polarised ET.