Spin Polarization Driven by Molecular Vibra-tions Leads to Enantioselectivity in Chiral Mol-ecules
Miwa and Inoue Groups
Chirality is a pseudo-scalar that changes its sign with mirror reflection and lacks mirror symmetry. This intri-guing property is widely recognized and studied in diverse fields, including physics, chemistry, biology, and as-tronomy. Recently, numerous phenomena related to chi-rality-induced spin selectivity (CISS) have been reported in the field of physical chemistry [1]. A key observation in this area was the detection of photoelectrons that exhibited finite spin angular momentum upon passing through double-stranded DNA molecules in a Mott polarimeter. This discovery sparked a series of studies and reports on various CISS-related phenomena. Noteworthy phenomena include magnetoresistance in junctions involving chiral molecules and ferromagnetic electrodes, as well as the enantiomer separation using ferromagnetic substrates. These findings are intriguing from a scientific standpoint and have substantial potential for practical applications, particularly in enantioselective synthesis.
In the context of CISS-related phenomena, electrons trav-ersing chiral materials are considered to gain orbital angular momentum due to their helical motion. Subsequently, they acquire spin angular momentum via spin–orbit interaction. However, the model attributing CISS primarily to electric current remains a subject of debate. For example, reports of exceptionally large CISS-induced magnetoresistance in materials with relatively low spin polarization challenge the validity of current-induced spin polarization and its tun-neling-based explanation. Consequently, achieving a comprehensive and unified understanding of CISS-related phenomena remains challenging, hindering its practical application and further advancement in the field. In this study, we employed the prototypical chiral elec-trolyte (1S)-(+)- or (1R)-(−)-camphor-10-sulfonic acid [2] ((S)- or (R)-CSA) in a custom-made electrochemical cell with precisely engineered ferromagnetic CoPt/Au elec-trodes to conduct time-resolved magnetoresistance obser-vations (Fig. 1).
To evaluate the magnetoconductance (MC) effect, which is the variation in current as a function of the magnetization direction of CoPt, the voltage was set at the reduction voltage. The electric current was then measured under this constant voltage setting. A constant magnetic field of 0.6 T was applied perpendicular to the electrode surface. The polarity of the magnetic field was alternated every 300 seconds. Figure 2(a) shows that switching the polarity of the magnetic field from negative to positive (and vice versa) leads to a gradual decrease (or increase) in current until a new steady state is established, with a relaxation time of about 50 seconds. The MC effect diverges from predictions made by traditional theories, such as spin-dependent tun-neling similar to the tunneling magnetoresistance effect. If changes in interface conductance due to spin-dependent tunneling were responsible, we would expect an immediate response in current following a change in the polarity of the magnetic field, with relaxation trends counteracting this change. However, our experimental results indicate a monotonous decrease or increase in conductance following a change in the polarity of the magnetic field. This suggests that a mechanism other than spin-dependent tunneling conduction is required to explain the MC effect.
From these results, we find that the essence of CISS lies in the magnetic interaction between chiral molecules and the ferromagnetic electrode, which is analogous to the inter-layer exchange coupling. Our findings reveal a critical insight: in CISS-related phenomena, the electric current does not polarize the spins; rather it merely probes the system. Spin alignment driven by the vibration of the chiral molecules plays a pivotal role [3].
References
- [1] R. Naaman et al., Nat. Rev. Chem. 32, 056302 (2024).
- [2] T. S. Metzger et al., Angew. Chem. Int. Ed. 59, 1653 (2020).
- [3] S. Miwa et al., Sci. Adv. 11, eadv5220 (2015).