Spin dependent electrochemistry: Focus on chiral vs achiral charge transmission through 2D SAMs adsorbed on gold

Abstract

The efficiency of charge transmission in chiral compared to achiral molecular systems, of structurally related compounds, is probed via cyclic voltammetry measurements. For such a purpose five different thiols have been selected two achiral (i.e. 3-mercaptopropionic acid (MPA) and 8-mercapto-1-octanol (M8O)) and three chiral (i.e. L- and D-cysteine (Lcys, Dcys) and D-penicillamine (Dpen)). These compounds are used to form 2D self-assembled monolayers (SAM) on gold. Cyclic voltammetry (CV) measurements, by using the potassium hexacyanoferrate(III)/potassium hexacyanoferrate(II) redox couple (Fe3Fe2), are exploited to probe the charge transfer ability of the electrode|SAM|solution interface. In particular, MPA, Lcys and Dpen compounds are selected due to their quite similar structural and geometrical characteristics (virtually the same molecular length and terminal groups). M8O is a longer SAM forming thiol, which was selected as a reference blocking electrode compound. The comparison of the cyclic voltammetry data shows that a better ability in the charge transmission is obtained when chiral SAM are used; namely in the case of L-cysteine and D-penicillamine. This result can be related to the spin filtering ability of chiral compounds: chiral induced spin selectivity (CISS) effect. As rationalized on the basis of theoretical values of optical rotation (OR), calculated at the CAMB3LYP/6-31G(d,p) level of the theory.