ConspectusMolecular spintronics (spin + electronics), which aims to exploit
both the spin degree of freedom and the electron charge in molecular
devices, has recently received massive attention. Our recent experiments
on molecular spintronics employ chiral molecules which have the unexpected
property of acting as spin filters, by way of an effect we call “chiral-induced spin selectivity” (CISS). In this
Account, we discuss new types of spin-dependent electrochemistry measurements
and their use to probe the spin-dependent charge transport properties
of nonmagnetic chiral conductive polymers and biomolecules, such as
oligopeptides, L/D cysteine, cytochrome c, bacteriorhodopsin
(bR), and oligopeptide-CdSe nanoparticles (NPs) hybrid structures.
Spin-dependent electrochemical measurements were carried out by employing
ferromagnetic electrodes modified with chiral molecules used as the
working electrode. Redox probes were used either in solution or when
directly attached to the ferromagnetic electrodes. During the electrochemical
measurements, the ferromagnetic electrode was magnetized either with
its magnetic moment pointing “UP” or “DOWN”
using a permanent magnet (H = 0.5 T), placed underneath
the chemically modified ferromagnetic electrodes. The spin polarization
of the current was found to be in the range of 5–30%, even
in the case of small chiral molecules. Chiral films of the l- and d-cysteine tethered with a redox-active dye, toludin
blue O, show spin polarizarion that depends on the chirality. Because
the nickel electrodes are susceptible to corrosion, we explored the
effect of coating them with a thin gold overlayer. The effect of the
gold layer on the spin polarization of the electrons ejected from
the electrode was investigated. In addition, the role of the structure
of the protein on the spin selective transport was also studied as
a function of bias voltage and the effect of protein denaturation
was revealed. In addition to “dark” measurements, we
also describe photoelectrochemical measurements in which light is
used to affect the spin selective electron transport through the chiral
molecules. We describe how the excitation of a chromophore (such as
CdSe nanoparticles), which is attached to a chiral working electrode,
can flip the preferred spin orientation of the photocurrent, when
measured under the identical conditions. Thus, chirality-induced spin
polarization, when combined with light and magnetic field effects,
opens new avenues for the study of the spin transport properties of
chiral molecules and biomolecules and for creating new types of spintronic
devices in which light and molecular chirality provide new functions
and properties.