Spin selective charge recombination in chiral donor–bridge–acceptor triads

Abstract

In this paper, we outline a physically motivated framework for describing spin-selective recombination processes in chiral systems, from which we derive spin-selective reaction operators for recombination reactions of donor–bridge–acceptor molecules, where the electron transfer is mediated by chirality and spin–orbit coupling. In general, the recombination process is selective only for spin-coherence between singlet and triplet states, and it is not, in general, selective for spin polarization. We find that spin polarization selectivity only arises in hopping-mediated electron transfer. We describe how this effective spin-polarization selectivity is a consequence of spin-polarization generated transiently in the intermediate state. The recombination process also augments the coherent spin dynamics of the charge separated state, which is found to have a significant effect on the recombination dynamics and to destroy any long-lived spin polarization. Although we only consider a simple donor–bridge–acceptor system, the framework we present here can be straightforwardly extended to describe spin-selective recombination processes in more complex systems.

Conditions for EPR detection of chirality-induced spin selectivity in spin-polarized radical pairs in isotropic solution

“…where the zero-quantum operators are: 40 ZQx = ŜPx ŜQx + ŜPy ŜQy ; ZQy = ŜPy ŜQx − ŜPx ŜQy . (18) Discarding the coherent terms, Eq. ( 17) is identical to the form we have used, Eq.…”

“…In practice, the magnitude and sign of χ would be determined by a number of factors including spin-orbit coupling and the chiral properties of the medium through which the electron is transferred when the radical pair is formed. [16][17][18] For simplicity, the CISS polarization axis was assumed to be parallel to the dipolar axis (the vector connecting the centres of spin density of the two radicals). From Eq.…”

“…[10][11][12][13][14][15] In a typical experiment, injected electrons tunnel coherently through a layer of chiral material and emerge spinpolarized. Although there are good reasons to think that CISS polarization should also arise in radical pairs formed by photo-induced electron hopping, [16][17][18] and that it should be detectable by EPR, [19][20][21][22] this has yet to be demonstrated unambiguously. Using a spintronic detection device, Carmeli et al have reported spin-selective passage of electrons through a monolayer of oriented photosystem I reaction centres in what appears to be a CISS mechanism operating on the electron transfer pathway within the protein.…”