Quantum Coherence Enhances Electron Transfer Rates to Two Equivalent Electron Acceptors.

When a molecular electron donor interacts with multiple electron acceptors, quantum coherence can enhance the electron transfer (ET) rate. Here we report photodriven ET rates in a pair of donor-acceptor (D-A) compounds that link one anthracene (An) donor to one or two equivalent 1,4-benzoquinone (BQ) acceptors. Subpicosecond ET from the lowest excited singlet state of An to two BQs is about 2.4 times faster than ET to one BQ at room temperature, but about 5 times faster at cryogenic temperatures. This factor of 2 increase results from a transition from ET to one of two acceptors at room temperature to ET to a superposition state of the two acceptors with correlated system-bath fluctuations at low temperature.

Effect of the reflectional symmetry on the coherent hole transport across DNA hairpins.

The coherent hole transfer in three types of DNA hairpins containing strands with adenine (A) and guanine (G) nucleobases has been studied. The investigated hairpins involve An+1GGAn, AnGAGAn, or (AG)2nA strands that connect the hole donor and hole acceptor located on opposite ends of hairpins. The positive charge transfer from the photo-excited donor to the acceptor is shown to be slower for An+1GGAn in comparison with AnGAGAn and (AG)2nA sequences. We have revealed that this is due to the reflectional symmetry of the last two sequences with respect to the axis passing through the middle base. As has been demonstrated, the symmetry of the sequence structure manifests itself in the reflectional symmetry of the energy eigenstates. In addition, it has been shown that (AG)2nA is the only symmetric sequence with a zero energy state in the middle of the LUMO tight-binding energy band. Based on our theoretical findings, we predict that the hairpin with this sequence should have the fastest coherent hole transfer rate among the class of base sequences studied.

Spin polarization transfer by the radical pair mechanism.

In a three-site representation, we study a spin polarization transfer from radical pair spins to a nearby electron or nuclear spin. The quantum dynamics of the radical pair spins is governed by a constant exchange interaction between the radical pair spins which have different Zeeman frequencies. Radical pair spins can recombine to the singlet ground state or to lower energy triplet states. It is then shown that the coherent dynamics of the radical pair induces spin polarization on the nearby third spin in the presence of a magnetic field. The spin polarization transfer depends on the difference between Zeeman frequencies, the singlet and triplet recombination rates, and on the exchange and dipole-dipole interactions between the different spins. In particular, the sign of the polarization depends on the exchange coupling between radical pair spins and also on the difference between singlet and triplet recombination rate constants.

Spin dynamics of radical pairs with restricted geometries and strong exchange coupling: the role of hyperfine coupling.

Subnanosecond radical pair (RP) formation by electron transfer from an excited singlet state or by bond breaking produces two correlated spins coupled by their spin-spin exchange (J) and magnetic dipole (D) interactions. In the high magnetic field limit, the two-spin system can be described by a singlet state (S) and three triplet states (T0, T(+1), T(-1)). When J is small relative to the electron Zeeman interaction, |T0⟩ is the only triplet state that is populated by coherent spin mixing with the |S⟩ state because the |T(+1)⟩ and |T(-1)⟩ states are well-separated from |S⟩ by a large energy gap. Herein, we describe the spin dynamics for RPs having restricted geometries in which J is similar in magnitude to the electron Zeeman interaction and does not fluctuate significantly. Under these circumstances, depending on the sign of J, the energies of |T(+1)⟩ or |T(-1)⟩ are close to that of |S⟩ so that weak isotropic electron-nuclear hyperfine coupling leads to population of |T(+1)⟩ or |T(-1)⟩. An approximate relationship for the triplet quantum yield is developed for a RP in the large J regime, where one or both electrons interact with nearby spin-1/2 nuclei. This relationship also yields the net spin polarization transfer to the nuclear spins.

Electron transfer in a two-level system within a Cole-Davidson vitreous bath.

We study electron transfer (ET) in a two level quantum system coupled to a glassy viscous bath. The bath is modeled by the Cole-Davidson (CD) spectral density. The ET in this model is compared to the ET in a normal Drude-Debye (DD) model. It is shown that at low temperatures and when the coupling to the bath is weak, the viscous bath preserves the quantum coherence for a longer time. However in the strong coupling regime, the tunneling rate is higher in the CD. In the classical high temperature limit the difference between the CD and DD models is negligible.