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2018 | Buch

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Spin Dynamics in Radical Pairs

verfasst von: Dr. Alan Lewis

Verlag: Springer International Publishing

Buchreihe : Springer Theses

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik" , Springer Professional "Wirtschaft"

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Über dieses Buch

This book sheds new light on the dynamical behaviour of electron spins in molecules containing two unpaired electrons (i.e. a radical pair). The quantum dynamics of these spins are made complicated by the interaction between the electrons and the many nuclear spins of the molecule; they are intractable using analytical techniques, and a naïve numerical diagonalization is not remotely possible using current computational resources. Hence, this book presents a new method for obtaining the exact quantum-mechanical dynamics of radical pairs with a modest number of nuclear spins. Readers will learn how a calculation that would take 13 years using conventional wavepacket propagation can now be done in 1 day, and will also discover a new semiclassical method for approximating the dynamics in the presence of many nuclear spins.

The new methods covered in this book are shown to provide significant insights into three topical and diverse areas: charge recombination in molecular wires (which can be used in artificially mimicking photosynthesis), magnetoelectroluminescence in organic light-emitting diodes, and avian magnetoreception (how birds sense the Earth’s magnetic field in order to navigate).

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Abstract

Radical pair reactions play an important role in a wide range of biological and technological processes. The overall rates and product yields of these reactions are strongly dependent on the spin dynamics of the radical pair, despite the fact that the magnetic interactions which produce these dynamics are far weaker than the thermal energy at room temperature. In this thesis we will outline the quantum mechanics which describes these radical pair reactions, before deriving semiclassical approximations which allow the computation of the ensemble averages of observables in realistic radical pairs. We will then apply these methods to three different problems in order to gain an insight into the physical processes which affect the rate and outcome of these radical pair reactions.

Alan Lewis

Chapter 2. Quantum Mechanics

Abstract

In this chapter, we shall outline the quantum mechanical description of the spin dynamics of a radical pair reaction. To begin, we will sketch the origin of the Hamiltonian which governs the evolution of the electron and nuclear spins in a radical pair. We will then describe the Haberkorn operator used to account for the recombination of radical pairs, and introduce the ensemble dynamics of the radical pair, which provide a connection to experimental measurements. Using this machinery, we shall demonstrate an efficient method of calculating these ensemble averages and discuss its limitations, which motivate the development of approximate semiclassical theories of spin dynamics in Chap. 3. We will then introduce spin correlation tensors, which may be used in the special case where there is no coupling between electron spins to further reduce the computational time required to simulate radical pair reactions. Finally, we shall discuss relaxation effects, noting that the difficulty of including them in fully quantum mechanical simulations provides additional motivation to find semiclassical models which can account for these effects.

Alan Lewis

Chapter 3. Semiclassical Approximations

Abstract

In Chap. 2, we outlined the quantum mechanics which describes radical pair reactions, and demonstrated an efficient way to perform fully quantum mechanical simulations. However, the computational time required for these calculations scales exponentially with the number of nuclear spins in the radical pair, meaning there is a limit to the size of radical pair which may practically be treated in this way. Furthermore, including spin relaxation in quantum mechanical simulations is an extremely complex task. In order to avoid the scaling problem, and to allow a straightforward phenomenological account of relaxation, we have considered two semiclassical models of radical pair reactions, which will be presented in this chapter.

Alan Lewis

Chapter 4. Molecular Wires

Abstract

In recent years, molecular wires have been the subject of significant interest and investigation [1, 2]. A common class of molecular wires are those with a D–B–A structure – an electron donor separated from an electron acceptor by a molecular “bridge”, typically an oligomer, which allows precise control over the separation between the electron donor and acceptor.

Alan Lewis

Chapter 5. Avian Magnetoreception

Abstract

In 2000, Ritz and Schulten proposed that a radical pair reaction could be responsible for the magnetoreception observed in some birds [1]. This suggestion gained further traction in 2008, when Maeda et al. showed that the recombination rate of a carotenoid-porphyrin-fullerene radical pair was affected by the application of an Earth-strength magnetic field [2]. At the same time, substantial circumstantial evidence for the involvement of a radical pair reaction in the avian compass was mounting. However, in order for such a reaction to act as a biological compass, it must have an anisotropic response to an Earth-strength magnetic field. This has not yet been observed experimentally, so theoretical studies of the cryptochrome-based radical pair thought to be responsible for magnetoreception are required to assess the likelihood of this mechanism being the basis of the magnetic compass of migratory birds.

Alan Lewis

Chapter 6. Magnetoelectroluminescence

Abstract

Electroluminescence is an important and much studied property of semiconducting films of conjugated organic polymers [1‐4], and is the basis of their commercial application in organic light emitting diodes (oLEDs) [5‐8]. These have the potential to be more efficient, more easily scalable, and more flexible than their inorganic counterparts [7, 9, 10]. oLEDs are constructed in four layers: a thin film of the semiconducting polymer is sandwiched between an electron-injecting metal cathode and a transparent hole-injecting layer, which is then covered by a transparent anode. Calcium and aluminium are commonly used for the cathode and indium tin oxide for the anode, with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) a typical hole-injecting layer [11‐15]. In this chapter, we are concerned with the properties of the semiconducting polymer layer which affect the efficiency of electroluminescence.

Alan Lewis

Chapter 7. Conclusions and Further Work

Abstract

There has been significant interest in radical pair reactions for a number of years [1‐5], due to their relevance to a number of biological and technological systems [6‐11]. In particular, the effect of magnetic fields on radical pair reactions has been widely studied [12‐16], with even very weak magnetic interactions able to dramatically change the rate and yield of a reaction [17, 18]. In this thesis, we have developed both quantum mechanical and semiclassical methods of simulating radical pair reactions, and then applied those methods to three real systems in order to obtain some physical insight into their behaviour. Here we shall summarise our findings, before suggesting two areas for further work where the application of the semiclassical theory introduced in Sect. 3.1 has shown promising early results.

Alan Lewis

Backmatter

Titel: Spin Dynamics in Radical Pairs
verfasst von: Dr. Alan Lewis
Verlag: Springer International Publishing
Electronic ISBN: 978-3-030-00686-0
Print ISBN: 978-3-030-00685-3
DOI: https://doi.org/10.1007/978-3-030-00686-0

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