Principles and Applications of Photochemistry

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1 Principles and Applications of Photochemistry Brian Wardle Manchester Metropolitan University, Manchester, UK A John Wiley & Sons, Ltd., Publication

3 Principles and Applications of Photochemistry

5 Principles and Applications of Photochemistry Brian Wardle Manchester Metropolitan University, Manchester, UK A John Wiley & Sons, Ltd., Publication

6 This edition first published John Wiley & Sons, Ltd Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the publisher nor the author shall be liable for any damages arising herefrom. Library of Congress Cataloging-in-Publication Data Wardle, Brian. Principles and applications of photochemistry / Brian Wardle. p. cm. Includes bibliographical references and index. ISBN (cloth) ISBN (pbk. : alk. paper) 1. Photochemistry. I. Title. QD708.2.W dc A catalogue record for this book is available from the British Library. ISBN CLOTH , PAPER Set in 10.5 on 13 pt Sabon by Toppan Best-set Premedia Limited Printed and bound in Great Britain by TJ International Ltd, Padstow, Cornwall

7 Dedication To my family, past and present and to my tutors, all of whom shed the light. And God said, Let there be light and there was light And God saw the light, that it was good and God divided the light from the darkness Genesis 1,3

9 Contents Preface xiii 1 Introductory Concepts 1 Aims and Objectives The Quantum Nature of Matter and Light Modelling Atoms: Atomic Orbitals Modelling Molecules: Molecular Orbitals Modelling Molecules: Electronic States Light Sources Used in Photochemistry The Mercury Lamp Lasers Efficiency of Photochemical Processes: Quantum Yield Primary Quantum Yield (φ) Overall Quantum Yield (Φ) 26 2 Light Absorption and Electronically-excited States 29 Aims and Objectives Introduction The Beer Lambert Law The Physical Basis of Light Absorption by Molecules Absorption of Light by Organic Molecules Linearly-conjugated Molecules Some Selection Rules Absorption of Light by Inorganic Complexes 43

10 viii CONTENTS 3 The Physical Deactivation of Excited States 47 Aims and Objectives Introduction Jablonski Diagrams Vibrational Relaxation Internal Conversion Intersystem Crossing Fluorescence Phosphorescence Excited-state Lifetimes Excited Singlet-state Lifetime Excited Singlet-state Radiative Lifetime Lifetimes of the T 1 Excited State 57 4 Radiative Processes of Excited States 59 Aims and Objectives Introduction Fluorescence and Fluorescence Spectra An Exception to Kasha s Rule Fluorescence Quantum Yield Factors Contributing to Fluorescence Behaviour The Nature of S Molecular Rigidity The Effect of Substituent Groups The Heavy Atom Effect Molecular Fluorescence in Analytical Chemistry Phosphorescence Delayed Fluorescence P-type Delayed Fluorescence (Triplet Triplet Annihilation) E-type Delayed Fluorescence (Thermallyactivated Delayed Fluorescence) Lanthanide Luminescence 74 5 Intramolecular Radiationless Transitions of Excited States 77 Aims and Objectives Introduction The Energy Gap Law The Franck Condon Factor Case (A): Both Electronic States Have a Similar Geometry, with a Large Energy Separation between the States 80

11 CONTENTS ix Case (B): Both Electronic States Have a Similar Geometry, with a Small Energy Separation between the States Case (C): The Electronic States Have Different Geometries, with a Large Energy Separation between the States Heavy Atom Effects on Intersystem Crossing El-Sayed s Selection Rules for Intersystem Crosssing 83 6 Intermolecular Physical Processes of Excited States 87 Aims and Objectives Quenching Processes Excimers Excimer Emission in Ca 2+ Sensing Exciplexes Exciplex Fluorescence Imaging Intermolecular Electronic Energy Transfer The Trivial or Radiative Mechanism of Energy Transfer Long-range Dipole Dipole (Coulombic) Energy Transfer Dynamic Processes within Living Cells Molecular Ruler Molecular Beacons Short-range Electron-exchange Energy Transfer Triplet Triplet Energy Transfer and Photosensitisation Singlet Oxygen and Photodynamic Therapy for Cancer Treatment Photoinduced Electron Transfer (PET) Fluorescence Switching by PET The Marcus Theory of Electron Transfer Experimental Evidence Relating to the Marcus Equation Evidence for the Inverted Region Some Aspects of the Chemical Properties of Excited States 119 Aims and Objectives The Pathway of Photochemical Reactions Differences between Photochemical and Thermal Reactions Photolysis 127

12 x CONTENTS Photohalogenation of Hydrocarbons The Stratospheric Ozone Layer: Its Photochemical Formation and Degradation Radicals in the Polluted Troposphere An Introduction to the Chemistry of Carbon-centred Radicals Photochemistry of the Complexes and Organometallic Compounds of d-block Elements The Photochemistry of Metal Complexes An Aside: Redox Potentials Involved in Photoredox Reactions Organometallic Photochemistry The Photochemistry of Alkenes 145 Aims and Objectives Excited States of Alkenes Geometrical Isomerisation by Direct Irradiation of C=C Compounds Phototherapy Vision Photosensitised Geometrical Isomerisation of C=C Compounds Synthesis Concerted Photoreactions Electrocyclic Reactions Sigmatropic Shifts Photocycloaddition Reactions Solar Energy Storage Photoaddition Reactions DNA Damage by UV The Photochemistry of Carbonyl Compounds 161 Aims and Objectives Excited States of Carbonyl Compounds α-cleavage Reactions Intermolecular Hydrogen-abstraction Reactions Intramolecular Hydrogen-abstraction Reactions Photocyloaddition Reactions The Role of Carbonyl Compounds in Polymer Chemistry Vinyl Polymerisation 170

13 CONTENTS xi Photochemical Cross-linking of Polymers Photodegradation of Polymers Investigating Some Aspects of Photochemical Reaction Mechanisms 173 Aims and Objectives Introduction Information from Electronic Spectra Triplet-quenching Studies Sensitisation Flash Photolysis Studies An Aside: Some Basic Ideas on Reaction Kinetics Flash Photolysis Studies in Bimolecular Electron-transfer Processes Photochemistry of Substituted Benzoquinones in Ethanol/Water Time-resolved Infrared Spectroscopy Femtochemistry Low-temperature Studies 195 Further Reading Semiconductor Photochemistry 197 Aims and Objectives Introduction to Semiconductor Photochemistry Solar-energy Conversion by Photovoltaic Cells Dye-sensitised Photovoltaic Cells Semiconductors as Sensitisers for Water Splitting Semiconductor Photocatalysis Semiconductor-photoinduced Superhydrophilicity 211 Further Reading An Introduction to Supramolecular Photochemistry 213 Aims and Objectives Some Basic Ideas Host Guest Supramolecular Photochemistry Micelles Zeolites as Supramolecular Hosts for Photochemical Transformations Cyclodextrins as Supramolecular Hosts Supramolecular Photochemistry in Natural Systems 221

14 xii CONTENTS Vision Photosynthesis Bacterial Photosynthesis Artificial Photosynthesis Photochemical Supramolecular Devices Devices for Photoinduced Energy or Electron Transfer Devices for Information Processing based on Photochemical or Photophysical Processes Devices Designed to Undergo Extensive Conformational Changes on Photoexcitation: Photochemically-driven Molecular Machines 235 Further Reading 238 Index 241

15 Preface Photochemistry is the branch of chemistry which relates to the interactions between matter and photons of visible or ultraviolet light and the subsequent physical and chemical processes which occur from the electronically excited state formed by photon absorption. The aim of this book is to provide an introduction to the principles and applications of photochemistry and it is generally based on my lectures to second and third - year undergraduate students at Manchester Metropolitan University (MMU). Chapters 1 and 2 give a general introduction to the concepts of light and matter and to their interaction resulting in electronically excited states. Chapters 3 to 6 relate to processes involving physical deactivation of the electronically excited states. Chapter 7 provides an overview of the chemical properties of excited states including their reaction pathways and the differences between photochemical reactions and the so - called thermal reactions. Chapters 8 and 9 relate to the photochemical reactions of two of the more interesting groups of organic compounds, namely alkenes and carbonyl compounds. Here the photochemical reactions provide an important extension to the reactions the ground state species. Chapter 10 considers some mechanistic aspects of photochemical reactions and looks at some techniques employed in this field. Chapters 11 and 12 cover areas where outstanding progress has been made in recent years. Chapter 11 considers semiconductor photochemistry whereas in Chapter 12 an introduction to supramolecular photochemistry is presented.

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