Chromic Phenomena: Technological Applications of Colour Chemistry

Chromic Phenomena Technological Applications of Colour Chemistry

Peter Bamfield Penarth, UK Email: [email protected] and

Michael Hutchings Holcombe, Bury, UK Email: [email protected] Print ISBN: 978-1-78262-815-6 PDF ISBN: 978-1-78801-284-3 EPUB ISBN: 978-1-78801-503-5

A catalogue record for this book is available from the British Library r Peter Bamfield and Michael Hutchings 2018

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Printed in the United Kingdom by CPI Group (UK) Ltd, Croydon, CR0 4YY, UK Preface to the Third Edition

As soon as we had been invited by the Royal Society of Chemistry to write a third edition of ‘‘Chromic Phenomena’’, following the first and second editions published in 2001 and 2010, the daunting scale of the new task became apparent. Later in the process we quantified exactly how daunting by researching the information explosion we have been faced with. A search of Royal Society of Chemistry publication data reveals about a dozen key topics where there are more than ten times the amount of publications recently than we dealt with in the preceding edition of this book. (Notwithstanding new arrivals with zero hits previously, the record appears to be for ‘‘upcon- version’’ which occurs in article titles an astonishing 48 times more frequently between 2010 and 2016 as the preceding seven-year period between 2003 and 2009.) Across all 50 or so topics we checked, the average increase is about eight times. We provide a more detailed breakdown of these publication statistics in Appendix 2. So from the outset we felt a radical revision was desirable. We have eliminated the original Chapter 2 which covered basic colouration phenomena and applications, mainly on the basis that new work in this area had atypically become limited. However, as an aide memoire for our read- ership, a description of the main chromophores used in classical dyes and pigments is given Appendix 1. Elsewhere we have also cut back topics dealing with more or less obsolete technology (e.g. the cathode ray tube), but even here not completely as we feel that putting the ongoing development of certain topics within an historical perspective is important, for example optical data storage. This still left diﬃcult decisions on how to give a fair and balanced coverage of new and expanding subjects. As we stand back and survey the finished manuscript, it is clear that while the use and development of the classical chromisms continues to grow, colour applications in the life sciences have required a much expanded coverage, from the point of view of what we can refer to as analysis (imaging, sensing) and treatment

Chromic Phenomena: Technological Applications of Colour Chemistry By Peter Bamfield and Michael Hutchings r Peter Bamfield and Michael Hutchings 2018 Published by the Royal Society of Chemistry, www.rsc.org

v vi Preface to the Third Edition (photodynamic and photothermal). In the seven years before 2010, there were no Royal Society of Chemistry titles containing the key words ‘‘theranostics’’, ‘‘nanoplasmonic’’, or ‘‘mechanochromism’’. Since then each has appeared in a substantial number of article titles. Other areas have required significant rewrites, including applications in solar energy, the next generation of display technologies, and bioinspired structural colours. It is also clear that our eﬀorts to restrict the size of this edition have not been entirely successful. It now extends to 782 pages compared with the 562 of the second edition. Despite this, no doubt we have unwittingly omitted significant science and technology, for which we can only apologise to the responsible scientists. Besides the sheer volume of published science, not to mention the devices and products that result from its application, colour-related topics continue to be recognised at the highest level by awards of Nobel Prizes. Following on from green fluorescent protein in 2008, 2010 saw the recognition of graphene (physics) whose optical properties bring it into this book as a new material. In 2014 innovation of the blue LED and super-resolution microscopy were recognised by the physics and chemistry awards, respectively, both again fea- turing in later chapters. The chemistry award in 2016 for molecular machines is particularly significant for us given that one of the prize winners, Sir Fraser Stoddart, was a one-time, if temporary, colleague of ours in ICI where he readily admits much of his thinking that led to molecular machines started. We gratefully acknowledge the expert advice and comment from willing and enthusiastic contacts in response to queries. Miss Susannah Penney, Consultant Surgeon at Central Manchester NHS Foundation Trust, provided valuable observations on image guided surgery. Noor van der Veeken and Dr Toon Coenen of DELMIC B. V., Delft, The Netherlands, gave important advice on cathodoluminescence in general, and their SPARC microscope in particular. Besides the production team at the Royal Society of Chemistry, we also thank Dr Aileen Day and Dr John Boyle and colleagues in the Data Science team at the Royal Society of Chemistry for carrying out user access searches, and for advising us on our own publication statistics searches. Now that the task of pulling this new edition together is complete, we shall be able to revert to our gentle part-time retirements and re-continue Alpine and Pyrenean cycling, poetry, bass-playing, golf, attending to two and a half thousand adolescent trees, and still carry out a professional half-time job on computer aided synthesis software which somehow happily continued along- side the distraction of what we call CP3. Above all, we shall especially be able to give more and over-due attention to our long-suﬀering wives Domini and Julie who, as before, have had to put up with our absences in front of the computer screens that tried to teach us rather more about modern colour chemistry and its applications than we were prepared for. Thanks and hugs to both.

Peter Bamfield Michael Hutchings [email protected] [email protected] Introduction

Most of us go about our daily activities without paying much conscious attention to colour, except on the occasions when we are choosing what to wear or making a choice of cosmetics, judging the ripeness of fruit and without fail whilst waiting for traﬃc signals to change. In fact a quick audit would show that colour plays an extremely important and essential role in our everyday lives. Fortunately, total colour blindness is a rarity, but think what it must be like for those people for whom it is an inherited condition.1 Why colour is so important has exercised the minds of some of our greatest philosophers for many centuries.2,3 What is colour, what role does it play in Nature, and in our social and intellectual environment? Even today the answers to these questions are far from known and arguments over the psychological and social impact of colour continue to rage.4,5 However, ever since Newton recognised the relationship between light and colour in the late 17th century, our understanding of the hard science behind colour and its many applications has been increasing gradually, until it has reached today’s level of sophistication.6–8 Rather interestingly, a marriage between our understanding of the physics and chemistry of colour and their utilisation in both art and technology has become a topic of interest to many authors.9,10 Obviously the main use of colour in technology is to impart a visual stimulus as part of the overall aesthetic of the object being viewed. Whether this is in the coloration of textiles, plastics, paper, and hair or on a myriad of surfaces including wood, paper and metals, etc., an essential factor is its long-term stability to light. Yet colour can have less obvious applications beyond its position on the colour spectrum and its stability to light. Colour can be an enabler. We shall meet many applications where the actual colour is unimportant. Its value now is in being present or absent, as a label or signal. Do any of the cells in a

vii viii Introduction tissue sample fluoresce when treated with a particular fluorescent agent? If so, the presence of colour signifies that cell is malignant and is a health alert. The actual colour of the fluorescent signal is immaterial. If it is there it signifies it is a harbinger of bad news, and no-one cares if it is red, green or whatever. Or a series of four objects can be distinguished by one of four colours. It is immaterial what the actual colour labelling object 1 is, as long as that colour reliably and inevitably represents object 1 and none of the other three, etc. This exact problem is central to the identification of the sequence of the four nucleotides making up any DNA strand, part of the multibillion dollar genomics industry. We can take this a step further. The fact that a material is coloured might be totally irrelevant to its further function, and thus application. The important feature might be that the physical properties of the material cause a desirable technological effect, and yet might simultaneously be responsible for colour as an unavoidable, perhaps even undesirable, by-product. This combination can turn up in some nonlinear optical materials, where the electron-donor/p-conjugation/electron-acceptor molecular design motif of many dyes, thus giving colour, is what is significant for imparting quadratic optical nonlinearity (see Chapter 35). It was once fashionable to refer to dyes (or colouring matters in general) whose primary purpose was not to act as colorants as ‘‘functional dyes’’. We do not use that descriptor, but we shall meet coloured materials which have non-colouring applications throughout this book. As stated in earlier editions, this book is concerned with the technological applications of colour chemistry and not the theoretical aspects. However, in some of the newer areas it has been considered necessary to provide a little more theoretical understanding to help put the technology into context, for example in describing the chromic phenomena deriving from excitonic coupling within dimers and aggregates of dyes, and plasmonic coupling amongst colloidal metal particles. Other specific aspects of colour theory are covered as needed in the main body of the text. For those requiring a more detailed treatment of the theoretical aspects of colour physics excellent texts are available,7,8 whilst they are summarised in very readable forms in others of a more general nature.10,11 There are many ways in which colour can be caused to arise both by chemical and physical means, all of which are used or have potential in technological applications. These different ways of producing colour can be grouped into five fundamental classes (Table I.1). These can be further split into what Nassau has called ‘‘the fifteen causes of colour’’.6 The main Nassau groupings of I and V can be considered as physical phenomena, II and IV are borderline between chemistry and physics and III covers purely chemical phenomena. The relationship between light and the optical properties of materials, using the fifteen causes of colour as a basis, has been covered in a text that is recommended reading.12 The ‘‘fifteen causes’’ can be rather prescriptive when it comes to pulling together those technologically important chromic phenomena which rely Introduction ix Table I.1 The five groupings of the causes of colour. Group Causes Examples I Vibrations and simple excitations Incandescence, vapour lamps (external heat or energy transfer some lasers within molecules) II Ligand field effects (from Phosphorescence, lasers unpaired electrons in transition metal ions and complexes) III Transition between molecular Absorbing dyes and pigments, orbitals both organic and inorganic, fluorescence IV Transition between energy bands Metals, pure and doped semi- conductors V Geometrical and physical optics Interference phenomena, iridescence, diffraction, liquid crystals largely on chemistry for their effect. An alternative method is to follow a technology/structure based classification.13–16 However, whilst this in turn has advantages, it fails to relate the chemistry/physics across particular technology boundaries, e.g. luminescence phenomena in pigments, sensors and probes. Since the main purpose of this book is to describe the technological applications of colour, via a variety of chromic phenomena, a somewhat different system was adopted for the first and second editions.17 This involved classification of the various chromic phenomena according to whether they fall into five broad groupings involving the following processes, where ‘‘light’’ obviously implies ‘‘colour’’:

Stimulated colour change, often reversible; absorption and reflection of light; absorption of energy followed by emission of light; absorption of light and energy transfer (or conversion); manipulation of light.

Whilst arguably this is an over-simplification of the colour-producing phenomena, it has proved to be an effective way of gathering together the vast array of colour-based technological applications into an understandable mosaic (Figure I.1). In practice we have found very few examples of chromic phenomena that do not conveniently fit into one of these categories. Research into all of these five areas has not been at the same level in the decades since the first edition and we have had to make painful decisions about which areas to no longer describe in detail. It is always difficult when dealing with frontier research to decide which will lead on to technologies with commercial applications, but we believe our choices are balanced. The most difficult decision was to leave out a detailed discussion on what was Chapter 2 in the second edition (now Part 2), namely classical dyes and pigments. This may seem strange since applications in this area still x Introduction

Figure I.1 Classification of chromic phenomena. Introduction xi represent the largest volume uses of coloured chemicals with an estimated CAGR of 5% to reach a world market of around US$ 66 bn in 2022. However, since the second edition there have been few chemical developments and practically no new applications for existing dyes and pigments of importance. One significant development is the rapidly growing area of smart textiles, but these utilise mainly technologies described elsewhere in this text, especially in the colour-change area. Nevertheless, concerned that some readers might not be aware of the chromophores that are used in classical dyes and pigments, we have added an Appendix in which they are listed, together with references to several excellent books on this topic. Therefore in this third edition there are now only four parts with an added Appendix; the main technologies dependent on chromic phenomena and their respective parts are shown in Figure I.2. Part 1 covers the chemistry and applications of the stimulated colour change grouping. This contains all the well-known ‘‘isms’’ of chromic phenomena: photochromism, thermochromism, ionochromism, electrochromism and solvatochromism, as well as the lesser known but increasingly important gasochromism, vapochromism and mechanochromism. ‘‘Chromisms’’ impinge on our everyday lives, as in photochromic spectacle lenses, thermochromic paints and temperature indicators, thermal papers, smart windows and mirrors, and in visual displays. Other lesser known chromisms, introduced in the second edition, have increased in research interest since then and hence have been expanded. These include those formed by excitonic coupling and aggregation, the colour change properties of plasmonic metallic nanoparticles and their many new applications as sensors. At this point it seems appropriate to make a general comment about the nomenclature which is being allowed into the colour chemistry literature. Those working on colour change phenomena are responsible for the in- vention of some of the ugliest compound names encountered in chemistry. All ‘‘chromisms’’ that we know are listed later in Table 1.1 of Part 1, but citation there should not be taken as our personal approval or sanction of their general use. In fact we would recommend the avoidance and aban- donment of the worst. Part 2 covers phenomena which result in and then apply the emission of light, or ‘‘luminescence’’ in its most general sense. Compared with the previous edition of this book, this part has been largely rewritten to include science and technology which has appeared since 2009. The first and major chapter on photo-initiated luminescence surveys known and new types of molecules and materials that underpin the topic. At the same time, we address various important luminescence phenomena, including a new section on the triplet state, phosphorescence, and persistent luminescence. Later chapters cover luminescence caused by electricity; by reactions of a chemical, biological or electrochemical nature; and by mechanical forces. In all cases, we relate the molecules and materials to their ultimate technological applications, some commonplace, others obscure. The major applications xii Introduction

Figure I.2 The main technologies dependent on chromic phenomena with corresponding Part numbers in this book. Introduction xiii can be generalised under three categories: to aid analysis in its most basic sense; as sources of lighting; and as the basis for display technologies. Under analysis, we give an overview of the massive area of life science-based application, covering biomedical imaging; cellular probes for biochemical entities and physical phenomena; the fluorescent dyes for new super- resolution microscopy; and various extra-cellular analysis. Besides that, there are of course increasingly urgent non-biological applications such as optical detection of explosives and other undesirable volatile organics, amongst others. Lighting is undergoing a revolution based on much more energy-efficient light-emitting devices, both semi-conductor, and more recently organic (OLEDs). Both are contributing to the rapid development of ever more sophisticated display systems prevalent across every-day life, from small smart phone screens to the increasingly large domestic TVs. The various external modulating influences of light-emission processes – their ‘‘chromisms’’ – provide an under-current through this part of the book, and lead us to address the question of ‘‘chromism’’ nomenclature across the multi-dimensional luminescence field. The availability of cheap solid-state lasers that cover a wide range of the spectrum has meant that the need for molecules that can absorb their energy and convert it for use in a variety of applications has continued to grow. Part 3 covers such light processing materials and their applications in large- scale uses include optical data storage discs and the photoreceptors for laser printers, but also in photomedicine such as the treatment of cancers via photodynamic and photothermal therapies. Solar energy conversion, via photovoltaic cells, or mimicking nature’s skill with photosynthesis for the production of useful chemicals has occupied a great deal of research energy which has subsequently produced a massive investment in the production of solar cells and their installation in solar farms. At the opposite end of commercial application is the conversion of light into kinetic energy via the frontier topic of molecular machines. The manipulation of light is the topic of the last part in the book. A now classical example of light manipulation is the use of liquid crystals in flat panel display screens for PC monitors, TVs and mobile devices such as phones and tablets. Combinations of reflection, refraction, diffraction, scattering and interference used in designing lustre and colour variable synthetic pigments are remarkably common as means of generating colour in nature. This has led to their use in producing bioinspired photonic structures. Chromic materials again result when the periodicity of the material composite is changed from two dimensions to three, as in colloidal photonic crystals formed from colloidal-sized particles to give synthetic opals and inverse opals. These show a parallel set of ‘‘chromisms’’ to con- ventional dyes (Part 1). Plasmonic nanoparticles provide examples of structural colour that are finding uses in colour printing, reflective displays and electronic paper. Whilst holography has its most popular manifestation in the graphic arts, this technology can also be used to give media for high- density 3D-holographic data storage and 3D displays. Nonlinear optical xiv Introduction materials comprising second order and third order materials of interest to readers of this text are the range of chromophores that are employed and these are listed. A major target for these materials is their use in all-optical signalling and hence their potential in optical computing. No classification system can hope to encompass all the possible components without any overlap from one group to another, and this is true with the one used in this book. To overcome these difficulties there is extensive cross-referencing within the body of the text, designed to help the reader see where these overlaps occur. Such a vast array of technologies is covered that it would have to run to a multi-volume work if each had been given full justice. The objective has been to provide enough material in each area for the reader to gain a basic understanding of the theory, with greater detail provided on the chemistry and applications of the technology. In fact even the smallest sub-section is sometimes served by a monograph or at least a hefty review. We have tried to identify these so that for each topic a major source text is suggested and the reader can obtain further detailed information as required. We commented already in the Preface about the increased number of relevant publications across the board in the most recent period compared with previously. Appendix 2 takes this a step further.

References 1. O. Sacks, The Island of the Colour-blind, Picador, London, 1996. 2. J. Gage, Colour and Culture: Practice and Meaning from Antiquity to Abstraction, Thames and Hudson, London, 1993. 3. J. Gage, Colour and Meaning: Art, Science and Symbolism, Thames and Hudson, London, New edn., 2000. 4. C. A. Riley II, Color Codes. Modern Theories in Philosophy, Painting and Architecture, Literature, Music and Psychology, UP of New England, Hanover, NH, 1995. 5. Readings on Color, in The Philosophy of Color, ed. A. Byrne and D. R. Hilbert, MIT Press, Cambridge, US, 1997, vol. 1. 6. K. Nassau, The Physics and Chemistry of Color, Wiley-Interscience, New York, 2nd edn., 2001. 7. Colour Physics for Industry, ed., R. McDonald, SDC, Bradford, UK, 2nd edn., 1997. 8. (a) A. Valberg, Light Vision and Color, John Wiley & Sons Ltd, Chichester, 2005;; (b) Readings on Color, in The Science of Color, ed. A. Byrne and D. R. Hilbert, MIT Press, Cambridge, US, 1997, vol. 2. 9. Colour: Art & Science, ed. T. Lamb and J. Bourriau, Cambridge University Press, Cambridge, UK, 1995. 10. Color for Science, Art and Technology, ed. K. Nassau, Elsevier, Amsterdam, North Holland, 1997. 11. H. Zollinger, Color: A Multidisciplinary Approach, Wiley-VCH Verlag GmbH, Weinheim, 1999. Introduction xv 12. R. J. D. Tilley, Colour and the Optical Properties of Materials, John Wiley & Sons Ltd, Chichester, 2011. 13. P. Gregory, High-Technology Applications of Colour, Plenum, New York, 1991. 14. H. Zollinger, Color Chemistry, Wiley-VCH Verlag GmbH, Weinheim, 3rd edn., 2003. 15. Industrial Dyes, ed. K. Hunger, Wiley-VCH Verlag GmbH, Weinheim, 2003. 16. W. Herbst and K. Hunger, Industrial Organic Pigments, Wiley-VCH Verlag GmbH, Weinheim, 3rd edn., 2004. 17. P. Bamfield in Proceedings of ChemiChromics USA ‘99, January 1999, New Orleans, Spring Innovations Ltd, UK.

Contents

Part 1: Colour Change Phenomena and their Applications Chapter 1 Introduction 3 References 7

Chapter 2 Photochromism 8 2.1 Main Chemical Classes 10 2.2 Spirobenzopyrans 10 2.3 Spironaphthoxazines 12 2.4 Benzo and Naphthopyrans (Chromenes) 14 2.5 Fulgides 18 2.6 Diarylethenes 22 2.7 Miscellaneous Photochromic Systems 28 2.7.1 Azobenzenes 28 2.7.2 Anils and Other Organic Photochromes 30 2.7.3 Multiphotochromic Molecular Systems 34 2.7.4 Inorganic Photochromes 36 2.7.5 Organic–Inorganic Hybrids, Transition Metal Complexes and Organometallics 37 2.7.6 Polymers, Matrices and Amorphous Materials 40 2.7.7 Couples with Carbon Nanomaterials 41 2.7.8 Biological Photochromes 43 2.8 Applications of Photochromic Materials 46 2.8.1 Applications in Ophthalmics 46 2.8.2 Novelty Items, Fashion, Cosmetics and Security 49

xvii xviii Contents 2.8.3 Molecular Photoswitches 50 2.8.4 Molecular Logic Gates, Circuits and Optical Computing 51 2.8.5 Optical Data Storage 54 2.8.6 Chemical Sensors 60 2.8.7 Biological and Medical Applications 63 2.8.8 Light-responsive Molecular Containers 67 References 67

Chapter 3 Thermochromism 75 3.1 Inorganic Thermochromism: Transition Metal Complexes and Organometallic Materials 75 3.2 Reversible Intrinsically Thermochromic Organic Systems 77 3.2.1 Molecular Rearrangements 77 3.2.2 Stereoisomerism 78 3.2.3 Macromolecular Systems 80 3.3 Reversible Indirect Thermochromic Systems 84 3.3.1 Composite Thermochromic Pigments 85 3.3.2 Chromogenic Gels 87 3.4 Commercial Applications of Thermochromic Materials 88 3.4.1 Paints, Plastics, Inks and Textiles 88 3.4.2 Architectural Uses 90 References 92

Chapter 4 Ionochromism: Halochromism, Acidochromism and Metallochromism 96 4.1 Halochromic and Acidochromic Compounds 96 4.1.1 Phthalides 97 4.1.2 Leuco Di- and Triarylmethanes 98 4.1.3 Fluorans 99 4.1.4 Azo and Styryl Dyes 99 4.2 Metallochromic Materials 100 4.2.1 Metallochromism in Chelates and Crown Ethers 100 4.2.2 Chromogenic Anion Sensors and Metallochromism 102 4.3 Applications of Ionochromism and Sub-chromisms 106 4.3.1 Analytical Chemistry 107 4.3.2 Absorbance Based Ion-selective Optical Sensors 108 Contents xix 4.3.3 Carbonless Copying Paper 111 4.3.4 Direct Thermal Printing 112 4.3.5 Functional Textile Sensors 114 References 114

Chapter 5 Electrochromism 117 5.1 Electrochromic Cells 117 5.2 Electrochrome Types 119 5.2.1 Solution Electrochromes (Type-I) 119 5.2.2 Solution-solid Electrochromes (Type-II) 119 5.2.3 Solid Electrochromes (Type-III) 119 5.3 Electrochromic Chemicals 120 5.3.1 Inorganic Oxides 120 5.3.2 Prussian Blue and Metal Hexacyanometallates 122 5.3.3 Metal Phthalocyanines 122 5.3.4 Viologens (4,40-Bipyridylium Salts) 123 5.3.5 Polymeric Electrochromes 125 5.3.6 Other Organic Electrochromes 130 5.3.7 Metal Oxide Nanocrystals – Plasmonic Electrochromism 130 5.3.8 Graphene and Related Polycyclic Aromatic Hydrocarbons 131 5.4 Applications of Electrochromism 132 5.4.1 Auto-dimming Rear-view Mirrors 133 5.4.2 Dimmable Aircraft and Automobile Windows 135 5.4.3 Smart Architectural Windows – Chromogenic Glazing 135 5.4.4 Photoelectrochromic/Photovoltachromic Windows 138 5.4.5 Flexible EC Devices in Solar Protection 140 5.4.6 Flexible EC Displays 141 5.4.7 Flexible ECs in Smart Textiles 143 5.4.8 Bioelectrochromic Devices 146 References 147

Chapter 6 Gasochromism 152 6.1 Windows and Reflective Mirrors 152 6.2 Gasochromic Hydrogen Sensors 154 6.3 Colorimetric Redox Based Gas Sensors 155 References 158 xx Contents Chapter 7 Solvatochromism 160 7.1 Solvent Polarity and Solvatochromic Shifts 160 7.2 Solvatochromic Materials 163 7.3 Applications of Solvatochromism 164 7.3.1 Analysis of Liquids 165 7.3.2 Solvatochromic Probes 166 References 167

Chapter 8 Vapochromism 168 8.1 Vapochromic Materials 168 8.2 Vapochromic Sensors for VOCs and Their Applications 170 References 172

Chapter 9 Mechanochromism 174 9.1 Piezochromism 174 9.2 Tribochromism 176 9.3 Applications of Mechanochromism 176 References 177

Chapter 10 Chromic Phenomena via Aggregation 179 10.1 Excitonic Coupling and Aggregation 179 10.2 Aggregachromism 180 10.3 Crystallochromism 184 10.4 Excitonic Eﬀects in Nature 186 References 187

Chapter 11 Miscellaneous Chromisms 189 11.1 Amorphochromism 189 11.2 Chronochromism 190 11.3 Radiochromism 192 11.4 Magnetochromism 193 11.5 Biochromism 196 11.6 Hydrochromism 196 11.7 Cryochromism 200 References 201

Chapter 12 Colour Change and Nanoplasmonics 203 12.1 Plasmonic Metallic Nanoparticles 203 12.1.1 Properties of Noble Metal Nanoparticles 206 Contents xxi 12.2 Colour Change Applications of Metallic Nanoparticles 209 12.2.1 Colorimetric Sensors Based on Metallic Nanoparticles 209 12.2.2 Colorimetric Nanoparticle Biosensors 209 12.2.3 Plasmonic Chromisms 212 12.2.4 Other Applications of NMNPs 218 References 218

Chapter 13 Electrophoretic Displays 221 References 224

Part 2: Luminescent Materials and their Applications Chapter 14 Introduction 227

Chapter 15 Photoluminescence 229 15.1 Photoluminescent Chromophores – Organic Fluorophores 231 15.1.1 Traditional Molecular Fluorophores 233 15.1.2 Fluorescent Proteins 241 15.1.3 Aggregation Induced Emission 245 15.1.4 Fluorescent Polymers 249 15.1.5 Fluorescent Dyes and Polymers in Nanoparticles 251 15.2 The Triplet State – Organic Phosphorescence and Persistent Luminescence 253 15.2.1 Persistent Luminescence of Organics 257 15.3 Photoluminescent Semi-conductors 258 15.3.1 Quantum Dots 258 15.3.2 Perovskites 261 15.3.3 Carbon 263 15.3.4 Silicon 264 15.4 Photoluminescence from Metals and Metal Ions 264 15.4.1 Metal Clusters 264 15.4.2 d-block Transition Metal Ion Complexes 266 15.4.3 Rare Earth Ions 267 15.4.4 Metal–Organic Frameworks 268 15.5 Inorganic Photoluminescent Pigments – Phosphors 270 15.5.1 Phosphor Materials 271 xxii Contents 15.5.2 Phosphor Applications 273 15.5.3 Phosphors in Cathode Ray Tubes – Cathodoluminescence 274 15.5.4 Persistent Luminescence 275 15.6 Up-conversion and Up-converters 278 15.6.1 Rare Earth Ion Up-converters 278 15.6.2 Hot-band Excitation – Optical Cryocooling 280 15.6.3 Triplet–Triplet Annihilation 283 15.6.4 Multi-photon Absorbers 285 15.7 Quantum Cutters – Down-conversion 290 15.8 Fluorochromisms and Chromic Luminescence 291 15.8.1 Electrofluorochromism 292 15.8.2 Mechanofluorochromism 296 15.8.3 Vapofluorochromism and Solvatofluorochromism 302 15.8.4 Other Fluorochromisms 303 15.9 Summary of Photoluminescent Molecules and Materials 303 15.10 Photoluminescent Probes and Imaging 304 15.11 Whole Body In Vivo Imaging 308 15.11.1 Fluorescence-guided Surgery 313 15.11.2 Imaging via Persistent Luminescent Inorganic Phosphors 316 15.12 Super-resolution Imaging 317 15.12.1 STED 317 15.12.2 PALM and STORM 320 15.12.3 SOFI 322 15.12.4 Fluorescent Biosensors for Single Molecule Counting 322 15.13 Luminescent Materials and Molecules as Cell Probes and for Cell Imaging 324 15.13.1 Molecular Beacons 332 15.13.2 Applications of Fluorescent Proteins 334 15.13.3 Physical Properties within Cells 336 15.13.4 Tumour Hypoxia 339 15.14 Luminescent Materials and Molecules for Extracellular Biomedical Analysis 340 15.14.1 Wound pH 340 15.14.2 Blood Analysis 340 15.14.3 Commercial Blood Analyte Sensors 343 15.15 Luminescent Molecules and Materials for Non-bio Analysis 343 Contents xxiii 15.15.1 Oxygen Sensing 343 15.15.2 Pressure Sensitive Paints in Aerodynamics 344 15.15.3 Marine Sciences 344 15.15.4 Detection of Explosives 346 15.15.5 Detection of Other Vapours – ‘‘Vapofluorochromism’’ 349 15.15.6 Securities and Anti-counterfeiting 351 15.15.7 Environmental Contaminants of Inland and Marine Waters 353 References 353

Chapter 16 Chemiluminescence 366 16.1 Chemiluminescent Reactions 366 16.1.1 Luminol Oxidation 366 16.1.2 Acridinium Compounds 368 16.1.3 Dioxetanes 368 16.2 Chemiluminescence Applications 370 16.2.1 Chemiluminescent Analysis 370 16.2.2 Chemiluminescent Lighting 372 References 372

Chapter 17 Bioluminescence 374 17.1 Bioluminescent Reactions 374 17.1.1 Firefly Bioluminescence and D-luciferin 375 17.1.2 Marine Bioluminescence and Coelenterazine 375 17.2 Applications of Bioluminescence 377 17.2.1 ATP Determination 377 17.2.2 Bioluminescence for In Vivo Imaging 378 References 380

Chapter 18 Electrochemiluminescence 381 18.1 The Chemistry of Electrochemiluminescence 381 18.2 Analytical Technology Based on Electrochemiluminescence 384 18.2.1 DNA Detection 384 18.2.2 Functional Nucleic Acid Sensors – Aptamers 385 18.2.3 Immunoassays 386 References 387 xxiv Contents Chapter 19 Electroluminescence 388 19.1 Light Emitting Diodes (LED) 390 19.1.1 Coloured LEDs 391 19.1.2 White LEDs – Lighting and Displays 392 19.1.3 Inorganic LEDs Going Forward 394 19.1.4 Micro-LEDs 394 19.1.5 QD LEDs 395 19.1.6 Perovskite LEDs 395 19.1.7 Other New Approaches to Self-shade White LEDs 397 19.2 Organic Light Emitting Diodes (OLED) 398 19.2.1 Small Molecule OLEDs 400 19.2.2 Polymer OLEDs 401 19.2.3 Phosphorescent OLEDs – The Second Generation 403 19.2.4 TADF and ‘‘Hot Exciton’’ OLEDs – The Third Generation 405 19.2.5 White OLEDs 407 19.2.6 OLEDs as Commercial Sources of Lighting 411 19.2.7 OLEDs in Displays 412 19.2.8 Medical Applications of OLEDs 413 19.3 Light-emitting Electrochemical Cells (LEC) 414 References 416

Chapter 20 Mechanoluminescence 420 References 423

Chapter 21 Incandescence 425 Reference 426

Part 3: Light Processing Materials in Biomedical, Energy and Other Applications Chapter 22 Introduction 429 Reference 431

Chapter 23 Near-Infrared Absorbers and Their Applications 432 23.1 Organic and Organometallic Near-infrared Absorbers 434 23.1.1 Polymethines (Cyanines) 434 23.1.2 Squaraines and Croconium Dyes 437 23.1.3 Iminium Salts 439 Contents xxv 23.1.4 Triphenylmethanes 439 23.1.5 Nickel Dithiolenes 440 23.1.6 Quinones and Indoanilines 441 23.1.7 Rylenes, Other Polycyclic Hydrocarbons and Perylenes 442 23.1.8 Phthalocyanines, Porphyrins and Analogues 444 23.1.9 Donor–Acceptor Extended Conjugated Molecules 449 23.1.10 Conjugated Polymers 450 23.2 Inorganic Near-infrared Absorbers 452 23.3 Applications of NIR Absorbers and Other Laser Addressable Compounds 452 23.3.1 Thermal Energy Conversion 453 23.3.2 Protection from IR Radiation and Camouflage 457 23.3.3 Security Outlets 459 References 459

Chapter 24 Optical Data Storage 464 24.1 Magneto-Optic and Phase Change Media 464 24.2 Optical Data Storage Using Dyes 466 24.2.1 Recordable Media 466 24.2.2 Dye Requirements 467 24.2.3 Dye Classes 468 24.2.4 Dyes for High Density Recording Media 469 24.3 Developments in Optical Data Storage 470 References 471

Chapter 25 Organic Photoconductors 472 25.1 Charge Generation Materials 475 25.1.1 Azo Pigments 475 25.1.2 Phthalocyanines 476 25.1.3 Other CGMs 477 25.2 Charge Transport Materials 478 References 479

Chapter 26 Photosensitisers 480 26.1 Applications of Photosensitisers 482 26.1.1 Generation of Singlet Oxygen for Synthetic Applications 482 26.1.2 Photosensitisers on Nanoparticles, Films and Fibres 483 References 485 xxvi Contents Chapter 27 Photosensitisers in Medicine and Chemical Biology 487 27.1 Photomedicine 489 27.2 Photodynamic Therapy (PDT) 489 27.2.1 Mechanism of PDT 490 27.2.2 Light Sources for PDT 492 27.2.3 Photosensitisers for PDT 492 27.2.4 Second Generation 494 27.2.5 Third Generation 497 27.2.6 Targeted Delivery 502 27.2.7 Photodynamic Molecular Beacons 505 27.3 Photothermal Therapy 506 27.3.1 Inorganic Photothermal Agents 507 27.3.2 Organic Photothermal Agents 509 27.4 Phototheranostics 510 27.5 Phototherapy Using Nitric Oxide 514 27.6 Photoinactivation of Microbes, Viruses and Parasites 516 27.6.1 Photobactericides 516 27.6.2 Photoantivirals 520 27.6.3 Photodynamic Inactivation of Fungi 520 27.7 Other Applications of Photodynamic Inactivation 521 27.7.1 Decontamination of Blood 521 27.7.2 Photodynamic Treatment of Blood Borne Parasites 522 27.7.3 Photoinsecticides 523 27.7.4 Photodecontamination in Non-medical Areas 525 References 526

Chapter 28 Solar Energy Utilisation 533 28.1 Solar Cells and Electrical Energy 534 28.2 Photovoltaic Cells – Materials and Construction 535 28.2.1 Single Junction Wafer Based Photovoltaics 539 28.2.2 Multi-junction II–V Wafer Solar Cells 539 28.2.3 Commercial Thin Film Solar Cells 542 28.3 Emerging Third-generation Technologies 543 28.3.1 Organic Photovoltaics (OPVs) 543 28.3.2 Polymer Based Solar Cells (PSCs) 545 28.3.3 Small Molecule Based Solar Cells 549 28.3.4 Quantum Dot Solar Cells 551 28.3.5 Dye Sensitised Solar Cells (DSSCs) 553 Contents xxvii 28.3.6 Quantum Dot Sensitised Solar Cells (QDSSCs) 561 28.3.7 Perovskite Solar Cells 561 28.3.8 Plasmonics in Photovoltaic Applications 565 28.3.9 Applications of Photovoltaics in Architecture, Transport, Fashion and Design 567 28.4 Artificial Photosynthesis 570 28.4.1 Artificial Light Harvesting Antenna 571 28.4.2 Artificial Reaction Centres 575 28.4.3 Artificial Reaction Centre-Antenna Couples 576 28.4.4 Transmembrane Pumping 576 28.5 The Production of Useful Chemicals and Fuels Using Solar Energy 578 28.5.1 Water Splitting 579 28.5.2 Carbon Dioxide Photoreduction 589 References 591

Chapter 29 Conversion of Light into Kinetic Energy 599 29.1 Light-activated Molecular Tweezers 600 29.2 Light Driven Rotors 600 29.3 Photoinduced Molecular Shuttles 603 29.4 Applications of Light Driven Molecular Machines 605 References 607

Part 4: Light Manipulation Materials, Structural Colours and Photonics Chapter 30 Introduction 611

Chapter 31 Liquid Crystal Materials and their Uses 612 31.1 Nematic Liquid Crystals and Their Applications 613 31.1.1 Nematic LC Displays 614 31.1.2 Nematic Liquid Crystal Materials 618 31.1.3 Colour Filters for Displays 620 31.2 Cholesteric/Chiral Nematic Liquid Crystals and Their Applications 622 31.2.1 Cholesteric Liquid Crystal Displays, Memory Panels and Electronic Paper 623 31.2.2 Sensing Applications 627 xxviii Contents 31.2.3 Protective Eyewear 629 31.2.4 Polymeric Cholesteric Liquid Crystals and Colour 630 31.2.5 Chiral Dopants as Light Driven Molecular Switches 633 31.3 Polymer-Dispersed Liquid Crystals 634 31.4 Polymer Stabilised Blue Phase LCs 635 31.5 Side Chain Polymeric Liquid Crystals 636 References 638

Chapter 32 Colours from Physical Eﬀects 641 32.1 Lustre and Colour Variable Pigments 642 32.1.1 Optical Basis of Pigments Based on Interference and Diﬀraction 642 32.1.2 Materials, Construction and Processes 643 32.1.3 Applications of Lustre/Colour Variable Pigments 646 32.2 Interferometric Modulator (IMOD) Technology 647 32.3 Iridescent Fibres 649 32.4 The Blackest Blacks – High Absorption and Low Reflection 649 32.5 Bioinspired Photonic Structures 651 32.5.1 Structural Colours in Nature 651 32.5.2 Biomimetic Templates for Photonic Structures 653 32.5.3 Colloidal Self-assembly of Photonic Crystals/Inverse Opals 656 32.5.4 Top-down Routes to Photonic Structures 659 32.5.5 Photonic Crystals Exhibiting Chromisms and their Applications 659 32.6 Plasmonic Structural Colour 669 32.6.1 Plasmonic Nanostructures 670 32.6.2 Plasmonic Colour Printing 673 32.6.3 Plasmonic Reflective Displays 676 32.6.4 Plasmonic Colour in Electronic Paper 676 32.6.5 Other Applications 677 References 679

Chapter 33 Holography 685 33.1 Principles of Holography 685 33.1.1 Full Colour Holography 686 Contents xxix 33.2 Materials Used in Holography 687 33.2.1 Photopolymers in Holography 688 33.2.2 Rewritable Holographic Media 690 33.3 Applications of Holography 694 33.3.1 Holographic Data Storage 694 33.3.2 Holographic Imaging Applications in Security 697 33.3.3 3D Displays 697 33.3.4 Holographic Sensors and Chromisms 698 References 702

Chapter 34 Laser Diodes 705 34.1 Organic Lasers 705 34.1.1 Small Fluorescent Dyes 707 34.1.2 Luminescent Linear Conjugated Polymers 708 34.1.3 Conjugated Dendrimers 709 34.1.4 Liquid Crystal Lasers 711 34.1.5 Colloidal Photonic Crystal Lasers 712 References 714

Chapter 35 Nonlinear Optics 716 35.1 Basis of Nonlinear Optics 716 35.2 Nonlinear Optical Materials 717 35.2.1 Design Principles for D–p–A NLO Chromophores 718 35.2.2 Ionic Chromophores 719 35.2.3 Twisted p-electron Systems 720 35.2.4 Metal Complexes 721 35.2.5 Dendrimers and Poled Polymers 722 35.2.6 Polymethines and All Optical Signal Processing 722 35.2.7 Octupolar Chromophores 725 35.2.8 Multi-photon Absorbing Dyes 725 35.2.9 Reverse Saturable Absorbers – Optical Limiting 727 References 728

Chapter 36 Photorefractive Polymers 730 36.1 Applications of Photorefractive Materials 733 References 733 xxx Contents Appendix 1 Organic Chromophores used as Commercial Dyes and Pigments 734 A1.1 Structural Classes used as Dyestuﬀs 735 A1.1.1 Azo Dyestuﬀs 736 A1.1.2 Cyclic and Polycyclic Quinones 745 A1.1.3 Azines, Oxazines and Thiazines 749 A1.1.4 Polymethines 749 A1.1.5 Triarylcarbenium Dyes 751 A1.1.6 Phthalocyanines 751 A1.1.7 Sulfur Dyes 752 A1.2 Structural Classes used as Pigments 752 A1.2.1 Azo Pigments 752 A1.2.2 Metal Complexes 755 A1.2.3 Benzimidazolones 755 A1.2.4 Isoindolinone Pigments 756 A1.2.5 Phthalocyanines 756 A1.2.6 Quinacridones 758 A1.2.7 Perylenes and Perinones 759 A1.2.8 Polycyclic Quinones 760 A1.2.9 Diketo-pyrrolopyrroles 761 A1.2.10 Triarylcarbeniums 761 A1.3 Solvent Dyes 761 References 763

Appendix 2 Increase in the Number of Relevant Scientific Publications 765 References 768

Subject Index 769 Part 1 Colour Change Phenomena and their Applications

CHAPTER 1 Introduction

Colour is a property we can discern directly using our eyes, and hence is described as being perceptually conspicuous. This means that a change in the colour of an object, whether this is achromatic from white to black, or chromatic from colourless to coloured or one colour to another, is easily detected directly by people with normal vision or, in a secondary way, by the use of simple spectrophotometric instruments. Such changes in colour provide very important visual signals that we can use to convey valuable information to an observer, the most obvious in everyday experience being the ubiquitous signals that are used to control traﬃc flow. When illumin- ated, red means stop, green go and amber take care, easily seen and unambiguously understood. In addition, by selective absorption or transmission of light by a material, it is possible to restrict the light energy im- pinging upon an observer, as experienced on sunny days by users of spectacles with darkened glass lenses. When a third parameter, namely an external stimulus, whether this is chemical or physical, is the cause of the change in colour or the restriction of light transmission, especially when this change is reversible, the potential applications significantly widen. Under- standably, research into chemicals that undergo changes in colour upon the application of an external stimulus, especially when this change can be ef- fected in real time, has been extensive and ongoing. Chemical and material products of this work have found uses in a wide variety of outlets, in both low and high technology areas, and the number of applications shows no sign of diminishing, as the contents of this Part will show. Some clarification is required on the terminology used in this area. Chromic materials is a term widely used to cover products, which exhibit chromic phenomena, finding applications in what are known as smart or intelligent materials.1a,b The term chromatic phenomena has also been suggested, but this has been avoided, since this it is already widely used in music, art and the optical sciences. Chromogenic phenomena, whilst

3 4

Table 1.1 List of chromisms.a–s Hybrids Electrochromism Ionochromism Mechanochromism Photochromism Solvatochromism Thermochromism (dual stimulus) Miscellaneous Gasochromism Acidochromism Piezochromism Diastereo- Aquachromism Diastereo- Photoelectro- Group A photochromism thermochromism chromism Hydrochromism Photovolta- Affinochromism chromism Hygrochromism Bioelectro- Affinitychromism chromism Biochromism Halochromism Tribochromism Heliochromism Rigidochromism Cryochromism Thermosolvato- Group B chromism Halosolvato- chromism Viscochromism Aggregachromism Amorphochromism Crystallochromism Concentrato- chromism Metallochromism Barochromism Chirochromism Sorptiochromism Electromechano- Group C chromism Electropiezo- Cathodochromism chromism Magnetochromism Radiochromism Alkalinochromism Rheochromism Aromachromism Chronochromism Goniochromism 1 Chapter Vapochromism aThere are six main classes of chromic phenomena, plus some hybrids and a miscellany of others. bThere are sub-chromisms in all of these especially Solvatochromism, many of which have been given alternative names historically. cHybrids are ones where more than one chromic phenomenon is at work, although quite commonly we have used the term ‘‘indirect chromism’’. Introduction dThe Miscellaneous ones can be put in three groups. A. where stimulus is biological/chemical, B. involving some change in shape or form, C. involving an external field or radiant energy. eThose in bold italics will have separate sections in Part 1. The rest will appear within the text of the main ones as appropriate. Goniochromism is covered in Part 4 (Chapter 32, Section 32.1.1). fRigidochromism and Viscochromism is usually reserved for luminescent behaviour as more obviously in Fluorochromism (Part 2). gGeneric term Chemichromism (hence chemichromics) and therefore Photochemichromism, electrochemichromism, etc. hMultichromism (multichromic) – commonly used term for multi-coloured forms, e.g. CP in polymers. iPolychromism (polychromics) – several coloured forms, can be related aggregachromism, crystallochromism etc. jOnly one or very few references in Google to Elastochromic but not chromism, colour change on deforming rubbery films of colloidal crystals, Lyochromic not chromism, Sonochromism and (Sonochromic), Tautochromism but not chromic, Varichromism and Waterchromism. kCryptochromism – camouflage (Cryptochromic). lBathochromism (bathochromic), Hyperchromism (hyperchromic), Hypochromism (Hypochromic), Hypsochromism (Hypsochromic), Ipsochromism (Ipsochromic) – all relate to effects on absorption bands. mPanchromism (Panchromic) – wide spectral response. nMonochromism (monochromic) having only one colour, Dichromism (dichromic) two colours in defective vision or having two distinct colours, Trichromism (trichromic) similarly three distinct colours, Tetrachromism (tetrachromic, e.g. bird vision) four distinct colours. No *chromisms of following but do have *chromic references, pentachromic, hexachromic, and heptachromic. Most relate to aspects of vision, but can be confused with salts of chromium in searches. oPleochromism – colour change in gemstones. pPhytochromism colour reaction of phytochromic pigments. qAchromism – colourless often used in a race context. Heterochromism (heterochromic) – medical term, e.g. differently coloured iris in eyes, but also skin. Homo- chromism (homochromic) having one colour (usually a medical term). Metachromism (metachromic) in evolutionary theory. Xanthochromism applied to birds, fish and other animals whose colouration is unusually yellow through an excess of yellow pigment. rSynchromism (synchromic) – idea in art that colour and sound are the same phenomenon. sThe following have only the *chromic and not *chromism ending. Antichromic (anticancer), Arkyochromic (medical), Autochromic (biochemical and in dimming mirrors), Cytochromic, Karyochromic, Lipochromic (lipid pigments), Normachromic or Normochromic (medical – blood count), Ommochromic (natural pigments), Optochromic, Quadrichromic – four colour printing technique, Somatochromic, Spectrochromic, Stichochromic, Superchromic, Transchromic (transgenic animals). 5 6 Chapter 1 synonymous with chromic phenomena, has been adopted as the preferred one in the automotive and architectural areas, hence chromogenic materials.2a,b There is a further complication in inorganic chemistry, especially where metal complexes are involved, as here the topic is called chromotropism, but definitely not chromotropic which is reserved for the naphthalene sulfonic acid of that name.2c These colour change phenomena, whether known as chromic or chromogenic, are classified and named after the stimulus that causes changes in the differential absorption, and reflection and/or scattering of white light. For this reason, stimuli-responsive materials is a generic name commonly applied to the products used in the various application areas for colour change.2d There are two basic types of chromisms; direct and indirect. Direct is obviously where the stimulus itself causes an almost instantaneous change in colour, e.g. photochromism by light, whereas with indirect chromisms there is not an immediate change in colour but one that only occurs via an intermediate index, which can be either chemical or physical, e.g. chronochromism which involves a passage of time. Some of the names of these chromisms are immediately obvious. Photo- chromism, as stated above, is a change in colour, usually colourless to coloured, brought about by light, and the material or chemicals undergoing this change are photochromic. Electrochromism is a reversible colour change upon oxidation or reduction brought about by an electrical current or potential, thermochromism is a colour change brought about by heat, solvatochromism by solvents and ionochromism by ions, etc. A long list of names, shown in Table 1.1, has been devised to describe such chromic phenomena, from the very specific to others which seem to have been invented on the whim of a particular researcher, e.g. waterchromism for which alternatives like aquachromism and hydrochromism already existed. We have made an attempt to rationalise the nomenclature applied to these chromic phenomena in Table 1.1. To date the most important commercially of these phenomena are photochromism, thermochromism, electrochromism, ionochromism and solvatochromism, and consequently these will be covered in some detail in the chapters below. Among the miscellaneous chromisms several have grown in commercial importance in recent years, namely gasochromism, vapochromism, mechanochromism, and those due to aggregation or morphological changes, called aggregachromism, and the more recent ones due to plasmonic effects in metal nanoparticles. We have adopted a novel approach in assigning chromisms to those colour change effects induced by a variety of external stimuli on photonic crystals (Chapter 35, Section 35.5.5) and on holographic sensors (Chapter 33, Section 34.3.4). Many of these chromisms have important luminescent equivalents, at least nominally. Some of these are referred to as fluorochromisms, for instance solvatofluorochromism, mechanofluorochromism, etc. However, the nature of the chromism and luminescence is a complex subject that we have attempted to further explain and rationalise in Part 2. Introduction 7 It is becoming increasingly difficult with many new developments to make a clean separation between the various chromisms and the respective chromic materials. For instance the exciting work that has been carried out in recent years on p-conjugated organophosphorus compounds has shown that these molecules are responsive to a wide range of stimuli, including electric current, light, solvent polarity, temperature, mechanical force, solvent vapour and pH.2e

References 1. (a) P. Talvenmaa, in Intelligent Textiles and Clothing, ed. H. Mattila, Woodhead Publishing, Cambridge, 2006; (b) R. M. Christie, in Advances in the Dyeing and Finishing of Technical Textiles, ed. M. Gulrajani, Woodhead Publishing, 2013. 2. (a) C. M. Lampert, Chromogenic smart materials, in Materials Today, March 2004, pp. 28–35; (b) S. A. Jenekhe and D. J. Kiserow, Chromogenic Phenomena in Polymers, American Chemical Society, Washington, 2004; (c) C. Y. Fukuda, Inorganic Chromotropism: Basic Concepts and Applications of Colored Materials, Springer, Berlin, 2007; (d) M. W. Urban, Stimuli- Responsive Materials: From Molecules to Nature Mimicking Materials Design, Royal Society of Chemistry, Cambridge, 2016; (e) C. Reus and T. Baumgartner, Stimuli-responsive chromism in organophosphorus chemistry, Dalton Trans., 2016, 45, 1850–1855. CHAPTER 2 Photochromism

Photochromism is a chemical process in which a compound undergoes a reversible change between two states having separate absorption spectra, i.e. diﬀerent colours, whether the compound is in a crystalline, amorphous or solution state. The change, as illustrated in Figure 2.1, from a thermodynamically stable form A to B occurs under the influence of electromagnetic radiation, sometimes called ‘‘actinic radiation’’, usually UV light, and in the reverse direction, B to A, by altering or removing the light source or alternatively by thermalmeans.Whenthebackreaction occurs photochemically this is known as P-type photochromism and when thermally as T-type photochromism. In P-type materials the colour change occurs when irradiated at a specific wavelength, and stays in that state until irradiated by light of a diﬀerent wavelength, because the potential energy barrier between B and A is high. In the case of T-type materials the converse is true, so that B is metastable, and the colour change after irradiation remains in that state only as long as the light source remains, otherwise they fade back to the original state by thermal means. The change in colour in the forward direction is usually to longer wavelength, or bathochromic, as shown in Figure 2.1. The reversibility of this distinct colour change is an essential key to many of the uses of photochromism. The assistance of heat in the reversion of colour could be regarded as an example of thermochromism, but in this text the term is reserved for those systems where heat is the main cause of the colour change (see Chapter 3). In many of the photochromic molecular systems, including spiropyrans, spirooxazines and naphthopyrans (chromenes), the back reaction is pre- dominantly thermally driven and hence T-type, but other important photochromic molecules, such as fulgides and diarylethenes, are P-types where the photochemically induced state is thermally stable and the back reaction

8 Photochromism 9

Figure 2.1 Absorption spectra of reversible photochromic species. must be driven photochemically. Where a coloured form is photobleached to a transient colourless form followed by a thermal back reaction, it is called negative photochromism. The types of processes that are commonly used to design photochromic material include pericyclic/electrocyclic reactions, cis–trans isomerisations, intermolecular hydrogen transfers and group transfers, dissociation processes and electron transfer. Photochromism is therefore a vast field and in this chapter we will only be able to describe in any detail the main classes of photochromic compounds, but we will include several new systems that have come to the fore in recent years. Again, whilst the main emphasis will be on the current commercial applications for these materials, this is a constantly changing field and so some applications of a more speculative nature, especially those having potential in the near future, will also be described. For more detailed accounts of the chemistry of the historically important classes of photochromic materials the reader should consult more comprehensive texts, such as the ones edited by Crano and Guglielmetti1,2 or those by Bouas-Laurent and Durr3,4 and in the more recent texts.5a,b 10 Chapter 2 2.1 Main Chemical Classes The chemical reactions involved in photochromic behaviour in the main families of organic photochromes can be classified as follows:5b

1. Cyclisations – e.g. spiropyrans, spirooxazines, chromenes and fulgides. 2. Cis–trans isomerisation – e.g. azobenzenes, acylhydrazones. 3. Proton transfer – e.g. anils. 4. Homolytic cleavage – e.g. triphenylimidazoyl dimer.

For most commercial applications the minimum properties required for practical use from any class of organic photochromic compounds are:

1. Colour development: The material must develop a strong colour rapidly upon irradiation with UV light. 2. Control of return back to colourless state: The fade rate back to the colourless state must be controllable. 3. Wide colour range: The range of colours must be across the visible spectrum. 4. Long life: The response must be constant through many colouration cycles. 5. Colourless rest state: The rest state must have as little colour as possible, preferably colourless.

These are generic requirements, of the chosen materials, but in the literature these are most often described using the following terms:

1. Thermal stability (of both isomers); 2. fatigue-resistant; 3. high sensitivity; 4. rapid response; 5. reactivity in the solid state (in polymer matrices or in the crystalline phase).

There are five main families of compounds which can approach these ideal requirements: the T-type spiropyrans, specifically spiroindolinobenzopyrans, spironaphthoxazines, naphthopyrans, and the P-type fulgides and diarylethenes, as already mentioned above.

2.2 Spirobenzopyrans Spirobenzopyrans are a very widely studied chemical class of compounds which exhibit T-type photochromism. They consist structurally of a pyran ring, usually a 2H-1-benzopyran, linked via a common spiro group to another heterocyclic ring (1.1). Irradiation of the colourless spirobenzopyran (1.1) with UV light causes heterolytic cleavage of the carbon–oxygen bond to form Photochromism 11 the ring opened coloured species, often called the ‘‘merocyanine’’ form (MC), which can be present as either cis- (1.2) or trans- (1.3) isomers (Figure 2.2). They can also be drawn in the ortho-quinoidal form, as represented by (1.4) for the trans-isomer (1.3). The structure of the ring-opened form is probably best represented by a delocalised system with partial charges on nitrogen and oxygen atoms. For simplicity we will use the equivalent of the trans merocyanine structure (1.4) in this text. A very large number of possibilities exist for varying the components of the spiropyran ring. The pyran ring is usually a substituted benzo or naphthopyran but the heterocyclic component can be chosen from a long list of ring systems including indole, benzthiazole, benzoxazole, benzselenazole, quinoline, acridine, phenanthridine, benzopyran, naphthopyran, xanthene, pyrrolidine and thiazolidine. The thiopyran analogues have attracted much interest, as on ring opening they absorb at longer wavelengths than the corresponding pyrans. Spiroindolinobenzopyrans are readily synthesised typically by reacting Fischer’s base or a derivative with aromatic hydro- xyaldehydes (salicylaldehydes). The open chain form of the spirobenzopyran shows a strong, intense absorption in the visible region of the spectrum typical of merocyanine dyes. Because of the thermal instability of the open chain form it is necessary to use a rapid scanning spectrophotometer to measure the absorption spectrum. The ring-opened form of spiroindolinobenzopyran (1.4) has absorption at lmax 531 nm in toluene. This class of compounds exhibits a strongly positive solvatochromic eﬀect (see Chapter 7), with the shape of the absorption curve changing and its position moving hypsochromically as the

Me Me Me Me 3

N O 6 N O Me Me 8 (1.1) (1.2) Colourless Coloured

Me Me Me Me

N N Me Me O O (1.4) (1.3)

Figure 2.2 Spiroindolinobenzopyran and ring-opened merocyanine quinonoid form. 12 Chapter 2 Table 2.1 Absorption maxima (ethanol) of the coloured form of substituted spiroindolinobenzopyrans.

Me Me R 4 R1 R2

N Me O R (1.4) 3

Compound R1 R2 R3 R4 lmax (nm)

(1.4a) Ph NO2 OMe H 625 (1.4b) H NO2 OMe Ph 568 (1.4c) H OMe NO2 Ph 625 (1.4d) H NO2 H H 532 (1.4e) H H NO2 H 544 solvent polarity increases.6 Studies of these and other closely related spiro- heterocycles have provided a theoretical understanding into the origins of their photochromic, thermochromic and solvatochromic properties.7a,b From the data given in Table 2.1, it can be seen that substituents in the 3, 6, and 8 positions of the original spiropyran ring (see 1.1) have the biggest influence on the spectral properties of the coloured form.8 The large bathochromic shift in (1.4a) versus (1.4b) is credited to steric hindrance caused by the group in position 3, whilst that from the nitro group at position 8 is considered to be due to interaction of the phenolate anion with the oxygen atom of the nitro group. Replacing the isoindoline group in (1.4a) with a benzoxazole ring causes a hypsochromic shift (600 nm) whilst a benzthiazole ring moves the absorption bathochromically (635 nm). However, in spiropyrans the two isomers possess very diﬀerent properties so that the reversible isomerisation is not only achievable by light and thermal means, as stated above, but also by using diﬀerent solvents, acids, bases, metal ions, redox potential and mechanical force.9 This makes them very useful as optical gates for switches and memories, as will be discussed later in Section 2.8.3.

2.3 Spironaphthoxazines Spirooxazines, the nitrogen-containing analogues of the spiropyrans, are very resistant to photodegradation. This fatigue resistance is an essential property for those photochromic materials designed for applications in solar protection uses, such as sun spectacles. The photochromic ring opening of the benzannelated spironaphthoxazine analogue to its coloured form is shown in Figure 2.3. Nitrosonaphthols (1.5), the precursors used in the synthetic route to spirooxazines, are much more stable than the nitrosophenols required for Photochromism 13

Me Me Me Me N

N O N N Me Me O

Colourless Coloured merocyanine (MC)

Figure 2.3 Spironaphthoxazine photochromic forms. the parent benzo analogue and hence all the commercially available products are based on the spiroindolinonaphthoxazines ring structure (1.6).

O Me Me N 5' N OH 1 N O Me 6

(1.5) (1.6)

The spiroindolinonaphthoxazine derivatives became commercially important compounds once detailed research work led to products which overcame many of their initial weaknesses, such as relatively poor fatigue resistance and a poor colour range (550–620 nm). The important positions for substitution in the ring of (1.6) are the 50-position which has a large effect on the colour; the 60-position, which has a major effect on both the colour and properties such as optical density (OD) and extinction coefficient; and the alkyl group on position 1, which has a kinetic effect on the rate of loss of colour back to the colourless state.10 Another group of commercially important spirooxazines are those where the naphthalene ring is replaced by quinoline to give the spiroindolinopyr- idobenzoxazines (1.7). 14 Chapter 2 Table 2.2 Substituent effects on the absorption maxima (acetone) of the coloured state of spiroindolinonaphthoxazines (1.6).

0 6-Substituent 5 -Substituent lmax (nm) H H 605 Indolino H 592 Indolino OMe 623 Indolino Piperidino 637 Piperidino H 578 Piperidino Cl 568 Piperidino CF3 560 Morpholino H 580 Diethylamino H 574

The spectrum of the ring-opened form of the parent spiroindolinonaphthoxazine (1.6) has lmax at 590 nm (acetone). Spiroindo- linonaphthoxazines also show a negative solvatochromic shift, the absorption moving hypsochromically (20–60 nm) in less polar solvents (e.g. toluene versus ethanol), which is the converse of what happens with spiroindolinobenzopyrans (see Section 2.2). Introduction of dialkylamino substituents at the 60-position of (1.6) causes a hypsochromic shift in the absorption maximum of the coloured state and also an increase in its intensity. This hypsochromic shift can also be increased by introducing electron-withdrawing groups at the 5-position of (1.6), whilst electron-donating groups move the absorption maximum in the opposite direction (Table 2.2).10,11 Changing the alkyl substituent at the 1-position of (1.6) has little or no effect on the absorption maxima and no effect on the fatigue resistance. However, there is a very marked and technically important effect on the loss in the initial optical density of the coloured state after activating with UV light. This is frequently reported as the percentage loss in initial optical density ten seconds after removing the UV source, the IODF10 value. The more highly branched the alkyl group the lower the IODF10; methyl shows an IODF10 of 29% whilst for neopentyl this drops to 9%. Additionally, increasing the branching causes a lowering of the temperature dependence of the thermal conversion back into the colourless state.10 There has also been a systematic study on the effect of changing the gem alkyl groups at the 3-position as well as in the aromatic rings of (1.6) and its structural analogues.12

2.4 Benzo and Naphthopyrans (Chromenes) The photochromic compounds of potential interest based on the 2H- chromene-ring system are the 2H-benzopyrans (1.8) or the three isomeric naphthopyrans (1.9–1.11). However, 2H-naphtho[2,3-b]pyrans (1.11) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R1 and R2 can be part of a carbocyclic spiro Photochromism 15

Figure 2.4 Photochromic behaviour of chromenes. ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups.

The photochromic mechanism for the chromenes is very similar to that for spiropyrans given in Figure 2.12. Under the influence of UV the C–O bond in the pyran ring is broken (as in Figure 2.4), where formation of the trans–cis quinoidal species occurs in picoseconds, followed by isomerisation to the trans–trans form in nanoseconds. The two naphthopyrans of interest, (1.9) and (1.10), show quite diﬀerent photochromic behaviour. Isomer (1.10; R1,R2 ¼ Ph) produces a more bath- 1 2 ochromic coloured state than (1.9; R ,R ¼ Ph), (lmax ¼ 481 nm versus 432 nm in toluene), a large increase in colouration, but a very slow fade rate back to the colourless state.13a Because of the slow kinetics, coupled with a greater ease in synthesis, most of the work, until the mid-1990s when these problems were overcome, concentrated on the 3H-naphtho[2,1-b]pyrans (1.9). Simple 2,2-dialkyl-2H-benzopyrans can be synthesised by several well- established routes.13–16 3H-Naphtho[2,1-b]pyrans (1.9) with an amino or alkoxy residue at the 6-position (1.14), which show particularly high colourability, are synthesised from the corresponding 1-amino- and 1-alkoxy- 3-hydroxynaphthalenes (1.12 and 1.13) (Figure 2.5).17 As mentioned in Sections 2.2 and 2.3, the photochromic reactions of spirobenzopyran and spironaphthoxazines show a marked solvent dependency and this is also the case with benzo- and naphthopyrans. 16 Chapter 2

OH SiMe3 OH

NR2 OR Ar Ar' (1.12) OH (1.13)

Ar R O Ar'

(1.14; R = NR2 or OR)

Figure 2.5 Synthetic route to 6-amino and 6-alkoxy-3,30-3H-naphtho[2,1-b]pyrans.

Table 2.3 Influence of substituents in the 3-phenyl rings on the properties of 3,3-diaryl-3H-naphtho[2,1-b]pyrans. Reproduced from ref. 17 with permission of Springer Science and Business Media.

O R2

1 2 a R R l (nm) DOD T1 (s) Solvent max 2 H H 430 — 11 A H 4-MeO 458 — 8 A H 4-CF3 422 — 28 A 4-MeO 4-MeO 475 — 3 A 4-MeO 4-CF3 440 — 4 A 4-MeO 4-NMe2 512 — 1 A 4-NMe2 4-NMe2 544 — 0.5 A H H 0.36 45 B 2-F 4-MeO 456 1.0 170 B 2-F 3,4-diMeO 472 1.05 203 B 2-Me 4-MeO 469 2.40 600 B 2,6-diF2 4-MeO 450 2.23 1800 B aA ¼ toluene. B ¼ imbibed into di(ethylene glycol) bis(allyl carbonate) polymer.

Consequently, spectral data collected from the literature are only com- parable within any one study or where the same solvent has been used. This accounts for any discrepancies between one set of results and any Photochromism 17 Table 2.4 Influence of substituents in the 6- and 8-positions on the properties of 3,3-diaryl-3H-naphtho[2,1-b]pyrans. Reproduced from ref. 13a with permission of the University of Leeds, Department of Colour Chemistry.

1 2 a R R lmax (nm) IOD IODF10% Solvent/host H H 475 0.20 50 A MeO H 456 1.89 7 A H MeO 502 0.55 41 A H H 475 0.12 45 B MeO H 456 1.42 10 B Piperidino H 452 1.95 11 B Morpholino H 452 1.95 13 B aA ¼ polyurethane. B ¼ Spectralite. other listed in this and related sections of the Part. The data normally quoted when discussing the properties of photochromic materials relate to the absorption maximum (lmax) of the coloured state, the change in optical density on exposure to the xenon light source (DOD) and the fade rate (T1), 2 which is the time in seconds for the DOD to return to half of its equi- librium value. Two other measurements, often quoted in the literature, are the IOD (initial optical density) at lmax and the IODF10 value already described in Section 2.3. The influences on the absorption spectra and the other photochromic properties of compounds with substituents in the 3H-naphtho[2,1,b]pyran ring and on the 3,30-aryl groups have been studied in detail.13,17 Electron- donating groups on one or both of the 3-phenyl groups, especially at the para-position, show a marked bathochromic shift in the absorption maxima of the coloured state, whilst electron-withdrawing groups have the opposite effect (Table 2.3). Substitutions in the ortho-position have little effect on the absorption maxima but have a very marked effect on the rate of return back to the colourless state, presumably due to stabilisation of the open chain form (Table 2.3). Substitution in the naphthopyran ring is most effective in the 6- and 8-positions, especially the former. Electron-donating groups in the 8- position cause a bathochromic shift and an increase in IOD. In position 6, electron-donating groups have the opposite effect on the absorption maximum, but more importantly produce an even larger increase in IOD and a dramatic reduction in the IODF10, an effect that is described as ‘‘hyperchromic’’ (Table 2.4).13 As mentioned above, the main problem with the 2H-naphtho[1,2-b]pyran ring system (1.10) is the very slow rate of fading back to the colourless state: 1 2 1 2 (1.10: R ,R¼ Ph), t1/2 ¼ 1800 s versus 45 s for (1.9; R ,R¼ Ph). These problems have largely been overcome to produce materials suitable for ophthalmic uses by introducing electron-withdrawing groups in the 5-position, as in (1.15), and, more intriguingly, by having an indeno group fused onto the 5- and 6-positions (1.16)), covered in many patents to Transitions–Optical (see also in Section 2.8.1).17,18 18 Chapter 2

Of particular interest for ophthalmic outlets are neutral grey coloured photochromics. To meet this objective 2H-naphtho[1,2-b]pyrans with matched twin absorption peaks have been synthesised which produce various shades of grey, for example (1.17) has lmax 490 and 581 nm in toluene (see also Section 2.8.1).13b Having very fast colour-fading in photochromics would enable naphthopyrans to be considered for use in openings such as dynamic holography and molecular actuation (Part 4). To this end it has been shown that putting large groups, e.g. bromine or aryl in position 2- or 10- of (1.9) oﬀers steric hindrance to the formation of the trans–trans isomer, thus favouring the trans–cis isomer and enabling very fast colour fading, decreasing from many minutes to microseconds, even in polymer matrices (Table 2.5).19

2.5 Fulgides Stobbe was the first to observe photochromism in fulgides (1.18) when he synthesised them by the condensation of an arylaldehyde or ketone with a substituted methylene succinate.20 However, it was not until the 1970s, during the course of the extensive work carried out by Heller and his col- laborators, that their chemistry and use in photochromism was truly exploited.21–23 Heller’s work showed, amongst many other things, the importance of R1 in (1.18) being a five-membered ring heterocycle, e.g. furan.

Fulgides can exist as (E)- or (Z)-isomers by virtue of rotation around the double bonds in (1.18). This is illustrated for the furano derivative in Figure 2.6. Isomerisation of the yellow (Z)-fulgide (1.19) to the (E)-fulgide Photochromism 19 Table 2.5 Fast colour-fading in naphthopyrans.21

R3 R4

R1 R3 UV R2 R2 R1 O 4 R Δ, Vis O

CF TC Δ, UV

Δ, Vis

R3 R4

TT R2 R1 O

1 2 3 NP : R =H,R =H,R = CH(CH3)2,R4=OCH3 1 2 3 10-Br-NP: R =H,R =Br,R =CH(CH3)2, R4 = OCH3 1 2 3 2-Br-NP: R =Br,R =H,R =CH(CH3)2,R4=OCH3 1 2 3 2,10-Br-NP: R =Br,R =Br,R = CH(CH3)2, R4 = OCH3 2,10-Ph-NP: R1 =Ph,R2 =Ph, R3 =H,R4 =H 2-Py-NP: R1 =Pyrenyl,R2 =H, R3 =H,R4 =H

Compound l (nm) t1 max 2 NP 465 34 min 10-Br-NP 455 1.4 min 2-Br-NP 415 2.5 ms 2,10-Br-NP 415 0.8 ms 2,10-Ph-NP 430 5.1 ms 2-Py-NP 452 46 ms

(1.20) and cyclisation of this to the red photochrome (1.21), designated as C here but often called the P state, occurs on irradiation with UV light. The coloured species (1.21) is converted back into the (E)-fulgide (1.20) by white light but not by heat. Thermally assisted reversion of coloured to colourless is not observed, because the interactions between the two syn methyl groups prevent the symmetry allowed, disrotatory mode of electrocyclic ring opening.24 Since both the forward and back reactions require light energy it is important that they both show good quantum yields. 20 Chapter 2

Me O Me O Me O Me O Me O Me Me O Me O O Me O Me Me O disrotatory Me O conrotatory Me Me O Me

(1.19) (1.20) (1.21) Z- E- C

Figure 2.6 Photochromic process for fulgides. Reproduced with permission from ref. 24.

The Stobbe condensation, as described above, is the preferred route to fulgides as exemplified by the synthesis of the five-membered ring analogues such as the anhydrides (1.22), which can be readily converted into the corresponding fulgimides (1.23), or further functionalised by reaction with, for example, malononitrile to give (1.24) from the corresponding (E)-fulgide and (1.25) from the (Z)-isomer.25

The main areas for molecular manipulation of the heterocyclic fulgides (1.26; Table 2.6) are the hetero atom X in the five-membered ring; the substituent R4 on this ring; R2 on the bridging methylene group; changes in the exomethylene group substituents R1 and R2; changes from fulgides (1.26, A ¼ O) to give fulgimides (1.26, A ¼ NR); and conversion of the ring carbonyl groups into imides (1.26; B or C ¼ NR) or methylene groups as in (1.24). The change from X ¼ OtoX¼ S and NR causes the absorption of the coloured state to move bathochromically, from red to purple and blue. Major bathochromic shifts are also observed when R4 is a phenyl group carrying electron-donating groups in the 4-position. The substituent R3 has a marked influence on the quantum eﬃciency of the colouration process; isopropyl has in some cases given a 20–60% increase over methyl. Again the main Photochromism

Table 2.6 Eﬀect of substituents of the properties of fulgides and derivatives.

4 R3 R R3 B B

X R4 Me A A Me X R2 R2 R1 C C R1

(1.26E) (1.26C)

1 2 3 4 XR R R R AB C lmax Col (nm) emax Col fEC fCE O Me Me Me Me O O O 496 8200 0.20 0.055 S Me Me Me Me O O O 525 8000 0.15 0.01 NPh Me Me Me Me O O O 630 7300 — — O Me Me Me Ph-4-NR2 O O O 588 26 500 — — O Me Me i-Pr Me O O O 500 9000 0.58 0.043 O Adamantyl i-Pr Me O O O 519 6900 0.51 0.26 O Cyclopropyl Cyclopropyl Me Me O C(CN)2 O 594 — — — O Me Me O O C(CN)2 633 — — — O Me Me Me Ph O O C(CN)2 669 — — — 21 22 Chapter 2 effect of changing substituents R1 and R2 is photochemical, where replacing isopropylidene with a spiro adamantyl group results in a six-fold increase in the decolouration efficiency. Changing from a fulgide to a fulgimide has little effect on the photochromic properties but does make the ring system more resistant to hydrolysis. Replacement of either of the ring carbonyls of the fulgide by NR gives the isofulgimides. When B ¼ NR the change in colour is hypsochromic but for C ¼ NR there is little change in the absorption maximum but a large increase in extinction coefficient. Both of the dicya- nomethylene derivatives (1.24) and (1.25) move bathochromically, the effect being most marked in the latter case. The thienyl analogue of (1.25) is even more bathochromically absorbing.26 The results from some of these key structural modifications are summarised in Table 2.6. Another modification involves extending the heteroaryl component. For instance the indolylfulgides (1.27, X ¼ O) and fulgimides (1.27, X ¼ NR) are of particular interest as they show high resistance to both photochemical and thermal stress.27 Where the R1 substituent is a trifluoromethyl group (1.27, 1 28 R ¼ CF3, X ¼ O or NR) there is a strong bathochromic shift. Introducing a fluorinated phenyl ring in the fulgimide, for example (1.28), is claimed to produce a highly photochemically stable product, which also demonstrates high hydrolytic stability.29 Another interesting development in this area has been the synthesis of stable water-soluble fulgimides (1.29), opening up their use in aqueous environments.30

2.6 Diarylethenes Pioneering work carried out by Irie in the 1980s led to the development of these most valuable photochromes, and significantly they have become the most studied family of photochromes since the turn of the century.5a Their synthesis design involved using as a base the well-known photoisomerisation of stilbene and modifying the structure by replacing the phenyl rings with thiophenes, and the bridging ethylene group by a maleic anhydride, imide or perfluorocyclopentene group (Figure 2.7). The thiophene ring can be annulated with a benzene ring or replaced with indoles, furans and thiazole rings. The reversible electrocyclic interconversion between the colourless ring opened state and the coloured ring closed state on irradiation with light occurs at well-separated wavelengths. The thermal conversion is not favoured and the compounds show very high fatigue resistance.31,32a It is these Photochromism 23

R R F F UV X F F Me O O 1 2 Vis R= or FF R Me R 1 2 SS R S Me MeS R

colourless coloured X=OorNR

Figure 2.7 Photochromic behaviour of dithiophenylethenes. properties, listed below, that have made this type of photochrome much studied for use in applications such as optical switches and memories:33

1. Both isomers are thermally stable: a well-designed derivative has a half- life at room temperature of many years. 2. Colouration/decolouration cycles can be repeated more than 105 times. 3. The quantum yield of colouration is close to 1 (100%). 4. Response times of both colouration and decolouration are less than 10 ps. 5. Many of diarylethene derivatives undergo photochromic reactions even in the single-crystalline phase.

Chemists working in the field of photochromism have a wish list of applications which they want to accomplish using high performance photochromic molecules, which include:

1. Detection and analysis of photochromic reactions at a single molecule level; 2. construction of molecular systems which exhibit macroscale mechanical movement based on photochromic reactions of individual molecules; 3. conductance photoswitching based on the photochromic reactions of a single photochromic molecule.

A wide variety of routes are available for the synthesis of both symmetrical and non-symmetrical diarylethenes and their further functionalisation.31–34 These have been exploited to produce a myriad of complex structures.35 The absorption spectra of symmetrical dithienylethenes (Table 2.7) are dependent on the substituents on the thiophene ring. Introduction of phenyl groups in the 2-position cause bathochromic shifts, and significantly higher molar extinction coefficients. These are further enhanced by electron- donating groups in the para-position of the phenyl ring. There is, however, a large drop in the quantum efficiency of the cyclo-reversion reaction (Table 2.7).31 A move to even longer wavelengths can be achieved by using non- symmetrical maleic anhydrides, especially those containing thiophenes and indole rings, having a coloured form of the general formula shown in Table 2.8. Bathochromic shifts, with a combined effect of 102 nm, can be achieved by replacing the 2-methyl group in the thiophene ring and putting a 24 Chapter 2 Table 2.7 Properties of symmetrical dithiophenylperfluorocyclopentenes. Repro- duced from ref. 31 with permission of Springer Science and Business Media.

F F 2 2 C C F2C CF2 F2C CF2 Me Me Me Me Me Me

R S Me S R R SSMe R

a R lmax COL (nm) emax COL Fcyc Fcycrev H 534 5050 0.21 0.13 Ph 562 11 000 0.46 0.015 C6H4-4-OMe 570 14 000 0.48 0.008 C6H4-4-NEt2 597 18 000 0.37 0.0025 aHexane.

Table 2.8 Absorption maxima of non-symmetrical maleic anhydride-based photochromes. Reproduced from ref. 31 with permission of Springer Science and Business Media.

O O O

3 R R2 Me

1 N Me S R Me

1 2 3 a R R R lmax (nm) Me Me H 578 Me Me OMe 611 CN Me H 628 CN Me OMe 680 aHexane. methoxy group in position 5 of the indole ring (Table 2.8).31,32a A more extensive list of the spectral properties of thermally irreversible diarylethenes is available.36 Good correlation is claimed between the calculated and observed colour of ring closed diarylethenes using ab initio calculations based on a time-dependent density functional theory approach.37 The fatigue resistance of materials of this class, measured as the number of photochromic cycles at which the absorbance of the coloured species decreases to 80% of the first cycle, can be raised to quite high levels. For instance, for the simple symmetrical maleic anhydride (1.30), the repeatable Photochromism 25 cycle number in air is only 70, but for the non-symmetrical benzothiophene analogue (1.31; R ¼ Me) this rises to 3700, and to greater than 10 000 for (1.31; R ¼ OEt).32a The figures are even higher in vacuum due to either the absence or lower formation of singlet oxygen. The thermal stability of (1.30) is over 90 days at 80 1C and at least five years at room temperature, whilst products like (1.31) are only quoted to be ‘‘stable’’, having lifetimes longer than 12 h at 80 1C.

Gated photochromism is the term used to describe a type of photochromism where the photochromic process is controlled by passing through a gate, the opening and closing of which is governed by a non- photochemical process, which can be chemical (e.g. protonation, esterification, oxidation–reduction) or physical (e.g. solvation or thermal). An example of such a system is that described by Irie involving a chemically gated diarylethene derivative containing a hydroxyphenyl group, as outlined in Figure 2.8.38 In the compounds containing the hydroxy group, the excited states of the molecules are eﬃciently quenched by intramolecular proton transfer and therefore they are not photochromic. However, when the hydroxyl group is masked by esterification the esterified diarylethenes undergo typical photochromic reactions. Therefore the photochromic reaction of diarylethene derivatives can be controlled by the addition of acid. An example where the gate is controlled by thermal means is shown in Figure 2.9. Here the ring-opening process is suppressed with decreasing temperature, leading to the complete absence of the photoreaction below a cut oﬀ temperature of around 120–130 K. By contrast, the reverse ring- closure process to the coloured form shows no significant temperature dependence. This means there is a temperature-dependent region where photochemical ring opening is observed and a temperature-independent region, below 130 K, where no photochemistry is observed. Reversibility of the photoprocesses can thus be controlled by temperature.39 The behaviour of photochromes in the solid state is another area of great significance.40 In this respect it has been shown that single crystals of a range of diarylethenes demonstrate a variety of reversible photochromic responses, as shown in Figure 2.10, and, most interestingly, other photo-responses, such as bending and twisting in the solid state.33 This type of reactivity and behaviour is indispensable if photochromism is to be applied in a variety of technologies, such as displays, optical memories 26 Chapter 2

OH OH hv N N O O O O

Me Me Me Me

Me Me Me R R R S S R S Me S

esterificationhydrolysis esterification hydrolysis

O O

OMe OMe hv N N O O O O Vis Me Me Me Me

Me Me R R R R S Me S S Me S R=MeorPh

Figure 2.8 Chemically gated diarylethene derivatives.

540 nm <130K X Me 540 nm Me RR' SSMe >130K RR' SSMe 313 nm SMe R=Ph R'= O

Figure 2.9 Gated photochromism controlled by thermal means. and molecular machines. Whilst methods have been attempted with other photochromes, some of the most successful to date have been using diarylethenes. This is because they show excellent thermal stability in both open and closed ring forms coupled with good fatigue resistance. The use of diarylethenes in memories and switches will be covered in greater detail in sections below on the use of photochromes in molecular switches, optical computing, data storage and sensors (Sections 2.8.3–2.8.6).41 Photochromism

Figure 2.10 Colour changes of several diarylethene single crystals upon photoirradiation. Reproduced from ref. 33 with the permission from The Royal Society of Chemistry. 27 28 Chapter 2 2.7 Miscellaneous Photochromic Systems Whilst the foregoing classes of photochromic compounds are the most widely studied and applied technically, there are others which have been shown to have significant and growing importance in this still developing field.

2.7.1 Azobenzenes Azobenzenes provide the classic example of cis–trans isomerisation in photochromism. They are T-type, undergoing photoinduced isomerisation from the trans (E) to the cis (Z) forms, which can be reversed either by photochemical or thermal means (Figure 2.11).42 Because of the thermodynamic stability of the trans isomer of azobenzene the thermal cis–trans isomerisation occurs spontaneously in the dark. Photochromism is a technical disadvantage in commercial textile dyes because of an undesirable loss of colour on the cloth as it is exposed to daylight, due to conversion of the more highly coloured (E)- into the (Z)- form. The presence of the cis-isomer in a series of commercial dyes, exemplified by (1.32), was demonstrated using 1H NMR spectroscopy coupled with in situ laser irradiation.43a

Photochromism in simple azobenzenes, whilst involving relatively small changes in colour, is accompanied, significantly, by other changes in the

hν 1 N N NN

hν 2 or Δ

E-form Z-form

Figure 2.11 Photoisomerisation of azobenzenes. Photochromism 29

ABn

N R=alkyl,aryl,halide, N keto, carboxylic acid, ester.amide nitrile, nitro, 3-amino, 3-alkoxy R

aAB

N D = 2- or 4- amino N 2- or 4- hydroxy 2- or 4- alkoxy

ppAB A

A=nitro,carboxylicacid N N D = amino, alkoxy, hydroxy

Figure 2.12 Types of substituted azobenzenes. chemical and physical properties of the isomers which make this molecular system suitable for use as switches in a variety of applications, for instance in polymers (see Section 2.7.6), as probes (Section 2.8.7), molecular machines (Part 3), photoactive liquid crystals and in holography (Part 4).43b,44 Whilst not only producing drastic effects on the absorption, emission and photochemical properties, ring substituents can make the cis isomer more thermodynamically stable than the trans isomer. The effect of substituents on the azobenzene rings on the spectroscopic properties and cis–trans photoisomerisation has been studied in detail for three main classes of these compounds, and mechanisms for their conversion proposed (Figure 2.12).45 The azobenzene chromophore is also proving useful in producing compounds which undergo switching behaviour in both the visible and infrared regions of the spectrum,46 and also with very fast thermal back reactions.47 These properties are most useful in molecular switch applications and where photochromes are being applied in biological environments. Both these areas will be discussed further in greater detail later (Sections 2.8.3 and 2.8.7). The chromophore has been widely exploited as the functional group in light activated polymers and their applications (Section 2.7.6). 30 Chapter 2 2.7.2 Anils and Other Organic Photochromes Anils (also known as Schiff’s Bases) are readily synthesised by reacting an aromatic aldehyde with a primary amine. The most studied are those made from salicylaldehydes, as the presence of an o-hydroxyl group is an essential structural element. The simplest derivative, obtained on condensation with methylamine, shows only minimal photochromism.48 Those formed from aromatic or heteroaromatic amines, especially sali- cylidenanilines, are one of the earliest recorded photochromic systems, whilst probably better known for their thermochromic behaviour (see Chapter 3), and also the most valuable.49 Of particular interest is the fact that these anils undergo photochromism in the solid state, a property they share with other photochromes including diarylethenes, making them potentially useful in a variety of outlets.50,51 The photochromic T-type behaviour of crystalline anils is based upon tautomerism involving intramolecular proton transfer from the ortho-hydroxy group to the nitrogen atom of the imine excited electronic state, via a six-membered ring transition state, producing the trans-keto form with the spectrum shifted bathochromically (Figure 2.13).50 The historical development of anils in both photochromism and thermochromism has been the subject of a recent review.52 There are several other photochromic systems which involve ring opening, cis–trans isomerisation, photochemical cleavage or chiroptical processes.53–55 Dihydroindolizine-based systems, containing a five-membered ring, undergo photochemical ring-opening in a conrotatory 1,5-electrocyclic reaction to give coloured betaines (1.33). The process can be reversed either photochemically but more commonly by thermal means.53,56 Dihydroazulenes undergo ring- opening on exposure to light to give vinylheptafulvalenes (1.34). The reaction being thermally reversible, the electronic character of the substituent at position 7 influences the rate of the thermal back reaction, which as with many other photochromes is solvent dependent.57,58a Photochemical cleavage of a C–N bond in perimidinespirocyclohexadienones, followed by intermolecular rearrangement, produces deeply coloured quinoneimines such as (1.35), similar to indophenol dyes in structure; decolourisation is achieved thermally.55

H N

N hv N H OH O O

trans-enol form (E-OH) cis-keto form (Z-NH) trans-keto form (E-NH)

Figure 2.13 Photoinduced isomerisation of salicylidenaniline. Photochromism 31

A well-studied class of molecular switches and sensors is acylhydrazones, which undergo photoswitching by isomerisation around the carbon– nitrogen double bond, oﬀering both T-type and P-type behaviour de- pending on the substituents.58b,c A new class of switches has also been described in which a coloured polymethine chain is cyclised to a colourless ring structure by visible light (Figure 2.14A).58d The cyclic isomers produced by the condensation of bi-indone with cinnamic aldehydes exhibit what is claimed to be unprecedented photochromic behaviour, as the coloured open forms are observed upon irradiation by light of any wavelength between 254 and 642 nm (Figure 2.14B).58e Hemi-thioindigos, consisting of a thioindigo and a stilbene residue (1.36), show only moderate photochromaticity, based on cis–trans isomerisation of the double bond. Although this photochromic property has been known since the 1960s, it is one of developing interest.59,60 This is because the absorption spectra of both the (Z)- and (E)-isomers are red-shifted, with high extinction coeﬃcients of e420 000, which means that the transformations can be carried out with visible light, and in addition they show very little fatigue over many switching cycles. Sterically crowded alkenes are another class of photochromic systems relying on cis–trans isomerisation, e.g. (1.37). The unique feature of these molecules is that of adopting a non-planar conformation because of the steric interactions between the groups attached to the central double bond. Having high fatigue 32 Chapter 2

A O O O X Visible lght Y R N X X ROH Δ O X Y O H N R R

B O O 254...642nm

O Δ O O O

Figure 2.14 Photoswitches operable in visible light. resistance and thermal stability the molecules have been of interest as chiroptical switches, in this case involving on-oﬀ switching of emission.61

Photochromic quinones and extended quinones undergo a colour change by photochemical migration of a proton or an R group across the peri Photochromism 33 positions to produce an extended quinonoid chromophore (1.38), and the process is photoreversible.54 Bianthrylidenes also show useful photochromism that has been exploited in light-triggered switches (see Section 2.8.3), but are more noted for their thermochromic behaviour (Chapter 3, Section 3.2.2). Negative T-type photochromism, the process which involves decolouration upon irradiation with light followed by thermal recolouration is a feature that is exploited in bulk materials for a variety of applications. With positive T-type photochromic processes deep UV penetration is inhibited due to reabsorption of the excitation light by the generated coloured isomer. Until recently the time scales for the thermal back reaction for decolouration in negative photochromism were of the order of minutes, limiting their use for deep penetration in solid materials. However, recent workers have developed novel naphthalene-bridged phenoxyl-imidazolyl radical complexes that show very fast switching thermal back reactions after UV light irradiation, as shown in Figure 2.15(a). This structure has been further modified

Figure 2.15 Photochromic reaction schemes for naphthalene-bridged phenoxyl- imidazoyl radical complex. Reproduced from ref. 62 with the permission from The Royal Society of Chemistry. 34 Chapter 2 by replacing the phenoxyl unit with a naphthoxyl unit so that the molecular structure has an asymmetric carbon atom, which provides fast chiroptical switching.62,63

2.7.3 Multiphotochromic Molecular Systems The presence of more than one photochromic unit in a single compound, hence one with two or more switching elements, offers the potential for such molecules to not only store more information but to offer a way into new phenomena. Consequently, a vast body of work has been carried out using all the photochromes described above to construct such molecules and study their behaviour. This extensive field has been reviewed recently.64 For dimers of photochromic compounds, to get full photochromism it is essential that the two units be non-equivalent, so that symmetry is broken and all four states can be accessed. Essentially these dimers must show additional performance above that exhibited by a simple mixing of the monomers. This is in reality very difficult to achieve, since the two sub-units must interact with each other either elec- tronically or sterically, but not so that the electronic interactions are so strong that they interfere with characteristic features of the photochromes. A representative sample of homo-photochromic dimers derived from some of the aforementioned photochromes is shown in Figure 2.16. It is possible to go beyond dimers and construct bridged trimers, tetramers and hexamers of photochromes, and so produce compounds which can show a sequential series of photochromic responses. The most extensive work has been done on diarylethenes and azobenzene multimers.64 A nice example is provided from Higashiguchi and co- workers, with their work on a trimer of a dithienylethene, where isomerisation in up to two non-consecutive units was achieved (Figure 2.17).65 Because of the asymmetry in this molecule, four distinct colours could be observed; blue, red, yellow and black, which together with the colourless open form, makesthisafivestateswitch(seealso Section 2.8.5). After 2000, new types of multiphotochromic systems started to appear. These are multimers made up from different families of photochromes and have been called ‘‘hybrid multiphotochromes’’.64 Most of these systems made to date contain at least one diarylethene unit. They all show complex behaviour that is still far from fully understood, but even so are providing leads into exciting new areas (see Section 2.8.3). For instance, the dithienylethene/bis-fulgimide (1.39) has been used to construct an all-photonic- multi-encoder and molecular logic gate using various isomers of the fully closed form (1.40).66 Photochromism CN F F2 2 O O

F2 S O O N N N N S O

O CN S S CN NC O O

NC N N F CN F2 2 F2 O O

Ph Ph S S

O2N

O N N O O Ph Ph

NO2 35 Figure 2.16 Selected examples of photochromic dimers. 36 Chapter 2

Figure 2.17 Colour changes on opening the three dithienylethene units (black is also possible, but is not shown). Reprinted with permission from ref. 65. Copyright (2005) American Chemical Society.

2.7.4 Inorganic Photochromes Silver halide crystals trapped in a glass matrix were used in the earliest successful ophthalmic lenses with photochromic properties. This type of material offered significant practical benefits over other systems, having remarkable fatigue resistance. However, poor darkening and slow fading was a problem, and when there was a switch to plastic lenses it became necessary to move to other more compatible materials (see Section 2.8.1). However, the high fatigue resistance of inorganic photochromes makes them ideal can- didates for photonic applications. Several transition-metal oxides exhibit photochromism upon bandgap excitation, including tungsten oxide, molybdenum oxide, titanium dioxide, vanadium pentoxide, niobium pentoxide and zinc oxide.67 When working with these oxides it is important to use visible light in order to differentiate between thermochromic and photochromic effects, as exemplified by the work on vanadium pentoxide.68 Continuing interesting developments in this field include visible light colouration in molybdenum oxide–titanium dioxide composites,69,70 and the use of silver nanoparticles deposited on Photochromism 37

Figure 2.18 Tungsten oxide/methylcellulose composite film. Reprinted with permission from ref. 72. Copyright (2015) American Chemical Society. titanium dioxide to give multicolour photochromism.71 A recent example is the formation of titanium oxide-based photochromic films that change reversibly in air between colourless and transparent in the dark, and dark blue under UV irradiation (Figure 2.18). They were prepared by using methylcellulose as a film matrix and polyols such as ethylene glycol (EG), propylene glycol (PG) and glycerine (Gly) as dispersing agents.72

2.7.5 Organic–Inorganic Hybrids, Transition Metal Complexes and Organometallics The combination of organic and inorganic components into a single material oﬀers the prospect of forming products which have the advantage of both materials. In these hybrids the organic components provide light weight, flexibility and versatility, whilst the inorganic oﬀer high thermal and mechanical resistance.73,74 First were those based on metal halides and cy- anides, templated on photoactive components, nano-scale hybrids of poly- oxometalates and metal chalcogenides.73 Photochromic hybrids of metal halides are most commonly formed by combining the halides of Bi, Sn or Pb with a bipyridinium cation (e.g. methyl viologen).73 A recent example is (1.41) from benzyl viologen and the novel 5 trinuclear [Bi3Cl14] , which is claimed to show an improved photochromic response over previous hybrids of this type.75 Cyanide hybrid complexes are exemplified by those consisting of polycyano–polycadmate host clathrates and photochromic active dications (MV21, 6,7-dihydrodipyridopyrazinium (DQ21) or 1,10-diethyl-4,40-bipyridinium (EV21)).73 A very active area of research for hybrids is that involving poly- oxometalates (POMs). POMs can accept electrons or protons, making them suitable for both photochromic and electrochromic materials.73,76 The 38 Chapter 2 range of tuning of the coordination nanospaces in POMs, because of the almost infinite choice of organic linkers and the variability of the oxidation states in the metal nodes, endows them with many advantages in architectural design.77 There is an increasing interest in grafting organic photochromes onto POMs, as exemplified by the material made from a spiropyran and polyoxotungstates, which shows solid-state photochromism.78 The grafting of organic photochromes onto metal chalco- genide nanoparticles, or quantum dots (QDs), gives materials which are photoluminescent. The colour change is controlled by the UV initiated ring-opening of the photochrome, for instance the spiropyran (1.42), coupled via a thiol linker.79

There is one specific group of hybrids based on embedding organic photochromes in sol–gel derived silicates and aluminosilicates. This has been the basis of much research as it oﬀers a rapid entry into those areas requiring the use of a solid matrix. The sol–gel procedure is a low

Figure 2.19 Preparation of photochromic hybrid coatings on glass. Reproduced from ref. 72a with the permission from The Royal Society of Chemistry. Photochromism 39

N N N

NH N

N N N O O N HN

S S S

S N

S R N N O N N N S

OC16H33

Figure 2.20 Selected photochromic organic ligands. temperature method, and therefore is more adaptable for the use of organic photochromes, such as the much explored spiropyrans, spirooxazines and especially naphthopyrans. The matrices are prepared by the use of mixtures of tetraacetoxysilane and silicon alkoxides modified with alkyl or aryl groups. Hybrid coatings on glass are made as outlined in Figure 2.19.74 Transition metal complexes with ligands containing different photochromes are a relatively recent addition to the armoury of application de- velopers.80 The coordinating components of the ligands include, for example, 2,20-bipyridyl, 2-(2-pyridyl)imidazole, porphyrins and phthalocyanines, the photochrome elements of diarylethenes, azobenzenes, spiropyrans and spirooxazines, and the metals ruthenium, rhenium and osmium. Selected structures of photochromic organic ligands are shown in Figure 2.20. The photochromic metal complexes offer a way to tune photochromic behaviour with relatively simple synthetic methods. Photochromic organometallics are those in which the metal is attached to a photochromic unit via a direct link to carbon [M-C(photochromic unit)]. Unlike the transition metal complexes, which as stated above are developed for their photophysical properties, the research areas for photochromic organometallics are catalysis and molecular devices such as wires.81 40 Chapter 2 2.7.6 Polymers, Matrices and Amorphous Materials The incorporation of photochromic groups into polymers whether covalently bonded as part of the backbone, as pendant groups, or in admixture has been extensively studied because they offer easy fabrication of materials of potential use in a great many application areas.82–84 Of the many photochromic polymeric systems those involving azobenzenes,85 spiropyrans,86 and diarylethenes87 have been the most studied. Some selected structures are shown in Figure 2.21. Conjugated polymers, e.g. polythiophenes and polydiacetylenes, also show photochromic behaviour, but they are more noted for their thermochromism (Chapter 3, Section 3.2.3) and electrochromism (Chapter 5, Section 5.3.5). The presence of photochromic groups in polymer matrices not only influences their colour change properties but also leads to novel photoinduced properties. The matrix has an effect on photochromism and, also, photochromism has an effect on the matrix. Ichimura suggests that these effects are best understood by considering them in terms of orderedness of molecular structures of matrices, as outlined in Table 2.9.82 Zero-dimensional phase materials and their applications will be considered further in Section 2.7.7, whilst those involving two-dimensional and three-dimensional phases will be dealt with in Part 4 under liquid crystals, holography and optoelectronics. The assembly of supramolecular archi- tectures from amphiphilic photochromic compounds, for example diarylethenes with oligo-(ethylene glycol) side chains, is an emerging area of research with many potential application areas which require structural photoregulation.88

R O n R R NO2

N S N O N R Me R n

F2 F2 R R

Me n S Me S

Figure 2.21 Selected examples of photochromic polymers. Photochromism 41 Table 2.9 Dimensional diﬀerentiation in molecular structures of matrices. Orderedness Matrix Optical properties Zero-dimensional Solution Absorption Refraction Amorphous polymer Emission Reflection One-dimensional Phase-separated state Light scattering Micelle Inclusion complex Two-dimensional Stretched film Birefringence Liquid crystal (N,Sm) Dichroism Bilayer membrane Langmuir–Blodgett films Three-dimensional Cholesteric liquid crystal Optical rotary power Multilayer Circular dichroism Single Crystal

Photochromic amorphous molecular materials have been suggested as alternatives to photochromic polymers and molecularly dispersed polymer systems for optical data storage and optical switching applications. These are a class of photochromic materials that form uniform amorphous films by themselves, do not crystallise at high concentrations, and there is no dilution of the photochromic chromophores.89 The materials are based upon azobenzene and diarylethene photochromes, as shown in examples (1.43) and (1.44). They readily form amorphous glasses with well-defined Tg values when the melt samples are cooled on standing in air, and show reversible photochromic behaviour.

2.7.7 Couples with Carbon Nanomaterials One of the most exciting developments in recent years has been that involving the coupling of photochromes with carbon nanomaterials.90 The carbon nanomaterials involved are most commonly fullerenes, CNTs and graphenes which are coupled with azobenzene, styrene, diarylethene and 42 Chapter 2 spiropyran photochromes. The strategies used for this functionalisation include both covalent and non-covalent methodologies. The favoured covalent methods are condensation, cycloaddition and radical polymerisation; the non-covalent ones, physical adsorption on the carbon mixtures by p–p stacking, hydrophobic interaction or electrostatic interaction. These functionalisation methods and components are illustrated in Figure 2.22. A whole raft of properties can be modulated by the formation of such photochrome-carbon-based nanocomposites which include the following: dipole moments, current charge, charge transfer separation, charge transport, magnetism, photoconductivity, fluorescence, dispersibility and morphology. These properties have been employed to devise materials that have potential applications as molecular junctions, field eﬀect transistors,

Figure 2.22 Carbon-based nanomaterials functionalised with photochromic molecules. Reprinted from ref. 90 under a Creative Commons Attribution 4.0 license http://creativecommons.org/licenses/by/4.0/. Photochromism 43

S S R O N R H N O H Switch Open

Visible Light UV Light

S S R O N R H N O H Switch Closed

Figure 2.23 Diarylethene photoswitch between graphene electrodes.91 solar thermal storage, memory devices, sensors and biological applications.90 One example from this list of possible applications is that of a single- molecule electrical switch formed by the coupling of a single molecule of a diarylethene with graphene electrodes via bridges of methylene groups (Figure 2.23). This single-molecule electrical switch is fully reversible, con- ducting under the influence of UV light and is transformed by visible light into an insulator, with high reproducibility and is stable for over a year.91

2.7.8 Biological Photochromes The visual system is highly dependent on a complex photochromic photocycle, where the chromophore 11-cis-retinal is attached to the protein opsin via a protonated Schiﬀ base of a lysine residue. This linkage is broken by light and reversed thermally as shown in Figure 2.24. 44 Chapter 2

Figure 2.24 Rhodopsin photochromic cycle.

Bacteriorhodopsin (BR) is a seven-helical transmembrane protein, also with the retinal co-factor, found in the so-called ‘‘purple membranes’’ of bacterium H. salinarium. It converts the energy of ‘‘green’’ light (500–650 nm, max 568 nm) into an electrochemical proton gradient, which in turn is used by a second membrane protein, ATP synthase, to generate chemical energy in the form of ATP. The photocycle is initiated by the absorption of a photon by the all-trans retinal cofactor. Photon absorption causes rapid rearrangement of the electronic structure of the extended conjugated retinal system which results in trans-cis isomerisation and deprotonation with consequent colour change from the purple B intermediate to the yellow M intermediate as shown schematically in Figure 2.25. This reversible colour change gives bacteriorhodopsin the potential for applications in a variety of outlets including optical memories and switches. However, the protein occurring in nature does not meet the standards of overall eﬃciency required for its use in commercial applications and all successful photonic devices use chemical or genetic variants of bacteriorhodopsin.92 Plant phytochromes (1.45), and their bacterial ancestors, are chromopro- teins, which in plants control a variety of photomorphogenic processes. There are two forms: phytochrome Pr absorbs red light and the biologically active Pfr far-red light. It is this property that leads to photoperiodism, for instance the photoperiodic induction of flowering (Figure 2.26). In this way the phytochrome is acting as a light-operated molecular switch. It is also involved in chloroplast development (not including chlorophyll synthesis), Photochromism 45

Figure 2.25 Simplified photochemical cycle of bacteriorhodopsin.

Figure 2.26 Phytochromes in the photomorphogenic process in plants. leaf senescence and leaf abscission. The phytochrome photocycle is a complicated process but the interconversion of Pr into Pfr involves isomerisation of the bond between rings C and D as shown in (1.45) and proton transfer. As with bacteriorhodopsin, the native protein will need either chemical or more likely genetic modification to render it of any use in technical applications outside the horticultural field. An example is the conversion of phytochromes into fluorescent probes for infrared imaging.93 46 Chapter 2

2.8 Applications of Photochromic Materials A reversible change in colour is not the only alteration in physical property experienced during photochemical transformations of photochromic materials. There are also changes in refractive index, dielectric constant, oxidation/reduction potentials, and of course geometrical structure. Besides the well-established use of photochromic materials in ophthalmics, especially in plastic lenses for sun glasses, there is therefore a growing list of other potential application areas, including cosmetics; actinometry and heat measurement; optical memories for data storage, both 3D and near-field; photo-optical switches, logic gates and computing, filters, displays; self- developing photography; and many others.32

2.8.1 Applications in Ophthalmics Spectacle lenses that darken when exposed to strong sunlight and reverse back to colourless in low light situations, such as indoors, oﬀer the wearer personal comfort and safety. During most of the last century spectacle lenses were made of glass and the photochromic systems of choice were inorganic systems, based on silver halide crystals trapped in a glass matrix. The first of these was developed and launched by Corning Inc. in 1966, and over the years proved to be quite satisfactory in protecting wearers, but their popu- larity was limited by the fact that they switched very slowly when going from sunlight into a darkened room. However, in the latter decades of the twen- tieth century, plastic lenses were introduced and rapidly replaced glass as the material of choice in the developed world. The established silver halide photochromic systems could not be adapted to work well in plastic and hence there was a need for alternative materials, and these have turned out to be photochromic dyes. Pioneers of the research into this application area were Pilkington in the UK and PPG industries in the USA with their Tran- sitions photochromic plastic lenses. Photochromism 47 The demands on the photochromic materials are quite severe. A major issue is the ability to obtain the ‘‘correct’’ colour and optical density in the lens. The generally preferred tints in photochromic eyewear are switchable neutral greys or browns, because they allow the sky to look blue and do not interfere with red danger signals. These shades are readily obtainable in glass because the silver halide system absorbs broadly in the visible and can be manipulated to obtain greys or browns. Until fairly recently there was no single photochromic compound that could do this and colour formulators needed to resort to mixtures of products to produce the desired colours in the lens. To be of use in these mixtures photochromic materials have to satisfy other criteria, including a matching of the activation and fade rates, showing a balanced response to UV absorbance, having matching temperature dependency on the change from coloured to colourless, and also matching fatigue rates and life expectancy in use. If this is not the case users will find that they are looking out at the world through variably coloured spectacles in changing light conditions. The requirements for use of photochromic materials in lens production can be summarised as follows:94

Weak visible absorption when unactivated so residual colour is low; quick response to an increase in illumination; have a strongly absorbing activated state, because even when irradiated with light of high intensity, only a relatively small proportion of color- ant will exist in this form; show a good compromise between depth of activated colour and rate of fade to ensure both are acceptable as the properties of high intensity and short half-life tend to be mutually exclusive; produce satisfactory performance over two years by having reasonably lightfast coloured and colourless forms which respond well to photostabilisers; resist the tendency to ‘‘fatigue’’, whereby during activation a proportion of the dye is undesirably and irreversibly converted into non- photochromic molecules, leading to gradual weakening of colour upon repeated activation; colour up in a manner that is not greatly aﬀected by the temperature of its environment; exhibit suﬃcient solubility in lens media to give solutions rather than dispersions because commercial T-type classes do not exhibit useful photochromism in solid form.

These requirements for the use of photochromic organic materials in ophthalmic outlets have also been described in both an interesting and entertaining form in an overview article by Towns.95 The three main classes that have been much studied for ophthalmic applications are spiroindolinonaphthoxazines (Section 2.3), diarylnaphtho- pyrans, indenonaphthopyrans (Section 2.4) and fulgides (Section 2.5). How 48 Chapter 2 Table 2.10 Photochromic properties of commercially available classes. Performance (OD, Temperature Class act/fade rate) dependency Fatigue Spiroindolinonaphthoxazines Good High Good Naphthopyrans Good Medium Good Fulgides Good Medium Fair

Table 2.11 Photochromic properties of naphthopyrans in ophthalmic plastic.

Me OH iPr CO2CH2CO2Et

OMe Ph O O Ph

(1.15) Me (1.16) OMe

Optical density

Compound l (nm) 30 s 120 s At saturation T1 (fade) (s) max 2 (1.15) 464 0.16 0.29 0.37 60 (1.16) 582 0.12 0.25 0.36 80 these different photochromic classes match up to the desired criteria for use in lenses is shown in Table 2.10. Fulgides exhibit only a fair fatigue resistance and spiroindolinonaphthoxazines suffer from a high temperature dependency. Consequently, for this photochromic outlet, naphthopyrans have become the molecules of choice. Typical compounds used to produce greys and browns are the yellow (1.15) and the blue (1.16), the properties of which are given in Table 2.11. This pair offer a well matched build up to saturation optical density but their fade rates, whilst close, are far from perfectly matched.96 Commercial products have been produced that do achieve the correct balance, but information on their actual composition is commercially sensitive and hence is not disclosed in detail in the open literature. In 1999 the James Robinson company in the UK (now Vivimed Labs) largely overcame the balanced mixture problem by developing single- molecule neutral grey colouring naphthopyrans, having the general structure (1.46).13b,97 By suitable molecular manipulation it is possible to balance the twin peaks in the absorption spectrum to get the desired colour and kinetics. From this they introduced three commercial products – Reversacol Midnight Grey, Misty Grey and Graphite – which were designed for use in commercial sunglasses and ski goggles. Photochromism 49

Ophthalmics is the most important outlet for photochromic materials and much research is carried by leading manufacturers in order to make sunglasses and sunwear more responsive to the specific needs of the range of customers. For instance Transitions claim to have developed more than 3500 diﬀerent photochromic dyes and many new application methods, which are covered by a raft of patents.98a,b There is a special requirement for a lens that protects the user from sun glare whilst driving behind the protective windshields of cars and other vehicles from which the UV is reflected. To this end Transitions and Younger Optics have jointly developed spectacle lenses that are made up of a combination of polarising and photochromic components that are claimed to have answered this need. The polariser element is active under every lighting condition, producing a light yellow/green lens colour, but when behind the windshield a photochrome that is now activated by visible light comes into play and the lens changes to a copper colour, whilst under daylight a photochrome is now darkened as normal by the incident UV producing a dark brown lens.99

2.8.2 Novelty Items, Fashion, Cosmetics and Security The uses of photochromics in both novelty printing and incorporation into plastics are significant commercial outlets for this colour change effect. For instance, it is possible to produce mass pigmented polypropylene by dis- solving different photochromic materials into molten polypropylene. By melt spinning, a thread is produced that is photoactive and is used to embroider designs on textile garments. Indoors, away from UV light, the design is white on the fabric but changes its colour on exposure to UV radiation in daylight.100 Typical uses for photochromic effects are on children’s toys and for logos on T-shirts (see Figure 2.27), but the list of items is extensive and in- cludes lunch boxes, crayons, jelly shoes, hair clips, hair combs, shoestrings, coasters, craft beads, PVC belts, watch bands, drinking straws, spoons, cups, frisbees, combs, greeting cards, stickers and business cards. In the cosmetic area a use has been found in nail polishes, light enhancing make-up and temporary hair colorants. 50 Chapter 2

Figure 2.27 Photochromic images on T-shirts.

A range of colours is available commercially, yellow through red to purple and green.101 The products are also available from several companies in a variety of forms for these outlets including inks and microencapsulated and plastic resin concentrates. The use of processes, other than mass pigmentation of polymers and pigment type printing from inks, the dyeing of traditional textiles such as cotton and wool, has shown little progress due to the limited availability of stable, water soluble photochromic colours. However, because of the potential commercial outlets for colour-change in this area, known as ‘‘smart’’ or ‘‘functional’’ textiles, e.g. inter-active camouflage, there has been an ongoing interest.102,103a,b,c Developments in both the dyeing and printing processes, especially for polyester, have been the subject of recent reviews. Especially notable is the one by Christie which covers developments in the applications of several chromisms to textiles.103b Photochromic materials have been used as anti-counterfeit markers on garments and as anti-tampering devices on packaging and printed documents. Security markers on nationally important documents, such as banknotes and passports, are examples. The use of photochromics and other chromisms in architecture, design and fashion is considerable and has been covered in books and also on several websites.104,105

2.8.3 Molecular Photoswitches Because photochromic compounds can switch reversibly from one state into another under the influence of light, they have been at the forefront of research into the development of molecular photoswitches for a variety of leading edge applications.106 Structural changes in photochromes are accompanied by a range of changes in both their physical and chemical properties.107 These include absorption spectra, fluorescence emission, electron conductivity, electrochemical, magnetic and coordination properties, refractive index and geometrical structure. Photochromic changes can also be complemented by other chromisms, for example by solvent (solvatochromic), by emission (fluorochromic), by pH (acidochromic), metals