Photochromic Polymers: The Application and Control of Photochromism through its Interaction with Polymers
Francesca Ercole
Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy at the University of NSW
March 2011 Abstract
This research highlights the application of polymer conjugation as a way to tailor photochromic performance. The main technology that was developed is based on targeting the local environment around the photochromic by attaching of one or more polymer tails to the dye molecule. Within the cured lens matrix this allows the dye’s mobility and switching speed to be affected in a predictable manner, as compared to the free dye. Importantly, the bulk lens matrix can be left unaltered in order to bring about the desired changes to photochromic performance. This methodology was initially applied to comprehensively tune naphthopyran switching behaviour. Naphthopyrans are a pertinent class of photochromic dyes since they are used commercially in lenses. ATRP (Atom Transfer Radical Polymerization) allowed the synthesis of photochromic-polymer conjugates based on a range of polymer types and chain lengths, and therefore with different Tg’s. For the most part, a photochromic-functionalized initiator was used for the construction of the conjugates so that each dye molecule is covalently bound to the end of a polymer chain. With control over the average molecular weight and their distributions, the photochromic-polymer conjugates inherit uniform characteristics which can therefore be targeted in order to control photochromic performance. An investigation of various ATRP-produced naphthopyran-poly(n-butyl acrylate) conjugates showed that the geometry of the polymer chain is also an important consideration. This study included random copolymers and a gradient copolymer system which incorporate dye units pendant along the chains. The best system which provided superior kinetics per chain length of conjugated polymer was a Y-branching approach. This was made possible using a 2-armed photochromic initiator which locates a dye unit both pendant and exactly in the middle of two polymer chains. A related system that demonstrated the importance of targeting the dye’s local environment as a way to control photochromic performance, were films composed of ABA triblock copolymers, in which photochromic units reside in the middle of the soft central section (B). Another simple chemical strategy for making photochromic-polymer conjugates, based on a convergent methodology, is to conjugate the dye to a preformed polymer of choice. A series of flexible naphthopyran-poly(dimethylsiloxane) oligomeric conjugates
iii Abstract ______were generated as such, which showed optimized performance in a lens matrix. In a separate chapter, polyethylene glycol oligomers were also conjugated to dyes in the same manner. Schemes to synthesize the appropriate hydroxyl naphthopyrans were described. A brief study was conducted to understand the outcome of having photochromic units bound to a crosslinked and branched polymer system and the underlying factors that affect photochromic behaviour. In order to do this, various polymerizable naphthopyran monomers were synthesized and reacted firstly with the curable lens composition to become part of the network structure. Then, various crosslinked hyperbranched polymer structures were also synthesized, via RAFT, incorporating photochromic dyes either as mono-bound pendant units or as bis-tethered crosslinking agents. The behaviour of these matrix-bound systems were also compared to linear polymers. The strong sensitivity of photochromic dyes to their surroundings makes them suitable for probing studies. This approach was applied to monitor the assembly of block-copolymer micelles in water, a process that leads to measurable changes in the dye’s environment and fade kinetics. The probing study also showed that the dyes do not necessarily need to be bound to the polymer chain in order for these changes to be detectable.
iv Declarations ______
Originality Statement
I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for an award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis.
I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged.
Signed Francesca Ercole
Date 13/03/2011
v
Declarations ______
Copyright Statement
I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or hereafter known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the abstract of my thesis in Dissertations Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.
Signed Francesca Ercole
Date 13/03/2011
Authenticity Statement
I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.
Signed Francesca Ercole
Date 13/03/2011
vii
Publications ______
Publications arising from this thesis
The Application of a Photochromic Probe to Monitor the Self-Assembly of Thermosensitive Block Copolymers. Ercole F.; Harrisson S.; Davis T.P.; Evans R.A.; Soft Matter, DOI: 10.1039/C0SM00746C, 2011.
Photo-responsive systems and biomaterials: photochromic polymers, light-triggered self-assembly, surface modification, fluorescence modulation and beyond. Ercole F.; Davis T.P.; Evans R.A..; Polymer Chem., 1, 37-54, 2010. (Front Cover of Inaugural Issue.)
Photochromic Polymer Conjugates: The Importance of Macromolecular Architecture in Controlling Switching Speed within a Polymer Matrix. Ercole F.; Malic N.; Harrisson S.; Davis T.P.; Evans R.A..; Macromolecules, 43, 249-261, 2010.
Optimizing the Photochromic Performance of Naphthopyrans in a Rigid Host Matrix using Poly(dimethylsiloxane) Conjugation. Ercole F.; Malic N.; Davis T.P.; Evans R.A.; J. Mater. Chem., 19, 5612-5623, 2009.
Comprehensive Modulation of Naphthopyran Photochromism in a Rigid Host Matrix by Applying Polymer Conjugation. Ercole F.; Davis T.P.; Evans R.A..; Macromolecules, 42, 1500-1511, 2009.
Publications arising outside the scope thesis
Living spontaneous gradient copolymers of acrylic acid and styrene: one-pot synthesis of pH-responsive amphiphiles. Harrisson, S.; Ercole, F.; Muir B., Polymer Chem., 1, 326-332, 2010.
Noncovalent Liposome Linkage and Miniaturization of Capsosomes for Drug Delivery. Hosta-Rigau L.; Chandrawati R.; Saveriades E.; Odermatt P.D.; Postma A.; Ercole F.; Breheney K.; Wark K.L.; Stadler B.; Caruso F. Biomacromolecules, 11, 3548-3555, 2010.
ix
Contents ______
Table of Contents
Abstract ...... iii
Declarations ...... v
Publications ...... ix
Table of Contents ...... xi
1 Introduction ...... 1 1.1 General Introduction and Aims ...... 1 1.2 Outline of Thesis ...... 2
2 Literature Review ...... 9 2.1 Photochromism – Definition and Description ...... 9 2.1.1 Mechanisms Involved in the Transformations ...... 10 2.1.2 General Photochemistry ...... 14 2.1.3 Techniques for Investigating Photochromic Transformations ...... 15 2.1.4 Photochromic Behaviour and Spectrokinetic Properties...... 17 2.1.5 Interconverting Open Isomers ...... 18 2.2 Photochromic Polymers ...... 19 2.2.1 Matrix effect ...... 19 2.2.2 Kinetic Interpretations ...... 20 2.2.3 Historical Perspective ...... 21 2.2.4 Aggregation ...... 23 2.2.5 Polarity ...... 23 2.2.6 Inorganic Hosts ...... 25 2.2.7 Method of Incorporation ...... 27 2.3 Applications ...... 29 2.3.1 Optical Data Storage and Ophthalmic Lenses ...... 29 2.3.2 Photo-responsive Polymers ...... 30 2.3.2.1 Photo-orientation ...... 30 2.3.2.2 Photo-responsive Liquid Crystals ...... 31 2.3.2.3 Photo-responsive Biomaterials ...... 34 2.3.2.4 Photo-regulation of Fluorescence ...... 43
xi Contents ______
2.4 Photochromic Polymers via Controlled Radical Polymerization ...... 44 2.5 Controlled Radical Polymerization - CRP ...... 48 2.5.1 Atom Transfer Radical Polymerization - ATRP ...... 50 2.5.2 Reversible Addition Fragmentation and chain Transfer - RAFT ...... 53 2.6 References ...... 57
3 Comprehensive Modulation of Naphthopyran Photochromism in a Rigid Host Matrix by Applying Polymer Conjugation...... 71 3.1 Introduction ...... 71 3.2 Results and Discussion ...... 74 3.2.1 Polymerization - Conjugate Synthesis ...... 75 3.2.2 Optical Clarity of Test Samples ...... 79 3.2.3 Photochromic Kinetics ...... 80 3.2.4 Tuning of Decolouration Rate: Chain Length and Rigidity Affects ...... 82 3.2.5 Concurrent Influence of Local Rigidity: Colourability Evaluation ...... 86 3.2.6 Copolymerized Naphthopyran versus End-functional ...... 88 3.3 Conclusion ...... 90 3.4 Experimental Details ...... 90 3.5 References ...... 99
4 Photochromic Polymer Conjugates: The Importance of Macromolecular Architecture in Controlling Switching Speed within a Polymer Matrix...... 103 4.1 Introduction ...... 103 4.2 Results and Discussion ...... 105 4.2.1 Naphthopyran-Polymer Conjugate Synthesis...... 105 4.2.2 Evaluation of Photochromic Performance: Cast-in Lenses...... 115 4.2.3 Photochromic Films: ABA Triblock Copolymers...... 119 4.3 Conclusion ...... 122 4.4 Experimental Details ...... 123 4.5 References ...... 132
5 Optimizing the Photochromic Performance of Naphthopyrans in a Rigid Host Matrix using Poly(dimethylsiloxane) Conjugation...... 135 5.1 Introduction ...... 135 5.2 Results and Discussion ...... 136 xii Contents ______
5.2.1 Naphthopyrans ...... 136 5.2.2 Starting Materials ...... 139 5.2.2.1 Naphthols ...... 139 5.2.2.2 Propynols ...... 142 5.2.2.3 Conjugated Naphthopyrans ...... 144 5.2.3 Photochromic Properties ...... 145 5.3 Conclusion ...... 152 5.4 Experimental Details ...... 152 5.5 References ...... 175
6 Photochromic Behaviour within Polymer Matrices Part 1: Highly Crosslinked Networks ...... 179 6.1 Introduction ...... 179 6.2 Results and Discussion ...... 181 6.2.1 Photochromic Kinetics ...... 182 6.2.2 Photochromic Behaviour within Network Structure ...... 185 6.3 Conclusion ...... 191 6.4 Experimental Details ...... 192 6.5 References ...... 200
7 Photochromic Behaviour within Polymer Matrices Part 2: Hyperbranched Polymers ...... 203 7.1 Introduction ...... 203 7.2 Results and Discussion ...... 205 7.2.1 Polymer Synthesis ...... 207 7.2.2 Photochromic Behaviour ...... 211 7.3 Conclusion ...... 215 7.4 Experimental Details ...... 215 7.5 References ...... 220
8 The Application of a Photochromic Probe to Monitor the Self-Assembly of Block Copolymers in Water...... 223 8.1 Introduction ...... 223 8.2 Results and Discussion ...... 225 8.2.1 RAFT Synthesis of Polymers...... 226
xiii Contents ______
8.2.2 Thermally-Induced Self-Assembly...... 228 8.2.3 Thermal Decolouration Kinetics...... 229 8.2.4 Photochromic Probing...... 230 8.3 Conclusion ...... 238 8.4 Experimental Details ...... 238 8.5 References ...... 252
9 General Conclusions ...... 257
Acknowledgements ...... 261
Appendix 1 (Supplementary Info. for Chapter 3) ...... A1
Appendix 2 (Supplementary Info. for Chapter 4) ...... A6
Appendix 3 (Supplementary Info. for Chapter 5) ...... A14
Appendix 4 (Supplementary Info. for Chapter 6) ...... A16
Appendix 5 (Supplementary Info. for Chapter 7) ...... A18
Appendix 6 (Supplementary Info. for Chapter 8) ...... A20
xiv 1 Introduction
1.1 General Introduction and Aims
Photochromism has become an important and integral area of research, attracting interest for many high-end applications such as stimuli-responsive materials, high- density optical data storage, optical memories and switches, optical displays, non-linear optics, to name a few. The worldwide commercialization of T-type dyes in ophthalmic lenses is probably the most notable application as well as surface coatings, novelty items and various other light transmissible materials where dyes are added to provide UV protection and striking visual effects. A considerable amount of effort has been devoted to developing the chemistry and synthesis of photochromic dyes and certainly the last decade has seen a sharp rise in the number of publications containing comprehensive studies in solution which demonstrate the dependence of photochromic properties on electronic structure of dyes. Overall this has promoted a deeper understanding and appreciation of their unique properties. Unfortunately, an often overlooked issue is that of practically utility. For most applications, dyes normally require a solid format that is optically clear and mechanically viable. Polymers are an excellent option here since they offer robustness and clarity as a matrix with the possibility of forming films, beads, fibres, gels and mouldable items. Continued studies into photochromic polymers have been invaluable, proposing new concepts, solutions and applications for photochromism. These have evolved from a deeper understanding of how photochromics interact in a polymeric environment. The behaviour of the photochromics and polymers can be thought of as being inter- connected on many levels: the characteristics of the polymer and the mode of incorporation, whether covalent attachment or dissolution into a polymer host, can have an influence on photochromic behaviour. Micro and nano-environmental properties of polymer matrices, such as local rigidity, polarity and free volume, as well as their intermolecular interactions can all affect the efficiency and properties of the photochromic interconversions. Photo-repsonsive polymers have also shown that the photochromic transformations can also be used to influence the properties and behaviour of the polymer which incorporates them. Lastly, photochromism has also been applied to probe environmental characteristics since their behaviour is greatly influenced by what surrounds them. Chapter 1. Introduction ______
As a general concept, photochromic transitions are slower in a polymer matrix, as compared to solution. This effect is broadly attributable to the reduced segmental motion of the macromolecules and the limited free volume available in a polymeric medium. The matrix environment imposes steric constraints by limiting the mobility of the molecules and therefore their ability to isomerize. Furthermore, aggregation of molecules within solid media can also influence kinetic and spectral properties. When pertinent dyes like naphthopyran and spirooxanine dyes are included into a rigid lens matrix, this represents a very severe test of their photochromic performance. Important photochromic characteristics such as the ability to display intense and desirable colours with excellent fatigue resistance are often compromised by inefficient switching. The development of more rapid and efficient fading lenses is an area that is both commercially enticing and attracts attention from a material standpoint. Our solution to overcoming the matrix effect has been to apply concepts developed in drug and gene delivery, where polymer conjugates are used to protect the drug from a harsh biological environment. It has been found that when dyes have oligomers covalently attached to them, they are essentially insulated from the rigid lens matrix environment, since the entanglement and partitioning of polymer tails around attached dye molecules is able to provide a local environment of controlled viscosity. This technology was demonstrated for the first time by Such et al in 2004 where the photochromism of a spirooxazine was able to be controlled in a lens matrix through local environment effects. Polymer conjugation was applied using living radical polymerization (ATRP) to grow polymer chains from a spirooxazine initiator. In this thesis many of the original concepts will be extended with new ideas and methodologies by focusing on naphthopyran (chromene) dyes which are used commercially in ophthalmic lenses. The general aim of the studies will be to broaden an understanding of how these molecules interact with respect to their local environment, centralizing on how to control and apply their photochromism logically with polymers.
1.2 Outline of Thesis
Chapter 2. Literature Review. This chapter reviews the area of photochromism. It begins with a broad description into the main concepts, such as the mechanisms, the different classes of dyes, the photochemistry involved, the techniques used for investigating photochromic reactions and the analysis and interpretation of 2 Chapter 1. Introduction ______photochromic behaviour. The next section is an in-depth discussion into photochromic polymers which extends these main concepts but focuses on photochromic behaviour in polymeric environments. Important considerations are the effect of the matrix properties, such as rigidity, polarity and the method of incorporation which are all discussed. Finally, the application of photochromic polymers in literature is reviewed at length with many examples of photo-responsive systems. Overall, the discussion emphasizes the multi-faceted relationship between the photohchromic dye and the polymer that incorporates it. The use of controlled radical polymerization and its application to the synthesis of photochromic polymers is also discussed. Some of this chapter has been published as part of a review article (Photo-responsive Systems and Biomaterials: photochromic polymers, light-triggered self-assembly, surface modification, fluorescence modulation and beyond. Ercole F.; Davis T.P.; Evans R.A..; Polymer Chem., 1, 37-54, 2010, Front cover of inaugural issue).
Chapter 3. Comprehensive Modulation of Naphthopyran Photochromism in a Rigid Host Matrix by Applying Polymer Conjugation. This chapter introduces the technique of tailoring photochromic performance in a lens matrix by using novel photochromic-polymer conjugates, synthesized using atom transfer radical polymerization (ATRP), using an essentially divergent (grow-from) approach for polymer synthesis. The concept of creating a customized local environment for dye within a host matrix is applied to naphthopyrans, an important class of photochromic dyes currently dominating the commercial ophthalmic lens market. In this study naphthopyran polymer conjugates of various rigidities were synthesized by ATRP and their photochromic properties were tested within a rigid host matrix (lens). Broad tuning of photochromic kinetics was displayed as a result of polymer conjugation because of its ability to alter the local environment of the naphthopyran within the host. End- functionalized conjugates, synthesized from a naphthopyran functionalized ATRP initiator, allowed systematic tuning of kinetics via modulation of chain length of attached polymer. Reducing the rigidity of the conjugate resulted in an acceleration of kinetics and an increase in colorability. Pronounced chain lengths of poly(methyl methacrylate) (>18,000 g/mol) were required for decoloration kinetics to be effectively lowered compared with an unconjugated naphthopyran control. Random in-chain incorporation of the naphthopyran was afforded by copolymers made with naphthopyran-functionalized monomers. At the expense of a defined placement of the
3 Chapter 1. Introduction ______dye moiety with respect to the conjugated polymer chain, such conjugates displayed a pronounced ability to influence the kinetics. Persistent color due to thermally stable isomer populations was observed for all samples and found to be uninfluenced by polymer conjugation. This content of this chapter has been published (Comprehensive Modulation of Naphthopyran Photochromism in a Rigid Host Matrix by Applying Polymer Conjugation. Ercole F.; Davis T.P.; Evans R.A..; Macromolecules, 42, 1500-1511, 2009).
Chapter 4. Photochromic Polymer Conjugates: The Importance of Macromolecular Architecture in Controlling Switching Speed within a Polymer Matrix. This chapter continues with the concept of applying photochromic-polymer conjugates to control photochromic behaviour but expands this idea by applying different architectures. Naphthopyran-poly(n-butyl acrylate) conjugates with different geometries were assembled using ATRP. First, within the rigid lens matrix, an investigation of the photochromic behavior of various poly(n-butyl acrylate), p(n-BA), homopolymers showed that mid-placement of a single dye moiety, made possible using a Y-branching difunctional photochromic initiator, gave superior fade kinetics per chain length of conjugated polymer compared to end-functionalized homopolymers. Furthermore, having the dye pendant from the chain opposed to directly within the chain was also found to be advantageous. Fading kinetics became faster when chain length was increased, except in the case of linear random copolymers synthesized by copolymerization of n-butyl acrylate with a naphthopyran acrylate. A gradient copolymer made with a non-photochromic difunctional initiator and a naphthopyran methacrylate displayed superior kinetics. Films consisting of ABA triblock copolymers, incorporating the photochromic in the middle of a soft p(n-BA) section, gave slower switching speeds compared to lens samples, with responses that were highly tunable and dependent on the amount of soft section inhabited by the photochromic moiety. The content of this chapter has been published (Photochromic Polymer Conjugates: The Importance of Macromolecular Architecture in Controlling Switching Speed within a Polymer Matrix. Ercole F.; Malic N.; Harrisson S.; Davis T.P.; Evans R.A. Macromolecules, 43, 249-261, 2010).
4 Chapter 1. Introduction ______
Chapter 5. Optimizing the Photochromic Performance of Naphthopyrans in a Rigid Host Matrix using Poly(dimethylsiloxane) Conjugation. This chapter introduces a simple chemical strategy for making photochromic-polymer conjugates. This is essentially a convergent approach, whereby the dye is directly conjugated to a pre-formed polymer. A series of different methoxy substituted 2,2-diaryl-2H- naphthopyran photochromic dyes were assembled incorporating hydroxyl functionality to allow their subsequent attachment to flexible poly(dimethylsiloxane) oligomers. The photochromic performance of the generated PDMS-naphthopyran conjugates was studied in a thermoset host matrix (the lens) and compared to non-conjugated, electronically equivalent control dyes. Both coloration and decolouration speeds were found to be greatly improved with critical T1/2 decolouration times reduced by 42-80%. The extent of solution-like performance provided by PDMS conjugation in the rigid host was examined with reference to the fade performance of control dyes in solution, and found to range from 20-90%. These measures are believed to be influenced by the electronic nature and steric interactions of the photochromic dyes. The content of this chapter has been published (Optimizing the Photochromic Performance of Naphthopyrans in a Rigid Host Matrix using Poly(dimethylsiloxane) Conjugation. Ercole F.; Malic N.; Davis T.P.; Evans R.A.; J. Mater. Chem., 19, 5612- 5623, 2009).
Chapter 6. Photochromic Behaviour within Polymer Matrices. Part 1: Highly Crosslinked Networks. This chapter focuses on the host matrix itself. Instead of doping a specialized photochromic-polymer into the matrix mixture, various naphthopyran dye monomers were reacted with the lens matrix composition to become part of the final crosslinked network structure and their photochromic behaviour subsequently investigated. Compared to unbound dyes (either free or polymer- conjugated), direct tethering to the rigid network was found to restrict the ability of the dyes to move and universally caused a decrease in colouration and decolouration rates. Tethering to the network structure via two reactive points located on opposite sides of the dye molecule caused a further reduction in switching speed resulting in very low levels of colouration. The fade kinetics displayed by matrix tethered dyes were also found to be more complex indicating that their local environment is less homogenous overall in the host matrix. A PEG spacer separating one tethering point from the dye allowed fade kinetics to approach those of un-tethered controls. An EG-succinate spacer
5 Chapter 1. Introduction ______that directly separates the tethering point/s from the dye itself was found to have the largest impact with longer spacers offering less improvement in speed. The position of the tethering point with respect to the dye, whether attached through the top or bottom section of the molecule, was also found to be an added complexity, significantly influencing both fade kinetics and colourability.
Chapter 7. Photochromic Behaviour within Polymer Matrices. Part 2: Hyperbranched Polymers. This chapter focuses on another crosslinked matrix system - hyperbranched polymers. Again, naphthopyran monomers were reacted within the matrix, ending up as part of the final crosslinked branched structure and their photochromic behaviour subsequently investigated. Photochromic performance was used to probe and speculate on the nature of the hyperbranched pMA and pMMA structures. The slower kinetics indicated a denser structure than expected. Longer and more flexible crosslinks are expected to provide an environment more suitable for faster switching. The content of Chapter 6 and Chapter 7 are being compiled into a combined manuscript for publication.
Chapter 8. The Application of a Photochromic Probe to Monitor the Self- Assembly of Block Copolymers in Water. The thermal decolouration response of a spirooxazine dye incorporated within several amphiphilic polymers was used to probe their self-assembly behaviour in water. Various thermally responsive poly(N-isopropyl acrylamide)-block-poly(N-acryloyl morpholine) copolymers were synthesized by RAFT polymerisation which contained the photochromic entity either as a end group or side pendant. The Arrhenius plots of thermal decolouration for each of the block copolymers in water displayed a deviation from linearity which correlated well to their thermal self- assembly profiles, as measured by DLS. The block copolymers containing spirooxazine units within the polyNIPAM section displayed the most notable deviations. The results were interpreted in terms of the effect of the environment on the transition state of the open form on progression to the closed form. The incorporation of the free dye, SOX- PROP, within the core of micelles based on poly(n-butyl acrylate)-block-poly(N- acryloyl morpholine) was also apparent from the measured decolouration kinetics. This was thought to result from the change in the local environment of the dye which occurs during encapsulation into the micelles.
6 Chapter 1. Introduction ______
The content of this chapter has been published (The Application of a Photochromic Probe to Monitor the Self-Assembly of Thermosensitive Block Copolymers. Ercole F.; Harrisson S.; Davis T.P.; Evans R.A.; Soft Matter., DOI: 10.1039/C0SM00746C, 2011.
Chapter 9. General Conclusions
This chapter collectively summarises results obtained in previous experimental chapters.
7
2 Literature Review
2.1 Photochromism – Definition and Description
The phenomenon of photochromism is defined as a reversible transformation of a chemical species between two isomeric forms, induced by the absorption of light, which results in a change of absorption spectra (colour).1 This change is reversible so that the original isomeric form can be restored by exposure to light or heat. The photochromic interconversion between isomeric forms is often referred to as switching. The system can be represented by a reversible transformation between forms A and B as shown in Figure 1.
h ( ) 1 A 1 B ( 2 ) h 2 /
Figure 1. Photoisomerization bewteen isomeric forms A and B, each having different absorption spectra.
Chemical processes involved in organic photochromism include pericyclic reactions, intramolecular hydrogen, group and electron transfers and dissociation processes. The most common photochromic systems are based on unimolecular processes like cis-trans isomerizations, compared to bimolecular systems, such as photo-dimerization reactions.2 Photochromic molecules are considered dyes due to their colourabilty and can be suitably divided into two classes depending on how form B reverts back (or decolouration) to the colourless form A: with P-type dyes the back reaction occurs exclusively with light; with T-type dyes the back reaction can also occur thermally. Other descriptive classifications are commonly used. For example, when form A is colourless or pale yellow and form B is more strongly coloured and red shifted (i.e. red Chapter 2. Literature Review ______
or blue coloured), this is referred to as positive photochromism. When max(A) >