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3.

Introduction

This chapter will introduce the phenomenon of photochromism, including a short overview of some of the classes of materials that exhibit this effect. The main focus is upon the triphenylimidazolyl radical-dimer systems and dihydropyrenes that feature in the results and discussion of Chapter 4.

3.1.

Photochromic compounds

Photochromism describes a reversible colour change of a material when exposed to light.1 This phenomenon has been recognised for some 145 years, with the earliest known report attributable to Fritzsche, who noted that when a solution of tetracene was exposed to daylight its usual orange colour bleached.2 This was later demonstrated to be a result of photodimerisation of the acene that disrupts its conjugation and shifts the absorption maximum into the ultraviolet region.3

Photochromism has been defined above as being a reversible process; therefore, irreversible photochemical reactions, even if they include a change in colour, such as the conversion of white AgCl to colloidal brown/black Ag, are not considered forms of photochromism. A second stimulus is required to induce the reverse process: this can either be via a thermal (T-type) or photo (P-type) mechanism, with the latter typically at a different wavelength to that used for the forward reaction. Reversibility is also a desirable quality for technological applications. These applications range from the relatively mundane, such as spectacles which change colour upon exposure to bright light,4 to more advanced, prototypically realised, optoelectronic memory devices for data storage wherein a molecule is ‘written’ into one state by an input photon and ‘read’ by a second (for instance, by a change in its UV-visible absorption or infra-red spectrum).5 The use of TPA could even result in 3D optical data storage due to localised excitation of the photochromic material.6

If a photochromic compound is to find use in a particular application then the material has to be stable upon transformation between the two or more forms of interest, typically for extended periods of time and over multiple cycles.1 A complicating factor is that of additional photochemical pathways that deplete the sample by producing non- photochromic side-products. Such fatigue pathways affect many photochromic systems: if the alternative processes are sufficiently competitive with the desired conversion, the

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Figure 3.1. Common photochromic materials and their classifications. The compounds bound

within the blue rectangle to the lower right corner of the figure are those that are explored in this work. The photochromic mechanism of each class is indicated by the colour of their name:

trans-cis isomerism (red), cycloaddition (black), electrocyclisation (blue), excited state

intramolecular proton transfer (green) and radical-dimer equilibrium (orange).

material is rendered essentially useless under certain operating conditions. These pathways depend on the molecule of interest, but may include reaction with oxygen or adventitious water, proton abstraction from solvent or the formation of non- photochromic isomers. It is therefore important to study not only the properties of direct importance to the photochromism, such as the quantum yield, rate of transformation and net change in colouration, but also to identify and understand such deleterious pathways.

There are several general classes of molecules that undergo photochromism, with some of the most common illustrated in Figure 3.1. The tetracene dimerisation observed by Fritzsche is an example of a cycloaddition. Photochemical inter- or intramolecular concerted cycloaddition reactions are observed for [2n+2m] processes, where n and m are the number of π-electrons of the two components and are positive integers having an even sum. These reactions are a subset of the processes that can occur according to the Woodward-Hoffmann Rules. The inherent reversibility of

124 cycloaddition reactions makes them suitable as a mechanism for photochromic materials. The dimerisation of tetracene described above is a [4+4] cycloaddition. The extension of this concept to other acenes, such as anthracene, is obvious.7 Acene dimerisation still receives attention with new derivatives of tetracene being investigated.8 The dimerisation of tetracene has the complication that it can produce a mixture of two isomers. Of the other possible photocycloadditions that could lead to photochromism, the [2+2] reaction of two alkenes is the most studied. Such studies include pre-organised tethered systems and solid-state structures that orientate the two molecules into a suitable mutual arrangement to facilitate a topochemical cycloaddition.9

A somewhat related process to that described above is electrocyclisation, wherein one new bond is formed in an intramolecular reaction. Some examples of molecules that undergo photochromic isomerisation through this mechanism are indicated in Figure 3.1. These include the spiropyran,10 fulgide,11 diarylethene12 and dihydropyrenei (DHP)13 classes, with the last of these occurring via a retro- electrocyclisation in the forward direction of the photochromism.13 Arguably, the most well studied class are the diarylethene derivatives developed by Irie.12 The bisthiophene analogue shown is the most common, with the perfluorocyclopentene ring required for both rigidity, to hinder isomerism, and for inhibition of fatigue by oxidation at the pseudo-benzylic positions. In the open form, conjugation is limited to each individual thiophene ring, whereas upon electrocyclisation, conjugation occurs also through the bridge. The diarylethene photochromic-unit has been incorporated into many organic14-16 and organometallic17-19 systems to induce a change in electronic communication between two moieties. Furthermore, development of the system through variation of the arene has lead to tuning of both the colour and switching rates of diarylethene molecules.12 The spiropyran and fulgide systems are similar in their mechanism and utility, although they perhaps have more restricted opportunity for derivatisation. DHPs will be considered in more detail later in Section 3.3.

Another mechanism for photochromic materials to operate via is a trans to cis isomerism, such as the azo dyes.20 Excitation to the S1 π*←π state of a diarylazo compound reduces the bond order of the azo linkage, allowing rotation about this bond and formation of the metastable cis form that has a different absorption spectrum. The

i

In full, the unsubstituted derivative is trans-10b,10c-dimethyl-10b,10c-dihydropyrene. Numbering of substituents is the same as for pyrene used in Chapter 2.

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cis form can be reverted to the thermodynamically more stable trans form by either

irradiation or heat. Schiff bases (imines) may also undergo a similar isomerism; however, in the case of substituted 2-phenolimines, excited state intramolecular proton transfer from the hydroxyl group to the nitrogen atom of the imine is a competitive process, with concomitant redistribution of the π-system and a change in conjugation.21 Thus, Schiff bases are an interesting example whereby two different mechanisms could result in different photochromic behaviour.

The final mechanism of photochromism to be discussed here is the dimerisation of a radical, as exemplified by the triphenylimidazolyl radical (TPIR). Radicals, in general, have low energy absorption bands due to the small energy gap between their highest doubly occupied molecular orbital and their yet higher energy singly occupied molecular orbital (SOMO). Upon dimerisation, a closed-shell species is formed that has an absorption spectrum more akin to a typical organic material. Thus, a significant colour change is observed upon dimerisation. If the radical dimerisation is reversible, then photochromism can be achieved. The TPIR system is considered in some more detail in the next section.