Photochromic systems, absorption

A photochromic system based on the reversible photooxidation of the leuco forms of certain organic dyes has been described (B-76MI11401). Under appropriate conditions compounds such as the phenothiazine (150) can exist in different oxidation states (Scheme 17), each having significantly different absorption spectra. 

Most photochromic systems are not reversible indefinitely. However, very little careful analytical data have been accumulated to characterize the nature of the degradation products or to specify the degree of quantitative reversibility. The reasons for side reactions are inherent in the high photochemical reactivities of the systems. First of all, there must be an excited state formed by absorption this state is then transformed into other excited states or reactive species. The latter may include triplet states, carbonium ions, carbanions, free radicals, or other highly reactive intermediates. Certain of these are oxygen sensitive so that exclusion of the atmosphere and other potential sources of contaminants during irradiation is necessary. A second major route of degradation involves the excited state of the colored form which may already be... 

Photochromism has also been observed when two porphyrinic groups are linked to a dithienylethene scaffold. The closed form showed an absorption band at 560 nm (01JA1784). The same behavior was observed in the system 318-319 (02AM918). Dithienylethene photochromic systems have also been described to be linked to single-walled nanotubes (07JA12590). 

Photochromism is a reversible transformation of a single chemical species between two states, the absorption spectra of which are clearly different, the transition in at least one direction being induced by electromagnetic radiation [1], The widest and most important group of the photochromic system is based on electrocyclic reactions [2,3] a few have been commercially successful (polymer-based photochromic eyewear, novelty items and security printing inks). Several other photochromic systems based on E,Z-isomerization, cycloaddition reaction, electron or proton transfer have potential industrial applications [4],... 

There are different types of photochromic systems (5), e.g. triplet-triplet absorption, cis-trans isomerization or homolytic dissociation reactions. Since lignin has a very heterogenous and complex structure there is probably not a single responsible system, but rather a mixture of systems. 

The observed photochromic effect can be due to a triplet-triplet absorption which is the simplest photochromic system in materials containing molecules in the stable ground state and in a metastable photochemically excited state. 

Situations where the absorption spectra of reactants and products are not conducive to high yield solid-state reactions may occur in classic photochromic systems. In a recent study on the crystalline photochromism of l,2-bis(2,4-dimethyl-3-thienyl)perfluorocyclopentane (55, Scheme 35) by Irie et al., it was... 

Ordinarily, the photochromic reaction involves a reversible transformation between two species with B having at least one absorption band appearing at longer wavelength than those of A. The activating radiation generally is in the UV region (300 to 400 mm) but could be in the visible (400 to 700 nm). The most prevalent photochromic systems are established to be unimolecular reactions (A — B) and those described in this book correspond to this type. 

K. Uchida, S. Nakamura, and M. Irie, Thermally irreversible photochromic systems. Substituent effect on the absorption wavelength of 1 l,12-dicyano-5a,5b-dihydro-5a,5b-dimethylbenzo [1,2-6 6,5-6 ]bis[l]benzothiophene, Bull. Chem. Soc. Jpn. 65, 430-435 (1992). 

It is worth noting that the longwave absorption bands of 21bH+ and 22bH + are shifted bathochromically with respect to those of 21b and 22b. Table 8.7 contains some data on spectral and photochemical parameters of the novel photochromic systems described in this section. Photochromic behavior of one of the compounds is portrayed in Figure 8.10. 

Crystalline-state photochromism usually proceeds with considerably lower interconversion ratios of less than 15% because the light penetration into the bulk crystal is prohibited by the absorption of the photo-generated isomer (inner-filter effects) [3,4]. The fully reversible crystalline-state photochromism of 1 can be partly attributed to its photochromic property. The rhodium dithionite complex 1 belongs to a unique class of photochromic compounds, which exhibits a unimolecular type T inverse photochromism [13]. The type T inverse photochromism means that the back reaction occurs thermally and the A.max of the absorption spectrum of 1 is longer than that of 2. If the back reaction occurs photochemically and the XmaK of the initial absorption spectrum is shorter than that of the photo-generated isomer, it is called type P positive photochromism and is known as a common photochromic system. 

Fig. 4.5 Absorption spectra of a unimolecular two-state photochromic system. Figure adapted from Ref. [51]...

The previous section describes photochromic systems in which interconversion between two forms can be induced by absorption of light. However, more complex scenarios also exist and some have particular practical importance. With 2-(2, 4 -dinitrobenzyl)pyridine (DNBP), photochromism involves phototautomerisation with hydrogen transfer [69, 70]. However, this can either be transferred to the pyridine nitrogen giving the blue NH form or to the oxygen of the nitro group to give the yellow OH form (Fig. 4.7). These can revert thermally or photochemi-cally to the most stable colourless CH form. 

This chapter focuses on this particular family of photochromic systems. As presented elsewhere in this book, azobenzene-containing materials experience reorganizations at different length scales in response to light of the appropriate wavelength. The photochemistry of azobenzene molecules will be described. The main types of azobenzene polymeric architectures will be presented as well as associated synthetic methods. A brief overview of the photoresponse of azobenzene polymers when exposed to linearly polarized light in their absorption bands will be presented. Special emphasis will be... 

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