Benzopyrans photochromism

The photochromic compounds of potential interest, based on the 2//-chromene ring system, are the 2//-benzopyrans (1.18) or the three isomeric naphthopyrans (1.19-1.21). However, 2H-naphtho[2,3-( ]pyrans (1.21) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R and can be part of a carbocyclic spiro ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups. 

Simple 2,2-dialkyl-2/f-benzopyrans, or chromenes as they are very often called in the photochromic hterature, can be synthesised by several well established routes, a selection of which are shown in Figure 1.7. ... 

The photochromic chromophores can be classified into two categories, depending on the thermal stability of the photogenerated isomers. When photogenerated isomers are unstable and revert thermally to their initial isomer state in the dark, the chromophores are classified as T-type (thermally reversible type). Most photochromic chromophores belong to this type. The photogenerated blue color of 6-nitro-l, 3, 3 -trimethylspiro-[2H-l-benzopyran-2,2 -indoline], for example, disappears in less than half an hour even in high Tg polymer matrices.181 Such thermally unstable photochromic chromophores cannot be applied in photoswitchable molecular systems, because the switched states are unstable. For those applications, the characteristic of persistence, or in other words thermal irreversibility, is indispensable. 

His research interests include the chemistry of benzopyrans and benzothiopyrans and various aspects of color chemistry. These have resulted in the development of a range of photochromic naphthopyrans which are in commercial use in ophthalmic lenses, security materials, and novelty items. 

Twenty-two derivatives of spiro[(27/-l-benzopyran)-2,2 -benzo-l, 3 -dithiole] (117) were prepared (35-80% yield) and studied. Most existed in solution only in the spiro form, and were fast faders so that photochromism was observable only in frozen solutions at 77 K. The colored forms (Amax usually in the 550-600-nm region) were stabilized by 7-diethylamino and 7,8-benzo groups. The benzodithiolane and benzoxathiolane spiropyrans exhibit positive solvatochromism, rather than the... 

M. A. Halberstam and M. B. Gordin, Kinetics of reversible photochromic reactions in the series of 1,5-disubstituted 3,3-dimethyl-6 -nitro-8 -bromospiro-[(2 /7,l -benzopyran)-2,2 -indolines], Photo-... 

S.-R Heum, M.-S. Hur, P. M. Kazmaier, and E. Buncel, Thermo- and photochromic dyes Indolino-benzospiropyrans. Part 1. UV-VIS spectroscopic studies of 1, .3,.3-spiro(2 //-1 -benzopyran-2,2 -indolines) and the open-chain merocyanine forms solvatochromism and medium effects on spiro ring formation, Can. J. Chem., 69, 1940-1947 (1991). 

K. Matsui and S. Yoshida, Photochromic film of 6-nitro-r,3, 3 -trimethylspiro[2/7-l-benzopyran-2,2 -indoline] prepared by plasma polymerization, J. Appl. Phys., 64, 2607-2610 (1988). 

H. Takahashi, H. Murakawa, Y. Sakaino, T. Ohzeki, J. Abe, and O. Yamada, Time-resolved resonance Raman studies of the photochromic reaction of 6-nitro-l, 3, 3 -trimcthylspiro 2 //- 1 -benzopyran-2,2 -indoline], J. Photochem. Photobiol. A Chem., 45, 233-241 ( 1988). 

In 1966, Becker and Michl8 reported for the first time on the photochromism of 2//-l-benzopyrans. Becker investigated more than 25 compounds and found that in... 

The photochromic properties of a number of naturally occurring 27/-1-benzopyrans were studied at low temperatures by Kolc and Becker.65 A few examples of this structurally diverse class of compounds are shown in Figure 3.18. 

The now classical route to 2/7-1 -benzopyrans, cyclisation of aryl propargyl ethers, has been utilised in the synthesis of mollugin <06T8419> and to prepare novel photochromic naphtho[2,l-b]pyrans <06T737>. 

Smets and coworkers worked with stretched spirobenzopyran rubber networks obtained by copolymerization of ethyl aaylate with variaWe amounts of a bis-photochrome dimethacrylate as cross-linking agent, namely l,l -(a,a -p-xylyl)-bis-[3, 3 -dimethyl-8-methacryloyloxymethyl-6-nitro-spiro(2H-l-benzopyran-2,2 -indoline)]. The chemical structure of these DIPS-rubbers is given in Fig. 11. 

Resonance Raman studies of the photochromism of 1 .3, 3trimethyl spiro - [2H-1 -benzopyran-2,2 -indoline] show at least four... 

T. Yoshida, A. Morinaka, and N. Funakoshi, Photochromism of a vacuum-deposited l, 3, 3 -trimethyl-6-hydroxyspiro[2/7-l-benzopyran-2,2 -indoline] film, J. Chem. Soc., Chem. Commun. 1986,437-438. 

Spiro compounds other than spirofindoline-benzopyrans] have been synthesized with the aim of finding photochromic systems endowed with both good color-ability and long durability. 

We have shown38 that 8-methoxy-6-nitro-BIPS (11) is the only photochrome able to sensitize the formation of (h with a quantum yield A = 0.15 that is about one-fourth that of a commonly used singlet-oxygen sensitizer, i.e. methylene blue, for which 0A = 0.57.39 This benzopyran behaves as a good quencher of l02 as well. Unfortunately, in the paper by Guglielmetti and co-workers an analysis of the... 

P. Appriou, R. Guglielmetti, and F. Gamier, Study of the photochemical processes in the benzopyranic ring opening reaction of photochromic spiropyrans, 7. Photochem. 8, 145-165 (1978). 

T. Yoshida and A. Morinaka, Decomposition process ofthe photochromic compound spiro[l,3,3-trimethylindoline-6 -hydroxy benzopyran] in the solid state under UV irradiation, 7. Photochem. Photobiol. A 63, 227-234 (1993). 

Spiro[indoline-benzopyran] 458 is a highly photodegradable photochromic molecule.59,60... 

T. Bercovici, R. Heiligman-Rim, and E. Fischer, Photochromism in spiropyrans, Part VI Trimethylindolino-benzopyran and its derivatives, Mol. Photochem. 1,23-55 (1969). 

This chapter, far from being exhaustive, is aimed at outlining the properties of the radical ions and of the free radicals derived from photochromic compounds independently of whether or not these paramagnetic species are involved in the photochromic process itself. In particular, it will deal with the EPR of dihydro-and dialkyldipyridyl radical cations (viologens) and of the radical anions of a variety of nitro-substituted spirofmdoline-benzopyrans], spiro[indoline-naphtho-pyrans], and spiro[indoline-naphthoxazines] as well as their triphenylgermyloxy nitroxides and with the triplet spectra observed upon irradiation of 2,2,4,4-tetrachloro-1 -keto-1,4-dihydronaphthalene. 

Spirofindoline-benzopyrans], spiro[indoline-naphthopyrans], and spiro[indo-line-naphthoxazines] can rather readily undergo electron-transfer processes in either direction i.e they can be readily oxidized and reduced. Because the presence of nitro groups in their molecular framework normally enhances the photochromaticity of these compounds, several combined electrochemical and EPR studies have been carried out on a variety of mono- and dinitro-substituted spiroindolinic photochromes.52 56-60... 

S. Hashimoto, A. Shimojima, T. Yuzawa, H. Hiura, J. Abe, and H. Takahashi, Time-resolved resonance Raman and molecular orbital studies ofthe structure ofthe transient species involved in the photochromic reaction of 2,2/-Spirobi 2//-l-benzopyran, ./. Mol. Struct. 242, 1-14 (1991). 

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