Spiro compounds spiropyrans

The first chapter of the book deals with leuco-spiropyrans and related spiro compounds, which constitute one of the classes of leuco compounds not of the redox type. Such materials are photochromic, and are of major technical importance. The author, Hiroyuki Nakazumi of the Department of Applied Chemistry at the University of Osaka Prefecture, is well known for his researches in functional dye chemistry, particularly photochromic materials, and he provides a very useful update of the field, covering mechanisms, synthesis, spectra and applications, together with a useful section on approaches to near-infrared absorbing photochromic dyes. 

Spiropyrans are also used in conjunction with organic acids. When the reagents are brought into contact with each other by heating, the change in pH transforms the spiro compound into a coloured form (see Section 1.14.3.2). 

In the early seventies, Arnold and Paal published a detailed IR study on a series of 16 photochromic compounds.8 In this paper the authors reported a comparative assignment of the main vibrations of indolino- and benzothiazolino-spiropyrans along with the corresponding spirooxazines. IR data from benzox-azolinospiropyrans were also included for comparison. Thus, this study provided an interesting data base which includes the vibrational characteristics of the two popular parent spiro compounds (since they have been used so often in various... 

The reaction between isatin derivatives 44, malonodini-trile 21, and CH acids 43 led to the formation of spiropyran compounds 46 by the application of a variety of catalysts. Yuan et al. reported the use of 10mol% cupreine 45 as an organocatalyst that led to spiro compounds with enhanced enantiomeric excess (Scheme 13.15) [23]. The reactions are conducted in dichloromethane or 1,2-dichloroethane as solvents at 0 °C, while the addition of 4 A molecular sieves provides shorter reaction times and higher ee s. 

Instead of metal chelation, an intramolecular hydrogen bonding between the oxygen atom of phenolate and a hydrogen atom of a carboxylic acid in the 8-position leads to stabilization of the colored form, such as compound 12.20,21 This spiropyran exhibits reversed photochromism, which means that thermally stable species change from the spiro form to the colored form, and thus the colorless form produced by photoirradiation soon converts to thermally stable colored form. 

Indolines, benzoxazole, and benzothiazole are possible as 2-methylene heterocycles. The number of known spirooxazine derivatives is much less than for the spiropyrans. This may be partly due to lack of many substituted o-nitrosonaphthols and partly due to lack of sufficient stability of spiro-oxazines. The structures of parent spirooxazines and the Xmax of their photomerocyanine forms are listed in Table 5. The Xmax of the colored forms of compounds 41-43 are not described in the literature. 

Chapter 4 is concerned with a technically important group of leuco compounds which like the spiropyrans are not formed by reduction of the parent dye, but by formation of a spiro structure from the dye in such a way that the newly created sp3 center destroys the conjugation, and hence, the color of the chromophore. These are the phthalides (spirolactones) and the position of equilibrium is determined by pH rather than a redox process. Such materials are used mainly as color formers in pressure-sensitive... 

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

Spiropyrans refers in general to a (substituted) 27/-pyran having a second ring system, usually (but not necessarily) heterocyclic, attached to the 2-carbon atom of the pyran in a spiro manner as shown in structure 1 i.e., a carbon atom is common to both rings. Compounds in which the second ring system is merely a saturated carbocycle such as cyclohexyl or adamantyl are better considered simply as 2,2-dialkylpyrans, and are discussed in Chapter 3. 

The second method corresponds to moving the formyl group from the aldehyde intermediate to the methylene base, and is a standard method for preparing a merocyanine (the open form of a spiropyran). This method is useful for the reaction of the easily obtained and stable Fischer s aldehydes (2-formylmethyleneindolines) (1) with ketomethylene compounds such as 2-hydroxybenzofuran (2) or 2-hydroxybenzothiophene where the corresponding hydroxyaldehyde is difficult to obtain (Scheme 4). These two methods include the several routes to various symmetrical and unsymmetrical spiro(dipyrans).6... 

The various techniques of Raman spectroscopy have recently been applied to the spiropyrans, and this is discussed more extensively in Chapter 8 in Volume 2. BIPS and its 6-nitro-, 6-nitro-8-methoxy-, and 6-nitro-5,7-methoxy derivatives were studied by surface-enhanced (SERS) and resonance-enhanced Raman spectroscopy in neutral, acidic, and basic solutions. The complex results are too numerous to summarize here, but two unusual ones are that even in acid solutions the tested compounds gave the SERS spectra of the unprotonated open forms, and that an aged solid sample of 6-nitroBIPS gave results very different from a fresh sample. This was attributed to the existence of two enantiomers of the spiro form, which are slowly interconvertible in the solid state and which have greatly different photocoloration quantum yields.167... 

Upon treatment with acid (or on contact with an acidic surface), many spiropyrans give the salt of the open form or the open form itself, depending upon the relative base strengths of the spiro and open forms. Thus, treatment of several BIPS (7-diethylamino, 6-nitro, and 5 -nitro) with trifluoroacetic acid in the nonprotic solvents acetonitrile and chloroform gave the protonated merocyanine form, which upon neutralization with base gave the open colored form.180 This sequence of operations causes coloration by a non-thermal, non-photochemical route the adsorption coloration was utilized in the early applications of spiropyrans in carbonless (pressure-sensitive) copy papers. In this application, dialkylamino-substituted spirodi(benzopyrans) were preferred paper containing BIPS compounds turned pink on storage. 

This chapter is divided into four sections devoted to, respectively, spiro-heterocyclic compounds (e.g., spiropyrans and spirooxazines), Schiff bases and related nitrogen-containing molecules, bianthrones and other overcrowded ethenes, and, finally, miscellaneous compounds. 

Spiropyrans are an important class of materials that show interesting properties such as photochromism and acidichromism. Similar to azo compounds, photoirradiation or the addition of acidic molecules to spiropyrans lead to drastic changes in physical and chemical properties due to spiro-merocyanine isomerization. Hexagonal columnar LC complexes of a spiropyran derivative with 4-methylbenzenesulfonic acid were formed due to the acid-induced spiro-merocyanine isomerization [100], The columnar LC mixture aligned uniaxially on a substrate can serve as one-dimensional ion conductors. When poly(styrenesulfonate) was employed as a complexing agent for spiropyran, no mesophase was observed. 

When spiropyrans are treated with acids they are converted into spiropyran salts , which exhibit photochromic behavior differing from that of the parent spiropyran compounds. The gross mechanism proposed is illustrated in Figure 14.9. In the dark at room temperature, the compounds give colored solutions due to the presence of the 0-protonated merocyanine species III. The open form is converted by irradiation with visible light to the iV-protonated spiro form IV. As spiropyrans are fairly strong bases in the open form but very weak bases in the clos spiro form, the charged species IV can lose a proton and the neutral species I can be actually formed. 

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