Spiropyran colored form

The photochromism of the spiropyran depends on the structure of heterocyclic parts, the medium such as solvent or plastic films, temperature, and light energy. Though the actual mechanisms may be more complex, a simple photochromic behavior in the spiropyrans is illustrated in Scheme 1. Initially, a spiropyran is excited by photoirradiation, and then a cisoid isomer arises after dissociation of the C—O bond. Finally, the cisoid form changes to the thermodynamically stable transoid form. The equilibrium between the cisoid and transoid forms largely depends on the substituent groups. The reversal of the colored form to the colorless spiropyran occurs by thermal or photochemical energy. More detailed mechanisms will be described in Section 1.2.1.6. 

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. 

The photocoloring reaction for spiroindolinobenzopyrans with a nitro group proceeds mainly via the formation of the excited triplet state of the molecule. The reaction proceeds partly from the triplet state [(SP )3] of the spiropyran to the triplet state (X)3 of the cis-cisoid isomer which subsequently transforms into the CF and partly from (SP )3 to the CF. This process from (X)3 to the colored form is accelerated by the presence of atmospheric oxygen (Scheme 6).2,28 For the photocoloring reaction, the participation of singlet or triplet state depends not only on the substituent but also on the nature of the heterocyclic component. 

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. 

In contrast to normal spiropyrans, in spironaphthooxazine series, as the polarity of the solvent decreases, a hypsochromic shift of the of the colored form is observed, except for spiropiperidinonaphthooxazine.79 For example, the of 33 shifts to shorter wavelength of ca. 20-60 nm in less polar solvents, such as toluene and cyclohexane, compared to ethanol. This result may suggest that the ground state of the photomerocyanine form in spironaphthooxazine is less polar than the excited state and the neutral quinoid form largely contributes to the photomerocyanine form in the ground state. 

Unlike spiropyran, 2 -substitution in spirooxazine has no effect on Alkoxy, chloro, and nitro substituents at 5-position, and alkyl substituent at 1-position in the indoline component have small effects on the Xmix of the colored form. 

Like other spiropyrans, the colored form of spirooxazines generated by UV irradiation, reconverts to the colorless form. However, it is possible to measure the thermal decay rates and activation energies at ambient temperature, since this fading reaction obeys first-order kinetics in solution. The thermal decay rate constant for spiroindolinonaphthooxazine has been found to be 0.02-0.15s 1 in ethanol and 0.1-1.4s 1 in toluene, although this may vary according to the substituent groups.72,77 However, these values are smaller than those of the spironaphthopyran series. 

The colored form of spiropyrans 10 presented in Table 3, which shows Vax in the near IR, has been prepared using similar molecular design.13 In contrast to spiropyran 10, the merocyanine form 45 is unstable, and quickly changed to the spiro form 45. The thermal stability is affected by presence... 

In the spirothiopyran 45b, the spiro form has two absorptions in the visible region (Vax 490 and 474nm) due to a polyene chromophore from IV-vinyl group to oxygen of the benzopyrylium component.90 The colored form of 45b produced by visible light irradiation shows the Vax at 570 nm. This colored form 45b" was confirmed by characteristic 1H-NMR spectra, as well as that of spiropyran. 

Much interest has been devoted to the photochromic behaviour of simple chromenes, especially because of practical applications of spiropyrans, particularly indolinospirans.5 Kolc and Becker30 have been able to demonstrate the o-quinoneallide structure of the colored form 4, by producing it in THF at -75° and trapping by reduction with LiAlHj. It is concluded31,32 that the same intermediates occur when spiropyrans are irradiated, because only the pyran moiety has an... 

Reversible photocoloration is attributed to an equilibrium between the spiropyran ( closed, colorless ) form and a merocyanine ( open, colored ) form, as shown in Scheme 1 for an indolinospirobenzopyran. The merocyanine itself is an equilibrium mixture of geometrical conformations, and its electronic distribution varies from highly zwitterionic to an essentially nonionic ortho-quinoidal structure. 

The series of spiropyrans (90) was prepared in 54-65% yield from 4-methyl-7-hydroxy-8-formylcoumarin and cationic salts. The absorption maximum of the colored forms of the dyes shown in Figure 1.2 were (Z, nm given) (91),... 

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... 

Another approach to obtaining spiropyrans whose colored forms absorb in the infrared is to use a heterocyclic base, such as those displayed in Figure 1.4, based upon benz[ctf]indole (121), 2-azaazulene (122),139-141 pyrroloanthrone (123),142 and other ring systems (124, 125)143-... 

A similar study of the photooxidation of some spiropyrans and spironaphthox-azines indicates that the spiro and open forms of these dyes are singlet oxygen quenchers and that the colored form does not act as a sensitizer. A mechanism is proposed that involves the formation of a superoxide radical anion by photoinduced electron transfer to oxygen from a merocyanine form of the dye, followed by nucleophilic attack of the radical anion on the radical cation of the dye.174... 

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. 

Spiropyrans show promise for optical recording, three-dimensional optical memories,214 and holography.215 The dyes currently under study for these applications very probably will not be used merely dissolved in a bulk polymer matrix, but will be oriented in films and membranes, or adsorbed or vapor deposited on solid substrates to take advantage of the nonlinear optical properties of the colored forms. For example, thick (0.5 mm) PMMA films of 6-nitro-thiaBIPS can be used to record wavelength-multiplexed volume holograms with an infrared diode laser. This system is impractical at present because of fatigue and poor diffraction efficiencies.216... 

The number of cycles in DHI 7 increases since oxygen is excluded, which, after activation to 102, causes destruction of the colored betaine 9.3 The number of Z cycles measured (e.g., in Saran) exceeds Z50 = 600. The Z50 equals the number of cycles leading to 50% of the original extinction of the colored form (Z 80 equals 80% of the original E). Similar matrix effects have been observed with spiropyrans.57 Typical values are given in Table 6.4. In a poly(vinyl butyral) (TrosifolR) matrix, the number of cycles for DHI 7 —> 9 exceeds Z50 = 50 00.3-7... 

For any application of photochromic molecules, a discussion of the thermal stability of the colored form is in order. We will discuss two types of photochromic molecules (1) those that operate by a photon-heat mode (photochemical forward reaction and a thermal reverse reaction), such as spiropyran derivatives, and (2) those that exhibit a photon-photon mode, such as diarylethene derivatives. 

The investigation of the colored form of spiropyrans by theoretical calculations started at the dawn of studies on photochromic compounds. The pioneering studies of a benzothiazolinic spiropyran were published by Guglielmetti and coworkers, who investigated the different planar forms of the open-form stereoisomers by calculations of the atomic interaction energies,31 the extended Hiickel method,3133 the PPP method,32 and theCNDO/2 method.34 The results of molecular mechanical (MM) calculations on spiropyrans and spirooxazines were compared with the X-ray structures and used to explain H NMR data.35... 

For many organic photochromic compounds (e.g., spiropyrans, anils, and hydrazones) for which the color change is photochemically induced, the bleaching reaction is at least partly thermally controlled. Most of these systems are discussed in other chapters of this monograph or have been discussed elsewhere2 among these, we will only review those systems for which the thermal equilibrium between the colorless and colored forms has been studied. 

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