Spiropyran, merocyanine form

The C3—H is labile and can be replaced by deuterium from D2O or from MeOD. [36,55,94,95]. The fact that the C3 proton can undergo facile isotopic exchange in nonacidic conditions is further evidence for the negative charge on C3. Protonation of the spiropyran merocyanine forms appears to occur at C3... 

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... 

A spiropyran compound bearing a pyridinium group and a long alkyl chain behaves as a surfactant. The components shown in Scheme 1 exhibit reverse photochromism in polar solvents. The colored merocyanine form is more stable than the spiropyran form in the dark. Upon photoirradiation at A>510 nm, the polar merocyanine form is converted to the hydrophobic spiropyran form so that the CMC (critical micelle concentration) of the surfactant decreases. Consequently, when the initial concentration is set between the CMC of the two forms, photoirradiation induces a sudden formation of micelles at a certain conversion to the spiropyran form corresponding to the CMC of the mixed micelle of the two forms. 

When the initial concentration of the merocyanine form is lower than the CMC of the spiropyran form, the change in surface tension is gradual all through the progression of photoreaction. The value of Ajjq/Acqq remains constant during photoirradiation. Unfortunately, reversibility of this photochromism is poor and the micelle formation/dissociation cycle deteriorates rapidly. 

Figure 2 FUR spectta of 6-nitto-BIPS spiropyran closed form, 6-nitto-BIPS photoaggregate, and 6,8-dinitro-BIPS merocyanine.

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

Spiropyran merocyanine specffa shift markedly to the blue as the solvent polarity increases [4,25], as shown in Fig. 7a for 6,8-dinitro-BIPS merocyanine. This is generally accepted to imply that they have a zwitterionic character caused by the donation of electron density from the indoline nitrogen to the phenolic C9 oxygen [4,25]. The rational behind this assignment to the zwitterion is based simply on the fact that if the zwitterion is in the highest occupied molecular orbital (HOMO) state, then the corresponding quinoidal resonant form is in the lowest unoccupied molecular orbital (LUMO) state and changing the solvent to a more polar one... 

Clearly, the TC isomer has its spectrum red-shifted compared to the TT isomeric form. Although spectra for individual isomeric forms of spiro-oxazine and spiropyran merocyanines are not available, it has been shown that the different isomers have very different spectra. In fact, Abe and co-workers [89] have shown that the merocyanine of spiro-oxazine can be converted photochemically between two states having different absorption spectra. Even earlier work carried out at low temperature and or with visible-light irradiation suggested that the spiropyran merocyanine isomers also exhibit significant differences between their absorption spectra [6-8]. 

Two permanent merocyanines have been reported for the spiro-oxazines [85]. These were NOSH heteroanellated by imidazo [l,2-a]pyridine and imidazo [l,2-a]pyrimadine. Several tests have been conducted to determine the nature of these species. H -NMR data show that the indoline nitrogen is not highly charged and the crystal structure indicates that the ground state is essentially the quinoidal form. The most stable form was found to be the TTC isomeric form by x-ray analysis. The dipole moment of these permanent spiro-oxazine merocyanines was around 3.84 D, which is much lower than the values reported for spiropyran merocyanines. 

The mechanism in Scheme 15 suggests the rapid formation of an intermediate that is cis about the central (3-methine bond, which leads eventually to the ring-closed form but which can also re-form the state as the equilibrium concentration depletes. Scheme 16 is based on the fact that two possible conformers for the merocyanine have been identified (TTC and TTT) [36] and these may have different ground-state recovery times, but TTC and TTT generate the closed form with equal efficiency. Both of these schemes have merit and both have possible flaws however, what is clear is that there is no triplet-state involvement and that the bleaching reaction generates the spiropyran closed form essentially within 1.5 nsec. 

Nitro-substitution especially at the 6-position of BIPS opens up a triplet pathway for photo-isomerization. This pathway runs in parallel to the singlet manifold. This increases the yield and, in turn, may lead to photo-aggregation that is observed for these compounds. Photochemical ring closure to the spiropy-ran form is more efficient for these 6-nitro-substituted compounds. The photochemistry of 6-nitro-BIPS merocyanine is similar to that of unsubstituted BIPS(s) however, the 6,8-dinitro compound efficiently cyclizes upon excitation to form the spiropyran closed form via a singlet manifold. 

Spiropyrans, and spirooxazines, better known for their photochromic behaviour (see sections 1.22 and 1.23), also exhibit thermochromism. The ring opening to produce the highly coloured merocyanine form is induced by heating either the solid or... 

Photochromism of Spiropyrans. The reversible photochromism of spiro-pyrans has been exhaustively studied by Fischer and his colleagues58 and shown to be a reversible transformation between the spiropyran form A (colorless) and the merocyanine form B (colored). 

To explain their results obtained in the study of photochromism of 1,3,3-trimethylindolino-6-nitro-8-methoxybenzospiropyrane adsorbed on silicic acid, Balny et al.57 have proposed that energy transfer takes place between the singlet states of A (spiropyrane) and B (merocyanine form). 

The thermal equilibrium (Scheme 3) between the open and closed forms of a spiropyran is the basis of a thermographic system by NCR Corp.232 The spiropyran (109) is coated with a metal salt of a fatty add and a binder. On heating, the spiropyran is converted into the open merocyanine form, while the melting of the salt allows formation of a complex (110), preventing return to the spiro form. 

Fedorova, O.A., Koshkin, A.V., Gromov, S.P., Avakyn, V.G., Nazarov, V.B., Brichkin, S.B., Vershinnikova, T.G., Nikolaeva, T.M., Chemih, L.A., Alfimov, M.V. (2002) Crown-containing spironaphthoxazines and spiropyrans. 3. Synthesis and investigation of die open merocyanine form of the crown-containing spirobenzothiazolylnaphthoxazine, Russ. Chem. Bull., 51, 1441-1450. 

Following the pioneering work of Hirshberg,[421 the photochromism of spiropyrans has been extensively studied.[43] The photochromic and thermochromic behavior of this class of compounds is due to the interconversion of the closed spiropyran form to the open merocyanine form (Scheme 15). UV irradiation of the closed form 28 results in ring-opening to the zwitterionic form 29, which reverts to the closed form either thermally or on irradiation with visible light. 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

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). 

Fig. 5 Smart UV-responsive coating on silica nanoparticles with PNIPAM brushes functionalized with FRET donors, 4-(2-acryloyloxyethylamino)-7-nitro-2,l,3-benzoxadiazole (NBDAE), and photoswitchable acceptors, l -(2-methacryloxyethyl)-3, 3 -dimethyl-6-nitro-spiro(2//-l-benzo-pyran-2,2 -indoline) (SPMA). The UV radiation induces the change from colorless spiropyran derivatives in the outer part of the coating (7) to the fluorescent merocyanine form (2). Thus, FRET with the benzoxadiazole moieties in the inner part of the coating is enabled and the fluorescence color changes from green to red. By variation of the temperature and induction of a collapse of the PNIPAM chains (3), the FRET efficiency can be tuned (4). Reprinted, with permission, from [70], Copyright (2009) American Chemical Society...

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. 

In the case of a typical spiropyran, the coloration-bleach cycle may be repeated anywhere from 100 to 1000 times. Typical values for the molar absorptivity of the colored merocyanine form range from 10,000 to 50,000 L/mol/cm. A few spiropy-... 

Consequently, the main class of photochromic compounds that shows change in chirality by photochromic transformation is that based on electrocyclization. Fulgides [11-13], diarylethenes [14-16], and spiropyrans [17,18] are included in this class. It may be somewhat strange to put spiropyrans into this category because the colored merocyanine forms. ure known to take zwitterionic structures, which do not cyclize through the electrocyclization mechanism. However, in order to simplify the classification, we consider spiropyrans to cyclize from the neutral dienone structures as the merocyanine forms. 

Spiropyrans SP-2, reported by Miyashita et al., forming chromium complexes on their indoline phenyl ring, can possibly take two diastereomeric spiro forms >[47]. Because of the favorable interaction of the pyran oxygen atom and the chromium atom, the thermal ring closure occurred diastereomerically to give only one diastereomer from the planar merocyanine form generated by photoirradia-... 

Like azobenzenes (III.A), polypeptides carrying spiropyrans have been studied to control the conformations of polypeptides. In polar solvents such as hexafluoropropanol, the colored merocyanine form is thermally more stable than the spiro form. Therefore it shows reverse (or negative) photochromism irradiation by visible light on the colored species bleaches it, while the spiro form formed... 

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