Merocyanine “colored” form

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 case of photoexcitation by an intense femtosecond laser pulse the irradiated powder becomes more permanendy colored, gaining an absorption spectrum comparable to a usual merocyanine form. This photo-coloration is threshold dependent and does not occur at fluences below 0.5 mJ/cm. Thermally, this colored form bleaches with complex kinetic components that exist for between minutes and hours. 

Observation of the spectral evolution with time was revealing. A species with an absorption maximum around 650 nm formed in times of 10 nsec. This transient was similar to that which formed in 1-butanol within 2 psec after femtosecond excitation [26], but thereafter rapidly evolved to its final colored form. In the crystalline state, however, this transient clearly had a longer lifetime. This transient absorption change was assigned to a nonplanar photo-product transforming to a metastable merocyanine in both the solution study and the crystalline-state study. The main difference between photolysis in the crystal as compared to that in solution is that the time scales for photo-transformation are drastically increased. 

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. 

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. 

Malatesta et al.36 disclosed that spirooxazines react easily in their open merocyanine (MC) forms with free radicals to give deeply colored, reduced, free-radical adducts (FRA) that are devoid of photochromic activity. The radicals attack the C5I=C6 double bond of MC and yield stable, deeply colored, free-radical adducts (compound 29) that can no longer close back to the corresponding spiro form. The adducts absorb in the 510-560-nm region and are characterized by high molar absorptivities. 

First, the first-order fading kinetics are monitored after the colored form has been photochemically induced, leading to kob = (kf + kb) = 9.S 10 2s l. Second, the rate constant kf is measured by taking advantage of the ability of the merocyanine form to complex Cu(II), leading to a new absorption band. 

With compounds exhibiting negative thermochromism (i.e the colored form is more stable than the colorless form), the concentration of the merocyanine form is high enough to allow the relative concentration of each species to be measured by 1H - NMR spectroscopy. Using this technique, Nishimura et al46 have estimated the equilibrium constant for the spiropyran 54 to be 2.83 at 25°C in DMSO-ck as solvent. 

The wavelength of the absorption maximum of the colored form of spiropyrane bound on poly(methyl methacrylate) differs from that of the maximum of a low-molecular weight model in the same solvent. Similarly to Irie et al.,39 we have observed that the absorption maximum of the merocyanine form bound to PMMA in benzene, dioxane, tetra-hydrofurane, ethyl acetate, and chloroform solutions is shifted to shorter wavelengths compared with the maximum of absorption of the low-... 

For thermographic recording materials, thermochromic properties of the spiroindolino- and spirobenzothiazolino-benzopyrans have been utilized. As an example, thermal paper patented by National Cash Register38 can be cited. In this paper, the colored merocyanine form is fixed by reacting with phenols or metallic salts.2... 

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. 

Spiro-oxazine (NOSH) photo-induced ring closure reactions were first described by Bohne et al., who used two-laser two-color excitation in the UV and visible regions [71,72]. In this work, they found that photoexciting the merocyanine in cyclohexane leads to a bleached product which recovers quantitatively to the merocyanine form over 30 xsec. The transient bleach state had an absorbance... 

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). 

For the spironaphthoxazines conjugated with aza-15(18)-crown-5(6)-ether moieties at 6 -position of naphthalene fragment (13a,b) it was found that the addition of Li+ and alkaline earth (Mg2+, Ca2+, Sr2 and Ba2+) metal cations to 13a,b solutions results in a hypsochromic shift of the UV absorption band of the spiro form and a bathochromic shift of the absorption band of the merocyanine form in the visible region [36], In addition, the equilibrium shifts to the merocyanine form, and the lifetime of the photoinduced merocyanine form increases (Scheme 15). The isomerization of crown-containing compound 13a,b to the colored merocyanine form was promoted most strongly by the presence of metal ions, which are expected to be the best recognized by the crown ether ring (Scheme 15). 

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...

Unlike the still-unknown 277-pyran which exists exclusively in the ring-opened form, 277-thiopyran is a well-characterized molecule. Nevertheless, the S-C(2) bond can be cleaved and this is the basis of the photochromic properties observed with spirobenzothiopyrans. Irradiation at 365 nm of the spiro[2/7-l-benzothiopyran-2,2 -indoline] 273 in both the solid state and in solution results in opening of the thiopyran ring and the formation of a colored metastable zwitterionic merocyanine (Equation 21). The open form exhibits solvatochromism, with Amax 588 nm in methanol and 673 nm in acetone. In solution, the thiopyran unit reforms rapidly when irradiation ceases, but continuous irradiation leads to the growth of crystals of the open form <2002JOC533>. 

The time-resolved resonance Raman spectrum of the colored species produced during a 30-ns laser pulse indicated that a short-lived intermediate, almost certainly the cis form, is a precursor to the stable trans form of the merocyanine.163... 

Y. Onai, K. Kasatani, M. Kobayashi, H. Shinohara, and H. Sato, Long-lived colored merocyanine conformers in the aggregates formed on UV irradiation of spiropyran. A Raman spectroscopic study, Chem. Lett., 1990, 1809-1812. 

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