Merocyanine form

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... 

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... 

Photoinduced, spontaneous aggregation processes have been shown to occur when indolinobenzopyrans are irradiated in aliphatic solvents. The aggregates which are globular in appearance, consist of submicron cores of crystalline materials with an amorphous exterior and are termed "quasicrystals" (L-3). Spectroscopic studies by Krongauz and coworkers (1) indicate that the composition of the cores are AnB (n=2,3) and the amorphous exteriors AB. The most stable quasicrystals have been derived from 1-(/ -metha-crolyloxyethyl)-3,3 dimethyl-61-nitrospiro- (indoline-2,2 —[2H-1 ] benzopyran (SP-A) and its associated merocyanine form (SP-B). 

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. 

Thermal ring opening to form the merocyanine form is less dependent upon the type of substituent keeping a AG of near to 101 kJmol for BIPS, 6-nitro-BIPS and 6,8-dinitro-BIPS. [37] The ring opening reaction s rate-determining step is probably a rotation cis to trans about the central p-methine bond and this is also consistent with the polar CCC or CCT transition state which have a rather double p-methine bond. 

Another explanation for their resonance Raman results could be a change in the zwitterionic nature of the merocyanine isomers in the different solvents which may result in changes in the Raman transition probabilities, or the spectral changes could be due to solvent shifts of the absorption spectrum, resulting in a change in the relative contribution of the different vibrational modes to each resonance Raman spectrum. We note that in the same article, the authors report the transient absorption spectra of the merocyanine forms, which clearly show that the BIPS spectrum in cyclohexane has more discrete vibrational modes than are observed in the polar solvents, which show more spectral broadening. Al-... 

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],... 

Based on fluorescence lifetime measurements, the merocyanine form appeared to be a mixture of two or three isomers. The fluorescence had two or three component decays. Their CARS investigation backed up this conclusion of isomeric variance. 

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 thermal reverse reaction s value of AG increased from 77 to 92 to 111 kJ mol respectively for DIPS, 6-nitro-BIPS, and 6,8-dinitro-BIPS probably due to stabilization of the merocyanine form by delocalization of the negative charge on the C9 oxygen [37]. 

Monti et al. [105] also used fluorescence lifetimes to monitor the merocyanine forms of NOSH. In ethanol, they identified two components to the decay having lifetimes of 15 and 700 psec. The longer-lifetime decay is a very minor component. In the case of NOSH in ethanol, 85% of the decay was attributed to the fastest component with a lifetime of 20 psec, but to fit the decay, it was necessary to use a further two components. This is in agreement with Wilkinson et al.[64], who suggested three components to the merocyanine formation from the NOSH closed form based on picosecond time-resolved resonance Raman which they attributed to equilibration of three merocyanines trans about the (3-bond. Monti et al. further found that in acetonitrile, NOSH had one component decay with a lifetime of 20 psec. Clearly, solvent and substiments are important factors. 

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... 

Overall, the quantum yield of ring closure is reported to be low when measured in toluene [100] and this is in accordance with the transient studies. Further, in the transient study by Bohne et al. [71,72], the dye laser used to bleach the merocyanine form was rather powerful at 100-300 mJ/pulse. Even using this strong laser pulse on a merocyanine solution with an absorbance of 0.25, they only permanently bleached the mero-form absorbance by one-third in acetonitrile and one can, therefore, imagine that the bleaching yield is also rather low in acetonitrile. 

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. 

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... 

The formation of the merocyanine form 119 can be induced by addition of heavy metal cations (Pb, La, Eu, Tb ) to a solution of a spirooxazine 118 containing a crown ether group in the B-ring (Equation 1). The chelation occurs first to the crown ether and then to the negatively charged oxygen. In contrast, 118 does not react upon addition of alkaline earth metal cations (Mg, Ca, Ba ) <2005JP0504>. 

The merocyanine form of spirooxazines can react with free radicals, which is important as it causes degradation of the photochromic materials <1995JOC5446>. 

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. 

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). 

The UV-induced isomerisation of 13a,b into the merocyanine form causes a decrease of the cation binding ability (Scheme 16). 

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