Merocyanine, formation

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

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. 

The photochromic reaction leading to the blue merocyanine formation results in a narrowing of the reflection bandwidth. 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

The various transitions of triafulvenes to pentafulvenes achieved by addition of electron-rich double bonds is complemented by the reaction of triafulvenes with ynamines and yndiamines299, which gives rise to 3-amino fulvenes 539. This penta-fulvene type deserves some interest for its merocyanine-like inverse polarization of the fulvene system and its formation is reasonably rationalized by (2 + 2) cycloaddition of the electron-rich triple bond to the triafulvene C /C2 bond (probably via the dipolar intermediate 538) ... 

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. 

Changes in the shape of the absorption spectrum correspond very well with micelle formation. The ratio of absorbance at 550 nm to that at 500 nm(both are absorptions of merocyanine) is constant below the CMC whereas the value increases continuously with concentration above CMC. This indicates that the merocyanine is a sensitive probe to detect micelle formation. During the photoirradiation experiment shown in Figure 2, the ratio of absorbance started to increase at the A /Aq value where the surface tension showed a sudden drop. 

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. 

Much discussion about the photo-formation of the merocyanines is based on the possibility of four stable merocyanine isomers that are trans about the 13-bond on the methine bridge. Commonly, these are referred to as TTC, CTT, CTC, and TTT [6-8,28,36] and they are shown in Scheme 4. Note that these are possible or hypothesized structures. Corner et al. even suggest that isomers cis about the central 3-methine bond may have stability and equilibrate with the more planar isomers [14,46-51]. Certainly, the four isomers cis about the cental (3-methine bond could have transient stability. 

Photo-aggregation of the nitro-BIPS compounds continues to fascinate researchers and articles are being produced still regarding this phenomenon [24,38-45,60-62]. It is, of course, noteworthy before we continue with this discussion that spiropyran merocyanines are zwitterions possessing a significant dipole moment and that their stability in a nonpolar solvent will therefore be minimal. Spontaneous precipitation upon formation may be expected as the final outcome of the photo-formation of the 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],... 

Tamai and Masuhara [26] also worked on NOSH, but in 1-butanol. They could examine femtosecond dynamics for the C—O bond breaking and formation of a primary photo-product X, which formed within 1 psec and had a broad absorption with peaks at 450 and 700 nm. The spectrum of X then evolved, forming a broad merocyanine-type spectrum, which itself evolved with time to form the usual merocyanine spectrum in that solvent after less than 400 psec. The spectral broadening was said to be either due to the formation of a vibrationally hot ground state or to an equilibration between isomeric forms because the spectrum that formed at early times was similar to the spectrum usually obtained in cyclohexane. Tamai s spectra are shown in Fig. 3. 

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. 

Recall the Aramaki and Atkins ons result [52] that pumping the A-methoxy-ethyl NIPS closed form with 287-nm light and the merocyanine with 574-nm light in polar solvents both resulted in the formation of what they assigned as... 

It is possible to produce sequential colour changes using bis-spiropyrans. The colourless bis-spiropyran (1.60), in Figure 1.16, when heated in n-propanol to 60 °C changes to a red colour, due to the formation of the mono-merocyanine (1.61), and at 70 °C it becomes blue as the bis-merocyanine (1.62) appears. 

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 2 1 complexes ZnL2 (LH = 67) have been investigated by multinuclear (15N, 13C and 3H) NMR techniques 469 complex formation causes considerable change of the jt-electron structure in the merocyanin part of the ligand. 

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. 

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>. 

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... 

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