Bis-spiropyrans

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. 

AminoFischer s base was condensed with the diacid chlorides of glutaric, pimelic, and azelaic acids to give in 70-92% yields the bis-amides, which upon reaction with 5-nitrosalicylaldehyde gave 70-88% of the bis-spiropyrans (72, n = 3,5,7). The alternative route, reaction of the acid chlorides with 5 -amino-6-nitroBIPS, is impractical because of the difficulty of obtaining this BIPS. The bis-BIPS exhibit strong photochromism. The open forms precipitate in nonpolar... 

Although the thermochromic properties ofmonospiropyrans (with one pyran ring) have been described extensively, bis-spiropyrans with various structures have also been studied. For bis-spiropyrans in which the merocyanine fragments are... 

SP-MC form. At temperatures up to 70°C, the color changes from red to blue apparently, the second chromophore in the bis-spiropyran opens during this process, and the equilibrium represented by Scheme 5 is established, where MC-MC represents a molecule with two open pyran rings. 

The conformation of 17-cp/-deoxysalinomycin (a close relative of the antibiotic) has a definite form in solution but it cannot exist as a head-to-tail hydrogen-bonded structure. The relevance of these deductions to its biological role has been discussed. One of the constituents of the secretion of the bee Andrena wilkella is the bis-spiropyran (7), and this has been synthesized from ethyl (+)-(5 )-3-hydroxybutenoate, obtained from the reduction of ethyl acetoacetate by yeast. ... 

The naphthopyran ring-opening reactions have not been as well studied as they have for the spiropyrans and spiro-oxazines. Aubard et al. [75] recently reported that in acetonitrile and hexane, irradiation of CHRl resulted in a broad transient spectrum after 0.8 psec, having three maxima at 360-370 nm, 500 nm, and 650 nm. At 1.8 psec, a well-defined band forms at 425 nm. From 10 to 100 psec, there is little further evolution except for a continued growth in the peak at 425 nm of about 15%. There is also a decrease in the overall bandwidth. The mechanism in Scheme 9 has been proposed, where B2 and B, are isomers of the mero-form. Three isobestic points were identified in the transient spectra at different times, suggesting four transient states. Forming between 0.8 and 1.6 psec, the Bi state was assigned as the cis isomer. This had a spectrum similar to that obtained for Tamai s X transient of the spiro-oxazine NOSH, which was obtained at subpicosecond time scales [26]. 

The structural formula of l,2-dicyano-l,2-bis-(2,4,5-trimethyl-3thienyl)ethene, and 6-nitro-l, 3, 3 -trimethylspiro[2H-l-benzopyrane-2,2 -indoline], referred to here as DE and SP, respectively, and their photochemical isomers are shown in Figure 3.14. The DE and SP chromophores have two photochemical isomers, a stable isomer and a thermally unstable isomer, namely the open-ring and close-ring forms for DE and the spiropyran and photomerocyanine for SP. The stable and thermally unstable isomers are henceforth referred to here as the A and B isomers, respectively- Light irradiation produces photoreaction in both the A—>B and A< B directions, and the thermal reaction proceeds in the A<—B direction. In contrast to the colored photomerocyanine form, which usually fades after several minutes at room temperature, the colored close-ring form of DE is stable for more than three months at Both the A and B isomers of DE and SP can... 

There continues to be a high level of interest shown in photochromic systems. The optically active l,T-bi-2-naphthol gives an optically active intramolecular addition product (37) on irradiation, and on prolonged exposure, cycloreversion occurs which gives evidence for a photoequilibrium between the asymmetric molecules (Cavazza et al). A number of reports within the year describe a variety of aspects of the well-known thermally-reversible spiroindoline-oxazine to photo-merocyanine conversion. For example, the process is sensitised by triplet cam-phorquinone (Favaro et al.), the influence of complexation on indoline and phenanthroline spiropyrans with transition and rare earth metal ions has been described (Atabekyan et al.), and the photochromism of other derivatives in water using vesicles and y-cyclodextrin is reported to be faster than in methanol with the process most favoured in the vesicles (Ishiwatari et al.). 

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