Bis-merocyanines

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. 

Fig. 18 Bis(merocyanine)-based gelator and its supramolecular polymerization process...

Lohr, A., Griine, M., 8c Wiirthner, F. (2009). Self-assembly of Bis(merocyanine) Tweezers into discrete bimolecular 7t-stacks. Chemistry - A European Journal, 15, 3691-3705. 

More recently, a spirobenzopyran dimer bridged by a diaza-18-crown-6 moiety through the 8-position (67) was developed by Kimura and co-workers.119 121 Crowned bis(spirobenzopyran) 67 shows a similar coloration selectivity to that of 63. Complexation of multivalent metals, especially Ca2+ and La3, by 67 enhanced the isomerization of the spirobenzopyran moiety to the corresponding merocyanine form due to an effective intramolecular interaction between a crown-complexed cation and the two phenolate anions in the cation complexes of the merocyanine form. 

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.). 

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... 

Figure 38 Photochromism-based 3D optical data storage, (a) The read-write mechanism using two different wavelengths (1064 and 532 nm) proposed by Renzepis etal. [54] (b) Spirobenzopyran absorbs a 1064 nm photon and a 532 nm photon to become merocyanin. (c and d) gives 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene and 2-(1,2-dimethyl-3-indolyl)-3-(2,4,5-trimethyl-3-thienyl)maleic anhydride which are highly stable molecules with potential application in 3D optical data storage. The illustration of the write/read optics is reproduced with permission from ACS.

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