Spiropyran-merocyanine system

T. Bercovici, and E. Fischer Cations of the Spiropyran - Merocyanine System and their Photochromie Properties. Lecture at the Symposium on Photochemistry, Rochesters, 1963,... 

Another typical photoresponsive material for preparation of switchable surfaces is the spiropyran-merocyanine system. The spiropyran isomerizes to zwitterionic merocyanine conformation by UV exposure, and the reverse reaction can be triggered by irradiation with visible light as well as azobenzene. The changes in hydrophilic/hydrophobic properties through the isomerization of spiropyran groups also enable the control of cell adhesion/ detachment. Edahiro et al. reported photoresponsive cell culture substrates grafted... 

Fig. 4 Chemical structures of the 11-cis retinal (a), p-hydrojdcynnamoyl anion (b), azobenzene (c), and spiropyran-merocyanine systems (d). The bond thatisomerizes after absorption of light in the first three molecules and the photo-induced reaction in the latter system are shown.

Levitus, M., TaUiavini, M., Negri, R.M., Atvars, T.D.Z., and Aramendia, P.F., Novel kinetic model in amorphous polymers. Spiropyran-merocyanine system revisited, /. Phys. Chem. B., 101, 7680,1997. 

In contrast to the previous discussion, some photonic protein switches are reversible. Reminiscent of several retinylidene-based rhodopsins, such as Sensory Rhodopsin most of these systems are based on reversible photoisomerization. In this case, however, the chromophore is based on the azobenzene, cinnamate, or spiropyrane-merocyanine system. The photochemical reactions for these three classes of photonic switches are illustrated in Figure 133.3. 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

The Zv - complex of the merocyanine system (57) releases the zinc when it is irradiated with visible light and this results in the formation of the colourless closed spiropyranindoline (58, R = H). When the irradiation is stopped the Zn " complex reforms but this does not happen with the nitro derivative (58, R = NO2) in which it is thought that the nitro group stabilises the phenoxide ion in the open form. Other workers have also studied the complexation of spiropyran based merocyanines with transition and rare earth metal ions. An investigation of the influence of Lewis acids (hexa-fluoropropanol, trifluoroethanol and 2-fluoroethanol) on the stability of the coloured form of spiropyran and spirooxazines has been reported. Protonation of the open system produces a form that is photochemically inert and the behaviour of these acids is markedly different from that of acetic acid with such systems. 

Scheme 7.1 Control of threading-dethreading jaecesses in pseudorotaxane [C AH] by means of light-induced proton exchange with a spiropyran-merocyanine photochromic system...

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. 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

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.). 

Interest continues in the photochromic behaviour of the spiropyran and related systems both in the patent and the basic research literature. Nanosecond laser kinetic spectroscopy has been used to investigate the intermediates involved in the interconversions between the colourless spiropyrans (35) and (36) and the coloured merocyanines present in solution at room temperature as the -isomers (37) and (38). On excitation (530 nm) of nitro derivative (37) in acetone, two transients were observed, the triplet excited state (lifetime 6ps) and the Z-isomer (lifetime 0.3ms). The same transients were observed on excitation (353 nm) of the spiropyran form (35). Naphthalene or benzophenone sensitisation of the photo-... 

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