Isomerization spiropyran-merocyanine

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]. 

Clearly, the TC isomer has its spectrum red-shifted compared to the TT isomeric form. Although spectra for individual isomeric forms of spiro-oxazine and spiropyran merocyanines are not available, it has been shown that the different isomers have very different spectra. In fact, Abe and co-workers [89] have shown that the merocyanine of spiro-oxazine can be converted photochemically between two states having different absorption spectra. Even earlier work carried out at low temperature and or with visible-light irradiation suggested that the spiropyran merocyanine isomers also exhibit significant differences between their absorption spectra [6-8]. 

Two permanent merocyanines have been reported for the spiro-oxazines [85]. These were NOSH heteroanellated by imidazo [l,2-a]pyridine and imidazo [l,2-a]pyrimadine. Several tests have been conducted to determine the nature of these species. H -NMR data show that the indoline nitrogen is not highly charged and the crystal structure indicates that the ground state is essentially the quinoidal form. The most stable form was found to be the TTC isomeric form by x-ray analysis. The dipole moment of these permanent spiro-oxazine merocyanines was around 3.84 D, which is much lower than the values reported for spiropyran merocyanines. 

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 . 

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... 

Cottone G, Noto R, La Manna G (2004) Theoretical study of spiropyran-merocyanine thermal isomerization. Chem Phys Lett 388 218-222... 

Poly(methacrylic acid) with spirobenzopyran pendant groups is soluble in polar solvents, such as methanol or water, when the content of spirobenzopyran is less than 20 mol%. A methanol solution of the polymer has a weak red color under thermal equilibrium conditions, which indicates the presence of merocyanine form in equilibrium with spiropyran form. Visible irradiation completely bleached the absorption (merocyanine - spiropyran) after the light was removed, the absorption gradually reappeared in the dark (spiropyran merocyanine). Ultraviolet irradiation caused enhancement of the absorption near 530 nm (merocyanine merocyanine) the intense absorption again returned to the thermal equilibrium intensity after removal of the light (merocyanine - spiropyran). Together with these isomerizations. 

Nitro-substitution especially at the 6-position of BIPS opens up a triplet pathway for photo-isomerization. This pathway runs in parallel to the singlet manifold. This increases the yield and, in turn, may lead to photo-aggregation that is observed for these compounds. Photochemical ring closure to the spiropy-ran form is more efficient for these 6-nitro-substituted compounds. The photochemistry of 6-nitro-BIPS merocyanine is similar to that of unsubstituted BIPS(s) however, the 6,8-dinitro compound efficiently cyclizes upon excitation to form the spiropyran closed form via a singlet manifold. 

Photochemical Generation of an Interfacial Shock Wave. Both the an-thocyanidine and the thioindigo monolayers showed a decrease in surface pressure at constant area during the photoisomerization reaction. A different behavior is observed with mixed monolayers of the surface active spiropyran SP and octadecanol (OD), molar ratio SP 0D = 1 5, on illumination with UV radiation. The isomerization of the spiropyran to the merocyanine MC causes an increase in surface pressure at constant area (5, 14). This is shown in Figure 4, where the sudden rise in surface pressure it upon repeated 0.5 s exposures (as indicated by the arrows) can be seen to occur in a wide surface pressure range (15). The kinetics of the relaxation process following the surface pressure increase depends on the surface pressure. 

The fast isomerization of the spiropyran to the merocyanine provides a possibility of generating an interfacial shock wave. The methods used so far in studying the transmission of waves in mono-layers and the adjacent bulk phases require mechanical (16) or electrocapillary (17) excitation of the interface which involves the displacement of the aqueous bulk phase. In addition, the range of frequencies accessible to the investigation of interfacial waves by the conventional techniques is very limited. The fast photochemical generation of an interfacial shock wave is strictly occurring in the monolayer and provides a larger spectrum of frequencies which can be fully explored only after the development of appropriate detection methods. 

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

The photochromism of spirobenzopyrans is a well-documented phenomenon that arises from the photoinduced reversible isomerization between spiropyran and merocyanine forms . In spirobenzopyrans carrying a crown ether moiety (e.g., Ill), this interconversion process is affected by metal ion complexation. A strong interaction of the crown ether unit with a metal ion caused the thermal isomerization of the spirobenzopyran residue to the corresponding merocyanine form with simultaneous suppression of the UV-induced isomerization process (negative photochromism) (Scheme 3). Conversely, a weak metal ion interaction induced a positive photochromism <2001JOC1533, 2002EJ0655>. 

The photoreaction of spiropyrans 4 comprises a bond cleavage resulting in a zwitterionic species, the merocyanine, and subsequent cis to trans isomerization (see Figure 6.4). There are several species in equilibrium, so the chemistry and photochemistry of the spiropyrans are rather complex. The photoreaction of spiropyrans is stericaliy quite demanding and has been reported to be restricted in LBK films. ... 

The immobilization of photoisomerizable host molecules onto an electrode surface can be used for the construction of novel ion-selective electrodes. A photoisomerizable calix[4]arene derivative was incorporated into a polymeric membrane on an electrode surface and the two different isomeric states of the host molecule provided responses selective to Li" or Na" ions depending on the state.Another photoisomerizable host molecule has been constructed from spiropyran and crown-ether subunits. Different binding affinities for Li were found depending on the isomeric state of the photoisomerizable component (26). The researchers suggested that this effect was caused by the coordination of the Li ion to the 0 of the zwitterionic merocyanine-form (Figure 7.29). The photocontrolled host molecules were immobilized in a polymeric film onto an electrode surface, resulting in a photochemically switchable ion-selective electrode. 

Spiropyrans are an important class of materials that show interesting properties such as photochromism and acidichromism. Similar to azo compounds, photoirradiation or the addition of acidic molecules to spiropyrans lead to drastic changes in physical and chemical properties due to spiro-merocyanine isomerization. Hexagonal columnar LC complexes of a spiropyran derivative with 4-methylbenzenesulfonic acid were formed due to the acid-induced spiro-merocyanine isomerization [100], The columnar LC mixture aligned uniaxially on a substrate can serve as one-dimensional ion conductors. When poly(styrenesulfonate) was employed as a complexing agent for spiropyran, no mesophase was observed. 

Fig. 3A-C. Some examples of photoisomerizable molecules A trans-cis Isomerization around the azobenzene double bond. B Isomerization between spiropyran and merocyanine states. C The reversible electrocyclic ring closure of diarylethenes...

In another example, a mixed monolayer composed of a photoisomerizable component and an electrochemical catalyst was applied to switch the electrocatalytic properties of a modified electrode between ON - and OFF -states. A gold electrode surface functionalized with a spiropyran-monolayer and pyrroloquinoline quinone (PQQ) moieties incorporated into the mono-layer was applied to control the electrocatalytic oxidation of NADH by light [92]. The positively charged merocyanine-state interface resulted in the repulsion of Ca2+ cations (promoters for the NADH oxidation by the PQQ), thus resulting in the inhibition of the electrocatalytic process. In the nitrospiropyran-state the monolayer does not prevent association of the PQQ-catalyst and Ca2+-promoter, so provides efficient electrocatalytic oxidation of NADH. Similar results have been achieved by a combination of the photo- and thermal effects resulting in the isomerization of the spiropyran-monolayer with the incorporated PQQ-catalyst [93], Other photoisomerizable materials such as an azobenzenealkanethiol derivative mixed with a ferrocene-redox component have also been used to control the electrocatalyzed electron transfer process between a command interface and a dissolved redox probe [94]. 

Many examples of photochromic compounds involve charge separation, for instance, the classical isomerization of a spiropyran in a highly colored merocyanine dye. 

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