Isomerization spiropyran

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. 

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. 

Photoinduced unimolecular reactions often have kinetics of the order of ps. One example of isomerization in ps times is shown in Figure 8.9. This is the photochromic reaction of a spiropyran. The photoinduced process takes place... 

Figure 8.9 (a) Kinetics of absorbance changes of a spiropyran in various alcohols. Horizontal axis, time in ps vertical axis, absorbance, (b) Structures of a spiropyran and its photochemical intermediate, (c) Potential energy diagram of the isomerization reaction... 

There are other examples of such fast bond dissociations in the liquid phase, not necessarily hydrogen bonds. Thus the first step in the photo-chromic isomerization of spiropyrans (the ps events are shown in section 8.1.) is complete within 100 fs, that is within a single vibration of the bond. 

A number of photochromic systems have been extensively investigated that undergo cis-trans isomerization (indigos, azo compounds) cleavage (spiropyrans), electrocyclic processes (fulgides, 1,2-diarylethenes) [8.229, 8.244, 8.245], For instance, cis-trans isomerization of a thio-indigo derivative allows the reading of pyrene excimer or monomer fluorescence [8.246]. The 1,2-dithienylethene system presents particularly attractive interconversion properties by photoreversible cyc-litation [8.245],... 

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 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 . 

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 structures of spiropyran-modified poly(L-glutamate)s are strongly affected by light or dark conditions, as demonstrated by the CD spectra in Figure 10. Before irradiation, the colored solutions show the CD spectrum of a random coil conformation. After exposure to sunlight, the colorless solutions display the typical CD pattern of the a-helix, thus indicating that the isomerization of the side chains causes a transition from coil to helix in the polypeptide chains. The photoinduced conforma-... 

Trimethylindolenine and its 5-methyl-, 5-chloro- and 7-chloro derivatives react with phenyl isocyanate and various acyl isocyanates and isothiocyanates to give 75-97% of the corresponding l-carbamoyl-2-methyleneindolines (52), which isomerize to the 2-(carbamoylmethylene) indolines (53) (41-85% yield).52 Whether these Fischer s base derivatives after further N-alkylation will condense with salicylaldehydes to form spiropyrans is not known. 

Not all heterocyclic methylene bases give spiropyrans (at least under the usual conditions). l,3,4,5,5-Pentamethyl-2-methylene-2,5-dihydropyrrole (69)77 did not condense with 5-nitrosalicylaldehyde to give any photochromic material.7 However, l-methyl-2-methylene-3,3,4,5-tetraphenyl-2,3-dihydropyrrole (70), prepared by reaction of methyllithium with 1 -methyl-3,3,4,5-tetraphenylcrotonolactam, did condense with 5-nitrosalicylaldehyde to give a product exhibiting reverse photo-chromism the unexposed form was red and the exposed form yellow. This behavior probably represents a trans to cis isomerization of the open form 7... 

This type of photocyclization is also found in the unimolecular open form/ closed form isomerism of photochromic systems [70] as, for example, spiropyrans and spirodihydroindolazines, fulgides. The chirality in such photochromic reactions will be covered by Chap. 9 of this book an example is 47 in Scheme 9. 

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>. 

Equation 3.22. This type of experiment will be discussed eventually for spiropyran and diarylethene chromophores in films of PMMA. Next, I compare reorientation observations after cis—>trans thermal isomerization of azobenzene to the theoretical developments in Section 3.2.3.2. 

FIGURE 3.14 Chemical structures and isomerization of (top) diarylethene and (bottom) spiropyran isomers. 

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