Isomer closed-ring

Compound Open-ring isomer Closed-ring isomer ... 

Open-ring form isomer Closed-ring form isomer... 

It is possible for long chains of carbons to loop around and form a closed ring structure. If you take the linear isomer of hexane in Figure 6-9... 

Ring-closing metathesis, which has proved to be a popular route to the marine toxins, has found a further application as the key step in the synthesis of the pheromone (-)- and ( )-frontalin <99TL1425>. The precursor in this reaction is a mixture of the syn- and anri-isomers 39. Ring closure in the presence of a ruthenium benzylidene catalyst occurs within minutes at room temperature when only the syn-isomer cyclises to 40. The unreacted anri-isomer can be re-equilibrated for a further cyclisation. 

The conversion from the open- to the closed-ring isomer in the photo-stationary state under irradiation with 405 nm light decreased with increasing solvent polarity. The cyclization quantum yield decreased to 0.097 in THF, 0.029 in ethanol and 0.013 in acetonitrile, respectively. 

Compound 311 underwent a thermally irreversible photochromic reaction in solution (08JCS(CC)335). Upon alternate irradiation with UV and visible light, a hexane solution of 311 reversibly changed its color from colorless to bluish purple due to the isomerization between the open- and closed-ring isomers, 311 and 312. 

The absorption spectrum at 0° has a maximum at 600 nm. Upon rotation of the direction of the incident polarized light by as much as 90°, the absorption intensity decreases. The anisotropy of the absorption spectra reflects the regular orientation of the photogenerated closed-ring isomers and indicates that the photochromic reaction occurred in the single-crystalline phase. The blue color disappeared by irradiation with visible light a > 480 nm). Anthracene-substituted derivatives also showed photochromic properties (01JPC(A)1741). 

The blue color was disappeared by irradiation with visible (X > 500 nm) light. In the dark, however, the blue color remained stable and at room temperature never reverted to the colorless form. In toluene, the colored isomer was found to be stable even at 100 °C. The stable, colored isomer was isolated by HPLC and its molecular structure was analyzed by NMR and X-ray crystallography. Both indicated that the blue colored isomer was the closed-ring form. Therefore, the photochromism of the diarylethene derivative was ascribed to the following photocydization and cycloreversion reactions. 

The thermal stability of 3a above 150 °C was measured in crystalline and melt states.1211 Its half-life at 150 °C was determined to be 3.3 h. The activation energy was determined as 139 kj mol-1 from the temperature-dependence of the thermal cycloreversion rates. This value suggests that the half-life of the closed-ring isomer is 1900 years at 30 °C. The closed-ring isomer is stable enough for practical application. 

On the other hand, photogenerated closed-ring isomers of diarylethenes with pyrrole, indole, or phenyl rings, which have rather high aromatic stabilization energy, are thermally unstable.1221 The photogenerated, blue, closed-ring isomer of l,2-bis(2-cyano-l,5-dimethyl-4-pyrrolyl)perfluorocyclopentene 11a disappeared in 37 s (= t1/2 ) at 25 °C. 

Regarding the closed-ring isomers, the difference in behavior between those diarylethenes with furan, thiophene, or thiazole rings and those with pyrrole, indole, or phenyl rings agrees well with the theoretical prediction that the thermal stability depends on the aromatic stabilization energy of the aryl group.1201... 

Some diarylethene derivatives that possess strongly electron-withdrawing substituents deviate from the general ruleJ5,23 The closed-ring isomers of 12b and 13b, possessing dicyanoethylene substituents, reverted to the open-ring isomers in 3.3 min and 186 min, respectively, at 60 °C. The dithienylethenes 14b, with pyridinium ion substituents, and 15b, with formyl residues, also underwent thermally reversible photochromic reactions. 

The thermal instability of these closed-ring isomers is ascribed to the fact that the photogenerated central carbon-carbon bonds in the closed-ring isomers are weakened by the electron-withdrawing substituents. 

When bulky substituents are introduced at the 2-positions of the benzothiophene rings, as in 16a, the red closed-ring isomers were found to become thermally unstable.1241... 

The red color disappeared in 20 h at 70 °C, while no such instability was observed for the closed-ring isomer 7b, with methyl groups at its 2-positions. It is considered that the bulky substituents at the reactive carbons also weaken the photogenerated central carbon-carbon bond. 

The open-ring isomer was excited with a 355 nm laser pulse (fwhm 22 ps) and the formation of the closed-ring isomer was followed at 560 nm in hexane. A rapid spectral evolution in a few tens of picoseconds was observed, and attributed to the photocycliza-tion reaction. The rise curve was reproduced by taking into account the pulse duration and the time constant of formation (t = 8 ps). Taking the rather long pulse duration into account, it was concluded that the switching time is shorter than 10 ps. 

When the twisting power of the closed-ring isomer is larger than that of the openring isomer, it is expected that UV irradiation should induce the phase change from the nematic to chiral nematic phases.1481 A diarylethene 31a, with two diarylethene units in a chiral cyclohexane, was incorporated into K15 and the phase change concomitant with photoisomerization was measured. 

In the open-ring isomer, free rotation is possible between the ethene moiety and the aryl group. Therefore, the open-ring isomer is non-planar and the it-electrons are localized in the two aryl groups. On the other hand, the closed-ring isomer has a bond-alternative polyene structure and the jt-electrons are delocalized throughout the molecule. These geometric and electronic structural differences result in some differences in their physical properties. For example, the closed-... 

Fig. 9.2 The open-ring isomer 8a and the closed-ring isomer 8b of the radical-substituted diarylethene. Two SOMOs of 8a and HOMO of 8b are also shown.

In solution, 10a showed typical photochromic behavior on irradiation with UV and visible light (Fig. 9.4). Although the radical moiety has a weak absorption in the region from 550 to 700 nm, this did not prevent the photochromic reaction. Almost 100 % photochemical conversions were observed in both the cyclization from the open-ring isomer 10a to the closed-ring isomer 10b and the cycloreversion from 10b to 10a. For the practical use of photochromic devices, high conversion is one of the most important characteristics. 

Although the switching of exchange interaction was detected by a susceptibility measurement of biradical 10, both open- and closed-ring isomers 10a and 10b had nine-line ESR spectra because the exchange interaction between the two radicals was much stronger than the hyperfme coupling constant in both isomers. For one... 

X 1(T6 M). Bottom (dark) line, open-ring irradiation with 313-nm light, isomer 16a top (pale) line, closed-ring iso-... 

---