Spiropyrans ring opening

B. Photochemical Ring Opening 1. Spiropyran Ring Opening... 

Spectral Transparence Starting from 230 nm R R Spiropyran Light-Induced Ring-Opening Reactions Photochromism ... 

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

The naphthopyran ring-opening reactions have not been as well studied as they have for the spiropyrans and spiro-oxazines. Aubard et al. [75] recently reported that in acetonitrile and hexane, irradiation of CHRl resulted in a broad transient spectrum after 0.8 psec, having three maxima at 360-370 nm, 500 nm, and 650 nm. At 1.8 psec, a well-defined band forms at 425 nm. From 10 to 100 psec, there is little further evolution except for a continued growth in the peak at 425 nm of about 15%. There is also a decrease in the overall bandwidth. The mechanism in Scheme 9 has been proposed, where B2 and B, are isomers of the mero-form. Three isobestic points were identified in the transient spectra at different times, suggesting four transient states. Forming between 0.8 and 1.6 psec, the Bi state was assigned as the cis isomer. This had a spectrum similar to that obtained for Tamai s X transient of the spiro-oxazine NOSH, which was obtained at subpicosecond time scales [26]. 

Unsubstimted spiropyrans similarly exhibit rapid ring-opening photochemistry to the spiro-oxazines also via a singlet state. One transient state has been identified also in polar solvents. This was assigned as 5i, but by analogy with the spiro-oxazines, this may be similar to X that was proposed by many workers. Photochemical ring closure appears to be inefficient and we often observe non-... 

Spiropyrans, and spirooxazines, better known for their photochromic behaviour (see sections 1.22 and 1.23), also exhibit thermochromism. The ring opening to produce the highly coloured merocyanine form is induced by heating either the solid or... 

The reverse reaction, the photochemical ring opening of spiropyranes (22b), takes place by absorption in the short-wave uv region of the spectrum and the merocyanine isomer (22a) is obtained. The electron transition of (22a) is in the visible spectral region, whereas (22b) is colorless. As a result, the dye solution can change from colodess to a colored solution (87,88). These photochromic reactions can be used for technical applications (89). 

Following the pioneering work of Hirshberg,[421 the photochromism of spiropyrans has been extensively studied.[43] The photochromic and thermochromic behavior of this class of compounds is due to the interconversion of the closed spiropyran form to the open merocyanine form (Scheme 15). UV irradiation of the closed form 28 results in ring-opening to the zwitterionic form 29, which reverts to the closed form either thermally or on irradiation with visible light. 

The inclusion complexation of spiropyrans in cyclodextrins has also been explored as a means to control photochromic reactions.1591 Distinct differences in complexation of sulfonic acid-modified spiropyrans to various cyclodextrins were observed and the closed spiropyran form bound to (3-cyclodextrin was stable towards photochemical ring-opening. 

Quantum mechanical calculations on the 2//-pyran ring opening and closing agree with the different mechanisms for photocoloration and photodecoloration experimentally observed in spiropyrans. The photobleaching proceeds from the 7 state, and mechanistically is very different from thermal bleaching, which proceeds from the S0 state and has a much lower activation of energy barrier. 

The first example of optical resolution of a photochromic compound was di with spiropyrans SP-1. During the photochemical ring opening and subseqi thermal ring closing reactions, SP-1 preserved chirality, although it had a chi to racemize when it took merocyanine structures [44-46]. 

Perhaps the most promising materials for all-optical write-read-erase switches are systems in which ultraviolet irradiation induces ring opening. One of the most studied systems are spiropyrans in which writing and nondestructive reading are based on successive two-photon and one-photon processes [46, 205 209]. 

Figure 2. Two-photon excitation of the closed spiropyran to the Si or S2 state results in ring opening, and fluorescence from the Si state of the open-ring merocyanine species. The latter is stable and can be accessed for numerous read cycles via two-photon excitation with 1064 nm light. Switching back to the closed-ring spiropyran is achieved with mild heat.

P. Appriou, R. Guglielmetti, and F. Gamier, Study of the photochemical processes in the benzopyranic ring opening reaction of photochromic spiropyrans, 7. Photochem. 8, 145-165 (1978). 

The finding of a AG value of 81.5 kJ mol" for 56 (Table 12) means that its half-life,/0.5, before thermal ring opening at room temperature is of the order of only 10 s. Similarly low barriers were seen for several spirobipyrans in Tables 6 and 8. Therefore, the thermal ring-opening reactions of spiropyrans represent further examples of fast isomerizations. 

P. N. Day, Z. Wang, and R. Pachter, Ab initio study of the ring-opening reactions of pyran, nitrochromene, and spiropyran, J. Phys. Chem, 99, 9730-9738 (1995) and references therein. 

Attempts to detect the main structural changes occurring in the ring-opening/ closing process in spiropyrans, using IR vibrational spectroscopy, were reported by Schiele and Arnold for a series of indolinonitrospiropyrans12 and later by... 

Photochemically induced ring opening is responsible for the photochromism observed in indolinespiropyrans. Factors affecting the photostability of these spiropyrans and their behaviour in organised molecular assemblies have been investigated, and new idolinespiropyrans with 7r-acceptor substituents in position 8 have... 

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