Spiropyrans, irradiation

Like other spiropyrans, the colored form of spirooxazines generated by UV irradiation, reconverts to the colorless form. However, it is possible to measure the thermal decay rates and activation energies at ambient temperature, since this fading reaction obeys first-order kinetics in solution. The thermal decay rate constant for spiroindolinonaphthooxazine has been found to be 0.02-0.15s 1 in ethanol and 0.1-1.4s 1 in toluene, although this may vary according to the substituent groups.72,77 However, these values are smaller than those of the spironaphthopyran series. 

In the spirothiopyran 45b, the spiro form has two absorptions in the visible region (Vax 490 and 474nm) due to a polyene chromophore from IV-vinyl group to oxygen of the benzopyrylium component.90 The colored form of 45b produced by visible light irradiation shows the Vax at 570 nm. This colored form 45b" was confirmed by characteristic 1H-NMR spectra, as well as that of spiropyran. 

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]. 

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]. 

Takeda et al. [104] found that the steady-state irradiation of A-octadecyl 6,8-dinitro-BlPS resulted in the formation of the closed form. This is in agreement with the femtosecond studies. However, the fade reaction showed three components when monitored at 500 nm in 1,2-dichloroethane. The first component was manifest as a bleach that occurred within the instrument response time of 25 nsec. This was followed by two longer components with 240-nsec and 3.4 psec lifetimes. The spiropyran ring-closed form evolved with two exponential components with lifetimes of 620 nsec and 4 p-sec. The Af-octadecyl group may affect the outcome compared to the A-methylated form. 

The spiropyran derivative incorporated RMs showed good photo-controUed extraction of zwitter-ionic amino acid under UV irradiation and release under visible irradiation Unlike native enzyme, the modified counterpart tended to form reversible oligomeric structures in RMs, which allowed in realizing the role of carbohydrate moieties in the functioning of glycoproteins... 

The photochemical reaction of halo-1,4-naphthoqutnones with 1,1-diphenylallenes affords spiropyran adducts, nevertheless, irradiation of 2,3-dichloro-1,4-naphthoquinone (7) and phenylallcne (8) gives a mixture of cyclobutanes 9 in 74% isolated yield.13... 

Much interest has been devoted to the photochromic behaviour of simple chromenes, especially because of practical applications of spiropyrans, particularly indolinospirans.5 Kolc and Becker30 have been able to demonstrate the o-quinoneallide structure of the colored form 4, by producing it in THF at -75° and trapping by reduction with LiAlHj. It is concluded31,32 that the same intermediates occur when spiropyrans are irradiated, because only the pyran moiety has an... 

Irradiation of spiropyrans of the type 343 establishes a photostationary state between Formulas 343 and 344 (147,148). 

We have also demonstrated self-held ultrafast parallel optical switching based on the same geometry and using photochromic compounds instead of phthalocyanines [81-84], Absorption spectra of spiropyran derivative (SP) dispersed in polystyrene with a weight ratio of 1 10 are shown in Fig. 28 before and after UV irradiation for 5 sec. Strong absorption in the visible region due to photomerocyanine (PM) can be held for a long time and be reverted to that of... 

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 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 ... 

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-... 

Fig. 10 Effect of irradiation and dark-adaptation on CD spectra of poly(L-glutamic acid) containing 85 mol%spiropyran units, in HFP 1) Kept in the dark 2) exposed to sunlight dashed lines intermediate spectra during decay in the dark over 8 h.

Fig. 11 Poly(L-glutamic acid) incorporating 85 mol% spiropyran units in the side chains. Effect of irradiation on CD spectra in various HFP/MeOH solvent mixtures in the presence...

The effect of light on CD spectra at various solvent compositions, for a polymer containing 85 mol% spiropyran units, is shown in Figure 11. When methanol concentration is below 5%, both the dark-adapted and the irradiated samples show the typical CD pattern of disordered polypeptides. In HFP/MeOH = 90/10, the sample kept inthedark is random coil, whereas the sample exposed to light displays the standard CD pattern of the a-helix. The intensity of the bands indicates that under these conditions light causes the full conversion from random coil to 100% a-helix. With increasing methanol con-... 

Fig. IS Poly(L-lys ine) containing 46 mol% spiropyran units (XVI) in HFP/ NEt3. Variation of ellipticity at 222 nm as a function oftriethylamine concentration forthe sample kept in the dark (continuous line) and after irradiation (dashed line).

Photomechanical effects have been also observed in monolayers obtained from poly(L-glutamic acid) modified with carbocyanine1831 and spiropyran dyes.184 In the latter case, irradiation at 254 nm produced changes in the molecular conformation, which in turn caused photomodulation of the surface pressure and surface area of the films. From all these examples, it appears that photoresponsive monolayers are quite fascinating systems, which may eventually come to be regarded as a machine to transform light into mechanical energy . 81 ... 

Many recent papers are ignored or mentioned only briefly because they are discussed elsewhere in this book. The biological applications of spiropyrans incorporated into membranes and their use as specific ion recognition sensors and signal transducers is discussed in Volume 2, Chapter 9. Spiropyran-modified artificial monolayer, bilayer, and multilayer membranes whose physical and chemical properties can be controlled by irradiation are discussed in Chapter 1 in Volume 2. The X-ray structures of numerous spiropyrans, the nature of the molecular packing, and the relationship between structure and photochemical behavior is reviewed in Volume 2, Chapter 7. 

Amino-6-nitro-Fischer s base (36, R = N02) was reduced to the 5,6-diamino compound (36, R = NH2) in 80% yield, which (as its hexachlorostannate salt) was condensed with benzoic acid and with benzil to give, respectively, a 68% yield of 5,7,7-trimethyl-6-methylene-6,7-dihydro-5 7/-pyrrolo[2,3-/]-benzimidazole (37) and a 62% yield of 6,8,8-trimethyl-7-methylene-2,3-diphenyl-7,8-dihydro-6 7/-pyrrolo[2,3-g]quinoxaline (38).42 Spiropyrans from these linearly annellated Fischer s bases were not reported they likely would color upon irradiation with blue light. 

Three spiropyrans, each molecule having a long alkyl chain located in a different relative position (l -hexadecyl, 3-decyl, and 8-dodecanoyloxymethyl), were spread as monolayers on water under irradiation and the changes of surface area and surface pressure with time were measured both when the lamp was turned on and after it was turned off. The results were especially striking observations of a reversible and an irreversible photoconversion of a surface-inactive system into a surface-active one.154... 

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