Spiropyran chromophore

FIGURE 3.15 UV-vis absorption spectra of PMMA films containing (A) diarylethene and (B) spiropyran before (I) and after (2,3) linearly polarized UV irradiation.The probe light was also linearly polarized, and spectra were obtained for both parallel, Abs//, and perpendicular Abs., orientations. The insets in (A) are e>q anded views of both the UV and the visible absorptions of the diarylethene chromophore. identical spectra were obtained for both Abs// and Abs before UV irradiation. For both the diarylethene and spiropyran chromophores, inasmuch as the orientational hole burning and orientational redistribution processes occur with Abs > AbS// within the irradiation wavelength band, note the inversion of the sign of the dichroism between the UV and visible bands of the spectra. After reference 29, redrawn by permission. 

Photoinduced mechanical response of a monolayer consisting of poly(methyl methacrylate) with spirobenzopyran pendant groups was examined at an air-water interface [55]. Irradiation with UV light pulses isomerized the spiropyran chromophore and led to a relative surface pressure increase of up to 10%. The pressure decreased progressively in the dark, and a typical decay time was estimated as being approximately 20 min. The decay can be accelerated by use of visible light pulses (Fig. 26). The whole process can be repeated several times. 

For two-photon memories, a number of media types and reading mechanisms have been used (165). Generally, media comprise two photon-absorbing chromophores dissolved within a soHd polymer matrix. Suitable reversible photochromic dyes are, for example, spiropyrans. Although photochromic materials often suffer from photobleaching, as well as from instability leading to self-erasure, new materials and host environments are under development (172). Bacteriorhodopsin (BR) also has been proposed as a two-photon memory material. 

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. 

Chapter 4 is concerned with a technically important group of leuco compounds which like the spiropyrans are not formed by reduction of the parent dye, but by formation of a spiro structure from the dye in such a way that the newly created sp3 center destroys the conjugation, and hence, the color of the chromophore. These are the phthalides (spirolactones) and the position of equilibrium is determined by pH rather than a redox process. Such materials are used mainly as color formers in pressure-sensitive... 

J-C. LeDuc, F. Gamier, and R Guglielmetti, Synthesis and properties of azaheterocyclic spiropyrans containing a chromophoric group of the arylazo type on the benzopyran nucleus, Compt. Rend. Acad. Set., Ser. C, 282, 691-694 (1976). 

Section 3.2 of this chapter recalls the pure photochemical point of view of photoisomerization of azobenzene derivatives. Section 3.3 discusses the theory of photo-orientation by photoisomerization and gives analytical expressions for the measurement of coupled photoisomerization and photo-orientation parameters. Sections 3.4 and 3.5 review observations of photo-orientation in azobenzene and push-pull azobenzene derivatives, respectively. Among other things, these sections address photo-orientation in both cis and trans isomers and discuss the effect of trans<->cis cycling, i.e., the photochemical quantum yields, on photo-orientation. Section 3.6 discusses the effect of the symmetry of photochemical transitions on photo-orientation in spiropyran and diarylethene-type chromophores. Finally, I make some concluding observations in Section 3.7. 

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. 

The structural formula of l,2-dicyano-l,2-bis-(2,4,5-trimethyl-3thienyl)ethene, and 6-nitro-l, 3, 3 -trimethylspiro[2H-l-benzopyrane-2,2 -indoline], referred to here as DE and SP, respectively, and their photochemical isomers are shown in Figure 3.14. The DE and SP chromophores have two photochemical isomers, a stable isomer and a thermally unstable isomer, namely the open-ring and close-ring forms for DE and the spiropyran and photomerocyanine for SP. The stable and thermally unstable isomers are henceforth referred to here as the A and B isomers, respectively- Light irradiation produces photoreaction in both the A—>B and A< B directions, and the thermal reaction proceeds in the A<—B direction. In contrast to the colored photomerocyanine form, which usually fades after several minutes at room temperature, the colored close-ring form of DE is stable for more than three months at Both the A and B isomers of DE and SP can... 

SP-MC form. At temperatures up to 70°C, the color changes from red to blue apparently, the second chromophore in the bis-spiropyran opens during this process, and the equilibrium represented by Scheme 5 is established, where MC-MC represents a molecule with two open pyran rings. 

FIGURE 3.16 Dependence of the absorbance of PMMA films containing (left) diarylethene and (right) spiropyran on the angle, 4, between UV irradiation and probe beam polarizations. The normalized absorbance is defined as (Abs - Abs n)/(Abs - Abs ,, where Abs j and Abs , are the maximum and minimum absorbances, respectively. The H and signs stand for the directions parallel and perpendicular to the UV polarization, respectively. The markers are experimental data points, and the full and dashed lines are cos 4 theoretical fits. The analysis wavelengths are indicated. Note the apparent n/2 angle-shift between the UV and visible orientational distributions of the diarylethene chromophore. After reference 29, redravm by permission. 

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