Donor photoisomerization

In calcium chelators Indo-1 (17) and Fura-3 (18b) (Figure 2.9),(18) the fluoropho-res have donor-acceptor stilbene-like structures rigidified so as to avoid photoisomerization. Based on the same principle, Fura-2 (18a)a8) is one of the most popular calcium indicator for microscopy of individual cells because, in contrast to Quin-2 (see Section 2.2.5.), the excitation spectrum is blue shifted on cation binding, thus allowing intensity-ratio measurements. On the other hand, there is almost no shift of the emission spectrum, which can be interpreted along the same line as DCM-crown (see earlier in this section). 

Scheme 10 Stereoselective photoisomerization of donor—acceptor-substituted molecular switch 17.

A major advantage is the potential to lock (and protect) written information in the photobistable material. A number of chemical gated systems involving mutual regulation of the photochromic event and, for instance, fluorescence, ion binding, or electrochemical properties have been reported.1501 Scheme 19 illustrates a chiral gated response system based on donor-acceptor substituted alkene 17.[511 The photochemical isomerization process of both the M-ds and the P-trans form was effectively blocked by the addition of trifluoroacetic acid. Protonation of the dimethyl-amine donor unit of M-rfs-17a and P-trons-17b resulted in an ineffective acceptor-acceptor (nitro and ammonium) substituted thioxanthene lower half. Since the stereoselective photoisomerization of 17 relies on the presence of both a donor and acceptor unit, photochemical switching could be restored by deprotonation by the addition of triethylamine. 

The presence of the electron-donor substituents in the migration group of 11-aryloxy derivatives of 6-amino-5,12-naphthacenequinone was favorable for photochromic transformations.52 Aryloxynaphthacenequinones with electron-acceptor substituents in the phenoxy group were photoisomerized less efficiently. 

Skibsted and co-workers have recently presented several studies which rigorously test these stereochemical models. Complexes of the form cis- and trans-[Rh(NN)2XY]"+, (NN = en or tn (1,3-diaminopropane) X = H20, OH Y = H20, OH), plus cis and rra s-[Rh(tn)2(H20)Z]2+ (Z = Cl, Br), display similar photochemistries (a) cis to trans isomerization dominates for the diaqua and aquachloro ions (in which the H20 or halide, poorer a donors than en, occupy the axial site in the SP (square planar) intermediate) (b) trans to cis photoisomerization dominates for the hydroxoaqua species, as the OH ion is a better a donor than the en nitrogens, and hence occupies an equatorial site and (c) no photochemistry is observed for the dihydroxo ions.626 667,724 These results are summarized in Scheme 24 and Table 50. 

Photolysis of [Rh(tfacac)3] (tfacac is the unsymmetrically substituted 1,1,1-trifluoromethyl-acac) reveals the existence of two photoinduced reaction paths the relative efficiency of the two paths is dramatically solvent dependent.1140 In cyclohexane, mer- cis isomerization is the only observed photoreaction, but if ethanol or 2-propanol is added to the solvent, the photoisomerization efficiency decreases, and photodecomposition occurs. The nature of the photodecomposition products is not specified, but the enhanced photoreactivity in the presence of tri(n-butyl)stannane, a hydrogen atom donor, and flash and continuous photolysis studies in mixed-solvent systems strongly implicate hydrogen atom abstraction from the solvent as a key step in the photodecomposition of wer-[Rh(tfacac)3] and suggests that the photo reactive states have considerable radical character .1140 Analysis of quantum efficiencies implies that at least two distinct photoproduced excited states must be involved. 

One concludes from these facts that pseudo-stilbenes are not suitable for persistent switching of the molecular form. Any information based on E-Z isomerization is quickly lost. If, however, fast interconversion of E- and Z-forms is the aim, as it is in the alignment of the higher-order polarizability tensor of donor/acceptor azobenzenes, then thermal isomerization supports the photoisomerization process. 

This chapter is organized as follows. Section 4.2 addresses the study of photoisomerization and photoinduced orientation of azobenzene molecules at the molecular level in SAMs of azo-silane molecules. Section 4.3 discusses photoinduced effects in supramolecular assemblies, i.e., LBK multilayer structures containing azobenzene molecules, and compares the photoinduced movement of azobenzenes in these structures to that observed in spin-cast films. Section 4.4 focuses on the isomerization and sub-Tg photoinduced orientation in a series of very high Tg (up to 350°C) nonlinear optical polyimide and thermoplastic donor-embedded polyurethane polymers containing azo dye, especially focusing on polymer structure-Tg-photoinduced molecular movement relationships. Section 4.5 describes pressure effects on photoisomerization and photo-orientation in films of a PMMA polymer containing azo dye. Finally, we make some concluding remarks in Section 4.6. 

Similar PID results have been reported in very high Tg (up to SSO C) azo-polyimides. It has been shown previously that irradiation of the donor-embedded polyimide derivatives with polarized light alone at room temperature induces a quasi-permanent nonpolar orientation, which can be thermally erased only by heating the polymer above Tg. While, the lifetimes of the polar order generated by thermal poling of the donor-embedded polyimides were found to be on the order of tens of years to centuries at room temperature, photoisomerization can efficiently depole these polymer films in a matter of minutes at room temperature. Indeed, Figure 8.6 shows the effect of p-polarized irradiation on the SH signal of PI-2, which had been previously thermally... 

Another approach to the organization of integrated optoelectronic switches is schematically detailed in Fig. 23, and involves the organization of a photoisomerizable command interface on the solid support [86]. The command surface controls the interfacial electron transfer to a solution-state redox species. In one photoisomeric state, electron transfer to a redox probe solubilized in the electrolyte solution is prohibited (e.g. by repulsive interactions), whereas in the complementary state of the monolayer the interfacial electron transfer is allowed (e.g. because of associative interactions). Various interactions, such as electrostatic interactions, host-guest or donor-acceptor interactions, contribute to the selective contacting of the redox probe to one state of the photoisomerizable monolayer. 

The formation of a complex between the eosin-functionalized Au/quartz crystal and 53a results in a mass increase on the transducer, leading to a decrease in the crystal frequency. Photoisomerization of the electron acceptor to the 53b-state, Fig. 28B, yields an acceptor of lower affinity for the eosin interface. Thus, dissociation of the n donor-acceptor complex occurs, causing a decrease in the mass on the crystal (i.e. an increase of the crystal s resonant frequency). Figure 28C shows the frequency changes of the eosin monolayer-functionalized Au/quartz crystal upon its interaction with 53a and 53b. While the association of 53a is accompanied by a frequency decrease of Af — —25 Hz, the binding of 53b to the modified surface results in a frequency change of... 

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