Isomerization photoinduced

Next, we consider Model III, which describes an ultrafast photoinduced isomerization process. Figure 26 shows quantum-mechanical results as well as results of the ZPE-corrected mapping approach for three different observables ... 

In solution the photochromism of coordination compounds can result from photoinduced isomerization (e.g. metal dithizonates186-188), substitution (e.g. iron(II) phthalocyanine in the presence of benzyl isocyanide189) and oxidation—reduction (e.g. tris(diethyldithiocarbamato)-nickel(IV) bromide190). 

Aromatic Nitro Compounds. The photoinduced isomerization of crystalline 2(2, 4 -dinitrobenzyl) pyridine was first reported by Tchit-chibabine et al. (29) who proposed the following tautomeric equilibrium ... 

Like proton transfer, photoisomerization is a fundamentally important photochemical process. The two most important forms of photoisomerization are valence isomerization and stereoisomerization. The latter is probably the most common photoinduced isomerization in supramolecular chemistry. It may occur in systems in which the photoactive component has unsaturated bonds which can be excited, and this effect may be exploited for optical switching applications. A number of interfacial supramolecular complexes capable of undergoing cis-trans photoisomerization have been studied from this perspective - some examples are outlined in Chapter 5. 

Figure 2.20 Potential energy diagram illustrating the room-temperature photoinduced isomerization of stilbene in solution wavy lines represent non-radiative transitions, curved arrows isomerizations and straight lines radiative transitions...

Since then, several other examples of this photoinduced isomerization (though not always of synthetic utility) have been reported, sometimes as a typical photochemical reactivity of the isoxazole ring [2, 3]. Representative examples include (i) the synthesis of 2-trifluoromethyloxazole 77 from a preparative-scale irradiation at 254 nm in acetonitrile of the easily accessible 3-trifluoromethylisoxazole 76 [51] and... 

These results have been explained through a photoinduced isomerization of the (Z,Z)-isomers in the presence of both tetraphenylporphine and... 

The complexation/decomplexation of a linear guest by a macrocyclic host can be exploited [ref. 7] (Fig. 10.2) to switch reversibly between two states. When the two components are dissociated, the system is in its State 0. When they are associated in the form of a [2]pseudorotaxane, the system is in its State 1. Switching between these two states can be achieved [ref. 7] by protonation/ deprotonation, by oxidation/reduction, or by photoinduced isomerization of the guest. Alternatively, the addition/removal of a sequestering agent able to... 

Figure 9-10. Examples of photoinduced reactions in drug molecules. (I) Photoinduced isomerization (Ib), cyclization and enol-keto isomerization (Ic) of stilboestrol (la). (II) Photoinduced reactions of ketoprofen (Ila) and its degradation product (Ilb) decarboxylation (Ilb), reduction (lie), and dimerization (IIc) products of the drug. (Ill) A -dealkylation of methotrexate followed by oxidation. (IV) Dehalogenation (IVb) and photohydrolysis (IVc) of frusemide (IVa) [44].

Isomerization of safrole and eugenol is reported to occur when these alkenes are irradiated in the presence of iron carbonyls. Co-ordinatively unsaturated iron carbonyls are first formed and subsequently give iron carbonyl-alkene complexes whose alkene ligands are then isomerized. A further reported photoinduced isomerization involves Fe(fnb-Ti -cot)(CO)3 (cot = cyclo-octatetraene), which on irradiation in a CH4 matrix at 12 K is converted into Fe(cfiair-Ti -cot)(CO)3 via Fe(T -cot)(CO)3 as intermediate. The results of these studies are related to the fluxional behaviour of Fe(i7 -cot)(CO)3 in solution. ... 

UV irradiation of the MgO catalyst in the presence of trans-l-C Hs at 273 K led to a marked enhancement in the formation of ds-l-C Un as well as in the formation of I-C4H8. The activity of the MgO catalyst for such photoinduced reactions depends strongly on the evacuation temperature of the MgO. When MgO was degassed at higher temperatures, the yields of the photoinduced isomerization reactions increased linearly with the UV irradiation time, indicating that the reactions proceeded catalytically under UV irradiation at 273 K 96-98). 

To summarize, there are three types of photoinduced motions at the molecular level, at the nanometer, or domain, level, and at the micrometer (macroscopic) level. All are the result of photoinduced isomerization of the azobenzene groups. One interesting direction of our research was to try to exploit these phenomena for their photonic applications. We have demonstrated, at least as proof of principle, a few possible photonic functions for the new materials we synthesized. Some of these are summarized in publications they will also be reviewed here. 

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