Photoinduced processes isomerization

Various mechanisms for the photoinduced geometrical isomerization of 9-styrylanthracenes have been proposed recently [90-93]. From steady-state and time-resolved fluorescence measurements on the parent 9-styrylanth-racenes 68a/69a and their formyl and 4-phenylsulfonyl derivatives (681/691 68j/69j), it has been concluded that the geometrical isomerization is an adiabatic process which occurs both in the excited singlet and triplet states [90]. Thus, for the conversion of the parent cis compound 68a into 69a, 85%... 

Photoinduced unimolecular reactions often have kinetics of the order of ps. One example of isomerization in ps times is shown in Figure 8.9. This is the photochromic reaction of a spiropyran. The photoinduced process takes place... 

The most prevalent photoinduced processes in supramolecular and interfacial systems are electron transfer, energy transfer and nuclear motion, such as proton transfer and isomerization. Before discussing these processes, it is important to outline the fundamental properties of electronically excited states. 

The photochromism of spirobenzopyrans is a well-documented phenomenon that arises from the photoinduced reversible isomerization between spiropyran and merocyanine forms . In spirobenzopyrans carrying a crown ether moiety (e.g., Ill), this interconversion process is affected by metal ion complexation. A strong interaction of the crown ether unit with a metal ion caused the thermal isomerization of the spirobenzopyran residue to the corresponding merocyanine form with simultaneous suppression of the UV-induced isomerization process (negative photochromism) (Scheme 3). Conversely, a weak metal ion interaction induced a positive photochromism <2001JOC1533, 2002EJ0655>. 

Photoinduced valence isomerization has been observed. An example is the process given in Eq. 333. ... 

A molecular-level machine is a particular type of molecular-level device in which the component parts can display changes in their relative positions as a result of some external stimulus.13,9 131 Although there are many chemical compounds whose structure and/or shape can be modified by an external stimulus (see, e.g., the photoinduced cis-trans isomerization processes), the term molecular-level machines is only used for systems showing large amplitude movements of molecular components. 

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 ... 

The ultrafast photoreactions in PNS of these proteins take place immediately after conversion from the FC state to vibrationally unrelaxed or only partially relaxed FI state [1-3]. For PYP [1] and Rh [3], the primary process is twisting of the chromophore, which causes the ultrafast fluorescence quenching, in the course of the isomerization, while the primary process for FP [2] is the ultrafast electron transfer leading to the fluorescence quenching reaction in PNS. Thus, in spite of the different molecular structures of PYP, Rh and FP chromophores and different kind of photoinduced reactions, these photoresponsive proteins show ultrafast and highly efficient photoreactions from FI state of similar nature (vibrationally unrelaxed or only partially relaxed), suggesting the supremely important role of the PNS controlling the reactions. 

Photoinduced and chemical oxidation of coordinated imines to amides in isomeric osmium(II) complexes of AT-arylpyridine-2-carboxaldimines has been studied by Ghosh and coworkers [34]. The complexes [Os(Br)2(Rimpy)2] (Scheme 1) were prepared and characterized structurally, indicating the pyridines of the two Rimpy ligands are trans to one another and the bromides are cis. Broadband photolysis of the complex in aerated acetonitrile resulted in oxidation of one Rimpy ligand to yield the RimOpy ligand and oxidation of the osmium to Os(III). The overall yield of the photochemical process is reported to be greater than 95%. The authors propose a mech-... 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

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. 

The extremely rapid rate of formation of bathorhodopsin as compared to isomerization rates observed in model protonated Schiff bases (a factor of 103) suggested the idea that electron transfer between an amino acid residue (e.g., tyrosine or tryptophan) in the protein and the chromophore may catalyse isomerization. Thus, a photoinduced electron transfer leading to a radical anion chromophore, instead of complete cis-trans isomerization, was considered as a plausible alternate mechanism for the primary event [200], This mechanism, however, is difficult to reconcile with the known photoreversibility but thermal irreversibility of the bleaching process. Thermal irreversibility of the light-induced electron transfer would require geometrical separation of donor and acceptor moieties which would then not allow photoreversibility [201]. 

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