Chromophore acyclic

Desaturation is easily inhibited and is a major target site for bleaching herbicides. The short chromophore acyclic intermediates such as phytoene which accumulate are unable to quench the energy of Chl or 02/ and thus cannot protect the plant against photosensitized... 

Considering an olefinic functionality as a chromophore, the absolute configuration of cyclic allylic alcohols can be determined using a method that involves the conversion of the alcohol to the corresponding benzoate.60 This can also be extended to acyclic alcohols where the conformations are dynamic (see Fig. 117). Interested readers may consult the literature for details.61... 

Figure 21. CD spectra of acyclic D-tetrose diastcreomers derivatized with the anthroate/methoxycinnamate exciton chromophores. Reprinted from Croat. Chem. Acta 62, 211 (1989).

The bipyridyl chromophore has been extensively used in lanthanide coordination chemistry. In addition to those based on the Lehn cryptand (see Section IV.B.4), a number of acyclic ligands have also employed this group. One such ligand is L17, which binds to lanthanide ions such that one face of the ligand is left open (Scheme 3) (60). As expected, luminescence is extremely weak in water and methanol, but stronger in acetonitrile ( = 0.30, 0.14 for europium and terbium, respectively). In addition, the nature of the counter ion can... 

The base Amax value for the former is taken as 214 nm and for the latter as 253 nm. To these values are added increments according to the nature of the substituent present and other structural features as shown in the table. In those cases where homoannular and heteroannular chromophores coexist in conjugation, the base value for the homoannular contribution is used, to which is added the appropriate increment for extended conjugation. It should also be noted that homoannular dienes in ring systems other than the six-membered ring do not give a satisfactory correlation between calculated and experimentally determined Amax values. The rules however are adequate for acyclic or heteroannular... 

Alternatively to the DNA modifications in the previous two sections where the chromophore was attached to one of the four DNA bases, chromophores can be incorporated as artificial DNA bases substituting a natural base or even a whole base-pair. There is a large number of recently reported syntheses of chromophores as DNA base surrogates, e.g. flavine derivatives [26] and thiazole orange derivatives [42]. Additionally, a variety of phosphoramidites as DNA building blocks for the introduction of fluorophores into DNA are commercially available, e.g. acridine derivatives. Clearly, the synthetic protocols for this kind of DNA modification do not follow a principle strategy which can be applied in a versatile fashion, as is the case for the DNA base modifications mentioned in the previous sections. It is important to point out that in many cases it turned out to be useful to replace the 2 -deoxyribose moiety with acyclic linker systems. This was also the case during our attempts to synthesize ethidium-modified DNA, which will be described here briefly. 

The photocycloaddition of vinylogous amides is not limited to substrates in which the chromophore is constrained in a five- or six-membered ring, and several groups have recently described the photocycloaddition of acyclic vinylogous amides. Tietze and coworkers reported that irradiation of 92 gives the bicyclic product 95 in quantitative yield (Scheme 22)43. The authors proposed that the observed product 95 could be the result of cycloaddition from the tautomeric form of the chromophore, followed by fragmentation of the resulting cycloadduct 93 to the imino-aldehyde 94, which condensed to the observed product. 

High quantum and chemical yields of ODPM rearrangements are obtained with substrates in which the ly-enone chromophore is part of a conformationally rigid molecular assembly which at the same time guarantees adequate orbital overlap of the C=C and rr-bonds. Whereas in bicyclic and bridged 3, y-unsaturated ketones these prerequisites are widely met, acyclic p,7-unsaturated ketones usually rearrange inefficiently since other channels of energy dissipation from the triplet state predominate. Exceptions to this rule are substrates in which the C= ir-bond is part of a styrene or an a, -enone moiety (Section 2.6.3.1). In this context it should also be noted that p,7-unsaturated aldehydes, except for one case, are ODPM unreactive (Section 2.6.3.2). 

In cases where either the C=C, the (3=0 or both moieties of a p,7-unsatuiated enone are part of a cyclic system, the ODPM reactivity is noticeably enhanced and less substituent-dependent compared with the previously discussed acyclic arrangement of the chromophore. Two examples of this category have already been given earlier in Section 2.6.2.1 (Scheme 1) the transformations of (3a-d) to a mixture of (4a-d) + (5a-d) and (6c,e,f) to (7c,e,f) which proceed in yields as high as 65% and 84%, respectively. 

To determine the ACs of acyclic 1,2-glycols, the CD exciton chirality method has been applied to their dibenzoates or bis(2-anthroates), which show typical bisignate Cotton effects (see Section 9.04.4) as exemplified in Figures 27 and 28.72,73 Acyclic dibenzoates or bis(2-anthroates) can rotate around the bond connecting two benzoate or 2-anthroate chromophores, and therefore the CD sign depends on the conformational equilibrium. From the data of many examples, general rules were derived as shown in Figures 27 and 28. 

The fact that the formazan is formed ehminates structures XVII, XVIII, and XXI, which do not meet the conditions for this reaction. This fimiishes dired evidence in place of the indirect evidence on which Hardegger and Schreier had discarded structures XVIII and XXI. The bicyclic structure XIX, and XX, proposed by Percival" can be dismissed on comparison of the ultraviolet spectra. This compound, after coupling, should contain the two chromophoric groups of the simple formazans, whereas osazone formazans contain three of them. Anhydro- D-glucose phenylosazone formazan and its diacetate display practically the same maxima (maxima at 335 and 405 ran and 340 and 405 m/x, respectively) as does the indisputably-acyclic pyruvaldehyde phenylosazone formazan (maxima at 335 and 410 m/t), and these peaks differ substantially from the values for the simple sugar for-... 

This chapter describes photoreactions of nitrogen containing chromophores which are different from the principal reactions of electronically excited carbonyl compounds. Five groups of substrates were included cyclic and acyclic imines, nitriles, amine oxides, azo and diazo compounds, and enamides. 

In contrast to these efficient intramolecular reactions most intermolecular photoreactions involving acyclic imines proceed with very low quantum yields. This is probably due to the rapid deactivation of the excited imine chromophore by C-N bond rotation. Cyclic imines, however, which lack this possibility, can undergo photoaddition to alkenes. 

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