Candidate Chromophores

Figure 5.3.5-1 The devolution of the properties of candidate chromophoric materials. The shaded area on the right highlights the unique properties of the chromphores of vision. This is the only state in which the functional properties of the chromophores can be determined in the laboratory. The shaded area in the upper left is only shown for completeness and will not be discussed in detail. See text.

Figure 5.3.5-1 The devolution of the properties of candidate chromophoric materials. 26...

The light emitter in Latia luminescence. The purple protein is strongly fluorescent in red. Thus, at first glance, it would appear to be a most probable candidate for the light emitter or its precursor. However, this possibility was ruled out when we found that there is no way to relate the fluorescence of the purple protein to the bioluminescence spectrum. Thus, the luciferase must contain a chromophore that produces the light emitter. 

In this section, we have demonstrated that a single organic dye nanocrystal comprised of many chromophores shows photon antibunching when the size is sufficiently small. The present results indicate that molecular assemblies can also be considered as candidates for new single-photon sources. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

The presence of a covalent acyl-enzyme intermediate in the catalytic reaction of the serine proteases made this class of enzymes an attractive candidate for the initial attempt at using subzero temperatures to study an enzymatic mechanism. Elastase was chosen because it is easy to crystallize, diffracts to high resolution, has an active site which is accessible to small molecules diffusing through the crystal lattice, and is stable in high concentrations of cryoprotective solvents. The strategy used in the elastase experiment was to first determine in solution the exact conditions of temperature, organic solvent, and proton activity needed to stabilize an acyl-enzyme intermediate for sufficient time for X-ray data collection, and then to prepare the complex in the preformed, cooled crystal. Solution studies were carried out in the laboratory of Professor A. L. Fink, and were summarized in Section II,A,3. Briefly, it was shown that the chromophoric substrate -carbobenzoxy-L-alanyl-/>-nitrophenyl ester would react with elastase in both solution and in crystals in 70 30 methanol-water at pH 5.2 to form a productive covalent complex. These... 

Moreover, a chromophore surrounded by a perfectly defined environment might be regarded as a potential candidate for SMS, allowing study of the interactions of a fluorophore with the surrounding matrix with a very high spatial resolution, which is currently under investigation. 

The lack of selectivity and efficiency with which a single carboxylic acid binds nucleotide bases makes it an unlikely candidate for incorporation into a host for nucleotide guests. Nonetheless, it proved synthetically expedient to do so with molecular tweezers such as 11 and 21. The issue addressed was whether the aromatic chromophores in the molecular tweezer represented in Fig. 6 could cooperate with the acid group to increase the binding efficiency and selectivity of the host. 

Acifluorfen (5-(2-chloro-o , a, a ,-trifluoro-p-tolyl)-2-nitrobenzoic acid) (IV) mainly undergoes photodecarboxylation upon irradiation at 254 or 313 nm. This compound was exposed to solar light, first in the presence of humic acids (lOmglr1), and then dissolved in a natural water sampled in an artificial lake [68]. No effect of humic acids was observed. The lack of a photo-inductive effect of humic substances can be explained by the fact that acifluorfen bears electron-withdrawing substituents rendering it poorly oxi-dizable by reactive triplet states. In contrast, the half-life of IV was found to decrease by a third in the natural water. This photo-inductive effect might be due to the photogeneration of hydroxyl radicals by chromophores other than humic components of DOM nitrate and nitrite ions are probable candidates. 

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