Chromophores absorbing

Molecules with two or more isolated chromophores (absorbing groups) absorb light of nearly the same wavelength as does a molecule containing only a single chromophore of a particular type. The intensity of the absorption is proportional to the number of that type of chromophore present in the molecule. Representative chromophores are given in Table 7.9. 

Different light-absorbing groups, called chromophores, absorb characteristic wavelengths, opening the possibility of qualitative analysis based on the location of an absorption peak. 

Like group vibrations, the wavelength at which a chromophore absorbs can be employed as an analytical tool, but a rather less useful one. 

Benzoin and a wide variety of related compounds (e.g. 12, 70-74) have been extensively studied both as initiators of polymerization and in terms of their general photochemistry.271 2 3 The acetophenone chromophore absorbs in the near UV (300-400 nm). In the absence of hydrogen atom donors the mechanism of... 

Gruehn et al. [42] have reported on the isomorphy between modifications of LaTa309 and CeTajOg. Both contain trivalent lanthanide ions. The former is white, the latter is orange (P-modification), brass (O-modification) or yellow (M-modification). These colors are due to Ce(III)-Ta(V) MMCT absorptions, the Ce(III) and tantalate chromophores absorbing only in the ultraviolet. No doubt there are many more examples of this type in the literature. 

Most chromophores absorb light along a preferred direction1 (see Chapter 2 for the definition of absorption transition moment, and for examples of transition moments of some fluorophores, see Figure 2.3), depending on the electronic state. In contrast, the emission transition moment is the same whatever the excited state reached by the molecule upon excitation, because of internal conversion towards the first singlet state (Figure 5.2). 

Chromophores absorbing in the region between 1900 and 2600 cm-4 The absorptions listed in Table 3.3 often yield useful information because, even though some are of only weak or medium intensity, they occur in regions largely devoid of absorption by other commonly occurring chromophores. 

Transient absorbance difference spectra measured following 532-nm laser excitation, where both Ru(II) and Os(II) chromophores absorb, reveal the typical... 

These features are qualitatively discussed as follows. When the gel is illuminated, the chromophore absorbs the light energy and the local temperature of the polymer network rises. The increment of the temperature should be proportional to the intensity of the incident light and also to the concentration of the chromophore, and therefore, to the polymer network density <)>. It may thus be assumed that... 

Metal phthalocyanines (MPc) are a very attractive choice of dye since they are stable and the chromophores absorb strongly in the solar spectrum. Also a wide variety of metal phthalocyanines are known thus a wide range of redox potentials is available, and indeed some are semiconductors in their own right. 

Reactions analogous to these are observed with the polymers that contain metal-metal bonds along their backbones. Because the metal-metal bond chromophore absorbs in the visible region, the photochemical reactions of the polymers can be conveniently monitored by electronic absorption spectroscopy. The quantum yields for the reactions are in the range 0.1 to 0.6, depending on the specific polymer and the M-M bond.14 Sample reactions of the polymers are shown in equations 20-22. 

Creed et al. [125]. The 2,6-di-substitution pattern was chosen so that the anthracene moiety would be more likely to be elongated and to act as a mesogen. This polymer has an LC mesophase that is most probably N. Evidence was reported for chromophore association in this polymer even when it is highly diluted in a good solvent such as dichloromethane, but spectral perturbations due to aggregation are especially noticeable in films. Aggregation, as evidenced by a strongly blue-shifted band at 235 nm (the isolated chromophore absorbs at... 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

Deoxyribose reacts with hydroxyl radical with a rate constant of 3.1 x 109 M 1s 1. Deoxyribose degraded by hydroxyl radical produces malonyldialdehyde which reacts with thiobarbituric acid (TBA) to produce a pink chromophore absorbing at 532 nm (Halliwell and Gutteridge, 1981). 

The biochemistry that mediates photon absorption in the disk membranes and closing of the ionic channels in the plasma membrane is summarized in Fig. 2 and discussed in detail in reviews that provide access to the original literature (Helmreich and Hofmann, 1996 Menon et al, 2001 Hamm 1998, 2001 Bunemann and Hosey, 1999 Krupnick and Benovic, 1998). In the dark (inactive) state, rhodopsin (R) contains a covalently bound 11 -cis retinal chromophore buried within the protein core. In rhodopsin, the chromophore absorbs maximally around 500 nm, and absorption of a photon isomerizes the retinal to the all-trans form within about 200 fs (Peteanu et al, 1993) with a quantum efficiency of 0.67 (Dartnall, 1972). The change in chromophore shape creates strain, and subsequent thermal relaxation of both chromophore... 

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