Reversible chromophores

Chromophores can be classified in two categories reversible and irreversible. Reversible chromophores, often named molecular switches, undergo... 

For two-photon memories, a number of media types and reading mechanisms have been used (165). Generally, media comprise two photon-absorbing chromophores dissolved within a soHd polymer matrix. Suitable reversible photochromic dyes are, for example, spiropyrans. Although photochromic materials often suffer from photobleaching, as well as from instability leading to self-erasure, new materials and host environments are under development (172). Bacteriorhodopsin (BR) also has been proposed as a two-photon memory material. 

The color and constitution of cyanine dyes may be understood through detailed consideration of their component parts, ie, chromophoric systems, terminal groups, and solvent sensitivity of the dyes. Resonance theories have been developed to accommodate significant trends very successfully. For an experienced dye chemist, these are useful in the design of dyes with a specified color, band shape, or solvent sensitivity. More recendy, quantitative values for reversible oxidation—reduction potentials have allowed more complete correlation of these dye properties with organic substituent constants. 

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

Second, reducing bleaches such as lemon juice (in combination with sunlight) or sulfur dioxide convert the double bonds in the chromophore into single bonds, eliminating its ability to absorb visible light. Sometimes the reaction is reversible, where oxygen in the air reacts with the molecule to repair the chromophore, and the stain returns. 

The physical properties of the expanded radialenes were greatly enhanced upon donor functionalization, leading to the stable derivatives 76-78 with fully planar conjugated rr-chromophores [110]. These compounds exhibit large third-order nonlinear optical coefficients, can be reversibly reduced or oxidized, and... 

Anthocyanins were decolorized at pH 3.0 by the addition of sodinm sulfite at the C-2 or C-4 of the chromophore, a reaction that was rapidly reversible on acidification. Snlfnr dioxide, EDTA, and a combination of snlfnr dioxide and EDTA exerted very small effects on the losses of anthocyanins in strawberry pnrees and juices during 10 wk of storage at -20°C. Conversely, the addition of snlfnr dioxide and storage at 20°C slowed the anthocyanin losses and concnrrently decreased the formation of polymeric componnds, especially in pnrees. EDTA had a slight effect on color stability. ... 

The identification and quantification of potentially cytotoxic carbonyl compounds (e.g. aldehydes such as pentanal, hexanal, traw-2-octenal and 4-hydroxy-/mAW-2-nonenal, and ketones such as propan- and hexan-2-ones) also serves as a useful marker of the oxidative deterioration of PUFAs in isolated biological samples and chemical model systems. One method developed utilizes HPLC coupled with spectrophotometric detection and involves precolumn derivatization of peroxidized PUFA-derived aldehydes and alternative carbonyl compounds with 2,4-DNPH followed by separation of the resulting chromophoric 2,4-dinitrophenylhydrazones on a reversed-phase column and spectrophotometric detection at a wavelength of378 nm. This method has a relatively high level of sensitivity, and has been successfully applied to the analysis of such products in rat hepatocytes and rat liver microsomal suspensions stimulated with carbon tetrachloride or ADP-iron complexes (Poli etui., 1985). 

The interest in low-valent Ni complexes in S-rich environments has been stimulated by the presence of Ni in [Ni,Fe] hydrogenase and CODH. While thiolate ligation usually favors higher oxidation states, thioethers stabilize Ni1 and Ni°. In most cases, however, Ni1 ions of an NiS4 chromophore are unstable with respect to disproportionation. The cyclic voltam-mogram of square planar (983) with homoleptic thioether coordination exhibits a quasi-reversible wave at —0.42V (vs. NHE), which on the basis of the rhombic EPR spectrum (gi 2.27, g2 2.11, and g3 2.03) of the chemically reduced species (Na/Hg) is assigned to metal-centered reduction. 8... 

Solvatochromic probes have been used for a variety of applications like the study polarity of pure and mixed solvents [99], and the retention behavior in reverse-phase liquid chromatography [100] among other applications. Frechet et al. used 4-(N-methylamino)-l-nitrobenzene (p-MANB), as the chromophore, to probe the microenvironment of polyaromatic ether based dendrimers [101]. 

Dehydration converting the imidazolone ring to imidazolinone seems to be sensitive to the aromatic nature of residue 66 [61, 62]. This step is thought to lead to the formation of an enolate intermediate, which can be trapped by reverse anaerobic chemical reduction of the mature chromophore using dithionite and other reducing agents [63]. 

Olsen S, Lamothe K, MartiAAnez TJ (2010) Protonic gating of excited-state twisting and charge localization in GFP chromophores a mechanistic hypothesis for reversible photoswitching. J Am Chem Soc 132 1192-1193... 

Violot S, Carpentier P, Blanchoin L, Bourgeois D (2009) Reverse pH-dependence of chromophore protonation explains the large stokes shift of the red fluorescent protein mReima. J Am Chem Soc 131 10356-10357... 

As with the oxo-Mov complexes mentioned in the previous section, the NIR transitions become far more impressive when two or more of these chromophores are linked by a conjugated bridging ligand, as in [ Ru(bipy)2 2(/u-L21)]"+ ( = 0-4), which exhibits a five-membered redox chain, with reversible... 

Trivially, photo-excitation will drastically enhance the oxidation potential of the flavin chromophore and might give rise to a great variety of reversible chemical reactions, depending on the structure of the environment and/or the pathway of potential e - as well as H -conductivity. It must be emphasized, that the oxidative action of the flavin triplet Tj is by no means confined to 1 e -uptake from suitable aromatic... 

Fe(isoxazole)6]2+ and [Fe(isoxazole)4(H20)2]2+, together with four non-co-ordinated tetrafluoroborate anions (Fig. 15). The structural details for the low-field trans [Fe(isoxazole)4(H20)2]2+ are consistent with a high-spin Fe(II) chromophore (average Fe-O=2.09 A and Fe-N=2.19 A), whereas those for [Fe(isoxazole)6]2+ show a marked temperature dependence (average Fe-N=1.98 A at 115 K and 2.17 A at 231 K) related to the reversible low-spin to high-spin transition. From magnetic susceptibility measurements, the transition temperature has been found to be T1/2=137 K. 

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