Photochemical perturbation

Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

The lability inherent in the planar, tetrahedral equilibria which nearly all involve Ni(II) requires that nmr line broadeningor photochemical perturbation methods be used for their kinetic resolution. First-order interconversion rate constants for... 

Photochemical perturbation of equilibrium (42) using a pulsed neodymium laser has been reported116 [dpp = Ph2P(CH2)3PPh2]. Irradiation at 1060 nm,... 

The photochemical perturbation of the system Ni and PADA produces two relaxation effects in both aqueous and micellar media characterised by widely differing relaxation times. The faster process is independent of the concentration of both Ni and PADA but is dependent on temperature. In bulk water at 298.2 K the fast (F) relaxation time Tp has a value of 35( 4) ys, which gives a first order rate constant of 2.9( 0.3) x 10 s . This fast process is identified with ring closure (step 3 of figure 2) following photochemical generation of the monodentate complex. The activation energy (E )p for this process is found to be 40 4 k J mol . ... 

Gonzalez, D. S., Sawyer, A., and Ward, W. W. (1997). Spectral perturbations of mutants of recombinant Aequorea victoria green-fluorescent protein (GFP). Photochem. Photobiol. 65 21S. 

We have considered the factors determining the participation of PPA in photochemical processes40- 42, 45). Poly(propynoic acid) has served as an example of the perturbation of a conjugated system resulting from photoinitiated oxidation. 

Yushchenko DA, Shvadchak W, Bilokin MD, Klymchenko AS, Duportail G, Mely Y, Pivovarenko VG (2006) Modulation of dual fluorescence in a 3-hydroxyquinolone dye by perturbation of its intramolecular proton transfer with solvent polarity and basicity. Photochem Photobiol Sci 5 1038-1044... 

Ward WW, Prentice HJ, Roth AF, Cody CW, Reeves SC (1982) Spectral perturbations of the aequoria green-fluorescent protein. Photochem Photobiol 35 803-808... 

Finally, in many of the perturbation calculations of the effect of substituents and other structural changes, an important tacit assumption is made and it is far from obvious that it is always fulfilled. As already discussed, the physical argument on which the calculation is based is that the value of the initial slope, or the height of a small barrier along the way, determine the rate at which the photochemical reaction occurs. However, the experimental value with which comparison is made usually is not the reaction rate but the quantum yield, which of course also depends on rates of other competing processes and these may be affected by substitution as well. For instance, the rate at which fluorescence occurs is related to the absorption intensity of the first transition, the rate of intersystem crossing may be affected by introduction of heavy atoms... 

Organic Molecules in Adsorbed or Other Perturbing Polar Environments, Photochemical and Spectroscopic Properties of (Nichollas and... 

Cyclohexadienones. The perturbation of the electronic absorption spectra by adsorption on silica gel can have a significant effect on subsequent photochemical reactions. For example, Hart has shown that 2,4-cyclo-hexadienones photochemically degrade cleanly in nonpolar solvents to a ketene (1) but in highly polar solvents or adsorbed on silica gel bicyclic ketone (2) is the predominant product.33 The absorption spectra indicate that in nonpolar solvents the lowest singlet state is the n,n state, from which the formation of the ketene proceeds. This n -n band is obscured by the band in polar media, inversion of the energy levels of the n,n, and the first... 

We consider first how surfaces which are themselves not photosensitive can perturb chemical reactivity. First, the surface can influence diffusional motion of adsorbed substrates, intermediates or products. With preadsorbed substrates, one can probe the nature of motion of intermediates generated on the surface and search for differences in reactivity caused by surface confinement . When several photochemical precursors to benzyl radicals, e.g., benzyl phenylacetate, a dibenzyl ketone, or a dibenzyl sulfone, are irradiated as adsorbates on dry silica gel, singlet and triplet radical pairs are generated, Eq. (7). The extent of radical recombination observed requires... 

Such solvent perturbations are largely responsible for the variation of photochemical behaviour of a molecule in different solvents, because the order of energy levels may change with change of solvent. Many such examples will be presented in appropriate places. 

Rate constants of unimolecular processes can be obtained from spectral data and are useful parameters in photochemical kinetics. Even the nature of photoproducts may be different if these parameters change due to some perturbations. In the absence of bimolecular quenching and photochemical reactions, the following reaction steps are important in deactivating the excited molecule back to the ground state. 

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