Quinones, photochemical kinetic study

An extensive review of 2 + 2-photocycloadditions of dienones and quinones has been published. The photocycloaddition of heterocyclic 2,3-diones (18) with electron-rich alkenes in the presence of the photosensitizer benzophenone yields 2-1-2-cycloadducts (19) and (20) with high regio- and stereo-selectivity (Scheme 1) Time-resolved spectroscopy has shown that in the photochemical cycloaddition between stilbenes and quinones, the (singlet) ion-radical pair [S ", Q+ ] is the primary reaction intermediate and therefore establishes the electron-transfer pathway for this typical Paterno-Biichi transformation. A kinetic study of the Paterno-Biichi cycloaddition of stilbene to chloranil shows that solvent polarity and donicity control the formation as well as the reaction path of the ion-radical. The photoirradiation of chloranil with... 

Recently, 2-hydroxypropyl cellulose (HPC) was demonstrated to be a very suitable matrix for UV/Vis absorption study of photoyellowing of milled-wood lignin in the solid state [10] where clean kinetics of discolouration were established after different chemical treatments of the lignin in solution. This indicates the importance in the discolouration process of phenolic structures without carbonyl groups and conjugated double bonds. The present communication describes a similar photochemical study on quinones and hydroquinones incorporated in HPC films under comparative concentrations ( 6.5xl0"5 mol/g HPC). For comparison, we also use UV/Vis reflectance spectroscopy to examine the behaviour of these compounds adsorbed on filter paper after light-exposure. 

Femtosecond kinetics of photochemical charge separation in photosynthetic bacteria at low temperatures (25 K) was studied by Lautwasser, Finkele, Scheer and Zinth using Rb. sphaeroides RCs depleted of quinones. Fig. 9 (C, a) shows the initial formation of P at 920 nm followed by a very rapid relaxation with a Ti of 1.4 0.3 ps. At 794 nm and 25 K, the absorption increases very rapidly to a maximum in about 0.1 and then decays to a minimum at tD 0.5 ps. This is followed by a slow rise and a plateau after 5 ps. The early rapid-decay component with a time constant of 0.3 0.15 ps appears to be the counterpart of the 0.9-ps component at room temperature. The data points in Fig. 9 (C, b) can be fitted by a model with three time constants, namely 0.3ps, l.Aps and 1 ns. 

Analysis of mutations at Trp so indicates that this residue plays an important role in tight binding of Q, but that is not essential for photochemical reduction of the quinone. Mutant RCs which have been quinone-depleted or reconstituted will be useful for future studies of the dynamics of charge separation and recombination. Measurements of the kinetics of electron transfer from to in some of these mutants are currently in progress. 

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