Quenching and Back Reactions

The quenching and back reaction rate constants as determined by flash photolysis, are collected in Table 10. Noteworthy here is the quenching by S032-, which proceeds by two parallel routes. The normal dynamic quenching (reaction 54) has a rate constant kq = 3 x 10s mol-1 dm3 s with... 

Photoinduced Electron-Transfer Reactions 13.4.3. Quenching and Back Reactions... 

The quenching and back reactions are not uncoupled processes, and the back reaction of the primary products of the quenching reaction can occur before they have diffused out of the solvent cage in which they form. This is shown in Scheme 1, where S is the ES of the sensitizer that undergoes reaction with the quencher, Q, and is the lifetime of the excited state in the absence of the quencher ... 

For systems such as these, which consist of electron transfer quenching and back electron transfer, it is in general possible to determine the rates both of quenching and of the back reaction. In addition to these aspects of excited state chemistry, one can make another use of such systems. They can be used to synthesize other reactive molecules worthy of study in their own right. The quenching reaction produces new and likely reactive species. They are Ru(bpy)3+ and Ru(bpy)j in the respective cases just shown. One can have a prospective reagent for one of these ions in the solution and thereby develop a lengthy and informative series of kinetic data for the transient. 

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. 

Table 1. Quenching (k ) and Back Reaction (kp Rate Constants and Cage Escape Yields Reactions of [ Ru(bipy)j] ...

Quenching of the ( CT)[Ru(bipy)3] by [Cr(bipy)]3 has been studied. This is via electron transfer to the Cr complex and a rapid back reaction. The ruthenium complex will also quench the 727 nm emission of the metal-centred doublet excited state of the chromium species, by a similar mechanism. Evidently both ligand- and metal-centred excited states can be quenched by bimolecular redox processes. A number of Ru complexes, e.g. [Ru(bipy)3] and [Ru(phen)3] also have their luminescence quenched by electron transfer to Fe or paraquat. Both the initial quenching reactions and back reactions are close to the diffusion-controlled limit. These mechanisms involve initial oxidation of Ru to Ru [equation (1)]. However, the triplet excited state is more active than the ground state towards reductants as well as... 

Oxidative quenching of complexes such as Ru(bipy) and back electron transfer (Reactions 1 and 2) occurs with a variety of reagents. One of the most widely used oxidants in these studies is Paraquat (PQ ). For Ru(bipy)3 rate constants for the quenching and back-electron-transfer reactions in acetonitrile are, respectively, 2.8 X 10 and 8.1 X 10 M" sec (39,40), When the hydrophobic complexes are used as substrate, both forward and back reactions are retarded for example with 1 the corresponding rate constants are 1.2 X 10 and 1.8 X 10 M" sec" (39, 40), As pointed out previously with Ru(bipy)3 and PQ the electron-transfer products can be intercepted as illustrated in Reactions 5, 6, and 7, but in this case no permanent chemistry occurs. When solutions of 1 and Paraquat are irradiated in dry acetonitrile, we also find that reverse electron transfer is efficient enough so that no permanent chemistry... 

Fig. 7 Dependence of log with the driving force AG for excited state quenching and back electron transfer reactions involving Cr(bpy)3 , Rh(phen)3 and lr(5,6-Me2-phen)2Cl2], methoxybenzenes and aromatic amines.

These are chemically independent of Eqs. (11-51) and (11-52). Each follows in time after the quenching event. The back reactions do not involve luminescent quenching, and so they are generally studied by absorption measurements. In these cases the values17 are 53 = 5 x 106 Lmol-1 s-1 and 54 = 4.5 X 107 Lmol-1 s-1. 

The incident shock wave moves down the tube, heating and accelerating the test gas. In RST mode, the shock hits an end plate and reflects back to the test gas, further heating the gas and initiating stagnation conditions. Subsequently, a rarefaction wave travels down the tube and quenches all further reactions. 

The ionic domain of polyelectrolytes also affects the photoinduced charge separation between the coexisting low molecular charged compounds38). The quenching of Ru(bpy)2 + by a zwitterion compound, dibenzylsulphonates viologen (18, BSV), was enhanced about 4 times by poly(vinylsulphate) (PVS)39), and the back reaction... 

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