Reductive quenching of excited states

The other diad system, consisting of sensitizer 1 deposited on Cab-O-Sil and electron donor 3 in solution, shows more interesting photochemistry. Referring to Figure 5, the redox potential of 3 allows for the reductive quenching of excited-state Ru(II) via reaction 2. The formal potential of 3 is +0.50 V versus SCE (standard calomel electrode) in aqueous solution (0.1 M KCl, Pt working electrode) by way of comparison, the formal potential of the Ru-(II)-Ru(I) couple is ca. +0.6 V (97, 98). 

Ru(bipy)3] can be generated by reductive quenching of excited state [Rufbipy)]] " (see below) or by controlled potential electrolysis of [Ru(bipy)3] " at — 1.42 V. [Ru(btpy)3] have been generated electrochemically and their absorption spectra are consistent with the formation of charge localized ruthenium(II)-bipy radical species. Comparison of the electronic spectra for [Ru (bipy)3] shows the progressive growth of a band near 340 nm which is observed also for free (bipy ) radical anion, and the concomitant collapse of the (bipy ) transition near... 

FIGURE 12.18 An alternative mechanism for excited-state sensitization of Ti02 by Ru(dcb)(bpy)22 + wherein a donor first reductively quenches the excited state, followed by thermal injection by the reduced sensitizer. A potential advantage of this mechanism is that the reduced sensitizer is a stronger reductant than the excited state, 380 mV in this case. 

As outlined in the introduction, quenching of excited states of complexes such as Ru(bipy)3 by reductants such as triphenylamine in a one-electron transfer step (Reaction 3) is a well-established process. With most complexes examined to date, back-electron transfer is both rapid and efficient so that net chemical change is seldom observed. The hydro-phobic Complexes 1, 2, and 3 were found to have absorption spectra, luminescence spectra, and excited-state lifetimes similar to those of Ru(bipy)3 however, their solubility and redox behavior is somewhat different (39, 40). While Ru(bipy)3 is soluble in water and a few polar organic solvents. Complexes 1, 2, and 3 are water insoluble but rather widely soluble in a variety of nonaqueous solvents. Redox potentials of Complexes 1 and 2 are shifted more anodic as shown in Scheme 1 the... 

Other groups within the protein may affect excited states. Disulfide bonds quench the excited states of tryptophan. For instance, at 77 K the phosphorescence lifetime of native lysozyme is low, 1.4s reduction of the disulfide bonds or denaturation gave the typical phosphorescence lifetime of 5.6 s.(49) Therefore, the absence of phosphorescence at room temperature from this protein is likely to be due to quenching of both the singlet and the triplet state. 

Some polymers show discoloration as well as reduction of the mechanical properties (e.g. aromatic polyesters, aromatic polyamides, polycarbonate, polyurethanes, poly (phenylene oxide, polysulphone), others show only a deterioration of the mechanical properties (polypropylene, cotton) or mainly yellowing (wool, poly(vinyl chloride)). This degradation may be less pronounced when an ultraviolet absorber is incorporated into the polymer. The role of the UV-absorbers (usually o-hydroxybenzophenones or o-hydroxyphenylbenzotriazoles) is to absorb the radiation in the 300-400 nm region and dissipate the energy in a manner harmless to the material to be protected. UV-protection of polymers can be well achieved by the use of additives (e.g. nickel chelates) that, by a transfer of excitation energy, are capable of quenching electronically excited states of impurities (e.g. carbonyl groups) present in the polymer (e.g. polypropylene). 

There is a preliminary report of an excited state photoelectrochemical cell based on the reductive quenching of the MLCT excited state of [Os(bipy)diars2]2+ by N(p-C6H4Br)3 in acidic, aerated acetonitrile it produces H202 and bromine with high quantum efficiency.177... 

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