Quenching reaction processes

Fig. 6.3. The VIR Model of Horie considers the chemical reaction process in terms of three components void, inert, and reactants. The influence of the inerts is critical as that component causes thermal quenching of incipient reactions.

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

The ratio of the enantiomeric benzyl amide products was determined by analyzing a diluted aliquot of the quenched reaction mixture by HPLC using a chiral stationary phase column (Chiralcel OD, Daicel Chemical Co.). Since racemization is a pseudo-first-order kinetic process, these data (along with the time zero value) are sufficient for determination of the intrinsic rate of racemization kR. The half-life for racemization lRU2 can be directly calculated from the l/d ratio (or % enantiomeric excess, %ee) where t was the time of benzylamine addition (the delay time) ... 

The reaction process can be monitored by TLC or GC. The unsaturated ketoester is generally consumed within 1 h. Once the starting material is consumed, the reaction can be quenched with 1 N HCl. The double bond in the product will be slowly reduced at elevated temperature (80 °C) and prolonged reaction time. However, it is relatively stable at room temperature under the catalysis of Ru(p-cymene)(TsDPEN) no significant amount (<1 %) of double bond reduced product was detected 5h after the consumption of the starting material. 

Some solvents containing heavy atoms can induce enhancement of phosphorescence at the expense of fluorescence, e.g. ethyl iodide, nitro-methane, CS2 (external heavy atom effect). Irreversible conversion to ionic or radical products is often observed. Hence the system changes with time and the process should be classed a photochemical reaction distinct from the reversible quenching reactions discussed above. For example for anthracene and carbon tetrachloride ... 

The ultrafast photoreactions in PNS of these proteins take place immediately after conversion from the FC state to vibrationally unrelaxed or only partially relaxed FI state [1-3]. For PYP [1] and Rh [3], the primary process is twisting of the chromophore, which causes the ultrafast fluorescence quenching, in the course of the isomerization, while the primary process for FP [2] is the ultrafast electron transfer leading to the fluorescence quenching reaction in PNS. Thus, in spite of the different molecular structures of PYP, Rh and FP chromophores and different kind of photoinduced reactions, these photoresponsive proteins show ultrafast and highly efficient photoreactions from FI state of similar nature (vibrationally unrelaxed or only partially relaxed), suggesting the supremely important role of the PNS controlling the reactions. 

An internal conversion process recently discovered in our laboratory may shed light on the subject. Murovla found that quadricyclene, 6, is a powerful quencher of the excited singlet states of naphthalene and other aromatic hydrocarbons. In the course of the quenching reaction, the quencher was extensively converted to bicyclo [2.2.1] hepta-2,5-diene, 7. The following mechanism was suggested. 

We arrive here at the limit of quenching considered as a purely photophysical process, involving no permanent chemical change. Electron transfer is in fact a chemical reaction which leads to new, distinct species M + and N - these may separate and react to form new molecules, in which case we enter the realm of photochemistry. Photoinduced electron transfer must be considered to be a photochemical reaction (see chapter 4), but in some cases it may appear to be a quenching reaction when the reactants are restored to their initial state (Figure 3.38). 

Before concluding this chapter, we wish to mention very briefly a number of interesting beam experiments studying processes that are—to some extent—inverse quenching reactions. A first group of experiments... 

The best evidence for a CT process rather than direct hydrogen abstraction involves the values of kT s-butyl- and ferf-butylamine display much the same value 156> triethylamine and ferf-butyldimethylamine are equally reactive and some 50 times more so than primary amines 155>. Thus the rate constant for reaction is independent not only of the type of C—H bond a to the nitrogen but also of the presence or absence of a-hydrogens. Such evidence demands that abstraction of an a-hydrogen not be involved in the rate-determining quenching reaction. Moreover, the relative reactivity of amines (tertiary > secondary > primary) is proportional to the ease with which they are oxidized. 

In the quenching reaction of A-[Ru(bpy)3]2+ by [Co(ox)3]3- and Co(acac)3, only the homochiral preference was observed in water, whereas the stereoselectivity of the quenching by [Co(ox)3]3 becomes reverse in 80% methanol-water. These results suggest that the stereoselectivity is determined not only by the photoin-duced electron transfer but also by the different elementary step such as the reverse reaction. The photoreduction of the cobalt(III) complex by the ruthen-ium(II) complex involves various elementary steps, as shown in Scheme 11. Considering this scheme, one can easily understand that the overall photoreduction of the cobalt(III) complexes is determined by not only the quenching process but also the reverse reaction between the reduced Co(II) complexes and the oxidized ruthenium(III) complex. This conclusion is essentially the same as that reported by Ohkubo and his collaborators. 

---