Chiral quencher

Later in this chapter we will describe a series of recent experiments in which non-racemic excited states were obtained from racemic mixtures by adding an optically active quencher molecule. The chiral quencher changes the excited state lifetimes of the enantiomers, and makes them unequal. We designate the new lifetimes as k+ and k, and relate them to the concentration of chiral quencher, [QaJ, and bimolecular quenching constants, k and k as follows... 

The second application of time-resolved CPL is in the study of enantioselective (or more properly, di-astereomer-selective) quenching. In these experiments an enantiopure quencher molecule is added to a racemic solution. The interaction of the chiral quencher with the individual enantiomers is such that the excited state of one of the enantiomers is quenched more rapidly than the other. This process is depicted schematically in Figure 2, where we have assumed the quencher is an R enantiomer, and and denote the quenching rate constants. This process may be described using the following simple kinetic model... 

The photoresolution of JIN proceeds most efficiently In chiral media without singlet quenchers and with macroscopic order. Several factors may contribute to the necessity of long-range order. One of these Is the ability of cholesteric media to... 

A significant ability to discriminate between chiral amines based on the quenching of S-di-2-naphthylprolinol fluorescence emission was reported by Diamond et al. [32], fl-Phenylethylamine (PEA) was seen to have a much greater efficiency as a quencher than the S-enantiomer. l- and D-norephedrine, which have structural conformation similarities to PEA, were also observed to have an enantiomeric selectivity. The mechanism of chiral recognition is proposed to be a combination of hydrogen bonding and 3D chirally restricted space. 

The third example clarifies the special feature of singlet photochemistry. In contrast with 34 and 36, the a-ketoester 39 reacts from the singlet state if irradiated in the presence of naphthalene as triplet quencher. Despite the low diastereoselectiv-ity, the chirality of the alanine derivative 39 is completely conserved during cycli-zation to the pyrrolidines 40a and 40b. This approach has been called the memory effect of chirality [16]. 

The vibrational deactivation of 102 has been elegantly exploited to achieve highly stereoselective photooxygenation of alkenes in which one site is protected by a nearby C H bond acting as a quencher.146 Replacement of the protective C H group by C D significantly reduces the selectivity of attack. The authors anticipate that this new concept of stereocontrol may prove to be general and may facilitate chiral control in a variety of phototransformations. 

L-Trp-OMe 2-N-dansylethyl 3,3-dimethylacrylate EDMA MeCN Chiral fluorescence sensor new functional monomer competitive binding with a quencher 4-nitrobenzaldehyde [69]... 

The concept of intramolecular alkylation of AT-substituted amino acid derivatives via 1,5-diradicals also turned out to be an excellent system for studying the different stereochemical course of spinisomers as discussed in Section 6.2.2. Thus, the a-ketoester 9, which contains an alanine moiety, was prepared. In contrast to aryl ketones, a-ketoesters are not completely converted into the triplet state after photochemical excitation. Upon addition of either a triplet quencher (naphthalene) or a triplet sensitizer (benzophenone), each of the two spin states may be forced (Scheme 3, Table 1). The chiral center at the d-position with respect to the keto carbonyl group raises the question whether a memory effect of chirality may be observed during the cyclization. The results summarized in Table 1 amply demonstrate the specific properties of spinisomeric biradicals. In the presence of naphthalene, which probably acts not only as a triplet quencher but also as a singlet sensitizer, the chiral information of the reactant 9 is almost entirely conserved in the helical diradical 10 because of its very short lifetime. In contrast, the addition of benzophenone results in almost complete racemization, and also the cis/trans selectivity is... 

DNA is an interesting supramolecular structure, as it provides different complexation sites for molecules. Planar and relatively small molecules intercalate between base pairs, whereas larger molecules can bind to the minor or major grooves of the DNA helix. Interaction can be rather specific due to the chirality of the helix [147,150]. In addition, the negatively charged phosphate backbone is able to anchor or bind cationic probes or quenchers. Most studies have been performed with calf thymus DNA in its native double helical form. 

Each binding site in BSA and HSA exhibits very different photochemical behaviour. Thus, the HT/HH ratio and enantioselectivity obtained in albumin-mediated AC photocyclodimerization are dynamic functions of the AC/BSA or AC/HSA ratio. For example, the HT dimers are produced as the major products in the photodimerization of AC in host-free bulk water and also in the presence of HSA, whereas the HH dimers become the dominant products in the BSA-mediated photodimerization. In the presence of BSA (AC/BSA = 1.3), chiral HT and HH dimers, 44 and 45, were obtained in 29% and 41% ee, respectively. Interestingly, the ee s for 44 and 45 were further improved up to 38% and 58% by adding nitromethane as a site-selective quencher of AC located in such binding sites that are more accessible but less enantioselective. 

The first quencher used for this purpose was the resolved tris(l,10-phenanthroline)m-thenium(II) complex, [Ru(phen)3] " (Metcalf et al., 1990a, 1989). The enantioselective excited-state quenching of [Tb(DPA)3] , [Eu(DPA)3] , and Py(DPA)3] by resolved-[Rulphenls] " " has provided important information regarding the stracture and excited state energetics of the racemic donor and chiral acceptor molecules (e.g., size, shape, electrostatic... 

Chiral amines have been shown to be able to quench the fluorescence of several fluorophores. The first cases were reported by Irie and coworkers, who described that the collisional quenching of (/ )-binaphthyl by A, A -dimethyl-l-phenylethyl-amine was dependent on the stereochemistry of the latter [50]. The mechanism of this process corresponds to a series of subsequent events formation of an encounter complex between the excited fluorophore and the quencher, which can then give rise to a tight exciplex ([A-F ] in Fig. 2a) or to an electron transfer ion pair [ A -F ] (Fig. 2a). This can explain the strOTig dependence of both quenching and enantios-electivity uprm the polarity of the solvent observed in these studies [126]. 

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