Excimer intramolecular

Excitation fluorescence is the principle of the fluorescence techniques used for studying polymer blends. The method comprises of three steps incorporation of an excimer, its excitation, and recording the excitation delay. The excimer can be an aromatic polymer component of the blend (viz., PS, poly(viny 1-dibenzyl), polyvinylnaphthalene, an aromatic group grafted onto the macromolecular chain, etc.), or it can be added as probe molecule (e.g., anthracene). There are three possibilities for the aromatic rings to form excimers intramolecular adjacent, intramolecular nonadjacent, and intermolecular types. Each of these types is sensitive to different aspects of the chain conformation and environment, thus, sensitive to blend miscibility effects. The most important of these for studies of polymer blends is the intermolecular, usually identified from concentration measurements (Wiruiik et al. 1988). 

A way of avoiding these uncertainties is to use probes in which the chromophores are linked together by short methylene chains, O linkages or N atoms. Such probes may form excimers intramolecularly by a rearrangement or bending of the molecule. 

Hydroxy Benzophenone P-Naphthoxy Energy Transfer Processes Triplet State Intramolecular Excimers" ... 

Replacing the nitrile group by a benzothiazole produces an important subclass of fluorescent compounds represented by thioflavin T (25, Fig. 10). It is not clear if this compound undergoes deactivation via intramolecular rotation that would meet the criterion for a molecular rotor. The steady-state absorption and emission properties of thioflavin T has been attributed to micelle formation [53, 54], dimer and excimer formation [55, 56], and deactivation through intramolecular rotation [57]. 

Lehrer, S. S. (1997). Intramolecular pyrene excimer fluorescence A probe of proximity and protein conformational change. Fluorescence Spectroscopy 278, 286-295. 

Bodenant B, Fages F, Delville MH (1998) Metal-induced self-assembly of a pyrene-tethered hydroxamate ligand for the generation of multichromophoric supramolecular systems. The pyrene excimer as switch for iron(III)-driven intramolecular fluorescence quenching. J Am Chem Soc 120 7511-7519... 

When the two monomers are linked by a short flexible chain, intramolecular excimers can be formed. This process is still diffusion-controlled, but in contrast to the preceding case, it is not translational it requires a close approach between the two molecules via internal rotations during the excited-state lifetime. Equations (4.44), (4.45), (4.47) to (4.49) are still valid after replacing k [M] by k because intramolecular excimer formation is independent of the total concentration. Estimation of the local fluidity of a medium can be achieved by means of probes capable of forming intramolecular excimers (see Chapter 8). 

The viscosity dependence of intramolecular excimer formation is complex. As in the case of molecular rotors (Section 8.2), most of the experimental observations can be interpreted in terms of free volume. However, compared to molecular rotors, the free volume fraction measured by intramolecular excimers is smaller. The volume swept out during the conformational change required for excimer formation is in fact larger, and consequently these probes do not respond in frozen media or polymers below the glass transition temperature. 

Intramolecular excimers have been used for probing bulk polymers, micelles, vesicles and biological membranes (Bokobza and Monnerie, 1986 Bokobza, 1990 Georgescauld et al., 1980 Vauhkonen et al., 1990, Viriot et al., 1983 Zachariasse et al., 1983). In particular, this method provides useful information on the local dynamics of polymer chains in the bulk (see Box 8.2). 

In conclusion, the method of intramolecular excimer formation is rapid and convenient, but the above discussion has shown that great care is needed for a reliable interpretation of the experimental results. In some cases it has been demonstrated that the results in terms of equivalent microviscosity are consistent with those obtained by the fluorescence polarization method (described in Section 8.5), but this is not a general rule. Nevertheless, the relative changes in fluidity and local dynamics upon an external perturbation are less dependent on the probe, and useful applications to the study of temperature or pressure effects have been reported. 

Box 8.2 Intramolecular excimer fluorescence for probing the mobility of bulk polymers... 

The correlation time tc of the motions involved in intramolecular excimer formation is defined as the reciprocal of the rate constant ki for this process. Its temperature dependence can be interpreted in terms of the WLF equationb) for polymers at temperatures ranging from the glass transition temperature Tg to roughly Tg +100° ... 

The choice of method depends on the system to be investigated. The methods of intermolecular quenching and intermolecular excimer formation are not recommended for probing fluidity of microheterogeneous media because of possible perturbation of the translational diffusion process. The methods of intramolecular excimer formation and molecular rotors are convenient and rapid, but the time-resolved fluorescence polarization technique provides much more detailed information, including the order of an anisotropic medium. 

De Schryver F., Collart P., Vandendriessche J., Goedeweeck R., Swinnen A. and Van der Auweraer M. (1987) Intramolecular Excimer Formation in Bichromophoric Molecules Linked by a Short Flexible Chain, Acc. Chem. Res. 20, 159-166. 

Georgescauld D., Desmasez J. P., Lapouyade R., Babeau A., Richard H. and Winnik M. (1980) Intramolecular Excimer Fluorescence A New Probe of Phase Transitions in Synthetic Phospholipid Membranes, Photochem. Photobiol. 31, 539-545. 

Vauhkonen M., Sassaroli M., Somerharju P. and Eisinger J. (1990) Dipyrenyl-phosphatidylcholines as Membrane Fluidity Probes. Relationship between Intramolecular and Intermolecular Excimer Formation Rates, Biophys. J. 57, 291-300. 

The non-cyclic ethers E-2 (Figure 10.26), with two pyrenes linked at both ends of the chain, show strong intramolecular excimer formation. Addition of alkaline earth metal ions leads to an increase in monomer emission at the expense of the excimer band. The helical structure of the 1 1 complexes is supported by NMR spectra. Thanks to the pseudocyclic structure, the stability constants of the complexes with Ca2+, Sr2+ and Ba2+ in acetonitrile are quite high (106-107 for n — 5), but the selectivity is poor as a consequence of the flexibility of the oxyethylene chain. 

Intramolecular Folding The Excimer/Exciplex Mechanism and Dewar Isomerization (Butterfly Mechanism)... 

Figure 5.5. The formation of intramolecular excimers usually involves the folding of a flexible chain, and the displacement of the large chromophores into a sandwich-type arrangement necessitates a large reaction volume.

After the initial use of pyrene and other excimer-forming molecules in biological applications,(54) it was soon recognized that excimer formation can be strongly enhanced and the probe concentrations therefore strongly reduced if the reaction partners are chemically linked, e.g., by a methylene chain. Thus, intramolecular excimers were developed, some of which are collected in Figure 5.9. 

Figure 5.9. Chemical structure of some intramolecular excimers used as free volume fluorescence probes.

Highly selective fluorometric determination of polyamines based on intramolecular excimer-forming derivatization with a pyrene-labeling reagent. Anal Chem 72 ... 

Yoshitake T, Ichinose F, Yoshida H, Todoroki K, Kehr J, et al. 2003. A sensitive and selective determination method of histamine by HPLC with intramolecular excimer-forming derivatization and fluorescence detection. Biomed Chromatogr 17 509-516. 

PC3P is a sensitive probe for local viscosity measurement by forming an intramolecular excimer [35,36]. The extent of excimer emission depends upon the rate of conformational change of the chain linking the two... 

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