Excimers intramolecular

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

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. 

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. 

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... 

Zachariasse, K. A., Vaz, W. L. C., Sotomayor C., and Kiihnle, W. Investigation of human erythrocyte ghost membrane with intramolecular excimer probes, Biochim. Biophys. Acta, 1982, 688, 323-332. 

Melnick, R. L., Haspel, H. C., Goldenberg, M., Greenbaum, M., and Weinstein, S. Use of fluorescent probes that from intramolecular excimers to monitor structural changes in model and biological membranes, Biophys. J., 1981, 34,499-515. 

Fig. 49 Schematic representation of intramolecular excimer formation of dianaph-thypropane (DNP)...

Baker and coworkers [16] reported on a self-referencing luminescent thermometer designed around the temperature-dependent intramolecular excimer formation/dissociation of the molecular probe l,3-Ws(l-pyrenyl)pro-pane (BPP) dissolved in 1-butyl-l-methylpyrrolidinium bjs(trifluoromethyl-sulfonyl)imide, [C4Cipyr][Tf2N]. Upon an increase in temperature, and hence a decrease in the IL s bulk viscosity, the excimer-to-monomer fluorescence... 

Karmakar, R., Samanta, A., Intramolecular excimer formation kinetics in room temperature ionic liquids, Chem. Phys. Let., 376, 638-645, 2003. 

Oyama et al. [23] utilized fluorescence spectroscopy to study how the molar masses of both PAA and PEO affect complexation. Actually, they used the exci-mer formation between the pyrene groups attached to the chain ends of PEO as a molecular probe. The molecular weights of PAA used were 1850, 4600 and 890,000, and those of PEO were 4800 and 9200. The latter are much lower than the PEO molecular weight in the work of Bednar et al. [19]. In order to monitor both intramolecular and infermolecular excimer formation, it was necessary to distinguish the two types of excimers clearly, so Oyama et al. used two kinds of solutions. One contained 99% of untagged PEO and only 1% PEO, where PEO refers to PEO whose chain ends are tagged by pyrene. In this solution, PEO is believed to behave Hke an individual PEO chain, which provides only intramolecular excimer. The other contained fully tagged PEO, which provides both intramolecular and intermolecular excimers. 

Frank et al. [29] studied the effect of hydrophobic interaction by comparing the fluorescent properties of PMAA/PEO and with those of PAA/PEO . Here PEO denotes pyrene end-labeled PEO. Figure 3 shows the intensity ratio le/Im of inframolecular excimer pyrene for PMAA/PEO (9200) and PAA/PEO (9200). It is seen that when added, PMAA more markedly reduces intramolecular excimer formation in PEO than does PAA. This difference is thought to be due to a stronger abihty of PMAA to combine PEO and the consequent suppression of intramolecular cychzation of PEO. 

How is the monomer fluorescence of aryl vinyl polymers or intramolecular excimer-forming compounds distinguished from that of monochromophoric compounds ... 

The first question cannot be answered directly even for a bichromophoric compound capable of forming intramolecular excimers. The simplest kinetic scheme in fluid solution involves the six rates kFM, kM, kFD, kD, khM, and k(,D. The latter two... 

In all the above studies 4-9-37 41> the 1,3-diarylpropanes had a reduced monomer quantum yield due to substantial intramolecular excimer formation, but the monomer fluorescence spectrum was unchanged from that of the analogous monochromophoric... 

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