Excimer forming

Masuko M, Ohtani H, Ebatal K, Shimadzu A (1998) Optimization of excimer-forming two-probe nucleic acid hybridization method with pyrene as a fluorophore. Nucleic Acids Res 26 5409-5416... 

Fig. 8.3. Excimer-forming bifluorophores for the study of fluidity. 1 ,co-di-(l-pyrenyl)-propane 2 a,co-di-(l-pyrenyl)-methylether 3 10,10 -diphenyl-bi s-9-anth ryl methyloxide (DIPHANT) 4 meso-2,4-di-(N-carbazolyl) pentane.

The method is based on the fact that the rate of conformational change required for excimer formation depends on the free volume induced by the segmental motions of the polymer occurring above the glass transition. DIPHANT (compound 3 in Figure 8.3) was used as an excimer-forming probe of three polymer samples consisting of polybutadiene, polyisoprene and poly(dimethylsiloxane).a)... 

This investigation shows that it is indeed possible to study the flexibility of polymer chains in polymer matrices by means of excimer-forming probes and that the rotational mobility of these probes reflect the glass transition relaxation phenomena of the polymer host matrix, in agreement with the appropriate WLF equation. 

Fig. 10.26. Excimer-forming cation sensors (E-1 Bouas-Laurent H. et al. (1986) J. Am. Chem. Soc. 108, 315. Marquis D. and Desvergne J.-P. (1994) Chem. Phys. Lett. 230, 131. E-2 Suzuki...

It seems to be natural to suspect if these excimer forming sites were the effective trapping center also for hole carriers. 

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. 

The diffusion-related molecular processes occurring within a Cig stationary-phase have also been investigated using pyrene as a fluorescent probe [169]. Particular spectral bands were attributed to pyrene excimers formed in a diffusion-limited reaction. Rate constants for this formation were then used to estimate the microviscosity of the stationary-phase. A similar application of total internal reflection fluorescence... 

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. 

Excimer-forming site (EFS) in ground singlet state... 

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

One method of directly determining QM and Q for excimer-forming compounds involves the relationship direct method requires a number of samples of an aryl vinyl polymer with differing molecular weights. If QM and Q are assumed to be independent of molecular weight, then they can again be obtained from the (pD vs. polymer samples. The rate can be determined if FM is assumed to be identical to that of the monochromophoric compound. It follows that kM = kFM/QM. While these methods are rarely used in the literature, it is worthwhile to review the assumptions that Qm and Q are independent of solvent, or of the molecular weight and structure of the compound. 

Phane compounds, i.e. two chromophores held face-to-face by at least two hydrocarbon links (4) Sandwich dimers, which are chromophore pairs produced by photolysis of photodimers in rigid matrices and (5) Bichromophoric compounds having a single saturated hydrocarbon linkage, which form intramolecular excimers as allowed by the rotational isomers of the linkage. In each case, we will utilize the intermolecular excimer formed in solution as the standard against which the properties of constrained excimers will be measured. 

It is clear that the sandwich-dimer studies discussed above apply to P1VN, not P2VN, since no photodimerization has been observed in bis(2-naphthyl)alkanes 10 and ethers 39). Nevertheless, the UV absorbance of naphthyl sandwich dimers, like that expected for [3.4] or [3.5] naphthalenophanes, differs from that of isolated molecules only for X > 325 nm. The same slight difference in UV absorbance probably occurs for excimer-forming sites. 

The conformational statistics of asymmetric vinyl chains such as P2VN are well-known 126). The rotational conformers of isotactic (meso) dyads are entirely different from those of syndiotactic (dl) dyads. Frank and Harrah132) have described each of the six distinct conformers for meso and dl dyads, using the t, g+ and g nomenclature of Flory 126). Excimer-forming sites (EFS) are found in the tt and g g+ meso states, and in the degenerate tg , g t dl state. Because the rotational conformers of compounds such as l,3-bis(2-naphthyl)propane do not match those of either the iso-or syndiotactic dyads of P2VN, the propane compounds make poor models of aryl vinyl polymers. However, the rate constants of fluorescence and decay of the intramolecular excimer in polymers can usually be determined from the propane compounds (but see the exceptional case of PVK and its models133)). 

To determine whether QD mid QD/QM for the intramolecular excimer differ from the corresponding inteimolecular values, the fluorescence behavior of excimer-forming bis(2-naphthyl) compounds has been collected in Table 9. The experimentally-measured excimer and monomer quantum yields [Pg.64]

For excimer-forming bichromophoric compounds in solution, the probability M of ultimate monomer decay in Eq. (1) is given by 37)... 

The analysis of fluorescence quenching in excimer-forming polymers embodied by Eqs. (7)—(9) contains a number of unresolved flaws. First, the decay schemes of P2VN U,19S) and other polymers 44 45 151,158,195) in solution require at least three decay terms to adequately interpret the fluorescence behavior over a range of temperatures. While additional work on the decay response of polymers as a function of [Q] is needed, the current quenching scheme is probably too simplistic. Second, an... 

Quenching rates derived for excimer-forming polymers are questionable, given that the decay kinetics of unquenched samples are uncertain. More reliable values of kQM can be obtained for non-excimeric polymers. 

The dynamic RIS model, which was proposed before to investigate the dynamics of local conformational transitions in polymers, is elaborated to formulate the increase in the number of excimer-forming sites through rotational sampling. Application of the model to the meso and racemic diads in PS confirms the fact that conformational mobility of the chain plays a major role in intramolecular exclmer formation. Comparison with experiments demonstrates that the decay of the monomer fluorescence in styrene dimers is predominantly governed by the process of conformational transitions. 

The dynamic RIS formalism is used to calculate the rate of first passage from non-excimer-forming conformations to excimer-formlng conformations in seven aromatic polyesters with different flexible spacers between the aromatic rings. The equilibrium chain statistics provides a good description of the relative excimer population for these polyesters, even at times where the dynamic contribution is significant. 

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