Excimer-monomer ratio

Table 1 Excimer/monomer ratio for extended nanostar 27 [48,49]...

Exitation wavelength, nm Dendrimer generation Excimer/ monomer ratio... 

Aqueous solutions of polymethylene-bis-/ -naphthoates of the general formula shown in 31 (Fig. 7), showed excimer emission owing to the addition of 5 x 10" M y-CD whatever the value of n. On addition of p-CD, both monomer and excimer fluorescence appeared, the excimer prevailing for compounds with n = 3 and 4. The y-CD complexes afforded better protection against Oj than P-CD complexes. An increase of P and y-CD concentration decreased the excimer/monomer ratio. On the basis of these results, the formation of two types of complexes, shown in Figure 7, was proposed. 

Based on the interplay between the complexing ability of crown-ethers or cry tands and the dual fluorescence (monomer/excimer) of aromatic rings (especially anthracene), new supramolecular receptors were designed and synthesized. Modulation of the fluorescrace (wavelengA, quantum yield, excimer/monomer ratio) of different photoactive systems is observed as a specific response to the complexation of metal (Na ", K" ", Ag" ", Tl", Rb" ") or molecular Ql3N -(CH2)n- NH3) cations. These properties could be used for light frequency conversion (anthraceno-coronand), detection and estimation of cations at low concentration (craterophanes, tonnelet) and evaluation of the solvent polarity (semaphorene). 

Table 1. Excimer/monomer ratios for extended nanostar 14. ...

The fluorescence properties of pyrene and its derivatives depend on the microscopic concentration of the fluorophore in the lipoprotein, thus allowing both continuous monitoring of the kinetics of transfer and quantification of changes in mass, as well as obviating physical separation of reactants and products. The excimer/monomer ratio is linear with concentration (Pownall and Smith, 1973) and decreases with probe transfer from a donor to an acceptor lipoprotein (Fig. 16). 

Fig. 19 Excimer to monomer ratio, le/Im of pyrene-labeled poly(acrylic acid), py-PAA, as a function of DTAB concentration in 1 g/L sodium polyacrylate and 0.03 M NaCl...

For this study, we maintained the pyrene probe concentrations constant, while we varied sulfonate concentration. The measured excimer to monomer ratios as a function of the molar ratio of probe to sulfonate for sulfonates A and B are shown in Figure 3. 

Excimer Emission and CPF Spectra Fluorescence spectra of the two polypeptides in TMP solution are shown in Figure 7 (lower curves). Small excimer emissions are observed in the two polymers. The monomer/excimer intensity ratio was independent of the polymer concentration at least down to [pyr] = 1x10 mol L, suggesting an intramolecular character of the excimer. Since the interchromo-phore distances in the most probable conformations predicted from the conformational energy calculation are much longer than the exci-mer-forming distance, the excimers should be formed at the point where conformations of the main chain and/or the side chain are largely distorted. 

Fig. 4.22. Coiling presented in terms of excimer to monomer ratio, le/Im, for alumina with 20 ppm polyacrylic acid as a function of final pH under shifted pH conditions.

The fluidity in the neighborhood of probe molecules can be tested by use of probes capable of intramolecular excimer formation. The probe molecules contain the two excimer-forming moieties linked by an alkyl chain. The extent of excimer formation depends on the viscosity of the environment and can be monitored by measuring the excimer/monomer fluorescence intensity ratio. The dependence of this ratio on reciprocal viscosity for the probe molecule dipyrenylpropane is shown in Fig. 18, in which the obtained microfluidities for surfactant systems are indicated. The fluidities decrease in the order SHS microemulsion, SDS, CTAC, Triton X-100 cf. Ref. 167 (for abbreviations see Tables 6 and 7). The same sequence order was found by Kano et al. (68). In systems containing heavy counterions the method leads to data that must be evaluated carefully, since heavy atom interactions may be different with excited monomers and excimers. The intramolecular excimer technique is also useful in biological studies. For instance, Almeida et al. investigated the sarcoplasmic reticulum membrane in which the activity of the Ca -pumping enzyme is modulated by the membrane fluidity (197). 

Figure 18. Excimer/monomer fluorescence intensity ratio 77/ at 30°C of l,3-di(l-pyrenyl)propane [Py(3)Py] as a function of the reciprocal viscosity for a series of hexadecane-liquid paraffin mixtures (curve). The 77/ ratios obtained with Py(3)Py in aqueous micellar solutions of sodium dodecyl sulfate (SDS, 0.1 Af), cetyltrimethyl-ammonium chloride (CTAC, O.OS Af), and Triton X-100 (0.004 Af), and in the microemulsion SHS(M) (see Table 7) are indicated. The fluidities determined were 3.1 mPa s (SHS(M)], 11 mPa s [SDS] 37 mPa s [CTAC], and 105 mPa s [Triton X-100]. Redrawn after Ref. 167.

In many instances, it is acceptable to have an observable which reflects the local viscosity of the membrane. The excimeF-to-mon( ner ratios have been widely used for this purpose. Emission spectra of pyrene and its excimer were shown in Hgure 1.11. The ratio of excimer to monomer emission increases with increasing pyrene concentration and with increasing rates of diffusion. Typical data are shown in Figure 9.20 for rene in DMPC vesicles. As the temperature is increased, the excimer-to>monomer ratio increases. The presence of cholesterol results in a lower ratio, reflecting slower pyrene diffusion. By compuisonof... 

If lifetime measurements are availaUe, it is not necessary to use the excimer-to-monomer ratios. TMs can be seen by considering the monoiner lifetime at low concentrations, where excimer formation does not occur (Tqm). and in the presence of excimer formation (Tm),... 

Polypeptide-grafted cyclodextrin bioconjugates Fluorescence 1 1 and 1 2 Excimer/ monomer emission bands ratio nd" 2012 [104]... 

If a Stokes law expression is assumed for k, the viscosit dependence of the excimer to monomer ratio should be R h, This is consistent with photostationary fluorescence measurements on P2VN in toluene under hydrostatic pressure yielding an empirical expression of the form... 

The fluorescent probe methods were used to determine miCToviscosity (T ,) of potassium A-acylalaninates and potassium A-acylvalinates micelles. The results obtained show that 11 on the micellar surface is larger in A-acylalaninates than in 7V-acylvalinates, whereas in the micellar core remains same in the micelles of the two surfactant series. The values of 11 of SDS, lithium dodecyl sulfate (LDS), CTACl, and CjjEg, determined at 15°C by monomer/excimer intensity ratio and excimer lifetime of dipyrenylpropane dissolved in micelles, are 19, 19, 39, and 57 cP, respectively. The fluorescent probe technique was used to determine r m values of SDS, CTABr, and CTACl micelles at different applied pressure. The derived values of SDS, CTABr, and CTACl micelles at 25°C and atmospheric pressure are 12,47, and 27 cP, respectively. The addition... 

Fluorescence Measurement Fluorescence spectra were measured on a Spex Fluorolog 212 spectrofluorometer equipped with a 450 W xenon arc lamp and a Spex DM1B data acquisition station. Spectra were recorded in the front-face illumination mode using 343 nm as the excitation wavelength. Single scans were performed using a slit width of 1.0 mm. PDA fluorescence emission spectra were recorded from 360 to 600 nm, with the monomer and excimer fluorescence measured at 376.5 and 485 nm, respectively. Monomer and excimer peak heights were used in the calculation of the ratio of excimer to monomer emission intensities (Ie/Im). Excitation spectra were recorded from 300 nm to 360 nm and monitored at 376.5 and 500 nm for the monomer and excimer excitation, respectively. 

Figure 6. Ratio of excimer (Ie) to monomer (Im) fluorescence intensities of 5 mol% PDA in monolayer LB PMMA film as a function of transfer pressure.

The steady-state fluorescence intensities are obtained by integration of Eqs (4.43) and (4.44). The ratio of the fluorescence intensities of the excimer and monomer bands, Ie/Im (Figure 4.6), is often used to characterize the efficiency of excimer formation. This ratio is given by... 

The method using intermolecular excimer formation is based on the same principle because this process is also diffusion-controlled. Excimers should, of course, be formed during the monomer excited-state lifetime. In Section 4.4.1, it was shown that the ratio Ie/Im of the intensities of the excimer and monomer bands is proportional to ki provided that the transient term can be neglected. When the dissociation rate of the excimer is slow with respect to de-excitation, the relationship is... 

Figure B8.2.1 shows the fluorescence spectra of DIPHANT in a polybutadiene matrix. The h/lu ratios turned out to be significantly lower than in solution, which means that the internal rotation of the probe is restricted in such a relatively rigid polymer matrix. The fluorescence intensity of the monomer is approximately constant at temperatures ranging from —100 to —20 °C, which indicates that the probe motions are hindered, and then decreases with a concomitant increase in the excimer fluorescence. The onset of probe mobility, detected by the start of the decrease in the monomer intensity and lifetime occurs at about —20 °C, i.e. well above the low-frequency static reference temperature Tg (glass transition temperature) of the polybutadiene sample, which is —91 °C (measured at 1 Hz). This temperature shift shows the strong dependence of the apparent polymer flexibility on the characteristic frequency of the experimental technique. This frequency is the reciprocal of the monomer excited-state...

When a fluoroionophore contains two fluorophores whose mutual distance is affected by cation complexation, recognition of this cation can be monitored by the monomer/excimer fluorescence-intensity ratio (see Chapter 4, Section 4.4.1 for ex-rimer formation). Cation binding may favor or hinder exrimer formation. In any case, such a ratiometric method allowing self-calibration measurement is of great interest for practical applications. 

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