Ratio of excimer to monomer fluorescence intensities

Lateral Mobility(Fluidity) of Sulfonate A and B Micelles. The ratio of excimer to monomer fluorescence intensity of pyrene had previously been used to measure the fluidity of biological membranes (8). The ease of excimer formation was correlated with the fluidity of the membrane. The same principle may be applied to the measurement of fluidity in inverted micelles. To this end, we used three pyrene carboxylic acid probes of varying chain length PVA, PNA and... 

De Schryver and co-workers u> have confirmed Chandross result for the UV absorbance of l,3-bis(2-naphthyl)propane. Nishijima et al.12) have stated that the absorbance spectrum of meso- and dl-2,4-bis(2-naphthyl)pentane and of the compounds l,3-bis(2-naphthyl)A, where A = propane, butane, pentadecane, and 5-phenylpentane, is similar to the absorbance spectrum of 2-ethylnaphthalene. Finally, an unusual result has been obtained by De Schryver et al.13> for the compound bis(l-(2-naphthyl)ethyl)ether. The meso compound gave a lower value of ID/IM, the ratio of excimer to monomer fluorescence intensities, under excitation at 304 nm relative to excitation at 285 nm, while the dl compound had no such excitation dependence. The UV absorbance spectra of these compounds were not reported, however. 

The excimer fluorescence has been used to characterize the miscibility of the guest and host polymers [Xie et al., 1993]. Since the excimer forming site concentration depends on the extent of the guest polymer aggregation, the ratio of excimer to monomer fluorescence intensity, is related to blend miscibility. 

Aromatic hydrocarbons such as pyrene have also been employed as a luminescence probe of polarity and microviscosity in a variety of organized assemblies (109). Pyrene is a good excimer-forming probe due to the long lifetime of fluorescence and formation of excited-state dimers (excimers) at low concentration. Figure 9 shows an example pyrene luminescence spectrum. The ratio of excimer to monomer fluorescence intensity is often utilized as a measure of pyrene mobility and proximity. The vibronic fine structure of the pyrene monomer is sensitive... 

Effect of PS Molecular Weight. The fluorescence results for 5% PS/PVME blends are shown in Figure 5. The ratio of excimer to monomer fluorescence intensities is given as a function of temperature for monodisperse PS molecular weights ranging from 2,200 to 390,000. The PVME was a polydisperse sample having 47,000. 

Figure 6 Ratio of excimer to monomer fluorescence intensities plotted as a function of 1-Np content in poly(A/l-Np(x)) in aqueous solution. (From Ref. 25.)...

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. 

Configurational aspects are apparent in publications concerning the fluorescence characteristics of polymers of differing tacticity and of model compounds. It has been shown that the intensity ratio of excimer to monomer fluorescence is greater in fluid solutions of isotactic polystyrene [62,63] and poly(p-methyl-styrene) [64,65] relative to that of the atactic polymers. This phenomenon has been attributed in the case of poly(p-methy1styrene) to the existence of a lesser energy barrier to excimer formation in meso dyads compared to the racemic dyad [65]. Similar conclusions of direct relevance to excimer formation in polystyrene were made by Bokobza et al [55] in studies of intramolecular excimer formation in model compounds. 

It can be seen that the excimer to monomer fluorescence intensity ratios for the same molar ratio of probe to sulfonate are much smaller in the sulfonate A system than in the sulfonate B system. For both sulfonates A and B, the intensity ratio tends to Increase with the chain length of the carboxylic acid. The variation is distinctly established for sulfonate B micelles, but less so for sulfonate A micelles. The results indicate that the internal fluidity of the micelles decreases from the edge of the polar core to the continuous hydrocarbon medium the gradient is steeper for sulfonate B. 

Having established the existence of the excimer emission of NDI based polyurethanes in solution, and realizing that the intramolecular excimer forming naphthyl carbamate groups are located on the backbone of the polymer, it becomes apparent that an excellent opportunity exists for chain conformational studies as a function of solvent. Figure 10 shows the steady-state fluorescence spjectra of NDI-650 in four solvents with distinctively different solvating power. In each case (curves a-d) both monomer and excimer emission are observed however, tlie ratios of excimer to monomer emission reflect conformational differences of the NDI-650 polymer in the solvent employed. The excimer to monomer intensity ratio... 

Dependence of the ratio of relative intensity of excimer to monomer fluorescence, le/Ij,/ on the pH of aqueous solutions of PMA. 

From the temperature dependence of the excimer-to-monomer fluorescence intensity ratio I /I, for Py and IPy(3)IPy on reversed-phase Si-C. g, the activation energy E of surface diffusion was determined, using [2]. Values for E of 19 and 40 kJ/mol were found... 

Figure 4.14 Time dependence of the ratio of the intensity of excimer to monomer fluorescence emission associated to the release of pyrene from pyrene-loaded micelles of poly(methacrylic acid)-poly(styrene)-poly(methacrylic acid). Reproduced from Reference 87 with permission of the American Chemical Society.

For solutions of flexible chain polymers two sharp transitions at concentration values and c+ could be easily located in the log-log plot of the excimer to monomer fluorescence intensity ratio < vs the concentration c,... 

The experimental method to assess the phase behavior is to measure the ratio of excimer to monomer intensity, la/lm- Samples of the polymer blend are cast on sapphire disks at thicknesses of the order to 10-25 pm, and the fluorescence spectra are obtained. In a study of the excimer fluorescence of poly(2-vinyl naphthalene) (P2VN) in polystyrene, the monomer emission peak at 337 nm was compared with the excimer emission peak at 398 nm [334]. The ratio of the peaks versus polystyrene molecular weight is illustrated in Fig. 5.32 (a and b). Phase separation (at 0.3 wt% P2VN addition) was observed at PS Mn of 17,500. It was noted that the results show the onset of phase separation before any visual phase separation was observed. 

In contrast, the fluorescence spectra of the parent y-cyclodextrins (compounds y-CD1, y-CD2, y-CD3, y-CD4) exhibit both monomer and excimer bands in the absence of guests because the cavity is large enough to accommodate both fluorophores (Figure 10.38). The ratio of excimer and monomer bands changes upon guest inclusion. The ratio of the intensities of the monomer and excimer bands was used for detecting various cyclic alcohols and steroids (cyclohexanol, cyclo-dodecanol, i-borneol, 1-adamantanecarboxylic acid, cholic acid, deoxycholic acid and parent molecules, etc.). 

Fluorescent hydrophobes (naphthyl and pyrenyl groups) incorporated into the polysulfobetaines are a powerful tool for studying the formation of intra-and interpolymer aggregates in aqueous and aqueous salt solutions [85,229-231]. Intermacromolecular hydrophobic association is observed as an increase in the excimer emission relative to that of the monomer emission, where h/Iu is the ratio of intensities of excimer and monomer fluorescence which reflects the extent of inter/intra macromolecular interactions. Intramolecular micellization is easily monitored by the quenching efficiency of the thallium ions. The decrease of h/Iu reflects the breaking of the intra- and interchain associations in aqueous salt solutions, leading to chain expansion. 

Steady state absorption spectra and emission spectra were recorded on a Perkin-Elmer 552 UV-Vis and MPF-44B fluorescence spectrophotometer respectively. The ratio of Ig/I is the ratio of the intensity of excimer (A 480 nm) to monomer fluorescence (A 377 nm). The ratio of I3/I1 is the ratio of the intensity of the pyrene monomer fluorescence intensity of peak 3 (A 384 nm) to peak 1 (A 373 nm). 

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.

S-2, in which the spacer between the two boronic acids is flexible, has the additional capability of forming excimers. The 1 1 binding of a saccharide leads to an increase in the monomer fluorescence intensity. This increase has two origins the decrease in excimer formation, and the increase in fluorescence quantum yield resulting from suppression of the PET process. The 1 1 complex is formed at low saccharide concentrations, but increasing the concentration leads to the formation of the 1 2 complex, as revealed by the increase in the ratio of the intensities of the excimer band to the monomer band. The selectivity of S-2 was found to be similar to that of S-l. 

Dependence of the observed excimer to monomer intensity ratio on polystyrene concentration for PS/PVME blends cast from tetrahydrofuran (circles) and toluene (squares) with the fluorescence spectra measured before (open symbols) and after (filled symbols) annealing at 383 K for 10 hours,... 

Pyrene groups were attached to poly(ethylene glycol) (PEG) at both chain ends to allow pyrene excimer fluorescence to be used as a molecular probe of the complexation between PEG and poly(acrylic acid) (PAA). The excimer to monomer intensity ratio, was... 

Excimer fluorescence of pyrene attached to synthetic polymer chains can be used to study polymer conformation in solution and on particle surfaces. In this case, fluorescence spectroscopy involves the measurement of the emission intensity of monomer (/ , observed at 375 mn) and excimer (/ at 480 nm). The ratio of f to I is related to coiling/stretching behavior of a labeled polymer and we have called it the coiling index. In the absence of intermolecular interactions (the polymer concentration used is usually below this limit), a high value of IJI can be considered the result of a coiled conformation whereas a low value is associated with a stretched conformation. 

The bis-arylammonium chlorides 24 and 25 emit excimer fluorescence in dilute aqueous solutions. No efiect of the addition of a- or P-CD was observed on the fluorescence of 24, whereas the addition of y-CD increased the intensity ratio of the excimer to monomer. The addition of P-CD to 25 caused the disappearance of the excimer while, after the addition of y-CD, the monomer peak almost disappeared. The authors proposed that the inclusion of only one aryl moiety inhibits the excimer formation, whereas the larger y-cavity can host two biphenyl groups, which facilitates excimer formation. According to the authors, formation of the eclipsed conformers of 24 and 25, which alone can give excimer, occurs in the excited state [138]. 

---