Ratio of excimer to monomer fluorescence

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. 

Analysis of this scteme ows that for steady-state excitation, the Stern-Volmer relationship (62) and quantum yield ratio of excimer to monomer fluorescence (63) are obtained. 

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. 

Description of the excimer photophysics for a two-phase system presented by Wandelt and co-workers [66] are based on both the two-phase model assumptions and the experimental results. The two-phase model describes the results of the experimental studies of photoenergy migration in heterogeneous solid-state polymer blends by Frank and collaborators [68, 69]. Tao and Frank [69] used three-dimensional electronic excitation transport to interpret the ratio of excimer to monomer fluorescence for poly-2-vinylnaphthalene with polycyclohexyl methacrylate. The assumptions of the two-phase model are ... 

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... 

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. 

P2VN). The treatment of the polymer photophysics will be phenomenological it will only be necessary to assume that the experimentally observed ratio of excimer to monomer fluorescence is proportional to the local density of aromatic rings in the blend. More sophisticated treatments that incorporate a detailed consideration of energy migration will be examined in later chapters. 

Vala [11] observed that the ratio of excimer to monomer fluorescence was independent of PS concentration in dilute solution but Increased after the PS content reached a certain value. Similar observations have been made in other studies [20-22]. The conclusion drawn from these experiments is that excimer formation may be intermolecular as... 

In addition, there have been a number of studies of the steady state fluorescence properties of styrene random copolymers in dilute solution as a function of copol3nner composition [57-60] In these studies, the conclusion that energy migration is important in PS has generally been reached based on correlations between the ratio of excimer-to-monomer fluorescence and various parameters that describe copolymer composition ... 

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. 

If and are the probabilities of eventual monomer decay given that the photon is absorbed by the rich or lean phase, respectively, then the ratio of excimer to monomer fluorescence is given by a simple weighted average of fluorescence contributions from the rich and lean phases ... 

Alternatively, other researchers have used the tendency of pyrene to form excimers to study its CD inclusion complexes [74,81]. In fact, pyrene in y-CD was one of the first systems in which 2 2 complexation was reported to occur this results in a very different broad, red-shifted excimer fluorescence, and the ratio of excimer to monomer fluorescence as a function of y-CD concentration has been used to determine the binding constants. 

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. 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

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. 

Picosecond fluorescence studies were applied by Winnik and co-workers [72] for studies of temperature-induced phase transition of pyrene-labelled hydroxypropylcellulose (HPC-Py) in water. Temperature dependence of the fluorescence emission ratio of excimer to monomer emission (Ie/Im) showed a significant increase of excimer emission in a temperature range 283-313 K, then a decrease to a constant value at 319 K. Two excimer bands were observed when time-resolved spectroscopy was used i) a broad, structureless band with a maximum at 420 nm and a corresponding lifetime of 250 ps and ii) the well-known band of pyrene excimer, with a maximum at 470 nm and a lifetime of 68 ns. In the initial time region, 0-150 ps, monomer emission was observed, with a simulation by a superposition of three components (377, 398 and 421 nm). They observed only one excimer emission above the LCST and that was with a maximum at 470 nm. They concluded that the LCST implies a complete disruption of the ordered microstructures, which were created in cold water. 

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.

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.). 

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. 

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. 

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... 

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,... 

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.

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