Intramolecular excimer-monomer ratio

Fig. 6 Ratio of intramolecular excimer (/ ) and monomer (/) emission intensities (a) and energy of excimer emission (b), as function of n in a,co-di(l-pyrenyl) alkanes, Pyr-(CHa) -Pyr. (Reproduced with permission from Zacchariasse and Kuhnle, 1976)...

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... 

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. 

To distinguish the two types of excimers clearly a solution of 1% of PEG (both chain ends labeled by chromophores) and 99% of PEG (unlabeled) was also prepared, fixing the total polymer concentration to be 1x10 M. Under this condition, only intramolecular excimer is formed because the excimer to monomer Intensity ratio, I /I, was observed to be constant regardless of small variation of PEG (labeled) concentration in the mixture with unlabeled PEG. We assumed that this allowed the behavior of an individual PEG chain to be examined as discussed in the following section. Furthermore, fully labeled PEG chains in 1x10 M... 

Figure 2 Change in the intramolecular excimer to monomer intensity ratio with the addition of PAA at 303K.

Information on microviscosity is obtained by studying the excimer forming capabilities of suitable fluorescent probes. The excimer, which is a complex of a ground state and excited state monomer, has a characteristic emission frequency. The intramolecular excimer formation for example, of 1,3-dinaphthyl propane (DNP), is a sensitive function of the microviscosity of its neighborhood. This property, expressed as the ratio of the excimer and monomer yield (/e//m) for DNP, has been determined for dodecyl sulfonate solutions and its adsorbed layer for the various regions of the adsorption isotherm (Fig. 4.18) (Somasundaran et al., 1986). Comparing the ratios thus obtained to the /e//m values of DNP in mixtures of ethanol and glycerol of known viscosities, a microviscosity value of 90 to 120 cPs is obtained for the adsorbed layer in contrast to a value of 8 cPs for micelles. The constancy of microviscosity as reported by DNP is indicative of the existence of a condensed surfactant assembly (solloids) that holds the probe. 

Figure 1. Intramolecular excimer to monomer emission intensity ratio as a function of the molar ratio of poly carboxylic acid) to PEG for Py-PEG-Py of weight-average molecular weights 4800 and 9200. All data are nomwlized by the Id/Im value for a 1% pyrene-tagged PEG solution with no polyacid added.

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). 

Intramolecular excimers are formed in bichromophores (equation 2) and the ratio of monomers over excimer fluorescence is controll by the conformational mobility of the chain linking the two chromophores. 

Rollins et al. investigated the intramolecular excimer formation of 1,3-di(2-naphthyl)propane in supercritical CO2 (172) and compared the results with intermolecular pyrene-excimer formation recorded under similar conditions (46). Their results show that the ratio of excimer emission to monomer emission decreases gradually with increasing CO2 density (Figure 23), in a pattern that agrees well with that predicted from viscosity changes in terms of the classical photophysical model for excimer formation (35). In a comparison of l,3-di(2-naphthyl)propane and pyrene in the same fluid, the ratio of excimer emission to... 

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. 

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. 

To establish the nature of the excimer formation as either intramolecular or intermolecular, an extremely dilute solution (1 X 10 g/dL) of NDI-650 was prepared. Even in this ultra-dilute solution excimer emission was present. Figure 7 shows a plot of the excimer (430 nm) to monomer (350 nm) intensity ratio as a function of polymer concentration. Not until a concentration of 1.0 g/dL is reached does intermolecular excimer formation between naphthyl carbamate groves on different polymer backbones become important. 

The essential feature of the intramolecular cyclization of short polymer chains containing terminal pyrene groups is that the process is difiiision controlled at room temperature. As such, the cyclization process is expected to obey an Einstein relation with both the rate constant for cyclization and the ratio of excimer to monomer emission intensities proportional to... 

Excimer to Monomer Intensity Ratio. Our approach toward the determination of this information is to focus on the configurational behavior of the pyrene-terminated PEG. Two sets of experiments were performed to separate the intramolecular behavior of the isolated Py-PEG-Py chains from the intermolecular interactions that occur between labeled PEG chains. First, we performed photostationary fluorescence measurements of IJI for fully labeled PEG as a function of the molar ratio of the poly(carboxylic acid) to that of the PEG. These results will contain both intermolecular and intramolecular contributions to the excimer fluorescence. Next, we performed analogous measurements with the same total amount of PEG but with only 1% having pyrene labels. The selection of the relative amount of the tagged polymer is somewhat arbitrary, but it appears to be sufficiently low to provide a reasonable approximation to the behavior of the isolated, labeled PEG chain. These fluorescence results are then subtracted from those for the fully tagged system, leaving just the intermolecular contribution. 

Figure 5. Excimer to monomer intensity ratio relative to that in free solution for Py-PEG-Py 4250) (+) and Py-PEG-Py 8650) (A) as a function of the number of colloidal silica particles available per chain on average. All initial solutions were in the dilute (intramolecular) regime before the addition of...

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