Intramolecular excimer fluorescence studies

In conclusion, the method of intramolecular excimer formation is rapid and convenient, but the above discussion has shown that great care is needed for a reliable interpretation of the experimental results. In some cases it has been demonstrated that the results in terms of equivalent microviscosity are consistent with those obtained by the fluorescence polarization method (described in Section 8.5), but this is not a general rule. Nevertheless, the relative changes in fluidity and local dynamics upon an external perturbation are less dependent on the probe, and useful applications to the study of temperature or pressure effects have been reported. 

Oyama et al. [23] utilized fluorescence spectroscopy to study how the molar masses of both PAA and PEO affect complexation. Actually, they used the exci-mer formation between the pyrene groups attached to the chain ends of PEO as a molecular probe. The molecular weights of PAA used were 1850, 4600 and 890,000, and those of PEO were 4800 and 9200. The latter are much lower than the PEO molecular weight in the work of Bednar et al. [19]. In order to monitor both intramolecular and infermolecular excimer formation, it was necessary to distinguish the two types of excimers clearly, so Oyama et al. used two kinds of solutions. One contained 99% of untagged PEO and only 1% PEO, where PEO refers to PEO whose chain ends are tagged by pyrene. In this solution, PEO is believed to behave Hke an individual PEO chain, which provides only intramolecular excimer. The other contained fully tagged PEO, which provides both intramolecular and intermolecular excimers. 

Frank et al. [29] studied the effect of hydrophobic interaction by comparing the fluorescent properties of PMAA/PEO and with those of PAA/PEO . Here PEO denotes pyrene end-labeled PEO. Figure 3 shows the intensity ratio le/Im of inframolecular excimer pyrene for PMAA/PEO (9200) and PAA/PEO (9200). It is seen that when added, PMAA more markedly reduces intramolecular excimer formation in PEO than does PAA. This difference is thought to be due to a stronger abihty of PMAA to combine PEO and the consequent suppression of intramolecular cychzation of PEO. 

In all the above studies 4-9-37 41> the 1,3-diarylpropanes had a reduced monomer quantum yield due to substantial intramolecular excimer formation, but the monomer fluorescence spectrum was unchanged from that of the analogous monochromophoric... 

Despite the technical problems in the latter film study, we conclude that there is no intramolecular excimer formation in the compounds of Richards et al.143, and probably little intermolecular excimer formation in the pure films. The absence of an effect of solvent power 25) on the possible excimer fluorescence of the R = CH3 polymer may not be significant, since little change in the coil dimensions would be expected for the short ( 300 backbone atoms) polymers 143> which were studied. Additional work is needed on the fluorescence of such polymers having higher molecular weights, different aryl substituents (R = 2-naphthyl, for example), and fewer adventitious impurities. 

The prospensity for the formation of bends in the flexible spacer in polyesters containing naphthyl units is examined by the study of the fluorescence of a series of diesters. The dependence of the degree of intramolecular excimer formation on the length of the aliphatic spacer, under circumstances where the dynamics of rotational isomerism in the flexible spacer is suppressed, is evaluated by extrapolation of the measurements to infinite viscosity n. The extrapolated results exhibit an odd-even effect, with the more intense excimer emission being observed when the number of methylene groups is odd. The odd-even effect is rationalized by an RIS analysis of the diesters. 

Steady-state fluorescence spectroscopy has also been used to study solvation processes in supercritical fluids. For example, Okada et al. (29) and Kajimoto and co-workers (30) studied intramolecular excited-state complexation (exciplex) and charge-transfer formation, respectively, in supercritical CHF3. In the latter studies, the observed spectral shift was more than expected based on the McRae theory (56,57), this was attributed to cluster formation. In other studies, Brennecke and Eckert (5,31,44,45) examined the fluorescence of pyrene in supercritical CO2, C2HSteady-state emission spectra were used to show density augmentation near the critical point. Additional studies investigated the formation of the pyrene excimer (i.e., the reaction of excited- and ground-state pyrene monomers to form the excited-state dimer). These authors concluded that the observance of the pyrene excimer in the supercritical fluid medium was a consequence of increased solute-solute interactions. 

The importance of steric effects shows up in the entropy of formation for excimers. Zachariasse et al. (1978) have studied the temperature dependence of intramolecular excimer fluorescence of 1,3-di(4-biphenyl)propane, and obtained a value of —64.8 J K mol- for —AS. The intramolecular excimer of 1,3-di( 1-naphthyOpropane has a —AS value of —41.8 J K- m-. The much higher AS value for the biphenyl system can be attributed to its non-planar aromatic framework which has to become planar for excimer formation. 

Intramolecular Excimer Fluorescence Studies in Polymers Carrying Aromatic Side Chains. Some years ago, it was shown that certain excited aromatic molecules may form a complex with a similar molecule in the ground state, which is characterized by a structureless emission band red-shifted relative to the emission spectrum of the monomer. The formation of such complexes, called "exclmers", requires the two chromophores to lie almost parallel to one another at a distance not exceeding about 3.5A° (11). Later, it was found that Intramolecular excimer formation is also possible. In a series of compounds of the type C5H (CH2)jiC H5, excimer fluorescence, with a maximum at 340nm, was observed only for n 3 -all the other compounds had emission spectra similar to toluene, with a maximum at about 280nm (12). Similar behavior was observed in polystyrene solutions, where the phenyl groups are also separated from one another by three carbon atoms (13). 

Chandross and Depster (16) studied the temperature dependence of the rate constant of excimer formation for l,3-bls(a-naphtyl) propane and for l,3-bls(3-naphtyl)-propane. They found an activation energy of 4 kcal/mol, comparable to the energies related to the rotation barrier in a methylene chain. Recently Avourls, Kordas, and El-Bayouml studied the time dependence of the intramolecular excimer fluorescence of 1,3-dlnaphtylpropane. They determined rate constants kg and k j at different viscosities (37). 

Intramolecular excimer emission, the polarization of fluorescence, nonradiative energy transfer and the use of medium-sensitive fluoropliores has been used to study the conformational mobility of polymers in dilute solution, the interpenetration of drain molecules, the association of polymers with each other or with small species and the cooperative transition of certain poly-carboxylic acids from a compact to an expanded state. 

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... 

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. 

Photolysis of [2,2]paracyclophane in glassy solvents at 77 K produces a species with two benzyl radicals linked by an ethylene bridge." From studies of excitation spectra and emission lifetime a broad band was observed, which is attributed to intramolecular excimer fluorescence of this radical pair. 

The results of intramolecular excimer fluorescence studies indicate that the helical polypeptide is a rigid enough molecular framework to hold chromophores in solution without large fluctuations for about 25 ns at 20°C and 40 ns at — 40°C. 

With the increased level of understanding of the cyclization dynamics as monitored by intramolecular excimer fluorescence, it is now possible to extend this probe to the study of systems more complex than dilute solutions. One such situation involves the structure and dynamics of macromolecular complexes formed between polymeric proton donors and acceptors in aqueous solution. For example, there has been widespread interest in the complexes formed between poly(ethylene glycol) and poly(acrylic acid) or poly(methacrylic acid) in aqueous solution (34, 35). A second, complicated morphological problem is to describe the configurational behavior of polymer chains adsorbed on colloidal particles. This research has relevance to the understanding of steric stabilization. One system of particular interest is the interaction of poly(ethylene glycol) and colloidal silica (36-40). 

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). 

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