Intramolecular excimer fluorescence

Box 8.2 Intramolecular excimer fluorescence for probing the mobility of bulk polymers... 

Georgescauld D., Desmasez J. P., Lapouyade R., Babeau A., Richard H. and Winnik M. (1980) Intramolecular Excimer Fluorescence A New Probe of Phase Transitions in Synthetic Phospholipid Membranes, Photochem. Photobiol. 31, 539-545. 

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). 

Ve have described three fluorescence techniques for monitoring the cure of epoxy resins. The first one is based on intramolecular excimer fluorescence, the second one is based on the enhancement of fluorescence intensity with the medium viscosity, and the third one is based on the measurement of the translational diffusion coefficient of a fluorescent probe. Finally, we have demonstrated the fluorescence monitoring of the formation of a polyimide polymer. 

Di(1-pyieny1)propane. Bauer et al. (32) observed an excimer-like emission in the fluorescence spectrum of 1Py(3)1Py on dry silica, which was attributed to intramolecular interactions in the ground state. The excimer emission disappeared on adsorption of 1-decanol to the surface. In contrast, strong intramolecular excimer fluorescence was found by Avnir et al. for 1Py(3)1Py adsorbed on a silica surface with up to a double-layer equivalent of 1-octanol (38), see Section 4.3.2. Excimer formation was also detected on reversed-phase Si-C g and on untreated silica, the ratio I /I depending on the amount of 1Py(3)1Py adsorbed (38), see Fig. 3. 

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. 

A similar situation occurs in the interaction of a-, p-, and y-CD with diphenylphosphate, 33. In HjO, 33 has two emission bands peaking at 286 and 330 nm. The band at 330 nm is assigned to intramolecular excimer fluorescence. Addition of a- and p-CD to the 33 solution increased the monomer and decreased the excimer band the opposite phenomenon... 

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). 

Intramolecular excimer fluorescence involves a conformational transition furing the lifetime of an excited chromophore and it can, therefore, used to study conformational transitions on the timescale of 10 ° sec. We have used this technique to study the behavior of copolymers and to define the dependence of the rate of conformational transitions on the viscosity of the medium. 

The objective of this section is to demonstrate that intramolecular excimer fluorescence may be used in conjunction with rotational isomeric state theory to characterize aspects of the conformational structure of polystyrene. In Section 2.1 data on polystyrene and model compounds are analyzed to show the geometrical requirements for an excimer forming site (EFS) and to clarify the dissociation behavior. Rotational isomeric state theory is briefly described in Section 2.2 and then employed in Section 2.3 to calculate the EFS population for the isolated PS chain. 

Nishikawa, H. Tabata, T. Kitani, S. Simple detection method of biogenic amines in decomposed fish by intramolecular excimer fluorescence. Food Nutr. Set 2012,3, 1020-1026. 

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