Excimer: Examples

Since the special structure of the excimer requires very close approach of two chromophores, excimer emission can be used to study close approach of two parts of a polymeric system.

Excimers in small molecules

Intramolecular

• The classic experimental work on intramolecular excimer formation was performed over 30 years ago, (Hirayama, F. "Intramolecular Excimer Formation. I. Diphenyl and Triphenyl Alkanes" J. Chem. Phys. 1965 42, 3163-3171). It called attention to the importance of 1,3 attachment of two chromophores, as in 1,3-diarylpropane and 2,4-diarylpentane.

• Measurements with dilute solutions of 2,4-diphenylpentane show two bands, the excimer band (340 nm) and the monomer band (280 nm). Tacticity matters. Excimer emission is stronger from meso 2,4-diphenylpentane than from racemo-2,4-diphenylpentane. The tt conformation of the meso stereoisomer is the dominant excimer; it places both aromatic rings very close to one another. The excimer band is more sensitive to quenching by oxygen than is the monomer band. This class of molecules is reviewed in De Schryver, F. C.; Collart, P.; Vandendriessche, J.; Goedeweeck, R.; Swinnin, A.; Van der Auweraer, M. "Intramolecular Excimer Formation in Bichromophoric Molecules Linked by a Short Flexible Chain" Acct. Chem. Res. 1987, 20, 159-166.

• The ratio of the intensity of excimer and monomer emission depends on the local viscosity of the medium. Since the excimer is not a stable structure in the ground state, it must be formed by a conformational change after excitation has occurred (but before the excitation is lost as normal, monomer fluorescence). If the medium is too viscous, the necessary conformational change cannot occur during the lifetime of the electronic excited state. Small molecules have been used as probes of the local mobility in polymers, by dispersing them at very low concentration in the polymer, and then monitoring their fluorescence. See, for example, Jing, D. P.; Bokobza, L.; Monnerie, L.; Collart, P.; De Schryver, F. C. "Investigation of local molecular motions in bulk polymers through the emission properties of an intramolecular excimer-forming probe" Polymer 1990, 31, 110-114.

Intermolecular

• Solutions of pyrene exhibit fluorescence emission spectra with a shape that is highly dependent on concentration. In dilute solution, the emission (near 400 nm) is from isolated molecules of pyrene (monomer emission). Another band, red-shifted (to 500 nm) from the monomer emission, appears at higher concentration. In sufficiently concentrated system, the excimer band is dominant. See G Fig. 7.3

• Pyrene has been used as a probe to monitor rates of polymerization, by taking advantage of the fact that the viscosity increases as polymerization occurs, and the ease of the formation of excimers by pyrene depends on the viscosity of the medium. For an application to poly(methyl methacrylate, see: Valdes-Aguilera, O.; Pathak, C. P.; Neckers, D. C. "Pyrene as a Fluorescent Probe for Monitoring Polymerization Rates" Macromolecules 1990, 23, 689-692.

Excimers in polymers

Intramolecular

• The excimer fluoresence of dilute solutions of aromatic vinyl polymers depends on their stereochemical composition. The ratio of the intensities of excimer and monomer fluorescence, I_E/I_M is larger in dilute solutions of isotactic polystyrene than in similar solutions of atactic polystyrene.

• In aromatic polyesters, the intramolecular excimers are dominated by interaction of nearest neighbor chromophores in conformations that require a tight bend of the spacer between them. Therefore measurement of I_E/I_M in dilute solution from a family of polyesters with flexible spacers of different lengths gives information about the effect of the spacer length on the ability of the molecule to make a tight bend. Polyesters with terephthalate or isophthalate separated by short -(CH₂)_x- spacers show a strong odd-even effect, with I_E/I_M being larger when x is odd. See Mendicuti, F.; Patel, B.; Waldeck, D. H.; Mattice, W. L. "Intramolecular excimer formation by phthaloyl, isophthaloyl, and terephthaloyl groups in polyesters with different numbers of methylene and oxyethylene spacer" Polymer 1989, 30, 1680-1684.

• Sometimes chromophores are intentionally attached to two specific sites on a chain so that fluorescence can be used to determine how frequently these two sites come into contact. For example, pyrene has been convalently attached to the two ends of polystyrene chains of different molecular weight. The intensity of pyrene excimer emission in dilute solution is then used to learn about the close approach of one end of the chain to its other end, a process called macrocyclization. This approach has been used to study the conformations of end-capped poly(ethylene oxide, Duhamel, J.; Yekta, A.; Hu, Y. Z.; Winnik, M. A. "Evidence for Intramolecular Hydrophobic Association in Aqueous Solution for Pyrene-End-Capped Poly(ethylene oxide)" Macromolecules 1992, 25, 7024-7030.

• Poly(vinyl naphthalene) exhibits fluorescence from the naphthalene units, which can be either monomer emission or excimer emission. When this polymer is mixed, at low concentration, with a variety of methacrylate polymers, I_E/I_M from the poly(vinyl naphthalene) can vary by an order of magnitude, depending on the structure of the methacrylate. When I_E/I_M is plotted against the solubility parameter for the host methacryalate polymer, this ratio passes through a pronounced minimum when the solubility parameter of the host matches that of the poly(vinyl naphthalene). Presumably this effect arises from changes in the expansion and contraction of the poly(vinyl naphthalene) chain in the various matrices, with the expansion being greatest in the host with solubility parameter matching that of the poly(vinyl naphthalene). The expanded polymer cannot form intramolecular excimers as easily as the contracted conformations of the same polymer. Hence I_E/I_M is related to the miscibility, on a very intimite scale, of the two components of the blends. See Gashgari, M. A.; Frank, C. W. "Excimer Fluorescence as a Molecular Probe of Blend Miscibility. 4. Effect of Temperature in Solvent Casting" Macromolecules 1981, 14, 1558-1567 and Tao, W. C.; Thomas, J. W.; Frank, C. W. "Excimer fluorescence as a molecular probe of polymer blend miscibility: 8. Polymeric and glassy solvent host matrices" Polymer 1988, 29, 1625-1634.

Intermolecular

• Chains labelled with pyrene have been used to study phase separation of polymers in aqueous solution: Winnik, F. M. "Fluorescence Studies of Aqueous Solutions of Poly(N-isopropylacrylamde) below and above Their LCST" Macromolecules 1990, 23, 233-242.

• End-labelled chains have been used as a model for the termination process in free-radical polymerization: Strukelj, M.; Martinho, J. M. G.; Winnik, M. A.; Quirk, R. P. "Intermolecular Excimer Formation for Pyrene-End-Capped Polystyrene: A Model for the Termination Process in Free-Radical Polymerization" Macromolecules 1991, 24, 2488-2492.

• Excimer emission is exploited in polymers used for the construction of light-emitting devices.

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