Intramolecular excimer emission

The fluorescence spectra of the N-P -N were examined in order to obtain further information on the cavities experienced by the N-P -N molecules. At a loading level of 1 X 10-3 mol/g-film, the N-P -N show monomeric and weak intramolecular excimer emissions. Because the rate of translational diffusion of the naphthoate groups in the LDPE films is slow on the timescale of the decay of the excited singlet states [32,35,112-114], the excimers probably form from chro-mophores that are at or near excimerlike conformations prior to excitation. On the basis of the relative weakness of the intramolecular excimer emissions, it is pro-... 

Lariat crown ethers with two terminal pyrenyl sidearms connected to the same carbon 103 (/= 0, 1 m = 0-2 n = 0-2) or to two different carbon atoms 104 (m = 0-2 n 1,2) and 105 (m 0, 1) showed intramolecular excimer emission in the free state (Jt-Jt-stacking of the pyrene rings), whose intensity decreases with the increase of monomer emission intensity upon metal ion complexation <20020L2641, 2004JOC4403>. This response has been ascribed to the cooperative participation of one of the two sidearms in the complexation of the crown ring with the metal ion, which renders inoperative the Jt-Jt-stacking of aromatic rings. Most of these fluorophores show alkaline earth over alkali metal ion selectivities. 

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. 

Liu S-Y, He YB, ChanWHetal (2006) Cholic acid-based high sensitivity fluorescent sensor for a, )-dicarboxylate an intramolecular excimer emission quenched by complexation. Tetrahedron 62 11687-11696... 

Intramolecular excimer emission is also a valuable tool for the study of the dependence of the rate constant for a conformational transition kt on the properties of the solvent medium. This rate constant is related to the concept of the "internal viscosity" x] of a polymer chain which opposes the separation of the chain ends at a rate dh/dt in response to a force F so that dh/dt= F/ni Kuhn and Kuhn who originally introduced the concept s assumed that the internal viscosity was a function of the potential energy barriers characterizing hindered rotation around the bonds of the chain backbone it was, therefore, assumed to be an intrinsic property of the chain, independent of the solvent medium. More recently, it has been pointed out that the viscosity r of the medium must necessarily contribute to the resistance to conformational changes, so that Tii should be the sum of two contributions, one due to the height of the potential energy barriers, the other proportional to the viscosity of the solvent, i.e., rii= A+Bn. A similar reasoning would lead us to expect k to be of the form kt= kT/(A +B n) and studies of the dependence of... 

The non-cyclic ethers E-2 (Figure 10.26), with two pyrenes linked at both ends of the chain, show strong intramolecular excimer formation. Addition of alkaline earth metal ions leads to an increase in monomer emission at the expense of the excimer band. The helical structure of the 1 1 complexes is supported by NMR spectra. Thanks to the pseudocyclic structure, the stability constants of the complexes with Ca2+, Sr2+ and Ba2+ in acetonitrile are quite high (106-107 for n — 5), but the selectivity is poor as a consequence of the flexibility of the oxyethylene chain. 

PC3P is a sensitive probe for local viscosity measurement by forming an intramolecular excimer [35,36]. The extent of excimer emission depends upon the rate of conformational change of the chain linking the two... 

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. 

The excited state properties of bis-a-9-anthrylmethyl ethers 23 closely resemble those of l,3-di-9-anthrylpropane derivatives. The photoexcited parent compound 23a deactivates by fluorescence (0 = 0.03) from the locally excited state only, and it isomerizes by intramolecular 4ji+4ji cycloaddition with a quantum yield of 0.32 [66]. By contrast, excimer emission (see Table 4) does characterize the excited state properties of the 10,10 -diphenyl derivative 23b, which does not undergo intramolecular cycloaddition for steric reasons [66,67]. 

Exceptional fluorescence properties also characterize the ri.s-isomer 38e. Unsubstituted cis-l,2-di-9-anthrylethylene 38a and its monosubstituted derivatives such as 38b are nonfluorescent at room temperature. By contrast, cis-dianthrylethylene 38e does fluoresce with quantum yields of 0.0018, 0.0042, and 0.0064 in cyclohexane, dichloromethane, and acetonitrile, respectively. The emission is structureless (see Figure 18), and is associated with a solvent-independent Stokes shift of about 6000cm-1. As the molecular geometry of 38e is characterized by overlapping anthracene systems [80], the structureless emission may be attributable to an intramolecular excimer state. 

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)...

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