Naphthalene excimer emission

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

Since Forster s original work, a large number of aromatic compounds, including benzene, naphthalene, and anthracene, have been found to have concentration-dependent fluorescence spectra under some conditions. Most of these excimers are not as stable a.s the pyrene prototype, and require lower temperatures or higher concentrations to be observed.29 Some crystals also exhibit excimer emission. Crystalline pyrene, for example, has only a single structureless fluorescence band of the same energy as its excimer emission in solution.30... 

Ueno et al. found that two naphthalene derivatives can be included in the large cavity of y-CD [29], This result was obtained by observing enhanced excimer emission in the solution of y-CD and 2-naphthaleneacetic acid. On this basis, y-CD with one appending naphthalene moiety was shown to form a complex in... 

As very different systems, Ueno et al. prepared a-helix peptides bearing y-cyclodextrin and one (16,17) or two (18) naphthalene units in their side chains (Fig. 2) [38], They are alanine-based peptides composed of 17 amino acid residues. The peptides 16 and 17, which have y-CD and one naphthalene unit, display a simple monomer emission and the fluorescence intensity slightly decreases with increasing guest concentration. On the other hand, the peptide 18, which has y-CD and two naphthalene units on both sides of the y-CD unit, exhibits considerable excimer emission in addition to the predominant monomer emission and the excimer emission decreases with increasing guest concentration. This indicates that 18 excludes the two naphthalene units from the y-CD cavity to the opposite directions upon guest accommodation. 

The CD architecture is particularly fitting for excimer-based reporter sites. As long as the bucket is sufficiently large to accept two guests, monomers will cofacially organize within the interior of a CD to form excimer. Early spectroscopic studies revealed an enhanced excimer emission upon the addition of y-CD to solutions of naphthalene [361], a result that was subsequently observed for anthracene [362] and pyrene [363,364], Pyrene excimer formation has been examined in particular detail. Proper concentrations of pyrene and y-CD afford 2 1 or 2 2 excimer inclusion complexes. In both cases, pyrenes interact in a cofacial manner and excimer efficiently forms. Alcohol addition can disrupt the excimer by the preference to form a 1 1 1 complex of alcohol, pyrene, and CD. 

Another approach to studying conformational effects upon excimer formation has been to synthesise cyclophanes. The rigidity of the structure of these compounds enables one to probe very precisely the effect of the relative orientation of the two groups upon excimer emission. Thus the [3,3]-cyclophanes [18] and [19] (in which, for clarity, the unsaturation of the naphthalene rings is not shown) interconvert, and each has its own photo-chemistry (Kawabata... 

The series of copolymers of 1-vinyl naphthalene with methyl methacrylate used in a recent study is shown in Table 3 ete the fonctions are the mole fraction of naphthalene chromophores in the copolymer, fn the fraction of linkages between naphthalene species in the polymer chain, f n, and the mean sequence length of aromatic species, 1 . Fluorescence decays in the monomer and excimer emission... 

The fluorescence of the phenyl polymer is similar in shape to the fluorescence from the alkyl polymers and the similar shape of the phosphorescence spectrum, as well, suggests that the origins of the electronic spectrum are also much the same. The apparent increased quantum yield for phosphorescence in poly(phenyl methyl silylene) probably reflects a mixing of the ring electronic levels with the levels of the chain. Both the fluorescence and phosphorescence of the naphthyl derivative are substantially altered relative to the phenyl polymer. Fluorescence resembles that of poly(B Vinyl naphthalene) (17,29) which is attributed to excimer emission. Phosphorescence is similar to naphthalene itself. These observations suggest that the replacement of an alkyl with phenyl moiety does not change the basic nature of the electronic state but may incorporate some ir character. Upon a naphthyl substitution both the fluorescence and phosphorescence become primarily tt-tt like. 

Radio, chemi and thermoluminescence studies on polymers continues to attract much interest as an analytical probe. The t-irradiation of pyrene and naphthalene doped polyethylene gives excimer emission in the range of the high temperature radiothermoluminescence peak whereas other workers have used the technique to establish changes in the glass transition of the polymer 40 particular feature of radiothermoluminescence is... 

The addition of -CD to an aqueous naphthalene solution caused the growth of its molecular fluorescence and the appearance of excimer emission [130]. By lowering the temperature of the solution, the excimer intensity grew at the expense of that of the monomer. The excimer emission was attributed to the association of 1 1 complexes to give 2 2 / -CD-naphthalene inclusion compounds. In air-saturated solutions, the three species have the following lifetimes 40 ns (free naphthalene), 48 ns (1 1 complex), and 68 ns (2 2 complex). The quenching rate constants derived from these lifetimes by the addition of I" were 6x 10 dm mol s , 3.9x 10 dm mol s , and 1.8 x 10 dm mol s , respectively, which confirmed the protection furnished by the cavity to the included molecules. 

The fluorescence of 61 in HjO/ethylene glycol (90/10) showed both monomer- and excimer-like components [212] the relative intensity of excimer emission decreased with increasing [glycol] and almost pure monomer fluorescence was observed in pure ethylene glycol. On the basis of the emission characteristics and of the pattern of ICD signals, which indicate the inclusion of two naphthalene groups in the y-CD cavity, it was proposed that two different structures are in equilibrium, as shown in Scheme 8. Form I should be present in pure ethylene glycol, while forms II and III should be present in water-rich solutions. 

If the guest molecules are prone to form excimers, occurrence of the corresponding emission band in the presence of CyDs can be a source of information about the complex stoichiometry. The naphthalene/)9-CyD system gives a good example [21, 31], Figure 10.3.2 shows the fluorescence spectra of naphthalene in water and with added 0.01-M j8-CyD the band at 410 nm has been assigned to the naphthalene excimer in a 2 2 guest host complex. 

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