Naphthalene excimer

The appearance of the naphthalene excimer band in the thermoluminescence130 or infrared stimulated luminescence8 of irradiated naphthalene-saturated glasses is therefore attributed to the recombination of this dimer cation with released electrons... 

The literature review on excimers will be directed along the two rather interdependent lines of structure and properties. In this section, the structure and symmetry of naphthalene excimers will be considered, and the number of intramolecular EFS... 

The theoretical approaches taken to calculate the binding energy of the excimer have been reviewed 68-70). Most authors have assumed a sandwich structure for the excimer in which the ring planes are parallel and the molecular axes are aligned. By matching the calculated and experimental values of the excimer fluorescence peak, the interplanar distance of the excimer can be computed. All such calculations yield values of the interplanar distance which are 0.2-0.5 A less than the ground-state van der Waals ring separation. For the naphthalene excimer, an interplanar distance of 3.3 0.3 A has been computed. 

The results of the studies (a) - (c) described above are not in agreement with the existence of triplet state excimers of naphthalene. It is concluded that previous reports in the literature(25-29) are erroneous and that it is likely that some other type of complex such as an "oxyplex" is responsible for the observed behaviour and would be consistent with the fact that Pratte et al(30) did not observe triplet naphthalene excimers in bichromophoric systems under highly degassed fluid solutions. 

Figure 2 raises the question of how the electronic interaction depends on the orientation. For instance, three different orientations are shown for the naphthalene excimer (Figure 2a). In the amine-anthracene exciplex (Figure 2b), the nitrogen can be located over one of the outer six-membered rings or over the inner six-membered ring of anthracene. At the bottom of Figure 2b, an orientation is shown in which the nitrogen is located directly...

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. 

Some evidence for the process (AQ) - (AQ) when QsA, or for photoassociation of the triplet A + A - (AA)(, has been provided by Hoytink and co-workers,<1B8) who reported excimer phosphorescence from cooled ethanolic solutions of phenanthrene and naphthalene. 

Different aromatic hydrocarbons (naphthalene, pyrene and some others) can form excimers, and these reactions are accompanying by an appearance of the second emission band shifted to the red-edge of the spectrum. Pyrene in cyclohexane (CH) at small concentrations 10-5-10-4 M has structured vibronic emission band near 430 nm. With the growth of concentration, the second smooth fluorescence band appears near 480 nm, and the intensity of this band increases with the pyrene concentration. At high pyrene concentration of 10 2 M, this band belonging to excimers dominates in the spectrum. After the act of emission, excimers disintegrate into two molecules as the ground state of such complex is unstable. 

Photoinduced excited states of the naphthalene derivatives included in the amphiphilic p-CD LB films were found to be stablized by measurements of the fluorescence lifetimes and the excimer formation of the naphthalene derivatives adsorbed by the CD monolayer occured mainly between the adjacent layers [29]. 

In acetonitrile-dichloromethane 1 1 v/v solution, their absorption spectra are dominated by naphthalene absorption bands and they exhibit three types of emission bands, assigned to naphthyl localized excited states (/Wx = 337 nm), naphthyl excimers (Amax ca. 390 nm), and naphthyl-amine exciplexes (/lmax = 480 nm) (solid lines in Fig. 3). The tetraamine cyclam core undergoes only two protonation reactions, which not only prevent exciplex formation for electronic reasons but also cause strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers. 

Many aromatic hydrocarbons such as naphthalene or pyrene can form excimers. The fluorescence band corresponding to an excimer is located at wavelengths higher than that of the monomer and does not show vibronic bands (see Figure 4.6 and the example of pyrene in Figure 4.7). 

An alternative mechanism for the formation of a two hosts-two guests complex has been suggested. From equilibrium studies of the concentration-dependence of the absorption spectra and excimer fluorescence intensities of naphthalene in the presence of beta cyclodextrin, Hamai ° concluded that the excimer fluorescence is due to a two hosts-two guests complex, formed by the association of two 1 1 beta cyclodextrin-naphthalene complexes. Thus, the mechanism can be described as follows. 

Evidence for photoassociation in the triplet manifold is at present inconclusive. Although Hoytink et al.20 have reported excimer phosphorescence from cooled ethanolic solutions of phenanthrene and naphthalene, concentration and temperature-dependent studies of the emission characteristics must be extended in order to distinguish photoassociation of the triplet state from intersystem crossing of the singlet excimer and possible triple-triplet annihilation. Certainly the decay constant of the molecular triplet state in fluid media is relatively insensitive to solute concentration21 although this... 

Simple MO considerations show that the dimer cation of an aromatic hydrocarbon M, with one less antibonding electron than the ground excimer configuration, should be stable with respect to its constituents M+ + M this is confirmed by esr129 and optical absorption8 studies of y-irradiated solutions of benzene, naphthalene, and anthracene in low temperature glasses. 

How many stable excimer structures exist for naphthalene and its alkyl-substituted derivatives ... 

The volume change AV associated with intermolecular excimer formation has been determined for naphthalene and various alkyl derivatives through the application of pressure 74). For naphthalene and the two methylnaphthalene isomers, the value of AV = — 16cm3/mole was measured at room temperature. Assuming the sandwich structure for the excimer, and taking the projected area of the naphthalene molecule... 

There is a substantial entropy decrease AS associated with intermolecular excimer formation, as given in the tabulation by Birks 71). For all solvents (except 95% ethanol 75), the value AS ss —20 cal/mole-K was observed for naphthalene and its derivatives. For comparison, the entropy of fusion of unsubstituted aromatic hydrocarbons such as naphthalene falls in the range of —8 to —15 e.u. The large loss of entropy in the intermolecular excimer formation process indicates a very constrained symmetric structure. 

The rate constant kTD for fluorescence of the pyrene intermolecular solution excimer has been found to follow the relation kFD = n2(kFD)n=I, where n is the the refractive index of the solvent69 . The values of kTO for the 1-methylnaphthalene excimer in ethanol at various temperatures are also consistent with the above relation 76). The fact that (kFD)n=I is independent of solvent and temperature indicates that the excimer has a specific structure, according to Birks 69,71). Experimentally, it was observed much earlier that kFM = n2(kFM)n=i for the polycyclic aromatic hydrocarbons, and that k /kp is independent of solvent and temperature. Table 5 shows that agreement between independent investigators of the excimers of naphthalene compounds is not always good, as in the case of 1-methylnaphthalene. 

In summary, all available evidence suggests that the intermolecular excimers of naphthalene compounds have a sandwich structure in which the ring planes are parallel and the molecular axes are aligned. While the intermolecular excimer appears to adopt the eclipsed sandwich structure in solution, there may be differences in the structure of excimers constrained by hydrocarbon links or by rigid matrices. These constrained excimers will be considered next. 

Birks 68) has proposed that the only change between the unexcited and excited pyrene pair is a reduction in the interplanar distance from 3.53 to 3.37 A, i.e. that the pyrene excimer is not a completely eclipsed sandwich pair either in solution or in the crystal. This proposal is consistent with the observed similarity of the excimer band position for the crystal and solution environment, and with the emission of excimer fluorescence from the crystal even at 4 K. For naphthalene, the greater separation and the nonparallel structure of nearest-neighbor pairs in the crystal apparently prohibits the formation of the sandwich excimer during the naphthalene singlet monomer lifetime. Thus, no excimer fluorescence is observed from defect-free naphthalene crystals. 

Kawakubo s fluorescence results 86> for methyl- and dimethylnaphthalene solids can be similarly related to the crystal structure. Both 2-and 2,6-substituted naphthalenes retain the same close-packed layer structure as seen in naphthalene. The only effect of the methyl substitution is to increase the crystal dimension along the naphthalene long axis87 . Less is known about the crystal structures of 1- and 1,6-substituted naphthalenes, except that the 1-substituent requires a different packing pattern than naphthalene and that 1- and 1,6-substituted naphthalenes have much lower melting points than the 2-substituted naphthalenes. The absence of sandwich pairs in 2- and 2,6-substituted naphthalene crystals certainly explains the lack of excimer fluorescence in the crystal spectra. Presumably, such pairs are also absent in crystalline 1-methylnaphthylene, but they seem to be present in glassy 1-methyl-naphthalene and in 1,6-dimethylnaphthalene solid. 

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