Excimers sandwich structure

Theoretical estimates of the excimer stabilization energy W = — AHa (Table IX) have been made on the assumption of a symmetrical sandwich structure of one planar identical molecule exactly superimposed on the other this configuration is based on the observation of characteristic emission bands... 

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 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... 

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. 

In conclusion, there is much to be done in characterizing the photophysics of naphthalenophanes. The fact that the eclipsed isomers emit at lower energies relative to the noneclipsed isomers is in accord with the assignment of the parallel-plane sandwich structure to the naphthalene solution excimer. 

Excimer formation is observed quite frequently with aromatic hydrocarbons. Excimer stability is particularly great for pyrene, where the enthalpy of dissociation is A// = 10 kcal/mol (Fbrster and SeidI, 1965). The excimers of aromatic molecules adopt a sandwich structure, and at room temperature, the constituents can rotate relative to each other. The interplanar separation is 300-350 pm and is thus in the same range as the separation of 375 pm between the two benzene planes in 4,4 -paracyclophane (13), which exhibits the typical structureless excimer emission. For the higher homologues, such as 5,5 -paracylophane, an ordinary fluorescence characteristic of p-dialkyl-benzenes is observed (Vala et al., 1965). 

It seems reasonable to assume that the dimeric benzene ion has a sandwich conformation. A similar structure has been invoked to explain excimer spectra from neutral benzene molecules in solution where stable dimers are observed (27). Also Howarth and Fraenkel (II) have suggested that the two hydrocarbon moieties of the dimeric ion of naphthalene lie in parallel planes. Ishitani and Nagakura (12) have suggested a sandwich structure for paracyclophane anions but they also consider the possibility of a diphenyl type of structure. 

Fig. 1.5. Typical sandwich structure (of poly(2-vinylnaphthalene)) responsible for the excimer formation.

The structure of the pyrene excimer is sandwich-shaped, with the distance between the planes of the two rings being of the order of 0.35nm (Figure 6.3). 

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. 

There is ample evidence that the formation of excimers and exciplexes in covalently linked, a, co-diarylalkanes is optimum for a chain length comprising three methylene groups, since this configuration allows a face-to-face sandwich-type structure to be achieved without imposing severe entropic penalties [58, 59], A two-carbon atom chain is rather too short for optimal exciplex formation, but it facilitates TB mediated ET from an aW-trans conformation, as shown in Figure 16b. 

Some aromatic hydrocarbons, for example pyrene and perylene, also crystallise in a herringbone pattern however, the structural units of these lattices are pairs of molecules which form sandwich-like dimers (see Figs. 2.12 and 2.13 as well as Table 2.4). The two partners are bound together only weakly and absorb light like monomers, but they flouresce as dimers. This interesting phenomenon is called excimer emission (excimer stands for excited dimer ) more will be said about this topic later in Chap. 5. By the way, perylene (see also Table 2.4) crystallises not only in the dimer-like a phase but also in a jS phase (Fig. 2.13). In the latter, which... 

There are also crystals of aromatic hydrocarbons whose fluorescence is an excimer emission. For this to occur, the molecules in the crystal must be ordered pairwise in parallel planes, with a small distance between neighbouring planes. In the pyrene crystal, this distance is 3.53 A. The molecules are thus already ordered as physical sandwich dimers within the crystal. Prime examples of this - as also in solution - are pyrene and perylene (cf the crystal structures in Chap. 2, Figs. 2.11 and 2.12). 

Basically, in the extended conformation as well as in the three-fold helical conformation of the iPS chain, the phenyls do not form excimer states, because the distance between parallel phenyls in the three-fold helical conformation is 0.665 nm, as reported by Natta [62] and Sundararajan and co-workers [54]. This distance is too large for excimer formation of the sandwich type (which have a distance of 0.3-0.35 nm). In this situation, the excimer can be formed only outside the crystalline region, e.g., in the region of the lamellar borders, because in these areas some deformation of the helical conformation of the PS chain makes the excimer structure formation more probable. The excitation energy can effectively migrate along the helical structure [60, 63] to the lamellar border, where... 

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