Pyrene excimer ‘sandwich’ structure

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. 

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). 

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). 

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