Paracyclophanes, excimer formation

The inter-ring separation in [4.4] paracyclophane has been calculated to be 3.73 A, assuming normal bond angles and planar benzene rings. At this distance, there is no ground-state overlap, and the UV absorbance does not extend past 280 nm. Nevertheless, the peak of the excimer fluorescence intensity of [4.4] paracyclophane is red-shifted 1900 cm"1 relative to the peak of the solution excimer of toluene at 31,300 cm-1. Neither the excimer lifetime nor the excimer fluorescence response function have been reported for any of the exrimer-forming paracyclophanes, so little is known about the kinetics of excimer formation in these compounds. 

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). 

In the paracyclophane series, Hilller, Glass, and Rice (39) found no evidence for triplet excimer formation in the (4,4)-derlvative, while the (2,2)-derlvative showed a phosphorescence that could be attributed to triplet excimer. However, as already mentioned, the 2,2-paracyclophane shows a strong ground state interaction. 

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