Intramolecular Electron Transfer between 2,5-Dimethoxy-1,4-phenylene Units in [n.n]Paracyclophane Radical Cations

A range of [n.n]paracyclophane radical cations (4(.+)-12(.+)), in which two 2,5-dimethoxy-1,4-phenylene units are connected by alkano bridges of varying length, have been studied by ESR and ENDOR spectroscopy. In the [2.2]- and [3.3]paracyclophane radical cations 4(.+)-6(.+), 10(.+) and 11(.+) the delocalization of the unpaired electron over both pi-moieties and the distinct difference between the first and second oxidation potentials, Delta E = E-2(0) - E-1(0), are evidence for a strong intramolecular electronic interaction between the two electrophores. The [5.5] and [7.7] species (8(.+) and 9(.+)) are localized radical cations at low temperature (ca. 220 K). At room temperature, the higher molecular flexibility leads to a significant increase in the number of internal collisions between the electrophores, resulting in a fast (ESR time scale) intramolecular electron transfer. The intermediate [4.4]paracyclophane radical cations 7(.+) and 12(.+) are apparently also localized radical cations. The close interplanar distance between the two pi-moieties, however, facilitates their mutual contacts. In 7(.+), the intramolecular electron transfer becomes fast on the ESR time scale at room temperature; in 12(.+) the transfer is fast over the temperature range 200-300 K.