Ar. Wartini et al., INTRAMOLECULAR ELECTRON-TRANSFER BETWEEN 2,5-DIMETHOXY-1,4-PHENYLENE UNITS IN [N.N]PARACYCLOPHANE RADICAL CATIONS, EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, (1), 1998, pp. 139-148
A range of [n.n]paracyclophane radical cations (4(.+)-12(.+)), in whic
h two 2,5-dimethoxy-1,4-phenylene units are connected by alkano bridge
s 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 p
i-moieties and the distinct difference between the first and second ox
idation potentials, Delta E = E-2(0) - E-1(0), are evidence for a stro
ng 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 highe
r 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 sc
ale at room temperature; in 12(.+) the transfer is fast over the tempe
rature range 200-300 K.