| Abstract
| - Unusual dimers with wide interplanar separations, that is, very long bonds with dD ≈ 3.05 Å, arecommon to the spontaneous self-association of various organic π-radicals in solution and in the crystallinesolid state, independent of whether they are derived from negatively charged anion radicals of planar electronacceptors (TCNE-•, TCNQ-•, DDQ-•, CA-•), positively charged biphenylene cation-radical (OMB+•), or neutralphenalene radical (PHEN•). All dimeric species are characterized by intense absorption bands in the near-IR region that are diagnostic of the charge-transfer transitions previously identified with intermolecularassociations of various electron-donor/acceptor dyads. The extensive delocalizations of a pair of π-electronsaccord with the sizable values of (i) the enthalpies (−ΔHD) and entropies (−ΔSD) of π-dimerization measuredby quantitative UV−vis/EPR spectroscopies and (ii) the electronic coupling element Hab evaluated fromthe strongly allowed optical transitions, irrespective of whether the diamagnetic dimeric species bear adouble-negative charge as in (TCNE)22-, (TCNQ)22-, (DDQ)22-, (CA)22- or a double-positive charge as in(OMB)22+ or are uncharged as in (PHEN)2. These long-bonded dimers persist in solution as well as in thesolid state and suffer only minor perturbations with ΔdD< 10% from extra-dimer forces that may be imposedby counterion electrostatics, crystal packing, and so forth. The characteristic optical transitions in suchdiamagnetic two-electron dimers are shown to be related to those in the corresponding paramagnetic one-electron pimers of the same π-radicals with their parent acceptor, both in general accord with Mullikentheory.
|
