Stable (long-bonded) dimers via the quantitative self-association of different cationic, anionic, and uncharged pi-radicals: structures, energetics, and optical transitions

Unusual dimers with wide interplanar separations, that is, very long bonds with d(D) approximately 3.05 A, are common to the spontaneous self-association of various organic pi-radicals in solution and in the crystalline solid state, independent of whether they are derived from negatively charged anion radicals of planar electron acceptors (TCNE-*, TCNQ-*, DDQ-*, CA-*), positively charged biphenylene cation-radical (OMB+*), or neutral phenalene 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 intermolecular associations of various electron-donor/acceptor dyads. The extensive delocalizations of a pair of pi-electrons accord with the sizable values of (i) the enthalpies (-Delta H(D)) and entropies (-Delta S(D)) of pi-dimerization measured by quantitative UV-vis/EPR spectroscopies and (ii) the electronic coupling element H(ab) evaluated from the strongly allowed optical transitions, irrespective of whether the diamagnetic dimeric species bear a double-negative charge as in (TCNE)(2)(2-), (TCNQ)(2)(2-), (DDQ)(2)(2-), (CA)(2)(2-) or a double-positive charge as in (OMB)(2)(2+) or are uncharged as in (PHEN)(2). These long-bonded dimers persist in solution as well as in the solid state and suffer only minor perturbations with Delta d(D) < 10% from extra-dimer forces that may be imposed by counterion electrostatics, crystal packing, and so forth. The characteristic optical transitions in such diamagnetic two-electron dimers are shown to be related to those in the corresponding paramagnetic one-electron pimers of the same pi-radicals with their parent acceptor, both in general accord with Mulliken theory.