Crystal Coulomb Energies. II. The Electrostatic Binding Energy of Four Organic Donor-Acceptor Crystals by Ewald's Method and Charge Transfer in Organic Lattices

In an effort to confirm the classification of four organic donor-acceptor crystals into the almost fully ionic (``holoionic'') class, and the nearly neutral (``nonionic'') class, their electrostatic binding energies EC, based on the point-charge model, have been calculated rigorously. The EC were found to be model independent, and were compared with 2ε0, the cost of ionizing the lattices. EC+ 2ε0>0 was expected for the two crystals which are experimentally known to be nonionic: for (1:1)-(hexamethylbenzene: para-chloranil) the result was EC=−4.4 eV against 2ε0=6.6 eV, and for (1:1)-(naphthalene:tetracyanoethylene) EC=−4.3 eV vs 2 ε0=6.5 eV. For the two crystals which are holoionic by experiment EC+ 2ε0<0 was expected, but for (1:1)-(N, N, N′, N′, -tetramethyl-para-phenylenediamine+ 7, 7, 8, 8-tetracyanoquinodimethan−) the result was EC=−3.9 eV vs 2ε0=4.6 eV, and for (1:1)-(N, N, N′, N′-tetramethyl-para-phenylenediamine+ -para-chloranil−) EC=−4.4 and 2ε0=4.9 eV. Clearly in these crystals the total binding energy must include a nonnegligible exchange Coulomb energy.