3,5,7,9-Substituted Hexaazaacridines: Toward Structures with Nearly Degenerate Singlet−Triplet Energy Separations

Toward the goal of preparing stable, neutral open-shell systems, we synthesized a novel series of p-phenyl-substituted 3,5,7,9-hexaazaacridine and 3,5,7,9-hexaazaanthracene derivatives. The effects of substitution on the molecular electronic properties were probed both experimentally and computationally [B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p)]. While the experimentally prepared structures already have small (20−25 kcal/mol) singlet−triplet energy gaps, systems with even smaller (<9 kcal/mol) singlet−triplet energy separations can be realized through systematic variation of the substituent numbers, types, and patterns. Hexaazaanthracenes show generally smaller singlet−triplet energy gaps than hexaazaacridines. Nitrogen-bonded σ- and π-acceptor substituents that cause positive inductive and mesomeric effects as well as carbon-bonded σ-donor substituents make substituted hexaazaanthracenes promising candidates for purely organic high-spin systems.