Solution-Synthesized, Crystallographically Characterized [3]Triangulene Dimer Tetraradical with an Open-Shell Singlet Ground State

Abstract

Triangulene, known as Clar’s hydrocarbon, is one of the most iconic non-Kekulé polycyclic benzenoid hydrocarbons, yet solution-phase triangulene dimers have remained experimentally inaccessible despite extensive surface studies. Here, we report the solution synthesis, isolation, and single-crystal structural characterization of a kinetically stabilized β–β-linked triangulene dimer, TRI-d. A β-position covalent linkage combined with bulky substituents inhibits undesired covalent radical recombination and provides kinetic protection for the open-shell framework, resulting in enhanced stability of TRI-d under ambient conditions. In the solid state, TRI-d adopts a distinctly twisted geometry composed of two well-defined triangulene motifs, in contrast to the largely planar surface-bound dimers. Pulsed EPR and magnetic measurements reveal weak antiferromagnetic coupling between the two S = 1 units (J = −3.47 meV), producing a singlet ground state and a quintet excited state separated by a small gap (ΔES-Q = −0.16 kcal mol–1), consistent with theory and significantly weaker than in surface-supported analogues. Because of this weak interaction, the S = 2 state is directly detectable by cw-EPR spectroscopy. TRI-d further exhibits millisecond spin–lattice relaxation (T1 = 5.6 ms at 10 K) and microsecond coherence (Tm = 2.24 μs), highlighting its promise for constructing higher-order triangulene oligomers and exploring quantum spin functionalities.