Acetylene-Linked Phenalenyl Oligomers as a Creative Source of Extended Polyradical Character

Phenalenyl is known for its highly delocalized radical structure, making it a fundamental building block in the construction of polyradical compounds. This study explores how different connection topologies between phenalenyl units via acetylenic bridges modulate the polyradical character, as well as the electronic and magnetic properties of the resulting systems. The connection type depends on the atom occupation pattern of the phenalenyl singly occupied orbital (SOMO). Three types of connections are defined that induce different π conjugation strength. Linear di- and tetra-phenalenyl chains and cyclic tri- and tetra-phenalenyl aggregates have been investigated. High-level multireference averaged coupled cluster (MR-AQCC) calculations were performed to describe the electronic structures of these compounds. The polyradical character of the oligomers is assessed using descriptors such as singlet-triplet splitting, effectively unpaired electrons (N_U). Additionally, the harmonic oscillator model of aromaticity (HOMA), multicenter index (MCI), fluctuation index (FLU), nucleus-independent chemical shifts (NICS (1)), and the anisotropy of the current-induced density (ACID) analysis are employed to characterize the influence of the phenalenyl linkages on aromaticity. Results indicate that bridges enabling stronger interaction between the SOMOs of phenalenyl units lead to a reduction in polyradical character. Aromaticity analysis corroborates these findings, revealing decreased aromaticity in rings where electron interaction occurs through the bridge. On the contrary, choosing bridging types of weak interaction leads to strong open shell character providing candidates for molecular magnetism. A comparison with the predictions of Ovchinnikov's rule is carried out both to rationalize the outcomes of the quantum chemical calculations and to highlight limitations of the rule, particularly in the treatment of quasi-degenerate states.