Computational study of Organic Magnetism in Triangulene-Based Diradicals through pi-Conjugated Linker design

Abstract

This work presents a computational study of organic magnetism in triangulene-based diradicals connected through π-conjugated organic linkers. The main objective is to identify structural and electronic factors that favour the stabilization of high-spin states and enhance ferromagnetic coupling between two persistent radical centers. Density Functional Theory calculations were performed to evaluate different linker structures, substitution patterns and radical-linker connections. The magnetic behaviour was analysed using the singlet-triplet energy difference, defined as ΔES-T = ESinglet - ETriplet, as a comparative descriptor of triplet-state stabilization. The results show that the linker plays an active role in the magnetic communication pathway, rather than acting only as a passive spacer between the radical units. Cyano-substituted non-Kekulé linkers were first investigated, showing that electron-withdrawing groups strongly influence the stabilization of the triplet state. Among the systems studied, the disubstituted linker was selected as a suitable compromise between ferromagnetic coupling and synthetic feasibility. The effect of additional substituents was also analysed, revealing that halogen substitution, especially fluorine, increases the singlet-triplet energy difference, whereas electron-donating groups such as hydroxyl and amino groups lead to lower values. Frontier orbital analysis showed that favourable alignment and spatial similarity between the linker LUMO and the triangulene SOMO promote spin delocalization through the π-conjugated bridge. The influence of the radical–linker distance was studied by comparing direct connections with acetylene and diacetylene spacers. Although shorter connections generally provide stronger coupling, acetylene-based bridges remain attractive due to their linear geometry and synthetic accessibility. Finally, carboxylic acid groups were explored as promising alternatives to cyano substituents. These systems retain significant triplet stabilization and show a strong dependence on the orientation of the COOH group, suggesting that local polarization and weak intramolecular interactions may provide additional ways to tune magnetic coupling. Overall, this work provides useful structure-property relationships for the rational design of future high-spin organic materials with potential applications in molecular electronics, spintronics and organic magnetism.