A new architecture for high spin organics based on Baird’s rule of 4n electron triplet aromatics

The quest for high-spin organic molecules is still an outstanding issue in chemistry. These molecules are potential elements of organic ferromagnets, molecular electronics, and spintronics. High-spin organic molecules have low density, are transparent and flexible when compared with transition metal magnets. Unfortunately, ground state organic molecules with high-spin multiplicity are insufficient.

The observed high multiplicity ground state in these molecules is resulting from attractive interactions among the aligned spins. However, the coupling is normally weak owing to the large spatial separation of the spins leading to premature spin alignment break down at temperatures near absolute zero, therefore, hindering practical application. Above all, there exists no driving force for the high spin state to be ground state other than the unpaired electrons exchange coupling. This makes the high spin state sensitive to perturbations and this could favor spin frustrated systems.

Aromaticity remain to be the most efficient stabilizing force entailing cyclic electron delocalization. Baird’s rule indicates that 4n electron completely conjugated molecules should become aromatic in triplet electronically excited state. Therefore, scientists Michael Mauksch and Svetlana Tsogoeva at Friedrich-Alexander University Erlangen-Nürnberg in Germany demonstrated computationally that triads and dyads of the 4n electron triplet aromatic cyclopentadienyl cation benefit from mutual stabilization of constituent rings. This resulted in stabilized ground states of maximum spin multiplicity via ferromagnetic coupling of the unpaired spins. Their work is now published in Physical Chemistry Chemical Physics.

Aromaticity, together with ferromagnetic coupling and conjugation, is a discriminating factor in the stability of spin isomers. This is in contrast to the high-spin molecules where all spin isomers are stabilized by conjugation. Magnetic coupling of the unpaired spins would be strong owing to proximity of atoms with spin density. Not only does the spin density reside in the π-orbitals. In fact, cyclic π-MO delocalization in triplet aromatic rings would even be damaged if spins were paired.

Cation spin coupling when all the rings are aromatic, can be ferromagnetic or antiferromagnetic. The former dominates. The findings of the study suggested an approach for magnetic materials produced from organic elements that may give way for building 3-dimensional organic high-spin networks by combining linear and branched connectivities. This may result in high Curie temperature when compared to the 1-dimensional systems. “For the first time”, says Mauksch, “have we achieved to link the high-spin ground state directly to its aromaticity. In principle, there is no limit to the maximum total spin when more units get connected. Applications are conceivable where robust permanent magnetism is sought after, as e.g. in electric motors, loudspeakers or sensors.”

Synthesizing these compounds is possible because of the strong energetic preference of spin isomers where all rings are 4n electron triplet aromatic. To get a neutral polyadic magnetic material diamagnetic as well as non-coordinating anions should be employed in a practical sense in order to compensate for the positive charge of the carbocation. “Work along these lines is already in progress in our laboratory. In addition, we are about to find neutral systems that do not require presence of gegenions”, says Tsogoeva.

The authors computationally demonstrated that non-conjugated cyclopentadienyl cation polyads that are linked by an sp³ carbon atom could be essential building blocks for high spin organic molecules that are neither polycarbenes nor polyradicals. Open shell aromaticity was found to be the principle driving force to realizing defined high multiplicity ground states in molecules.