Generation and Characterization of a Tetraradical Embedded in a Curved Cyclic Paraphenylene Unit

Unique spin-spin (magnetic) interactions, ring-size effects on ground-state spin multiplicity, and in-plane aromaticity has been found in localized 1,3-diradicals embedded in curved benzene structures such as cycloparaphenylene (CPP). In this study, we characterized the magnetic interactions in a tetraradical consisting of two localized 1,3-diradical units connected by p-quaterphenyl within a curved CPP skeleton by electron paramagnetic resonance (EPR) spectroscopy and quantum chemical calculations. Persistent triplet species with zero-field splitting parameters similar to those of a triplet 1,3-diphenylcyclopentane-1,3-diyl diradical were observed by continuous wave (CW) or pulsed X-band EPR measurements. The quintet state derived from the ferromagnetic interaction between the two triplet diradical moieties was not detected at 20 K under glassy matrix conditions. At the B3LYP/6-31G(d) level of theory, the singlet state was lower in energy than the triplet and quintet states. These findings will aid in the development of open-shell species for material science application.

Automated summary

This study characterized the magnetic interactions in a tetraradical embedded in a curved benzene structure and found unique spin-spin interactions, ring-size effects, and in-plane aromaticity. Triplet species with similar properties as a diphenylcyclopentane diradical were observed, but the quintet state resulting from ferromagnetic interaction was not detected. The singlet state was found to be lower in energy than the triplet and quintet states at a theoretical level. These findings are important for the development of open-shell species in material science.