How Spin Conductive are Oligo(p-phenylenes) in Trityl-Based Biradicals

The relationship between structure and exchange coupling constant J is of interest for the rational design of molecular open-shell systems in the context of molecular magnetism, dynamic nuclear polarization, and quantum information science. However, even for the basic relationship between J and the distance r between two spin centers for the archetypal oligo(p-phenylene) bridge, there are only a few and contradicting experimental and theoretical data. We therefore synthesized a series of six trityl-[(p-phenylene)_n=1-5]-trityl biradicals and used Electron Paramagnetic Resonance methods to extract J values in the range from 17.5 MHz to 235 GHz. We find a distance dependence parameter β of 0.11 Å^-1 for unsubstituted and 0.67 Å^-1 for substituted oligo(p-phenylene) bridges. This places unsubstituted oligo(p-phenylene)-bridges into the spin-conductor regime and highlights the tunability of conductance by substitution, i.e., increase of the dihedral angle between the phenylene units. DFT calculations employing low Fock-exchange fraction functionals exhibit some limitations in reproducing the experimental J value, particularly in the weak coupling regime. Furthermore, biradicals with n = 3 - 5 exhibit spin coherence of 1.2 μs at room temperature in the solution state. These long coherence times, combined with the modular bridge design, position bistrityl systems as promising candidates for applications in quantum information technology.