On the magnetic exchange-vibronic coupling of pancake-bonded organic diradicals

Although pancake-bonded organic diradicals are promising systems for molecular spintronics, their magnetic exchange couplings, 2J values, are sensitive to thermal fluctuations due to their highly delocalized, multicenter bonds. Here, we present a systematic evaluation of 2J values and the corresponding mode-specific exchange-vibronic couplings, d(2J)/dQ_i, across a set of covalently tethered bi-Blatter diradicals. Specifically, we varied the peri-fused bridging scaffold that connects the monomer radicals to 1,8-naphthalene, 5,6-acenaphthylene, 1,8-anthracene and 4,5-phenanthrene, allowing us to investigate how strain tunes magnetic interactions. To do this, we first resolve the methodological challenge of calculating accurate singlet-triplet gaps, by demonstrating that spin-flip time-dependent DFT, SF-TDDFT, and multireference single-point calculations performed atop broken-symmetry DFT, BS-DFT, geometries yield excellent quantitative agreement with experimental 2J benchmarks. The bridge topology dictates the static ground state, switching the coupling from strongly antiferromagnetic in the phenanthrene and naphthalene scaffolds to weak coupling in the anthracene analogue. We probe the magnetic exchange response by displacing the geometries along low-frequency normal modes. For these dynamic properties, BS-DFT provides overly-large and unphysical exchange-vibronic gradients as well as thermally-induced 2J fluctuations. By contrast, SF-TDDFT accurately tracks the multireference methods, both in sign and magnitude. We find that the exchange-vibronic coupling is tied to only a few low-frequency lateral shearing and inter-deck compression modes. By connecting the d(2J)/dQ_i gradients with harmonic thermal displacement models, we provide insights for suppressing thermally driven fluctuations in organic spintronic architectures.