1-, 2-, and 3-Electron Pancake Bonds in Covalent Scaffolds: Redox Modulation of Through-Space Exchange in Blatter...

Pancake-bonded organic diradicals are rare platforms where the magnetic exchange coupling, multicenter bonding, and spectroscopic properties can be simultaneously tuned by redox chemistry. Here, we computationally map the complete redox series, from dianion (-2) to dication (+2), of two covalently tethered bi-Blatter diradicals. The first is a [2,2]-paracyclophane-bridged syn-conformer, and the other is a one-sided peri-naphthalene-bridged analogue. Using a multireference protocol, CASSCF/NEVPT2, supplemented by DFT-based methods, we show that the bridging topology dictates redox-induced structural reorganization, magnetic exchange, and optical spectroscopy. For the peri-naphthalene system, calculated singlet-triplet gaps and UV-vis-NIR spectra are in good agreement with experimental data. Mayer bond orders from CASSCF densities show that [2,2]-paracyclophane has a more uniform interdeck covalency across the redox series than the peri-naphthalene analogue. This is because the two ethano bridges of the paracyclophane restrict lateral slippage and vertical expansion, thereby suppressing structural reorganization and preserving interdeck communication. By contrast, the peri-naphthalene charged species undergo significant structural expansion, up to 0.6 Å, as well as lateral slippage, up to 2.28 Å, a fact that affects their electronic structures and optical responses. Upon single-electron oxidation or reduction, both scaffolds yield mixed-valence states with intense Charge Resonance (CR) bands in the near-IR at 1000-1500 nm. Even at compressed neutral geometries, the oscillator strengths of the CR bands increase 4-fold, while the excitation energies are unchanged. Our work establishes a quantitative structure-coupling-spectroscopic relationship across a five-state redox series, with direct implications for the design and performance of redox-switchable organic spin systems.