Reversible Oxygen-Switchable Spin Dynamics in a Purely Organic Bicyclic Expanded Porphyrin

Abstract

The emulation of metalloprotein functions—specifically the reversible oxygen binding and spin-crossover (SCO) capabilities of hemoglobin—in purely organic scaffolds remains a "holy grail" in bio-inspired chemistry. This challenge stems from the inherent kinetic and thermodynamic instability of organic diradicals toward molecular oxygen. Herein, we report the synthesis and characterization of a doubly N-fused bicyclic expanded porphyrin (1) that defies this paradigm. Through a unique topological design fusing a bithiophene bridged bicyclic octaphyrin, we engineered a system that exhibits "inverse" spin-crossover behavior dependent on protonation state. In its neutral form, 1 exists as a paramagnetic diradical that switches to a diamagnetic singlet upon oxygen binding. Conversely, the di-protonated congener is a diamagnetic Hukel-Möbius hybrid (anti)aromatic system that switches to a thermally accessible paramagnetic triplet state (Δ2J/kB = 9.6 Kelvin) upon binding O2. Detailed spectroscopic, magnetic (EPR, SQUID), and computational (NICS, ACID, EDDB) analyses reveal that this reversible, room-temperature oxygen gas binding is driven by a synergistic switching between Hückel/Möbius aromatic topologies and its spin state.