Axially Chiral Bifluorenylidene Radical Anions with Long Spin–Lattice Relaxation Times at Room Temperature in Fluid Solution

Despite the recent interest in magnetochiral phenomena, well-characterized radicals that possess both molecular chirality and spin remain scarce. Herein, we report the synthesis of a series of axially chiral bifluorenylidene (BF) molecules and their singly reduced radical anions with varying degrees of fjord-region benzannulation. Our systematic structural perturbations provide insight into how the twist angle across their central C═C bond influences their redox state, electronic structure, stereochemical dynamics, and spin properties. Variable-temperature nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) studies are used to explore the barriers to major racemization processes. UV–vis–NIR absorption and magnetic circular dichroism spectroscopies provide insight into the electronic structures of the systems and are combined with multireference computational techniques to characterize the potential energy surfaces. Pulsed EPR measurements revealed that the BF radical anions displayed notably long spin–lattice (T1) relaxation times that approached 0.1 ms in fluid 2-methyltetrahydrofuran solution at room temperature, nearly 2 orders of magnitude longer than typical organic radicals. Remarkably, we found the unique combination of π delocalization, electronic structure, and low anisotropy insulates the spin from longitudinal relaxation due to molecular tumbling. The T1 temperature dependence is instead consistent with relaxation through thermally activated local-mode processes, suggesting that molecular design to control vibrations may be just as important in extending solution-phase T1 times as in the solid state. Our results establish BF radical anions as a versatile framework to explore spin–chirality interactions and to achieve long-lived spin states in nonviscous fluid solution at room temperature.