Experimental and computational studies reveal mechanistic differences in the photophysics of an open- versus closed-shell π-conjugated redox pair.

By stabilizing unpaired spin in the ground state, open-shell π-conjugated molecules can achieve optoelectronic properties that are inaccessible to closed-shell compounds. Here, we report the synthesis and characterization of a N-substituted, bisphenalenyl π-radical cation [3(OTf)] that shows antiambipolar charge transport and fluorescence via anti-Kasha doublet emission. 3(OTf) produces a red emission (634-659 nm) by radiative decay from β-LUMO to β-SOMO, based on density functional theory and configuration interaction singles calculations, and records one of the highest photostabilities (t1/2 = 9.5 × 104 s) among fluorescent radicals. Characterization of 3(OTf)-based field-effect transistors reveals that the observed electrical conductivity (σRT ≤ 1.3 × 10-2 S/cm) is enabled by hole and electron transport (μe/μh ≤ 5.70 × 10-5 cm2 V-1 s-1) that is most efficient in the absence of gating, which represents the first example of antiambipolarity in a molecular material.

Open-shell, pi-conjugated molecules represent exciting next-generation materials due to their unique optoelectronic and magnetic properties and their potential to exploit unpaired spin densities to engineer exceptionally close pi-pi interactions. However, prior syntheses of ambient stable, open shell molecules required lengthy routes and displayed intermolecular spin-spin coupling with limited dimensionality. Here we report a general fragment-coupling strategy with phenalenone that enables the rapid construction of both biradicaloid (Ph-2-s-IDPL, 1) and radical [10(OTf)] bisphenalenyls in <= 7 steps from commercial starting materials. Significantly, we have discovered an electronically stabilized pi-radical cation [10(OTf)] that shows multiple intermolecular closer-than-vdW contacts (<3.4 angstrom) in its X-ray crystal structure. DFT simulations reveal that each of these close pi-pi interactions allows for intermolecular spin-spin coupling to occur and suggests that 10(OTf) achieves electrostatically enhanced intermolecular covalent-bonding interactions in two dimensions. Single crystal devices were fabricated from 10(OTf) and demonstrate average electrical conductivities of 1.31 X 10(-2) S/cm. Overall, these studies highlight the practical synthesis and device application of a new pi-conjugated material, based on a design principle that promises to facilitate spin and charge transport.