Anthra/Tetra/Pentaquinodimethane-Supported Organoboranes with Blue-to-Red Emissions and Responsive Transformations to Dication Species and Open-Shell Diradical States

Structurally-modified acenes with a linear fusion of π-extended systems have shown highly attractive properties and promising applications in semiconductor materials, optoelectronic materials and others due to their unique electronic structures. We have accessed a series of anthra/tetra/pentaquinodimethane-supported organoboranes, Mes*B-A, Mes*B-T and Mes*B-P, with highly tunable emissions from blue to red (~680 nm) by controlling the number of fused benzene rings of the [n]acene core (n = 3–5). Interestingly, these redox-switchable quinoid systems have chemically and electrochemically enabled two-electron oxidations, leading to dicationic anthracene, tetracene and pentacene segments (Mes*B-A²⁺, Mes*B-T²⁺ and Mes*B-P²⁺) as evidenced by new absorption bands in the UV−vis−NIR spectra and spectroelectrochemical studies. Meanwhile, all the molecules feature a π-conjugated, overcrowded ethylene structure that allows for a spin-state transition from closed-shell to the open-shell diradicals (Mes*B-A^2•, Mes*B-T^2• and Mes*B-P^2•) under thermal conditions. This can further be confirmed by the variable-temperature (VT) ¹H NMR and electron spin resonance (ESR) spectroscopy. These organoboranes also experienced an emission change in response to fluoride binding with electron-deficient boron centers. Our current work demonstrates not only the synthetic contribution to [n]acene-based luminescent materials, but also showcases multistate transformations for potential applications depending on well-tuned electronic, magnetic, electron transfer and charge transport mechanisms.