Temperature-Responsive Near-Infrared Emission Enabled by Reversible π-Umpolung with an Alkenyl-Strapped Diarylboryl Unit

Abstract

Coordination of a Lewis base to a tricoordinate boryl group generates a tetracoordinate species, thereby inverting the electronic character of the boryl substituents from electron-accepting to electron-donating. Utilizing this π-umpolung strategy, we report temperature-responsive fluorophores that emit in the near-infrared (NIR) region. To achieve reversible π-umpolung, we designed a diarylboryl unit in which two aryl rings are tethered by an alkenyl linker. This alkenyl-strapped scaffold engages in a weak olefin–borane interaction and undergoes frustrated Lewis pair (FLP)-type addition even with bulky neutral Lewis bases such as tricyclohexylphosphine (PCy3). When boryl groups are installed at both termini of 4,7-di(2-thienyl)-2,1,3-benzothiadiazole, coordination of PCy3 induces pronounced red-shifts in the emission spectra. In polar acetonitrile, the emission maximum reaches 732 nm, entering the NIR region. This red shift arises from the strong σ-donating character of the resulting tetracoordinate boron centers, which enhance intramolecular charge transfer (ICT) character in the excited state. Unlike conventional dimesitylborane–fluoride complexes, the FLP-type adducts exhibit reversible, temperature-dependent shifts in the dissociation/association equilibrium. Although the solvent polarity influences the equilibrium, modulation of phosphine Lewis basicity enables reversible dissociation even in polar media, allowing this system to display large emission changes spanning the visible to NIR region.