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

## TL;DR

This paper introduces a new temperature-responsive material that emits near-infrared light through a reversible chemical process involving boron and phosphine.

## Contribution

The novel contribution is a temperature-responsive fluorophore using reversible π-umpolung with an alkenyl-strapped diarylboryl unit.

## Key findings

- Coordination of a Lewis base to a boryl group causes a red shift in emission spectra, reaching up to 732 nm in polar solvents.
- The alkenyl-strapped diarylboryl unit enables reversible π-umpolung and temperature-dependent fluorescence modulation.
- Phosphine-induced transformations allow emission changes spanning the visible to near-infrared region.

## 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.

Coordination of a Lewis base switches an electron‐accepting tricoordinate boron center into an electron‐donating tetracoordinate one. An alkenyl‐strapped diphenylthienylboryl π‐system undergoes phosphine‐induced reversible π‐umpolung, where transformation from an acceptor–acceptor–acceptor framework to a donor–acceptor–donor system gives rise to temperature‐responsive fluorescence changes in the red‐to‐NIR region.