From Terminal to Spiro-Phosphonium Acceptors, Remarkable Moieties to Develop Polyaromatic NIR Dyes

Phosphonium-based compounds gain attention as promising photofunctional materials. As a contribution to the emerging field, we present a series of donor-acceptor ionic dyes, which were constructed by tailoring phosphonium (A) and extended & pi;-NR2 (D) fragments to an anthracene framework. The alteration of the & pi;-spacer of electron-donating substituents in species with terminal -+PPh2Me groups exhibits a long absorption wavelength up to & lambda;(abs)=527 nm in dichloromethane and shifted the emission to the near-infrared (NIR) region (& lambda;=805 nm for thienyl aniline donor), although at low quantum yield (& phi;<0.01). In turn, the introduction of a P-heterocyclic acceptor substantially narrowed the optical bandgap and improved the efficiency of fluorescence. In particular, the phospha-spiro moiety allowed to attain NIR emission (797 nm in dichloromethane) with fluorescence efficiency as high as & phi;=0.12. The electron-accepting property of the phospha-spiro constituent outperformed that of the monocyclic and terminal phosphonium counterparts, illustrating a promising direction in the design of novel charge-transfer chromophores.

Automated summary

Phosphonium-based compounds are gaining attention as promising photofunctional materials. This study presents a series of donor-acceptor ionic dyes constructed by tailoring phosphonium and extended π-NR2 fragments to an anthracene framework. The introduction of a P-heterocyclic acceptor substantially narrows the optical bandgap and improves the efficiency of fluorescence. The phospha-spiro moiety allows for near-infrared emission with high fluorescence efficiency.