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 π‐NR2 (D) fragments to an anthracene framework. The alteration of the π‐spacer of electron‐donating substituents in species with terminal −+PPh2Me groups exhibits a long absorption wavelength up to λabs=527 nm in dichloromethane and shifted the emission to the near‐infrared (NIR) region (λ=805 nm for thienyl aniline donor), although at low quantum yield (Φ<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 Φ=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. [ABSTRACT FROM AUTHOR]