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

Abstract

Phosphonium-based compounds gain attention as a promising photofunctional materials. As a contribution to the emerging field, we present a series of donor-acceptor ionic dyes, which were constructed by tailoring the phosphonium (A) and extended π-NR2 (D) fragments to the 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 of as high as Φ = 0.12. The electron-accepting property of phospha-spiro constituent outperformed that of the monocyclic and terminal phosphonium counterparts, illustrating a promising direction in the design of novel charge transfer chromophores.