Intramolecular Phosphacyclization: Polyaromatic Phosphonium P‐Heterocycles with Wide‐Tuning Optical Properties

Abstract

Rationally designed cationic phospha‐polyaromatic fluorophores were prepared through intramolecular cyclization of the tertiary ortho ‐(acene)phenylene‐phosphines mediated by Cu II triflate. As a result of phosphorus quaternization, heterocyclic phosphonium salts 1 c – 3 c , derived from naphthalene, phenanthrene, and anthracene cores, exhibited very intense blue to green fluorescence ( Φ em =0.38–0.99) and high photostability in aqueous medium. The structure–emission relationship was further investigated by tailoring the electron‐donating functions to the anthracene moiety to give dyes 4 c – 6 c with charge‐transfer character. The latter significantly decreases the emission energy to reach near‐IR region. Thus, the intramolecular phosphacyclization renders an ultra‐wide tuning of fluorescence from 420 nm ( 1 c ) to 780 nm ( 6 c ) in solution, extended to 825 nm for 6 c in the solid state with quantum efficiency of approximately 0.07. The physical behavior of these new dyes was studied spectroscopically, crystallographically, and electrochemically, whereas computational analysis was used to correlate the experimental data with molecular electronic structures. The excellent stability, water solubility, and attractive photophysical characteristics make these phosphonium heterocycles powerful tools in cell imaging.

Cationic Organophosphorus Chromophores: A Diamond in the Rough among Ionic Dyes

“…The extension of the acene framework from naphthalene to chrysene ( 51 ) however induces blue shift of the emission ( λ em =560 nm, Φ em =0.06 in CH 2 Cl 2 ). Similarly, the binaphthyl dication 28 [56] reveals further blue shift and emits comparably to 49 ( λ em =453 nm, Φ em =0.11 in water) [57b] . On the downside, 52 possessing anthracene motif shows deep‐red fluorescence ( λ em =672 nm, Φ em =0.04 in CH 2 Cl 2 ) even without involving strong donor groups, which confirms the importance of P + ‐PAH connectivity but not only the size of the aromatic framework.…”

“…It was also noted that stoichiometric amount of PhI . Cl 2 can oxidize λ 3 σ 3 phosphine into a reactive intermediate, which quantitatively cyclizes into phosphaphenalene cation 31 (Scheme 10); [59] however this method was inefficient for the preparation of 29 ‐type salts [57b] …”

“…The naphthalene‐based fluorophores 29 a , 47 and 48 a – g (Figure 16) reveal strong impact of non‐acene fragment of the backbone on the photophysical performance. Relatively weak deep blue emission of thienyl compound 47 ( λ em =418 nm, Φ em =0.07 in CH 2 Cl 2 ) [49b] is significantly improved in its phenylene congener 29 a ( λ em =418 nm, Φ em =0.3 in CH 2 Cl 2 ) [57b] . Salts 48 , obtained via alkyne insertion into phospharuthenacycles followed by P‐C reductive elimination, are luminescent for aromatic R substituents only [74] .…”

“…The employment of different acene cores together with donor‐acceptor architecture in 29 a – h allows to alter the fluorescence through the entire visible range ( λ em =418–780 nm in CH 2 Cl 2 , Figure 16 B) with good quantum yield even in the near‐IR region ( 29 h , λ em =780 nm, Φ em =0.18 in CH 2 Cl 2 ) [57b] . Due to the ionic nature, most of salts 29 are moderately soluble in water.…”

Ligand-enforced intimacy between a gold cation and a carbenium ion: impact on stability and reactivity

“…52 A parallel also exists with the work of Romero-Nieto on the synthesis phosphonium-containing heterocycles by cyclization. 53 Hydrogen atom migration can be confirmed by the appearance of an aliphatic signal in the 1 H NMR of the compounds.…”