Synthesis, Post-Functionalization and Properties of Diphosphapentaarenes.

Linearly-fused polyarenes are an important class of compounds with high relevance in materials science. While modifying the shape and size represents a common means to fine-tune their properties, the precise placement of heteroatoms is a strategy that is receiving an increasing deal of attention to overcome the intrinsic limitations of all-carbon structures. Thus, linearly-fused diphosphaarenes recently emerged as a novel family of molecules with striking optoelectronic properties and outstanding stability. However, the properties of diphosphaarenes are far from being benchmarked. Herein, we report the synthesis, phosphorus post-functionalization and properties of new diphosphapentaarene derivatives. We describe their synthetic limitations and unveil their potential for optoelectronic applications.

Two-Fold Intramolecular Phosphacyclization: From Fluorescent Diphosphapyrene Salts to Pentavalent Derivatives.

The synthesis of π-extended pyrene-based luminescent compounds containing two six-membered phosphacycles has been realized through a two-step synthesis. It involves a Cu(II)-mediated double cyclization of tertiary diphosphane derivatives to afford dicationic molecules with quaternized phosphorus centers. Subsequent transformation of diphosphonium species into the corresponding P-oxide derivatives has been successfully achieved through Pd(0)-assisted cleavage of the P-Ph bonds, which opens a promising way for the functionalization of polyaromatic P-systems.

Controlling the molecular arrangement of racemates through weak interactions: the synergy between π-interactions and halogen bonds.

POX and NX halogen bonds in combination with π-stacking interactions lead to the sorting of π-extended R- and S-isomers. Theoretical calculations point to a positive synergistic effect between the π-interactions and the halogen bonds to be the origin of such phenomena. As a result, enantiomeric building blocks form homoleptically connected quadrangular structures.

En Route Towards the Control of Luminescent, Optically-Active 3D Architectures.

π-Extended systems are key components for the development of future organic electronic technologies. While conceiving molecules with improved properties is fundamental for the evolution of materials science, keeping control over the 3D arrangement of molecules represents an ever-expanding challenge. Herein, a synthetic protocol to replace carbon atoms of π-systems by dissymmetric phosphorus atoms is reported; in particular, it allowed for conceiving new fused phosphapyrene derivatives with improved properties. The presence of dissymmetric phosphorus atoms precluded the formation of excimers. X-ray diffraction revealed that, meanwhile, strong intermolecular interactions are taking place in the solid state. The phosphapyrenes photoluminesce in the visible region with high quantum yields; importantly, they are CD-active. In addition, the unique non-planar features of phosphorus atoms allowed for the control of the 3D arrangement of molecules, rendering lemniscate-like structures. Based on our discoveries, we envisage the possibility to construct higher-order, chiral 3D architectures from larger phosphorus-containing π-systems.

Phosphorus Post-Functionalization of Diphosphahexaarenes.

Diphosphahexaarenes are highly stable π-extended frameworks containing two six-membered phosphorus heterocycles that have emerged recently. Herein, we present a detailed investigation on the post-functionalization reactions of their phosphorus centers with special emphasis on the selectivity of the processes and the impact of the phosphorus functionalizations into the physicochemical properties. These studies reveal that indeed the phosphorus atoms of the diphosphahexaarenes are readily available to be functionalized with quaternizing and oxidizing agents as well as borane groups without compromising the stability of the system. In addition, the optoelectronic properties of the diphosphahexaarenes are impacted by the phosphorus post-modifications.

Lighting with organophosphorus materials: solution-processed blue/cyan light-emitting devices based on phosphaphenalenes.

A family of electroluminescent organophosphorus materials for solution-processed organic light-emitting diodes is reported. The investigated systems present a six-membered phosphorus heterocycle fused with a pyrrole or benzopyrrole moiety. The materials exhibit high thermal stability and are soluble in a variety of organic solvents. Already in very simple OLED architectures, the novel electroluminescent materials display blue or cyan luminescence; i.e. color coordinates x = 0.18; y = 0.21 and x = 0.22; y = 0.30, respectively, with performances that are in the range of state-of-the-art phosphazene materials. In electrofluorochromic devices, the color emission coordinates of the organophosphorus materials are tuned upon applying an electric potential. These findings set the bedrock for the development of new generations of organophosphorus light-emitting devices.

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

Rationally designed cationic phospha-polyaromatic fluorophores were prepared through intramolecular cyclization of the tertiary ortho-(acene)phenylene-phosphines mediated by CuII 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 (Φem =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.

Diphosphahexaarenes as Highly Fluorescent and Stable Materials.

Oligoarenes are regarded as subunits of π-extended carbon nanoforms, such as graphene and nanotubes, with exceptional technological importance. Fused arenes can thus provide fundamental insight into the nature of the electronic properties of fused polyaromatic rings and pave the way for the design of extended graphene-like materials. However, large π-extended arenes often show low stability. Herein we report the straightforward preparation of linearly fused diphosphahexaarenes containing two six-membered phosphorus heterocycles. They are highly stable towards air, water, and light over months in both solution and the solid state. Single-crystal X-ray crystallography confirmed the molecular structure of all derivatives. Investigations of their optoelectronic properties revealed that the diphosphahexaarenes exhibit ambipolar redox behavior and high fluorescence quantum yields. Embedding six-membered phosphorus rings into large acenes thus opens up new opportunities for the investigation of polyaromatic systems.

A Guide for the Design of Functional Polyaromatic Organophosphorus Materials.

The impact of integrating six-membered phosphorus heterocycles into a poly(hetero)aromatic materials is investigated. Mechanistic studies reveal the key synthetic requirements to embed the latter phosphorus heterocycles in polyaromatic molecules. DFT calculations indicate that introducing six-membered phosphorus rings into π-extended molecules induces a particular electron distribution over the π-extended system. Electrochemical investigations confirm that inserting six-membered phosphacycles into polyaromatics triggers ambipolar redox behavior. Steady-state spectroscopy reveals that fusing pyrroles with phosphorus-containing polyaromatic molecules induces fluorescence quantum yields as high as 0.8, whereas transient absorption spectroscopy demonstrates that fusing thiophenes promote the formation of non-emissive triplet-excited states. As a whole, the optoelectronic properties of fused phosphorus-containing materials give rise to promising performances in photoelectrochemical cells. Moreover, X-ray analyses confirm that the 3D arrangement in the solid state of polyaromatic systems containing six-membered phosphorus rings can be tailored through post-functionalization of the phosphorus center. Altogether, this investigation sets the bedrock for the design of a new generation of highly functional polyaromatic organophosphorus materials, keeping control over their electrochemical properties, fluorescence features, photo-induced excited states, and 3D molecular arrangement.