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 -+ PPh2 Me 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.

Boundaries of the Hyperconjugation from π-Extended Six-Membered Phosphorus Heterocycles.

In the context of materials science, six-membered phosphorus heterocycles are intriguing building blocks due to their tunable properties through phosphorus post-functionalization and their unique hyperconjugative effects arising from the phosphorus substituents that contribute to further tuning the optoelectronic properties of the system. Seeking for the discovery of improved materials, the latter features have triggered an astonishing evolution of molecular architectures based on phosphorus heterocycles. Theoretical calculations showed that the hyperconjugation causes a reduction in the S0-S1 gap, which strongly depends on the nature of both the P-substituent and the π-conjugated core, but where are the limits? Outlining the hyperconjugative effects of six-membered phosphorus heterocycles would allow scientists to know how to design next-generation organophosphorus systems with enhanced properties. Herein, we discovered that, in cationic six-membered phosphorus heterocycles, an increase in the hyperconjugation does not affect the S0-S1 gap anymore; i.e., quaternizing the phosphorus atoms leads to properties that go beyond those provoked by hyperconjugative effects. DFT calculations revealed that the latter is particularly marked in phosphaspiro derivatives. Our detailed investigations spotlight the potential of π-extended systems based on six-membered phosphorus spiroheterocycles for accessing properties beyond those achieved to date through hyperconjugative effects, thus laying the groundwork for new research possibilities toward improved organophosphorus systems.

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.

Boron- and phosphorus-containing molecular/nano platforms: exploiting pathological redox imbalance to fight cancer.

Cancer is currently the second leading cause of death globally. Despite multidisciplinary efforts, therapies to fight various types of cancer still remain inefficient. Reducing high recurrence rates and mortality is thus a major challenge to tackle. In this context, redox imbalance is an undervalued characteristic of cancer. However, it may be targeted by boron- and phosphorus-containing materials to selectively or systemically fight cancer. In particular, boron and phosphorus derivatives are attractive building blocks for rational drug discovery due to their unique and wide regioselective chemistry, high degree of tuneability and chemical stability. Thus, they can be meticulously employed to access tunable molecular platforms to selectively exploit the redox imbalance of cancer cells towards necrosis/apoptosis. This field of research holds a remarkable potential; nevertheless, it is still in its infancy. In this mini-review, we underline recent advances in the development of boron- or phosphorus-derivatives as molecular/nano platforms for rational anticancer drug design. Our goal is to provide comprehensive information on different methodologies that bear an outstanding potential to further develop this very promising field of research.

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.

Luminescent Pyrrole-Based Phosphaphenalene Gold Complexes: Versatile Anticancer Tools with Wide Applicability.

Invited for the cover of this issue are the groups of Christel Herold-Mende and Carlos Romero-Nieto at the Universities of Heidelberg and Castilla-La Mancha. The image depicts the use of phosphaphenalene gold(I) complexes for the treatment of brain cancer. Read the full text of the article at 10.1002/chem.202104535.

Luminescent Pyrrole-Based Phosphaphenalene Gold Complexes: Versatile Anticancer Tools with Wide Applicability.

Brain cancer, one of the most lethal diseases, urgently requires the discovery of novel theranostic agents. In this context, molecules based on six-membered phosphorus heterocycles - phosphaphenalenes - are especially attractive; they possess unique characteristics that allow precise chemical engineering. Herein, we demonstrate that subtle structural modifications of the phosphaphenalene-based gold(I) complexes lead to modify their electronic distribution, endow them with marked photophysical properties and enhance their efficacy against cancer. In particular, phosphaphenalene-based gold(I) complexes containing a pyrrole ring show antiproliferative properties in 14 cell lines including glioblastomas, brain metastases, meningiomas, IDH-mutant gliomas and head and neck cancers, reaching IC50 values as low as 0.73 µM. The bioactivity of this new family of drugs in combination with their photophysical properties thus offer new research possibilities for both the fundamental investigation and treatment of brain cancer.

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.

Correction: Gold(I) complexes based on six-membered phosphorus heterocycles as bio-active molecules against brain cancer.

Correction for 'Gold(i) complexes based on six-membered phosphorus heterocycles as bio-active molecules against brain cancer' by Saskia Roesch et al., Chem. Commun., 2020, DOI: 10.1039/d0cc05761d.

Gold(i) complexes based on six-membered phosphorus heterocycles as bio-active molecules against brain cancer.

π-Systems based on six-membered phosphorus heterocycles possess structural and electronic characteristics that clearly distinguish them from the rest of the organophosphorus molecules. However, their use in cancer therapy has been uninvestigated. In particular, glioblastoma is one of the most lethal brain tumors. The development of novel and more efficient drugs for the treatment of glioblastoma is thus crucial to battle this aggressive disease. Herein, we report a new family of gold(i) complexes based on six-membered phosphorus heterocycles as a promising tool to investigate brain cancer. We discovered that the latter complexes inhibit the proliferation, sensitize to apoptosis and hamper the migration of not only conventional but also stem-like glioblastoma cells. Our results unveil thus new research opportunities for the treatment of glioblastoma.

Synthesis of Blue-Luminescent Seven-Membered Phosphorus Heterocycles.

A facile synthetic procedure to prepare π-extended seven-membered phosphorus heterocycles, both symmetric and asymmetric, is reported. The prepared molecules present a persistent nonplanar framework and are soluble in a wide variety of solvents. The seven-membered phosphorus heterocycles can be electrochemically reduced and oxidized, and photoluminesce with a blue color.

Extraction of 2'-O-apiosyl-6'-O-crotonic acid-betanin from the ayrampo seed (Opuntia soehrensii) cuticle and its use as an emitting layer in an organic light-emitting diode.

The molecule 2'-O-apiosyl-6'-O-crotonic acid-betanin (called Achkiy) was obtained after an ecofriendly and low-cost purification process of the extract from the ayrampo seed cuticle. Results from EDS give us an idea of the organic elements present in the ayrampo cuticle layer composed of carbon, oxygen and nitrogen. Further characterization analysis of ayrampo extract by Fourier Transform Infrared Spectrophotometry (FTIR) corroborated the presence of characteristic functional groups corresponding to carboxyl, carbonyls, hydroxyls and secondary amines. On the other hand, we have confirmed by absortion peak the glucose, apiosyl, crotonic acid and betanin at 227 nm, 276 nm, 291 nm and 534 nm bands respectively. Mass Spectrometry (MS) characterization was used finally to identify the electroactive Achkiy molecule. This molecule was tested in an Organic Light Emitting Diode (OLED) achieving a luminance of 4.8 Cd m-2 when bias voltage of 16.5 V and a current of 34.1 mA was applied. In addition, the irradiance generated by the Achkiy layer reaches a value of ≈ 113.3 µW m-2 emitting light with a λ ≈ 390.10 nm. These preliminary results report an interesting molecule extracted from a natural pigment wich emits light in the blue region.

Organophosphorus-B(C6F5)3 adducts: towards new solid-state emitting materials.

The coordination of B(C6F5)3 to materials based on six-membered phosphorus heterocycles via P[double bond, length as m-dash]O bonds tunes their physicochemical properties both in solution and in the solid state, remarkably improving their performances in light-emitting layers.

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.

Dismantling the Hyperconjugation of π-Conjugated Phosphorus Heterocycles.

The development of π-extended phosphorus heterocycles has been rapidly increasing because of their unique optoelectronics properties, which are very often considered to be a consequence of special hyperconjugative interactions. However, the latter interactions have primarily been investigated within the five-membered species, phospholes, and they are often conceptually extrapolated to the rest of π-extended phosphorus heterocycles (including six-membered P-heterocycles) despite evident structural differences. Herein, we report, for the first time, a detailed investigation that sheds light on the hyperconjugative effects of a series of phosphorus heterocyclic systems by means of EDA and NBO calculations within a DFT framework. Our results lay the foundations for the future design of π-extended phosphorus heterocycles with improved optoelectronics properties.

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.

Highlights on π-systems based on six-membered phosphorus heterocycles.

π-Extended molecules based on six-membered phosphorus heterocycles are exceptional systems due to their unique structural and chemical properties. Even though the first six-membered phosphorus heterocycles were synthesized in the 1950s-1960s, their chemistry is currently experiencing a renaissance with the focus on materials science. In this perspective, we highlight relevant systems that emphasize the potential of these particular organophosphorus materials for different fields of research, from bio- to optoelectronic applications. We also discuss their properties and outline our personal outlook on the future efforts toward the next generation of improved organophosphorus materials.