Advancement in Synthetic Strategies of Phosphorus Heterocycles: Recent Progress from Synthesis to Emerging Class of Optoelectronic Materials

Organophosphorus heterocycles have long been acknowledged for their significant potential across diverse fields, including catalysis, material science, and drug development. Incorporating phosphorus functionalities into organic compounds offers a means to effectively tailor their medicinal properties, augment biological responses, and enhance selectivity and bioavailability. The distinctive physical and photoelectric characteristics of phosphorus-containing conjugated compounds have garnered considerable interest as promising materials for organic optoelectronics. These compounds find extensive utility in various applications such as light-emitting diodes, photovoltaic cells, phosphole-based fluorophores, and semiconductors.

Illuminating the Path to Enhanced Bioimaging by Phosphole-based Fluorophores

As the research of biological systems becomes increasingly complex, there is a growing demand for fluorophores with a diverse range of wavelengths. In this study, we introduce phosphole-based fluorophores that surpass existing options like dansyl chloride. The reactive S-Cl bond in chlorosulfonylimino-5-phenylphosphole derivatives allows rapid and direct coupling to peptides making the fluorophores easily introducible to peptides. This coupling process occurs under mild conditions, demonstrated for [F7 ,P34 ]-NPY and its shorter analogues. Peptides linked with our fluorophores exhibit similar receptor activation to the control peptide, while maintaining high stability and low toxicity, making them ideal biolabeling reagents. In fluorescence microscopy experiments, they can be easily visualized even at low concentrations, without suffering from the typical issue of bleaching. These phosphole-based fluorophores represent a significant leap forward in the field. Their versatility, ease of modification, superior performance, and applicability in biological labeling make them a promising choice for researchers seeking advanced tools to unravel the details of complex biological systems.

Facile modification of phosphole-based aggregation-induced emission luminogens with sulfonyl isocyanates

Phosphole oxides undergo a highly chemoselective reaction with sulfonyl isocyanates forming sulfonylimino phospholes in high yields. This facile modification proved to be a powerful tool for obtaining new phosphole-based aggregation-induced emission (AIE) luminogens with high fluorescence quantum yields in the solid state. Changing the chemical environment of the phosphorus atom of the phosphole framework results in a significant shift of the fluorescence maximum to longer wavelengths.

Luminescent 1H-1,3-benzazaphospholes

2-R-1H-1,3-Benzazaphospholes (R-BAPs) are an interesting class of σ²P heterocycles containing P[double bond, length as m-dash]C bonds. While closely related 2-R-1,3-benzoxaphospholes (R-BOPs) have been shown to be highly photoluminescent materials depending on specific R substituents, photoluminescence of R-BAPs has been previously limited to an example having a fused carbazole ring system. Here we detail the synthesis and structural characterization of a new R-BAP (3c, R = 2,2'-dithiophene), and compare its photoluminescence against two previously reported R-BAPs (3a, R, R' = Me and 3b, R = 2-thiophene). The significant fluorescence displayed by the thiophene derivatives 3b (φ = 0.53) and 3c (φ = 0.12) stands in contrast to the weakly emissive methyl substituted analogue 3a (φ = 0.08). Comparative computational investigations of 3a-c offer insights into the interplay between structure-function relationships affecting excited state relaxation processes.

Yellow to blue switching of fluorescence by the tuning of the pentaphenylphosphole structure: phosphorus electronic state vs. ring conjugation

The interaction between diphenylacetylene and dichlorophenylphosphine under various conditions is a simple method for the preparation of pentaphenylphosphole derivatives exhibiting fluorescence properties. Depending on the electronic state of the various centers of the phospholic structure, it was possible to obtain molecules with fluorescence, as in the blue area for 1,2,3,4,5-pentaphenyl-2,5-dihydro-phosphole-1-oxide (H₂PPPO), in the yellow area for 1,2,3,4,5-pentaphenylphosphole-1-oxide (PPPO) and in the cyan area for 1,2,3,4,5-pentaphenylphosphole (PPP). The effect of the structure and π-conjugation on the optical properties of these compounds was studied using PPP derivatives as examples. Unusual changes in the optical properties of PPP derivatives in solution and in the crystalline state are explained. In the case of agglomeration of PPPO and PPP molecules, the effect of aggregation-induced emission (AIE) was observed to have weak fluorescence in solution and strong fluorescence in the aggregated state. However, for H₂PPPO, the AIE effect remains mild. With the help of experimental studies, supported by theoretical calculations, the main mechanism of the optical properties of pentaphenylphosphole derivatives has been revealed. It was observed that the intramolecular motions of PPPO and PPP are more limited in the solid state than the motions of H₂PPPO, which is associated with less conjugation of the phenyl rotors of H₂PPPO. The analysis of the structure and distribution of electron density showed why hydrogenation of the phosphole ring leads to a sharp change in the optical properties of pentaphenylphosphole derivatives, while the oxidation of phosphorus does not lead to the disappearance of the AIE effect and to a lesser extent affects the change in the fluorescence wavelength. Thus, it was shown how the regulation of various structural features of the phospholic ring helps to control the optical properties of such compounds.

Cationic-palladium catalyzed regio- and stereoselective syn-1,2-dicarbofunctionalization of unsymmetrical internal alkynes

π-Extended tetrasubstituted olefins are widely found motifs in natural products, leading drugs, and agrochemicals. Thus, development of modular strategies for the synthesis of complex all-carbon-substituted olefins always draws attention. The difunctionalization of unsymmetrical alkynes is an attractive approach but it has remained faced with regioselectivity issues. Here we report the discovery of a regio- and stereoselective syn-1,2-dicarbofunctionalization of unsymmetrical internal alkynes. A cationic Pd-catalyzed three-component coupling of aryl diazonium salts, aryl boronic acids (or olefins) and yne-acetates enables access to all-carbon substituted unsymmetrical olefins. The transformation features broad scope with labile functional group tolerance, building broad chemical space of structural diversity (94 molecules). The value of this synthetic method is demonstrated by the direct transformation of natural products and drug candidates containing yne-acetates, to enable highly substituted structurally complex allyl acetate analogues of biologically important compounds. Synthetic versatility of the carboxylate bearing highly substituted olefins is also presented. The reaction outcome is attributed to the in situ formation of stabilized cationic aryl-Pd species, which regulates regioselective aryl-palladation of unsymmetrical yne-acetates. Control experiments reveal the synergy between the carboxylate protecting group and the cationic Pd-intermediate in the regioselectivity and reaction productivity; density functional theory (DFT) studies rationalize the selectivity of the reaction.

Synthesis and photophysical properties of donor-substituted phenyl-phosphachromones as potential TADF materials

A series of novel arylamine-substituted phenyl-phosphachromones were constructed via post-functionalization.

Toward N,P-Doped π-Extended PAHs: A One-Pot Synthesis to Diannulated 1,4,2-Diazaphospholium Triflate Salts

A novel method for the one-pot synthesis of diannulated 1,4,2-diazaphospholium triflate salts by a Me₃SiOTf-mediated self-condensation of dichlorophosphaneyl aza-(poly)cyclic aromatic hydrocarbons (aza-(P)AHs; namely, pyridine, quinoline, phenanthridine, and benzo[d]thiazole) is reported. The diannulated 1,4,2-diazaphospholium triflate salts are characterized by multinuclear NMR spectroscopy, X-ray analysis, as well as their calculated NICS values, underlining their aromatic character. Quantum mechanical calculations shed light on the intermolecular reaction mechanism. Follow up chemistry, such as the halogenation reaction with XeF₂ or SO₂Cl₂ with the dipyridinium derivative selectively yields the respective dihalo-σ⁴,λ⁵- and tetrahalo-σ⁵,λ⁶-diazaphospholium triflate salts. The dihalo-σ⁴,λ⁵-diazaphospholium triflate salt serves well as a surrogate for the introduction of the cationic 2-(1,2'-bipyridin)-1-iumyl ligand (1,2'-bipyl), the monocationic structural isomer of the prototypical 2,2'-bipyridine ligand (2,2'-bipy).

Facile Synthesis of Nitrogen-Containing Six-Membered Benzofuzed Phenophosphazinine Oxides and Studies of the Photophysical Properties

A facile synthesis of nitrogen-containing six-membered benzofuzed phosphacycles has been described. This cyclization reaction triggered by Tf₂O provides an expedient protocol of phosphacycles with a simple operation at mild conditions. The preliminary photophysical studies of these novel materials reveal that the fluorescence intensity enhances with the molecular aggregation in the solid state and that the introduction of the sterically bulky phenanthrene group into the phosphacycles allows a significant increase in the fluorescence quantum yield.

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

The suitability of phosphaphenalene derivatives for light-emitting diodes and electrofluorochromic devices is herein demonstrated.

Effects of counter anions, P-substituents, and solvents on optical and photophysical properties of 2-phenylbenzo[b]phospholium salts

The effects of counter anions, P-substituents, and solvents on the optical and photophysical properties of 2-phenylbenzo[b]phospholium salts were studied.

π-Conjugated phospholes and their incorporation into devices: components with a great deal of potential

This review serves as a brief introduction to phospholes and discusses their unique favorable properties for application in organic electronic materials. Over the past several years, π-conjugated phospholes have been slowly making their way into devices. We report here the mode of synthesis of these π-conjugated phospholes as well as discuss the performances of the devices.

Stimuli-responsive chromism in organophosphorus chemistry

The last five years have seen a huge advance in organophosphorus chemistry with respect to compounds with strong chromic responses to external stimuli. This Frontier article briefly summarizes these trends in order to especially highlight the broad versatility of phosphole-based scaffolds in chromic materials.

Phosphole-based ligands in catalysis

This review provides an overview of phosphole-based ligand families (monophospholes, multidentate hybrid phosphole ligands, diphosphole and 2,2′-biphosphole-based ligands) and their potential in metal- and organo-catalyzed reactions (asymmetric reactions included).

Electroluminescence and fluorescence response towards acid vapors depending on the structures of indole-fused phospholes

New isomers of phosphole heteroacenes 2-DIPO and 3-DIPO, in which indoles were fused with phospholes in different manners, have been synthesized.

Preparation of dithienylphospholes by 1,1-carboboration

In this study the scope of the 1,1-carboboration reaction was extended to the preparation of mixed heterole-based conjugated π-systems. Two arylbis(alkynyl)phosphane starting materials 2 were synthesized bearing two thiophene isomers at the alkyne units and the bulky tipp-substituent (tipp=2,4,6-triisopropylphenyl) at the phosphorous atom. The bis(thienylethynyl)phosphanes 2 were converted into the corresponding 2,5-thienyl-substituted 3-borylphospholes 4 in a double 1,1-carboboration reaction sequence employing the strongly electrophilic B(C6 F5 )3 reagent under mild reaction conditions. Subsequent Suzuki-Miyaura type cross-coupling yielded the corresponding 3-phenylphospholes 7 in a one-pot procedure from phosphanes 2 in high yields. Phospholes 7 were converted into the respective phosphole oxides 8. A photophysical characterization of derivatives 7 and 8 was carried out. The results presented here demonstrate the suitability of the 1,1-carboboration reaction for the preparation of phosphole-/thiophene-based, light-emitting systems.

1,2-dihydrophosphete: a platform for the molecular engineering of electroluminescent phosphorus materials for light-emitting devices

The discovery and molecular engineering of novel electroluminescent materials is still a challenge in optoelectronics. In this work, the development of new π-conjugated oligomers incorporating a dihydrophosphete skeleton is reported. Variation of the substitution pattern of 1,2-dihydrophosphete derivatives and chemical modification of their P atoms afford thermally stable derivatives, which are suitable emitters to construct organic light-emitting diodes (OLEDs). The optical and electrochemical properties of these new P-based oligomers have been investigated in detail and are supported by DFT calculations. The OLED devices exhibit good performance and current-independent CIE coordinates.

C,C-diacetylenic phosphaalkenes in palladium-catalyzed cross-coupling reactions

The reactivity of bis-TMS-substituted C,C-diacetylenic phosphaalkene (A(2)PA) 1 in Sonogashira-Hagihara cross-coupling reactions has been examined. The selective and successive deprotection of the two silyl groups in 1 is enabled by the steric bulk of the Mes* group which renders the acetylene trans to Mes* more reactive and thereby facilitates selective and consecutive couplings with iodoarenes. In situ transformation of the TMS-protected acetylenes into Cu(i)acetylides is the key step in the synthetic sequence and enables the preparation of the first dimeric A(2)PA linked by a phenylene spacer. cis-trans Isomerization across the P[double bond, length as m-dash]C bond is triggered by a tertiary amine and exclusively observed in the case of nitrophenyl-substituted A(2)PAs. The introduced aryl groups are integral parts of the entire π-system as evidenced by spectroscopic and electrochemical studies.