Theoretical Insight into the Effect of Phosphorus Oxygenation on Nonradiative Decays: Comparative Analysis of P-Bridged Stilbene Analogs

Incorporation of the phosphorus element into a π-conjugated skeleton offers valuable prospects for adjusting the electronic structure of the resulting functional π-electron systems. Trivalent phosphorus has the potential to decrease the LUMO level through σ*-π* interaction, which is further enhanced by its oxygenation to the pentavalent P center. This study shows that utilizing our computational analysis to examine excited-state dynamics based on radiative/nonradiative rate constants and fluorescence quantum yield (ΦF) is effective for analyzing the photophysical properties of P-containing organic dyes. We theoretically investigate how the trivalent phosphanyl group and pentavalent phosphine oxide moieties affect radiative and nonradiative decay processes. We evaluate four variations of P-bridged stilbene analogs. Our analysis reveals that the primary decay pathway for photoexcited bis-phosphanyl-bridged stilbene is the intersystem crossing (ISC) to the triplet state and nonradiative. The oxidation of the phosphine moiety, however, suppresses the ISC due to the relative destabilization of the triplet states. The calculated rate constants match an increase in experimental ΦF from 0.07 to 0.98, as simulated from 0.23 to 0.94. The reduced HOMO-LUMO gap supports a red shift in the fluorescence spectra relative to the phosphine analog. The thiophene-fused variant with the nonoxidized trivalent P center exhibits intense emission with a high ΦF, 0.95. Our prediction indicates that the ISC transfer is obstructed owing to the relatively destabilized triplet state induced by the thiophene substitution. Conversely, the thiophene-fused analog with the phosphine oxide moieties triggers a high-rate internal conversion mediated by conical intersection, leading to a decreased ΦF.

Three-Component Synthesis of Dioxaphosphorane-Fused Diphosphacycles Exhibiting Unique Dynamic Fluorescence "On/Off" Properties

Rigidly planar polycyclic phosphacycles featuring an internal dioxaphosphorane are promising photofunctional materials. However, the lack of efficient synthetic methods resulted in limited structural diversities which significantly hampered extensive study. Herein, we report a straightforward three-component synthesis of novel dioxaphosphorane-fused diphosphacycles with distinctive photophysical properties. Control experiments and theory calculations were performed to account for a plausible reaction mechanism. We also systematically investigated the structure-property relationships of these unprecedented platforms by combining experiments (X-ray analysis, optical and redox properties) and theoretical computations. Based on their unique structure and properties, a novel fluorescent switch for pH sensing was revealed by a dynamic ring-opening/ring-closing process.

Synthesis and Optical Properties of 1,2,5,10-Tetraphenylanthra[2,3-b]phosphole Derivatives

1,2,5,10-Tetraphenylanthra[2,3-b]phosphole oxides and 1-methyl-1,2,5,10-tetraphenylanthra[2,3-b]phospholium salts were prepared, and their optical properties were investigated. The substituent at the para position and the fused anthracene moiety were found to exert significant impacts on the fluorescence properties of the P-bridged 2-styrylanthracene skeleton.

Synthesis of (Thio)Furan-Fused Phospholes via Phosphonation Cyclization and a Base-Promoted Phospha-Friedel-Crafts Reaction

Herein, we developed a novel strategy for synthesizing ladder (thio)furan-fused phospholes via intermolecular phosphonation cyclization and a base-promoted phospha-Friedel-Crafts reaction under mild conditions. The starting substrates are readily available phosphinic acids and easy-to-handle alkynes. The details of the reaction mechanism were further rationalized using theoretical calculations. This protocol can be widely applied to synthesize furan- and thiofuran-fused phospholes as well as the corresponding large π-extended derivatives, which are of great interest in the domain of organic functional materials.

Palladium-catalysed C–H arylation of benzophospholes with aryl halides

A palladium-catalysed C–H arylation of benzophospholes with aryl halides has been developed. The reaction with aryl iodides and bromides proceeds well even under phosphine ligand-free Pd(OAc)₂ catalysis whereas the Pd(PCy₃)₂ is effective for the coupling with less reactive aryl chlorides. The optimal conditions are also applicable to the double arylations with organic dihalides and annulation reaction with ortho-dihalogenated benzenes, making the corresponding benzophosphole-based acceptor–donor–acceptor-type molecules and highly condensed heteroacene-type molecules of potent interest in materials chemistry. Although there are many reports of catalytic C–H functionalisations of related benzoheteroles such as indoles, benzothiophenes, and benzofurans, this is the first successful example of the catalytic direct C–H transformation of benzophospholes, to the best of our knowledge. The preliminary optoelectronic properties of some newly synthesized benzophosphole derivatives are also investigated.

A palladium-catalysed C–H arylation of benzophospholes with aryl halides has been developed to form the corresponding acceptor–donor–acceptor-type molecules and highly condensed heteroacene-type molecules of potent interest in material chemistry.

One-pot heteroannulation toward phosphaperylene diimides with high luminescence and out-of-plane anisotropy

This work presents a P-heteroannulation strategy on perylene diimides in one pot. The resulting phosphaperylene diimides demonstrate an out-of-plane dipole moment of 8.10 D and high fluorescence quantum yields of up to 94%.

Formal Transition-Metal-Catalyzed Phosphole C-H Activation for the Synthesis of Pentasubstituted Phospholes

Unprecedented formal transition-metal-catalyzed phosphole C-H functionalization is described in this paper. Pentasubstituted phospholes were prepared via the copper-catalyzed reaction of 1,3,4-trisubstituted phosphole with aryl iodides or bromides under distinct conditions. The developed methodology is able to accommodate a wide variety of substituents, including aryl, heteroaryl, and alkenyl.

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.

Near-Infrared Phosphorus-Substituted Rhodamine with Emission Wavelength above 700 nm for Bioimaging

Phosphorus has been successfully fused into a classic rhodamine framework, in which it replaces the bridging oxygen atom to give a series of phosphorus-substituted rhodamines (PRs). Because of the electron-accepting properties of the phosphorus moiety, which is due to effective σ*-π* interactions and strengthened by the inductivity of phosphine oxide, PR exhibits extraordinary long-wavelength fluorescence emission, elongating to the region above 700 nm, with bathochromic shifts of 140 and 40 nm relative to rhodamine and silicon-substituted rhodamine, respectively. Other advantageous properties of the rhodamine family, including high molar extinction coefficient, considerable quantum efficiency, high water solubility, pH-independent emission, great tolerance to photobleaching, and low cytotoxicity, stay intact in PR. Given these excellent properties, PR is desirable for NIR-fluorescence imaging in vivo.

Insights on the design and electron-acceptor properties of conjugated organophosphorus materials

The development of conjugated organic materials has become a rapidly evolving field of research, particularly with a view toward practical applications in so-called organic electronics that encompass a variety of device types, such as OLEDs, OPVs, and OFETs. Almost all of these devices minimally require the presence of electron-donor and -acceptor components that act as p- and n-type semiconductors, respectively. Research over the past two decades has shown that while there is an abundant resource of organic p-type materials, suitable n-type species are few and far between. To overcome this severe bottleneck for the further development of organic electronics, researchers have identified organo-main-group avenues as valuable alternatives toward organic electron-acceptor materials that may ultimately be used as n-type components in practical devices. One particular element of interest in this context is phosphorus, which at first glance may not necessarily suggest such properties. In this Account, I provide detailed insights on the origin of the electron-acceptor properties of organophosphorus-based conjugated materials and include an overview of important molecular species that have been developed by my group and others. To this end, I explain that the electron-acceptor properties of conjugated organophosphorus materials originate from an interaction known as negative hyperconjugation. While this particular interaction creates a simply inductively withdrawing phosphoryl substituent for π-conjugated scaffolds, incorporation of a phosphorus atom as an integral part of a cyclic substructure within a π-conjugated system provides a much more complex, versatile, and consequently highly valuable tool for the tuning of the electron-acceptor properties of the materials. Notably, the degree of negative hyperconjugation can effectively be tailored in various ways via simple substitution at the phosphorus center. This is now well established for phosphole-based molecular materials, in which the electron-acceptor properties are also mirrored by the degree of antiaromaticity of the system. Particularly, fused and π-extended phosphole materials show appreciable electron-acceptor properties, evident in low reduction potentials and corresponding LUMO levels. But these features do not always translate into powerful n-type materials. My group and others have thus recently been focusing on molecular organophosphorus scaffolds that also involve incorporation of imino or carbonyl groups, next to the incorporation of low coordinate phosphorus centers, to achieve superior electron-acceptor features. This state-of-the-art research has confirmed the great potential of the organophosphorus route toward powerful electron-acceptor materials, but further work is required to also establish these species as functional n-type materials.