P-Protected Diphosphadibenzo[a,e]pentalenes and Their Mono- and Dicationic P-Bridged Ladder Stilbenes

Dithieno[3,2-b; 2',3'-f]phosphepinium-Based Near-Infrared Fluorophores: px-π* Conjugation Inherent to Seven-Membered Phosphacycles

Positively charged phosphorus-containing heterocycles are characteristic core skeletons for functional molecules. While various phosphonium-containing five- or six-membered-ring compounds have been reported, the seven-membered-ring phosphepinium have not been fully studied yet. In this study, dithieno[3,2-b; 2',3'-f]phosphepinium ions containing electron-donating aminophenyl groups were synthesized. An X-ray crystallographic analysis of the resulting donor-acceptor-donor dyes revealed a bent conformation of the central seven-membered ring. These compounds exhibit fluorescence in the near-infrared region with a bathochromic shift of ca. 70 nm compared to a phosphepine oxide congener and a large Stokes shift. High fluorescence quantum yields were obtained even in polar solvents due to the suppression of the nonradiative decay process. A theoretical study revealed that the phosphepinium skeleton is highly electron-accepting owing to the orbital interaction between a px orbital of the phosphonium moiety and a π* orbital of the 1,3,5-hexatriene moiety. Due to the lower-lying px orbital in the phosphonium moiety compared to that of the phosphine oxide and the bent conformation of the seven-membered ring, the phosphepinium ring permits effective px-π* conjugation. A large structural relaxation with a contribution of a quinoidal resonance structure is suggested in the excited state, which should be responsible for the bright emission with a large Stokes shift.

Triplet quenching pathway control with molecular dyads enables the identification of a highly oxidizing annihilator class

Metal complex - arene dyads typically act as more potent triplet energy donors compared to their parent metal complexes, which is frequently exploited for increasing the efficiencies of energy transfer applications. Using unexplored dicationic phosphonium-bridged ladder stilbenes (P-X2+) as quenchers, we exclusively observed photoinduced electron transfer photochemistry with commercial organic photosensitizers and photoactive metal complexes. In contrast, the corresponding pyrene dyads of the tested ruthenium complexes with the very same metal complex units efficiently sensitize the P-X2+ triplets. The long-lived and comparatively redox-inert pyrene donor triplet in the dyads thus provides an efficient access to acceptor triplet states that are otherwise very tricky to obtain. This dyad-enabled control over the quenching pathway allowed us to explore the P-X2+ photochemistry in detail using laser flash photolysis. The P-X2+ triplet undergoes annihilation producing the corresponding excited singlet, which is an extremely strong oxidant (+2.3 V vs. NHE) as demonstrated by halide quenching experiments. This behavior was observed for three P2+ derivatives allowing us to add a novel basic structure to the very limited number of annihilators for sensitized triplet-triplet annihilation in neat water.

Bis-benzofulvenes: Synthesis and Studies on Their Optoelectronic Properties

We report the synthesis of bis-benzofulvenes and the studies on their optical and redox properties. Bis-benzofulvenes were achieved through the Pd-catalyzed intramolecular Heck coupling followed by Ni⁰-mediated C(sp²)-Br dimerization. Low optical and electrochemical energy gaps of 2.05 and 1.68 eV were achieved by tuning the substituent on the exomethylene unit and the aromatic ring. The observed trends in the energy gaps were compared, and the frontier molecular orbitals were visualized using the density functional theory.

Selective oxidation of benzylic alcohols via synergistic bisphosphonium and cobalt catalysis

A synergistic photocatalytic system using a bisphosphonium catalyst and a cobalt catalyst has been developed, enabling the selective oxidation of benzylic alcohols under oxidant-free and environmentally benign conditions. High efficiencies have been obtained for a variety of alcohol substrates, and exclusive selectivity for aldehyde products has been achieved across the board. Furthermore, this photocatalytic system proved to be efficient when performed under continuous-flow conditions, even using a simple and easily assembled continuous-flow setup.

Straightforward Access to Multifunctional π-Conjugated P-Heterocycles Featuring an Internal Ylidic Bond

We report the straightforward one-pot synthesis of novel 5- or 6-membered P-heterocycles featuring an internal ylidic bond: P-containing acenaphthylenes and phenanthrenes. The stability of the compounds tolerates post-functionalization through direct arylation to introduce electron-rich/poor substituents and the synthetic strategy is also compatible with the preparation of more elaborate polyaromatic scaffolds such as acenes and helicenes. Using a joint experimental (X-ray analysis, optical and redox properties) and theoretical approach, we perform a full structure-property relationships study on these new platforms. In particular, we show that molecular engineering allows not only tuning their absorption/emission across the entire visible range but also endowing them with chiroptical or non-linear optical properties, making them valuable dyes for a large panel of photonic or opto-electronic applications.

Phosphorus radicals and radical ions

Synthesis and characterization of isolable radicals of main-group elements have been a long-pursued quest. Although there has been considerable progress in this area, particularly in isolating carbon- radicals, the isolation...

A 2,2'-diphosphinotolane as a versatile precursor for the synthesis of P-ylidic mesoionic carbenes via reversible C-P bond formation

A metal-templated synthetic route to cyclic (aryl)(ylidic) mesoionic carbenes (CArY-MICs) featuring an endocyclic P-ylide is presented. This approach, which requires metal templates with two cis-positioned open coordination sites, is based on the controlled cyclisation of a P,P'-diisopropyl-substituted 2,2'-diphosphinotolane (1) and leads to chelate complexes coordinated by a phosphine donor and the CArY-MIC carbon atom. The C-P bond formation involved in the former partial cyclisation of 1 proceeds under mild conditions and was shown to be applicable all over the d-block. In the presence of a third fac-positioned open coordination site, the P-C bond formation was found to be reversible, as shown for a series of molybdenum complexes. DFT modelling studies are in line with an interpretation of the target compounds as CArY-MICs.

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

Tunable electron-accepting properties of the cationic phosphorus center, its geometry and unique preparative chemistry that allows combining this unit with diversity of π-conjugated motifs, define the appealing photophysical and electrochemical characteristics of organophosphorus ionic chromophores. This Minireview summarizes the achievements in the synthesis of the π-extended molecules functionalized with P-cationic fragments, modulation of their properties by means of structural modification, and emphasizes the important effect of cation-anion interactions, which can drastically change physical behavior of these two-component systems.

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.

Isolable Dibenzo[a,e]disilapentalene with a Dichotomic Reactivity toward CO2

The first dibenzo[a,e]disilapentalene with two Si═C moieties in the heteropentalene core has been prepared. Its solid-state structure and density functional theory (DFT) calculations revealed that the Si═C bonds are involved in an expanded π-conjugated system. The Si═C bonds show a distinguished reactivity toward CO₂, depending on the reaction conditions. While one product results from fixation of two CO₂ molecules across one Si═C bond, two different products could be isolated from the reaction of three CO₂ molecules with both Si═C bonds. The mechanism has been uncovered by DFT calculations.