A Guide for the Design of Functional Polyaromatic Organophosphorus Materials

Abstract

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.

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

“…Molecular structureso f1c, 2c and 4c determined by singlecrystal X-ray diffraction analysisa re shown in Figures 3a nd S3; the relevant data are given in Tables S1 and S2 (Supporting Information). In contrastwith previously reported fused six-membered P-heterocycles, [19] the phosphorusc enters are slightly out-of-plane with one phenylr ing being nearly perpendicular to the main scaffold. In compounds 1c, 2c and 4c,t he heteroatom adopts as omehow distorted tetrahedral geometry;t he C-P-C angles are close to the idealized value of 109.58 (Table S2,S upporting Information), although larger deviations are found inside the P-cycles [103.4(2)-105.1(6)8]i ndicating certain geometric strain.T he hydrocarbon backbones in 1c, 2c, and 4c are visibly twisted due to intramolecular steric repulsion;t he dihedral angles between the corresponding C 6 H 4 and PAHp lanes are 27.0 (1c), 32.3 (2c), and 37.18 (4c).…”

“…Supporting our strategy to obtain organophosphorus NIR dyes, compounds 1 c – 6 c clearly demonstrate decreasing optical band gaps (Figure S7 and Table S3, Supporting Information). Although different methods were utilized for the DFT calculations, the frontier orbital energies of 1 c (−2.559 eV for LUMO and −6.534 eV for HOMO) significantly differ from the ones of the phosphaphenalene oxide congener (−2.165 eV for LUMO and −6.162 eV for HOMO) . This indicates that the introduction of a quaternized phosphorus atom into a fused P‐heterocyclic systems substantially reduces both the LUMO and HOMO energies compared with the λ 5 σ 4 oxide analogue.…”

“…In dichloromethane solution, the emission maximum is found at 418 nm with a Φ em of 0.30. Compared to its P‐oxide congener ( λ em =389, Φ em =0.14 in CH 2 Cl 2 ), the luminescence of the phosphonium salt 1 c shows two‐fold higher Φ em (in CH 2 Cl 2 ) together with a noticeable bathochromic shift. The latter is presumably attributed to a larger delocalization of the frontier orbitals over the PR 4 + versus PR 3 =O group.…”

“…[4d, 18] One quite generalp reparative method of electrophilici ntramolecular cyclizationw as discovered for naphthalene chlorophosphane precursors to give as eries of neutral phosphaphenalene Poxides. [19] The latter exhibited remarkable photophysical properties;they mostly emitted in the blue/green region of the visible spectrum and reached fluorescenceq uantum yields (F em ) up to 0.8. Remarkably,t wo different copper(II)-mediated phosphaannulation reactions,w hich lead to phosphonium salts, have been realized for the anthracene-based tertiary phosphanes.…”

Phosphorus Post‐Functionalization of Diphosphahexaarenes

“…To that end, phosphorush eterocycles are particularly attractiveo wing to their uniquep ost-syntheticp ossibilities at the phosphorusa tom. [12,[14][15][16] Typical reactions involve the oxidation/reductiono ft he phosphorus centers as well as coordinationo ft he trivalents tate. As an important requirement to fulfill, however,t he post-modified areness hould preservet heir stabilitya nd solubilityt oa llow the processingo ft he materials.…”