Diphosphahexaarenes as Highly Fluorescent and Stable Materials

Abstract

Oligoarenes are regarded as subunits of p-extended carbon nanoforms,s uch as graphene and nanotubes,w ith exceptional technological importance.F used arenes can thus providef undamental 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 pextended arenes often show lowstability.Herein we report the straightforwardp reparation 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.S ingle-crystal X-ray crystallography confirmed the molecular structure of all derivatives.I nvestigations 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. Angewandte ChemieCommunications

From Phosphaphenalenes to Diphosphahexaarenes: An Overview of Linearly Fused Six‐Membered Phosphorus Heterocycles

“…As a result, large laterally fused aromatic systems are rather challenging to characterize, especially by spectroscopic techniques. With this in mind, we envisaged the synthesis of novel diphosphahexaarenes, formed by six linearly fused six‐membered rings bearing two phosphorus centers (Figure ) . It is worth noting that, strictly speaking, the diphosphahexaarenes cannot be considered as acenes since they possess two off‐line rings.…”

“…In addition, all derivatives were soluble in a series of organic solvents including ethanol, acetonitrile, chloroform, dichloromethane, and tetrahydrofuran. X‐ray diffraction studies revealed an essentially planar scaffold for all diphosphahexaarenes, although 52a exhibited a slightly curved structure . In the solid state, the diphosphahexaarenes formed dimeric structures by π‐stacking with different degrees of overlap.…”

“…In particular, compound 52a revealed to form dimers with perfect face‐to‐face stacking from the outer rings with a short intermolecular distance of 3.29 Å, although the shortest distance was found for 51a ; i.e. 3.24 Å. Quantum chemical calculations revealed that the HOMO (–5.84 eV) and LUMO (–2.41 eV) of 52 were higher than those of 51 , –5.96 and –2.32 eV, respectively . Calculated NICS(1) values pointed to a higher aromaticity of the outer rings, even though the frontier molecular orbitals were delocalized over the whole systems.…”

“…Interestingly, we observed that the phosphorus heterocycles of the diphosphahexaarenes, in agreement with our previously reported phosphaphenalenes, revealed anti‐aromatic character. Regarding the electrochemical properties, all derivatives presented ambipolar redox behavior . Both 51a , b undergo irreversible oxidation at +1.77 V and two reduction processes at –1.43 and –1.71 V. In turn, 52a , b present one irreversible oxidation at +1.79 V and three reduction processes at –1.41, –1.52 and –1.68 V. Derivatives 51 and 52 are outstandingly photostable; we did not observe any decomposition after several months upon repeated cycles of illumination.…”

“…the development of novel systems based on six‐membered phosphorus heterocycles and their application in materials science, we have very recently reported the largest linearly fused systems developed to date; i.e. the diphosphahexaarenes, which contain two six‐membered phosphorus heterocycles …”