Phosphaphenalenes are a class of

heterocyclic A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and ...

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

compounds containing

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

with

molecular formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

. It is related to

phenalene 1''H''-Phenalene, often called simply phenalene is a polycyclic aromatic hydrocarbon (PAH). Like many PAHs, it is an atmospheric pollutant formed during the combustion of fossil fuels. It is the parent compound for the phosphorus-containing phos ...

, where one of its carbons is replaced by phosphorus (phenalene is a

polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

). Phosphaphenalene can exist in a number of

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

s depending on where in the ring system the phosphorus atom is placed. The phosphorus atom in these molecules has unusual properties, in particular changing its affinity as an electron acceptor when exposed to certain wavelengths of light. This makes phosphaphenalenes useful in optoelectronic applications such as temperature-dependent

luminescence Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light". It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a cryst ...

and

electrochromism Electrochromism is a phenomenon in which a material displays changes in color or opacity in response to an electrical stimulus. In this way, a smart window made of an electrochromic material can block specific wavelengths of ultraviolet, visib ...

(reversibly changes colour when electricity is applied).

References

Phosphorus heterocycles {{aromatic-stub