Phosphaphenalene

Phosphaphenalenes are a class of heterocyclic aromatic compounds containing phosphorus with molecular formula C
12H
9P, related to the phenalene polycyclic aromatic hydrocarbons, C
13H
10. Phosphaphenalene can exist in a number of isomers 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 and electrochromism (reversibly changes colour when electricity is applied).

...
Wikipedia