Dianene was a deadly poison. On Pluto, Megropolis One was fitted with a sprinkler system that released dianene as part of "Contingency Plan A". This would kill all non-Usurians within the city in ten seconds.
The Collector planned to use it after the Citizens' Revolution saw mass dissent against the Company. The release was controlled by a lever in the palace, but Leela stopped him from activating it. (TV: The Sun Makers)