Fluorescence of dicyanovinyl julolidine in a room-temperature ionic liquid

9-Dicyanovinyl julolidine is an adequate probe for free volume in the ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate. Evidence is activation energy of 11 kJ/mol for the radiationless deactivation of the fluorescence involving the dicyanovinyl moiety, compared to 33 kJ/mol of the activation energy of the solvent viscosity.