We synthesized a new room temperature ionic liquid, [2PA-Hmim][Pf2N], and characterized the photochromic behavior by absorption spectroscopy. [2PA-Hmim][Pf2N] was found to be a photochromic liquid where solvent itself undergoes photochromic reaction.

Photoisomerization quantum yields (ΦE–Z) of phenylazo-substituted imidazolium cations, which are counterparts of recently developed photochromic ionic liquids, were measured under S1(n, π*) excitation. The ΦE–Z values are lower in imidazolium-type ionic liquids than those in conventional solvents and in tert-alkylammonium-type ionic liquids. This observation was argued based on solute–solvent complex formation at imidazolium rings.