Fluorescence switching in 4-amino-1,8-naphthalimides; "on¯off¯on" operation controlled by solvent and cations

The fluorescence properties of 4-(2ʹ-N,N-dimethylaminoethyl)amino-9-butylnaphthalimide (1) are so strongly affected by the nature of the solvent medium in which the molecule is situated that the emission can be considered to be "switched off" when a certain solvent polarity is reached. The mechanism appears to involve the formation of an exciplex between the naphthalimide ring and the distal dimethylamino group which is stabilised by electron transfer. Kinetic parameters for the exciplex formation in 1 and its N,N-dimethylaminopropyl analogue are derived in a range of solvents. The fluorescence can be "switched on" again by metal ions and protons.