Three perylene tetracarboxylic diimide (PDI) compounds, namely, N N'-di(2-N '',N ''-dimethylamino)ethylperylene-3,4,9,10-tetracarboxylic diimide (1), N N'-di(2-N '',N ''-dimethylamino)propylperylene-3,4,9,10-tetracarboxylic diimide (2), and N,N'- dicyclohexyl-1,7-pyrrolidinylperylene-3,4 9,10-tetracarboxylic diimide (3), have been desgined and prepared Their photophyscial properties in room-temperature ionic liquid (RTILs) were studied by steady-state absorption and emission spectra and fluorescence lifetime measurements The intramolecular photoinduced electron transfer from dimethylamine to PDI in 1 and 2 has been efficiently hindered because of the solvation of RTILs A two-conformation mechanism for the PET in 1 and 2 is proposed, which explains the results of the fluorescence lifetime measurements well The solvation of RTILs to 3 resembled that of a normal polar organic solvent with polarity larger than that of DMF