What is described herein are cationic lipids that facilitate the intracellular delivery of biologically active molecules, comprising a compound of formula Iwherein R1 is a linear alkyl, alkenyl, or alkynyl of 2 to 12 carbons, or a branched alkyl with 7-25 R2 is a branched alkenyl with 7-25 carbons, L1 and L2 are independently a linear alkenylene with 2-18 carbons, X is S or O, L3 is a bond, and R3 R4, and R5 are each a linear or branched alkylene with 1-6 carbons.
