What is described is a compound of formula I wherein R is a linear alkyl of 1 to 12 carbons, or a linear alkenyl or alkynyl of 2 to 12 carbons; L is a linear alkylene or alkenylene of 5 to 18 carbons; X is —CO—O— or —O—CO—; Y is S or O; R1 is a linear or branched alkylene consisting of 1 to 6 carbons; and R2 and R3 are the same or different, consisting of a hydrogen or a linear or branched alkyl consisting of 1 to 6 carbons; and n is 1-6; or a salt, a solvate, or a pharmaceutical formulation thereof.