What is described is a compound of formula I wherein X is an ethene, or an unsubstituted or substituted arene or heteroarene; Y is a bond, an ethene, or an unsubstituted or substituted arene or heteroarene; Z is S or O; L is a linear or branched alkylene of 1 to 6 carbons; R1 and R2 are independently a linear or branched alkyl or alkenyl of 1 to 18 carbons; R3 and R4 are independently a linear or branched alkyl of 1 to 6 carbons; n is 0 to 6; and m, p, q, and r are independently 1-18; or a pharmaceutically acceptable salt thereof.