Aspects of the present invention are directed to working fluids and their use in processes wherein the working fluids comprise compounds having the structure of formula (I): wherein R1, R2, R3, and R4 are each independently selected from the group consisting of: H, F, Cl, Br, and C1-C6 alkyl, at least C6 aryl, at least C3 cycloalkyl, and C6-C15 alkylaryl optionally substituted with at least one F, Cl, or Br, wherein formula (I) contains at least one F and at least one Cl or Br, provided that if any R is Br, then the compound does not have hydrogen. The working fluids are useful in Rankine cycle systems for efficiently converting waste heat generated from industrial processes, such as electric power generation from fuel cells, into mechanical energy or further to electric power. The working fluids of the invention are also useful in equipment employing other thermal energy conversion processes and cycles.