Electrolyte solutions for flow batteries and other electrochemical systems can contain an active material that is capable of transferring one or more electrons per molecule during an oxidation-reduction cycle. Doubly bridged aromatic groups or their coordination compounds can be particularly suitable active materials. Flow batteries can include a first half-cell containing a first electrolyte solution, and a second half-cell containing a second electrolyte solution, in which at least one of the first electrolyte solution and the second electrolyte solution contains an active material having at least two aromatic groups doubly bridged by a carbonyl moiety and a bridging moiety containing a bridging atom selected from carbon, nitrogen, oxygen, sulfur, selenium and tellurium. Such bridged compounds can directly function as the active material, or coordination compounds containing the bridged compounds as at least one ligand can serve as the active material.