ustainable energy conversion, photocatalysis and electrochemical energy storage require molecular systems capable of efficient electron-transfer processes in daily life. This dissertation explores such processes through studies in tetrapyrrole chemistry, indium coordination chemistry and aqueous flow batteries including synthetic, spectroscopic, electrochemical and computational approaches. The first chapter covers chlorin-based tetrapyrroles and their photochemical applications. Publication I presents that a water-soluble tin(IV) chlorin can function as a red light-responsive photocatalyst for nicotinamide and flavin cofactors recycling in aqueous media. Spectroscopic and computational investigations provided evidence for the formation of chlorin–phlorin intermediates involved in proton-coupled electron-transfer and hydride-transfer reactions, offering new insights into artificial photosynthesis and light-driven redox chemistry. The second chapter covers indium coordination chemistry. Publication II presents the first successful synthesis and characterization of indium(III) corrole complexes, overcoming a longstanding challenge in metallocorrole chemistry. Publication III presents the structural characterization of a novel deuterated pyridine indium(III) chloride complex. Together, these studies broaden the understanding of indium-containing tetrapyrrole and coordination compounds. The third chapter covers aqueous redox flow batteries from fundamental to advanced aspects. Publication IV presents phenoxazine radicals as stable positive electrolyte materials for neutral-pH aqueous flow batteries. Publication V reveals a capacity-recovery development for chloranilate-based quinonoid in alkaline flow batteries, and Publication VI examines Eu³⁺/TEMPTMA⁺ battery system operating at neutral-pH. All together, these studies advance the understanding of electrolyte stability, charge-storage mechanisms, and overall battery performance in aqueous flow battery systems.