(Ru(bpy) 3 ) 2+ * and other remarkable metal-to- ligand charge transfer (MLCT) excited states*

In 1974, the metal-to-ligand charge transfer (MLCT) excited state, (Ru(bpy) 3 ) 2+ *, was shown to undergo electron transfer quenching by methylviologen dication (MV 2+ ), inspiring a new approach to artificial photosynthesis based on molecules, molecular-level phenomena, and a "modular approach". In the intervening years, application of synthesis, excited-state measurements, and theory to (Ru(bpy) 3 ) 2+ * and its relatives has had an outsized impact on photochemistry and photophysics. They have provided a basis for exploring the energy gap law for nonradiative decay and the role of molecular vibrations and solvent and medium effects on excited-state properties. Much has been learned about light absorption, excited-state electronic and molecular structure, and excited-state dynamics on timescales from femtoseconds to milliseconds. Excited-state properties and reactivity have been exploited in the investigation of electron and energy transfer in solution, in molecular assem- blies, and in derivatized polymers and oligoprolines. An integrated, hybrid approach to solar fuels, based on dye-sensitized photoelectrosynthesis cells (DSPECs), has emerged and is being actively investigated.