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 (MV²⁺), 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 assemblies, 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.

Time-dependent perturbation theory and application of the Golden Rule have been shown to be quantitatively applicable to electron transfer in the inverted region, energy transfer, and excited-state decay based on spectroscopic measurements on dπ⁶ polypyridyl complexes of Ru(II), Os(II), and Re(I).

Synthesis and photophysical properties of the highly emissive complex [Ir(Fppy)₂(dmb)]⁺ are reported along with those of additional heteroleptic cyclometalated Ir(III) complexes, [Ir(ppy)₂(NN)](PF₆): FppyH = 2-(2,4-difluorophenyl)pyridine; ppyH = 2-phenylpyridine; NN = 4,4′-dimethyl-2,2′-bipyridine (dmb), 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (Ph₂phen). TD-DFT calculations and Franck–Condon emission spectral band shape analyses show that the broad and structureless emission from [Ir(Fppy)₂(dmb)]⁺ in acetonitrile at 298 K mainly arises from a triplet metal-to-ligand charge-transfer excited state, ³MLCT_Ir(ppy)→NN. The emission maximum varies systematically with variations in electron-donating or -withdrawing substituents on both the NN and the Xppy ligands, and emission efficiencies are high, with an impressive ϕ ≈ 1 for [Ir(Fppy)₂(dmb)]⁺. At 77 K in propionitrile/butyronitrile (4/5, v/v), emission from [Ir(Fppy)₂(dmb)]⁺ is narrow and highly structured consistent with a triplet ligand-centered transition (³LC_NN) and an inversion in excited-state ordering between the ³MLCT_Ir(ppy)→NN and ³LC_NN states. In a semirigid film of the poly(ethyleneglycol)dimethacrylate with nine ethylene glycol spacers, PEG-DMA550, emission from [Ir(Fppy)₂(dmb)]⁺ is MLCT-based. The thermal sensitivity of the photophysical properties of this excited state points to a possible application as a temperature sensor in addition to its more known use in light-emitting devices.