Radical Cation Probes for Photoinduced Intramolecular Electron Transfer in Metal−Organic Complexes

Two transition metal complexes of the type fac-(bpy)ReI(CO)3(DA)+ (where bpy = 2,2‘-bipyridine and DA is a pyridine ligand that is substituted with a 1,2-diamine electron donor) have been prepared. The 1,2-diamine serves as a “reactive donor ligand” owing to its propensity to undergo rapid C−C bond fragmentation when activated by single electron transfer oxidation. Photoexcitation of the diamine complexes affords a ligand-to-ligand charge transfer (LLCT) state via intramolecular electron transfer quenching of a metal-to-ligand charge transfer (MLCT) state, [(bpy)ReI(CO)3(DA)]+ + hν → [(bpy•-)ReII(CO)3(DA)]+*(MLCT) → [(bpy•-)ReI(CO)3(DA•+)]+*(LLCT). Photochemical product and quantum efficiency studies indicate that the diamine reactive donor ligand undergoes photoinduced C−C bond fragmentation with high efficiency, presumably via the radical cation (DA•+) which is present in the LLCT excited state. Laser flash photolysis allows direct detection of the metal complex based radicals that are formed by C−C bond fragmentation. Quantitative kinetic information gathered through luminescence, laser flash photolysis, and quantum yield studies allows estimation of the rates for formation of the LLCT state by forward electron transfer (kFET), decay of the LLCT state by back electron transfer (kBET), and the rate of diamine radical cation bond fragmentation in the LLCT state (kBF). The relationship between these kinetic parameters and the driving force for electron transfer and bond fragmentation as well as the structure of the reactive donor ligands is discussed

Bonded Exciplex Formation: Electronic and Stereoelectronic Effects

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes. Examples of competing reactions to form both conventional and bonded exciplexes are presented, which illustrate the delicate balance between these two processes when their reaction energetics are similar. Experimental and computational evidence is provided for the formation of a bonded exciplex in the reaction of the singlet excited state of 2,6,9,10-tetracyanoanthracene (TCA) with an oxygen-substituted donor, dioxane, thus expanding the scope of bonded exciplexes

Bonded Exciplexes. A New Concept in Photochemical Reactions

Charge-transfer quenching of the singlet excited states of cyanoaromatic electron acceptors by pyridine is characterized by a driving force dependence that resembles those of conventional electron-transfer reactions, except that a plot of the log of the quenching rate constants versus the free energy of electron transfer is displaced toward the endothermic region by 0.5−0.8 eV. Specifically, the reactions with pyridine display rapid quenching when conventional electron transfer is highly endothermic. As an example, the rate constant for quenching of the excited dicyanoanthracene is 3.5 × 109 M-1 s-1, even though formation of a conventional radical ion pair, A•-D•+, is endothermic by ∼0.6 eV. No long-lived radical ions or exciplex intermediates can be detected on the picosecond to microsecond time scale. Instead, the reactions are proposed to proceed via formation of a previously undescribed, short-lived charge-transfer intermediate we call a “bonded exciplex”, A-−D+. The bonded exciplex can be formally thought of as resulting from bond formation between the unpaired electrons of the radical ions A•- and D•+. The covalent bonding interaction significantly lowers the energy of the charge-transfer state. As a result of this interaction, the energy decreases with decreasing separation distance, and near van der Waals contact, the A-−D+ bonded state mixes with the repulsive excited state of the acceptor, allowing efficient reaction to form A-−D+ even when formation of a radical ion pair A•-D•+ is thermodynamically forbidden. Evidence for the bonded exciplex intermediate comes from studies of steric and Coulombic effects on the quenching rate constants and from extensive DFT computations that clearly show a curve crossing between the ground state and the low-energy bonded exciplex state

Excited-State Structure and Delocalization in Ruthenium(II)−Bipyridine Complexes That Contain Phenyleneethynylene Substituents

A comprehensive photophysical study has been carried out on the two complexes [(bpy)2Ru(4,4‘-PE-bpy)]2+ and [(bpy)2Ru(5,5‘-PE-bpy)]2+ (44Ru and 55Ru, respectively, where bpy = 2,2‘-bipyridine and PE = phenyleneethynylene). The objective of this work is to determine the effect of the phenyleneethynylene substituents on the properties of the metal-to-ligand charge-transfer excited state. The complexes have been characterized by using UV−visible absorption, photoluminescence, and UV−visible and infrared transient absorption spectroscopy. The results indicate that the MLCT excited state is localized on the PE-substituted bpy ligands. Moreover, the photophysical data indicate that in the MLCT excited state the excited electron is delocalized into the PE substituents and the manifestations of the electronic delocalization are larger when the substituents are in the 4,4‘-positions