Russell H. Schmehl

The photophysical behavior of d6 complexes having nearly isoenergetic MLCT and ligand localized excited states

Abstract A large number of second- and third-row d6 transition metal complexes with N-heterocyclic ligands have been prepared in which the lowest energy excited state is a metal-to-ligand charge transfer (MLCT) state. In some of these complexes ligand localized (IL) excited states exist which are of the same spin multiplicity as the MLCT state and are energetically accessible from the 1MLCT state. Several complexes have been examined which exhibit luminescence from two states in frozen matrices. In fluid solutions, however, only a few examples exist of complexes having two non-equilibrated triplet excited states. This article provides a brief overview of the experimental methods commonly used in the analysis of triplet excited state formation and relaxation in transition metal complexes having coexisting triplet excited states. In addition, descriptive examples are given of complexes for which both 3IL and 3MLCT states can be populated following excitation into the 1MLCT state.

Straightforward Self-Assembly of Porphyrin Nanowires in Water: Harnessing Adamantane/β-Cyclodextrin Interactions

A convenient approach for the self-assembly of well-defined porphyrin nanowires in water, wherein the individual monomers do not aggregate via pi-pi interactions, is disclosed. These unidirectional and heteromeric assemblies are instead composed of robust beta-CD/adamantane host/guest interactions. A combination of surface microscopies and fluorescence energy transfer experiments were conducted on the nanowires demonstrating their stability and resistance to disassembly.

The effect of redox site concentration on the rate of mediated oxidation of solution substrates by a redox copolymer film

Abstract Copolymer films of the complexes [Ru(II)(bpy) 2 (p-cinn) 2 ] 2+ and [Os(II)(bpy) 2 (p-cinn) 2 ] 2+ (p-cinn = N -(4-pyridyl)cinnamamide) are formed by reductive electrochemical polymerization of solutions of the two complexes at platinum electrodes. The films are redox active and mediate the oxidation of solution substrates via Os(III) ( E 0′ =0.64 V) and Ru(III) ( E 0 =1.13 V) sites in the film. At potentials where only Os(II) is oxidized the rates of oxidation of [Ru(bpy) 2 (NO)Cl] + and of [Ru(i-pr-bpy) 2 Cl 2 ] (i-pr-bpy= 4,4′-di-iso-propylcarboxy-2,2′-bipyridine) depend on the concentration of Os(III) sites in the copolymer film. Lowering the mole fraction of Os(II) in the copolymer results in decreases in the mediated oxidation rate. At low Os(II) fractions, oxidation of [Ru(i-pr-bpy) 2 Cl 2 ] by Os(III) is limited exclusively by the rate of transport of holes through the film. As the fraction of Os(II) in the copolymer increases, the limiting rate of substrate oxidation is influenced by several processes including diffusion of the substrate within the film and the rate of the electron exchange reaction. Rotated disk voltammetric data for substrate oxidation by the films are analyzed to obtain kinetic parameters for the competing processes.

Mechanistic details for cobalt catalyzed photochemical hydrogen production in aqueous solution: efficiencies of the photochemical and non-photochemical steps.

A detailed examination of each step of the reaction sequence in the photochemical sacrificial hydrogen generation system consisting of [Ru(bpy)3](2+)/ascorbate/[Co(DPA-bpy)OH2](3+) was conducted. By clearly defining quenching, charge separation, and back electron transfer in the [Ru(bpy)3](2+)/ascorbate system, the details necessary for evaluation of the efficiency of water reduction with various catalysts are provided. In the particular Co(III) catalyst investigated, it is clear that the light induced catalytic process is considerably less efficient than the electrocatalytic process. A potential source of catalyst inefficiency in this system is reduction of the products formed in oxidation of the sacrificial electron donor. The data provided for excited state quenching and charge separation in this particular aqueous system are meant to be used by others for thorough investigation of the quantum efficiencies of other aqueous homogeneous and nanoheterogeneous catalysts for water reduction.

Photochemistry and Photophysics of Coordination Compounds: Osmium

This review provides an overview of research since the year 2000 relating to photochemical and photophysical processes involving complexes of osmium. The review is broken into four focus subjects. The photophysical investigations of Os(II) imine complexes and other classical transition metal complexes is presented, including a brief discussion of theoretical approaches to understanding electronic structure. An overview of light induced energy and electron transfer reactions involving Os complexes is also covered. Photoreactions of organometallic Os complexes are discussed, focusing on mechanistic aspects of metal–metal cleavage in diosmium and triosmium carbonyl complexes. Finally, reports of applications of Os complexes to sensors, dye sensitized solar cells, electroluminescent devices, and biological systems are presented.

Preferential solvation of an ILCT excited state in bis(terpyridine–phenylene–vinylene) Zn(II) complexes

The excited state of terpyridine derivatives of phenylene-vinylene fragments chelating Zn(II) show a strong solvatochromism (up to 56 nm) upon preferential solvation by polar solvents of an intraligand charge transfer state.

A Fluorescent 18-Crown-6 Based Luminescence Sensor for Lanthanide Ions

Abstract The synthesis and photophysical behavior of a luminescent chemosensor for the early lanthanide elements is described. The sensor is a 1,4-diphenylethynyl-benzene chromophore having 18-crown-6 moieties bound to the outer phenyl rings. The chromophore is luminescent and the emission is quenched by lanthanide (Ln3+) ions with larger ionic radii and existing f–f transitions (Ce3+, Pr3+ and Nd3+). Alkali, alkaline earth ions and lanthanides with smaller radii (Ge3+, Tb3+, Dy3+, and Yb3+) do not affect the emission.

The influence of bridging ligand electronic structure on the photophysical properties of noble metal diimine and triimine light harvesting systems.

This manuscript discusses the photophysical behavior of transition metal complexes of Ru(II) and Os(II) employed in development of light harvesting arrays of chromophores. Particular emphasis is placed on the relationship between the photophysical behavior of complexes having metal-to-ligand charge transfer (MLCT) excited states and the electronic characteristics of bridging ligands used in preparing oligometallic complexes. Examples are presented that discuss intramolecular energy migration in complexes having two distinct MLCT chromophores with bridging ligands that only very weakly couple the two chromophores. In addition, systems having bridging ligands with localized triplet excited states lower in energy than the MLCT state of the metal center to which they are attached are discussed. These systems very often have excited states localized on the bridging ligand with excited state lifetimes on the order of tens of microseconds. Finally, systems having Fe(II) metal centers, with very low energy MLCT states, are discussed. In complexes also containing bridging ligands with low energy triplet states, energy partitioning between the Fe center MLCT state (or Fe localized ligand field states) and the ligand triplet state is observed; the two states relax to the ground state via parallel pathways, but the Fe(II) center does not serve as an absolute excitation energy sink.

pH control of intramolecular energy transfer and oxygen quenching in Ru(II) complexes having coupled electronic excited states.

This work illustrates the control of excited state energy transfer processes via variation of pH in transition metal complexes. In these systems a Ru(II) complex having two carboxylated bipyridyl ligands is covalently linked to pyrene via one of two different pyrene derivitized bipyridyl ligands. The energy of the Ru to carboxy-bipyridine (3)MLCT state is pH dependent while the pyrene triplet energy remains unchanged with solution acidity. At pH 0 the (3)MLCT state is the lowest energy state, and as the pH is raised and the carboxy-bipyridyl ligands are successively deprotonated, the energy of the (3)MLCT state rises above that of the pyrene triplet, resulting in a significant increase in the lifetime of the observed emission. Detailed analysis of ultrafast and microsecond time-resolved excited state decays result in a description of excited state decay that involves initial equilibration of the (3)MLCT and pyrene triplet states followed by relaxation to the ground state. The lifetime of excited state decay is defined by the position of the equilibrium, going from 2 μs at pH 0 to >10 μs at higher pH as the equilibrium favors the pyrene triplet. In addition, quenching of the excited state by dissolved oxygen exhibits a pH dependence that parallels that of the excited state lifetime. The results illustrate the utility of exploiting excited state equilibria of this type in the development of highly effective luminescent oxygen sensors.

An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy

This article is a short review that presents a short review of photosubstitution reactions of Ru(II) imine complexes and illustrates their use in the development of therapeutic agents. The review begins with an overview of the photophysical behavior and common photoreactions of Ru(II) imine complexes, with select examples from the literature since the 1960s. It is followed by a more detailed picture of the application of knowledge gained over the years in the development of Ru(II) complexes for photobiology and photodynamic therapy.