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Nanosecond time-resolved absorption studies in a magnetic field ranging from zero to 3.0 T have been performed on a series of covalently linked donor-Ru(bipyridine)3-acceptor complexes (D-C2+-A2+). In these complexes the electron donor is a phenothiazine moiety linked to a bipyridine by a (-CH2-)p (p = 1, 4, 5, 7) chain, and the electron acceptor is an N,N'-diquaternary-2,2'-bipyridinium moiety, linked to a bipyridine by a (-CH2-)2· chain. On the nanosecond time scale the first detectable photoinduced electron-transfer product after exciting the complex C2+ is the charge-separated (CS) state, D+-C2+-A+, where an electron of the phenothiazine moiety, D, has been transferred to the diquat moiety, A2+. In zero field the lifetime of the CS state is about 150 ns. At low fields (B0 < 0.5 T) the magnetic field strongly affects the decay kinetics, splitting it up into a major component, the rate constant of which decreases by a factor of about 10 at fields of several 100 mT, and a minor component with an approximately field independent rate constant. At high fields (B0 > 0.5 T) the total amplitude of the CS absorption signal decreases and the relative contribution of the fast decaying component increases. The magnetic field effects can be consistently interpreted and quantitatively modeled by taking into account the mechanisms and kinetics of the spin multiplicity changes in the CS state and its precursor, a short-lived CT state (D-C3+-A+) formed upon primary electron transfer from the triplet excited complex to the diquat moiety. Exploiting the magnetic field dependent kinetics, the rate constants of the triplet-singlet transitions in the two types of linked radical pairs and of all the electron-transfer processes following the primary one can be assessed. Magnetic-field-dependent investigations thus can be essential for the understanding of the complex kinetics in supramolecular systems with sequential cyclic electron transfer.

The spectral parameters for the optically induced intervalence charge transfer and the rates of thermal electron transfer as a function of temperature have been measured for a rigid, triply linked mixed-valence dinuclear tris(2,2‘-bipyridine)iron complex. The total reorganizational energy associated with the intramolecular electron exchange in this complex is almost exclusively outer-sphere in nature and comes from thermal fluctuations of the solvent. Thus, the system can be treated rigorously at the classical level, where in this context classical refers to treatments of the nuclear modes. The theories developed to describe the optical electron transfer and the thermal electron transfer are evaluated by analysis of the spectral and rate data, respectively. The quantities common to both theories are the donor−acceptor coupling matrix element, H12, and the total reorganizational energy. Applying the respective theories to the appropriate corresponding sets of data yields reorganizational energies that are ...

A polymerizable ester-substituted trisbipyridine complex of ruthenium(II) has been synthesized and the solid-state electrochemiluminescence (ECL) properties of its polymer evaluated. A collection of 12 “sandwich-type” solid-state cells were investigated each having a ∼0.3 μm thick film of the polymer sandwiched between a transparent indium/tin oxide (ITO) anode and a porous Au cathode. The ECL of these devices was evaluated in the solid state. Despite considerable variability in performance, the most efficient of these devices exhibited ECL quantum yields matching the efficiency of the best organic polymer based light-emitting devices.

Coupling between donor and acceptor orbitals for optically-induced intervalence electron transfer processes has been considered for a series of rigid mixed-valent dinuclear tris(2,2‘-bipyridine)iron complexes. Each of the four complexes considered contains three saturated bridges which link the two tris(2,2‘-bipyridine)iron moieties. The bridging linkages are −CH2CH2−, −CH2CH2CH2−, −CH2OCH2−, and −CH2SCH2−. Despite differences in the composition of the bridges X-ray diffraction and/or molecular dynamics calculations show that the metal−metal separation and relative bipyridine orientations among all four complexes are nearly identical. Consequently, the only factor which differs significantly among these complexes and which might affect the donor−acceptor coupling in the mixed-valent forms is their connectivity. These complexes thus provide a unique opportunity to focus on potential superexchange coupling in the absence of ambiguities introduced by other structural and energetic considerations. Theories de...

A molecular triad assembly consisting of an electron donor, a bis(phenanthroline)copper(I) chromophore, and an electron acceptor has been prepared. Under visible-light excitation, this assembly undergoes efficient (ca. 50%) photoinduced, multistep formation of a diradical cation charge-separated state that has a lifetime of >100 ns and stores >1.0 eV of energy. This system constitutes an earth-abundant functional analogue of related Ru(bpy)3 triad systems.

Intramolecular photoinduced electron transfer in a series of linked ruthenium(II) diimine donor-diquat acceptor complexes was examined as a function of the nature of the bridge between the donor and acceptor centers. Bridging groups include polymethylene chains (-(CH{sub 2}){sub n-}, n=2-7,12) and phenyl- or cyclohexyl-containing spacers (p-(-CH{sub 2}-Ph-CH{sub 2}-), p-(-CH{sub 2}-trans-cyclohexyl-CH{sub 2}-), and p-(-CH{sub 2}-cis-cyclohexyl-CH{sub 2}-)). The rate constant for electron transfer decreases sharply as the number of methylenes separating the donor and acceptor increases in straight chain alkyl bridged complexes. An odd-even alternation in the electron-transfer rate constant is observed in the straight chain series. Rate constants for electron transfer in the series of complexes bridged by the cyclic spacers having a fixed number of bonds but differing steric environments varied by only a factor of 2. Results are discussed in terms of possible through-space and through-bond interactions. The combined results suggest that the bridging ligands may serve to mediate the electron transfer in these systems. 49 refs., 7 figs., 3 tabs.

Supramolecular triad assemblies consisting of a central trisbipyridine ruthenium(II) chromophore (C2+) with one or more appended phenothiazine electron donors (D) and a diquat-type electron acceptor (A2+) have been shown to form long-lived photoinduced charge separated states (CSS) with unusually and consistently high quantum efficiency. Up to now, there has been no understanding for why these large efficiencies (often close to unity) are achieved across this entire class of triads when other, seemingly similar systems are often much less efficient. In the present study, using a bimolecular system consisting of a chromophore-acceptor diad (C2+-A2+) and an N-methylphenothiazine donor, we demonstrate that a ground-state association exists between the RuL3(2+) and the phenothiazine prior to photoexcitation. It is this association process that is responsible for the efficient CSS formation in the bimolecular system and, by inference, also must be an essential factor in the fully intramolecular process occurring with the D-C2+-A2+ triad analogues.

A series of conducting polymers have been prepared through thermal polymerization of transition-metal diimine complexes. The as-polymerized material is electrochemically converted into its formally zerovalent form. Due to the proximity of the half-wave potentials of the formal 1+/0 and 0/1- couples, there is substantial disproportionation of the redox sites at room temperature, resulting in a conductive tervalent mixed-valent material. The redox processes that give rise to this mixed-valent material are predominantly ligand-based, and therefore are highly sensitive to substitution on the ligand periphery. Solution redox chemistry of the monomer can be used to accurately predict the work function of the corresponding zerovalent conducting polymer, which has been verified by ultraviolet photoelectron spectroscopy. Many of these materials have especially low work functions (

A new electrochemical approach for detecting target DNA is described. The scheme involves the catalytic oxidation of a cobalt bipyridine “reporter molecule” in solution. Probe/target DNA duplexes (ds-DNA), bound on an ITO electrode, selectively recruit redox catalyst molecules from solution. This surface-bound catalyst “turns on” the redox chemistry of the reporter molecule which is otherwise kinetically inert to oxidation on ITO. The mode of selective catalyst binding is intercalation between base pairs of ds-DNA. With this approach, we demonstrate detection of a 20-mer DNA target oligonucleotide at picomolar concentrations with outstanding signal-to-noise.

Methode de preparation et etude des proprietes electrochimiques, spectroelectrochimiques et photophysiques des composes du titre

Examples are known of chromophore-quencher complexes where following optical excitation of a metal-to-ligand charge transfer (MLCT) or a porphyrin ..pi.. ..-->.. ..pi..* chromophore, oxidative or reductive intramolecular electron-transfer quenching occurs. In a recent example, photolysis of a porphyrin-based system containing both an electron-transfer donor and an acceptor led to a relatively long-lived (3 ..mu..s) photoinduced charge separation onto peripheral donor and acceptor redox sites where in terms of the redox potential stored, ..delta..G/sup 0/ > 1.0 eV. One value of such systems is that they begin to reveal how oxidative and reductive equivalents can be generated photochemically and stored within the same molecule. They report here that MLCT-based excitation of the complex (Ru(Me(bpy)-3DQ/sup 2 +/)(Me(bpy)-PTZ)/sub 2/)/sup 4 +/ is followed by a sequence of intramolecular events which lead with relatively high efficiency to a charge-separated state based on the PTZ and -DQ/sup 2 +/ redox sites for which the transiently stored free energy is ..delta..G/sup 0/' approx. 1.29 eV.