Multiphoton, Multielectron Transfer Photochemistry in a Soluble Polymer

[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 (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.

On the Quenching of MLCTRe→bpy Luminescence by Cu(II) Species in Re(I) Polymer Micelles

Transmission electron microscopy (TEM) and dynamic light scattering (DLS) studies on acetonitrile solutions of the polymer {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 demonstrated that the Re(I) polymer molecules aggregate to form spherical micelles of radius R = 156 nm. Coordination of Cu(II) species to the Re (I) polymer causes a decrease in the micelle radius and a distortion from the spherical shape. Besides, the coordination of Cu(II) species to the {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 polymer produces the quenching of the metal to ligand charge transfer (MLCT) excited state by energy transfer processes that are more efficient than those in the quenching of the monomer pyRe(CO)3bpy+ luminescence by Cu(II). Moreover, the kinetics of the quenching by Cu(II) do not follow a Stern-Volmer behavior. Conversely, the quenching of the MLCT luminescence of the Re(I) polymer by the sacrificial electron donor 2,2',2' '-nitrilotriethanol, TEOA, follows a Stern-Volmer kinetics. A comparison is made between the quenching by CuX2 (X = Cl or CF3SO3) and TEOA.

Contrasting Intrastrand Photoinduced Processes in Macromolecules Containing Pendant −Re(CO)3(1,10-phenanthroline)+: Electron versus Energy Transfer

The photochemical and photophysical properties of the polymers [(vpy-CH3+)2-vpyRe(CO)3(phen)+]200 (vpy = vinyl pyridine, phen = 1,10-phenanthroline) have been investigated in solution phase and compared to those of a related polymer, [(vpy)2-vpyRe(CO)3(phen)+]200, and monomer, pyRe(CO)3(phen)+. Irradiations at 350 nm induce intrastrand charge separation in the peralkylated polymer, a process that stands in contrast with the energy migration observed with [(vpy)(2)-vpyRe(CO)3(phen)+]200. Electronically excited -vpyRe(CO)3(phen)+ chromophores and charge-separated intermediates react with neutral species, e.g., 2,2',2' '-nitrilotriethanol, and anionic electron donors, e.g., SO3(2-) and I-. The anionic electron donors react more efficiently with the metal-to-ligand charge transfer excited state of these polyelectrolytes than with the excited state of pyRe(CO)3(phen)+.

Metal‐Containing Polymers

The focus of this article is on the synthesis, properties, and applications of metal‐containing polymers. These polymers may be classified based on the types of metal–ligand bonding, and whether the metal is an integral part of the polymer backbone or pendent to the polymer backbone. The first class of polymers discussed are those with metals σ‐bonded to organic spacers in their backbones. Examples of metal–metal single and multiple bonds are also described. Metallocenes and coordination polymers are two classes of polymers where the metal is π‐coordinated to the organic ligands. The coordination of metals to porphyrins, phthalocyanines, Schiff base ligands, as well as to many other types of ligands are described. Another major class of polymers are those where the metal is pendent to the polymer backbone. These polymers include materials with metallic moieties π‐coordinated to unsaturated ligands in their backbones. Polymers with metallic moieties in their side chains are also described. A brief overview of star polymers and dendrimers is also included.

DNA photocleavage by a supramolecular Ru(II)-viologen complex

A novel Ru(II) complex possessing two sequentially linked viologen units, Ru-V(1)-V(2)(6+), was synthesized and characterized. Upon excitation of the Ru(II) unit (lambda(exc) = 532 nm, fwhm approximately 10 ns), a long-lived charge-separated (CS) state is observed (tau = 1.7 micros) by transient absorption spectroscopy. Unlike Ru(bpy)(3)(2+), which cleaves DNA upon photolysis through the formation of reactive oxygen species, such as (1)O(2) and O(2)(-), the photocleavage of plasmid DNA by Ru-V(1)-V(2)(6+) is observed both in air and under N(2) atmosphere (lambda(irr) > 395 nm).

Electronic energy transfer and collection in luminescent molecular rods containing ruthenium(II) and osmium(II) 2,2':6',2"-terpyridine complexes linked by thiophene-2,5-diyl spacers

The electronic absorption spectra, luminescence spectra and lifetimes (in MeCN at room temperature and in frozen n-C3H7CN at 77 K), and electrochemical potentials (in MeCN) of the novel dinuclear [(tpy)Ru(3)Os(tpy)]4+ and trinuclear [(tpy)Ru(3)Os(3)Ru(tpy)]6- complexes (3 = 2,5-bis(2,2':6',2''-terpyridin-4-yl)thiophene) have been obtained and are compared with those of model mononuclear complexes and homometallic [(tpy)Ru(3)Ru(tpy)]4+, [(tpy)Os(3)Os(tpy)]4+ and [(tpy)Ru(3)Ru(3)Ru(tpy)]6+ Complexes. The bridging ligand 3 is nearly planar in the complexes, as seen from a preliminary X-ray determination of [(tpy)Ru(3)Ru(tpy)][PF6]4, and confers a high degree of rigidity upon the polynuclear species. The trinuclear species are rod-shaped with a distance of about 3 nm between the terminal metal centres. For the polynuclear complexes, the spectroscopic and electrochemical data are in accord with a significant intermetal interaction. All of the complexes are luminescent (phi in the range 10(-4)-10(-2) and tau in the range 6-340 ns, at room temperature), and ruthenium- or osmium-based luminescence properties can be identified. Due to the excited state properties of the various components and to the geometric and electronic properties of the bridge, Ru --> Os directional transfer of excitation energy takes place in the complexes [(tpy)Ru(3)Os(tpy)]4+ (end-to-end) and [(tpy)Ru(3)Os(3)Ru(tpy)]6+ (periphery-to-centre). With respect to the homometallic case, for [(tpy)Ru(3)Os(3)Ru(tpy)]6+ excitation trapping at the central position is accompanied by a fivefold enhancement of luminescence intensity.

Mimicking the antenna-electron transfer properties of photosynthesis

A molecular assembly based on derivatized polystyrene is described, which mimics both the light-harvesting and energy-conversion steps of photosynthesis. The system is unique in that the two key parts of a photosynthetic system are incorporated in a functional assembly constructed from polypyridine complexes of Ru(II). This system is truly artificial, as none of the components used in construction of the assembly are present in a natural photosynthetic system. Quantitative evaluation of the energy and electron transfer dynamics after transient irradiation by visible light offers important insights into the mechanisms of energy transport and electron transfer that lead to photosynthetic light-to-chemical energy conversion.

Ground-State Properties and Excited-State Reactivity of 8-Quinolate Complexes of Ruthenium(II)

In an effort to explore new systems with highly reducing excited states, we prepared a series of Ru(II) complexes of the type Ru(L)(2)quo(+) (L = bpy (2,2'-bipyridine), phen (1,10-phenanthroline), dmphen (4,7-dimethyl-1,10-phenanthroline), tmphen (3,4,7,8-tetramethyl-1,10-phenanthroline); quo(-) = 8-quinolate) and investigated their photophysical and redox properties. The absorption and emission spectra of the Ru(L)(2)quo(+) are significantly red-shifted relative to those of the parent complexes Ru(L)(3)(2+), with emission maxima in the 757-783 nm range in water. The Ru(L)(2)quo(+) systems are easily oxidized with E(1/2)(Ru(III/II)) values ranging from +0.62 to +0.70 V vs NHE, making the emissive Ru --> phen MLCT (metal-to-ligand charge transfer) excited states (E(00) approximately 1.95 eV in CH(3)CN) of the Ru(L)(2)quo(+) complexes significantly better reducing agents than the MLCT states of the parent Ru(L)(3)(2+) complexes. Emission lifetimes of 17.0 and 32.2 ns were measured for Ru(phen)(2)quo(+) in water and acetonitrile, respectively, and 11.4 ns for Ru(bpy)(2)quo(+) in water. Transient absorption results are consistent with the formation of reduced methyl viologen upon Ru(phen)(2)quo(+) excitation with visible light in water. The possibility of observing the Marcus inverted region in the forward bimolecular electron transfer reaction from the highly reducing Ru(phen)(2)quo(+) excited state was explored with neutral electron acceptors with reduction potentials ranging from +0.25 to -1.15 V vs NHE.