Spin-chemical effects on intramolecular photoinduced charge transfer reactions in bisphenanthroline copper(i)-viologen dyad assemblies.

Two covalently linked donor-acceptor copper phenanthroline complexes (C-A dyads) of interest for solar energy conversion/storage schemes, [Cu(i)(Rphen(OMV)2 4+)2]9+ = RC+A4 8+ with RC+ = [Cu(i)Rphen2]+ involving 2,9-methyl (R = Me) or 2,9-phenyl (R = Ph)-phenanthroline ligands that are 5,6-disubstituted by 4-(n-butoxy) linked methylviologen electron acceptor groups (A2+ = OMV2+), have been synthesized and investigated via quantum chemical calculations and nanosecond laser flash spectroscopy in 1,2-difluorobenzene/methanol (dfb/MeOH) mixtures. Upon photoexcitation, charge transfer (CT) states RC2+A+A3 6+ are formed in less than one ns and decay by charge recombination on a time scale of 6-45 ns. The CT lifetime of RC2+A+A3 6+ has a strong dependence on MeOH solvent fraction when R = Me, but is unaffected if R = Ph. This solvent effect is due to coordination of MeOH solvent in MeC+A4 8+ (i.e. exciplex formation) allowed by conformational flattening of the ligand sphere, which cannot occur in PhC+A4 8+ having bulkier Phphen ligand framework. Interestingly, the decay time of the CT state increases for both species at low magnetic fields with a maximum increase of ca. 30% at ca. 150 mT, then decreases as the field is increased up to 1500 mT, the highest field investigated. This magnetic field effect (MFE) is due to magnetic modulation of the spin dynamics interconverting 3CT and 1CT states. A quantitative modeling according to the radical pair mechanism involving ab initio multireference calculations of the complexes revealed that the spin process is dominated by the effect of Cu hyperfine coupling. The external magnetic field suppresses the hyperfine coupling induced spin state mixing thereby lengthening the CT decay time. This effect is counteracted by the field dependent processes of T0-S mixing through the Δg-mechanism and by a local mode spin-orbit mechanism. Further, the maximum MFE is limited by a finite rate of direct recombination of 3CT states and the spin-rotational mechanism of spin relaxation. This study provides a first comprehensive characterization of Cu(ii)-complex spin chemistry and highlights how spin chemistry can be used to manipulate solar energy harvesting and storage materials.

Cyclic voltammetric study of cobalt poly-4-t-butylpyridine ligand complexes on glassy carbon electrodes: electrolyte dependence and mechanistic considerations.

Polypyridyl complexes of Co(II/III) have been gaining prominence as potential replacements for I(-)/I(3)(-) as mediators in dye sensitized solar cells. In that regard, homoleptic pseudo-octahedral complexes of 4,4'-di-t-butyl-2,2'-bipyridine, [Co(DTB)(3)](2+/3+), and 4,4',4″-tri-t-butyl-2,2':6,2″-terpyridine, [Co(TTT)(2)](2+/3+), have been of particular interest. These complexes show extreme electrode surface and electrolyte dependent electrochemical behavior. Below, we report on the cyclic voltammetric behavior of these two complexes at glassy carbon electrodes in two different electrolytes. The electrochemical data suggests that the Co(II/III) electron transfer is significantly nonadiabatic, especially for [Co(DTB)(3)](2+/3+) in LiClO(4) electrolyte.

Photoinduced multistep charge separation in a heteroleptic Cu(I) bis(phenanthroline)-based donor-chromophore-acceptor triad.

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.

Enhanced photocatalytic properties of nanoclustered P-doped TiO2 films deposited by advanced atmospheric plasma jet.

A facile preparation of P-doped TiO2 nanoclusters onto fluorine-doped tin oxide (FTO) glass by an advanced atmospheric plasma jet (AAP jet) is reported here. Titanium tetrachloride (TiCl4) and phosphorus trichloride (PCl3) were used as precursors. Radio frequencies were used to generate plasma at fix powder with Argon as carrier gas. Films were deposited at 500 degrees C for 10 minutes. For comparison, as-prepared, annealed and deposited at 500 degrees C samples were studied for chemical/physical properties by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Optical properties were studied by using UV-Vis spectroscopy which indicated a reduction in optical band with P-doping. The rhodamine B (Rh-B) degradation by P-doped TiO2 deposited at 500 degrees C showed enhanced degradation efficiency than that of annealed TiO2. The suggested deposition method appears to be suitable for the synthesis of photocatalyst with proper control over dopants.

Fast one-step method to synthesize TiO2 nanoparticle clusters for dye sensitized solar cells.

We have successfully designed a rapid method for producing dye sensitized solar cells (DSSC) using TiO2 films prepared by a modified dielectric barrier discharge jet (m-DBD jet) method which uses a DBD jet with elevated substrate temperatures from room temperature (RT) to 500 degrees C for approximately 10 min. This facile process has several advantages over other methods such as (1) eliminating additional coating and annealing steps, (2) creating films with high speed electron mobility via hierarchical pore clusters, and (3) allowing controlled TiO2 bandgap by N doping using atmospheric nitrogen instead of supplying N2 gas. Depending on reaction conditions, the resulting nanostructured materials have various sizes and shapes, with those deposited at the highest substrate temperatures displaying hierarchical walnut-shaped morphology as revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A possible growth mechanism of TiO2 nanoparticle clusters (TNC) is presented and discussed. Finally, this m-DBD jet method produces TNC films that exhibit approximately 4 times higher photo-conversion efficiency than the nanoparticle films by the unmodified DBD jet method.

Establishment of structure-conductivity relationship for tris(2,2'-bipyridine) ruthenium ionic C(60) salts.

Three ionic C(60) salts with [Ru(bpy)(3)](m+) (bpy = 2,2'-bipyridine) as cations were synthesized. The UV-vis-NIR spectra, XPS spectra, and elemental analysis have demonstrated their compositions: [Ru(bpy)(3)](2)(C(60)) (1), [Ru(bpy)(3)](C(60)) (2), and [Ru(bpy)(3)](C(60))(2) (3). Single crystals of polycrystalline compounds 1 and 2 were obtained as solvates. At room temperature, all three salts are semiconducting with the highest four-probe conductivity observed for compound 1 at ∼10 S m(-1). The electronic conduction mechanisms can be described appropriately by an electron hopping model in this immobilized polyvalent redox system.

Mesoporous TiO2 films fabricated using atmospheric pressure dielectric barrier discharge jet.

TiO2 nanoparticles were synthesized by a facile method of dielectric barrier discharge jet (DBD jet) for the dye-sensitized solar cell (DSSC) and other potential applications. DBD jet is utilized as a method for deposition of TiO2 nanoparticles with a 9 µm/min growth rate which is more than ×25 faster than reported previously. Their performance was compared with cells fabricated using commercial TiO2 nanoparticles (P25). The crystallinity and chemical bonding states of samples were characterized by XRD and XPS. Photoanodes fabricated by the DBD jet method resulted in approximately 50% higher photoconversion efficiency than ones prepared from P25 nanoparticles.

cis-dichloro-bis(4,4'-dicarboxy-2,2-bipyridine)osmium(II)-modified optically transparent electrodes: application as cathodes in stacked dye-sensitized solar cells.

An optically transparent cathode was developed for use in dye-sensitized solar cells. Fluorine-doped tin oxide conducting glass modified with a monolayer of cis-dichloro-bis(4,4'-dicarboxy-2,2'-bipyridine)osmium(II) efficiently reduces the oxidized form of the redox mediator. This transparent cathode transmits light that is not absorbed by the dyed TiO2; consequently, more than one cell can operate in optical tandem (i.e., stacked). Two types of stacked cells are considered: one where both photoanodes are dyed with the same dye and one where the photoanodes are dyed with different, spectrally complementary dyes. Overall behavior of these tandem cells is compared with single-cell analogs.

Spin chemical control of photoinduced electron-transfer processes in ruthenium(II)-trisbipyridine-based supramolecular triads: 2. The effect of oxygen, sulfur, and selenium as heteroatom in the azine donor.

Nanosecond time-resolved absorption studies in a magnetic field ranging from 0 to 2.0 T have been performed on a series of covalently linked donor(PXZ)-Ru(bipyridine)3-acceptor(diquat) complexes (D-C2+-A2+). In the PXZ moiety, the heteroatom (X = O (oxygen), T (sulfur), and S (selenium)) is systematically varied to study spin-orbit coupling effects. On the nanosecond time scale, the first detectable photoinduced electron-transfer product after exciting the chromophore C2+ is the charge-separated (CS) state, D+-C2+-A+, where an electron of the PXZ moiety, D, has been transferred to the diquat moiety, A2+. The magnetic-field-dependent kinetic behavior of charge recombination (monoexponential at 0 T progressing to biexponential for all three complexes with increasing field) can be quantitatively modeled by the radical pair relaxation mechanism assuming creation of the CS state with pure triplet spin correlation (3CS). Magnetic-field-independent contributions to the rate constant kr of T+/- --> (T0,S) relaxation are about 4.5 x 10(5) s-1 for DCA-POZ and -PTZ (due to a vibrational mechanism) and 3.5 x 10(6) s-1 for DCA-PSZ (due to spin rotational mechanism). Recombination to the singlet ground state is allowed only from the 1CS spin level; spin-forbidden recombination from 3CS seems negligible even for DCA-PSZ. The field dependence of kr (field-dependent recombination) can be decomposed into the contributions of various relaxation mechanisms. For all compounds, the electron spin dipolar coupling relaxation mechanism dominates the field dependence of tau(slow) at fields up to about 100 mT. Spin relaxation due to the g-tensor anisotropy relaxation mechanism accounts for the field dependence of tau(slow) for DCA-PSZ at high fields. For the underlying stochastic process, a very short correlation time of 2 ps has to be assumed, which is tentatively assigned to a flapping motion of the central, nonplanar ring in PSZ. Finally, it has been confirmed by paramagnetic quenching (here Heisenberg exchange) experiments of the magnetic-field effects with TEMPO that all magnetic-field dependencies observed with the present DCA-PSZ systems are indeed due to the magnetic-field dependence of spin relaxation.

High energy and quantum efficiency in photoinduced charge separation.

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.

Efficient non-corrosive electron-transfer mediator mixtures for dye-sensitized solar cells.

The electrochemical and photoelectrochemical study of noncorrosive electron-transfer mediator mixtures for DSC is reported. These mixtures, characterized by the presence of kinetically fast organic or metal-organic redox couples in conjunction with a Co(II)/(III) relay, exhibited maximum photon-to-current conversion efficiencies (IPCE %) exceeding 80% when employed in the presence of a suitably designed MLCT sensitizer, rivaling with the performances of the classical I-/I3- electrolyte.

Electrochemical olefin epoxidation employing an iron(IV)porphyrin/trisbipyridineruthenium(III) polymer catalyst.

A porphyrin molecule containing four meso-appended 2,2'-bipyridyl ligands has been prepared. Each bipyridine is attached to the porphyrin core at the 4-position (pseudo-para to one of the pyridine nitrogens). Subsequently, each of the four bipyridines was complexed with a RuL2(2+) moiety and iron coordinated in the porphyrin core. When L = 4-vinyl-4'-methyl-2,2'-bipyridine, reduction of the ruthenium centers resulted in the formation of robust electroactive polymer films which deposited on the electrode surface. In the presence of aqueous acid, these films electrocatalytically epoxidize cyclohexene at positive potentials from the formal iron(IV) oxidation state. Although product analysis has only been conducted for cyclohexene, the catalytic activity extends to a large variety of olefins including ethylene and propylene.

Indium tin oxide electrodes modified with tris(2,2'-bipyridine-4,4'-dicarboxylic acid) iron(II) and the catalytic oxidation of tris(4,4'-di-tert-butyl-2,2'-bipyridine) cobalt(II).

Indium tin oxide (ITO) electrodes modified by attachment of tris(2,2'-bipyridine-4,4'-dicarboxylic acid) iron(II) are examined. The mode of attachment is believed to be via the COOH functions in a manner similar to attachment of similar carboxylate-containing compounds to TiO2 surfaces. On the surface the complex resides as a stable electrochemically active monolayer. These modified electrodes can efficiently catalyze the oxidation of certain cobalt complexes, specifically, tris(4,4'-di-tert-butyl-2,2'-bipyridine) cobalt(II). On the unmodified ITO surfaces this cobalt complex is essentially electrochemically inert. The catalytic process approaches diffusional control at very slow scan speeds. Also, the electro-catalysis is sufficiently efficient that the peak oxidation current for Co2+, under certain conditions, exceeds the i(p) for the surface oxidation of the adsorbed Fe2+ by >x100 and the current for the uncatalyzed oxidation of Co2+ by considerably more than that.

Substituted polypyridine complexes of cobalt(II/III) as efficient electron-transfer mediators in dye-sensitized solar cells.

A number of cobalt complexes of substituted polypyridine ligands were synthesized and investigated as possible alternatives to the volatile and corrosive iodide/triiodide redox couple commonly used as an electron-transfer mediator in dye-sensitized solar cells (DSSCs). The extinction coefficients in the visible spectrum are on the order of 10(2) M(-1) cm(-1) for the majority of these complexes, diminishing competition with the light-harvesting dye. Cyclic voltammetric studies revealed a dramatic surface dependence of the heterogeneous electron-transfer rate, which is surprisingly different for gold, carbon, and platinum electrodes. DSSCs were assembled using a mediator that consisted of a mixture of Co(II) and Co(III) complexes in a 10:1 ratio. DSSCs containing these mediators were used to characterize incident photon-to-current conversion efficiency and photoelectrochemical responses. The best performing of these mediators were identified and subjected to further study. As suggested by electrochemical results, gold and carbon are superior cathode materials to platinum, and no evidence of corrosion on any cathode material was observed. Addition of lithium salts to the mediator solution resulted in a dramatic improvement in cell performance. The observed Li(+) effect is explained in terms of the recombination of injected electrons in the photoanode with the oxidized mediator. The best mediator, based on tris(4,4'-di-tert-butyl-2,2'-dipyridyl)cobalt(II/III) perchlorate, resulted in DSSCs exhibiting efficiencies within 80% of that of a comparable iodide/triiodide-mediated DSSC. Due to the commercial availability of the ligand and the simplicity with which the complex can be made, this new mediator represents a nonvolatile, noncorrosive, and practical alternative as an efficient electron-transfer mediator in DSSCs.