Search

The organization of photoresponsive molecular systems and nano-materials on semiconductor surface holds great potential in the building of solar energy conversion devices where efficient energy conversion results from the optimized cooperation of several subsystems (semiconductor, dye sensitizers, redox mediator, hole transport medium), whose properties can be finely tuned through rational synthetic design. This chapter will review the fundamentals of semiconductor sensitization, a process relying on the quenching by charge transfer of molecular excited states coupled to semiconductor surfaces, and will move on by describing the structural and electronic properties of some of the most successful dye designs, used in conjunction with new electron transfer mediators in liquid electrolytes. From liquid electrolytes, a step forward is made by developing solid state hole conductors, which found their best employment in hybrid junctions with organo-halide lead perovskites, representing, at present, the most promising materials for solar-to-electric power conversion in mesoscopic solar cells. Finally, one of the most challenging tasks which can find solution by exploiting molecular level sensitized materials is discussed in detail through meaningful case studies: the production of solar fuels by photoelectrochemical water splitting.

Doping hematite with different elements is a common strategy to improve the electrocatalytic activity towards the water oxidation reaction, although the exact effect of these external agents is not yet clearly understood. Using a feasible electrophoretic procedure, we prepared modified hematite films by introducing in the deposition solution Ti(IV) butoxide. Photoelectrochemical performances of all the modified electrodes were superior to the unmodified one, with a 4-fold increase in the photocurrent at 0.65 V vs. SCE in 0.1 M NaOH (pH 13.3) for the 5% Ti-modified electrode, which was the best performing electrode. Subsequent functionalization with an iron-based catalyst led, at the same potential, to a photocurrent of ca. 1.5 mA?cm-2 , one of the highest achieved with materials based on solution processing in the absence of precious elements. AFM, XPS, TEM and XANES analyses revealed the formation of different Ti(IV) oxide phases on the hematite surface, that can reduce surface state recombination and enhance hole injection through local surface field effects, as confirmed by electrochemical impedance analysis.

A new hole-transport material (HTM) based on the 1,3,4-oxadiazole moiety (H1) was prepared through a single-step synthetic pathway starting from commercially available products. Thanks to a deep HOMO level, H1 was used as HTM in CH3 NH3 PbBr3 perovskite solar cells yielding an efficiency of 5.8%. The reference HTM (Spiro-OMeTAD), under the same testing conditions, furnished a lower efficiency of 5.1%. Steady-state and time-resolved photoluminescence of the thin films showed good charge-extraction dynamics for H1 devices. In addition, H1 shows a large thermal stability and completely amorphous behavior (as evaluated by thermal gravimetric analysis and differential scanning calorimetry).

The charge transfer dynamics involving a new Ru(II) polypyridine complex (1), developed to generate highly oxidizing photoholes for water oxidation, was studied by electrochemical, photoelectrochemical and spectroscopic means. Mesoporous TiO2 electrodes sensitized with complex 1, under 1 sun illumination (420 nm cut-off filter) and a moderate applied bias (0.3 V vs. SCE), in ACN/0.1 M LiI as a sacrificial electron donor reach an anodic photocurrent of similar to 0.2 mA cm(-2) with 3% photon-to-current conversion efficiency. When 0.1 M aqueous sodium ascorbate (pH 3) is used instead of iodide, the photocurrent increases to similar to 0.7 mA cm(-2) and up to 1 mA cm(-2) if the concentration of ascorbate is increased to 0.5 M, explainable with a modification of the charge injection mechanism. This is the photoelectrochemical evidence, in the heterogeneous phase, of the so-called "anti-biomimetic" pathway, confirmed in transient absorption spectroscopy by a long lived sharp bleaching at 480 nm and a narrow absorption between 500 and 550 nm, characteristic fingerprints of the photogenerated reduced state (1(-)). After the formation of *1/TiO2, reductive quenching by ascorbate occurs, not observed in LiI where the classic oxidative quenching takes place. Due to the modest excited state oxidation potential, electron transfer to TiO2 is thermodynamically more favorable from 1(-) than *1. Lastly, experiments performed with sensitized SnO2 photoanodes, where *1 undergoes the usual oxidative quenching, by charge transfer to the conduction band of the metal oxide allowed us to verify the interaction between 1(+) and IrO2 nanoparticles, grafted onto the surface in order to drive photoinduced water oxidation.

A series of mono- and di-branched donor-π-acceptor charge-separated dyes incorporating triphenylamine as a donor and either Dalton’s or benzothiadiazole group as strong acceptors was synthesized and its fundamental properties relevant to the sensitization of nanocrystalline NiO investigated. The dyes exhibited an intense visible absorption band with a strong charge transfer character favorable to NiO sensitization, shifting the electron density from the donor to the acceptor branches. Nevertheless, the computed exciton binding energy is circa twice that of a common literature standard (P1), suggesting a more difficult charge separation. When tested in p-type dye-sensitized solar cells the dyes successfully sensitized NiO electrodes, with photocurrent densities about half than that of the reference compound. Being recombination kinetics comparable, the larger photocurrent generated by P1 agrees with the superior charge separation capability originating by its smaller exciton binding energy.

The potentiostatic anodization of metallic tungsten has been investigated in different solvent/electrolyte compositions with the aim of improving the photoelectrochemical performances of the tungsten oxide layer. Among the explored electrolytes, the anodization in the NMF/H2O/NH4F solvent mixture was found to produce the most efficient WO3 photoanodes, which, combining spectral sensitivity, high electrochemically active surface and improved charge transfer kinetics, outperform, under simulated solar illumination, most of the reported nanocrystalline substrates produced by anodization in aqueous electrolytes and by sol gel methods. While the preparation of the photoelectrodes is a slow process at room temperature (20 degrees C), it could be greatly accelerated (x 10) by carrying out the anodization at 40-50 degrees C, thus proving to be a fast and convenient approach to the production of high performing WO3 photoactive substrates directly connected to a metal electron collector.

Different chemical routes for obtaining nanostructured WO 3 on transparent conductive oxides have been tried by considering the Santato-Augustynski recipe as a main guideline. The preparation started from tungstate salts as precursors and involved different organic dispersing agents. Calcination in air at 550 °C produced the final oxide. Aim of this work is to test the oxide materials as photoanodes for the photo-electrochemical water splitting. WO 3 is a wide bandgap semiconductor, able to transport photogenerated carriers under irradiation, provided that it has a good crystalline structure. The morphology and the crystal size of the obtained WO 3 nanoparticles have been investigated by SEM and XRD. This work shows that the particle size of the WO 3 film decreases with increase in molecular weight of the organic dispersing agents. The current–voltage characteristic curves of WO 3 -based electrodes have been obtained by biasing a photo-electrochemical cell with an external voltage ramp, both in the dark and under simulated solar irradiation. Results showed that under incident irradiance a noticeable photocurrent density is obtained with a reasonably steep current–voltage slope and with a plateau current of the order of 1.6–1.8 mA/cm 2 under incident 0.14 W/cm 2 AM 1.5 irradiation.

Dye-sensitized solar cells (DSSCs) were assembled by using the bougainvillea flowers, red turnip and the purple wild Sicilian prickly pear fruit juice extracts as natural sensitizers of TiO(2) films. The yellow orange indicaxanthin and the red purple betacyanins are the main components in the cocktail of natural dyes obtained from these natural products. The best overall solar energy conversion efficiency of 1.7% was obtained, under AM 1.5 irradiation, with the red turnip extract, that showed a remarkable current density (Jsc = 9.5 mA/cm(2)) and a high IPCE value (65% at lambda = 470 nm). Also the purple extract of the wild Sicilian prickly pear fruit showed interesting performances, with a Jsc of 9.4 mA/cm(2), corresponding to a solar to electrical power conversion of 1.26%.

Objectives Zirconium is widely used as material for prosthetic devices because of its good mechanical and chemical properties. When exposed to oxygen, zirconium becomes zirconium oxide (ZO, chemically ZrO2) which is biocompatible. ZO can be also prepared as a colloidal suspension and then used to coat surfaces. Zirconium oxide coating (ZOC) can potentially have specific biologic effects. Methods The effect of ZOC on bone throughout an in vivo study using dental implants covered with ZOC and then inserted in rabbit tibia was tested in this study. Results The histologic analysis demonstrated that (1) bone growth is more evident around ZOC fixtures than in controls and (2) a more mature bone is present in the peri-implant ZOC surface than in controls. Significance ZOC can enhance implant osseointegration.

Dye-sensitized photo-electrochemical cells based on a solid poly-(3,4-(ethylenedioxy)pyrrole) (PEDOP) electron mediating matrix were assembled and photo-electrochemically characterized. The conductive polymer was directly grown on the sensitized TiO2 surface exploiting a photo-assisted electropolymerization promoted by a properly designed ruthenium dye bearing pyrrole functionalities. J-V characteristics and photoaction spectra indicated very low efficiencies mainly determined by an efficient recombination of photo-injected electrons with both PEDOP acceptor states and oxidized sensitizers. (c) 2007 Elsevier B.V. All rights reserved.

The effect of pyrrole- and pyrrolidine-containing ligands (L) on the properties of heteroleptic [RuL2dcbpy]2+ complexes has been investigated. TiO2 electrodes modified with the new complexes exhibited extended absorption domains and high absorbances. Providing that a cobalt-based mediator was used for regeneration of the RuII state, good incident photon-to-current efficiency (near 80%) values were obtained in the pyrrole series.

Different approaches have been explored to increase the water oxidation activity of nanostructured hematite (alpha-Fe2O3) photoanodes, including doping with various elements, surface functionalization with both oxygen evolving catalysts (OEC) and functional overlayers and, more recently, the introduction of ultrathin oxide underlayers as tunneling back contacts. Inspired by this latter strategy, we present here a photoanode design with a nanometric spin-coated iron oxide underlayer coupled with a mesoporous hematite film deposited by electrophoresis. The electrodes equipped with the thin underlayer exhibit a four-fold improvement in photoactivity over the simple hematite porous film, reaching a stable photocurrent density of ca. 1 mA cm(-2) at 0.65 V versus the saturated calomel electrode (SCE) at pH 13.3 (NaOH 0.1 M) under air mass (AM) 1.5G illumination. A further improvement to 1.5 mA cm(-2) is observed after decoration of the hematite surface with a Fe(III)-OEC. These results demonstrate that by combining different iron oxide morphologies, it is possible to improve the selectivity of the interfaces towards both electron collection at the back contact and hole transfer to the electrolyte, obtaining an efficient all-iron based photoelectrode entirely realized with simple wet solution scalable procedures.

[(N,N?-Bis(2-(trimethylammonium)ethylene) perylene 3,4,9,10-tetracarboxylic acid bisimide)(PF6)2] (1) was observed to spontaneously adsorb on nanocrystalline WO3 surfaces via aggregation/hydrophobic forces. Under visible irradiation (? > 435 nm), the excited state of 1 underwent oxidative quenching by electron injection (kinj > 108 s-1) to WO3, leaving a strongly positive hole (Eox ? 1.7 V vs SCE), which allows to drive demanding photo-oxidation reactions in photoelectrochemical cells (PECs). The casting of IrO2 nanoparticles (NPs), acting as water oxidation catalysts (WOCs) on the sensitized electrodes, led to a 4-fold enhancement in photoanodic current, consistent with hole transfer from oxidized dye to IrO2 occurring on the microsecond time scale. Once the interaction of the sensitizer with suitable WOCs is optimized, 1/WO3 photoanodes may hold potentialities for the straightforward building of molecular level devices for solar fuel production.

As the greenhouse effect increases, the development of systems able to convert with high efficiency CO2 to energetically rich molecules owns a crucial weight in the technological and environmental domain. As catalyst, rhenium complexes, of the type fac-[Re(L)(CO)3Cl] (i.e. L = 2,2′-bipyridyl or 4,4′-bipyridyl), have attracted a large interest demonstrating promising catalytic properties. fac-[Re(v-bpy)(CO)3Cl]-based polymer deposited onto a solid support has been already investigated as heterogeneous catalyst in the reduction of CO2. Here, we deposited by electrochemical polymerization fac-[Re(v-bpy)(CO)3Cl] onto a nanocrystalline TiO2 film on glass and we investigated by cyclic voltammetry the properties of such heterogeneous catalyst in the electrochemical reduction of CO2. We demonstrated that the nanoporous nature of the substrate allows to increase the two-dimensional number of redox sites per surface area and hence to get a significant enhancement of the catalytic yield.

The electrochemical behaviour and the electrochromic properties of a film of poly-bithiophene (PBT) deposited onto a mesoporous TiO2 surface layered on an optically transparent electrode (OTE) were investigated. The polymeric film was prepared by anodic polymerisation of 2,2'-bithiophene (BT) and, interestingly, a cyclic voltammetric (CV) behaviour very similar to that observed on platinum or other conducting substrates was obtained. Furthermore, both the CV pattern and the absorption properties of the film grown on TiO2 indicated that a significantly larger amount of PBT could be deposited onto the mesoporous TiO2 layer with respect to Pt or conducting SnO2 substrates having the same geometric area. This suggested that PBT growth, besides at the exposed SnO2 surface within the pores of the mesoporous network, might also occur onto the (very large) surface of the TiO2 nanocrystals. Finally, the PBT/TiO2 heterojunction was investigated for its electrochromic properties. Because of the very large specific surface concentrations of PBT and the high volume density of surface-related defects in the mesoporous TiO2 film, the hetrojunction was shown to undergo sharp optical transition upon either oxidation of the PBT film or intercalation of Li+ ions in the reduced TiO2 film. (C) 2005 Elsevier B.V. All rights reserved.

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+ byx100 and the current for the uncatalyzed oxidation of Co2+ by considerably more than that.

The synthesis of a series of four complexes of the type Os(bpy-4,4′PO3H2)2L2 (bpy-4,4′PO3H2 = 2,2′-bipyridine-4,4′-bisphosphonic acid and L2 = Cl2, (CN)2, 2,2′-bipyridine-4,4′-bisphosphonic acid or 2,2′-bipyridine-4,4′-dimethoxy) is reported and the spectroscopic, photophysical, and photoelectrochemical properties are presented and compared to the cis-Ru(H2dcb)2(NCS)2 (H2dcb = 2,2′-bipyridine-4,4′-dicarboxylic acid).

New ruthenium complexes based on bipyiridine and terpyridyne type ligands, containing boronic acid functions, of formulae: [Ru(bpy-Φ-B(OH)2)2(CN)2], 6, (bpy-Φ-B(OH)2 = 4-(2,2′-dipyridil)phenylboronic acid), [Ru(trpy-B(OH)2)(tb2bpy)Cl](PF6), 11, (trpy-B(OH)2 = 4-[4′-(2,2′:6′,2″-terpyridil]-boronic acid), [Ru(trpy)-Φ-B(OH)2)(tb2bpy)Cl] (PF6), 12, (trpy)-Φ-B(OH)2 = 4-[4′-(2,2′:6′,2″-terpyridil]-phenylboronic acid), have been prepared, characterized, and tested in dye-sensitized solar cells (DSSCs). The results of this first study indicate that: (i) the boronic acid function acts as interlocking group between sensitizer and mesoporous titanium dioxide; (ii) only the presence of two B(OH)2 groups/molecule allow to obtain high photoanode surface coverages with optical densities > 1; (iii) The presence of the phenyl spacer in 12 reduces the photocurrent efficiency with respect to complex 11, most probably for a reduced electronic coupling between excited sensitizer and semiconductor conduction band. The insights gained from this work suggest that better photoelectrochemical performances should be obtained with dye molecules based on bpy type ligands bearing two boronic acid functions directly bound to the aromatic ring.

The trinuclear [{Ru(NH3)4(py-COOH)}(μ-CN){Ru(bpy)2}(μ-CN){Ru(NH3)4(py-COOH)}]6+ (py-COOH = isonicotinic acid, bpy = 2,2'-bipyridine) (1) and dinuclear [{Ru(dcb)2}2L]7- (dcb = 2,2'-bipyridine-4,4'-dicarboxylic acid, H3L = 9-phenyl-2,3,7-trihydroxy-6-fluorone) (2) complexes were prepared and their spectroelectrochemical behaviour characterised in solution and on conductive glass electrodes layered with Sb-doped nanocrystalline SnO2. The systems display a reversible redox response in a narrow potential range (-0.5 to +0.5 V vs SCE) and offer the possibility of controlling colour changes of electrochromic devices through variations of the coordinated and bridging ligands at the metal centres. SnO2(Sb) electrodes loaded with these dyes show switching times of the order of ms.

The complex cis-[(dcbH2)2Ru(ppy)2]2+, in which dcbH2 is 4,4′-(CO2H)2-2,2′-bipyridine and ppy is 4-phenylpyridine, was anchored to nanocrystalline TiO2 films deposited onto FTO substrate. Transient UV–visible absorption spectra with the resulting electrodes were carried out in order to investigate the electron-transfer process across the excited dye/semiconductor interface, as well as the charge recombination and quenching processes. The formation of the oxidized complex [(dcbH2)2RuIII(ppy)2]3+ upon light excitation was monitored by transient absorption difference spectra. The compound performs successful conversion of visible light into electricity as a molecular sensitizer of nanocrystalline n-type TiO2 in photoelectrochemical solar cells. A fast quenching of the oxidized complex in the presence of iodide emphasizes the importance of a proper concentration of donor species in the redox mediator for the effective regeneration of the oxidized sensitizer. Kinetics data of electron injection obtained by time-resolved experiments are discussed in parallel with photoelectrochemical properties of the dye-sensitized anatase TiO2.

Amorphous iron oxide nanoparticles were synthesized by pulsed-laser deposition (PLD) for functionalization of indium–tin oxide surfaces, resulting in electrodes capable of efficient catalysis in water oxidation. These electrodes, based on earth-abundant and nonhazardous iron metal, are able to sustain high current densities (up to 20 mA/cm2) at reasonably low applied potential (1.64 V at pH 11.8 vs reversible hydrogen electrode) for more than 1 h when employed as anodes for electrochemical water oxidation. The good catalytic performance proves the validity of PLD as a method to prepare nanostructured solid-state materials for catalysis, enabling control over critical properties such as surface coverage and morphology.

Bipyridine bearing pi-delocalized dithienypyrrole moieties (DTP) have been involved as ligands (L) in the preparation of RuL2(dcbpy)(2+) and RuL(dcbpy)(NCS)(2) dyes. These dyes exhibited wide absorption domains in solution and when adsorbed on TiO2 photoanodes, light harvesting was found to be excellent (absorbance>1 in the 350-600 nm range). Careful investigations, combining transient spectroscopy and ab initio computations evidenced the capture of the photoexcited electron by low lying orbitals centred on the DTP moiety thus limiting the efficiency in DSSC despite favourable redox potentials for both injection and ground state regeneration. (C) 2013 Elsevier Ltd. All rights reserved.

Hematite photoelectrodes prepared via a hydrothermal route are functionalized with a water oxidation catalyst consisting of amorphous Fe(III) oxide, obtained by successive ionic layer adsorption and reaction. The performances of the catalyst-modified photoanodes are considerably higher than those of the parent electrodes, resulting in a nearly doubled photoanodic current in all the basic aqueous electrolytes explored in this study. The combination of electrochemical impedance spectroscopy and laser flash photolysis indicates that the presence of the catalyst results in enhanced hole trapping in surface reactive states exposed to the electrolyte, allowing for a more successful competition between charge transfer and recombination.

In the context of the development of new electron mediators for dye-sensitized solar cells that could efficiently substitute the more common I3(-)/I(-) redox couple, we have prepared a series of Cu(I)- phenanthroline complexes through an easy synthetic route. The novel [Cu(2-mesityl-4,7-dimethyl-1,10-phenanthroline)2]PF6 (3) is of particular interest because the presence of the bulky mesityl groups protects as a kiss lock the Cu center, leading to superior dye regeneration kinetics with respect to simpler phenanthroline-based Cu(I) complexes and to a lower dark current with respect the I3(-)/I(-). The exploitation of a dual-component electrolyte constituted by (3) and a Fe(II) comediator, in combination with the [Ru(4,4'-dicarboxy-2,2'-bipyridine)2(4,4'-dinonyl-2,2'-bipyridine)](PF6)2 sensitizer, allowed us to increase the performance of the cell, reaching Jsc values of 4.0 mA cm(-2), comparable to that recorded with I3(-)/I(-) (3.8 mA cm(-2)).

A series of bimetallic complexes of general formula [(phen)Re(CO)(3)LOs(trpy)(bpy)](3+) (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, trpy = 2,2':6',2' '-terpyridine, and L = 4,4'-bipyridine (4,4'-bpy), trans-1,2-bis(4-pyridyl)ethylene (t-bpe), or 1,2-bis(4-pyridyl)ethane (bpa)) and the model mononuclear species [(phen)Re(CO)(3)L](+) and [Os(trpy)(bpy)L](2+) have been synthesized and their photophysical and photochemical properties studied. In the binuclear species an efficient Re(I)-Os(II) energy transfer is observed, which is analyzed in terms of Förster theory. In the binuclear [(phen)Re(CO)(3)(t-bpe)Os(trpy)(bpy)](3+) complex, the trans to cis isomerization of the coordinated t-bpe ligand, characteristic of the [(phen)Re(CO)(3)(t-bpe)](+) subunit, is inhibited by competitive intramolecular energy transfer.

The complex fac-[Re(CO)3(phen)(t-bpe)]PF6 (t-bpe=trans-1,2-bis(4-pyridyl)ethylene, phen=1,10-phenanthroline) was synthesized, purified and characterized by UV–Vis, 1H NMR, Raman and IR spectroscopy. 1H NMR resonance spectrum of fac-[Re(CO)3(phen)(t-bpe)]+ reveals a clear electronic communication of the metal center across the coordinated t-bpe with upfield shift of the ligand signals upon complexation. The CO stretching frequencies are in accord with facial geometry of the rhenium complex. The photochemical behavior of free trans-1,2-bis(4-pyridyl)ethylene and of its protonated form is also reported. The quantum yield for the trans→cis photoinduced isomerization of free ligand in methanolic solutions at 313 nm irradiation is 0.17±0.03. The average values for the photoisomerization of the protonated ligand (pH 1.9) are 0.19±0.02 and 0.15±0.01, respectively at 313 and 334 nm excitation. The photoassisted trans→cis isomerization of t-bpe in the fac-[Re(CO)3(phen)(t-bpe)]+ complex was achieved with higher quantum yields at lower energy irradiation by exploiting the red shifted absorption upon formation of the complex. The photoproducts cis-1,2-bis(4-pyridyl)ethylene and fac-[Re(CO)3(phen)(c-bpe)]PF6 were synthesized and identified by UV–Vis, 1H NMR and luminescence properties.

A new series of panchromatic ruthenium(II) sensitizers derived from carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) have been developed. Black dye containing different degrees of protonation [(C(2)H(5))(3)NH][Ru(H(3)tcterpy)(NCS)(3)] 1, [(C(4)H(9))(4)N](2)[Ru(H(2)tcterpy)(NCS)(3)] 2, [(C(4)H(9))(4)N](3)[Ru(Htcterpy)(NCS)(3)] 3, and [(C(4)H(9))(4)N](4)[Ru(tcterpy)(NCS)(3)] 4 (tcterpy = 4,4',4' '-tricarboxy-2,2':6',2' '-terpyridine) have been synthesized and fully characterized by UV-vis, emission, IR, Raman, NMR, cyclic voltammetry, and X-ray diffraction studies. The crystal structure of complex 2 confirms the presence of a Ru(II)N6 central core derived from the terpyridine ligand and three N-bonded thiocyanates. Intermolecular H-bonding between carboxylates on neighboring terpyridines gives rise to 2-D H-bonded arrays. The absorption and emission maxima of the black dye show a bathochromic shift with decreasing pH and exhibit pH-dependent excited-state lifetimes. The red-shift of the emission maxima is due to better pi-acceptor properties of the acid form that lowers the energy of the CT excited state. The low-energy metal-to-ligand charge-transfer absorption band showed marked solvatochromism due to the presence of thiocyanate ligands. The Ru(II)/(III) oxidation potential of the black dye and the ligand-based reduction potential shifted cathodically with decreasing number of protons and showed more reversible character. The adsorption of complex 3 from methoxyacetonitrile solution onto transparent TiO(2) films was interpreted by a Langmuir isotherm yielding an adsorption equilibrium constant, K(ads), of (1.0 +/- 0.3) x 10(5) M(-1). The amount of dye adsorbed at monolayer saturation was (n(alpha) = 6.9 +/- 0.3) x 10(-)(8) mol/mg of TiO(2), which is around 30% less than that of the cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) complex. The black dye, when anchored to nanocrystalline TiO(2) films achieves very efficient sensitization over the whole visible range extending into the near-IR region up to 920 nm, yielding over 80% incident photon-to-current efficiencies (IPCE). Solar cells containing the black dye were subjected to analysis by a photovoltaic calibration laboratory (NREL, U.S.A.) to determine their solar-to-electric conversion efficiency under standard AM 1.5 sunlight. A short circuit photocurrent density obtained was 20.5 mA/cm(2), and the open circuit voltage was 0.72 V corresponding to an overall conversion efficiency of 10.4%.

Time-resolved infrared measurements indicate ultrafast

Two novel Ru(II)-Rh(III) polypyridine dyads, containing carboxylic functions at the Rh(III) unit, RhIII(dcb)2-(BL)-RuII(dmp)2 and RhIII(dcb)2-(BL)-RuII(bpy)2 (bpy = 2,2‘-bipyridine; dcb = 4,4‘-dicarboxy-2,2‘-bipyridine; dmp = 4,7-dimethyl-1,10-phenanthroline; BL = 1,2-bis[4-(4‘-methyl-2,2‘-bipyridyl)]ethane), have been synthesized. Their photophysical behavior in solution, compared with that of the mononuclear RuII(dcb)2(dmb) model (dmb = 4,4‘-dimethyl-2,2‘-bipyridine), indicates the occurrence of fast (108−109 s-1) and efficient (>95%) Rh(III)-*Ru(II) → Rh(II)-Ru(III) photoinduced electron transfer. These species adsorb firmly on nanoporous TiO2 films, via the dcb ligands of the Rh(III) units. The behavior of the adsorbed species has been studied by means of nanosecond time-resolved emission and absorption measurements, as well as by photocurrent measurements. Photocurrent action spectra demonstrate that light absorption by the Ru(II) chromophore leads to electron injection into the semiconductor. A deta...

The complexes [Ru(dcb)2(L)] (L = 3-(2-phenol)-5-(pyridin-2-yl)-1,2,4-triazole (2-ppt), 3-(4-phenol)-5-(pyridin-2-yl)-1,2,4-triazole (4-ppt), 3,5-bis(pyrazin-2-yl)-1,2,4-triazole (bpzt), 3-(2-phenol)-5-(pyrazin-2-yl)-1,2,4-triazole (2-ppzt) and dcb = 4,4’-(CO2H)2-2,2’-bipyridine) have been synthesized, spectroscopically characterized and anchored to nanocrystalline TiO2 electrodes for the conversion of light into electricity in regenerative solar cells. The different efficiencies observed have been rationalized on the basis of an analytical expression relating the incident photon-to-current-conversion efficiency (IPCE) to the kinetic parameters of the relevant electron transfer processes involved in the solar cell.

The photophysical properties of mono- and dinuclear complexes based on the bridging ligand tpphz (tpphz = tetrapyrido[3,2-a:2‘,3‘-c:3‘ ‘,2‘ ‘-h:2‘ ‘‘-3‘ ‘‘-j]phenazine) were investigated. The complexes are of general formula [M(bpy)2(tpphz)]2+ [M = Ru(II), Os(II)] and [(bpy)2M1(tpphz)M2(bpy)2]n+ [M1= M2 = Ru(II), n = 4; M1= M2 = Os(II), n = 4; M1= Ru(II), M2 = Os(II), n = 4; M1= Ru(II), M2 = Os(III), n = 5]. The tpphz bridging ligand, being aromatic, rigid, and planar, has interesting structural features for the design of covalently linked donor−acceptor systems. In this work particular attention was devoted to the electronic properties of this bridge and their effect on the photophysical behavior. All of the results are consistent with direct involvement of the tpphz bridge in the photophysically active, lowest MLCT excited states. Relevant findings are as follows: (i) in mononuclear complexes, MLCT excited-state energies are highly sensitive to interactions at the free bpy-like end of the tpphz ligand,...

The kinetic study of interfacial electron transfer in sensitized nanocrystalline semiconductor is essential to the design of molecular devices performing specific light induced functions in a microheterogeneous environment. A series of molecular assemblies performing direct and remote charge injection to the semiconductor have been discussed in the context of artificial photosynthesis. A particular attention in this article has been paid to the factors that control the interfacial electron transfer processes in nanocrystallineTiO2films sensitized with mononuclear and polynuclear transition metal complexes.

Porous nanostructured photoanodes prepared from colloidal solutions of DeGussa or sol−gel processed TiO2 material have been derivatized with cis-M(dcb)2(CN)2, where M = Ru(II) or Os(II) and dcb is 4,4‘-(COOH)2-2,2‘-bipyridine. When employed in a regenerative solar cell with iodide donors, cis-Ru(dcb)2(CN)2/TiO2 converts absorbed photons to electrons nearly quantitatively while cis-Os(dcb)2(CN)2/TiO2 does not. In-situ time-resolved diffuse reflectance measurements reveal a sluggish iodide oxidation rate is responsible for the low photocurrent efficiency of cis-Os(dcb)2(CN)2/TiO2. Transient absorption measurements support this conclusion and allow full wavelength characterization of the charge-transfer processes.

Resonance raman spectra were measured for a series of Os, Ru, and Re dinuclear and trinuclear complexes in resonance with their low-energy metal-to-metal or intervalence transfer absorption band. The spectra typically demonstrate a series of enhanced low-wavenumber Raman bands due to metal-ligand modes along with enhancement of v (CN) bands. The spectra also demonstrate low-wavenumber polypyridine bands which gain enhancement from the intervalence transfer transition. One notable exception for this enhancement pattern is [(bpy) 2 Ru 11 ((CN)Ru III (NH 3 ) 5 ) 2 ] 6+ (where bpy is 2,2′-bipyridine) which demonstrates enhanced low-wavenumber bands, but no enhancement of the Raman band of the bridging cyanides.

In this work, the novel molecular sensitizers cis -[(dcbH 2 ) 2 Ru(ppy) 2 ] 2+ and cis -[(dcbH 2 ) 2 Ru(ppy)(H 2 O)] 2+ where dcbH 2 =4,4′-(CO 2 H) 2 -2,2′-bipyridine and ppy = 4-phenylpyridine, have been prepared and their effectiveness for sensitization of n-type TiO 2 electrodes in thin-layer transparent solar cells has been investigated. Spectral and photoelectrochemical measurements show that both dyes, when anchored to nanocrystalline TiO 2 , present good light harvesting properties and provide enhanced spectral response of the TiO 2 electrodes to visible light in regenerative solar cells. The derivative with one coordinated ppy, cis -[(dcbH 2 ) 2 Ru(ppy)(H 2 O)] 2+ resulted in considerably higher incident photon-to-current conversion efficiency values, as well as in a broader spectral response in longer wavelengths than the corresponding derivative with two coordinated ppy, cis -[(dcbH 2 ) 2 Ru(ppy) 2 ] 2+ . The results show that the number of azine ligands coordinated to the non-attached side of the (dcbH 2 ) 2 RuLL' dye can be controlled and plays an important role to extend spectral sensitivity to the visible light, as well as in tuning the overall properties of the system.

The electrochemical behavior of the family of mononuclear Re(I) complexes, [ReI(CO)3LX]n+, with L = 2,2‘-bipyridine (bpy), 1,10-phenanthroline (phen) and X = Cl, CN (n = 0); ACN (n = 1) and that of the metal−metal-bonded dimeric complexes [(L)(CO)3Re−Re(CO)3(L)] (L = bpy, phen) has been studied in various aprotic solvents using cyclic voltammetry (CV), chronoamperometry, and spectroelectrochemistry. The wide cathodic potential window investigated has permitted the observation of the largest number of redox processes so far obtained for these species. A detailed description of the kinetics of both oxidation and reduction processes of such species is given. The electrochemistry of the complexes [Re(CO)3(L)CN] (L = bpy, phen) is reported here for the first time. The electrochemical behavior of the monomeric species is greatly influenced by the nature of the monodentate ligand X. The concentration dependence of the cyclic voltammetric reductive behavior has evidenced the occurrence of bimolecular homogeneous ...

The bridging ligand 2,2‘:3‘,2‘‘:6‘‘,2‘‘‘-quaterpyridine contains two inequivalent metal binding sites (A and B) which perturb the metal-to-ligand charge transfer (MLCT) energy levels of the attached Re(I) or Ru(II). Time-resolved infrared difference spectra of [(bpy)2RuIIABReI(CO)3Cl]2+ are consistent with a lowest lying RuIII−AB•- MLCT state, while spectra of [(CO)3ClReIABRuII(bpy)2]2+ demonstrate the presence of both ReII−AB•- and RuIII−AB•- MLCT excited states. The latter result suggests the presence of an equilibrium between these two excited states at room temperature.

Time-resolved resonance Raman and infrared spectroscopies have been applied to a variety of problems arising in excited states and molecular assemblies based on polypyridyl complexes of Re(I), Ru(II) and Os(II). Application of transient resonance Raman spectroscopy has been used to assign the acceptor ligand in heteroleptic Ru(II) complexes and to characterize molecular structure in acceptor ligands. It has been applied to intramolecular electron transfer in chromophore-quencher complexes and molecular assemblies and to intramolecular energy transfer in polynuclear complexes. Time-resolved infrared spectroscopy has opened new avenues for the study of excited-state electronic structure and dynamics by the observation of ν(CO) and ν(CN) bands in transient absorption difference spectra. This technique has been applied to distinguishing between metal-to-ligand charge transfer and ligand-based ππ ∗ states in Re(I) complexes and to intramolecular energy transfer in cyano-bridged oligomers.

Long-lived interfacial charge-separated pairs, [TiO2(e-), D+], have been created by visible light excitation of Ru(II) polypyridyl compounds anchored to TiO2 particles in the presence of phenothiazine donors, D. The kinetic aspects of the formation and recombination have been kinetically resolved for analogous colloidal TiO2 solutions and films. Charge-separated pair lifetimes are shortened in colloidal films due to their high local concentrations. Less than 1% incident photon-to-current efficiency is observed when PTZ (phenothiazine) derivatives are employed as donors in regenerative solar cells. Light excitation of Ru(4,4‘-(CO2H)2-2,2‘-bipyridine)2(4-CH3,4‘-CH2-PTZ-2,2‘-bipyridine)]2+, anchored to TiO2, results in rapid intramolecular electron transfer from PTZ to the ruthenium metal center which efficiently translates the hole away from the chromophoric unit to the pendant PTZ group. The net result is the formation of a remarkably long-lived charge-separated pair, TiO2(e-)|−RuII−PTZ+, that lives for ∼ ...

Some recent studies mainly addressing the preparation and the modification of nanostructured thin films based on WO(3) and their application to photoelectrolysis of aqueous electrolytes are reviewed with the aim of rationalizing the main factors at the basis of an efficient photoanodic response. WO(3) represents one of the few materials which can achieve efficient water photo-oxidation under visible illumination, stably operating under strongly oxidizing conditions; thus the discussion of the structure-related photoelectrochemical properties of WO(3) thin films and their optimization for achieving almost quantitative photon to electron conversion constitutes the core of this contribution.

The application of time-resolved vibrational (Raman and infrared) spectroscopy to the study of excited-state intercomponent processes in inorganic chemistry represents a powerful new application of these techniques. The development of vibrational spectroscopy with time resolution from microseconds to femtoseconds to characterize the photochemistry and photophysics of metal-containing supramolecular systems provides a mechanism for understanding the fundamental processes which occur following photolysis. The utility and potential of this approach in the elucidation of excited-state processes is demonstrated with studies of excited-state localization, excited-state structure, and photoinduced intramolecular electron and energy transfer.

A number of mixed-valence complexes containing Ru III (NH 3 ) 4 (X) X = NH 3 , py) units cyano bridged to −Ru II (bpy) 2 or −Os II (bpy) 2 moieties have been examined by positive electrospray (ES) and Cs + fast ion bombardment (FAB) mass spectrometry. Both sets of data yielded a substantial [ M -PF 6 ] + signal and a number of characteristic fragment ions involving the multiple loss of the PF 6 − anion. ES data provided clear information on multiply charged ions as well as a strong indication of Ru-NH 3 bond weakening when Ru III (NH 3 ) 5 units are cyano bridged to Os II . The advantages of ES mass spectrometry for this class of compounds and the gas-phase redox chemistry of the investigated ions are discussed.

Correction for ‘On the stability of manganese tris(β-diketonate) complexes as redox mediators in DSSCs' by Stefano Carli et al., Phys. Chem. Chem. Phys., 2016, 18, 5949–5956.

Anodically grown WO(3) photoelectrodes prepared in an N-methylformamide (NMF) electrolyte have been investigated with the aim of exploring the effects induced by anodization time and water concentration in the electrochemical bath on the properties of the resulting photoanodes. An n-type WO(3) semiconductor is one of the most promising photoanodes for hydrogen production from water splitting and the electrochemical anodization of tungsten allows very good photoelectrodes, which are characterized by a low charge-transfer resistance and an increased spectral response in the visible region, to be obtained. These photoanodes were investigated by a combination of steady state and transient photoelectrochemical techniques and a correlation between photocurrent produced, morphology, and charge transport has been evaluated.

The first dithienylpyrrole (DTP)-based bipyridine ligands has been prepared and coordinated with ruthenium to give the corresponding homoleptic complexes. Bipyridine was bound at pyrrole (DTP(1)) or thiophene (DTP(2)) ring. A strong bathochromic effect was obtained by switching from pyrrole to thiophene for ligands and complexes. Interestingly the DTP(2) series offered a wide absorption window from UV to visible domain with an almost constant absorbance. These effects are due to a larger extent of delocalization as supported by DFT calculations and photophysical measurements.

Photoinduced electron transfer and charge recombination can be studied in metal-polypyridine polynuclear complexes based on the (MebpyCH 2 CH 2 Mebpy) (Me, methyl; bpy,2,2′-bipyridine) bridging ligand. An example is provided by the binuclear complex [Ru(Me 2 phen) 2 (MebpyCH 2 CH 2 Mebpy)Rh(Me 2 bpy) 2 ] 5+ (phen, 1,10-phenanthroline) (abbreviated as Ru(II)Rh(III)). In this system, photoinduced electron transfer processes originating from both local excited states (*Ru(II)Rh(III)→Ru(III)Rh(II) and Ru(II)*Rh(III)→Ru(III)Rh(II)), as well as charge recombination (Ru(III)Rh(II)→Ru(II)Rh(III)), have been resolved in the nanosecond and picosecond time domains using transient absorption and emission measurements. An energy transfer process (Ru(II)*Rh(III)→*Ru(II)Rh(III) can also be observed in rigid media, where the competing electron transfer process is suppressed. The factors affecting the kinetics of the various electron transfer steps can be discussed in terms of current transfer theories. Polychromophoric complexes suited for the study of intercomponent energy transfer can be designed using Re(I) and Ru(II) polypyridine units as molecular components and cyanide bridges as connectors. The energy flow in such systems is determined by the relative energy ordering of the metal-to-ligand charge transfer (MLCT) excited states of the various units, which can be controlled synthetically through the type of metal, the type of polypyridine ligand and the binding mode of the bridging cyanide(s). For example, in [NCRu(bpy) 2 CNRu(bpy) 2 CN] + , the flow is from the C-bonded to the N-bonded (to bridging cyanide) unit; in [(CO) 3 Re(phen)NCRu(bpy) 2 CN] + , the flow is from Re(I) to Ru(II); in [NCRu(bpy) 2 CNRu(bpy(COO) 2 ) 2 NCRu(bpy) 2 CN] 2− , the flow is from the bpy-containing units to the units containing dicarboxy—bpy. With regard to the identification of the lowest energy state in such systems, valuable information can be obtained using vibrational spectroscopies, such as time-resolved IR and resonance Raman. Compounds of this series behave in many respects as supramolecular antenna systems. Applications of the antenna effect to the spectral sensitization of semiconductors are discussed.