pH-dependent electron transfer from re-bipyridyl complexes to metal oxide nanocrystalline thin films

Photoinduced interfacial electron transfer (ET) from molecular adsorbates to semiconductor nanoparticles has been a subject of intense recent interest. Unlike intramolecular ET, the existence of a quasicontinuum of electronic states in the solid leads to a dependence of ET rate on the density of accepting states in the semiconductor, which varies with the position of the adsorbate excited-state oxidation potential relative to the conduction band edge. For metal oxide semiconductors, their conduction band edge position varies with the pH of the solution, leading to pH-dependent interfacial ET rates in these materials. In this work we examine this dependence in Re(L(P))(CO)3Cl (or ReC1P) [L(P) = 2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] and Re(L(A))(CO)3Cl (or ReC1A) [L(A) = 2,2'-bipyridine-4,4'-bis-CH2COOH] sensitized TiO2 and ReC1P sensitized SnO2 nanocrystalline thin films using femtosecond transient IR spectroscopy. ET rates are measured as a function of pH by monitoring the CO stretching modes of the adsorbates and mid-IR absorption of the injected electrons. The injection rate to TiO2 was found to decrease by 1000-fold from pH 0-9, while it reduced by only a factor of a few to SnO2 over a similar pH range. Comparison with the theoretical predictions based on Marcus' theory of nonadiabatic interfacial ET suggests that the observed pH-dependent ET rate can be qualitatively accounted for by considering the change of density of electron-accepting states caused by the pH-dependent conduction band edge position.     

Comparison of electron injection dynamics from re-bipyridyl complexes to TiO2 nanocrystalline thin films in different solvent environments

Factors that control photoinduced interfacial electron transfer (ET) between molecular adsorbates and semiconductor nanoparticles have been intensely investigated in recent years. In this work, the solvent dependence of interfacial ET was studied by comparing ET rates in dye sensitized TiO2 nanocrystalline films in different solvent environments. Photoinduced ET rates from Re(LA)(CO)3Cl [LA=dcbpy=4,4'-dicarboxy-2,2'-bipyridine] (ReC1A) to TiO2 nanocrystalline thin films in air, pH buffer, MeOH, EtOH, and DMF were measured by femtosecond transient IR spectroscopy. The ET rates in these solvent environments were noticeably different. However, differences between the rates in pH buffer and nonaqueous solvents (MeOH, EtOH, and DMF) were much smaller than the values expected from much more negative TiO2 conduction band-edge positions in the latter solvents under anhydrous conditions. It was suggested that the presence of adsorbed water, which was evident in FTIR spectra, lowered the band edge of TiO2 in these solvents and reduced the rate differences. The important effect of adsorbed water was verified by comparing two samples of Re(LP)(CO)3Cl [LP=2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] sensitized TiO2 in DMF, in which the presence of a trace amount of water was found to significantly increase the injection rate.     

Electron-transfer dynamics from Ru polypyridyl complexes to In2O3 nanocrystalline thin films

Photoinduced electron injection dynamics from Ru(dcbpy)(2)(X)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; X(2) = 2SCN(-), 2CN(-), and dcbpy; referenced as RuN3, Ru505, and Ru470) to In(2)O(3) nanocrystalline thin films were studied using ultrafast transient IR absorption spectroscopy. After 532 nm excitation of the adsorbates, the dynamics of electron injection from their excited states to In(2)O(3) were studied by monitoring the IR absorption of the injected electrons in the semiconductor. The injection kinetics were non-single-exponential. For samples exposed to air, the half rise times, defined as the time of 50% injection yield, were 5 +/- 0.8, 85 +/- 20, and >200 ps for RuN3, Ru505, and Ru470, respectively. For samples in pH 2 buffer, the corresponding half time for injection from these complexes became 6 +/- 1, 105 +/- 20, and 18 +/- 5 ps. The injection kinetics from RuN3 to In(2)O(3) was found to be similar to that to SnO(2). These kinetics traces showed a negligible <100 fs injection component and were very different from those to TiO(2). The dependences of the injection kinetics on adsorbate energetics and the nature of the semiconductors are discussed.     

Covalent attachment of a rhenium bipyridyl CO2 reduction catalyst to Rutile TiO2

We have characterized the covalent binding of the CO(2) reduction electrocatalyst ReC0A (Re(CO)(3)Cl(dcbpy) (dcbpy =4,4'-dicarboxy-2,2'-bipyridine)) to the TiO(2) rutile (001) surface. The analysis based on sum frequency generation (SFG) spectroscopy and density functional theory (DFT) calculations indicates that ReC0A binds to TiO(2) through the carboxylate groups in bidentate or tridentate linkage motifs. The adsorbed complex has the dcbpy moiety nearly perpendicular to the TiO(2) surface and the Re exposed to the solution in a configuration suitable for catalysis.     

Electron injection dynamics of Ru polypyridyl complexes on SnO2 nanocrystalline thin films

Ultrafast infrared spectroscopy was utilized to investigate the electron-transfer dynamics from Ru(dcbpy)(2)(X)(2) complexes (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; X(2) = SCN(-), 2CN(-), and dcbpy; referenced as RuN3, Ru505, and Ru470, respectively) to nanocrystalline SnO(2) films. For both films exposed to air (dry) and submerged in a pH 2 buffer solution, all traces show biphasic dynamics with a small ultrafast component (less than 10%) and nonexponential slow component, indicating that most injection occurs from thermalized excited state of the dye. In the dry film, the injection rate becomes slower, comparing RuN3, Ru505, and Ru470, correlating with decreasing excited-state oxidation potentials in these dyes. However, the variation of injection rate with dye potential is less noticeable at pH 2. The possible reason for the different injection dynamics in these dyes and under different environments are discussed. These injection dynamics are also compared with those on TiO(2) and ZnO.     

Ultrafast electron transfer between molecule adsorbate and antimony doped tin oxide (ATO) nanoparticles

Ultrafast transient IR spectroscopy has been used to examine the effect of doping on interfacial electron transfer (ET) dynamics in Re(dpbpy)(CO)(3)Cl (dpbpy = 4,4'-(CH(2)PO(OH)(2))2-2,2'-bipyridine) (ReC1PO(3)) sensitized ATO (Sb:SnO(2)) nanocrystalline thin films. In films consisting of particles with 0%, 2% and 10% Sb dopant, the rates of electron injection from the adsorbate excited state to ATO were independent of and the rates of the recombination increased with the doping level. The observed similar forward electron injection rates were attributed to negligible changes of available accepting states in the conduction band at the doping levels studied. The dependence of the recombination rate on conduction band electron density and a possible mechanism for the recombination process were discussed.     

Photoinduced electron injection from Ru(dcbpy)2(NCS)2 to SnO2 and TiO2 nanocrystalline films

Photoinduced electron injection from the sensitizer Ru(dcbpy)2(NCS)2 (RuN3) into SnO2 and TiO2 nanocrystalline films occurs by two distinct channels on the femto- and picosecond time scales. The faster electron injection into the conduction band of the different semiconductors originates from the initially excited singlet state of RuN3, and occurs in competition with intersystem crossing. The rate of singlet electron injection is faster to TiO2 (1/55 fs-1) than to SnO2 (1/145 fs-1), in agreement with higher density of conduction band acceptor states in the former semiconductor. As a result of competition between the ultrafast processes, for TiO2 singlet, whereas for SnO2 triplet electron injection is dominant. Electron injection from the triplet state is nonexponential and can be fitted with time constants ranging from approximately 1 ps (2.5 ps for SnO2) to approximately 50 ps for both semiconductors. The major part of triplet injection is independent of the semiconductor and is most likely controlled by intramolecular dynamics in RuN3. The overall time scale and the yield of electron injection to the two semiconductors are very similar, suggesting that processes other than electron injection are responsible for the difference in efficiencies of solar cells made of these materials.     

Effect of the anchoring group in Ru-bipyridyl sensitizers on the photoelectrochemical behavior of dye-sensitized TiO2 electrodes: carboxylate versus phosphonate linkages

The effects of the number of anchoring groups (carboxylate vs phosphonate) in Ru-bipyridyl complexes on their binding to TiO(2) surface and the photoelectrochemical performance of the sensitized TiO(2) electrodes were systematically investigated. Six derivatives of Ru-bipyridyl complexes having di-, tetra-, or hexacarboxylate (C2, C4, and C6) and di-, tetra-, or hexaphosphonate (P2, P4, and P6) as the anchoring group were synthesized. The properties and efficiencies of C- and P-complexes as a sensitizer depended on the number of anchoring groups in very different ways. Although C4 exhibited the lowest visible light absorption, C4-TiO(2) electrode showed the best cell performance and stability among C-TiO(2) electrodes. However, P6, which has the highest visible light absorption, was more efficient than P2 and P4 as a sensitizer of TiO(2). The surface binding (strength and stability) of C-complexes on TiO(2) is highly influenced by the number of carboxylate groups and is the most decisive factor in controlling the sensitization efficiency. A phosphonate anchor, however, can provide a stronger chemical linkage to TiO(2) surface, and the overall sensitization performance was less influenced by the adsorption capability of P-complexes. The apparent effect of the anchoring group number on the P-complex sensitization seems to be mainly related with the visible light absorption efficiency of each P-complex.     

Co[HO2C(CH2)3NH(CH2PO3H)2]2: a new canted antiferromagnet

A new cobalt(II) carboxylate-phosphonate, namely, Co[HO2C(CH2)3NH(CH2PO3H)2]2, with a layered architecture has been synthesized by hydrothermal reactions. The Co(II) ion in the title compound is octahedrally coordinated by six phosphonate oxygen atoms from four carboxylate phosphonate ligands. Neighboring CoO6 octahedra are interconnected by phosphonate groups into a 2D layer with a 4,4-net topology. Adjacent layers are further cross-linked via hydrogen bonds between the noncoordinate carboxylate groups and noncoordinate phosphonate oxygens. The ac and dc magnetic susceptibility and magnetization measurements indicate that Co[HO2C(CH2) 3NH(CH2PO3H)2]2 is a canted antiferromagnet with T(c) = 8.75 K.     

Tuning open circuit photovoltages with tripodal sensitizers

The sensitizers [Ru(bpy)2(deeb)](PF6)2 (1), [Ru(bpy)2(bpy)-(E-Ph)-Ad](PF6)2 (2), and [Ru(bpy)2(bpy)-(E-Ph)2-Ad](PF6)2 (3), where deeb is 4,4'-(COOCH2CH3)2-2,2'-bipyridine, E-Ph is phenylethynyl, and Ad are tripod shaped bpy ligands based on 1,3,5,7-tetraphenyladamantane, were anchored to mesoporous nanocrystalline (anatase) TiO2 thin films and studied in regenerative solar cells with 0.1 M LiI/0.005 M I2 dichloromethane electrolyte. Over three decades of 488 nm irradiance, the open circuit photovoltage increased markedly with the distance between the Ru center and the surface binding groups, 1 (7 A) < 2 (18 A) < 3 (24 A). The diode equation accurately models the irradiance dependent data and indicates that the TiO2(e-) --> I3- (and/or I2) charge recombination rate constants were decreased by a factor of 20 for 2/TiO2 and 280 for 3/TiO2 relative to 1/TiO2. The results suggest that control of the sensitizer-TiO2 orientation is important for efficient power optimization.     

Intermolecular energy transfer across nanocrystalline semiconductor surfaces

The yields and dynamics for energy transfer from the metal-to-ligand charge-transfer excited states of Ru(deeb)(bpy)(2)(PF(6))(2), Ru(2+), and Os(deeb)(bpy)(2)(PF(6))(2), Os(2+), where deeb is 4,4'-(CH(3)CH(2)CO(2))(2)-2,2'-bipyridine, anchored to mesoporous nanocrystalline (anatase) TiO(2) thin films were quantified. Lateral energy transfer from Ru(2+)* to Os(2+) was observed, and the yields were measured as a function of the relative surface coverage and the external solvent environment (CH(3)CN, THF, CCl(4), and hexanes). Excited-state decay of Ru(2+)*/TiO(2) was well described by a parallel first- and second-order kinetic model, whereas Os(2+)*/TiO(2) decayed with first-order kinetics within experimental error. The first-order component was assigned to the radiative and nonradiative decay pathways (tau = 1 micros for Ru(2+)*/TiO(2) and tau = 50 ns for Os(2+)*/TiO(2)). The second-order component was attributed to intermolecular energy transfer followed by triplet-triplet annihilation. An analytical model was derived that allowed determination of the fraction of excited-states that follow the two pathways. The fraction of Ru(2+)*/TiO(2) that decayed through the second-order pathway increased with surface coverage and excitation intensity. Monte Carlo simulations were performed to estimate the Ru(2+)* --> Ru(2+) intermolecular energy transfer rate constant of (30 ns)(-1).     

Step-scan FTIR time-resolved spectroscopy study of excited-state dipole orientation in soluble metallopolymers

Step-scan FTIR time-resolved spectroscopy (S2FTIR TRS) in acetonitrile-d3 has been used to probe the acceptor ligand in metal-to-ligand charge transfer (MLCT) excited states of amide-substituted polypyridyl complexes of RuII and in analogues appended to polystyrene. On the basis of ground-to-excited state shifts in v(C = O) of -31 cm-1 for the amide group in [RuII(bpy)2(bpyCONHEt')]2+ (bpyCONHEt' = 4'-methyl-2,2'-bipyridine-4-carboxamide-Et'; Et' = -CH2CH2BzCH2CH3) (1) and in the derivatized polystyrene abbreviated [PS-[CH2-CH2NHCObpy-RuII(bpy)2]20]40+ (3), the excited-state dipole is directed toward the amide-containing pyridyl group in the polymer side chain. Smaller shifts in v(C = O) of -17 cm-1 in [RuII(4,4'-(CONEt2)2bpy)2-(bpyCONHEt')]2+ (2) and in the derivatized polystyrene abbreviated [PS-[CH2CH2NHCObpy-RuII(4,4'-(CONEt2)2bpy)2]20]40+ (4) indicate that the excited-state dipole is directed toward one of the diamide bpy ligands. The nearly identical results for 1 and 3 and for 2 and 4 show that the molecular and electronic structures of the monomer excited states are largely retained in the polymer samples. These conclusions about dipole orientation in the polymers are potentially of importance in understanding intrastrand energy transfer dynamics. The excited-state dipole in 3 is oriented in the direction of the covalent link to the polymer backbone, and toward nearest neighbors. In 4, it is oriented away from the backbone.     

Novel Ru-dioxolene complexes as potential electrochromic materials and NIR dyes

A series of Ru(bpy)(2)-dioxolene complexes 1-4 (bpy = 2,2'- bipyridine) and corresponding Ru(dcb)(2)-dioxolene complexes 5-8 (dcbH(2) = 2,2'-bipyridine-4,4'-dicarboxylic acid) have been prepared, and their spectroelectrochemical behavior in solution has been investigated. The complexes show reversible electrochemical behavior accompanied by a strong NIR absorption in their semiquinone forms due to a Ru(dpi) --> sq(pi) MLCT band. Complete quenching of the NIR absorption occurs both upon oxidation (to the quinone form) and upon reduction (to the catechol form) very close to 0 V. The color of the systems can be tuned by using a wide range of ligands. The complexes 5-8 can be anchored onto nanocrystalline inorganic semiconductors allowing incorporation into potential electrochromic devices. As a proof of principle, compound 8 has been adsorbed on nanocrystalline Sb-doped SnO(2) supported on FTO glass, and it displays reversibly switchable electrochromic behavior in the NIR.     

A nuclear isotope effect for interfacial electron transfer: excited-state electron injection from Ru ammine compounds to nanocrystalline TiO2

The coordination compounds Ru(deeb)(NH3)4(PF6)2 and Ru(deeb)(NH2(CH2)2NH2)4(PF6)2, where deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, were synthesized and attached to optically transparent nanocrystalline (anatase) TiO2 films. The compounds were found to be nonemissive in fluid acetonitrile and when attached to TiO2 with excited-state lifetimes <10 ns. Infrared measurements showed the expected isotopic substitution of the deuterated compounds on TiO2 thin films. A small 10-15 mV shift in the RuIII/II reduction potentials was measured upon deuteration. Metal-to-ligand charge-transfer (MLCT) excitation resulted in interfacial electron transfer into the TiO2 semiconductor with quantum yields that were dependent on the excitation wavelength and deuteration of the ammine ligands. The quantum yields were optimized with blue light excitation (417 nm) and deuterium substitution. In contrast, the kinetic rate constants for charge recombination were insensitive to deuteration and the excitation wavelength. Control experiments with Ru(deeb)(bpy)2(PF6)2 indicated that deuteration of the TiO2 surface alone does not affect the injection or recombination processes. A model is proposed wherein electron injection occurs in competition with vibrational relaxation and/or intersystem crossing of the excited states. Exchange of hydrogen by deuterium slows vibrational relaxation and/or intersystem crossing, resulting in higher injection yields.     

Electrochromic devices based on binuclear mixed valence compounds adsorbed on nanocrystalline semiconductors

A series of cyano-bridged binuclear mixed valence complexes of the general formula M-Ru(III)(NH(3))(4)pyCOOH [pyCOOH = isonicotinic acid; M = cis-Ru(bpy)(2)(CN)(2), 1 (bpy = 2,2' bipyridine); trans-Ru(py)(4)(CN)(2), 2 (py = pyridine); [Ru(CN)(6)](4)(-), 3; [Fe(CN)(6)](4)(-), 4] have been prepared and anchored through the carboxylic function to nanocrystalline TiO(2) or SnO(2) electrodes. The complexes display a reversible electrochromic behavior in the range of applied potential from -0.5 to +0.5 V, versus SCE. Tuning of the electronic transitions in the visible and near-infrared spectral regions is achieved through changes of the solvent and of the cyano-bridged metal moiety M.     

Adjacent- versus remote-site electron injection in TiO2 surfaces modified with binuclear ruthenium complexes

Nanocrystalline thin films of TiO2 cast on an optically transparent indium tin oxide glass were sensitized with ruthenium homo- and heterobinuclear complexes, [LL'Ru(BL)RuLL']n+ (n = 2, 3), where L and L' are 4,4'-dicarboxy-2,2'-bipyridine (dcb) and/or 2,2'-bipyridine (bpy) and BL is a rigid and linear heteroaromatic entity (tetrapyrido[3,2-a:2',3'-c:3",2"-h:2'",3'"-j]phenazine (tpphz) or 1,4-bis([1,10]phenanthroline[5,6-d]imidazol-2-yl)benzene (bfimbz)). The photophysical behavior of the RuII-RuII diads in solution indicated the occurrence of intercomponent energy transfer from the upper-lying Ru --> bpy charge-transfer (CT) excited state of the Ru(bpy)(2) moiety to the lower-lying Ru --> dcb CT excited state of the Ru(bpy)(dcb) (or Ru(dcb)(2)) subunit in the heterobinuclear complexes. These sensitizer diads adsorbed on nanostructured TiO2 surfaces in a perpendicular or parallel attachment mode. Adsorption was through the dcb ligands on one or both chromophoric subunits. The behavior of the adsorbed species was studied by nanosecond time-resolved transient absorption and emission spectroscopy, as well as by photocurrent measurements. In the TiO2-adsorbed samples where BL was bfimbz, the electron injection kinetics was very fast and could not be resolved because an electron is promoted from the metal center to the dcb ligand directly linked to the semiconductor. In the TiO2-adsorbed samples where BL was tpphz, for which, in the excited state, a BL localization of the lowest-lying metal-to-ligand charge transfer (MLCT) is observed, slower injection rates (9.5 x 10(7) s(-1) in [(bpy)(2)Ru(tpphz)Ru(bpy)(dcb(-))](3+)/TiO2 and 5.5 x 10(7) s(-1) in [(bpy)(dcb)Ru(tpphz)Ru(bpy)(dcb(-))](3+)/TiO2) were obtained. Among the systems, the heterotriad assembly [(bpy)(2)Ru(bfimbz)Ru(bpy)(dcb(2-))](2+)/TiO2 gave the best photovoltaic performance. In the first case, this was attributed to a fast electron injection initiated from a dcb-localized MLCT; in the second case, this is attributed to improved molecular orientation on the surface, which was due to rigidity and, at the same time, linearity of the heterotriad system, resulting in a slower charge recombination between the injected electron and the hole.     

Toward exceeding the Shockley-Queisser limit: photoinduced interfacial charge transfer processes that store energy in excess of the equilibrated excited state

Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine, bq is 2,2'-biquinoline, and deeb and deebq are 4,4'-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (+/- 2) x 10(-8) mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were phi(417 nm) = 0.18 +/- 0.02, phi(532.5 nm) = 0.08 +/- 0.02, and phi(683 nm) = 0.05 +/- 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.     

Impact of ligand modification on hydrogen photogeneration and light-harvesting applications using cyclometalated iridium complexes

To explore structure-activity relationships with respect to light-harvesting behavior, a family of bis-cyclometalated iridium complexes [Ir(C^N)(2)(Hbpdc)] 2-5 (where C^N = 2-phenylbenzothiazole and its functionalized derivatives, and H(2)bpdc =2,2'-bipyridine-4,4'-dicarboxylate) was synthesized using a facile method. The photophysical and electrochemical properties of these complexes were investigated and compared to those of analogue 1 (C^N = (4-trifluoromethyl)-2-phenylbenzothiazole); they were also investigated theoretically using density functional theory. The molecular structures of complexes 2-4 were determined by X-ray crystallography, which revealed typical octahedral coordination geometry. The structural modifications involved in the complexes were accomplished through the attributes of electron-withdrawing CF(3) and electron-donating NMe(2) substituents. The UV-vis spectra of these species, except for that of 5, displayed a broad absorption in the low-energy region, which originated from metal-to-ligand charge-transfer transitions. These complexes were found to exhibit visible-light-induced hydrogen production and light-to-electricity conversion in photoelectrochemical cells. The yield of hydrogen production from water using these complexes was compared, which revealed substantial dependences on their structures, particularly on the substituent of the cyclometalated ligand. Among the systems, the highest turnover number of 1501 was achieved with complex 2, in which the electron-withdrawing CF(3) substituent was connected to a phenyl ring of the cyclometalated ligand. The carboxylate anchoring groups made the complexes highly suitable for grafting onto TiO(2) (P25) surfaces for efficient electron transfer and thus resulted in an enhancement of hydrogen evolution compared to the unattached homogeneous systems. In addition, the combined incorporation of the electron-donating NMe(2) group and the electron-withdrawing CF(3) substituent on the cyclometalated ligand caused complex 5 to not work well for hydrogen production. Their incorporation, however, enhanced the performance of 5 in the light-harvesting application in nanocrystalline TiO(2) dye-sensitized solar cells, which was attributed to the intense absorption in the visible region.     

Ligand-localized electron trapping at sensitized semiconductor interfaces

Nanocrystalline (anatase), mesoporous TiO2 thin films were derivatized with [Ru(bpy)2(deebq)](PF6)2 or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine and deebq is 4,4'-diethylester-2,2'-biquinoline. Both compounds bind to the nanocrystalline TiO2 films with typical limiting surface coverages of 7 (+/-2) x 10-8 mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.60 V vs SCE) occurs prior to TiO2 reduction. Steady-state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced compound, MII(deebq-)(bpy)2+/TiO2. Neither the photoluminescent excited states or the reduced forms of these compounds inject electrons efficiently into TiO2. Transient absorption measurements after a approximately 10-ns laser pulse, reveal greater than 80% MLCT excited states and a smaller fraction of extremely long-lived charge-separated state intermediates assigned to equal concentrations of MII(deebq-)(bpy)2+/TiO2 and MIII(deebq)(bpy)23+/TiO2. The results are consistent with a mechanism of ultrafast electron injection followed by ligand-localized trapping on a second compound. The quantum yield for formation of the charge-separated states (phiCSS) is excitation wavelength dependent. With 417 nm excitation, phiCSS(417) = 0.14 +/- 0.03, and this decreases with 532.5 nm excitation, phiCSS(532.5) = 0.08 +/- 0.03, and 683 nm excitation for M = Os, phiCSS(683) = 0.05 +/- 0.01. Electron transfer to yield ground-state products, MII(deebq-)(bpy)2+/TiO2 + MIII(deebq)(bpy)23+/TiO2 --> 2 MII(deebq)(bpy)22+/TiO2, occurs with a driving force of 2.05 eV for Ru/TiO2 and 1.64 eV for Os/TiO2. The dynamics of this process were quantified on a millisecond time scale and were found to follow second-order kinetics. The intermediates are sufficiently long-lived that continued pulsed excitation at 10 Hz leads to high concentrations and the formation of transient images on the semiconductor surface that are easily observed by the naked eye.     

Synthesis and Characterization of Heteroleptic Ru(II) Complexes Based on 4,4′‐Bis((E)‐styryl)‐2,2′‐bipyridine as Ancillary Ligand and Application for Dye‐Sensitized Solar Cells

Heteroleptic Ru(II) complexes were designed based on 4,4′‐bis((E)‐styryl)‐2,2′‐bipyridine (bsbpy) as an ancillary ligand for dye‐sensitized solar cells (DSSCs), and those Ru(II) sensitizers, [Ru(L)(bsbpy)(NCS)₂][TBA] (TBA; tetrabutylammonium), were synthesized according to a typical one‐pot reaction of [RuCl₂(p‐cymene)]₂ with the corresponding anchoring ligands (where L = 4,4′‐dicarboxy‐2,2′‐bipyridine (dcbpy), 4,4′‐bis((E)‐carboxyvinyl)‐2,2′‐bipyridine (dcvbpy), 4,7‐dicarboxy‐1,10‐phenanthroline (dcphen), or 4,7‐bis((E)‐carboxyvinyl)‐1,10‐phenanthroline (dcvphen)). The new Ru(II) dyes, [Ru(L)(bsbpy)(NCS)₂][TBA] that incorporated vinyl spacer(s) into ancillary and/or anchoring ligand displayed red‐shifted bands over the overall UV/VIS region relative to the absorption spectra of N719 . A combination of bsbpy ancillary and dcphen anchoring ligand showed the best result for the overall power conversion efficiency (η); i.e., a DSSC fabricated with [Ru(dcphen)(bsbpy)(NCS)₂][TBA] exhibited a power conversion efficiency (η) of 2.98% (compare to N719 , 4.82%).