Squarylium Cyanine Dyes for Photoelectric Conversion

Asanimportantclassoffunctionalmaterials,squaryliumcyaninedyespossessmanyexcellentpropertiessuchasstablestructures,outstandingphotoconductivityaswellassmallthermalconductivity,andexhibitintenseandsharpabsorptionbandsinthevisibleandnearinfraredregionwithhighlightharvestingcapacity,whichopenupextensiveapplicationsinthevariousfieldsl-3.Inthispaper,twosymmetricalsquaryliumindocyanineswiththeintroductionofhydroxyethylandsulfopropylgroupsontheheterocyclicnitrogenweresynthesizedandcharacterizedbyUV,I…     

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).     

Synthesis and photophysical properties of ruthenium-based dendrimers and their use in dye sensitized solar cells

First- and second-generation dendrimers (Ru3 and Ru6) have been synthesized, and their photophysical properties were investigated in solution and when adsorbed on the nanocrystalline TiO2 surface. The performance of Ru3 and Ru6 as charge transfer photosensitizers in nanocrytalline TiO2 based solar cells was also investigated. The best photovoltaic performance was obtained by the Ru3 based solar cell yielding a short circuit current of J sc = 5.52 mA.cm (-2) and an open circuit voltage of V oc = 626 mV, corresponding to an overall conversion efficiency of eta = 1.80% that is approximately double the conversion efficiency of the reference compound Ru1 (eta = 0.91%) and of the second generation dendrimer Ru6 (eta = 0.95%). The particular efficiency of the first generation dendrimer, Ru3, is attributed to the better light-harvesting properties of the doped nanocrystalline TiO2 film when compared to Ru1, whereas the poor performance of the second generation dendrimer, Ru6, is attributed to the uneven adsorption of all of the ruthenium moieties to the nanocrystalline TiO2 surface at the same time.     

中位四(1-苯基吡唑-4-基)卟啉掺杂空穴传输材料的红色有机电致发光器件(英文)

使用中位-四(1-苯基吡唑-4-基)卟啉(TPPyPH2)掺杂空穴传输材料N,N′-二苯基-N,N′-双(4-甲苯基)-1,1′-二苯基-4,4′-二胺(TPD)制备了红色有机电致发光器件.因为TPD的发射光谱与TPPyPH2的吸收光谱具有更大的光谱重叠,为了得到更为有效的从主体材料TPD向红光染料TPPyPH2的能量传递,我们使用TPD代替传统的8-羟基喹啉铝(Alq3)作为主体发光材料.器件在680nm处具有纯的红光发射峰;通过使用Alq3电子传输层以及使用Alq3共掺杂发光层的方法,使器件的发光性能得到了改善,结构为ITO/Alq3+TPPyPH2+TPD(50nm)/Alq3(30nm)/Al的器件的最大发光亮度为177cd/m2.     

Semiconductor photoinduced cycloreversion of the dimer of methyl 2-naphthoate

The dimer of methyl 2-naphthoate (1) has been found to undergo efficient cyclorever-sion to its monomer, methyl naphtha)ene-2-carboxylate (2) on illuminated ZnO and TiO2 but not on CdS. An electron transfer and cation radical chain mechanism is proposed. Quantum yields, solvent effect, the role of oxygen, and the quenching of the reaction were investigated, and were consistent with the proposed mechanism.     

Light-driven tyrosine radical formation in a ruthenium-tyrosine complex attached to nanoparticle TiO2

We demonstrate a possibility of multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystalline TiO(2). The complex mimics the function of the tyrosine(Z)() and chlorophyll unit P(680) in natural photosystem II (PSII). A ruthenium(II) tris(bipyridyl) complex covalently linked to a L-tyrosine ethyl ester through an amide bond was attached to the surface of nanocrystalline TiO(2) via carboxylic acid groups linked to the bpy ligands. Synthesis and characterization of this complex are described. Excitation (450 nm) of the complex promotes an electron to a metal-to-ligand charge-transfer (MLCT) excited state, from which the electron is injected into TiO(2). The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 5 micros with a yield of 15%. This rather low yield is a result of a fast back electron transfer reaction from the nanocrystalline TiO(2) to the photogenerated Ru(III).     

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.     

Rationale for kinetic heterogeneity of ultrafast light-induced electron transfer from Ru(II) complex sensitizers to nanocrystalline TiO2

Because of their successful use in dye-sensitized solar cells, Ru(II) polypyridyl complex dyes adsorbed on nanocrystalline TiO2 films have been regarded as model systems for the experimental study of the ultrafast dynamics of interfacial light-induced electron transfer. Most studies have reported charge injection kinetics from Ru(dcbpyH2)2(NCS)2 (N3) to take place with a fast (sub-100 fs) phase, followed by a slower (0.7-100 ps) multiexponential component. This complex, multiphasic behavior observed for the electron injection process has prevented the development of a satisfying kinetic model and has led to often contradicting conclusions. Here, we show that the observed kinetic heterogeneity can result from the aggregation of sensitizer molecules on the surface. Carefully controlled deposition of Ru(II) complex dye molecules onto nanocrystalline titania consistently yields a monophasic injection dynamics with a time constant shorter than 20 fs. The latter figure suggests the process is beyond the scope of vibration-mediated electron transfer kinetic models and might be controlled by the electron dephasing in the solid.     

Synthesis and characterization of dinuclear ruthenium complexes covalently linked to Ru(II) tris-bipyridine: an approach to mimics of the donor side of photosystem II

To mimic the electron-donor side of photosystem II (PSII), three trinuclear ruthenium complexes (2, 2a, 2b) were synthesized. In these complexes, a mixed-valent dinuclear Ru2(II,III) moiety with one phenoxy and two acetato bridges is covalently linked to a Ru(II) tris-bipyridine photosensitizer. The properties and photoinduced electron/energy transfer of these complexes were studied. The results show that the Ru2(II,III) moieties in the complexes readily undergo reversible one-electron reduction and one-electron oxidation to give the Ru2(II,III) and Ru2(III,III) states, respectively. This could allow for photooxidation of the sensitizer part with an external acceptor and subsequent electron transfer from the dinuclear ruthenium moiety to regenerate the sensitizer. However, all trinuclear ruthenium complexes have a very short excited-state lifetime, in the range of a few nanoseconds to less than 100 ps. Studies by femtosecond time-resolved techniques suggest that a mixture of intramolecular energy and electron transfer between the dinuclear ruthenium moiety and the excited [Ru(bpy)3]2+ photosensitizer is responsible for the short lifetimes. This problem is overcome by anchoring the complexes with ester- or carboxyl-substituted bipyridine ligands (2a, 2b) to nanocrystalline TiO2, and the desired electron transfer from the excited state of the [Ru(bpy)3]2+ moiety to the conduction band of TiO2 followed by intramolecular electron transfer from the dinuclear Ru2(II,III) moiety to photogenerated Ru(III) was observed. The resulting long-lived Ru2(III,III) state decays on the millisecond timescale.     

Characterization of photoinduced self-exchange reactions at molecule-semiconductor interfaces by transient polarization spectroscopy: lateral intermolecular energy and hole transfer across sensitized TiO2 thin films

Transient anisotropy measurements are reported as a new spectroscopic tool for mechanistic characterization of photoinduced charge-transfer and energy-transfer self-exchange reactions at molecule-semiconductor interfaces. An anisotropic molecular subpopulation was generated by photoselection of randomly oriented Ru(II)-polypyridyl compounds, anchored to mesoscopic nanocrystalline TiO(2) or ZrO(2) thin films, with linearly polarized light. Subsequent characterization of the photoinduced dichromism change by visible absorption and photoluminescence spectroscopies on the nano- to millisecond time scale enabled the direct comparison of competitive processes: excited-state decay vs self-exchange energy transfer, or interfacial charge recombination vs self-exchange hole transfer. Self-exchange energy transfer was found to be many orders-of-magnitude faster than hole transfer at the sensitized TiO(2) interfaces; for [Ru(dtb)(2)(dcb)](PF(6))(2), where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine and dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, anchored to TiO(2), the energy-transfer correlation time was θ(ent) = 3.3 μs while the average hole-transfer correlation time was <θ(h+)> = 110 ms, under identical experimental conditions. Monte Carlo simulations successfully modeled the anisotropy decays associated with lateral energy transfer. These mesoscopic, nanocrystalline TiO(2) thin films developed for regenerative solar cells thus function as active "antennae", harvesting sunlight and transferring energy across their surface. For the dicationic sensitizer, [Ru(dtb)(2)(dcb)](PF(6))(2), anisotropy changes indicative of self-exchange hole transfer were observed only when ions were present in the acetonitrile solution. In contrast, evidence for lateral hole transfer was observed in neat acetonitrile for a neutral sensitizer, cis-Ru(dnb)(dcb)(NCS)(2), where dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine, anchored to TiO(2). The results indicate that mechanistic models of interfacial charge recombination between electrons in TiO(2) and oxidized sensitizers must take into account diffusion of the injected electron and the oxidized sensitizer. The phenomena presented herein represent two means of concentrating potential energy derived from visible light that could be used to funnel energy to molecular catalysts for multiple-charge-transfer reactions, such as the generation of solar fuels.     

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.     

TiO2纳米颗粒对钌(II)三联吡啶配合物光损伤小牛胸腺DNA能力的影响

由于极短的激发态寿命, 钌(II)三联吡啶配合物对脱氧核糖核酸(DNA)的光损伤能力低下. 设计合成了三个钌(II)三联吡啶配合物[Ru(ttp)(tpy)]^2＋ (1), [Ru(ttp-COOH)(tpy)]^2＋ (2)和[Ru(ttp-COOH)(tpy-pyr)]^2＋ (3), 其中tpy为2,2':6',2"-三联吡啶, ttp为4′-(4-甲苯基)-2,2':6',2"-三联吡啶, ttp-COOH为4′-(4-羧基苯基)-2,2':6',2"-三联吡啶, tpy-pyr为4'-(1-芘基)-2,2':6',2"-三联吡啶. 比较了TiO₂纳米颗粒对它们光损伤小牛胸腺DNA的影响. 发现TiO₂纳米颗粒在空气和氩气条件下均可显著提高配合物3光损伤DNA的能力. TiO₂纳米颗粒和配合物3间的光诱导电子转移作用及其该作用生成的钌(III)物种可能是促进配合物3对DNA光损伤的主要原因.     

Charge transfer dynamics of model charge transfer centers of a multicenter water splitting dye complex on rutile TiO2(110)

Charge transfer dynamics between an adsorbed molecule and a rutile TiO(2)(110) surface have been investigated in three organometallic dyes related to multicenter water splitting dye complexes: Ru 535 (cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)), Ru 455 (cis-bis(2,2'-bipyridyl)-(2,2'-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(II)), and Ru 470 (tris(2,2'-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(II)). The adsorption of the dye molecules on the rutile TiO(2)(110) surface has been studied using core-level and valence photoemission. Dye molecules were deposited in situ using ultrahigh vacuum electrospray deposition. Core-level photoemission spectra reveal that each complex bonds to the surface via deprotonation of two carboxylic groups. All three dye complexes show evidence of ultrafast charge transfer to the TiO(2) substrate using the core-hole clock implementation of resonant photoemission spectroscopy.     

Structural and photophysical properties of adducts of [Ru(bipy)(CN)4]2- with different metal cations: metallochromism and its use in switching photoinduced energy transfer

We show in this paper how the 3MLCT luminescence of [Ru(bipy)(CN)4]2-, which is known to be highly solvent-dependent, may be varied over a much wider range than can be achieved by solvent effects, by interaction of the externally directed cyanide ligands with additional metal cations both in the solid state and in solution. A series of crystallographic studies of [Ru(bipy)(CN)4]2- salts with different metal cations Mn+ (Li+, Na+, K+, mixed Li+/K+, Cs+, and Ba2+) shows how the cyanide/Mn+ interaction varies from the conventional "end-on" with the more Lewis-acidic cations (Li+, Ba2+) to the more unusual "side-on" interaction with the softer metal cations (K+, Cs+). The solid-state luminescence intensity and lifetime of these salts is highly dependent on the nature of the cation, with Cs+ affording the weakest luminescence and Ba2+ the strongest. A series of titrations of the more soluble derivative [Ru(tBu2bipy)(CN)4]2- in MeCN with a range of metal salts showed how the cyanide/Mn+ association results in a substantial blue-shift of the 1MLCT absorptions, and 3MLCT energies, intensities, and lifetimes, with the complex varying from essentially non-luminescent in the absence of metal cation to showing strong (phi = 0.07), long-lived (1.4 micros), and high-energy (583 nm) luminescence in the presence of Ba2+. This modulation of the 3MLCT energy, over a range of about 6000 cm-1 depending on the added cation, could be used to reverse the direction of photoinduced energy transfer in a dyad containing covalently linked [Ru(bipy)3]2+ and [Ru(bipy)(CN)4]2- termini. In the absence of a metal cation, the [Ru(bipy)(CN)4]2- terminus has the lower 3MLCT energy and thereby quenches the [Ru(bipy)3]2+-based luminescence; in the presence of Ba2+ ions, the 3MLCT energy of the [Ru(bipy)(CN)4]2- terminus is raised above that of the [Ru(bipy)3]2+ terminus, resulting in energy transfer to and sensitized emission from the latter.     

Conduction band mediated electron transfer across nanocrystalline TiO2 surfaces

Mesoporous thin films comprised of interconnected nanocrystalline (anatase, 20 nm) TiO2 particles were functionalized with [Ru(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine and deebq is 4,4'-diethylester-2,2'-biquinoline, or iron(III) protoporphyrin IX chloride (hemin). These compounds bind to TiO2 with saturation surface coverages of 8 (+/-2)x10(-8) mol/cm2. Electrochemical measurements show that the compounds first reduction occurs prior to or commensurate with the reduction of the TiO2 electrode. Apparent diffusion constants, Dapp, abstracted from chronoabsorption data measured in acetonitrile were found to be dependent on the applied potential and the electrolyte used. The Dapp values for reduction of Ru(dcbq)(bpy)2/TiO2, where dcbq is 4,4'-(COO-)2-2,2'-biquinoline, increased with decreasing surface coverage. At near saturation surface coverage, the apparent diffusion constant was 9.0 x 10(-12) m2/s after a potential step from -0.61 to -1.31 vs Fc+/0. The Dapp varied by over a factor of six with applied potential for the oxidation of [Ru(dcbq-)(bpy)2]-/TiO2 to Ru(dcbq)(bpy)2/TiO2. Complete reduction of hemin/TiO2 to heme/TiO2 was observed under conditions where the heme surface coverage was about 1/100 of that expected for monolayer surface coverage. The hemin reduction rates were strongly dependent on the final applied potential. The rates for heme to hemin oxidation were less than or equal to the hemin to heme rates in the presence and absence of pyridine. This behavior was opposite to that observed with Ru(dcbq)(bpy)2/TiO2 where reduction was slower than oxidation. A Gerischer-type model was proposed to rationalize the rectifying properties of the interface.     

Synthesis and characterisation of ruthenium complexes containing a pendent catechol ring

A series of [Ru(bipy)2L]+ and [Ru(phen)2L]+ complexes where L is 2-[5-(3,4-dimethoxyphenyl)-4H-1,2,4-triazol-3-yl]pyridine (HL1) and 4-(5-pyridin-2-yl-4H-1,2,4-triazol-3-yl)benzene-1,2-diol (HL2) are reported. The compounds obtained have been characterised using X-ray crystallography, NMR, UV/Vis and emission spectroscopies. Partial deuteriation is used to determine the nature of the emitting state and to simplify the NMR spectra. The acid-base properties of the compounds are also investigated. The electronic structures of [Ru(bipy)2L1]+ and Ru(bipy)2HL1]2+ are examined using ZINDO. Electro and spectroelectrochemical studies on [Ru(bipy)2(L2)]+ suggest that proton transfer between the catechol and triazole moieties on L2 takes place upon oxidation of the L2 ligand.     

Solvent effects on interfacial electron transfer from Ru(4,4'-dicarboxylic acid-2,2'-bipyridine)2(NCS)2 to nanoparticulate TiO2: spectroscopy and solar photoconversion

Resonance Raman spectra are reported for Ru(4,4'-dicarboxylic acid-2,2'-bipyridine)2(NCS)2 (commonly called "N3") in ethanol solution and adsorbed on nanoparticulate colloidal TiO2 in ethanol (EtOH) and in acetonitrile (ACN), at wavelengths within the visible absorption band of the dye. Raman cross sections of free N3 in EtOH are found to be similar to those of N3 adsorbed on colloidal TiO2 in EtOH, and are generally lower than those of N3 on TiO2 in ACN. Strong electronic coupling mediated by surface states results in red-shifted absorption spectra and enhanced Raman signals for N3 adsorbed on nanocolloidal TiO2 in ACN compared to EtOH. In contrast, the absorption spectrum of N3 on nanocrystalline TiO2 in contact with solvent is similar for ACN and EtOH. Wavelength-dependent depolarization ratios for N3 Raman bands of both free and adsorbed N3 reveal resonance enhancement via two or more excited electronic states. Luminescence spectra of N3 adsorbed on nanocrystalline films of TiO2 and ZrO2 in contact with solvent reveal that the quantum yield of electron injection phi(ET) into TiO2 decreases in the order ACN > EtOH > DMSO. Dye-sensitized solar cells were fabricated with N3 adsorbed on nanocrystalline films of TiO2 in contact with ACN, EtOH, and DMSO solutions containing LiI/LiI3 electrolyte. Photoconversion efficiencies eta were found to be 2.6% in ACN, 1.3% in DMSO, and 0.84% in EtOH. Higher short circuit currents are found in cells using ACN, while the maximum voltage is found to be largest in DMSO. It is concluded that the increased photocurrent and quantum yield of interfacial electron transfer in acetonitrile as compared to ethanol and DMSO is primarily the result of faster electron injection of N3 when adsorbed on TiO2 in the presence of ACN as opposed to EtOH or DMSO.     

Singlet and triplet energy transfer processes in ruthenium(II) bipyridine complexes containing covalently bound arenes

Singlet and triplet energy transfer processes in [Ru(bipy)₂(4-methyl-4′-(2-arylethyl)-2,2′-bipyridine)]²⁺ have been investigated, where ARYL = 2-naphthyl (Ru-Naph), 9-anthryl (Ru-Anth) and 1-pyrenyl (Ru-Pyrene). In each case fluorescence from the aromatic chromophore is quenched by intramolecular energy transfer to Ru(bipy)₃²⁺ whereas emission from the Ru(bipy)₃²⁺ moiety is controlled by the relative energy of its ³MLCT state and the pendant arene triplet states. Consequently ³MLCT emission is observed for Ru-Naph whereas it is fully quenched for Ru-Anth. When the two states are isoenergetic (e.g. Ru-Pyrene) a long-lived ³MLCT emission is observed which delays with the same lifetime as the pyrene triplet state (5.23 μs).     

On the mechanism of d-f energy transfer in RuII/LnIII and OsII/LnIII dyads: Dexter-type energy transfer over a distance of 20 A

We have used time-resolved luminescence methods to study rates of photoinduced energy transfer (PEnT) from [M(bipy)3]2+ (M=Ru, Os) chromophores to Ln(III) ions with low-energy f-f states (Ln=Yb, Nd, Er) in d-f dyads in which the metal fragments are separated by a saturated -CH2CH2- spacer, a p-C6H4 spacer, or a p-(C6H4)2 spacer. The finding that d-->f PEnT is much faster across a conjugated p-C6H4 spacer than it is across a shorter CH2CH2 spacer points unequivocally to a Dexter-type energy transfer, involving electronic coupling mediated by the bridging ligand orbitals (superexchange) as the dominant mechanism. Comparison of the distance dependence of the Ru-->Nd energy-transfer rate across different conjugated spacers [p-C6H4 or p-(C6H4)2 groups] is also consistent with this mechanism. Observation of Ru-->Nd PEnT (as demonstrated by partial quenching of the RuII-based 3MLCT emission (MLCT=metal-to-ligand charge transfer), and the growth of sensitised NdIII-based emission at 1050 nm) over approximately 20 A by an exchange mechanism is a departure from the normal situation with lanthanides, in which long-range energy transfer often involves through-space Coulombic mechanisms.     

The influence of dye structure on charge recombination in dye-sensitized solar cells

Replacing the nonyl groups on the solar cell dye Ru(4,4'-carboxylic acid-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(NCS)(2) (Z-907) with amino groups results in a marked decrease in solar cell performance. This is despite the fact that the amino derivative (Z-960) has more favourable light absorption characteristics than Z-907 when used with thick nanocrystalline TiO(2) layers. Electron transfer to the electrolyte from the exposed fluorine-doped tin oxide (FTO) substrate is particularly fast in cells employing the Z-960 dye if a compact TiO(2) blocking layer is not used. The kinetics of electron transfer from the nanocrystalline TiO(2) layer in DSCs employing Z-960 are comparable to those of bare TiO(2) and ca. 2 to 5 times faster than for cells employing Z-907. The faster charge recombination in cells employing Z-960 lowers open-circuit photovoltage and results in very significant charge collection losses that lower short-circuit photocurrent. Voltammetric measurements show that surface modification of FTO electrodes with Z-960 results in slightly more facile charge transfer to acceptor species in triiodide/iodide electrolytes in the dark. A simpler molecule, p-aminobenzoic acid, more dramatically catalyses this charge transfer reaction. Conversely, chemical modification of FTO electrodes with Z-907 or p-toluic acid retards charge transfer kinetics. Similar results are obtained for nanocrystalline TiO(2) electrodes modified with these benzoic acid derivatives. These results strongly imply that surface adsorbed molecules bearing amino groups, including dye molecules, can catalyse charge recombination in dye-sensitized solar cells.