Competitive Intermolecular Energy Transfer and Electron Injection at Sensitized Semiconductor Interfaces

Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges

A family of three ruthenium bipyridyl rigid-rod compounds of the general form [Ru(bpy)₂(LL)](PF₆)₂ were anchored to mesoporous thin films of tin-doped indium oxide (ITO) nanocrystals. Here, LL is a 4-substituted 2,2-bipyridine (bpy) ligand with varying numbers of conjugated phenylenethynylene bridge units between the bipyridine ring and anchoring group consisting of a bis-carboxylated isophthalic group. The visible absorption spectra and the formal potentials, E^o(Ru^III/II), of the surface anchored rigid-rods were insensitive to the presence of the phenylene ethynylene bridge units in 0.1 M tetrabutyl ammonium perchlorate acetonitrile solutions (TBAClO₄/CH₃CN). The conductive nature of the ITO enabled potentiostatic control of the Fermi level and hence a means to tune the Gibbs free energy change, -ΔG^°, for electron transfer from the ITO to the rigid-rods. Pseudo-rate constants for this electron transfer reaction increased as the number of bridge units decreased at a fixed -ΔG^°. With the assumption that the reorganization energy, λ, and the electronic coupling matrix element, H_ab, were independent of the applied potential, rate constants measured as a function of -ΔG^° and analyzed through Marcus-Gerischer theory provided estimates of H_ab and λ. In rough accordance with the dielectric continuum theory, λ was found to increase from 0.61 to 0.80 eV as the number of bridge units was increased. In contrast, H_ab decreased markedly with distance from 0.54 to 0.11 cm^-1, consistent with non-adiabatic electron transfer. Comparative analysis with previously published studies of bridges with an sp³-hybridized carbon indicated that the phenylene ethynylene bridge does not enhance electronic coupling between the oxide and the rigid-rod acceptor. The implications of these findings for practical applications in solar energy conversion are specifically discussed.

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.

Large footprint pyrene chromophores anchored to planar and colloidal metal oxide thin films

Sensitization and binding of a large footprint pyrene chromophore to planar (sapphire) and colloidal metal oxide films (TiO2 and ZrO2) is investigated. The model compound combines a 1-pyrenyl-ethynylenephenylene unit with a new adamantane-tripodal linker that binds to the surface. The linker design, combining a large footprint (approximately 2 nm2) of the tripodal linker with the meta position of the COOH anchoring groups, was suggested from atomistic models, and it aims to provide improved spacing control. The pyrene chromophore unit provides a probe of sensitizer-sensitizer interactions through its propensity to form excimers, unless neighboring pyrene units are sufficiently spaced (>or=3.5 A). Absorption and fluorescence studies, and a comparison with a pyrene-rigid rod model compound, suggest that the new tripodal anchor group allows spacing control on planar surfaces. On colloidal films, the linker provides spacing control at low surface coverage but sensitizer-sensitizer interactions are still observed on colloidal films at high surface coverage. Implications for the functionalization of metal oxide films in hybrid molecule-metal oxide semiconductor material systems are discussed.

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).

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.

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).

Remote and Adjacent Excited-State Electron Transfer at TiO2 Interfaces Sensitized to Visible Light with Ru(II) Compounds

The ruthenium polypyridyl compounds, Ru(dpp)2(deeb)(PF6)2 (Ru-deeb) and cis-Ru(dpp)2(eina)2(PF6)2 (Ru-eina), where dpp is 4,7-diphenyl-1,10-phenanthroline, deeb is 4,4'-diethyl ester-2,2'-bipyridine, and eina is 4-ethyl ester pyridine, have been prepared and characterized to sensitize nanocrystalline TiO2 (anatase) thin films. In neat acetonitrile at room temperature, Ru-deeb was emissive with lambdaem=675 nm, tau=780 ns, and emission quantum yield phiem=0.067, whereas Ru-eina was nonemissive with tau<10 ns. The short lifetime and observed photochemistry for Ru-eina are consistent with the presence of low-lying ligand-field (LF) excited states. The metal-to-ligand charge transfer (MLCT) excited states of Ru-deeb were found to be localized on the surface-bound deeb ligand, and on the remote dpp ligand for Ru-eina. Interfacial proton concentration was employed to tune the relative sensitizer-semiconductor energetics. Injection quantum yields, phiinj, varied from approximately 0.2 at pH=5 to approximately 1 at pH=1, with a slope of approximately 0.15/pH for both compounds. At pH=12, long-lived excited states were observed with phiinj<0.05. At pH<or=2, phiinj became temperature-dependent for Ru-eina, but not for Ru-deeb. A mechanism is proposed wherein population of LF states at elevated temperatures lowers phiinj.

Molecular Approaches to Solar Energy Conversion with Coordination Compounds Anchored to Semiconductor Surfaces

Strategies toward the realization of molecular control of interfacial charge transfer at nanocrystalline semiconductor interfaces are described. Light excitation of coordination compounds, based on (dpi)6 transition metals, anchored to wide band-gap semiconductors, such as TiO2, can initiate electron-transfer processes that ultimately reduce the semiconductor. Such photoinduced charge-separation processes are a key step for solar energy conversion. The thermodynamics and kinetic rate constants for three different interfacial charge separation mechanisms are discussed. Tuning the energetic position of the semiconductor conduction band relative to the molecular sensitizer has provided new insights into interfacial charge transfer. Supramolecular compounds that efficiently absorb light, promote interfacial electron transfer, and feature additional functions such as intramolecular electron transfer when bound to semiconductor surfaces have also been studied. New approaches for enhancing charge-separation lifetimes for solar energy conversion are presented.

Dye-Sensitized SnO2 Electrodes with Iodide and Pseudohalide Redox Mediators

Dye-sensitized mesoporous nanocrystalline SnO2 electrodes and the pseudohalogen redox mediator (SeCN)2/SeCN- or (SCN)2/SCN- or the halogen redox mediator I3-/I- were implemented for regenerative solar cell studies. Adsorption isotherms of the sensitizers Ru(deeb)(bpy)2(PF6)2, Ru(deeb)2(dpp)(PF6)2, and Ru(deeb2(bpz)(PF6)2, where deeb is 4,4'-diethylester-2,2'-bipyridine, dpp is 2,3-dipyridyl pyrazine, and bpz is bipyrazine, binding to the SnO2 surface were well described by the Langmuir model from which the saturation coverage, Gamma0 = 1.7 x 10(-8) mol/cm2, and surface-adduct formation constant, Kad = 2 x 10(5) M(-1), were obtained. Following excited-state interfacial electron transfer, the oxidized sensitizers were reduced by donors present in the acetonitrile electrolyte as shown by transient absorption spectroscopy. With iodide as the donor, a rate constant k > 10(8) s(-1) was measured for sensitizer regeneration. In regenerative solar cells, it was found that the incident photon-to-current conversion efficiencies and open circuit voltages (Voc) were comparable for (SeCN)2/SeCN- and I3-/I- for all three sensitizers. The Voc varied linearly with the logarithm of the short circuit photocurrent densities (Jsc), with typical correlations of approximately 50-60 mV/decade. Capacitance measurements of the SnO2 electrode in the presence of I3-/I-, (SeCN)2/SeCN- or (SCN)2/SCN- are reported.

Solvent-induced modulation of collective photophysical processes in fluorescent silica nanoparticles

In this paper we show how it is possible to control the nature and the efficiency of collective photophysical processes in a network composed of two different fluorescent units organized on the surface of silica nanoparticles. Such a structure is obtained by covering nanoparticles with a layer of dansyl moieties (Dns) and by partially protonating them in solution. The two fluorophores Dns and Dns.H(+) have very different photophysical properties and can be selectively excited and detected. The interaction between the two units Dns and Dns.H(+) has been first investigated in a reference compound obtained by derivatizing 1,6-hexanediamine with two dansyl units. The photophysical characterization of this compound (absorption spectra, fluorescence spectra, quantum yield, and lifetime) showed that the two moieties can be involved both in energy and electron-transfer processes. Dansylated nanoparticles were prepared by modifying preformed silica nanoparticles with dansylated (3-aminopropyl)trimethoxysilane. Photophysical studies indicated that protonation has a dramatic effect on the fluorescence of the nanoparticles, leading to the quenching of both the protonated units and the surrounding nonprotonated ones. This amplified response to protonation, due to charge-transfer interactions, is solvent-dependent and is less efficient in pure chloroform with respect to acetonitrile/chloroform (5/1 v/v) mixtures. The reduced efficiency of the electron-transfer processes responsible for the quenching makes energy transfer competitive to such an extent that in pure chloroform excitation energy migration takes place from Dns.H(+) to Dns with great efficiency.

Long-range electron transfer across molecule-nanocrystalline semiconductor interfaces using tripodal sensitizers

Four tripodal sensitizers, Ru(bpy)(2)(Ad-tripod-phen)(2+) (1), Ru(bpy)(2)(Ad-tripod-bpy)(2+) (2), Ru(bpy)(2)(C-tripod-phen)(2+) (3), and Ru(bpy)(2)(C-tripod-bpy)(2+) (4) (where bpy is 2,2'-bipyridine, phen is 1,10-phenanthroline, and Ad-tripod-bpy (phen) and C-tripod-bpy (phen) are tripod-shaped bpy (phen) ligands based on 1,3,5,7-tetraphenyladamantane and tetraphenylmethane, respectively), have been synthesized and characterized. The tripodal sensitizers consist of a rigid-rod arm linked to a Ru(II)-polypyridine complex at one end and three COOR groups on the other end that bind to metal oxide nanoparticle surfaces. The excited-state and redox properties of solvated and surface-bound 1-4 have been studied at room temperature. The absorption spectra, emission spectra, and electrochemical properties of 1-4 in acetonitrile solution are preserved when 1-4 are bound to nanocrystalline (anatase) TiO(2) or colloidal ZrO(2) mesoporous films. This behavior is indicative of weak electronic coupling between TiO(2) and the sensitizer. The kinetics for excited-state decay are exponential for 1-4 in solution and are nonexponential when 1-4 are bound to ZrO(2) or TiO(2). Efficient and rapid (k(cs) > 10(8) s(-)(1)) excited-state electron injection is observed for 1-4/TiO(2). The recombination of the injected electron with the oxidized Ru(III) center is well described by a second-order kinetic model with rate constants that are independent of the sensitizer. The sensitizers bound to TiO(2) were reversibly oxidized electrochemically with an apparent diffusion coefficient approximately 1 x 10(-11) cm(2) s(-)(1).

Charge-transfer studies of iron cyano compounds bound to nanocrystalline TiO(2) surfaces

Nanocrystalline (anatase) titanium dioxide films have been sensitized to visible light with K(4)[Fe(CN)(6)] and Na(2)[Fe(LL)(CN)(4)], where LL = bpy (2,2'-bipyridine), dmb (4,4'-dimethyl-2,2'-bipyridine), or dpb (4,4'-diphenyl-2,2'-bipyridine). Coordination of Fe(CN)(6)(4-) to the TiO(2) surface results in the appearance of a broad absorption band (fwhm approximately 8200 cm(-1)) centered at 23800 +/- 400 cm(-1) assigned to an Fe(II)-->TiO(2) metal-to-particle charge-transfer (MPCT) band. The absorption spectra of Fe(LL)(CN)(4)(2-) compounds anchored to TiO(2) are well modeled by a sum of metal-to-ligand charge-transfer (MLCT) bands and a MPCT band. Pulsed light excitation (417 or 532 nm, approximately 8 ns fwhm, approximately 2-15 mJ/pulse) results in the immediate appearance of absorption difference spectra assigned to an interfacial charge separated state [TiO(2)(e(-)), Fe(III)], k(inj) > 10(8) s(-1). Charge recombination is well described by a second-order equal concentration kinetic model and requires milliseconds for completion. A model is proposed wherein sensitization of Fe(LL)(CN)(4)(2-)/TiO(2) occurs by MPCT and MLCT pathways, the quantum yield for the latter being dependent on environment. The solvatochromism of the materials allows the reorganization energies associated with charge transfer to be quantified. The photocurrent efficiencies of the sensitized materials are also reported.