Interfacial electron transfer of perylenes: Influence of the anchor binding mode.

Interfacial electron transfer (IET) through saturated single-linker and dual-linker groups from a perylene chromophore into nanostructured TiO2 films was studied by ultrafast spectroscopy. Perylene chromophores with one and two propanoic acid linker groups in the peri and ortho positions were investigated. In comparison to previously studied perylenes bound via unsaturated acrylic acid linkers, the chromophores with saturated linkers showed bi-exponential IET dynamics. Two distinct transfer times were observed that indicate the presence of two concurrent binding modes. A comparison between ortho- and peri-substituted sensitizers resulted in slower IET dynamics and weaker electronic coupling for ortho substitution. Finally, IET from sensitizers with saturated linker groups is neither promoted nor hindered by a second linker group. This indicates that only one of the two linkers binds covalently to the surface. This study reveals the importance of the anchor-binding mode and design considerations of the linker for regulating IET.

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, Eo(RuIII/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, Hab, were independent of the applied potential, rate constants measured as a function of -ΔG° and analyzed through Marcus-Gerischer theory provided estimates of Hab 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, Hab 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.

Self-Assembled Monolayers for Improved Charge Injection of Silver Back Electrodes in Inverted Organic Electronic Devices.

Self-assembled monolayers (SAMs) formed from thiol compounds bound to Ag and Au electrodes have been used as an important strategy in improving the stability and efficiency of optoelectronic devices. Thiol compounds provide only one binding site with the metal electrode which limits their influence. Dithiolane/dithiol compounds can provide multiple binding sites and could be useful in enhancing the performance of the device. In this study, inverted organic semiconducting hole-only devices were fabricated by using Ag back electrodes in conjunction with SAMs formed from disulfide lipoic acid-based compounds and were compared to a long aliphatic chain thiol. The binding and the electronic properties as well as electrical characteristics of the SAMs on silver were studied to look at the influence of their structure on charge injection in the organic semiconductor devices. It was found that the SAMs formed with (±)-α-lipoic acid, isolipoic acid, and (±)-4-phenylbutyl 5-(1,2-dithiolan-3-yl) pentanoate significantly improved the charge injection by either changing the work function of the Ag or altering the physical interaction between the polymer and the metal surface. This study may lead to an understanding of how the nature of the functional groups of the SAM and the number of bonds formed between each SAM molecule and the metal electrode influence the contact resistance and the performance of organic semiconductor devices.

Controlling and Optimizing Photoinduced Charge Transfer across Ultrathin Silica Separation Membrane with Embedded Molecular Wires for Artificial Photosynthesis.

Ultrathin amorphous silica membranes with embedded organic molecular wires (oligo(p-phenylenevinylene), three aryl units) provide chemical separation of incompatible catalytic environments of CO2 reduction and H2O oxidation while maintaining electronic and protonic coupling between them. For an efficient nanoscale artificial photosystem, important performance criteria are high rate and directionality of charge flow. Here, the visible-light-induced charge flow from an anchored Ru bipyridyl light absorber across the silica nanomembrane to Co3O4 water oxidation catalyst is quantitatively evaluated by photocurrent measurements. Charge transfer rates increase linearly with wire density, with 5 nm-2 identified as an optimal target. Accurate measurement of wire and light absorber densities is accomplished by the polarized FT-IRRAS method. Guided by density functional theory (DFT) calculations, four wire derivatives featuring electron-donating (methoxy) and -withdrawing groups (sulfonate, perfluorophenyl) with highest occupied molecular orbital (HOMO) potentials ranging from 1.48 to 0.64 V vs NHE were synthesized and photocurrents evaluated. Charge transfer rates increase sharply with increasing driving force for hole transfer from the excited light absorber to the embedded wire, followed by a decrease as the HOMO potential of the wire moves beyond the Co3O4 valence band level toward more negative values, pointing to an optimal wire HOMO potential around 1.3 V vs NHE. Comparison with photocurrents of samples without nanomembrane indicates that silica layers with optimized wires are able to approach undiminished electron flux at typical solar intensities. Combined with the established high proton conductivity and small-molecule blocking property, the charge transfer measurements demonstrate that oxidation and reduction catalysis can be efficiently integrated on the nanoscale under separation by an ultrathin silica membrane.

Synthesis and Properties of Perylene-Bridge-Anchor Chromophoric Compounds.

The quest to control chromophore/semiconductor properties to enable new technologies in energy and information science requires detailed understanding of charge carrier dynamics at the atomistic level, which can often be attained through the use of model systems. Perylene-bridge-anchor compounds are successful models for studying fundamental charge transfer processes on TiO2, which remains among the most commonly investigated and technologically important interfaces, mostly because of perylene's advantageous electronic and optical properties. Nonetheless, the ability to fully exploit synthetically the substitution pattern of perylene with linker (= bridge-anchor) units remains little explored. Here we developed 2,5-di-tert-butylperylene (DtBuPe)-bridge-anchor compounds with t-Bu group substituents to prevent π-stacking and one or two linker units in both the peri and ortho positions, by employing a combination of Friedel-Crafts alkylations, bromination, iridium-catalyzed borylation, and palladium-catalyzed cross-coupling reactions. Photophysical characterization and computational analysis by density functional theory (DFT) and time-dependent DFT (TD-DFT) were carried out on four DtBuPe acrylic acid derivatives with a single or a double linker in peri (12b), ortho (15b), peri,peri (18b), and ortho,ortho (21b). The energies of the unoccupied orbitals {LUMO, LUMO + 1, LUMO + 2} are strongly affected by the presence of a π-conjugated linker, resulting in a stabilization of these states and a red shift of their absorption and emission spectra, as well as the loss of vibronic structure in the spectrum of the peri,peri compound, consistent with the strong bonding character of this substitution pattern.

Helical Peptides Design for Molecular Dipoles Functionalization of Wide Band Gap Oxides.

The use of helical hexapeptides to establish a surface dipole layer on a TiO2 substrate, with the goal of influencing the energy levels of a coadsorbed chromophore, is explored. Two helical hexapeptides, synthesized from 2-amino isobutyric acid (Aib) residues, were protected at the N-terminus with a carboxybenzyl group (Z) and at the C-terminus carried either a carboxylic acid or an isophthalic acid (Ipa) anchor group to form Z-(Aib)6-COOH or Z-(Aib)6-Ipa, respectively. Using a combination of vibrational and photoemission spectroscopies, bonding of the two peptides to TiO2 surfaces (either nanostructured or single-crystal TiO2(110)) was found to be highly dependent on the anchor group, with Ipa establishing a monolayer much more efficiently than COOH. Furthermore, a monolayer of Z-(Aib)6-Ipa on TiO2(110) was exposed for different binding times to a solution of a zinc tetraphenylporphyrin (ZnTPP) derivative terminated with an Ipa anchor group (ZnTPP-P-Ipa). Photoemission spectroscopy revealed that ZnTPP-P-Ipa partly displaced Z-(Aib)6-Ipa, forming a coadsorbed monolayer on the oxide surface. The presence of the peptide molecular dipole shifted the HOMO levels of the ZnTPP group to lower energy by ∼300 meV, in accordance with a simple parallel plate capacitor model. These results suggest that a mixed-layer approach, involving coadsorption of a strong molecular dipole compound with a chromophore, is a versatile method to shift the energy levels of such chromophores with respect to the band edges of the substrate.

Comparison of ZnO surface modification with gas-phase propiolic acid at high and medium vacuum conditions.

Recent advances in preservation of the morphology of ZnO nanostructures during dye sensitization required the use of a two-step preparation procedure. The first step was the key for preserving ZnO materials morphology. It required exposing clean ZnO nanostructures to a gas-phase prop-2-ynoic acid (propiolic acid) in vacuum. This step resulted in the formation of a robust and stable surface-bound carboxylate with ethynyl groups available for further modification, for example, with click chemistry. This paper utilizes spectroscopic and microscopic investigations to answer several questions about this modification and to determine if the process can be performed under medium vacuum conditions instead of high vacuum procedures reported earlier. Comparing the results of the preparation process at medium vacuum of 0.5 Torr base pressure with the previously reported investigations of the same process in high vacuum of 10-5 Torr suggests that both processes lead to the formation of the same surface species, confirming that the proposed modification scheme can be widely applicable for ZnO sensitization procedures and does not require the use of high vacuum. Additional analysis comparing the computationally predicted surface structures with the results of spectroscopic investigations yields the more complete description of the surface species resulting from this approach.

Vibrational Spectroscopy on Photoexcited Dye-Sensitized Films via Pump-Degenerate Four-Wave Mixing.

Molecular sensitization of semiconductor films is an important technology for energy and environmental applications including solar energy conversion, photocatalytic hydrogen production, and water purification. Dye-sensitized films are also scientifically complex and interesting systems with a long history of research. In most applications, photoinduced heterogeneous electron transfer (HET) at the molecule/semiconductor interface is of critical importance, and while great progress has been made in understanding HET, many open questions remain. Of particular interest is the role of combined electronic and vibrational effects and coherence of the dye during HET. The ultrafast nature of the process, the rapid intramolecular vibrational energy redistribution, and vibrational cooling present complications in the study of vibronic coupling in HET. We present the application of a time domain vibrational spectroscopy-pump-degenerate four-wave mixing (pump-DFWM)-to dye-sensitized solid-state semiconductor films. Pump-DFWM can measure Raman-active vibrational modes that are triggered by excitation of the sample with an actinic pump pulse. Modifications to the instrument for solid-state samples and its application to an anatase TiO2 film sensitized by a Zn-porphyrin dye are discussed. We show an effective combination of experimental techniques to overcome typical challenges in measuring solid-state samples with laser spectroscopy and observe molecular vibrations following HET in a picosecond time window. The cation spectrum of the dye shows modes that can be assigned to the linker group and a mode that is localized on the Zn-phorphyrin chromophore and that is connected to photoexcitation.

Morphology-Preserving Sensitization of ZnO Nanorod Surfaces via Click-Chemistry.

Films of ZnO nanorods grown by chemical vapor deposition were functionalized with a chromophore in a stepwise process that preserves the surface morphology. In the first step, the ZnO nanorods were functionalized by exposure to prop-2-ynoic acid (propiolic acid) in vacuum, which did bind through the COOH group leading to a ZnO surface functionalized with ethyne moieties (ethyne/ZnO). In the second step, 9-(4-azidophenyl)-2,5-di-tert-butylperylene (DTBPe-Ph-N3) was reacted with the ethyne/ZnO surface via copper-catalyzed azide-alkyne click reaction (CuAAC) in solution to form the DTBPe-functionalized surface (DTBPe/ZnO). The ZnO morphology was preserved after each step, as demonstrated by scanning electron microscopy (SEM). Each step was probed by X-ray photoelectron spectroscopy (XPS), and transient absorption spectroscopy (TA) of the resulting DTBPe/ZnO surface shows interfacial electron transfer following visible light excitation. As expected, attempts to bind the reference compound 1-(4-(8,11-ditert-butylperylen-3-yl)-phenyl)-1H-1,2,3-triazole-4-carboxylic acid (DTBPe-Ph-Tz-COOH) directly from solution lead to etched surfaces (confirmed by SEM) and undefined binding modes (confirmed by TA).

Functionalization of MgZnO nanorod films and characterization by FTIR microscopic imaging.

Metal organic chemical vapor deposition grown films consisting of MgxZn1-xO (4% < x < 5%) nanorod arrays (MgZnOnano) were functionalized with 11-azidoundecanoic acid (1). The MgZnOnano was used instead of pure ZnO to take advantage of the etching resistance of the MgZnOnano during the binding and subsequent sensing device fabrication processes of sensor devices, while the low Mg composition level ensures that selected ZnO properties useful for sensors development, such as piezoelectricity, are retained. Compound 1 was bound to the MgZnOnano surface through the carboxylic acid group, leaving the azido group available for click chemistry and as a convenient infrared spectroscopy (IR) probe. The progress of the functionalization with 1 was characterized by FTIR microscopic imaging as a function of binding time, solvents employed, and MgZnOnano morphology. Binding of 1 was most stable in solutions of 3-methoxypropionitrile (MPN), a non-protic polar solvent. This occurred first in µm-scale islands, then expanded to form a rather uniform layer after 22 h. Binding in alcohols resulted in less homogenous coverage, but the 1/MgZnOnano films prepared from MPN were stable upon treatment with alcohols at room temperature. The binding behavior was significantly dependent on the surface morphology of MgZnOnano. Graphical abstract The functionalization of MgZnO nanorod films with a click-ready linker and its dependence on bidning conditions and morphology has been studied by FTIR microscopic imaging using the azido group as the IR tag.

Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups.

Heterogeneous electron transfer (HET) between photoexcited molecules and colloidal TiO2 has been investigated for a set of Zn-porphyrin chromophores attached to the semiconductor via linkers that allow to change level alignment by 200 meV by reorientation of the dipole moment. These unique dye molecules have been studied by femtosecond transient absorption spectroscopy in solution and adsorbed on the TiO2 colloidal film in vacuum. In solution energy transfer from the excited chromophore to the dipole group has been identified as a slow relaxation pathway competing with S2-S1 internal conversion. On the film heterogeneous electron transfer occurred in 80 fs, much faster compared to all intramolecular pathways. Despite a difference of 200 meV in level alignment of the excited state with respect to the semiconductor conduction band, identical electron transfer times were measured for different linkers. The measurements are compared to a quantum-mechanical model that accounts for electronic-vibronic coupling and finite band width for the acceptor states. We conclude that HET occurs into a distribution of transition states that differs from regular surface states or bridge mediated states.

Photoelectrochemical properties of porphyrin dyes with a molecular dipole in the linker.

The electronic properties of three porphyrin-bridge-anchor photosensitizers are reported with (1a, 1e, 3a and 3e) or without (2a and 2e) an intramolecular dipole in the bridge. The presence and orientation of the bridge dipole is hypothesized to influence the photovoltaic properties due to variations in the intrinsic dipole at the semiconductor-molecule interface. Electrochemical studies of the porphyrin-bridge-anchor dyes self-assembled on mesoporous nanoparticle ZrO2 films, show that the presence or direction of the bridge dipole does not have an observable effect on the electronic properties of the porphyrin ring. Subsequent photovoltaic measurements of nanostructured TiO2 semiconductor films in dye sensitized solar cells show a reduced photocurrent for photosensitizers 1a and 3a containing a bridge dipole. However, cooperative increased binding of the 1a + 3a co-sensitized device demonstrates that dye packing overrides any differences due to the presence of the small internal dipole.

Synthesis of Zinc Tetraphenylporphyrin Rigid Rods with a Built-In Dipole.

Three Zn(II) tetraphenylporphyrins (ZnTPP) were synthesized to study the influence of a molecular dipole on the energy level alignment of a chromophore bound to a metal oxide semiconductor: ZnTPP-PE(DA)-IpaOMe (1), ZnTPP-PE-IpaOMe (2), and ZnTPP-PE(AD)-IpaOMe (3). Each contained a rigid-rod linker made of a p-phenylene ethynylene (PE) moiety terminated with the methyl ester of an isophthalic acid unit (Ipa). Porphyrins 1 and 3 contained an intramolecular dipole in the central phenyl ring, which was built by introducing electron donor (D, NMe2) and acceptor (A, NO2) substituents in para position to each other. In 1 and 3, the relative position of the D and A substituents, and therefore the dipole direction, was reversed. Porphyrin 2, without substituents in the linker, was synthesized for a comparison. The structures of precursors to 1 and 3 and the structure of 1 were determined by single crystal X-ray analysis. Solution UV-vis and steady-state fluorescence spectra of 1-3 were identical to each other and exhibited the spectral features typical of the ZnTPP chromophore and their electrochemical properties were also very similar. Methyl esters 1-3 were hydrolyzed to the corresponding carboxylic acids for binding to metal oxide semiconductors.

Synthesis and electronic properties of 1,2-hemisquarimines and their encapsulation in a cucurbit[7]uril host.

The synthesis of a new class of robust squaraine dyes, colloquially named 1,2-hemisquarimines (1,2-HSQiMs), through the microwave-assisted condensation of aniline derivatives with the 1,2-squaraine core is reported. In CH3CN, 1,2-HSQiMs show a broad absorption band with a high extinction coefficient and a maximum at around λ=530 nm, as well as an emission band centered at about λ=574 nm, that are pH dependent. Protonation of the imine nitrogen causes a redshift of both absorption and emission maxima, with a concomitant increase in the lifetime of the emitting excited state. Encapsulation of the chromophore into a cucurbit[7]uril host revealed fluorescence enhancement and increased photostability in water. The redox characteristics of 1,2-HSQiMs indicate that charge injection into TiO2 is possible; this opens up promising perspectives for their use as photosensitizers for solar energy conversion.

Homoleptic "star" Ru(II) polypyridyl complexes: shielded chromophores to study charge-transfer at the sensitizer-TiO2 interface.

Three homoleptic star-shaped ruthenium polypyridyl complexes, termed Star YZ1, Star YZ2, and Star YZ3, where the Ru(II) center is coordinated to three bipyridine ligands each carrying two oligo(phenylene ethynylene) (OPE) rigid linker units terminating with isophthalic ester (Ipa) groups for binding to metal-oxide surfaces were synthesized. In Star YZ3, each OPE linker was substituted with two n-butoxy (n-BuO) solubilizing groups. Star complex YZ4, which is homoleptic but lacks the octahedral symmetry, was synthesized as a reference compound. The Star complexes were synthesized using two approaches: in the first, Ru(4,4'-(Br)2-2,2'-bpy)3 was reacted in a Sonogashira cross coupling reaction with the ethynyl-OPE-Ipa linkers; in the second, the 2,2'-bpy-OPE-Ipa ligands were reacted with Ru(DMSO)4(PF6)2. The photophysical behavior of the Star complexes were studied in fluid solution and anchored to the surface of mesoporous nanocrystalline TiO2 thin films (Star/TiO2). To a first approximation the excited state behavior in CH3CN was unchanged when the compounds were anchored to a TiO2 thin film, indicating that the highly symmetrical (octahedral) and rigid molecular structure of the ligands shielded the chromophoric core from the TiO2 semiconductor. Inefficient excited state injection, φ(inj) < 0.05, was observed to occur on a nanosecond time scale with slow recombination. In addition, the presence of n-BuO groups on the linker unit gave a large increase in the extinction coefficient of YZ3, which allows for enhanced harvesting of sunlight. The results indicate that molecular design on the nanometer length scale can be utilized to control excited state relaxation pathways at semiconductor surfaces.

Functionalization of nanostructured ZnO films by copper-free click reaction.

The copper-free click reaction was explored as a surface functionalization methodology for ZnO nanorod films grown by metal organic chemical vapor deposition (MOCVD). 11-Azidodecanoic acid was bound to ZnO nanorod films through the carboxylic acid moiety, leaving the azide group available for Cu-free click reaction with alkynes. The azide-functionalized layer was reacted with 1-ethynylpyrene, a fluorescent probe, and with alkynated biotin, a small biomolecule. The immobilization of pyrene on the surface was probed by fluorescence spectroscopy, and the immobilization of biotin was confirmed by binding with streptavidin-fluorescein isothiocyanate (streptavidin-FITC). The functionalized ZnO films were characterized by Fourier transform infrared attenuated total reflectance (FTIR-ATR), steady-state fluorescence emission, fluorescence microscopy, and field emission scanning electron microscopy (FESEM).