Interrogating Light-initiated Dynamics in Metal-Organic Frameworks with Time-resolved Spectroscopy.

Time-resolved spectroscopy is an essential part of both fundamental and applied chemical research. Such techniques access light-initiated dynamics on time scales ranging from femtosecond to microsecond. Many techniques falling under this description have been applied to gain significant insight into metal-organic frameworks (MOFs), a diverse class of porous coordination polymers. MOFs are highly tunable, both compositionally and structurally, and unique challenges are encountered in applying time-resolved spectroscopy to interrogate their light-initiated properties. These properties involve various excited state mechanisms such as crystallographically defined energy transfer, charge transfer, and localization within the framework, photoconductivity, and structural dynamics. The field of time-resolved MOF spectroscopic studies is quite nascent; each original report cited in this review was published within the past decade. As such, this review is a timely and comprehensive summary of the most significant contributions in this emerging field, with focuses on the overarching spectroscopic concepts applied and on identifying key challenges and future outlooks moving forward.

Ultrafast terahertz spectroscopy provides insight into charge transfer efficiency and dynamics in artificial photosynthesis.

Terahertz (THz) spectroscopy provides a noncontact method to measure the ultrafast dynamics and photoconductivity of mobile carriers in semiconducting materials. This has proven useful in studying artificial photosynthesis devices which use semiconductor photoelectrodes. We present a brief introduction to optical-pump THz-probe (OPTP) spectroscopy, a technique that provides unique and useful insight into interfacial electron transfer (from the surface-attached dye to the conduction band of the semiconductor) in dye-sensitized photoelectrochemical cells. Compared with more familiar methods like visible transient absorption spectroscopy, OPTP spectroscopy stands out in offering both sub-picosecond time resolution as well as sensitivity to mobile carriers (electrons and holes) in the semiconductor portion of artificial photosynthesis devices. The mobile carriers are crucial to device performance as only they pass to the other half cell to complete the reaction. In order to highlight these advantages and illustrate the types of questions OPTP spectroscopy can address, we discuss three case studies. In the first, OPTP spectroscopy is used to measure the injection rates from a set of six different dyes, revealing the effect of the energetics and lifetimes of the dye excited states on interfacial electron transfer. The subsequent case studies investigate the influence of varying the moieties which bind to the surface (anchors), as well as the moieties that connect the chromophore with these anchors (linkers). OPTP spectroscopy was used to measure the interfacial electron transfer rate as these moieties were varied.

Nelly: A User-Friendly and Open-Source Implementation of Tree-Based Complex Refractive Index Analysis for Terahertz Spectroscopy.

Terahertz (THz) spectroscopy is a powerful tool for unambiguously extracting complex-valued material properties (e.g., refractive index, conductivity, etc.) from a wide range of samples, with applications ranging from materials science to biology. However, extracting complex refractive indices from THz time-domain spectroscopy data can prove challenging, especially for multilayer samples. These challenges arise from the large number of transmission-reflection paths the THz pulse can take through the sample layers, leading to unwieldy strings of Fresnel coefficients. This issue has often been addressed using various approximations. However, these approximations are only applicable to specific classes of samples and can give erroneous results when misapplied. An alternative to this approach is to programmatically model all possible paths through the sample. The many paths through the sample layers can be modeled as a tree that branches at every point where the paths diverge, i.e., whenever the pulse can either be transmitted or reflected. This tree can then be used to generate expressions relating the unknown refractive index to the observed time domain data. Here, we provide a freely available open-source package implementing this method as both a MATLAB library and a corresponding graphical user interface, which can also be run without a MATLAB license (https://github.com/YaleTHz/nelly). We have tested this method for a range of samples and compared the results to commonly used approximations to demonstrate its accuracy and wide applicability. Our method consistently gives better agreement than common approximations.

Direct Evidence of Photoinduced Charge Transport Mechanism in 2D Conductive Metal Organic Frameworks.

Conductive metal organic frameworks (MOFs) represent a promising class of porous crystalline materials that have demonstrated potential in photo-electronics and photocatalytic applications. However, the lack of fundamental understanding on charge transport (CT) mechanism as well as the correlation of CT mechanism with their structure hampered their further development. Herein, we report the direct evidence of CT mechanism in 2D Cu-THQ MOFs and the correlation of temporal and spatial behaviors of charge carriers with their photoconductivity by combining three advanced spectroscopic methods, including time resolved optical and X-ray absorption spectroscopy and terahertz spectroscopy. In addition to Cu-THQ, the CT in Cu/Zn-THQ after incorporating Zn2+ guest metal was also examined to uncover the contribution of through space pathway, as the presence of the redox inactive 3d10 Zn2+ is expected to perturb the long range in-plane CT. We show that the hot carriers in Cu-THQ generated after photoexcitation are highly mobile and undergo fast localization to a lower energy state (cool carriers) with electrons occupying Cu center and holes in ligands. The cool carriers, which have super long lifetime (>17 ns), are responsible for the long-term photoconductivity in Cu-THQ and transport through the O-Cu-O motif with negligible contribution from interlayer ligand π-π stacking, as incorporation of Zn2+ in Cu-THQ significantly reduced photoconductivity. These unprecedented results not only demonstrate the capability to experimentally probe CT mechanism but also provide important insight in the rational design of 2D MOFs for photoelectronic and photocatalytic applications.

Suspensions of Semiconducting Nanoparticles in Nafion for Transient Spectroscopy and Terahertz Photoconductivity Measurements.

The characterization of emerging materials is crucial for the experimentally driven design of next-generation technologies. We describe a cost- and time-effective method for suspending nanoparticles and other photoactive materials in Nafion for transient spectroscopy and time-resolved terahertz (THz) photoconductivity measurements. Nafion is an ideal suspension matrix because it has high transparency throughout the UV/vis/near-IR and THz regions of the spectrum. Suspensions of nanoparticles in Nafion require only small amounts of sample (<5 mg) and can be prepared and deposited in ∼1 h. The suspension is well-suited for transient THz measurements, which can be used to determine the photoconductivity spectrum of the embedded nanoparticles. In this work, we used silicon nanoparticles as a model material to demonstrate the efficacy of Nafion suspensions for transient THz spectroscopy. This methodology can be used for rapid and cost-effective measurements of emerging materials such as solar materials, electrocatalysts, and nanomaterials.

Collaboration between experiment and theory in solar fuels research.

As the challenges in science increase in scope and interdisciplinarity, collaboration becomes increasingly important. Our groups have maintained close collaborations for solar fuels research over the past decade. Based on this experience, we discuss strategies for collaboration between experiment and theory including facilitation of effective communication and navigation of problems that arise. These strategies are illustrated by case studies of collaborative efforts in solar fuels research pertaining to interfacial electron transfer in dye-sensitized metal oxides and the design and mechanism of water-oxidation catalysts.

Metal-Organic Framework Photoconductivity via Time-Resolved Terahertz Spectroscopy.

While metal-organic frameworks (MOFs) have been under thorough investigation over the past two decades, photoconductive MOFs are an emerging class of materials with promising applications in light harvesting and photocatalysis. To date, there is not a general method to investigate the photoconductivity of polycrystalline MOF samples as-prepared. Herein, we utilize time-resolved terahertz spectroscopy along with a new sample preparation method to determine the photoconductivity of Zn2TTFTB, an archetypical conductive MOF, in a noncontact manner. Using this technique, we were able to gain insight into MOF photoconductivity dynamics with subpicosecond resolution, revealing two distinct carrier lifetimes of 0.6 and 31 ps and a long-lived component of several ns. Additionally, we determined the frequency dependent photoconductivity of Zn2TTFTB which was shown to follow Drude-Smith behavior. Such insights are crucially important with regard to developing the next generation of functional photoconductive MOF materials.

Highly Active NiO Photocathodes for H2O2 Production Enabled via Outer-Sphere Electron Transfer.

Tandem dye-sensitized photoelectrosynthesis cells are promising architectures for the production of solar fuels and commodity chemicals. A key bottleneck in the development of these architectures is the low efficiency of the photocathodes, leading to small current densities. Herein, we report a new design principle for highly active photocathodes that relies on the outer-sphere reduction of a substrate from the dye, generating an unstable radical that proceeds to the desired product. We show that the direct reduction of dioxygen from dye-sensitized nickel oxide (NiO) leads to the production of H2O2. In the presence of oxygen and visible light, NiO photocathodes sensitized with commercially available porphyrin, coumarin, and ruthenium dyes exhibit large photocurrents (up to 400 µA/cm2) near the thermodynamic potential for O2/H2O2 in near-neutral water. Bulk photoelectrolysis of porphyrin-sensitized NiO over 24 h results in millimolar concentrations of H2O2 with essentially 100% faradaic efficiency. To our knowledge, these are among the most active NiO photocathodes reported for multiproton/multielectron transformations. The photoelectrosynthesis proceeds by initial formation of superoxide, which disproportionates to H2O2. This disproportionation-driven charge separation circumvents the inherent challenges in separating electron-hole pairs for photocathodes tethered to inner sphere electrocatalysts and enables new applications for photoelectrosynthesis cells.

A Terahertz-Transparent Electrochemical Cell for In Situ Terahertz Spectroelectrochemistry.

Terahertz spectroscopy is broadly applicable for the study of a wide variety of materials, but spectroelectrochemistry has not been performed in the THz range because of the lack of a THz-transparent electrochemical cell. While THz-transparent electrodes do exist, they have never been utilized in a complete three-electrode cell, which is the configuration required for accurate potential control in aqueous media. We have designed and constructed a THz-transparent three-electrode electrochemical cell and have performed THz spectroelectrochemistry of a SnO2 thin film. The cell utilizes a custom-made reference electrode and tubing which allows the composition of electrolyte to be changed during an experiment. THz spectroelectrochemical measurements show a decrease in THz transmission at potentials where SnO2 conduction band states are potentiostatically filled. We also describe a simple method for measuring the uncompensated resistance and RC time constant.

Terahertz Spectroscopy and Density Functional Theory Calculations of dl-Norleucine and dl-Methionine.

We present terahertz (THz) measurements and density functional theory (DFT) calculations of two amino acid crystals: dl-norleucine and dl-methionine. Their molecular structures are very similar and therefore also their crystal structures. We report the absorption spectra for both amino acids, which have a strong resonance at 1.87 THz in dl-norleucine and 1.94 THz in dl-methionine. In addition, we find a higher frequency resonance at 2.49 THz in dl-methionine, which has no corresponding mode in dl-norleucine. The experimental data are supported by DFT calculations, which show that the origin of the two strongest vibrational modes in dl-norleucine and dl-methionine are based on the same underlying vibrational motions, whereas the 2.49 THz resonance in dl-methionine is due to the motion of the sulfur atom, which is not present in dl-norleucine.

Exploring the solid state phase transition in dl-norvaline with terahertz spectroscopy.

dl-Norvaline is a molecular crystal at room temperature and it undergoes a phase transition when cooled below 190 K. This phase transition is believed to be Martensitic, thus making it of particular interest for molecular machines. In this paper we investigate this phase transition by measuring its terahertz (THz) spectrum over a range of temperatures. Temperature-dependent THz time-domain spectroscopy (THz-TDS) measurements reveal that the transition temperature (Tß→α) is 190 K. The influence of nucleation seeds was analyzed by determining the Tß→α of molecular crystals with varying grain size. Grains of 5 µm or less result in a lower transition temperature (Tß→α = 180 K) compared to larger grains of 125-250 µm (Tß→α = 190 K). Additionally, we gain insight into the physical process of the phase transition via temperature-dependent THz-TDS spectra of doped and mixed molecular crystals. The addition of molecular dopants, which differ from dl-norvaline only at the end of the side chain which resides in the hydrophobic layers of the crystal, decreases Tß→α. This is consistent with a solid-solid phase transition in which the unit cell shifts along this hydrophobic layer, and it leads us to believe that the phase transition in dl-norvaline is Martensitic in nature.

Electron injection dynamics in high-potential porphyrin photoanodes.

There is a growing need to utilize carbon neutral energy sources, and it is well known that solar energy can easily satisfy all of humanity's requirements. In order to make solar energy a viable alternative to fossil fuels, the problem of intermittency must be solved. Batteries and supercapacitors are an area of active research, but they currently have relatively low energy-to-mass storage capacity. An alternative and very promising possibility is to store energy in chemical bonds, or make a solar fuel. The process of making solar fuel is not new, since photosynthesis has been occurring on earth for about 3 billion years. In order to produce any fuel, protons and electrons must be harvested from a species in its oxidized form. Photosynthesis uses the only viable source of electrons and protons on the scale needed for global energy demands: water. Because artificial photosynthesis is a lofty goal, water oxidation, which is a crucial step in the process, has been the initial focus. This Account provides an overview of how terahertz spectroscopy is used to study electron injection, highlights trends from previously published reports, and concludes with a future outlook. It begins by exploring similarities and differences between dye-sensitized solar cells (DSSCs) for producing electricity and a putative device for splitting water and producing a solar fuel. It then identifies two important problems encountered when adapting DSSC technology to water oxidation-improper energy matching between sensitizer energy levels with the potential for water oxidation and the instability of common anchoring groups in water-and discusses steps to address them. Emphasis is placed on electron injection from sensitizers to metal oxides because this process is the initial step in charge transport. Both the rate and efficiency of electron injection are analyzed on a sub-picosecond time scale using time-resolved terahertz spectroscopy (TRTS). Bio-inspired pentafluorophenyl porphyrins are promising sensitizers because their high reduction potentials are compatible with the energy requirements of water oxidation. TRTS of free-base and metalated pentafluorophenyl porphyrins reveal inefficient electron injection into TiO2 nanoparticles but more efficient electron injection into SnO2 nanoparticles. With SnO2, injection time scales depend strongly on the identity of the central substituent and are affected by competition with excited-state deactivation processes. Heavy or paramagnetic metal ions increase the electron injection time scale by roughly one order of magnitude relative to free-base or Zn(2+) porphyrins due to the possibility of electron injection from longer-lived, lower-lying triplet states. Furthermore, electron injection efficiency loosely correlates with DSSC performance. The carboxylate anchoring group is commonly used to bind DSSC sensitizers to metal oxide surfaces but typically is not stable under the aqueous and oxidative conditions required for water oxidation. Electron injection efficiency of several water-stable alternatives, including phosphonic acid, hydroxamic acid, acetylacetone, and boronic acid, were evaluated using TRTS, and hydroxamate was found to perform as well as the carboxylate. The next challenge is incorporating a water oxidation catalyst into the design. An early example, in which an Ir-based precatalyst is cosensitized with a fluorinated porphyrin, reveals decreased electron injection efficiency despite an increase in photocurrent. Future research will seek to better understand and address these difficulties.

Molecular design of light-harvesting photosensitizers: effect of varied linker conjugation on interfacial electron transfer.

Interfacial electron transfer dynamics of a series of photosensitizers bound to TiO2via linkers of varying conjugation strength are explored by spectroscopic and computational techniques. Injection and recombination depend on the extent of conjugation in the linker, where the LUMO delocalization determines the injection dynamics but both the HOMO and HOMO-1 are involved in recombination.

Terahertz spectroscopic polarimetry of generalized anisotropic media composed of Archimedean spiral arrays: Experiments and simulations.

Terahertz time-domain spectroscopic polarimetry has been used to measure the polarization state of all spectral components in a broadband THz pulse upon transmission through generalized anisotropic media consisting of two-dimensional arrays of lithographically defined Archimedean spirals. The technique allows a full determination of the frequency-dependent, complex-valued transmission matrix and eigenpolarizations of the spiral arrays. Measurements were made on a series of spiral array orientations. The frequency-dependent transmission matrix elements as well as the eigenpolarizations were determined, and the eigenpolarizations were found be to elliptically corotating, as expected from their symmetry. Numerical simulations are in quantitative agreement with measured spectra.

Facet-dependent photoelectrochemical performance of TiO2 nanostructures: an experimental and computational study.

The behavior of crystalline nanoparticles depends strongly on which facets are exposed. Some facets are more active than others, but it is difficult to selectively isolate particular facets. This study provides fundamental insights into photocatalytic and photoelectrochemical performance of three types of TiO(2) nanoparticles with predominantly exposed {101}, {010}, or {001} facets, where 86-99% of the surface area is the desired facet. Photodegradation of methyl orange reveals that {001}-TiO(2) has 1.79 and 3.22 times higher photocatalytic activity than {010} and {101}-TiO(2), respectively. This suggests that the photochemical performance is highly correlated with the surface energy and the number of under-coordinated surface atoms. In contrast, the photoelectrochemical performance of the faceted TiO(2) nanoparticles sensitized with the commercially available MK-2 dye was highest with {010}-TiO(2) which yielded an overall cell efficiency of 6.1%, compared to 3.2% for {101}-TiO(2) and 2.6% for {001}-TiO(2) prepared under analogous conditions. Measurement of desorption kinetics and accompanying computational modeling suggests a stronger covalent interaction of the dye with the {010} and {101} facets compared with the {001} facet. Time-resolved THz spectroscopy and transient absorption spectroscopy measure faster electron injection dynamics when MK-2 is bound to {010} compared to other facets, consistent with extensive computational simulations which indicate that the {010} facet provides the most efficient and direct pathway for interfacial electron transfer. Our experimental and computational results establish for the first time that photoelectrochemical performance is dependent upon the binding energy of the dye as well as the crystalline structure of the facet, as opposed to surface energy alone.

Ultrafast carrier dynamics in nanostructures for solar fuels.

Sunlight can be used to drive chemical reactions to produce fuels that store energy in chemical bonds. These fuels, such as hydrogen from splitting water, have much larger energy density than do electrical storage devices. The efficient conversion of clean, sustainable solar energy using photoelectrochemical and photocatalytic systems requires precise control over the thermodynamics, kinetics, and structural aspects of materials and molecules. Generation, thermalization, trapping, interfacial transfer, and recombination of photoexcited charge carriers often occur on femtosecond to picosecond timescales. These short timescales limit the transport of photoexcited carriers to nanometer-scale distances, but nanostructures with high surface-to-volume ratios can enable both significant light absorption and high quantum efficiency. This review highlights the importance of understanding ultrafast carrier dynamics for the generation of solar fuels, including case studies on colloidal nanostructures, nanostructured photoelectrodes, and photoelectrodes sensitized with molecular chromophores and catalysts.

Linker rectifiers for covalent attachment of transition-metal catalysts to metal-oxide surfaces.

Linkers that favor rectification of interfacial electron transfer are likely to be required for efficient photo-driven catalysis of multi-electron reactions at electrode surfaces. Design principles are discussed, together with the synthesis and characterization of a specific pair of molecular linkers, related by inversion of the direction of an amide bond in the heart of the molecule. The linkers have a terpyridyl group that can covalently bind Mn as in a well-known water oxidation catalyst and an acetylacetonate group that allows attachment to TiO2 surfaces. The appropriate choice of the sense of the amide linkage yields directionality of interfacial electron transfer, essential to enhance electron injection and slow back-electron transfer. Support comes from electron paramagnetic resonance and terahertz spectroscopic measurements, as well as computational modeling characterizing the asymmetry of electron transfer properties.

Hydroxamate anchors for improved photoconversion in dye-sensitized solar cells.

We present the first analysis of performance of hydroxamate linkers as compared to carboxylate and phosphonate groups when anchoring ruthenium-polypyridyl dyes to TiO2 surfaces in dye-sensitized solar cells (DSSCs). The study provides fundamental insight into structure/function relationships that are critical for cell performance. Our DSSCs have been produced by using newly synthesized dye molecules and characterized by combining measurements and simulations of experimental current density-voltage (J-V) characteristic curves. We show that the choice of anchoring group has a direct effect on the overall sunlight-to-electricity conversion efficiency (η), with hydroxamate anchors showing the best performance. Solar cells based on the pyridyl-hydroxamate complex exhibit higher efficiency since they suppress electron transfer from the photoanode to the electrolyte and have superior photoinjection characteristics. These findings suggest that hydroxamate anchoring groups should be particularly valuable in DSSCs and photocatalytic applications based on molecular adsorbates covalently bound to semiconductor surfaces. In contrast, analogous acetylacetonate anchors might undergo decomposition under similar conditions suggesting limited potential in future applications.

Light-driven water oxidation for solar fuels.

Light-driven water oxidation is an essential step for conversion of sunlight into storable chemical fuels. Fujishima and Honda reported the first example of photoelectrochemical water oxidation in 1972. In their system, TiO2 was irradiated with ultraviolet light, producing oxygen at the anode and hydrogen at a platinum cathode. Inspired by this system, more recent work has focused on functionalizing nanoporous TiO2 or other semiconductor surfaces with molecular adsorbates, including chromophores and catalysts that absorb visible light and generate electricity (i.e., dye-sensitized solar cells) or trigger water oxidation at low overpotentials (i.e., photocatalytic cells). The physics involved in harnessing multiple photochemical events for multielectron reactions, as required in the four-electron water oxidation process, has been the subject of much experimental and computational study. In spite of significant advances with regard to individual components, the development of highly efficient photocatalytic cells for solar water splitting remains an outstanding challenge. This article reviews recent progress in the field with emphasis on water-oxidation photoanodes inspired by the design of functionalized thin film semiconductors of typical dye-sensitized solar cells.

Computational Prediction and Experimental Realization of Earth-Abundant Transparent Conducting Oxide Ga-Doped ZnSb2O6.

Transparent conducting oxides have become ubiquitous in modern optoelectronics. However, the number of oxides that are transparent to visible light and have the metallic-like conductivity necessary for applications is limited to a handful of systems that have been known for the past 40 years. In this work, we use hybrid density functional theory and defect chemistry analysis to demonstrate that tri-rutile zinc antimonate, ZnSb2O6, is an ideal transparent conducting oxide and to identify gallium as the optimal dopant to yield high conductivity and transparency. To validate our computational predictions, we have synthesized both powder samples and single crystals of Ga-doped ZnSb2O6 which conclusively show behavior consistent with a degenerate transparent conducting oxide. This study demonstrates the possibility of a family of Sb(V)-containing oxides for transparent conducting oxide and power electronics applications.