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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Self‐Standing Chiral Covalent Organic Framework Thin Films with Full‐Color Tunable Guest‐Induced Circularly Polarized Luminescence

Abstract Exploring self‐standing chiral covalent organic framework (COF) thin films with controllable circularly polarized luminescence (CPL) is of paramount significance but remains a challenging task. Herein, we demonstrate the first example of self‐standing chiral COF films employing a polymerization‐dispersion‐filtration strategy. Pristine, low‐quality chiral COF films were produced by interfacial polymerization and then re‐dispersed into COF colloidal solutions. Via vacuum assisted assembly, these COF colloids were densely stacked and assembled into self‐standing, pure chiral COF films ( L‐ / D‐ CCOF‐F) that were transparent, smooth, crack‐free and highly crystalline. These films were tunable in thicknesses, areas, and roughness, along with strong diffuse reflectance circular dichroism (DRCD) and cyan CPL signals, showing an intrinsic luminescence asymmetric factor ( g lum ) of ~4.3×10 −3 . Furthermore, these COF films served as host adsorbents to load various achiral organic dye guests through adsorption. The effective chiral transfer and energy transfer between CCOF‐F and achiral fluorescent dyes endowed the dyes with strong chirality and tunable DRCD, resulting in intense, full‐color‐tunable solid‐state CPL. Notably, the ordered arrangement of dye guest molecules within the preferentially oriented chiral pores of CCOF‐F contributed to an amplified | g lum | factor of up to 7.2×10 −2 , which is state‐of‐the‐art for COF‐based CPL materials. This work provides new insights into the design and fabrication of self‐standing chiral COF films, demonstrating their great potential for chiroptical applications.

Tang, Xihao↗
DOE OSTI2019

New hints on Maya Blue formation process by PCA-assisted in situ XRPD/PDF and optical spectroscopy

The exact recipe to prepare the ancient Maya Blue (MB), an incredibly resistant and brilliant pigment prepared from indigo (dye) and Paligorskite (clay), is lost to the ages. To unravel the key features of the MB formation process, several inorganic-dye couples were heated to 200°C and cooled to RT, to investigate their reactivity and the diffusion and degree of sequestration of the dye into the inorganic host. In situ XRPD/PDF and fiber optic reflectance spectroscopy (FORS) data, sided by TGA, provided a comprehensive overview on MB formation mechanism. XRPD/PDF gave information on long/short range behaviors of water desorption/adsorption and indigo sequestration, while TGA and in situ FORS gave information on mass and optical changes within temperature. Ex situ dye removal was used to understand the sample stability after the thermal treatments. A statistical approach based on Principal Component Analysis was exploited to efficiently and jointly analyze the ~3000 collected patterns. MB formation starts below 110°C with disordered distribution of indigo within the channels, reaching maximum reaction speed and higher ordering at 150°C. Above 175°C, color changes and a stronger sequestration of indigo into framework channels are observed, while the affinity for water is dramatically reduced. The origin of different colors, hues and stability in historical MB samples can then explained in term of different thermal history of the starting mechanical indigo-palygorskite mixture.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fluorescent Benzocoumarin-π-Extended Styryl Hybrids: Solvatochromism, Excess Dipole Moment, NLO Properties and DFT Study

Four benzo[h]chromen-3-yl)ethylidene) malononitrilestyryl hybrid dyes are presented for their potential non-linear optical properties using solvatochromism and density functional theory. The dyes are characterized by {sup 1}H and {sup 13}C NMR and elemental analysis. They have different donor and same acceptor as dicyanovinyl coumarin. Absorption and emission properties are highly dependent on substituents and environment. Dyes 3a and 3c show absorption solvatochromism while 3b and 3d show emission solvatochromism. Solvatochromism is used for the evaluation of excited state dipole moment and nonlinear optical properties which are comparable with the computed values. Limits of hyperpolarizability are close to the experimental and calculated values. Results shows that these dyes having strong intramolecular charge transfer characters proposing their potential nonlinear optical character.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Indigo-Based Acceptor Type Small Molecules: Synthesis, Electrochemical and Optoelectronic Characterizations

In this paper, we report design and synthesis of novel low bandgap small molecules, indigo-benzimidazole (Tyr-3) and indigo-schiff base (Tyr-4) type acceptors. In these structures, tert-butoxycarbonyl (t-BOC) group has been attached to indigo nitrogen atom in order to increase the solubility. UV-vis absorption spectra of Tyr-3 and Tyr-4 dyes exhibit wide absorption bands ranging from 350 to 600 nm, indicating the relatively low bandgap giving around 2.07 eV for each. Excitation of both Tyr-3 and Tyr-4 dyes at 485 nm displays characteristic emission features of indigo moiety and also intramolecular charge transfer complex (ICT) related with their subunits. Besides increasing fluorescence quantum yields as compared to model compound, biexponential decay times for fluorescence life times were also obtained for Tyr-3 and Tyr-4 dyes. Their appropriate energy levels along with low HOMO levels are desired for light harvesting acceptors for organic solar cells when blended with poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole] (PCDTBT) as donor polymer. Photovoltaic behavior of the synthesized dyes were examined in bulk heterojunction concept and achieved photovoltaic conversion efficiencies were discussed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Optimization of Beidellite/Polyaniline Production Conditions by Central Composite Design for Removal of Acid Yellow 194

In the adsorption process, the adsorbent structure and its interaction with wastewater are the main determinant of the process efficiency. For this reason, the correct selection of the production conditions for the produced adsorbents is critical. In this study, the reaction time and the concentration of acid, oxidant, substrate, and monomer were selected as independent variables which effect the structure of the polyaniline based composite material. These production variables of polyaniline/beidellite (PANI/BEI) composites were designed by response surface methodology (RSM). According to RSM, 46 different PANI/BEI composites were synthesized and were used as an adsorbent in the removal of acid yellow 194. Thus, the optimum PANI/BEI production conditions were determined. After the identification of the optimum PANI/BEI production conditions, the effect of adsorption conditions such as pH, mixing time, and dye concentration were studied by batch experiments. The maximum adsorption capacity was obtained as 123 (mg g{sup −1}) at the T = 25 °C, t = 24 h, pH 3, m{sub s} = 0.15 g adsorbent, V{sub s} = 50 ml dye solution, and ω = 200 rpm. The results presented in this study revealed that the central composite design is a suitable method to optimize the production conditions of PANI/BEI and the prepared PANI/BEI composites has an important potential as an adsorbent for the removal of metal-complex dyes such as AY194. In addition, the polymerization of aniline and the removal of dye were confirmed by different analytical techniques such as BET, XRD, SEM, and EDAX.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

The Removal of Acid Violet 90 from Aqueous Solutions Using PANI and PANI/Clinoptilolite Composites: Isotherm and Kinetics

Polyaniline (PANI) and polyaniline/Gördes-clinoptilolite (PANI/GC) composite materials were synthesized by the chemical oxidative polymerization technique and used in the adsorption of Acid Violet 90 metal-complex dye (AV 90). The samples were characterized by X-ray diffractions, nitrogen adsorption–desorption isotherms, scanning electron microscopes and Fourier transform infrared. The effect of initial pH (2–8), sorbent dosage (0.5–4.0 g/L) and initial dye concentrations (50–400 mg/L) on adsorption onto PANI and PANI/GC were examined in a batch system. Langmuir, Freundlich and Temkin isotherm models were used to investigate the adsorption mechanism of AV 90 on PANI and PANI/GC. Langmuir isotherm model for PANI/GC and Freundlich isotherm model for PANI were fitted well with the experimental data. The highest dye uptake capacities were obtained with Langmuir isotherm model as 153.85 mg/g and 72.46 mg/g for PANI and PANI/GC, respectively. In order to determine the adsorption kinetics, pseudo first-order and second-order kinetic models were studied. As a result, the adsorption of AV 90 dye on PANI and PANI/GC was better identified with Pseudo second-order kinetic model than the first one.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Cell culture medium and nano-confined water on nonlinear optical properties of Congo Red

Optical properties of Congo Red in the solvents and nano-confined water can be studied using techniques, including Z-scan, spectrophotometer and photon correlation spectroscopy The absorbance of Congo Red depends on environmental conditions that were considered as the rules of dielectric constant and solubility. The solubility of Congo Red in the solvent and of dye-water in nano-confined water determines the value of both nonlinear absorption and refractive index. The nonlinear optical properties of AOT/n-Decane/H{sub 2}O are higher than AOT/Toluene/H{sub 2}O. The photon correlation spectroscopy findings show higher dye aggregation in AOT/Toluene/H{sub 2}O in comparison with AOT/n-Decane/H{sub 2}O. According to the results, the nonlinear optical properties of Congo Red are enhanced by increasing the dye solubility in the nano-confined water, as well as the dye solubility changes in microemulsion by oil length scale and polarity. Incremental concentration of cell culture medium enhanced the nonlinear optical properties of Congo Red in nano-confined water.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

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. In this work, 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.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Pore-scale supercritical CO 2 dissolution and mass transfer under drainage conditions

Abstract: Recently, both core- and pore-scale imbibition experiments have shown non-equilibrium dissolution of supercritical CO 2 (scCO 2 ) and a prolonged depletion of residual scCO 2 . In this study, pore-scale scCO 2 dissolution and mass transfer under drainage conditions were investigated using a two-dimensional heterogeneous micromodel and a novel fluorescent water dye with a sensitive pH range between 3.7 and 6.5. Drainage experiments were conducted at 9 MPa and 40 °C by injecting scCO 2 into the sandstone-analogue pore network initially saturated by water without dissolved CO 2 (dsCO 2 ). During the experiments, time-lapse images of dye intensity, reflecting water pH, were obtained. These images show non-uniform pH in individual pores and pore clusters, with average pH levels gradually decreasing with time. Further analysis on selected pores and pore clusters shows that (1) rate-limited mass transfer prevails with slowly decreasing pH over time when the scCO 2 -water interface area is low with respect to the volume of water-filled pores and pore clusters, (2) fast scCO 2 dissolution and phase equilibrium occurs when scCO 2 bubbles invade into water-filled pores, significantly enhancing the area-to-volume ratio, and (3) a transition from rate-limited to diffusion-limited mass transfer occurs in a single pore when a medium area-to-volume ratio is prevalent. The analysis also shows that two fundamental processes – scCO 2 dissolution at phase interfaces and diffusion of dsCO 2 at the pore scale (10-100 µm) observed after scCO 2 bubble invasion into water-filled pores without pore throat constraints – are relatively fast. The overall slow dissolution of scCO 2 in the millimeter-scale micromodel can be attributed to the small area-to-volume ratios that represent pore-throat configurations and characteristics of phase interfaces. This finding is applicable for the behavior of dissolution at pore, core, and field scales when water-filled pores and pore clusters of varying size are surrounded by scCO 2 at narrow pore throats.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2012

Synthesis of MSnO{sub 3} (M = Ba, Sr) nanoparticles by reverse micelle method and particle size distribution analysis by whole powder pattern modeling

Highlights: ► BaSnO{sub 3} and SrSnO{sub 3} nanoparticles synthesized using the reverse micelle method. ► Particle size and size distribution studied by whole powder pattern modeling. ► Nanoparticles are of optimal size for investigation in dye-sensitized solar cells. -- Abstract: Light-to-electricity conversion efficiency in dye-sensitized solar cells critically depends not only on the dye molecule, semiconducting material and redox shuttle selection but also on the particle size and particle size distribution of the semiconducting photoanode. In this study, nanocrystalline BaSnO{sub 3} and SrSnO{sub 3} particles have been synthesized using the microemulsion method. Particle size distribution was studied by whole powder pattern modeling which confirmed narrow particle size distribution with an average size of 18.4 ± 8.3 nm for SrSnO{sub 3} and 15.8 ± 4.2 nm for BaSnO{sub 3}. These values are in close agreement with results of transmission electron microscopy. The prepared materials have optimal microstructure for successive investigation in dye-sensitized solar cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Improvement of charge separation in TiO{sub 2} by its modification with different tungsten compounds

Highlights: • Ammonium m-tungstate doped to TiO{sub 2} highly improved charge separation in TiO{sub 2}. • Negative electrokinetic potential of TiO{sub 2} facilitates holes migration to its surface. • Fast migration of holes to TiO{sub 2} surfaces increased yield of OH radicals formation. • Adsorption of dyes on photocatalyst increased its decomposition under visible light. - Abstract: Three different tungsten precursors were used for TiO{sub 2} modification: H{sub 2}WO{sub 4}, WO{sub 2}, and ammonium m-tungstate. It was proved that modification of TiO{sub 2} with tungsten compounds enhanced its photocatalytic activity through the improvement of charge separation. This effect was obtained by coating of TiO{sub 2} particles with tungsten compound, which changed their surficial electrokinetical potential from positive onto negative. The most efficient tungsten compound, which caused enhanced separation of free carriers was ammonium m-tungstate (AMT). Two dyes with different ionic potential were used for the photocatalytic decomposition. It appeared that cationic dye—Methylene Blue was highly adsorbed on the negatively charged surface of TiO{sub 2} modified by AMT and decomposed, however this photocatalyst was quickly deactivated whereas anionic dye—acid red was better adsorbed on the less acidic surface of TiO{sub 2} and was rapidly decomposed with almost the same rate in the five following cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Dry synthesis of water lily flower like SrO{sub 2}/g-C{sub 3}N{sub 4} nanohybrids for the visible light induced superior photocatalytic activity

Highlights: • Water lily flower like SrO{sub 2}/g-C{sub 3}N{sub 4} nanohybrids have been successfully synthesized by a dry synthesis method. • TEM images confirmed that the water lily like morphology of SrO{sub 2}/g-C{sub 3}N{sub 4} nanohybrids. • The photodegradation of RhB dye solution was performed under the visible light illumination. • The SrO{sub 2}/g-C{sub 3}N{sub 4} nanohybrids showed excellent photocatalytic activity compared to pure g-C{sub 3}N{sub 4} and SrO{sub 2}. • Most important reactive oxidative species were found out by trapping experiment. - Abstract: In the present study strontium oxide/graphitic carbon nitride (SrO{sub 2}/g-C{sub 3}N{sub 4}) nanohybrids was successfully synthesized by a facile simple dry synthesis and used for the photocatalytic degradation of organic contaminants for the very first time. The SrO{sub 2}/g-C{sub 3}N{sub 4}nanohybridsare having a superior crystallinity and crystallite size was ∼7.1 nm. The photodegradation efficiency of nanohybrids was tested on the degradation of RhB dye solution under visible light irradiation. The SrO{sub 2}/g-C{sub 3}N{sub 4}nanohybrids exhibited excellent photocatalytic activity, which may be attributed to the improved charge separation and complete degradation of the dye solution is achieved within 60 min. Moreover, the synthesized nanohybrids exhibited a good stability towards the photodegradation of RhB dye. Finally, the possible mechanism for the charge separation in thephoto degradationprocess wasexplored. Therefore, the nanohybrids are possessing potential interest in waste water treatment and environmental remediation under visible light irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fe-doped ZnO nanoellipsoids for enhanced photocatalytic and highly sensitive and selective picric acid sensor

Highlights: • Low-temperature synthesis of Fe-doped ZnO nanoellipsoids. • Used as efficient photocatalyst for photocatalytic degradation of various dyes (RhB, MB, MO). • Potential scaffold for fabrication of highly sensitive and selective picric acid sensor. - Abstract: Herein, we report the synthesis, characterization, and photocatalytic and sensing capabilities of crystalline Fe-doped ZnO nanoellipsoids (NEs). These NEs were synthesized by a low-temperature simple solution process and were characterized with respect to their structural, morphological, optical, compositional, photocatalytic and sensing properties. The detailed studies confirmed that the synthesized NEs were grown in high density possessing wurtzite hexagonal phase and exhibiting good optical properties. Further, the NEs were applied as a photocatalyst for the photocatalytic degradation of various harmful organic dyes such as methyl orange (MO), rhodamine B (RhB) and methylene blue (MB). Interestingly, it was observed that MB and RhB dyes undergo almost 100% photodegradation with 15–60 min, however, MO dye requires 90 min for the degradation. It is also found that picric acid quenches the luminescence of the NEs (with excitation/emission maxima of 380/392 nm). This finding is exploited in a luminescent assay for picric acid with a 2.9 μM lower detection limit and a linear range that extends from 5 to 60 μmol L{sup −1}. The results show that simply synthesized Fe-doped ZnO nanomaterials can efficiently be used for the photocatalytic and sensing applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Photoannealing of Merocyanine Aggregates

In this study we elucidate the fundamental difference between aggregate formation of donor–π–acceptor merocyanines in their electronic ground and excited states. While increasing the π-bridge size favors formation of π-stacked aggregates in the dark, irradiation with visible light causes reorientation of the dyes to form prototype H-aggregates with compensating dipole moments. This photoannealing changes the supramolecular structure and its UV–vis spectroscopic properties dramatically, thus being of importance for the function of active layers composed of these dyes. Aggregates of the ground state dyes are bound cooperatively through ππ-London dispersion interactions and hydrogen bonds between the polar α-cyano-carboxylic acid groups. However, charge transfer upon photoexcitation leads to repulsion of the polar acid groups. Electronic excitation of the dyes approximately doubles the ground state dipole moment, thus driving molecular reorientation into prototype H-aggregate structures. We show that this photoinduced supramolecular rearrangement can disrupt the large polymeric aggregates formed in the dark. The photoinduced supramolecular structural changes reported in this work will influence the performance of optoelectronic devices composed of these structures and must be controlled to avoid morphological decomposition of active layers upon operation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Vibronic Effects in the Ultrafast Interfacial Electron Transfer of Perylene-Sensitized TiO 2 Surfaces

We combine ultrafast transient absorption (TA) spectroscopy and nonadiabatic quantum dynamics simulations to describe the real-time unfold of vibronic effects on the photoabsorption of TiO 2 anatase sensitized with the (perylen-9-yl)carboxylate dye (Pe-COOH/TiO2). The excited state is mapped in time and frequency by ultrafast broadband spectroscopy while atomistic quantum dynamics is used to simulate the self-consistent vibronic effects. The TA map shows the lifetime of the electronic population generated in the S 1 state of the dye and the rise of the absorption D 0 –D 1 of the cation. The theoretical analysis reveals that the electron transfer from perylene into TiO2 is complete within 20 fs, in agreement with the 12 fs experimental measurement. Because of the structural relaxation produced by the photoinduced electron transfer, the optical gap decreases by 390 meV, in agreement with the D 0 –D 1 transition band. Furthermore, the reorganization energy estimated to be around 220 meV is mostly due to the energy shift of the HOMO level, since the electron transfer occurs in the wide-band limit with little dependence on reorganization energy modes. By assuming the Condon approximation and by making use of the mixed quantum/classical trajectories of the Pe-COOH/TiO 2 system, the absorption spectrum is calculated, and the broad features of the transient absorption spectrum are correlated to excited-state nuclear reorganization effects of the adsorbate dye. The reorganization energy modes are identified by the power spectrum of the velocity autocorrelation function, which shows the occurrence of nonequilibrium modes within the range 1000–1800 cm –1 as in-plane asymmetric C–C vibrations in the perylene dye. The vibrational modes with the strongest influence on the optical gap contribute to shifting the absorption spectrum up in energy by ~2000 cm –1 . Finally, the overall agreement between theory and experiment reveals the capabilities of both methods to study vibronic effects in molecular and extended systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Diffusion Behavior of Differently Charged Molecules in Self-Assembled Organic Nanotubes Studied Using Imaging Fluorescence Correlation Spectroscopy

The diffusion behavior of fluorescent molecules within bolaamphiphile-based organic nanotubes (ONTs) was systematically explored using imaging fluorescence correlation spectroscopy (imaging FCS). Anionic sulforhodamine B (SRB), zwitterionic/cationic rhodamine B (RB) or cationic rhodamine 123 (R123) was loaded into ONTs having cylindrical hollow structures (ca. 10 nm in inner diameter) with amine and glucose groups on the inner and outer surfaces, respectively. Wide-field fluorescence video microscopy was used to acquire imaging FCS data for dye-doped ONTs in aqueous solutions of different ionic strengths (1 – 500 mM) at different pH (3.4 – 8.4). The diffusion behavior of these dyes was discussed on the basis of their apparent diffusion coefficients (D) that were determined by autocorrelating the time transient of fluorescence intensity at each pixel on an ONT. Molecular diffusion in the ONTs was significantly slowed by molecule-nanotube interactions, as shown by the very small D (10-1 – 10-2 µm2/s). The pH-dependence of D revealed that dye diffusion was basically controlled by electrostatic interactions associated with the protonation of the amine groups on the ONT inner surface. The pH-dependent change in D was observed over a wide pH range, possibly due to electrostatically induced variations in the pKa of the densely packed ammonium ions on the ONT inner surface. On the other hand, the influence of ionic strength on D was relatively unclear, suggesting the involvement of non-coulombic interactions with the ONTs in molecular diffusion. Notably, individual ONTs of different lengths (1 – 5 µm) afforded similar diffusion coefficients for each type of dye at each solution condition, implying that the properties of ONTs were uniform in terms of solute loading and release. These conclusions highlight the characteristics of molecular diffusion behavior within the ONTs, and will help in the design of organic nanotubes better suited for use as drug vehicles and contaminant adsorbents.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new low-spin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from –200 to –430 mV versus Fc +/0 . The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron self-exchange rate constants of 2.2–4.2 M –1 s –1 compared to the high-spin to low-spin redox couple [Co(bpy) 3 ] 3 + /2 + at 0.27 M –1 s –1 , which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy) 3 ] 3 + /2 + redox shuttle (19.8 vs 18.0 mA/cm 2 ). Finally, future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗