Halogenated (F, Cl, Br, or I) Diphenylhexatrienes: Crystal Structures, Fluorescence Spectroscopic Properties, and Quantum Chemical Calculations

A series of halogenated compounds, (<i>E</i>,<i>E</i>,<i>E</i>)-1,6-di­(4-X-phenyl)­hexa-1,3,5-trienes (<b>1</b>: X = F, <b>2</b>: X = Cl, <b>3</b>: X = Br, <b>4</b>: X = I), were synthesized and their crystal structures and fluorescence emission properties were systematically investigated. Single-crystal X-ray analysis reveals that molecules are arranged via X/X halogen bonds and/or CH/X-type hydrogen bonds to form a herringbone structure in <b>1</b> and π-stacked structures in <b>2</b>–<b>4</b>. In the structures of <b>2</b> and <b>3</b>, which are almost isomorphous, the distance and displacement for the nearest stacking molecules are smaller than those in <b>4</b>. Although the structures of <b>2</b>–<b>4</b> are basically not greatly different from each other, the nearest-neighbor arrangements are π-stacked in <b>2</b> and <b>3</b>, but herringbone in <b>4</b>. Steady-state and time-resolved measurements show that the solid-state fluorescence properties also strongly depend on the halogen size. The fluorescence spectra are red-shifted and the Stokes shifts are large in <b>2</b> and <b>3</b> relative to those in <b>1</b> and <b>4</b>, resulting from the difference in molecular arrangement in the crystal structure. The experimentally observed clear correlation between crystal structure and optical transition energy is reproduced fairly well by quantum chemical calculations for the excited states of molecular pairs in the X-ray determined structures of <b>1</b>–<b>4</b>

Dye Aggregation Effect on Interfacial Electron-Transfer Dynamics in Zinc Phthalocyanine-Sensitized Solar Cells

Aggregation of adsorbed dye molecules on TiO₂ electrode typically decreases the yield of photoinduced charge separation at the dye/TiO₂ interface. The decreased yield could be caused by the alternations of energy levels and/or adsorption geometry of sensitizers by the aggregation. We investigated the origin of the decreased yield for the aggregated sensitizers by employing zinc phthalocyanine-sensitized TiO₂ electrode in redox-containing electrolytes. The degree of aggregation was controlled by the amount of coadsorbent, the addition of bulky molecular unit to phthalocyanine cores, and the alternation of the adsorption angle by changing the position of adsorption site. Femtosecond transient absorption measurements showed that injection yield was not significantly influenced by the aggregation but by dye adsorption angle and by the amount of dye. On the other hand, aggregation induced subnanosecond charge recombination, and the recombination seemed independent of the adsorption angle. These results appear to be not consistent with an interpretation where flat adsorption geometry enhances fast recombination. Here we interpreted the results with the dye-adsorption-density-dependent tunneling barrier height

Trapped State Sensitive Kinetics in LaTiO₂N Solid Photocatalyst with and without Cocatalyst Loading

In addition to the process of photogeneration of electrons and holes in photocatalyst materials, the competitive process of trapping of these charge carriers by existing defects, which can both enhance the photocatalytic activity by promoting electron–hole separation or can deteriorate the activity by serving as recombination centers, is also very crucial to the overall performance of the photocatalyst. In this work, using femtosecond diffuse reflectance spectroscopy we have provided evidence for the existence of energetically distributed trapped states in visible-light responsive solid photocatalyst powder material LaTiO₂N (LTON). We observe trapped state sensitive kinetics in bare-LTON. CoO_<i>x</i> cocatalyst loading (2 wt % CoO_<i>x</i>-LTON) shows effect on the kinetics only when presence of excess energy (for above bandgap excitation) results in the generation of surface carriers. Thus, the kinetics show appreciable excitation wavelength dependence, and the experimental results obtained for different λ_exc have been rationalized on this basis. In an earlier work by Domen and co-workers, the optimized CoO_<i>x</i>/LTON has been reported to exhibit a high quantum efficiency of 27.1 ± 2.6% at 440 nm, the highest reported for this class of photocatalysts (<i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 8348–8351). In the present work, the mechanism is addressed in terms of picosecond charge carrier dynamics

Direct Aqueous Dispersion of Carbon Nanotubes Using Nanoparticle-Formed Fullerenes and Self-Assembled Formation of p/n Heterojunctions with Polythiophene

Single-walled carbon nanotubes (SWCNTs) have received much attention because of their potential in optoelectronic applications. Pristine SWCNTs exhibit substantial van der Waals interactions and hydrophobic characteristics, so precipitation occurs immediately in most organic solvents and water. Highly toxic and hazardous chemicals are often used to obtain well-dispersed SWCNTs. Developing environmentally friendly processing methods for safe and practical applications is a great challenge. Here, we demonstrate direct exfoliation of SWCNTs in pure water only with n-type semiconducting fullerene nanoparticles. The resultant SWCNTs can be well-dispersed in water, where they remain essentially unchanged for several weeks. Adding an aqueous solution of p-type semiconducting water-soluble polythiophene yields self-assembled p/n heterojunctions between polythiophene and the nanoparticles. The aqueous-dispersed SWCNTs yield photocurrent responses in solution-processed thin films as a potential application of water-dispersed carbon nanomaterials

Spectroscopic Characterization of Nanohybrids Consisting of Single-walled Carbon Nanotubes and Fullerodendron

<div><p>Hydrogen gas, which can be used in fuel cells to generate electricity, is considered the ultimate clean energy source. Recently, it was reported that a photo-induced electron transfer system consisting of single-walled carbon nanotubes (SWCNTs) and fullerodendrons shows photo-catalytic activity with a very high quantum yield for splitting water under visible light irradiation. However, the mechanism of high efficiency hydrogen generation is not yet clearly understood. We report here the spectroscopic characterizations of the SWCNT-fullerodendron composites. The results indicate two important fundamental properties of the composite system. First, fullerodendrons preferentially interact with the semiconducting SWCNTs instead of with their metallic counterparts. Second, the photo-induced electron transfer process from the C₆₀ moiety of fullerodendrons to SWCNTs occurs more efficiently with an increasing tube diameter.</p> </div

Driving Force Dependence of Electron Transfer Kinetics and Yield in Low-Band-Gap Polymer Donor–Acceptor Organic Photovoltaic Blends

The rate of photoinduced electron transfer (PET) (κ_PET), quantum yield of PET (QY_PET), and charge extraction yield (EQE) are determined for a series of donor–acceptor (DA) organic photovoltaic systems, comprising low-band-gap polymer donors and the phenyl-C₆₁-butyric acid methyl ester (PCBM) acceptor. The energetic alignment of these polymer donors relative to PCBM provides driving forces for PET (Δ<i>G</i>_PET) in the range of 0.18–0.57 eV. Femtosecond transient absorption (TA) spectroscopy was used to assess the PET kinetics and QY_PET, while time-resolved charge extraction (TRCE) measurements were employed to assess EQE. Near unity QY_PET was observed in DA blend films with a Δ<i>G</i>_PET of 0.57 and 0.30 eV, whereas no resolvable PET was observed with a Δ<i>G</i>_PET of 0.18 eV. For the DA blends that exhibit PET, both κ_PET and QY_PET appear independent of Δ<i>G</i>_PET, with an average κ_PET of 420 fs for the 70% PCBM blends. An increase in nanosecond charge separation yield (TA) and EQE (TRCE) between DA systems was observed, which appears not to be due to the PET process but rather the subsequent recombination processes. DA systems should be designed to minimize Δ<i>G</i>_PET, minimizing associated losses in device open-circuit potential; however, picosecond bimolecular recombination severely limits achievable charge extraction yields in these DA systems

Cation Exchange at Semiconducting Oxide Surfaces: Origin of Light-Induced Performance Increases in Porphyrin Dye-Sensitized Solar Cells

The origin of simultaneous improvements in the short-circuit current density (<i>J</i>_sc) and open-circuit voltage (<i>V</i>_oc) of porphyrin dye-sensitized TiO₂ solar cells following white light illumination was studied by systematic variation of several different device parameters. Reduction of the dye surface loading resulted in greater relative performance enhancements, suggesting open space at the TiO₂ surface expedites the process. Variation of the electrolyte composition and subsequent analysis of the conduction band potential shifts suggested that a light-induced replacement of surface-adsorbed lithium (Li⁺) ions with dimethylpropylimidazolium (DMPIm⁺) ions was responsible for an increased electron lifetime by decreasing the recombination with the redox mediator. Variation of the solvent viscosity was found to affect the illumination time required to generate increased performance, while similar performance enhancements were not replicated by application of negative bias under dark conditions, indicating the light exposure effect was initiated by formation of dye cation molecules following photoexcitation. The substituents and linker group on the porphyrin chromophore were both varied, with light exposure producing increased electron lifetime and <i>V</i>_oc for all dyes; however, increased <i>J</i>_sc values were only measured for dyes containing binding moieties with multiple carboxylic acids. It was proposed that the initial injection limitation and/or fast recombination process in these dyes arises from the presence of lithium at the surface, and the improved injection and/or retardation of fast recombination after light exposure is caused by the Li⁺ removal by cation exchange under illumination

Alternation of Charge Injection and Recombination in Dye-Sensitized Solar Cells by the Addition of Nonconjugated Bridge to Organic Dyes

Most metal-free organic dyes for dye-sensitized solar cells are designed by following a donor conjugated-bridge acceptor structure with a carboxyl acid as an anchoring unit. In this work, we examined the influence of a nonconjugated methylene unit between the cyano group and carboxyl acid by applying it to a previously reported carbazole dye, <b>MK-2</b>. Two dyes, <b>MKZ-35</b> and <b>-36</b>, were synthesized with glycine and β-alanine, respectively. Dye-sensitized TiO₂ solar cells (DSSCs) with <b>MKZ-35</b> and <b>-36</b> showed smaller values in the short-circuit current (<i>J</i>_sc) and higher values in open-circuit voltage (<i>V</i>_oc) compared with the values with <b>MK-2</b>. The lower <i>J</i>_sc was due to less injection efficiency and fast geminate recombination while the higher <i>V</i>_oc was attributed to longer lifetime of the injected electrons in the DSSCs. DFT calculations showed that <b>MKZ-35 </b>dyes interact with each other. One possible explanation for the longer electron lifetime is that the interacted molecules may act as a 3D enlarged dimer molecule or form an induced quadrupole, reducing the interaction between the dyes and acceptor species. On the other hand, the longer electron lifetime with <b>MKZ-36 </b>than that with <b>MK-2</b> seems to be due to the longer distance between the TiO₂ surface and conjugated framework of the dye

Plate-like Sm₂Ti₂S₂O₅ Particles Prepared by a Flux-Assisted One-Step Synthesis for the Evolution of O₂ from Aqueous Solutions by Both Photocatalytic and Photoelectrochemical Reactions

Sm₂Ti₂S₂O₅ (STSO) is a visible-light-responsive oxysulfide semiconductor photocatalyst with applications to water splitting. In this work, plate-like STSO particles were synthesized through a flux-assisted one-step method at various temperatures. The activities of these materials during photocatalytic and photoelectrochemical O₂ evolution from aqueous solutions were investigated. Single-phase STSO with a single crystal habit was produced at 923 K, which is approximately 200 K lower than the temperatures required for previously reported methods, such as solid-state reactions and thermal sulfurization under a H₂S flow. The STSO sample synthesized at the optimal temperature exhibited an AQE of 1.3 ± 0.2% at 420 nm during photocatalytic sacrificial O₂ evolution. This efficiency is twice the values reported for specimens prepared using conventional methods. An STSO/Ti/Sn electrode fabricated by the particle transfer method generated a photoanodic current and evolved O₂ by water oxidation with a Faradaic efficiency of approximately 70 ± 7%. The synthesis temperature yielding the highest activity was lower for photocatalytic O₂ evolution than for photoelectrochemical O₂ evolution. This work demonstrates the applicability of the flux method to the synthesis of well-crystallized oxysulfides having various particle sizes and intended for different uses

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La₅Ti₂CuS₅O₇ Photocathodes by Surface Modification

Particulate La₅Ti₂CuS₅O₇ (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs RHE for the photocathodic current in photoelectrochemical (PEC) H₂ evolution. However, the low photocathodic current imposes a ceiling on the solar-to-hydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO₂, Nb₂O₅, and Ta₂O₅ by radio frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment