Nature-Inspired Photocatalytic Hydrogen Production with a Flavin Photosensitizer.

Green hydrogen, by definition, must be produced with renewable energy sources without using fossil fuels. To transform the energy system, we need a fully sustainable production of green and renewable energy as well as the introduction of such "solar fuels" to tackle the chemical storage aspect of renewable energies. Conventional electrolysis of water splitting into oxygen and hydrogen gases is a clean and nonfossil method, but the use of massive noble-metal electrodes makes it expensive. Direct photocatalytic hydrogen evolution in water is an ideal approach, but an industrial scale is not available yet. In this paper, we intend to introduce flavins as metal-free organic photosensitizers for photoinduced reduction processes. Specifically, a flavin photosensitizer was employed for the photocatalytic evolution of hydrogen gas in aqueous media. The ratio of photosensitizer to cocatalyst concentration has been found to affect the efficiency of the hydrogen evolution reaction. Since flavins are nature-inspired molecules (like vitamin B2) with easily tunable properties through structure modification, this family of compounds opens the door for new possibilities in sustainable green hydrogen production.

Effect of Chlorine Substituents on the Photovoltaic Properties of Monocyanated Quinoxaline-Based D-A-Type Polymers.

A string of monocyanated quinoxaline (Qx)-based D-A-type polymers systematically decorated with electron-attracting chlorine (Cl) atoms was created for use in non-fullerene polymer solar cells (PSCs). First, coupling of the benzodithiophene (BDT) donor and Qx acceptor with the strong electron-attracting cyano (CN) unit at its 5-position yielded the monocyanated reference polymer PB-CNQ. Subsequently, the additional Cl atoms were separately or simultaneously incorporated into the thiophene side groups of the BDT donor and Qx acceptor to create other objective polymers, PBCl-CNQ, PB-CNQCl, and PBCl-CNQCl. The Cl substituents on the BDT donor and Qx acceptor are represented by the names of the polymers. Owing to the favorable contributions of Cl substituents, the inverted-type non-fullerene PSCs based on partially chlorinated PBCl-CNQ (12.80%) and PB-CNQCl (13.93%) exhibited better power conversion efficiencies (PCEs) than the device based on unchlorinated reference PB-CNQ (11.19%). However, a significantly reduced PCE of 9.84% was observed for the device based on PBCl-CNQCl, in which Cl atoms were loaded on both the BDT donor and Qx acceptor at the same time. Hence, these results reveal that optimization of the number and position of Cl substituents in monocyanated Qx-based polymers is essential for enhancing their photovoltaic nature through the synergistic effects between two strong electron-attracting CN and Cl substituents.

Effect of Electron-Withdrawing Chlorine Substituent on Morphological and Photovoltaic Properties of All Chlorinated D-A-Type Quinoxaline-Based Polymers.

The choice of the chlorine (Cl) atom as an electron-withdrawing substituent in conjugated polymers leads to a higher potential in the commercialization of polymer solar cells than its fluorine counterpart because of the versatility and cost-effectiveness of the chlorination process. In addition, the population and location of Cl substituents can significantly influence the photovoltaic characteristics of polymers. In this study, three chlorinated quinoxaline-based polymers were invented to examine the numerical and positioning effects of the Cl atom on their photovoltaic characteristics. The number of Cl substituents in the reference polymer, PBCl-Qx, was adjusted to three: two Cl atoms in the benzodithiophene-type D unit and one Cl atom in the quinoxaline-type A unit. Subsequently, two more Cl atoms were selectively introduced at the 4- and 5-positions of the alkylated thiophene moieties at the 2,3-positions of the quinoxaline moiety in PBCl-Qx to obtain the additional polymers PBCl-Qx4Cl and PBCl-Qx5Cl, respectively. The conventional PBCl-Qx4Cl device exhibited a better power conversion efficiency (PCE) of 12.95% as compared to those of PBCl-Qx (12.44%) and PBCl-Qx5Cl (11.82%) devices. The highest PCE of the device with PBCl-Qx4Cl was ascribed to an enhancement in the open-circuit voltage and fill factor induced by the deeper energy level of the highest occupied molecular orbital and the favorable morphological features in its blended film with Y6.

Effect of electron-withdrawing fluorine and cyano substituents on photovoltaic properties of two-dimensional quinoxaline-based polymers.

In this study, strong electron-withdrawing fluorine (F) and cyano (CN) substituents are selectively incorporated into the quinoxaline unit of two-dimensional (2D) D-A-type polymers to investigate their effects on the photovoltaic properties of the polymers. To construct the 2D polymeric structure, electron-donating benzodithiophene and methoxy-substituted triphenylamine are directly linked to the horizontal and vertical directions of the quinoxaline acceptor, respectively. After analyzing the structural, optical, and electrochemical properties of the resultant F- and CN-substituted polymers, labeled as PBCl-MTQF and PBCl-MTQCN, respectively, inverted-type polymer solar cells with a non-fullerene Y6 acceptor are fabricated to investigate the photovoltaic performances of the polymers. It is discovered that the maximum power conversion efficiency of PBCl-MTQF is 7.48%, whereas that of PBCl-MTQCN is limited to 3.52%. This significantly reduced PCE of the device based on PBCl-MTQCN is ascribed to the formation of irregular, large aggregates in the active layer, which can readily aggravate the charge recombination and charge transport kinetics of the device. Therefore, the photovoltaic performance of 2D quinoxaline-based D-A-type polymers is significantly affected by the type of electron-withdrawing substituent.

Tunable Properties of Nature-Inspired N,N'-Alkylated Riboflavin Semiconductors.

A series of novel soluble nature-inspired flavin derivatives substituted with short butyl and bulky ethyl-adamantyl alkyl groups was prepared via simple and straightforward synthetic approach with moderate to good yields. The comprehensive characterization of the materials, to assess their application potential, has demonstrated that the modification of the conjugated flavin core enables delicate tuning of the absorption and emission properties, optical bandgap, frontier molecular orbital energies, melting points, and thermal stability. Moreover, the thin films prepared thereof exhibit smooth and homogeneous morphology with generally high stability over time.

Immobilization of molecular catalysts for artificial photosynthesis.

Artificial photosynthesis offers a way of producing fuels or high-value chemicals using a limitless energy source of sunlight and abundant resources such as water, CO2, and/or O2. Inspired by the strategies in natural photosynthesis, researchers have developed a number of homogeneous molecular systems for photocatalytic, photoelectrocatalytic, and electrocatalytic artificial photosynthesis. However, their photochemical instability in homogeneous solution are hurdles for scaled application in real life. Immobilization of molecular catalysts in solid supports support provides a fine blueprint to tackle this issue. This review highlights the recent developments in (i) techniques for immobilizing molecular catalysts in solid supports and (ii) catalytic water splitting, CO2 reduction, and O2 reduction with the support-immobilized molecular catalysts. Remaining challenges for molecular catalyst-based devices for artificial photosynthesis are discussed in the end of this review.

Mechanically Interlocked Carbon Nanotubes as a Stable Electrocatalytic Platform for Oxygen Reduction.

Mechanically interlocking redox-active anthraquinone onto single-walled carbon nanotubes (AQ-MINT) gives a new and advanced example of a noncovalent architecture for an electrochemical platform. Electrochemical studies of AQ-MINT as an electrode reveal enhanced electrochemical stability in both aqueous and organic solvents compared to physisorbed AQ-based electrodes. While maintaining the electrochemical properties of the parent anthraquinone molecules, we observe a stable oxygen reduction reaction to hydrogen peroxide (H2O2). Using such AQ-MINT electrodes, 7 and 2 µmol of H2O2 are produced over 8 h under basic and neutral conditions, while the control system of SWCNTs produces 2.2 and 0.5 µmol, respectively. These results reveal the potential of this rotaxane-type immobilization approach for heterogenized electrocatalysis.

High Efficiency Doping of Conjugated Polymer for Investigation of Intercorrelation of Thermoelectric Effects with Electrical and Morphological Properties.

Intercorrelation of thermoelectric properties of a doped conjugated semiconducting polymer (PIDF-BT) with charge carrier density, conductive morphology, and crystallinity are systematically investigated. Upon being doped with F4-TCNQ by the sequential doping method, PIDF-BT exhibited a high electrical conductivity over 210 S cm-1. The significant enhancement of electrical conductivity resulted from a high charge carrier density, which is attributed to the effective charge-transfer-based integer doping between PIDF-BT and dopant molecules. Based on the systemic characterization on the optical, electrical, and structural properties of doped PIDF-BT annealed at different temperatures, we investigated the characteristic correlations between thermoelectric properties of PIDF-BT films and their four-probe electrical conductivity, charge carrier density, and charge carrier mobility obtained from AC Hall effect measurements. This study revealed that exercising fine control over the crystallinity and conductive migration of the conjugated polymer films can be a strategic approach to suppressing the degradation of the Seebeck coefficient at high charge carrier density and ultimately to maximizing the power factors of organic thermoelectric devices.

Synthesis of Trifluoromethylated Quinoxaline-Based Polymers for Photovoltaic Applications.

A series of quinoxaline-based conjugated polymers, in which the electron-donating benzodithiophene (BDT) unit is linked to the electron-accepting 6,7-difluorinated quinoxaline (DFQ) derivatives by a thiophene bridge, is synthesized. To investigate their effects on the intrinsic properties of polymers, strong electron-withdrawing trifluoromethyl (CF3 ) groups were incorporated into the meta-position of the phenyl ring at the 2,3-positions of the DFQ unit of the reference polymer, labelled PEhB-FQx, to yield the target polymer PEhB-FQxCF3. In addition, the 2-ethylhexyloxy substituents on the BDT donor in PEhB-FQxCF3 are changed to the more planar 2-ethylhexyl thiophene units to produce another target polymer PThB-FQxCF3. Owing to the significant contributions of the CF3 moiety, PEhB-FQxCF3 exhibits quite discernible optical and electrochemical properties along with significant enhancement in photovoltaic performances compared to the reference polymer PEhB-FQx. Furthermore, the incorporation of the alkylthienyl side chains on the BDT moiety confers on the resultant PThB-FQxCF3 to possess the maximum power conversion efficiency of 7.26% with an open circuit voltage of 0.88 V, short-circuit current density of 12.20 mA cm-2 , and fill factor of 67.80%.

Highly Luminescent 2D-Type Slab Crystals Based on a Molecular Charge-Transfer Complex as Promising Organic Light-Emitting Transistor Materials.

A new 2:1 donor (D):acceptor (A) mixed-stacked charge-transfer (CT) cocrystal comprising isometrically structured dicyanodistyrylbenzene-based D and A molecules is designed and synthesized. Uniform 2D-type morphology is manifested by the exquisite interplay of intermolecular interactions. In addition to its appealing structural features, unique optoelectronic properties are unveiled. Exceptionally high photoluminescence quantum yield (ΦF ≈ 60%) is realized by non-negligible oscillator strength of the S1 transition, and rigidified 2D-type structure. Moreover, this luminescent 2D-type CT crystal exhibits balanced ambipolar transport (µh and µe of ≈10-4 cm2 V-1 s-1 ). As a consequence of such unique optoelectronic characteristics, the first CT electroluminescence is demonstrated in a single active-layered organic light-emitting transistor (OLET) device. The external quantum efficiency of this OLET is as high as 1.5% to suggest a promising potential of luminescent mixed-stacked CT cocrystals in OLET applications.

Bistable Solid-State Fluorescence Switching in Photoluminescent, Infinite Coordination Polymers.

Photo-functional infinite coordinated polymers (ICPs) were synthesized that consist of the photochromic dithienylethene (DTE) and a luminescent bridging unit to give enhanced fluorescence in the solid state. We could fabricate well-ordered micropatterns of these ICPs by a soft-lithographic method, which repeatedly showed high contrast on-off fluorescence switching.

Manipulation of Chain Conformation for Optimum Charge-Transport Pathways in Conjugated Polymers.

A pair of different diketopyrrolopyrrole-based conjugated polymers (CPs) were designed and synthesized to investigate the effect of chain conformation on their molecular assembly. Conformation management was achieved by the incorporation of different linkers during polymerization. Through the use of computational calculations and UV-vis absorption measurements, the resulting CPs (PDPP-T and PDPP-BT) were found to exhibit partly modulated chain geometry. Grazing incident X-ray diffraction experiments with a two-dimensional detector revealed that PDPP-T having a planar chain conformation exhibited an edge-on type molecular arrangement, which evolved to a face-on type chain assembly when the planar geometry was altered to a slightly twisted one as in PDPP-BT. In addition, it was verified that the directional electric carrier mobility of CPs was critically distinguished by the distinctive chain arrangement in spite of their similar chemical structure. Concentration-dependent absorption measurements could provide an improved understanding of the assembly mechanism of CP chains: the planar conformation of PDPP-T facilitates the formation of preassembled chains in a concentrated solution and further directs the edge-on stacking, while the twisted dihedral angle along the benzothiophene in PDPP-BT prevents chain assembly, resulting in the face-on stacking. Because CP chain conformation is inevitably connected with the generation of preassembled chains, manipulating CP geometry could be an efficient tool for extracting an optimum chain assembly that is connected with the principal charge-transport pathway in CPs.

Designing Highly Efficient CuI Photosensitizers for Photocatalytic H2 Evolution from Water.

A series of CuI photosensitizers was synthesized, characterized, and investigated for photocatalytic H2 evolution from water. A structure-property correlation was established for their catalytic activity and photophysical properties, which was further elaborated by DFT calculations. A new CuI photosensitizer (Cu-TPAPhen) with triphenylamine-substituted phenanthroline ligands showed unprecedentedly high turnover numbers of 19 000 when tested in combination with triethylamine as a sacrificial reagent and colloidal Pt as a H2 evolution catalyst. This work paves the way toward cheap metal-based photosensitizers which can replace noble-metal complexes in photocatalytic systems.

Smart Fluorescent Nanoparticles in Water Showing Temperature-Dependent Ratiometric Fluorescence Color Change.

We synthesized two different amphiphilic small molecules 1 and 2 by attaching the same oligo(ethylene glycol) (OEG) unit to the same dicyanodistyrylbenzene (DCS) fluorophore but at different positions. These molecules self-assemble into nanoparticles in water and show lower critical solution temperature (LCST) at 26 and 58 °C, respectively. Upon heating, the transition of hydrophilic coils to hydrophobic globules of the OEG unit leads to the change in the stacking structure of the luminescent DCS cores. As a result, it shows significant ratiometric fluorescence color changes from excimeric yellow emission to monomer-dominated green emission. Interestingly, the coassembly of 1 and 2 exhibits single transition temperature between the transition temperatures of the two components. Moreover, it is demonstrated that the transition temperature of the coassembly is delicately tuned over 26-58 °C by varying the molar mixing ratio of them.

Highly Enhanced Fluorescence of Supramolecular Polymers Based on a Cyanostilbene Derivative and Cucurbit[8]uril in Aqueous Solution.

Supramolecular polymers (SPs) have received great attention because of their potential for various practical applications. As part of our search for SPs that are highly fluorescent in aqueous media, we designed a system based on a cucurbit[8]uril (CB[8]) host and a newly designed cyanostilbene guest. Fluorescence quantum yields of ≈0 % in the disassembled monomer state and 91 % in the CB[8]-induced SP state were obtained. The intriguing photophysical properties of the SP are elucidated through detailed experimental and computational analysis, paving the way towards a fascinating class of water-soluble fluorescent SPs.

Self-Healing of Molecular Catalyst and Photosensitizer on Metal-Organic Framework: Robust Molecular System for Photocatalytic H2 Evolution from Water.

Inspired by self-repair mechanism of PSII in plants, we report a self-healing system which spontaneously repairs molecular catalyst and photosensitizer during photocatalytic H2 evolution. A bipyridine-embedded UiO-type metal-organic framework (MOF), namely Ptn_Ir_BUiO, which incorporated H2-evolving catalyst and photosensitizer, was synthesized and subject to photocatalytic H2 evolution reaction (HER). Impressively, HER with Pt0.1_Ir_BUiO showed very stable molecular photocatalysis without significant decrease in its activity and colloidal formation for 6.5 days at least; in the homogeneous counterpart, the molecular catalyst became a colloid just after 7.5 h. It was revealed that the arrangement of diimine sites which closely and densely surrounded the H2-evolving catalyst and photosensitizer in the MOF enabled such a highly efficient self-healing.

Self-Assembled Organic Single Crystalline Nanosheet for Solution Processed High-Performance n-Channel Field-Effect Transistors.

Submillimeter sized n-channel organic single crystalline nanosheet based on dicyanodistyrylbenzene derivative, (2E,2'E)-3,3'-(2,5-dimethoxy-1,4-pheny-lene)bis(2-(5-(4-(trifluoromethyl)phenyl)thiophen-2-yl)acrylonitrile) (Me-4-TFPTA), is developed. Strong π-π interaction, hydrogen bonding interactions derived from cyano group (CN) as well as solvent inclusion along the lateral direction play a key role in forming nanosheet morphology. Me-4-TFPTA nanosheets exhibit excellent field-effect electron mobility of up to 7.81 cm(2) v(-1) s(-1) .

Stimuli-Responsive Reversible Fluorescence Switching in a Crystalline Donor-Acceptor Mixture Film: Mixed Stack Charge-Transfer Emission versus Segregated Stack Monomer Emission.

We report on a molecularly tailored 1:1 donor-acceptor (D-A) charge-transfer (CT) cocrystal that manifests strongly red-shifted CT luminescence characteristics, as well as noteworthy reconfigurable self-assembling behaviors. A loosely packed molecular organization is obtained as a consequence of the noncentrosymmetric chemical structure of molecule A1, which gives rise to considerable free volume and weak intermolecular interactions. The stacking features of the CT complex result in an external stimuli-responsive molecular stacking reorganization between the mixed and demixed phases of the D-A pair. Accordingly, high-contrast fluorescence switching (red↔blue) is realized on the basis of the strong alternation of the electronic properties between the mixed and demixed phases. A combination of structural, spectroscopic, and computational studies reveal the underlying mechanism of this stimuli-responsive behavior.