A Coupled System of Ni3S2 and Rh Complex with Biomimetic Function for Electrocatalytic 1,4-NAD(P)H Regeneration.

NAD(P)H cofactor is a critical energy and electron carrier in biocatalysis and photosynthesis, but the artificial reduction of NAD(P)+ to regenerate bioactive 1,4-NAD(P)H with both high activity and selectivity is challenging. Herein, we found that a coupled system of a Ni3S2 electrode and a Rh complex in an electrolyte (denoted as Ni3S2-Rh) can catalyze the reduction of NAD(P)+ to 1,4-NAD(P)H with superior activity and selectivity. The optimized selectivity in 1,4-NADH can be up to 99.1%, much higher than that for Ni3S2 (80%); the normalized activity of Ni3S2-Rh is about 5.8 times that of Ni3S2 and 13.2 times that of the Rh complex. The high performance of Ni3S2-Rh is attributed to the synergistic effect between metal sulfides and Rh complex. The NAD+ reduction reaction proceeds via a concerted electron-proton transfer (CEPT) mechanism in the Ni3S2-Rh system, in which Ni3S2 acts as a proton and electron-transfer mediator to accelerate the formation of Rh hydride (Rh-H), and then the Rh-H regioselectively transfers the hydride to NAD+ to form 1,4-NADH. The artificial system Ni3S2-Rh essentially mimics the functions of ferredoxin-NADP+ reductase in nature.

Hydrogen Sulfide Splitting into Hydrogen and Sulfur through Off-Field Electrocatalysis.

Hydrogen sulfide (H2S), a toxic gas abundant in natural gas fields and refineries, is currently being removed mainly via the Claus process. However, the emission of sulfur-containing pollutants is hard to be prevented and the hydrogen element is combined to water. Herein, we report an electron-mediated off-field electrocatalysis approach (OFEC) for complete splitting of H2S into H2 and S under ambient conditions. Fe(III)/Fe(II) and V(II)/V(III) redox mediators are used to fulfill the cycles for H2S oxidation and H2 production, respectively. Fe(III) effectively removes H2S with almost 100% conversion during its oxidation process. The H+ ions are reduced by V(II) on a nonprecious metal catalyst, tungsten carbide. The mediators are regenerated in an electrolyzer at a cell voltage of 1.05 V, close to the theoretical potential difference (1.02 V) between Fe(III)/Fe(II) and V(II)/V(III). In a laboratory bench-scale plant, the energy consumption for the production of H2 from H2S is estimated to be 2.8 kWh Nm-3 H2 using Fe(III)/Fe(II) and V(II)/V(III) mediators and further reduced to about 0.5 kWh Nm-3 H2 when employing well-designed heteropolyacid/quinone mediators. OFEC presents a cost-effective approach for the simultaneous production of H2 and elemental sulfur from H2S, along with the complete removal of H2S from industrial processes. It also provides a practical platform for electrochemical reactions involving solid precipitation and organic synthesis.

Type-I Photodynamic Therapy Induced by Pt-Coordination of Type-II Photosensitizers into Supramolecular Complexes.

Platinum supramolecular complexes based on photosensitizers have garnered great interest in photodynamic therapy (PDT) due to Pt (II) centers as chemotherapeutic agents to eliminate tumor cells completely, which greatly improve the antitumor efficacy of PDT. However, in comparison to precursor photosensitizer ligand, the formed platinum supramolecular complexes typically exhibit inferior outcomes in terms of reactive oxygen species (ROS) generation. How to boost ROS generation in the formed platinum supramolecular complexes for enhanced PDT is an enticing yet highly challenging task. Here we report a Pt-coordination-based dimeric photosensitizer complex (Cz-BTZ-Py)2Pt(OTf)2. It is found that comparing with photosensitizer ligand Cz-BTZ-Py, the formed supramolecular complex exhibit redshifts of absorption wavelength as well as enhanced ROS generation efficiency. Moreover, type-I ROS generation (O2⋅-) is produced in the formed platinum supramolecular complexes mainly due to a reduced energy gap ΔEST resulting from exciton coupling between two photosensitizer ligands. And type-I ROS (O2⋅-) generation significantly amplifies the photodynamic therapy (PDT) outcomes. In vitro evaluation shows excellent photochemotherapy performance of (Cz-BTZ-Py)2Pt(OTf)2 nanoparticles. We anticipate this work would provide a novel approach to design type-I photosensitizers for efficient PDT.

Theoretical Investigation on the "ON-OFF" Mechanism of a Fluorescent Probe for Thiophenols: Photoinduced Electron Transfer and Intramolecular Charge Transfer.

In this study, the sensing mechanism of (2E,4E)-5-(4-(dimethylamino)phenyl)-1-(2-(2,4dinitrophenoxy)phenyl)penta-2,4-dien-1-one (DAPH-DNP) towards thiophenols was investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT). The DNP group plays an important role in charge transfer excitation. Due to the typical donor-excited photo-induced electron transfer (d-PET) process, DAPH-DNP has fluorescence quenching behavior. After the thiolysis reaction between DAPH-DNP and thiophenol, the hydroxyl group is released, and DAPH is generated with the reaction showing strong fluorescence. The fluorescence enhancement of DAPH is not caused by an excited-state intramolecular proton transfer (ESIPT) process. The potential energy curves (PECs) show that DAPH-keto is less stable than DAPH-enol. The frontier molecular orbitals (FMOs) of DAPH show that the excitation process is accompanied by intramolecular charger transfer (ICT), and the corresponding character of DAPH was further confirmed by hole-electron and interfragment charge transfer (IFCT) analysis methods. Above all, the sensing mechanism of the turn-on type probe DAPH-DNP towards thiophenol is based on the PET mechanism.

Cs2 SnCl6 : To Emit or to Catalyze? Te4+ Ion Calls the Shots.

A low concentration of Te4+ doping is found to be capable of endowing the lead-free Cs2 SnCl6 perovskites with excellent photoluminescence quantum yield (PLQY), while further increasing Te4+ concentration leads to PLQY deterioration. The mechanism behind the improved PLQY is intensively studied and reported elsewhere. However, little work is conducted to understand the decreased PLQY at high doping levels and to explore its implications for non-PL-related applications. Here, it is demonstrated that the Te4+ -incorporated Cs2 SnCl6 can be promising candidate for efficient CO2 photocatalysis. An optimum photocatalytic performance is achieved when Te4+ concentration reaches as high as 50%, at which point significant PL quenching has occurred. Through a detailed spectral characterization, such concentration-dependent functionality is attributed to systematic changes in both electronic and local crystal structure, which allow a robust regulation of excitation energy relaxation channels. These findings expand the scope of available photocatalysts for CO2 reduction and also inform synthetic planning for the preparation of multifunctional Pb-free metal halide perovskites.

Unraveling the Mechanism of ACQ-to-AIE Transformation of Fluorescein Derivatives.

Although fluorescein derivatives have excellent properties and strong practicability, they are typical aggregation-induced quenching (ACQ) molecules, which are not conducive to working in the solid state. Recently, the fluorescein derivative Fl-Me with aggregation-induced emission (AIE) property was synthesized, which brought a new dawn for the research and development of fluorescein-based materials. In this study, the AIE mechanism of Fl-Me was investigated based on time-dependent density functional theory and the ONIOM method. The results revealed that an effective dark-state deactivation pathway leads to the fluorescence quenching of Fl-Me in a solution environment. Accordingly, the AIE phenomenon originates from the closure of the dark-state quenching channel. It is worth emphasizing that we found that the carbonyl group of molecular Fl-Me has intermolecular hydrogen bonding interaction with the adjacent molecules, which caused the increase of the dark-state energy in the crystalline state. Moreover, the restriction of the rotational motion and the nonexistence of the π-π stacking interaction are beneficial to the enhancement of fluorescence upon aggregation. Finally, the ACQ-to-AIE transformation mechanisms of fluorescein derivatives have been discussed. This work provides deeper insight into the photophysical mechanism for the fluorescein derivatives Fl-Me with AIE feature and eventually is expected to help researchers to develop more fluorescein-based AIE materials with remarkable properties for various fields.

Enhanced Fluorescence by Inter/Intramolecular Hydrogen Bonding in Si-Substituted Coumarins.

Introducing heteroatoms in organic fluorophores offers a unique strategy to tune their photophysical properties without dangling structural decorations. Silicon-substituted coumarins (Si-coumarins) are the analogues of coumarin with the substitution of ester oxygen atoms by silicon atoms. In Si-coumarins, significant fluorescence enhancement in protic solvents through the formation of inter/intramolecular hydrogen bonds (H-bonds) offered great potential in various aspects with many unique photophysical properties. The energies of nπ* and ππ* states in Si-coumarins are elaborately tuned by inter/intramolecular H-bonds and solvents after incorporating silicon atoms. For example, the inter/intramolecular H-bonds elevate the energy of the nπ* state in protic solvents, leading to an enlarged energy gap between the nπ* and ππ* states. Thus, fluorescence is enhanced by reducing the nonradiative transition through the nπ* state in coumarins, resulting in many unique photophysical properties. The understanding of H-bonds in Si-coumarins offers more potential strategies for the design of novel fluorophores.

N-Protonation as a Switch of the Twisted Excited States with ππ* or nπ* Character and Correlation with the π-Electrons Characteristic of Rotatable Bonds.

N-protonation for numerous fluorophores is widely known as an efficient switch for the fluorescence turn-on/off in acidic conditions, which has been applied in various scenarios that involve pH monitoring. Yet the universal mechanism for fluorescence regulation through N-protonation is still elusive. Herein, the excited state deactivation processes are systematically investigated for a series of nitrogen-containing fluorescent probes through theoretical approaches. Two types of mechanisms for the complex fluorescent phenomena by N-protonation are concluded: one is through the regulation for the transition to a ππ* twisted intramolecular charge transfer (TICT) state; the other one applies for the case when nonradiative decay pathway is predominant by a dark nπ* state, which is also accompanied by an evident structural twisting and can be regarded as another kind of TICT state. More generally, the formation of the TICT state is closely related to the conjugated π-electrons on the single bond that links the acceptor and donor part of fluorophores, which provides a simple strategy for evaluating the occurrence of the TICT process. The current contributions can bring novel insights for the rational design of functional fluorophores that involve TICT process in the excited states.

Theoretical Investigations on the Sensing Mechanism of Phenanthroimidazole Fluorescent Probes for the Detection of Selenocysteine.

The level of selenocysteine (Sec) in the human body is closely related to a variety of pathophysiological states, so it is important to study its fluorescence sensing mechanism for designing efficient fluorescent probes. Herein, we used time-dependent density functional theory to investigate the fluorescence sensing mechanism of phenanthroimidazole derivates A4 and B4 for the detection of Sec, which are proposed to be designed based on excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) mechanisms. The calculation results show that the fluorescence quenching mechanism of A4 and B4 is due to the photo-induced electron transfer (PET) process with the sulfonate group acts as the electron acceptor. Subsequently, A4 and B4 react with Sec, the sulfonate group is substituted by hydroxyl groups, PET is turned off, and significant fluorescence enhancement of the formed A3 and B3 is observed. The theoretical results suggest that the fluorescence enhancement mechanism of B3 is not based on ICT mechanism, and the charge transfer phenomenon was not observed by calculating the frontier molecular orbitals, and proved to be a local excitation mode. The reason for the fluorescence enhancement of A3 based on ESIPT is also explained by the calculated potential energy curves.

Highly Efficient and Ultralong Afterglow Emission with Anti-Thermal Quenching from CsCdCl3 : Mn Perovskite Single Crystals.

Triplet exciton-based long-lived phosphorescence is severely limited by the thermal quenching at high temperature. Herein, we propose a novel strategy based on the energy transfer from triplet self-trapped excitons to Mn2+ dopants in solution-processed perovskite CsCdCl3 . It is found the Mn2+ doped hexagonal phase CsCdCl3 could simultaneously exhibit high emission efficiency (81.5 %) and long afterglow duration time (150 s). Besides, the afterglow emission exhibits anti-thermal quenching from 300 to 400 K. In-depth charge-carrier dynamics studies and density functional theory (DFT) calculation provide unambiguous evidence that carrier detrapping from trap states (mainly induced by Cl vacancy) to localized emission centers ([MnCl6 ]4- ) is responsible for the afterglow emission with anti-thermal quenching. Enlightened by the present results, we demonstrate the application of the developed materials for optical storage and logic operation applications.

Spatiotemporal imaging of charge transfer in photocatalyst particles.

The water-splitting reaction using photocatalyst particles is a promising route for solar fuel production1-4. Photo-induced charge transfer from a photocatalyst to catalytic surface sites is key in ensuring photocatalytic efficiency5; however, it is challenging to understand this process, which spans a wide spatiotemporal range from nanometres to micrometres and from femtoseconds to seconds6-8. Although the steady-state charge distribution on single photocatalyst particles has been mapped by microscopic techniques9-11, and the charge transfer dynamics in photocatalyst aggregations have been revealed by time-resolved spectroscopy12,13, spatiotemporally evolving charge transfer processes in single photocatalyst particles cannot be tracked, and their exact mechanism is unknown. Here we perform spatiotemporally resolved surface photovoltage measurements on cuprous oxide photocatalyst particles to map holistic charge transfer processes on the femtosecond to second timescale at the single-particle level. We find that photogenerated electrons are transferred to the catalytic surface quasi-ballistically through inter-facet hot electron transfer on a subpicosecond timescale, whereas photogenerated holes are transferred to a spatially separated surface and stabilized through selective trapping on a microsecond timescale. We demonstrate that these ultrafast-hot-electron-transfer and anisotropic-trapping regimes, which challenge the classical perception of a drift-diffusion model, contribute to the efficient charge separation in photocatalysis and improve photocatalytic performance. We anticipate that our findings will be used to illustrate the universality of other photoelectronic devices and facilitate the rational design of photocatalysts.

Unraveling the Key Role of the Benzyl Group in the Synthesis of CL-20 Precursor HBIW.

As one of the most important energetic material molecules, hexanitrohexaazaisowurtzitane (CL-20) can only be synthesized using an amine with a benzyl group. Moreover, the reaction mechanism remains unexplored and the special role of the benzyl group has not been revealed. To address these issues, we perform an extensive theoretical study to investigate the synthesis mechanism of CL-20 precursor HBIW by employing density functional theory. Our calculated results demonstrate that the benzyl group can reduce the energy of the intermediate and the energy barrier of the rate-determining step due to the π-π stack interaction between two benzene rings of the benzyl group. For the first time, we revealed that the reactions can produce 16 intermediates with different chiralities during the formation of the first two side five-membered rings and only two of which can further form the bottom six-membered ring and finally obtain the product HBIW. The steric hindrance effect of the benzyl group leads to the formation of a higher-energy intermediate first, thereby providing an opportunity to correct the wrong chirality. All of these factors make the diimine with the benzyl group the most suitable reactant for the synthesis of CL-20.

Mechanistic Investigation on the Initial Thermal Decomposition of Energetic Materials FOX-7 and RDX in the Crystal and Gas Phase: An MM/DFT-Based ONIOM Calculation.

Interpreting the initial decomposition mechanism is important for evaluating the thermal stability of explosives. In this study, we theoretically investigated the initial thermal decomposition reactions for two typical energetic materials, FOX-7 and RDX, in both the gas phase and crystal phase. Single molecular decomposition pathways in the gas phase are calculated using the density functional theory (DFT) method, and the crystal phase reactions are simulated through the MM/DFT-based ONIOM method. The calculation results indicate that the crystal environment has a significant influence on the initial thermal decomposition mechanism of FOX-7 and RDX. The cage effect induced by the crystal environment greatly confines molecular mobility and diffusion, rendering the generated small molecules to react with the remaining fragment and yield new decomposition channels compared with the gas phase condition. The crystal packing structures and intermolecular interactions (hydrogen bonds/π-π stacking) significantly increase the reaction barriers of FOX-7 and RDX, leading to the crystal phase reactions being more difficult to occur than in the gas phase. Since the practical application of explosives is mostly in the crystal state, it is important to consider the environmental effects on the initial decomposition reactions. The same insight can also be relevant for other energetic materials.

Dual fluorescence of 2-(2'-hydroxyphenyl) benzoxazole derivatives via the branched decays from the upper excited-state.

As a special fluorescence phenomenon, double fluorescence has been widely developed and applied in various fields. Nevertheless, most of the research on fluorescence emission channels focuses on the first excited state, while the research on how to control the fluorescence emission channel through the upper excited state is relatively under-explored. Here, we use the time-dependent density functional theory method and consider the 2-(2'-hydroxyphenyl) benzoxazole (HBO) derivative system as an example to study the effect of upper excited states on double fluorescence. According to the calculation results, a new mechanism for the dual fluorescence was proposed, which involved the different decay pathways from the upper excited-state, the internal conversion through vibrational relaxation, and conical intersection, respectively. This research has potential value and can help in determining how to control the fluorescence emission channel through the upper excited state.

New Cy5 photosensitizers for cancer phototherapy: a low singlet-triplet gap provides high quantum yield of singlet oxygen.

Highly efficient triplet photosensitizers (PSs) have attracted increasing attention in cancer photodynamic therapy where photo-induced reactive oxygen species (ROSs, such as singlet oxygen) are produced via singlet-triplet intersystem crossing (ISC) of the excited photosensitizer to kill cancer cells. However, most PSs exhibit the fatal defect of a generally less-than-1% efficiency of ISC and low yield of ROSs, and this defect strongly impedes their clinical application. In the current work, a new strategy to enhance the ISC and high phototherapy efficiency has been developed, based on the molecular design of a thio-pentamethine cyanine dye (TCy5) as a photosensitizer. The introduction of an electron-withdrawing group at the meso-position of TCy5 could dramatically reduce the singlet-triplet energy gap (ΔE st) value (from 0.63 eV to as low as 0.14 eV), speed up the ISC process (τ ISC = 1.7 ps), prolong the lifetime of the triplet state (τ T = 319 µs) and improve singlet oxygen (1O2) quantum yield to as high as 99%, a value much higher than those of most reported triplet PSs. Further in vitro and in vivo experiments have shown that TCy5-CHO, with its efficient 1O2 generation and good biocompatibility, causes an intense tumor ablation in mice. This provides a new strategy for designing ideal PSs for cancer photo-therapy.

Marcus inverted region of charge transfer from low-dimensional semiconductor materials.

A key process underlying the application of low-dimensional, quantum-confined semiconductors in energy conversion is charge transfer from these materials, which, however, has not been fully understood yet. Extensive studies of charge transfer from colloidal quantum dots reported rates increasing monotonically with driving forces, never displaying an inverted region predicted by the Marcus theory. The inverted region is likely bypassed by an Auger-like process whereby the excessive driving force is used to excite another Coulomb-coupled charge. Herein, instead of measuring charge transfer from excitonic states (coupled electron-hole pairs), we build a unique model system using zero-dimensional quantum dots or two-dimensional nanoplatelets and surface-adsorbed molecules that allows for measuring charge transfer from transiently-populated, single-charge states. The Marcus inverted region is clearly revealed in these systems. Thus, charge transfer from excitonic and single-charge states follows the Auger-assisted and conventional Marcus charge transfer models, respectively. This knowledge should enable rational design of energetics for efficient charge extraction from low-dimensional semiconductor materials as well as suppression of the associated energy-wasting charge recombination.

Stacking Engineering: A Boosting Strategy for 2D Photocatalysts.

Two-dimensional (2D) photocatalytic material is a vital project for modern solar energy conversion and storage. Despite a vast family of potential 2D photocatalysts that is demonstrated, their commercial applications are severely limited because of fast photogenerated electron-hole recombination. Here, based on first-principles, we propose a general paradigm to boost the separation of photoexcited charge carriers in 2D photocatalysts by stacking engineering. Taking the emerging water splitting photocatalyst MoSi2N4 as an example, we show that specific interlayer stacking-induced electric polarization plays a significant role in altering the electronic properties and thus the suppressed recombination rate of photoexcited carriers. Moreover, we find that the catalytic performance can be further controlled by vertical strain. These generalized findings not only highlight the importance of stacking-induced electric polarization but also offer new prospects for the design and application of 2D photocatalysts.

A combined experimental and theoretical investigation of the excited-state dynamics of 2,4,6-trinitrotoluene (TNT) in DMSO solvent.

In the present contribution we carried out a TDDFT and femtosecond transient absorption study of the excited state dynamics of TNT in DMSO solvent. Vertical excitation and excited state relaxation were calculated at the SMD/M06-2X/TZVP level of theory. The electron absorption spectrum for the DMSO solvated TNT was calculated and compared with the experimental results. The results of the electronic excitation energies and the spin-orbital constants imply an intersystem crossing for the S1-T2 transition. The femtosecond time-resolved transient absorption measurements of the TNT in DMSO show the presence of two absorption signals around 650 nm and 540 nm, which are assigned to the population in the lowest singlet and triplet excited states, S1 and T1, respectively. The fast decay of the S1 state population is assigned to an efficient S1-T2 intersystem crossing, which soon internally converts to the T1 state. The slow decay of the T1 population is attributed to the nonradiative transition to the S0 state. The combined theoretical and experimental results present a mechanistic view of the photophysical dynamics of TNT in DMSO solution.

Controlling Photoluminescence and Photocatalysis Activities in Lead-Free Cs2 Ptx Sn1-x Cl6 Perovskites via Ion Substitution.

Lead-free halide perovskites have triggered interest in the field of optoelectronics and photocatalysis because of their low toxicity, and tunable optical and charge-carrier properties. From an application point of view, it is desirable to develop stable multifunctional lead-free halide perovskites. We have developed a series of Cs2 Ptx Sn1-x Cl6 perovskites (0≤x≤1) with high stability, which show switchable photoluminescence and photocatalytic functions by varying the amount of Pt4+ substitution. A Cs2 Ptx Sn1-x Cl6 solid solution with a dominant proportion of Pt4+ shows broadband photoluminescence with a lifetime on the microsecond timescale. A Cs2 Ptx Sn1-x Cl6 solid solution with a small amount of Pt4+ substitution exhibits photocatalytic hydrogen evolution activity. An optical spectroscopy study reveals that the switch between photoluminescence and photocatalysis functions is controlled by sub-band gap states. Our finding provides a new way to develop lead-free multifunctional halide perovskites with high stability.

Effects of Intermolecular Hydrogen Bonding and Solvation on Enol-Keto Tautomerism and Photophysics of Azomethine-BODIPY Dyads.

Boron-dipyrromethene derivatives (BODIPYs) are a category of molecules with excellent photophysical properties and can be applied to various fields. This work investigates the fluorescent properties of two azomethine-BODIPY dyads in different solvents based on the time-dependent density functional theory (TD-DFT) method. The potential energy curves (PECs) show that the polar protic solvent and the enhanced π-conjugation effect can lower the proton-transfer (PT) barriers, causing the main configuration of NA-BODIPY in methanol to be the keto form, while the main configuration of NA-BODIPY in toluene and SA-BODIPY in methanol and toluene is the enol form. The keto forms of the two compounds possess the twisted intramolecular charge transfer (TICT) decay pathway in the excited state identified by the optimized twisted configurations and the appropriate barriers of the TICT process, whereas the twisted configurations of the enol forms are nonexistent. TICT successfully competes with excited-state proton transfer (ESIPT) of the keto form, which leads to the fluorescence quenching of NA-BODIPY in methanol. This work provides new ideas for the influence of enol-keto tautomerism and the competitiveness of TICT and ESIPT on the photophysical properties of BODIPYs and is expected to provide guidance for the design of new BODIPY functional molecules.