Local Charge Transfer Research Articles

Frenkel and Charge-Transfer Excitonic Couplings Strengthened by Thiophene-Type Solvent Enables Binary Organic Solar Cells with 19.8 % Efficiency.

Overcoming the trade-off between short-circuited current (Jsc) and open-circuited voltage (Voc) is important to achieving high-efficiency organic solar cells (OSCs). Previous works modulated the energy gap between Frenkel local exciton (LE) and charge-transfer (CT) exciton, which served as the driving force of exciton splitting. Differently, our current work focuses on the modulation of LE-CT excitonic coupling (tLE-CT) via a simple but effective strategy that the 2-chlorothiophene (2Cl-Th) solvent utilizes in the treatment of OSC active-layer films. The results of our experimental measurements and theoretical simulations demonstrated that 2Cl-Th solvent initiates tighter intermolecular interactions with non-fullerene acceptor in comparison with that of traditional chlorobenzene solvent, thus suppressing the acceptor's over-aggregation and retarding the acceptor crystallization with reduced trap. Critically, the resulting shorter distances between donor and acceptor molecules in the 2Cl-Th treated blend efficiently strengthen tLE-CT, which not only promotes exciton splitting but also reduces non-radiative recombination. The champion efficiencies of 19.8 % (small-area) with superior operational reliability (T80: 586 hours) and 17.0 % (large-area) were yielded in 2Cl-Th treated cells. This work provided a new insight into modulating the exciton dynamics to overcome the trade-off between Jsc and Voc, which can productively promote the development of the OSC field.

Regulating Planarized Intramolecular Charge Transfer for Efficient Single-Phase White-Light Emission in Undoped Metal-Organic Framework Nanocrystals.

The technology of combining multiple emission centers to exploit white-light-emitting (WLE) materials by taking advantage of porous metal-organic frameworks (MOFs) is mature, but preparing undoped WLE MOFs remains a challenge. Herein, a pressure-treated strategy is reported to achieve efficient white photoluminescence (PL) in undoped [Zn(Tdc)(py)]n nanocrystals (NCs) at ambient conditions, where the Commission International del'Eclairage coordinates and color temperature reach (0.31, 0.37) and 6560 K, respectively. The initial [Zn(Tdc)(py)]n NCs exhibit weak-blue PL consisting of localized excited (LE) and planarized intramolecular charge transfer (PLICT) states. After pressure treatment, the emission contributions of LE and PLICT states are balanced by increasing the planarization of subunits, thereby producing white PL. Meanwhile, the reduction of nonradiative decay triggered by the planarized structure results in 5-fold PL enhancement. Phosphor-converted light-emitting diodes based on pressure-treated samples show favorable white-light characteristics. The finding provides a new platform for the development of undoped WLE MOFs.

Chemical states of corrosion products in liquid lead from ab initio molecular dynamics

Liquid lead (Pb) is a promising coolant for lead-cooled fast reactors, and has advantages over lead–bismuth eutectic (LBE) in many aspects. But it can also cause severe corrosion to structural materials at high temperatures. In this study, the chemical states of 10 alloying elements as well as H and O in liquid Pb were investigated via ab initio molecular dynamics, and the local structure, dissolution and charge transfer were analyzed systematically. In liquid Pb, the coordination numbers (CNs) of Fe and Ni are the smallest, with the value of 7.72 and 7.01, respectively, and Al, Mn, Mo, Nb, Ti, and Si have the biggest CNs with about 12. This is correlated with the effective atomic radii in liquid Pb. Judged by the dissolution energy, the tendency of alloying elements to dissolve in liquid Pb is Al > Si > Ni > Ti > Cu > Fe > Mn > Nb > Cr > Mo. The calculated Bader charges show that the alloying atoms undergo minimal electron transfer in liquid Pb except Ti. H2, H2O, and O2 will decompose quickly and cannot exist in liquid Pb. Then, the alloying element will form a strong binding with O atom, especially Al and Si. O atoms can lead to an increase in the coordination of alloying atoms in liquid Pb and obviously reduce their diffusion coefficients in most cases. These findings can provide guidance for predicting material corrosion and designing future high-performance corrosion-resistant materials.

Stepwise Charge/Energy Transfer in MR-TADF Molecule-Doped Exciplex for Ultralong Persistent Luminescence Activated with Visible Light.

Organic long-persistent luminescence (OLPL), which relies on energy storage for delayed light emission by the charge separation state, has attracted intense attention in various optical applications. However, charge separation (CS) is efficient only under ultraviolet excitation in most OLPL systems because it requires a driving force from the large energy difference between the local excited (LE) and charge transfer (CT) states. In this study, a multiresonance thermally activated delayed fluorescence (MR-TADF) molecule is incorporated into an exciplex system to achieve efficient OLPL in a composite material activated by visible light via a stepwise charge/energy transfer process. The enhanced absorption of the composite material facilitated a tenfold increase in the duration of the OLPL, which can last for several hours under visible light excitation. The excited state of the MR-TADF molecule tends to charge transfer to the acceptor, followed by energy transfer to the exciplex, which benefits from the small difference between the LE and CT states owing to the inherent CS characteristics of the opposing resonance effect. Afterglow displays of these composite materials are fabricated to demonstrate their considerable potential in encryption patterns and emergency lights, which take advantage of their excellent processability, visible light activation, and tunable luminescence properties.

Local Charge Transfer Unveils Antideactivation of Ru at High Potentials for the Alkaline Hydrogen Oxidation Reaction.

The activity of Ru-based alkaline hydrogen oxidation reaction (HOR) electrocatalysts usually decreases rapidly at potentials higher than 0.1 V (vs a reversible hydrogen electrode (RHE)), which significantly limits the lifetime of fuel cells. It is found that this phenomenon is caused by the overadsorption of the O species due to the overcharging of Ru nanoparticles at high potentials. Here, Mn1Ox(OH)y clusters-modified Ru nanoparticles (Mn1Ox(OH)y@Ru/C) were prepared to promote charge transfer from overcharged Ru nanoparticles to Mn1Ox(OH)y clusters. Mn1Ox(OH)y@Ru/C exhibits high HOR activity and stability over a wide potential range of 0-1.0 V. Moreover, a hydroxide exchange membrane fuel cell with a Mn1Ox(OH)y@Ru/C anode delivers a high peak power density of 1.731 W cm-2, much superior to that of a Pt/C anode. In situ X-ray absorption fine structure (XAFS) analysis and density functional theory (DFT) calculations reveal that Mn in Mn1Ox(OH)y clusters could receive more electrons from overcharged Ru at higher potentials and significantly decrease the overadsorption of the O species on Ru, thus permitting the HOR on Ru to proceed at high potentials. This study provides guidance for the design of alkaline HOR catalysts without activity decay at high potentials.

Surface-enhanced Raman spectroscopy sensing of Huangjiu's total antioxidant capacity utilizing polyoxometalate-based frameworks nanozymes

The ability to accurately assess the total antioxidant capacity (TAC) of Huangjiu is essential for the comprehensive assessment of its nutritional and medicinal benefits. Consequently, developing a rapid, sensitive and specific analytical methodology to determine TAC is an urgent research need. In this work, we explored the potential of a one-step solvothermal process, leading to the synthesis of polyoxometalates-metal-organic frameworks (POMs-MOFs), H3PW12O40 (HPW)-CuBTC materials, which possess a high surface area and considerable active sites. Our results demonstrated that HPW-CuBTC materials exhibit remarkable surface-enhanced Raman scattering (SERS) activity, capable of detecting 10–8 M methylene blue (MB). SERS activity was attributed to both the localized surface plasmon resonance (LSPR) and charge transfer (CT) synergistic enhancement mechanisms. Furthermore, HPW-CuBTC materials exhibited peroxidase (POD)-like activity, with a catalytic mechanism involving a synergistic catalytic effect between HPW and CuBTC. We developed a catechin sensor based on the POD-like properties of HPW-CuBTC, with a linear range from 4 μM to 8 μM (R2=0.98). Additionally, we utilized the HPW-CuBTC materials to construct a TAC sensing platform in Huangjiu. This work introduces new technologies and methodologies for rapid trace detection of TAC in Huangjiu, providing a valuable tool for evaluating its nutritional and medicinal benefits.

Controlled Self‐Assembly of Zn–Tetraphenylporphyrins for Efficient Photocatalytic Solar H2 Production and Simultaneous Organic Transformation to Valuable Chemicals

Herein, we have designed aqueous dispersed self‐assembled nanostructures with diverse morphologies from the zinc tetraphenyl porphyrin (ZnTPP) monomer employing a simple solution‐based coprecipitation methods. Detail morophological studies have been carried out by various electron microscopy techniques. Finally, the structural features were correlated with the underpinning photophysical processes using steady‐state and time‐resolved spectroscopy. Detailed studies suggest that control morphology and highly defined intermolecular interactions affect the overall photoinduced charge transfer process. Based on the fundamental investigations, all these different types of nanostructures have been utilized as photocatalysts for solar hydrogen production, without using any cocatalysts, and it was found that the spherical nanostructure exhibits significantly higher H2 production rates of ~1682 m mole/ g, which is few folds higher than other 1D and 2D nanostructured materials. The experimental findings were further supported by the TD‐DFT study. Furthermore, the detailed computational studies suggest that the spherical aggregates exhibited a more vital interaction between the ZnTPP molecules, causes significant electronic coupling between bright local excited and charge transfer states, which supports our experimental findings. Finally, we have selectively utilized the oxidative half‐reaction for the simultaneous transformation of glycerol to valuable chemicals along with photocatalytic H2 production through reductive half‐reaction.

Electronic Couplings and Conversion Dynamics between Localized and Charge Transfer Excitations from Many-Body Green's Functions Theory.

We investigate the determination of electronic coupling between localized excitations (LEs) and charge-transfer (CT) excitations based on many-body Green's functions theory in the GW approximation with the Bethe-Salpeter equation (GW-BSE). Using a small molecule dimer system, we first study the influence of different diabatization methods, as well as different model choices within GW-BSE, such as the self-energy models or different levels of self-consistency, and find that these choices affect the LE-CT couplings only minimally. We then consider a large-scale low-donor morphology formed from rubrene and fullerene and evaluate the LE-CT couplings based on coupled GW-BSE-molecular mechanics calculations. For these disordered systems of bulky molecules, we observe differences in the couplings based on the Edmiston-Ruedenberg diabatization compared to the more approximate Generalize Mulliken-Hush and fragment charge difference diabatization formalisms. In a kinetic model for the conversion between LE and CT states, these differences affect the details of state populations in an intermediate time scale but not the final populations.

A Hot Exciton Luminogen Constructed by an o‐Carborane Scaffold

AbstractEmitters with the combination of an electron donor (D) and an acceptor (A) have been widely studied and usually exhibited interesting photophysical properties. Hot exciton materials involving an ordinary D‐A structure typically undergo a hybridized local charge transfer (HLCT) process. However, the hot exciton molecules containing more D/A fragments are much less explored. In this study, a new hot exciton material based on an inorganic boron cluster is designed and synthesized, which possesses a V‐type D–A–A′–D′ molecular architecture. The emitter shows an impressive photoluminescence quantum yield (PLQY) of up to 90% and demonstrates multiple emissions, excitation‐wavelength‐, solvent‐, and temperature‐dependent emissions. These characteristics are attributed to the coexistence of HLCT and the through‐space charge transfer process (TSTC) triggered by an o‐carborane scaffold in the excited state. Besides, the interchangeable HLCT/TSCT/hRISC processes are demonstrated by temperature‐dependent spectra and femtosecond time‐resolved transient absorption spectra. These findings confirm a novel hot exciton mechanism, simultaneously induced by HLCT and TSCT. This is also the first example of a carborane‐based hot exciton molecule. The current study not only reports a new molecular architecture for a hot exciton material but also gives rise to a new insight into the hot exciton mechanism.

Disentangling Acid-Base Chemistry through Blue Shifting Hydrogen Bond Contributions.

The blue shifting of vibrational frequencies in hydrogen bonded molecules, as observed in aqueous environments, has been attributed to local partial charge transfer from solvation. Here, we extrapolate the blue shift model to the stronger ionic interactions between hydrogen bond acceptors associated with protonation through augmented pH levels and competitive interactions with counter ion pairing. The chemical model we utilize in this work is the aqueous pyridine-pyridinium equilibrium to characterize the blue shifts observed in the pyridinium chloride ionic system. The observed agreement between observed experimental and calculated spectral shifts shows that the blue shifting model can be extrapolated to stronger interactions and accurately describe the nature of the hydrogen bond.

Symmetry-breaking charge separation in a null-excitonic 3-dimensional rigid nonconjugated trimer.

Photoinduced symmetry-breaking charge separation (SB-CS) has been extensively observed in various oligomers and aggregates, which holds great potential for robust artificial solar energy conversion systems. It attaches great importance to the precise manipulation of interchromophore electronic coupling in realizing efficient SB-CS. The emerging studies on SB-CS suggested that it could be realized in null-excitonic aggregates, and a long-lived SB-CS state was observed, which offers an advanced platform and has gathered immense attention in the SB-CS field. Here, we unveiled the null-exciton coupling induced ultrafast SB-CS in a rigid polycyclic aromatic hydrocarbon framework, triperyleno[3,3,3]propellane triimides (TPPTI), in which three chromophores were attached through a nonconjugated bridge. Through a combination of theoretical calculations and steady-state absorption results, we demonstrated that this nonconjugated TPPTI possesses negligible exciton coupling. Increased solvent polarity was found to significantly enhance state mixing between local excited and charge transfer states. Using transient absorption spectroscopy, ultrafast SB-CS was observed in highly polar dimethylformamide, facilitated by a selective hole-transfer coupling and a favorable charge separation free energy (ΔGCS). Additionally, the rate ratio between SB-CS and charge recombination was at least high to 1800 in dimethylformamide. This investigation provides profound insights into the role of null-exciton coupling in dominating ultrafast SB-CS in multichromophoric systems.

Multifunctional applications of organic light-emitting diodes and room temperature phosphorescence based on benzoyl-fluorene hybrid molecules

Organic light-emitting diodes (OLEDs) and room temperature phosphorescence (RTP) materials have attracted great interests for their wide application in lighting and other fields. How to efficiently utilize triplet excitons to achieve the multifunctionality of OLED and RTP simultaneously is a major challenge. Herein, two blue molecules, 3PCZFB and 3PCZBF were reported. Theoretical calculations indicate that both molecules possess hybrid localized charge transfer (HLCT) characteristic. More importantly, both can simultaneously obtain electro-fluorescence and RTP properties by multiple harvesting of triplet excitons with high spin-orbit coupling matrix element (SOCME). The introduction of benzoyl fluorene increased the contribution of the n-orbital, resulting in a high SOCME, which is beneficial to both high exciton utilization efficiency (EUE) and RTP. The non-doped OLED of 3PCZFB achieved maximum external quantum efficiency (EQEmax) of 4.9 % and EUE of 58.6 % at 459 nm. Besides, 0.1 % 3PCZFB and 3PCZBF doped in PMMA sheets achieve RTP efficiencies of 6.6 % at 524 nm and 25.9 % at 506 nm, as well as lifetimes of 379.8 ms and 204.8 ms. This is the first time Benzoyl fluorene-based molecules achieved dual-function applications in OLED and RTP by utilizing and recycling triplet excitons of T1.

The in-plane band bending effect around Au islands grown on the Bi2Te3 topological insulator

We have utilized scanning tunneling microscopy and spectroscopy (STM/STS) as well as x-ray linear dichroism (XLD) along with DFT calculations to determine the structural and electronic changes of the gold-covered nonstoichiometric Bi2Te3 single crystal surface. XLD spectra supported by a numerical simulation of several configurations show that at low coverage (< 1.0 ML) gold forms insoluble islands, while for higher coverage (> 1.0 ML) mixing at the Au/Bi2Te3 interface may occur. For local electronic characterization around stand-alone Au islands, small coverages were selected for which the inter-island distance is large enough to avoid overlapping island-wide effects. STS spectra reveal that for small coverages the topological surface states (TSS) are preserved. However, at a distance of about 10 nm from an Au island, by approaching the islands, the electronic structure gradually changes. This is manifested by a gradual shift of the STS spectra to more negative energies with respect to the pristine surface. The results are discussed in view of possible Au-Bi2Te3 mixing, a local charge transfer and the presence of in-plane downward band bending.

Derivatives of Phenyl Pyrimidine and of the Different Donor Moieties as Emitters for OLEDs.

Two derivatives of phenyl pyrimidine as acceptor unit and triphenylamino or 4,4'-dimethoxytriphenylamino donor groups were designed and synthesized as emitters for organic light-emitting diodes (OLEDs) aiming to utilize triplet excitons in the electroluminescence. Thermogravimetric analysis revealed high thermal stability of the compounds with 5% weight loss temperatures of 397 and 438 °C. The theoretical estimations and photophysical data show the contributions of local excited and charge transfer states into emission. The addition of the methoxy groups led to the significant improvement of hole-transporting properties and the bathochromic shift of the emission from blue to green-blue spectral diapason. It is shown that mixing of the compounds with the organic host results in facilitation of the delayed emission. The singlet-triplet energy splitting was found to be too big for the thermally activated delayed fluorescence. No thermal activation of the long-lived emission was detected. No experimental evidence for triplet-triplet annihilation and room temperature phosphorescence were detected making the hot exciton mechanism the most probable one. The OLEDs based on the compounds reached the maximum external quantum efficiency of up to 10.6%.

Room-Temperature Phosphorescence Materials Featuring Triplet Hybrid Local Charge Transfer Emission.

Room-temperature phosphorescence materials have found important applications in dissolved oxygen sensing, temperature monitoring, anticounterfeiting, etc., because of their prolonged phosphorescence lifetime. However, the known systems mainly utilize the triplet local excited state emission, which is generally less sensitive to microenvironment perturbation. In this work, we designed a series of 4-phenyl-1,8-naphthalimide (NMI) derivatives containing different numbers of carbazole (Cz) units (denoted as NMI-Cz, NMI-2Cz, and NMI-3Cz). Steady state and time-resolved spectroscopy studies determined that the compounds undergo intramolecular through-space charge transfer in solution, yielding a triplet hybrid local charge transfer state. Room-temperature phosphorescence emission was observed in compound-doped poly(methyl methacrylate) thin films upon ammonia treatment. Interestingly, emission from different films exhibited different persistence times. We believe a film-based, time-resolved luminescent ammonia sensor could be developed by making a device of the emissive films as fabricated.

Modulation of hydroxymethyl/aldehyde groups activation on V2O3 decorated CuCo for 5-hydroxymethylfurfural electrooxidation

5-Hydroxymethylfurfural electrooxidation reaction (HMFOR) has attracted increasing attention due to energy conservation and value-added production. It is crucial to design highly active catalysts with fast kinetics and reveal the catalytic mechanism. Herein, the activation of hydroxymethyl/aldehyde groups is tuned on V2O3-modificated CuCo for efficient HMFOR. Experimental and theoretical results reveal that strong electron transfer occurs on the heterointerface between V2O3 and CuCo, endowing its unique electronic structure and conductivity. Meanwhile, CuCo-V2O3 heterostructure facilitates the modulation of local electron density, charge transfer, and adsorption behavior to promote the activation of hydroxymethyl/aldehyde groups. The higher activation of hydroxymethyl groups than that of aldehyde groups over CuCo-V2O3 induces an altered rate-determining step for HMFOR. Consequently, CuCo-V2O3 presents enhanced HMFOR activity (E10, 1.27 VRHE), reaction kinetics (38 mV dec−1), 2,5-furandicarboxylic acid selectivity (99.1%), and stability (40 successive cycles). This work provides a strategy for designing highly active catalysts for synthesizing value-added products.

Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems.

Energy transfer across the donor-acceptor interface in organic photovoltaics is usually beneficial to device performance, as it assists energy transport to the site of free charge generation. Here, we present a case where the opposite is true: dilute donor molecules in an acceptor host matrix exhibit ultrafast excitation energy transfer (EET) to the host, which suppresses the free charge yield. We observe an optimal photochemical driving force for free charge generation, as detected via time-resolved microwave conductivity (TRMC), but with a low yield when the sensitizer is excited. Meanwhile, transient absorption shows that transferred excitons efficiently produce charge-transfer states. This behavior is well described by a competition for the excited state between long-range electron transfer that produces free charge and EET that ultimately produces only localized charge-transfer states. It cannot be explained if the most localized CT states are the intermediate between excitons and the free charge in this system.

Exploring the Optical and Photovoltaic Changeover for Carbazole-Bridge Push-Pull Switches Against Their Local Excitations and Charge Transfers

This research investigates characterization of low lying excitations of newly designed organic dyes for their local excited and charge transfer-related molecular switching attributes such as donor-acceptor relations against the carbazole bridge. The eight new dyes, having 4 symmetrical (Syn-A-D) and 4 antisymmetric (Anti-A-D) arrangements showed how the push-pull effect can operate to create the charge transfer phenomenon. Their absorption energies (E) for Syn dyes ranged from 2.98 eV to 4.07 eV, with corresponding wavelengths (λmax) ranging from 304 nm to 415 nm. While for their Anti arrangements, it ranged from 1.62 eV to 3.99 eV, with corresponding λmax values as 310-617 nm. Syn-C1 with an ionization potential (IP) of 6.59 eV and an electron affinity (EA) of 2.04 eV can be predicted to be more chemically stable compared to Anti-C2 with an IP of 5.16 eV and an EA of 3.86 eV. Syn-C also has the highest Voc (3.24 eV) and Pmax (22.35 W), indicating that it has the potential to be more chemically stable as compared to the other dyes. The findings of this research contributed to a better understanding of the molecular switching attributes of carbazole-based organic dyes, which could have implications for various applications, including optoelectronics and energy conversion devices.

Promising sensors for pharmaceutical pollutant adsorption using Clar’s goblet-based 2D membranes

This study focuses on the design of new 2D membranes from connected Clar’s Goblet as a potential sensor for pharmaceutical pollutants, specifically the painkiller drugs aspirin, paracetamol, ibuprofen, and diclofenac. The electronic, optical, and interaction properties are investigated using density functional theory calculations. The Clar’s Goblet membranes (CGMs) that were chosen are semiconductors with an energy gap of around 1.5 eV, according to energy gap calculations and density of states. Molecular electrostatic potential (ESP) analysis shows that CGMs have electrophilic and nucleophilic sites, suggesting their suitability for interacting with pharmaceutical pollutants. The adsorption energies confirm the chemical adsorption of pharmaceutical pollutants with diclofenac showing the strongest adsorption. The UV–Vis absorption spectra of CGMs-drug complexes are analyzed, revealing a redshift compared to the absorption spectrum of CGMs alone, confirming the adsorption of these drugs. Further analysis using hole/electron examinations indicates that the type of excitation is local excitation rather than charge transfer excitation. This study quantitatively characterized hole and electron distribution in excited states using various indices. The analysis revealed local excitation transitions and significant charge transfer between the CGMs molecule and pharmaceutical pollutants. Additionally, non-covalent interaction analysis indicates the presence of van der Waals interactions, highlighting the adsorption behavior of the drugs. These results demonstrate the potential of CGMs as a highly sensitive sensor for pharmaceutical pollutants.

Tuning Rashba-Dresselhaus effect with ferroelectric polarization at asymmetric heterostructural interface.

The spin-orbit interaction (SOI) plays an essential role in materials properties, and controlling its intensity has great potential in the design of materials. In this work, asymmetric [(La0.7Sr0.3MnO3)8/(BaTiO3)t/(SrTiO3)2]8 superlattices were fabricated on (001) SrTiO3 substrate with SrO or TiO2 termination, labelled as SrO-SL and TiO2-SL, respectively. The in-plane angular magnetoresistance of the superlattices shows a combination of two- and four-fold symmetry components. The coefficient of two-fold symmetry component has opposite sign with current I along [100] and [110] directions for TiO2-SL, while it has the same sign for SrO-SL. Detailed study shows that the asymmetric cation inter-mixing and ferroelectricity-modulated electronic charge transfer induce asymmetric electronic potential for SrO-SL with dominating Rashba SOI, and symmetric electronic potential for TiO2-SL with dominating Dresselhaus SOI induced by BaTiO3. This work shows that the Rashba and Dresselhaus SOIs are sensitive to the ferroelectric polarization in the asymmetric structure.