Low Potential Energy Barrier Research Articles

Jellyfish-inspired bistable piezoelectric-triboelectric hybrid generator for low-frequency vibration energy harvesting

In pursuit of enhancing the output power of energy harvesters, this paper introduces a novel jellyfish-inspired bistable piezoelectric-triboelectric hybrid generator (JIB-PTHG) with low potential energy barrier characteristics and low-frequency, broadband energy harvesting capabilities. The JIB-PTHG comprises two flexible beams, and two rigid links, connected via a spring. Utilizing the harmonic balance method and Runge-Kutta fourth-order algorithm, a comprehensive analysis of the mechanical properties of the low-barrier bistable structure is conducted. Static equilibrium bifurcation and dynamic response analysis are performed. Subsequently, an electromechanical coupling model incorporating piezoelectricity and triboelectricity is developed to theoretically predict the output voltage of the two modules, revealing a positive correlation between displacement and output voltage. Experiment validation demonstrated that adjusting the length of the rigid links can effectively increase the deformation of the piezoelectric beam and the relative displacement of the triboelectric layers, thus improving the generator's output performance. Specifically, the TENG module exhibited exceptional energy harvesting performance within the frequency range of 3.5–6.5 Hz, achieving a voltage of 1050 V, a power of 1.84 mW, and the ability to illuminate 70 LED lights under an excitation amplitude of 17.5 mm and a frequency of 5.5 Hz. The PEG module, displayed high output performance within the frequency range of 4-7 Hz, reaching a voltage of 6.2 Vs and a power of 12.8 μW at an excitation frequency of 7 Hz and an amplitude of 17.5 mm. Both theoretical and experimental findings indicate that the JIB-PTHG has achieved improved output power within the frequency range of 3.5–6.5 Hz. Furthermore, the JIB-PTHG has the potential to harness bridge vibration energy and convert it into useful electrical energy for powering wireless sensor nodes in bridge health monitoring systems.

Reconfigurable stochastic neurons based on strain engineered low barrier nanomagnets

Stochastic neurons are efficient hardware accelerators for solving a large variety of combinatorial optimization problems. ‘Binary’ stochastic neurons (BSN) are those whose states fluctuate randomly between two levels +1 and −1, with the probability of being in either level determined by an external bias. ‘Analog’ stochastic neurons (ASNs), in contrast, can assume any state between the two levels randomly (hence ‘analog’) and can perform analog signal processing. They may be leveraged for such tasks as temporal sequence learning, processing and prediction. Both BSNs and ASNs can be used to build efficient and scalable neural networks. Both can be implemented with low (potential energy) barrier nanomagnets (LBMs) whose random magnetization orientations encode the binary or analog state variables. The difference between them is that the potential energy barrier in a BSN LBM, albeit low, is much higher than that in an ASN LBM. As a result, a BSN LBM has a clear double well potential profile, which makes its magnetization orientation assume one of two orientations at any time, resulting in the binary behavior. ASN nanomagnets, on the other hand, hardly have any energy barrier at all and hence lack the double well feature. That makes their magnetizations fluctuate in an analog fashion. Hence, one can reconfigure an ASN to a BSN, and vice-versa, by simply raising and lowering the energy barrier. If the LBM is magnetostrictive, then this can be done with local (electrically generated) strain. Such a reconfiguration capability heralds a powerful field programmable architecture for a p-computer whereby hardware for very different functionalities such as combinatorial optimization and temporal sequence learning can be integrated in the same substrate in the same processing run. This is somewhat reminiscent of heterogeneous integration, except this is integration of functionalities or computational fabrics rather than components. The energy cost of reconfiguration is miniscule. There are also other applications of strain mediated barrier control that do not involve reconfiguring a BSN to an ASN or vice versa, e.g. adaptive annealing in energy minimization computing (Boltzmann or Ising machines), emulating memory hierarchy in a dynamically reconfigurable fashion, and control over belief uncertainty in analog stochastic neurons. Here, we present a study of strain engineered barrier control in unconventional computing.

Pyrolysis of cashew nutshell residues for bioenergy and renewable chemicals: Kinetics, thermodynamics, and volatile products

The present study's motivation and novelty are related to the potential of raw (RCNS) and pressed (PCNS) cashew nutshell residues for producing bioenergy and renewable chemicals through their physicochemical characterization and pyrolysis processing (multicomponent kinetic analysis, thermodynamic study, and volatile product analysis). A thermogravimetric analyzer and an analytical pyrolyzer coupled with gas chromatographymass spectrometry (Py–GC/MS) were used to perform the pyrolysis reactions. The pyrolysis behavior of RCNS and PCNS was accurately modeled with the help of the Asym2sig deconvolution function through five and four parallel devolatilization events, respectively. The average activation energies for the pyrolysis of RCNS and PCNS fell in the ranges of 63.8 − 249.3 and 91.1 − 167.4 kJ mol−1, respectively, as determined by four isoconversional methods (Friedman, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Starink). Pre-exponential factors ranging from 4.2 × 108 to 6.9 × 1016 min−1 (RCNS) and 4.8 × 108 to 3.2 × 1011 min−1 (PCNS) were estimated from the kinetic compensation effect. The master plots method evidenced that the most likely reaction models involved in the pyrolysis pertain to the nucleation-growth and n-order reaction mechanisms. Based on the multiple kinetic triplets acquired, the verification step of the summative rate expressions indicated excellent agreement between the simulated behavior and the experimental data, with a minimum quality of fit of 93.1%. The pyrolysis route can valorize both cashew nutshell residues to obtain renewable chemicals promoting the circular economy, as verified by Py-GC/MS analysis. Aliphatic hydrocarbons were the dominant components of the condensable volatile products at 650 °C, while reaction temperatures of 450 and 550 °C favored the production of oxygenated compounds. Due to the low potential energy barrier, the thermodynamic study attests to the viability of converting the studied residues into valuable products. The present results play an essential role in the utility of both cashew nutshell residues as inexpensive feedstocks for pyrolysis, possibly leading to bioenergy and biobased chemicals, which fall under the principle of valorization of lignocellulosic residues.

Enhanced proton conductivity of SPEEK membranes by constructing hydrogen bond network through doping ionic liquid and graphene oxide

AbstractSo as to improve proton conductivity at medium temperatures while reducing the loss of ionic liquids (ILs), this article fabricated a series of hybrid membranes with sulfonated poly (ether ether ketone) (SPEEK) as their base material, 1‐allyl‐3‐methylimidazolium chloride ([AMIM][Cl]) as their primary proton conductor, and graphene oxide (GO) as their filler. The characterization results showed that the –SO3H groups of SPEEK formed acid–base pairs with the imidazole ring of [AMIM][Cl] at the interface where electrostatic attraction inhibited the main‐chain migration of the SPEEK. At the same time, the large hydrogen bond network formed between [AMIM][Cl], GO and SPEEK provided more jump sites for proton transfer. Proton conduction of the prepared membranes was enhanced by way of low potential energy barriers. At 120°C, SPEEK/IL/GO‐2% exhibited a proton conductivity of 21.35 mS/cm, and the IL loss rate was 20.17%. The characterization results demonstrated that the prepared SPEEK/IL/GO composite membranes had excellent physicochemical properties, dimensional stability, high proton conductivity, and low IL loss, which were expected to be used in practical conditions at medium temperature.Highlights Large hydrogen bonding network provides additional proton hopping sites. Physical cross‐linking limits the leaching of ionic liquid. SPEEK/IL/GO‐2% membrane achieves a conductivity of 21.35 mS/cm at 120°C.

Interface engineering of Mo-doped Ni9S8/Ni3S2 multiphase heterostructure nanoflowers by one step synthesis for efficient overall water splitting

Accelerating charge transfer efficiency by constructing heterogeneous interfaces on metal-based substrates is an effective way to improve the electrocatalytic performance of materials. However, minimizing the substrate-catalyst interfacial resistance to maximize catalytic activity remains a challenge. This study reports a simple interface engineering strategy for constructing Mo-Ni9S8/Ni3S2 heterostructured nanoflowers. Experimental and theoretical investigations reveal that the primary role assumed by Ni3S2 in Mo-Ni9S8/Ni3S2 heterostructure is to replace nickel foam (NF) substrate for electron conduction, and Ni3S2 has a lower potential energy barrier (0.76 to 1.11 eV) than NF (1.87 eV), resulting in a more effortless electron transfer. The interface between Ni3S2 and Mo-Ni9S8 effectively regulates electron redistribution, and when the electrons from Ni3S2 are transferred to Mo-Ni9S8, the potential energy barriers at the heterogeneous interface are 1.06 eV, lower than that between NF and Ni3S2 (1.53 eV). Mo-Ni9S8/Ni3S2-0.1 exhibited excellent oxygen evolution reaction (OER)/hydrogen evolution reaction (HER) bifunctional catalytic activity in 1 M KOH, with overpotentials of only 223 mV@100 mA cm−2 for OER and 116 mV@10 mA cm−2 for HER. Moreover, when combined with an alkaline electrolytic cell, it required only an ultra-low cell voltage of 1.51 V to drive a current density of 10 mA cm−2. This work provides new inspirations for rationally designing interface engineering for advanced catalytic materials.

Applicability of the Thawed Gaussian Wavepacket Dynamics to the Calculation of Vibronic Spectra of Molecules with Double-Well Potential Energy Surfaces.

Simulating vibrationally resolved electronic spectra of anharmonic systems, especially those involving double-well potential energy surfaces, often requires expensive quantum dynamics methods. Here, we explore the applicability and limitations of the recently proposed single-Hessian thawed Gaussian approximation for the simulation of spectra of systems with double-well potentials, including 1,2,4,5-tetrafluorobenzene, ammonia, phosphine, and arsine. This semiclassical wavepacket approach is shown to be more robust and to provide more accurate spectra than the conventional harmonic approximation. Specifically, we identify two cases in which the Gaussian wavepacket method is especially useful due to the breakdown of the harmonic approximation: (i) when the nuclear wavepacket is initially at the top of the potential barrier but delocalized over both wells, e.g., along a low-frequency mode, and (ii) when the wavepacket has enough energy to classically go over the low potential energy barrier connecting the two wells. The method is efficient and requires only a single classical ab initio molecular dynamics trajectory, in addition to the data required to compute the harmonic spectra. We also present an improved algorithm for computing the wavepacket autocorrelation function, which guarantees that the evaluated correlation function is continuous for arbitrary size of the time step.

Computation of global reactivity descriptors along the proton transfer co-ordinate of 9-Hydroxy-phenalen-1-one and 6-Hydroxy-benzo[de]anthracen-7-one: a DFT-based comparative study

Potential energy (PE) curves along the proton transfer co-ordinate of ground and excited states of 9-Hydroxy-phenalen-1-one (9HPO) and 6-Hydroxy-benzo[de]anthracen-7-one (6HBAO) have been studied using DFT-CAM-B3LYP/6-311G(d,p) and TD-DFT-CAM-B3LYP/6-31G(d) level of theory, respectively. The choice of these two molecules is based on their symmetric and asymmetric nature for their intramolecular H-bonded six-membered ring systems. Ground and excited states of PE calculations show that due to lower potential energy barrier, 9-Hydroxy-phenalen-1-one (9HPO) and 6-Hydroxy-benzo[de]anthracen-7-one (6HBAO) can be in equilibrium with their enol and keto tautomeric forms. Computed values of global reactivity descriptors, that is, average polarisability (α av) and chemical hardness (η) along the proton transfer co-ordinate of both the molecules are very small although their variations conform with their respective optimum principles. Inspired by the significant variation of the first hyperpolarisabilty along the proton transfer co-ordinate of both the compounds, it is concluded that the first hyperpolarisability can serve as a better parameter to identify enol or keto tautomeric forms in enol keto equilibrium. The variation of average first hyper polarisability (β av) along the proton transfer co-ordinate follows the optical gap rule in both the molecules.

Theoretical Investigation of the Excited-State Dynamics Mechanism of the Asymmetric Two-Way Proton Transfer Molecule BTHMB.

An asymmetric two-way proton transfer molecule 3-(benzo[d]-thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) with the function of white-light emission was synthesized in a recent experiment (Bhattacharyya, A.; Mandal, S. K.; Guchhait, N. J. Phys. Chem. A 2019, 123, 10246). The particularity of this molecule is that there are two possible forms, one of which contained a six-membered H-bonded network toward a N atom (BTHMB-NH) present in the molecule as a proton acceptor and the other was toward an O atom (BTHMB-OH). Unfortunately, the experimental work lacked the theoretical explanation about the determination of the BTHMB-NH form and its excited-state intramolecular proton transfer (ESIPT) process under different solvents. Therefore, this study has explored these two points by means of the time-dependent density functional theory (TDDFT) method. The calculated relative energy and potential energy profile (PEP) of the transformation between BTHMB-NH and BTHMB-OH forms illustrated that BTHMB-NH was more stable, and the transfer from BTHMB-NH to BTHMB-OH was almost impossible at both S0 and S1 states under all solvents due to high potential energy barriers (PEBs) (11.67-21.59 kcal/mol). These calculated results provided the theoretical explanation and verification for the conclusion that the BTHMB molecule exists in the BTHMB-NH form in the experiment. Subsequently, the constructed PEPs of the ESIPT process for BTHMB-NH have proved that it was prone to the ESIPT process due to low PEBs (0.11-0.28 kcal/mol) at the S1 state. In particular, as the solvent polarity increased, the intensity of the intramolecular hydrogen bond (IHB) (O3-H4···N5) increased and the ESIPT process was more likely to occur. In addition, the twisted intramolecular charge-transfer (TICT) process was studied to explore the possible fluorescence quenching pathway of BTHMB-NH. Based on the PEPs of BTHMB-NH-T as a function of the N5-C6-C7-C8 dihedral angle at the S0 and S1 states, it is seen that the S0 state TICT process was inhibited due to the large PEBs (16.45-23.93 kcal/mol). Although the S1 state PEBs have been greatly reduced, they were still maintained at about 3.60 kcal/mol (3.60-3.84 kcal/mol), and hence, this process was still relatively difficult to occur. Due to the fact that BTHMB can be regarded as a standard in future designs involving red light and solvent-specific white-light emitters, a certain amount of investigative work on the ESIPT process was done in detail, and it paved the way for future research on the directionality of ESIPT in double ESIPT probes.

Synergistic nano-tribological interaction between zinc dialkyldithiophosphate (ZDDP) and methyl oleate for biodiesel-fueled engines

In biodiesel-fueled compression-ignition (CI) engines, dilution by unburned biodiesel has been found to have adverse effects on the boundary lubrication properties of additives in fully formulated engine lubricants. Such dilution of engine lubricants could be even more pronounced for CI engines running on higher blend concentrations of biodiesel. Given the nanoscopic nature of the interaction, this study seeks to determine the nano-tribological properties of an engine lubricant additive (e.g., zinc dialkyldithiophosphate (ZDDP)) when diluted with a fatty acid methyl ester (e.g., methyl oleate). Using lateral force microscopy (LFM) together with a fluid imaging technique, the lowest nanoscopic friction forces and coefficient of friction values (0.068–0.085) were measured for ZDDP when diluted with 70 vol% of methyl oleate. These values are also observed to be lower than those measured for neat ZDDP and neat methyl oleate, respectively, under similar conditions. Subsequently, interpreting the data with the Eyring thermal activation energy approach, it could then be elucidated that the lower frictional losses observed for the contact lubricated with this volumetric mixture are a result of the lower potential energy barrier and activation energy required to initiate sliding. These energy values are approximated to be 2.6% and 28.9% (respectively) lower than that of the contact lubricated with neat ZDDP. It was also found that the mixture, at this volumetric concentration, possesses the highest possible pressure activation energy (load-carrying capacity) along with the lowest possible shear activation energy (shearing), potentially indicating optimum tribological conditions for boundary lubrication. Thus, the findings of this study suggest that an optimum concentration threshold exists in which a synergistic nano-tribological interaction between additives and fatty acid methyl esters can be attained, potentially reducing boundary frictional losses of lubricated conjunctions. Such findings could prove to be essential in effectively formulating synergistic additive concentrations for engine lubricants used in biodiesel-fueled CI engines.

Unveiling the effect of solvent polarity on the excited state intramolecular proton transfer mechanism of new 3-hydroxy-4-pyridylisoquinoline compound.

The new 3-hydroxy-4-pyridylisoquinoline compound is attractive and promising lead structure in drug discovery. The pronounced sensitivity of its emission property toward solvent polarity effect was presented in experiment (J. Org. Chem, 2019, 84, 3011). Nevertheless, the experiment was lack of solvent polarity effect on the excited state intramolecular proton transfer (ESIPT) mechanism in detail. In this study, the ESIPT process of this molecule in different polarity solvents were comprehensively expounded by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In order to ensure the accuracy of the experiment and roundly explore in theoretical level, two ESIPT pathways (1 and 2) based on the N1 and N2 forms of studied molecule were proposed, among which the ESIPT pathway 1 was derived from experiment. The calculated electronic spectrum of both N1 and N2 forms were rather comparable with the experiment. The calculated intramolecular hydrogen bond (IHB) parameters and infrared (IR) vibration spectra determined the enhancement of IHBs at the S1 state under different solvents for both N1 and N2 forms. The frontier molecular orbitals (FMOs) analysis proved that the intramolecular charge transfer (ICT) taken place during photoexcitation. The potential energy curves (PECs) at the S0 and S1 states were constructed to illustrate the solvent polarity effect on ESIPT mechanism. According to potential energy barriers (PEBs) on the PECs at S1 state, it is concluded that the ESIPT pathway 1 was forbidden with exceedingly high PEBs (24.585-25.322 kcal/mol), while the ESIPT pathway 2 was feasible with enough low PEBs (0.100-0.510 kcal/mol), which suggested the inconsequence of the experiment. Based on the PEBs of ESIPT pathway 2 in different solvent, the effect of solvent polarity on ESIPT mechanism was depicted. The results are as follows: the S1 state IHB intensity was enhanced with increasing solvent polarity; the extent of ICT was decreased with the increment of solvent polarity; the S1 state PEB was decreased as the solvent polarity increased. Indeed in short, the ESIPT reaction became more and more likely as the solvent polarity enhanced. We believe that this investigation will be useful to the utilization and development of property for such photochemical substances.

Investigation on the excited state intramolecular proton transfer process for the novel 2‐(3,5‐dichloro‐2‐hydroxy‐phenyl)‐benzooxazole‐5‐carboxylicacid system

AbstractIn view of the importance of excited state hydrogen bonding interactions and excited state proton transfer (ESPT) dynamical behaviors, in this work, we theoretically explore the excited state process for a novel 2‐(3,5‐dichloro‐2‐hydroxy‐phenyl)‐benzooxazole‐5‐carboxylicacid (DHPBC) system. On the basis of discrete Fourier transform (DFT) and time‐dependent density functional theory (TDDFT) methods, we reappear previous experimental reports with checking the reasonability of our theoretical level. We calculate and compare the primary geometrical parameters and hydrogen bonding energies around hydrogen bonding moieties, which reveals that the intramolecular hydrogen bond OH···N of DHPBC is enhanced in the S1 state. The increased electron densities around N atom contribute to attracting proton of hydroxyl upon the photoexcitation, which plays important roles in strengthening hydrogen bond and in facilitating excited state intra‐ or intermolecular proton transfer (ESIPT) process. Via constructing potential energy curves along with the ESIPT path, we verify that the ESIPT process of DHPBC system is ultrafast because of the low potential energy barrier. That should be the reason why the normal emission of DHPBC cannot be detected in previous experiment. This work not only clarifies the excited state hydrogen bonding interactions and fills the vacancy of specific ESIPT mechanism for DHPBC system in experiment, but also explains the fluorescence quenching phenomenon.

Charge confinement and thermal transport processes in modulation-doped epitaxial crystals lacking lattice interfaces

Heterogeneous nanosystems offer a robust potential for manipulating various functional material properties, beyond those possible from their individual constituent materials. We demonstrate the formation of a class of materials with a homogeneous lattice but spatially heterogeneous electrical functionality; specifically, we develop epitaxial modulation-doped thin films in which the spatial separation of electronic charge densities is achieved without perturbing the parent crystal's compositional or structural homogeneity. Unlike the previous realizations of modulation doping in crystals, our materials demonstrate periodic layering of spatially segregated, varying electronically donor-doped regions in a single compositionally and structurally homogenous single-crystalline lattice. We demonstrate the formation of ``modulation-doped epitaxial crystals'' (MoDECs) using alternating layers of doped cadmium oxide, and the ability to spatially confine regions of variable carrier concentration via low potential-energy barriers in a spatially homogeneous, epitaxial crystal with a chemically and structurally homogenous lattice (i.e., no chemical or structural lattice interfaces). The low potential energy that confines electrons within the doped layers coupled with the crystalline nature of the MoDECs and lack of lattice interfaces presents a platform to study the electron thermal boundary resistances at low-energy electronic barriers. We find that the electron interfacial density does not impede thermal conductivity, despite evidence that the doped layers retain their carrier concentrations. Thus, the negligible thermal boundary resistances at the electronic interfaces result in the thermal conductivities of the MoDECs being related to only a series resistance sum of the thermal resistances of each of the individual layers, with no thermal resistances from the electronic boundaries that maintain charge separation. This is in stark contrast with other nanoscale multilayer materials, where thermal boundary resistances at the internal material interfaces reduce the thermal conductivity of the multilayer compared to that of the parent materials. The ability to modulation dope epitaxially grown films with no structural heterogeneity in the lattice will further enable unique platforms for mid-IR photonics, such as hyperbolic metamaterials, optical filters with spatially discrete optical absorption, or energy harvesting based on charge injection across modulation-doped interfaces.

Synergistic integration of Bi metal and phosphate defects on hexagonal and monoclinic BiPO4: Enhanced photocatalysis and reaction mechanism

Bi metal deposited hexagonal BiPO4 with the exposure of {102} facet (Bi-HBPO-102) and Bi metal deposited monoclinic BiPO4 with the exposure of {120} facet (Bi-MBPO-120) were prepared by chemical deposition method and solvothermal approach, respectively. The as-prepared catalysts presented more efficient photocatalytic activity of NOx removal than pure BiPO4 (2.0% for BiPO4, 51.4% for Bi-HBPO-102 and 36.2% for Bi-MBPO-120) under visible light irradiation, which can be attributed to the synergistic effects endowed by the phosphate defect, the surface plasmon resonance (SPR) effect of Bi metal and the facet effect. The existence of phosphate defect was confirmed by the XPS and solid state EPR technique. The DFT calculation revealed the position of phosphate and the phosphate defect induced the formation of an intermediate level within the forbidden band to allow efficient charge transfer from valence band to conduction band. Moreover, the Bi metal would act as the electron contributor and electron conductor which facilitated the charge carriers separation. Therefore, a new charge transfer pathway can be certified on account of the fact that the covalent loop was evidently generated both at the interface and along with the path of [Bi2O2]2+ → Bi metal → PO43− on the Bi@BiPO4. More importantly, the Bi-HBPO-102 with exposure of {102} facet exhibited higher photocatalytic activity than the Bi-MBPO-120 with exposed {120} facet. The {102} facet with the stronger distorted PO4 tetrahedron and the lower potential energy barrier (-17.5eV) contributed to the contacted interface with the more efficient charge transfer, which promoted the generation of active radicals on {102} facet. Additionally, for Bi-HBPO-102, the reaction intermediate NO+ can be observed with in situ DRIFTS, which facilitated the activation of NO via the formation of NO+ to promote the oxidation of NO into final products. Herein, a new strategy for tailoring the charge transfer pathway was developed to enhance the photocatalytic performance and a new photocatalytic reaction mechanism for photocatalytic NOx removal was proposed. This work could provide new insights into the modification of photocatalysts and mechanistic understanding of the gas-phase photocatalytic reaction mechanism.

Excited‐state intramolecular proton transfer of 6‐amino‐2‐(2′‐hydroxyphenyl) benzoxazole (6A‐HBO) in different solvents

AbstractThe excited‐state intramolecular proton transfer (ESIPT) process of 6‐amino‐2‐(2′‐hydroxyphenyl) benzoxazole (6A‐HBO) was investigated using density functional theory and time‐dependent density functional theory methods with B3LYP and TZVP basis sets. n‐Heptane, dichloromethane, methanol, and acetonitrile were chosen as a series of polar solvents in calculations using the IEFPCM model. To obtain a more comprehensive ESIPT mechanism, we constructed the S0 and S1 states' potential energy surfaces (PESs) by incrementally twisting the ─OH bond and increasing the distance of O–H bond. Based on the analysis of the bond lengths, the IR vibrational spectra, and the frontier molecular orbits (MOs), the intramolecular hydrogen bonding (O─H...N) is clearly strengthened, and the charge is redistributed in the S1 state. The results of calculated absorption spectrum are in accord with the experimental data. The fluorescence spectrum of 6A‐HBO‐enol showed a normal red shift, but the red shift of the 6A‐HBO‐keto is larger and increases with the solvent polarity, indicating a charge transfer. Analysis of the PESs indicates a lower potential energy barrier in S1 state for the proton transfer from the O atom to the N atom, with the excited state potential barrier slightly decreasing with the increase of the solvent polarity.

The excited state hydrogen bond and proton transfer mechanism of a novel dye CS‐Azine

AbstractIn this work, based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we theoretically explore the excited state behavior for a novel dye molecule 3,3′‐(5,5′‐((1E,1E′)‐hydrazine‐1,2‐diylidenebis (methanylylidene))bis(2‐morpholinothiazole‐5,4‐diyl))bis(4‐hydroxy‐2H‐chromen‐2‐one) (CS‐Azine). Coupling with atoms in molecules, we investigate the intramolecular dual hydrogen bonds of CS‐Azine system and verify the formation of them. Via study the primary bond distances, bond angle, and infrared vibrational spectra involved in hydrogen bonding moieties, we find O1‐H2···N3 and O4‐H5···N6 of CS‐Azine should be strengthened in the S1 state. When exploring the photo‐excitation process, we confirm that the charge redistribution around hydrogen bonding moieties reveals the tendency of ESIPT reaction. To further investigate whether single or double proton transfer occurs in the S1 state, we consider two kinds of reaction paths (ie, the stepwise and synergetic ESIPT reactions). And the constructed potential energy curves demonstrate that only the single proton transfer reaction should be the most supported in the S1 state from CS‐Azine to CS‐Azine‐SPT form due to the low potential energy barrier. This work not only clarifies the excited state behavior and mechanism about CS‐Azine system but also paves the way for further applying CS‐Azine dye in future.

Generating conformational transition paths with low potential-energy barriers for proteins.

The knowledge of conformational transition paths in proteins can be useful for understanding protein mechanisms. Recently, we have introduced the As-Rigid-As-Possible (ARAP) interpolation method, for generating interpolation paths between two protein conformations. The method was shown to preserve well the rigidity of the initial conformation along the path. However, because the method is totally geometry-based, the generated paths may be inconsistent because the atom interactions are ignored. Therefore, in this article, we would like to introduce a new method to generate conformational transition paths with low potential-energy barriers for proteins. The method is composed of three processing stages. First, ARAP interpolation is used for generating an initial path. Then, the path conformations are enhanced by a clash remover. Finally, Nudged Elastic Band, a path-optimization method, is used to produce a low-energy path. Large energy reductions are found in the paths obtained from the method than in those obtained from the ARAP interpolation method alone. The results also show that ARAP interpolation is a good candidate for generating an initial path because it leads to lower potential-energy paths than two other common methods for path interpolation.

Theoretical explorations about the excited state behaviors for two novel high efficient ESIPT compounds

Two high efficient excited state intramolecular proton transfer (ESIPT) compounds (i.e., 3-(5-([1,1′-biphenyl]-4-yl)oxazol-2-yl)-4′-(N,N-diphenylamino)-[1,1′-biphenyl]-4-ol (1) and 4′-(N,N-diphenylamino)-3-(5-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)oxazol-2-yl)-5-methyl-[1,1′-biphenyl]-4-ol (2)) are explored theoretically. Based on DFT and time-dependent DFT (TDDFT) methods, we investigate the hydrogen bonding interactions and ESIPT mechanism. Via B3LYP/TZVP/IEFPCM (toluene) theoretical level, we reappear the experimental steady-state spectra, which demonstrate that the theoretical manner is reasonable and effective. Based on reduced density gradient (RDG) versus sign(λ2)ρ analyses, we confirm intramolecular hydrogen bond for both 1-enol and 2-enol. Investigating geometrical parameters and infrared (IR) vibrational spectra, we verify the O-H···N should be strengthened in the S1 state for 1-enol and 2-enol systems. Exploring frontier molecular orbitals (MOs) and charge density difference (CDD) maps, we find charge redistribution provides the tendency of ESIPT. The constructed potential energy curves demonstrate that the proton transfer should happen in the S1 state. Particularly, the low potential energy barriers of forward and backward ESIPT process for both 1 and 2 systems, the dynamical equilibrium could be verified, which means 1 and 2 systems should be potential for novel white light LEDs materials. This work not only explores and explains previous experimental phenomenon, but also makes a reasonable assignment about the ESIPT mechanism.

Theoretical insights into the excited state intramolecular proton transfer mechanism for a novel 4‐methoxy‐3‐hydroxyflavone system

A novel 3‐hydroxyflavone derivative, 4‐methoxy‐3‐hydroxyflavone (4M3HF), is theoretically investigated regarding its excited state intramolecular proton transfer (ESIPT) mechanism based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods. By comparing the solvent polarity, we selected acetonitrile and toluene in this work. First, based on atoms in molecules (AIM) analyses, we verified the formation of intramolecular hydrogen bond of the 4M3HF structure. On investigating the geometrical parameters (i.e., bond lengths and bond angles), we found that the hydrogen bond strength should be enhanced in the S1 state. By calculating the infrared vibrational spectra, we confirmed the strengthening of the intramolecular hydrogen bond in the S1 state. We further studied the excitation process using the frontier molecular orbitals method, based on which we concluded that the charge redistribution in solvents facilitates the ESIPT tendency. For exploring the ESIPT behavior, we constructed the potential energy curves along with ESIPT paths in both solvents (acetonitrile and toluene). We found that low potential energy barriers facilitate the ESIPT process in the 4M3HF system, which was also verified by searching the transition state (TS) structure. Comparing these two solvents, we concluded that solvent effects play little role in the ESIPT reaction in the 4M3HF system. And we could successfully explain the previous experimental result.

Structurally Flexible Oxocarbenium/Borohydride Ion Pair: Dynamics of Hydride Transfer on the Background of Conformational Roaming.

We apply Born-Oppenheimer molecular dynamics to the practically significant [dioxane-H(+)-acetone][(C6F5)3B-H(-)] and [Et2O-H(+)-OCPr2][(C6F5)3B-H(-)] ion pair intermediates. Dynamics of hydride transfer in cation/anion ion pair takes place on the background of large-amplitude configurational changes. Geometry of oxocarbenium/borohydride ion pairs is flexible, meaning that we uncover significant actual structural disorder at a finite temperature. Therefore, although the starting structure can be fairly close to the configurational area of the hydride transfer transition state (TS) and despite a low potential energy barrier (ca. 1.5 kcal/mol, according to the literature), already at T ≈ 325 K the system can remain ignorant of the TS region and move round and about ("roam") in the configurational space for a period of time in the range between 10 and 100 ps. This indicates structural flexibility of oxocarbenium/borohydride ion pair on apparently a flat potential energy "landscape" of cation/anion interaction, and this has not been taken into consideration by the free energy estimations in static considerations made thus far. The difference between the dynamics-based representation of the system versus the static representation amounts to the difference between quasi-bimolecular versus unimolecular descriptions of the hydride transfer step.

Different ESIPT Mechanisms forAngular‐Shaped Quinacridone in Toluene and Dimethyl Formamide (DMF) Solvents: A Theoretical Study

Adopting density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we investigat and present two different excited‐state intramolecular proton transfer (ESIPT) mechanisms ofangular‐quinacridone (a‐QD) in both toluene and DMF,theoretically. Comparing the primary structural variations of a‐QD involved in the intramolecular hydrogen bond, we conclude that N1–H2⋯O3 should be strengthened in the S1state, which may facilitate the ESIPT process. Particularly, in toluene, the S1‐state‐stable a‐QD enol* could not be located because of the non‐barrier ESIPT process. Concomitantly, infrared vibrational spectral analysis further verified the stability of the hydrogen bond. In addition, the role of charge–transfer interaction has been addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. The potential energy curves according to variational N1–H2 coordinate demonstrates that the proton transfer process should occur spontaneously in toluene; however, in DMF, a low potential energy barrier of 0.493 kcal/mol is needed to complete the ESIPT reaction. Although this barrier of 0.493 kcal/mol is too low to make an important impact on the ESIPT reaction, just because of the existence of barrier, ESIPT mechanisms in toluene and DMF are different.