Change In Dipole Moment Research Articles

CN− and NH2− co-decorated graphdiynes: A theoretical investigation

In this work, the cyano (CN−) and amino (NH2−) groups are used to co-decorate the monolayer graphdiyne (GDY) resulting in two new GDY derivative systems (i.e., A1C2- and A2C1-GDYs). The structural stabilities of these two systems are carefully verified. Due to the local dipole moments introduced by the CN− and NH2− groups, as well as the breaking central symmetry, the corresponding mechanical, piezoelectric, and electronic features, even the electrical transport properties, are significantly altered comparing with the pristine GDY, F-GDY, and CN-GDY systems. The predicted band gaps of A1C2- and A2C1-GDYs are around 1.25 eV and 1.33 eV at the PBE level, respectively, which are obviously larger than that of the pristine GDT monolayer (0.53 eV at the same theoretical level). There are also observable negative piezoelectric effects in both new GDY derivatives due to the change of the local dipole moment under strains. Moreover, the negative electrical conductivity constants σxy are revealed in both co-decorated monolayer GDY systems at certain chemical potentials, indicating the switch of the major carrier types according to the Hall effect. This theoretical study not only expands the GDY family, but also provides a new opportunity for switching carrier types in 2D materials using the in-plane electric field.

Insights of the pH-dependent emission of carbon dots: a density functional theory study

The mechanism of luminescence in carbon dots has long been a topic of controversy, as accurately characterizing their structure has proven difficult. This has made it challenging to guide the design and synthesis of carbon dots, hindering the development of highly efficient fluorescent varieties. Understanding the mechanism and regulatory mechanisms of carbon dot luminescence at the molecular level is important not only from a theoretical perspective but also has practical value. Previous studies have suggested that the fluorescence emission wavelength of carbon dots can be altered by changing the pH of the carbon dot solution, but the mechanism behind this modulation is not yet fully understood. In this study, we used density functional theory calculations and excited state simulations with 2, 3-Diaminophenazine, a representative luminescent molecule found in molecular state luminescent carbon dots, to investigate the effect of protonation on the absorption and fluorescence emission spectra of carbon dots. We analyzed the electronic structure and spatial distribution of the luminescent molecules before and after protonation and found that the change in molecular dipole moment caused by protonation was responsible for the significant red shift in the fluorescence spectrum. To further verify our findings, we modified the substituted functional groups with different electron-pulling abilities in the corresponding luminescent molecules to modulate the molecular dipole. The results showed that the larger the dipole moment in the luminescent molecules with different substituted functional groups, the longer the fluorescence emission wavelength. Therefore, the protonation-induced change in molecular dipole is the main reason for the pH-dependent modulation of luminescence in molecular state luminescent carbon dots.

Computational Analysis of Excited-State Properties of MoS2 Nanosheet Modified with a Pyrene Derivative

MoS2, a representative transition metal dichalcogenides (TMD), is known to undergo the hydrogen evolution reaction in the exfoliated state, and its use as an auxiliary catalyst in place of platinum has been widely reported. Recently, the exfoliation of bulk MoS2 and efficient covalent modification of MoS2 nanosheets with N-bromobenzylmaleimide was successfully achieved using a planetary ball mill. For the photofunctionalization of MoS2, further coupling reaction with a pyrene derivative produced pyrene-benzyl-maleimide-MoS2 (Py-Bn-Mal-MoS2) nanosheet. The absorption spectrum of Py-Bn-Mal-MoS2 shows peaks similar to that of 1-phenylpyrene. On the other hand, the emission spectrum of Py-Bn-Mal-MoS2 is much different from that of 1-phenylpyrene. Furthermore, the emission peak of Py-Bn-Mal-MoS2 is more redshifted with increasing solvent polarity. However, the detailed mechanism is unclear. In this study, we investigated the excited-state properties of Py-Bn-Mal-MoS2 with theoretical methods. First, we constructed model systems of Py-Bn-Mal-MoS2 nanosheet and performed time-dependent density functional theory (TDDFT) calculations. The calculated molecular orbitals and dipole moment changes demonstrated that the emission is a charge transfer transition from the pyrene moiety to MoS2 nanosheet. Next, we performed three-dimensional reference interaction site model (3D-RISM) calculations, which is based on the integral equation theory of molecular liquids, to assess the solvent dependence of emission energies. The calculated results reproduced the experimental solvent dependence well. The distributions of polar solvent molecules showed strong interaction between the excited state of Py-Bn-Mal-MoS2 and solvent molecules. This study supports the potential of covalent modification of MoS2 for photocatalytic applications.

Fascinating Pathway to Facilitate the Photoisomerization of Spiropyran-Based Nanocomposites.

Recently, spiropyran-based composites have gained more attention on account of their stimuli-responsive essence, especially of the fascinating and green photo stimulus. However, the great dipole moment change between the ring-opened merocyanine and ring-closed spiropyran requires a large free volume available for isomerization, which significantly restrains the photoisomerization of spiropyran-based nanocomposites. Herein, a fascinating pathway by regulating the states both of spiropyran and the immobilized nanoparticle supports was put forward to facilitate the photoisomerization. The results demonstrated that the spiropyran grafting percentage of 5.18% and immobilized supports with less aggregation, high specific surface area, large pore size, and noncrystalline structure were suitable to fabricate spiropyran-based nanocomposites, which showed a significant improvement for Pb2+ and Cr3+ removal from aqueous solution on account of free photoisomerization of spiropyran on the support's surface. This work will pave the pathway to extend the exploitation of spiropyran-based nanocomposites in various fields such as biotechnology, physiology, and electronics to photonics and environmental-friendly fields.

Estimation of dipole moments by Solvatochromic shift method, spectroscopic analysis of UV–Visible, HOMO-LUMO, ESP map, Mulliken atomic charges, NBO and NLO properties of benzofuran derivative

Solvatochromic shift method has been used to estimate ground state (μg) and excited state (μe) dipole moments of benzofuran derivative (5NFMOT) at room temperature indifferent solvents. This method involves Lippert, Bakhshiev and Kawski-Chamma-Viallet equations. Reichardt microscopic solvent polarity parameter was used to calculate the change in dipole moment. Effect of solvents on absorption and fluorescence spectra was investigated using Kamlet and Catalan's MLR approach. Computational studies of benzofuran derivative were carried out using Gaussian 16. Geometry optimization was performed using DFT method at 6–311++G (d, p)/B3LYP basis set. UV–Visible spectra have been studied by using TD-DFT method for the same basis set. The μg was obtained from DFT method, whereas μe from TD-DFT method. It has been observed that both from experimentally and theoretically the ground state dipole moment is higher than the excited state dipole moment. HOMO-LUMO surfaces and Mulliken atomic charges have been studied in gas phase and solution. Furthermore, 3D plots of electrostatic potential map are used to identify reactive centers such as electrophilic and nucleophilic sites. NBO analysis is helpful to obtain stability and charge delocalization of the title molecule. Polarizablity, and hyper polarizability values were used to study the NLO properties.

Impact of the Protein Environment on Two-Photon Absorption Cross-Sections of the GFP Chromophore Anion Resolved at the XMCQDPT2 Level of Theory.

The search for fluorescent proteins with large two-photon absorption (TPA) cross-sections and improved brightness is required for their efficient use in bioimaging. Here, we explored the impact of a single-point mutation close to the anionic form of the GFP chromophore on its TPA activity. We considered the lowest-energy transition of EGFP and its modification EGFP T203I. We focused on a methodology for obtaining reliable TPA cross-sections for mutated proteins, based on conformational sampling using molecular dynamics simulations and a high-level XMCQDPT2-based QM/MM approach. We also studied the numerical convergence of the sum-over-states formalism and provide direct evidence for the applicability of the two-level model for calculating TPA cross-sections in EGFP. The calculated values were found to be very sensitive to changes in the permanent dipole moments between the ground and excited states and highly tunable by internal electric field of the protein environment. In the case of the GFP chromophore anion, even a single hydrogen bond was shown to be capable of drastically increasing the TPA cross-section. Such high tunability of the nonlinear photophysical properties of the chromophore anions can be used for the rational design of brighter fluorescent proteins for bioimaging using two-photon laser scanning microscopy.

Tuning of environment-sensitive 3-hydroxychromone fluorophores based on strong donor substituents in positions 2 or 7

We have synthesized a series of environmentally sensitive fluorophores based on 3-hydroxychromone with systematically varied aromatic groups at positions 2 and 7. The two families of 2,7-diaryl-3-hydroxychromones were synthesized using Suzuki coupling from the corresponding 2-aryl-7-bromo-3-hydroxychromones. These chromophores exhibit a dual fluorescent emission and a strong response to changes in environmental polarity in organic solvents. Both experimental determination and calculations of dipole moment changes show stronger charge transfer in compounds with an electron donor group at position 7 compared to position 2. Their emission is almost totally quenched in the aqueous medium, while the increase in quantum yield upon binding to biological molecules serves as a useful characteristic. Particularly, compounds with strong donors in position 7 are sensitive to diverse lipid environments, whereas those with strong donors in position 2 have different emission profiles for distinct protein interactions. The complex behavior of the probes was evaluated using spectral phasor analysis. Altogether, these novel families of probes may help monitor several biological processes in vitro and in cell cultures.

Study of Optical Properties of PVC/Ag2O Nano-Composites by Dispersing Spherical Nanofillers into Polymer Matrix under Normal and Heat Treated Conditions

In the present work a novel polymer/metal oxide composites containing Polyvinyl Chloride (PVC) and Ag2O nanoparticles (NP) were fabricated under room and elevated temperatures. The PVC/Ag2O NC has been fabricated to enhance the electronic application such as memory devices, sensors, electronic luminescent devices and medical applications. The Ag2O NP was synthesized by dropwise mixing process having 25 nm in size with spherical surface morphology. The Fourier transform infrared spectroscopy (FTIR) was taken for (1) virgin PVC film (2) PVC/Ag2O NC and PVC/Ag2O (heat treated) Composites. The formation of absorbance peaks, their shifting, intensity and maximum absorbance values were considered in all the three C C, C-Cl and C-H classified stretching modes of vibrations for different composites. It was found in C-C stretch range the virgin PVC back bone chain occurs at 1101 cm-1 which shifted to 1076 cm-1 for PVC/Ag2O NC, further shift is evident for PVC/Ag2O (heat treated) Composites at 1151 cm-1. In addition rocking vibration and wagging of methylene group changes in the same fashion. The change in spectral feature takes places due to filling of Ag2O NP in PVC matrix, indicates that changes in dipole moments occur as a result of molecular vibrations. It is also observed the absorbance peaks value is high for virgin PVC film for all modes of stretching vibration, which decreases for Ag2O/PVC (NC) further decrease is evident for Ag2O/PVC (NC) heat treated composite.

The mechanism of pyroelectricity in polar material hemimorphite

It is known that a crystal structure and symmetry determine the physical properties of materials. Lattice distortion can strongly affect the symmetry of the crystal structure. Polar materials show changes in polarization with temporal fluctuations of temperature due to the asymmetry. As a polar crystal, hemimorphite shows excellent pyroelectric properties. However, to date, there are a few studies on its intrinsic physical properties, and the mechanism of its pyroelectricity remains unclear. In this paper, single-crystal x-ray diffraction measurement was carried out to obtain the atomic positions at 100–400 K. Furthermore, the electric dipole moments of [ZnO4] and [SiO4] polyhedrons along a, b, and c axes have been calculated. The calculated pyroelectric coefficient derived from the intrinsic electric dipole moment was compared with the experimental measurement. The results indicate that the pyroelectric coefficients of hemimorphite at different temperatures mainly come from the variation of the electric dipole moment of [ZnO4] and [SiO4] polyhedrons along the c axis. The electric dipole moment changes as a function of temperature from 100 to 400 K, which is induced by the random lattice distortion. It is found that pyroelectricity is strongly correlated with the random lattice distortion. The establishment of the relationship between lattice distortion and pyroelectricity helps us to regulate the specific electrical parameters of the material, which may lead to future work in energy harvesting and further properties.

Improved Structural Order and Exciton Delocalization in High-Member Quasi-Two-Dimensional Tin Halide Perovskite Revealed by Electroabsorption Spectroscopy

Engineering of quasi-two-dimensional (quasi-2D) tin halide perovskite structures is a promising pathway to achieve high-performance lead-free perovskite solar cells, with recently developed devices demonstrating over 14% efficiency. Despite the significant efficiency improvement over the bulk three-dimensional (3D) tin perovskite solar cells, the precise relationship between structural engineering and electron-hole (exciton) properties is not fully understood. Here, we study exciton properties in high-member quasi-2D tin perovskite (which is dominated by large n phases) and bulk 3D tin perovskite using electroabsorption (EA) spectroscopy. By numerically extracting the changes in polarizability and dipole moment between the excited and ground states, we show that more ordered and delocalized excitons are formed in the high-member quasi-2D film. This result indicates that the high-member quasi-2D tin perovskite film consists of more ordered crystal orientations and reduced defect density, which is in agreement with the over 5-fold increase in exciton lifetime and much improved solar cell efficiency in devices. Our results provide insights on the structure-property relationship of high-performance quasi-2D tin perovskite optoelectronic devices.

Exploring Bethe-Salpeter Excited-State Dipoles: The Challenging Case of Increasingly Long Push-Pull Oligomers.

The change of molecular dipole moment induced by photon absorption is key to interpret the measured optical spectra. Except for compact molecules, time-dependent density functional theory (TD-DFT) remains the only theory allowing to quickly predict excited-state dipoles (μES), albeit with a strong dependency on the selected exchange-correlation functional. This Letter presents the first assessment of the performances of the many-body Green's function Bethe-Salpeter equation (BSE) formalism for the evaluation of the μES. We explore increasingly long push-pull oligomers as they present an excited-state nature evolving with system size. This work shows that BSE's μES do present the same evolution with oligomeric length as their CC2 and CCSD counterparts, with a dependency on the starting exchange-correlation functional that is strongly decreased as compared to TD-DFT. This Letter demonstrates that BSE is a valuable alternative to TD-DFT for properties related to the excited-state density and not only for transition energies and oscillator strengths.

Realizing Photoswitchable Mechanoluminescence in Organic Crystals Based on Photochromism.

Organic mechanoluminescent (ML) materials possessing photophysical properties that are sensitive to multiple external stimuli have shown great potential in many fields, including optic and sensing. Particularly, the photoswitchable ML property for these materials is fundamental to their applications but remains a formidable challenge. Herein, photoswitchable ML is successfully realized by endowing reversible photochromic properties to an ML molecule, namely 2-(1,2,2-triphenylvinyl) fluoropyridine (o-TPF). o-TPF shows both high-contrast photochromism with a distinct color change from white to purplish red, as well as bright blue ML (λML = 453nm). The ML property can be repeatedly switched between ON and OFF states under alternate UV and visible light irradiation. Impressively, the photoswitchable ML is of high stability and repeatability. The ML can be reversibly switched on and off by conducting alternate UV and visible light irradiation in cycles under ambient conditions. Experimental results and theoretical calculations reveal that the change of dipole moment of o-TPF during the photochromic process is responsible for the photoswitchable ML. These results outline a fundamental strategy to achieve for the control of organic ML and pave the way to the development of expanded smart luminescent materials and their applications.

Encapsulating Ne inside C20H20. A molecular dynamics study on the stability of the endohedral system

The central collision between a neon atom and a dodecahedrane cage has been investigated within the framework of ab initio Hartree-Fock molecular dynamics simulations for various initial velocities of the incident atom. Four scenarios regarding the interaction’s outcome have been identified. A reaction channel involving the encapsulation of the projectile inside the molecular cage has been studied for a simulation time of 4.8 ps. The analysis is focused on: the rate of energy exchange between Ne and C20H20, the mechanical oscillation frequencies of the atom and the cage, and also, on the frequency with which the dipole moment changes. The results suggest that inside the molecular cage, neon is a stable atomic vibrator, which oscillates with the highest peak frequency of 16 THz. Furthermore, the same frequency characterizes the dipole moment oscillation, suggesting a strong coupling between the atom and the cage, making the endohedral system suitable for a wide range of applications in nano-electronics.

Effect of the π-bridge structure on the intramolecular charge transfer of push–pull 2-phenylthiophene and 2-(furan-2-yl)pyridine derivatives

Effect of the bridge structure on the photophysical properties of D-π-A of structurally similar 2,5-diphenylthiophene (cpd. I) and 2-(5-phenylfuran-2-yl)pyridine (cpd. 2) derivatives having the same electron donating and electron withdrawing groups were studied in different solvents. Solvent effect on the absorption and fluorescence emission spectra as well as their excited state decay were studied in detail. Introduction of 2,5-diphenylthiophene and 2-(5-phenylfuran-2-yl)pyridine between methoxy and carbonitrile groups lead to a shift of about 20 nm in both the absorption and fluorescence emission maxima of the former. Stokes shift of about 90 nm was observed for both compounds in protic solvents and polar aprotic solvents and decreases as the solvent polarity decreases. The solvatochromism in the fluorescence emission spectra was higher than the observed in absorption spectra, indicating higher dipole moment in the excited state than in the ground state for both compounds. DFT and TD-DFT optimized geometry calculations show that (I) is planar in the excited state while (II) is planar in both the ground and excited states reflecting the importance of the bridge structure on the geometry in the ground and excited states. Theoretical calculations of the dipole moment change from the ground to the excited states were consistent with the values obtained from Lippert-Mataga relationship and higher than values obtained from Reichardt-Ravi relationship. It has been found that the fluorescence lifetime of both compounds was found to decrease as the emission energy increases and being higher for (II) than for (I), and also was found to increase as the solvent’s dielectric constant increases. All photophysical properties were found to correlate well with Reichardt’s solvent polarity parameter, ETN. We have employed Kamlet-Taft, Catalán and Laurence et al. multi-parametric relationships to assign the contribution of the specific and non-specific solute–solvent interactions. Non-specific interactions were found to be mostly dominating in all cases.

Coherent exciton coupling in fluorescent protein dimers probed by two-photon polarization ratio

Coherent excitonic coupling of two chromophores in a dimer of a GFP-analogue, VenusA206, was discovered previously by using circular dichroism and a combination of photon antibunching and fluorescence correlation spectroscopy. To further elucidate this effect, here we use a new approach based on the measurements of two-photon polarization ratio (Ω) as a function of excitation wavelength. Theoretically, Ω depends on the angle between the vectors of absorption transition dipole moment and permanent dipole moment change upon excitation. Generally, excitonic coupling of two chromophores should provide a change of this angle. We measured Ω for the tandem dimer of Venus (tdV) with two chromophores per protein chain and, as a control - for the tandem dimer with one of the chromophores knocked out by point mutation (tdVx). Compared to tdVx, we observe in tdV a significant increase of Ω in the red side of the pure electronic transition and a significant decrease of Ω in the blue side. At the same time, the two-photon absorption spectra of the two systems virtually coincide. Our observations are consistent with a model where only a minority of dimers are coupled coherently and the two permanent dipoles are oriented in an oblique head-to-head arrangement with an angle ∼400 between them. For multiphoton microscopy applications, it is interesting to investigate if the coherently coupled GFP dimer can provide a cooperative enhancement of the two-photon cross section, i.e., absorb stronger than two uncoupled monomers. This possibility can come from the “motional narrowing” of excitonic transition and will be discussed.

Effects of donor and acceptor substituents on the photophysics of 4-ethynyl-2,1,3-benzothiadiazole derivatives.

The present work explores the photophysical, electrochemical, and fluorescence polarization properties of a group of π-conjugated phenylethynyl-2,1,3-benzothiadiazole derivatives (BTDs) bearing different electron-donating (ED) or electron-withdrawing (EW) substituents at the para position of the phenylethynyl moiety. The BTDs were synthesized through the Sonogashira cross-coupling reaction between 4-bromo-2,1,3-benzothiadiazole and the respective para-substituted phenylethynyl derivatives. The BTDs with the EW-substituents show relatively weak solvatochromic behavior, while the BTDs with the strong ED-substituents like methoxy and N,N-dimethylamino-based substituents (BTDPhOMe and BTDPhNMe2) exhibit a pronounced solvatochromic behavior. The change in dipole moments in the excited states of the derivatives was calculated using Lippert-Mataga plots. The conclusions drawn on the spectral behavior of the molecules could be rationalized by TD-DFT calculations involving electron density difference (EDD) maps that correlate with the ICT characteristics of the molecules. The experimental and theoretical calculations reveal that the BTDs with the strong ED-substituents (strong push-pull type BTDs) have a strong ICT character in the excited state. These strong push-pull type BTDs show high fluorescence quantum yield (ΦF) in apolar solvents and low ΦF in polar solvents. In contrast, the BTDs with the weak ED-substituents (weak push-pull type BTDs) and EW-substituents (pull-pull type BTDs) have a weaker ICT character with low ΦF in apolar and high ΦF in polar solvent media. There is good a agreement among the HOMO-LUMO band gaps obtained from absorption spectroscopy and electrochemical studies and theoretical calculations. The fluorescence anisotropy measurement in the glycerol medium shows that the studied BTDs generally exhibit higher sensitivity towards microviscosity than the traditional DPH fluorescence anisotropy probe.

Combination of practical and theoretical measurements of albumin egg as an eco-friendly inhibitor for copper corrosion in alkaline solutions.

Utilizing environmentally acceptable substances as inhibitors of metal corrosion is one of the most important strategies to reduce corrosion. In alkaline solutions (1.0 M KOH), the influence of albumin egg as a green corrosion inhibitor for copper was studied via a mix of experimental and theoretical investigations. Cyclic voltammetry (CV), open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), AFM, and SEM/EDX methods were all utilized to examine the inhibitory effect of albumin egg. By increasing the amount of albumin egg in the corrosive solution, the inhibition efficiency is increased. The albumin egg is a highly effective cathodic type inhibitor, according to electrochemical tests, with an inhibition efficiency of up to 94%. It also follows the Langmuir isotherm during adsorption. Investigations using SEM/EDX and AFM show that the albumin egg can create an adsorption layer on the surface enabling the shielding of the copper surface from harmful ions. In order to better understand the molecular structure of the albumin egg and its inhibitory action against corrosion, computational and molecular dynamics simulation techniques were also employed for calculating the electronic characteristics of inhibitor molecules. Calculations were made for total energy (TE), change in total energy (DET), energy gap (ΔE), ELUMO, EHOMO, dipole moment (D), and softness (δ). Utilizing the Monte Carlo simulation, the mechanism of albumin egg adsorption on the surface of Cu was investigated. The theoretical outcomes were found to confirm the empirical results.

How far the chemistry of self-assembled monolayers on gold surfaces affects their work function?

Self-assembled monolayers composed of various long-chain aliphatic molecules and different tail functional groups have been synthesized on the Au(111) surface and characterized by Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Carboxy, amino, thio and methyl terminal groups have been considered in the design of self-assembled monolayers with different aliphatic chain lengths (from C6 to C16). Work function measurements by Kelvin probe force microscopy have been carried out under a controlled and room atmosphere. Remarkably, a reduction of the relative humidity from 40% to 3% has induced a work function shift of up to 0.3 eV. As expected, the changes of the chain length of the aliphatic moiety and of the tail group have a significant impact on the tuning of the measured work function (3.90 eV for dodecanethiol versus 4.57 eV for mercaptohexadecylamine). Surprisingly, the change of the net dipole moment of the tail group (sign and amplitude) does not dominate the work function variations. In contrast, the change of the chain length and the possibility of the tail group to form a complex hydrogen bond network between molecules lead to significant modulations of the work function. In order to interpret these original findings, density functional theory models of equivalent self-assembled monolayers adsorbed on the Au(111) surface have been developed at an unprecedented level of description with large supercells including simultaneously 27 co-adsorbed molecules and weak van der Waals interactions between them. Such large systems have allowed the theoretical modeling of complex hydrogen bond networks between molecules when possible (carboxy tail group). The comparison between computed and measured work functions shows a striking agreement, thus allowing the disentanglement of the previously mentioned competing effects. This consistency between experiment and theory will help in designing the electronic properties of self-assembled monolayers in the context of molecular electronics and organic transistors.

Origin of the superior oxygen reduction activity of zirconium nitride in alkaline media.

The anion exchange membrane fuel cell (AEMFC), which can operate in alkaline media, paves a promising avenue for the broad application of earth-abundant element based catalysts. Recent pioneering studies found that zirconium nitride (ZrN) with low upfront capital cost can exhibit high activity, even surpassing that of Pt in alkaline oxygen reduction reaction (ORR). However, the origin of its superior ORR activity was not well understood. Herein, we propose a new theoretical framework to uncover the ORR mechanism of ZrN by integrating surface state analysis, electric field effect simulations, and pH-dependent microkinetic modelling. The ZrN surface was found to be covered by ∼1 monolayer (ML) HO* under ORR operating conditions, which can accommodate the adsorbates in a bridge-site configuration for the ORR. Electric field effect simulations demonstrate that O* adsorption on a 1 ML HO* covered surface only induces a consistently small dipole moment change, resulting in a moderate bonding strength that can account for the superior activity. Based on the identified surface state of ZrN and electric field simulations, pH-dependent microkinetic modelling found that ZrN reaches the Sabatier optimum of the kinetic ORR volcano model in alkaline media, with the simulated polarization curves being in excellent agreement with the experimental data of ZrN and Pt/C. Finally, we show that this theoretical framework can lead to a good explanation for the alkaline oxygen electrocatalysis of other transition metal nitrites such as Fe3N, TiN, and HfN. In summary, this study proposes a new framework to rationalize and design transition metal nitrides for alkaline ORR.

Modification of optical properties in helicenes via construction of phosphine oxide-based push-pull systems

Six [6]helicene based push-pull systems combining three diarylamines as donors and two phosphine-oxides as acceptors were designed and synthesized in order to modify optical properties of the helicene skeleton. The properties of the target compounds were studied by DFT calculations and spectroscopic methods in both solution and solid film and compared to those of purposely synthesized reference compounds. It was found that the optical properties change substantially upon formation of the push-pull system and show dependence on the withdrawing ability of the acceptor. Furthermore, the introduction of the diphenylaminophenylene group as a donor led to significant increase of fluorescence quantum yield and also a significant dependence of the fluorescence maxima on the polarity of solvent – solvatochromism. The Lippert-Mataga plots were used to estimate the change in dipole moments (Δμ).