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This study investigates the design and analysis of symmetric and asymmetric non-fullerene acceptors (NFAs), focusing on the burgeoning interest in asymmetric NFAs due to their exceptional solar cell properties. Our approach involves modifying the core and introducing two distinct terminal endgroups to the π-core system. Through rigorous first-principle simulations, we systematically explore the solar cell parameters of the designed NFAs when combined with the PM6 polymer. Surprisingly, our results demonstrate that incorporating the A5 acceptor, alongside other end-group acceptors (A1-A5), leads to a significant increase in the difference between ground and excited dipole moment (∆µ), enhanced charge separation rates (kCS), and notably reduced energy losses (< 0.35 eV) compared to other complexes. Furthermore, our findings challenge the conventional wisdom that asymmetric compounds consistently outperform symmetric ones. We identify specific symmetric configurations, particularly those paired with A5 acceptors, that exhibit substantial improvements in solar cell properties. This study emphasizes the critical importance of thoughtful material design, providing valuable insights for researchers striving to develop next-generation small-molecule acceptors for organic solar cells
We present a comprehensive analysis of the structure–property relationship in small molecule non-fullerene acceptors (NFAs) featuring an acceptor–donor–acceptor configuration employing state-of-the-art quantum chemical computational methods. Our focus lies in the strategic functionalization of halogen groups at the terminal positions of NFAs as an effective means to mitigate non-radiative voltage losses and augment photovoltaic and photophysical properties relevant to organic solar cells. Through photophysical studies, we observe a bathochromic shift in the visible region for all halogen-functionalized NFAs, except type-2, compared to the unmodified compound. Most of these functionalized compounds exhibit exciton binding energies below 0.3 eV and ΔLUMO less than 0.3 eV, indicating their potential as promising candidates for organic solar cells. Selected candidate structures undergo an analysis of charge transport properties using the semi-classical Marcus theory based on hopping transport formalism. Molecular dynamics simulations followed by charge transport simulations reveal an ambipolar nature of charge transport in the investigated NFAs, with equivalent hole and electron mobilities compared to the parent compound. Our findings underscore the crucial role of end-group functionalization in enhancing the photovoltaic and photophysical characteristics of NFAs, ultimately improving the overall performance of organic solar cells. This study advances our understanding of the structure–property relationships in NFAs and provides valuable insights into the design and optimization of organic solar cell materials.
Multiresonant thermally activated delayed fluorescence (MR-TADF) emitters have recently attracted great interest for application in organic light-emitting diodes due to their remarkable electroluminescent efficiency and narrow emission spectra. It is therefore essential to establish computational methodologies that can accurately model the excited states of these materials at manageable computational costs. With regard to MR-TADF design and their associated photophysics, previous works have highlighted the importance of wave function-based methods, at much higher computational costs, over the traditional time-dependent density functional theory approach. Herein, we employ two independent techniques built on different quantum mechanical frameworks, highly correlated wave function-based STEOM-DLPNO–CCSD and range-separated double hybrid density functional, TD-B2PLYP, to investigate their performance in predicting the excited state energies in MR-TADF emitters. We demonstrate a remarkable mean absolute deviation (MAD) of ∼0.06 eV in predicting ΔEST compared to experimental measurements across a large pool of chemically diverse MR-TADF molecules. Furthermore, both methods yield superior MAD in estimating S1 and T1 energies over earlier reported SCS-CC2 computed values [J. Chem. Theory Comput. 2022, 18, 4903]. The short-range charge-transfer nature of low-lying excited states and narrow fwhm values, hallmarks of this class of emitters, are precisely captured by both approaches. Finally, we show the transferability and robustness of these methods in estimating rates of radiative and nonradiative events with adequate agreement against experimental measurements. Implementing these cost-effective computational approaches is poised to streamline the identification and evaluation of potential MR-TADF emitters, significantly reducing the reliance on costly laboratory synthesis and characterization processes.
Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials have acquired substantial attention due to their high electroluminescence efficiency with narrow emission spectra. However, the existing MR-TADF emitters suffer from substantial efficiency roll-off due to insufficient rate constants of the reverse intersystem crossing (kRISC) process compared to the traditional TADF materials. Herein, we employ the DLPNO-STEOM-CCSD method, which is computationally less expensive than the wave function-based EOM-CCSD method, to evaluate the electronic and photophysical properties of MR-TADF materials accurately. The predicted singlet–triplet energy gap (ΔEST), one of the critical parameters governing the TADF efficiency, exhibits remarkable agreement with the experimental measurement, with a standard deviation value of 0.026 eV (obtained based on five experimentally synthesized MR-TADF systems). The proposed technique was utilized to determine the suitability of 15 newly designed MR-TADF emitters via their computed radiative and nonradiative rates, luminescence efficiencies, and exciton characteristics. Moreover, most conceived molecules exhibit blue emission with decent to strong oscillator strengths, making them potential candidates for practical light-emitting applications. The proposed computational route will undoubtedly accelerate the designing and prescreening of potential MR-TADF emitters before their expensive laboratory synthesis and characterization.
Contact resistance (RC) in organic devices originates from a mismatch in energy levels between injecting electrodes and organic semiconductors (OSCs). However, the microscopic effects governing charge transfer between electrodes and the OSCs have not been analyzed in detail. We fabricated transistors with different OSCs (PTB7, PCDTBT, and PTB7–Th) and electrodes (MoO3, Au, and Ag) and measured their contact resistance. Regardless of the electrodes, devices with PTB7–Th exhibit the lowest values of RC. To explain the trends observed, first-principles computations were performed on contact interfaces based on the projector operator diabatization method. Our results revealed that differences in energy levels and the electronic couplings between OSCs’ highest occupied molecular orbitals and vacant states on the electrodes influence device RC. Further, based on values obtained from the first-principles, the rate of charge transfer between OSCs and electrodes is calculated and found to correlate strongly with trends in RC for devices with different OSCs. We thus show that device RC is governed by the feasibility of charge transfer at the contact interface and hence determined by energy levels and electronic coupling among orbitals and states located on OSCs and electrodes.
Photoexcitation of noble metal nanoparticles creates surface plasmons which further decay to form energetic charge carriers. These charge carriers can initiate and/or accelerate various chemical processes at nanoparticle surfaces, although the efficiency of such processes remains low as a large fraction of these carriers recombine before they can reach the reaction sites. Thus efficient utilization of these charge carriers requires designing nanostructures that promote the separation of charges and their transport toward the reaction sites. Here we demonstrate that covalently bound surface-coating ligands with suitable orbital alignment can provide electron transport channels boosting hot electron extraction from a gold nanostructure leading to a huge enhancement in the rate of hydrogen evolution reaction (HER) under NIR excitation. A (p)Br-Ph-SH substituted gold nanoprism (AuTP) substrate produced ∼4500 fold more hydrogen compared to a pristine AuTP substrate under 808 nm excitation. Further experimental and theoretical studies on a series of substituted benzene-thiol bound AuTP substrates showed that the extent of the ligand-mediated HER enhancement depends not only on the polarity of the ligand but on the interfacial orbitals interactions.
Interlayers at electrode interfaces have been shown to reduce contact resistance in organic devices. However, there still needs to be more clarity regarding the role of microscopic properties of interlayer functionalized interfaces on device behavior. Here, we show that the impact of functionalized electrodes on device characteristics can be predicted by a few critical computationally derived parameters representing the interface charge distribution and orbital interactions. The significant influences of interfacial orbital interactions and charge distribution over device and interface properties are exhibited. Accordingly, a function is developed based on these parameters that capture their effect on the interface resistance. A strong correlation is observed, such that enhanced orbital interactions and reduced charge separation at the interface correspond to low resistance regardless of the individual molecules utilized as the interlayer. The charge distribution and orbital interactions vary with the molecular structure of the interlayer, allowing the tuning of device characteristics. Hence, the proposed function serves as a guideline for molecular design and selection for interlayers in organic devices.
The molecular engineering of small molecule non-fullerene acceptors (NFAs) is central to enhancing organic solar cell (OSC) performance. One of the effective strategies is the chemical tailoring of the ladder-type central π-core unit of NFAs. Especially boron-nitrogen (B-N) functionalized heterocycles in ladder-type π-cores is considered a promising approach to boost the device performance by regulating energy levels, band gap, and photovoltaic properties of organic materials. Here, we employ a multiscale computational workflow to design acceptor-donor-acceptor (A-D-A) type B-N functionalized NFAs starting from well-defined building blocks representing the donor and acceptor units. Initial assessment of the dataset generated via quantum mechanical calculations revealed that B-N functionalization in the designed NFAs leads to a bathochromic shift in the absorption maximum in the near-infrared region with ∆LUMO below 0.3 eV required for improved solar cell efficiency. Further, crucial threshold parameters are imposed on an initial database of 120 NFAs to screen and identify five potential candidate structures on which molecular dynamics simulations are performed to generate amorphous morphologies. Charge transport simulations on these morphologies exhibit ambipolar character with superior mobilities for holes and electrons compared to the parent molecule. Our design principle guides us in identifying novel NFAs with promising photovoltaic characteristics and highlights that precisely manipulating boron-nitrogen functionalization is a possible way toward high-efficiency OSCs.
We investigated the optoelectronic and photovoltaic properties of three types of acceptor-donoracceptor-based non-fullerene acceptor (NFA) molecules for organic solar cell (OSC) applications. Density functional theory and its time-dependent variant were employed to compute the quadrupole moment perpendicular to π-system (Q20), open circuit voltage (VOC), and other relevant solar cell parameters. The role of functionalization in the acceptor unit on the overall device performance was explored by incorporating halogen and methoxy-based electron-withdrawing groups. The electronegativity differences between halogen atoms and the methoxy group demonstrated contrasting effects on the energy levels, molecular orbitals, and absorption maximum. We observed a trade-off between short-circuit current (JSC) and VOC, which was further substantiated by an inverse correlation between Q20 and VOC. We found an optimum value of Q20 in the range 80 to 130 ea02 to achieve an optimized solar cell performance. Among the designed systems, Se-derived NFAs with a small band gap, red-shifted absorption maximum, high-oscillator strength, small exciton binding energy, and optimum Q20 turned out to be the potential candidates for future applications. These criteria can be generalized to design and screen next-generation non-fullerene acceptors to achieve improved OSC performance.
Mono metal (Ni, Co)-substituted (in) and supported (on) CeO2 catalysts were prepared by using solution combustion synthesis and formaldehyde reduction methods. The catalysts were completely characterized by both bulk and surface techniques. Both supported and substituted catalysts show distinct differences in the dry reforming of methane (DRM) activity. Co-substituted CeO2 showed the highest stability under the DRM reaction conditions at 800 °C. Detailed kinetic investigations were also carried out to estimate the apparent activation energy. Carbon deposition on the spent catalysts was investigated by thermal gravimetric analysis (TGA) and TEM which shows that the deactivation is due to the presence of amorphous and graphitic carbon. Transient studies on a mass spectrometer indicate that the prominence of the reaction CO2 + C → 2CO is responsible for the catalyst's stability. Surface lattice oxygen reactivity is a vital factor in catalytic stability and its action decides the reaction steps. DFT further verifies that the energy of vacancy formation is significantly lower in Co-substituted CeO2 as compared to Ni-substituted CeO2. This confirms that the Co-substituted catalyst favors oxidation due to higher availability of surface oxygen, while in contrast Ni hinders oxidation by decreasing the availability of surface oxygen for the reaction.
Organelles are the working hubs of the cells. Hence, visualizing these organelles inside the cells is highly important for understanding their roles in pathological states and development of therapeutic strategies. Herein, we report the development of a novel highly substituted oxazoles with modular scaffolds (AIE-ER, AIE-Mito and AIE-Lyso) which can home into endoplasmic reticulum (ER), mitochondria and lysosomes inside the cells. These oxazoles showed remarkable aggregation-induced emission (AIE) property in water and in solid state due to dual intramolecular H-bonding which was confirmed by the pH and temperature dependent fluorescence studies followed by molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Confocal laser scanning microscopy studies revealed that AIE-ER, AIE-Mito and AIE-Lyso efficiently homed into ER, mitochondria and lysosomes respectively in the HeLa cervical cancer cells and non-cancerous human retinal pigment epithelial RPE-1 cells within 3h without showing any toxicity to the cells with high sub-cellular photostability. To the best of our knowledge, this is the first report of highly substituted oxazole-based small molecule AIEgens for organelle imaging. We anticipate these novel AIEgens have promise to image sub-cellular organelles in different diseased states as well as understanding the inter-organelle interactions towards the development of novel therapeutics.
Mitochondrion has been implicated in the development and progression of breast cancer, making it an unconventional target for the anti-cancer therapy. As a result, there is a need to explore novel small molecules for next generation breast cancer therapy. Towards this endeavour, herein, we have identified a novel 3-methoxy-pyrrole-based small molecule from a synthetic library. Confocal fluorescence microscopy studies revealed that, this lead small molecule induced mitochondrial damage and generated reactive oxygen species (ROS) in the MCF7 breast cancer cells. Small molecule based mitochondrial impairment subsequently triggered cell cycle arrest and nuclear DNA damage followed by apoptosis leading remarkable cell killing and inhibition in the colony formation in MCF7 cells. Moreover, this 3-methoxy-pyrrole killed drug resistant MDA-MB-231 triple negative breast cancer cells and OVCAR8 ovarian cancer cells efficiently. This novel small molecule can open a new direction towards mitochondria targeted breast cancer therapy in future.
Zinc(II)porphyrin catalyzed light induced C-H arylation of heteroarenes from anilines is discussed. The method is nontoxic and efficient, using only 0.5 mol % of porphyrin catalyst to produce bi(hetero)aryls in good yields. This work demonstrates the potential use of porphyrin photocatalysts as efficient and robust alternatives to organic dyes.
In this study, we fabricated CuO thin films using the sol-gel spin coating method. The fabricated thin films were utilized for electrocatalytic reduction of CO2 (CO2ER). Fabrication of thin film is vital to provide a large surface area and a more exposed (111) crystal plane for ethanol selectivity. This is verified by comparing it with bulk powder material which does not give such activity. CO2ER over thin film electrode specifically forms CO(g) and Ethanol(l), 2 and 12 electron reduction products, and eliminates the possibility of unwanted HER as a side reaction in the CO2 saturated NaHCO3 electrolyte. We achieved significant product selectivity and faradaic efficiency, utilizing the very low potential for both CO and ethanol. Specific formation of only CO and ethanol makes the process efficient as the separation of gas and liquid is easy. Results based on density functional theory calculations suggest that CuO (111) and CuO (-111) surfaces promote CO2 adsorption and subsequent formation of CO. However, a direct CO-dimerization is observed only on the CuO (111) surface that facilitates the formation of ethanol as the C2 product. This comparative study of bulk and thin-film opens new insight and highlights the importance of catalyst fabrication for the specific product formation utilizing significantly less energy.
We developed a deep potential machine learning model for simulations of chemical reactions in molten alkali carbonate-hydroxide electrolyte containing dissolved CO2, using an active learning procedure. We tested the deep neural network (DNN) potential and training procedure against reaction kinetics, chemical composition, and diffusion coefficients obtained from density functional theory (DFT) molecular dynamics calculations. The DNN potential was found to match DFT results for the structural, transport, and short-time chemical reactions in the melt. Using the DNN potential, we extended the time scales of observation to 2 ns in systems containing thousands of atoms, while preserving quantum chemical accuracy. This allowed us to reach chemical equilibrium with respect to several chemical species in the melt. The approach can be generalized for a broad spectrum of chemically reactive systems.
Endoplasmic reticulum (ER) has emerged as one of the interesting sub-cellular organelles due to its role in myriads of biological phenomena. Subsequently, visualization of the structure-function and dynamics of ER remained a major challenge to understand its involvement in different diseased states including cancer. To illuminate the ER, herein we have designed and synthesized γ-resorcyclic acid-based small molecules which showed remarkable aggregation induced emission (AIE) property in water. This AIE property was originated from the dual intramolecular H-bonding leading to the self-assembled 2D aggregation confirmed by the pH and temperature dependent fluorescence quenching studies as well as scanning electron microscopy. These small molecules illuminated the sub-cellular ER in HeLa cervical cancer cells as well as non-cancerous RPE-1 human retinal epithelial cells within 1h. These novel small molecules have the potential to light up ER chemical biology in diseased states.
Amorphous small-molecule organic materials are utilized in organic light emitting diodes (OLEDs), with device performance relying on appropriate chemical design. Due to the vast number of contending materials, a symbiotic experimental and simulation approach would be greatly beneficial in linking chemical structure to macroscopic material properties. We review simulation approaches proposed for predicting macroscopic properties. We then present a library of OLED hosts, containing input files, results of simulations, and experimentally measured references of quantities relevant to OLED materials. We find that there is a linear proportionality between simulated and measured glass transition temperatures, despite a quantitative disagreement. Computed ionization energies are in excellent agreement with the ultraviolet photoelectron and photoemission spectroscopy in air measurements. We also observe a linear correlation between calculated electron affinities and ionization energies and cyclic voltammetry measurements. Computed energetic disorder correlates well with thermally stimulated luminescence measurements and charge mobilities agree remarkably well with space charge–limited current measurements. For the studied host materials, we find that the energetic disorder has the greatest impact on the charge carrier mobility. Our library helps to swiftly evaluate properties of new OLED materials by providing well-defined structural building blocks. The library is public and open for improvements. We envision the library expanding and the workflow providing guidance for future OLED material design.
We performed ab initio molecular dynamics simulations of a molten [Li0.6K0.4]3CO3OH electrolyte containing dissolved CO2 and confirmed the presence of pyrocarbonate, bicarbonate, and water along with the constituent ions and molecular CO2. Our calculations indicate kinetics-driven formation of pyrocarbonate whereas bicarbonate and water are thermodynamically favored. Our results also demonstrate the presence of water at higher concentrations (double or more) than that of CO2, which reinforces the conclusions in our earlier work [AIChE J.2020, e16988] based on chemical reaction equilibrium simulations. Structural analysis indicates a larger distortion in water geometry, due to its higher polarizability compared to the nonpolar CO2, explaining the higher reactivity and smaller average lifetime of H2O in the melt. The computed lifetime distributions of the reaction products reveal that the bicarbonate ion lives the shortest among all the species present in the system. It initiates a sequence of successive proton exchange events; such sequences of exchanges along a hydrogen-bonded network gives the Grotthuss mechanism for proton transport in liquid water. The estimated proton diffusion, based on a random walk model, is about 30 times faster than the hydroxide diffusion obtained from classical molecular dynamics simulations. We believe that the presence of proton transfer events in the system has a large impact on the overall ion dynamics and electrical conductivity of the medium.
It has been recently suggested that hydroxide ions can be formed in the electrolyte of molten carbonate fuel cells when water vapor is present. The hydroxide can replace carbonate in transporting electrons across the electrolyte, thereby reducing the CO2 separation efficiency of the fuel cell although still producing electricity. In this work, we obtain the equilibrium concentration of hydroxide in five molten alkali carbonate salts from molecular simulations. The results reveal that there can be a substantial amount of hydroxide in the electrolyte at low partial pressures of CO2. In addition, we find that the equilibrium concentration of molecular water dissolved in the electrolyte is over two orders of magnitude higher than that of CO2. Increasing the size and polarizability (or in other words reducing the "hardness") of the cations present in the electrolyte can reduce the hydroxide fraction, but at the cost of lowering ionic conductivity.
We use molecular dynamics simulations based on a recently developed force field to obtain the viscosity, ionic conductivity, and liquid–vapor surface tension of molten alkali-metal carbonate–hydroxide mixtures over a range of cation and hydroxide compositions. Recent experimental and simulation studies have suggested that molten carbonates contain non-negligible amounts of hydroxide ions in the presence of water at low partial pressures of CO2. However, due to the high temperatures (~600 0C or higher) required to melt pure alkali carbonates and their mixtures, there is a lack of experimental thermodynamic and transport data for these molten phases. Here, we deploy a recently parametrized force field for molten alkali carbonates and hydroxides [ J. Chem. Theory Comput. 2020, 16, 5736−5746, DOI: 10.1021/acs.jctc.0c00285] to simulate some physical properties using atomistic molecular dynamics. Our predictions show a consistent decrease in viscosity and an increase in ionic conductivity with increasing hydroxide fractions, whereas a higher Li+ mole fraction leads to an increase in both viscosity and ionic conductivity. The computed surface tension values exhibit an upward trend with higher asymmetry in cation and anion sizes. Structural analysis suggests that in the carbonate–hydroxide melts the smaller/harder ions, OH- and Li+, are more favored at the interface than the larger/softer ions, CO32- and K+/Na+. The current approach provides a systematic route to obtaining physicochemical properties of molten alkali carbonate and hydroxide mixtures over a large domain of chemical compositions and thus can steer future experimental research for molten carbonates and their applications.
Scaled-charge models have been recently introduced for molecular simulations of electrolyte solutions and molten salts to attempt to implicitly represent polarizability. Although these models have been found to accurately predict electrolyte solution dynamic properties, they have not been tested for coexistence properties, such as the vapor pressure of the melt. In this work, we evaluate the vapor pressure of a scaled-charge sodium chloride (NaCl) force field and compare the results against experiments and a non-polarizable full-charge force field. The scaled-charge force field predicts a higher vapor pressure than found in experiments, due to its overprediction of the liquid-phase chemical potential. Reanalyzing the trajectories generated from the scaled-charge model with full charges improves the estimation of the liquid-phase chemical potential but not the vapor pressure.
Organic semiconductors, which serve as the active component in devices, such as solar cells, light-emitting diodes and field-effect transistors, often exhibit highly unipolar charge transport, meaning that they predominantly conduct either electrons or holes. Here, we identify an energy window inside which organic semiconductors do not experience charge trapping for device-relevant thicknesses in the range of 100 to 300 nm, leading to trap-free charge transport of both carriers. When the ionization energy of a material surpasses 6 eV, hole trapping will limit the hole transport, whereas an electron affinity lower than 3.6 eV will give rise to trap-limited electron transport. When both energy levels are within this window, trap-free bipolar charge transport occurs. Based on simulations, water clusters are proposed to be the source of hole trapping. Organic semiconductors with energy levels situated within this energy window may lead to optoelectronic devices with enhanced performance. However, for blue-emitting light-emitting diodes, which require an energy gap of 3 eV, removing or disabling charge traps will remain a challenge.
Unicolored phosphor‐sensitized fluorescence (UPSF) is a dual emitting concept proposed for improving efficiencies and operational lifetimes of blue organic light emitting diodes (OLEDs). To overcome the limitations of the individual emitters, it uses a phosphorescent donor to sensitize a fluorescent acceptor. To quantify the potential of the concept, a multiscale model of a UPSF OLED is developed. It starts from atomistic morphologies, the rates of all processes on the available experimental data are parameterized, and the respective master equation is solved with the help of the kinetic Monte Carlo algorithm. The simulations show that the energy transfer between donor molecules is essential to reproduce the results of the time‐resolved photoluminescence experiment. The scope of the experiment is expanded by studying the effect of the acceptor concentration, as well as Förster and (parasitic) Dexter energy transfer from the donor to acceptor, on the characteristics of the UPSF OLED. The study shows that an appropriate material design can further improve efficiency by more than 30% and at the same time achieve radiative decay times below 0.02 µs, thus significantly extending OLED operational lifetime.
Electron trapping is a well-recognized issue in organic semiconductors, in particular in conjugated polymers, leading to a significant electron mobility reduction in materials with electron affinities smaller than 4 eV. Space-charge limited current measurements in diodes indicate that these traps have similar molecular origin, while calculations show that hydrated molecular oxygen is a plausible molecular candidate, with the tail of the solid-state electron affinity distribution reaching values as high as 4 eV. By decreasing the trap density by mixing conjugated polymers with an insulating polymer matrix, one can fill the traps with charges and hence eliminate their effect on electron mobility. Trap dilution not only improves transport but also reduces trap-assisted recombination, boosting the efficiency of polymer light emitting diodes.
A series of conjugated polymers comprising polythiophene, polyselenophene, and polytellurophene with branched 3,7‐dimethyloctyl side chains, well‐matched molecular weight, dispersity, and regioregularity is synthesized. The ionization potential is found to vary from 5.14 to 5.32 eV, with polytellurophene having the lowest potential. Field‐effect transistors based on these materials exhibit distinct hole transport mobility that varies by nearly three orders of magnitude, with polytellurophene having the highest mobility (2.5 × 10−2 cm2 V−1 s−1). The large difference in mobility demonstrates the significant impact of heteroatom substitution. Although the series of polymers are very similar in structure, their solid‐state properties are different. While the thin film microstructure of polythiophene and polyselenophene is identical, polytellurophene reveals globular features in the film topography. Polytellurophenes also appear to be the least crystalline, even though their charge transport properties are superior to other samples. The torsional barrier and degree of planarity between repeat units increase as one moves down group‐16 elements. These studies show how a single atom in a polymer chain can have a substantial influence on the bulk properties of a material, and that heavy group‐16 atoms have a positive influence on charge transport properties when all other variables are kept unchanged.
Amorphous organic semiconductors based on small molecules and polymers are used in many applications, most prominently organic light emitting diodes (OLEDs) and organic solar cells. Impurities and charge traps are omnipresent in most currently available organic semiconductors and limit charge transport and thus device efficiency. The microscopic cause as well as the chemical nature of these traps are presently not well understood. Using a multiscale model we characterize the influence of impurities on the density of states and charge transport in small-molecule amorphous organic semiconductors. We use the model to quantitatively describe the influence of water molecules and water-oxygen complexes on the electron and hole mobilities. These species are seen to impact the shape of the density of states and to act as explicit charge traps within the energy gap. Our results show that trap states introduced by molecular oxygen can be deep enough to limit the electron mobility in widely used materials.
Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices, such as organic light-emitting diodes, solar cells, and field-effect transistors. Here, we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV.
Improving lifetimes and efficiencies of blue organic light-emitting diodes is clearly a scientific challenge. Towards solving this challenge, we propose a unicolored phosphor-sensitized fluorescence approach, with phosphorescent and fluorescent emitters tailored to preserve the initial color of phosphorescence. Using this approach, we design an efficient sky-blue light-emitting diode with radiative decay times in the submicrosecond regime. By changing the concentration of fluorescent emitter, we show that the lifetime is proportional to the reduction of the radiative decay time and tune the operational stability to lifetimes of up to 320 h (80% decay, initial luminance of 1000 cd/m2). Unicolored phosphor-sensitized fluorescence provides a clear path towards efficient and stable blue light-emitting diodes, helping to overcome the limitations of thermally activated delayed fluorescence.
Amorphous small‐molecule hole‐transporting materials are commonly used in organic light‐emitting diodes and perovskite solar cells. Characterization of their main functionality, hole transport, has been complicated by the presence of large contact barriers. Using a recently developed technique to establish Ohmic hole contacts, the bulk hole transport in a series of molecules with a broad range of ionization energies is investigated. The measured charge‐carrier mobility dependence on charge concentration, electric field, and temperature is used to extract the energetic disorder and molecular site spacing. Excellent agreement of these parameters as well as ionization energies with multiscale simulations paves the way to predictive charge‐transport simulations from the molecular level.
The extent of charge transfer between the cation and the anion in a room-temperature ionic liquid depends on the basicity of the anion. Ion charges determined in the condensed state via density functional theory calculations capture this effect rather well, and charges derived in such a manner have been employed in force field-based molecular dynamics simulations to quantitatively reproduce several physical properties of the liquids. However, the issue of transferability of cation charges in mixtures of ionic liquids, say with one type of cation and two different anion types needs to be addressed. Herein, we demonstrate that the cation charge in such a mixture varies linearly with anion composition, a result that ties in rather well with X-ray photoelectron spectroscopic experiments. The variation in cation charge with bulk anion composition is shown to be a result of changes in its coordination environment. Cations surrounded by a higher proportion of more basic anions possess lower charges than those surrounded by less basic anions. Time scales for the exchange of anion types for the occupation of hydrogen bonding sites around the cation have been determined and are seen to be constituted by three processes–breakage of existing hydrogen bond, diffusion to the hydrogen bonding site and displacement of the incumbent anion from its site in the cation coordination shell. These time scales explain the differences observed between infrared and NMR spectroscopic experiments in ionic liquid mixtures rather well.
The variation of the center atom in the cation from an N to a P-atom leads to improved physiochemical properties of protic ionic liquids (PILs) which are suitable for electrolyte applications. We present an atomistic simulations study to compare the effect of an alkyl or aryl group on trioctylammonium triflate ([HN(Oct)3][TFO]) and triphenylammonium triflate ([HN(Ph)3][TFO]) with their phosphonium analogues. We have computed the binding energy from quantum chemical calculations and physical properties such as the viscosity and the electrical conductivity of PILs from molecular dynamics simulations. The influence of the aromatic character in PILs is found to be significant to the physical properties. Gas phase quantum chemical calculations on clusters of ion pairs have revealed the presence of C–H/π interactions in aromatic PILs along with hydrogen bonding. The variation in strength of the ion-pair affinities is examined using electric-current correlation and velocity autocorrelation functions. The qualitative differences observed are due to the aromatic rings and change in the central atom of the quaternary cation from an N to a P-atom, substantiated quantitatively by diffusion coefficients and electrical conductivities. The relatively weaker ion-pair interactions and low binding energy (−73.34 kcal mol−1) lead to the highest electrical conductivity in [HP(Ph)3][TFO].
Molecular modeling of environmentally benign solvents is essential for a microscopic understanding of interactions, processes and phenomena which contribute to their properties in bulk and at interfaces. Challenges in their modeling arise from their constitution – mixing of organic and inorganic components, their nano and possibly mesostructural order, and the quantum nature of interactions. This short review provides an overview of recent work on four categories of such solvents – room temperature ionic liquids, supercritical carbon dioxide, organic carbonates and deep eutectic solvents, with emphasis on their applications in electrochemistry and biomolecular dissolution.
We elucidate the dynamics and mechanism of proton transport in a protic organic ionic plastic crystal (POIPC) [TAZ][pfBu] by means of Born–Oppenheimer molecular dynamics simulations at 400 K and zero humidity. The arrangement of ionic species in the crystal offers a two-dimensional hydrogen bond network along which an acidic proton can travel from one cation to another through a sequence of molecular reorientations. The results suggest spontaneous autodissociation of the N–HN bond in the cation and multiple proton shuttle events from the cation’s nitrogen to the anion’s oxygen site in a native crystal. A complete proton transfer event is observed in simulations of a defective crystal with a single proton hole created in the cation. The barrier for proton transfer is determined using ab initio metadynamics simulations to be 7 kcal/mol, in agreement with experimental conductivity data. Using gas phase quantum chemical calculations, we propose [TAZ][CF3CF2CH2CF2SO3] as a compound that can show enhanced conductivity compared to that of [TAZ][pfBu].
Ionic liquids have generated interest for efficient SO2 absorption due to their low vapor pressure and versatility. In this work, a systematic investigation of the structure, thermodynamics, and dynamics of SO2 absorption by ionic liquids has been carried out through quantum chemical calculations and molecular dynamics (MD) simulations. MP2 level calculations of several ion pairs complexed with SO2 reveal its preferential interaction with the anion. Results of condensed phase MD simulations of SO2–IL mixtures manifested the essential role of both cations and anions in the solvation of SO2, where the solute is surrounded by the "cage" formed by the cations (primarily its alkyl tail) through dispersion interactions. These structural effects of gas absorption are substantiated by calculated Gibbs free energy of solvation; the dissolution is demonstrated to be enthalpy driven. The entropic loss of SO2 absorption in ionic liquids with a larger anion such as [NTf2]− has been quantified and has been attributed to the conformational restriction of the anion imposed by its interaction with SO2. SO2 loading IL decreases its shear viscosity and enhances the electrical conductivity. This systematic study provides a molecular level understanding which can aid the design of task-specific ILs as electrolytes for efficient SO2 absorption.
Critical aspects of thermal behavior and the electrolytic properties of solid-state Protic Organic Ionic Plastic Crystals (POIPCs) are unknown. We present molecular dynamics (MD) simulations on a perfect crystal and a vacancy model to probe such physical phenomena in POIPCs using 1,2,4-triazolium perfluorobutanesulfonate ([TAZ][pfBu]) as an example. The results show the existence of a rotator phase wherein the cations, although translationally ordered are disordered rotationally and exhibit a tumbling motion which significantly affects hydrogen bond lifetimes. van Hove correlation functions characterize the concerted hopping of ions (cation or anion) at 500 K. These results are substantiated by calculated free energy barriers (cation = 2.5 kcal mol−1 and anion = 6 kcal mol−1 ) and suggest that proton and ion transport influenced by facile hydrogen bond dynamics in the rotator phase contribute to the solid-state conductivity of POIPCs.
Ionic liquids are appropriate candidates for the absorption of acid gases such as SO2. Six anion-functionalized ionic liquids with different basicities have been studied for SO2 absorption capacity by employing quantum chemical calculations and molecular dynamics (MD) simulations. Gas phase quantum calculations unveil that the high uptake of SO2 in these ionic liquids originates from the basicity of the anions and the consequent enhanced anion-SO2 interactions. MD simulations of SO2–IL mixtures reveal the crucial role of both cations and anions in SO2 dissolution. Multiple-site interactions of SO2 with the anions have been identified. The calculated solvation free energy substantiates these observations. The order of computed Henry’s law constant values with change in the anion is in fair agreement with experimentally determined SO2 solubility order.
A refined set of potential parameters for 1-alkyl-3-methylimidazolium based room temperature ionic liquids with anions such as acetate, dicyanamide, and thiocyanate has been obtained. Site charges of ions were derived through the density-derived electrostatic and charge partitioning (DDEC/c3) method utilizing periodic density functional theory calculations of these liquids. Intermolecular structure and dynamics, in particular, the collective quantities predicted by the refined force field, match experimental results quantitatively.
Hydrogen bonding in alkylammonium based protic ionic liquids was studied using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Normal-mode analysis within the harmonic approximation and power spectra of velocity autocorrelation functions were used as tools to obtain the vibrational spectra in both the gas phase and the crystalline phases of these protic ionic liquids. The hydrogen bond vibrational modes were identified in the 150–240 cm–1 region of the far-infrared (far-IR) spectra. A blue shift in the far-IR mode was observed with an increasing number of hydrogen-bonding sites on the cation; the exact peak position is modulated by the cation–anion hydrogen bond strength. Sub-100 cm–1 bands in the far-IR spectrum are assigned to the rattling motion of the anions. Calculated NMR chemical shifts of the acidic protons in the crystalline phase of these salts also exhibit the signature of cation–anion hydrogen bonding.
Protic ionic liquids (PILs) are of great interest as electrolytes in various energy applications. Molecular dynamics simulations of trialkylammonium (with varying alkyl group such as methyl, ethyl, and n-propyl) triflate PILs are performed to characterize the influence of the alkyl group on the acidic site (N–H) of the ammonium cation. Spatial distribution function of anions over this site on the cation reveals significant influence of the length of alkyl tail on intermolecular structure. Vibrational density of states and normal modes are calculated for bulk liquids to probe atomic displacements in the far infrared region. The observed N–H⋯O hydrogen bond stretching vibration in 155–165 cm−1 frequency region agrees well with experiments. Trends in electrical conductivity calculated using Nernst–Einstein and Green–Kubo relation are in qualitative agreement with experiments. The self-diffusion coefficient and the electrical conductivity is highest for N,N-dimethyl-N-ethylammonium triflate ([N112][TfO]) and is lowest for N,N-di-n-propyl-N-methylammonium triflate ([N133][TfO]) IL.
The dissolution of cellulosic biomass in room temperature ionic liquids (RTILs) is studied through free energy calculations of its monomer, viz., cellobiose, within a molecular dynamics simulation approach. The solvation free energy (SFE) of cellobiose in ionic liquids containing any of seven different anions has been calculated. The ranking of these liquids based on SFE compares well with experimental data on the solubility of cellulose. The dissolution is shown to be enthalpically dominated, which is correlated with the strength of intermolecular hydrogen bonding between cellobiose and the anions of the IL. Large entropic changes upon solvation in [CF3SO3]− and [OAc]− based ionic liquids have been explained in terms of the solvent-aided conformational flexibility of cellobiose.
Quantitative prediction of physical properties of room temperature ionic liquids through nonpolarizable force field based molecular dynamics simulations is a challenging task. The challenge lies in the fact that mean ion charges in the condensed phase can be less than unity due to polarization and charge transfer effects whose magnitude cannot be fully captured through quantum chemical calculations conducted in the gas phase. The present work employed the density-derived electrostatic and chemical (DDEC/c3) charge partitioning method to calculate site charges of ions using electronic charge densities obtained from periodic density functional theory (DFT) calculations of their crystalline phases. The total ion charges obtained thus range between −0.6e for chloride and −0.8e for the PF6 ion. The mean value of the ion charges obtained from DFT calculations of an ionic liquid closely matches that obtained from the corresponding crystal thus confirming the suitability of using crystal site charges in simulations of liquids. These partial charges were deployed within the well-established force field developed by Lopes et al., and consequently, parameters of its nonbonded and torsional interactions were refined to ensure that they reproduced quantum potential energy scans for ion pairs in the gas phase. The refined force field was employed in simulations of seven ionic liquids with six different anions. Nearly quantitative agreement with experimental measurements was obtained for the density, surface tension, enthalpy of vaporization, and ion diffusion coefficients.
Transport properties of five room-temperature ionic liquids based on the 1-butyl-3-methylimidazolium cation with any of the following anions, [PF6]−, [BF4]−, [CF3SO3]−, [NTf2]−, and [NO3]−, were determined from classical molecular dynamics simulations. The force field employed fractional ion charges whose magnitude were determined using condensed phase quantum calculations. Integrals of appropriate equilibrium time correlation functions within the Green–Kubo approach were employed to predict shear viscosity and electrical conductivity of these liquids. Computed shear viscosity values reproduce experimental data with remarkable accuracy. Electrical conductivity calculated for [BMIM][PF6] and [BMIM][BF4] showed impressive agreement with experiment while for [BMIM][CF3SO3] and [BMIM][NTf2] the agreement is fair. The current approach shows considerable promise in the prediction of collective transport quantities of room temperature ionic liquids from molecular simulations.
Tri-n-butyl phosphate (TBP) is an important extractant for heavy metal ions. The microscopic structure of TBP/n-octane mixtures as a function of concentration of TBP is examined through atomistic molecular dynamics simulations. A weak association between TBP molecules both in pure TBP as well as in the octane solution is established. In dilute TBP/n-octane solutions, TBP molecules are inhomogeneously distributed. Structural results from simulations are compared with experimental X-ray and neutron scattering data. Features are assigned through calculations of partial structure factors.