Preferred Molecular Orientation Research Articles

Thermally activated delayed fluorescence tetradentate ligand-containing gold(III) complexes with preferential molecular orientation and their application in organic light-emitting devices.

A new class of thermally activated delayed fluorescence (TADF) pyridine-/pyrazine-containing tetradentate C^C^N^N gold(III) complexes have been designed and synthesized. Displaying photoluminescence quantum yields (PLQYs) of up to 0.77 in solid-state thin films, these complexes showed at-least a six-fold increase in the radiative decay rate constant (kr) in toluene upon increasing temperature from 210 to 360 K. Using variable-temperature (VT) ultrafast transient absorption (TA) spectroscopy, the reverse intersystem crossing (RISC) processes were directly observed and the activation parameters were determined, in line with the results of the Boltzmann two-level model fittings, in which the energy separation values between the lowest-lying singlet excited state (S1) and the lowest-lying triplet excited state (T1), ΔE(S1-T1), of these complexes were estimated to be in the range of 0.16-0.18 eV. Through strategic modification of the position of the electron-donating -tBu substituent in the cyclometalating ligand, the permanent dipole moments (PDMs) of these tetradentate gold(III) emitters could be manipulated to enhance their horizontal alignment in the emitting layer of organic light-emitting devices (OLEDs). Consequently, the resulting vacuum-deposited OLEDs demonstrated a 30% increase in the theoretical out-coupling efficiency (ηout), as well as promising electroluminescence (EL) performance with maximum external quantum efficiencies (EQEs) of up to 15.7%.

Composition Dictates Molecular Orientation at the Heterointerfaces of Vapor-Deposited Glasses.

Physical vapor deposition (PVD) can prepare organic glasses with a preferred molecular orientation. The relationships between deposition conditions and orientation have been extensively investigated in the film bulk. The role of interfaces on the structure is less well understood and remains a key knowledge gap, as the interfacial region can govern glass stability and optoelectronic properties. Robust experimental characterization has remained elusive due to complexities in interrogating molecular organization in amorphous, organic materials. Polarized soft X-rays are sensitive to both the composition and the orientation of transition dipole moments in the film, making them uniquely suited to probe molecular orientation in amorphous soft matter. Here, we utilize polarized resonant soft X-ray reflectivity (P-RSoXR) to simultaneously depth profile the composition and molecular orientation of a bilayer prepared through the physical vapor deposition of 1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene (DSA-Ph) on a film of aluminum-tris(8-hydroxyquinoline) (Alq3). The bulk orientation of the DSA-Ph layer is controlled by varying deposition conditions. Utilizing P-RSoXR to depth profile the films enables determination of both the bulk orientation of DSA-Ph and the orientation near the Alq3 interface. At the Alq3 surface, DSA-Ph always lies with its long axis parallel to the interface, before transitioning into the bulk orientation. This is likely due to the lower mobility and higher glass transition of Alq3, as the first several monolayers of DSA-Ph deposited on Alq3 appear to behave as a blend. We further show how orientation at the interface correlates with the bulk behavior of a codeposited glass of similar blend composition, demonstrating a straightforward approach to predicting molecular orientation at heterointerfaces. This work provides key insights into how molecules orient during vapor deposition and offers methods to predict this property, a critical step toward controlling interfacial behavior in soft matter.

Hydrogen diffusion in phase clathrate hydrate

Recently, a new phase H2 hydrate was experimentally identified. In this work, the diffusive behaviors of H2 in phase clathrate hydrate are explored using classic molecular dynamics (MD) simulations. It reveals that the cage occupancy by H2 molecule negligibly influences the phase clathrate structure but greatly dictates the diffusion coefficient of H2 molecule. Due to the small cage size and small windows connecting the neighboring cages in phase clathrate, non-occupancy of the neighboring cages is demanded to enable the diffusion of H2 molecule that is primarily dominated by hopping mechanism. Moreover, the analysis of diffusive free energy landscape reveals lower energy barrier of H2 molecule in phase clathrate hydrate than that of other gases in conventional clathrate hydrates, and that H2 molecule travels through the windows between neighboring cages with preferential molecular orientation. This study provides critical physical insights into the diffusion behaviors of H2 in the phase clathrate hydrate, and implies that the clathrate hydrate is a promising solid structure for the next-generation H2 storage.

Interaction of Cyanoacrylate with Metal Oxide Surfaces (Cu, Al).

Cyanoacrylates are an extremely reactive class of adhesives. Despite their commercial use as instant adhesives, the adhesion mechanism, especially to technically relevant oxidized metal surfaces, has not yet been sufficiently investigated. In the present work, ultra-thin ethyl cyanoacrylate films are deposited on copper oxide and aluminum oxide by spin coating and cured there. Various surface sensitive spectroscopy methods are used to identify possible interactions. X-Ray photoelectron spectroscopy (XPS) indicates, among other information, hydrogen bonding of the carbonyl group to the oxidized surfaces. Metastable induced electron spectroscopy (MIES) measurements support the theory of this preferential molecular orientation. In addition, XPS shows the presence of an ionic carboxylate (COO- ) species at the interface. Infrared reflection adsorption spectroscopy (IRRAS) measurements confirm this ionic interaction and furthermore allow to investigate the influence of water on the reaction. A possible interaction mechanism of cyanoacrylates with metal oxides could be proposed. The formation of a carboxylate species probably occurs by hydrolysis of the ethyl group via the intermediate of a carboxyl (COOH) species.

Overcoming Disordered Preaggregation in Liquid State for Highly Efficient Organic Solar Cells Printed from Nonhalogenated Solvents

AbstractThe current power conversion efficiencies of laboratory‐sized organic solar cells (OSCs), based on the spin‐coating process with halogenated solvents, have exceeded 19%. Environmentally friendly printing is needed to bridge the gap between laboratory and industrialization by being compatible with roll‐to‐roll large‐area production. Here, the molecular design rules are revealed for enhancing the green printing potential of the state‐of‐the‐art photovoltaic martial systems by investigating the detailed structure formation dynamic and the key determining factors. By comparing two model systems based on D18:Y6 and D18:BTP‐eC9, it is found that disordered preaggregation in liquid state can result in over‐sized domains with reduced crystallinity and disordered molecular orientation, which significantly limits device performance. By systematically tuning the length of the inner alkyl side chains with multiple Y‐series materials, the authors demonstrate that molecular side‐chain engineering can effectively supress the detrimental disordered preaggregation in liquid state during environmentally friendly printing process, leading to enhanced crystallization with preferential faceon molecular orientation, more efficient exciton dissociation and charge carrier transport, and finally high upscaling potential. The work provides deeper insights into molecular engineering and structure formation dynamics toward environmentally friendly production of OSCs.

Preparation and characterization of polythiophene/gold nanoparticles/carbon nanotubes nanocomposites thin films: Spectroscopy and morphology

Hybrid nanocomposite films containing polythiophene/gold nanoparticles and polythiophene/gold nanoparticles/carbon nanotubes, synthesized through a liquid-liquid interfacial polymerization route at different reaction times and amounts of carbon nanotubes, were investigated through spectroscopic and morphological techniques. X-ray photoelectron spectroscopy showed the formation of polythiophene aggregates and the presence of large gold particles, corroborating the results of transmission and scanning electron microscopy. Near-edge X-ray absorption fine structure showed films without any preferential molecular orientation of the polymer chains. The bandgap obtained by reflection electron energy loss spectroscopy is reduced with the increase in the synthesis reaction time and the decrease in the amount of carbon nanotubes, which is followed by ultrafast electron dynamics studied with the core-hole clock method. These films were heat-treated at 200 °C for 30 min and faster charge transfer times were observed. Current x Voltage curves showed a photoactive response of the materials.

Thermally activated delayed fluorescence materials for nondoped organic light‐emitting diodes with nearly 100% exciton harvest

AbstractHigh‐performance nondoped organic light‐emitting diodes (OLEDs) are promising technologies for future commercial applications. Herein, we synthesized two new thermally activated delayed fluorescence (TADF) emitters that enable us, for the first time, to combine three effective approaches for enhancing the efficiency of nondoped OLEDs. First, the two emitters are designed to have high steric hindrances such that their emitting cores will be suitably isolated from those of their neighbors to minimize concentration quenching. On the other hand, each of the two emitters has two stable conformations in solid films. In their neat films, molecules with the minority conformation behave effectively as dopants in the matrix composing of the majority conformation. One hundred percent exciton harvesting is thus theoretically feasible in this unique architecture of “self‐doped” neat films. Furthermore, both emitters have relatively high aspect ratios in terms of their molecular shapes. This leads to films with preferred molecular orientations enabling high populations of horizontal dipoles beneficial for optical out‐coupling. With these three factors, OLEDs with nondoped emitting layers of the respective emitters both achieve nearly 100% exciton utilization and deliver over 30% external quantum efficiencies and ultralow efficiency roll‐off at high brightness, which have not been observed in reported nondoped OLEDs.

Orientation analysis of polymer thin films on metal surfaces via IR absorbance of the relative transition dipole moments

Modern technology has increasing requirements for thin film based electronic devices. Many surface and electronic properties strongly depend on the orientation of the material. Industrial development has a need for cheap, non-destructive, and fast orientation analysis in different conditions. Infrared-orientation analysis is less represented in literature compared to other orientation methods, while offering a lot of unique advantages. In this respect for thin polymeric films, we propose infrared (IR) absorbance-spectroscopy near metal surfaces to distinguish the orientation of different molecular segments. By use of the ratio method, the calculation of the orientation becomes mostly independent from the angle of incidence and averages out aperture errors of the measurement setup, hence the given solutions are generally applicable for many setups. For the most common point groups, the hereby-presented mathematical solution is valid for thin films cast on metal surfaces. Extensive quantum mechanical calculations of the transition dipole moments are avoided for known IR spectroscopy bands. The detailed application of the given IR orientation analysis is demonstrated for the polymers polystyrene, polyethylene glycol, poly(methyl methacrylate) and polyvinyl acetate. Each segment of these polymers is analyzed for the preferred molecular orientation in respect to the surface.

A Closer Look at Tip-Enhanced Raman Chemical Reaction Nanoimages.

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique that enables ultrahigh spatial resolution and ultrasensitive chemical imaging. This technique's ability to track plasmon-induced/enhanced chemical reactions in real space has gained increasing popularity in recent years. In this study, we expose inherent difficulties associated with assigning TERS signatures that accompany chemical transformations. Namely, distinct selection rules as well as the possibility of multiple physical processes/chemical reaction pathways complicate spectral assignments and necessitate caution in assigning the experimental observables. We illustrate the latter using 4,4'-dimercaptostilbene-functionalized plasmonic silver nanocubes, wherein we identify the TERS signatures of product formation, molecular charging, multipolar Raman scattering, and preferred molecular orientations that all lead to distinct and assignable spectral patterns.

Spectroscopic study of D1–A–D2–A terpolymer films for optoelectronic applications

Several strategies have been considered in search of more efficient organic materials for charge transfer in photovoltaic devices. Among them, the integration of donor-acceptor (D-A) functional units on a conjugated copolymer has been widely applied. In this framework, we evaluated four terpolymers made up of donor moieties derived from 9,9-dioctylfluorene and 9-(heptadecan-9-yl)-9H-carbazole combined with 2,1,3-benzothiadiazole, the acceptor moiety, in different monomer ratios and polymerization routes (block and random microstructures). The preferred molecular orientation and charge transfer dynamics of the polymeric films were assessed by near edge X-ray absorption fine structure spectroscopy (NEXAFS) and resonant Auger electron spectroscopy (RAES) around the sulfur K-edge. Charge transfer times (τCT) were estimated by the Core-Hole Clock (CHC) method. Films with a high degree of organization were identified for the block terpolymer and random terpolymers with uneven amounts of donor units, showing a preferred orientation of the benzothiadiazole (BT) molecular plane parallel to the substrate surface. The values of τCT measured for all terpolymers were higher than those for typical polymers used in photovoltaic devices, which is not desirable for this type of optoelectronic application, but this may be correlated to the strong acceptor character of BT, the unit probed. To investigate the effect of film formation on the excited state behavior, steady-state and time-resolved photoluminescence measurements were also conducted. X-ray photoelectron spectroscopy (XPS) was employed to characterize the surface chemical composition of the terpolymer films. Based on the spectroscopic data the block copolymer appears to be the most suitable for the desired application.

Molecular orientation and thermal stability of thin-film organic semiconductors

Molecular orientation in organic semiconductors plays a critical role in maximizing external quantum efficiencies of organic light-emitting diodes. It was generally believed that the molecular packing of organic semiconductors is either amorphous or liquid-crystal-like with a preferred molecular orientation distributed uniformly throughout the film. In this paper, however, we report that the orientation of organic molecules in physical-vapor deposited films varies drastically depending on thickness. The thermal stability of the molecular network, measured by its characteristic glass transition temperature, also varies as a function of the film thickness. Based on a two-layered film-structure model, we propose a simple function to quantify the molecular dipole orientation S parameter as a function of film thickness. This function describes well experimental data. In addition to contributing to external quantum efficiency, the molecular orientation parameter S is found to have a strong impact on disruptive change in material density after thermal anneal and glass transition.

Ion transfer and adsorption of water-soluble metal complexes of 8-hydroxyquinoline derivatives at the water|1,2-dichloroethane interface

The transfer mechanism and adsorption state of water-soluble 8-quinolinolate complexes were studied at the water|1,2-dichloroethane interface by electrochemical and spectroelectrochemical techniques. The interfacial affinities of the metal complexes of 8-hydroxyquinoline-5-sulfonate (QS) were estimated as AlQS33−, CuQS22− >ZnQS22−. Potential modulated fluorescence spectroscopy revealed the potential-driven process of fluorescent QS complexes, where Al(III) and Zn(II) complexes were transferred across the interface accompanied by the adsorption at the aqueous side of the interface. The adsorption state and preferential molecular orientation of these complexes were analyzed in detail by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. The PM-TIRF results showed that the square-planar 1:2 complexes, AlQS2− and ZnQS22−, were oriented relatively in parallel to the interface and approximately identical to the aqueous species. The adsorption behavior of the Zn(II) complex of tridentate 8-hydroxyquinoline-2-carboxylate (QC) ligand was also investigated, and ZnQC22− exhibited a strong interfacial affinity with intermediate spectral features between the aqueous and organic species.

Deciphering photocarrier dynamics for tuneable high-performance perovskite-organic semiconductor heterojunction phototransistors

Looking beyond energy harvesting, metal-halide perovskites offer great opportunities to revolutionise large-area photodetection technologies due to their high absorption coefficients, long diffusion lengths, low trap densities and simple processability. However, successful extraction of photocarriers from perovskites and their conversion to electrical signals remain challenging due to the interdependency of photogain and dark current density. Here we report hybrid hetero-phototransistors by integrating perovskites with organic semiconductor transistor channels to form either “straddling-gap” type-I or “staggered-gap” type-II heterojunctions. Our results show that gradual transforming from type-II to type-I heterojunctions leads to increasing and tuneable photoresponsivity with high photogain. Importantly, with a preferential edge-on molecular orientation, the type-I heterostructure results in efficient photocarrier cycling through the channel. Additionally, we propose the use of a photo-inverter circuitry to assess the phototransistors’ functionality and amplification. Our study provides important insights into photocarrier dynamics and can help realise advanced device designs with “on-demand” optoelectronic properties.

Effects of Solvent Stabilization on Pharmaceutical Crystallization: Investigating Conformational Polymorphism of Probucol Using Combined Solid-State Density Functional Theory, Molecular Dynamics, and Terahertz Spectroscopy.

Solid-state density functional theory (DFT), molecular dynamics (MD), and terahertz (THz) spectroscopy were used to study the formation of enantiotropically related conformational Form I and Form II polymorphs of the pharmaceutical compound, probucol. DFT calculations were performed on the crystal systems to compare relative lattice energies and the solvent stabilization of the metastable Form II structure. The thermodynamics of solvent inclusion in the Form II·MeOH crystal system were determined from MD simulations, as was the favored conformation of molecular probucol in methanol and ethanol solutions. The findings from both solid-state DFT and MD calculations suggest that the preferred molecular orientations of the probucol molecule in solution and the probable inclusion of methanol in the crystal lattice during the crystallization process lead to the solvent selectivity of the probucol polymorph formation. The additional stabilization energy provided by the crystallization solvent facilitates the nucleation and growth of the Form II polymorph under conditions that favor this metastable crystal form over the thermodynamically stable Form I, despite the higher energy molecular and crystalline configurations of probucol Form II. We demonstrate the influence of solvent on the formation of pharmaceutical polymorphs and provide a molecular-level view of complex interactions leading to polymorphism using a combination of computational methods and THz spectral data.

Switching the Electronic Properties of ZnO Surfaces with Negative T‐Type Photochromic Pyridyl‐dihydropyrene Layers and Impact of Fermi Level Pinning

AbstractRemote control of the electronic energy levels by external stimuli such as light will enable optoelectronic devices with improved or additional functionalities. Here, it is demonstrated that the electronic properties of ZnO interfaced with negative T‐type photoswitches, that is, pyridyl‐dihydropyrene (Py‐DHP), can indeed be photomodulated. The process of forward switching of Py‐DHP with green light from an isomer with a low energy gap to an isomer with a wider one is followed by a thermally activated backward transfer. Using photoemission spectroscopy and density functional theory modeling, it is shown that Py‐DHP ring closure/opening reactions result in a reversible shift of frontier occupied molecular levels by 0.7 eV with respect to the Fermi level. Notably, in both molecular configurations, the energy level alignment at ZnO/Py‐DHP interfaces is governed by a Fermi level pinning at the lowest unoccupied molecular level. Moreover, upon switching, an increase in the ionization energy for Py‐DHP multilayers compared to that of a monolayer is observed. This is attributed to a different preferred molecular orientation in monolayer versus multilayers. The results show that a dynamic tuning of the energy level alignment at inorganic/organic interfaces by external stimuli is feasible and will aid the development of photoprogrammable optoelectronic devices.

1]Benzothieno[3,2-b]benzothiophene (BTBT) derivatives: Influence in the molecular orientation and charge delocalization dynamics

Abstract Using near-edge X-ray absorption fine structure (NEXAFS) and resonant Auger spectroscopy (RAS) in conjunction with the core-hole clock methodology the electronic structure, molecular ordering and orientation and charge transfer dynamics in the femtosecond time scale of 2,7-di-tert-pentyl[1]benzothieno[3,2-b]benzothiophene (di(Me)2Pr-BTBT), 2,7-di-iso-propyl[1]benzothieno[3,2-b]benzothiophene (diiPr-BTBT) and 2,7-di-tert butyl[1]benzothieno[3,2-b]benzothiophene (ditBu-BTBT) films were investigated. Total electron yield (TEY) and fluorescence yield (FY) NEXAFS spectra were recorded with the aim of determining the preferred molecular orientation of the oligomers at the surface and in the bulk. Angular dependent sulfur 1s NEXAFS spectra for diiPr-BTBT and ditBu-BTBT films deposited onto FTO (fluorine doped tin oxide) point to well-organized films with a preferred edge-on geometry, while for the di(Me)2Pr-BTBT film little variation is seen, indicating that the film matches its herringbone crystal packing. Films prepared on ITO (indium tin oxide) and silicon were also investigated by TEY and FY NEXAFS. Greater film ordering is observed in the bulk. The electron charge transfer time following sulfur K-edge main resonance was calculated. The ditBu-BTBT films showed the lowest charge transfer time as well as greater molecular organization, pointing to an increased coupling, as compared to the other oligomers.

The growth and assembly of organic molecules and inorganic 2D materials on graphene for van der Waals heterostructures

The unique properties of graphene have made it a promising material for integration in future electronic applications. The idealized surface of graphene, atomically-flat and without dangling bonds, offers the opportunity to understand the assembly of organic and inorganic molecules to form a wide range of ordered architectures and functional graphene-based heterostructures. In this review, we summarize recent progress in the growth of hierarchical nanostructures on graphene. The self-assembly of organic molecules and inorganic two-dimensional (2D) layers on graphene for the construction of various types of heterostructures are highlighted. Van der Waals interactions between the assembled molecules and graphene are shown to allow the formation of highly-ordered structures with preferred molecular orientations and stacking configurations that circumvent the strict lattice-matching requirements in traditional epitaxial growth. Finally, we briefly discuss representative applications of graphene-based heterostructures in electronic and optoelectronics.

Morphology‐Driven Sensitivity Enhancement of MEH‐PPV Langmuir‐Blodgett Films on Plastic Substrates for NO2 Gas

Abstract3D ordered Langmuir–Blodgett (LB) films of poly[2‐methoxy‐5‐ (2‐ethylhexyloxy)‐1, 4‐phenylenevinylene] (MEH‐PPV) were prepared via thermal annealing. Surface properties of MEH‐PPV prepared by using different solvents, through both LB and Spin Casting techniques, were examined at the air‐water interface by measuring surface pressure‐area isotherms and Brewster Angle Microscopy. Dramatically different surface morphologies were obtained for spin cast and LB films, both prepared by using different solvents. The annealed LB films showed enhanced NO2 gas sensing as compared to spin cast films. Spectroscopy and morphological analyses reveal that enhanced response of LB films toward NO2 gas is due to their 3D ordered morphology, preferred molecular orientation for in‐plane transport as well as higher surface area.

High pump efficiency of a second-order distributed feedback laser based on holographic polymer dispersed liquid crystals with preferred liquid crystal molecular orientation

We report on the fabrication and characterization of a surface-emitting distributed feedback (DFB) organic semiconductor laser based on a holographic polymer dispersed liquid crystal (HPDLC) transmission grating.

Modeling the polarized X-ray scattering from periodic nanostructures with molecular anisotropy.

There is a need to characterize nanoscale molecular orientation in soft materials, and polarized scattering is a powerful means to measure this property. However, few approaches have been demonstrated that quantitatively relate orientation to scattering. Here, a modeling framework to relate the molecular orientation of nanostructures to polarized resonant soft X-ray scattering measurements is developed. A variable-angle transmission measurement called critical-dimension X-ray scattering enables the characterization of the three-dimensional shape of periodic nanostructures. When this measurement is conducted at resonant soft X-ray energies with different polarizations to measure soft material nanostructures, the scattering contains convolved information about the nanostructure shape and the preferred molecular orientation as a function of position, which is extracted by fitting using inverse iterative algorithms. A computationally efficient Born approximation simulation of the scattering has been developed, with a full tensor treatment of the electric field that takes into account biaxial molecular orientation, and this approach is validated by comparing it with a rigorous coupled wave simulation. The ability of various sample models to generate unique best fit solutions is then analyzed by generating simulated scattering pattern sets and fitting them with an inverse iterative algorithm. The interaction of the measurement geometry and the change in orientation across a periodic repeat unit leads to distinct asymmetry in the scattering pattern which must be considered for an accurate fit of the scattering.