Guest Concentration Research Articles

Adamantyl and carbazole containing trans-poly(norbornene-dicarboximide)s as electro-optic chromophore hosts

A selection of trans-poly(norbornene-dicarboximide) random copolymers incorporating adamantyl and carbazole groups have been prepared using the Grubbs 1st generation catalyst. The copolymer with the greatest carbazole content was employed as a host for a high activity chromophore guest. The adamantyl groups were able to elevate the glass transition and thermal decomposition temperatures of the host copolymers, while the carbazole groups were included to facilitate the dispersion of the added chromophores and enhance the poling process. The mass of the adamantyl pendant monomer was kept constant as the mass of the carbazole pendant monomer was increased gradually. The successful incorporation of the carbazole moiety into the copolymers was confirmed by 1H NMR integration. The polymers had a predominantly trans microstructure and the cis/trans ratio did not change significantly. The polymer molecular weights increased linearly from 25,758 g/mol to 56,827 g/mol as more carbazole-pendant monomer was incorporated. Polydispersities ranged from 1.29 to 1.70 and elemental analysis confirmed an ascending carbon% in the polymers from 72.29% to 76.26%. The glass transition temperatures decreased from 240 °C to 126 °C, with the thermal decomposition temperatures (10% loss) remaining relatively constant at between 423 °C to 394 °C, indicating the polymers had a good thermal stability. The chromophore guest concentration was increased gradually into the copolymer host from 15 wt% to 65 wt%. When poled at a field of 50 V/μm in thin film it was found that the optimum poling temperature decreased linearly from 150 °C to 90 °C as the chromophore concentration was increased, a maximum poling efficiency of 1.02 was achieved at a chromophore concentration of 55 wt%.

Multivalency in a Dendritic Host–Guest System

Multivalency is an important instrument in the supramolecular chemistry toolkit for the creation of strong specific interactions. In this paper we investigate the multivalency effect in a dendritic host–guest system using molecular dynamics simulations. Specifically, we consider urea–adamantyl decorated poly(propyleneimine) dendrimers that together with compatible mono-, bi-, and tetravalent ureidoacetic acid guests can form dynamic patchy nanoparticles. First, we simulate the self-assembly of these particles into macromolecular nanostructures, showing guest-controlled reduction of dendrimer aggregation. Subsequently, we systematically study guest concentration dependent multivalent binding. At low guest concentrations multivalency of the guests clearly increases relative binding as tethered headgroups bind more often than free guests’ headgroups. We find that despite an abundance of binding sites, most of the tethered headgroups bind in close proximity, irrespective of the spacer length; nevertheless, longer spacers do increase binding. At high guest concentrations the dendrimer becomes saturated with bound headgroups, independent of guest valency. However, in direct competition the tetravalent guests prevail over the monovalent ones. This demonstrates the benefit of multivalency at high as well as low concentrations.

How does the complexation ability between host endo-functionalized molecular tube and strongly hydrophilic guest molecules in water depend on guest concentration?

We have already shown in our previous work (R. Paul, S. Paul, Synergistic Host-Guest Hydrophobic and Hydrogen Bonding Interactions in the Complexation between Endo-Functionalized Molecular Tube and Strongly Hydrophilic Guest Molecules in Aqueous Solution, Phys. Chem. Chem. Phys. 20 (2018) 16540–16550.) the main factors responsible in the complexation between an endo-functionalized molecular tube and different strongly hydrophilic guest molecules in water. These types of molecular tubes have gained a significant importance for encapsulating many environmentally and biologically important molecules. Here, we have examined how does this type of complexation ability depend on concentration of guest molecules? We use classical molecular dynamics together with MM-PBSA method for this purpose. We examine the effect of concentration of different guest molecules on the host-guest complexation ability by determining different types of hydrogen bond properties, preferential interaction parameter, water-inserted guest interaction energy parameter, host-guest binding free energy and potential of mean force. From the calculated free energy values obtained from MM-PBSA and PMF calculation, it is revealed that as the concentration of guest decreases the free energy becomes less favorable.

Understanding charge transport in Ir(ppy)3:CBP OLED films.

Ir(ppy)3:CBP blends have been widely studied as the emissive layer in organic light emitting diodes (OLEDs), yet crucial questions about charge transport within the layer remain unaddressed. Recent molecular dynamics simulations show that the Ir(ppy)3 molecules are not isolated from each other, but at concentrations of as low as 5 wt. % can be part of connected pathways. Such connectivity raises the question of how the iridium(iii) complexes contribute to long-range charge transport in the blend. We implement a kinetic Monte Carlo transport model to probe the guest concentration dependence of charge mobility and show that distinct minima appear at approximately 10 wt. % Ir(ppy)3 due to an increased number of trap states that can include interconnected complexes within the blend film. The depth of the minima is shown to be dependent on the electric field and to vary between electrons and holes due to their different trapping depths arising from the different ionization potentials and electron affinities of the guest and host molecules. Typical guest-host OLEDs use a guest concentration below 10 wt. % to avoid triplet-triplet annihilation, so these results suggest that optimal device performance is achieved when there is significant charge trapping on the iridium(iii) complex guest molecules and minimum interactions of the emissive chromophores that can lead to triplet-triplet annihilation.

Effect of exciton diffusion on the triplet-triplet annihilation rate in organic semiconductor host-guest systems

We study the contribution of triplet exciton diffusion to the efficiency loss resulting from F\orster-type triplet-triplet annihilation (TTA) in organic phosphorescent semiconductor host-guest systems, using kinetic Monte Carlo (KMC) simulations. Our study focusses on diffusion due to F\orster-type guest-guest transfer, but includes also a comparison with simulation results for the case of Dexter-type guest-guest transfer. The simulations are carried out for a wide range of F\orster radii, and for guest concentrations up to 100 mol%, with the purpose to support analyses of time-resolved photoluminescence experiments probing TTA. We find that the relative contribution of diffusion to the TTA-induced efficiency loss may be deduced quite accurately from a quantitative experimental measure for the shape of the time-dependent photoluminescence intensity, the so-called $r$ ratio. For small guest concentrations and F\orster radii that are most relevant to organic light-emitting diodes (OLEDs), the diffusion contribution is in general quite small. Under these weak-diffusion conditions, the absolute diffusion contribution to the TTA-induced efficiency loss can be understood quantitatively using a capture radius formalism. The effective guest-guest diffusion coefficient that follows from the TTA simulations, using the capture radius formalism, agrees well with the diffusion coefficient that follows from direct KMC diffusion simulations. The simulations reveal that the diffusion coefficient is strongly affected by the randomness of the distribution of guest molecule locations.

Fluorinated Bambusurils as Highly Effective and Selective Transmembrane Cl−/HCO3− Antiporters

Summary The exchange of chloride and bicarbonate across lipid bilayers is an important biological process. Synthetic molecules can act as mobile carriers for these anions, although most show little selectivity. Here we report on three bambus[6]uril macrocycles functionalized with fluorinated benzyl groups, which are able to exchange Cl− and HCO3− efficiently. Remarkably, rates for Cl−/NO3− exchange are two orders of magnitude lower. The higher rates of Cl−/HCO3− transport can be explained by the ability of the bambusurils to complex Cl− and HCO3− simultaneously, facilitating their exchange at the bilayer interface. Furthermore, the exceptionally high affinity and selectivity of these systems for NO3− appear to contribute to the poor Cl−/NO3− exchange. This work not only demonstrates the importance of anion binding characteristics on anion transport but also the potential relevance of bambusurils for anion transport applications considering the high rate observed for Cl−/HCO3− exchange.

Effect of Triplet Confinement on Triplet–Triplet Annihilation in Organic Phosphorescent Host–Guest Systems

AbstractThe efficiency of phosphorescent organic light emitting diodes (OLEDs) shows a decrease with increasing luminance (“roll‐off”). One of the contributions to the roll‐off is triplet–triplet annihilation (TTA). TTA is the process of energy transfer from one triplet exciton to another, after which the excited exciton decays nonradiatively to the lowest triplet state. In this study, the TTA‐rate is measured for a large number of emissive materials consisting of a small concentration of phosphorescent “guest” molecules, with emission colors across the entire visible range, embedded in various host materials. It is found that the TTA‐rate does not only depend on the direct interaction rate between the excitons on the guest molecules, but also on the difference in triplet energy ΔE T of the host and guest molecules: when ΔE T is smaller than about 0.20 eV, diffusion of excitons via the host molecules leads to a significant enhancement of the TTA‐rate. By varying the guest concentration and using kinetic Monte Carlo simulations, the roles of the direct interaction, guest‐mediated diffusion, and host‐mediated diffusion are disentangled.

Adsorption of 1-Propanol in the Channel-Like InOF-1 Metal–Organic Framework and Its Influence on the CO2 Capture Performances

The n-PrOH adsorption properties of the one-dimensional-channel-like metal–organic framework InOF-1 were first explored and evidenced a high affinity for this host–guest system owing to the shape of the adsorption isotherms at a low relative pressure, the presence of a hysteresis loop during the desorption process, and a relatively high isosteric heat of adsorption. Monte Carlo simulations revealed that this thermodynamic behavior is related to a preferential interaction between n-PrOH and the μ2-OH groups of the InOF-1 surface at low loading, whereas n-PrOH self-aggregates at higher guest concentration to first form dimers and then clusters. In complement to this thermodynamic exploration, the kinetics of n-PrOH were further characterized and the mobility of the guests was shown to be slow most probably due to the formation of these guest clusters. Finally, we revealed that in contrast to other solvents we have reported in the past, the n-PrOH confinement does not enhance the CO2 capture in InOF-1. This ...

Fabrication of solution-processed pure blue fluorescent OLED using exciplex host

Exciplex host in organic light-emitting diodes (OLEDs) can use radiative energy of upconverted singlet excitons by reverse intersystem crossing and thus transfer the energy to guest emission to effectively enhance the device performance. However, investigation of OLED with pure blue emission using exciplex host has rarely been reported. In this contribution, solution-processed OLEDs using exciplex host with pure blue fluorescence-type guest emission have been achieved. Through optimization of guest concentration, improved efficiency and blue-shifted electroluminescence spectrum of the OLED device was obtained. This study provides a method to fabricate efficient OLEDs with deeper blue emission for application in full-color display and white lighting source with high color rendering index.

Left- and right-circularly polarized light-sensing based on colored and mechano-responsive chiral nematic liquid crystals.

A liquid crystal host-guest system composed of achiral organic molecules (host) and colored chiral metal complexes (guest) was fabricated to sense both right- and left-handed circularly polarized light (r- and l-CPL), depending on the guest (dopant) concentration. The CPL-sensing can be reversibly turned on and off upon mechanical stress and heating.

Bridging solution properties to gas hydrate nucleation through guest dynamics.

By investigating the aqueous solution properties of several hydrate guests with molecular simulations, we find that with increasing guest concentration, the guest's hydration shell becomes more ordered and the system entropy decreases. A common critical value of the self-diffusion coefficient of different guest molecules is identified, below which hydrates will nucleate very readily.

Guest–Host Complexes of TCNQ and TCNE with Cu3(1,3,5-benzenetricarboxylate)2

A combined spectroscopic and structural study was undertaken to investigate the nature of the incorporation of the electron acceptor guest 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the closely related guest tetracyanoethylene (TCNE) into the host porous framework [Cu3(BTC)2] (BTC = 1,3,5-benzenetricarboxylate)—a guest–host system recently shown to be highly conductive. We find that the guest concentration in the system can be modulated via the synthesis reaction time and temperature. A suite of spectroscopic, X-ray and neutron powder diffraction, and density functional theory techniques revealed the mechanism of guest binding within the framework host, including the guest redox states. This work provides insights into the way that electrical conductivity arises in porous framework host–guest systems and contributes to understanding how fine-tuning framework properties influences conductivity.

Surface chemistry effects on heterogeneous clathrate hydrate nucleation: A molecular dynamics study

We report results from a molecular dynamics study of clathrate hydrate nucleation near model hydrophilic and hydrophobic surfaces. –CH3 and –OH terminated self-assembled monolayers (SAMs) are used as model surfaces. We study the nucleation of a soluble, structure II forming guest molecule with a coarse-grained model compatible with monatomic water. Despite the presence of SAMs, we show that nucleation occurs through a homogeneous mechanism in OHSAM and CH3SAM systems. Formation of ice-like or intermediate water structure is not observed near either surface prior to nucleation. Nucleation occurs more quickly in OHSAM systems than CH3SAM systems. However, the faster nucleation is driven by a partitioning of guest molecules which results in higher bulk guest concentration in OHSAM systems compared with CH3SAM systems. Despite significant aggregation of guest molecules near CH3SAM, no nucleation is observed near the surface. The formation of guest contact pairs, facilitated by the presence of CH3SAM, may prevent nucleation in this region. Our results highlight the numerous routes by which surfaces can affect hydrate nucleation due to the multicomponent nature of the phenomena.

Lattice response of the porous coordination framework Zn(hba) to guest adsorption

Analysis of in situ neutron powder diffraction data collected for the porous framework material Zn(hba) during gas adsorption reveals a two-stage response of the host lattice to increasing CO2 guest concentration, suggesting progressive occupation of multiple CO2 adsorption sites with different binding strengths. The response of the lattice to moderate CH4 guest concentrations is virtually indistinguishable from the response to CO2, demonstrating that the influence of host–guest interactions on the Zn(hba) framework is defined more strongly by the concentration than by the identity of the guests.

Host proficiency of N,N′-bis(9-phenyl-9-thioxanthenyl)ethylenediamine for pyridine and the methylpyridine guests – a competition study

N,N′-Bis(9-phenyl-9-thioxanthenyl)ethylenediamine proved to be an extremely efficient host compound for pyridine and the isomeric methylpyridines. Furthermore, this host displayed selective behaviour during equimolar guest competitions, consistently favouring 3-methylpyridine in binary, ternary and quaternary experiments. Selectivity orders were 3-methylpyridine >> 4-methylpyridine > 2-methylpyridine, and 3-methylpyridine > pyridine > 4-methylpyridine > 2-methylpyridine, for equimolar ternary and quaternary solutions, respectively. When guest concentrations in binary competitions were varied, 3-methylpyridine remained the favoured guest, even at low 3-methylpyridine concentrations. Single crystal X-ray diffraction showed that all four complexes were isostructural (monoclinic, P21/n) while guests occupy discrete cavities in the crystal. Only 3-methylpyridine experiences (guest methyl)C–H∙∙∙π(host) and (guest methyl)C–H∙∙∙H–Ar(host) interactions, explaining the observed affinity of the host for this guest. DSC analyses provided further affirmation for the host preference: endotherm peak temperatures for the guest release processes correlated directly with the selectivity order for the three methylpyridine isomers.

Conformational Selection as the Mechanism of Guest Binding in a Flexible Supramolecular Host.

This study offers a detailed mechanistic investigation of host-guest encapsulation behavior in a new enzyme-mimetic metal-ligand host and provides the first observation of a conformational selection mechanism (as opposed to induced fit) in a supramolecular system. The Ga4L4 host described features a C3-symmetric ligand motif with meta-substituted phenyl spacers, which enables the host to initially self-assemble into an S4-symmetric structure and then subsequently isomerize to a T-symmetric tetrahedron for better accommodation of a sufficiently large guest. Selective inversion recovery 1H NMR studies provide structural insights into the self-exchange behaviors of the host and the guest individually in this dynamic system. Kinetic analysis of the encapsulation-isomerization event revealed that increasing the concentration of guest inhibits the rate of host-guest relaxation, a key distinguishing feature of conformational selection. A comprehensive study of this simple enzyme mimic provides insight into analogous behavior in biophysics and enzymology and aims to inform the design of efficient self-assembled microenvironment catalysts.

Novel blue light emitting poly(arylene ether)s under the influence of guest concentrations in host-guest systems: Synthesis, electrochemical and photophysical properties

Novel blue light emitting poly(arylene ether)s under the influence of guest concentrations in host-guest systems: Synthesis, electrochemical and photophysical properties

Guest concentration, bias current, and temperature-dependent sign inversion of magneto-electroluminescence in thermally activated delayed fluorescence devices

Non-emissive triplet excited states in devices that undergo thermally activated delayed fluorescence (TADF) can be up-converted to singlet excited states via reverse intersystem crossing (RISC), which leads to an enhanced electroluminescence efficiency. Exciton-based fluorescence devices always exhibit a positive magneto-electroluminescence (MEL) because intersystem crossing (ISC) can be suppressed effectively by an external magnetic field. Conversely, TADF devices should exhibit a negative MEL because RISC is suppressed by the external magnetic field. Intriguingly, we observed a positive MEL in TADF devices. Moreover, the sign of the MEL was either positive or negative, and depended on experimental conditions, including doping concentration, current density and temperature. The MEL observed from our TADF devices demonstrated that ISC in the host material and RISC in the guest material coexisted. These competing processes were affected by the experimental conditions, which led to the sign change of the MEL. This work gives important insight into the energy transfer processes and the evolution of excited states in TADF devices.

Self-assembly in a model colloidal mixture of dimers and spherical particles.

We investigate the structure of a dilute mixture of amphiphilic dimers and spherical particles, a model relevant to the problem of encapsulating globular "guest" molecules in a dispersion. Dimers and spheres are taken to be hard particles, with an additional attraction between spheres and the smaller monomers in a dimer. Using the Monte Carlo simulation, we document the low-temperature formation of aggregates of guests (clusters) held together by dimers, whose typical size and shape depend on the guest concentration χ. For low χ (less than 10%), most guests are isolated and coated with a layer of dimers. As χ progressively increases, clusters grow in size becoming more and more elongated and polydisperse; after reaching a shallow maximum for χ≈50%, the size of clusters again reduces upon increasing χ further. In one case only (χ=50% and moderately low temperature) the mixture relaxed to a fluid of lamellae, suggesting that in this case clusters are metastable with respect to crystal-vapor separation. On heating, clusters shrink until eventually the system becomes homogeneous on all scales. On the other hand, as the mixture is made denser and denser at low temperature, clusters get increasingly larger until a percolating network is formed.

Charge Transport by Superexchange in Molecular Host-Guest Systems.

Charge transport in disordered organic semiconductors is generally described as a result of incoherent hopping between localized states. In this work, we focus on multicomponent emissive host-guest layers as used in organic light-emitting diodes (OLEDs), and show using multiscale abinitio based modeling that charge transport can be significantly enhanced by the coherent process of molecular superexchange. Superexchange increases the rate of emitter-to-emitter hopping, in particular if the emitter molecules act as relatively deep trap states, and allows for percolation path formation in charge transport at low guest concentrations.