Quadrupolar Molecules Research Articles

Spectroscopic Investigation and Theoretical Modeling of Benzothiadiazole-Based Charge-Transfer Chromophores: From Solution to Nanoaggregates

Experimental and theoretical investigations are presented on four quadrupolar molecules constituted of a central electron-acceptor 2,1,3-benzothiadiazole conjugated at both sides with electron-donating thiophene (or 2,2′-bithiophene) substituted groups. Previous investigation of their physicochemical properties demonstrated that thermally induced self-organization might be achieved for one of them. In this study, we further explore the structure–properties relationships for these quadrupolar chromophores also examining their aggregation behavior in organic nanoparticles. Strong spectroscopic evidence of J-aggregates formation is obtained, favored by the presence of branched terminal alkyl chains. The surprisingly low fluorescence quantum yield of these J-aggregates is rationalized within the essential-state model approach.

A simple model of solvent-induced symmetry-breaking charge transfer in excited quadrupolar molecules.

A simple model has been developed to describe the symmetry-breaking of the electronic distribution of AL-D-AR type molecules in the excited state, where D is an electron donor and AL and AR are identical acceptors. The origin of this process is usually associated with the interaction between the molecule and the solvent polarization that stabilizes an asymmetric and dipolar state, with a larger charge transfer on one side than on the other. An additional symmetry-breaking mechanism involving the direct Coulomb interaction of the charges on the acceptors is proposed. At the same time, the electronic coupling between the two degenerate states, which correspond to the transferred charge being localised either on AL or AR, favours a quadrupolar excited state with equal amount of charge-transfer on both sides. Because of these counteracting effects, symmetry breaking is only feasible when the electronic coupling remains below a threshold value, which depends on the solvation energy and the Coulomb repulsion energy between the charges located on AL and AR. This model allows reproducing the solvent polarity dependence of the symmetry-breaking reported recently using time-resolved infrared spectroscopy.

Femtosecond to nanosecond studies of octupolar molecules and their quadrupolar and dipolar analogues.

The photophysical properties of two octupolar (T) molecules and of their linear (L) and quadrupolar (Q) analogues are studied by means of steady state and femtosecond to nanosecond spectroscopy. The compounds bear a triphenylamine donor, cyano acceptors and acetylenic (series 1) or olefin (series 2) π-bridges. In the octupolar compound of series 2 (2T), fluorescence is emitted from an excited state localized on a single branch, while in that of series 1 (1T), the emitting state is delocalized among branches pointing to a reduced excited state polarity. Excited state dynamics in series 1 has shown an increase of lifetime with solvent polarity. In the branched compounds of series 2, multiexponential dynamics in polar solvents is exhibited indicating a distribution of emitting geometries. Femtosecond anisotropy in 1T indicates incoherent excitation transfer on the timescale of a few ps, in agreement with the hopping time predicted by the Förster model. However, no hopping mechanism is observed in 2T possibly because of an increased intramolecular charge transfer leading to a low energy relaxed excited state localized on a single branch.

Phase behavior of a binary fluid mixture of quadrupolar molecules.

We propose a model molecule to investigate microscopic properties of a binary mixture with a closed-loop coexistence region. The molecule is comprised of a Lennard-Jones particle and a uniaxial quadrupole. Gibbs ensemble Monte Carlo simulations demonstrate that the high-density binary fluid of the molecules with the quadrupoles of the same magnitude but of the opposite signs can show closed-loop immiscibility. We find that an increase in the magnitude of the quadrupoles causes a shrinkage of the coexistence region. Molecular dynamics simulations also reveal that aggregates with two types of molecules arranged alternatively are formed in the stable one-phase region both above and below the coexistence region. String structures are dominant below the lower critical solution temperature, while branched aggregates are observed above the upper critical solution temperature. We conclude that the anisotropic interaction between the quadrupoles of the opposite signs plays a crucial role in controlling these properties of the phase behavior.

Modulation of Symmetry-Breaking Intramolecular Charge-Transfer Dynamics Assisted by Pendant Side Chains in π-Linkers in Quadrupolar Diketopyrrolopyrrole Derivatives.

The effect of the length of pendant side chains in centrosymmetric quadrupolar molecules on dynamics of their most perplexing photophysical phenomenon, i.e., symmetry-breaking intramolecular charge transfer, has been discovered. Unexpectedly, considerable influence of length of these pendant side chains in π-linkers arose as a structural factor enabling the control of the degree of fluorescence solvatochromism. The symmetry-breaking intramolecular charge-transfer dynamics has been described on quadrupolar diketopyrrolopyrrole derivatives possessing fluorene moieties as π-linkers and diarylamino groups as electron donors. On the basis of the evolution of transient fluorescence spectra obtained by a femtosecond broadband fluorescence up-conversion spectroscopy, it was found that the relative contribution of diffusive solvation and torsional relaxation in overall spectral relaxation can be modulated by the length of pendant side chain in π-linkers. Consequently, we demonstrated that this modulation plays a significant role in determining the photophysical properties of diketopyrrolopyrroles in a polar medium.

Lanthanide-Potassium Biphenyl-3,3'-disulfonyl-4,4'-dicarboxylate Frameworks: Gas Sorption, Proton Conductivity, and Luminescent Sensing of Metal Ions.

A novel sulfonate-carboxylate ligand of biphenyl-3,3'-disulfonyl-4,4'-dicarboxylic acid (H4-BPDSDC) and its lanthanide-organic frameworks {[LnK(BPDSDC)(DMF)(H2O)]·x(solvent)}n (JXNU-2, where JXNU denotes Jiangxi Normal University, DMF indicates dimethylformamide, and Ln = Sm(3+), Eu(3+), and Pr(3+)) were synthesized and structurally characterized. The three isomorphous lanthanide compounds feature three-dimensional frameworks constructed from one-dimensional (1D) rod-shaped heterometallic Ln-K secondary building units and are an illustration of a Kagome-like lattice with large 1D hexagonal channels and small 1D trigonal channels. The porous material of the representive JXNU-2(Sm) has an affinity to quadrupolar molecules such as CO2 and C2H2. In addition, the JXNU-2(Sm) compound exhibits humidity- and temperature-dependent proton conductivity with a large value of 1.11 × 10(-3) S cm(-1) at 80 °C and 98% relative humidity. The hydrophilic sulfonate group on the surface of channels facilitates enrichment of the solvate water molecules in the channels, which enhances the proton conductivity of this material. Moreover, the JXNU-2(Eu) material with the characteristic bright red color shows the potential for recognition of K(+) and Fe(3+) ions. The enhancing Eu(3+) luminescence with the K(+) ion and quenching Eu(3+) luminescence with the Fe(3+) ion can be associated with the functional groups of the organic ligand.

Direct Visualization of Excited-State Symmetry Breaking Using Ultrafast Time-Resolved Infrared Spectroscopy.

Most symmetric quadrupolar molecules designed for two-photon absorption behave as dipolar molecules in the S1 electronic excited state. This is usually explained by a breakup of the symmetry in the excited state. However, the origin of this process and its dynamics are still not fully understood. Here, excited-state symmetry breaking in a quadrupolar molecule with a D-π-A-π-D motif, where D and A are electron donating and accepting units, is observed in real time using ultrafast transient infrared absorption spectroscopy. The nature of the relaxed S1 state was found to strongly depend on the solvent polarity: (1) in nonpolar solvents, it is symmetric and quadrupolar; (2) in weakly polar media, the quadrupolar state observed directly after excitation transforms to a symmetry broken S1 state with one arm bearing more excitation than the other; and (3) in highly polar solvents, the excited state evolves further to a purely dipolar S1 state with the excitation localized entirely on one arm. The time scales associated with the transitions between these states coincide with those of solvation dynamics, indicating that symmetry breaking is governed by solvent fluctuations.

Highly efficient, rapid and selective CO2 capture by thermally treated graphene nanosheets

Graphene, by virtue of its unique molecular structure and many interesting properties, is receiving considerable attention as an adsorbent for separation and purification of gases. In this study, graphene nanosheets were subjected to heat treatment in the temperature range of 200–800°C under flowing N2 to improve their textural characteristics (surface area, pores size, and total pore volume) for efficient removal of CO2 from flue gases. The resulting graphene materials showed highly ordered structure, large surface area (up to 484m2g−1) and interconnected hierarchical pore networks with narrow pore size distribution in the large-micropore/small-mesopore range. As a consequence, the heat-treated graphene samples demonstrated significantly greater CO2 uptake capabilities (up to 2.89mmolg−1) compared to pristine graphene (0.81mmolg−1) at 0°C and 1bar. More importantly, the materials displayed rapid adsorption kinetics with ultrahigh selectivity for CO2 over N2, as well as stable and readily reversible adsorption/desorption cycling behavior. The isosteric heat of adsorption had an unusual dependence on surface loading because of the presence of attractive intermolecular forces between the adsorbed quadrupolar CO2 molecules. These findings demonstrate for the first time that thermal treatment at high-temperatures can have a positive influence on the single component CO2 adsorption characteristics of graphene sheets and should be explored further as an effective strategy in the design and development of graphene-based porous solid adsorbents for CO2 abatement.

Pyrrolo[3,2-b]pyrroles-From Unprecedented Solvatofluorochromism to Two-Photon Absorption.

A combined experimental and theoretical study of the two-photon absorption (2PA) properties of a series of quadrupolar molecules possessing a highly electron-rich heterocyclic core, pyrrolo[3,2-b]pyrrole, is presented. In agreement with quantum-chemical calculations, large 2PA cross-section values, σ2PA ≈10(2) -10(3) GM (1 GM=10(50) cm(4) s photon(-1) ), are observed at wavelengths of 650-700 nm, which correspond to the two-photon allowed but one-photon forbidden transitions. The calculations also predict that increased planarity of this molecule through removal of two N-substituents leads to further increase in the σ2PA values. Surprisingly, the most quadrupolar pyrrolo[3,2-b]pyrrole derivative, containing two 4-nitrophenyl substituents at positions 2 and 5, demonstrates a very strong solvatofluorochromic effect, with a fluorescence quantum yield as high as 0.96 in cyclohexane, whereas the fluorescence vanishes in DMSO.

Molecular dynamics as a tool to study heterogeneity in zeolites – Effect of Na+ cations on diffusion of CO2 and N2 in Na-ZSM-5

Zeolites typically contain extra-framework cations to charge-compensate for trivalent Al atom substitutions in the SiO2 framework. These cations, such as Na+, directly interact with quadrupolar guest molecules, such as CO2 and N2, which move through their micropores, causing energetic heterogeneity. To assess the effects of heterogeneity in Na-ZSM-5 on diffusion of CO2 and N2, molecular dynamics (MD) simulations are carried out. In silicalite-1, the pure-silicon form of ZSM-5, the self-diffusivity exhibits a monotonic decrease with molecular loading, while the corrected diffusivity shows a relatively constant value. In contrast, the Na+ cations cause a maximum or a flat profile over molecular loading for the self- and corrected diffusivities of CO2 at T=200 and 300K, while the cations only have minimal impact on the diffusivity of N2. The MD simulations allow us to identify energy basins or sites at which guest molecules spend a relatively long time, and construct a coarse-grained lattice representation for the pore network. Average residence times at these sites are calculated for both species. The trends observed in the residence times correlate to the trends observed in the diffusivity. The residence times for CO2 at T=200K are long at low loading, but decrease with loading as additional CO2 molecules compete to stay close to a cation. In contrast, the residence times for N2 are relatively insensitive to the cations, only mildly increasing near a cation. This difference in behavior can be associated to the quadrupole moments of these molecules.

Separation of Carbon Dioxide from Nitrogen or Methane by Supported Ionic Liquid Membranes (SILMs): Influence of the Cation Charge of the Ionic Liquid

Supported ionic liquid membranes (SILMs) are promising tools for the separation of carbon dioxide from other gases. In this paper, new imidazolium, pyrrolidinium, piperidinium, and morpholinium ionic liquids with a triethylene glycol side chain and tosylate anions, as well as their symmetrical dicationic analogues, have been synthesized and incorporated into SILMs. The selectivities for CO2/N2 and CO2/CH4 separations have been measured. The selectivities exhibited by the dicationic ionic liquids are up to two times higher than the values of the corresponding monocationic ionic liquids. Quantum chemical calculations have been used to investigate the difference in the interaction of carbon dioxide with monocationic and dicationic ionic liquids. The reason for the increased gas separation selectivity of the dicationic ionic liquids is two-fold: (1) a decrease in permeance of nitrogen and methane through the ionic liquid layer, presumably due to their less favorable interactions with the gases, while the permeance of carbon dioxide is reduced much less; (2) an increase in the number of interaction sites for the interactions with the quadrupolar carbon dioxide molecules in the dicationic ionic liquids, compared to the monocationic analogues.

Structure and orientational ordering in a fluid of elongated quadrupolar molecules

A second-order density-functional theory is used to study the effect of quadrupolar interactions on the isotropic–nematic transition in a system of fluids of elongated molecules interacting via the Gay–Berne potential. The direct pair-correlation functions of the coexisting isotropic fluid that enter in the theory as input information are obtained by solving the Ornstein–Zernike equation using the Percus–Yevick integral equation theory in the (reduced) temperature range of 1.6≤T∗≤3.0 for different densities, temperatures and quadrupole moments. Using the harmonic coefficients of the direct pair-correlation functions, isotropic–nematic phase coexistence and thermodynamic parameters have been calculated. The theoretical results have been compared with the available computer simulation results.

A study of isotropic–nematic transition of quadrupolar Gay–Berne fluid using density-functional theory approach

The effects of quadrupole moments on the isotropic–nematic (IN) phase transitions are studied using the density-functional theory (DFT) for a Gay–Berne (GB) fluid for a range of length-to-breadth parameters in the reduced temperature range . The pair-correlation functions of the isotropic phase, which enter into the DFT as input parameters are found by solving the Percus–Yevick integral equation theory. The method used involves an expansion of angle-dependent functions appearing in the integral equations in terms of spherical harmonics and the harmonic coefficients are obtained by an iterative algorithm. All the terms of harmonic coefficients which involve l indices up to less than or equal to 6 are considered. The numerical accuracy of the results depends on the number of spherical harmonic coefficients considered for each orientation-dependent function. As the length-to-breadth ratio of quadrupolar GB molecules is increased, the IN transition is seen to move to lower density (and pressure) at a given temperature. It has been observed that the DFT is good to study the IN transitions in such fluids. The theoretical results have also been compared with the computer simulation results wherever they are available.

Double Lactonization in Triarylamine-Conjugated Dimethyl Diethynylfumarate: Formation of Intensely Colored and Luminescent Quadrupolar Molecules Including a Missing Structural Isomer of Pechmann Dyes

Acid-induced double lactonization in triarylamine-conjugated dimethyl diethynylfumarate E-1 opens up a new synthetic route to Pechmann dyes. This one-pot reaction affords three donor-acceptor-donor quadrupolar molecules (P(55)-1, P(66)-1, and P(56)-1); P(56)-1 comprises a missing structural isomer of Pechmann dyes. They are intensely colored and brightly luminescent. An organic field-effect transistor device fabricated with P(66)-1 functions as a p-type semiconductor.

Two-photon absorption properties of proquinoidal D-A-D and A-D-A quadrupolar chromophores.

We report the synthesis, one- and two-photon absorption spectroscopy, fluorescence, and electrochemical properties of a series of quadrupolar molecules that feature proquinoidal π-aromatic acceptors. These quadrupolar molecules possess either donor-acceptor-donor (D-A-D) or acceptor-donor-acceptor (A-D-A) electronic motifs, and feature 4-N,N-dihexylaminophenyl, 4-dodecyloxyphenyl, 4-(N,N-dihexylamino)benzo[c][1,2,5]thiadiazolyl or 2,5-dioctyloxyphenyl electron donor moieties and benzo[c][1,2,5]thiadiazole (BTD) or 6,7-bis(3',7'-dimethyloctyl)[1,2,5]thiadiazolo[3,4-g]quinoxaline (TDQ) electron acceptor units. These conjugated structures are highly emissive in nonpolar solvents and exhibit large spectral red-shifts of their respective lowest energy absorption bands relative to analogous reference compounds that incorporate phenylene components in place of BTD and TDQ moieties. BTD-based D-A-D and A-D-A chromophores exhibit increasing fluorescence emission red-shifts, and a concomitant decrease of the fluorescence quantum yield (Φ(f)) with increasing solvent polarity; these data indicate that electronic excitation augments benzothiadiazole electron density via an internal charge transfer mechanism. The BTD- and TDQ-containing structures exhibit blue-shifted two-photon absorption (TPA) spectra relative to their corresponding one-photon absorption (OPA) spectra, and display high TPA cross sections (>100 GM) within these spectral windows. D-A-D and A-D-A structures that feature more extensive conjugation within this series of compounds exhibit larger TPA cross sections consistent with computational simulation. Factors governing TPA properties of these quadrupolar chromophores are discussed within the context of a three-state model.

Localized excitation effect on dipole moments of oligophenylenevinylenes in their excited Franck–Condon state

The results from electrooptical absorption measurements (EOAM) on the ground and excited Franck–Condon state dipole moments of oligophenylenevinylenes in 1,4-dioxane and cyclohexane are presented. The dipole moments of quadrupolar dyes in the ground state are not equal zero. This means that one of the functional end groups of these molecules is not on the axis or on the plane with the central part of the molecule. Such a conclusion is confirmed by the quantum-chemical and semi-empirical recalculations. Owing to disorder in the ground-state geometry of the dyes the π–π conjugated system is located on mostly the plane part of the molecule and in the main this part absorbs the light. The localized excitation effect causes the large change of the dipole moments of quadrupolar molecules upon excitation to the Franck–Condon excited state. Due to an efficient intramolecular charge transfer the new oligophenylenevinylenes must have significant non-linear optical properties.

Aggregation-enhanced two-photon absorption and up-converted fluorescence of quadrupolar 1,4-bis(cyanostyryl)benzene derivatives showing solvatochromic fluorescence

This paper reports synthesis and semiempirical modeling of 1,4-bis(cyanostyryl)benzene (CSB)-based quadrupolar isomeric molecules (α- and β-CSB-TPs), designed to produce enhancement in fluorescence quantum yield and two-photon absorption cross-sections for the nanoaggregate form. Fluorescence yield together with high two-photon optical properties of the isomers have been fine-tuned by moving the cyano group from α to β position, which results in longer absorption-fluorescence wavelengths, and higher one- and two-photon absorptivities for the β-CSB-TP. In nonpolar toluene solution, both isomers exhibit strong one- and two-photon induced fluorescence. Both isomers followed a trend of strong solvatochromisms which was gradually on increasing solvent polarity. Aqueous dispersions of nanoparticles with diameters of ca. 150 nm have been prepared by aggregation of each isomer. The quenched fluorescence in polar media was greatly intensified followed by a 21 fold increase in TPA cross-sections. This helped achieve intense up-converted fluorescence by two-photon absorption of excited β-CSB-TP organic nanoparticles. Coaggregation-enhanced tunable fluorescence has also been demonstrated as a possible application of the isomeric α- and β-CSB-TP mixture.

DFT and MP2 calculations of the static dipole hyperpolarizabilities of DAAD Quadrupolar Molecules

We report a theoretical study of the electron correlation, basis sets and substituent effects on the average of the dipole polarizability and second hyperpolarizability, and , respectively, of a pair of DAAD quadrupolar isomers. The geometric optimization calculations were carried out at the HF, MP2 and DFT (B3LYP and BHHLYP) levels of theory using different basis sets. The results show that the electron correlation effects for the dipole polarizability are consistent between the MP2 and DFT levels, whereas for the second hyperpolarizability the DFT performance shown significant differences with respect to the corresponding MP2 results. This behavior does not change with the variation of the employed basis sets. Regarding to the substituent effects, the results show that electron donor groups in conjunction with extended basis sets increase the values of and at all levels of theory. In particular, -N(CH$_{3})_{2}$ and -OCH$_{3}$ groups are the substituents that lead to the highest values for these properties. We have also found that there exists a geometry dependency associated to the type of isomer on the values.

Excluded volume contribution to deviations from ideality in liquid mixtures: Consequences for engineering models

The contribution from volume exclusion to departures from ideality in binary liquid mixtures is assessed in the case of mixtures of noble gases ( Ar + Kr and Kr + Xe ), and of ‘simple’, i.e. symmetric-top, molecules ( chloromethane + fluoromethane : CH 3 Cl + CH 3 F and trichloromethane (chloroform) + acetonitrile: CHCl 3 + CH 3 CN ). In the first case, this is done by estimating the equivalent hard sphere diameter of the atoms. In the second case, a description of the liquid mixtures is developed within the Padé approximation applied to dipolar–quadrupolar molecules. Excess chemical potentials and activity coefficients of the species in the mixture are determined using data for vapor–liquid equilibria. It is found that volume exclusion constitutes a large contribution to deviations from ideality in the mixtures studied.

Conformational dependence of the second hyperpolarizability of quadrupolar molecules

Hatree–Fock calculations at ab initio and semiempirical levels were carried out for the averaged polarizability 〈 α〉 and second hyperpolarizability 〈 γ〉 of two pairs of quadrupolar isomers with different donor and acceptor groups. These properties were correlated with the antibonding/bonding π occupation number (π∗/π ratio). It was found that isomers with extended π systems had low π∗/π ratios and high 〈 α〉 and 〈 γ〉 values, while low 〈 α〉 and 〈 γ〉 values were obtained for isomers with large π∗/π ratios and no extended π system. The PM3 and PM6 〈 α〉 values were found to be in excellent agreement with the HF/6-31+G(d,p) ones. The PM3 values for 〈 γ〉 were significantly larger than those calculated by HF/6-31+G(d,p), with an average PM3/HF ratio of 1.43. The PM6 results were noticeably better with a ratio of 0.85. The calculation of 〈 α〉 and 〈 γ〉 at MP2/6-31+G(d,p) level for representative isomers showed that the contribution of the electron correlation to their values was small and that the HF/6-31+G(d,p) method provides reliable values at much lower computational cost.