Quadrupolar Compounds Research Articles

Symmetry‐Broken Intermolecular Charge Separation of Cationic Radicals

AbstractA quadrupolar compoundPyr‐BAwith two pyrrole‐type nitrogen atoms doped externally was prepared in this work. In high contrast with other π ionic radicals, its cationic radicalPyr‐BA⋅+undergoes unusual symmetry‐broken charge separation (SB‐CS), generating the mixed valence complex ofPyr‐BA+1−q⋅⋅⋅Pyr‐BA+1+q, whereqis the degree of charge transfer. Variable‐temperature (VT) single‐crystal analysis, absorption and EPR experiments all confirmed that aggregation and lower temperature would help to facilitate this SB‐CS process. Gibbs energy calculations and gauge‐including magnetically induced current simulation both validate that, forPyr‐BA⋅+, SB‐CS behavior is more favorable than the conventional dimerization mode. To the best of our knowledge, this is the first study that shows solid single‐crystal evidence for spontaneous SB‐CS between identical ionic radicals. Such a unique phenomenon is of great significance both in terms of fundamental aspects and uncharted material science.

Symmetry-Broken Intermolecular Charge Separation of Cationic Radicals.

A quadrupolar compound Pyr-BA with two pyrrole-type nitrogen atoms doped externally was prepared in this work. In high contrast with other π ionic radicals, its cationic radical Pyr-BA⋅+ undergoes unusual symmetry-broken charge separation (SB-CS), generating the mixed valence complex of Pyr-BA+1-q ⋅⋅⋅Pyr-BA+1+q , where q is the degree of charge transfer. Variable-temperature (VT) single-crystal analysis, absorption and EPR experiments all confirmed that aggregation and lower temperature would help to facilitate this SB-CS process. Gibbs energy calculations and gauge-including magnetically induced current simulation both validate that, for Pyr-BA⋅+ , SB-CS behavior is more favorable than the conventional dimerization mode. To the best of our knowledge, this is the first study that shows solid single-crystal evidence for spontaneous SB-CS between identical ionic radicals. Such a unique phenomenon is of great significance both in terms of fundamental aspects and uncharted material science.

Anisotropic Magnetic Field Dependence of Specific Heat in the Cubic Symmetric Quadrupolar Kondo Compound Pr0.05La0.95Pb3

We studied the dependence of the electronic specific heat Cel/T on the magnetic field up to H = 8 T in a dilute quadrupolar compound Pr0.05La0.95Pb3 with a non-Kramers Γ3 doublet in the crystal-ele...

Enhancement of Two-Photon Absorption Parallels Intramolecular Charge-Transfer Efficiency in Quadrupolar versus Dipolar Cationic Chromophores

State-of-the-art femtosecond spectroscopies and quantum-chemical methods were used to investigate the excited-state dynamics of D−π–A+ (C1) and D−π–A+–π–D (C2) methylpyridinium (acceptor unit, A) derivatives bearing dibutylamino groups as strong electron donors (D) and bithiophenes as highly effective π-rich spacers. The absorption spectra of C1 and C2 are broad and shifted to the red side of the visible spectral range. A significant negative solvatochromism was observed for the absorption bands of the investigated salts with increasing solvent polarity that was rationalized in terms of the change in electron density upon excitation. The absorption spectra of C2 are red-shifted with respect to those of C1, whereas the emission bands of the two compounds overlap, suggesting a localization of the excitation on just one branch of the quadrupolar compound, which becomes the fluorescent portion. This is in agreement with our quantum-mechanical calculations, which predict that the symmetry of C2 is broken in th...

Synthesis and Characterization of Novel 1,4-Bis(carbazolyl)benzene Derivatives with Blue-Violet Two-Photon-Excited Fluorescence

Novel fluorescent two-photon absorption molecules containing bis(carbazolyl)benzene as the central unit and phenylethynyl moieties as peripheral groups (Cz-Ps) are synthesized and characterized. In these molecules, two carbazolyl moieties are linked with benzene at the 9-position and synthesized by a concise process involving nucleophilic substitution between the cyclopentadienyliron complexes of dichloroarenes and phenylethynyl carbazole, followed by photolysis. The optimal structures of Cz-Ps reveal that two carbazolyl rings linked by a benzene ring are not planar. This feature prevents the electron conjugation of the molecule from extending throughout the whole molecule and allows Cz-Ps to realize blue-violet emissions and high fluorescence quantum yield. With increasing number of conjugated phenylacetylene structures in Cz-Ps, the maximal absorption and emission peaks were red-shifted. The quadrupolar compound DMoCz-P shows strong one-photon and two-photon activities. The resulting molecules are also ...

Electronic Excitations in Solution: The Interplay between State Specific Approaches and a Time-Dependent Density Functional Theory Description.

We critically analyze the performances of continuum solvation models when coupled to time-dependent density functional theory (TD-DFT) to predict solvent effects on both absorption and emission energies of chromophores in solution. Different polarization schemes of the polarizable continuum model (PCM), such as linear response (LR) and three different state specific (SS) approaches, are considered and compared. We show the necessity of introducing a SS model in cases where large electron density rearrangements are involved in the excitations, such as charge-transfer transitions in both twisted and quadrupolar compounds, and underline the very delicate interplay between the selected polarization method and the chosen exchange-correlation functional. This interplay originates in the different descriptions of the transition and ground/excited state multipolar moments by the different functionals. As a result, the choice of both the DFT functional and the solvent polarization scheme has to be consistent with the nature of the studied electronic excitation.

Simulating the capture of CO2 from natural gas: New data and improved models for methane + carbon dioxide + methanol

The simulation of carbon capture unit operations often involves predicting the vapor liquid equilibrium (VLE) for mixtures containing polar, non-polar and quadrupolar compounds. In this work, we investigate how well a simple cubic equation of state (EOS) can predict the results of new low temperature, high-pressure VLE measurements of the ternary methane+carbon dioxide+methanol system, which is important to the Rectisol process used for capturing CO2 from natural gas. The ternary p, T, x measurements presented here are the first such data for this system reported in the open literature. First, predictions made with the Peng Robinson (PR) EOS as implemented in a commercial process simulator were compared to binary p, T, x data measured in this work and also taken from the literature. Significant deviations were found between the measured liquid mole fractions and those predicted with the EOS using the default binary interaction parameters (BIP): the relative standard errors were 39%, 77% and 17% for the methane+methanol, methane+carbon dioxide and methanol+carbon dioxide binaries, respectively. Regression of the PR EOS to the binary VLE data by adjusting the BIPs improved the liquid phase mole fraction predictions for the ternary mixture data by a factor of about 2.5 for methane and methanol. However, improvement by a factor of 4.4 in the carbon dioxide liquid mole fraction was achieved by describing the carbon dioxide+methanol binary with an asymmetric composition and temperature dependent mixing rule and tuning the BIPs therein to VLE data for this binary over a wide temperature range. This reduced the standard error in the liquid phase CO2 mole fractions predicted for the ternary mixture using the optimized model by 79% relative to the default model.

Photoinduced symmetry-breaking intramolecular charge transfer in a quadrupolar pyridinium derivative.

We report here a joint experimental and theoretical study of a quadrupolar, two-branched pyridinium derivative of interest as a potential non-linear optical material. The spectral and photophysical behaviour of this symmetric system is greatly affected by the polarity of the medium. A very efficient photoinduced intramolecular charge transfer, surprisingly more efficient than in the dipolar asymmetric analogue, is found to occur by femtosecond resolved transient absorption spectroscopy. TD-DFT calculations are in excellent agreement with these experimental findings and predict large charge displacements in the molecular orbitals describing the ground state and the lowest excited singlet state. The theoretical study also revealed that in highly polar media the symmetry of the excited state is broken giving a possible explanation to the fluorescence and transient absorption spectra resembling those of the one-branched analogous compound in the same solvents. The present study may give an important insight into the excited state deactivation mechanism of cationic (donor-π-acceptor-π-donor)(+) quadrupolar compounds characterised by negative solvatochromism, which are expected to show significant two-photon absorption (TPA). Moreover, the water solubility of the investigated quadrupolar system may represent an added value in view of the most promising applications of TPA materials in biology and medicine.

Octupolar chimeric compounds built from quinoline caged acetate moieties: a novel approach for 2-photon uncaging of biomolecules

The present study describes the synthesis and investigation of chimeric structures where 6-quinoline and 8-quinoline caging units are integrated in multipolar systems to yield “hybrid” molecular structures for two-photon uncaging. These systems were demonstrated to exhibit strikingly enhanced (up to more than 2 orders of magnitude larger for octupolar derivatives) two-photon absorption responses in the NIR region compared to common caging groups. Whereas the quadrupolar compound shows the lowest two-photon uncaging cross-section (δu), octupolar chimeric derivatives display one-order larger δu values than their dipolar analogues. This opens a promising route for the design of efficient octupolar type derivatives for two-photon uncaging of biomolecules.

THEORETICAL STUDY OF ONE- AND TWO-PHOTON ABSORPTION PROPERTIES FOR THREE SERIES OF DIPHENYLAMINE AND DIFLUORENYLAMINE SUBSTITUTED CONJUGATED COMPOUNDS

The one-photon (OPA) and two-photon (TPA) absorption properties of three series of symmetrically substituted quadrupolar compounds with structure of donor-π bridge-donor (D–π–D) were investigated by time-dependent density functional theory (TDDFT) based on the Hartree–Fock (HF)-optimized geometrical structures. These compounds were constructed with either phenyl or fluorenyl groups connected by vinylene unit as the central π-conjugated bridges and either diphenylamine or difluorenylamine groups as terminal electron donors. The calculated OPA spectra are dominated by two strong transitions which are attributed to the charge transfers from the donor groups to central conjugated chains. The OPA and TPA transition strength all increase with the extension of conjugated chain length in each series and the corresponding wavelength shifts red in general. The transition strength in either OPA or TPA process also increases from series one to series two or three by replacing the phenyl groups with fluorenyl groups. The intramolecular charge transfers make significant contributions to the TPA activity. According to the three-state model, the enhanced TPA activity comes from the enhancement of transition moment between states–states as conjugated chain increases.

The synthesis and single and two-photon excited fluorescence of a new quasi-quadrupolar organoborane compound

A new acceptor-π-acceptor quadrupolar compound with a dimesitylboryl as acceptor and 2,7-dithienylfluorene as the conjugated bridge has been synthesized using the Suzuki–Miyaura coupling reaction. Its single and two-photon related photo-physical properties were experimentally examined. The combination of a large two-photon cross-section ( δ = 1150 GM at 730 nm in hexane), high emission quantum yield ( Ф = 0.81 in hexane) and a strong binding constant with fluoride anions ( K 1 = 3.0 × 10 5 mol −1 L) make this compound attractive for application as a two-photon excited fluorescent chemosensor for fluoride anions.

A density functional theory for vapor-liquid interfaces using the PCP-SAFT equation of state

A Helmholtz energy functional for inhomogeneous fluid phases based on the perturbed-chain polar statistical associating fluid theory (PCP-SAFT) equation of state is proposed. The model is supplemented with a capillary wave contribution to the surface tension to account for long-wavelength fluctuations of a vapor-liquid interface. The functional for the dispersive attraction is based on a nonlocal perturbation theory for chain fluids and the difference of the perturbation theory to the dispersion term of the PCP-SAFT equation of state is treated with a local density approximation. This approach suggested by Gloor et al. [Fluid Phase Equilib. 194, 521 (2002)] leads to full compatibility with the PCP-SAFT equation of state. Several levels of approximation are compared for the nonlocal functional of the dispersive attractions. A first-order non-mean-field description is found to be superior to a mean-field treatment, whereas the inclusion of a second-order perturbation term does not contribute significantly to the results. The proposed functional gives excellent results for the surface tension of nonpolar or only moderately polar fluids, such as alkanes, aromatic substances, ethers, and ethanoates. A local density approximation for the polar interactions is sufficient for carbon dioxide as a strongly quadrupolar compound. The surface tension of acetone, as an archetype dipolar fluid, is overestimated, suggesting that a nonisotropic orientational distribution function across an interface should for strong dipolar substances be accounted for.

Syntheses, structures, two-photon absorption cross-sections and computed second hyperpolarisabilities of quadrupolar A–π–A systems containing E-dimesitylborylethenyl acceptors

A series of bis(E-dimesitylborylethenyl)-substituted arenes, namely arene = 1,4-benzene, 1,4-tetrafluorobenzene, 2,5-thiophene, 1,4-naphthalene, 9,10-anthracene, 4,4′-biphenyl, 2,7-fluorene, 4,4′-E-stilbene, 4,4′-tolan, 5,5′-(2,2′-bithiophene), 1,4-bis(4-phenylethynyl)benzene, 1,4-bis(4-phenylethynyl)tetrafluorobenzene and 5,5″-(2,2′:5′,2″-terthiophene), have been synthesised viahydroboration of the corresponding diethynylarenes with dimesitylborane. Their absorption and emission maxima, fluorescence lifetimes and quantum yields are reported along with the two-photon absorption (TPA) spectra and TPA cross-sections for the 5,5′-bis(E-dimesitylborylethenyl)-2,2′-bithiophene and 5,5′-bis(E-dimesitylborylethenyl)-2,2′:5′,2″-terthiophene derivatives. The TPA cross-section of the latter compound of ca. 1800 GM is the largest yet reported for a 3-coordinate boron compound and is in the range of the largest values measured for quadrupolar compounds with similar conjugation lengths. The X-ray crystal structures of 1,4-benzene, 2,5-thiophene, 4,4′-biphenyl and 5,5″-(2,2′:5′,2″-terthiophene) derivatives indicate π-conjugation along the BCC–arene–CCB chain. Theoretical studies show that the second molecular hyperpolarisabilities, γ, in each series of compounds are generally related to the HOMO energy, which itself increases with increasing donor strength of the spacer. A strong enhancement of γ is predicted as the number of thiophene rings in the spacer increases.

Validation of the COSMO-RS electrostatics by Monte-Carlo simulations

The conductor-like screening model for real solvents (COSMO-RS) meanwhile is widely accepted as a novel and efficient tool for the predictive simulation of fluid phase thermodynamics based on quantum chemical calculations for the individual compounds. In COSMO-RS all molecular interactions are described as local interactions of surface segments and quantified by the local surface polarization charge densities which are known from the quantum chemical simulation of the compounds in a virtual conductor (COSMO). The statistical thermodynamics of the interacting segments is solved in COSMO-RS by an accurate and efficient self-consistency algorithm. While the treatment of hydrogen bonding and dispersive interactions as local surface interactions appears to be justified by the local nature of these interactions, the local description of the electrostatics in COSMO-RS is somewhat counter-intuitive, because the Coulomb electrostatics is longer ranged and non-local in its nature. This has been a frequent topic of criticism of the COSMO-RS theory. In order to check the validity of the local description of electrostatics we compare the results of MC simulations and COSMO-RS calculations for dipolar and quadrupolar compounds of the same shape and very similar σ-profiles. According to the COSMO-RS theory such two compounds should have very similar interactions in the liquid phase, which is a bit counterintuitive from the perspective of the conventional molecular simulation. For this purpose different simplified CO 2-shaped and spherical pseudo-molecules with a variety of different charge distributions have been consistently treated by COSMO-RS and by MC simulation applying a 2-center or 1-center Lennard–Jones model, respectively, in the MC-simulations. The comparison reveals that the results for the average electrostatic interaction energies and for the vapour pressures resulting from both simulation methods are in satisfying agreement. This indicates the validity of the local description of electrostatics and hence gives further confirmation of the fundaments of the COSMO-RS theory.

Possible observation of quadrupolar Kondo effect in Pr-based dilute quadrupolar compounds

We have studied the low-temperature properties of Pr x La 1 - x Pb 3 and Pr x La 1 - x lnAg 2 with the crystal-electric-field (CEF) ground state of a non-Kramers Γ 3 doublet. In Pr x La 1 - x Pb 3 , non-Fermi-liquid (NFL) behavior has been observed for Pr concentrations x ≤ 0.05 in the specific heat C / T , susceptibility and electrical resistivity at low temperatures. Moreover, application of magnetic field raises C / T rapidly, which demonstrates the presence of the residual entropy at lower temperatures. In Pr x La 1 - x lnAg 2 , characteristic features have been seen up to x=0.8 in the susceptibility and specific heat below 15 K. These behaviors can be caused by the quadrupolar Kondo effect arising from the correlation between the dilute Γ 3 moments and the conduction electrons.

Two-photon absorption in two-dimensional conjugated quadrupolar chromophores

We present ultrafast z-scan measurements of the two-photon absorption (TPA) spectra of a pair of two-dimensionally conjugated quadrupolar donor/acceptor (D/A) chromophores. The all-donor substituted species displays a peak TPA cross section [sigma(2)=520+/-30 GM] that is more than twice that of the D-A species [sigma(2)=240+/-20 GM]. Unlike previous structure-property studies that have evaluated TPA behavior for D/A molecules through the comparison of dipolar and quadrupolar compounds, both molecules investigated herein are quadrupolar, ultimately providing a more consistent evaluation of the effects of donor and/or acceptor substitution on the TPA of conjugated chromophores.

Theoretical studies on two-photon absorption properties of newly synthesized triaryl boron-based A-π-A and triaryl nitrogen-based D-π-D quadrupolar compounds

The nonlinear optical properties of newly synthesized triaryl boron-based A-π-A and triaryl nitrogen-based D-π-D compounds have been studied with hybrid density functional theory. The generalized few-state model is employed to calculate two-photon absorption cross sections of the compounds. It is found that boron-based A-π-A compounds have large two-photon absorption cross sections. Moreover, the length of π center has larger effect on the one- and two-photon absorption properties of boron-based A-π-A compounds than those of nitrogen-based D-π-D compounds. The numerical simulation shows that boron-based A-π-A compounds have similar performance as nitrogen-based D-π-D analogues in regard to nonlinear optical properties. Boron-based A-π-A compounds are promising candidates for large two-photon absorption materials. The theoretical results are in good agreement with the available experimental measurements.

Triaryl boron-based A-pi-A vs triaryl nitrogen-based D-pi-D quadrupolar compounds for single- and two-photon excited fluorescence.

Three new A-pi-A-type compounds with trivalent boron, protected by two mesityl groups, as electron acceptor have been synthesized and investigated together with their two diphenylamino-ended D-pi-D analogues. These boranes exhibit large two-photon absorption cross sections and high fluorescence quantum yields.

The influence of sigma and pi acceptors on two-photon absorption and solvatochromism of dipolar and quadrupolar unsaturated organic compounds.

Two-photon absorption cross sections delta and solvatochromic properties were determined for a series of quadrupolar and dipolar compounds by using femtosecond excitation in the spectral range between 710 and 960 nm. The compounds investigated were distyrylbenzenes and polyenes bearing appropriate pi or sigma acceptors. The delta values for the centrosymmetric compounds trans, trans- 1,4-bis[2-(2',5'-dihexyloxy)phenylethenyl]-2,3,5,6-tetrafluorobenzene (6), trans, trans-1,4-bis[2-(4'-dibutylamino)phenylethenyl]- 2,3,5,6-tetrafluorobenzene (2), trans, trans-1,4-bis[2-(4'dimethylamino)phenylbutadienyl]- 2,3,5,6-tetrafluorobenzene (7), trans,-trans-1,4-bis[2-(4'-dimethylamino)phenylethenyl]2,5- dicyanobenzene (4) and trans,trans-1,4-bis[2-(4'-dimethylamino)phenylethenyl]-2- propylsulfonyl-5-(2-ethylhexyl)sulfonylbenzene (3) are on the order of 600, 1400, 1700, 3000, and 4100 x 10(-50) cm4 s photon-1, respectively. The corresponding dipolar compounds trans-2-(4'- dimethylaminophenyl)ethenyl-2,3,4,5,6-pentafluorobene (8), trans-4-(4'-dimethylaminophenyl)butadienyl-2,3,4,5,6-pentafluorobenzene (9), trans-6-(4'-dimethylaminophenyl)hexatrienyl-2,3,4,5,6- pentafluorobenzene (10) were additionally investigated. All centrosymmtric compounds are good fluorescent materials, while the dipolar chromophores 8-10 exhibit low fluorescence quantum yields. Solvatochromism was also observed for the fluorophores 2-10 as a result of intramolecular charge transfer (ICT). Furthermore, a reasonable correlation was obtained between measured and calculated delta. Quantum chemical calculations were performed by using the INDO Hamiltonian with a MRDCI scheme. The results show that the sum over states (SOS) expression for the second hyperpolarizability gamma is appropriate to describe the mechanism of two-photon absorption. Mechanistic investigations of quadrupolar compounds showed that the energy of the two-photon excited state is higher than S1.

Magnetic structures in the quadrupolar compound TmAu2

The occurrence of a quadrupolar ordering within the γ orthorhombic symmetry at 7 K in TmAu2 (MoSi2-type tetragonal structure) was recently discovered. The existence of an antiferromagnetic ordering around 3 K in the quadrupolar phase leads to complex magnetic phase diagrams in the (H,T) plane. From neutron diffraction studies on a single crystal, a magnetic structure is determined for each of the five phases observed for a magnetic field applied along the twofold orthorhombic axis. The complexity of the phase diagram can be understood as the result of a conflict, under field, between quadrupoles and antiferromagnetic moments, associated with a rather degenerated maximum for the J(Q) Fourier transform of the exchange couplings. Then classical arrangements perpendicular to the field and Ising-type behaviours are observed; in the latter case, the incommensurate nature of the magnetic interactions leads to behaviours of devil's staircase type.